子計畫一:兆位元時代光電科技之基礎研究(2/4)
計畫類別： 整合型計畫
計畫編號： NSC94-2752-E-009-007-PAE
執行期間： 94 年 04 月 01 日至 95 年 03 月 31 日
執行單位： 國立交通大學光電工程學系(所)
計畫主持人： 潘犀靈
共同主持人： 許根玉，王興宗
計畫參與人員： 潘犀靈 , 許根玉 , 王興宗 , 林恭如 , 林烜輝 , 李柏璁 ,
郭浩中 , 許晉瑋 ,
報告類型： 完整報告
處理方式： 本計畫可公開查詢
中 華 民 國 95 年 3 月 29 日
COVER
Program for Promoting Academic Excellence of Universities（Phase II）
Midterm Report
建構兆位元紀元的光電科技-子計畫一：兆位元時代光電科技之基礎研究 Photonic Sciences and Technologies for the Tera Era:
Subproject 1: Fundamental Studies on Photonic Science and Technology for the Tera Era
NSC 94-2752-E-009-007-PAE
Overall Duration: Month 4 Year 2004 - Month 3 Year 2008 Midterm Duration: Month 4 Year 2004 - Month 3 Year 2006
National Chiao Tung University 2006.02.26
I.BASIC INFORMATION OF THIS SUB-PROJECT (FORM 1)
Project Title: Photonic Sciences and Technologies for the Tera Era--Subproject 2:
Next Generation Optical Communication and Optical Storage Technologies 建構兆位元紀元的光電科技-子計畫一：下世代光通訊與儲存技術
Serial No.: NSC 94-2752-E-009-007-PAE Affiliation National Chiao Tung University
國立交通大學 Name Ci-Ling Pan
潘犀靈 Name Gong-Ru Lin 林恭如 Tel: (03)5712121-31921 Tel: (03)5712121-56376 Fax: (03)5716631 Fax: (03)5716631 Princ ipal I n ves tigator
E-mail [email protected] Project Coo
rdinator
E-mail [email protected] Expenditures1 (in NT$1,000) Manpower2:Full time/Part time(Person-Months)
Projected Actual Projected Actual
FY2004
FY2005 12,947,000 7,504,215 165 165
FY2006 - -
FY2007 - -
Overall
Notes: 1,2 Please explain large differences between projected and actual figures. Principal Investigator’s Signature:
II.EXECUTIVE SUMMARY ON RESEARCH OUTCOMES OF THIS PROJECT (FORM 2) (PLEASE STATE THE FOLLOWING CONCISELY AND CLEARLY)
1. GENERAL DESCRIPTION OF THE PROJECT:INCLUDING OBJECTIVES OF THE PROJECT (MAXIMUM 3 PAGES)
The main goals of this project are to design, construct and characterize new optical and
optoelectronic functional devices and modules to meet the challenge of the tera-bit
information era. To achieve these goals, we focus our research on the following fundamental
research topics:
(I)
Coherent and THz Photonics;
(II)
Quantum (Photonic Crystal) structures and Enabling devices;
(III) Volume Holographic Materials, Technology and Enabling devices
(I) Coherent and THz Photonics
One of the current trends in photonics is the development of a technology based
with better control of the light-matter interaction. Employing advanced laser-based
techniques, novel design concept, and fabrication technologies of novel photonic
structures from potential photonic materials, we shall be able to steer photon energies
into specific degrees of freedom of complex systems or materials, to create new
materials, to generate new functionality from a device. One of the goals of the present
project is thus the development and employment of advanced laser technology, in
particular, ultrafast-laser-based techniques such as coherent control, spatially,
temporally, and spectrally resolved real-time imaging, and laser-assisted fabrication and
properties modification for fundamental studies of photonic properties of various novel
photonic materials, structures and devices.
In view of the emerging applications of electromagnetic waves at millimeter-wave
or THz frequencies in remote sensing, imaging, and communication, we will conduct
studies on various aspects of THz photonics and applications, employing the coherent
photonic tools developed in our laboratories over the years.
Our main objectives are studies of photonics-based ultra-wideband (THz) wireless
communication and frequency measuring technologies for the next generation. Two
emerging technologies will be explored: (1) coherent THz communication; The
frequency of the carrier wave for this technology is in the sub-millimeter wave (0.1 to
10 THz, 1 THz = 1×1012 Hz) band; and (2) optical-impulse THz radio communication,
which is carrier free. In both approaches, photonics-based technologies will be used
for the generation and detection of THz radiation. Sub-millimeter wave or THz
communication combines the merits of optical and wireless communication
technologies. It is particularly suitable for short-distance, point-to-point, high
information-data-rate (multi-Mb/s to Gb/s), and secure communication channels. A
related but critical technology is precision measurement of the frequency of the THz
band of the electromagnetic wave. Possible THz frequency standards might emerge
from this research. This technology would be based on the femtosecond optical
frequency comb generator spanning hundreds of THz of bandwidth (1200 – 1600 nm)
and traceable to known frequency standards. I note that the femtosecond frequency
comb is one the subject cited for the 2005 Nobel laureates in physics, Prof. Ted Hansch
and Dr. John Hall. Our work would also allow precision measurement of present and
future communication channels (the International Telecommunication Union or ITU
grid) for (ultra) dense wavelength division multiplexed (DWDM) optical
communication.
The advances in THz applications would also require concurrent progress in THz
photonic elements, such as generators, detectors, polarizers, attenuators, modulators and
phase shifters. Novel materials and structures need be explored to address this need.
We have made good progress in broad-band THz receivers and liquid crystal THz
optics in the past. In this work, we would like to investigate (1) highly efficient THz
emitters and detectors, (2) explore the possibility of combining liquid crystals with
photonic crystals and meta-materials for tunable THz optics. With the structured
material or meta-material and highly birefringent materials such as liquid crystals for
added functionality, new possibilities arise for novel optical elements because of the
strong coupling of these novel materials with the electromagnetic wave. Starting from
the theoretical analysis, we will work on design and fabrication of various THz optical
components. Our long-range goal would be highly directional and intense THz
sources, taking advantage of the unique properties of photonic crystals or
meta-materials. The technologies developed in this project would also make possible
advances in other important applications of THz science and technology, e.g.,
biomedical sensing and imaging.
(II) Quantum (Photonic Crystal) structures and Enabling devices
The main objectives of this research project will be focus on 3 parts. First,
Development and study of novel blue and UV-LED and surface emitting laser, the
specific objectives of this proposal include (1) to development nitride-based blue and
UV material and optoeletronic device; (2) to development novel process for obtaining
high performance of blue and UV LED and LD. Second, to investigate nanotechnology
and nano-photonics. This part of the object will focus on investigating the optical
properties of mesoscopic GaN-based quantum confined structures and to achieve
controlled photon emission from the GaN-based quantum confined structures. The
specific objectives of this proposal include (1) establishment of the fabrication
technology of GaN quantum confined structures such as quantum dots and
nanostructures; (2) simulation and modeling of the op;tical properties of microcavity
quantum confined structures and development of device design guidelines for
fabrication of microcavity quantum confined structures; (3) fabrication of devices that
incorporate the quantum confined structures into a microcavity such as vertical cavity
surface emitting laser (VCSEL) structures; (4) investigation of the optical properties of
the fabricated quantum confined structures and microcavity structures; and (5)
investigation and demonstration of the controlled photon emission from the microcavity
quantum confined structures or devices. Third, for the fabrication of long wavelength
VCSEL (LW-VCSEL) and high speed VCSEL for communication, the specific
objectives of this proposal include (1) fabrication single mode high speed GaAs or InP
-based VCSEL; fabrication of InP based 1300 nm or 1500nm Long Wavelength
VCSEL; (2) VCSEL Arrays Chip and Multiple-Wavelength or tunable Source.
