A. Coherent and THz Photonics

Profs. Ci-Ling Pan, Ru-Pin Chao Pan, Jung Y. Huang, Gong-Ru Lin and Jin-Wei Hsu

Major research outcomes in this area include generation of sub-single-cycle optical pulses, adaptic coherent control, dipole antennas with detection bandwidth exceeding 30 THz, a record for ion-implanted photoconductors (OptExp’04, selected by the AIP virtual journal), first directly-modulated THz communication link for audio and burst signals (Opt Exp’05). Prof. Pans’ group also pioneered the field of Liquid Crystal THz Photonics, achieving the first room-temperature, 0-2p tunable THz phase shifter [OptExp04, selected by the AIP Virtual Journal, Taiwan Patent 200186, US patent pending], an important milestone for THz phased array applications. The work on other liquid-crystal-enabled THz functional devices such as a tunable THz Lyot filter (APL’06, Taiwan and U.S. patents pending) was highlighted by SPIE Newsroom (http://spie.org/x14608.xml). In collaboration with ITRI, the NCTU team has developed a THz System for Detecting of biological tissue burn trauma (Taiwan patent I276425, U. S. patent 7307258 B2). In collaboration with Prof. Jin-Wei Shi, Prof. Pan and co-workers have developed high-speed optical detectors and THz photonic transmitters with bandwidth beyond several hundred GHz (APL’06, PTL’07, PTL’08). In collaboration with Prof. Chi-Kuang Sun (NTU), we have reported low-loss hollow-core THz fiber wave guide [APL’08, highlighted by Nature Photonics, April 2008].  Scanning and interferometric THz fiber endoscopic imaging was also demonstrated (APL’08, OptExp’08).

1.     Ultrabroad band THz field detector based on Arsenic-ion-implanted GaAs and proton-bombarded InP (Prof. Ci-Ling Pan):

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 J. of Ultrafast Sci., August 2004].  This is an extension of our previous work on Arsenic-ion-implanted GaAs [APL 83(7)1322, 2003, selected by the Virtual J. of Ultrafast Sci., 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. A photoconductive THz Spiral Antenna fabricated on multi-Energy Arsenic-Ion-Implanted GaAs also was well-received [JAP 98:013711, 2005. Selected by the Virtual J. of Ultrafast Sci., August 2005]. Such antennas were used for the first directly-modulated THz communication link for audio and burst signals (Opt Exp 13, 10416-10423, 2005) In collaboration with ITRI, the NCTU team has developed a THz System for Detecting of biological tissue burn trauma (Taiwan patent I276425, U. S. patent 7307258 B2).

2.     Novel Photonic THz Transmitters (Profs. Jin-Wei Shi and Ci-Ling Pan)A detection bandwidth exceeding

We have investigated two types of sub-THz Photonic-Transmitters.  The first design is based on Separated-Transport-Recombination Photodiodes (STR-PD) based on low-temperature MBE-grown GaAs (LTG-GaAs) and a Micromachined Slot Antenna [PTL 19:840, 2007].  Under femtosecond optical pulse illumination, this device radiates strong electrical pulses (4.5-mW peak power) without the use of a Si-lens.  The peak power is as high as 300 mW, occuring at 500 GHz, which corresponds to the designed resonant frequency of the slot antenna. The saturation problem related to the output terahertz power that occurs with the traditional LTG-GaAs-based photonic-transmitters when operated under high external applied electrical fields ( 50 kV/cm) has been eliminated by the use of our device.

We have investigated two types of sub-THz Photonic-Transmitters.  The first design is based on Separated-Transport-Recombination Photodiodes (STR-PD) based on low-temperature MBE-grown GaAs (LTG-GaAs) and a Micromachined Slot Antenna [PTL 19:840, 2007].  Under femtosecond optical pulse illumination, this device radiates strong electrical pulses (4.5-mW peak power) without the use of a Si-lens.  The peak power is as high as 300 mW, occuring at 500 GHz, which corresponds to the designed resonant frequency of the slot antenna. The saturation problem related to the output terahertz power that occurs with the traditional LTG-GaAs-based photonic-transmitters when operated under high external applied electrical fields ( 50 kV/cm) has been eliminated by the use of our device.

 An alternative design, appropriate for wireless THz impulse-radio (IR) communication, is realized by monolithic integration of a GaAs/AlGaAs based uni-traveling-carrier (UTC) photodiode with a substrate-removed broadband antenna. The device can radiate strong sub-THz pulses (20mW peak-power) with a narrow pulse-width (<2ps) and wide bandwidth (100~250GHz).  The maximum average power emitted by our device, under the same THz time-domain spectroscopic setup, is around 10 times higher than that of the low-temperature-grown GaAs based photoconductive antenna, whilst with a much lower DC bias (9V vs. 35V). [PTL, to appear 2008].

3.     Liquid crystal THz photonics (Prof. Ci-Ling Pan and Ru-Pin Pan) 

We have pioneered this field.  The optical constants of several important liquid crystals were determined in the THz regime for the first time [Appl. Opt., 42(13): 2372, 2003 and J. Biological Phys. 29(2-3):335, 2003, J. Appl. Phys. 103: 101809, 2008, Ferroelectrics, to appear 2008]. 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, 0-2p tunable THz phase shifter [Opt. Exp. 12(12): 2625, 2004, Selected by the Virtual J. Ultrafast Sci., 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.  Recently, we also reported 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, collected by the Nanostructured Surfaces Web].  The first ever THz Lyot filter [APL 88:101107, 2006, collected by the Virtual J. of THz Sci. and Technol.], electrically switchable THz quarter-wave plate [OL 31(8):1112, 2006, collected by the Virtual J. of THz Science and Technology, OSA Virtual J. Biomed. Opt.] and electrically tunable room-temperature 2p Liquid Crystal Terahertz Phase Shifter [IEEE PTL 18(14): 1488, July 15, 2006, collected by Virtual J. of THz Sci. and Technol., July 2006] were demonstrated recently.  Our work on THz photonic elements with liquid-crystal-enabled functionalities was highlighted by SPIE Newsroom (http://spie.org/x14608.xml) in 2007. Other novel devices such as polarizers, phase gratings, Solc birefringent filters have also been demonstrated [OL, to appear 2008, Opt. Exp. 16(5):2995, 2008; OL, to appear 2008].

