Craig J. Hamilton

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.

Compact ultrafast semiconductor disk laser: targeting GFP based nonlinear applications in living organisms

We present a portable ultrafast Semiconductor Disk Laser (SDL) (or vertical extended cavity surface emitting laser—VECSELs), to be used for nonlinear microscopy. The SDL is modelocked using a quantum-dot semiconductor saturable absorber mirror (SESAM), delivering an average output power of 287 mW, with 1.5 ps pulses at 500 MHz and a central wavelength of 965 nm. Specifically, despite the fact of having long pulses and high repetition rates, we demonstrate the potential of this laser for Two-Photon Excited Fluorescence (TPEF) imaging of in vivo Caenorhabditis elegans (C. elegans) expressing Green Fluorescent Protein (GFP) in a set of neuronal processes and cell bodies. Efficient TPEF imaging is achieved due to the fact that this wavelength matches the peak of the two-photon action cross section of this widely used fluorescent marker. The SDL extended versatility is shown by presenting Second Harmonic Generation images of pharynx, uterus, body wall muscles and its potential to be used to excite other different commercial dyes. Importantly this non-expensive, turn-key, compact laser system could be used as a platform to develop portable nonlinear bio-imaging devices.

Localized Kerr‐type nonlinearities in GaAs/AlGaAs multiple quantum well structures at 1.55 μm

We report the use of a novel impurity free vacancy disordering technique which has been used to produce waveguides with different Kerr‐type nonlinear coefficients. The technique relies on standard SiO2 dielectric caps to promote disordering and Ga2O3 caps to suppress disordering. Band‐gap shifts of around 40 nm and consequent changes in n2 of more than 60% are reported.

Quantum well intermixing in material systems for 1.5 μm (invited)

Precise control over local optical and electrical characteristics across a semiconductor wafer is a fundamental requirement for the fabrication of photonic integrated circuits. Quantum well intermixing is one approach, where the band gap of a quantum well structure is modified by intermixing the well and barrier layers. Here we report recent progress in the development of intermixing techniques for long wavelength applications, discussing two basic techniques. The first is a class of laser disordering techniques which take place in the solid state. The second is a novel intermixing technique involving plasma induced damage. Both techniques enable large band gap shifts to be achieved in standard GaInAsP multiple quantum well laser structures. The potential of both techniques for photonic integration is further demonstrated by the fabrication and characterisation of extended cavity lasers.

Quantum Dot Based Semiconductor Disk Lasers for 1–1.3 μm

Optically pumped quantum dot (QD)-based semiconductor disk lasers (SDLs) have been under intense research after their first demonstration and important enhancements of their parameters have been achieved since then. In this paper, we present recent developments in QD-based SDLs emitting in the 1-1.3 μm spectral region. Three different wavelength ranges of 1040, 1180, and 1260 nm were explored. Power scaling up to 6 W was achieved for 1040 and 1180 nm devices and up to 1.6 W for 1260 nm device. New spectral regions were covered by direct emission and frequency doubling was used to demonstrate spectral conversion into visible region with green, orange, and red light. Also, the broad gain bandwidth of QD materials was explored and wavelength tuneability up to 60 nm around 1040 nm, 69 nm around 1180 nm, and 25 nm around 1260 nm was demonstrated. The efficiency of excited and ground state emission in QDs was also compared. All these improvements allow new possibilities in applications of QD SDLs, reveal their potential, and suggest the aims for future research in the field.

85.7 MHz repetition rate mode-locked semiconductor disk laser: fundamental and soliton bound states.

Mode-locked optically pumped semiconductor disk lasers (SDLs) are in strong demand for applications in bio-medical photonics, chemistry, space communications and non-linear optics. However, the wider spread of SDLs was constrained as they are operated in high repetition rates above 200 MHz due to short carrier lifetimes in the semiconductors. Here we demonstrate experimentally and theoretically that it is possible to overcome the limitation of fast carrier relaxation and show significant reduction of repetition rate down to 85.7 MHz by exploiting phase-amplitude coupling effect. In addition, a low repetition rate SDL serves as a test-bed for bound soliton state previously unknown for semiconductor devices. The breakthrough to sub-100 MHz repetition rate will open a whole new window of development opportunities.

The influence of single-photon absorption on the performance of the two-photon waveguide autocorrelator

The propagation loss and the single-photon absorption coefficients in a two-photon waveguide autocorrelator are measured as a function of wavelength. The propagation loss was as low as 1.37 cm/sup -1/ at a wavelength of 940 nm in a device with an Al/sub 0.2/Ga/sub 0.8/As waveguide core. Using a photocurrent technique, band-to-band absorption was measured for photon energies well below that of the bandgap. It was demonstrated that, although the band-to-band absorption coefficient is small (/spl sim/10/sup -2/ cm/sup -1/ at a wavelength of 1 /spl mu/m), it is responsible for reducing the contrast ratio of the waveguide autocorrelator. It is suggested that the single-photon absorption takes place via deep levels with relatively long carrier lifetimes.

Modification of the second-order optical nonlinearities in AlGaAs asymmetric multiple quantum well waveguides by quantum well intermixing

We demonstrate that a quantum well intermixing technique can be used to control the second-order nonlinearity χzzz(2) in an AlGaAs asymmetric coupled quantum well waveguide structure at 1.52 μm. Photoluminescence measurements also indicate that the spatial resolution of the impurity-free vacancy disordering process used for quantum well intermixing is better than 1.5 μm which should be sufficient for first-order quasiphase-matched second harmonic generation.

Quantum-well intermixing in GaAs-AlGaAs structures using pulsed laser irradiation

We report the use of a pulsed laser irradiation technique, using multiphoton absorption, to promote quantum-well intermixing (QWI) in double-quantum-well GaAs-AlGaAs laser structures. The process requires neither ion implantation nor the deposition of dielectric caps. Differential bandgap shifts of up to 40 meV have been obtained between the control and the laser irradiated samples. Bandgap tuned lasers were fabricated from the intermixed samples and exhibited negligible changes in slope efficiency and only small increases (15%) in threshold current compared to as-grown devices.