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Latest News

  Vice Chairman Sheng-Lung Huang engaged as the Topic Editor of the world-

      renowned journal Optics Letters.

  Professor Sheng-Lung Huang is reappointed as the Vice Chairman of GIEOE~

Professor Sheng-Lung Huang has fulfilled the role of Vice Chairman of GIEOE for half a year and provided exemplary performance. With the Dean’s approval, Professor Huang has been reappointed as the Vice Chairman for another year.

  Spotlight of New Faculty Member: Prof. Gong-Ru Lin

Curriculam Vitae and Professionalism of Prof. Gong-Ru Lin

  Dr. Y. K. Chen Visits~ (July 7, 2006)

  Professor Pak-Lim Chu Visits~ (July 7, 2006)

  Professor Pochi Yeh Visits~ (July 14, 2006)

  Professor Simon Fleming Visits~ (July 10, 2006)

  Professor Anshi Xu Visits~ (July 25, 2006)

Prof. Anshi Xu, Chairman of Department of Electronics, Peking University, visited our Institute on July 25, accompanied by Prof. Shien-Kuei Liaw of the National Taiwan University of Science and Technology.  Prof. Xu also serves as the Director of the National Laboratory on Local Fiber-Optic Communication Networks & Advanced Optical Communication Systems (Beijing Section) and the Director of the Photonics & Communication Technology Institute of Peking University.  His research areas include WDM automatic optical communication networks, high-speed large-capacity WDM fiber-optic communication systems, optical internet networks, coherent optical communication, and photonic devices. He was invited as a visiting
	scholar to stay in the Department of Electronic Engineering of NTUST during the whole July.  We were glad to have the opportunity to invite him to our campus and meet our faculty members. After viewing the research posters of the photonics laboratories in EE Building II, Prof. Xu commented that the research in our Institute was more focused on photonic devices.  In Peking University, similar works have been mainly conducted in the physics related departments and institutes.  He met and had joyful discussions with several colleagues having research activities related to fiber-optic communication technologies in the lunch hosted by Chairman Yang.  It is expected that more academic exchanges between our Institute and Prof. Xu’s Department can be arranged in the near future.
Photo of Prof. Anshi Xu (second from the right) taken with Prof. Shien-Kuei Liaw (first from the left), Chairman C. C. Yang (second from the left) and Prof. Hung-Chun Chang

Special Report

  Workshop on Nitride Compounds and their Applications

May 12~13, 2006

Barry Lam Hall, National Taiwan University

The Workshop on Nitride Compounds and their Applications, organized by the Graduate Institute of Electro-Optical Engineering, National Taiwan University and National Science and Technology Program for Nanoscience and Nanotechnology Program Office, was held in the Barry Lam Hall, National Taiwan University on May 12^th and 13^th 2006. Totally, 15 international and domestic scholars were invited to present their research accomplishments, including professors Ian Ferguson (Georgia Institute of Technology, US), Yasuhiko Arakawa (Tokyo University, Japan), Li-Chyong Chen, Shangjr Gwo, Yan-Kuin Su, Jen-Inn Chyi, Dong-Sing Wuu, Lung-Han Peng, Li-Wei Tu, Shing-Chung Wang, Ching-Ting Lee, Jenq-Dar Tsay, C. C. Yang, and W. N. Wang (Bath University, UK).

In the two-day workshop, there were totally 317 participants. They came from universities, industries, high schools, and research institutes. At the end of the workshop, Dr. W. N. Wang from Bath University was invited to share his experiences of representing UK and EU to visit the LED-related companies and research institutes in the United States and Japan. With the workshop, all the participants would have better understandings of the current development of the white-light emitting-diode technology.

  College of Electrical Engineering and Computer Science, National Taiwan University

Commencement of Academic Year 2005–Highlights

June 3, 2006

Barry Lam Hall Square, National Taiwan University

This year, the commencement of National Taiwan University took place on the morning of June 3. In the College of EECS, the “academic group” of Electrical Engineering, including Department of Electrical Engineering, Graduate Institute of Electro-Optical Engineering, Graduate Institute of Communication Engineering, and Graduate Institute of Electronics Engineering, also held a graduation ceremony on the same day, beginning at three o’clock in the afternoon at Barry Lam Hall square. President Si-Chen Lee was specially invited to deliver the opening remarks. President Chao-Han Liu of the University System of Taiwan was also invited to address the graduates, offering advice and good-wishes. Dean of the College of EECS, Soo-Chang Pei and graduate institute chairmen also provided remarks, encouraging the graduates to advance bravely and pursue their dreams after leaving NTU.

Next, after Kung-An Lin of GIEOE delivered a speech expressing the gratitude of all the graduates, the ceremony’s main event— conferral of diplomas and righting of the tassels commenced. As the students ascended the stage, accepted their diplomas, and took photographs with their advisors, the families and friends looking on from below stage, holding cameras and bouquets, must have felt happiness and extreme pride.

After the rituals of giving thanks to professors, then tossing caps, and cheering, the commencement neared its end. In the flowing music provided by the orchestra, the atmosphere of commencement was pervaded not only by a sense of nostalgia for the just-ended student career, but also by an exciting anticipation for the beginning of a new life.

