Office of Naval Research. MURI Quarterly Report for. Sep. 1 - Dec. 31, For Dr. Michele Anderson and Dr. V. Browning on DOD/ONR MURI

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1 Office of Naval Research MURI Quarterly Report for Sep. 1 - Dec. 31, 003 For Dr. Michele Anderson and Dr. V. Browning on GRANT NUMBER: N DOD/ONR MURI Scalable and Reconfigurable Metamaterials Principle Investigator: Xiang Zhang Co-PIs: G. Chen, T. Itoh, E. Yablonovitch, J. D. Joannopoulos, J. Pendry, D. Smith, and S. Schultz University of California in Los Angeles Engineering IV, 40 Westwood Plaza Los Angeles, CA

Summary Based on the feedbacks from the second year annual review, the program roadmap and the progress of last quarter, we have been optimizing plasmonic hole-array lithography and self roll-up

Continuous progress has been made in theoretical and systematical level design for plasmonic tapered metal/dielectric photolithography for the upcoming experiments.

2 Summary Based on the feedbacks from the second year annual review, the program roadmap and the progress of last quarter, we have been optimizing plasmonic hole-array lithography and self roll-up method for 3D metamaterials. Studies have been also continued on novel lens design and superlensing in photonic crystals. Continuous progress has been made in theoretical and systematical level design for plasmonic tapered metal/dielectric photolithography for the upcoming experiments. The physical understanding of magnetic response from metameterials has been continued. Some of the potential devices working in microwave range have been explored and interesting simulation results have been obtained. Significant interactions and collaborations have been made between team members. Work in progress: Meta-materials synthesis: Plasmonic Lithography using Hole-array: (Zhang, UCLA) A novel plasmonic lithography utilizing surface plasmon polaritons is proposed for an ultimate high resolution and high density nanofabrication method. Sub-wavelength features patterning (~10 nm) using the proposed method has been reported in the last quarter review. By optimizing the exposure procedures, the resolution has been improved drastically. The exposure result of ~90 nm dot array pattern has been observed at 170 nm period of aluminum hole-array. Further extension of the optical limit (high resolution and high density) has been explored. Through the considering of the surface plamons property of good conductive metals (Ag, Al, Au and Cu), silver (Ag) has significant largest surface plamon wave vector among others at the designed exposure wavelength of 364 nm (i-line). The surface plamon wavelength in this case is equivalent to λ/3 where λ is the wavelength of light in the glass media. Numerical study of silver hole array at the exposure condition shows significant contrast which implies the possibility to extend the optical limit of the plasmonic lithography. Fig. 1. Left: AFM image of the transferred dot array pattern. The lattice constant is 170 nm, the aperture diameter is 40 nm, the spacer layer thickness is 30 nm, and the exposure time is 9 sec at power of 8 mw/cm. Right: 3D topography of the left image.

3 Fig... Left: surface plasmon wave vector of good conductive metals (Ag, Al, Au, and Cu) at the glass interface. Right: numerical study of -D Ag hole array; silver film thickness 40 nm, hole diameter 60 nm, lattice constant 10 nm, and wave length 364 nm. Microscale 3-D Roll-up Structure: (Chen, MIT) The best materials system for producing consistent nanoscroll structures has been (from the bottom up) silicon dioxide/ chrome/ gold / chrome. Silicon dioxide serves as a relatively stress-free insulating layer. The first (very thin) chrome layer serves as an adhesion layer for the gold. The gold provides high conductivity (. µω-cm resistivity) and tends to make the structures more resistant to breakage when rolling up. The second chrome layer is highly tensile and provides the force required for rollup. In addition, all of these materials have strong chemical resistance to nitric acid, which is used to dissolve an underlying sacrificial layer of copper. Using this system, nanoscrolls have been fabricated with 4 µm diameter tubes when the sacrificial layer is copper, and with 8 µm diameter tubes when the sacrificial layer is photoresist. In both cases, sample uniformity is excellent when the structures are completely released from the substrate, with only a few defects observed over a thousand structures. For partial release from the substrate, the uniformity is poor since some structures (especially those near the edges) completely release from the substrate before others have completely rolled up. Several different samples were assembled from nanoscrolls released from the substrate and tested on the FTIR. We hoped to find a narrow peak in the reflectance spectrum to verify our calculation of the effective magnetic permeability of the structures. No such peak has been found yet. This may be due to poor quality of the samples tested, or the actual resonant frequency of the nanoscrolls may be out of the testable range of the FTIR. Metamaterials Physics Study THz magnetic response from artificial materials: (Zhang, UCLA) Last quarter Zhang s lab (in corporation with UCSD and Imperial College) reported the experimental demonstration of an artificial magnetic metamaterial at THz frequencies. The further effort towards thorough physical understandings of this artificial magnetism is ongoing in Zhang s lab. For example, our preliminary FTIR experiments on the MMR samples (Fig. 3) reveal a distinct bianisotropic characteristics: an electric polarization as a

