Synchronously pumped femtosecond optical parametric oscillator of congruent and stoichiometric MgO-doped periodically poled lithium niobate

A synchronously pumped femtosecond optical parametric oscillator based on congruent MgO-doped periodically poled lithium niobate (c-MgO:PPLN) is reported. The system, operating at room temperature, was pumped by a mode-locked Ti:sapphire laser. The wavelengths of the signal and idler waves were tuned from 870 nm to 1.54 μm and 1.58 to 5.67 μm, respectively, by changing the pump wavelength, the grating period or the cavity length. Pumped by 1.1 W of 755 nm laser radiation, the OPO generated 310 mW of 1080 nm radiation. This signal output corresponds to a total conversion efficiency of 50%. Without dispersion compensation the OPO generated phase-modulated signal pulses of 200 fs duration. Besides the OPO of c-MgO:PPLN, an OPO of stoichiometric (s) MgO:PPLN was investigated. Because of the reduced sensitivity to photorefractive damage, both crystals allowed efficient OPO operation at room temperature. Received: 19 August 2002 / Revised version: 11 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-631/205-3906, E-mail: andres@physik.uni-kl.de     

High-repetition-rate eye-safe optical parametric oscillator intracavity pumped by a diode-pumped Q-switched Nd:YVO<Subscript>4</Subscript> laser

A high-repetition-rate eye-safe optical parametric oscillator (OPO), using a non-critically phase-matched KTP crystal intracavity pumped by an acousto-optically (AO) Q-switchedNd:YVO₄ laser, is experimentally demonstrated. It is found that the average OPO signal power at 1573 nm can be efficiently increased by increasing the pulse repetition rate. Moreover, the intracavity OPO process effectively shortens the pulse width so that it is in the range 5∼8 ns for pulse repetition rates of 10 to 80 kHz. As a result of the relatively short pulse, the peak power at 1573 nm is higher than 2 kW at a pulse repetition rate of 80 kHz. Received: 10 July 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +886-35/729-134, E-mail: yfchen@cc.nctu.edu.tw     

Tuning and stability of a continuous-wave mid-infrared high-power single resonant optical parametric oscillator

A 2.2-W continuous-wave, continuously tunable, single-frequency OPO has been developed in the 3.0–3.8 μm wavelength range for the detection of molecular trace gasses. The oscillation threshold, output power and stability of the single resonant OPO were improved by optimizing pump beam waist and OPO cavity length. Both air-spaced and solid etalons were tested to frequency stabilize and tune the OPO, from which the solid etalon gave a better performance. Temperature oscillations in the PPLN crystal caused oscillations in the idler wavelength of less than 200 MHz over 300 s; the short-term stability was less than 3 MHz over 1 s. The high laser power, in combination with photoacoustic spectroscopy, achieved a detection limit of 10 parts-per-trillion for ethane in nitrogen. Received: 9 April 2002 / Revised version: 14 June 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax:+31-24/3653311, E-Mail: maartenh@sci.kun.nl     

Investigation of the spatial beam quality of pulsed ns-OPOs

This paper reports on an experimental investigation of the spatial beam quality of a pulsed nanosecond optical parametric oscillator (OPO). The OPO is a 355 nm pumped type-I BBO-OPO. The spatial quality of the OPO output is characterized by the M²-parameter in dependence of other experimental parameters such as resonator length, pump-beam diameter, pump-pulse duration and pump energy. The results obtained indicate, that an appropriate choice of these parameters substantially improves the spatial beam quality of the OPO output. Received: 19 November 2002 / Revised version: 3 February 2003 / Published online: 5 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-631/205-3906, E-mail: anstett@rhrk.uni-kl.de     

Application of mid-infrared cavity-ringdown spectroscopy to trace explosives vapor detection using a broadly tunable (6–8 μm) optical parametric oscillator