- The GaN-based UV LD have applicatics to the high density storge in the storge
project..
- The Long Wavelength VCSEL will be useful to the optical communication
project.
(IV) (III) Volume Holographic Materials, Technology and Enabling devices
Volume holographic technology and applications have been explored for past 50
years but still have not yet achieved significant breakthrough. The development of the
proper recording material is a fundamental key to the success for the holographic
systems. Therefore, in this sub-project, we plan to develop novel volume holographic
materials and explore its applications on novel information processing with ultrahigh
density (1 Tbits/in2) and ultrafast fast (Tbps). Through the innovative researches and
international collaborative efforts, we anticipate becoming a world class leader in the
field of parallel information photonic system.
2. BREAKTHROUGHS AND MAJOR ACHIEVEMENTS
(I) Coherent and THz Photonics
1. Freezing phase scheme for fast adaptive coherent control:
The operational principle is based on a concept that the highest peak intensity will
correspond to a frozen phase state of all spectral components involved in a coherent optical
pulse. It is fast and immune to the noise and laser power fluctuation, and useful for a
variety of applications that require complete-field characterization and adaptive coherent
control on the same setup [JOSA B 22:1134 (2005), selected by the Virtual Journal of
Ultrafast Sciences, Vol. 4, No. 6, June 2005].
2. Ultrafast Photoconductive Switch and THz Spiral Antenna Fabricated on Multi-Energy
Arsenic-Ion-Implanted GaAs:
With multi-energy implantation and post annealing, the dark current of a GaAs:As+ PCS
antenna is reduced to 24 A/cm2. The device exhibits a nearly identical rising and falling
response and exhibits 2.7-ps switching response with operational bandwidth exceeding 150
GHz. This multi-energy implantation results in a shorter THz emission pulse with central
frequency and spectrum linewidth extending to 0.2 THz and 0.18 THz, respectively [JAP
98:013711 (2005), selected by the Virtual Journal of Ultrafast Sciences, Vol. 4, No. 8,
August 2005].
3. Directly modulated THz Communication link:
We have demonstrated for the first time transmission of audio and burst signals through a
prototype THz analog communication link, employing dipole antenna as THz emitter and
receiver. The transmission distance is about 100 cm. By using a direct voltage
modulation format, we observed a clearly demodulated burst signal with a rising time of 32
µs. The highest audio modulating bandwidth achieved was 30 kHz in this first experiment.
The transmission of a six-channel analog and burst audio signal with least distortion is also
demonstrated [Opt. Exp., 13:10416, 2005].
4. A powerful THz emitter in the 800nm wavelength regime:
In this project, a GaAs/AlGaAs based Unitraveling Carrier Photodiode (UTC-PD) at a
wavelength of around 830nm is demonstrated. Compared with the performance of control
GaAs based p-i-n photodiode, UTC-PD can attain resembling external efficiency with
much better electrical bandwidth performance under higher output photocurrent.
Significant bandwidth enhancement can occur in a much lower photocurrent density
(0.3mA/µm2 vs. 0.017 mA/µm2) than that of the reported InP-InGaAs based UTC-PD with
a similar degree of enhanced optical-to-electrical (O-E) bandwidth (~10GHz) by
optimizing the p-type doping profile in the absorption region. The results indicate that the
demonstrated device structure has the potential to increase the linear operation regime of
UTC-PD and serve as a powerful THz emitter in the 800nm wavelength regime.
The conceptual band diagram and cross-sectional view of demonstrated UTC-PD device is
shown in Figure 1 (a) and (b), respectively. The inset of Figure 1 (a) shows the top-view of
a fabricated device. In order to accelerate the photo-generated electron, in Figure 1 (b), we
adopted a graded p-type doping profile in the GaAs based photo-absorption layer with
160nm total thickness.
To characterize the dynamic performance of our devices under continuous-wave (CW)
operation, a heterodyne-beating system and a lightwave-component-analyzer (LCA)
system have been established to measure the frequency responses of microwave and
optical-to-electrical (O-E) scattering (S) parameters of devices. Figure 2 (a) and (b)
represents the measured frequency responses of demonstrated UTC-PD and the control
p-i-n under a fixed 50_ load (RF spectrum analyzer), the same optical pumping power
(15mW), and different reverse bias voltages (-1V, -3V, and -5V). It is clear that UTC-PD
has much better high-speed performance than the control p-i-n PD especially under high
reverse bias voltages (-5V) and high output photocurrent (~1mA), and both devices exhibit
nearly similar responsivity performance (~0.07A/W). Such measured result indicates that,
compared with control p-i-n, the efficiency performance of our UTC-PD will not be
sacrificed for its better high-power and high-speed performance.
The measured f3dB bandwidths of similar two devices under different operation conditions
are shown in Figure 3. Under the high reverse bias voltage (-5V), significant bandwidth
enhancement of UTC-PD has appeared under ~1mA output photocurrent.
A GaAs/AlGaAs based UTC-PD at an 830nm wavelength has been demonstrated.
Compared with the control p-i-n PD, our device can have superior speed and power
performance without sacrificing its responsivity performance. According to the O-E
measurement result, the self-induced field and optimized p-type doping profile in the
absorption layer causes significant bandwidth enhancement .The demonstrated device has
the potential to serve as a powerful THz emitter under 800nm wavelength.
Fig. 1a Fig. 1b
Fig.3
(II) Quantum (Photonic Crystal) structures and Enabling devices
1. Development and study of novel blue and UV-LED and surface emitting laser
(a) We successfully improved the performance of LEDs using two methods: (1)micro-hole
array LED (2) undercut LED
(b) The process of p-side down GaN LEDs on Cu substrate using Laser lift-off was
established.
(c) The LLO LEDs on Cu substrate showed linearly increased light output-power as the
driving current was increased up to 1A with large emitting area of 1mm×1mm.
(d) Demonstration of laser action in GaN vertical micro-cavity under optical pumping at
room temperature
(f) Demonstration of high reflectivity and crack-free AlN/GaN DBR
2. Development and study of nanotechnology and nano-photonics
(a) Fabrication InGaN/GaN MQW nanorods of 100 nm in diameter by ICP etching for the
first time
(b) We present a novel method to fabricate High density (3.0×1010 cm-2) GaN-based
nanorod -LED with controllable dimension and density using Ni mask.
(c) About 5 times enhancement of intensity from the InGaN/GaN nanorod was also
observed
(d) Fabrication first GaN nanorod with MQW structures and showed light emission
enhancement in GaN light emitting devices
(e) Fabrication of nano structures and demonstration of high Q micro-cavity
3. Development and study of long-wavelength VCSEL
(a) We fabricated the PC-VCSEL by proton implant and ICP etching
(b) single mode output with SMSR > 50 dB was obtained by proton implanted PC-VCSEL.
(c) Demonstration of singlemode Quantum-Dot VCSEL in 1.3 um with Side-mode
Suppression Ratio over 30dB
External Light Injection
(e) Demonstration of singlemode InAs quantum dot photonic crystal VCSELs
World Class results
GaN VCSEL
4. Develop high-quality 1.55µm InGaAsP/InP MQW epitaxial structures as the active region
of the 2D photonic crystal defect laser cavities.
5. Develop wafer bonding technique to integrate InGaAsP/InP MQW wafer with Sapphire
substrate that has higher thermal conductivity than air. Achieve the air bubble-free and
stress-free bonding quality by using a pre-made channel structure on wafer.
