Session H32: Focus Session: Photonic Crystals, Metamaterials and Other Optical Systems

Disordered Hyperuniform Photonic Band Gap Materials

Until recently, the only materials known to have complete photonic band gaps were photonic crystals, periodic structures, and it was generally assumed that long-range periodic order was instrumental in the band gap formation. We have shown that there exists a more general class of systems, called hyperuniform photonic structures, which exhibit large and complete photonic band gaps. This classification includes not only crystalline structures, but also non-crystalline materials, ranging from quasicrystals with crystallographically-forbidden rotational symmetries to isotropic, translationally-disordered structures. Remarkably, we find that the photonic band gaps in hyperuniform disordered structures are not only comparable to those found in photonic crystals, but also display a high degree of isotropy. These new materials possess unique photonic and physical properties that provide important advantages for applications. Our results show that hyperuniform disordered structures enable the realization of optical cavities with ultimate isotropic confinement of the electromagnetic radiation, lossless waveguides with arbitrary bending angles and flexible optical insulator platforms.   

Experimental observation of photonic bandgaps in two dimensional hyperuniform disordered materials

We report the first experimental demonstration of photonic bandgaps (PBGs) in 2D hyperuniform disordered materials and show that is possible to obtain isotropic, disordered, photonic materials of arbitrary size with complete PBGs. There are only limited numbers of allowed rotational symmetries in periodic or quasiperiodic structures. Periodicity and Bragg scattering lead to different stop gap center frequencies in different directions, since periodicities change in different directions. Hyperuniformity together with short range geometric order and uniform local topology are enough to give raise to an isotropic PBG. Hyperuniform systems have a variance in mass or particle number which varies with distance, r, from an arbitrary point less rapidly than the d dimensional volume. We present a new class of photonic materials posessing PBGs that have a number of advantages, including: isotropy, robustness against disorder (they are already disordered), flexibility (can fit arbitrary regions of space in which one may have trouble putting a periodic system), and possibly lower minimum dielectric contrast.   

Photonic density of states of 2D quasicrystals from level set equations and decorated quasiperiodic tiling patterns

The TE and TM photonic band gaps (PBG) of 8mm, 10mm, and 12mm rotationally symmetric 2D quasicrystals (QCs) were numerically investigated for families of morphologies generated from level set equations and from quasiperiodic tiling patterns decorated with cylindrical rods, respectively. We discovered a 12mm QC with 56.5{\%} TE PBG, which is the largest reported TE PBG for aperiodic crystals to date. Further, we find that the TM PBG of 2D QC is highly related to the shape of the structural features comprised by the QC morphology. Two physical models are presented to explain the decrease of the center frequency of PBG as dielectric filling ratio increases.   

Measurement of photonic band diagram in non-crystalline photonic band gap (PBG) materials

Non-crystalline PBG materials have received increasing attention recently and sizeable PBGs have been reported in quasi-crystalline structures or even in disordered structures. Band calculations for periodic structures produce accurate dispersion relations in them and refraction properties at their surfaces. However, band calculations for non-periodic structures employ large super-cells of N $>$100 building blocks, and provide little useful information other than the PBG frequency and width. Since band is folded into N bands, within the first Brillouin zone of the supper-cell. Using stereolithography, we construct various quasi-crystalline or disordered PBG materials and perform transmission measurements. The dispersion relations of EM wave (band diagrams) are reconstructed from the measured phase data. Our experiments not only verify the existence of sizeable PBGs in these structures, but also provide detailed information of the effective band diagrams, dispersion relation, group velocity vector, and their angular dependence. Slow light phenomena are also observed in these structures near gap frequencies. This study presents a powerful tool to investigate photonic properties of non-crystalline structures and provides important dispersion information, which is otherwise impossible to obtain.   

MWIR tunable polarmetric scatterometery applied to a fishnet structure

Understanding how light is scattered by a material, such as a metamaterial, which is engineered to have specific optical properties is necessary for a better understanding of the design parameters and for refining designs. Because of their high irradiance and small spot size, lasers are an ideal light source for these scatter measurements. However, lasers are highly monochromatic and it can be very difficult to manufacture metamaterials to resonate at such specific wavelengths. By modifying a Schmitt Measurement System's Complete Angle Scatter Instrument (CASI) with the addition of 6 external cavity Quantum Cascade Lasers by Daylight Solutions we were able to have a tunable laser light source from 4.35 to 9.71 $\mu $m with a small exclusion from 6.54 to 7.40 $\mu $m. The CASI system was further modified with the addition of a dual rotating retarder which allows the full Mueller matrix to be calculated for both specular scatter and off specular scatter. This makes the system unique to commercially available systems like Woollam's IR-VASE which can only measure the Mueller matrix elements for the specular reflection. This unique system was used to measure a fishnet structure at both resonate and off resonate frequencies. The fishnet sample was also measured using an IR-VASE system to compare specular results.   

