UNCLASSIFIED//FOR OFFICIAL UDE ONL¥ Defense Intelligence R eference Document j^^^^^^JJ Acquisition Threat Support 6 April 2010 ICO D: 1 Decem ber 2009 DIA-08-1004-006 Metamaterials for Aerospace Applications UNCLASSIFIED//FOR OFFICIAL USE OHL¥ U N C LA S SIFIE D / ZfOLOEHaAMfi^dfttY Metamaterials for Aerospace Applications Prepared by: Acquisition Support Division (DW O-3) Defense W arning Office Directorate for Analysis Defense Intelligence Agency Author: AAP Person 78 Administrative Note COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized. This product is one in a series of advanced technology reports produced in FY 2009 under the Defense Intelligence Agency, Defense W arning O ffice's Advanced Aerospace W eapon System Applications ( AAW SA) Program . Com m ents or questions pertaining to this docum ent should be addressed to|aap Person 1 | AAW SA Program Manager, Defense Intelligence Agency, ATTN: CLAR/DW O -3, Bldg 6 000, W ashington, DC 20340-5100. ii UNCLASSIFIED / /FOP .................. RIIH ^ UNCLASSIFIED/ /FOP QEH6JA I Mfr nm^ Contents Definition of Metamaterials............................................. 1 Applications to Sub-Diffraction Imaging: Super-Lens and Hyper-Lens...................6 Applications to Circuits and W aveguide Miniaturization: Slowing Down and Manipulating Electromagnetic Pulses (EMP) Using Advanced Metamaterials......16 Metamaterials for Energy Harvesting.................................... 20 Nonlinear Non-R eciprocal Chiral Metamaterials: For Developing Novel Optical Isolators and "One-W ay" Microwave Mirrors ................................................... 27 Tunable Switchable Metamaterials.................................. 30 Summary and Conclusions................................................................................ 31 R eferences.................................................................................. 31 Figures Figure 1. Example of a Metamaterial Component: The Magnetic Split R ing R esonator (SR R ) Design.................................. 2 Figure 2. Example of Another Metamaterial Component: Electric R ing R esonator (ER R )............................................................................................2 Figure 3. G eometry of Original Planar Metamaterial Unit Cells (OE1-OE6) and Their Complements (CE1-CE6)............................................................................3 Figure 4. R ecent Optical Metamaterials for Telecommunication W avelength and Mid-Infrared Indefinite Permittivity Material . 5 Figure 5. Schematic of The Super-lens W ith n=-l R efractive Index Corresponding to ( Surrounded by Vacuum................................... 7 Figure 6. Schematic of the SiC-based Super-lens W hich is Imaging Sub-wavelength Holes Buried Under the SiO2 Layer................................... 8 Figure 7. Theoretical Concepts (left panel) and Experimental Implementation (right panel) of an Optical Hyperlens Capable of Magnifying Sub­ Diffraction Objects to Observable (larger than Size.................9 Figure 8. Hyperlens Based on a Converging Array of Metal W ires.................... 10 Figure 9. Far-Field Super-lens (FSL) Based on an Indefinite Permittivity Metamaterial Placed Between the Object and the Image-R eleasing G rating..................................................... 12 Figure 10. Tomographic Multi-Beam Multi-Detector Holography of Sub-W avelength Objects Using Indefinite Permittivity Medium (IPM).............................12 Figure 11. First Experimental Demonstration of Propagating Sub-Diffraction W aves in the Indefinite Permittivity Medium (IPM).......................13 Figure 12. Schematic for 2-Beams/2-Detectors Interferometric Measurement and Numerical Simulation........... 14 Figure 13. Experimental Setup for 2-Beams/2-Detectors Interferometric Measurement in the Lab and Preliminary Experimental R esults...........15 iii U N CLASSI FI ED/^EOUmCMMMMNtr""" U NCLASSI FI ED/ / FOK OFFICIAL UDE ONLY Figure 14. Schematic of Pulse Compression in Magnetized Plasma.............. 16 Figure 15. Trapped R ainbow: A W aveguide W ith Negative Index Core Can Stop Light.......................................................................................17 Figure 16. "Plasmonic Molecule" Exhibiting EIT......................................................18 Figure 17. True Multi-Layer Metamaterial W ith a Unit Cell Shown in Figurel6: R adiative Antenna (Single Metal Strip) Coupled to a Dark Antenna (Two Perpendicular Metal Bars)...........................................19 Figure 18. "Perfect" Narrow-Band Microwave Absorber.........................................20 Figure 19. W ide-Angle Plasmonic Absorber Based on Negative Index Metamaterial..........................................................................21 Figure 20 . Specific Design of a W ide-Angle Plasmonic Absorber Based on Negative Index Metamaterial Operating at A=1550 nm ................ 22 Figure 21. Experimental R eflectivity vs. W avelength and Theoretical Plot of R eflectivity Contours........................... 23 Figure 22. Preliminary Attempts to Design a Better Absorber Using Complementary MetaMaterials (U-shaped C-MM)............................................................25 Figure 23. Engineering the Complex R eflectivity Coefficient Using the Concept of a MetaMirror.............................................................................26 Figure 24. Example of a G eneric Chiral Metamaterial............................ 28 Figure 25. Example of Time-Irreversibility of Light Propagation Inside the Twisted Fiber Core................................................................. 29 Figure 26. THz Properties of an Electric Split R ing R esonator................................31 IJNCI A g<^TFn/J'FPP ^r^^*' UJL - ULI "" iv UNCLASSIFIED/ /FOP OFFTCT"' nrr n»i »f Definition of Metamaterials A m etam aterial is defined as an artificial m edium whose properties ( m echanical, optical, m agnetic, or other) cannot be found in naturally-occurring m aterials. The em phasis of this study will be on electrom agnetic and optical m etam aterials. Such m etam aterials can exhibit rather extrem e properties, such as negative refractive index, which im plies that both electric perm ittivity and m agnetic perm eability m ust be negative ( f<( ) //<0) ( Reference 1). Such m etam aterials used to be called "left-handed" because of the unusual phase relationship between the electric and m agnetic fields. Specifically, in m ost ( positive index, including vacuum ) m edia one uses the right-hand rule to define the relationship between electric field ( ^) m agnetic field ( A), and the propagation wavenum ber (k ). The physical basis of the right-hand rule is that the direction of energy propagation defined by the Poynting vector ^ - c.^ x A / 4^ and the direction of the phase velocity ( defined by the wavenum ber ^) m ust coincide. That does not hold true for negative index m etam aterials where the two directions are opposite, therefore, the left-handed relationship m ust hold for the three vectors. Nevertheless, the "left-handed" designation did not withstand the test of tim e because it was causing confusion and creating irrelevant allusions to helical ( a.k.a. chiral) structures. Although chiral structures can indeed exhibit negative index behavior ( Reference 2), chirality is not necessary. A typical m etam aterial consists of resonant elem ents such as Split Ring Resonators ( SRR). An exam ple of an SRR is shown in Figure 1. The m ain function of the SRR is to enable strong m agnetic response of the structure. A sim ple em pirical form ula exists for the m agnetic perm eability of a m etam aterial com prised of the SRRs: „=i-A <». ® " - ® M (i) where a>M is the resonant frequency of the SRR, and F is proportional to the volum e filling factor of SRRs. It is noteworthy that SRRs are designed in such a way that it has a large capacitance. As the result, the resonant frequency of an SRR is sm all, ( that is, the SRR-containing cell is very sub-wavelength). In the exam ple shown in Figure 1 ( taken from Reference 6 ), the unit cell operated at ) = 1 - 2 2 ■ fy -coR + icoy ( 2) where a^is the resonant frequency and / is the loss coefficient. Negative index m etam aterials are by no m eans the only potentially useful m etam edia. Several new concepts such as Indefinite Perm ittivity Metam aterials ( IPM) ( References 3, 4) and Epsilon-Near-Zero ( ENZ) m etam aterials ( Reference 5) have recently em erged and found som e exciting applications that will be reviewed below. IPMs can be used as ultra-com pact spatial filters ( both high-pass and low-pass) whereas ENR m etam aterials can be used for m aking sub-wavelength waveguides capable of coupling close to 100 percent of the incident radiation ( Reference 8), as well as directing