台大光电所所讯

Wire Electrical Discharge Machined Folded-Up Corner Cube Retroreflector

Professor Jui-che Tsai

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

台湾大学光电所蔡睿哲教授

We have developed an approach to construct a corner cube retroreflector (CCR). A two-dimensional cutout pattern is first fabricated with wire electrical discharge machining process. It is then folded up into a three-dimensional CCR suspended on a cantilever beam. The folded-up CCR may be driven through external actuators for optical modulation; it can also mechanically respond to perturbation, acceleration, etc., to function as a sensor. Mechanical (static and dynamic modeling) and optical (ray tracing) analyses are also performed.

Yu-Fan Chen, Yen-Hung Wang, Jui-che Tsai, “Study of wire electrical discharge machined folded-up corner cube retroreflector with a tunable cantilever beam,” Opt. Eng. 57(3), 035104 (2018)

Three Dimensional Simulation on the Transport and Quantum Efficiency of UVC-LEDs with Random Alloy Fluctuations

Professor Yuh-Renn Wu's laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

台湾大学光电所吴育任教授

The active regions of ultraviolet light emitting diodes (UVLEDs) for UVB and UVC wavelengths are composed of AlGaN alloy quantum barriers (QBs) and quantum wells (QWs). The use of alloy QBs and QWs facilitates the formation of percolative paths for carrier injection but also decreases carrier confinement within the QWs. We applied the recently developed Localization Landscape (LL) theory for a full 3D simulation of the LEDs. LL theory describes the effective quantum potential of the quantum states for electrons and holes in a random disordered system with a high computational speed. The results show that the potential fluctuations in the n-AlGaN buffer layer, QWs and QBs provide percolative paths for carrier injection into the top (p-side) QW. Several properties due to compositional disorder are observed: (1) The peak internal quantum efficiency (IQE) is larger when disorder is present, due to carrier localization, than for a simulation without fluctuations. (2) The droop is larger mainly due to poor hole injection and weaker blocking ability of the electron blocking layer (EBL) caused by the fluctuating potentials. (3) Carriers are less confined in the QW and extend into the QBs due to the alloy potential fluctuations. The wave function extension into the QBs enhances TM emission as shown from a k.p simulation of wave-functions admixture, which should then lead to poor light extraction. The work is published in Applied Physics Letter.

FIG.1. (a) Illustration of UVC-LED layer structures. Contacts are put on the top and bottom surfaces. The simulated device area is 30nm x 30nm. The thickness of QW and QB is 2 and 10nm, respectively. (b) Computed Al compositions in the AlGaN layers obtained by assuming a random alloy. (c) Emission peak calculated by averaging 10 different random maps for a single QW. (d)The band profile of UVC-LEDs without considering composition fluctuation.

FIG. 2. Side view of (a) conduction band potential; (b) radiative recombination rate; (c) non-radiative recombination rate; (d) electron current density; (e) hole current density; and (f) ε_xx. All maps at J=(4.7V,20A/cm²). The z direction is the growth direction of the epitaxial stack crystal c-axis.