Shawn-Yu Lin

A three-dimensional photonic crystal operating at infrared wavelengths

The ability to confine and control light in three dimensions would have important implications for quantum optics and quantum-optical devices: the modification of black-body radiation, the localization of light to a fraction of a cubic wavelength, and thus the realization of single-mode light-emitting diodes, are but a few examples. Photonic crystals — the optical analogues of electronic crystal — provide a means for achieving these goals. Combinations of metallic and dielectric materials can be used to obtain the required three-dimensional periodic variations in dielectric constant, but dissipation due to free carrier absorption will limit application of such structures at the technologically useful infrared wavelengths. On the other hand, three-dimensional photonic crystals fabricated in low-loss gallium arsenide show only a weak ‘stop band’ (that is, range of frequencies at which propagation of light is forbidden) at the wavelengths of interest. Here we report the construction of a three-dimensional infrared photonic crystal on a silicon wafer using relatively standard microelectronics fabrication technology. Our crystal shows a large stop band (10–14.5 μm), strong attenuation of light within this band (∼12 dB per unit cell) and a spectral response uniform to better than 1 per cent over the area of the 6-inch wafer.

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.

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.

Three-dimensional photonic-crystal emitter for thermal photovoltaic power generation

A three-dimensional tungsten photonic crystal is experimentally realized with a complete photonic band gap at wavelengths λ⩾3 μm. At an effective temperature of 〈T〉∼1535 K, the photonic crystal exhibits a sharp emission at λ∼1.5 μm and is promising for thermal photovoltaic (TPV) power generation. Based on the spectral radiance, a proper length scaling and a planar TPV model calculation, an optical-to-electric conversion efficiency of ∼34% and electrical power of ∼14 W/cm2 is theoretically possible.

Three-dimensional photonic crystal with a stop band from 1.35 to 1.95???m

A combination of advanced silicon-processing techniques was used to create three-dimensional (3D) photonic crystals with a 180-nm minimum feature size. The resulting 3D crystal displayed a strong stop band at optical wavelengths from lambda=1.35 microm to lambda=1.95 microm . This is believed to be the smallest 3D crystal with a complete 3D photonic bandgap ever created.

Realization of a near-perfect antireflection coating for silicon solar energy utilization

To harness the full spectrum of solar energy, Fresnel reflection at the surface of a solar cell must be eliminated over the entire solar spectrum and at all angles. Here, we show that a multilayer nanostructure having a graded-index profile, as predicted by theory [J. Opt. Soc. Am. 66, 515 (1976); Appl. Opt. 46, 6533 (2007)], can accomplish a near-perfect transmission of all-color of sunlight. An ultralow total reflectance of 1%-6% has been achieved over a broad spectrum, lambda = 400 to 1600 nm, and a wide range of angles of incidence, theta = 0 degrees-60 degrees . The measured angle- and wavelength-averaged total reflectance of 3.79% is the smallest ever reported in the literature, to our knowledge.

Highly dispersive photonic band-gap prism

We propose the concept of a photonic band-gap (PBG) prism based on two-dimensional PBG structures and realize it in the millimeter-wave spectral regime. We recognize the highly nonlinear dispersion of PBG materials near Brillouin zone edges and utilize the dispersion to achieve strong prism action. Such a PBG prism is very compact if operated in the optical regime, ~20 mm in size for lambda ~ 700 nm, and can serve as a dispersive element for building ultracompact miniature spectrometers.

A Surface Plasmon Enhanced Infrared Photodetector Based on InAs Quantum Dots

In this paper, we report a successful realization and integration of a gold two-dimensional hole array (2DHA) structure with semiconductor InAs quantum dot (QD). We show experimentally that a properly designed 2DHA-QD photodetector can facilitate a strong plasmonic-QD interaction, leading to a 130% absolute enhancement of infrared photoresponse at the plasmonic resonance. Our study indicates two key mechanisms for the performance improvement. One is an optimized 2DHA design that permits an efficient coupling of light from the far-field to a localized plasmonic mode. The other is the close spatial matching of the QD layers to the wave function extent of the plasmonic mode. Furthermore, the processing of our 2DHA is amenable to large scale fabrication and, more importantly, does not degrade the noise current characteristics of the photodetector. We believe that this demonstration would bring the performance of QD-based infrared detectors to a level suitable for emerging surveillance and medical diagnostic applications.

A three-dimensional optical photonic crystal

We report on the successful fabrication of a working three-dimensional (3D) crystal operating at optical wavelength /spl lambda/. The minimum feature size of the 3D structure is 180 nm. The 3D crystal is free from defects over the entire 6-inch silicon wafer and has an absolute photonic bandgap (PBG) centered at /spl lambda//spl sim/1.6 /spl mu/m. Our data provides the first conclusive evidence for the existence of a complete 3D PBG in optical /spl lambda/. This development will pave the way to tinier, cheaper, more effective waveguides, optical switches and lasers.

Very low-refractive-index optical thin films consisting of an array of SiO 2 nanorods

The refractive-index contrast in dielectric multilayer structures, optical resonators, and photonic crystals is an important figure of merit that creates a strong demand for high-quality thin films with a low refractive index. A SiO2 nanorod layer with low refractive index of n = 1.08, to our knowledge the lowest ever reported in thin-film materials, is grown by oblique-angle electron-beam deposition of SiO2. A single-pair distributed Bragg reflector employing a SiO2 nanorod layer is demonstrated to have enhanced reflectivity, showing the great potential of low-refractive-index films for applications in photonic structures and devices.