4 response to an applied magnetic field and vice versa. We have simulated the dispersion diagram of the split ring resonators, which indeed display a remarkable anisopotry of the fundamental bands (Fig. 4) and an analytical model is sought to quantify the bianisotropic coefficient from the geometric factors. Reflection GHz P-Sym S-Sym Frequency (THz) P-Asym S-Asym Fig. 3. Orientation issue upper panel shows when the applied fields are perpendicular to the symmetric axis of split ring structures (i.e., asymmetric cases), we observe the reflection peaks in FTIR oblique angle measurement. On the other hand, when the applied fields are parallel to the symmetric axis of split ring structures (i.e., symmetric cases), the reflection peaks aforementioned vanish. Fig 4. The computed dispersion diagram of sample D1, for TE polarization. On the ΓX and ΓY direction, we can find an discrepancy of resonance ~140GHz. Field radiated by an array of point sources in a composite comprising alternating layers of positive and negative refracting index material (Pendry, IC) In a previous report we described a new formulation of imaging by negative media. It was realised that two adjacent slabs of materials, of equal thickness, optically annihilate one another provided that εµin, the one is the negative mirror image of εµ, in the other. Although this result is quite plausible where rays follow a simple distorted trajectory in each medium, in some instances the theorem has startling consequences. Consider figure 5, a system drawn to my attention by David Smith: the mirror theorem applies but a ray construction contradicts the theorem. Applying the laws of refraction to ray implies that the ray is rejected by the system instead of being transmitted through to the other side and a dark shadow behind the cylinders is predicted by the ray picture. In fact a full solution of Maxwell s equations shows that ray is transmitted and emerges through the system just like ray 1 and no shadow is formed. The apparent paradox is resolved by recognising that a series of resonances form on the surfaces of the cylinders and these resonances enable radiation to tunnel across the gap between the cylinders. A clue to the nature of these resonances is given by the closed loop of dotted rays in the centre of the figure which indicates the presence of a state which traps radiation i.e. we have a resonance.

5 ε 1 µ 1 1 ε=+ 1 µ=+ =+11 1 Fig. 5. The left and right media in this D system are negative mirror images and therefore optically annihilate one another. However a ray construction appears to contradict this result. Nevertheless the theorem is correct and the ray construction erroneous. Note the closed loop of rays indicating the presence of resonances. One important restriction of the new lenses as currently formulated is that they produce images of exactly the same size as the objects. To do otherwise we must introduce curved surfaces and this is most easily done through coordinate transformations and, again in a previous report, we described one way in which a spherical lens may be constructed. Using the new theorem we have now realised that yet another formulation of the spherical can be made, this time with even more novel properties than the previous one. The following is the new prescription for a spherical lens: 3 r r ε x =ε y =ε z = +, 0< r< r r ε x =ε y =εz, r < r< r r ε x =ε y =ε z = + 1, r < r< µ =ε, µ =ε, µ =ε x x y y z z 3 3 In other words outside radius r the structure is empty space, between r and r 3 lies the negatively refracting material, though now more structured than before, and finally inside the annulus is a material of constant high permittivity, high permeability. This structure has the unusual property that viewed from beyond a radius, 1 = 3 r r r the contents of the inner sphere (any electrical or magnetic sources of radiation) appear to be expanded to fill a sphere radius r 1 filled with ε =µ= 1 material i.e. magnified by a factor r r 3. The region of space between r 1 and r 3 has vanished and is not visible.

