Chengao Wang

Development of high quantum efficiency GaAs/GaInP double heterostructures for laser cooling

We report on the growth and characterization of high external quantum efficiency (EQE) GaAs/GaInP double heterostructures. By properly treating the GaAs/GaInP interface, we are able to produce structures measuring a record EQE of 99.5% ± 0.1% in GaAs. This efficiency exceeds the requirement for achieving laser cooling in GaAs. However, net cooling has not yet been realized due to residual below gap background absorption.

Differential luminescence thermometry in semiconductor laser cooling

We demonstrate a non-contact, spectroscopic technique to measure the temperature change of semiconductors with very high precision. A temperature resolution of less than 100 μK has been obtained with bulk GaAs. This scheme finds application in experiments to study laser cooling of solids. We measure a record external quantum efficiency of 99% for a GaAs device.

Precision, all-optical measurement of external quantum efficiency in semiconductors

External quantum efficiency of semiconductor photonic devices is directly measured by wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent temperature or power-dependent photoluminescence. Time-resolved photoluminescence lifetime and power-dependent photoluminescence measurements are used to evaluate unprocessed heterostructures for critical performance parameters. The crucial importance of parasitic background absorption is discussed.

Influence of coulomb screening on lateral lasing in VECSELs

Parasitic lateral lasing in certain optically pumped semiconductor disc lasers drains the gain of the vertical mode and thus causes power scaling degradation and premature rollover in surface emitting operation. We have observed this effect in both multiple quantum wells (MQW) (GaInAs/GaAs) and double heterostructures (DHS) (GaInP/GaAs/GaInP) under pulsed excitation even when the gain chip lateral dimensions are much larger than the diameter of the pump laser. Lateral lasing occurs persistently between cleaved facets at a band-tail wavelength much longer than the peak of the gain. We show that the effect of bandgap renormalization due to Coulomb screening explains this phenomena. Exploiting the simple analytical plasma theory of bulk semiconductors (Banyai & Koch, 1986), we can account for such an effect in double heterostructures.

Characterization of external quantum efficiency and absorption efficiency in GaAs/ InGaP double heterostructures for laser cooling applications

The state of current research in laser cooling of semiconductors is reviewed. Emphasis is placed on the characterization of external quantum efficiency and absorption efficiency in GaAs/InGaP double heterostuctures. New experimental results will be presented that characterize device operation as a function of laser excitation power and temperature. Optimum carrier density is obtained independently and used as a screening tool for sample quality. The crucial importance of parasitic background absorption is discussed.

GaAs/GaInP double heterostructure characterization for laser cooling of semiconductors

External quantum efficiency of semiconductor photonic devices is directly measured by wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent photoluminescence. Differential power-dependent photoluminescence measurement is used to quickly screen the sample quality before processing.

99% External quantum efficiency from a GaAs heterostructure at 100 K

Record external quantum efficiency (99%) is obtained for a GaAs/InGaP heterostructure bonded to a dome lens at 100 K. This was measured using a differential luminescence thermometry technique with temperature resolution ~ 30 muK.

Tip-enhanced stimulated Raman scattering with ultra-high-aspect-ratio tips and confocal polarization Raman spectroscopy for evaluation of sidewalls in Type II superlattices FPAs

Actoprobe team had developed custom Tip Enhancement Raman Spectroscopy System (TERS) with specially developed Ultra High Aspect Ratio probes for AFM and TERS measurements for small pixel infrared FPA sidewall characterization. Using this system, we report on stimulated Raman scattering observed in a standard tip-enhanced Raman spectroscopy (TERS) experiment on GaSb materials excited by 637-nm pump laser light. We explain our results by TERS-inherent mechanisms of enormous local field enhancement and by the special design and geometry of the ultrahigh-aspect-ratio tips that enabled conditions for stimulated Raman scattering in the sample with greatly enhanced resonance Raman gain when aided by a microcavity to provide feedback mechanism for the Raman emission. The approach has great potential for further, orders-of-magnitude, progress in TERS enhancement by significantly increasing its nonlinear component. We report development of novel class of probes for atomic force microscopy (AFM active optical probe - AAOP) by integrating a laser source and a photodetector monolithically into the AFM probe. The AAOPs are designed to be used in a conventional AFM and would enhance its functionality to include that of the instruments (NSOM, TERS, hybrid AFM).

Determination of external quantum efficiency in semiconductors using pulsed power-dependent photoluminescence

Abstract. External quantum efficiency (EQE) is a parameter widely used in various photonic devices. In laser refrigeration of solids, materials with high EQE are essential for achieving net cooling. Pulsed power-dependent photoluminescence measurement is developed and demonstrated to be a rapid and efficient tool to determine the EQE and screen the sample quality before the fabrication process for the application of laser cooling in semiconductors. EQE values obtained from this technique are shown to be consistent with results from other more precise, but time-consuming measurements for various samples at different temperatures.

High sensitivity background absorption measurements in semiconductors

Laser cooling in InGaP|GaAs double heterostructures (DHS) has been a sought after goal. Even though very high external quantum efficiency (EQE) has been achieved, background absorption has remained a bottleneck in achieving net cooling. The purpose of this study is to gain more insight into the source of the background absorption for InGaP|GaAs DHS as well as GaAs|AlGaAs DBRs by employing an excite-probe thermal Z-scan measurement.