President Si-Chen Lee	President Chao-Han Liu of the University System of Taiwan
Dean of the College of EECS, Soo-Chang Pei	Kung-An Lin of GIEOE

  OECC 2006 Report

July 3~7, 2006

Splendor Kaohsiung Hotel, Kaohsiung

OECC 2006 was held at the Splendor Kaohsiung Hotel from July 3 to July 7 in Kaohsiung.  Last year, it was in Seoul, Korea, and will be held in Yokohama, Japan next year.  This is the first time that Taiwan hosted OECC, which attracted scholars from 15 countries with near 450 paper submissions.  About 65% of the submissions were from overseas.

Three plenary speeches were arranged.  The first by Dr. Takashi Nakashima, NTT, Japan, on "FTTH Expansion in Japan."  He presented the latest penetration of FTTH in Japan where the number of FTTH customers had already overtook that of CATV customers.  The second by Dr. Victor Klimov, Los Alamos National Lab., U.S.A. He introduced how nanocrystals can dramatically enhance the spectral tunability and photoluminescence quantum yields, which make nanocrystals attractive for use in a variety of light-emitting technologies.  Dr. Yung S. Liu, the former director of Opto-Electronics and Systems Labs. of ITRI delivered the third plenary speech.  He emphasized that nanotechnologies have made light sources viable commercial products that impact immensely to our life, such as optical fiber communication, lasers, CD/DVD, etc.

Our faculty and students actively participated the conference in organizing, and in reviewing and submitting papers (See http://www.oecc2006.org for details).  Professors C. C. Yang and Sheng-Lung Huang were co-chairs of the Technical Program Committee.  Professor Hung-Chun Chang co-chaired the category of Optical Fibers, Fiber Devices, and Optical Sensing.  Professors Hao-Hsiung Lin and Keanpgo Ricky Ho were committee members.  The conference also invited Professor C. C. Yang and Chung-Chih Wu to deliver invited talks.  Among the 12 student paper awards, 4 of them are from domestic works including 2 from our Institute by Mr. Sen-Ming Hsu and Min-Yann Hsieh.

OECC 2006 is the second large-scale international optoelectronics conference held in Taiwan (The first is the 5^th Pacific Rim Conference on Lasers and Electro-Optics organized by Professor C. C. Yang in 2003), a summary will be posted in the November issue of IEEE Communications Magazine.

  NTU–Chi Mei Research Collaboration Midterm Meeting

July 20, 2006

Barry Lam Hall, National Taiwan University

The Project, “Research Collaboration between National Taiwan University and Chi Mei Optoelectronics,” with Chairman C.C. Yang serving as the Principal Investigator, held its midterm meeting on July 20 in room 201 of Barry Lam Hall. On the meeting day, Chi Mei Optoelectronics Corporation’s (part of Chi Mei Group) technological development Deputy Head Director Dr. Chung-Kuang Wei led thirteen participants to visit and discuss the project. During the meeting, already-involved professors and junior professors reported on relevant research, eliciting enthusiastic discussions. Further advancement of the collaboration between both parties is eagerly anticipated. The following professors participated in the conference: Chairman C. C. Yang of the GIEOE; Yun-Shiuan Liao of the Department of Mechanical Engineering; Wei-Fang Su of the Institute of Polymer Science and Engineering; Feng-Yu Tsai, Miin-Jang Chen of the Department of Materials Science and Engineering; Tzong-Lin Wu of the GICE, Hoang-Yan Lin, Chih-I Wu, Jiun-Haw Lee, Jian-Jang Huang, Ding-Wei Huang, Guo-Dung Su, Yun-Li Li, Jui-Che Tsai of the GIEOE. Photographs of the meeting are as follows:

The meeting in progress	Deputy Head Director Chung-Kuang Wei (left) and Director Kun-Feng Huang (right) of Chi Mei Optoelectronics Corporation

Prof. Ian T. Ferguson: Sentiments on Visiting Taiwan∼

Profiles--Faculty Members

Laboratory Descriptions

(1) Nanoscience and nanotechnologies：

1. Near-field optics

The atomic force microscope (AFM), just like a human finger, can “sense” the surface morphology and “feel” the stiffness of materials with a nanometer spatial resolution. The near-field optical microscope, on the other hand, can resolve the nanoscopic spatial distribution of optical fields with a tapered fiber tip. Combining these two apparatus and femtosecond laser pulses, we can perform experiments and obtain imaging with high spatial (nanometer) and high temporal (femtosecond) resolutions. Under the funding of National Science and Technology Program for Nanoscience and Nanotechnology, we are currently conducting related researches. Topics include: (1) carrier and phonon dynamics of nanoparticles, (2) High spatial and temporal resolution of nano-ultrasonics with the aids of optical second and third harmonic generations.

2. Nano-ultrasonics

Acoustic waves have been wildly utilized for applications such as sonar and ultrasonic imaging. However, the frequency of acoustic waves for typical applications is less than 1GHz, and the corresponding wavelength is longer than 1 mm. Consequently, nanometer resolutions can not be achieved by low-frequency ultrasound. Recently, we have successfully generated acoustic waves with a wavelength shorter than 10 nm, which are phrased as nanoacoustic waves or acoustic nanowaves. In solids, this acoustic wavelength is only as long as tens of atom spacing and the acoustic frequency is up to the THz regime. For acoustic waves with such a short wavelength and high frequency, their physical properties are intriguing and we are investigating several physical issues related to nanoacoustic waves, such as propagation loss, scattering, and dispersion.

