Optics Express

Published by Optical Society of America
Online ISSN: 1094-4087
Publications
Article
Surface phonon polariton (SPP) characteristics of In(0.04)Al(0.06)Ga(0.90)N/AlN/Al(2)O(3) heterostructure are investigated by means of p-polarized infrared (IR) attenuated total reflection spectroscopy. Two absorption dips corresponding to In(0.04)Al(0.06)Ga(0.90)N SPP modes are observed. In addition, two prominent dips and one relatively weak and broad dip corresponding to the Al(2)O(3) SPP mode, In(0.04)Al(0.06)Ga(0.90)N/Al(2)O(3) interface mode, and Al(2)O(3) bulk polariton mode, respectively, are clearly seen. No surface mode feature originating from the AlN layer is observed because it is too thin. Overall, the observations are in good agreement with the theoretical predictions.
 
Article
Electro-optic modulation at lambda=1.5 mum has been demonstrated for the first time to the best of our knowledge in a ridge waveguide phase modulator produced in cubic potassium sodium tantalate niobate thin films epitaxially grown on potassium tantalate substrates exploiting the large quadratic electro-optic Kerr coefficient of R11 = 8.2x10(-17) m(2)/V(2). The relative permittivity, Kerr coefficient, and refractive index have been evaluated for the thin film crystal and are compared to the values measured in bulk crystals. The half-wave voltage times length figure of merit of the modulator has been measured to be Vpil=38 Vcm at room temperature.
 
Article
The effect of ultra-thin inserting layer (UIL) on the photovoltaic performances of InGaN/GaN solar cells is investigated. With UIL implemented, the open-circuit voltage was increased from 1.4 V to 1.7 V, short-circuit current density was increased by 65% and external quantum efficiency was increased by 59%, compared to its counterparts at room temperature under 1-sun AM1.5G illumination. The improvements in electrical and photovoltaic properties are mainly attributed to the UIL which can boost the crystal quality and alleviate strain. Moreover, it can act as a transition layer for higher indium incorporation and an effective light sub-absorption layer in multiple quantum wells.
 
Article
Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub>:0.06Ce<sup>3+</sup>, xMn<sup>2+</sup> (YAG:0.06Ce,xMn) phosphors have been synthesized and the effect of different charge compensators on the color adjustment has been investigated for the first time. The luminescence properties of Mn<sup>2+</sup> singly doped and co-doped with Ce<sup>3+</sup> into YAG host have been discussed. It is observed that in singly doped sample, Mn<sup>2+</sup> ions not only occupy two kinds of Al<sup>3+</sup> sites to generate a yellow and a deep red emission bands, but also occupy Y<sup>3+</sup> sites to obtain a green emission band in YAG host. Considering Mn<sup>2+</sup> substitution for Al<sup>3+</sup>, quadrivalence ions including Zr<sup>4+</sup>, Ge<sup>4+</sup> and Si<sup>4+</sup> ions are introduced to balance the charge difference. The results show that Si<sup>4+</sup> as charge compensator exhibits the best tunable effect on controlling the Mn<sup>2+</sup> emissions in YAG:0.06Ce, xMn. In Si<sup>4+</sup>-Mn<sup>2+</sup> co-doped samples, the emission color can be tuned from greenish-yellow to red with increasing the content of Mn<sup>2+</sup>. The Commission International de L'Eclairage (CIE) chromaticity coordinates are also investigated.
 
Article
A series of single-phased emission-tunable Na<sub>0.34</sub>Ca<sub>0.66</sub>Al<sub>1.66</sub>Si<sub>2.34</sub>O<sub>8</sub>:Eu<sup>2+</sup>,Mn<sup>2+</sup> phosphors were successfully synthesized by a wet-chemical synthesis method. Photoluminescence excitation (PLE) spectra indicate that the phosphor can be efficiently excited by UV radiation from 250 to 420 nm. Also, NCASO:Eu<sup>2+</sup>,Mn<sup>2+</sup> phosphor exhibit a broad blue emission band at 440 nm and an orange emission band at 570 nm, which originate from Eu<sup>2+</sup> and Mn<sup>2+</sup> ions, respectively. Therefore, overall emission color can be tuned from blue to white by increasing the concentration of Mn<sup>2+</sup> ions in the host lattice utilizing energy transfer from Eu<sup>2+</sup> to Mn<sup>2+</sup> ions. This energy transfer phenomenon was demonstrated to be a resonant type through dipole-dipole interaction determined with the help of PL spectra, decay time measurement, and energy transfer efficiency of the phosphor. These results indicate that NCASO:Eu<sup>2+</sup>,Mn<sup>2+</sup> can be a promising single-phased white-emitting phosphor for white-light UV LEDs.
 
Article
Fiber dispersion and nonlinearity management strategy based on a modification of a photonic-crystal fiber (PCF) core with an air hole is shown to facilitate optimization of PCF components for a stable soliton frequency shift and subpetahertz sideband generation through four-wave mixing. Spectral recoil of an optical soliton by a red-shifted dispersive wave, generated through a soliton instability induced by high-order fiber dispersion, is shown to stabilize the soliton self-frequency shift in a highly nonlinear PCF with an air-hole-modified core relative to pump power variations. A fiber with a 2.3-microm-diameter core modified with a 0.9-microm-diameter air hole is used to demonstrate a robust soliton self-frequency shift of unamplified 50-fs Ti: sapphire laser pulses to a central wavelength of about 960 nm, which remains insensitive to variations in the pump pulse energy within the range from 60 to at least 100 pJ. In this regime of frequency shifting, intense high- and low-frequency branches of dispersive wave radiation are simultaneously observed in the spectrum of PCF output. An air-hole-modified-core PCF with appropriate dispersion and nonlinearity parameters is shown to provide efficient four-wave mixing, giving rise to Stokes and anti-Stokes sidebands whose frequency shift relative to the pump wavelength falls within the subpetahertz range, thus offering an attractive source for nonlinear Raman microspectroscopy.
 
Article
We have investigated the absorption spectra of seventeen explosives and related compounds (ERCs) by using terahertz time-domain spectroscopy in the 0.1-2.8 THz region. Most of these substances show characteristic absorption features in this frequency range. The measured absorption coefficients of these ERCs form a database, which is of great importance for biochemical, defense and security related applications.
 
