Andrew W. Jackson

A traveling-wave THz photomixer based on angle-tuned phase matching

A traveling-wave THz photomixer based on a free-space optical-THz phase-matching scheme is proposed. A dc-biased coplanar strip line fabricated on low-temperature-grown GaAs serves as the active area of the device, and is illuminated by two noncollinear laser beams which generate interference fringes that are accompanied by THz waves. The device with the laser-power-handling capability over 300 mW and a 3-dB bandwidth of 1.8 THz was experimentally demonstrated. The results show that traveling-wave photomixers have the potential to surpass small-area designs.

High-power 1320-nm wafer-bonded VCSELs with tunnel junctions

A new long-wavelength vertical-cavity surface-emitting laser structure is described that utilizes AlGaAs-GaAs mirrors bonded to AlInGaAs-InP quantum wells with an intracavity buried tunnel junction. This structure offers complete wavelength flexibility in the 1250-1650 nm fiber communication bands and reduces the high free-carrier losses and bonded junction voltage drops in previous devices. The intracavity contacts electrically bypass the bonded junctions to reduce threshold voltage. N-type current spreading layers and undoped AlGaAs mirrors minimize optical losses. This has enabled 134/spl deg/C maximum continuous-wave lasing temperature, 2-mW room-temperature continuous-wave single-mode power, and 1-mW single-mode power at 80/spl deg/C, in various devices in the 1310-1340 nm wavelength range.

Self-assembled ErAs islands in GaAs for optical-heterodyne THz generation

We report photomixer devices fabricated on a material consisting of self-assembled ErAs islands in GaAs, which is grown by molecular beam epitaxy. The devices perform comparably and provide an alternative to those made from low-temperature-grown GaAs. The photomixers frequency response demonstrates that the material is a photoconductor with subpicosecond response time, in agreement with time-resolved differential reflectance measurements. The material also provides the other needed properties such as high photocarrier mobility and high breakdown field, which exceeds 2×10^5 V/cm. The maximum output power before device failure at frequencies of 1 THz was of order 0.1 µW. This material has the potential to allow engineering of key photomixer properties such as the response time and dark resistance.

Electrical design optimization of single-mode tunnel-junction-based long-wavelength VCSELs

We present principles for tunnel-junction (TJ) design optimization for use in intracavity contacted long-wavelength vertical-cavity surface-emitting lasers (LW-VCSELs). Using the WKB approximation, we find that layer thicknesses of 10 nm on the n++ side and 10 nm on the p++ side are large enough to maximize quantum tunneling probability and small enough to yield low optical free-carrier absorption loss. We also conjecture that our experimental test structures and actual devices have far lower active acceptor concentration than we expect based on an analytical model. Finally, we calculate the necessary doping levels to enable single-mode operation of LW-VCSELs and incorporate these conditions into a complete optimized model of our VCSELs. Based on optimal I-V curves, we can expect an increase in single-mode output power from 2 to 3.5 mW

FUSED VERTICAL COUPLERS

A vertical directional coupler fabricated using wafer fusion is demonstrated with a very short coupling length of 62 μm. The optical propagation loss introduced by the fused layer is investigated. An excess loss of 1.1 dB/cm at 1.55 μm was measured for waveguides which incorporate a fused junction near the core region. Fused vertical couplers make it possible to realize three-dimensional waveguide structures and compact switching arrays and they solve some of the topology problems of large switch arrays.

Reduced thermal conductivity in low-temperature-grown GaAs

Thermal conductivity of low-temperature-grown GaAs(LT GaAs) was measured at room temperature using a self-heated photolithographically patterned platinum wire on the surface of the sample. Finite element calculations were performed to extract the thermal conductivity from the nonlinear I–V characteristic of the wires. For LT GaAs grown at a substrate temperature of 240 °C, the thermal conductivity was found to be only 23% of the value for stoichiometric GaAs. Rapid thermal annealing of the sample at 650 °C for 30 s increased the thermal conductivity to 46% of the GaAs value. Strong phonon scattering by point defects could account for reduced thermal conductivity in the as-grown material. The reduced thermal conductivity in the annealed material, however, is not consistent with our current understanding of the defects in annealed LT GaAs.

Real‐time simultaneous optical‐based flux monitoring of Al, Ga, and In using atomic absorption for molecular beam epitaxy

We have developed a multichannel atomic absorption measurement system for real‐time simultaneous monitoring of Al, Ga, and In molecular beam fluxes. In our configuration, distinct atomic emission lines from three hollow cathode lamps are combined into one beam, thus requiring only one pair of through view ports for the optical probe beam. Based on the dual beam optical configuration, the reference arm compensates for intensity drift of the light sources. In this work, we demonstrate the use of reflection high energy electron diffraction oscillations for calibrating the absorption signal.

Wafer-fused optoelectronics for switching

Wafer fusion technique for realization of compact waveguide switches and three-dimensional (3-D) photonic integrated circuits is investigated theoretically and experimentally. Calculations based on beam propagation method show that very short vertical directional couplers with coupling lengths from 40 to 220 /spl mu/m and high extinction ratios from 20 to 32 dB can be realized. These extinction ratios can be further improved using a slight asymmetry in waveguide structure. The optical loss at the fused interface is investigated. Comparison of the transmission loss in InGaAsP-based ridge-loaded waveguide structures with and without a fused layer near the core region, reveals an excess loss of 1.1 dB/cm at 1.55 /spl mu/m wavelength. Fused straight vertical directional couplers have been fabricated and characterized. Waveguides separated by 0.6 /spl mu/m gap layer exhibit a coupling length of 62 /spl mu/m and a switching voltage of about 2.2 V. Implications for GaAs-based fused couplers for 850 nm applications will also be discussed.

In situ monitoring and control for MBE growth of optoelectronic devices

Improved control over layer thickness has been realized using optical interference techniques such as reflectance spectroscopy. It is now common to observe spectra of distributed-Bragg-reflector (DBR) mirrors during growth to make corrections for growth rate drifts. Real-time optical flux monitoring (OFM) by atomic absorption allows precise layer control by measuring group III fluxes continuously during growth. The flux information can be used to operate growth shutters and to control effusion cell heaters in a feedback loop. Improved substrate temperature measurement by diffuse reflectance spectroscopy (DRS) allows precise measurement of substrate temperature. DRS is not subject to the same errors encountered in pyrometer or thermocouple measurements of substrate temperature.

Self-assembled ErAs islands in GaAs for THz applications

This paper concerns self-assembled ErAs islands in GaAs grown by molecular beam epitaxy. The nucleation of ErAs on GaAs occurs in an island growth mode leading to spontaneous formation of nanometer-sized islands. Pump–probe measurements indicate that the ErAs islands capture photogenerated carriers on a subpicosecond time scale. This together with the high resitivity of the material allows us to use it as a fast photoconductor. The performance of photomixer devices made from this material is discussed.