Prof. Pan,Ci-Ling

I have several ongoing projects, ranging from THz radio-over-fiber communication and sensing technology, nano-structured silicon solar cells to attosecond science and technology. The last of which is a new initiative and I expect to devote considerable time for its execution in the next five years.

Synopses of these ongoing projects are summarized below:

[ 1 ] Studies of Key Components and Technologies for Radio and Ultra-Wideband Over Fiber Communication System at V- and Beyond W-bands (Main) (NSC, Aug. 1, 2009-July 31, 2012)

Terahertz (THz) wave, the relatively less explored electromagnetic radiation in a frequency interval from 0.1 to 10 THz, has traditionally only been the domain of astronomers and molecular spectroscopists. Relatively few applications relevant to the society in general have been explored, however. The so-called THz gap of the electromagnetic spectrum is illustrated below:

Relatively few applications relevant to the society in general have been explored, however.

Spurred by discovery of new physical principles and innovative technologies in photonics, advanced materials and nanotechnology, novel applications of THz waves (also called T-rays) in basic science, security, medical diagnostics and treatment, nondestructive inspection, and manufacturing quality-control have recently been demonstrated. THz sensing, in particular, has attracted lots of attention due to it is molecular unambiguousness. THz imaging potentially can even allow spotting the onset of cancer and is complementary to widely used techniques such as optical coherence tomography (OCT). As a result, companies such as Nikon, Coherent, Picometric and Teraview are pursuing commercialization of THz technology. Marketing opportunities for security screening in airport, counterfeit prevention in drugs/pharmaceuticals, defect detection in semiconductor manufacturing, and other high-value areas appear promising. Nonetheless, these efforts are hampered for the lack of room-temperature THz emitters or THz receivers that cost just a few dollars and are the size of a coin.

Information and communications technology should also benefit from advances in THz science and technology. The synergy of research areas such as wireless communication, biomedical and environmental sensing, as well fiber-optic communication with THz science and technology will generate a wealth of applications in, for example, biometrics — the development of recognition techniques based on unique human physical or behavioral traits — using a THz camera, massive sensor networks and selective communication. Taiwan is actively pursuing Wimax technology (2-11 GHz), while a Wireless HD (WiHD) Interest Group (http://www.wirelesshd.org/) was recently established by companies like NEC, Sony, Samsung and LG. Its goal is to establish a wireless digital interface to combine uncompressed high-definition video, multichannel audio data toward the unlicensed 60 GHz band.

Extending wireless communication to the THz frequency band is one of the holy grails for researchers in THz science and technology. A roadmap of THz Science and Technology in information and Communication technology was recently put forward by Tonouchi [Nature Photonics, 1(2):97, 2007]：

A roadmp of THz Science and Technology in Information Technology

Recently we have made significant breakthroughs in THz communication and sensing, e.g., the demonstration of directly modulated THz audio and burst communication link and integrated THz biosensing chip. This is possible only through emergence of novel THz functional elements such Photonic THz transmitters and liquid crystal tunable THz photonic devices developed by the PIs. In this project, we propose to demonstrate certain milestones of ultra-wide-band (UWB) THz impulse radio communication and sensing network for future ultra-broadband media, data and biomedical applications using the discoveries mentioned above.

For the near-term (five to 10 years), however, it is more practical to consider a V-band or W-band ROF or ultrawide band over fiber (UOF) system. The broadband data signals are first transmitted in an optical fiber that has low propagation loss and high data capacity. Then, the signal is radiated in free space over the last mile to the end user via a photonic transmitter. The system can thus provide broadband wireless communication services for the user.

The figure below shows the conceptual setup of the radio-over-fiber communication system and its key components. By using such system, we can distribute the W-band optical signal through the nearly lossless optical fiber and radiate it for the end users. Hence, the problem of huge free-space propagation loss of W-band signal can be minimized.

The proposed Radio-Over-Fiber Communication system and its key components

This project is an integrated project joining the efforts of teams from NTHU, NCU, and NCTU. We will develop the key components and related technology, including the optical sources, active components, and high-speed circuits for the applications radio-over-fiber (ROF) and ultra-wideband (UWB)-over-fiber (UOF) communication systems at V- and beyond W-bands. We will also integrate the developed components by use of flip-chip bonding technology and test the transmission characteristics, which include eye-patterns, bit-error-rate, and error-vector-magnitude, of such integrated module. Regarding the optical sources for ROF communication system, a clean and continuous-wave (CW) optical MMW sources with over 100GHz operating frequency will be demonstrated. On the other hand, the shaped optical pulse trains with special coding scheme, whose envelop cover all the available W-bands (75-110GHz), with high repetition rate (>10GHz) will also be developed for the applications of UOF system. In order to convert the high-power optical signals from UOF or ROF central stations, we will also demonstrate novel high-power photodiodes with over (120GHz) bandwidth and high saturation current performance (20mA). Furthermore, in order to further amplify the radiated CW or peak power and reduce their phase noise, we will also demonstrate some novel nonlinear circuits, such as GaN based piezoelectric oscillator at W-band, injection-locked oscillator array, and novel printed-circuit antenna array with high directivity (~20dB) at V- and W-bands. Nitride-based quantum cascade structures can potentially be developed as room-temperature THz laser devices. Overall, this project will cover key active/passive devices, circuits, optical sources, and the test beds for ROF and UOF communication system at V- and beyond the W-bands.