4.          Adaptive coherent control: Technology and Applications (Profs. Jung Y. Huang, Chuck Chao-Kuei Lee and Ci-Ling Pan)

 Emplying the freezing phase algorithm, we also investigated the enhancement of broadband THz radiation using femtosecond pulse shaping.  Over 60% radiation enhancement in output power and two-fold broadening of bandwidth were found for optimized positively chirped optical pulses.  We have tentatively attributed the phenomon to the increasing saturation fluences from competition between band-filling and pump-dump processes during excitation.  In addition, pump power dependence of THz radiation and enhancement factor, which is defined as ratio of peak amplitude of the radiated THz pulse before and after adaptive control.  With fixed probe beam power while reducing the pump power from 45mW to 5mW, we observed an increase in the enhancement factor from 40% to 60%.  A model of enhancement based on higher saturation flurence for positively-chirped optical excitation is proposed.  Other factors such as difference in absorption by leading waves in for positive or negative chirped pulse could also contribute to the observed phenomenon [CLEO’08, Opt. Exp., submitted, 2008].

5.            Femtosecond LaserAnnealing: A novel approach for dopant profile engineering and fabrication of poly-Si TFT (Prof. Ci-Ling Pan)

Amorphous silicon (a-Si) for TFT applications was crystallized by femtosecond laser annealing (FLA) using a near-infrared (800 nm) ultrafast Ti:sapphire laser system for the first time.  We found that FLA assisted by spatial scanning of laser strip spot can crystallize a-Si films with largest grains of ~800 nm, requiring laser fluence as low as ~45 mJ/cm2, and low laser shots. Moreover, the optimal annealing conditions are observed with a significant laser-fluence window (~30%) [reported at CLEO2003 as a news story; APL 85(7):1232, 2004, selected by the Virtual J. Ultrafast Sci., September 2004, ROC patent I245321]. We also demonstrated dopant profile engineering by near-infrared femtosecond laser activation [APL 88:1311104, March 27, 2006, selected by Virtual J. of Nanoscale Sci. and Technology, Vol. 13, No. 14, April 10, 2006 and Virtual Journal of Ultrafast Science, Vol. 5, No. 4, April 2006]. Preamorphizing implantation is not required. We find dopant profiles in FLA-activated samples essentially duplicate those of as-implanted ones even for junctions as deep as 100 nm below the surface. Laser-recrystallized material was used successfully for fabricating thin film transitors [Opt. Exp., 15: 6981, 2007, selected by Virtual J. of Ultrafast Sci., July 2007]. THz spectroscopic techniques were employed for diagnostics of the fs-laser-annealed poly-Si material [Photonics Asia, invited talk, 2007, Opt. Exp. Submitted, 2008].  It is shown that The transient mobilities of poly-Si with large (~ 500 nm) and small (~ 50 nm) grain sizes, fitted by the Drude model, are 175.0±19.4 cm2/V s and 94.5±20.2 cm2/V s, respectively. We proposed that higher mobility in large-grain poly-Si by femtosecond laser annealing is due to reduction of deep state density rather than tail state density.

6.            Tunable Lasers and Electro-Optic Devices for DWDM and Attosecond Generation with Liquid Crystal (LC) Enabled Functionalities and other applications (Profs. Ci-Ling Pan, Ru-Pin Pan, Andy H. Kung):

A specially designed SLM was developed and used in frequency synthesis of attosecond pulses, in collaboration with Prof. Andy H. Kung [Phys. Rev. Lett. 100: 163906, 2008].  Using 7 Raman sidebands generated by molecular modulation in H₂, we achievd the synthesis of periodic waveforms consisting of a train of pulses that are 0.83 cycles long and have an electric field pulse width of 0.44 fs.

The SLM composed of a row of five 14 mm high by 4 mm wide by 0.022 mm thick liquid-crystal panels. The size and location of each panel is designed to match the sideband beam size and to allow unimpeded passage of five sidebands. With this new modulator, a total of 7 sidebands can now be employed for waveform synthesis. The total bandwidth is 24931.2 cm^-1 or 2 octaves. We verify by optical correlation that the carrier-envelope phase is constant in these waveforms when they are synthesized from commensurate sidebands. The estimated overall shift of the carrier-envelope phase is less than 0.18 cycles from the first to the last pulse of nearly 10⁶ pulses in the pulse train.

7.            Generation of coherent mid- and far- infrared radiation in GaSe (Profs. Ci-Ling Pan and Jung Y. Huang)

A table-top infrared light source with high intensity and wide tunability is constructed by use of difference-frequency-mixing in GaSe nonlinear optical crystal. Tuning wavelengths from 2.4μm to 28μm are obtained with highest energy output ~13μJ at 3.5μm. The output characteristics are compared among pure and erbium doped crystals. Second-order nonlinear coefficient d_eff⁽²⁾ of the Er:GaSe crystals reveal a d_eff⁽²⁾ of 55.3 pm/V, which is about 24% larger than that of pure GaSe. The improvement of d_eff⁽²⁾ can be attributed to the substitutive and interstitial doping of Er ion in GaSe unit cell. [Opt. Exp. 14:5484, 2006, selected by Virtual J. of Ultrafast Sci., August 2006) and Virtual J. of Biomed. Opt.]. We also report a study of the effect of optical absorption on generation of coherent infrared radiation from mid-IR to THz region from GaSe crystal. The infrared-active modes of e-GaSe crystal at 236 cm^-1 and 214 cm^-1 were found to be responsible for the observed optical dispersion and infrared absorption edge. Based upon phase matching characteristics of GaSe for difference-frequency generation (DFG), new Sellmeier equations of GaSe were proposed. The output THz power variation with wavelength can be properly explained with a decrease of parametric gain and the spectral profile of absorption coefficient of GaSe. The adverse effect of infrared absorption on (DFG) process can partially be compensated by doping GaSe crystal with erbium ions. [Opt. Exp. 14:10636, 2006, selected by Virtual J. of Ultrafast Sci., January 2007, listed in Virtual J. of THz Sci. and Technol., October 2006].  Recently, we proposed and demonstrated coherent generation and spectral synthesis of terahertz radiation with multiple stages of optical rectification [Opt. Exp., submitted, 2008]. This approach can potentially be useful for the generation of single-cycle high-amplitude terahertz pulses, which is currently limited by the pulse walk-off effect from group velocity mismatch.

