F. Genty

Single-frequency tunable Sb-based VCSELs emitting at 2.3 /spl mu/m

We present a comparison between two kinds of single-frequency Sb-based semiconductor VCSELs operating at 2.3 /spl mu/m in continuous-wave regime at room temperature. These lasers are studied in view of application to spectroscopy or trace gas detection. Both are based on a molecular beam epitaxy grown half-VCSEL. In the first configuration, a dielectric mirror is deposited on top to form a microcavity, while in the second a concave mirror is used to form an external cavity. The external cavity VCSEL exhibits 5-mW output power, a narrow linewidth (<<20 kHz), and 50-GHz continuous frequency tunability.

Photothermal investigations of thermal and optical properties of GaAlAsSb and AlAsSb thin layers

Investigation of the optical and thermal properties of GaAlAsSb/GaSb and AlAsSb/GaSb heterostructures by photothermal deflection spectroscopy (PDS) is presented. From a comparison of the amplitude and phase of the experimental PDS signal with the corresponding theoretical ones, we have determined the thermal conductivities and thickness, as well as the variations versus wavelength of the optical absorption coefficients and refractive indices of the GaAlAsSb and AlAsSb layers. The refractive indices measured in the transparency region and the small values of the thermal conductivity are in good agreement with theoretical interpolation reported in previous literature.

Room-Temperature Continuous-Wave Operation of 2.3-

Room-temperature continuous-wave (CW) operation of GaSb-based monolithic microcavity vertical-cavity surface-emitting lasers operating near 2.3 mum is presented. These devices were composed of two n-doped AlAsSb-GaSb Bragg mirrors, a type-I GaInAsSb-AlGaAsSb multiquantum-well active region, and an n++-InAsSb/p++-GaSb tunnel junction. CW laser operation was observed up to 294 K. A CW threshold current density as low as 1.1 kA ldr cm-2 was obtained at 284 K for 60-mum-diameter devices (20-mum-diameter emitting area).

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We present the study of thermal and optical properties of 15.5 pair of GaAlAsSb/AlAsSb layers deposited upon a GaSb substrate by photothermal deflection spectroscopy. This stacking of layers constitutes a distributed Bragg mirror.

m Sb-Based Electrically Pumped Monolithic Vertical-Cavity Lasers

In this chapter, we tried to give a precise picture of the current state of the art of VCSEL technology in the 2–3 µm spectral range. We described classical microcavities but also structures with external-cavity geometry which allows one to fabricate devices with improved output beam properties (single transverse mode TEM00 operation) at high power. The electrically-pumped microcavities with highest performance in the 2–3 µm range are obtained from the AlGaInAs/InP materials system. But λ = 2.1 µm seems to represent the high wavelength limit of such devices. Concerning optically-pumped devices, Sb-based structures seem to have the best properties. Circular TEM00 output beam and single frequency laser operation was obtained above room temperature from both microcavities and external cavity devices with Type-I GaInAsSb/AlGaAsSb MQW active regions.

Investigation of thermal and optical properties of distributed Bragg reflectors by photothermal deflection spectroscopy

In this paper, we present a new Photothermal Deflection Technique (PTD) to determine thermal properties of bulk doped or undoped semiconductor such as GaAs, GaSb, InAs, etc. The method proposed here consists in covering the sample with a thin graphite layer in order to increase the photothermal signal and to ovoid any reflection on the sample surface. This method deals with the analysis of the logarithm of amplitude and phase variation of the photothermal signal versus square root modulation frequency where the sample placed in air is heated by a modulated light beam coming from a halogen lamp. So the best coincidence between experimental curves and corresponding theoretical ones gives simultaneously the best values of thermal conductivity and thermal diffusivity of the sample. These obtained values are in good agreement with those found in literature. The advantage of applying this method in this way lies in its simplicity and its sensibility to both thermal conductivity and thermal diffusivity.

VCSELs Emitting in the 2–3 µm Wavelength Range

Mid-infrared laser diodes are highly attractive sources for gas spectroscopy applications due to the presence of strong absorption lines of many gaseous species in this spectral range. Particularly, the 2.3 – 3.3 µm wavelength range contains strong absorption lines of some pollutants as CH4, NH3 or HF while CO2 and H2O interference absorption lines are very low which renders possible the highly selective and sensitive detection of these pollutants. Vertical-cavity surface-emitting lasers (VCSELs) appear especially well adapted to be used as laser sources for absorption spectroscopy due to several intrinsic characteristics that they offer. The low threshold, single-mode operation, the circular output beam with low divergence are only some of their well-known advantages.

New Photothermal Deflection Method to Determine Thermal Properties of Bulk Semiconductors

Vertical External-Cavity Surface-Emitting Lasers (VECSEL) exhibit narrow linewidth single-frequency operation and wide mode-hope-free tuning range, particularly well adapted for trace gas spectroscopy application, especially in the 2-2.5 /spl mu/m atmospheric transparency window (CH4, NH3, CO2...). VECSELs over come beam quality limitations of laser diodes and power restrictions of microcavity VCSELS.