Evidence for energy relaxation via a radiative cascade in surface-passivated PbS quantum dots

Multiple emission peaks have been observed from surface passivated PbS nanocrystals displaying strong quantum confinement. The emission spectra are shown to be strongly dependent on the excited-state parity. We also find that intraband energy relaxation from initial states excited far above the band-edge is nearly three orders of magnitude slower than that found in other nanocrystal quantum dots, providing evidence of inefficient energy relaxation via phonon emission. The initial-state parity dependence of the photoluminescent emission properties suggests that energy relaxation from the higher excited states occurs via a radiative cascade, analogous to energy relaxation in atomic systems. Such radiative cascade emission is possible from ideal zero-dimensional semiconductors, where electronic transitions can be decoupled from phonon modes.

Direct observation of mixed-parity excited states in surface-passivated PbS nanocrystals

Photoluminescent emission is observed from surface-passivated PbS nanocrystals following the two-photon excitation of high-energy excitonic states. The emission appears directly at the excitation energy with no detectable Stokes-shift for a wide range of excitation energies. The observation of direct emission from states excited by two-photon absorption indicates that the parity of the excited states of surface-passivated PbS nanocrystals is partially mixed.

COMMAD 04 proceedings

The following topics were dealt with: semiconductor growth (MBE, PECVD, MOCVD, MOVPE) and characterizations; high-electron mobility transistors (HEMTs); microcavity organic light emitting diode (MOLED); semiconductor superlattices; photodiode arrays; MEMS structures; lithography;semiconductor lasers; semiconductor optical amplifiers; surface treatment and annealing

Novel spatial scanning technique for surface roughness measurement

We present the first spatial scanning system using wavelength-spatial transformation of chromatic dispersion device. Optical probe used in fiber optic interferometer for surface measurement is demonstrated by using diffraction grating and wavelength scanning technique.

Novel evaluation procedure for internal and extraction efficiency of high-power blue LEDs

Internal quantum efficiency (IQE) of a high-brightness blue LED has been evaluated from the external quantum efficiency measured as a function of current at room temperature. Processing the data with a novel evaluation procedure based on the ABC-model, we have determined separately IQE of the LED structure and light extraction efficiency (LEE) of UX:3 chip. Full text Nowadays, understanding of LED efficiency behavior at high currents is quite critical to find ways for further improve­ment of III-nitride LED performance [1]. External quantum ef­ficiency ηe (EQE) provides integral information on the recom­bination and photon emission processes in LEDs. Meanwhile EQE is the product of IQE ηi and LEE ηext at negligible car­rier leakage from the active region. Separate determination of IQE and LEE would be much more helpful, providing correla­tion between these parameters and specific epi-structure and chip design. In this paper, we extend the approach of [2,3] to the whole range of the current/optical power variation, provid­ing an express tool for separate evaluation of IQE and LEE. We studied an InGaN-based LED fabricated by Osram OS. LED structure grown by MOCVD on sapphire substrate was processed as UX:3 chip and mounted into the Golden Dragon package without molding. EQE was measured with Labsphere CDS-600 spectrometer. Plotting EQE versus output power P and finding the power Pm corresponding to EQE maximum ηm enables comparing the measurements with the analytical rela­tionships ηi = Q/(Q+p1/2+p-1/2) ,p = P/Pm , and Q = B/(AC) 1/2 where A, Band C are recombination constants [4]. As a result, maximum IQE value equal to QI(Q+2) can be found from the ratio ηm/ηe plotted as a function of p1/2 +p1-1/2 (see Fig.la) and then LEE calculated as ηext = ηm (Q+2)/Q . Experimental EQE as a function of normalized optical power p is shown in Fig. 1 b along with the analytical approximation based on the ABC­model. The approximation fits perfectly the measurements in the range of the optical power (or operating current) variation by eight orders of magnitude. In conclusion, new express method for separate evaluation of IQE and LEE of III-nitride LEDs is suggested and applied to characterization of a high-brightness blue LED. With this method, we obtained LEE from the free chip surface to the air as 69.8% and IQE as 85.7% at the maximum and 65.2% at the operation current 350 rnA. [I] G. Verzellesi, D. Saguatti, M. Meneghini, F. Bertazzi, M. Goano, G. Meneghesso, and E. Zanoni, "Efficiency droop in InGaN/GaN blue light-emitting diodes: Physical mechanisms and remedies," 1. AppL Phys., vol. 114, no. 7, pp. 071101, Aug., 2013. [2] C. van Opdorp and G. W. 't Hooft, "Method for determining effective non radiative lifetime and leakage losses in double-heterostructure las­ers," 1. AppL Phys., vol. 52, no. 6, pp. 3827-3839, Feb., 1981. [3] M. Meneghini, N. Trivellin, G. Meneghesso, E. Zanoni, U. Zehnder, and B. Hahn, "A combined electro-optical method for the determination of the recombination parameters in InGaN-based light-emitting diodes," 1. AppL Phys., vol. 106, no. II, pp. 114508, Dec., 2009. [4] Qi Dai, Qifeng Shan, ling Wang, S. Chhajed, laehee Cho, E. F. Schubert, M. H. Crawford, D. D. Koleske, Min-Ho Kim, and Yongjo Park, "Carrier recombination mechanisms and efficiency droop in GalnN/GaN light-emitting diodes," App/. Phys. Leu., vol. 97, no. 13, pp. 133507, Sept., 2010. © 2014 IEEE.

