We fabricated current-injection InGaAs quantum-dot microdisk lasers with benzocyclobutene cladding successfully. The benefits of this device planarization process are the better thermal conductivity and the less fragility. With this structure, low threshold current of 0.45 mA is achieved at room temperature from a device of 6.5 μm in diameter with single-mode emission from quantum-dot ground states. Through a temperature dependent measurement, the low loss level of our microdisk cavities has been confirmed. These microdisk lasers also show a characteristic temperature significantly higher than that of the edge-emitting lasers from the same wafer. Therefore, single-mode current-injection QD microdisk lasers with very low threshold current are demonstrated and they can serve as very suitable light sources in a compact integrated optical communication system. This work has been published in Opt. Express, Vol. 19, No. 15, pp. 14145, (2011).

Fig. 1. The cross-section SEM image of a current-injection microdisk laser device. The microdisk diameter is 10.5 μm.

Fig. 2. (a) The mode spectrum of a lasing 6.5-μm-diameter microdisk. The WGM linewidth is 0.28 nm. (b) The L-I curve of the 6.5-μm-diameter microdisk, the threshold current is 0.45 mA.

Professor Guo-Dung Su

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Micro-lens array (MLA) is an important component and is used widely in many applications, such as increasing the light extraction efficiency, beam shaping for illumination or light gathering, wavefront sensor, and information displays. There have been many techniques to fabricate micro-lens array, including thermal reflow, laser ablation, direct laser writing, direct e-beam writing, deep lithography with protons, ink-jet printing, and gray-scale mask methods. However, there are still some drawbacks of these techniques. Thermal reflow method requires high temperature and the need for an etch-transfer process. The limited numerical aperture is also concerned. In the laser ablation, the problems are high facility cost and high energy consumption. For the ink-jet printing, the lens size and alignment accuracy are limited. It is difficult to fit the shape precisely and to distinguish the gray levels in a sharp edge by gray-scale mask method. Therefore, a fast, cost effective, stable, and focal length controllable method is desirable to fabricate MLAs.

There is another approach to fabricate MLAs using hydrophobic effect. This approach offers an attractive alternative because it allows accurate and direct fabrication of polymer microlenses without heating or post etching of the substrate. The approach is based on patterning hydrophobic molecules on a substrate and self-assembly of a liquid pre-polymer. It was affected by different surface energy on the hydrophilic domains. The techniques to create patterns include micro-contact printing (µCP) of self-assembled mono layers (SAMs) and adhesive lithography. It provided a simpler method to fabricate MLAs using the hydrophobic effect. However, it still needs either an etch-transfer process of µCP stamp or lithography for an adhesive substrate. That increases the cost and process complexity.

SU-8 photoresist (MicroChem, MA) has high optical transmittance from visible to near-infrared wavelength and high refractive index (~1.63). Furthermore, it has better chemical resistance and mechanical strength than other common polymers such as polycarbonate or PMMA. As a result, SU-8 photoresist is ideal for microlens array fabrication. In this work done in Micro Optical Device Lab (MODEL), a transparent and self-assembled MLA fabricated by the hydrophilic effect through ultra-violet (UV)/ozone treatment is presented. A base layer of SU-8 photoresist can be surface modified to become more hydrophilic using UV/ozone cleaner (model UV-1, Samco). Contact angles on the modified surface were ranged from 27˚ to 15˚ depending on the processing time, UV power and ozone concentration. The light source of UV/ozone cleaner is 110W with primary process wavelength of 254 and 182 nm. The oxygen flow rate is 0.5 L/min. The MLA was made of diluted SU-8 photoresist, which has refractive index mismatch from a glass substrate. This method provides a fast, low cost, no etch-transfer, no lithography fabrication processes. The focal length of MLA is controllable based on UV/ozone treatment.

Figure 1. The fabrication process of SU-8 photoresist MLA using hydrophilic effect under UV/ozone treatment.	Figure 2. The SEM of (a) microlenses of 100 μm diameter under one minute UV/ozone treatment time. (b) the cross section profile of one minute treated microlens.

Professor Snow H. Tseng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The total scattering cross-section (TSCS) is a useful parameter in analyzing light scattering properties. Here we derive a pragmatic relationship of the optical theorem to be employed in a Finite-difference time-domain (FDTD) simulation of light scattering problems. Specifically, the accuracy of the proposed TSCS calculation method is analyzed and compared with other methods. This research solidifies the theoretical basis for light scattering simulation studies.

Fig. 1. Compare TSCS calculated via various methods in an FDTD simulation with grid resolution Ds = 1/60 mm. Methods (A): Optical theorem, (B): sum of far-field RCS at all angles, (C): near-field outgoing Poynting vector flux, and (D): definition of ECS.

Fig. 2. Compare RMS error of the TSCS calculation in an FDTD simulation.

Fig. 3. Error of TSCS calculation via various methods compared with Mie theory. The FDTD simulation is performed with an FDTD grid resolution of Ds = 1/60 mm. (a) the error of TSCS calculated via Optical theorem is larger than others; (b) after compensating for the numerical dispersion of a frequency of 750 THz at 0 degree, the accuracy of Optical theorem calculated TSCS exceeds others.

