J.H. Marsh

A UNIVERSAL DAMAGE INDUCED TECHNIQUE FOR QUANTUM WELL INTERMIXING

We report a novel technique for quantum well intermixing which is simple, reliable and low cost, and appears universally applicable to a wide range of material systems. The technique involves the deposition of a thin layer of sputtered SiO2 and a subsequent high temperature anneal. The deposition process appears to generate point defects at the sample surface, leading to an enhanced intermixing rate and a commensurate reduction in the required anneal temperature. Using appropriate masking it is possible to completely suppress the intermixing process, enabling large differential band gap shifts (over 100 meV) to be obtained across a single wafer.

Monolithic integration via a universal damage enhanced quantum-well intermixing technique

A novel technique for quantum-well intermixing is demonstrated, which has proven a reliable means for obtaining postgrowth shifts in the band edge of a wide range of III-V material systems. The technique relies upon the generation of point defects via plasma induced damage during the deposition of sputtered SiO/sub 2/, and provides a simple and reliable process for the fabrication of both wavelength tuned lasers and monolithically integrated devices. Wavelength tuned broad area oxide stripe lasers are demonstrated in InGaAs-InAlGaAs, InGaAs-InGaAsP, and GaInP-AlGaInP quantum well systems, and it is shown that low absorption losses are obtained after intermixing. Oxide stripe lasers with integrated slab waveguides have also enabled the production of a narrow single lobed far field (3/spl deg/) pattern in both InGaAs-InAlGaAs, and GaInP-AlGaInP devices. Extended cavity ridge waveguide lasers operating at 1.5 /spl mu/m are demonstrated with low loss (/spl alpha/=4.1 cm/sup -1/) waveguides, and it is shown that this loss is limited only by free carrier absorption in waveguide cladding layers. In addition, the operation of intermixed multimode interference couplers is demonstrated, where four GaAs-AlGaAs laser amplifiers are monolithically integrated to produce high output powers of 180 mW in a single fundamental mode. The results illustrate that the technique can routinely be used to fabricate low-loss optical interconnects and offers a very promising route toward photonic integration.

Monolithic integration in InGaAs-InGaAsP multiple-quantum-well structures using laser intermixing

The bandgap of InGaAs-InGaAsP multiple-quantum-well (MQW) material can be accurately tuned by photoabsorption-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. Bandgap shifts larger than 100 meV are obtainable using laser power densities of around 5 W/spl middot/mm/sup -2/ and periods of illumination of a few minutes to tens of minutes. This process provides an effective way of altering the emission wavelengths of lasers fabricated from a single epitaxial wafer. Blue shifts of up to 160 nm in the lasing spectra of both broad-area and ridge waveguide lasers are reported. The bandgap-tuned lasers are assessed in terms of threshold current density, internal quantum efficiency, and internal losses. The ON/OFF ratios of bandgap-tuned electroabsorption modulators were tested over a range of wavelengths, with modulation depths of 20 dB obtained from material which has been bandgap-shifted by 120 nm, while samples shifted by 80 nm gave modulation depths as high as 27 dB. Single-mode waveguide losses are as low as 5 dB/spl middot/cm/sup -1/ at 1550 mm. 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 demonstrated.

The two-photon absorption semiconductor waveguide autocorrelator

In this paper, we report a novel semiconductor waveguide autocorrelator that employs two-photon absorption (TPA), rather than the more commonly used second-harmonic generation, as its nonlinear mechanism. An introduction to TPA is given, and the reasons for its use in the autocorrelator are explained with reference to nonlinear absorption experiments. We then describe the fabrication of the p-i-n GaAs/AlGaAs waveguide. The initial experiments carried out with a single beam in the waveguide are described and fitted to theory. Autocorrelation traces for both a CW modelocked Nd/sup 3+/:YAG laser at 1.06 /spl mu/m and a Q-switched InGaAsP/InP laser at 1.3 /spl mu/m are presented and their pulsewidths obtained by means of a theoretical model. The sensitivity and resolution of the TPA waveguide autocorrelator are then discussed, as is a proposal for a multiple-contact TPA waveguide autocorrelator, designed so that the two pulse trains are counter-propagating. This would result in an autocorrelator from which the laser pulsewidth could be obtained in a single measurement, with no moving parts, and which could be integrated with a semiconductor laser. >

Postgrowth control of GaAs/AlGaAs quantum well shapes by impurity-free vacancy diffusion

The control of quantum well shapes in GaAs/AlGaAs material after growth has been investigated both theoretically and experimentally. Double quantum well samples capped either by SiO/sub 2/ or fluorides of the group IIA elements were annealed, and energy gap shifts were measured by photoluminescence. These experimental energy shifts were compared to a theoretical model to obtain the diffusion coefficient of aluminum into the quantum wells. Fluorides were found to inhibit the intermixing process almost completely, whereas SiO/sub 2/ is known to enhance it. The aluminum diffusion coefficients for samples annealed at 920/spl deg/C for 30 s are 4.0/spl times/10/sup -17/ cm/sup 2//s and 2.1/spl times/10/sup -15/ cm/sup 2//s for SrF/sub 2/ and SiO/sub 2/ caps, respectively. The activation energies found were 4.09 and 6.40 eV for the same two caps. >

Quasi phase matching in GaAs--AlAs superlattice waveguides through bandgap tuning by use of quantum-well intermixing.

We report the observation of second-harmonic generation by type I quasi phase matching in a GaAs-AlAs superlattice waveguide. Quasi phase matching was achieved through modulation of the nonlinear coefficient chi((2))(zxy), which we realized by periodically tuning the superlattice bandgap. Second-harmonic generation was demonstrated for fundamental wavelengths from 1480 to 1520 nm, from the third-order gratings with periods from 10.5 to 12.4microm . The second-harmonic signal spectra demonstrated narrowing owing to the finite bandwidth of the quasi-phase-matching grating. An average power of ~110 nW was obtained for the second harmonic by use of an average launched pump power of ?2.3mW .

Design and fabrication of low beam divergence and high kink-free power lasers

We report the design and fabrication of high performance high power lasers with emission wavelength from 800 to 1000 nm using a novel wafer structure, in which a graded V-shape layer was incorporated, to reduce the vertical far field (wafer growth direction) and to suppress higher order mode lasing. The structure offers the freedom to independently design the vertical far field and optical overlap with the quantum wells. An extremely low far field can be achieved, which still retains high optical overlap, allowing a low threshold current to be maintained. In addition, the structure can greatly enhance the laser kink-free power by suppressing or even completely eliminating higher order mode lasing, an extremely desirable property for high power single mode lasers.

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.

Improved catastrophic optical damage level from laser with nonabsorbing mirrors

The authors demonstrate an improved catastrophic optical damage (COD) level from a ridge laser with nonabsorbing mirrors (NAMs) fabricated by quantum well intermixing. Under destructive testing conditions, the COD level of the NAM laser was improved by a factor of 2.6 compared to the standard laser, attributed to reduced absorption induced facet degradation. Verification of the degradation mechanism was confirmed by inspection and removal of damaged facets.

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>>