Gangyi Xu

Low threshold current density distributed feedback quantum cascade lasers with deep top gratings

The authors report on pulsed and continuous-wave operation of single mode tunable distributed feedback quantum cascade lasers at λ∼7.7μm. A deep top grating and a thin heavily doped InP layer beneath the grating are formed in the upper cladding to obtain simultaneously a strong feedback effect and a low waveguide loss. A “three-well vertical transition” active region is used with a low doped injector. In pulsed operation, a very low threshold current density of 970A∕cm2 and a peak power of 75mW are achieved at 300K. Reliable single mode emission with a side mode suppression ratio of ∼30dB is achieved in a large temperature range (155–320K) with a tuning range of 90nm. The lasers operate in continuous-wave mode up to 130K.

Band alignment at the interface of PbTe/SnTe heterojunction determined by X-ray photoelectron spectroscopy

We report the determination of band alignment of PbTe/SnTe (111) heterojunction interfaces using X-ray photoelectron spectroscopy (XPS). Multiple core levels of Pb and Sn were utilized to determine the valence band offset (VBO) of the heterojunction. The XPS result shows a type-III band alignment with the VBO of and the conduction band offset (CBO) of . The experimental determination of the band alignment of the PbTe/SnTe heterojunction shall benefit the improvement of PbTe/SnTe-related optoelectronic and electronic devices.

Mid-Infrared Tunable Diode Laser Absorption Spectroscopy for Gas Sensing

Most of gaseous materials show characteristic optical absorptions, especially in the mid-infrared (2-25 mum) band. The optical fingerprints of different gases have made the spectral absorption a unique method of gas analysis. The tunable diode laser absorption spectroscopy (TDLAS) method use a compact single mode diode laser tuning over an interested wavelength range swiftly can fulfill the function of spectroscopy scan. In this talk the TDLAS experiments using home made mid-infrared diode lasers for gas sensing are represented, and various effects affecting the performance of the sensor have been discussed.

Room Temperature, Low Threshold Distributed Feedback Quantum Cascade Lasers at 7.7 μm

In this paper, we present low threshold DFB QCLs (λ ≡ 7.7 μm) with top grating by innovating the waveguide structure and the method of grating fabrication. Highly doped InP cladding layer was taken as the host layer for the grating, and was anisotropically etched along [O-11] direction with the lateral etching being negligible. This method releases the limitation on the grating depth. By careful design and control a deep first-order grating, with a thin highly doped InP layer beneath it, was formed in the plasmon enhanced cladding layers. The deep grating ensures strong feedback effect and the thin highly doped InP layer separates the high loss metals in the groove of the grating from the active region, suppressing the increase of the waveguide loss. It allows the fabrication of DFB QCLs with low threshold current density.

Highly efficient power extraction in terahertz quantum cascade laser via a grating coupler

We demonstrate the highly efficient power extraction from terahertz quantum cascade laser (THz-QCL) via a grating coupler (GC). The GC, formed in the top metallization of a Fabry-Perot (FP) THz-QCL with a metal-metal (MM) waveguide, diffracts the THz wave into the free space when it oscillates between the two cleaved facets. The radiation loss can be tailored in a wide range by changing the structure of the GC. The asymmetric positioning of the GC enables near-unidirectional emission, the enlarged emission surface reduces the beam divergence, and the MM waveguide keeps the optical confinement factor near unity. Experimental results confirm that, when the operating frequency is near 2.7 THz, the laser with a GC shows superior performances than the conventional FP lasers in terms of output power and operation temperature.We demonstrate the highly efficient power extraction from terahertz quantum cascade laser (THz-QCL) via a grating coupler (GC). The GC, formed in the top metallization of a Fabry-Perot (FP) THz-QCL with a metal-metal (MM) waveguide, diffracts the THz wave into the free space when it oscillates between the two cleaved facets. The radiation loss can be tailored in a wide range by changing the structure of the GC. The asymmetric positioning of the GC enables near-unidirectional emission, the enlarged emission surface reduces the beam divergence, and the MM waveguide keeps the optical confinement factor near unity. Experimental results confirm that, when the operating frequency is near 2.7 THz, the laser with a GC shows superior performances than the conventional FP lasers in terms of output power and operation temperature.

The effect of (NH/sub 4/)/sub 2/S passivation treatments on the dark current-voltage characteristics of InGaAsSb PIN detector

InGaAsSb/GaSb detector is very promising for novel applications in long-wavelength range (>2 /spl mu/m). Dark current, composed of surface and volume currents, is a leading source of the electronic noise. Due to a large density of surface states of the InGaAsSb surface, the surface recombination current is the dominant factor in the whole dark current. In this paper, the influence of alkali and neutralized (NH/sub 4/)/sub 2/S passivation on the dark current density of InGaAsSb PIN detector has been studied. A new modified neutralized (NH/sub 4/)/sub 2/S passivation method was proposed. Results show a drastic improvement of the device characteristics was obtained by the modified neutralized (NH/sub 4/)/sub 2/S passivation. In our experiment, the PIN detectors were taken from the same epitaxial wafer and prepared by same fabrication process. In order to compare the effect of different (NH/sub 4/)/sub 2/S passivation solution, the devices were divided into two groups. The results indicate that the sulfur-passivation can remove native oxide and depress surface stats of InGaAsSb effectively.