it around tight bends with negligible bending losses. Yet another class of planar m etam aterials, com plem entary m etam aterials ( CMMs), has recently em erged ( Reference 7). Instead of using m etallic structures deposited on a substrate ( left panel of Figure 3), CMMs consist of slits in the continuous m etal screen ( right panel of Figure 3). The shape of the slits coincides with that of the m aterials them selves. Such com plem entary m etam aterials have been recently used for m aking epsilon-near zero waveguides ( Reference 8). Figure 3. G eometry of Original Planar Metamaterial Unit Cells (OE1-OE6) and Their Complements (CE1-CE6). The polarization of norm ally incident electrom agnetic radiation is configured as shown in O E1 and CE1 for the original and com plem entary m etam aterials, respectively. ( Reference 9) 3 U N CLASSI FI ED/^EOUHaCMMSMNET""" U N CLASSI FI ED/ZEfiaJJEHOAW SW fttr In general, m etam aterials offer a new way of designing electrom agnetic structures with arbitrary values of perm ittivity/perm eability tensors, as well as other param eters ( such as bi-anisotropy coefficient). In m any instances, m etam aterials enable us to considerably m inim ize sizes of resonators, transm ission lines, and so forth. Such m iniaturization is possible due to the resonant nature of the individual unit cells. Specifically, the structures shown in Figure 3 have high capacitance; therefore, their individual sizes are very sub-wavelength. That enables arrangem ent within sub­ wavelength units that can be densely packed and result in strongly m iniaturized com ponents. It is this m iniaturization that m akes m etam aterials interesting for aerospace application where sm all weight and size are essential. W hile the m ost spectacular progress in the field of electrom agnetic m etam aterials has so far occurred in the m icrowave range, it is the optical ( visible, infrared, m id-infrared) spectral regions that hold m ost prom ise for revolutionary applications. Electrom agnetic m etam aterials have a trem endous potential for revolutionizing propagation, storage, and conversion of electrom agnetic waves across the entire Electrom agnetic Spectrum . In our opinion, the m ost exciting applications that are relevant for aerospace applications include energy harvesting, developing novel optical devices with unusual yet practically im portant capabilities ( for exam ple, non-reciprocal devices), enhancing the efficiency of nonlinear optical devices, developing novel im aging m odalities capable of breaking the diffraction lim it ( for exam ple, super-lenses, hyper-lenses, far field super-lenses), and developing novel lithographic techniques. O ptical m etam aterials are still a very new area. Just a handful of experim ental dem onstrations of m ulti-layer ( truly bulk) optical m etam aterials exist at the m om ent. Am ong the m ost recent ones are ( a) dem onstration of the negative index optical m etam aterial at the telecom m unications wavelength ( Reference 10) that used the so- called fishnet structure shaped as a prism for dem onstrating Snell's Law, and ( b) dem onstration of the Indefinite Perm ittivity Material ( IPM) and negative refraction ( which, in the context of anisotropic m etam aterials, is not the sam e as negative refractive index) in the m id-infrared part of the spectrum ( Reference 11). These structures have the distinction of being m ulti-layer ( or bulk). Most previous exam ples of optical m etam aterials have dealt with single or double-layer substances which cannot be, strictly speaking, characterized as m etam aterials. The difficulty in obtaining strong m agnetic activity in optical m etam aterials has been explained in several recent reviews ( References 12, 13). In a nutshell, the issue is that the m agnetic m om ent of m ost structures ( including atom ic system s) is very sm all, m uch sm aller than the electric m om ent. Therefore, it is difficult to observe any optical effects that can be clearly assigned to m agnetic activity. This is especially true for the structures that are m uch sm aller than one wavelength. Exceptions, such as artificially constructed split rings, are possible. H owever, such structures cannot be operated at very high frequency because of the excitation of electrostatic resonances ( Reference 12). In other words, when the resonant frequency is too high ( or the dielectric perm ittivity of a m etal is not sufficiently large), electrostatic resonances disrupt m agnetic activity. More specifically, the energy inside and in the vicinity of a m etam aterial elem ent ( for exam ple, Split Ring Resonator) becom es predom inantly electrostatic, ( that is, in the form of the kinetic energy of oscillating electrons). The recently described m ulti-layer fishnet ( Reference 10) is not an exception: its unit cell ( that is, the lateral period) is only one-half of the operating wavelength. 4 U N C LASSI FI ED/2EQUK *G MMfSMflEr"""" UNCLASSIFIED/ /FOP OFFTCT"' nrr n»i »f Figure 4. R ecent Optical Metamaterials for Telecommunication W avelength A = 1.5 pm (Left and Middle) and Mid-Infrared IPM. The m ulti-layer fishnet is m ade of silver film s separated by a dielectric spacer. A focused ion beam was used to produce the prism -shaped fishnet. The IPM was obtained by depositing interleaved 80 nm layers of Ino.53G ao.47As and Alo.4sIno.52As. The layers, approxim ately 8.1 pm thick, grown by m olecular beam epitaxy on lattice-m atched InP substrates. The InG aAs layers were uniform ly doped to create different values of perm ittivity in alternating layers. ( Reference 10 and 11) That is not to say that there is not ongoing theoretical and experim ental work on designing optical m etam aterials for practical applications. The author's research group at UT- Austin, has designed the first Plasm onic Negative Index Metam aterials ( P-NIM) super-lens ( Reference 14), developed novel techniques for analyzing optical properties of plasm onic nanostructures, ( including band-structure calculations of periodic nanostructures) ( Reference 15) and quasi-static calculations of plasm onic resonances ( Reference 16 ). The UT-Austin group has also designed a num ber of unique sub­ wavelength P-NIMs in the optical part of the spectrum ( References 14, 17, 18), and has recently published a review of optical P-NIMs ( Reference 12). The group has also contributed to developing and experim entally im plem enting the concept of the "perfect lens" ( Reference 19) based on plasm onic/polaritonic m aterials. A perfect lens enables im aging of sub-wavelength objects in the infrared part of the spectrum , including objects buried under the surface. Also developed is a W ide-Angle "Perfect" Absorber of Mid-Infrared Radiation ( W APAMIR) ( Reference 20) based on the negative index m etam aterial whose im pedance is perfectly m atched to vacuum . Below is a concise sum m ary of various topics/applications that are especially suitable for the aerospace industry. This study concentrates on the facility of m etam aterials to m iniaturize various optical and m icrowave com ponents. Metam aterials can also be used for im aging very sm all ( sub-wavelength) objects without resorting to costly and space­ consum ing near-field scanning optical m icroscopy. Also described are the ongoing efforts in the field to m ake extrem ely com pact m etam aterials-based lasers. Sm aller lasers m ean sm aller weight and m ore room for other diagnostic devices and useful payload within the confines of a space vehicle. Applications of m etam aterials to photon harvesting is especially fitting for advanced aerospace platform s because of the necessity to collect electrom agnetic energy for battery recharging, diagnostic spectroscopy, and other vital functions of a space vehicle. • Complementary Metamaterials for Energy Harvesting. Developm ent of ultra­ thin photovoltaic and therm o-photovoltaic cells is ham pered by weak photon absorption in sem iconductors. Metam aterials can m odify absorption m aking it wavelength-selective ( tunable), highly efficient, and, if desired, wide-angle. Recently a way has been found for creating quarter-wavelength resonators backed by leaky m irrors m ade out of CMMs. 5 U N C LASSI FI ED/^EQUmCMMMMNET""" UNCLASSI FIE D77FOR OFFICIAL USE ONLY • Far Field Super-Lens Based on the Interferometry of Sub-Diffraction W aves. Sub-diffraction im aging has long been considered to be possible only using near­ field m icroscopes. Those are fairly com plex, slow-scanning, and large devices that are not appropriate for advanced aerospace platform s. Metam aterials enable new im aging m odalities: super-lenses, hyper-lenses, and far-field super-lenses. In addition to a survey of the existing scientific literature, novel ideas on developing a new interferom etric Far-field Super-Lens ( FSL) based on the m ulti-beam m ulti­ detector technique utilizing m aterials with Indefinite Perm itivity Tensor are presented. Fabrication of such Indefinite Perm ittivity Materials ( IPMs) for the m id­ infrared part of the spectrum is achieved and dem onstrates the capabilities of transm itting electrom agnetic waves with the spatial period m uch sm aller than the vacuum wavelength of light. Interference between sub-diffraction waves enables disentangling m ultiple diffractive orders and extracting their am plitudes. • Nonlinear Non-R eciprocal Chiral Metamaterials: Developing Novel Optical Isolators and "One-W ay" Microwave Mirrors. These developm ents are m otivated by the need to construct one-way "light diodes" for com pact optical isolators. Presently there are two approaches to optical isolation: the m ost com m on using m agnetic fields, and the less developed based on using nonlinearities. A different approach relies on the phenom enon of adiabatic m ode conversion in nonlinear chiral m etam aterials. Prelim inary theoretical results for a sim ple chiral fiber with a variable twist period ( pitch) that enables full transm ission of a tightly confined core m ode in the forward direction and full m ode-conversion of the core m ode into a cladding m ode for the backwards propagation is obtained. • Slowing Down Light and Miniaturizing Optical Components Using the Phenomenon of Electromagnetically Induced Transparency in Metamaterials. The speed of light places a natural lim it on the size of optical/m icrowave com ponents. Metam aterials offer an exciting opportunity to slow down light. This has two m ajor im plications: ( a) light can be stored/m anipulated in sm aller volum es, and ( b) nonlinear effects are strongly enhanced by the resulting energy com pression. Applications to Sub-Diffraction Imaging: Super-Lens and Hyper-Lens The super-lens is one of the earliest applications of m etam aterials ( Reference 21), and its principle is shown is Figure 5. W ithout the super-lens, all inform ation about sub­ diffraction ( or sub-wavelengths, which is equivalent) features of the periodic object would have been lost. The reason for the inform ation loss is evanescent decay of the large spatial wavenum bers. The only m ethod of accessing/m easuring these features would be to scan the object using a near field scanning optical m icroscope. By inserting a super-lens between the object and the im aging plane, evanescent waves m ay be am plified and the im age transferred forward. Unfortunately, this approach by itself does not rem ove the need for a near-field scanning device; the im age that is recreated in the im aging plane is still sub-wavelength, and needs to be read out. 6 UNC LASSI FI ^//FO0'^^"lL UjL UI1LI ~ UNCLASSIFIED / ZEfi&A EEICMMfiMNtr Figure 5. Schematic of the Super-Lens with n=-l R efractive Index Corresponding to (e = -1, p = -1) Surrounded By Vacuum. Super-lens' presence enables im aging sub-diffraction objects such as the periodic grating shown here. There are, however, interesting circum stances when it is very im portant to transport the im age towards the scanning device. O ne such special circum stance is spatially- resolved spectroscopy of sm all ( for exam ple, cellular) structures. O ne can envision space expeditions to other planets that could, potentially, result in finding som e evidence of prim itive cellular-level life. It would then be highly desirable to exam ine the structure of the living cell in its natural environm ent. In all likelihood, that environm ent would be liquid. Therefore, it would be very desirable to exam ine the cell without actually touching it with a tip of a near-field optical m icroscope. Thus, the sub-surface im aging of a sm all object which is buried underneath a liquid layer would be necessary. No such experim ents have so far been conducted. H owever, several years ago there was an experim ent dem onstrating im aging of sub-diffraction objects buried under the layer of silicon dioxide. The schem atic and experim ental results from the experim ent ( Reference 19) are shown in Figure 6 . In this experim ent the sub-wavelength objects were sim ple holes that were m illed in the m etal using an FIB. They were buried underneath the super-lens consisting of SiC ( negative epsilon m aterial for m id-infrared frequencies) and silicon dioxide ( positive epsilon m aterial). Note that this configuration ( m aterials with ^ >0 and t2 ~-fj <0 joined together: sandwiched or positioned next to each other) is typical for a near-field super-lens. The difference between the near-field super-lens shown in Figure 6 and the "ideal" super-lens shown in Figure 5 is that the ideal also requires a m aterial with a negative value of m agnetic perm eability. 7 U N C LASSI FI ^//FO0 ^^^"lL U_iL UI1LI ~ UNCLASSIFIED / ZEfi&AEEICMMfiMNtr Figure 6. Schematic of the SiC-Based Super-Lens W hich is Imaging Sub-W avelength Holes Buried Under the SiOz Layer. The im aged objects are A/20 holes m illed in gold using FIB. The scattered signal is picked up by the tip of an NSO M and directed towards the IR detector. Depending on the im aging wavelength, either am plitude ( e) or the phase ( f,g) of the signal are prom inent. As the laser beam scatters off the sub-wavelength holes, its electric field is picked up by the tip of a Near-Field Scattering O ptical Microscope ( NSO M) and re-scattered into the far-field. There it is interfered with the reference pulse and picked up by an infrared detector. Note that this interferom etric technique enables one to extract both the phase and am plitude of the field, as shown in Figure 6 . This significantly broadens the spectral range over which the super-lens yields m eaningful inform ation. For exam ple, the am plitude contrast is highest at A = 10.85 pm shown in panel ( e) while the phase contrasts are the highest at A = 11.03 pm and A = 10.6 5 pm . 8 U N C LASSI FI ED/XBOEEK W W W MfrtF- UNCLASSI FI ED//FOA OFFICIAL USE ONLY Despite the convenience of the near-field super-lens, ( that is, its ability to transport the im age) it still requires an NSO M to read out the im age. W ithin the confines of an advanced aerospace platform such device ( with its necessary auxiliaries) m ay not fit. Therefore, one has to consider alternative m etam aterials-based ideas for sub-diffraction im aging. O ne such idea, the hyper-lens, has been proposed recently by two groups ( References 22-24), and already experim entally tested by another group ( Reference 25). The principle of the hyper-lens is very sim ple: to use an indefinite perm ittivity m edium ( som etim es referred to as the hyperbolic m edium because the relationship between the propagation wavenum bers and the frequency, also known as the constant frequency contour, has a hyperbolic nature) in a tapered form at. Several conceptual im plem entations such as the spoke-like structure and the cylindrical m ulti-layer structure ( see Figure 7, left panel) have been suggested. The hyper-lens works on two principles: ( a) indefinite perm ittivity m aterials ( of which the super-lens is one exam ple) are capable of propagating sub-diffraction waves, and ( b) the expanding nature of the hyper-lens can m agnify im ages to the A/2 size, at which point they becom e observable in a conventional m icroscope. O ne recent experim ental im plem entation of the hyper­ lens in UV is shown in the right panel of Figure 7. The hyper-lens is m ade of 16 layers of Ag/AhO s. This specific hyper-lens was used for im aging a line pair object with line width of 35 nm and spacing of 150 nm and was operated at A = 400 nm . The m agnified im age ( 350 nm spacing) can be clearly resolved with an optical m icroscope [num erical aperture ( NA) = 1.4], thus dem onstrating m agnification and projection of a sub­ diffraction-lim ited im age into the far field. Jacob, Alekseyev, Narimanov, Opt.Exp.'0 6 Salandrino & Engheta, PR B'0 6 G ovyadinov & Podolsky, PR B'0 6 Liuet.al, Science'0 7 Figure 7. Theoretical Concepts (left panel) and Experimental Implementation (right panel) of an Optical Hyper-lens Capable of Magnifying Sub-Diffraction Objects to Observable (larger than A/2) Size 9 U N CLASSI FI ED/^EQ&HEElG MMSe'efrtr"""" UNCLASSIFIED / ZEfi&A EEIfiMMMMNtr Note that the hyper-lens concept does not require em ploying a bulky near-field scanning optical m icroscope. H owever, the practical im plem entation of the hyper-lens is by no m eans sim ple. The original im plem entation required depositing the sam ple on the curved surface of the hyper-lens. A m ore practical im plem entation of the super-lens has been theoretically proposed by another group ( Reference 26 ). The concept is shown in Figure 8. The hyper-lens involves an array of thin m etallic wires converging towards the tip. As is dem onstrated, a dense array of