Professor Chinlon Lin visited GIPO on March 9^th (Tuesday) and lectured in Auditorium 101, Barry Lam Hall. His lecture, “From G. Marconi to Charles K. Kao--100 Years of Nobel Prize (Physics, 1909-2009) and A Century of Modern Marvels of EECS” was attended by GIPO professors and students with enthusiasm. Everyone learned much.

Speaker, Professor Chinlon Lin

Professor Stephen E. Saddow attended the GIPO Photonics Forum on March 12^th, 2010 (Friday). GIPO professors and students who attended his lecture “Silicon Carbide: A Robust Semiconductor Material for Harsh Environment MEMS and Biomedical Applications” participated with enthusiasm. Professor Saddow’s lecture was fascinating and thorough, and he interacted with his audience throughout the lecture.

Speaker, Professor Stephen E. Saddow

March 26^th, 2010 (Friday) 2:30 PM

Director-general Zhu-Shun Zhuo of Value Creation Center, Wistron Corporation

The Transformation and Development of the Photonics and Information and  Communication Industries

Director-general Zhu-Shun Zhuo visited GIPO on March, 26^th, 2010 (Friday) to speak at GIPO’s Photonics Forum. His lecture, “The Transformation and Development of the Photonics and Information and Communication Industries” was attended with enthusiasm by GIPO professors and students. Director-general Zhu-Shun Zhuo’s lecture was both informative and fascinating and he interacted with his audience throughout the lecture. Everyone who attended learned much.

Speaker, Director-general Zhu-Shun Zhuo

Speaker, Professor Wood-Hi Cheng

The host of the lecture, Professor Zhe-Chuan Feng (left) pictured with Professor Chi-Shun Tu (right).

After the fire broke out, a command zone, a rescue zone, an assembly zone were formed immediately.

The broadcasting unit proceeded to announce the danger in the building.

The evacuation assistance unit evacuating personnel on the spot.

Experts teaching participants how to use fire extinguishers after the drill.

The intense singles championship match

Awarding - third place, singles: Ying-Yuan Huang (left)

Awarding - third place, singles: Yang-Kai Wu (left)

Awarding - second place, singles: Wei-Chung Chen (left)

Awarding - champion, singles: Agnes Lee (left)

Awarding - second place, doubles: Wei-Hsiuan Tseng (left) / Po-Sheng Wang (middle)

Awarding - champions, doubles: Agnes Lee (left) / Yu-Ta Wang (middle)

All participants, at the 2^nd GIPO Students’Association Badminton Tournament closing ceremony

Research Accomplishments in 2009, Wide Gap Semiconductor Laboratory

Professor Zhe-Chuan Feng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

l“Raman scattering study on anisotropic property of wurtzite GaN”, Hung Chiao Lin, Z.C. Feng, et al., JOURNAL OF APPLIED PHYSICS 105, 036102 (2009)

l“Temperature dependent and time-resolved photoluminescence studies of InAs self-assembled quantum dots with InGaAs strain reducing layer structure”, Lingmin Kong, Zhe Chuan Feng, et al., JOURNAL OF APPLIED PHYSICS 106, 013512 (2009)

l“Suppression of phase separation in InGaN layers grown on lattice-matched ZnO substrates”, N. Li, … Z.C. Feng, et al., Journal of Crystal Growth 311 (2009) 4628–4631.

l“Rapid thermal annealing effects on the structural and optical properties of ZnO films deposited on Si substrates”, Y.C. Lee, … Z.C. Feng, et al., J. Luminescence 129 (2009) 148–152.

l“Metalorganic chemical vapour deposition of GaN layers on ZnO substrates using α-Al2O3 as a transition layer”, S.J. Wang, … Z.C. Feng, et al., J. Phys. D: Appl. Phys. 42 (2009) 245302 (5pp).

l“Temperature-Dependent Excitonic Luminescence in ZnO Thin Film Grown by Metal Organic Chemical Vapor Deposition”, Y.C. Lee, … Z.C. Feng, et al., Japanese Journal of Applied Physics 48 (2009) 112302.

Enhancing InGaN-based solar cell efficiency through localized surface plasmon interaction by embedding Ag nanoparticles