6 r 1 r r 1 r 3 Fig. 6. It is possible to design a spherical annulus of negative material lying between r and r 3 that acts like a magnifying glass. To the outside world the contents of the sphere radius r 3 appear to fill the larger sphere radius r 1 with proportionate magnification. Viewed from inside radius r 3 the world outside radius r 1 appears to be shrunken by a factor r3 r and now reaches to radius r 3 and appears to have ε=µ= r r3 i.e. the same as the filling of the internal sphere. Again the region of space between r 1 and r 3 has vanished and is not visible. This result is remarkable because the region between r 1 and r is simply empty space, and yet according to our theory appears to be filled! We plan to elaborate on this result in the next phase of our work. Photonic Crystals as Alternates for Metamaterial System (Joannapoulos, MIT) Joannapoulos s group is continuing their studies of photonic crystals as alternates for metamaterial systems at optical frequencies. In this regard they have been studying the intriguing possibility of subwavelength imaging, or what has come to be known as superlensing. In particular, they have investigated the possibility of photonic-crystal superlensing in detail by studying the transmission of evanescent waves through a slab of a photonic crystal that includes material losses. It is important to note that the transmission considered here differs fundamentally from its conventional implication of energy transport, since evanescent waves need not carry energy in their decaying directions. Fig. 7. (Color) Calculated transverse intensity distribution for imaging with lossy photonic crystals. Each inset number correspond to the permittivity of the dielectric host for the curve of the same color.

7 Thus, it is possible to obtain transmission amplitudes for evanescent waves greatly exceeding unity without violating energy conservation. In general, appreciable material losses will impose severe limitations to the transmission coefficient of evanescent waves, in a manner similar to that of the intrinsic energy leakage rate of a crystal mode above the light line, which in turn reduces the superlensing effect. However, it is also expected that, in the limit of extremely small material loss, in the sense implied by the original proposal of a perfect lens, their findings about the image of a superlens will remain valid. As an example, they show the calculated focusing effect in a series of slightly lossy photonic crystals in Fig. 7. The losses are modeled as a positive imaginary part on the permittivity of the dielectric host. As the losses increase, the strength of the transmitted near-fields is attenuated, and the subwavelength features in the central image peak gradually disappear. The effects of surface imperfections on subwavelength imaging can also be qualitatively analyzed. They consider these defects to occur only on a length scale that is smaller than a lattice constant, and thus much smaller than the operating wavelength, with correspondingly little influence on propagating waves. Since the transmission of evanescent waves depends sensitively on the bound surface photon states, which in turn depend sensitively on the surface structure, imperfections are expected to be most influential on the crystal surface. Their effects may thus be minimized by improving the surface quality. These considerations suggest that the predictions of this work should be observable in realistic situations. Thus an ongoing effort involving future work is the close collaboration with Schultz/Smith and Chen groups to explore and verify these predictions experimentally. They are also continuing our studies of negative refraction of electromagnetic waves in photonic crystal systems. In a collaborative effort with Pendry, the Schultz/Smith and Chen groups, they have recently designed a high-index dielectric d periodic photonic crystal structure that can exhibit single-beam negative refraction in for all incoming angles in a regime of positive effective index of refraction. The frequency range is chosen so that for all incident angles, one obtains a single negative-refracted beam. This structure has now been fabricated by Chen group and is currently being tested by the Schultz/Smith groups. Surface Plasmon Imaging (Yablonovitch, UCLA) Moving forward on our goal to employ surface plasmons as an intermediary allowing photolithography at X-ray wavelengths with optical frequencies, our group has continued its theoretical and systems level work, as well as pushed the groundwork for upcoming experiments. Achieving imaging capabilities in the plane of the plasmon will require engineering advancements at many levels. To this end, we have expanded material trade studies, computationally modeled novel slab geometries, constructed power budgets for the device as well as overall experiment conditions, and begun designing grating structures for in-coupling light into the plasmon mode. Perhaps most importantly, we have laid out a path to production for plasmonic lithography. Details of selected results are listed below. A. Plasmons on Dielectric Waveguides Deposited on Silver Our original plan for plasmonic focusing involved a tapered silver film with the film thickness decreasing as the plasmons approach the line image. The waning film thickness causes a decrease in plasmon wavelength, allowing the increase in resolution near the

