Rajat Sharma

Bio: Rajat Sharma is an academic researcher from University of California, Santa Barbara. The author has contributed to research in topics: Light-emitting diode & Epitaxy. The author has an hindex of 12, co-authored 22 publications receiving 752 citations.

Room-temperature continuous-wave lasing in GaN/InGaN microdisks

Abstract: Microdisk lasers feature low-loss, high-quality whispering gallery modes1,2,3 that offer the potential for ultralow-threshold lasing4,5,6,7 that is not limited by challenges in mirror fabrication. Here we report fabrication and optical measurements of GaN-based microdisk lasers with a very low threshold of 300 W cm−2—orders of magnitude lower than any previous GaN microdisk laser. This is also the first report of room-temperature, continuous-wave (CW) lasing in a GaN microdisk.

Roughening Hexagonal Surface Morphology on Laser Lift-Off (LLO) N-Face GaN with Simple Photo-Enhanced Chemical Wet Etching

Abstract: A photo-enhanced chemical wet etching technique is presented to form a roughened surface morphology with hexagonal symmetry on laser lift-off (LLO) N-face GaN grown by metalorganic chemical vapor deposition (MOCVD) An aqueous solution of KOH was used as etch electrolyte The etched surface showed cones with hexagonal pyramid structures bound by {1011} facets A detailed analysis of the etch rates and time-evolution of the surface morphology is described as a function of KOH concentration (125 M to 88 M) The comparison between (0001) N-face and Ga-face GaN etch morphology is discussed This roughened hexagonal surface morphology can be applied to enhance the external efficiency in GaN based light-emitting diodes (LEDs)

292 nm AlGaN Single-Quantum Well Light Emitting Diodes Grown on Transparent AlN Base

Abstract: We report the use of a novel transparent AlN base layer to achieve sub-300 nm electroluminescence from an AlGaN p-n junction single quantum well light emitting diode grown on sapphire by metal organic chemical vapor deposition. For unpackaged devices tested on wafer at a continuous current of 120 mA, an optical power of 2.4 µW was achieved. Peak emission was 292 nm, with very little secondary wavelength emission.

Photonic crystal-assisted light extraction from a colloidal quantum dot/GaN hybrid structure.

Abstract: We present a study of the light extraction from CdSe/ZnS core/shell colloidal quantum dot thin films deposited on quantum well InGaN/GaN photonic crystal structures. The two-dimensional photonic crystal defined by nanoimprint lithography is used to efficiently extract the guided light modes originating from both the quantum dot thin films and the InGaN quantum wells. Far-field photoluminescence spectra are used to measure the extraction enhancement factor of the quantum dot emission (×1.4). Microphotoluminescence measurements show that the guided mode effective extraction lengths range between 70 and 180 μm, depending on the wavelength of light.

Impact of strain on free-exciton resonance energies in wurtzite AlN

Abstract: The strain dependence of the free-exciton resonance energies in AlN epilayers is presented and the values are analyzed using an appropriate Hamiltonian assuming equibiaxial stress for the wurtzite crystal structure in order to obtain valence band parameters. Based on the results, we study the strain dependence of the valence band ordering, optical transition probability, and free-exciton binding energy. As a result of these calculations, the following strain-free values are obtained for the energy gap, averaged dielectric constants, and ordinary and extraordinary dielectric constants: Eg=6.095 eV at T=11 K, ϵ=7.87, ϵ⊥=7.33, and ϵ∥=8.45, respectively. A brief discussion of the valence band ordering in bulk AlxGa1−xN is also presented.

Monolayer excitonic laser

Abstract: A laser is made from the two-dimensional material WS2 by embedding it into a microdisk resonator.

Whispering gallery microcavity lasers

Abstract: Whispering gallery mode (WGM) optical microresonators have attracted intense interests in the past decades. The combination of high quality factors (Q) and small mode volumes of modes in WGM resonators significantly enhances the light-matter interactions, making them excellent cavities for achieving low threshold and narrow linewidth lasers. In this Review, the progress in WGM microcavity lasers is summarized, and the laser performance considering resonator geometries and materials as well as lasing mechanisms is discussed. Label-free detection using WGM resonators has emerged as highly sensitive detection schemes. However, the resolution is mainly limited by the cavity Q factor which determines the mode linewidth. Microcavity lasers, due to their narrow laser spectral width, could greatly improve the detection resolution. Some recent developments in sensing using microcavity lasers are discussed.

Comparison between blue lasers and light-emitting diodes for future solid-state lighting

Abstract: Solid-state lighting (SSL) is now the most efficient source of high color quality white light ever created. Nevertheless, the blue InGaN light-emitting diodes (LEDs) that are the light engine of SSL still have significant performance limitations. Foremost among these is the decrease in efficiency at high input current densities widely known as “efficiency droop.” Efficiency droop limits input power densities, contrary to the desire to produce more photons per unit LED chip area and to make SSL more affordable. Pending a solution to efficiency droop, an alternative device could be a blue laser diode (LD). LDs, operated in stimulated emission, can have high efficiencies at much higher input power densities than LEDs can. In this article, LEDs and LDs for future SSL are explored by comparing: their current state-of-the-art input-power-density-dependent power-conversion efficiencies; potential improvements both in their peak power-conversion efficiencies and in the input power densities at which those efficiencies peak; and their economics for practical SSL.

Continuous-wave biexciton lasing at room temperature using solution-processed quantum wells

Abstract: Solution-processed inorganic and organic materials have been pursued for more than a decade as low-threshold, high-gain lasing media, motivated in large part by their tunable optoelectronic properties and ease of synthesis and processing. Although both have demonstrated stimulated emission and lasing, they have not yet approached the continuous-wave pumping regime. Two-dimensional CdSe colloidal nanosheets combine the advantage of solution synthesis with the optoelectronic properties of epitaxial two-dimensional quantum wells. Here, we show that these colloidal quantum wells possess large exciton and biexciton binding energies of 132 meV and 30 meV, respectively, giving rise to stimulated emission from biexcitons at room temperature. Under femtosecond pulsed excitation, close-packed thin films yield an ultralow stimulated emission threshold of 6 μJ cm(-2), sufficient to achieve continuous-wave pumped stimulated emission, and lasing when these layers are embedded in surface-emitting microcavities.

III-Nitride UV Devices

Abstract: The need for efficient, compact and robust solid-state UV optical sources and sensors had stimulated the development of optical devices based on III–nitride material system. Rapid progress in material growth, device fabrication and packaging enabled demonstration of high efficiency visible-blind and solar-blind photodetectors, deep-UV light-emitting diodes with emission from 400 to 250 nm, and UV laser diodes with operation wavelengths ranging from 340 to 350 nm. Applications of these UV optical devices include flame sensing; fluorescence-based biochemical sensing; covert communications; air, water and food purification and disinfection; and biomedical instrumentation. This paper provides a review of recent advances in the development of UV optical devices. Performance of state-of-the-art devices as well as future prospects and challenges are discussed.