Professor Yean-Woei Kiang's Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We have demonstrated the simulation results of using the interaction of metal NP induced LSP with an InGaN absorbing layer for enhancing the efficiency of an InGaN/GaN-based solar cell. The absorption increase is implemented by embedding Ag NPs in InGaN for inducing LSP. The effective LSP absorption and NP scattering lead to the enhancement of InGaN absorption. The embedment of metal NPs in semiconductor may affect carrier transport. However, we show that such an effect is small unless the surface recombination velocity at the interface between a metal NP and surrounding InGaN is extremely high. Fig. 1 shows the photon absorption rates (the left ordinate) of the InGaN layer and Ag NP and the short-circuit current densities (the right ordinate) of the solar cell as functions of wavelength for the cases with and without Ag NP when the surface recombination velocity, S, is 10 m/s. For comparison, the incident photon flux under the condition of AM1.5G (the curve labeled by “incidence”) is also plotted in Fig. 1. The significant absorption enhancement on the long-wavelength side by embedding the Ag NP can be clearly seen. In Fig. 1, the curve labeled by “metal dissipation” represents the part of photons absorbed by the Ag NP and turned into dissipation heat. The embedment of the Ag NP results in an increase of integrated photon absorption rate by 28.44 %. The two curves of short-circuit current density (per unit spectral width), i.e., J_SC (without NP) and J_SC (with NP), which essentially follow the oscillatory behaviors of the two curves for InGaN absorption show the enhancement of photo-generated current. With the embedded Ag NP, the integrated J_SC is increased by 27.87 %. Fig. 2(a) shows the distribution of electrical field magnitude at 580 nm in the region around the Ag NP by assuming that the magnitude of the incident field is unity. Here, one can see that strong near field is generated around the Ag NP. Also, certain backscattered field is distributed in the n-GaN layer. Fig. 2(b) shows the stream line distribution of static electric field inside the solar cell. Here, one can see the distortion of static electric field by the embedded Ag NP. Fig. 3 shows the integrated current densities (the left ordinate) and the output power densities (the right ordinate) as functions of applied voltage for the cases with NP and without NP corresponding to the results shown in Fig. 1. The first and second numbers in the parentheses represent the voltage for the maximum output power (in V) and the maximum output power density (in mW/cm²). Here, one can see that the open-circuit voltage of ~1.51 V is not affected by the embedment of the Ag NP. The integrated current density, J, is significantly increased from 8.08 to 10.17 mA/cm² by embedding the Ag NP. The maximum output powers of both cases are achieved at 1.4 V in applied voltage. The maximum output power is increased from 10.59 to 13.53 mW/cm² (roughly from 10.59 to 13.53 % in efficiency), corresponding to an increase of 27.76 %.

Flexible ZnO transparent thin film transistor using solution-based process

Professor Ching-Fuh Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Currently, transparent electronics based on a flexible substrate are one of the most crucial technologies for the next generation of optoelectronic devices, such as displays, home electronic appliances, and photovoltaics. Hence, transparent thin-film transistors (TFTs) on flexible substrates are key devices for realizing these transparent electronic products. Although organic TFT and its manufacturing process are suitable for flexible substrates, their low mobility and extreme sensitivities to oxygen and moisture overshadow their device performance attributes. TFTs that use ZnO have the advantages of high charge mobility, excellent environmental stability, and high transparency, in comparison to TFTs that are based on organic semiconductors.

In this study, we have fabricated a highly transparent ZnO-TFT by a hydrothermal method using a polymeric PMMA film as the gate insulating film. An O₂-plasma treatment was applied to the PMMA dielectric to enhance compatibility between the ZnO active layer and the PMMA dielectric at the interface. In order to investigate the effects of grain boundaries on the flexible ZnO-TFTs, we studied ZnO-TFTs that had three kinds of ZnO channel morphologies, which grew for 50 min at ZnO solution concentrations of 110, 80, and 50 mM, respectively. The field effect mobility (μ) and threshold voltage (V_T) values were calculated from the slope of a plot of the square root of the drain-source current (I_DS) versus the gate voltage (V_G) in the regime of the drain-source voltage (V_DS)=20 V, using the equation I_DS= (WCi/2L)μ(V_G-V_T)². The saturation mobility was highest at 7.53 cm²/Vs for the 110 mM preparation and lowest at 0.097 cm²/Vs for the 50 mM preparation, with 0.69 cm²/Vs for the 80 mM preparation.

Enhancements of direct band radiative recombination from Ge

Professor Chee-Wee Liu's group

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Due to the high carrier mobility, strong photon absorption, and possible integration with Si, the indirect band gap Ge is used for both scientific interest and optoelectronic applications. The methods to enhance the direct bandgap transition include the high pumping level, the elevated temperature, the high concentration of n type doping, and the biaxial tensile strain. Significant enhancement of direct transition fraction in both photoluminescence (PL) and electroluminescence (EL) is demonstrated. The physical recombination models are developed for both direct and indirect transitions.

Fig. 1 and Fig. 2 show the PL infrared emission from the n⁺p Ge structure. The electrons in Γ valley and L valley recombine with holes in the valence band and emit infrared at the 795meV peak (direct bandgap transition) and the 695meV peak (indirect bandgap transition), respectively. By increasing pumping power and temperature, the electron fraction in Γ valley increases and the electron Fermi level moves upwards. The indirect and direct bandgap transitions are also observed in EL, measured (Fig. 3). The spectra of indirect and direct bandgap emission can be fitted by using the electron-hole-plasma (EHP) recombination model and the direct bandgap recombination model, respectively. To have a reasonable fit, the band tail of absorption edge in the direct bandgap recombination model is taken into consideration. The intensity fraction of direct bandgap transition increases with increasing excitation level and reaches 39% and 22% at EL excitation of 600mA and PL excitation of 360mW, respectively. The intensity fractions of direct bandgap transition are enhanced by 4 times and 1.1 times for EL excitation from 100mA to 600mA and PL excitation from 120mW to 360mW, respectively. By applying biaxial tensile strain, the tensile strain shrinks the direct bandgap more than the indirect bandgap. The direct bandgap transition becomes more and more significant (1.8X) by increasing the biaxial tensile strain up to 0.37% (Fig. 4).