Electromagnetism in multicoaxial negative-index metamaterial cables

By using an elegant Green [or response] function theory, which does not require matching of the messy boundary conditions, we investigate the surface plasmon excitations in the multicoaxial cylindrical cables made up of negative-index metamaterials. The multicoaxial cables with {\em dispersive} metamaterial components exhibit rather richer (and complex) plasmon spectrum with each interface supporting two modes: one TM and the other TE for (the integer order of the Bessel function) $m \ne 0$. The cables with {\em nondispersive} metamaterial components bear a different tale: they do not support simultaneously both TM and TE modes over the whole range of propagation vector. The computed local and total density of states enable us to substantiate spatial positions of the modes in the spectrum. Such quasi-one dimensional systems as studied here should prove to be the milestones of the emerging optoelectronics and telecommunications systems.   

Universal shift of the Brewster angle in stratified random media

We study theoretically the propagation and the Anderson localization of p-polarized electromagnetic waves incident obliquely on randomly stratified dielectric media with weak uncorrelated Gaussian disorder. Using the invariant imbedding method, we calculate the localization length and the disorder-averaged transmittance in a numerically precise manner. We find that the localization length takes an extremely large maximum value at some critical incident angle, which we call the generalized Brewster angle. The disorder-averaged transmittance also takes a maximum very close to one at the same incident angle. Even in the presence of an arbitrarily weak disorder, the generalized Brewster angle is found to be substantially larger than the ordinary Brewster angle in uniform media. It is a rapidly increasing function of the average dielectric permittivity and approaches 90 degrees when the average relative dielectric permittivity is slightly larger than two. We find that the dependence of the generalized Brewster angle on the average dielectric permittivity is universal in the sense that it is independent of the strength of disorder and the wave frequency. We also make a surprising observation that when the p wave is incident from an optically denser region, the localization length and the average transmittance become larger for stronger disorder in a wide range of incident angle.   

Unidirectional suppresion of Bragg reflection in grated PT-symmetric media

We study the scattering properties of light through optical fibers with grating that involves gain/loss modulation that respect Parity-Time (PT) symmetry. We derive analytical expressions for transmission and reflection coefficients both in the presence and absence of Kerr non-linearity. At the spontaneous PT-symmetric point we have found that Bragg reflection is suppressed once the light is injected from the left, while it is amplified (with respect to the passive medium) if the fiber is illuminated from the right. Our results are robust for a large interval of the detuning parameter away from the Bragg wavelength.   

Application of Iterative Time-Reversal for Electromagnetic Wave Focusing in a Wave Chaotic System

Time-reversal mirrors exploit the time-reversal invariance of the wave equation to achieve spatial and temporal focusing, and they have been shown to be very effective sensors of perturbations to wave chaotic systems. The sensing technique is based on a classical analogue of the Loschmidt echo [1]. However, dissipation results in an imperfect focusing, hence we created a sensing technique employing exponential amplification to overcome this limitation [1,2]. We now apply the technique of iterative time-reversal, which had been demonstrated in a dissipative acoustic system, to an electromagnetic time-reversal mirror, and experimentally demonstrate improved temporal focusing. We also use a numerical model of a network of transmission lines to demonstrate improved focusing by the iterative technique for various degrees and statistical distributions of loss in the system. The application of the iterative technique to improve the performance and practicality of our sensor is explored.\\[4pt] [1] B. T. Taddese, et al., Appl. Phys. Lett. 95, 114103 (2009).\\[0pt] [2] B. T. Taddese, et al., J. Appl. Phys. 108, (2010) in press; arXiv:1008.2409.   

