Jukka Viheriälä

Inhomogeneities in the nonlinear tensorial responses of arrays of gold nanodots

We use second- and third-harmonic-generation microscopy to address the tensorial nonlinear responses of individual particles in an array of cylindrical gold nanodots. The responses in both orders exhibit widely-variable, polarization-dependent differences between individual nanodots and thereby indicate tensorial inhomogeneities in the sample. The result provides clear evidence that the second-order response, which is forbidden by symmetry for ideal particles, must arise from small-scale, symmetry-breaking features. A similar result for the third-order response, which is allowed for ideal particles, suggests that both nonlinear responses are dominated by strong variations in field localization around the small-scale features differing among individual nanodots.

Soft stamp ultraviolet-nanoimprint lithography for fabrication of laser diodes

We investigate a novel nanofabrication process called soft ultraviolet (UV) nanoimprint lithography (NIL), for nanopatterning of compound semiconductors. We use flexible stamps with three layers and analyze their performance with wafers composed of III-V semiconductors. The developed stamp configuration is in many ways advantageous for the fabrication of precise gratings for various applications in photonics. We describe how to handle the deformation in both lateral and vertical directions by tuning the softness of the stamp and using a two step imprint process. As an application of the UV-NIL, we demonstrate a fabrication process for a laterally corrugated distributed feedback laser. Our laser fabrication process is free from regrowth and therefore easily adaptable to various material compositions and emission wavelengths. Because of the cost-effective full-wafer NIL, these lasers are attractive in various applications where low-cost, single-mode laser diodes are required. Our development work improves the design freedom of the NIL fabrication process of the laser diodes and improves the quality of the transferred patterns. To the best in our knowledge, this is the first demonstration of a single-mode laser diode fabricated by soft UV-NIL.

Soft stamp UV-nanoimprint lithography for fabrication of laser diodes

In this paper, we investigate a novel nanofabrication process called soft UV nanoimprint lithography, for nanopatterning of compound semiconductors. We use flexible stamps with three layers and analyze their performance with wafers composed of III-V semiconductors. The developed stamp configuration is in many ways advantageous for the fabrication of precise gratings for various applications in photonics. We describe how to handle the deformation in both lateral and vertical directions by tuning the softness of the stamp and using a two step imprint process. As an application of the UV-NIL, we demonstrate a fabrication process for a laterally corrugated distributed feedback laser. Our laser fabrication process is free from regrowth and therefore easily adaptable to various material compositions and emission wavelengths. Due to the cost effective full wafer NIL, these lasers are attractive in various applications where low cost, single-mode laser diodes are required. Our development work improves the design freedom of the NIL fabrication process of the laser diodes, and improves the quality of the transferred patterns. To the best of our knowledge, this is the first demonstration of a single-mode laser diode fabricated by soft UV-NIL.

Optically Pumped Edge-Emitting GaAs-Based Laser With Direct Orange Emission

Room temperature lasing operation at 599 nm for a AlGaInP/AlInP/GaAs edge-emitting laser structure is reported. The structure was grown on GaAs substrate and pumped optically with a 532 nm Q-switched laser. The lasing threshold for a 2 mm long and 25 μm wide ridge waveguide structure was 30 mW of average pump power. The orange output beam had an optical spectral width of 1.7 nm.

AlInP–AlGaInP Quantum-Well Lasers Grown by Molecular Beam Epitaxy

We have examined a possibility to use an AlxIn1-x P layer as an active region of a 650-nm semiconductor laser. Encouraging results have been obtained with compressively strained oxide-stripe AlInP-AlGaInP quantum-well lasers, which operated in continuous-wave mode at room temperature, producing an optical power of 460 and 320 mW per uncoated facet at 10 degC and 20 degC, respectively. In pulsed mode, a power level of 780 mW/facet was achieved at 2-A drive current at 5 degC. The results indicate that wide-bandgap AlInP affords an opportunity to develop lasers for the wavelengths 600leslambdales650 nm, which is difficult to achieve by any semiconductor heterostructure

High-power temperature-stable GaInNAs distributed Bragg reflector laser emitting at 1180 nm.