The high PL/EL pumping level, n-type doping concentration, and high temperature can enhance the direct bandgap transition. The strain can be an extra factor to enhance the direct transition of Ge up to ~ 1.8 times with 0.37% strain. The progressive improvement of the radiative recombination makes it possible to have Ge-based light emitting devices for practical applications.

Efficient and compact design of up-converted 435nm blue lasers

Professor Lung-Han Peng's group

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We reported a design of up-conversion 435nm blue lasers by simultaneously fulfilling the nonlinear processes of 1^st-order quasi-phase-matching optical parametric oscillation (QPM-OPO) with 2^nd-order second harmonic generation (QPM-SHG) in a periodically poled lithium tantalate (PPLT) at a single-period of 7.9μm and 75% domain duty cycle (Fig.1).  An optimum PPLT device of 15 mm length, albeit facet uncoated, exhibits a low threshold of 150mW and a differential slope efficiency of 22.6%, rendering a 56mW blue generation when pumped by a pulsed 532nm green laser with an average power rating of 400mW (Fig.2). We noted that the transmitted green power approached saturated values of 108 and 200 mW at PPLT crystal lengths of 20 and 10 mm, respectively.  This phenomenon resembles the optical power limiting effect earlier reported on the doubly- and singly-resonant oscillators (DRO/SRO). In a steady state operation, it signifies a pinning process of the oscillator gain at a threshold value and suggests that excess energy from the pump beam would be transferred to the phase-matched nonlinear optical process.

Glass-Clad Crystal Fibers Based Ultrahigh Resolution Optical Coherence Tomography

Professor Sheng-Lung Huang's group

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Optical coherence tomography (OCT) has evolved as a powerful diagnostic modality for non-invasive medical applications. With the great success of OCT in recent years, high resolution and high image fidelity are getting more importance. The axial resolution is inversely proportional to the light bandwidth. So far, several approaches have been proposed to achieve high axial resolution, such as multiplexed superluminescent diode, femtosecond laser, continuum generation from photonic crystal fiber, and xenon illuminator. In general, these solutions either have high cost or exhibit bumpy spectrum. In contrast, spontaneous emission light can generate broadband and near-Gaussian spectrum, but the power level is often very weak. Using waveguide to collect the amplified spontaneous emission (ASE) can improve brightness of the source for OCT system. A double-clad crystal fiber (DCF) growth technique has been developed to guide and amplify the spontaneous emission. Using active medium as the single-crystalline core, the DCF can maintain the high cross section of the crystal environment, and effectively collect the ASE.

By means of the laser-heated pedestal growth (LHPG) method, single crystalline fibers of various active media and diameters were first obtained from 500-μm-diameter source rods with a few diameter-reduction steps to become 30 ~ 100 μm in diameters. They were then gone through a codrawing LHPG process to form DCFs. During the codrawing process, the DCF cores were further reduced to 5 to 20 μm depending on applications. At visible wavelength range, a Ce³⁺:YAG DCF generating 560-nm center wavelength with a 3-dB bandwidth of 98 nm was fabricated. At near IR wavelength ranges, the active media were Ti³⁺:sapphire and Cr⁴⁺:YAG for a center wavelength of 770 nm and 1380 nm, respectively. For Ce³⁺:YAG DCF, the broadband emission and short central wavelength of this light source enable the realization of 1.5-μm axial resolution in air or 1.1 μm in bio tissues. The relatively smooth spectrum reduced the side lobe of its point spread function, and therefore, facilitated the generation of a high quality image with less crosstalk between adjacent image pixels.

As a demonstration, an Aplocheilus Lineatus Gold fish was adopted to map out the stroma of its cornea in vivo as shown in Fig. 1. The layers inside the cornea were clearly identified. As time went on, the cornea became thinner and more atrophic because of water dissipation in the dry atmosphere. This OCT system may be useful for the detection of early stage cornea disorders, such as Fuch’s and keratoconus dystrophies. Active broadband DCFs are promising for high resolution and high image fidelity OCT systems. Micron resolution can be achieved with low image pixel cross talk. These CW light sources can enable low-cost OCT systems with cellular resolution for various in-vivo bio-medical applications that fulfill the safety requirement of 20 mJ/cm² set by US Food and Drug Administration.

Acknowledgment

The authors would like to thank the support from the National Taiwan University and Foresight Taiwan Project Office, National Science Council under grant number NSC 98 2627-E-002-001.