Generation of nano-acoustic waves not only opens the passage to a new field but also creates the possibilities for innovative applications and technologies in the future. In respect of applications, we have studied phonon mirrors (which are made from phononic bandgap crystals) that selectively reflect nanoacoustic waves at specific frequencies. Besides, we have demonstrated frequency-tunable nanoacoustic waves in the sub-THz regime by optical coherent control techniques. Furthermore, we have also demonstrated that nanoacoustic waves can be utilized for ultrasonic nanoimaging. This developing nanoultrasonic technique has the advantages of non-destructive measurement, three-dimensional sub-surface imaging capability, and nanometer resolutions, of which other state-of-the-art imaging technology does not possess all. Nanoultrasonic imaging technology has great potential for nondestructive investigations of three-dimensional nanostructures, elasticity in the nanometer domain, and the forth.

Figure 1：The nanoultrasonic imaging technology is based on a “pulse-and-echo” method to examine the subsurface                    nanostructures. As shown in the left figure, two echoed peaks correspond to the echo time from different                    interfaces in the samples to be examined. From the information of sound velocities and echo time, we can                    nondestructively obtain the subsurface structures of the sample as shown in the right figure.

3. Nano-particles and nano-plasmonics

When material dimension reduces to the sub-micron or even to the nanometer regime, its optical and material properties would behave much differently. This nano-effect creates new application possiblilities and rooms for optimization.

Currently we are conducting research on the nonlinear optical properties and acoustic behavior of quantum dots and nanoparticles. In quantum dots, quantum confinement effects can be observed because of their size; their discrete energy levels, their optical and acoustic behaviors when doped in different matrices are one of the topics we are intensively studying. In nanoparticles, their ratio of surface to volume is much larger than their bulk counterparts and their sizes are on the same order of or less than the optical wavelengths; consequently resonance, magnification and other behaviors of electric fields on their surfaces can be evidently observed and a research field called “plasmonics” is established. We are conducting experiments on nanoparticles of metal, alloy, composite materials and biological molecules, aiming to decide their properties of higher-harmonic generation and to evaluate them as contrast agents for biophotonic applications.

Figure 2：The third harmonic generation (THG) ratio of silver nanoparticles at different THG wavelengths (in solid squares)                    and the theoretical curve (in solid line). The inset shows the χ(3) value of Ag nanoparticles calculated from                    experimental data at different excitation wavelengths.

4. Nano-biophotonics and nano-toxicology

Recently, nanotechnology has been widely used in many commercial products in daily life. In our lab, we are trying to deliver metal nanoparticles and quantum dots into cells as contrast agents. Compared with traditional staining, using nanoparticles or quantum dots can avoid photo bleaching effects. On the other hand, toxicity of these applied nanoparticles has gradually become a serious issue.  Through collaboration, we are also conducting nano-toxicology studies on specific nano-particles. These studies could provide answers to future nano-medicine.

(2) Biophotonics：

1. Nonlinear optical microscopy

Compared with traditional reflection/fluorescence confocal microscopy, nonlinear optical microscopy can avoid out-of-focus photodamage and is with an intrinsically three dimensional sectioning capability. Combined with a laser source with an emission wavelength at the biological penetration window (1200-1300nm), it can further reduce photodamage and increase the penetration depth in live biological specimens. In our lab, we focus on the development of multimodal nonlinear microscopy combining second harmonic generation (SHG) and third harmonic generation (THG) signals, while minimizing the usage of multi-photon fluorescence signals. SHG microscopy is generally used to observe non-centrosymmetric structures, THG microscopy is generally used to provide morphological information, and multiphoton fluorescence microscopy is generally used to provide molecular images. In our previous nonlinear optical microscopy studies of in vivo vertebrate embryos based on a femtosecond Cr:forsterite laser with a central wavelength at 1230nm, complex developmental processes throughout the 1.5-mm-thick zebrafish embryos from initial cell proliferation, gastrulation, to tissue formation could all be continuously visualized in vivo without any treatment on live specimens. We also demonstrated the first in vivo optical virtual biopsy based on epi-THG microscopy using Syrian hamster oral mucosa as a model system. Without complex physical biopsy procedures, epi-THG microscopy can provide high-spatial-resolution dynamic images of oral mucosa and sub-mucosa in all three dimensions. The demonstrated intravital epi-THG microscopy provides high resolution observation of blood flow in capillaries and could be a promising tool to image angiogenesis, which is an important feature of many human diseases including malignancies. We also demonstrated the first four photon fluorescence microscope.

2. Nonlinear endoscope

An endoscope is an important tool for noninvasive medical diagnosis. For future clinical applications, we are trying to set up several nonlinear endoscopes based on near-IR nonlinear microscopy that has 3D sub-micron resolution and high penetration in bio-tissues, including fiber-bundle-based and MEMS-mirror-based endoscopes. The project is sponsored by the National Health Research Institute.

3. Optical molecular imaging

Optical molecular imaging is a technique that combines optical microscopy with molecular biology techniques to in vivo observe signal transmission, gene expression, and so on. Our laboratory collaborates with the Institute of Molecular and Cellular Biology, National Taiwan University, to develop a new kind of transgenic fluorescent zebrafish, which can be two-photon excited by our infrared femtosecond laser. Combined with harmonic optical microscopy, tracking the embryonic development of zebrafish heart in vivo and noninvasively can be achieved. We also collaborates with National Taiwan University Hospital and National Cheng Kung University Hospital to develop various nanoparticles, which can be used to in vivo target certain molecules or genes to enhance harmonic signals, that is, to act as the contrast agent of harmonic signals. In the future, our laboratory will keep developing techniques like FRET, FLIM techniques, etc., and our final goal is to apply these techniques for clinical applications and to provide useful information for clinical diagnoses.

