July 2010 - September 2010
Publisher: Chairman Ching-Fuh Lin  Editors: Prof. I-Chun Cheng, Ms. Hsiao-wen Lin  October 15, 2010

Congratulations! GIPO Professor Gong-Ru Lin receives the title of “Fellow of The Institute of Physics in UK, FInstP”.


June “Photonics Forum” Lecture Highlights


June 4th, 2010 (Friday) 2:30 PM
Speaker: Prof. Marek Osinski (University of New Mexico)
Topic: Biomedical Applications of Colloidal Nanocrystals

Professor Marek Osinski visited GIPO on June 4th (Friday) and spoke at Auditorium 101, Barry Lam Hall, on the topic of “Biomedical applications of colloidal nanocrystals”. GIPO professors and students attended with enthusiasm and were benefit from the lecture greatly.



June 11th, 2010 (Friday) 2:30 PM
Speaker: Prof. Kuang-Chien Hsieh (Department of Electrical and Computer Engineering, University of Illinois, Urbana-Champaign)
Topic: Low-Temperature Grown Compound Semiconductors for Optoelectronic Device Applications: Distributed Bragg Reflectors and Wafer Bonding Agents

Professor Kuang-Chien Hsieh visited GIPO on June 11th (Friday) and gave a presentation at the GIPO Photonics Forum on the topic of “Low-temperature grown compound semiconductors for optoelectronic device applications: distributed Bragg reflectors and wafer bonding agents”. GIPO professors and students attended the talk with enthusiasm. Professor Hsieh’s talk was fascinating and exciting, and he interacted with his audience throughout the presentation.

Speaker of the forum, Professor Kuang-Chien Hsieh



June 18th, 2010 (Friday) 2:30 PM


Prof. Ray-Hua Horng (Department of Electro-Optical Engineering, National Cheng Kung University)


The Development of Wide-Bandgap Indium Gallium Nitride (InGaN) Solar Cell Technology


Professor Ray-Hua Horng visited GIPO and delivered a speech on “The development of wide- bandgap indium gallium nitride (InGaN) solar cell technology” at the Photonics Forum on June 18th, 2010 (Friday). Her speech was informative and she interacted with the audience throughout the speech. GIPO professors and students were benefit greatly from her interesting presentation.

Host, Professor Gong-Ru Lin (left) with Professor Ray-Hua Horng


July “Photonics Forum” Lecture Highlights


July 8th, 2010 (Thursday) 2:30 PM
Speaker: Prof. Keh-Yung ChengDepartment of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign
Topic: Hyper-Uniform Nanophotonic Technology for Ultra-Fast Optical Systems - A Review of Activities at UIUC
  Professor Keh-Yung Cheng visited GIPO on July 8th (Thursday), and spoke at Auditorium 101, Barry Lam Hall. His talk on “Hyper-uniform nanophotonic technology for ultra-fast optical systems- A review of activities at UIUC” was interesting. GIPO professors and students attended the presentation with enthusiasm and learned much from it.

Speaker of the forum, Professor Keh-Yung Cheng




Dr. Rick Tsai, President of the New Businesses Department, Taiwan Semiconductor Manufacturing Company Limited, visited GIPO (August 27th, 2010)

Dr. Rick Tsai, President of the New Businesses Department of TSMC, led TSMC executives department head Chi-Guang Liu, vice department head Jin-Hau Tseng, and manager Lie-Heng Wei visiting GIPO on August 27th, 2010. They met with GIPO Chairman Ching-Fuh Lin, Professor Gong-Ru Lin, Professor Chih-I Wu, Professor Hoang-Yan Lin, Professor Yih-Peng Chiou, and Professor Ding-Wei Huang to exchange opinions and ideas on the research cooperation and personal training between these two parties.



High-order Rational Harmonic Mode-locking and Pulse-amplitude Equalization of SOAFL under Optical Injection

Professor Gong-Ru Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

First of all, we demonstrate the 20th-order rational harmonic mode-locking (RHML) semiconductor optical amplifier fiber laser (SOAFL) pulses by using 1 GHz backward dark-optical comb injection,  and  discuss  the  competition  between  mode-locking mechanisms in the SOAFL at high-gain and strong optical injection condition at higher RHML orders.  As the rational harmonic order increases up to 20, the auto-correlation traces and optical spectra of the RHML-SOAFL at different RHML orders are characterized.  The Fig. 1. shows that the normalized auto-correlation traces of the SOAFL at 1st, 5th, 10th, and 20th RHML orders gradually changes the pulsewidth from 13.5 to 35 ps.  The Fig. 2. exhibits that the SOAFL spectrum with red-shifted wavelength and reduced linewidth from 12 to 3 nm at RHML order increases to >8 reveal a less pronounced high-order RHML mechanism when competing with the continuous-wave lasing mechanism.  Furthermore, in order to solve the problem of uneven RHML pulse-amplitude, we use the reshaped 10-GHz gain-switching FPLD double-peak pulse to reconstruct the gain profile of SOA in time domain, the 4th-order RHML-SOAFL is demonstrated for achieving 40-GHz RHML pulse-train with optimized performance of pulse-amplitude equalization (PAE).  The Fig. 3. shows the auto-correlation traces of 4th-order RHML pulse-train without and with PAE.  Such the indirect gain compensation further balances the amplitude fluctuation from 45% to 3.5% when obtaining 40-GHz RHML pulse-train.  After initiating the PAE, the uncorrelated phase noise contributed by the residual ASE noise of the RHML-SOAFL is significantly decreased, which leads to a timing jitter reducing from 0.5 to 0.28 ps as shown in Fig. 4. and provides an amplitude-equalization pulse-train repeated at 40 GHz to meet the demand of being a perfect RZ pulsed carrier for its future application in OTDM network.

