GIPO newsletter

Wire Electrical Discharge Machined Folded-Up Corner Cube Retroreflector

Professor Jui-che Tsai

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

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

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.

Investigating the opacity of corneal and scleral scattering medium via numerical solutions of Maxwell’s equations

Professor Snow H. Tseng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Similar in structure, the cornea and sclera both consist of collagen fibrils, yet, the opacity differs significantly. Here we employ numerical solutions of Maxwell’s equations to model light propagation through cornea-like and sclera-like scattering medium, respectively. By varying the specific structure of the scattering medium, we investigate geometrical factors that can account for the opacity, including wavelength, size and the position of the dielectric cylinders. The reported numerical analysis may shed light to understanding the opacity of scattering medium.

Fig. 1. Schematics of the geometrical structure of cornea and sclera of a human eye. The scleral collagen fibrils are irregular in shape and position, whereas the corneal collagen fibrils are smaller and regular in position.

[http://www.nikon-lenswear.com.tw/zh/eyes-and-vision/how-your-eyes-work] [http://www.oculist.net/downaton502/prof/ebook/duanes/pages/v4/v4c023.html]

Fig. 2. Light transmission through scattering medium diminishes with increased constituent dielectric cylinder diameter d: (a) 120 nm, (b) 200 nm, (c) 280 nm, and (d) 360 nm. The amplitude and direction propagation of light is randomized for each wavelength.

Broadband Silicon Photonics Polarization Beam Splitters Designed Using Genetic Algorithm

Professor Ding-Wei Huang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

In this work, we present a design method for a directional coupler-type polarization beam splitter (PBS) for ultrabroadband operation based on a genetic algorithm (GA). The PBS is discretized into short sections, with the geometric parameters of each section optimized using a GA. For PBSs with an operating wavelength range λ = 1400–1650 nm, the insertion losses are less than 0.14 dB and 0.58 dB, and the maximum crosstalk is −20.6 and −16.2 dB for the TE and TM modes, respectively. Under the fabrication error of +/-10nm for the waveguide widths, the device shows negligible performance variation.

Fig. 1. Schematic of the Broadband silicon photonics polarization beam splitter.

Fig. 2. Output power at the Bar and Cross ports for the TE and TM modes calculated by 3D-FDTD simulation.

High Speed 850-nm Vertical Cavity Surface Emitting Lasers (VCSELs)

Professor Chao-Hsin Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

850 nm vertical-cavity surface-emitting lasers (VCSELs) have been extensively applied on high-speed short reach optical network systems due to their characteristics like low divergence angle, cost-effective fabrication, small footprint, low power consumption and high modulation speed at low currents, which can be applied on high-speed datacenter and giga bit Ethernet.

We accomplished a 50 Gb/s operation of an 850 nm oxide-confined VCSEL with a 3.3 μm aperture diameter. The light- current- voltage curve of 850nm VCSEL with a 0.8 mA threshold current (I_th) and the 0.6 W/A slope efficiency, and the high differential resistance of 135 Ω due to the small aperture, which is shown in Fig.1. Fig.2 shows the optical response of the VCSEL with injected current of 4, 6, and 8mA, respectively, and the modulation bandwidth increases from 22.7 to 25 GHz. Fig.3(a) shows 50 Gb/s BER measurement at I/I_th of 13. The error-free data transmission (BER < 10^-12) is achieved with received optical power of 1.7 mW and the corresponding eye diagram is shown in Fig. 3(b). The VCSEL shows an open eye at 50 Gb/s data rate without overshoot which means it can provide higher signal integrity without using pre-emphasis, equalization, and forward error correction. The energy/data efficiency is 748 fJ/bit at 50 Gb/s at room temperature.

Fig. 1. The light-current-voltage (L-I-V) characteristics of 850 nm VCSEL.

Fig. 2. Optical modulation of the VCSEL at various bias current. A 25 GHz bandwidth is obtained at I = 8 mA.

Fig. 3. (a) The 50 Gb/s bit-error-rate-test (BERT) results of the 850 nm VCSEL biased at 11 mA (I/I_th~ 13) with PRBS7 bit sequence and V_pp = 1252 mV. (b) is the corresponding eye diagram.

Three Dimensional Corrugated Electrode Structure for Low-voltage High-transmittance Blue-Phase Liquid Crystal Displays

Professor Wing-Kit Choi

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We have recently proposed new electrode designs for corrugated Polymer-Stabilized Blue-Phase Liquid Crystal Display (PSBP-LCD). These new designs are based on the concept of three dimensional (3D) electrode structure which can help create extra electric field in the transverse direction (y-axis). These new designs can help enhance the potential transmission by reducing the dead zones that exist along the turning edges of these devices. By using suitable device parameters, it is possible to obtain high transmission T of > 90% at low operation voltage of ~10V. PSBP-LCD has many attractive features such as sub-millisecond fast response time, no need of alignment layer, wide-viewing angle etc., and has been actively researched as a potential technology for next-generation displays in recent years. Results of this research have recently been published in Liquid Crystals [1].

[1] Bing-Han Chan & Wing-Kit Choi (2018): Three-dimensional corrugated electrode structure for low-voltage high-transmittance blue-phase liquid crystal displays, Liquid Crystals, DOI: 10.1080/02678292.2018.1530385