Janne Puustinen

11 W single gain-chip dilute nitride disk laser emitting around 1180 nm

We report power scaling experiments of a GaInNAs/GaAs-based semiconductor disk laser operating at ~1180 nm. Using a single gain chip cooled to mount temperature of ~10 °C we obtained 11 W of output power. For efficient thermal management we used a water-cooled microchannel mount and an intracavity diamond heat spreader. Laser performance was studied using different spot sizes of the pump beam on the gain chip and different output couplers. Intracavity frequency-doubling experiments led to generation of ~6.2 W of laser radiation at ~590 nm, a wavelength relevant for the development of sodium laser guide stars.

Formation and phase transformation of Bi-containing QD-like clusters in annealed GaAsBi

We report the formation and phase transformation of Bi-containing clusters in GaAs(1-x)Bi(x) epilayers upon annealing. The GaAs(1-x)Bi(x) layers were grown by molecular beam epitaxy under low (220 °C) and high (315 °C) temperatures and subsequently annealed using different temperatures and annealing times. Bi-containing clusters were identified only in the annealed samples that were grown at low temperature, revealing a relatively homogeneous size distribution. Depending on the annealing temperature and duration, the clusters show different sizes ranging from 5 to 20 nm, as well as different crystallographic phase, being coherently strained zincblende GaAs(1-x)Bi(x) (zb Bi-rich Ga(As, Bi)) clusters or rhombohedral pure Bi (rh-Bi) clusters. We found that: (1) the formation of the zb Bi-rich Ga(As, Bi) clusters is driven by the intrinsic tendency of the alloy to phase separately and is mediated by the native point defects present in the low temperature grown epilayers; (2) the phase transformation from zb Bi-rich Ga(As, Bi) to rh-Bi nucleates in zincblende {111} planes and grows until total consumption of Bi in the GaAs matrix. We propose a model accounting for the formation and phase transformation of Bi-containing clusters in this system. Furthermore, our study reveals the possibility to realize self-organized zb Bi-rich Ga(As, Bi) clusters that can exhibit QD-like features.

Power-scalable 1.57 μm mode-locked semiconductor disk laser using wafer fusion

We report the first (to our knowledge) wafer-fused high-power passively mode-locked semiconductor disk laser operating at 1.57 microm wavelength. An InP-based active medium was fused with GaAs/AlGaAs distributed Bragg reflector on a 2 inch wafer level, resulting in an integrated monolithic gain mirror. An intracavity wedged diamond heat-spreader capillary bonded to the gain chip provides efficient heat removal from the gain structure without disturbing the spectrum of the mode-locked laser. The laser produces over 0.6 W of average output power at 15 degrees C with 16 ps pulse width. The total output power accounting for all output beams emerging from the cavity was 0.86 W. The results reveal an essential advantage of wafer fusion processing of disparate materials over monolithically grown InP-based gain structures and demonstrate the high potential of this technique for power scaling of long-wavelength semiconductor disk lasers.

Postgrowth annealing of GaInAs∕GaAs and GaInAsN∕GaAs quantum well samples placed in a proximity GaAs box: A simple method to improve the crystalline quality

The effects of thermal annealing on GaInAs∕GaAs and GaInAsN∕GaAs quantum wells, grown by molecular beam epitaxy, were investigated. Optical and structural properties were examined upon annealing when the samples had a 200nm thick SiO2 cap layer, or were placed in a so-called GaAs box or were left uncapped. The GaAs box gave rise to the strongest photoluminescence without significant blueshift or structural changes at moderate annealing temperature. Capping with SiO2 impaired the samples and caused a more pronounced blueshift for the GaInAs quantum wells than for the GaInAsN ones. These results consolidate our understanding of the blueshift mechanisms.

Pulse dynamics of a passively mode-locked Bi-doped fiber laser

The pulse evolution in Bi-doped soliton fiber laser with slow and fast saturable absorber has been studied both experimentally and numerically. Semiconductor saturable absorbers with balanced slow and fast absorption recovery mechanisms exhibit a bi-temporal recovery dynamics which permits both reliable start-up of passive mode-locking and short pulse generation and stabilization. The pulse dynamics within the Bi fiber laser cavity have been investigated.

