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

Professor Marek Osinski visited GIPO on June 4^th (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.

Professor Kuang-Chien Hsieh visited GIPO on June 11^th (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 18^th, 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 18^th, 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

Speaker of the forum, Professor Keh-Yung Cheng

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 20^th-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 1^st, 5^th, 10^th, and 20^th 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 4^th-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 4^th-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.

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.

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 2^nd 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.

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 (ΔV_th) 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/SiN_x interface quality. Finally, the characteristic trapping time was extracted at different temperatures for device C. We obtain an average effective energy barrier E_τ of 0.57eV. The results presented in this work suggested that excellent τ and E_τ can be obtained from ZnO TFTs on the glass substrate following our fabrication steps.