8.            Nonlinear optical studies of Silicon nanocrystals and Nano-Silicon-based optoelectronics (Profs. Jung Y. Huang, Ci-Ling Pan, and Dr. Jia-Min Shieh)

A novel material of Si nanocrystals embedded in a three-dimensional array of mesoporous silica matrix has been studied by nonlinear optical techniques. We report sum-frequency generation spectroscopic studies of Si-O polar nanostructures embedded in a three dimensional array of mesoporous silica (MS) matrix by use of different frequency combinations with picosecond and femtosecond configurations. Such unique electronic structure of Si nanocrystals (nc-Si) embedded in SiO₂ is opening up wide applications to flash memory and photonic devices. The effective second-order nonlinear coefficient and Curie temperature of nc-Si are determined by surface sum frequency generation spectroscopy. A resonance feature around 480 nm was observed. The effective second-order nonlinear coefficient is estimated to be d_eff =3.7 pm/V. Nonlinearity is tentatively attributed to Si-O nanostructures in this novel material.  The effect of heating and cooling cycle on SFG signals provides evidence of ferroelectricity for nc-Si embedded mesoporous silica. The Curie temperature of the material is estimated to be 567K.

A two-terminal metal-oxide-semiconductor photodetector for which light is absorbed in the nano-Si material described above as a capping layer on p-type silicon substrates was fabricated.  Operated at reverse bias, enhanced photoresponse from 300 to 700 nm was observed. The highest optoelectronic conversion efficiency is as high as 200%.  The enhancements were explained by a transistorlike mechanism, in which the inversion layer acts as the emitter and trapped positive charges in the mesoporous dielectric layer assist carrier injection from the inversion layer to the contact, such that the primary photocurrent could be amplified [APL 90: 051105 2007, selected by Virtural J. Nanoscale Sci. & Technol.2007].  This paper was at one time among the top 20 most downloaded papers for the APL issue..

9.            Femtosecond Fiber Lasers and Applications (Profs. Gong-Ru Lin and Ci-Ling Pan)

   9-1  Self-Steepening of Prechirped Amplified and Compressed 29-fs Fiber Laser Pulse in Large-Mode-Area Erbium-Doped Fiber Amplifier

Prechirped amplification, soliton compression, and self-pulse-steepening of a 300-fs stretch-pulse mode-locked erbium-doped fiber laser (EDFL) pulse in an ultrashort length large-mode-area erbium-doped fiber amplifier (LMA-EDFA) and large-effect-area fiber (LEAF) link are investigated. In situ amplified compression of the single-mode-fiber prechirped EDFL pulse (broadened to 1.2 ps) is initiated in the LMA-EDFA at a pumping power of > 160 mW, which provides a 20-fold pulsewidth compressing ratio for the incoming EDFL pulse and supports a maximum output power of > 20 dBm. With an extremely short LEAF-based fifth-order soliton stage, the amplified EDFL pulse can further be compressed down to a pulsewidth of 29 fs, which gives rise to a total pulsewidth-compressing ratio of as high as 40. The LMA-EDFA-based prechirped and amplified soliton compression leaves a small pedestal on the EDFL pulse with an energy confinement ratio of 74%, providing a 20-dB magnified pulse energy of 2.3 nJ and a 10-dB spectral linewidth of 150 nm.  The self-steepening-induced blue-side spectral stretch by 1.3 THz is elucidated.

9-2  Dynamic chirp control of all-optical format-converted pulsed data from a multi-wavelength inverse-optical-comb injected semiconductor optical amplifier

By spectrally and temporally reshaping the gain-window of a traveling-wave semiconductor optical amplifier (TWSOA) with a backward injected multi- or single-wavelength inverse-optical-comb, we theoretically and experimentally investigate the dynamic frequency chirp of the all-optical 10GBit/s Return-to-Zero (RZ) data-stream format-converted from the TWSOA under strong cross-gain depletion scheme. The multi-wavelength inverse-optical-comb injection effectively depletes the TWSOA gain spectrally and temporally, remaining a narrow gain-window and a reduced spectral linewidth and provide a converted RZ data with a smaller peak-to-peak frequency chirp of 6.7 GHz. Even at high inverse-optical-comb injection power and highly biased current condition for improving the operational bit-rate, the chirp of the multi-wavelength-injection converted RZ pulse is still 2.1-GHz smaller than that obtained by using single-wavelength injection at a cost of slight pulsewidth broadening by 1 ps.

9-3  All-Optical Decision-Gating of 10-Gb/s RZ Data in a Semiconductor Optical Amplifier Temporally Gain-Shaped With Dark-Optical-Comb

We demonstrate a novel all-optical noninverted OC-192 return-to-zero (RZ) decision-gate by using a semiconductor optical amplifier (SOA) which is gain-controlled to achieve an extremely high cross-gain-modulation depth and a narrow gain window. A dark-optical-comb generated by reshaping the optical clock RZ data in a Mach–Zehnder intensity modulator is employed as an injecting source to temporally deplete most of the gain in the SOA. Such a dark-optical-comb injected SOA decision-gate exhibits improved 3R regeneration performances such as a timing tolerance of 33.5 ps, a Q-factor of 8.1, an input dynamical tolerance of 14 dB, and an extinction ratio (ER) of 14 dB. The deviation between the wavelengths of backward injected dark-optical-comb and input RZ data for optimizing the ER of the decision-gate is determined as Δλ = 19 nm. Under a threshold operating dark-optical-comb power of 7 dBm, such a decision-gate can recover the −18.5-dBm degraded RZ data with a bit-error-rate of less than 10−9 at 10 Gb/s. A negative power penalty of −4.2 dB is demonstrated for the RZ data after 50-km propagation and decision gating.

9-4  Simultaneous pulse amplification and compression in all-fiber-integrated pre-chirped large-mode-area Er-doped fiber amplifier

A large-mode-area Erbium-doped fiber amplifier (LMA-EDFA) based all-fiber-integrated amplified compressor with ultrashort length of 5.37 m and ultralow pumping power (260 mW) is proposed. The LMAEDFA suppresses nonlinear soliton-self-frequency-shift effect happened during femtosecond pulse amplification, in which the fiber laser pulse is reshaped to a low-pedestal hyperbolic-second shape with nearly 100% energy confinement. The pre-chirped amplification from 0.96 to 104 mW and the simultaneous compression of a passively mode-locked fiber laser pulse from 300 to 56 fs is demonstrated. The input pulse energy of 24 pJ is amplified up to 2.6 nJ with shortened pulsewidth of 56 fs and peak power as high as 46 kW.