m etal wires separated by m uch less than the wavelength constitutes a m etam aterial with the indefinite perm ittivity tensor. Specifically, the tensor com ponent along the wires is given by: fa = 1 - a>2 Kar - k2c2) and i- = 1, ( 3) where the z com ponent is along the wires and perpendicular direction is norm al to the wires. Because the only propagating waves are the TEM waves satisfying the ar = & ?c2dispersion relation, this m eta-m edium is strongly anisotropic and supports sub-wavelength waves which perform im aging. The spatial resolution is given by the spacing between wires. Figure 8 ( right panel) shows the m agnified im age of a sm all ( A/25) object placed at the tip of the hyper-lens. The m agnification factor is 5x. 30 .. Figure 8. Hyper-Lens Based on a Converging Array of Metal W ires. A sm all object can be placed at the tip, illum inated from the top, and m agnified by the expanding array of wires. Left panel: schem atic. Right panel: A/25 object m agnified by a factor 5x by the expanding hyper-lens. This hyper-lens can operate at m id-IR frequencies. ( Reference 26) Another concept for sub-wavelength im aging em ploying m etam aterials is the so-called Far-Field Super-Lens ( FSL). The concept is pioneered in Reference 27. The idea is 10 U N CLASSI FI ED/^EOUHaCMMSMNET""" UNCLASSIFIED / ZEfi&AEEIfiMMMMNtr described in Figure 9. A sub-wavelength object ( for exam ple, two slits) is located at the bottom of a m ulti-layer super-lens. Another sub-wavelength grating is deposited on top of the super-lens. Because the super-lens ( and forthat m atter, any indefinite perm ittivity m aterial) is capable of propagating sub-diffraction waves, the electrom agnetic perturbations created by the object are propagated through the super­ lens upwards, until they encounter the sub-wavelength grating. At that point these sub­ wavelength perturbations are diffracted on the im age-releasing grating and converted into the far-field electrom agnetic waves. Those far-field waves are collected by the objective of a m icroscope and observed through the eyepiece. The schem atic is shown in Figure 9( a). Note that, again, there is no need for NSO M. The actual im plem entation of the FSL used the following object: a nanowire pair with 50 nm wide slit and 70 nm gap inscribed by focused ion beam on a 40 nm thick Cr film on the quartz substrate. Diffraction-lim ited im age from a conventional optical m icroscope cannot resolve the two nanowires ( NA = 1.4, Ao = 377 nm ) as can be seen in Figure 9( c), but the FSL-equipped m icroscope can as shown in Figure 9( d). Despite the success of this dem onstration, there are serious issues involved in im aging sub-wavelength objects. Specifically it is pointed out in Reference 27 that m ultiple diffractive orders can becom e entangled, ( that is, launched in the sam e direction into the far field). Disentangling these diffraction orders is very im portant. The payoff would be im aging of fully 2-D ( flat) objects with a resolution sm aller than the period of the im age-releasing grating. More precisely, this am biguity is illustrated by the right panel in Figure 10. If the sub-wavelength object is represented by the continuous spectrum ( blue line), then the spectrum can be sam pled within the discrete set of "zones" which are defined by the diffractive orders of the im age-releasing grating. The width of each zone is 2a>/c, and they are labeled as 1st order, 2nd order, and so forth. W ave num bers belonging to the different zones can be diffracted onto the sam e far-field detector as explained in Figure 10. In order to disentangle the 1st and the 2nd zones, a single detector cannot provide sufficient inform ation. It turns out that using two detectors ( A and B) and two laser beam s ( Beam A and Beam B) provides additional inform ation that is sufficient to disentangle the two zones. This additional inform ation is obtained by com paring the intensity on the two detectors A and B. Another advantage of this im aging technique is that it is interferom etric in nature. Therefore, even if the contribution of som e of these spectral zones' orders is very weak, it can still be detected because of the high sensitivity of the interferom etric techniques. W hat m akes this interference special is that it involves sub-diffractive waves propagating through the indefinite perm ittivity m etam aterial. Below som e of the experim ents conducted in the laboratory that dem onstrate such interference are discussed. 11 U N C LASSI FI ED/XBOEEK W W W MfrtF- UNCLASSI FIED//EQB OFFICIAL USE ONL* Figure 9. F5L Based on an Indefinite Permittivity Metamaterial Placed Between the Object and the Image-R eleasing G rating. The grating releases into the far field sub-diffraction waves produced by light scattering off the object. The role of the m etam aterial is to propagate sub-diffraction waves from object to grating. ( Reference 27) Ever since Merlin's invention ( Reference 28) of the sub-diffraction near-field plate, it has becom e clear that the interference between sub-diffraction electrom agnetic fields can result in the form ation of a deeply sub-wavelength im age. The near-field plate, however, is not an im aging device; its purpose is to create a well-defined im age using an elaborate pre-fabricated sub-wavelength structure on the plate's surface. The goal for this study is to observe an a priori unknown sub-wavelength im age using a near­ field structure. In the past, successes ( Reference 19) in retrieving im ages of sub­ wavelength objects ( such as A/20 holes) using an NSO M for radiation detection are achieved. An NSO M is a near-field instrum ent, therefore, a m uch m ore desirable detection m ethod would involve far-field detection. To advance this goal, and to develop a tool som etim es referred to as the FSL, we've initiated research on m ulti­ beam m ulti-detector sub-wavelength holography illustrated in Figure 10. Figure 10 . Tomographic Multi-Beam Multi-Detector Holography of Sub-W avelength Objects Using Indefinite Permittivity Medium (IPM). Incident beam ( s) scatter off the sub-A object, propagate through the IPM, and then get re-scattered into the far field by the grating with the period D. The purpose of the m ulti-detector arrangem ent is to disentangle the ki and kz spatial wave num bers in the object's spectrum ( shown in the left panel). Beam s A and B are phase-shifted with respect to each other. 12 U N C LASSI FI ED/2E0 UK *G MMMMNEr"""" U N CLASSI FI ED/ZEfiaJJEHOAW SW fttr O ur im plem entation of the FSL utilizes an IPM whose dielectric perm ittivity tensor is anisotropic and contains positive and negative com ponents:^ >0,fn <0, where parallel refers to the IPM/object interface. W e have already fabricated such IPMs using S1O 2- SiC-SiO 2 m ulti-layer, and are investigating other approaches involving selectively-doped sem iconductor m ulti-layers sim ilar to the ones used by G m achl's group at Princeton. SiO 2-SiC-SiO 2 m ulti-layers are produced in-house ( with SiC film s shipped by Professor Ferro from University of Lyons, France). The m ain function of the IPM is to propagate sub-diffraction waves (knxi)/c) with as little decay as possible. That happens because these sub-diffraction waves are no longer evanescent: k^ »-k^£^/e1 >0. It is dem onstrated experim entally that there exists a frequency range for which sub­ diffraction waves propagate through the IPM with less attenuation than the radiation­ zone waves ( ^ /c), see Figure 11. M CT detector on optical rail Lock-in am plifier M id-IR CO 2 laser 10.66 pm - 11.31 pm O ptical chopper Figure 11. First Experimental Demonstration of P ropagating Sub-Diffraction W aves in the IPM. ( Left): Experim ental setup dem onstrating how sub-diffraction waves are launched into the IPM using FIB-fabricated sub­ wavelength grating. Because the periods of the bottom ( "launching") and top ( "transform ing") gratings are different^ far-field observation of different harm onics of the bottom grating is enabled. ( Right): Experim ental results: the first sub-diffraction harm onic of the grating ( green line) becom es stronger than the zeroth radiation­ zone harm onic of the grating ( blue line) in the region where ^ >0,^ < 0. O nce sub-diffraction waves propagate through the IPM, they can diffract on the im age­ releasing grating ( see Figure 10) and be radiated out into the far field. A detector array can be used to collect the signal and reconstruct the im age. Unfortunately, different wave num bers kv of the object are directed into the sam e detector and produce an am biguity in extracting their respective am plitudes A(kn). This am biguity is illustrated by Figure 10: wave num bers k}= Ak + 2nID and k2 = Ak + 4^/ D are directed to the sam e far-field detector. Sim ply put, a single num ber ( intensity of light