4. Gene engineering and stem cell research

A stem cell is a non-specialized, generic cell that can make copies of itself and differentiate into specialized cells for various tissues in the body. Collaborating with NTU Center for Genomic Medicine, our lab is in charge of the optical imaging system and participates in many gene engineering and stem cell researches. We have observed the development of pluripotent stem cells and neurons after the gene function was blocked.

5. Noninvasive early diagnosis of cancers

Cancer is the first cause of death in the past 23 years in Taiwan. If cancer can be diagnosed as early as possible, the death rate can be significantly reduced. In the past, physical biopsy is traditionally recognized as the final standard tool to diagnose cancer. However, physical biopsy requires removal, fixation, and staining of tissues, cells, or fluids from the lesions of patients. Such histological procedures are not only time-consuming but also invasive and painful. In addition, traditional physical biopsy procedures per se may potentially put patients in the risk of spreading tumor cells. Moreover, unless tedious serial sectionings are performed, there is no guarantee for a missing diagnosis due to a local invasion not present in the given histological sectioning examined by the pathologists. Therefore, a non-invasive in vivo optical virtual biopsy, which can provide highly penetrative three-dimensional (3D) images with a sub-micron spatial resolution to assist the real physical biopsy, is highly desired. In our lab, we developed nonlinear optical microscopy including SHG, THG and multi-photon fluorescence microscopy to study morphological and molecular differences between cancerous and normal tissues. The following figure shows one example that we use nonlinear optical microscopy to study oral cancer in a hamster.

(3) Ultrahigh frequency photonics:

1. Ultrahigh speed optical communication and photodetectors

In applications of ultrahigh-speed communication systems, photodetectors with high power-bandwidth products play an important role in photoreceiver circuits or in sub-millimeter wave signals generation. Our group has devoted to developing high-bandwidth and high-power photodetectors, while our photodetectors achieved a record-high electrical bandwidth (570GHz) and a record power-bandwidth product as large as 5.7 THz-V. Currently we are developing higher output power and higher bandwidth photodetectors by using all sorts of nano-optics effects.

2. Terahertz photonic transmitters

Terahertz (THz) waves are defined as electromagnetic or optical waves at frequencies ranging from 100 GHz to 10 THz. THz waves can be applied to ultra-high speed communication systems, molecular spectral imaging, national defense security, and other various novel applications. Currently generation of THz waves is still a challenge and we have successfully developed various THz photonic transmitters based on existing processing technologies. With the support of National Science Council in Taiwan, we successfully developed THz transmitters that can generate tunable THz waves at room temperature and are also easily integrated with other semiconductor devices. Moreover, our proposed device attains a record-high light-THz power conversion efficiency of 0.33% at 404.5 GHz and is the first optical device with an equivalent external quantum efficiency over 100%. Collaborating with Prof. Reey-Bei Wu of Graduate Institute of Communication Engineering, we are now developing THz antenna array technologies.

3. Terahertz fiber systems

In the past twenty years, scientific investigations on terahertz phenomena were intensively conducted based on the rapid developments of terahertz generation and detection techniques. However, most of today's THz setups still feature planar or curved metal reflectors. Such setups are rigid and vulnerable to environmental disturbances. We keenly need a suitable terahertz waveguide for more advanced THz applications.

We proposed and demonstrated a subwavelength PE fiber for terahertz wave guiding. This makes THz waveguiding more controllable, reliable and flexible compared with free-space propagation. The only sustained HE₁₁ mode will allow a large portion of energy outside the fiber and thus the THz wave propagates with a low attenuation constant on the order of or less than 0.01 cm^-1. Besides, the measured free-space coupling efficiency of the fiber is up to the order of 20% by using an off-axis parabolic mirror. With this direct coupling capability, our proposed THz fibers can be easily integrated with current THz systems.

4. THz microchip and biosensing

Sensing devices designed to identify minute amounts of biochemical substances accurately and noninvasively are strongly desired for many applications, such as in lab-on-a-chip (or biochip) development. Current analytic methods, including fluorescence based techniques, require modification of the surroundings of molecules and thus disturb the nature of the target molecules and could lower the precision of detection. Besides, it could increase additional preparation steps of sample testing. One alternative without disturbance of target molecules is the terahertz (THz)-based biosensing technique. Based on molecular fingerprints, direct THz-probing is a sensitive, label-free, and noninvasive way for biomolecule detection.

For the first time, we directly integrated an optoelectronic micro-THz source into a glass-substrate microchip for direct, sensitive and localized sensing of biomolecules, which are preserved in their native states. The capability to identity different illicit drug powders based on their spectral characteristics was successfully demonstrated with a < 10 nano-gram sensitivity. This THz microchip system has the advantages of label-free detection (needs no probing molecules), high selectivity (based on spectral characteristics of molecules), no sample preparation, repeatable usage (just replace the PE film), and ease of parallel integration with other biochip functionality modules, which are desirable for future lab-on-a-chip applications. Our demonstration shows the possibility to integrate optoelectronic-based photonic transmitters with current biochip technologies for various biosensing applications, including DNA sequencing and detections of explosive and virus. Due to the unique capability to detect the biomolecules in their natural states, the THz biochip should also enable future studies of molecular conformational dynamics.