9-5  Femtosecond mode-locked Erbium-doped fiber ring laser with intra-cavity loss controlled full L-band wavelength tunability

By using a tunable-ratio optical coupler (TROC) to adjust the wavelength dependent intra-cavity loss, a L-band mode-locked erbium-doped fiber-ring laser (ML-EDFL) is demonstrated for generating wavelength-tunable femtosecond pulses. The change of output coupling ratio introduces different intra-cavity loss and shifts the peak of mode-locked gain profile to provide continuous detuning on wavelength of the ML-EDFL. A maximum tuning range of about 40 nm (from 1565.1 to 1605.3 nm) by decreasing the output coupling ratio from 95% to 5% is obtained, corresponding to a wavelength tuning slope of 2.25 nm/dB. The ML-EDFL exhibits a super-mode suppressing ratio as high as 47 dB and a pulsewidth of <5 ps at repetition frequency of 1 GHz. Nearly transform-limited pulsewidth of 580 fs is generated by linear dispersion compressing the EDFL pulses with a 32.5m-long single-mode fiber under an output coupling ratio of 10%.

10. GaN-based Vertical Cavity Surface Emitting Laser and Light Emitting Diodes

（Prof. Hao-Chung Kuo and Tien-Chang Lu）

10-1. Study of high reflectivity mirror for blue high quality light emitter

In this part, we develop the high reflectivity epitaxially grown nitride mirror, usually in the form of distributed Bragg reflector (DBR), using MOCVD epitaxy technique. The nitride material system usually has a serious strain problem for the epitaxy of such multi-film structure. Therefore, the fabrication of high reflectivity mirror for blue light emitter is a difficult topic. In this study, we have developed a solution for the epitaxy of high-reflectivity reflector.

A crack-free GaN/AlN DBR incorporated with GaN/AlN superlattice (SL) layers was successfully grown on a c-plane sapphire substrate (Figure 1(a)). We inserted three sets of half-wave layers consisting of 5.5 periods of GaN/AlN SL layers and GaN layer in every five pairs of the 20 pair GaN/AlN DBR structure to suppress the crack generation. The grown GaN/AlN DBRs with SL insertion layers showed no observable cracks in the structure and achieved high peak reflectivity of 97% at 399 nm with a stop band width of 14 nm(Figure 1(b)). Based on the x-ray analysis (Figure 1(c)), the reduction in the in-plane tensile stress in the DBR structure with insertion of SL layers could be responsible for the suppression of crack formation and achievement of high reflectivity.

Figure 1 (a) The TEM image of GaN/AlN DBR; (b) The reflectivity spectrum of DBRs with and without superlattice; (c) the Reciprocal space maps of non-SL and SL samples.

10-2. Emission characteristics of optically pumped GaN-based vertical-cavity surface-emitting lasers

The laser emission characteristics of a GaN-based vertical-cavity surface-emitting laser with two dielectric distributed Bragg reflectors were investigated under optically pumped operation at room temperature. The laser emitted wavelength at 415.9 nm with an emission linewidth of 0.25 nm and threshold pumping energy of 270 nJ. The laser has a high characteristic temperature of about 278 K and high spontaneous emission coupling factor of 10−2. The laser emission showed single and multiple spot emission patterns with spectral and spatial variations under different pumping conditions.

Figure 2 (a) Schematic setup of pumping and μ-PL scanning. (b) Emission pattern of the VCSEL at pumping energy of 1.15 Eth with single laser emission spot and 1.12 Eth with two laser spots. The arrows indicate the position of the first and second emission spots. Emission spectrum at pumping energy of 1.15 Eth and 1.12 Eth, respectively. (c) PL spectra of bright (point A) and dark (point B) areas. (d) Laser emission intensity versus pumping energy in semilogarithmic scale. The b value estimated from the difference between the two dash lines is about 2x10-2. The inset shoes he spectrum of the laser emission with a wavelength of 415.9nm.

10-3. Study of characteristics of GaN vertical cavity surface emitting laser (VCSEL)

Following the success of laser action of GaN VCSEL using optical pumping, we further investigated the characteristics and performance of GaN blue VCSEL. The structure of GaN VCSEL is formed by a 3λ cavity sandwiched by a 25 pairs AlN/GaN distributed Bragg reflector (DBR) and an eight pairs Ta2O5–SiO2 DBR (Figure 3(a)). The pumping condition could be monitored by a CCD. The near field image was shown in figure 3(b) and laser occurred in the form of spot emission at the center of pumping area. The GaN VCSEL emits a blue wavelength at 448 nm with a linewidth of 0.17 nm (Figure 3(c)) with a near-field emission spot diameter of about 3μm. The laser beam has a near linear polarization with a degree of polarization of about 84%. The laser shows a high spontaneous emission coupling efficiency (β) of about  5×10-2 (Figure 3(d)) and a high characteristic temperature of about 244 K. The high beta value also implies the thresholdless laser for the nitride material system is highly possible.

10-4 Successfully fabricated low-temperature electrical pumping InGaN-MQW VCSELs by hybrid mirrors

The GaN-based VCSEL structure was grown by MOCVD (EMCORE D-75). We use the polished c-face (0001) 2-inch-diameter sapphire as a substrate for the epitaxial growth. The VCSEL structure composed of a 5λ cavity, a 29 pairs AlN/GaN DBR as bottom mirror and an eight pairs Ta₂O₅/SiO₂ dielectric mirror as the top DBR reflector. By using lithograph technology, etching by RIE and deposed contact metals on the substrate, we can successfully fabricate low-temperature electrical pumping InGaN-MQW VCSELs by hybrid mirrors. The schematic diagram of the full structure is shown in figure 4(a).

Figure 4(b) shows the photoluminescence emission intensity as a function of wavelength at low temperature condition (77K). From the PL spectrum, we can find the center wavelength at 465nm and a distinct narrow linewidth of the peak nearluy 5.2Å which can be calculated the cavity quality factor (Q) about 894. Figure 4(c) shows the variation of injection current with the voltage and pumping energy.