8 output coupler. This geometry, however, does present various fabrication difficulties. Not least of which is producing an adiabatic taper while keeping < 5nm smoothness on both sides of the film. An alternative approach has been investigated in which a thin sapphire layer is deposited on a silver substrate (see dispersion relations below). Most of the focusing could then be done in the low loss/high group velocity region of the dispersion curve. Near the line focus, a coupler would then couple the plasmon to a negative plasmon wave-vector with positive group velocity and small wavelength for the final focusing. As illustrated below, this could alleviate some of the fabrication concerns. Fig. 8. Numerical results of dispersion relations for the silver-sapphire-air plasmon geometry at various sapphire film thicknesses. The thick black arrow indicates a coupling from a large wavelength positive wave-vector to a short wavelength negative wave-vector, both with the same sapphire film thickness. B. Path to Large Scale Lithography using Plasmonic Imaging Recently we have put forth preliminary designs and calculations which will yield a path from theory to full scale production for plasmonic lithography. By leveraging off long established computer hard disk technology, we have been able to show that it is theoretically possible to write over the surface of a 1 inch wafer in under one hour. We are fortunate that modern hard drives work on an air bearing slider which floats 30nm ± 0.3nm (with technology rapidly moving to 5nm spacing), which is excellent for exposing photoresist using the near field. For our application, motion and dimensions must be controlled to ~10nm accuracy. To solve this, we propose using separate heads reading servo code, as well as several interferometers measuring position at all times, it would be possible to scan the write head over the substrate platter spinning at several thousand rpm.

9 image from modulator array SIDE VIEW Fig. 9. Geometry for scanning plasmon lens using a slider analogous to conventional hard disks scanning over a spinning substrate slider suspension photoresist silicon sapphire slider plasmon lens Metamaterials Devices A. Generalized surface plasmons (SP) (Itoh, UCLA) We have worked out the first full-wave demonstration of microwave SP between arbitrary metamaterials, and in particular at the interface between a RH and a LH media, as illustrated in Fig. 10, using the following simulation scheme. The wave is incident at the left arm of the L-shaped structure and hits the oblique surface with the triangular shaped part filled with air at an angle large than the critical angle. Consequently, only evanescent waves reach the RH/LH horizontal interface, which excite the surface plasmon. Fig. 10. Full-wave simulated RH/LHinterface SP(electric field magnitude) in parallel-plate waveguide structure loaded by 3 different materials. (a) Large wavelength. (b) Smaller wavelength. (c) Intersection with the air line. (d) Above the air line (simple negative refraction) In this last case, we have only two media (RH and LH), because coupling occurs to a propagating incident wave.

The characteristics of this RH/LH-interface SP were studied rigorously and a transmission line mushroom-structure implementation was proposed.

10 The characteristics of this RH/LH-interface SP were studied rigorously and a transmission line mushroom-structure implementation was proposed. Several original effects have been discovered: 1) this SP can cross the air line (never the case in the conventional SP); ) in that case, this radiative SP, since it is associated with the Brewster angle phenomenon of zero-reflection, is totally refracted; 3) depending on the metamaterials parameters, two SP resonances can exist, and very complex and unusual SP diagrams can appear. These new microwave SP have great potential in miniaturized microwave antennas, components and beam-formers. B. Parabolically-shaped metamaterial interface (Itoh, UCLA) We have introduced the novel idea of a refractive parabolic interface as opposed to the conventional reflective parabolic structure. This idea and effective-medium proof of concept are shown in Fig. 11. As for the SP, the transmission line mushroom CRLH structure excited in its LH range can be used for practical implementations. In the case of a partially reflective interface, a beam splitter with source at focus is obtained. This structure has potential for planar rectennas and various plane wave to cylindrical wave transformers. Fig. 11. Parabolic refractive RH/LH interface. Principle and full-wave simulated magnitude/phase in effective medium approach. C. Fixed-frequency arbitrary-angle electronically-tuned directive reflector (Itoh, UCLA) This reflector is a new extremely simple and flexible reflecto-directive system, illustrated in Fig. 1. It uses our recently developed fixed-frequency electronically-scanned CRLH leaky-wave antenna. The frequency of operation is fixed. An incident signal is received from a given direction by a quasi-omnidirectional patch-antenna antenna, amplified by an LNA (power boosting), filtered by a band-pass filter (noise reduction), and re-rediated by the antenna. By varying the bias voltage of the varactors integrated in this antenna, the dispersion characteristic is altered and consequently the direction of radiation is changed. This system was studied rigorously from the point of view of transmission lines and dispersion diagrams, and demonstrated experimentally.