6. Evelop fabrication technology of 2D photonic crystal defect cavity structures which is based
on electron beam lithography, RIE SiNx etch, and HDP InGaAsP/InP etch
7. Establish the measurement setup for defect cavities based on an infrared micro-PL system to
characterize and analyze the basic characteristics of 2D photonic crystal lasers including PL
spectra near and above threshold and L-L curve. We also study the thermal effects including
the red shifts of lasing wavelength and the threshold dependences under different substrate
temperatures and different pumping conditions. An ultra-low threshold pump power of 3.4µW
is obtained under 1% duty cycle pumping condition.
(III) Volume Holographic Materials, Technology and Enabling devices
Our comprehensive studies on doped photopolymer can provide researchers invaluable
guidance for the design, fabrication and characterization of novel holographic materials. The
methodology of our investigation can also be excellent reference for developing new recording
materials. Those thick holographic materials can open new widows for innovative applications
in optical information processing.
3. CATEGORIZED SUMMARY OF RESEARCH OUTCOMES.IN EACH RESEARCH AREA, PLEASE GIVE A BRIEF SUMMARY OF THE RESEARCH OUTCOMES ASSOCIATED WITH THE AREA.NOTE THAT THE SUMMARIES SHOULD BE CONSISTENT WITH THE STATISTICS GIVEN IN FORM 3.PLEASE LIST AND NUMBER OF EACH RESEARCH OUTCOMES IN ORDER IN APPENDIX II, AND LIST ALL THE PUBLICATIONS IN TOP CONFERENCES AND JOURNALS IN APPENDIX III.
A. Prof. Ci-Ling Pan
I.
THz Photonics
A detection bandwidth exceeding 30 THz was reported for THz dipole antenna fabricated
on InP:H+ [Opt. Exp. 12(13):2954, 2004, selected by the Virtual Journal of Ultrafast
Science, August 2004]. This is an extension of our previous work on
Arsenic-ion-implanted GaAs [APL 83(7)1322, 2003, selected by the Virtual Journal of
Ultrafast Science, September, 2003]. Both types of devices exhibit the broadest
bandwidth reported for THz antennas based on ion-implanted photoconductors and
comparable to that of LT-GaAs, the current state-of-art material for such applications. Our
most recent work in this area was the report of multi-Energy Arsenic-Ion-Implanted GaAs
Photoconductive THz Spiral Antenna [JAP 98:013711, 2005. Selected by the Virtual
Journal of Ultrafast Science, Vol. 4, No. 8, August 2005]. In order to generate higher
THz power, we experimented on coherent array of antennas [Opt2005]. Enhancement of
THz amplitude by 2.2 times by a 2-element array was achieved. Using photoconductive
antenna technology, we also report the first directly modulated THz communication link.
The transmission distance is about 100 cm. By using a direct voltage modulation format,
we observed a clearly demodulated burst signal with a rising time of 32 µs. The highest
audio modulating bandwidth achieved was 30 kHz in this first experiment. The
transmission of a six-channel analog and burst audio signal with least distortion is also
demonstrated [Opt. Exp., 13:10416, 2005]. We also report exploratory work on biomedical
applications of THz technology at a local conference [OPT2005]. On another front, we
have successfully generated and detected CW THz radiation by photomixing of two laser
diodes. Using an external cavity (ECL) configuration, the two laser diodes can be phase
locked to a femtosecond laser frequency comb. The beat note of the 2 ECLs locked at
0.7131000 THz and 0.4571000 THz were demonstrated. This is important for our goal of
establishment of THz frequency standard using the femtosecond frequency comb.
1. S. L. Wu (吳勝隆) and T. A. Liu (劉子安) and C. L. Pan (潘犀靈), “Spectroscopy of flour, lactose and starch in the THz range,” paper PE64, presented at Annual Meeting of the Physical Society, Feb. 9-11, 2004, Kaoshiung, Taiwan, in Conference Proceedings, 物理雙月刊, Vol. 27, No. 1, February 2005, p. 217.
2. Tze-An Liu, Gong-Ru Lin, Yen-Chi Lee, Shing-Chung Wang, M. Tani, Hsiao-Hua Wu, and Ci-Ling Pan, “Multi-Energy Arsenic-Ion-Implanted GaAs Photoconductive THz Spiral Antenna with Suppressed Dark Current and Trailing Edge,” J. Appl. Phys., Vol. 98, 013711-1 to -4, July 15, 2005, selected by the Virtual Journal of Ultrafast Science, Vol. 4, No. 8, August 2005.
3. Tze-An Liu, Gong-Ru Lin, Yung-Cheng Chang, Ci-Ling Pan, “A wireless audio and burst communication link with directly modulated THz photoconductive antenna,” Optics Express, Vol. 13, No. 25, pp. 10416-10423, 12 December, 2005.
4. Tze-An Liu(劉子安) , Chao-Jen Huang, Teh-Ho Tao, Ci-Ling Pan, “Thz Radaition From An Array Of Three Photoconductive Dipole Antennas,” C-SA-V4-1, presented at OPT2005 (Optics and Photonics Taiwan), Dec. 9-10, 2005, Tainan, Taiwan.
5. Tze-An Liu(劉子安) , Sheng-Lung Wu, Ci-Ling Pan, “Birefringence Measurement In Burned And Unburned Porcine Skin By THz Time Domain Spectroscopy,” C-SA-V5-4, presented at OPT2005 (Optics and Photonics Taiwan), Dec. 9-10, 2005, Tainan, Taiwan.
6. Tze-An Liu(劉子安) , Sheng-Lung Wu, Ci-Ling Pan, “Burn-Depth Detection Of Pork With T-Ray Technology,” PF-FR2-30, presented at OPT2005 (Optics and Photonics Taiwan), Dec. 9-10, 2005, Tainan, Taiwan.
7. Cheng-Yao Kao(高禎佑),Chih-Yu Wang, Yu-Ping Lan, Chao-kuei Lee, Jin-Long Peng, Ci-Ling Pan, “Towards Thz Frequency Metrology III: Frequency Locking Of Two Laser Diodes To The Femtosecond Frequency Comb,” C-SA-V6-7, presented at OPT2005 (Optics and Photonics Taiwan), Dec. 9-10, 2005, Tainan, Taiwan.
II.
Liquid crystal THz photonics:
We have pioneered this field. Previously, we reported for the first time optical constants
of several important liquid crystals in the THz regime [Appl. Opt., 42(13): 2372, 2003 and
J. Biological Phys. 29(2-3):335, 2003]. Unexpected large birefringence was observed for
the liquid crystals 5CB and E7 in the nematic phase. These properties were utilized to
demonstrate both magnetically and electrically controlled THz phase shifters [APL 83(22):
4497, 2003; IEEE MWCL 14(2):77, 2004], culminating in the first room-temperature, 0-2π
tunable THz phase shifter [Opt. Exp. 12(12): 2625, 2004, Selected by the Virtual Journal
of Ultrafast Science, September 2004, Taiwan Patent 200186, US patent filed]. The device
operates at room temperature, as opposed to previous devices needing liquid N2 for
cooling and achieving phase shift of a few degrees at best. Important applications such as
THz phased arrayed radar would be possible. Due the impact of our work, Prof. Pan was
asked to present several invited talks, including a keynote speech on the subject. Recently,
we have made several advances in THz Liquid crystal photonics.
(a) Control of enhanced THz transmission through 2-D metallic hole arrays using
magnetically controlled birefringence in a nematic liquid crystal cell. [Opt. Exp.
13(11):3921, 2005].