Figure 3 (a) The schematic diagram, (b) The near field image, (c) The threshold characteristics, and (d) The beta performance of of GaN VCSEL under optical pumping

10-4 Successfully fabricated low-temperature electrical pumping InGaN-MQW VCSELs by hybrid mirrors

The GaN-based VCSEL structure was grown by MOCVD (EMCORE D-75). We use the polished c-face (0001) 2-inch-diameter sapphire as a substrate for the epitaxial growth. The VCSEL structure composed of a 5λ cavity, a 29 pairs AlN/GaN DBR as bottom mirror and an eight pairs Ta₂O₅/SiO₂ dielectric mirror as the top DBR reflector. By using lithograph technology, etching by RIE and deposed contact metals on the substrate, we can successfully fabricate low-temperature electrical pumping InGaN-MQW VCSELs by hybrid mirrors. The schematic diagram of the full structure is shown in figure 4(a).

Figure 4(b) shows the photoluminescence emission intensity as a function of wavelength at low temperature condition (77K). From the PL spectrum, we can find the center wavelength at 465nm and a distinct narrow linewidth of the peak nearluy 5.2Å which can be calculated the cavity quality factor (Q) about 894. Figure 4(c) shows the variation of injection current with the voltage and pumping energy.

Fig. 4(a). The schematic diagram of the electrical pumping VCSEL structure. (b) The PL spectrum of the structure has the center wavelength 465nm and a narrow linewidth 5.2A. (c) LIV curves of the VCSEL structure has lower turn-on voltages 3.4V.

10-5 Successfully achieved the Lasing Action of GaN-based Two Dimensional Surface-emitting Photonic Crystal Laser

    The nitride heterostructure in this experiment was grown by the metal-organic chemical vapor deposition (MOCVD) system on sapphire substrate. The epitaxial structure consists of a 25 pairs AlN/GaN DBR and a 5λ cavity. The 2D PCSEL was fabricated by following steps. First, 200 nm Si3N4 film was deposited as a hard mask using PECVD and spun PMMA by spinner which was patterned using an e-beam lithography. The lattice constants of PCs were in the range between 180 nm and 300 nm. The diameter of each device was 50 μm. Second, the sample was performed a dry etching in an ICP-RIE system to etch GaN as deep as 400 nm. Finally, the sample was dipped in BOE to remove the hard mask to complete 2D PCSEL. Figure 5(a) and (b) show the schematic diagram of our 2D PCSEL, and the SEM image of fabricated 2D PCSEL in top view, respectively.

   The threshold characteristics of PCSEL were also measured. Taking one of them for example (a = 290 nm), the laser emission intensity from the PCSEL as a function of the exciting energy density is shown in figure 5(c). The threshold energy density (E_th) was observed to be around 3.5 mJ/cm². The light intensity increased rapidly and linearly as the excitation energy density was above the threshold. Figure 5(d) shows the lasing spectra at different pumping energy. A sharp and narrow laser emission was then clearly observed as the pumping energy increased above the threshold energy. The lasing wavelength located at 424.3 nm, and the FWHM of the laser is around 0.11 nm. Other devices also could be observed the lasing actions occur at the similar threshold energy but different lasing wavelength.

Figure 5 (a) The schematic diagram of the overall photonic crystal surface emitting laser structure. (b) The SEM image of the full structure in top view. (c) The light output intensity as a function of the pumping energy density at room temperature. The threshold energy density was about 3.5 mJ/cm². (d) The variation of the laser emission spectrum with increasing the pumping energy. The laser emission wavelength is 424.3nm with a linewidth of about 0.11nm

10-6. High Light-Extraction GaN-based Vertical LEDs With Double Diffuse Surfaces

We have demonstrated the high light-extraction (external quantum efficiency ~40%) 465-nm GaN-based vertical light-emitting diodes (LEDs) employing double diffuse surfaces. The high scattering efficiency of double diffused surfaces could be responsible for the high light output power. A schematic cross-section image of a GaN-based LED with double diffuse surfaces is shown in Figure 6(a) and (b) shows the light output power (L-I curve) of sample A, sample B and conventional LEDs. The sample B, the LED with double diffuse surfaces, and sample A, the LED with flat omnidirectional reflectors, produced much higher light output as compared with that of conventional LEDs under all our measurement condition. The calculated external quantum efficiency of our proposal LEDs with double diffuse surfaces is about 40% at 20mA (l~465 nm), which could compete with structures of state of the art.

Figure 6 (c) and (d) shows the cross-sectional transmission electron microscope (TEM) images of sample A and sample B, respectively. In Fig. 6(c), the top surface of p-type GaN was quite flat, as can be seen in conventional LEDs; however, lots of hexagonal V-shape pits was observed on p-type GaN surface of sample B, as shown in Fig. 6(d). Fig. 6(e) is an enlarged TEM image of one hexagonal V-shape pit. As can be seen in this figure, the hexagonal V-shape pit originated from threading dislocations and there is a thick dark band along the V-groove, being indicative of thickness variation.

Figure 6 (a) Schematic cross section of a GaN-based LED with double diffuse surfaces. (b) Output power of sample A, sample B, and conventional LEDs measured by an integral-sphere as a function of a forward current. Cross-sectional transmission electron microscope (TEM) images of (c) flat p-GaN surface (sample A) and (d) hexagonal V-shape roughened p-GaN surface (sample B). (e) is an enlarged TEM image of one hexagonal V-shape pit.

10 -7. Fabrication and Characterization of GaN-based LEDs Grown on Chemical Wet-etched Patterned Sapphire Substrates (CWE-PSS)

Characteristics of GaN-based LEDs grown on patterned sapphire substrate fabricated by the chemical wet etching were specifically analyzed. By chemical wet etching, the sapphire substrate exhibited a particular crystallography-etched facet of {1-102} R-plane with an inclined slope as large as 57o, facilitating a significant enhancement of the light extraction efficiency. An improvement of epitaxial quality was also achieved on CWE-PSS LEDs, according to device reliability testing results.