Passively mode-locked GaInNAs disk laser operating at 1220 nm

We report an optically-pumped semiconductor disk laser passively mode-locked with a semiconductor saturable-absorber mirror. Both the absorber and the gain media were made of dilute nitride compound semiconductor, GaInNAs, which enables operation around 1.2 microm wavelengths. The laser generated 5 ps optical pulses with an average output power up to 275 mW. Our demonstration provides an attractive approach for efficiently generating red-wavelengths through external cavity frequency doubling.

Observation of atomic ordering of triple-period-A and -B type in GaAsBi

We report the observation of atomic ordering of triple-period (TP)-A and -B type in low temperature (LT) grown GaAsBi alloy using transmission electron microscopy (TEM). In addition to previous reports, where only TP-A ordering was identified in III-V alloys, here, we confirm by electron diffraction, high-resolution (HR) TEM, and HR Z-contrast scanning TEM that two ordering variants coexists for LT-GaAsBi. We find that the TP-A ordering variant dominates over the TP-B variant. TP-A domains extend over 50–100 nm (projected lateral width) and are of higher perfection compared to TP-B domains. HR Z-contrast scanning TEM on different domains reveals a variation in the Bi occupancy in the {111} planes with triple period sequence. Since the formation of ordered phases has been directly linked to the occurrence of specific surface reconstructions, our results suggest a correlation between the TP-A and B type domains and the multiple stability of n × 3 and 3 × n reconstructions on the (001) surface of GaAsBi unde...

1.32 μm mode-locked bismuth-doped fiber laser operating in anomalous and normal dispersion regimes.

We demonstrate a 1.32 μm mode-locked bismuth fiber laser operating in both anomalous and normal dispersion regimes. In anomalous dispersion regime, achieved by using 13 nm/cm linearly chirped fiber Bragg grating, the laser exhibits multiple soliton operation with pulse duration of 2.51 ps. With the net normal cavity dispersion, the single-pulse operation with higher power has been obtained by avoiding the limitations generic to conservative soliton systems.

Spontaneous formation of three-dimensionally ordered Bi-rich nanostructures within GaAs1-x Bi x /GaAs quantum wells.

In this work, we report on the spontaneous formation of ordered arrays of nanometer-sized Bi-rich structures due to lateral composition modulations in Ga(As,Bi)/GaAs quantum wells grown by molecular beam epitaxy. The overall microstructure and chemical distribution is investigated using transmission electron microscopy. The information is complemented by synchrotron x-ray grazing incidence diffraction, which provides insight into the in-plane arrangement. Due to the vertical inheritance of the lateral modulation, the Bi-rich nanostructures eventually shape into a three-dimensional assembly. Whereas the Bi-rich nanostructures are created via two-dimensional phase separation at the growing surface, our results suggest that the process is assisted by Bi segregation which is demonstrated to be strong and more complex than expected, implying both lateral and vertical (surface segregation) mass transport. As demonstrated here, the inherent thermodynamic miscibility gap of Ga(As,Bi) alloys can be exploited to create highly uniform Bi-rich units embedded in a quantum confinement structure.

Spontaneous formation of nanostructures by surface spinodal decomposition in GaAs1−xBix epilayers

We report on the spontaneous formation of lateral composition modulations (LCMs) in Ga(As,Bi) epilayers grown by low-temperature (<300 °C) molecular beam epitaxy (MBE) on GaAs(001). Both cross-section and plan-view transmission electron microscopy techniques are used to investigate the nature of the LCMs, consisting of Bi-rich cylinder-like nanostructures lying along the [001] growth direction. The observed LCMs are the consequence of a two-dimensional phase separation process occurring at the surface of the growing epilayers, and their columnar nature is consistent with a surface-directed spinodal decomposition process. Although LCMs are thermodynamically driven, we show how they can be kinetically controlled, in particular, through the As/Ga flux ratio and the substrate temperature. This is a result of LCMs developing from surface atomic diffusion processes, since the atomic dimer configurations on the surface alter adatom diffusivity. The significant role of the surface reconstructions is also discussed.