(b) A liquid-crystal-based electrically tunable THz phase shifter and quarter-wave plate
[presented at LEOS’05, Opt. Lett., to be published].
(c) A tunable liquid crystal Lyot filter. This is the first reported birefringent THz filter, to
our knowledge [presented at LEOS’05, invited talk at the annual meeting of the Liquid
Crystal Society, Appl. Phys. Letter, to be published].
In related work, we demonstrated a THz plasmonic filter [J. Phys. D, and studied the effect
of hole materials in THz photonic crystals [presented at LEOS’05, submitted to Opt. Lett.].
1. Bor-Yuan Shew, Han-Chieh Li, Ci-Ling Pan and Cheng-Hao Ko, “ X-ray micromachining SU-8 resist for a terahertz photonic filter,” Journal Of Physics D-Applied Physics Vol. 38, No. 7, pp. 1097-1103, 7
April 2005.
2. Ci-Ling Pan, “Progress in Liquid Crystal THz Optics,” keynote speech, presented at Workshop On Global Perspectives In Frontiers Of Photonics: Computational Imaging, Biophotonics And Nanophotonics,” Durham, North Carolina, USA, May 18-19, 2005.
3. Ci-Ling Pan, Cho-Fan Hsieh, and Ru-Pin Pan, Masaki Tanaka, Fumiaki Miyamaru, Masahiko Tani, and Masanori Hangyo, “Control of enhanced THz transmission through metallic hole arrays using nematic liquid crystal,” Optics Express, Vol. 13, No. 11, pp. 3921 - 3930, May 30, 2005.
4. Ci-Ling Pan, “Recent Progress in Liquid Crystal THz Optics,” invited paper, presented at "Frontiers of Laser and Optical Sciences", October 1 - 2, 2005, Faculty of Science, Building No. 4, Room 1220 (2nd Floor), Hongo Campus, The University of Tokyo, Tokyo, Japan.
5. Ru-Pin Pan, Chao-Yuan Chen, Cho-Fan Hsieh, and Ci-Ling Pan, “A Liquid-Crystal-Based Terahertz Tunable Lyot Filter,” paper #ThCC3, presented at the 18th annual meeting of IEEE/LEOS, LEOS 2005, Sydney, Australia, October 23-27, 2005.
6. Cho-Fan Hsieh, Hung-Lung Chen, Chao-Yuan Chen, Ru-Pin Pan, and Ci-Ling Pan, “Voltage Controlled Liquid Crystal Terahertz Quarter Wave Plate,” paper # ThCC5, ibid..
7. Cheng Lo, Cho-Fan Hsieh, Ru-Pin Pan and Ci-Ling Pan, “Effects of Hole Material on Enhanced Terahertz Transmission through Metallic Hole Arrays,” paper # ThCC4, ibid..
8. Ci-Ling Pan, “A Liquid-Crystal-Based Terahertz Tunable Lyot Filter,” invited talk, presented at the annual meeting of the Liquid Crystal Society, Taiwan, Republic of China, December 30, 2005, Hsinchu, Taiwan.
9. Ci-Lin Pan and Ru-Pin Pan, “Recent progress in liquid crystal THz optics,” invited talk, presented at Photonics West 2006, San Jose, California, USA, Jan. 21-26, 2006, invited paper to be published in Proceedings of SPIE Vol. #6135, Liquid Crystal Materials, Devices, And Applications XI, Liang-Chy Chien, ed..
10. Ci-Ling Pan, “兆赫液晶光學,” invited talk, presented at 光學工程研討會 Optical Engineering Forum -- Meet SPIE Fellows, 14 February 2006, Chun-Li, Taiwan.
11. Chao-Yuan Chen, Cho-Fan Hsieh, Yea-Feng Lin, Ci-Ling Pan and Ru-Pin Pan, “A Liquid-Crystal-Based Terahertz Tunable Lyot Filter,” submitted to Appl. Phys. Lett., July 26, 2005 (accepted for publication, February 8, 2006).
12. Cho-Fan Hsieh and Ru-Pin Pan, Tsung-Ta Tang, Hung-Lung Chen, and Ci-Ling Pan, “Voltage-controlled liquid crystal terahertz phase shifter and quarter wave plate,” submitted to Optics Letters, November 5, 2005 (accepted for publication, 9 January, 2006).
B. Prof. Shiuan-Huei Lin
The main target of this project is to explore novel materials for volume and/or dynamic
holographic recording and its applications on ultrahigh density storage (1 Tbits/in2).
During the second year of project, we have investigated on the optimization of our doped
PMMA photopolymers. Experimentally, we have developed novel photopolymer materials,
such as doubly doped PMMA, quonone-based molecule doped PMMA, and doped
copolymer…etc. We have also performed holographic recording in these photopolymer
materials. In addition, the sample have been shaped as a 5-inch diameter disk with 2-mm
thickness. It was put into a shift-multiplexed holographic data storage system (shown in
Fig. 1.) and used to stored binary data as a computer data bank. The picture of disk sample
is shown in Figure 2. We have written ~57 holograms, at a storage density of ~ 45 bits/m2,
5 inches 2mm
Figure 2. The holographic disk
corresponding to ~ 50Gbytes of the storage capacity in this 5-inch disk. Raw bit error rate
has been estimated to be ~0.0015. This result demonstrates that our material can support
for the high-quality volume holographic storage applications.
C. Prof. Hao-Chung Kuo
I.
Development and study of novel blue and UV-LED and surface emitting laser
InGaN-based quantum-well (QW) light-emitting diodes (LEDs) are affecting the
development of full-color displays, illumination, and exterior automotive lighting over a
spectral range from near ultraviolet to green and amber. However, the internal quantum
efficiency for GaN-based LEDs is far smaller than 100% at room temperature due to the
activation of non-radiative defects. In addition, the external quantum efficiency of the
nitride-based LEDs is often low due to the large refractive index difference between the
nitride epitaxial layer and the air. In order to achieve high efficient light emitting diodes,
we developed some methods as following:
II.
Micro-hole array light emitting diode
The processing of the InGaN-based micro-hole array LEDs began with electron-beam
evaporated Ni (5 nm)/Au (8 nm) to form a high-transparency p-type Ohmic contact. The
holes and the rectangular mesa (360 µm × 250 µm) were fabricated simultaneously by
photolithographic patterning, the wet etching of Ni/Au layers and inductively coupled
plasma (ICP) self-aligned dry etching (SAMCO ICP-RIE 101iPH). The diameters of the
holes were 3, 7, 11, and 15 µm, as determined using a scanning electronic microscope
(SEM) measurement. Spacing between two holes was fixed at 25 µm. Thermal annealing
was applied to the p-type contact alloy at 500oC in air for 5 minutes. Finally, the trilayers
of Ti/Pt/Au (50 nm/20 nm/200 nm) for p-type pad were deposited. Fig. 1 shows an optical
microphotograph of the top of a micro-hole array LED chip and d = 7 µm, a bright
luminescence ring is observed at the periphery of the hole. Figure 2 plots the light
output-current density (L-J) curves. The micro-hole array LED with d = 7 µm has a light
output power of ~ 3.0 mW at 22.2 A/cm2 (corresponding to a driving current of 20 mA for
CCD Imaging Lens SLM Polymer Disk Stepping Motor Lens
Figure 1. The picture of holographic data storage system
the conventional BA LED), which is 36% greater than ~ 2.2 mW for the conventional BA
LED. Moreover, the light output power of the micro-hole array LEDs decreases as the d
increases above 11 µm and the light output power of the micro-hole array LED with d = 15
µm is less than that of the conventional BA LED. These facts are attributable to
combination of the enhancement in extraction efficiency by increasing the area of the
sidewall surfaces and the reduction of the active areas of the micro-hole array LEDs.
