Debashis Chanda

Polarization-independent actively tunable colour generation on imprinted plasmonic surfaces.

Structural colour arising from nanostructured metallic surfaces offers many benefits compared to conventional pigmentation based display technologies, such as increased resolution and scalability of their optical response with structure dimensions. However, once these structures are fabricated their optical characteristics remain static, limiting their potential application. Here, by using a specially designed nanostructured plasmonic surface in conjunction with high birefringence liquid crystals, we demonstrate a tunable polarization-independent reflective surface where the colour of the surface is changed as a function of applied voltage. A large range of colour tunability is achieved over previous reports by utilizing an engineered surface which allows full liquid crystal reorientation while maximizing the overlap between plasmonic fields and liquid crystal. In combination with imprinted structures of varying periods, a full range of colours spanning the entire visible spectrum is achieved, paving the way towards dynamic pixels for reflective displays.

Porosity control in metal-assisted chemical etching of degenerately doped silicon nanowires

We report the fabrication of degenerately doped silicon (Si) nanowires of different aspect ratios using a simple, low-cost and effective technique that involves metal-assisted chemical etching (MacEtch) combined with soft lithography or thermal dewetting metal patterning. We demonstrate sub-micron diameter Si nanowire arrays with aspect ratios as high as 180:1, and present the challenges in producing solid nanowires using MacEtch as the doping level increases in both p- and n-type Si. We report a systematic reduction in the porosity of these nanowires by adjusting the etching solution composition and temperature. We found that the porosity decreases from top to bottom along the axial direction and increases with etching time. With a MacEtch solution that has a high [HF]:[H(2)O(2)] ratio and low temperature, it is possible to form completely solid nanowires with aspect ratios of less than approximately 10:1. However, further etching to produce longer wires renders the top portion of the nanowires porous.

Coupling of plasmonic and optical cavity modes in quasi-three-dimensional plasmonic crystals.

The field of plasmonics has emerged as an interesting area for fundamental studies, with important application possibilities in miniaturized photonic components. Plasmonic crystals are of particular relevance because of large field enhancements and extraordinary transmission that arise from plasmonic interactions between periodic arrays of metallic elements. Here we report methods to enhance and modify the plasmonic resonances in such structures by strongly coupling them to optical modes of Fabry-Perot type cavities. First, we illustrate a type of plasmonic, narrow-band (~15 nm), high-contrast (>20 dB) absorber and an opto-fluidic modulator based on this component. Second, we use optimized samples as substrates to achieve strong amplification (>350%) and modulation (>4×) of surface-enhanced Raman scattering from surface-bound monolayers. Cavity-coupling strategies appear to be useful not only in these two examples, but also in applications of plasmonics for optoelectronics, photovoltaics and related technologies.

Performance of ultrathin silicon solar microcells with nanostructures of relief formed by soft imprint lithography for broad band absorption enhancement

Recently developed classes of monocrystalline silicon solar microcells can be assembled into modules with characteristics (i.e., mechanically flexible forms, compact concentrator designs, and high-voltage outputs) that would be impossible to achieve using conventional, wafer-based approaches. This paper presents experimental and computational studies of the optics of light absorption in ultrathin microcells that include nanoscale features of relief on their surfaces, formed by soft imprint lithography. Measurements on working devices with designs optimized for broad band trapping of incident light indicate good efficiencies in energy production even at thicknesses of just a few micrometers. These outcomes are relevant not only to the microcell technology described here but also to other photovoltaic systems that benefit from thin construction and efficient materials utilization.

Formation of High Aspect Ratio GaAs Nanostructures with Metal-Assisted Chemical Etching

Periodic high aspect ratio GaAs nanopillars with widths in the range of 500-1000 nm are produced by metal-assisted chemical etching (MacEtch) using n-type (100) GaAs substrates and Au catalyst films patterned with soft lithography. Depending on the etchant concentration and etching temperature, GaAs nanowires with either vertical or undulating sidewalls are formed with an etch rate of 1-2 μm/min. The realization of high aspect ratio III-V nanostructure arrays by wet etching can potentially transform the fabrication of a variety of optoelectronic device structures including distributed Bragg reflector (DBR) and distributed feedback (DFB) semiconductor lasers, where the surface grating is currently fabricated by dry etching.