A novel instrument, based on cavity-ringdown spectroscopy (CRDS), has been developed for trace gas detection. The new instrument utilizes a widely tunable optical parametric oscillator (OPO), which incorporates a zinc–germanium–phosphide (ZGP) crystal that is pumped at 2.8 μm by a 25-Hz Er,Cr:YSGG laser. The resultant mid-IR beam profile is nearly Gaussian, with energies exceeding 200 μJ/pulse between 6 and 8 μm, corresponding to a quantum conversion efficiency of approximately 35%. Vapor-phase mid-infrared spectra of common explosives (TNT, TATP, RDX, PETN and Tetryl) were acquired using the CRDS technique. Parts-per-billion concentration levels were readily detected with no sample preconcentration. A collection/flash-heating sequence was implemented in order to enhance detection limits for ambient air sampling. Detection limits as low as 75 ppt for TNT are expected, with similar concentration levels for the other explosives. Received: 1 April 2002 / Revised version: 13 June 2002 / Published online: 12 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-408/524-0551, E-mail: mtodd@picarro.com     

Generation of 100 μJ pulses at 82.8 nm by frequency tripling of sub-picosecond KrF laser radiation

. Investigations of the efficient generation of powerful coherent radiation at 82.8 nm by frequency tripling of short-pulse KrF laser radiation are presented. Argon gas is selected as nonlinear medium due to the resonantly enhanced 3rd-order susceptibility χ⁽³⁾(-3ω,ω,ω,ω). Pulse energies of 100 μJ at 82.8 nm have been measured for a pump pulse energy of 14 mJ. An upscaling to more than 500 μJ is expected with available more powerful pump lasers. Features of this XUV source and possible applications are discussed. Received: 26 July 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-511/7622211, E-mail: reinhardt@iqo.uni-hannover.de     

Resonances and spectral properties of detuned OPOs pumped by fluctuating sources

Twin-beam fluctuations are analyzed for detuned and mismatched OPO configurations. Resonances and frequency responses to the quantum noise sources (quantum and pump amplitude/phase fluctuations) are examined as functions of cavity decay rates, excitation parameters and detuning. The dependence of self- and mutual correlations of beam amplitudes and phases on detuning, mismatch and damping parameters is discussed. Received: 25 January 2002 / Revised version: 28 May 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +39-81/676-346, E-mail: alberto.porzio@na.infn.it     

Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection

The development of a compact tunable mid-IR laser system at 3.5 μm for quantitative airborne spectroscopic trace gas absorption measurements is reported. The mid-IR laser system is based on difference frequency generation (DFG) in periodically poled LiNbO₃ and utilizes optical fiber amplified near-IR diode and fiber lasers as pump sources operating at 1083 nm and 1562 nm, respectively. This paper describes the optical sensor architecture, performance characteristics of individual pump lasers and DFG, as well as its application to wavelength modulation spectroscopy employing an astigmatic Herriott multi-pass gas absorption cell. This compact system permits detection of formaldehyde with a minimal detectable concentration (1σ replicate precision) of 74 parts-per-trillion by volume (pptv) for 1 min of averaging time and was achieved using calibrated gas standards, zero air background and rapid dual-beam subtraction. This corresponds to a pathlength-normalized replicate fractional absorption sensitivity of 2.5×10^-10 cm^-1. Received: 29 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-1492, E-mail: dr@ucar.edu     

Table-top KrF amplifier delivering 270 fs output pulses with over 9 W average power at 300 Hz

Applying the combination of a solid-state Ti:Sa laser system and a newly developed wide-aperture, discharge-pumped KrF amplifier, output pulses with over 9 W average power at 300 Hz have been achieved in a single output beam. The frequency-tripled seed pulses of the Ti:Sa system – delivering approximately 10 μJ energy at 248 nm – were amplified to over 30 mJ using a 3-pass off-axis amplification scheme. The optical set-up has been fitted to the amplifier’s parameters, and stored-energy measurements were carried out with different parameters in order to optimize the operational conditions of the device for the highest average power. Received: 10 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-551/503-599, E-mail: jbekesi@llg.gwdg.de     