Optimally designed InGaN-based micro-hole array LEDs exhibit improved light output
efficiently and are candidate for white-light LEDs or high-power/ high-efficiency
large-area LEDs.
III.
Undercut LED
The process for conventional LED (LED I) and undercut LED (LED II) began with the
deposition of 0.6-µm-thick SiNx onto the sample surface using plasma enhanced chemical
vapor deposition (PE-CVD). Fig. 1 shows the schematic diagram of the undercut LED.
The mesa etching was then performed with Cl2/Ar as the etching gas in an ICP-RIE
system. An additional etching for LED II to form undercut side walls ~ 22o was carried out
after mesa etching with zero bias power. Finally, the metal contact layers, included
transparent contact and pad layers, were deposited onto samples using electron beam
0 50 100 150 200 250 300 0 2 4 6 8 10 12 14 16 18 20 22 Pow e r ( m W )
Current density (A/cm2)
Hole size (um) 15 12 9 7 5 3 Broad area
Fig. 1 The optical
microphotograph of the top of a micro-hole array LED chip and d = 7 µm
0 10 20 30 40 50 0 1 2 3 4 5 6 7 8 9 10 11 12 13 EL I n tensity (m W ) Current (mA) Conventional LED
LED with ~22o undercut angle
evaporation. Fig. 2 shows the SEM picture of side walls profile on LED II. Fig. 3 shows
the intensity–current (L–I ) characteristics of LED I and LED II. It can be seen that EL
intensity of the LED II is larger than that observed from the normal LED. With 20 mA
injection current, the light output power of LED I and LED II was about 3 mW and 5.1
mW, respectively. In other words, we could achieve a factor of 1.7 times output power
enhancement from the InGaN–GaN MQW LEDs by the introduction of the undercut side
walls. This simple and controllable method is beneficial to fabricate brighter LEDs.
IV.
Fabrication and performance of blue GaN-based vertical-cavity surface emitting laser
Our research group have recently successfully fabricated GaN-based micro-cavity VCSEL
and achieved laser operation under optical pumping conditions. These results were shown
in the following Figures and published in Applied Physics Letters. Fig.1 shows the laser
emission intensity versus pumping energy with clear threshold condition. The inset shows
the emission spectrum below and above threshold. Fig. 2 shows the laser emission image
Fig1.The schematic diagram of the undercut LED.
Fig. 2The SEM picture of side walls profile on LED II.
Fig. 3The intensity–current (L–I ) characteristics of LED I and LED II.
p-pad
Sapphire
n-GaN
MQW
p-GaN
n-pad
and beam intensity profile. we have investigated the performance of the GaN-based
VCSEL with emission wavelength at 448 nm under the optical pumping at room
temperature. The laser beam has a nearly linear polarization property with a degree of
polarization of about 84% as shown in Fig. 3. The laser has a high β value of about 5×10-2
indicating the coupling coefficient enhancement due to the laser microcavity, and a high
characteristic temperature of 244 K as shown in Fig. 4 suggesting potential for high
temperature applications.
V.
Enhanced light output of InGaN/GaN light emitting diode
The external quantum efficiency of GaN-based LEDs is low because the refractive index
of the nitride epitaxial layer differ greatly from that of the air, which limits the external
quantum efficiency of conventional GaN-based LEDs to only a few percent. The light
from LEDs can be enhanced either through the sample surface or through the side walls of
0 1 2 3 4 0 20 40 60 80 100 120 E m is sio n In ten s ity (a. u .)
Excitation Energy (µJ/pulse)
430 435 440 445 450 455 460 465 470 448nm 0.25nm 2.52 Eth 0.75 Eth 1.13 Eth Int e ns it y (a .u. ) Wavelength (nm)
Fig 1. The lasing characteristics of the GaN-based micro-cavity VCSEL.
Fig 2. The laser emission images of the GaN-based micro-cavity VCSEL.
10 Laser spot -150 -100 -50 0 50 100 150 0.0 0.2 0.4 0.6 0.8 1.0 experiment data N o rm a lize d I n te n s ity (a .u .)
Angle of Polarizer (degree)
-150 -100 -50 0 50 100 150 0.0 0.2 0.4 0.6 0.8 1.0 fitting curve 100 120 140 160 180 200 220 240 260 280 300 320 3.2 3.4 3.6 3.8 4.0 ln(E th ) Temperature (K) experiment data Linear fit of experiment data
Fig 3. The polarization characteristic of the laser emission at the pumping energy of 1.71Eth. The
solid dot shows the experiment data and the solid line is the fitting curve.
Fig 4. The semi natural-logarithm threshold energy as a function of the operation temperature. The solid dot shows the experiment data and the solid line is the linear fit of the experiment data.
the chip. This investigation describes the improvement of an InGaN/GaN MQW light
emitting diode by nano-roughening the p-GaN surface using Ni nano-mask and laser
etching as shown in Fig. 1. The nano-roughened surface improved the escape probability
of light output inside the LED structure, increasing by 55% the light output of InGaN/GaN
LED at 20 mA. As shown in Fig. 2, the operating voltage of the InGaN/GaN LED was
reduced from 3.54 to 3.27V at 20 mA and
the series resistance was reduced by 32% by the
increase in the contact area of the nano-roughened surface. The wall-plug efficiency of the
InGaN/GaN LED was increased by 68% by nano-roughening the
top p-GaN surface using
the Ni nano-mask and laser etching.
RMS roughness=5.8 nm
Fig. 1 AFM images of the top surface morphology of a LED sample with nano-roughened LED top p-GaN surface image.
0 20 40 60 80 100 0 5 10 15 20 25 30
Li
ght
out
put
p
o
w
e
r(
m
W
)
Current ( mA )
Conventional LED nano-roughened LEDVI.
Development and study of nanotechnology and nano-photonics
Recently, due to the fast development of the quantum electronics and nano-science,
fabrications and studies of quantum-confined structures have attracted a great deal of
interests for potential applications on optoelectronic devices such as quantum cryptography,
quantum information, single photon emitter and nano-light-emitting device. For
GaN-based materials, the low dimensional nanostructures have attracted many interests for
fundamental physical researches and potential applications. However, these GaN-based
nanostructures are mostly pure or single crystalline, and exhibit different electronic and
optical properties depending on their size and geometry. Many GaN-based devices must
take advantages of the multi-quantum-wells (MQWs) structure such as InGaN/GaN
MQWs. For this reason, it is necessary to fabricate the MQWs nanostructures and many of
their novel optical properties still remain a great challenge to be resolved. We have
successfully fabricated the nanorod composed of InGaN/GaN MQW structure using two
methods:
VII. Directly etching by ICP-RIE
The sample of grown wafer structure was subjected to dry etching technique for nanorods
formation using ICP system (SAMCO RIE-101iPH). The etching process of nanorods was
performed under an inductively coupled plasma produced by a gaseous mixture of Cl2/Ar
(10/25 sccm) at a chamber pressure of 20 mTorr. The ICP has a power of 200 W and a bias
power of 200 W. For PL measurement, a doubled Ti: Sapphire laser operating at 390 nm
with a spot diameter of 40 µm and a liquid helium flow cryostat for low temperature were
employed. Figure 1 displays a typical SEM image of In0.3Ga0.7N/GaN MQWs nanorods.