Fig. 7(a) schematically depicts the GaN-based LED grown on the CWE-PSS and the corresponding SEM micrograph of LED full structure is presented in Fig. 7(b). For fabricating the CWE-PSS, the SiO2 film with hole-patterns of 3-μm-diameter and 3-μm-spacing was deposited onto the sapphire substrate to serve as wet etching masks. The sapphire substrate was then wet etched using an H3PO4-based solution at an etching temperature of 300 oC. Fig.7 (c) and (d) show top and cross-section side views SEM images of the pattern sapphire substrate of etching time of 90s Fig.7 (e) and (f) show the evolution of CWE-PSS with the increase of sapphire etching time. With the increasing of the etching time, the total area of C-plane will decrease due to its relative faster etching rate than R-plane. Fig.7 (g) shows the measurement results of output power (L-I curves) of CWE-PSS LEDs with different sapphire etching times. According to this figure, the optimized CWE-PSS condition was achieved on the etching time of 90s, corresponding to an enhanced factor of 1.4. Better reliability characteristics were also observed on the CWE-PSS LEDs, as shown in Fig.7 (h).

Figure 7 (a) The schematic drawing of the device structure. (b) Cross- sectional side-view SEM images of the CWE-PSS LEDs Structure. (c)(d) The SEM images of the top and cross-section side views. (e)A top-view drawing depicts the evolution of the increasing etching time. (f)A schematic ray-tracing with increasing sapphire etching time. (g) Output power measurement and CWE-PSS LEDs. (h) Reliability test of the conventional and CWE-PSS LEDs under stress condition of 55℃ and 50 mA.

10-8. Fabrication of InGaN/GaN MQW Nanorods LED by ICP-RIE and PEC Oxidation Process with Self-Assembly Ni Metal Islands

We successful fabricated the InGaN/GaN MQW nanorods LED using Ni nano-masks, ICP-RIE etching and PEC oxidation process. The PEC oxidation process can produces better oxidation layer surrounding nanorod to isolate nanorods to electric pumping. A transparent contact layer was deposited to form a connection with the exposed p-type of individual nanorod. We estimate the mean dimension and density of the InGaN/GaN MQW nanorods LED as shown in Fig. 8(a) which shows the SEM images of InGaN/GaN MQW nanorods LED after ICP-RIE etching. The SEM image of in Fig. 8(b) shows the Ni/Au contact metal deposited on InGaN/GaN MQW nanorods LED after PEC oxidation process.

Fig. 8(c) shows the normalized PL intensity spectrum of the as-grown LED and nanorods LED with/without PEC. An enhancement by a factor of 6/5 times in photoluminescence intensities of nanorods with/without PEC process compared to that of as-grown structure. The peak wavelength observed from PL measurement shows a blue shift of 3.8 nm of the nanorods without PEC oxidation process and 8.6 nm of the nanorods with PEC oxidation process from that of the as-grown LED sample. The blue shift maybe is attributed to strain relaxation in the well for nanorods LED and intensity enhanced by scattering effect. The Fig. 8(d) shows the normalized EL spectrum of the as-grown LED and nanorods LED samples with PEC process at an injection current of 1mA. The EL spectrum shows 10.5 nm blue-shift of the nanorods with PEC from that of the as-grown LED sample.

Figure 8 The SEM images of (a) InGaN/GaN MQW nanorods LED after ICP-RIE etching. (b) InGaN/ GaN MQW nanorods LED after deposited contact metal. (c) Normalized PL intensity spectra for as-grown LED and nanorods LED with/without PEC at room temperature. (d) Normalized EL intensity spectra for as-grown LED and nanorods LED with PEC at room temperature.

10-9. Study of high Q micro-cavity light emitting diode (MCLED)

In this part, we mainly develop the micro-cavity light emitting devices with high quality factor. The fabricated structre of the high Q GaN-based micro-cavity light emitter is shown in figure 9(a). It also has a similar structure with the GaN VCSEL which is consist of a 25-pairs high-reflectivity AlN/GaN DBR (R = 98%), a 3λ InGaN/GaN active pn-junction region and an 8-pairs SiO2/Ta2O5 DBR (R = 99%). The MCLED shows that the emission intensity superlinearly increased with a very narrow linewidth of 0.52 nm equivalents to cavity Q value of 895 at driving current of 10 mA and a dominant emission peak wavelength at 465.3 nm (Figure 9(c). The quality factor is the best value compared to those previously published value. Moreover, the MCLED also shows an invariant emission peak wavelength with the varying current (Figure 9(d)). It means the photon emission could be highly control using this structure. The results should be promising for developing a number of high performance light emitters, including GaN-based VCSELs.

Figure 9 (a) The schematic diagram, (b) L-I-V curves, (c) The emission spectra, and (d) The variation of wavelength of GaN MCLED.

10-10. High-Performance GaN-based vertical-injection light-emitting diodes with TiO₂–SiO₂ Omnidirectional reflector and n-GaN roughness

We have designed and fabricated a new type of GaN-based thin-film vertical-injection light-emitting diode (LED) with TiO₂–SiO₂ omnidirectional reflector (ODR) and n-GaN roughness. The associated ODR designed for LED operation wavelength at 455 nm was integrated with patterned conducting channels for the purpose of vertical current spreading. With the help of laser lift-off and photo-electrochemical etching technologies, at a driving current of 350 mA and with chip size of 1 mm × 1 mm, the light–output power and the external quantum efficiency of our thin-film LED with TiO₂–SiO₂ ODR reached 330 mW and 26.7%. The result demonstrated 18% power enhancement when compared with the results from the thin-film LED with Al reflector replace.

Figure 10 Schematic diagram of a VLED structure (a) with Al mirror and roughness (b) TiO2–SiO₂ ODR and roughness. Inset in (a) shows the SEM image of surface roughness with PEC process. (c) I–V and (d) intensity–current (L–I) and EQE versus forward dc current for the LED with TiO2–SiO2 ODR and roughness, and for the LED with Al reflector and roughness fabricated in this letter. The inset shows the room-temperature EL spectrums at a driving current of 350 mA.

10-11. Trenched epitaxial lateral overgrowth of fast coalesced a-plane GaN with low dislocation density

We have grown high quality and fully coalesced a-plane GaN films at the thickness of 10 μm by using trenched epitaxial lateral overgrowth (TELOG) with a 2 μm seed/18 μm trench stripe pattern. Fig.1 shows the results of x-ray measurement. The FWHMs of x-ray rocking curves along (0001) c and (1-100) m directions were reduced from 973 to 385 arc sec and from 1811 to 260 arc sec, respectively, demonstrating the improvement of the crystal quality and the mitigation of the anisotropic in-plane strains between different crystal axes by TELOG. According to the results of μ-PL and TEM, the TDD can be reduced largely from 1×1010 to 3×107 cm−2 for the N-face GaN wing, which was shown in Fig.2. The Ga-face GaN could be much easily influenced by the thin GaN layer grown on the bottom of trench, indicating that a narrower stripped GaN seeds and deeper trench etched into the surface of sapphire can derive a better quality a-plane TELOG GaN film for the most of the area.