Compressed-pulse measurements. (a). Autocorrelation of 300 TW pulse, the experimental autocorrelation picture (insert) demonstrates that there is no amplitude front tilt or other spatial variations of the pulse arrival time; (b). Output spectrum: red curve experimental data, blue curve -Gaussian fit (FWHM=37nm); (c). Intensity profile reconstructed by the SPIDER (red curve), Gaussian-fit-curve of 30.4fs -FWHM (blue-points); d) Spectral phase reconstructed by the SPIDER.  
Article
We demonstrate the highest intensity - 300 TW laser by developing booster amplifying stage to the 50-TW-Ti:sapphire laser (HERCULES). To our knowledge this is the first multi-100TW-scale laser at 0.1 Hz repetition rate.
 
Article
We demonstrate a wafer-bonded silica-on-silicon planar waveguide platform with record low total propagation loss of (0.045 ± 0.04) dB/m near the free space wavelength of 1580 nm. Using coherent optical frequency domain reflectometry, we characterize the group index, fiber-to-chip coupling loss, critical bend radius, and propagation loss of these waveguides.
 
Article
Passive mode locking of a diode pumped Nd:La<sub>0.11</sub>Y<sub>0.89</sub>VO<sub>4</sub> mixed crystal laser with a semiconductor saturable absorber mirror (SESAM) was experimentally investigated for the first time to our knowledge. Stable CW mode-locking has been achieved on both a-cut and c-cut mixed crystals. In case of the a-cut crystal, when a 2% output coupler (OC) was used, the shortest pulse obtained was 4.5 ps and the highest output power was 0.94 W, while when a 6% OC was used, the shortest pulse obtained was 6.8 ps and the highest output power was 5.16 W. In the latter case the optical conversion efficiency is 38% and the slope efficiency is 40.3%, respectively; with the c-cut crystal the shortest pulse achieved was 5.5 ps. Moreover, simultaneous mode locking at two close wavelengths of 1064.3 nm and 1066.2 nm was observed on the c-cut crystal. The mode locked pulse beating generated temporal interference fringe of 0.5 THz repetition rate.
 
Concept of IEM algorithm. The decay of a molecule is discretized in time and for each time slot the corresponding photon count is measured, approaching for large values of M the decay function f ( t ). 
Number of bits required to store N c for (a) fixed accuracy mode and (b) fixed total count mode. 
Schematic and photo of the experimental setup. A standard Nikon B2-A filter cube was used for the fluorescence imaging.
(Media 1) Interface between two streams (100 μ M Rhodamine B from top inlet and 20 μ M Rhodamine 6G in water from the bottom inlet) changes as the flow rate is varied in a T- mixer (dimensions: W400 μ m × H200 μ m). Media at 5 times the actual speed (104s in real time): Initially the flow had been stopped, the flow was then re-started at time 5s, leading to a sharp interface between 2 well distinguished streams, but start to mix again after the flow has been stopped at time 45s. 
Article
A compact real-time fluorescence lifetime imaging microscopy (FLIM) system based on an array of low dark count 0.13microm CMOS single-photon avalanche diodes (SPADs) is demonstrated. Fast background-insensitive fluorescence lifetime determination is achieved by use of a recently proposed algorithm called 'Integration for Extraction Method' (IEM) [J. Opt. Soc. Am. A 25, 1190 (2008)]. Here, IEM is modified for a wider resolvability range and implemented on the FPGA of the new SPAD array imager. We experimentally demonstrate that the dynamic range and accuracy of calculated lifetimes of this new camera is suitable for widefield FLIM applications by imaging a variety of test samples, including various standard fluorophores covering a lifetime range from 1.6ns to 16ns, microfluidic mixing of fluorophore solutions, and living fungal spores of Neurospora Crassa. The calculated lifetimes are in a good agreement with literature values. Real-time fluorescence lifetime imaging is also achieved, by performing parallel 32 x 16 lifetime calculations, realizing a compact and low-cost FLIM camera and promising for bigger detector arrays.
 
Article
We present results for VCSEL based links operating PAM-4 signaling using a commercial 0.13microm CMOS technology. We perform a complete link analysis of the Bit Error Rate, Q factor, random and deterministic jitter by measuring waterfall curves versus margins in time and amplitude. We demonstrate that VCSEL based PAM-4 can match or even improve performance over binary signaling under conditions of a bandwidth limited, 100meter multi-mode optical link at 5Gbps. We present the first sensitivity measurements for optical PAM-4 and compare it with binary signaling. Measured benefits are reconciled with information theory predictions.
 
This figure is a revised version of Fig. 10 of [4 ]. “Spectral plots of the nea r-millimeter attenuation by the atmospheric H 2 O at sea level. H 2 O density = 5.9 g/m 3 . Curve A represents 
Atmospheric attenuation at sea level for different conditions of temperature, relative humidity (RH), fog, dust and rain. (STD: 20 °C, RH 44%), (Hu 35 °C, RH 90%), (Winter:  10 °C, RH 30%), (Fog, Dust, and Rain: 20 °C, RH 44%). Revised figure taken from Ref. 2. Six water windows (circled numbers) and 5 weak water lines (broad arrows) are marked for comparison with the THz-TDS measurements. 
Enclosed THz-TDS system for atmospheric measurements. 
(a). Measured THz reference pulse (lower trace) and measured THz sample pulse (upper trace). For clarity the THz sample pulse was shifted upwards by 500 pa. (b). Corresponding amplitude spectrum (upper curve) for the THz reference pulse and the amplitude spectrum for the sample pulse. 
The amplitude transmission through 6.18 m of atmosphere at 21 °C with RH 51%. The “real data points” are indicated by the larger open circles separated from each other by 6.1 GHz, and the interpolated points obtained from the zero-padding are separated from each other by 0.61 GHz and define the solid line. 10 water windows (circled numbers) and 8 weak water lines are marked to compare with previous predictions and measurements. 
Article
The attenuation of electromagnetic wave propagation in the clear atmosphere from low frequencies up to 2 THz is mainly caused by water vapor. Although there have been many numerical simulations and excellent early sub-mm and far-infrared measurements of this attenuation, there has remained controversy about the background absorption in the most promising windows of transparency below 1 THz. Here, we report an accurate terahertz time-domain spectroscopy (THz-TDS) characterization of water vapor from 0.2 to 2 THz. Our results agree with the previous predicted and measured attenuations for the weak water lines, but show more attenuation for the relatively transparent windows between these lines.
 
