Multilayer WS2 for low-power visible and near-infrared phototransistors.

Mechanically exfoliated multilayer WS2 flakes are used as the channel of field effect transistors for low-power photodetection in the visible and near-infrared (NIR) spectral range. The electrical characterization as a function of the temperature reveals devices with n-type conduction and slightly different Schottky barriers at the drain and source contacts. The WS2 phototransistors can be operated in self-powered mode, yielding both a current and a voltage when exposed to light. The spectral photoresponse in the visible and the NIR ranges shows a high responsivity (4.5 µA/W) around 1250 nm, making the devices promising for telecommunication applications.

Laser to laser power conversion with remote signaling.

We demonstrate laser power conversion using an edge-coupled waveguide configuration. A laser with an emission energy of 0.87 eV (1427 nm) optically pumps a second with an emission energy of 0.80 eV (1540 nm), achieving the maximum possible open circuit voltage of 0.83 V due to optically pumped lasing. A fiber to device power conversion efficiency of 33% is achieved with internal power conversion efficiency ranging from 57% to 51%. The voltage at maximum power is 0.6 V, which is a record for the wavelength range. The same optically pumped device is used for effectively power-free 500 Mbps upstream data transmission, enabling compact powering and signaling for emerging applications in minimally invasive medical interventions and remote photonics.

GHz bandwidth semipolar (112¯2) InGaN/GaN light-emitting diodes.

We report on the electrical-to-optical modulation bandwidths of non-mesa-etched semipolar (112¯2) InGaN/GaN light-emitting diodes (LEDs) operating at 430-450 nm grown on high-quality (112¯2) GaN templates, which were prepared on patterned (101¯2) r-plane sapphire substrates. The measured frequency response at -3 dB of the LEDs was up to 1 GHz. A high back-to-back data transmission rate of above 2.4 Gbps is demonstrated using a non-return-to-zero on-off keying modulation scheme. This indicates that (112¯2) LEDs are suitable gigabit per second data transmission for use in visible-light communication applications.

Stable locking phase limits of optically injected semiconductor lasers.

The stability of an optically injected laser is considered theoretically with an emphasis on the understanding of the locked phase whereas previous works focus primarily on the frequency detuning limits. Exemplary photon and carrier number curves for regions within and outside stable locking are presented. The dependence of the phase limits on injection ratio naturally divides into three regions with qualitatively different descriptions for the phase boundaries in each. Frequency detunings at which the locked phase is zero for different injection ratios are investigated. Using this zero phase point it is shown that the coupling rate between the injected and internal field as well as the linewidth enhancement factor can be determined in a single voltage measurement under weak injection. The modulation response parameters at these detunings are analysed and shown to be strongly interconnected.

Investigation of active filter using injection-locked slotted Fabry-Perot semiconductor laser.

We investigate the selective amplification and filtering of injection-locked slotted Fabry-Perot semiconductor lasers. Current and temperature tuning are used to selectively filter and amplify subcarriers of coherent optical combs with a selectivity of at least 10 GHz with an optical gain of up to 18 dB for filtered lines. A side mode suppression ratio in excess of 20 dB is also achieved.

Origin of power fluctuations in GaN resonant-cavity light-emitting diodes.

Resonant-cavity light-emitting diodes (RCLEDs) with multiple InGaN/GaN quantum wells have been grown on sapphire substrates. The emission was through the substrate, and the top contact consisted of a highly reflecting Pd/Ag metallization. The peak emission wavelength was measured to be 490 nm. Under constant current biasing, the intensity was observed to fluctuate irregularly accompanied by correlated variations in the voltage. To investigate this further, emission from the RCLED was focused through a GaAs wafer onto a Vidicon camera. This gave a series of infrared, near-field images, spectrally integrated over a wavelength range from 870 nm to 1.9 microm. Flashes from point sources on the RCLED surface were observed, indicating that short-lived, highly localized "hot spots" were being formed that generated pulses of thermal radiation. It is proposed that this phenomenon results from the migration of metal into nanopipes present in this material. The filled pipes form short circuits that subsequently fuse and are detected by bursts of infrared radiation that are recorded in real time.