with the wave num ber Ak/cincident on the detector) is insufficient for determ ining two scattering 13 U N C LASSI FI E D/^EQBJJEEi«A fe-tfS«WEr” UNCLASSIFIED / ZEfi&A EEICMMfiMNtr am plitudes ( ^)andA( t2)). Therefore, a new concept has to be developed, and the m ulti-detector technique is such a concept. The concept requires two detectors and two coherent laser beam s. The two beam s are form ed using a beam -splitter and a variable delay line im parting a phase shift to the two beam s ( see the actual experim ental photograph in Figure 13 where the beam ­ splitter BS and the Delay Line are shown). W e have theoretically dem onstrated that the intensity dependences of the two detector intensities /,( ^) and Z2( ^) as a function of the phase delay ^ provides enough inform ation to recover both A( ^) and A( £,). Figure 12. (Left): Schematic for 2-Beams/2-Detectors Interferometric Measurement. (R ight): Numerical Simulation: intensity on the two detectors as a function of the phase delay between beam s A and B produced by the interference between the zeroth and first diffractive orders of the bottom diffractive grating ( num erical sim ulation). The second detector provides the necessary second data point which is necessary for separating the contributions of different diffractive orders. Two sets of experim ents dem onstrating the feasibility of the concept are conducted. None of these experim ents constitutes im aging per se. H owever, without dem onstrating the two key m ilestones described below, proper im aging experim ents cannot be attem pted. The first m ilestone involves dem onstrating that IPM indeed supports propagating ( non­ evanescent) sub-diffraction waves. Figure 11 shows the experim ental schem atic ( left panel) and experim ental results. The bottom grating "im prints" its Fourier com ponents ( zeroth, first, second, third, and so forth) onto the incident laser pulse thereby generating electrom agnetic waves that are launched into the SiC-based IPM. The zeroth harm onic is inside the radiation zone ( that is, it is not sub-diffraction), while the first, second, and so forth sub-diffraction. These EM waves scatter off the top grating having a slightly different period and are released into the far field. Because the direction in which waves are released depend on the Fourier harm onic's num ber, we can experim entally separate and m easure them . Clearly, the relative m agnitudes of these diffractive orders dram atically vary as a function of the laser wavelength. For exam ple, the zeroth diffraction order clearly dom inates in the ^ <0,^ > O frequency range. H owever, in the s± > O ,^ < O frequency range the first diffractive order becom es larger 14 U N C LASSI FI ^//FO0'^^"lL U_iL UI1LI ~ UNCLASSIFIED / jmtfNCIA UJSUmX than the zeroth one. This confirm s the recently predicted effect that for IPMs one can indeed observe a very counterintuitive effect; sub-diffraction waves can indeed propagate with less loss than the diffraction-lim ited ones. The second m ilestone involves dem onstrating the possibility of observing the interference of sub-diffraction electrom agnetic waves inside the IPM using the two- beam /two-detector technique. Figure 10 dem onstrates this interference pattern which reveals the phase advance of the sub-diffraction waves inside the IPM. W hile we have so far dem onstrated the interference between the first Fourier com ponent of the grating ( sub-diffraction) and the zeroth Fourier com ponent, we see the possibility of interfering even m ore sub-diffraction waves ( 2nd and 3rd). Figure 13. (Left): Experimental Setup for 2-Beams/2-Detectors Interferometric Measurement in Our Lab. (R ight): Preliminary Experimental R esults: infrared intensity on two detectors ( red and black lines) are ( i) different from each other; ( ii) have a sinusoidal dependence on the delay line position ( in m icrons), which is equivalent to the phase delay between the two beam s; ( iii) are shifted in phase by the am ount equal to twice the phase difference between the 1st order ( sub-diffraction) and 0th order ( radiation zone) Fourier com ponents of the bottom grating. Measurem ents taken at A= 10.8 pm and A= 11.3 pm . W ith these two m ilestones established, it is now possible to conduct true sub­ wavelength im aging experim ents using two ( or m ore) far-field detectors and jointly processing their inputs. 15 U N C LASSI FI ED/^EOUHttGMMMMNET"""" UNCLASSIFIED/ /FOP OFFTCT"' nrr n»i »f Applications to Circuits and W aveguide Miniaturization: Slowing Down and Manipulating Electromagnetic Pulses (EMP) Using Advanced Metamaterials G iven the space constraints of an advanced aerospace platform and the am ount of the useful payload that has to be carried, it is very im portant that every optical and m icrowave com ponent be as sm all as possible. Because of the very large speed of light, there is a natural lim it to how sm all such com ponents can be m ade. Any structure capable of processing EMPs ( be those optical, TH z, or m icrowave) of tem poral duration r m ust be at least L -crlong. For exam ple, a 1 ns m icrowave pulse can be m anipulated inside a device that is at least 1 ft long. Pulse m anipulation can be understood very broadly by pulse com pression, frequency shifting, harm onics generation, or other, For aerospace com m unications system s, it m ay be very desirable to have the ability to m anipulate the form at of EMPs, ( that is, to change their frequency, duration, and repetition rate). Slowing down or even stopping the EMP can circum vent the length requirem ent if the group velocity is reduced to v «c, and thus the required length is L-v^r. Figure 14. Schematic of Pulse Compression in Magnetized Plasma. A radiation pulse with initial frequency ( W o and duration T slows down in the plasm a to a group velocity Vg0 « C . Adiabatic spatially uniform variation of the m agnetic field changes the radiation frequency to ^ and increases the group velocity to Vgl » vgG . The em erging pulse is com pressed to Tt = TVgQ/vg]. ( Reference 29) An exam ple of the pulse slowing down and subsequent m anipulation is first discussed in Reference 29 in the som ewhat esoteric context of m agnetized plasm a. Pulse duration, frequency, and ( for m ultiple pulses) repetition rate can be controlled by storing ( or slowing down) electrom agnetic waves and subsequently changing the system 's param eters. The essence of the com pact pulse m anipulator is shown in Figure 14. The pulse is slowed down inside the com pact plasm a device and m anipulated by changing the m agnitude of the m agnetic field. The advantage of slowing the pulses down is three-fold. First, the device can be m ade sm aller, resulting in size savings. Second, the tem poral scale on which the system has to be m anipulated is lengthened because the pulse is m oving slowly. Finally, the potentially large ratio between v , »vg0 results in the m ore dram atic dynam ic range of possible pulse com pression ratios. Plasm a-based 16 U N C LASSI FI ^//FO0'^^"lL U_iL UI1LI ~ UNCLASSIFIED / ZEfi&A EEIfiMMMMNtr devices m ay not be appropriate in the aerospace context because of their large size, power requirem ents, large m agnetic coils, and so forth. Fortunately, m etam aterials offer som e exciting opportunities for slowing down electrom agnetic waves as has been recently recognized ( Reference 30). Specifically, the authors have theoretically dem onstrated that an axially varying heterostructure with a m etam aterial core of negative refractive index can be used to efficiently and coherently bring light to a com plete standstill. O ne of the m ost rem arkable aspects of the approach is that it works for relatively broadband pulses. The broadband capability is achieved through "tapering" ( or axial variation) of a m etam aterial's param eters such as the effective e and p. Due to tapering, each frequency com ponent of the wave packet is stopped at a different guide thickness, leading to the spatial separation of its spectrum and the form ation of a 'trapped rainbow'. In Reference 30, the authors have actually opted for a physical tapering of the waveguide ( that is, reducing the thickness of the NIM waveguide along the length of the waveguide), although other approaches such as varying e and ^ will also work. Figure 15. Trapped R ainbow: A W aveguide with Negative Index Core Can Stop Light. A guided wave packet is efficiently injected from the ordinary waveguide to the left-handed heterostructure LH H ( see also Figure 4)r inside which it propagates sm oothly owing to the slow ( adiabatic) reduction in the thickness of the core. The sm allest ( red) frequency com ponents of the wave are stopped at the sm allest core thicknesses of the LH H , while the largest ( blue) com ponents stop at correspondingly larger core thicknesses. ( Reference 