Ann Catrina Bryce

Selective control of self-organized In0.5Ga0.5As/GaAs quantum dot properties: Quantum dot intermixing

Selective postgrowth control of the photoluminescence (PL) wavelength has been demonstrated for a single layer self-organized In0.5Ga0.5As/GaAs quantum dot (QD) structure. This was achieved by rapid thermal processing of dots using different dielectric caps. Selective band gap shifts of over 100 meV were obtained between samples capped with sputtered and plasma enhanced silica deposition, with the band gap shift under regions covered with plasma enhanced chemical vapor deposition SiO2 less than 70 meV. The effects of different caps on the PL linewidth were also observed. The differential band gap shift will enable the integration of passive and active devices in QD systems.

Fabrication of multiple wavelength lasers in GaAs-AlGaAs structures using a one-step spatially controlled quantum-well intermixing technique

We have applied a new technique, based on impurity-free vacancy diffusion, to control the degree of intermixing across a wafer. Bandgap tuned lasers were fabricated using this technique. Five distinguishable lasing wavelengths were observed from five selected intermixed regions on a single chip. These lasers showed no significant change in transparency current, internal quantum efficiency or internal propagation loss, which indicates that the material quality was not degraded after intermixing.<<ETX>>

Control of silica cap properties by oxygen plasma treatment for single-cap selective impurity free vacancy disordering

By exposing the SiO2 films used as annealing caps in the process of impurity free vacancy disordering (IFVD) to an oxygen plasma, which is produced in a reactive ion etching machine, the effect of the exposed caps on quantum well intermixing can be substantially controlled. The effect of the oxygen treatment is manifested in inhibiting the Ga outdiffusion from GaAs/AlGaAs heterostructures. A selective IFVD process using identical silica caps has been obtained by selective exposure of the caps to oxygen plasma. Differential band gap shifts in excess of 100 meV were achieved with control samples exhibiting band gap shifts less than 10 meV.

Engineering quantum-dot lasers

We discuss recent progress in the engineering of quantum-dot (QD) lasers, focusing on the spectral output, dynamics and techniques for integration. Two approaches to such engineering are discussed. Firstly, it is suggested that control of lasing spectra in QD lasers is possible by making use of waveguiding-related phenomena (substrate leakage and reflection) which, in unoptimised laser structures, result in the mode grouping effect (quasiperiodic spectral modulation). Secondly, first experimental studies of quantum dot intermixing are reported, suggesting that this technique is capable of both improving the performance of active QD media and integrating active QD sections with passive waveguides.

AlGaInAs/InP Monolithically Integrated DFB Laser Array

The monolithic integration of four 1.50-μm range AlGaInAs/InP distributed feed-back lasers with a 4 × 1 multimode-interference optical combiner, a curved semiconductor optical amplifier and an electroabsorption modulator using relatively simple technologies-sidewall grating and quantum well intermixing-has been demonstrated. The four channels span the wavelength range of 1530-1566 nm and can operate separately or simultaneously. The epitaxial structure was designed to produce a far field pattern at the output waveguide facet, which is as small as 21.2°× 25.1°, producing a coupling efficiency with an angled-end single mode fiber at twice that of a conventional device design.

Raman spectroscopy for characterizing compositional intermixing in GaAs/AlGaAs heterostructures

Compositional intermixing induced by the process of impurity-free vacancy (dielectric cap annealing induced) disordering in GaAs/AlGaAs is studied using Raman spectroscopy. The degree of intermixing in multiple-quantum-well structures was detected through the energy shift of certain Raman modes of the lattices. In addition, localized intermixing, with band-gap shifts as low as 6 nm realized in 1:1 band-gap grating patterns with different periods (⩾4 μm), was also detected through the energy shift and the full width at half maximum of the structures’s Raman modes.

Output Power Limitations and Improvements in Passively Mode Locked GaAs/AlGaAs Quantum Well Lasers

We report a novel approach for increasing the output power in passively mode locked semiconductor lasers. Our approach uses epitaxial structures with an optical trap in the bottom cladding that enlarges the vertical mode size to scale the pulse saturation energy. With this approach we demonstrate a very high peak power of 9.8 W per facet, at a repetition rate of 6.8 GHz and with pulse duration of 0.71 ps. In particular, we compare two GaAs/AlGaAs epilayer designs, a double quantum well design operating at 830 nm and a single quantum well design operating at 795 nm, with vertical mode sizes of 0.5 and 0.75 μm, respectively. We show that a larger mode size not only shifts the mode locking regime of operation toward higher powers, but also produces other improvements with respect to two main failure mechanisms that limit the output power, catastrophic optical mirror damage and catastrophic optical saturable absorber damage. For the 830-nm material structure, we also investigate the effect of nonabsorbing mirrors on output power and mode locked operation of colliding pulse mode locked lasers.

160-GHz 1.55-

A monolithic ~1.55-μm colliding-pulse mode-locked AlGaInAs/InP laser with a three-quantum-well active layer incorporating a passive far-held reduction layer has been demonstrated. The device emits pulses at 162 GHz, with a pulsewidth of 0.98 ps, a pulse energy of 0.13 pJ, and a time-bandwidth product of 0.52, while demonstrating a low divergence angle (12.7° × 26.3°) with a twofold improvement in butt coupling efficiency to a flat cleaved single-mode fiber, compared to the conventional mode-locked lasers.

\mu{\rm m}

Abstract The bandgap of InGaAsInGaAsP multiple-quantum well (MQW) material can be accurately tuned by photo-absorption induced disordering (PAID), using a Nd:YAG laser, to allow lasers, modulators and passive waveguides to be fabricated from a standard MQW structure. The process relies on optical absorption in the active region of the MQW to produce sufficient heat to cause interdiffusion between the wells and barriers. Blue shifts of up to 160 nm in the lasing spectra of both broad area and ridge waveguide lasers are reported. Bandgap tuned electro-absorption modulators were fabricated and modulation depths as high as 27 dB were obtained. Single mode waveguide losses are as low as 5 dB cm−1 at 1550 nm. Selective area disordering has been used in the fabrication of extended cavity lasers. The retention of good electrical and optical properties in intermixed material demonstrates that PAID is a promising technique for the integration of devices to produce photonic integrated circuits. A quantum well intermixing technique using a pulsed laser is also reported.

Colliding-Pulse Mode-Locked AlGaInAs/InP Laser With High Power and Low Divergence Angle

We report on the detailed characterization of ultrashort pulses emitted from a 1.5- AlGaInAs/InP semiconductor passively mode-locked laser, operating at a repetition frequency of 35 GHz. Both the temporal and phase profiles of the pulses are retrieved using a sonogram technique that utilizes a highly-sensitive two-photon absorption waveguide detector. The system enables full characterization of pulses with energy as low as 10 fJ and peak power level of 5 mW, which is only inaccessible by a limited number of high-sensitivity measurement approaches. We show that the pulses exhibit a prevailing positive linear chirp across a wide range of biasing conditions. Its high sensitivity to the gain section current proves the dominant contribution of the gain conditions to the group delay characteristics of the emitted pulses.