Figure 11 X-ray rocking curves of as-grown and TELOG a-plane GaN films (a) along (0001) direction (b) along (1-100) direction.(c) Top view μ-PL image of TELOG a-plane GaN film. (d) and (e) Cross-sectional TEM g=(0002) and g=(11-20) two beam bright field images.

10-12. InGaN/GaN nanostripe grown on pattern sapphire by metal organic chemical vapor deposition

We have used MOCVD to fabricate InGaN/GaN MQWs nano-stripes on trapezoidally patterned sapphire substrates. A series of special relations and planes of crystallization were defined by diffraction pattern analysis and TEM observations, which was shown as Fig. 12. The nano-stripe structures existed on top of the trapezoid pattern of the sapphire substrate in zone I. In the TEM images, the MQW structures appeared only in zones I and III. No MQW structures were detected from TEM observation of zone II, indicating that the growth direction might occur only toward the top and lateral facets of the trapezoidally patterned sapphire. Fig. 12(f) shows the results of μ-PL experiments which indicate that the intensity of the luminescence from the MQWs embedded in the nano-stripe structure was enhanced up to fivefold relative to those of regular thin film MQWs, probably as a result of much-improved internal and external quantum efficiency. Meanwhile, dislocations that stretched from the interfaces between the GaN and the substrates did not pass through the MQWs in the TEM observation (Fig. 12(g)). Therefore, these MQW nano-stripe arrays are capable of enhancing luminescence and appear to be suitable for application to the fabrication of high-efficiency light-emitting devices.

Figure 12 (a) Cross-sectional TEM image of the trapezoid structure. The crystalline orientation between GaN and sapphire in zone Ⅲ was defined by diffraction pattern as shown in the inset. (b)(c) High resolution TEM images with different magnifications of the MQWs in zone Ⅰ. (d)(e) High resolution TEM images with different magnifications of the MQWs in zone Ⅲ. (f) PL spectra of zone Ⅰ, zone Ⅲ and a conventional thin film 3 pairs MQWs. (g) The cross-section dark-field TEM images of the nano-stripe.

10-13. Enhancement of flip-chip light-emitting diodes with Omnidirectional reflector and textured micropillar arrays

The flip-chip light-emitting diodes (FC-LEDs) with a conductive omni-directional reflector and textured micropillar arrays were investigated. The micropillar arrays structure was formed on the bottom side of sapphire substrate by dry etching process to increase the light-extraction efficiency. Fig. 12(a) shows the schematic diagram of FC-LEDs structure with micropillar arrays surface. The surface morphology of the FC-LED with different etching condition sapphire surface was examined by scanning electron microscope as shown in Fig. 12(b). The corresponding current–voltage (I–V ) characteristics of flat surface FC-LEDs were also measured, respectively, as shown in Fig.12(c). The light output power of the FC-LED was increased by 65% for a 3.2-μm textured micropillar on the bottom side of the sapphire substrate as shown in Fig. 12(d). Our work offers promising potential for enhancing output powers of commercial light-emitting devices.

10-14 Enhancement of light output intensity by integrating ZnO nanorod arrays on GaN-based LLO vertical LEDs

    Enhancement of light output intensity for GaN-based vertical light-emitting diodes, combining wafer bonding and the laser lift-off (LLO) process, employing an omnidirectional extraction surface with synthesized single-crystal ZnO nanorod arrays in aqueous solution at room temperature is presented. Figure 13(a) to 13(d) shows the FESEM images of the synthesized ZnO nanorods on different surfaces. Figure 13(e) shows the current to voltage (I-V) curve of the VLED with and without ZnO nanorods. The light output intensity and wall-plug efficiency of the GaN-based LLO vertical LED with the omnidirectional extraction surface by ZnO nanorod arrays shows 38.9 and 41.2% increases, respectively, at 200 mA current injections compared to that of a vertical LED without ZnO nanorod arrays as shown in figure 13(f). The ZnO nanorod arrays not only support a current spreading layer but enhance the probability of photon escape through the omnidirectional extraction surface.

Figure 13. FESEM images of the GaN-based LEDs with ZnO nanorod arrays: (a) cross-sectional image of the synthesized ZnO nanorod arrays, (b) images of the n-GaN surface, (c) images of the bonding pad metal surface, and (d) images of the passivation SiO2 surface. (e) I-V and (f) L-I and WPE vs forward dc current for the GaN-based LLO LED with ZnO nanorod arrays and that without ZnO nanorod arrays fabricated in this letter.

11.  ctagonal Quasi-Photonic Crystal Nanocavity Lasers with Side-Mode Reduction and Condensed Device Size :（Po-Tsung Lee）

          We first propose a brand new single-defect nanocavity by using octagonal (8-fold) quasi-periodic photonic crystal (QPC) lattice. Both in finite-difference time-domain (FDTD) simulations and experimental measurements, we successfully confirm the resonance and lasing of WG mode with azimuthal number four in this nanocavity.

          In numerical simulations, we also identify all resonance modes in the nanocavity. We find that resonance modes are far away from each other in frequency. This is an advantageous property for reducing the influence of side modes in the nanocavity, which is better than other reported photonic crystal single-defect nanocavities.

          Due to the central zero-field distribution of WG mode profile, we successfully reduce the side mode by inserting a central air hole in the nanocavity without affecting the WG mode resonance. The side-mode suppression-ratio (SMSR) is increased up to larger than 30dB. We also investigate and discuss the possibility of electrical-driven structure based on this WG mode.

          Due to the isotropic photonic bandgap (PBG) effect of octagonal QPC lattice, we successfully obtain the WG mode lasing actions with very condensed device size of 3.5 μm × 3.5 μm and low effective threshold power of 0.2 mW. This indicates that this device can be easily integrated into PICs without affecting other integrated devices, which is a very important property.

Fig. 1: (Left) The scheme of single-defect nanocavity design and the scanning-electron microscope (SEM) pictures of fabricated devices. (Right) The WG mode lasing actions and its side mode reduction after inserting perturbation in the central of the nanocavity.