Transient reconfigurable subangstrom-precise photonic circuits at the optical fiber surface

Transient fully reconfigurable photonic circuits can be introduced at the optical fiber surface with subangstrom precision. A building block of these circuits - a 0.7Å-precise nano-bottle resonator - is experimentally created by local heating, translated, and annihilated.

Atténuation d'effets non linéaires dans les lasers fibrés de haute puissance opérés en régime continu

Les lasers à fibre de haute puissance sont maintenant la solution privilégiée pour les applications de découpe industrielle. Le développement de lasers pour ces applications n’est pas simple en raison des contraintes qu’imposent les normes industrielles. La fabrication de lasers fibrés de plus en plus puissants est limitée par l’utilisation d’une fibre de gain avec une petite surface de mode propice aux effets non linéaires, d’où l’intérêt de développer de nouvelles techniques permettant l’atténuation de ceux-ci. Les expériences et simulations effectuées dans ce mémoire montrent que les modèles décrivant le lien entre la puissance laser et les effets non linéaires dans le cadre de l’analyse de fibres passives ne peuvent pas être utilisés pour l’analyse des effets non linéaires dans les lasers de haute puissance, des modèles plus généraux doivent donc développés. Il est montré que le choix de l’architecture laser influence les effets non linéaires. En utilisant l’équation de Schrödinger non linéaire généralisée, il a aussi été possible de montrer que pour une architecture en co-propagation, la diffusion Raman influence l’élargissement spectral. Finalement, les expériences et les simulations effectuées montrent qu’augmenter la réflectivité nominale et largeur de bande du réseau légèrement réfléchissant de la cavité permet d’atténuer la diffusion Raman, notamment en réduisant le gain Raman effectif.

Design and Fabrication of Perovskite Micro-Cavity Lasers

Thesis (Master's)--University of Washington, 2016-08

Growth and application of planar III-V semiconductor nanowires grown with MOCVD

The semiconductor nanowire has been widely studied over the past decade and identified as a promising nanotechnology building block with application in photonics and electronics. The flexible bottom-up approach to nanowire growth allows for straightforward fabrication of complex 1D nanostructures with interesting optical, electrical, and mechanical properties. III-V nanowires in particular are useful because of their direct bandgap, high carrier mobility, and ability to form heterojunctions and have been used to make devices such as light-emitting diodes, lasers, and field-effect transistors. However, crystal defects are widely reported for III-V nanowires when grown in the common out-of-plane <111>B direction. Furthermore, commercialization of nanowires has been limited by the difficulty of assembling nanowires with predetermined position and alignment on a wafer-scale. In this thesis, planar III-V nanowires are introduced as a low-defect and integratable nanotechnology building block grown with metalorganic chemical vapor deposition. Planar GaAs nanowires grown with gold seed particles self-align along the <110> direction on the (001) GaAs substrate. Transmission electron microscopy reveals that planar GaAs nanowires are nearly free of crystal defects and grow laterally and epitaxially on the substrate surface. The nanowire morphology is shown to be primarily controlled through growth temperature and an ideal growth window of 470 +\- 10 °C is identified for planar GaAs nanowires. Extension of the planar growth mode to other materials is demonstrated through growth of planar InAs nanowires. Using a sacrificial layer, the transfer of planar GaAs nanowires onto silicon substrates with control over the alignment and position is presented. A metal-semiconductor field-effect transistor fabricated with a planar GaAs nanowire shows bulk-like low-field electron transport characteristics with high mobility. The aligned planar geometry and excellent material quality of planar III-V nanowires may lead to highly integrated III-V nanophotonics and nanoelectronics.

Design and development of distributed feedback transistor lasers

The transistor laser is a unique three-port device that operates simultaneously as a transistor and a laser. With quantum wells incorporated in the base regions of heterojunction bipolar transistors, the transistor laser possesses advantageous characteristics of fast base spontaneous carrier lifetime, high differential optical gain, and electrical-optical characteristics for direct “read-out” of its optical properties. These devices have demonstrated many useful features such as high-speed optical transmission without the limitations of resonance, non-linear mixing, frequency multiplication, negative resistance, and photon-assisted switching. To date, all of these devices operate as multi-mode lasers without any type of wavelength selection or stabilizing mechanisms. Stable single-mode distributed feedback diode laser sources are important in many applications including spectroscopy, as pump sources for amplifiers and solid-state lasers, for use in coherent communication systems, and now as TLs potentially for integrated optoelectronics. The subject of this work is to expand the future applications of the transistor laser by demonstrating the theoretical background, process development and device design necessary to achieve singlelongitudinal- mode operation in a three-port transistor laser. A third-order distributed feedback surface grating is fabricated in the top emitter AlGaAs confining layers using soft photocurable nanoimprint lithography. The device produces continuous wave laser operation with a peak wavelength of 959.75 nm and threshold current of 13 mA operating at -70 °C. For devices with cleaved ends a side-mode suppression ratio greater than 25 dB has been achieved.