11 Rx : Patch Antenna f in, θ in f out, θ ( V ) out LNA Tx : Electronically-Scanned LWA Fig. 1. Schematic of fixed-frequency arbitrary-angle electronically-tuned reflector. D. Nanoscale Optical Waveguides with Negative Dielectric Claddings (Chen, MIT) The dispersion properties of waveguides with negative dielectric cladding were investigated. It was found that TM1 modes can exist in the waveguides with the thickness of the guiding layer being ten times smaller than the working wavelength, i.e., the guiding layers could be in nanometer scale. The band-passing, wavelength-response, and transmission loss properties were analyzed. Artificial metal-dielectric composite materials, especially Prof. Pendry s metal-wire arrays with pre-controllable plasma frequency and low transmission losses, can be used for the claddings. The TM1 modes have a passing band, and hence the waveguides can also work as band-pass filters. The waveguides will not likely be subject to bending loss because the negative dielectric claddings theoretically cause total reflection for all incident angles. The waveguides are expected to find application in integrated optical devices, optical lithography, subwavelength optical imaging, and nonlinear optics due to the high energy density in an ultra-thin guiding layer. Work Planned for Next Quarter Metamaterial Synthesis Continue on metallization and molding development Continue on optimizing microstereolithography system Continue on developing NSOM for metamaterials characterization Chen s group will modify the process for keeping the 3D roll structures attached to the substrate even with complete removal of the sacrificial layer. Chen s group will collaborate with Zhang s group to carry out measurement and simulation on the roll up structure. Optimize the hole-array plasmonic lithography to have better resolution Yablonovitch s group will start to fabricate tapered structures and experimental verification. Metamaterial Physics.

12 A review article for Physics Today A book due to be delivered in the autumn of 004, on the subject of negative refraction. Joannapoulos group will continue their studies of negative refraction of electromagnetic waves in photonic crystal systems. Yablonovitch s group at UCLA will continue the current projects with an emphasis on extending their modeling capabilities using FDTD on the grating and taper region of the device. Metamaterial Devices Development of more practical excitation scheme and applications for the microwave SPs + theoretical exploration of different possible metamaterial SPs Practical implementation of the parabolic interface refractor Near-field characterization of CRLH TLs Collaboration between Itoh, Smith and Pendry on SPs: paper submitted to Antenna and Propagation Symposium, Monterey, 004. Collaboration between Itoh and Zhang for millimeter-wave implementation of the mushroom structure. Continue work on the modeling of guided waves with negative dielectric claddings. Synergy and Interactions: Pendry s group continues in collaboration with UCSD on the theory of negative refraction as applied to novel lens design and has produced some surprising results. A planned review of negative refraction will appear in Physics Today, and a book on the same topic will be delivered to the publishers later this year, authored jointly by David Smith and John Pendry. Joannapoulos s group works closely with Pendry, Schultz/Smith and Chen groups to explore new physics and applications using photonic crystal system. An ongoing effort involving future work brings a close collaboration between Zhang s group and Yablonovitch s group to design, verify and characterize plasmonic photolithography.

Office of Naval Research. MURI Annual Progress Report (Jun Aug. 2004) For Dr. Michele Anderson and Dr. V. Browning on DOD/ONR MURI

Office of Naval Research. MURI Annual Progress Report (Jun Aug. 2004) For Dr. Michele Anderson and Dr. V. Browning on DOD/ONR MURI Office of Naval Research MURI Annual Progress Report (Jun. 003 Aug. 004) For Dr. Michele Anderson and Dr. V. Browning on GRANT NUMBER: N0004-0--0803 DOD/ONR MURI Scalable and Reconfigurable Metamaterials