Schematic diagram of the system. : T. F. P., thinfilm polarizers; T. F. P. E. , thinfilm polarizer etalon; P. C. , Pockels cell; TiS, Ti:sapphires; CC1, concave mirror with a radius of curvature of 0.2 m; CC2, concave mirror with a radius of curvature of 0.3 m; F. R. , Faraday rotator; F. I. , Faraday isolator; CC3, concave mirrors with radii of curvature of 0.5 m; FM, flat mirrors; Pumping1, Second Harmonic of LD-pumped YAG laser with a power of 56 W; Pumping2, Second Harmonic of LD-pumped YAG laser with a power of 83 W; B. S. beam splitter with 30-% reflection. 
Average power of the amplified beam at each pass of 4-pass and single-pass amplifiers. 
Calculated and measured intensity profiles and phases of the compressed pulse. The measured traces were retrieved from SHG FROG, and the calculated ones were derived from the measured spectrum and calculated phase by ray tracing. R. En. , retrieved envelope (open circles); C. En. , calculated envelope (solid curve); R. Ph. , retrieved phase (open squares); C. Ph. , calculated phase (dashed curve). 
Article
An all solid state Ti:Sapphire amplifier system was developed consisting of a regenerative and multipass amplifiers with a peak power of 0.2 TW and a pulse width of 22 fs at 5 KHz. The pumping sources for the amplifiers are two diode pumped, Q-switched YAG lasers. The system consists of a mode locked oscillator, an Offner type stretcher, a regenerative amplifier, a four pass amplifier, a one pass amplifier and a compressor. A22.2 W average power is the highest ever obtained at any repetition rate in ultrashort pulse amplifiers.
 
Schematic overview of the ZGP-based MOPA. PBS: polarizer, λ /2: half-wave plate @ 2.05 µm, CaF 2 : CaF 2 wedge. The number of ZGP crystals in the OPA were 1-3 in the experiments. 
Article
Mid-infrared (3-5 μm) pulses with high energy are produced using nonlinear conversion in a ZnGeP<sub>2</sub>-based master oscillator-power amplifier, pumped by a Q-switched cryogenic Ho:YLF oscillator. The master oscillator is based on an optical parametric oscillator with a V-shaped 3-mirror ring resonator, and the power amplifier is based on optical parametric amplification in large-aperture ZnGeP<sub>2</sub> crystals. Pulses with up to 212 mJ energy at 1 Hz repetition rate are obtained, with FWHM duration 15 ns and beam quality M<sup>2</sup> = 3.
 
Article
We have demonstrated a laser-diode pumped continuous-wave (CW) and passively Q-switched laser with a Nd:Sc<sub>0.2</sub>Y<sub>0.8</sub>SiO<sub>5</sub> (Nd:SYSO) crystal for the first time. In the CW operation, the laser was found to oscillate in tri-wavelength regime at 1074.8 nm, 1076.6 nm and 1078.2 nm, respectively. The maximum CW output power of 1.96 W was obtained, giving an optical-to-optical conversion efficiency of 35% and a slope efficiency of 39%. Using either Cr<sup>4+</sup>:YAG or V<sup>3+</sup>:YAG crystal as saturable absorber, stable passively Q-switched laser was obtained at dual-wavelength of 1074.8 nm and 1078.2 nm with orthogonal-polarization. The maximum average output power, pulse repetition rate, and shortest pulse width were 1.03 W, 50 kHz, and 24 ns, respectively. The passively Q-switched dual-wavelength laser could be potentially used as a source for generation of terahertz radiation.
 
Article
We report on the design, implementation, and performance of an x-ray monochromator with ultra-high energy resolution (ΔE/E ≃ 2.7 × 10<sup>-8</sup>) and high spectral efficiency using x rays with photon energies E ≃ 9.13 keV. The operating principle of the monochromator is based on the phenomenon of angular dispersion in Bragg back-diffraction. The optical scheme of the monochromator is a modification of a scheme reported earlier [Shvyd'ko et al., Phys. Rev. A 84, 053823 (2011)], where a collimator/wavelength selector Si crystal was replaced with a 100-μm-thick type IIa diamond crystal. This modification provides a very-small-energy bandwidth ΔE ≃ 0.25 meV, a 3-fold increase in the aperture of the accepted beam, a reduction in the cumulative angular dispersion rate of x rays emanating from the monochromator for better focusing on a sample, a sufficient angular acceptance matching the angular divergence of an undulator source (≈ 10 μrad), and an improved throughput due to low x-ray absorption in the thin diamond crystal. The measured spectral efficiency of the monochromator was ≈ 65% with an aperture of 0.3 × 1 mm<sup>2</sup>. The performance parameters of the monochromator are suitable for inelastic x-ray spectroscopy with an absolute energy resolution ΔE < 1 meV.
 
Article
We demonstrate a very efficient high speed silicon modulator with an ultralow pi-phase-shift voltage-length product V(pi)L = 1.4V-cm. The device is based on a Mach-Zehnder interferometer (MZI) fabricated using 0.25microm thick silicon-on-insulator (SOI) waveguide with offset lateral PN junctions. Optimal carrier-depletion induced index change has been achieved through the optimization of the overlap region of carriers and photons. The 3dB bandwidth of a typical 1mm long device was measured to be more than 12GHz. An eye-diagram taken at a transmission rate of 12.5Gb/s confirms the high speed capability of the device.
 
Article
We propose a novel energy-efficient coherent-optical OFDM transmission scheme based on hybrid optical-electronic signal processing. We demonstrate transmission of a 0.26-Tb/s OFDM superchannel, consisting of 13 x 20-Gb/s polarization-multiplexed QPSK subcarrier channels, over 400-km standard single-mode fiber (SSMF) with BER less than 6.3x10(-4) using all-optical Fourier transform processing and electronic 7-tap blind digital equalization per subchannel. We further explore long-haul transmission over up to 960 km SSMF and show that the electronic signal processing is capable of compensating chromatic dispersion up to 16,000 ps/nm using only 15 taps per subchannel, even in the presence of strong inter-carrier interference.
 
