Prof. Gavin Conibeer visited GIPO on May 1^st (Friday), 2009, and lectured in room 105, the EE Building II. His lecture, “Third Generation Photovoltaics” elicited enthusiastic participation from GIPO professors and students, and everyone learned much.

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

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

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.

Aspect Ratio Dependent Ultra-low Reflection and Luminescence of Dry-Etched Si Nano-Pillars on Si Substrate

Professor Gong-Ru Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The Si nano-pillars with high aspect-ratio were fabricated by dry-etching the thin-SiO₂ covered Si substrate with rapidly self-assembled Ni nano-dot patterned mask (see Fig. 1). Aspect-ratio dependent ultra-low reflection and anomalous luminescence of Si nano-pillar are analyzed for applications in all-Si-based lighting and energy transferring systems. The Si nano-pillars induce an ultra-low reflectance and refractive index of 0.88% and 1.12, respectively, at 435 nm due to the air/Si mixed structure and highly roughened surface. The reflectance can be <10% with corresponding refractive index of <1.80 between 190 and 670 nm (see Fig. 2). Lengthening the Si nano-pillars from 150±15 to 230±20 nm further results in a decreasing reflectance, corresponding to a reduction in refractive index by Dn/n = 18% in visible and near-infrared wavelength region. After dry etching Si wafer into Si nano-pillars, the weak blue-green luminescence with double consecutive peaks at 418-451 nm is attributed to the oxygen defect (O^2-) induced radiation, which reveals less relevance with the ultra-low-reflective Si nano-pillar surface.

Partitioning Pixel of Organic Light-Emitting Displays with Patterned Microlens Array Films for Efficiency Improvement

Professor Hoang-Yan Lin’s group

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

A pixel partition scheme assisted with the hollow-arranged or patterned microlens array film (MAF) was proposed to improve the optical characteristics and the lifetime of OLEDs. In our simulation results, a OLED pixel of size 1 mm × 1 mm was equally divided into 10×10 smaller sub-pixels, which was of size 100 μm × 100 μm. The original pixel with emissive edge length of 1 mm covered with patterned MAF have 21% luminance enhancement in the surface normal or 0° direction as compared to bared device. After partitioning it into sub-pixels of 100 μm in edge length, the whole partitioned OLED with patterned MAF can have vast improvement up to 72% under the same total active area and optical power emitted from OLED. To gain higher efficiency, these encouraging results inspire us to apply the patterned MAF to small-size AM- and PM-OLED and to partition large pixel into smaller subpixels for lighting purposes!

Removing microlenses right above the emissive area from the single period or fully-filled MAF is defined as the hollow arranged MAF. Besides, the pixel spacing can be designed as 53 μm, 126 μm or 179 μm, which is corresponding to one, two or three rows of microlenses in between the sub-pixels. Figure 1 shows the pixel-partition configuration of pixel spacing by one row of microlenses, and note that the black circle in each configuration is the viewing aperture of luminance meter for measurement and is equal in size.

Hollow-arranged and fully-filled MAF with one, two and three rows of microlens in between the sub-pixel spacing were modeled. Considering normal direction luminance ratio, fully-filled MAF shows from 13% to 21% enhancement, which seems irrelevant to the unit edge length of sub-pixel. However, hollow-arranged MAF presents as much as 67% improvement. As the unit edge length of the sub-pixel decreases, the luminance ratio of hollow-arranged MAF will increase linearly; as increment of microlens rows inside the gap in between the sub-pixels, the luminance ratio of hollow-arranged MAF will also increase, as shown in Figure 2.

The Roles of Thermally Evaporated Cesium Carbonate to Enhance the Electron Injection in Organic Light Emitting Devices

Professor Chih-I Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The properties of thermally evaporated cesium carbonate (Cs₂CO₃) and its role as electron injection layers in organic light emitting diodes were investigated. According to the ultraviolet photoemission spectra (UPS), the Fermi level of Alq₃ after doped with Cs₂CO₃ shifts toward or into the lowest unoccupied molecular orbital (LUMO) as a result of chemical reaction and charge transfer between Cs₂CO₃ and Alq₃, which lowers the electron injection barrier and improves the current efficiency. As for whether Cs₂CO₃ being decomposed during the evaporation, we found that Cs₂CO₃ molecules were deposited on the substrates without decomposition, regardless the evaporation rates, based on the signature features of carbonate groups and ionization energies measured in UPS spectra and the binding energy shifts of core level electrons. The reaction mechanisms between Cs₂CO₃ and Alq₃ are also proposed. Since Cs₂CO₃ not only being used in the electron injection layer but also in converting high work function materials to cathodes, we further quantitatively investigated the work function modification of indium-tin-oxide (ITO) with deposition of Cs₂CO₃ at the surfaces. We found that while 0.5 angstrom thick of Cs₂CO₃ is sufficient to reduce the electron injection barrier of Alq₃, the thickness needed to convert ITO surface to low work function cathode is about 10 angstroms.