 

Emitting layer design of a white organic light-emitting device

Professor Jiun-Haw Lee

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

A white organic light-emitting device (WOLED) based on a phosphorescent blue and green emitter combined with red fluorescent one doped in a single host is presented. In such a device, efficient phosphorescent sensitization (PS) was achieved from the green phosphorescent emitter to the red fluorescent one, which was directly observed from transient electroluminescence. An undoped region was inserted between the green and blue dopant region to stabilize the emission spectra. In this configuration, the main recombination zone was at the blue-emitting region, and the minor one was located at the green one near the undoped region. To avoid carrier trapping, the red fluorescent emitter with a reasonably high concentration (0.5%) was doped away from the minor recombination zone. That WOLED exhibited a longer operation lifetime than the phosphorescent blue/green device, because the PS provided a radiative efficient energy relaxation from the green phosphorescent emitter to the red fluorescent one [published in Curr. Appl. Phys. 11, S183, 2011].

Fig. 1. Normalized EL spectra of the WOLED and blue/green OLED.

Fig. 2. Luminance decay curves of OLEDs.

 

Fabrication and characterization of a micro tunable cat’s eye retro-reflector

Professor Jui-che Tsai

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We developed a micro tunable cat’s eye retro-reflector. The tunability is obtained through the use of a smart film, a polymer dispersed liquid crystal device (PDLC). The cat’s eye device consists of three subassemblies, a front-side focusing unit, the smart film, and a back-side reflecting unit. Device characterization, considering possible fabrication inaccuracy such as misalignment, was performed (Fig. 1). The acceptance angle was measured to be 9°. Switching of an image pattern generated by a 2-D array of such tunable cat’s eye retro-reflectors was also demonstrated (Fig. 2). The tunable cat’s eye retro-reflectors can be used in a free-space communication system, or more specifically, an optical identification system.

Figure 1

Figure 2

© 2011 Elsevier B.V.
K. H. Chao, C. D. Liao, B. J. Yang, and J. C. Tsai, “Fabrication and characterization of a micro tunable cat’s eye retro-reflector,”Optics Communications, Vol. 284, pp. 5221-5224, Oct. 2011.

 

Enhanced optical absorption of dye-sensitized solar cells 
with microcavity-embedded TiO₂ photoanodes

Professor I-Chun Cheng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We report the enhanced performance of dye-sensitized solar cells (DSSCs) with microcavity-embedded nanoporous TiO₂ photoanodes. For DSSCs with photoanodes composed of a stack TiO₂ sublayers with microcavity concentrations arranged from low to high on the light illumination path, the short-circuit current density and the conversion efficiency were improved. A pronounced increase in optical absorption and incident monochromatic photon-to-current conversion efficiency in the long-wavelength region indicated that the enhancement of cell performance was due to the multiple scattering of light by the microcavities and the light confinement by the stack of TiO₂ sublayers with a high-to-low effective index of refraction. The internal resistances and the electron transport kinetics in the TiO₂ were studied by electrochemical impedance spectroscopy. The Nyquist plots of the electrochemical impedance spectra of the DSSCs (Figure 1) indicate that the presence of microcavities does not influence the electron lifetime in the TiO₂ film but does affect the charge transport resistances at the TiO₂/dye/electrolyte. The short-circuit current density and conversion efficiency (Figure 2) were improved by 26% and 20% for the DSSC with photoanode in a trilayer structure made using TiO₂ paste mixed with 2 wt.%, 5 wt.% and 10 wt.% PS microspheres diluted in DI water for the first, second and third sublayers, respectively.

Figure 1

Figure 2

©2012 Optical Society of America
D.-W. Liu, I-C. Cheng, J. Z. Chen, H.-W. Chen, K.-C. Ho, and C.-C. Chiang, “Enhanced optical absorption of dye-sensitized solar cells with microcavity-embedded TiO₂ photoanodes,” Optics Express, vol. 20, No. S2, pp. A168-A176, Mar. 2012

 

Direct Side Pumping of Double-Clad Fiber Laser by Laser Diode Array Through the Use of Sub-wavelength Grating Coupler

Professor Ding-Wei Huang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

An electron-beam fabricated sub-wavelength grating coupler for direct side coupling of light emission from a high-power laser diode array was studied theoretically and implemented experimentally. A gold-embedded silica-based design for grating coupler was employed to minimize the thermal expansion due to the accumulated heat from light absorption by metal part of the grating coupler. In addition, with the consideration of the backward diffraction loss and the groove wall non-verticality due to fabrication distortion, the grating pitch and groove width were optimized for the highest coupling efficiency. According to the experimental results, the grating coupler is capable of coupling light power up to 21 W from a 976-nm continuous-wave operated laser diode array into the inner clad of a 400-μm-diameter double-clad fiber with an overall coupling efficiency of 50%. Furthermore, a laser diode array side-pumped Ytterbium-doped double-clad fiber laser by using the grating coupler was demonstrated. By fine tuning the slow-axis collimation lens array, the laser pumping scheme can easily be switched between bi-directional pumping and uni-directional pumping. Compared with the uni-directionally pumped fiber laser of the same gain fiber length, the laser slope efficiency of the bi-directionally pumped fiber laser was increased by 18% due to a better gain distribution over the fiber length. Finally, the signal output power of 10 W with a slope efficiency of 61% was achieved for the bi-directional side-pumped fiber laser.

Fig. 1. Schematic diagram of side-pumped Yb-doped fiber laser (a) Bi-directionally pumped Yb-doped fiber and (b) Uni-directionally pumped Yb-doped fiber. FAC represents the fast-axis collimation lens, and SAC represents the slow-axis collimation lens.     Fig. 2. Schematic diagrams of the gold-embedded silica grating coupler with (a) normal incident pump light, and (b) an angle of incident pump light.

Fig. 3 (a) The measured laser signal output power against coupled pump power of bi-directional pumping (square points) and uni-directional pumping (triangular points) schemes, respectively. (b) Laser output spectrum for bi-directional pumping at 1 W (black curve) and 10 W (red curve) output power, respectively.