The nanorods fabricated by ICP dry etching were almost vertical and straight shape. The
nanorods have lengths up to 500 nm and diameters ranging from 60 to 100 nm. Nanorods
with diameters less than 55 nm were also observed. Structural characterization using TEM
confirmed that the MQWs structure in nanorods was intact in structure, as shown in figure
2. A typical PL spectrum of InGaN/GaN nanorods under an excitation density of 0.9
W/cm2 was measured at 4 K as shown in figure 3. It consists of several discrete emission
peaks whose positions are at 449, 453 and 457 nm respectively. The strong narrow
emission peak at 457 nm has a full width at half maximum (FWHM) of about 1.5 nm. The
position difference between each peak is estimated to be 4 nm (24 meV). The insert in
figure 3 is the spectrum from the as-grown bulk wafer before ICP etching, which was
measured at the same condition for the nanorods. It shows a typical InGaN/GaN MQWs
spectrum with a FWHM of about 26.5 nm and an undulation behavior which is probably
due to the Fabry-Perot interferences within the epitaxial layers. Indeed, fabrication of
nanorods structure from the In0.3Ga0.7N/GaN MQWs bulk wafer does exactly show the
different behavior than the typical PL emission spectra of bulk MQWs. This could be due
to the decrease of in-homogeneous broadening in wells of nanorods. Figure 4 shows a
series of spectra record at different excitation densities between 0.9 and 10.1 W/cm2 for
In0.3Ga0.7N/GaN MQWs nanorods at 4K. Under low excitation densities, the e1-h1 peak
at 457 nm is dominant. However, with increasing excitation density, the intensity of peak
on the high-energy side of the e1-h1 peak increases. Finally, this peak at 453 nm becomes
dominant over the e1-h1 emission. It have been demonstrated that the existence of
three-dimensionally localized, QDs-like states/structure from the appearance of individual
spectrally narrow emission lines. These experimental results suggest that excitons are
strongly localized or confined in QDs-like structure. Such a circumstance presents
interesting challenges to present efforts to develop blue nitride-based nano-optoelectronics
devices.
VIII. Etching with Ni mask
Fig. 1 shows the fabrication flowchart of the InGaN MQWs nanorods. First, a 3000
Å-thick Si3N4 thin film was deposited on the samples using the method of photo enhanced
chemical vapor deposition (PECVD), and then followed by the deposition of 50, 100, and
150 Å-thick Ni film respectively by electron-beam evaporation system. Then the samples
were treated with rapid thermal annealing (RTA) of 850 degree under nitrogen ambiance
for one minute to form self-assembled Ni nano-sized masks or clusters. In order to transfer
the nano-sized masks down to Si3N4 layer, a reactive ion etching (RIE) was conducted to
50 nm 50 nm 380 400 420 440 460 480 500 520 540 0.0 0.2 0.4 0.6 0.8 1.0 1.5 nm N o rm a lize d P L In te n s it y ( a .u .) Wavelength (nm) at 4K
after ICP etching
380400420440460480500520540 102 103 104 105 P L I n te n s it y ( a .u .) Wavelength (nm)
Before ICP etching
380 400 420 440 460 480 500 520 540 0.0 0.2 0.4 0.6 0.8 1.0 1.5 nm N o rm a lize d P L In te n s it y ( a .u .) Wavelength (nm) at 4K
after ICP etching
380400420440460480500520540 102 103 104 105 P L I n te n s it y ( a .u .) Wavelength (nm)
Before ICP etching
410 420 430 440 450 460 470 480 490 500 0 2000 4000 6000 8000 10000 12000 14000 at 4 K P L In te n s ity (a .u .) Wavelength (nm) 10.1 W /cm2 8.9 W /cm2 7.6 W /cm2 6.3 W /cm2 0.9 W /cm2
Fig. 1 Scanning electron microscopy image of In0.3Ga0.7N/GaN MQWs nanorods.
Fig. 2 Transmission electron micrograph of a single In0.3Ga0.7N/GaN MQWs
nanorod.
Fig. 3 Photoluminescence spectrum of In0.3Ga0.7N/GaN
MQWs nanorods excited under 0.9 W/cm2. The insert is a photoluminescence spectrum of the as-grown bulk sample.
Fig. 4 Excitation power dependent
photoluminescence spectra of In0.3Ga0.7N/GaN
etch Si3N4 film using mixture gases of CF4/O2. Then the samples were etched down to
the n-type GaN layer by ICP-RIE (SAMCO ICP-RIE 101iPH) with the nano-sized masks.
Finally, the remain of nano-masks were removed in buffer oxide etchant to expose the
InGaN/GaN MQW nanorods. Fig. 2 shows the mean dimension and density of InGaN
MQWs nanorods as a function of Ni-mask film thickness of 50 to 150 Å. The InGaN/GaN
MQW nanorods densities increase form 2.2x109 to 3x1010 cm-2 and the dimension
decrease from 150 to 60 nm as the Ni film thickness decrease from 150 to 50 Å. The
scanning electron microscope (SEM) image of the finished InGaN/GaN MQW nanorods
fabricated by the ICP-RIE dry etching using self-assembled Ni nano-masks is shown in Fig.
3. The transmission electron microscopy (TEM) (JEOL, JEM-200CX) image of a single
InGaN/GaN MQW nanorod is illustrated in Fig. 4. It shows clearly that the diameter and
height of a single nanorod are approximately 80 nm and 1 µm. The active region of
five-period MQW is also observed evidently from the TEM image. The width of the
quantum well and barrier are estimated to be about 5 and 25 nm. Fig 5 shows the emission
peaks of room temperature photoluminescence of the bulk and nanorods GaN LEDs at 451
and 446 nm. The blue-shift could be attributed to the partial strain relief in the well and
quantum confinement effect. In addition, the PL intensity in the nanorods is enhanced by a
factor of about 5 times than the bulk emission. The enhancement could be due to the better
overlap of the electron and hole wave functions with a reduced piezoelectric field, and
increasing of the radiative recombination rate. The light scattering off the etched sidewalls
of the nanorods could also increase the PL intensity. These results with MQW structure
should be applicable for fabrication of GaN-based light-emitting device.
850 ℃ N-GaN MQW P-GaN Sapphire InGaN/GaN x5 RTA ICP Ni RIE Si3N4
50 100 150 0 50 100 150 200 250 Nanor od densi ty ( x10 10 cm -2 ) Nanor od di am ensi on ( nm ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Thickness of Ni film 410 420 430 440 450 460 470 480 490 451.9nm 446.8nm Nanorods Bulk Nor m aliz ed I n tensity (a. u .) Wavelength (nm)
Fig. 3 The mean dimension and density of InGaN MQWs nanorods as a function of Ni-mask film thickness of 50 to 150 Å.
Fig. 4 The scanning electron microscope (SEM) image of the finished InGaN/GaN MQW nanorods.
Fig. 4 The emission peaks of room temperature photoluminescence of the bulk and nanorods GaN LEDs
Fig. 2 The mean dimension and density of InGaN MQWs nanorods as a function of Ni-mask film thickness of 50 to 150 Å.
IX.