Article
Site-selective spectroscopy and stimulated emission experiments performed in the (4)F(3/2)-->(4)I(11/2) laser transition of Nd(3+)-doped 0.8CaSiO(3-) 0.2Ca(3)(PO(4))(2) eutectic glass are presented. The spectral features of the excitation spectra and those of spontaneous and stimulated emissions reveal the existence of a very complex crystal field site distribution for Nd(3+) ions. As a consequence, the stimulated emission of Nd(3+) in this glass shows a tunability of about 10 nm as a function of excitation wavelength.
 
Article
In this work we report the study of energy transfer between Nd(3+) and Yb(3+) ions in glasses with the 0.8CaSiO(3)-0.2Ca(3)(PO(4))(2) eutectic composition at room temperature by using steady-state and time-resolved laser spectroscopy. The Nd(3+)?Yb(3+) transfer efficiency obtained from the Nd(3+) lifetimes in the single doped and codoped samples reaches 73% for the highest Nd(3+) concentration. The donor decay curves obtained under pulsed excitation have been used to establish the multipolar nature of the Nd(3+)-->Yb(3+) transfer process and the energy transfer microparameter. The nonradiative energy transfer is consistent with an electric dipole-dipole interaction mechanism assisted by energy migration among donors. Back transfer from Yb(3+) to Nd(3+) is also observed.
 
Cross-section (a) and longitudinal-section (b) of a sample grown at 100 mm/h with a diameter of 2.5 mm and doped with 1 wt% of Nd 2 O 3 . Longitudinal-section micrographs of 
Cross-section micrograph of a sample grown at 50 mm/h doped with 1 wt% of Nd 2 O 3 . 
Excitation spectra obtained at different emission wavelengths for the glass-ceramic samples grown at 50 mm/h (a) and 500 mm/h (b) and for the glass sample (c). 
Steady-state emission spectra of the 4 F 3/2 → 4 I 11/2 transition obtained under excitation at 
Experimental decays of the 4 F 3/2 level obtained under excitation at 802.2 nm at three emission wavelengths for the sample grown at 50 mm/h. 
Article
In this work we report the influence of the crystallization stage of the host matrix on the spectroscopic properties of Nd3+ ions in biocompatible glass-ceramic eutectic rods of composition 0.8CaSiO3-0.2Ca3(PO4)2 doped with 1 and 2 wt% of Nd2O3. The samples were obtained by the laser floating zone technique at different growth rates between 50 and 500 mm/h. The microstructural analysis shows that a growth rate increase or a rod diameter decrease leads the system to a structural arrangement from three (two crystalline and one amorphous) to two phases (one crystalline and one amorphous). Electron backscattering diffraction analysis shows the presence of Ca2SiO4 and apatite-like crystalline phases. Site-selective laser spectroscopy in the (4)I(9/2)→(4)F(3/2)/(4)F(5/2) transitions confirms that Nd(3+) ions are incorporated in crystalline and amorphous phases in these glass-ceramic samples. In particular, the presence of Ca(2)SiO(4) crystalline phase in the samples grown at low rates, which has an excellent in vitro bioactivity, can be unambiguously identified from the excitation spectra and lifetime measurements of the (4)F(3/2) state of Nd(3+) ions.
 
Article
Highly stable operation of a two-stage multipass Ti:sapphire amplifier (a four-pass pre-amplifier and a four-pass power amplifier) for a 100-mJ-class chirped-pulse amplification system has been demonstrated by passive stabilization. By optimizing the ratio of pump energies to the two amplifiers and the optical losses artificially inserted into the second power amplifier, a root-mean-square fluctuation in pulse energy of 0.3% was achieved, which was 5 times lower than that of the pump laser. This is the lowest pulse-to-pulse fluctuation, to the best of our knowledge, obtained by the 100-mJ-class Ti:sapphire amplifiers.
 
Article
This study reports fabrication of white-emissive, tandem-type, hybrid organic/polymer light-emitting diodes (O/PLED). The tandem devices are made by stacking a blue-emissive OLED on a yellow-emissive phenyl-substituted poly(para-phenylene vinylene) copolymer-based PLED and applying an organic oxide/Al/molybdenum oxide (MoO(3)) complex structure as a connecting structure or charge-generation layer (CGL). The organic oxide/Al/MoO(3) CGL functions as an effective junction interface for the transport and injection of opposite charge carriers through the stacked configuration. The electroluminescence (EL) spectra of the tandem-type devices can be tuned by varying the intensity of the emission in each emissive component to yield the visible-range spectra from 400 to 750 nm, with Commission Internationale de l'Eclairage chromaticity coordinates of (0.33, 0.33) and a high color rendering capacity as used for illumination. The EL spectra also exhibit good color stability under various bias conditions. The tandem-type device of emission with chromaticity coordinates, (0.30, 0.31), has maximum brightness and luminous efficiency over 25,000 cd/m(2) and approximately 4.2 cd/A, respectively.
 
Article
A miniaturized probe that possesses a diameter of 0.4 mm is developed for two-photon-excited fluorescence imaging. The miniaturized probe was manufactured by the collapse of air holes and the formation of a lens on the tip of a double-clad photonic crystal fiber (DCPCF) using electric arc discharging from a conventional fusion splicer. As a result, a femtosecond pulsed laser beam delivered by the DCPCF can be directly focused on a sample for two-photon fluorescence imaging. The numerical aperture of the lensed DCPCF is 0.12. The corresponding focal spot size is 6 microm, which is close to the diffraction limit. This 0.4-mm-diamter probe can provide clear two-photon-excited fluorescence images of 10-microm-diameter fluorescent microspheres.
 
Article
The optical properties of p-type GaSe and mixed GaSe(1-x)S(x), x=0.04, 0.023, 0.090, 0.133, 0.175, 0.216, 0.256, 0.362, 0.369, and 0.412, crystals were studied to reveal the potentials for phase matching and frequency conversion. Comparative experiment on Er3+:YSGG and CO2 laser SHG at identical experimental conditions is carried out at room temperature. Any change in polytype structure of GaSe1(1-x)S(x) was not found.
 
