Generalized homogenization method for subwavelength periodic lattices.

Periodic photonic lattices based on Guided-Mode Resonance (GMR) enable the manipulation of the incident light, making them essential components in a plethora of optical elements including filters, sensors, lasers, and polarizers. The GMR is regarded as a resonance phenomenon in the resonant-subwavelength regime of periodic lattices. We present a method that homogenizes these periodic structures in the subwavelength regime and provides an appropriate analytical interpretation of the resonance effect. Here, we propose a technique based on utilizing the dispersion relation for homogenization, which can be applied to multi-part period lattices under oblique incidence. The effect of asymmetry and emergence of the odd/even modes, not considered in previous methods, will also be taken into account and discussed. As a result of this analytical procedure, resonance lines are obtained, which are useful in designing optical elements such as wideband/narrowband reflectors and polarizers.

Tunable two-dimensional optical filter based on guided-mode resonance.

In this paper, a tunable, two-dimensional periodic structure based on guided-mode resonance (GMR) is proposed. This element can be employed as a tunable optical filter or a display pixel. Resonance tuning in this device is accomplished through perturbation in the refractive index profile and element's thickness, conceptually by a micro-electro-mechanical system mechanism. Simulation results show that resonance wavelength tunings of ∼48 nm (522-570 nm) and ∼17 nm (526-543 nm) are achieved by physical movement of 166 nm horizontally and 300 nm vertically, respectively. The proposed device can fairly maintain its tunability under ±5° deviations in the angle of incidence.

Properties of two-dimensional resonant reflectors with zero-contrast gratings.

The spectral properties of high-reflection mirrors imbued with two-dimensional (2D) subwavelength periodicity are investigated. The reflectors are designed in a silicon-on-glass film that is partially etched to implement a zero-contrast interface between the grating pillars and the sublayer, thereby annulling the local reflections and phase changes associated with hard interfaces. This approach has shown impressive results for 1D polarized reflectors; here we strive to discover analogous wideband unpolarized reflectors. Using particle swarm optimization, we report wideband unpolarized reflectors in the 1.4-2.0 µm wavelength band. A 2D reflector with square pillars exhibits 99% reflectance across a bandwidth exceeding 350 nm and possesses tolerance against angular deviations. The complementary structure with rectangular periodic voids achieves a bandwidth of 370 nm. A comparable, optimized 2D high-contrast grating reflector with grating pillars residing directly on the substrate yields a 99% bandwidth of 240 nm.

Dispersion engineering with leaky-mode resonant photonic lattices.

We investigate the dispersion properties of leaky-mode resonance elements with emphasis on slow-light applications. Using particle swarm optimization, we design three exemplary bandpass leaky-mode devices. A single-layer silicon-on-insulator leaky-mode element shows a time-delay peak of ~8 ps at the resonance wavelength. A double membrane element exhibits an average delay of ~6 ps over ~0.75 nm spectral bandwidth with a relatively flat dispersion response. By cascading five double-membrane elements, we achieve an accumulative delay of ~30 ps with a very flat dispersion response over ~0.5 nm bandwidth. Thus, we show that delay elements based on leaky-mode resonance, by proper design, exhibit large amount of delay yet very flat dispersion over appreciable spectral bandwidths, making them potential candidates for optical buffers, delay lines, and switches.

Leaky-mode resonant reflectors with extreme bandwidths.

We present ultrawide flat-band reflectors enabled with multilevel resonant leaky-mode structures. The reflectors are designed using particle swarm optimization. We show that three-level silicon-on-insulator structures provide bandwidths of approximately 840 nm for TE polarization and approximately 835 nm for TM polarization, across which reflectance is greater than 99% and 97.5%, respectively. For the germanium-on-insulator system, the 99% TE and TM bandwidths are approximately 1100 nm and approximately 1050 nm. The results indicate the potential of multilevel resonant leaky-mode elements in electromagnetics and photonics.

Guided-mode resonant wave plates.

We introduce half-wave and quarter-wave retarders based on the dispersion properties of guided-mode resonance elements. We design the wave plates using numerical electromagnetic models joined with the particle swarm optimization method. The wave plates operate in reflection. We provide computed results for reflectance and phase in the telecommunication spectral region near 1.55 microm wavelength. A surface-relief grating etched in glass and overcoated with silicon yields a half-wave plate with nearly equal amplitudes of the TE and TM polarization components and pi phase difference across a bandwidth exceeding 50 nm. Wider operational bandwidths are obtainable with more complex designs involving glass substrates and mixed silicon/hafnium dioxide resonant gratings. The results indicate a potential new approach to fashion optical retarders.