Optimizing energy transfer efficiency in highly branched nanoplasmonic waveguides

Energy transfer in highly branched nanoplasmonic particle waveguides is simulated and optimized by varying the waveguide branching geometry and composition. The periodically branched nanostructures provide a new route towards efficient nanoscale light concentration and local field enhancement. On the one hand, they mimick the analogous randomly branched plasmonic nanostructures which have been previously used for surface-enhanced optical spectroscopy such as SERS. On the other hand, the design is inspired by branched molecular aggregates used for energy funneling. The proposed nanostructures may find applications in sensing, light harvesting and nanophotonics.   

A Novel Nanoscale Coaxial Optical Microscope by Converging Array of Subwavelength Waveguides

A novel nanoscale coaxial optical microscope (NCOM) is proposed by constructing a converging array of coaxial subwavelength optical waveguides (nanocoax). This new design has potential for deep subdiffraction limit resolution, essentially independent of wavelength of the light source. The coaxial structure also has the capability of modal confinement, which can be utilized to extract phase information in the imaging plane. The transmittance and energy dissipation properties of a single nanocoax are obtained, in the visible light range, by numerical simulation. Optical properties of a converging nanocoax array are also investigated numerically. Finally, progress toward an experimental realization of this novel NCOM is discussed.   

Trap rainbow in a self-similar coaxial optical waveguide

We report in this work that the light waves with different frequencies can be selectively guided and spatially separated in a self-similar dielectric waveguide, where a hollow core is surrounded by a coaxial Thue-Morse multilayer. Due to the self-similar furcation feature in the photonic band structure, the transmission multibands are achieved. More interestingly, this dielectric waveguide supports cladding modes, which are spatially separated and confined along the waveguide. Consequently, a rainbow can be trapped (spatial confined but not stopped) in the Thue-Morse waveguide. The finding can be applied to designing miniaturized compact photonic devices, such as spectroscopy on a chip. Reference: Qing Hu, Jin-Zhu Zhao, Ru-Wen Peng, Feng Gao, Rui-Li Zhang, and Mu Wang, Appl. Phys. Lett. (2010) 96, 161101.   

Optical Transmission through Archimedeam Spiral Nanotrenches in Ti Film

We study the optical transmission of circularly polarized light through nanoscale Archimedean spiral trenches in Ti film through experiments and numerical simulations; the focus of these studies is on the effect of radial repetition of the spiral nanotrenches. Experimental measurements show that the left and right circularly polarized light exhibit different transmission through the spiral nanotrenches, and the transmission difference decays when the number of the radial periods of the spiral trenches is increased. Numerical simulations reproduce this interesting phenomenon. The underlying physical mechanism of the radial period dependence is attributed to the absorption difference at the center of the spirals.   

Bio inspired replication and mimicry of optical structure from nature

The optical response from some insects and animals is not from dye or pigment but from their complex structure. The so-called structural color involves interference, diffraction, scattering and photonic crystal effect in various combinations. Structures associated with the structural color have been invasively attended because they have been considered as essentials of optical and photonic devices. The diffraction grating was replicated from beetles by the atomic layer deposition (ALD), and the optical response of resulting structures was characterized. We also present our result on mimicry of the structure of Papilio butterfly. To mimic the structure in the butterfly, we created the basic cup-like structure from polymer films having ordered array of holes, and coated it with an alternating multilayer of the materials. The optical properties of the mimicked structures are also investigated.   

Carrier recombination lifetime in InGaN/GaN multiquantum well LED

Carrier dynamics in InGaN/GaN multiquantum well LED with 5{\%} In content in InGaN wells is studied by time-resolved photoluminescence (TRPL) using time-correlated single photon counting detection system. The excitation energy is 3.06 eV, frequency-doubled output of a Ti: sapphire laser operating at 808 nm (1.53 eV) with 100 fs pulse width and a repetition rate of 80 MHz. TRPL spectra are fitted biexponentially to obtain decay times. The fast decay process is carrier relaxation and slow decay is the carrier recombination process. The fast relaxation decay time shows insignificant variation with the photon energies and pump fluences. On the other hand carrier recombination time increases with increase of photon energies attaining maximum near photoluminescence peak energy and then decreases again on further increase of photon energies. The carrier recombination life time shows increasing behavior with increase of pump fluences and is obtained as long as $\sim $ 7 ns at pump fluence of 0.21 $\mu $J/cm$^{2}$ at room temperature. As the temperature decreases, the carrier recombination life time increases indicating the dominating nature of radiative decay process at low temperatures.