Article
A diode-pumped joule class in a 10 Hz output Nd:YLF ring amplifier has been developed. A phase conjugate plate was developed as a wavefront corrector for the residual wavefront distortion of an Nd:YLF rod. We have demonstrated a 0.46 J output of 10 ns pulse duration at 10 Hz repetition rate with 1.5 nJ of input energy. The effective gain of the ring amplifier system was 84.8 dB. To our knowledge, this is the highest magnification with joule-level output energy in a single-stage amplifier system that has ever been built. As a preamplifier system, this system contributed a demonstration of 21.3 J in a 10 Hz output diode-pumped Nd:glass zigzag slab laser system with a stimulated Brillouin scattering- phase conjugation mirror. We describe a robust and effective method of wavefront correction for high-energy laser systems.
 
Article
We present a detailed study of the effect of the carrier lifetime on the terahertz signal characteristics emitted by Br(+)-irradiated In(0.53)Ga(0.47)As photoconductive antennas excited by 1550 nm wavelength femtosecond optical pulses. The temporal waveforms and the average radiated powers for various carrier lifetimes are experimentally analyzed and compared to predictions of analytical models of charge transport. Improvements in bandwidth and in average power of the emitted terahertz radiation are observed with the decrease of the carrier lifetime on the emitter. The power radiated by ion-irradiated In(0.53)Ga(0.47)As photoconductive antennas excited by 1550 nm wavelength optical pulses is measured to be 0.8 muW. This value is comparable with or greater than that emitted by similar low temperature grown GaAs photoconductive antennas excited by 780 nm wavelength optical pulses.
 
(a) 480Gb/s WDM Experimental Setup, (b) 12 – Channel Spectrum before entering the chip, (c) 12 – Channel Spectrum directly at the chip’s output and Measured Spectral Response of the SOI – Plasmonic chip, (d) 12 – Channel Spectrum after being amplified in the receiver’s EDFA 
(a) BER curves for all 40Gb/s B2B and transmitted channels, and BER curves and Eye Diagrams for (b) channel #1 (Best – Performing Channel) and (c) channel #8 (Worst – Performing Channel) 
(a) SEM image of the dual-resonator Cycloaliphatic Acrylate Polymer-loaded DLSPPW switch, (b) Leakage Radiation Microscopy (LRM) Setup, LRM image in (c) “Cool” and (d) “Hot” state at λ = 1558nm 
(a) Transmission spectra for “Through” output port in “Cool” and “Hot” state, (b) Transmission spectra for “Drop” output port in “Cool” and “Hot” state, (c) Extinction Ratio values between “Cool” and “Hot” switching states at “Through” and “Drop” outputs 
Article
We demonstrate Wavelength Division Multiplexed (WDM)-enabled transmission of 480Gb/s aggregate data traffic (12x40Gb/s) as well as high-quality 1x2 thermo-optic tuning in Dielectric-Loaded Surface Plasmon Polariton Waveguides (DLSPPWs). The WDM transmission characteristics have been verified through BER measurements by exploiting the heterointegration of a 60 μm-long straight DLSPPW on a Silicon-on-Insulator waveguide platform, showing error-free performance for six out of the twelve channels. High-quality thermo-optic tuning has been achieved by utilizing Cycloaliphatic-Acrylate-Polymer as an efficient thermo-optic polymer loading employed in a dual-resonator DLSPPW switching structure, yielding a 9 nm wavelength shift and extinction ratio values higher than 10 dB at both output ports when heated to 90°C.
 
Article
A new Q-switched crystal Cr(5+):GdVO(4) was grown by the Czochralski method for the first time, to our knowledge. Polarized absorption spectra of Cr(5+)+:GdVO(4) were measured at room temperature. The results showed that the crystal has polarized absorption properties, and the absorption band of pi-polarized spectra located at 900 to 1300 nm should be suitable as a passive saturable absorber Q-switched laser at about 1 mum. With Cr(5+):GdVO(4) as a saturable absorber, the pulsed laser performance of Nd:Lu(0.5)Gd(0.5)VO(4) at 1.06 mum was demonstrated. The maximum average output power of 122 mW was obtained under a pump power of 3.79 W. The shortest pulse width and largest pulse energy obtained were 361 ns and 0.77 muJ, respectively. To our knowledge, it is the first time the absorption spectra of Cr(5+):GdVO(4) and a pulsed laser with the crystal as the saturable absorber have been obtained.
 
Giant-chirp all-normal dispersion LMA ytterbium fiber oscillator. 
(a) Optical pulse spectrum on logarithmic and linear (inset) scale. (b) Intensity autocorrelation of the compressed and chirped (inset) output pulses. 
(a) Oscilloscope traces of the output pulse train. (b) Radio-frequency spectrum at the fundamental repetition rate of 4.3 MHz and over the range of 200 MHz (inset). 
(a) Measured beam caustic (dotted) and corresponding fit with a M 2 of 1.07 (line). (b), 
Article
We present a mode-locked all-normal dispersion ytterbium fiber oscillator with output pulse energies beyond 0.5 µJ. The oscillator is mode-locked using nonlinear polarization rotation, and stable single-pulse operation is achieved by spectral filtering inside the resonator. The oscillator generates strongly chirped output pulses at a repetition rate of 4.3 MHz which can be compressed down to 760 fs.
 
kHz Ti:sapphire laser chirped pulse amplification system with a pressure-gradient hollow fiber pulse compressor. PC: Pockels cell, PID: proportional-integral-derivative controller, BS: beam splitter, PSD: position-sensitive detector, PM: power meter, CCD: charge coupled device. The spectrum broadened by the self-phase modulation in the fiber was monitored with a spectrometer. The compressed pulse duration was measured with SPIDER after dispersion compensation with chirped mirrors.  
Focusability of the beam after hollow fiber compression. The insert shows the focal spot image.  
Article
We have demonstrated the generation of 5 fs, 0.5 TW pulses at 1 kHz repetition rate using a pulse compression technique in a hollow fiber with a pressure gradient. Owing to the excellent beam quality by passing through the hollow fiber, the beam after pulse compression could be focused to a nearly diffraction-limited spot size. We obtained for the first time a peak intensity as high as 5x10(18) W/cm(2) in the 2-cycle regime.
 