5. THz molecular imaging

Many molecules have resonant absorptions at the THz spectral range and thus can be distinguished. Based on these molecular fingerprints, THz EM waves are used for imaging molecular distribution that can be applied to detection of virus and explosives hidden in plastics. Our recent research is devoted to the combination of compact THz photonic transmitters and THz subwavelenth fibers to realize a THz molecular imaging system with a miniature size, such as a THz endoscope system.

(4) Femtosecond optics:

1. Femtosecond laser

Femtosecond lasers can generate optical pulses with a duration on the order of 10^-15 seconds. It’s an important tool for the studying of ultrafast phenomena. In our lab, the operating femtosecond lasers include two home-built Cr:forsterite mode-locked lasers, three commercial Ti:sapphire mode-locked lasers, and one commercial optical parametric oscillator (OPO).

  The operation wavelengths of Ti:sapphire lasers range from 700 to 1000nm. The typical output power is 1.5W with a shortest pulse width of 30fs and a highest repetition rate of 2GHz. By frequency doubling with nonlinear crystals, blue or UV femtosecond pulses can be achieved from 350nm to 500nm. Combined with the OPO, the operation wavelengths of femtosecond pulses can be extend to 1 ~ 2mm.

  The home-built Cr:forsterite lasers operate around 1230nm with a 100fs pulse-width. At a 110MHz repetition rate, the output power could be as high as 300~500 mW. We successfully shrank the size of cavity through the use of double-chirped mirrors. Based on this system, Cr:forsterite lasers became more portable and could be widely applied to medical uses. In the future, our lab plans to build more portable Cr:forsterite femtosecond lasers with higher output power.

2. Ultrafast laser fields diagnostics

For ultrafast optical pulses, which are typically 100 femtosecond in duration, the variation of fields is faster than the speed of electronics. To obtain the complete field information, including amplitude and phase, we have developed a technique called "Triple-Optical Autocorrelation for Direct pulse shape measurement (TOAD)". From the measured triple-autocorrelation function by third-harmonic generation, we can analytically calculate the temporal shape of pulse intensity and uniquely determine the phase by a spectral measurement with Gerchberg-Saxton algorithm.

3. Photonic-crystal-fiber-based ultrafast source

Optical laser beams can be confined and guided within a small area by a photonic crystal fiber. When the optical intensity is getting higher and higher, the third order nonlinear effects such as self-phase modulation will appear. Balancing this effect with the dispersion of material, optical solitons can thus be formed, which can keep the pulse width unchanged for a long propagation length. In our lab, exploiting the self-frequency shift induced by optical soliton and Raman scattering effects, 800nm~1600nm ultrafast sources can be obtained by the excitation of 800nm femtosecond pulses from the Ti:sapphire laser. The wavelength can be tuned through the polarization of excitation, thus providing femtosecond sources with a broad tuning range. Under the funding of National Science Council and National Health Research Institute, our lab is now studying the ultrafast phenomenon of photonic crystals and planning to apply them to biology researches and optical control of pulse propagation.

4. Ultrafast carrier dynamis of wide-bandgap light-emitting semiconductors

The dynamic characteristics of carriers in semiconductors play an important role in the performance of the electronic and optoelectric devices. We use femtosecond-laser techniques to study the carrier dynamics within an ultrashort time domain, such as carrier-phonon interactions, radiative recombination processes from a signal quantum well, room-temperature exciton dynamics, and carrier dynamics in bandtail states. Besides wide-bandgap semiconductors such as GaN and ZnO, recently we also focus on the carrier dynamics in infrared materials such as InN.

We have also studied non-conventional optical elements such as gratings. An example is a color separation grating for separating different color lights into different diffraction orders. Application in display system is a high efficient color separation element for replacing the conventional color filters.

We have also studied non-conventional display systems such as auto-stereoscopic displays. An example is a large area system based on rear projection technology. We adopt two orthogonal circular polarized waves in stead of two orthogonal linear polarized ones to avoiding crosstalk effect. In such a system, we design a broadband retardation plate in order to minimize the chromatic dispersion of the system.

The above research results are accepted as four international conference papers and further results are preparing for patent filing and journal paper submission.

nanopyramids and nanopillars.  The targets are implementing Si nanophotonic devices based on quantum confinement effect, and discuss in more detail the origin of the enhancement on radiation/detection, solar energy conversion, and surface plasmonic resonance at blue-green wavelength region.  A future collaboration with Academia Sinica on the ab initio atomic potentials based boundary integral Green function for the quantum computation of Si quantum dots with atomic numbers ranging between 10³-10⁴ will be initiated to study the electrical property of Si nanocrystals.  These simulations will be helpful to elucidate the carrier transport and penetration between Si nanocrystals and neighborhood dopants, and to realize the surface plasmonic wave interaction between Si nanostructures and biomoleculars.  Several featured researches including ultra-low threshold ICP power PECVD synthesis for Si nanopyramids, CO₂ laser based rapid thermal annealing for precipitating Si nanocrystals, rapid self-aggregated metallic nanodot array for SPR or nano-lithographical applications will be introduced in the following sections.

