Graphene has great potential for fabrication of ultrafast opto-electronics, in which relaxation and transport of photoexcited carriers determine device performance. Even though ultrafast carrier relaxation in graphene has been studied vigorously, transport properties of photoexcited carriers in graphene are largely unknown. In this work, we utilize an ultrafast grating imaging technique to measure lifetime (τ r), diffusion coefficient (D), diffusion length (L) and mobility (µ) of photoexcited carriers in monoand multi-layer graphene non-invasively. In monolayer graphene, D~10,000cm 2 /s and µ~120,000cm 2 /Vs have been observed, both of which decrease drastically in multilayer graphene, indicating that the remarkable transport properties in monolayer graphene originate from its unique Dirac-Cone energy structure. Mobilities of photoexcited carriers measured here are several times larger than the Hall and Field-Effect mobilities reported in literature (<15,000cm 2 /Vs), due to the high energy of photoexcited carriers. Our results indicate the importance of obtaining monolayer graphene to realize high-performance graphene devices, as well as the necessity to use transport properties of photoexcited carriers for predicting the performance of graphene-based opto-electronics.

We describe how growth at low temperatures can enable increased active layer strain in GaSb-based type-I quantum-well diode lasers, with emphasis on extending the emission wavelength. Critical thickness and roughening limitations typically restrict the number of quantum wells that can be grown at a given wavelength, limiting device performance through gain saturation and related parasitic processes. Using growth at a reduced substrate temperature of 350 °°°°C, compressive strains of up to 2.8% have been incorporated into GaInAsSb quantum wells with GaSb barriers; these structures exhibited peak room-temperature photoluminescence out to 3.96 μm. Using this growth method, low-threshold ridge waveguide lasers operating at 20 °C and emitting at 3.4 μm in pulsed mode were demonstrated using 2.45% compressively strained GaInAsSb/GaSb quantum wells. These devices exhibited a characteristic temperature of threshold current of 50 K, one of the highest values reported for type-I quantum-well laser diodes operating in this wavelength range. This temperature stability is attributable to the increased valence band offset afforded by the high strain values, due to the simultaneously high quantum well indium and antimony mole fractions. Exploratory experiments using bismuth both as a surfactant during quantum well growth, as well as in dilute amounts incorporated into the crystal were also studied. Both methods appear promising avenues to surmount current strain-related limitations to laser performance and emission wavelength.

A novel ultrafast reflective grating-imaging technique has been developed to measure ambipolar carrier diffusion in GaAs/AlAs quantum wells and bulk GaAs. By integrating a transmission grating and an imaging system into the traditional pump−probe setup, this technique can acquire carrier diffusion properties conveniently and accurately. The fitted results of the diffusion coefficient and diffusion length in bulk GaAs agree well with the literature values obtained by other techniques. The diffusion coefficient and diffusion length of GaAs/AlAs quantum wells are found to increase with the well layer thickness, which suggests that interface roughness scattering dominates carrier diffusion in GaAs/AlAs quantum wells. With the advantages of simple operation, sensitive detection, rapid and nondestructive measurement, and extensive applicability, the ultrafast reflective grating-imaging technique has great potential in experimental study of carrier diffusion in various materials.

No abstract

Optical grating technique, where optical gratings are generated via light inference, has been widely used to measure charge carrier and phonon transport in semiconductors. In this paper, compared are three types of transient optical grating techniques: transient grating diffraction, transient grating heterodyne, and grating imaging, by utilizing them to measure carrier diffusion coefficient in a GaAs/AlAs superlattice. Theoretical models are constructed for each technique to extract the carrier diffusion coefficient, and the results from all three techniques are consistent. Our main findings are: (1) the transient transmission change ∆T/T0 obtained from transient grating heterodyne and grating imaging techniques are identical, even these two techniques originate from different detection principles; and (2) By adopting detection of transmission change (heterodyne amplification) instead of pure diffraction, the grating imaging technique (transient grating heterodyne) has overwhelming advantage in signal intensity than the transient grating diffraction, with a signal intensity ratio of 315:1 (157:1). I. INTRODUCTIONCarrier diffusion in semiconductors is crucial in electronic and opto-electronic devices, since it determines some key parameters of the devices, such as working frequency and response time.Studying carrier diffusion process can also reveal carrier scattering in semiconductors, assess carrier mobility with Einstein relation, and understand interactions between carriers and phonons, defects, and nanostructures. Currently, there are several optical techniques to measure the carrier diffusion coefficients nondestructively: transient grating [1,2], spatial scanning pump-probe [3,4], and grating imaging [5,6]. In the transient grating method, two pump beams overlap on the sample surface to generate a transient carrier density grating. A probe beam shines on the grating and the diffracted probe is taken as the signal, which reflects the decaying process of the carrier density grating. In the spatial scanning pump-probe technique, both the pump and probe beams are tightly focused onto the sample surface. The pump generates a Gaussian-shape carrier package and the probe is scanned spatially across the pump spot. By measuring the differential transmission or reflection (∆T/T0 or ∆R/R0) of the probe as a function of time and position, the evolution of the carrier package, which contains the information of carrier diffusion, is recorded. In the grating imaging technique, pump and probe beams overlap on a physical transmission amplitude grating (a photomask with metal strips patterned onto a glass substrate), whose image is formed by an objective lens onto the surface of the sample. The intensities of pump and probe beam on the sample are modulated in the same pattern as the transmission amplitude grating. The pump generates transient carrier grating in the sample, while the probe only detects the evolution of carrier density in the bright-strip regions. By measuring either ∆T/T0 or ∆R/R0 of the probe as a function o...