30) The schem atic of the light-stopping structure based on the waveguide with a negative index core ( dubbed left-handed heterostructure, or LH H , in Reference 30 is shown in Figure 14. Although light stopping is possible in other guided configurations that do not necessarily require // to be negative ( for exam ple, a m etal-dielectric-m etal waveguide would suffice), the key here is that perfect im pedance m atching can be achieved for the m etam aterials-based waveguides with the negative index core. That is very im portant for m axim izing the coupling efficiency from the regular waveguide to the LH H . Although Reference 30 does not present any specific ideas as to what could be done with the 17 U NCLASSI FI cr>//ffrp ^CCT^TA L UjL UI1LI ~ UNCLASSIFIED//rOn OFFICIAL USE ONLY slowed down and/or stopped light, the schem atic shown in Figure 13 provides som e key ideas. Moreover, the prospect of producing a low-loss negative index m aterial in the optical dom ain still rem ains som ewhat distant. Therefore, it m ay be worthwhile to exam ine other approaches to slowing down light that have em erged in the past few years. Stopping and/or slowing down light is an old idea originating from the atom ic concept of Electrom agnetically Induced Transparency ( EIT). The phenom enon has been considered to be purely quantum m echanical until several groups have dem onstrated that it has som e classical analogies ( Reference 31). Rem arkably, at least one group has dem onstrated in the past year that EIT can be achieved using plasm onic m etam aterials ( Reference 32). The idea is to create a plasm onic "m olecule" consisting of a radiative elem ent coupled with a subradiant ( dark) elem ent. The plasm onic m olecule showed electrom agnetic response that closely resem bles the electrom agnetically induced transparency in an atom ic system . Because of its subwavelength dim ension, this electrom agnetically induced transparency-like m olecule was shown to be suitable as a building block to construct a "slow light" plasm onic m etam aterial. The specific design of the plasm onic m olecule is shown in Figure 15. 10 .0 36.5 33.3 39.0 26.7 Z3.5 Z0 .Z 16.9 13.7 10 .4 7.14 3.88 0 Figure 16. "Plasmonic Molecule" Exhibiting EIT. Left: Radiative elem ent ( m etal strip) by itself gets strongly polarized by the incident EM wave, resulting in weak transm ission/strong reflection. Right: Radiative elem ent coupled to the "dark" elem ent ( two strips). Dark elem ent possesses a non-radiative quadrupole resonance which is excited by the radiative elem ent and de-polarizes the radiative elem ent. The result: vanishing reflection, high transm ission. Color bar: £ norm alized to the incident laser field at A = 700 nm . ( Reference 32) This specific plasm onic m olecule consists of the "dark state" ( two parallel plasm onic antennas oriented perpendicular to the incident vertical electric field) and the "radiative state" ( single plasm onic antenna oriented parallel to the electric field). The quality factor of the "dark antenna" state is an order of m agnitude higher than that of the "radiative" antenna. W hen the "radiative" antenna is spatially separated from the "dark" antenna ( or when the dark antennas are not present at all), all or m ost of the incident radiation is reflected from an array of "radiative" antennas whenever the resonance frequency of the antenna coincides with that of the laser. In this exam ple, the long antenna is 128 nm long, and the resonance wavelength is at A = 700 nm . The key effect here is that the resonance of the "dark" antenna should be at the sam e wavelength. 18 U N C LASSI FI ^/ZEO0 ^^^"lL UjL UI1LI ~ U N CLASSI FI ED/ZEfiaJJEHOAW SW fttr Because the exploited resonance has a quadrupole nature, it is slightly red-shifted. For that reason, the length of the "dark" antenna is 100 nm . W hen the two antennas are brought together, the radiative antenna polarizes the dark antenna, which, in turn, depolarizes the radiative antenna. As a result, the dipole m om ent of the coupled system is drastically reduced, the reflection drops and transm ission increases to alm ost 100 percent ( lim ited only by losses). Most of the energy is now stored inside the non- radiative ( dark) antenna. If m ultiple layers of dark/bright antennas are em ployed as shown in Figure 16, then one can achieve one of the m ost im portant m anifestations of EIT; "slow" light. Slow light can have m any interesting technological applications because ( a) slow light is easy to m anipulate by changing the structure's param eters ( as described in the section on tunable m etam aterials), and ( b) slow light has a high field intensity ( enhanced by the ratio of the free-space propagation speed to the slow propagation speed), therefore, all nonlinear processes are enhanced for slow light. Such nonlinear processes m ay include harm onics generation, optical diode action ( see the section on non-reciprocal optical elem ents), and m any others. Figure 17. True Multi-Layer Metamaterial W ith a Unit Cell Shown in Figure 15: radiative antenna ( single m etal strip) coupled to a dark antenna ( two perpendicular m etal bars). Such m etam aterial exhibits "slow" light propagation along the incidence direction ( slowed down by a factor 30 or m ore). ( Reference 32) It is im portant to realize that the geom etry suggested in Reference 32 is not unique. For exam ple, the dark and radiative antennas need not reside in the sam e plane. Nor is the effect of EIT ( and the related phenom enon of slow light) lim ited to the optical dom ain. Both infrared and m icrowave-range designs have started em erging. These frequency dom ains are likely to be of greater use for advanced aerospace platform s than the visible range targeted by m ost studies. 19 U N C LASSI FI ED/XBOEEK W W W MfrtF- UNCLASSIFIED/ /FOP fiFFTH"' nrr n»i »f Metamaterials for Energy Harvesting O ne of the m ost im portant applications of m etam aterials is related to developing "perfect absorbers" of infrared electrom agnetic radiation, be it in the m id-to-long infrared part of the spectrum ( m aking it relevant for night vision, harvesting of the Earth glow m id-infrared radiation, and so forth) or in the near-to-m id-IR spectrum ( m aking it relevant for day-tim e infrared photography of the earth terrain). For exam ple, day-tim e infrared photography relies on the different sunlight reflectivities of surfaces ( for exam ple, snow, brick walls, concrete walls, grass, and so forth), and can easily distinguish between those surfaces. This reflectivity differential tends to be the greatest between 2-3 m icrons, and rapidly decays toward longer wavelengths. O pen sky contains very little infrared radiation which explains why infrared im aging/photography is very im portant for aerial and satellite surveys. Because light scattering in the atm osphere scales as A’4, im aging through the atm osphere in the visible range is im possible, and infrared im aging becom es im portant. This is especially true for the 1 < A < 4 pm range. For longer wavelengths ( A>10 pm ) this brightness differential is largely gone because the em ission spectrum is dom inated by therm al em ission. In fact, the Earth glow m axim um is around A>10 pm , with m ost of the energy contained in the 3 pm < A < 14 pm range. This longer wavelength ( m id-to-far IR) spectral range is also very im portant. It can be used for night-tim e energy scavenging by high-altitude satellites and other aerospace platform s. There has been a surge of activity in this area, first in the m icrowave/TH z part of the electrom agnetic spectrum ( References 33, 34), and subsequently in m id-to-far infrared ( Reference 20). The concept of narrow-band m etam aterials-based absorbers introduced in Reference 33 has the potential for developing highly efficient bolom eter arrays. W hen applied to the infrared part of the spectrum , it can be used for space navigation, especially when weak infrared signals from specific stellar objects need to be picked up and discrim inated from other radiation sources. For such applications, the narrow-band "perfect" absorption is highly suitable. An array of such bolom eters would reject ( reflect) all undesirable frequencies and focus on the single wavelength characteristic of the source of interest. Moreover, if an array of different ( for exam ple, tuned to different frequencies) narrow band detectors can be deployed, then the hyper-spectral im aging capability could bring additional benefits. For exam ple, absolute tem peratures of a radiation source ( that is, stellar bodies) could be accurately determ ined, and could im prove the accuracy of space navigation further. Figure 18. "Perfect" Narrow-Band Microwave Absorber, ( a-c); Unit cell design* Right panel: sim ulated absorption/transm ission/refiection. ( Reference 33) Transmission 20 UNCLASSIFIED/ /FfiP OFFTCth ■ i|qn I INI I — UNCLASSIFIED//rOR OFFICIAL USE ONLY O ne possible design for the m icrowave frequency band is shown in Figure 17. H igh absorption is accom plished by reducing reflections to zero. This is accom plished by choosing m etam aterials param eters such that ^( = ck^ and the sam ple's thickness i/, we can plot the reflectivity as a function of re(n) and im(n). As can be clearly seen from Figure 20 ( right), there is a "sweet spot" corresponding to specific values of re(n) and im(n) that results in vanishing reflectivity ( or perfect absorption). R vanishes when the quarter-wavelength condition is approxim ately satisfied: re(n)kod = (2m+l)/2 ( 6 ) where m is an integer. Finite reflection and im perfect absorption result from lower ( or higher) values of im(n). Note that, coincidentally, for the case of heavily doped SiC predicted reflection is only 3 percent. H owever, for m ost m aterials ( that is, Si for visible light) absorption is too low for perfect absorption. Therefore, a question is posed: Can one m odify the structure of the m etal screen to enhance absorption? It is CMMs that enable such functionality? 