InGaN self-assembled quantum dots grown by metalorganic chemical-vapor deposition
We have successfully grown self-assembled InGaN QDs structures by MOCVD system as
shown in Fig. 1. The thermal budget control in the growth device, such as QDs LED or LD,
is an important issue due to the instability of InGaN under high process temperature. In
order to realize the effect of thermal annealing on QDs optical properties, the theoretical
and experimental study of QDs postgrowth annealing were carried out. Self-assembled
InGaN QDs structures were grown on sapphire substrates by MOCVD with growth
interruption. The flat GaN layer on the sapphire substrate with an average deviation Ra =
0.14 nm of roughness over an area of 1 µm square was used as the template to grow InGaN
QDs under a low V/III ratio (~8300), the low growth temperature (660oC) conditions and
various interruption time. Grown InGaN QDs at tint = 60s have a density of about 4.5 ×
1010 cm-2 with an average lateral size of 11.5 nm and an average height of 1.6 nm was
obtained. The interruption time on the morphological and optical properties of the InGaN
QDs suggest that the desorption effect during the growth interruption could decrease the
dimensions of the InGaN QDs structure, the surface diffusion effect during the growth
interruption could increase the QDs coverage occupied on the surface above the wetting
layer, and extend the emission wavelength to the short wavelength region as the increase of
the interruption time. By properly adjusting the interruption time, the uniformly distributed
InGaN QDs with small dimensions can be obtained and should applicable for the
applications of GaN-based light emitting device
X.
Development and study of long-wavelength VCSEL
Long-wavelength (1.3–1.5mm) vertical cavity surface emitting lasers (VCSELs) are
considered the best candidate for the future light sources in fiber communications. The
advantages of VCSELs include single longitudinal mode output, small divergence circular
emission beam profile, low power consumption and low-cost reliable productions. The
absence of high refractive index contrast in InP-lattice-matched materials impeded the
progress of the development of 1.3–1.5 mm VCSELs in comparison to the
short-wavelength (0.78–0.98 mm) VCSELs. Recently, long-wavelength VCSELs have
been successfully demonstrated with several different approaches, including wafer fusion
technique; the InGaNAs 1.3 mm VCSELs grown on GaAs substrates, but to extend the
InGaNAs gain peak to beyond 1.5 mm is rather difficult. Recently, the DBRs based on
relatively large refractive index contrast (∆=0.34) material combination of InP/InGaAlAs
have also been demonstrated. This material combination not only has a larger refractive
index contrast than the conventional InP/InGaAsP and InAlAs/In-GaAlAs material
systems, but it also has other benefits including the smaller conduction band discontinuity,
which is good for n-type DBRs, and the better thermal conductivity due to the binary alloy
of InP. So, we developed novel resonance cavity and distributed Bragg reflector.
XI.
Developing novel resonance cavity and distributed Bragg reflector
The InP and InGaAlAs belong to different group-V-based materials. Problems like the As
carry over, the transitional interface, and lateral uniformity will affect the quality of the
epitaxial layers and the reflectivity of the DBRs. As a result, the challenge of growing this
combination relies on perfect switching between InP and InGaAlAs. The growth
interruptions have been frequently used in the metal organic chemical vapor deposition
(MOCVD) growth of the InGaAs/InP or InGaAs/InGaAsP quantum wells in order to
obtain abrupt interface, but the growth of the InP/InGaAlAs DBRs using growth
interruptions has not been investigated. We report the effect of the growth interruptions on
fabrication of the InP/InGaAlAs DBRs. The lateral uniformity and the reflectivity of the
DBRs are very sensitive to the stabilization time of each terminated interface. We
incorporated an in situ laser reflectometry while growing DBRs with thickness more than 8
mm to insure minimum fluctuation in the center wavelength of the stopband. The optically
pumped 1.56 mm VCSELs with 35 pairs InP/InGaAlAs DBRs achieved stimulated
emission at room temperature with the threshold pumping power of 30mW.
In investigation of InP-based VCSEL, We have developed high quality active layer of
InGaAlAs and developed high reflective InP/InGaAlAs DBR, and this result was
published in Journal of Crystal Growth. We also developed InP/air-gap DBR and this
result was published in Solid-State Electronics.
XII. Singlemode Monolithic Quantum-Dot VCSEL in 1.3 um
We demonstrate monolithic quantum-dot vertical-cavity surface-emitting laser (QD
VCSELs) operating in the 1.3 µm optical communication wavelength. The QD VCSELs
have adapted fully doped structure on GaAs substrate. The output power is ~ 330 µW with
slope efficiency of 0.18 W/A at room temperature. Single mode operation was obtained
with side-mode suppression ratio of > 30 dB. The schematic diagram of the QD VCSEL is
shown in Fig. 1. The structure is grown on a GaAs (100) substrate using molecular beam
epitaxy (MBE) by NL Nanosemiconductor GmbH (Germany). The epitaxial structure was
as follows (from bottom to top) - n+-GaAs buffer, 33.5-pair n+-Al0.9Ga0.1As/n+-GaAs
(Si-doped) distributed Bragg reflector (DBR), undoped active region, p-Al0.98Ga0.02As
oxidation layer, 22-pair p+-Al0.9Ga0.1As/p+-GaAs DBR (carbon-doped) and p+-GaAs
(carbon-doped) contact layer. The graded-index separate confinement heterostructure
(GRINSCH) active region consisted mainly of five groups of QDs active region embedded
between two linear-graded AlxGa1-xAs (x = 0 to 0.9 and x = 0.9 to 0) confinement layers.
Fig. 2 plots curves of light output and voltage versus current (LIV). The threshold current
is ~ 1.8 mA and the threshold current density is 7.6 kA/cm2. The output power rollover
occurs as the current increases above 4mA with maximum optical output of 0.33mW at
20°C. Fig. 3 shows the typical emission spectra of the quantum-dot VCSELs, which
indicate single transverse mode operation in the whole operation range with a lasing
wavelength of ~1.278 µm and side mode suppression ratio (SMSR) > 30dB. To investigate
the temperature dependence of the QD VCSEL, LI curves were measured from room
temperature to 55oC with current step of 0.01 mA, as shown in Fig. 4. The threshold
current varies only 0.15 mA (< 10% of Ith ) with temperatures from 10°C to 45°C and the
slope efficiency drops from 0.18 to 0.1 W/A.
n-DBR Active layer Substrate p-DBR Bonding pad GaAs/Al0.90Ga0.10As 22 pairs Al0.98Ga0.02As layer InAs/InGaAs QDs x 5 GaAs/Al0.90Ga0.10As 33 pairs GaAs substrate n-DBR Active layer Substrate p-DBR Bonding pad GaAs/Al0.90Ga0.10As 22 pairs Al0.98Ga0.02As layer InAs/InGaAs QDs x 5 GaAs/Al0.90Ga0.10As 33 pairs GaAs substrate 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 2 4 6 8 10 12 14 16 18 20 Current (mA) V o ltag e (V ) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Powe r ( m W ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 2 4 6 8 10 12 14 16 18 20 Current (mA) V o ltag e (V ) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Powe r ( m W )
Fig. 2 L-I-V relationship of QD VCSEL. Inset is the near-filed pattern of the VCSELs at 3mA.
Fig. 1 QD VCSELs device structure.