On the other hand, we are interested in fundamental and technical study of fiber laser schemes for potential applications on ultrafast optoelectronic diagnosis, high-speed fiber-optic communications, and biophotonic or biomedical analysis, etc.  Over past years, we have constructed several types of continuous-wave and shorted-pulsed Erbium-doped fiber amplifier or semiconductor optical amplifier based fiber ring lasers.  In particular, we have theoretically simulated and experimentally established an optical injection mode-locked semiconductor optical amplifier fiber laser system, which exhibits femtosecond pulsewidth after external soliton compression and facilitates high-repetitive low-supermode-noise output.  The other milestones of our works include 40^th-oeder rational harmonic mode-locking of EDFL using a FPLD based mode-locker, mutual injection-locked EDFL/EDFL and FPLD link, tipped fiber based low-splicing-loss fusion technology between different fibers, >10GHz backward optical injection induced cross-gain-modulation for mode-locking, etc.  In brief, the distinguished researching fields on Si nanophotonics, fiber lasers, and all-optical communication data processing likes are emphasized in our laboratory, which include:

A. Femtosecond Mode-Locked Fiber Lasers and Soliton Compressors

B. Nanocrystallite Silicon LEDs and Solar Energy Photonics

C. All-Optical OC-192 NRZ/RZ Communication Data Format Processing

D. Mutually Injection-Locked Laser Diode Based Fiber-Optic Communication Link

E. Millimeter-wave Optoelectronic Phase-Locked Loops and Phase Shifters

F. GaAs:As+ Ultrafast Photoconductive Switches and Time-Resolved Optical Diagnostics

II. Silicon Nano-Photonics

II-1. Local Synthesis of Si Nanocrystals in SiO_x via CO₂ laser Rapid Thermal Annealing

Nowadays, the progressive development on Si nanostructures based photonic devices are particularly for the potential solution of novel optical interconnect between chip modules, which effectively integrate the waveguides, photonic crystal devices, and Si based electronic ICs on the same Si wafer to form a all-Si based photonic system on chip.  The key component to realize the Si based optical interconnect relies strictly on the implementation of fully compatible Si light emitting devices, however, which are still not a mature product at current stage.  Typically, the Si LEDs were fabricated using Si nanocrystals buried in the SiO_x file deposited by anomalous PECVD and post high-temperature annealing processes.  Furnace annealing at higher than 1000^oC and longer than 30 minutes usually degrades or damages the adjacent ICs, which in turn making the integration of Si LEDs and ICs impractical.  This urges us to seek alternative solution for locally synthesizing the Si nanocrystals in the Si-rich SiO_x film, and the best candidate is the focused laser annealing process.  Previously, excimer lasers, argon ion lasers, and Ti:sapphire lasers were ever employed as the common sources for annealing or refining versatile materials.  Nonetheless, these lasers experiences extremely low absorption procedure in general SiO_x materials, in which the optical energy is unable to efficiently convert into the heat during illumination.  Therefore, the precipitation of Si nanocrystals cannot be demonstrated using these visible or near-infrared lasers.  With the aid of a simulation model for three-dimensional thermal conductance, we propose for the first time the local synthesis of Si nanocrystals within SiO₂ matrix by using CO₂ laser based rapid thermal annealing procedure.  Experimental results reveal the generation of Si nanospheres with a concentration up to 10¹⁸ cm^-3 within a spot size of 50mm under a focused laser illumination intensity of 6 kW/cm², while the Si nanocrystal size can be well controlled at 3-8 nm.  The optimized laser annealing intensity is well below the laser ablation threshold, and a dehydrogenated process can also be implemented under a lower laser intensity of <4 kW/cm².  With such a CO₂ laser annealed SiO_x, we have successfully fabricated a MOS light emitting which exhibits similar characteristics with those fabricated using conventional furnace-annealed SiO_x films.

II-2. Si nanocrystals Based Light Emitting materials and Diodes

We also investigate the white-light and near-infrared electro-luminescent characteristics of the MOS diodes fabricated on the PECVD grown Si-rich SiOx film on Si substrate after high-temperature furnace annealing.  By changing the substrate temperature and induced couple plasma power during PECVD growth, the structure defects and Si quantum dots related PL mechanism have been studied in detail.  It is further observed that numerous Si nanopyramids can be grown along the (100)-oriented Si/SiO­x­ interface under the high substrate temperature and threshold ICP power conditions.  The power-variant and temperature-variant µ-PL spectra further interpret the contribution of the self-trapped exciton located at Si quantum dot surface to the PL at 700-850 nm.  It is confirmed by comparing the EL from the as-deposited and annealed SiO_x films that the Si nanocrystal contributes to the EL at 650-850 nm, whereas the EL at 400-650 nm is attributed to the Si and oxygen vacancy related radiative defects such as weak-oxygen bonds, neutral oxygen vacancy, and E’δ the precursor of Si nanocrystals, etc.  The preferred transport path of the injected carriers is through the defect sites, which can be avoided via the increasing substrate temperature and decreasing ICP power during growth.  Such a synthesizing process also facilitates the stable near-infrared EL and lengthened device lifetime.  In addition, the proposal on resonant-cavity enhanced EL device is very promising.