Reference:   G. -R. Lin et al., Optics Express, Vol. 18, No. 9, pp. 9570-9579, Feb. 2010.

Fig. 1. The auto-correlation traces of SOAFL at 1st, 5th, 10th, and 20th RHML orders. Fig. 2. Evolution on RHML-SOAFL spectra at different RHML orders.

Fig. 3. Auto-correlatted traces of 40-GHz RHML pulse-train without and with pulse-amplitude equalization. Fig. 4. The timing jitter of 40 GHz RHML pulse-train without and with pulse-amplitude equalization.


Emitter Apodization Dependent Angular Luminance Enhancement of Microlens-Array Film Attached Organic Light-Emitting Devices

Professor Hoang-Yan Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Taking organic emitter apodization calculated from electromagnetic theory as input, the angular luminance enhancement of a microlens-array-film (MAF) attached OLED (organic light-emitting device) can be further evaluated by ray-tracing approach. First, we assumed artificial emitters and revealed that not every OLED with MAF has luminance enhancement. Then, the OLEDs of different Alq3 thickness were fabricated and their angular luminance measurement validated simulation results. Mode analyses for different layers were performed to estimate the enhancement potential of the MAF attached devices. In conclusion, the organic emitters with higher off-axis-angle luminous intensity cause lower out-coupling efficiency but gain higher enhancement after the MAF attached.

Fig. 1. (a) The simulated source apodization varying Alq3 thickness from electromagnetic theory; (b) the simulated luminance from geometrical optics; (c) the validation of experimental results.

Fig. 2. (a) Mode ratios calculated by transfer matrix method with embedded sources. The blank area of each bar represented the optical power ratio of the surface plasmonic mode; (b) the experimental mode ratios.


A Novel Boundary-Confined Method for High Numerical Aperture Microlens Arrays Fabrication

Professor Guo-Dung John Su

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We present a technique to improve microlens arrays (MLAs) uniformity after the thermal reflow process. Microlens arrays (MLAs) usually form a layered structure in application-specific optical systems, such as backlight modules for liquid crystal displays (LCD), extraction improvement film for layered light emitting devices, wavefront sensors, image recorders, and a focusing component in the optical communication devices. It is hard to make small lenses and large arrays by traditional machining. Although several methods are proposed to replace the traditional machining, thermal reflow process is widely used to fabricate MLAs.

To overcome this difficulty, a novel method is proposed and demonstrated in this paper. It is called the boundary-confined method. A boundary between each PR cylinder is defined first by a thin negative tone PR. A thermal reflowing of a 2nd thick PR is halted at the boundary, as shown in Fig. 1. The uniformity can be improved without the cling phenomenon. Besides, the boundary is narrow and only a small amount of fill-factor is sacrificed. The height of the microlens is adjustable by the different diameter of PR cylinders inside the same boundary wall. We achieved high uniformity and high-NA (numerical aperture) simultaneously without sacrificing fill-factor too much. In order to improve fill-factor, residual PR (photoresist) between the photoresist cylinders are used to make photoresist flow outward in standard thermal reflow processes. PR cylinders, however, merge together easily due to an inexact reflow time and temperature distribution. This results in low uniformity and small lens height or low-NA. We proposed a boundary-confined method to pattern thin PR holes to prevent PR microlenses from merging together even after a long reflow time. Thick PR cylinders are patterned inside thin PR holes served as boundaries. PR microlenses are formed after reflowing the thick photoresist cylinders. Both the uniformity and the height of microlens can be well controlled. Besides, the fill-factor is high due to the high resolution at thin photoresist layer in photolithography. Our results show that the microlens is approximately a hemispherical profile. The gap between microlenses with 48 mm diameter in hexagonal arrangement is 2 mm and the height of microlens is 22 mm, as shown in Fig. 2. This work is also patterned under US 7,713,453 B2.

Figure 1. Schematic drawing of PR reflow by boundary-confined method.

Figure 2. The fabrication process sequence, and (a) PR microlens and SU-8 boundary, (b) PDMS mold captured by a microscope, (c) UV gel MLAs after releasing from PDMS mold.


