James A. Bur

Experimental observation of an extremely dark material made by a low-density nanotube array.

An ideal black material absorbs light perfectly at all angles and over all wavelengths. Here, we show that low-density vertically aligned carbon nanotube arrays can be engineered to have an extremely low index of refraction, as predicted recently by theory [Garcia-Vidal, F. J.; Pitarke, J. M.; Pendry, J. B. Phys. Rev. Lett. 1997, 78, 4289-4292] and, combined with the nanoscale surface roughness of the arrays, can produce a near-perfect optical absorption material. An ultralow diffused reflectance of 1 x 10(-7) measured from such arrays is an order-of-magnitude lower compared to commercial low-reflectance standard carbon. The corresponding integrated total reflectance of 0.045% from the nanotube arrays is three times lower than the lowest-ever reported values of optical reflectance from any material, making it the darkest man-made material ever.

Experimental observation of extremely weak optical scattering from an interlocking carbon nanotube array.

We experimentally demonstrate a nearly wavelength-independent optical reflection from an extremely rough carbon nanotube sample. The sample is made of a vertically aligned nanotube array, is a super dark material, and exhibits a near-perfect blackbody emission at T=450 K-600 K. No other material exhibits such optical properties, i.e., ultralow reflectance accompanied by a lack of wavelength scaling behavior. This observation is a result of the lowest ever measured reflectance (R=0.0003) of the sample over a broad infrared wavelength of 3 μm < λ < 13 μm. This discovery may be attributed to the unique interlocking surface of the nanotube array, consisting of both a global, large scale and a short-range randomness.

High-temperature metal coating for modification of photonic band edge position

Atomic-layer-deposited iridium is coated onto three-dimensional tungsten woodpile photonic crystals to modify the optical properties of the structure. As the lattice constant of a metallic photonic bandgap structure decreases to the scale of the near-IR wavelengths, the band edge becomes pinned and cannot be pushed to shorter wavelengths because of limitations inherent in the material. With a thin coating of iridium, the band edge of a pinned tungsten lattice is pushed from ~1.6 μm to below 1 μm. This shift in the reflectance band edge will be accompanied by a reduction in the absorptance of the structure in the 1-2 μm range.

Visible three-dimensional metallic photonic crystal with non-localized propagating modes beyond waveguide cutoff

We report experimental realization of a 5-layer three-dimensional (3D) metallic photonic crystal structure that exhibits characteristics of a 3D complete bandgap extending from near-infrared down to visible wavelength at around 650 nm. The structure also exhibits a new kind of non-localized passband mode in the infrared far beyond its metallic waveguide cutoff. This new passband mode is drastically different from the well-known defect mode due to point or line defects. Three-dimensional finite-difference-time-domain simulations were carried out and the results suggest that the passband modes are due to intra-structure resonances.

Achieving a photonic band edge near visible wavelengths by metallic coatings

A metallic coating method is used to modify the optical properties of a dielectric photonic lattice and to achieve a near visible photonic band edge. It is experimentally shown that the linear scaling rule of a metallic band edge versus lattice constant holds only for perfect conducting metals. When a metal deviates from a perfect conducting behavior near the plasma wavelength, the metallic photonic band edge is pinned and is nearly independent of lattice constant. For our tungsten photonic lattice, the pinning occurs at λ≈1.5–2μm. By using a thin copper coating (∼70nm) to a dielectric photonic lattice, a photonic band edge at λ∼750nm is observed. This achievement is made possible by the fact that copper is a good conductor at visible wavelengths and the linear scaling rule holds. Finally, this coating method allows for tailoring photonic properties through material engineering at the nanometer scale.

Direct observation of quasi-coherent thermal emission by a three-dimensional metallic photonic crystal

We report a direct observation of a quasi-coherent thermal emission from a heated three-dimensional photonic-crystal sample. While the sample was under Joule heating, we observed multiple oscillations in its emission interferogram and deduced a coherent length of L(coh)≅(20-40) μm, 5-10 times longer than that of a blackbody at comparable wavelengths. The observed, relatively long coherent length is attributed to coupling of thermal emission into lossy Bloch modes that oscillate coherently over a distance determined by decay length and the slow light nature of Bloch modes at the band-edges.

Optical Characterization of Bending Efficiency in On-Chip Hollow-Core Bragg Waveguides at

We present an experimental demonstration of high bending efficiency in on-chip hollow core of Bragg waveguides. Laser light in the wavelength range 1260-1630 nm is coupled into the hollow core of Bragg waveguides made of alternating silicon and silicon dioxide layers, and the transmission intensity of straight waveguides and waveguides with two bends are compared. Guiding by both types of waveguides is observed over a wide range of (nm).

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The use of metallic nanostructures for enhanced transmission and near field phenomena have been a topic of extensive research. Here we present integration of active media, consisting of InAs quantum dots (QD) embedded in quantum wells, with 2 dimensional metallic hole arrays (2DHA) leading to a strong interaction between resonant surface plasmons, excited at the metal-semiconductor interface, and intersubband transitions of quantum dots. The presence of a low-loss absorber within the enhanced near field region of 2DHA leads to an enhancement of photoresponse. The parameters of 2DHA were designed to overlap with absorption peaks of QDs. We present techniques of fabrication, accurate characterization of enhancement and efforts to optimize the 2DHA-QD coupling. Over an order of magnitude enhancement in photoresponse is observed due to spectral matching of intersubband absorption of quantum dots to that of 2DHA resonance, optimal placement of QD within the structure, and improved interaction lengths due to lateral propagation. This enhancement is also accompanied by significant narrowing of linewidth and the ability to tune the resonance by varying the 2DHA parameters. A hexagonal lattice with periodic circular holes on a thin gold film is used as the 2DHA. With further optimizations, these structures have significant applications in the mid-wave infrared (3-5 μm) and long-wave infrared (8-12 μm) regions for multispectral and polarization sensitive sensing

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Over the past two decades, the field of photonic-crystals has become one of the most influential realms of contemporary optics. In this paper, we will review two recent experimental progresses in three-dimensional photonic-crystal operating in optical wavelengths. The first is the observation of anomalous light-refraction, an acutely negative refraction, in a 3D photonic-crystal for light trapping, guiding and near-unity absorption. The second is the observation of quasi-coherent thermal emission from an all-metallic 3D photonic-crystal at elevated temperatures.

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A deformable photonic band gap (PBG) material is theoretically proposed as a transmissive element for beam steering at 77GHz. The deformation may be achieved by integrating microsprings (as spacers) into a one-dimensional PBG structure. This PBG material can produce a specific phase shift dependent on its spacer thickness. By varying the spacer thickness, we generate a continuous phase gradient across the element. Such a PBG device is experimentally realized, capable of beam deflection of up to ±15° and suitable for beam scanning for smart automobile radar application.