We report a single-mode 1180 nm distributed Bragg reflector (DBR) laser diode with a high output power of 340 mW. For the fabrication, we employed novel nanoimprint lithography that ensures cost-effective, large-area, conformal patterning and does not require regrowth. The output characteristics exhibited outstanding temperature insensitivity with a power drop of only 30% for an increase of the mount temperature from 20°C to 80°C. The high temperature stability was achieved by using GaInNAs/GaAs quantum wells (QWs), which exhibit improved carrier confinement compared to standard InGaAs/GaAs QWs. The corresponding characteristic temperatures were T0=110  K and T1=160  K. Moreover, we used a large detuning between the peak wavelength of the material gain at room temperature and the lasing wavelength determined by the DBR. In addition to good temperature characteristics, GaInNAs/GaAs QWs exhibit relatively low lattice strain with direct impact on improving the lifetime of laser diodes at this challenging wavelength range. The single-mode laser emission could be tuned by changing the mount temperature (0.1 nm/°C) or the drive current (0.5 pm/mA). The laser showed no degradation in a room-temperature lifetime test at 900 mA drive current. These compact and efficient 1180 nm laser diodes are instrumental for the development of compact frequency-doubled yellow-orange lasers, which have important applications in medicine and spectroscopy.

High power (60 mW) GaSb-based 1.9 μm superluminescent diode with cavity suppression element

The characteristics and the fabrication of a 1.9 μm superluminescent diode utilizing a cavity suppression element are reported. The strong suppression of reflections allows the device to reach high gain without any sign of lasing modes. The high gain enables strong amplified spontaneous emission and output power up to 60 mW in a single transverse mode. At high gain, the spectrum is centered around 1.9 μm and the full width at half maximum is as large as 60 nm. The power and spectral characteristics pave the way for demonstrating compact and efficient light sources for spectroscopy. In particular, the light source meets requirements for coupling to silicon waveguides and fills a need for leveraging to mid-IR applications photonics integration circuit concepts exploiting hybrid integration to silicon technology.

Distributed feedback lasers with photon-photon-resonance-enhanced modulation bandwidth

Multi-section distributed-feedback lasers with surface gratings have been fabricated without re-growth by employing ultraviolet nanoimprint lithography. High-frequency photon-photon resonance was exploited to extend the direct modulation bandwidth beyond the conventional limits set by the carrier-photon resonance.

2.34 μm electrically-pumped VECSEL with buried tunnel junction

Mid-infrared semiconductor laser are highly attractive sources for environmental monitoring since the spectral fingerprints of many environmentally important gases are located in the 2-3.3 μm wavelength regime accessible by gallium-antimonide technology. Here an electrically-pumped vertical-external-cavity surface-emitting laser (EP-VECSEL) was realized at 2.34 μm wavelength, using a gain mirror based on the GaSb material system. The gain mirror was grown by molecular beam epitaxy on an n-type GaSb substrate and it included a distributed Bragg reflector made of 24-pairs of AlAsSb/GaSb layers, and a gain region with 5 GaInAsSb quantum wells placed in a 3-λ thick micro-cavity. A structured buried tunnel junction (BTJ) with subsequent overgrowth was used in order to obtain efficient current confinement, reduced optical losses and increased electrical conductivity. Different components were tested with aperture sizes varying from 30 μm to 90 μm. Pulsed lasing was obtained with all tested components at 15 °C mount temperature. We obtained a maximum peak power of 1.5 mW at wavelength of 2.34 μm.

Bringing High-Performance GaInNAsSb/GaAs SOAs to True Data Applications

We experimentally demonstrate the high-speed data processing capabilities of a GaInNAsSb semiconductor optical amplifier operating at 1.55 μm. The investigated structure exhibits good thermal characteristics and fast gain dynamics with 10%-90% recovery time of 55 ps. Successful wavelength conversion of 10-Gb/s signals is reported. A maximum power penalty of <;2.4 dB for return to zero formatting and of 1.9 dB for nonreturn to zero is demonstrated.