The Correlation of Turn on Voltage and Band Alignment in Organic Light Emitting Diodes

Professor Chih-I Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Turn on voltage in the current density-voltage characteristics is one of the important factors to evaluate the performance of organic light emitting diodes (OLEDs). We report investigation of the origins of turn-on voltage, defined at where log J (current density) has a sharp rise and starts to increase dramatically. In OLEDs with NPB as the hole transport layer (HTL) and Alq3 as the electron transport layer (ETL), we find that the turn on voltage is always at 2V, regardless the cathode structures being used, such as Ca, Al, LiF/Al and Cs2CO3/Al. The turn on voltage is also independent on the thickness of organic layers (thickness varies from 30nm to 120 nm).

Beside NPB and Alq3, we also study the J-V characteristics on various OLEDs with T3/Alq3, NPB/T3, and NPB/Bphen as HTL/ETL, respectively. In all the devices mentioned above, the turn on voltage just equals to the difference between the LUMO of ETL and the HOMO of HTL, taking into consideration of vacuum level shift at organic interfaces measured from the ultraviolet photoemission spectroscopy (UPS). Combined with J-V characteristics of OLEDs and UPS measurement, we conclude that the turn on voltage of organic light emitting devices is determined by the difference between LUMO of ETL and HOMO of HTL and is independent of the cathode and thickness of organic layers. We also found that the charge transfers at the interface of ETL/HTL play an important role to the turn on voltage of OLEDs.


Emitting Layer Thickness Dependence of Color Stability in Phosphorescent Organic Light-Emitting Devices

Professor Jiun-Haw Lee

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We investigated the strong influence of the thickness of iridium(III)bis[(4,6-difluorophenyl) -pyridinato-N,C2’]picolinate (FIrpic) doped N,N’-dicarbazolyl-3, 5-benzene (mCP) blue emitting layer (B-EML) on color stability. The large voltage drop across the B-EML resulted in a higher sensitivity of the carrier transport and injection properties to the applied external voltage. According to carrier mobility measurements by the time-of-flight method, the electron mobility of the mCP exhibited a strong dependence on the electric field. Therefore, at a higher driving voltage, the more rapidly increasing electron mobility of the mCP and the decreasing energy barrier height on the electron transport path would extend the recombination zone from the B-EML to the tris(phenylpyridine)iridium (Ir(ppy)3) doped mCP green emitting layer (G-EML) in devices with thinner B-EMLs. Coupled with the fluctuations of the recombination zone, stronger triplet-triplet exciton annihilation occurring in the thinner B-EMLs led to an even more evident deterioration of the color stability. After circumventing these two negative factors, a green-blue organic light-emitting device (OLED) with ultra-high color stability was demonstrated, with the CIE coordinates slightly shifted from (0.256, 0.465) to (0.259, 0.467) with increased luminance from 48.7 to 12700 cd/m2. Further adding a red phosphorescent dopant into this green-blue EML backbone, we successfully fabricated a white OLED with high color stability, which exhibited a nearly invariant CIE coordinate throughout the practical luminance range from 1050 ((0.310, 0.441)) to 9120 cd/m2 ((0.318, 0.446)) and maximum efficiencies of 26.4 cd/A and 19.8 lm/W [published in Org. Electron. 11, 1500, 2010].


Effects of Gate Bias and Thermal Stress on ZnO Thin Film Transistors

Professor Jian-Jang Huang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The effects of gate bias and thermal stress on the threshold voltage shift were examined for ZnO TFTs fabricated on the glass substrate. We compared three samples with various post ZnO growth annealing durations. The results show that the threshold voltage shift (ΔVth) is only 2.2V after a 1.3×104s stress at the gate bias 20V for device C. And the threshold voltage shift can be correlated to the stress time following the charge trapping mechanism. The characteristic trapping time τ of device C was calculated to be 1.26×106 s. Further comparisons of the trap states and off currents reveal that device C has a better ZnO crystalinity and a better ZnO/SiNx interface quality. Finally, the characteristic trapping time was extracted at different temperatures for device C. We obtain an average effective energy barrier Eτ of 0.57eV. The results presented in this work suggested that excellent τ and Eτ can be obtained from ZnO TFTs on the glass substrate following our fabrication steps.

Fig. 1: Layer structure of the ZnO TFT on the glass substrate.

Fig. 2: Transfer curves at different stress time for device A(a), B(b) and C(c). The bias gate voltage is 20V.

Fig. 3: Time dependent ΔVth of device A, B and C under a gate bias 20V.

Fig. 4: Time evolution of transfer curve during the recovery phase of device C. The inset shows the ΔVth versus relaxation time.

Fig. 1  

Fig. 2(a) Fig. 2(b) Fig. 2(c)

Fig. 3 Fig. 4



Copyright © Graduate Institute of Photonics and Optoelectronics, National Taiwan University