II-3. Formation and Diagnosis of Si Nanopillars on Si Substrate

In particular, we have primarily demonstrated the rapid self-aggregation of metallic nanodots on Si substrate by using a less-adhesive and heat-accumulated SiO₂ buffered layer.  The self-assembly of metallic nanodots on Si wafer surface spends thermal processing time up to 10s minutes in previous work, which is mainly attributed the ultra-high thermal conductivity of 148 W/m-K in Si substrate.  This causes the metallic nanodots hardly to be congregated on the surface.  With the deposition of a 20nm thick low-conductivity SiO₂ buffered layer, the formation of NiSi₂ compounds restricted and the adhesion between Ni and Si can be released, the heat can therefore be accumulated at the Ni/SiO₂ interface to enhance the self-aggregation process.  This eventually results in a significant reduction on the formation duration of Ni nanodots from 10s min to 22 sec.  The average size and surface density of the formatted Ni nanodots are 30nm and 7´10¹⁰ cm^-2­, respectively.  Subsequently, the Ni nanodot array is employed as a mask for generating Si nanopillars via an ICP-RIE process.  The largest height-to-width aspect ratio ever obtained for the Si nanopillars in experiments is about 8-10 at a maximum height of 400 nm.  After removing the Ni and SiO₂ nanodots from the top of Si nanopillars, a visible PL is emitted from the Si at wavelength of 400-600nm.  Weak PL at near-infrared region (~750 nm) is also observed due to the occurrence of quantum confinement effect in Si nanopillars with size <10nm.  This has been verified in the experiment of saturating PL intensity with enlarging pumping power.  Furthermore, the blue-shift in PL wavelength of Si nanorods due to the persistent oxidation of Si nanopillars is reported.  Other effects such as faster charging/discharging, lower resistance and capacitance, and extremely low optical reflectivity are also observed.

III. Mode-Locked Fiber Laser Systems

III-1. 40GHz and 40^th Rational Harmonic Mode-Locking Fiber Laser

By using a 1GHz loss-modulated Fabry-Perot laser diode, we have implemented the rational harmonic mode-locking of an Erbium-doped fiber laser system.  The FPLD is not DC biased to remain unlasing condition, while a amplified RF signal is employed to sinusoidal modulated the FPLD, which facilitates the switching on the operation mode of the FPLD between loss modulation and gain switching.  In the proposed system, the FPLD is neither lasing nor gain-switching, which experiences a threshold modulating power of nearly 18 dBm to initiate the mode-locking of EDFL.  After a chirp compensation with dispersion managed fiber, the EDFL pulsewidth as short as 3 ps associated with a spectral linewidth of 1.3 nm can be obtained, corresponding to a transform-limited time-bandwidth product of 0.31.  If we denoteas the fundamental mode frequency of the EDFL, and define n and p as the harmonic and rational harmonic orders of the EDFL, it is seen that the rational harmonic mode-locking can be achieved when the modulating frequency of the FPLD is adjusted from = to =.  Therefore, the repetition frequency of the EDFL becomes p times of the FPLD modulation frequency.  Experimentally, a detuning frequency of 113 kHz will give rise to a 40^th ordered rational harmonic mode-locking output from the EDFL, in which the harmonic and rational harmonic orders are n=228 and p=40, respectively.

In opposite, the FPLD changes its operation function to generate gain-switched pulses when the DC biased current of the FPLD is increased to nearly threshold condition, while a dual pulse-train phenomenon revealed the such an evolution has been found during the current detuning procedure.  However, such kind of mutual injection-locking induced gain-switching inevitably broadens the EDFL pulsewidth to 26.4 ps, which is almost the same as that of a typical gain-switched FPLD (~21 ps).  Under such a strong competition, the mode-locking mechanism of the EDFL will be destroyed and the FPLD becomes lasing, the overall output stability of the mutually injection-locked gain-switching FPLD-EDFL link is relatively better than that of a FPLD loss-modulated rational harmonic mode-locked EDFL.  This is attributed to the narrow locking range of the mode-locking effect.  In addition, the unequivalent amplitude among adjacent pulse peaks needs to be eliminated using a intra-cavity Fabry-Perot etalon filter.  The effect of gain depletion in FPLD on its mutual injection locking and gain switching performances was realized to corroborate the diminishment of the rational harmonic mode-locking mechanism.

III-2. Backward Optical Injection Harmonic Mode-Locked SOA Fiber Laser

Alternatively, we perform a novel mode-locking mechanism for the semiconductor optical amplifier (SOA) based fiber ring lasers, which employs a modulated optical source to inject the SOA for gain depletion or cross-gain modulation.  By reshaping the gain window of the SOA with optical TTL signal of variable duty cycle, we theoretical simulated and experimentally demonstrate for the first time a 10GHz harmonic mode-locked SOAFL, in which the effect of cross-gain modulated gain window on the gain depletion level and the mode-locking dynamics was elucidated.  Such a high-speed backward injecting optical signal can be generated by passing a directly sinusoidal modulated distributed-optical-feedback laser diode output through an optical circulator to inject the SOA.  To reshape the injection optical wave and detune its duty cycle, a pseudo random binary sequence code is used to generate TTL signal for encoding the DFBLD in a traveling-wave Mach-Zehnder modulator.  Furthermore, the modulator has to be biased at nonlinear level for reshaping the optical TTL or pulse into a dark-optical-comb shape.  In principle, the SOA functions as a gain medium and a loss-modulator concurrently.  To improve the bandwidth of SOA and prevent the saturation of circulated mode-locking pulse-train in the SOA, an appropriate output coupling ratio for high output power should be decided.  Rational harmonic mode-locking output can also be obtained by slightly detuning the modulating frequency of the DFBLD.  To obtain a shortened or transform-limited pulse, the dispersion compensation and soliton compression techniques are introduced.  A dispersion compensated fiber loop are employed to achieve chirp-free SOAFL pulse and a high-gain EDFA is used to raise the peak power of the zero-chirp SOAFL pulse for high-order soliton generation in a single-mode or dispersion shifted fiber spool.  Consequently, the wavelength-tunable SOAFL pulse-train with average power of >1 Watt and pulsewidth of <100 fs can be obtained.