24 U N C LASSI FI ED//nnR CimG TAI UOE ONW UNCLASSIFIED / ZEfi&AEEIfiMMMMNtr Specifically, it has been found that by m aking the m irror slightly leaky, we can actually increase absorption. If the reflection coefficient of a m irror is given by r2, then the reflection/transm ission coefficients r,t through the structure are given by: A±/^l t = _&£L 1 + r^e 1 + r^e' ( 7) where r2 = 1 + r2and f0 -1 + ^. Note that Equation 7 turns into Equation 6 if r2 = -l ( perfectly reflecting m irror). From Equation 7 it follows that it m ay be possible to engineer the reflectivity r2in such a way that m inim izes reflection |r|2 while keeping transm ission |r| sm all. The rem ainder of the energy is guaranteed to be absorbed by the quarter-wavelength thick absorber. Figure 22. Preliminary Attempts to Design a Better Absorber Using Complementary MetaMaterials (U- shaped CMM). Note that only 40 percent absorptivity is possible with a sm ooth gold film . Paradoxically, this absorptivity increases to 75 percent when the m etal m irror is m ade "leaky" by perforating it with an array of CMMs ( left panel). The design process for engineering r2 using the sim plest CMMs, U-shaped apertures, has been started. Som e of the prelim inary results are shown in Figure 22. Figure 22 illustrates how the ( relatively low) 40 percent absorptivity of the SiC film covered by a sm ooth Au m irror ( black line) can be boosted up to 75 percent by patterning the m irror using CMMs. W e call such a "leaky m irror" patterned by CMMs a MetaMirror. It is clear from Figure 23 that a MetaMirror can be used for m aking absorptivity polarization­ dependent ( if that is desirable for applications dem anding a reflector-polarizer). MetaMirror can also be used for shifting the absorption wavelength which would be highly desirable for developing broadband absorbers. W e have found that there are two m echanism s capable of m aking MetaMirrors: ( a) excitation of the Long Range Surface Plasm on Polaritons ( LR-SPPs) on the patterned MetaMirror, and ( b) excitation of highly- localized ( shape-dependent but period-independent) SPPs. An exam ple of the m echanism ( a) is shown in Figure 23, but we also have prelim inary results indicating that both m echanism s can be operational in the sam e MetaMirror for close-by frequencies resulting in m ultiple dips of the reflectivity coefficient | r21. By com paring 25 U NCLASSI FI ED//rOn OrriCIAL USE ONLf U N CLASSI FI ED/ZEfiaJJEHOAW SW fttr Figure 23 with Figure 22, it is observed that the dips correlate with drops of the total reflectivity and increases of the total absorption of the absorber/MetaMirror structure. The MetaMirror approach to infrared energy harvesting is one of the very prom ising applications of m etam aterials. A num ber of aspects of MetaMirrors m ust be investigated and several im portant questions m ust be answered before practical applications can be pursued. Som e of those questions are: • W hat is the angular dependence of absorptivity, and can it be m ade wide-angle as we have recently dem onstrated in Reference 20 for negative-index m etam aterials? • Can absorptivity be m ade broad-bandwidth by com bining localized resonances with the LR-SPPs? That could be potentially accom plished by using U-shapes with different geom etries, yet spaced in a regular periodic pattern, or by using quasi- periodic arrangem ents of CMMs shapes. • W hat are the m ost prom ising polarization-independent unit cells of CMMs that result in enhanced absorptivity? • Is it possible to apply the MetaMirror concept in the visible and contribute to solar energy harvesting? As m ore researchers are investigating energy-harvesting applications of CMMs ( or MetaMirrors), it is believed that these questions will be answered very soon. Leaky Mirror Absorbing Material Figure 23. Engineering the Complex R eflectivity Coefficient T? Defined on the Left Panel Using the Concept of a MetaMirror. Dips of | ?; | shown in the right panel correspond to reflection dips ( and absorption peaks) in Figure 2. The physical reason for these dips is the excitation of long-range SPPs on the MetaMirror surface. Inset: Fabricated MetaMirror. 26 U NCLASSI FI cr>//fnp ^CCT^TA L UjL UIILI ~ UNCLASSI FIED//FOR OmCIAL USE ONLY Nonlinear Non-R eciprocal Chiral Metamaterials: For Developing Novel Optical Isolators and "One-W ay" Micro wave Mirrors O ptical isolators play a pivotal role in fiber-optic com m unication system s by protecting their active com ponents ( for exam ple, optical am plifiers) from unwanted reflected signals that could potentially destabilize them . Such protection is especially im portant in the context of advanced aerospace platform s, where repairs m ust be avoided at all costs. At the core of isolator design is an elem ent which provides non-reciprocity by breaking tim e reversal sym m etry. Non-reciprocity can only be caused by m agnetic fields or nonlinearities. The m ost com m on approach using m agnetic Faraday rotators results in a rather bulky im plem entation of an isolator. It is believed that m etam aterials are uniquely positioned to enhance the other approach of breaking non-reciprocity: use of nonlinear effects. It is very natural to use m etam aterial in the context of enhancing nonlinearity. As was explained previously, m etam aterials can be used to slow down light and, therefore, com press electrom agnetic energy. Any intensity enhancem ent increases nonlinear effects, and larger nonlinear effects translate into m ore com pact devices. Another aspect that m akes m etam aterials very appealing for non-reciprocal applications is the ability to m ake their properties tunable to alm ost any frequency range. O ne concept that is being explored ( still unpublished) relies on the nonlinearity and several other aspects of engineered chiral m etam aterials. A novel type of a nonlinear optical isolator based on adiabatic tim e-irreversible m ode conversion ( ATIMC) between two electrom agnetic m odes supported by the chiral m etam aterial is envisioned. As an exam ple of such m etam aterial, a twisted optical fiber shown in Figure 24 is used. It supports a tightly-confined core m ode ( CoM) which can be coupled to/converted into a loosely confined cladding m ode ( CIM) of the sam e fiber. Coupling and conversion between the core and cladding m odes is accom plished by twisting the fiber with a variable pitch A( z) = 2^7/3;,. Tim e irreversibility is achieved due to the com bination of the K err nonlinearity of the core m aterial ( resulting in the intensity-dependent propagation constant of the CoM) and sm all but finite loss of the CIM. As a result, the CoM, when injected in the forward direction, passes through the isolator with a negligible conversion into the CIM. If subsequently reflected back into the isolator ( this is equivalent to tim e reversal), it gets entirely converted into the CIM and subsequently dam ped as illustrated by Figure 25. Prelim inary sim ulations indicate that, for sufficiently large nonlinearity, one can find the loss rate a for the CIM such that two conditions are satisfied: ( a) a is sm all enough so that virtually no power is lost in the forward direction, and ( b) a is large enough so that the tim e-reversal is strongly violated, resulting in near-perfect optical isolation. 