Physical basis for wideband resonant reflectors.

In this paper, we address resonant leaky-mode reflectors made with a periodic silicon layer on an insulating substrate. Our objective is to explain the physical basis for their operation and to quantify the bandwidth provided by a single resonant layer by illustrative examples for both TE and TM polarized incident light. We find that the number of participating leaky modes and their excitation conditions affect the bandwidth. We show that recently reported experimental [1, 2] wideband reflectors operate under leaky-mode resonance. These compact reflectors are new elements with many potential applications in photonic systems. The results presented explaining their physical basis will aid in their continued development.

Wideband leaky-mode resonance reflectors: influence of grating profile and sublayers.

We apply inverse numerical methods to design compact wideband reflectors in which a periodic silicon layer supports resonant leaky modes. Using particle swarm optimization to determine appropriate device thickness, period, and fill factors, we arrive at example reflector designs for both TE and TM polarized input light. As a properly configured grating profile provides added design freedom, we design reflectors with two and four subparts in the period. In TM polarization, a particular single-layer two-part reflector has 520 nm bandwidth whereas the four-part device reaches 600 nm bandwidth. In TE polarization, the corresponding numbers are 125 nm and 495 nm, respectively. We provide a qualitative explanation for the smaller TE-reflector bandwidth. We quantify the effects of deviation from the design parameters and compute the angular response of the elements. As the angle of incidence deviates from normal incidence, narrow transmission channels emerge in the response yielding a bandpass filter with low sidebands. The effects of adding a silica sublayer between a silicon substrate and the periodic silicon layer is investigated. It is found that a properly designed sublayer can extend the reflection bandwidth significantly.

Widely tunable guided-mode resonance nanoelectromechanical RGB pixels.

Widely tunable display pixels are reported. The pixel consists of a subwavelength silicon-nitride/air membrane containing complementary fixed and mobile gratings. By altering the device refractive index profile and symmetry, using MEMS actuation methods, wavelength tuning across ~100 nm per pixel in the visible spectral region is shown to be possible. Initial results illustrating the influence of structural symmetry, pixel thickness, and polarization on the spectral response are provided. These pixels exhibit ~+/-4 degrees angular acceptance aperture. Applications in compact display systems are envisioned.

Particle swarm optimization and its application to the design of diffraction grating filters.

Particle swarm optimization (PSO) is an evolutionary, easy-to-implement technique to design optical diffraction gratings. Design of reflection and transmission guided-mode resonance (GMR) grating filters using PSO is reported. The spectra of the designed filters are in good agreement with the design targets in a reasonable computation time. Also, filters are designed with a genetic algorithm (GA) and the results obtained by the GA and PSO are compared.

Comparison of antireflection surfaces based on two-dimensional binary gratings and thin-film coatings.

A comparison of antireflection surfaces based on the two-dimensional binary gratings and thin-film coatings is presented. First, a two-dimensional hybrid binary grating is proposed and analyzed by use of a vector-based implementation of the rigorous coupled-wave analysis method. The optimum parameters of the structure are determined and the effects that changing them have on spectral characteristics of the structure are studied. Then this structure is compared with multilayer thin-film antireflection filters. These filters are designed by genetic algorithm and needle methods, which are powerful methods for multilayer filter design. The comparison results show that the sensitivity of the grating to changes in the incident wavelength is high. However, a reflectance of the order of 10(-3)% at the design wavelength can be achieved. The sensitivity of designed antireflection thin-film filters to wavelength changes is lower, however, and the minimum achievable reflectance is higher.

Integrated all-optical pulse regenerator in chalcogenide waveguides.

We report a fully integrated, passive, all-optical regenerator capable of terabit per second operation, based on a highly nonlinear chalcogenide (As2S3) glass rib waveguide followed by an integrated Bragg grating bandpass filter. We demonstrate a clear nonlinear power transfer curve with 1.4 ps optical pulses, capable of improving the signal-to-noise ratio and reducing the bit error rate for digital signals.