12.  High Quality Factor Dodecagonal QPC Microcavity Laser and Its Strong Mode Dependence : （Po-Tsung Lee）

          We propose and design a microcavity based on 12-fold QPC lattice with well-sustained WG mode and without any cavity modification, which is very different from octagonal QPC single-defect nanocavity.

          In numerical simulations, we successfully obtain the well-sustained WG mode with azimuthal number six and identify all resonance modes in the microcavity.

          We obtain a very high measured quality (Q) factor of 10,000 from well-fabricated devices.

          By randomly varying the outer and inner-most lattice positions of the microcavity, we propose and confirm a strong WG mode dependence on nearest air holes in theory and experiments. This is a very important conclusion and provides us the concept for our following researches to enhance WG mode in ordinary photonic crystal microcavities.

Fig. 2: (Left) The scheme of 12-fold QPC microcavity with WG mode and the simulated resonance modes properties. (Right) The SEM pictures of fabricated devices and its strong mode dependence under different cavity boundary conditions.

13.  Circular Photonic Crystal with Isotropic PBG Effect and Its High Q Microcavity Laser : （Po-Tsung Lee）

          We also investigate a novel QPC lattice structure named circular photonic crystal (CPC). By using FDTD simulations, we successfully confirm the better PBG isotropy compared with that of ordinary photonic crystals, including the variations of PBG width and boundary in different lattice directions.

          Using this isotropic PBG effect, we design a CPC microcavity with WG mode and high Q factor. In experiments, we successfully obtain measured Q factor as high as 11,000 and ultra-low effective threshold power smaller than 20 μW. It is worth to note that this is the highest Q factor ever reported in photonic-crystal-based microcavity by using multiple quantum wells (MQWs). Thus, the published results have been selected for Virtual Journal of Nanoscale Science & Technology in the issue of Apr. 30, 2007.

Fig. 3: (Left) The scheme of circular photonic crystal and its isotropic photonic bandgap effect. (Right) The SEM pictures of fabricated devices and its lasing action with high Q factor of 11,000.

14.  Enhanced WG Mode in Photonic Crystal Circular-Shaped Microcavity and Its Uniform Coupling Properties in Cavity-Waveguide System : （Po-Tsung Lee）

           We first propose the concept of combining topology and micro-gear lasers to enhance the WG mode in a photonic crystal microcavity by modifying the cavity boundary by repositioning the 12 nearest air-holes around the cavity.

           In numerical simulations, we successfully confirm and obtain the existence of enhanced WG mode. We also obtain a high measured Q factor of 7700 from well-fabricated devices.

           Both in simulations and experiments, we first investigate the uniform photonic crystal cavity-waveguide coupling property due to the presence of WG mode. This provides a promising solution for serious non-uniform cavity-waveguide coupling problems in most photonic crystal nano- and micro-cavities.

Fig. 4: (Left) The scheme of photonic crystal circular-shaped microcavity and its enhanced WG mode. (Right) The uniform coupling properties both in simulations and experiments are obtained when combining the cavity with external waveguides.

11.  Volume Holographic Data Storage（Profs. Ken Yuh Hsu and 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 (~Tbits/in²). During the forth year of project, we have investigated on the optimization of our doped PMMA photopolymers. In the holographic data storage experiments, we have fabricated a 5-inch diameter photopolymer 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. We have written ~57 holograms, at a storage density of ~ 175 bits/mm², corresponding to ~ 150GB 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. This system is suited for fundamental investigations of the material aspects of PQ:PMMA; however, as typical experimental setups with numerous facilities for mechanical and optical adjustments, they are not optimized in terms of system complexity. In order to improve the commercial prospect of a holographic mass storage based on doped photopolymers, read/write setups with reduced system complexity and with the potential to be fabricated at low cost are necessary. In forth year, we start to design a particularly promising system architecture that is based on the concept of planar-integrated free-space optics (PIFSO).

The idea of PIFSO is to miniaturize and “fold” a free-space optical system with a certain desired functionality into a transparent substrate of a few millimeters thickness in such a way that all optical components fall onto the plane-parallel surfaces. Passive components such as lenses or beam deflectors can then be integrated into the surfaces, for example, through surface relief structuring, and the implementation as diffractive optical components offers an almost unlimited design freedom. Active components such as optoelectronic I/O devices can be bonded on top of the plane-parallel substrates. Reflective coatings ensure that optical signals propagate along zigzag paths inside the substrate.  Since all passive components are arranged in a planar geometry, the optical system can be fabricated as a whole using mask-based techniques. Lithographic precision for the lateral positioning of components is thereby ensured.

Figure 2 Schematic setup of the PIFSO-type reflection holographic read/write system depicting it in the recording and in the read-out mode. Reference and address beams are exactly counterpropagating along zigzag paths inside the PIFSO substrate. The FT lens performs an optical Fourier transformation from the LCD and the CMOS sensor to the PQ:PMMA layer on the storage disk.

We apply the PIFSO principle for the construction of a read/write head for holographic storage disks. Figure 2 shows the proposed bidirectional Fourier optical system architecture in the recording and the readout mode.  One can recognize an orthogonal signal beam, skew reference, and address beam paths that intersect at a target position on the reflective lower side of the photosensitive layer of the storage disk on which the hologram is recorded.  All beams originate from the same laser source from which they are coupled into the PIFSO system by single-mode optical fibers.  The relay of the signal beam from the fiber end to the disk is carried out by a 4-f system; in its Fourier plane, the expanded beam is 2D spatially modulated by a LCD microdisplay. To be able to record a complete signal page without loss, the diameter of the reference beams has to be matched to the width of the signal spectrum at the disk.  Reference and address beams are furthermore perfectly collimated and counterpropagating so that they can be considered as mutually phase conjugate.  Hence, if the reference beam is used for the recording of a hologram, then a readout with the address beam will generate the phase-conjugate version of the original signal beam; this reconstructed beam propagates through the 4-f system in the opposite direction and is projected onto a CMOS sensor.

In summary, during this year a strategy using doped photopolymers to fulfill most of the material requirements has been proposed and demonstrated. The concept of using planar-integrated free-space optics to realize microintegration of the optical read/write head has been explored. With these two innovations, we anticipate further accelerated advances in page-oriented holographic data storage techniques in the near future that may eventually lead to a scientific breakthrough.