I. El-Kady

All-metallic three-dimensional photonic crystals with a large infrared bandgap

Three-dimensional (3D) metallic crystals are promising photonic bandgap structures: they can possess a large bandgap, new electromagnetic phenomena can be explored, and high-temperature (above 1,000 °C) applications may be possible. However, investigation of their photonic bandgap properties is challenging, especially in the infrared and visible spectrum, as metals are dispersive and absorbing in these regions. Studies of metallic photonic crystals have therefore mainly concentrated on microwave and millimetre wavelengths. Difficulties in fabricating 3D metallic crystals present another challenge, although emerging techniques such as self-assembly may help to resolve these problems. Here we report measurements and simulations of a 3D tungsten crystal that has a large photonic bandgap at infrared wavelengths (from about 8 to 20 µm). A very strong attenuation exists in the bandgap, ∼30 dB per unit cell at 12 µm. These structures also possess other interesting optical properties; a sharp absorption peak is present at the photonic band edge, and a surprisingly large transmission is observed in the allowed band, below 6 µm. We propose that these 3D metallic photonic crystals can be used to integrate various photonic transport phenomena, allowing applications in thermophotovoltaics and blackbody emission.

Photonic crystal enhanced narrow-band infrared emitters

We have experimentally and theoretically developed a unique thermally stimulated midinfrared source that emits radiation within a narrow range of wavelengths (δλ/λ⩽0.2). The emission wavelengths are defined by the periodicity of a metal coated silicon–air photonic crystal etched into the emitter surface. The lattice of the holes in the metal mediate the coupling of light into discrete surface plasmon states. This yields surfaces with spectrally nonuniform infrared reflection properties where over much of the IR 90+% of photons are reflected yet, in a narrow spectral region, 90% absorption is observed. Transfer matrix calculations simulate well the position and strength of the absorption features. This technology will afford tunable infrared emitters with high power in a narrow spectral band that are critical for sensing, spectroscopy, and thermophotovoltaic applications.

Origin of absorption enhancement in a tungsten, three-dimensional photonic crystal

A three-dimensional, metallic photonic crystal is realized and its absorption measured at infrared wavelengths. The metallic absorption rate is found to be suppressed in the photonic bandgap regime (λ∼8–20 μm). On the other hand, order-of-magnitude absorption enhancement is observed at the photonic band edge (λ∼5.8 μm). The enhancement is attributed to the slower group velocity of light at the photonic band edge, a longer photon–matter interaction length, and a finite intrinsic absorption of tungsten.

Dielectric waveguides in two-dimensional photonic bandgap materials

We investigate the performance and guiding properties of waveguides fabricated in a finite two-dimensional (2-D) photonic bandgap (PBG) structure. Confinement in the direction perpendicular to the plane of periodicity is achieved by fabricating the 2-D PBG structure in a high dielectric layer enclosed by two lower dielectric layers. Simulations using the finite-difference time-domain (FDTD) method are performed to investigate the energy transport in such waveguides. Good qualitative agreement is found with the experimental observations.

Enhanced complete photonic band gap in the optimized planar diamond structure

Abstract The diamond photonic crystal with dielectric rods has been modified to enlarge the fundamental band gap. By planarizing the diamond structure and reducing the thickness of the hexagonal meshes, the band gap can be increased substantially. The band gap is 29% for a refractive index contrast of 3.6. The modified structure is amenable to fabrication at optical and infrared wavelengths using state-of-the-art silicon-processing methods. Transfer matrix calculations demonstrate a large attenuation within the band gap.

Modeling of Photonic Band Gap Crystals and Applications

In this work, the authors have undertaken a theoretical approach to the complex problem of modeling the flow of electromagnetic waves in photonic crystals. The focus is to address the feasibility of using the exciting phenomena of photonic gaps (PBG) in actual applications. The authors start by providing analytical derivations of the computational electromagnetic methods used in their work. They also present a detailed explanation of the physics underlying each approach, as well as a comparative study of the strengths and weaknesses of each method. The Plane Wave expansion, Transfer Matrix, and Finite Difference time Domain Methods are addressed. They also introduce a new theoretical approach, the Modal Expansion Method. They then shift the attention to actual applications. They begin with a discussion of 2D photonic crystal wave guides. The structure addressed consists of a 2D hexagonal structure of air cylinders in a layered dielectric background. Comparison with the performance of a conventional guide is made, as well as suggestions for enhancing it. The studies provide an upper theoretical limit on the performance of such guides, as they assumed no crystal imperfections and non-absorbing media. Next, they study 3D metallic PBG materials at near infrared and optical wavelengths. The main objective is to study the importance of absorption in the metal and the suitability of observing photonic band gaps in such structures. They study simple cubic structures where the metallic scatters are either cubes or interconnected metallic rods. Several metals are studied (aluminum, gold, copper, and silver). The effect of topology is addressed and isolated metallic cubes are found to be less lossy than the connected rod structures. The results reveal that the best performance is obtained by choosing metals with a large negative real part of the dielectric function, together with a relatively small imaginary part. Finally, they point out a new direction in photonic crystal research that involves the interplay of metallic-PBG rejection and photonic band edge absorption. They propose that an absolute metallic-PBG may be used to suppress the infrared part of the blackbody emission and, emit its energy only through a sharp absorption band. Potential applications of this new PBG mechanism include highly efficient incandescent lamps and enhanced thermophotovoltaic energy conversion. The suggested lamp would be able to recycle the energy that would otherwise go into the unwanted heat associated with usual lamps, into light emitted in the visible spectrum. It is estimated this would increase the efficiency over conventional lamps by about 40%.

Photonic crystals for narrow-band infrared emission

MEMS silicon (Si) micro-bridge elements, with photonic band gap (PBG) modified surfaces are exploited for narrow-band spectral tuning in the infrared wavelength regime. Thermally isolated, uniformly heated single crystal Si micro-heaters would otherwise provide gray-body emission, in accordance with Plancks distribution function. The introduction of an artificial dielectric periodicity in the Si, with a surface, vapor-deposited gold (Au) metal film, governs the photonic frequency spectrum of permitted propagation, which then couples with surface plasmon states at the metal surface. Narrow band spectral tuning was accomplished through control of symmetry and lattice spacing of the PBG patterns. Transfer matrix calculations were used to model the frequency dependence of reflectance for several lattice spacings. Theoretical predictions that showed narrow-band reflectance at relevant wavelengths for gas sensing and detection were then compared to measured reflectance spectra from processed devices. Narrow band infrared emission was confirmed on both conductively heated and electrically driven devices.