Experimental Study of Design Parameters in Silicon Micropillar Array Solar Cells Produced by Soft Lithography and Metal-Assisted Chemical Etching

Solar cells, consisting of core-shell p-n junction silicon micropillars on a thin membrane fabricated using soft lithography and metal-assisted chemical etching, are studied as a function of geometrical designs. Significant enhancement in absorption rate is found without much dependence on the pillar diameters in the range of 0.5-2 μm. However, the short-circuit current increases continuously with diameter, which is inversely proportional to the total surface area for a fixed diameter/pitch pillar array. This study provides unambiguous evidence that surface recombination is the dominant loss mechanism in nanowire- or micropillar-based solar cells.

Roadmap on optical metamaterials

Optical metamaterials have redefined how we understand light in notable ways: from strong response to optical magnetic fields, negative refraction, fast and slow light propagation in zero index and trapping structures, to flat, thin and perfect lenses. Many rules of thumb regarding optics, such as mu = 1, now have an exception, and basic formulas, such as the Fresnel equations, have been expanded. The field of metamaterials has developed strongly over the past two decades. Leveraging structured materials systems to generate tailored response to a stimulus, it has grown to encompass research in optics, electromagnetics, acoustics and, increasingly, novel hybrid materials responses. This roadmap is an effort to present emerging fronts in areas of optical metamaterials that could contribute and apply to other research communities. By anchoring each contribution in current work and prospectively discussing future potential and directions, the authors are translating the work of the field in selected areas to a wider community and offering an incentive for outside researchers to engage our community where solid links do not already exist.

Multi-level diffractive optics for single laser exposure fabrication of telecom-band diamond-like 3-dimensional photonic crystals

We present a novel multi-level diffractive optical element for diffractive optic near-field lithography based fabrication of large-area diamond-like photonic crystal structure in a single laser exposure step. A multi-level single-surface phase element was laser fabricated on a thin polymer film by two-photon polymerization. A quarter-period phase shift was designed into the phase elements to generate a 3D periodic intensity distribution of double basis diamond-like structure. Finite difference time domain calculation of near-field diffraction patterns and associated isointensity surfaces are corroborated by definitive demonstration of a diamond-like woodpile structure formed inside thick photoresist. A large number of layers provided a strong stopband in the telecom band that matched predictions of numerical band calculation. SEM and spectral observations indicate good structural uniformity over large exposure area that promises 3D photonic crystal devices with high optical quality for a wide range of motif shapes and symmetries. Optical sensing is demonstrated by spectral shifts of the Gamma-Zeta stopband under liquid emersion.

One-dimensional diffractive optical element based fabrication and spectral characterization of three-dimensional photonic crystal templates.

We demonstrate improved fabrication precision and provide the first spectral characterization of Woodpile-type photonic crystal templates formed by one-dimensional diffractive optical elements. The three-dimensional periodic structures were produced in thick resist by sequential exposures of two orthogonal diffractive optical elements with an argon-ion laser. The observed crystal motif is shown to closely match the iso-intensity surfaces predicted by the interfering diffracted beams. Near-infrared spectroscopic observations reveal the presence of both low and high energy photonic stopbands that correspond with theoretical predictions in several crystal directions. Numerous high-energy stop bands are further reported along very narrow crystallographic angles that attest to the high periodicity and uniformity of the crystal motif through the full resist thickness and over the large sample area. The optical characterization demonstrates the precise control and facile means of diffractive-optical-element based holographic lithography in fabricating large-area three-dimensional photonic crystal templates, defining a promising medium for infiltration with high-refractive-index materials to create photonic bandgap devices.

Performance of clipped OFDM signal in fiber

Combined deployment of optical fiber technology and wireless networks has great potential for increasing the capacity and quality of service. By using radio-over-fiber (ROF) technology, the capacity of optical networks can be combined with the flexibility and mobility of wireless access networks without significant cost increment. The radio-over-fiber concept means to transport information over optical-fiber by modulating the light with the radio signal. This article discusses the effects of using fiber in conjunction with wireless local area network IEEE 802.11a standard (WLAN) to distribute RF signals. To achieve high throughput 802.11a LAN uses an orthogonal frequency division multiplexing (OFDM) based multicarrier wideband modulation technique. OFDM is one of the most favored modulation techniques in the WLAN scenario due to its efficient implementation and robustness against multipath and narrowband interference. One of the biggest drawbacks of OFDM is its high peak to average power ratio (PAPR). High PAPR of OFDM makes it unusable in nonlinear systems. In this article we discuss better ways to overcome the PAPR problem of the OFDM signal which will improve its performance in fiber.