1272 1274 1276 1278 1280 1282 -80 -70 -60 -50 -40 -30 -20 -10 4mA 1.6mA In ten s it y ( d B m ) Wavelength(nm) 1.6mA 2 mA 2.6mA 3.2mA 3.8mA 4 mA 1272 1274 1276 1278 1280 1282 -80 -70 -60 -50 -40 -30 -20 -10 4mA 1.6mA In ten s it y ( d B m ) Wavelength(nm) 1.6mA 2 mA 2.6mA 3.2mA 3.8mA 4 mA 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 50 oC 20 oC Po wer ( m W ) Current (mA) 20 oC 25 oC 30 oC 35 oC 40 oC 45 oC 50 oC
XIII. 1.3 µm Quantum Dot Vertical Cavity Surface Emitting Laser with External Light
Injection
This investigation presents and experimentally demonstrates an 1.3µm quantum dot
vertical-cavity surface-emitting laser (QD VCSEL) with external light injection. The 3 dB
frequency response of QD VCSEL based on TO-Can package is enhanced from the
free-running 1.75 GHz to 7.44 GHz with the light injection technique. Fig.1. shows the
experimental setup for the injection locking of QD VCSEL. The QD VCSEL is
hermetically sealed by a standard TO-Can laser package (TO-46) with a built-in lens. The
QD VCSEL TO-Can package and the single-mode fiber are assembled by laser welding
technique. Fig. 2 shows the frequency response of QD VCSEL. The 3 dB frequency
response is 1.75 GHz at operating bias of 4 mA. The inset of Fig. 2 shows the output
spectra of the QD VCSEL, which indicate single transverse mode operation and the side
mode suppression ratio over 30 dB. The QD VCSEL is used as the slave laser while a DFB
laser is used as the master laser. The QD VCSEL is biased at 4 mA. The injection power is
varied by a variable optical attenuator at the output of the DFB laser. The polarization of
the DFB laser is adjusted using a polarization controller before injecting into the QD
VCSEL. In the experiment, the polarization and the center wavelength of DFB laser are
adjusted that the QD VCSEL has the most significant enhancement in the frequency
response. Fig. 3 shows the frequency response of the QD VCSEL at different injection
powers. This figure clearly shows that external injection can achieve a significant
enhancement in frequency response. Moreover, when the QD VCSEL is injection locked,
as show in the inset of Fig. 3, its optical spectrum shifts a slightly longer wavelength. We
observe that the 3 dB frequency is over 7.1 GHz when injection power is more than 2
dBm.
OC Amp Network Analyzer PD QD VCSEL DC Bias PC Bias T DFB VA 1 2 3 DC Bias 6dBm ~ 1dBm OSA 0.0 2.5 5.0 7.5 10.0 -60 -50 -40 -30 -20 -10 0 10R
esp
o
n
se
(d
B
)
Frequency (GHz)
3mA 3.5mA 4mA 1276 1278 1280 1282 -70 -60 -50 -40 -30 -20 -10 0 In te n sit y ( 1 0 d B / d iv ) Wavelength (nm) 3mA 3.5mA 4mAFig.2. Small-signal frequency response of QD VCSEL at different bias currents.
Fig.1. Experimental setup for the injection locking of QD VCSEL (DFB: DFB laser, VA: variable optical attenuator, OC: optical circulator, OSA: optical spectrum analyzer, PC: polarization controller, PD: photodetector, Amp: electrical amplifier)
XIV. Singlemode InAs quantum dot photonic crystal VCSELs
An InAs quantum dot photonic crystal vertical-cavity surface-emitting laser (QD
PhC-VCSEL) for fibre-optic applications is first demonstrated. Single fundamental mode
CW output power of 0.2 mW has been achieved in the 1300 nm range, with a threshold
current of 4.75 mA. Side-mode suppression ratio larger than 40 dB has been observed over
the entire thermally limited operation range. The device structure is shown in Fig. 1. By
using two types of apertures in this device, we decouple the effects of the current
confinement from the optical confinement. To clarify the effect of the photonic crystal
index-guiding layer, a VCSEL with H+ implant aperture was also fabricated for
comparison. Fig. 2 shows the CW light-current-voltage (L–I–V) output and near-field
image operated at 6 mA (inset) of the PhC-VCSEL. The VCSEL emits 0.2 mW peak
power and exhibits single modes throughout the current range of operation. The threshold
current (Ith) of the PhC-VCSEL is 4.75mA. The I–V characteristics exhibit higher series
resistance for the PhC-VCSEL, which should be mainly due to proton implantation
through the p-ohmic contact of the device and blocking of the current flow in the region by
photonic crystal holes. The output power could be improved by reducing the series
resistance of the PhC-VCSEL. Lasing spectra of the PhC-VCSEL is shown in Fig. 3a,
confirming singlemode operation within the overall operation current. The peak lasing
wavelengths are 1268 and 1272 nm at 6 and 22 mA, respectively. The PhC-VCSEL
exhibits an SMSR > 40 dB throughout the current range. For comparison, a lasing spectra
of a QD VCSEL without photonic crystal holes shows multiple mode operation as the
driving current increased above 5 mA (Fig. 3b). The QD VCSEL showed multiple
transverse mode characteristics over a broader wavelength span.
0.0 2.5 5.0 7.5 10.0 -60 -50 -40 -30 -20 -10 0 10
R
esp
on
se(d
B
)
Frequency(GHz)
6dBm 4dBm 2dBm 1276 1278 1280 1282 -70 -60 -50 -40 -30 -20 -10 0 Int ens it y ( 10 dB / di v ) Wavelength (nm) 4 mA free running with 4 dBm injectionD. Prof. Gong-Ru Lin
I.
Erbium-doped fiber laser
Suppression of Phase and Supermode Noises in a Harmonic Mode-Locked
Erbium-Doped Fiber Laser with a Semiconductor Optical Amplifier Based High-Pass
Filter
By operating an intra-cavity semiconductor optical amplifier (SOA) based high-pass filter
at nearly transparent current condition, the supermode noise, the relaxation oscillation, and
the single-sided-band (SSB) phase noise can be simultaneously suppressed in an actively
mode-locked erbium-doped fiber laser (EDFL). The SOA at nearly transparent condition
enhances the SMN suppression ratio of the EDFL from 32 dB to 76 dB at a cost of phase
noise degrading from -114 dBc/Hz to -104.2 dBc/Hz and a broadening pulsewidth from 36
ps to 61 ps. With an optical bandpass filter (OBPF), the SSB phase noise and the SMN
suppression ratio can further be improved to -110 dBc/Hz and 81 dB, respectively. The
EDFL pulse can further be shortened to 3.1 ps with a time-bandwidth product of 0.63 after
compressing.
Fig. 1 Schematic of QD PhC-VCSEL
Fig. 2 CW L-I-V characteristics and near-field image (inset) PhC-VCSEL
Fig. 3 Spectra of QD a PhC-VCSEL
2 0 0 4 0 0 6 0 0 8 0 0 0 1 2 3 4 5 6 -1 0 -5 0 5 1 0 -1 2 0 -1 0 0 -8 0 -6 0 -4 0 1 2 3 4 5 6 -1 2 0 -1 0 0 -8 0 -6 0 -4 0 5 6 7 -1 40 -1 30 -1 20 T im e (p s ) (d ) F re q u e n c y (M H z) (c ) Pow e r ( m W ) (b ) Powe r ( d Bm ) (a ) 36 40 44 48 52 56 60 64 68 72 76 80 -110 -108 -106 -104 -102 -100 -98 -96 Without OBPF With OBPF Current (mA) P h ase N o is e (d Bc /H z ) 60 65 70 75 80 85 S M N S u p p re s s io n Ra ti o ( d B)
Fig. 1 The experimental setup of SOA filtered EDFL. MZM:
Mach-Zehnder intensity modulator; PC: polarization controller; OC: optical coupler; OBPF: optical band-pass filter; EDFA: erbium-doped fiber amplifier; SOA: semiconductor optical amplifier.
Fig. 2 Upper: (a) SMN spectrum (measured at VBW and RBW of 300 kHz) and (b) pulse shape of mode-locked EDFL without intra-cavity SOA filter. Lower: (a) SMN spectrum and (b) pulse shape of mode-locked EDFL with intra-cavity SOA and OBPF. Inset: the SMN spectrum measured at VBW and RBW of 1 Hz.
Fig. 3 The SMN suppression ratios and the SSB phase noises of the mode-locked EDFL with SOA filter (hollow and solid triangles) or with SOA and OBPF filters (hollow and solid squares) at different SOA currents.