[ 2 ] Photovoltaic technology with self-assembled nanostructures of silicon quantum-dots in mesoporous silica (NSC Nanoscience Program, Aug. 1, 2008- July 31, 2011).

The project team consists of Prof. C. T. Lee (NCKU), who acts as PI of the whole project, and members from NCTU and NDL. I am responsible for overseeing the NCTU (Profs. Jung Y. Huang, Ci-Ling Pan, Hao-Chung Kuo, Peichen Yu and Hyeyoung Ah) and NDL (Dr. Jia-Min Shieh) portions of the project and also lead the effort on investigating the underlying physical mechanisms in the novel nanostructured silicon material. Experimentally, various ultrafast and THz spectroscopic techniques will be employed, in additional to the more conventional spectroscopic methods, such as spectroscopic ellipsometry. Previously, enhanced UV-to-NIR photoresponse has been reported by the NCTU and NDL team for a photodiode with dense silicon quantum-dots embedded in MS. Phototransistor-like operation due to enhanced exciton resonant tunneling and injection was observed. We thus propose a related solar cell structure made of a superlattice of silicon quantum dots with gradually varying sizes inserted between p- and n-type tailored mesoporous materials. To enhance the PV efficiency, various approaches involving cost-effective methods for transmission enhancement of solar energy will be investigated. For example, a double-layer anti-reflective structure consisting of a dielectric film and a mesoporous layer of low refractive index will render the front surface of the device more effective in capturing photons. Preliminary results confirm the validity of this approach. Further, Indium-Tin Oxide (ITO) nanostructures not only offer broad angular and spectral anti-reflective characteristics, but also improve the electric properties of the cell, reducing the screen ratio of metal contacts. Initial work is promising (Adv. Mat., 2009). Moreover, surface plasmonic effects using periodic and random metal nano-particles can be designed to enhance the transmission in the vicinity of bandgap energy. Finally, it has been suggested that silicon nano-pillar structures can significantly alter the thermoelectric properties of bulk material. Hence, we will explore nanostructures with a high thermoelectric coefficient in order to harvest the waste heat from p-n junctions. In addition, a surface plasmonic coupling layer or even an optical antenna structure can be employed to offer free-space-to-device photon-capturing functionality without coupling loss. Finally, a wavelength up-conversion layer can be realized with appropriate nanocrystals embedded in MS to convert solar radiation from near IR to the visible region, promising efficient use of photons over the entire range of AM1 solar spectrum.

ResearchRoadmap for highly efficient nano-silcon solarcells

[ 3 ] Towards Tabletop Sub-Single-Cycle Attosecond Laser Sources (NSC, August 1. 2009-July 31, 2012)

Attosecond (1 as = 10^-18 sec) science is generally recognized as one of the frontier areas in science today. There are many potential applications. For example, Few cycle attosecond pulses have been used to create and then probe interference of vibrational wave functions in a molecule; control a simple chemical reaction [4] and to enable real-time observation of electron tunneling in atoms. The shortest pulse that have been generated to date is about 80 attosecond.

In this work, we propose a new scheme of sub-femtoseocond (attosecond) pulse generation by starting from a single nanosecond laser and phase lock the cascaded harmonics generated by second-order nonlinear optical processes. It is shown that sub-cycle attosecond pulses with carrier envelope phase (CEP) control can be generated in this manner. After full characterization such a light source will be used to study the generation of higher-order harmonics (HHG) with extreme ultraviolet (XUV) emission by half-cycle attosecond optical pulse and the generation of even shorter attosecond pulses. Such broad ultrashort pulses, spanning a spectrum from the near infrared to the XUV would be invaluable for dynamic studies of the condensed matter, including biomolecules. The availability of this source should stimulate both theoretical and experimental studies of light-matter interaction on the attosecond time scale. Our vision of attosecond science and technology is shown in the figure below.

A vision of attosecond science and technology

We have successfully completely the first phase of the construction of the Tsing Hua Attosecond Source. As a first demonstration, saw-tooth and square waveforms were synthesized. Initial results were presented at FiO2010.

The Tsing Hua Attosecond Source (Phase one)

[ 4 ] Industrial Collaborations:

There are two on-going projects:

In collaboration with CW Laser Corportation, we propose to improve the performance of traditional small form-factor diode-pumped and frequency-doubled solid-state green laser. The key task is the stabilization of the operation wavelength of the diode pump laser. Various approaches for wavelength locking will be explored. The operation temperature range of this laser is expected to extend from 0 to 50°C. Such lasers will be much more suitable for industrial applications such as instrumentation in outdoor environments and the laser display.

In collaboration with Prime Optical Fiber Corporation (POFC) and the CW Laser Corporation, we have been conducting research and development of advanced short-pulse fiber-based lasers. By frequency-doubling technique, we will also demonstrate high power green output. Such lasers would have immediate applications in scientific research, laser-based manufacturing, laser display and biomediciene. The tasks for the first year consist of the design, construction and optimization of mode-locked Yb-doped fiber lasers with average power > 5 Watts and pulse width <1ps. We also expect to generate green output exceeding 3 Watts by frequency doubling using efficient nonlinear crystals.

Professor Pan,Ci-Ling

Professor
Pan,Ci-Ling