Article
Optical properties of a lattice matched GaAs/In(0.5)(AlxGa(1-x))(0.5)P/GaAs heterostructure cavity have been characterized using piezoreflectance (PzR) measurements in the temperature range between 20 and 300 K. The measurements were carried out in the energy range of 1.3-6 eV. The PzR spectra of In(0.5)(AlxGa(1-x))(0.5)P at 20 and 300K clearly show a lot of interband transition features present at energies above the band edge. There is also a feature of interference-fringes oscillations observed in each PzR spectrum below band gap E0 of In(0.5)(AlxGa(1-x))(0.5)P. The oscillation period in between the PzR interference fringes can be utilized to determine the index of refraction (n) for the In(0.5)(AlxGa(1-x))(0.5)P at different temperatures. The Al composition x of In(0.5)(AlxGa(1-x))(0.5)P can be estimated from the evaluation value of E(0) by PzR. The obtained Al composition of x=0.691 is in good agreements with the original design for growing the quaternary compound. Electronic band structure of In(0.5)(AlxGa(1-x))(0.5)P is determined by the interband transitions from PzR. The temperature variations of the transition energies and index of refraction n for the In(0.5)(AlxGa(1-x))(0.5)P are analyzed discussed. The PzR is proven to be very sensitive in determining the optical parameters in III-V GaAs/InAlGaP/GaAs Fabry-Perot system.
 
Schematic view of the THz profilometer.
False-color plot of the topography of a nominally flat light-weight satellite telescope mirror.
Cyclic error (circles), correction of the displacement measured by the THz heterodyne profilometer (solid line) and the least-squares approximation of the cyclic error (dashed line). 
Article
Characterization of the topography of materials by interferometry in the visible or near-IR wavelength regime becomes difficult or impossible if the surface is rough on the length scale of a tenth of the wavelength and more. In this case, THz radiation can provide an interesting alternative. We demonstrate heterodyne profilometry at 600 GHz as a method for the accurate determination of surface topography with an achievable expanded standard uncertainty of 0.5 mu m. (C) 2008 Optical Society of America.
 
(a) Dual grating side-diffraction experiment setup. (b) Close-up of core region. Focused incoming probe beam (HeNe 632.8 nm) is first-order diffracted on two Bragg gratings in D-fibres. Solid line represent the extent of the probe beam, the short dashed line the diffracted light from the top fiber and the long dashed line the diffracted light from the bottom fiber.
Reproduction of coupler cross-cuts with corresponding side-diffraction interference pattern. (a) in-line and close cores, (b) transversely shifted and close cores and (c) in-line and slightly separated cores. 
(a) Diffraction interferometer for grating, or phasemask, comparison purposes. (b) Device for precision control of position. Inset: side-view with probe beam as thick line and diffracted light as thin line.
Article
We demonstrate a novel method that can detect period fluctuations of periodic structures such as fiber Bragg gratings at an accuracy of approximately 0.5 ppm. These fluctuations can consist of chirp rates, phase shifts etc. The method can also be used to measure phase masks or work as a position control device with spatial resolution in the order of 10 nm. The technique is a modified side diffraction method with interference between two diffraction orders.
 
Article
Previously, pulses shorter than 4 fs were generated by compressing white light from gas-filled hollow-core fibers with adaptive phase modulators; however, the energy of the few-cycle pulses was limited to 15 microJ. Here, we report the generation of 550 microJ, 5 fs pulses by using a liquid crystal spatial light modulator in a grating-based 4f system. The high pulse energy was obtained by improving the throughput of the phase modulator and by increasing the input laser energy. When the pulses were used in high harmonic generation, it was found that the harmonic spectra depend strongly on the high order spectral phases of the driving laser fields.
 
Article
In vivo photoacoustic (PA) flow cytometry (PAFC) has great potential for detecting disease-associated biomarkers in blood and lymph flow, as well as real-time control of the efficacy of photothermal (PT) and other therapies through the counting of circulating abnormal objects. We report on a high speed PAFC with a Yb-doped fiber laser having a 0.5-MHz pulse repetition rate at a wavelength of 1064 nm, pulse width of 10 ns, and energy up to 100 microJ. This is the first biomedical application of PA and PT techniques operating at the highest pulse repetition rate of nanosecond lasers that provide 100-fold enhancement in detection speed of carbon nanotube clusters, as well as real-time monitoring of the flow velocity of individual targets through the width of PA signals. The laser pulse rate limits for PT and PA techniques depending on the sizes of laser beam and targets and flow velocity are discussed. We propose time-overlapping mode and generation of periodic nano- and microbubbles as PA-signal and PT-therapy amplifiers, including discrimination of small absorbing targets among large ones. Taking into account the relatively low level of background signals from most biotissues at 1064 nm, our data suggest that a nanosecond Yb-doped fiber laser operating at high pulse repetition rate could be a promising optical source for time-resolved PA and PT cytometry, imaging, microscopy, and therapy, including detection of nanoparticles and cells flowing at velocities up to 2.5 m/s.
 
a) An optical image of a deformed doubled-stemmed silica microsphere. b) Schematic of the experimental setup for mechanically tuning the WGM resonances in a silica microsphere. c) A photo of the experimental setup. 
a) A schematic of the experimental setup used for the free space evanescent excitation and the detection of WGMs in a deformed silica microsphere. b) An example of a WGM resonance used in the experiment. The red curve shows a Lorentzian fit to the resonance with a linewidth of 65 MHz. 
Coarse tuning of the WGM resonance with the Attocube nano-positioner operating in a slip-stick (step) mode, with an external voltage per step of 25 volts. The red line is guide to the eye.
Fine tuning of the WGM resonance with the Attocube nano-positioner operating in a DC-offset mode. (a) 10 V per step. (b) 1 V per step. The red line is guide to the eye.
Tuning of the WGM resonance with the Attocube nano-positioner operating in a DC- offset mode, with 0.1 V per step, leading to a tuning resolution better than 10 MHz. The red line is guide to the eye. 
Article
We experimentally demonstrate the mechanical tuning of whispering gallery modes in a 40 μm diameter silica microsphere at 10K, over a tuning range of 450 GHz and with a resolution less than 10 MHz. This is achieved by mechanically stretching the stems of a double-stemmed silica microsphere with a commercially available piezo-driven nano-positioner. The large tuning range is made possible by the millimeter long slip-stick motion of the nano-positioner. The ultrafine tuning resolution, corresponding to sub-picometer changes in the sphere diameter, is enabled by the use of relatively long and thin fiber stems, which reduces the effective Poisson ratio of the combined sphere-stem system to approximately 0.0005. The mechanical tuning demonstrated here removes a major obstacle for the use of ultrahigh Q-factor silica microspheres in cavity QED studies of solid state systems and, in particular, cavity QED studies of nitrogen vacancy centers in diamond.
 
