Constant Optical Power Operation of an Ultraviolet LED Controlled by a Smartphone.

Constant light power operation of an ultraviolet (UV) LED based on portable low-cost instrumentation and a monolithically integrated monitoring photodiode (MPD) has been reported for the first time. UV light irradiation has become one of the essential measures for disinfection and sterilization. Monitoring and maintaining a specified light power level is important to meet the criteria of sterilization. We built a module composed of a monolithically integrated UV LED and MPD, a transimpedance amplifier, an Arduino Uno card, a digital-to-analog converter and a Bluetooth transceiver. An Android App that we wrote remotely controlled the UV LED module via Bluetooth. The Arduino Uno card was programmed to receive demands from the smartphone, sent a driving voltage to the LED and returned the present MPD voltage to the smartphone. A feedback loop was used to adjust the LED voltage for maintaining a constant light output. We successfully demonstrated the functioning of remote control of the App, and the resultant UV LED measured power remained the same as the setting power. This setup can also be applied to visible or white LEDs for controlling/maintaining mixed light's chromaticity coordinates or color temperature. With such controlling and internet capability, custom profiling and maintenance of precision lighting remotely would be possible.

Monolithic integration of GaN-based phototransistors and light-emitting diodes.

Monolithic integration of GaN-based phototransistors and light-emitting diodes (LEDs) is reported. Starting with an LED epitaxial wafer, selective Si diffusion was performed to produce an n-p-i-n structure for the phototransistor. A traditional AlGaN bulk electron-blocking layer (EBL) can block electron injection from an emitter to a collector, thereby hindering the photocurrent amplification process. We used an LED wafer with a superlattice EBL; blocking can be removed under a bias of approximately 7 V and above. External quantum efficiencies of more than 100% and 600% at approximately 380 nm and 330 nm, respectively, were achieved at room temperature and a bias of 11 V, corresponding to responsivities of 0.31 and 1.6 A/W, respectively, significantly higher than commercially available ultraviolet (UV) detectors. Furthermore, we demonstrated an integrated operation of the device. UV light was detected using a phototransistor that sent signals to drive an integrated LED as an indicator.

Light-scattering effectiveness of two-dimensional disordered surface textures in thin-film silicon solar cells.

To compare the light-scattering effectiveness of surface-textured solar cells of various design parameters such as density, diameter, refractive index, and location, this study used a new parameter, optical path length gain (OPLG), that is more sensitive than Haze. By modeling two-dimensional disordered textures as a structure that comprises many randomly distributed, small, spherical scatterers, ray-tracing simulations of surface-textured thin-film silicon solar cells were performed. The simulation results suggest that: (1) the optimal scatterer diameter for hydrogenated amorphous silicon (a-Si:H) solar cells is ~50 nm, producing an average OPLG of 3.5; and (2) the optimal scatterer diameter for a-Si:H/µc-Si:H (hydrogenated microcrystalline silicon) tandem cells is ~75 nm, producing an average OPLG of 3.4 and an increase in the bandwidth of the absorption spectrum of 14.5%.

Efficient and low-threshold Cr4+:YAG double-clad crystal fiber laser.

A significant advancement of cw lasing in Cr4+:Y3Al5O12 (Cr4+:YAG) double-clad crystal fiber grown by the codrawing laser-heated pedestal growth technique was demonstrated at RT. The optical-to-optical slope efficiency of 33.9% is the highest, to the best of our knowledge, among all Cr4+:YAG lasers, whether they are in bulk or fiber forms. The low-threshold lasing of 78.2 mW and high efficiency are in good agreement with the simulation. The keys to the high laser efficiency are twofold: one is the improved Cr4+ emission cross section and fluorescence lifetime due to release of the strain on the distorted Cr4+ tetrahedron, which also mitigates photobleaching in Cr4+:YAG; the other is the improved core uniformity at long fiber lengths. In addition, because of the low threshold, the impact of excited state absorption of the pump light is significantly reduced. The effects of crystal-orientation, self-selected, and pump-dependent linear polarization states were also addressed.