III-3. 10s Femtosecond High-Power Soliton Compression of Self-started Additive Pulse

         Mode-Locked Erbium-doped Fiber Laser

To further increase the peak power of ultrashort fiber laser pulses for versatile applications, the external cavity amplifier should be configured instead using an ultrahigh-gain laser cavity.  Among different approaches, the chirped amplification is a very useful technique, which induces a pre-chirp into the APM EDFL pulse via a nonlinear medium prior to the high-power amplification process, leading to a broadening of pulsewidth and a reduction in peak power and thus prevent gain saturation phenomenon.  Afterwards, the pre-chirped and amplified EDFL pulse is compressed again to reach high peak power.  However, the handling power-length products in conventional fibers are limited at about 1 kW×m due to their small mode fields.  The value is further reduced to below 0.5 kW×m in most fiber laser source with fiber length of several meters, which makes them being inappropriate as the dispersion compensation medium.  Besides, the compressed pulse quality is strongly correlated with the order of the soliton compression stage.  Our approach is using a large-mode-field-area EDFA to shorten the fiber length and to suppress the nonlinear self-phase modulation effect.  At last, a negative chirp is added into the EDFL pulse by passing through a segment of single-mode fiber, which compensates the positive chirp induced during the high-power amplification process and helps to continuously shorten the pulsewidth.  Such an all-in-one configuration decreases the soliton order and suppresses the pulse pedestal to obtain a pure Gaussian pulse shape with peak power of 46 kW and pulsewidth of 56 fs.  Extremely short pulsewidth of 30 fs can be achieved via a second-stage soliton compression process, the overall pulsewidth compressing ratio and peak-power amplification ratio are up to 10 and 700 times, respectively.

IV. All-Optical Communication Data Format Processing Technology

All Optical Networks (AONs) have been considered as the emerging solution for future high-speed fiber-optic communication link, which avoid the O-to-E and E-to-O transformation induced network bottleneck and establish O-to-O processing devices and techniques to overcome the speed limit of electronic carriers.  On the other hand, both the Fiber to the home (FTTH) and the Radio-on-Fiber (ROF) were already selected as the potential candidates for the last mile transmission solution.  The following illustration shows the configuration of AON link with backbone and ROF networks.  In particular, the highly repetitive and low duty-cycle return-to-zero (RZ) optical carrier source, NRZ-to-RZ and RZ-to-NRZ transformers are the immediate work to be developed in the AON fields.  To build the sub-Tbit/s pulsed optical carrier systems, we propose versatile mode-locking fiber laser configurations by using erbium-doped fiber amplifiers (EDFAs), traveling-wave semiconductor optical amplifiers (TW-SOAs), and fiber-based Raman amplifiers (FRA) as gain mediums, the self-developed novel low-loss fusion splicing technique, and the rational harmonic mode-locking technique to develop sub-Tbit/s (larger than 200 Gbit/s) fiber lasers.  Broad-band wavelength tenability, timing stabilization and nonlinear pulse compression are the key issues to be developed with these systems.  The construction of sub-Tbit/s all-optical diagnostic platform rely on the integration of photonic-crystal based nonlinear fiber loop mirror and 40-80GHz optoelectronic frequency regenerative system with the rational harmonic mode-locking fiber lasers.  We aim to investigate novel all-optical signal processing components/sub-system and corresponding testing infrastructure, which will be employed to build up a connection with the radio-on-fiber (ROF) based last-mile transporting networks, where the microwave signal is converted into the optical signal and distributed to the remote base stations (BSs) by fiber links.

Initially, a Fabry-Perot laser diode (FPLD) based all-optical data format transformer has be demonstrated for the first time, which can convert a >10 Gbit/s (OC-192) non-return-to-zero (NRZ) formatted data-stream into a return-to-zero (RZ) formatted data-stream.  The implementation of the FPLD-based NRZ-to-RZ data format converter helps to increase the transmission data rate between metropolitan area networks (MAN) with the integration of passive time-division-multiplexing (TDM) technique.  The FPLD-based NRZ-to-RZ data format converter will be used to link the AON and the ROF networks.  Meanwhile, the optical clock-data recovery of such a RZ-formatted data-stream will be achieved using a home-made semiconductor-optical-amplifier based fiber-ring laser system, some innovative architectures will be demonstrated to implement OC-768 (40 Gbit/s) all-optical data format conversion, logic or decision gate, injection-locked 3R regenerators, etc.  Our future goal will also involve the development of >200 Gbit/s high-speed pulsed optical carriers for high-bit-rate all-optical sampling test-bed.

Please send any comment to eoe5@cc.ee.ntu.edu.tw 

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Graduate Institute of Electro-Optical Engineering, National Taiwan University

 http://eoe.ntu.edu.tw/