Article
Femtosecond laser pulses, which are tunable from 440 to 990 nm, are generated at MHz repetition rates by noncollinear parametric amplification (NOPA). The pulses have durations of 20 to 30 fs over the major part of the tuning range and a high energy stability of 1.3% (rms). The NOPA is pumped with ultraviolet pulses from the third harmonic of an ytterbium doped fiber laser system and seeded by a smooth continuum generated in bulk sapphire. The residual second harmonic is used to pump an additional NOPA, which is independently tunable from 620 to 990 nm. Interference experiments show that the two NOPA systems have a precisely locked relative phase, despite of being pumped by different harmonics with a random phase jitter. This demonstrates that the phase of pulses generated by optical parametric amplification does not depend on the pump phase.
 
Article
Duobinary formats are today considered as being one of the most promising cost-effective solutions for the deployment of 40 Gb/s technology with direct detection on existing 10 Gb/s WDM long-haul (metropolitan and core) transmission infrastructures. Various methods for generating duobinary formats have been developed in the past few years but to our knowledge their respective performances for 40 Gb/s transmission have never been really compared experimentally. Here, we propose to evaluate at 40 Gb/s their respective robustness with respect to the most stringent transmission impairments, namely ASE noise, chromatic dispersion, polarization mode dispersion and nonlinear effects. We demonstrate that, owing to its enhanced resistance to intra-channel nonlinearities as compared to non-return-to-zero, duobinary can permit to reach transmission distances compliant with metropolitan and core applications on G.652 standard single mode fibre when quasi single-channel transmission conditions are met. We show furthermore that shifting optical duobinary filtering from the transmitter output to the receiver input can be of high interest to improve further the system maximum reach. We show also that phase-shaped binary transmission (PSBT) formats are fully compliant with 50-GHz channel spacing and that they are, in terms of transmission performance, as good as partial differential phase shift keying (Partial-DPSK), which is considered by equipment suppliers as the preferential transport solution for deployment of 40 Gb/s technology with direct detection on existing 10 Gb/s WDM metropolitan and core transmission infrastructures.
 
Schematic configuration of the Q-switched intracavity coherently combined fiber laser. 
Article
We demonstrate a high peak power, Q-switched pulsed, intracavity coherently combined fiber laser system. The system is based on two Yb-doped, rod-type, photonic crystal fibers which are passively phase-locked and combined into the single output beam in a power oscillator configuration. Experimental evidence indicate that this oscillator system provides record high peak power of ∼ 0.7 MW with pulse duration of ∼ 10 ns at 1 kHz repetition rate. The measured beam quality shows near-diffraction-limited operation of the coherently combined system.
 
Article
We report on the generation of wave breaking-free pulses from a passively mode-locked all-fiber Erbium oscillator at 1.55 microm pumped by a Raman fiber assembly operating at 1.48 microm. The extracted pulses show a self-similar behaviour and can be scaled up to an average power of 675 mW only limited by the pump power. The laser operates at the fundamental repetition rate of 108 MHz, therefore the pulse energy is 6.2 nJ. After external compression, the dechirped pulse duration is 64 fs.
 
(a) Isometric view of the hemispherical PA imaging array illustrating the transducer arrangement. Columns with transducers lightly shaded in green correspond to zenith angles of 22.5°, 45°, and 67.5° while columns with transducers lightly shaded blue correspond to zenith angles of 33.75°, 56.25°, and 78.75°. (b) Represents an unfolded schematic of (a) whereby each plane of 5 transducers is referenced P1 through P6, with P1 representing the bottom-most row and P6 the top-most row. (c) Summarizes the results of the estimate of effective matrix rank on the 6 different transducer arrangements. (d) Semi-logarithmic plot of the magnitude of the singular values versus singular value index for each of the 6 transducer arrangements. Left-most curve corresponds to the 5 transducer imaging operator while right-most to the 30 transducer imaging operator with intermediate curves represent increasing transducer count from left to right.
(a) Displays the estimated matrix rank for variable transducer count and arrangements. (b) Displays the estimated matrix rank for variable measurement space temporal sampling rates. Linear regression for (b) was performed only on the 4 data points contained within region (ii). The line is shown throughout the entire figure to show the expected value of the matrix rank. (c) Compares the expected matrix rank to the measured rank and is plotted as a percent error to highlight the deviation from linearity in regions (i) and (iii). (d) Provides a visual interpretation of the geometry associated with selected singular vectors for the imaging operator corresponding to the 30 transducer, 5 MHz temporal sampling rate. Images (i) through (iv) correspond to singular vectors of index 1, 10, 3632, and 4036.
Reconstruction of a simulated point, line, and multi-point source for each of the 6 transducer arrangements. The iterative technique was implemented on the system with 30 transducers and 5 MHz temporal sampling rate (shown in the column second from right). Ideal image based on phantom is shown in the right-most column.
Reconstruction of a simulated point, line, and multi-point source for each of the 8 measurement space temporal sampling rates (MHz).
4D real-time photoacoustic imaging experiment where data acquisition and image reconstruction were performed in real time. Images were captured from a photoacoustic point-like source translated in the negative x-direction at a velocity of 0.40 mm/s. The interval between 3D photoacoustic images was 1.4 s and rate-limited by the data acquisition transfer speed, computational, and data storage overhead. The first row shows an xy-plane (z = 4 mm). The second row shows the same reconstruction in a zy-plane (x = 7 mm) and the third row displays an xz-plane (y = 11 mm). Each column represents the 3D image data acquired at a specific point in time (indicated along bottom).
Article
Photoacoustic imaging is a non-ionizing imaging modality that provides contrast consistent with optical imaging techniques while the resolution and penetration depth is similar to ultrasound techniques. In a previous publication [Opt. Express 18, 11406 (2010)], a technique was introduced to experimentally acquire the imaging operator for a photoacoustic imaging system. While this was an important foundation for future work, we have recently improved the experimental procedure allowing for a more densely populated imaging operator to be acquired. Subsets of the imaging operator were produced by varying the transducer count as well as the measurement space temporal sampling rate. Examination of the matrix rank and the effect of contributing object space singular vectors to image reconstruction were performed. For a PAI system collecting only limited data projections, matrix rank increased linearly with transducer count and measurement space temporal sampling rate. Image reconstruction using a regularized pseudoinverse of the imaging operator was performed on photoacoustic signals from a point source, line source, and an array of point sources derived from the imaging operator. As expected, image quality increased for each object with increasing transducer count and measurement space temporal sampling rate. Using the same approach, but on experimentally sampled photoacoustic signals from a moving point-like source, acquisition, data transfer, reconstruction and image display took 1.4 s using one laser pulse per 3D frame. With relatively simple hardware improvements to data transfer and computation speed, our current imaging results imply that acquisition and display of 3D photoacoustic images at laser repetition rates of 10Hz is easily achieved.
 
