Farid Agahi

Microstructure of AlGaAs‐oxide heterolayers formed by wet oxidation

We have carried out a transmission electron microscopy based study of AlGaAs–Al(oxide) heterolayers created by lateral sidewall wet oxidation and identify the oxide phase formed as a consequence of the oxidation of AlAs to be γ‐Al2O3, with the cubic Fd 3m structure. The oxide‐semiconductor interface is weak and porous, possibly due to the high stress loads developed during oxidation, and we propose that the fast oxidation rates are a consequence of reactants transported to the oxidation front along the porous interface.

Recombination in GaAs at the AlAs oxide‐GaAs interface

Interface recombination in GaAs at the GaAs/AlAs interface has been investigated before and after selective ‘‘wet oxidation’’ of the AlAs layer. Time‐resolved photoluminescence of the band‐edge GaAs emission has been used to characterize the interface recombination. Prior to oxidation, the interface recombination is low. After oxidation, the interface recombination has greatly increased, and is comparable to a free GaAs surface in air. However, isolating the GaAs layer from the oxide by a 30 nm layer of Al0.3Ga0.7As allows the interface recombination to remain low after the oxidation. These results help explain the low threshold currents which have been observed in vertical cavity lasers which use wet oxidation of AlAs for current confinement.

A gratingless wavelength stabilized semiconductor laser

A single frequency laser structure is obtained by coupling a high order mode of a semiconductor waveguide to a low index polymer waveguide. The device does not require a grating or regrowth, emits in a mode compatible with optical fibers, and may be immune to catastrophic mirror damage. The epilayers of the semiconductor waveguide use quarterwave reflectors to support a mode with a low enough effective index to phase match to the polymer waveguide. The coupling between the two waveguides is highly frequency selective and therefore stabilizes the wavelength. Preliminary structures emit in a single longitudinal and spatial mode, have 30 dB of sidemode suppression, and emit about 6 mW into a fiber compatible mode.

WAVELENGTH-SELECTIVE WAVEGUIDE PHOTODETECTORS IN SILICON-ON-INSULATOR

We show that silicon‐on‐insulator substrates, combined with a low index waveguide, can yield intrinsically wavelength‐selective devices for multiwavelength applications. We demonstrate a simple wavelength‐selective photodetector that resembles an asymmetric directional coupler with a polymer waveguide coupled to a silicon waveguide. Only certain wavelengths are phase‐matched between the two guides and transfer from the polymer to the silicon. Operating the device at the band edge of silicon, the coupled radiation is absorbed and generates photocurrent. The 400 μm long detector exhibits a linewidth of a few nm, limited by the uniformity of the silicon‐on‐insulator material.

Waveguide version of an asymmetric Fabry–Perot modulator

We show that the high contrast property of vertical cavity asymmetric Fabry–Perot modulators can also be obtained in a waveguide geometry. Increasing the absorption in one arm of a waveguide directional coupler causes the transmission through the other arm to decrease, go to zero, and then increase again. Thus, by adjusting the coupling length and the absorption, an infinite contrast ratio can theoretically be obtained with a low insertion loss. The waveguide geometry is not only more convenient than normal incidence for fiber‐based devices, but also provides a larger optical bandwidth. We present the design equations and provide a first order model of device operation.

Movable‐mask reactive ion etch process for thickness control in devices

By moving the substrate relative to a shadow mask in a reactive ion etching system, we are able to precisely tailor the thickness of critical layers. To minimize disturbing the plasma, all the mechanical components are kept below the anode. The system is highly reproducible, and can be programmed to yield arbitrary vertical profiles along one horizontal axis. Using silicon‐on‐insulator substrates, the resonance wavelength was modified as a function of position with better than 1 nm control in the vertical dimension. This technique should prove useful for optical devices where the thickness of the layers controls the device characteristics.

Asymmetric waveguide devices with buried AlO x cladding layers

We demonstrate a highly asymmetric waveguide filter for wavelength division multiplexing applications using wet oxidation of AlAs to form low-index cladding layers. The dual waveguide structure consists of a low-index polymer waveguide coupled to a lossy high-index AlGaAs waveguide. The high asymmetry between the polymer and AlGaAs yields high resolution at small device lengths. The light is coupled in and out of the polymer guide and exhibits Fabry-Perot-like resonances as the light couples to different modes of the AlGaAs. With a device length of 400 /spl mu/m, we observe resonances with a width of about 1 nm at 800 nm, agreeing with theoretical expectations. In addition to the simple filter demonstrated here, such a structure could be used to form active light emitting or detector devices.

Near field microscopy of vertical cavity lasers

We show that near-field optical microscopy (NFOM) can be a powerful tool for the diagnosis and characterization of semiconductor laser structures, since it can obtain optical spectra with a spatial resolution better than 100nm. The capabilities of NFOM are demonstrated by characterizing a VCSEL in which spatial nonuniformities in the material cause a multi-mode emission pattern.