27 U N C LASSI FI ED/^EQUmCMMMMNtr""" UNCLASSIFIED/ /FOP QEHCttM nrr n»i »f Figure 24. Example of a G eneric Chiral Metamaterial: An O ptical Fiber with a Rectangular Cross Section Core Twisted During the Drawing Process Form s a Double H elix. If the core index is nonlinear, then one can engineer a variable helix pitch in such a way that the core ( localized) m odes can be selectively coupled to cladding m odes depending on the direction of propagation. Under a highly sim plified assum ption of just two interacting m ode ( core and cladding), we have developed a coupled-m ode theory describing the evolution of the m ode am plitudes a^and atl along the fiber axis z. The set of the generic equations for % and act is given by: ^-f^ + Xl^ |2 -SI2)am= iWa(:I, ( 8) ^-i^+SIVa^iW'a*, OT where /?0 = c(J3c0 + /Jd)/2a>\s the average norm alized propagation constant, 8(r) = ct/3M- /3d + 2^()/^y is the distance-dependent m ode detuning, y is the nonlinearity coefficient, W is the coupling strength between the m odes, and r-wz/cis the norm alized propagation distance. 28 U N C LASSI FI ^/./fn0 ^^^"lL UjL UI1LI ~ UNCUtSSIFIED/XEQUEEIG MM^ Figure 25. Example of Time-Irreversibility of Light Propagation Inside the Twisted Fiber Core: the core m ode propagates with alm ost no loss from left to right ( purple solid line), reflects back, and gets dissipated/m ode converted on its way back ( red line). Input m ode is assum ed to be right-hand circularly polarized ( RCP). Mode conversion: into LCP cladding m ode ( dashed line). Propagation from left to right is represented by the red lines, from right to left: by the purple lines. As an exam ple, we have used a Chiral Fiber ( CF) with the following properties: a 2 pm x 1.8 pm elliptical core with refractive index of na) = 2.2 surrounded by a round cladding with radius/? = 20 pm and refractive index h.; = 2.15. The helical pitch is assum ed to linearly vary over 7nBX = 500 m m by 6 percent around A = 16 6 pm . The assum ed y corresponds to the nonlinear refractive index m = 5.4 x 10’16 m 2/W at the operating vacuum wavelength A = 1.5 pm and the peak power P = 3.5 W . The cladding m ode was assum ed to be lossy with the loss coefficient ct = 10 dB/m . Results are shown in Figure 25. A core m ode injected from the left end of the fiber ( z = 0) propagates through the fiber without converting into the cladding m ode ( purple solid line) with m inim al losses. After getting reflected at z = 500 m m , the principal core m ode ( red solid line) gets converted into the delocalized cladding m ode ( red dashed line) and dam ped out. This exam ple clearly dem onstrates that the interplay between m ode-coupling, nonlinearity, and losses can result in the dram atic loss of tim e-reversal. Future research will be looking at other m etam aterial system s that support two distinct m odes ( one with a strongly nonlinear response and strong spatial localization, the other essentially linear and delocalized), orthogonal polarizations, and investigate non­ reciprocal wave propagation in such m etam aterials. Structures from the previous 29 U N C LASSI FI ED/XBOEEK W W W MfrtF- UNCLASSIFIED//FOR fiFFTH"! "nn W NfT sections, such as shown in Figures 15 and 16, will be prim ary candidates for im plem enting optical and m icrowave non-reciprocity. Such m etam aterials would be com prised of a unit cell containing a non-radiative elem ent ( that is, a two-strip capacitor-loaded antenna supporting a "dark" m agnetic m ode) and a single "bright" dipole antenna. Such a system exhibits EIT when the frequencies of the "dark" and "bright" resonances coincide. EIT results in energy com pression and enhanced nonlinearity. The source of the nonlinearity could be, for exam ple, a variable capacitance diode ( varactor) used as a capacitive load of the double-strip antenna. The second ( linear) m ode could have an orthogonal polarization, and the coupling between the two could be accom plished via spatially-periodic displacem ent of the single-strip and double-strip antennas with respect to each other. Tunable Switchable Metamaterials Electrom agnetic properties of m ost m etam aterials are "hard-wired", m eaning they are determ ined during fabrication. That can be a serious im pedim ent to using them in the context of space exploration. Being able to change optical/electrom agnetic properties of a m etam aterials-based device without having to re-m anufacture it would be highly desirable. Therefore, this survey is concluded by describing som e of the recent progress in m aking reconfigurable/switchable m etam aterials. This is a new exciting area of m etam aterials research that is worth watching for applications. O ne of the first electrically-controllable TH z m etam aterials has been reported in Reference 36 , where resonant properties of the electric split ring were controlled by applying reverse bias between m etal and highly-doped in G aAs layer. The schem atic of the experim ent is shown in Figure 25. W ithout reverse bias there is no resonant response of the split ring to incident TH z pulse because highly-conductive electrons of the n-G aAs layer are shorting the gap of the resonant split ring as schem atically indicated in Figure 26( b). W ith the applied reverse bias, electron density is depleted inside the gap. The resulting transm ission spectrum shows spectral dips which were converted into the effective dielectric perm ittivity £eff(a)) that exhibited resonant peaks. The strongest of the peaks corresponded to the Inductance-Capacitance ( LC) resonance of the split ring. O ne possible application of such electrically tunable m etam aterial suggested in Reference 36 was a m odulator. The authors claim that the perform ance of their device as a THz m odulator already exceeds current state-of-the-art electrical TH z m odulators ( based on sem iconductor structures) by one order of m agnitude on resonance. Moreover, their device operates at room tem perature. Needless to say, this m etam aterial-based m odulator can be im proved. For exam ple, configurations exploiting EIT could result in stronger m odulation strength. Another interesting possibility for tuning m icrowave m etam aterials has been suggested in Reference 37. Ferroelectrics ( such as BST) can be tuned by applying DC voltage which changes their dielectric perm ittivity. This property of BST was utilized to develop frequency tunable m agnetic m etam aterials using m etallic split rings loaded with barium -strontium titanate thin film capacitors. The resonant frequency of this m edium is voltage tunable across a 140 MH z band centered at 1.75 G H z. The effective relative perm eability of the slab was shown to have Lorentzian shape that reaches m inim um values between -2 and -3 for biases from 0 to 5 V. Therefore, perm eability of the slab can tune between positive and negative values, m aking it useful in applications requiring a state switchable m agnetic perm eability. 30 UNC LASSI FI ED/^EQUmCMMMMNET""" UNCLASSIFIED/ /FOP QEEWA I "CT nm^ Figure 26. TH z Properties of an Electric Split R ing R esonator, ( a) Are controlled by applying voltage between Schottky and O hm ic contacts ( c,d). ( b) Schem atic of circuit with inductance. Applied voltage controls charge density inside the split gap ( d). The structure is investigated using a single-cycle TH z pulse ( e). ( Reference 36 ) Summary and Conclusions W hile it is difficult to pinpoint the exact applications that m etam aterials will find in advanced aerospace industry, m etam aterials possess several features that uniquely suit them for aerospace applications. First, they enable m iniaturization of a variety of optical com ponents. Making space-born devices sm all and light-weight is essential. These opportunities have been covered in detail. Second, m etam aterials enable new m odalities for sub-diffraction im aging: super-lenses, hyperlenses, and far-field superlenses. Those m odalities dispense with the near-field scanning m icroscopes, which are com plex, slow-scanning, large devices that are not appropriate for advanced aerospace platform s. H arvesting infrared photons, whether from coherent laser sources on Earth ( for guidance, energy recharging, and so forth), from therm al Earth glow, or from the stars, is likely to be im portant for aerospace platform s. Metam aterials offer unique opportunities for m aking efficient wavelength-tunable, wide-angle absorbers. As discussed in the num erous exam ples in this report, m etam aterials are going to revolutionize the way light is captured, m anipulated, and used for im aging. Although m etam aterials are still an academ ic area of research, these exam ples illustrate that there is great potential for practical applications. R eferences [1] Sm ith D R, Padilla W J, Vier D C, Nem at-Nasser S C, Schultz S 2000 PRL 84 4184. [2] J. B. Pendry, "A Chiral Route to Negative Refraction", Science 306 , 1353 ( 2004). [3] D. Schurig and D. R. Sm ith, "Spatial filtering using m edia with indefinite perm ittivity and perm eability tensors", Appl. Phys. Lett., 82, 2215 ( 2003). [4] D. R. 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