Multiply scattered component of lidar returns

The results of statistical simulation of the spatiotemporal structure of the multiply scattered component of lidar returns by the Monte Carlo method are discussed for the case of monostatic sensing geometry. The spatial characteristics of the region of the medium where occurs the last scattering of photon before arriving at the reciever. This region of the medium is called the instantaneous brightness body of multiply scattered radiation. It is demonstrated that the instantaneous brightness body of multiply scattered radiation that propagates toward the receiver may occupy a large volume that does not necessarily coincide with the region of formation of the singly scattered component. The main factors influencing the spatial and brightness characteristics of this volume source are established. The effect of scattering order on the spatiotemporal structure of lidar returns is analyzed for the case of sensing of aerosol haze and advective and radiative fogs with optical thickness 2<τ<8. Received: 2 August 2001 / Revised version: 7 January 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +7-38/2225-8026, E-mail: belov@iao.ru     

GaN nanotweezers

A new form of GaN nanomaterial (nanotweezers) has been obtained by chemical vapor deposition on an etched cubic MgO (100) plane. The nanotweezers consist of a bottom rod and two arms. The bottom rods have diameters of about 100–150 nm and lengths of about 200–500 nm, on which two arms grow out. The bottoms of the arms are about 40–70 nm and the tops are about 15–30 nm in diameter, and 0.8–1.5 μm in length. X-ray and electron diffractions indicate the nanotweezers are zinc blende gallium nitride. We infer that the fabrication of the GaN nanotweezers is associated with small convex hillocks on the surface of the etched cubic MgO (100) single-crystal substrates and that the nanotweezers grow by a growth mechanism that is similar to vapor-phase heteroepitaxy. Received: 23 April 2002 / Accepted: 25 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/8264-9531, E-mail: xlchen@aphy.iphy.ac.cn     

Detection of N2O by photoacoustic spectroscopy with a compact, pulsed optical parametric oscillator

A pulsed optical parametric oscillator (OPO) operated in an optical cavity with a grazing-incidence grating configuration (GIOPO) was used for sensitive photoacoustic detection of trace quantities of dinitrogen oxide (N₂O). The (ν₁+ν₃) combination vibration band of N₂O was excited with the idler beam of the GIOPO at 2.86 μm using an optical cavity optimized for the idler beam. The linewidth of the GIOPO could be reduced to 0.4 cm^-1, allowing the rotational structure of the absorption spectrum to be resolved. A concentration sensitivity (signal-to-noise ratio=3) of 60 parts in 10⁹ by volume (60 ppb V) N₂O in synthetic air was obtained. This may be sufficient for continuous monitoring of N₂O in the atmosphere. Received: 29 April 2002 / Revised version: 4 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +49-6221/54-4255, E-mail: peter.hess@urz.uni-heidelberg.de     

A simple wet-chemical synthesis and characterization of CuO nanorods

Using a simple wet-chemical route, we synthesized CuO nanorods with diameters of ca. 5–15 nm and lengths of up to 400 nm. The purity, crystallinity, morphology, structure features, and chemical composition of the as-prepared CuO nanorods were investigated by powder X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Received: 22 March 2002 / Accepted: 12 June 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: wangqun@nju.edu.cn     

Low-threshold field electron emission of Si micro-tip arrays produced by laser ablation

Low-threshold field electron emission (FEE) is reported for periodic arrays of micro-tips produced by laser ablation of Si wafers. The best samples show emission at threshold fields as low as 4–5 V/μm for n-type Si substrates and of 1–2 V/μm for p-doped Si substrates, as measured with a flat-screen technique. Auger electron spectroscopy and X-ray electron spectroscopy reveal island-like deviation of the SiO₂ stoichiometry on the tip surfaces, with lateral dimensions of less than 100 nm. Microscopic studies using a special field-emission STM show that the emission originates from well-conducting regions of sub-micron size. The experimental data suggest FEE from the tip arrays by a geometric field enhancement of both the individual micro-tip and the narrow conducting channels in the tip body. Received: 3 May 2002 / Accepted: 1 July 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/135-82-34, E-mail: shafeev@kapella.gpi.ru     

