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Broad-band anti-reflection effects in self-assembled, subwavelength size of semiconductor nipple array

Professor Lung-Han Peng's group

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所 彭隆瀚教授

We reported the use of recessive size reduction in self-assembled polystyrene sphere mask with anisotropic etching to form lens-like nipple arrays onto the surface of silicon and gallium nitride.  These devices are shown to exhibit a filling factor near to an ideal close-packed condition and paraboloid-like etch profile with slope increased proportionally to the device aspect ratio.  Specular reflectivity less than 3% was observed over the visible spectral range for the 0.35μm-period nipple-lens arrays in Fig.1.  Using two-dimensional rigorous coupled-wave analysis the latter phenomenon can be ascribed to a gradual index matching mechanism accessed by a high surface-coverage semiconductor nipple array structure.

We further compared the weighted surface reflectivity of the following surface textures of (a) paraboloid-like nipple lens, (b) pillar with a cone-shape etch top and (c) pyramidal grating using two-dimensional rigorous coupled-wave analysis (2D-RCWA).  Our calculations show that the use of structure (a) of paraboloid-like nipple lens array renders the lowest reflectivity (<3%) over the 450-700nm spectral range.  The drastic reduction of reflectivity in the 450-900nm spectral range by texture change from pyramid to paraboloid can be ascribed, as referred from the inset of Fig.2, to a linear change in the effective refractive profile and thus a better gradual index matching mechanism provided by the latter.  The oscillatory behavior of reflectivity in the 450-700nm spectral range of the pillar structure (b) reveals subtlety of incomplete destructive interference for lack of continuous change in the effective reflective index.

Yb³⁺:YAG-silica fiber laser

Professor Sheng-Lung Huang's group

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所 黃升龍教授

We demonstrate a compact continuous-wave Yb³⁺:Y₃Al₅O₁₂–silica (Yb³⁺:YAG-silica) fiber laser grown by the codrawing laser-heated pedestal growth (CDLHPG) technique. The threshold is 25 mW, which is an order of magnitude lower than that of previous short-length fiber lasers (Yb³⁺-doped or Er³⁺/Yb³⁺-codoped). A slope efficiency of 76.3% was achieved from a 7-mm-short Yb³⁺:YAG-silica fiber, corresponding to an extracted power of nearly 1 W/cm. Additionally, low propagation loss and high emission cross section have been determined by analyzing the lasing thresholds and slope efficiencies, and there is an excellent agreement between theoretical results and experimental data.

The CDLHPG technique provides a simple yet effective way to incorporate glass network modifiers (Al₂O₃ and Y₂O₃) into SiO₂ host through interdiffusion process. Further, Cu-Al alloy packaging is employed to efficiently remove the heat generated inside the fiber compared with previous short-length fibers. To the best of our knowledge, this 7-mm-short Yb³⁺:YAG-silica fiber laser with record-low threshold down to 25 mW and record-high slope efficiency up to 76.3% is the shortest active fiber reported to date for any short-length fiber laser operated at room temperature.

A 68-μm-diameter Yb³⁺:YAG single crystal fiber was first grown by the LHPG method from a 20-mol.% doped <111> Yb³⁺:YAG single crystal with a cross section of 500 μm × 500 μm. The as-grown crystal fiber was then inserted into a fused silica capillary with 76 and 320 μm inner and outer diameters, respectively for redrawing with 5-mm/min growth speed by the CDLHPG technique. The silica diffuses completely into the Yb³⁺:YAG crystal fiber during redrawing to form the core of the Yb³⁺:YAG-silica fiber. After the CDLHPG process, the as-grown Yb³⁺:YAG-silica fiber was packaged by melting Al at 780 °C in a Cu holder to make the Cu-Al alloy. Finally, both ends of the Yb³⁺:YAG-silica fiber packaged by the Cu-Al alloy were fine polished for dielectric coating on the fiber end faces. The fiber compositions and microstructure were examined by an electron probe micro-analyzer and a high-resolution transmission electron microscopy, and the refractive index measurement was carried out by a home-made confocal microscope using a 635-nm distributed feedback laser. For laser characterization, the fiber was excited by a Ti³⁺:sapphire laser tuned to 915 nm. Four different output coupler transmittances of 0.7, 1.7, 8.9, and 12.5% were used for optimizing the output powers. The pump was first incident into a standard single-mode fiber (SMF-28) by a 10X objective, and was butt-coupled to the core of a 7-mm-long Yb³⁺:YAG-silica fiber through a dichroic-coated front face. The fiber laser output and the residual pump beam were collimated by an achromatic lens with 10-mm focal length and further filtered by a long-wavelength-pass filter before detection by a photo detector or an optical spectrum analyzer.

The performance of the Yb³⁺:YAG-silica fiber laser is shown in Fig. 1. The lasing threshold increases from 25 to 140 mW as the output coupler transmittance increases from 0.7% to 12.5%. The maximum output power of 670 mW was achieved with the output coupler transmittance of 12.5% under 1000-mW incident pump power, corresponding to a power yield of ~1 W/cm.

超短脈衝雷射在生醫影像、基礎科學研究、工程等眾多領域中，有許許多多不同的應用。而短脈衝雷射輸出端與眾多應用端中間，極需使用簡單、可靠之光學傳遞系統來連結。光纖及其相關元件具有體積小、簡單、高彎曲性、便宜、多功用性、高傳輸頻寬以及對電磁噪音高抵抗性的特性，極適合應用在光源輸出、傳遞以及作為應用端光學系統的骨幹。然而，當超短脈衝光源在光纖中傳遞，常常會因為一些線性及非線性光纖光學效應使得光纖輸出的脈衝質變差。吾人建立數套光纖輸出飛秒雷射光源(圖一)及其相關應用系統。為了非線性雷射顯微術及非線性光學內視鏡的應用，吾人建立了兩套工作在1.2微米波段的光纖輸出超短脈衝雷射光源，並且隨後建立相關的非線性影像系統，具有卓越的表現。在另一方面，利用光纖中的光固子自我頻率位移效應，吾人也建立一破世界紀錄之光纖輸出極寬頻可調雷射(圖二)。此雷射輸出波長可調範圍為1.2微米到2.2微米之間。與現有的光學參數震盪器比較，這個光纖輸出極寬頻可調雷射系統具有非常容易操作、大可調頻寬範圍、光纖輸出並且價格相對便宜的優點，深具未來發展的潛力。此外對於此種光纖輸出極寬頻可調雷射，吾人也提出並展示一種其在射頻時間延遲上的新穎應用。

© Photonics Technology Letters (PTL) 20, 900-902, 2008

http://www.ledinside.com.tw/node/9933

http://www.oled-display.net/iapp-dresden-announced-new-efficiency-record-in-oled-lighting

Nature 459, 234-238 (14 May 2009), “White organic light-emitting diodes with fluorescent tube efficiency”：http://www.nature.com/nature/journal/v459/n7244/full/nature08003.html