A typical Raman shift spectrum (a), and a scanning transmission electron microscopy image (b) taken in the phase-pure Cu 2 ZnSnS 4 areas of the thin film. Energy dispersive X-ray spectroscopy on the selected points marked in (b) are depicted in panels (c) - (e). Panel (e) was 
Article
A polycrystalline Cu2ZnSnS4 thin film was deposited on fused quartz by co-evaporation. The selected thickness was ~100 nm to avoid artifacts in its optical properties caused by thicker as-grown films. The composition and phase of the film were checked with x-ray fluorescence, Raman shift spectroscopy, scanning transmission electron microscopy, and energy dispersive x-ray spectroscopy. An improved spectroscopic ellipsometry methodology with two-side measurement geometries was applied to extract the complex dielectric function ε = ε1 + iε2 of the Cu2ZnSnS4 thin film between 0.73 and 6.5 eV. Five critical points were observed, at 1.32 (fundamental band gap), 2.92, 3.92, 4.96, and 5.62 eV, respectively. The ε spectra are in reasonable agreement with those from theoretical calculations.
 
Article
Straight single-line-defect photonic crystal (PC) waveguides on GaAs slabs with lengths of 1, 4, and 10 mm have been fabricated. By controlling the Al content of a sacrificial AlGaAs clad layer and the wet etching duration, a PC core layer with a very smooth surface was obtained. Atomic force microscope images indicate that the roughness on the top surface is less than 1 nm. An extremely low propagation loss of 0.76 dB/mm for the GaAs-based PC waveguide was achieved.
 
Article
Without using high speed RF feedback electronics, we successfully demonstrate a novel and economic long-term stabilization scheme for a 10 GHz 0.8 ps asynchronously mode-locked Er-fiber soliton laser by controlling the cavity length to lock the deviation frequency at 25 kHz. The required deviation frequency between the cavity harmonic frequency and the modulation frequency can be directly obtained from the low frequency electronic sideband of the laser output. The same feedback control unit is also useable for higher modulation frequencies, because the suitable deviation frequency always remains within the range of 15~40 kHz.
 
Article
We theoretically demonstrate a novel approach for generating Mid-InfraRed SuperContinuum (MIR SC) by using concatenated fluoride and chalcogenide glass fibers pumped with a standard pulsed Thulium (Tm) laser (TFWHM=3.5ps, P<sub>0</sub>=20kW, νR=30MHz, and Pavg=2W). The fluoride fiber SC is generated in 10m of ZBLAN spanning the 0.9-4.1μm SC at the -30dB level. The ZBLAN fiber SC is then coupled into 10cm of As<sub>2</sub>Se<sub>3</sub> chalcogenide Microstructured Optical Fiber (MOF) designed to have a zero-dispersion wavelength (λZDW) significantly below the 4.1μm InfraRed (IR) edge of the ZBLAN fiber SC, here 3.55μm. This allows the MIR solitons in the ZBLAN fiber SC to couple into anomalous dispersion in the chalcogenide fiber and further redshift out to the fiber loss edge at around 9μm. The final 0.9-9μm SC covers over 3 octaves in the MIR with around 15mW of power converted into the 6-9μm range.
 
Article
Here we show a significant advance in solar-pumped laser beam brightness by utilizing a 1.0 m diameter Fresnel lens and a 3 mm diameter Nd:YAG single-crystal rod. The incoming solar radiation is firstly focused by the Fresnel lens on a solar tracker. A large aspheric lens and a 2D-CPC concentrator are then combined to further compress the concentrated solar radiation along the thin laser rod within a V-shaped pumping cavity. 2.3 W cw TEM00 (M<sup>2</sup> ≤ 1.1) solar laser power is finally produced, attaining 1.9 W laser beam brightness figure of merit, which is 6.6 times higher than the previous record. For multimode operation, 8.1 W cw laser power is produced, corresponding to 143% enhancement in collection efficiency.
 
Article
In this paper we present an ultra high speed and highly phase sensitive line-field FD-OCT system for quantitative phase mapping. The system works with a maximum speed of 512 000 A-scan/s (1000 fps) in real time mode. Along the parallel recorded direction excellent phase stability corresponding to a path length variation of only 510 pm was measured. We demonstrate how to exploit this phase accuracy for fast chemical analysis of glucose mixture processes. The system has particular potential for studying micro-fluidic processes.
 
Article
Ge/Si heterojunction light emitting diodes with 20-bilayers undoped or phosphorus in situ doped GeSi islands were fabricated on n<sup>+</sup>-Si(001) substrates by ultrahigh vacuum chemical vapor deposition (UHV-CVD). Enhanced room temperature photoluminescence (PL) and electroluminescence (EL) around 1.5 ?m were observed from the devices with phosphorus-doped GeSi islands. Theoretical calculations indicated that the emission is from the radiative recombination in GeSi islands. The intensity enhancement of PL and EL is attributed to the sufficient supply of electrons in active layer for radiative recombination.
 
Top-cited authors
Benjamin Eggleton
  • The University of Sydney
Jens Limpert
  • Friedrich Schiller University Jena
Miles John Padgett
  • University of Glasgow
John Edward Bowers
  • University of California, Santa Barbara
Andreas Tünnermann
  • Fraunhofer Institute for Applied Optics and Precision Engineering IOF