Evidence for the existence of an unusual structure in the nanorods of InSb

Motivated by the recent prediction that InSb nanometer-diameter filaments can exhibit exotic physical properties, we have synthesized this material and have characterized the filaments. In this report, we describe the finding of an unusual phase of InSb. This new phase occurs when an InSb filament has a diameter of less than 100 nm. High-resolution X-ray-diffraction studies of InSb filaments, having radii ≅33.0 nm, indicate the InSb crystal lattice to be tetragonal, S.G. I4/mmm, with a unit-cell volume 12 times larger than that of the β-tin phase observed in bulk InSb above 2.0 GPa. The density of an InSb nanorod, as calculated from X-ray-diffraction data, is ∼22% more than that of the zinc-blende phase of InSb at ambient conditions, implying that the nanorod experiences a pressure of ∼2 GPa. In contrast, InSb nanorods with radii ≥50.0 nm are observed to show zinc-blende structure and densities nearly the same as that of the bulk. Received: 6 February 2002 / Accepted: 26 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-202/767-5301; E-mail: yousuf@seas.gwu.edu     

Large-scale synthesis of crystalline β-SiC nanowires

Large quantities of high-purity crystalline β-SiC nanowires have been synthesized at relatively low temperature via a new simple method, the chemical-vapor-reaction approach, in a home-made graphite reaction cell. A mixture of milled Si and SiC powders and C₃H₆ were employed as the starting materials. The results show that the nanowires with diameters of about 10–35 nm are single crystalline β-SiCwithout any wrapping of amorphous material, and the nanowire axes lie along the 〈111〉 direction. Some unique properties are found in the Raman scattering from the β-SiC nanowires, which are different from previous observations of β-SiC materials. A possible growth mechanism for the β-SiC nanowires is proposed. Received: 27 August 2002 / Accepted: 28 August 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-29/8491-000, E-mail: zjli-sohu@sohu.com     

Optical spectroscopy and diode-pumped laser characteristics of codoped Tm-Ho : YLF and Tm-Ho : BaYF : a comparative analysis

Single crystals of monoclinic BaY₂F₈ and tetragonal LiYF₄ codoped with the same Tm³⁺ and Ho³⁺ concentrations were successfully grown by the Czochralski method. Here we present a comparative analysis of the two hosts including spectroscopic characterization and cw diode-pumped laser experiments in the 2-μm wavelength region at room temperature. The main differences between the two hosts are a lower slope efficiency associated with a much wider tuning range (2005–2094 nm) of BaY₂F₈ with respect to LiYF₄. Received: 29 July 2002 / Published online: 11 December 2002 RID="*" ID="*"Corresponding author. Fax: +39-050/2214-333, E-mail: toncelli@df.unipi.it     

Efficient generation of blue light by frequency doubling of a Nd:YAG laser operating on 4 F 3/24 I 9/2 transitions

Efficient room-temperature operation of ⁴ F _3/2⁴ I _9/2 transitions in diode-end-pumped Nd:YAG lasers at 946 nm and 938.5 nm is reported. 7.0-W continuous-wave output power at 946 nm and 3.9 W at 938.5 nm have been obtained. An analytical model has been developed for the quasi-three-level laser including the influence of energy-transfer upconversion. Frequency doubling of these transitions in periodically poled KTP generated blue light at 473 nm and 469 nm. Both single-pass extra-cavity as well as intracavity schemes have been investigated. Received: 31 July 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +46-8/750-5430, E-mail: stefan.bjurshagen@acreo.se     

Micromagnetism of two-dimensional permalloy particles with different aspect ratios

Micromagnetic properties of the Fe₁₉Ni₈₁ (5 nm)/NiO (50 nm)/Fe₁₉Ni₈₁ (30 nm) structured system have been investigated in a photoemission electron microscope in the magnetic X-ray circular dichroism operating mode. The microstructured Fe₁₉Ni₈₁ (5 nm) film contained two-dimensional islands with the aspect ratio varying from 1:1 to 10:1, and the linear size of their long axis comprised 24, 12 and 6 μm. It is shown that the magnetic domains have the direction of magnetization preferentially parallel and antiparallel to the magnetic field direction in which this system was prepared. Their number is determined by the particles’ sizes, their shape as well as by the direction of the external magnetizing field and can be characterized by a non-monotonic size dependence. The magnetization of domains with different lateral sizes was found to be 0.4 T with an accuracy better than 20%. Received: 29 May 2002 / Accepted: 17 July 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-6131/392-3807, E-mail: nepijko@mail.uni-mainz.de