S. Karirinne

1.5-µm monolithic GaInNAs semiconductor saturable-absorber mode locking of an erbium fiber laser

We present a new monolithic GaAs-based semiconductor saturable absorber operating at 1.55 microm. An epitaxially grown absorber mirror in a GaInNAs/GaAs material system was successfully used to mode lock an erbium-doped fiber laser. The GaInNAs material system possesses intriguing physical properties and provides great potential for lasers and nonlinear optical devices operating at the 1.3-1.55-microm wavelength range.

Substrate-dependent crystallization and enhancement of visible photoluminescence in thermal annealing of Si/SiO2 superlattices

We study annealing of Si/SiO2 superlattices on fused quartz and crystalline Si substrates. Under annealing at 1200 °C, the superlattices on Si undergo partial crystallization involving clusterization of Si layers through ultrathin (1 nm) oxide, and visible photoluminescence (∼2.1 eV) strongly increases for the samples with thinner Si layers (⩽2.5 nm). The annealed superlattices on quartz exhibit a higher disorder, tensile stress, and weaker visible photoluminescence. The results do not support assignment of the observed visible photoluminescence to quantum confinement in Si crystallites but rather indicate that it originates from Si=O bonds stabilized in the Si/SiO2 network.

Growth-temperature-dependent (self-)annealing-induced blueshift of photoluminescence from 1.3 μm GaInNAs/GaAs quantum wells

Growing the capping layer of a GaInNAs/GaAs quantum well at typical substrate temperature for GaAs growth by molecular-beam epitaxy, like 580 °C, was found to induce a blueshift of the quantum-well emission whose magnitude significantly increased as the quantum-well growth temperature was decreased. The growth-temperature-dependent (self-)annealing-induced blueshift is correlated with the presence of indium and occurs without observable changes in alloy macroscopic composition or quantum-well structure. The underlying cause for the increase in blue shift with decreasing quantum-well growth temperature appears to be an enhancement in the amount of In–N bonds formed by (self-)annealing, likely through a defect-assisted mechanism.

Long-wavelength fast semiconductor saturable absorber mirrors using metamorphic growth on GaAs substrates

Metamorphic growth of InP on GaAs has been used to decrease the absorption recovery time of 1.55μm semiconductor saturable absorber mirrors. We show that the recovery time can be reliably controlled by changing the thickness of an InP “lattice reformation layer” grown between the GaAs-based distributed Bragg reflector and the active region. Semiconductor saturable absorber mirrors with a thickness of the InP reformation layer around 200 nm or smaller exhibit a recovery time short enough to reliably mode-lock fiber lasers.

Femtosecond neodymium-doped fiber laser operating in the 894-909-nm spectral range

We demonstrate a practical ultrafast Nd-doped fiber laser operating in the 894-909-nm spectral range. Using purposely designed semiconductor saturable absorbers, a truly self-started mode-locking regime of operation with clean transform-limited 360-fs pulses was achieved.

Annealing effects on optical and structural properties of 1.3-μm GaInNAs/GaAs quantum-well samples capped with dielectric layers

Effects of thermal annealing on photoluminescence (PL) and x-ray diffraction from metastable GaInNAs/GaAs quantum-well samples covered by dielectric layers have been studied. PL from uncoated samples exhibits a saturable blueshift of 22 meV relative to PL from the as-grown samples in these experiments. The shift is attributable to a change in the nearest neighbors of nitrogen in short-range-order N-InmGa4−m (0⩽m⩽4) clusters at a fixed composition with negligible Ga/In/N interdiffusion. A Si3N4 cap layer effectively prevents the blueshift in the early stage of annealing and improves emission intensity. Under severe annealing conditions (750 °C for 1500 s), the maximum blueshift for the Si3N4-covered samples is 31 meV. A SiO2 cap layer causes a large nonsaturable blueshift, almost 100 meV in these experiments. The large blueshift is assigned to the formation of defects (likely Ga vacancies) at the SiO2/GaAs interface. The defects are believed to diffuse into the bulk at elevated temperatures and to assist G...

Dilute nitride vertical-cavity surface-emitting lasers

A novel quaternary compound semiconductor material, Ga1 − xInxNyAs1 − y(0 < x, y < 1), was successfully used in demonstrating optically pumped continuous-wave vertical-cavity surface-emitting lasers emitting at 1280 nm. The epitaxial heterostructures of each laser wafer were grown in a single nucleation process by conventional molecular beam epitaxy using a nitrogen radio-frequency plasma source. The lasers consist of GaAs/AlAs distributed Bragg reflector mirrors and 6 or 15 Ga0.65In0.35N0.014As0.986/GaAs quantum wells with special strain-mediating layers. The laser characterization was carried out by using a fibre pigtailed 980 nm pump laser diode, 980/1300 nm wavelength division multiplexer and an optical spectrum analyser. A high optical output power of 3.5 mW was coupled lenslessly into a standard single-mode fibre.

The behaviour of optical and structural properties of GaInNAs/GaAs quantum wells upon annealing

Our experiments show that photoluminescence spectra from dilute nitride Ga1?xInxNyAs1?y/GaAs (y < 2%) quantum wells (QW) are independent of growth temperature of the QWs in the range from 430 to 470?C. Spectral blue shift upon annealing is large for the low-temperature QW (430?C), and emission intensity is more enhanced than that of the high-temperature QW (470?C). Raman scattering reveals that the 430?C QW contains more In?N bonds than does the 470?C QW. It seems, therefore, that the blue shift, which is proportional to a number of In?N bonds, originates from the presence of point-like defects of the alloy. Lower emission intensity from the annealed 470?C sample may be attributed to more pronounced alloy fluctuations and interface roughening, seen in cross-sectional transmission electron micrographs.

Long-wavelength nitride lasers on GaAs

This paper reviews recent studies of structural and optical properties of long wavelength GaInNAs/GaAs quantum well semiconductors and the performance features of GaInNAs/GaAs vertical cavity surface emitting lasers (VCSELs) and edge-emitting lasers. The studies forecast that GaInNAs VCSELs and possibly edge-emitting lasers will become a compliment, or even a replacement, to the now dominant InP-based devices at 1.3-µm short-haul data transmission systems, but whether GaInNAs lasers in general will ever be competitive with the InP lasers at 1.4-1.6 µm is an entirely open issue today. The dilute nitride technology has still some distance to go in addressing its remaining concerns; yet, the authors believe that it is still the best bet to bring about the long-waited breakthrough in the component technology for optical fibre networks.

Optimisation of growth temperature and post-growth annealing for GaInNAs/GaNAs/GaAs quantum-well structures emitting at 1.3 μm

Abstract Photoluminescence performances were studied on strain-compensated, GaInNAs/GaNAs/GaAs quantum-well laser-like structures grown by molecular beam epitaxy in an attempt to define optimal conditions for growth temperature and post-growth thermal treatment. The optimal process window is found to be narrow for light emission normal to the layer stack. The highest intensity, the narrowest line width and the longest wavelength were observed for the samples grown at approximately 450 °C, followed by annealing at 700 °C for 30 min.