Showing papers in "Applied Physics B in 2002"

Laser wake field acceleration: the highly non-linear broken- wave regime

Abstract: We use three-dimensional particle-in-cell simulations to study laser wake field acceleration (LWFA) at highly relativistic laser intensities. We observe ultra-short electron bunches emerging from laser wake fields driven above the wave-breaking threshold by few-cycle laser pulses shorter than the plasma wavelength. We find a new regime in which the laser wake takes the shape of a solitary plasma cavity. It traps background electrons continuously and accelerates them. We show that 12-J, 33-fs laser pulses may produce bunches of 3×1010 electrons with energy sharply peaked around 300 MeV. These electrons emerge as low-emittance beams from plasma layers just 700-μm thick. We also address a regime intermediate between direct laser acceleration and LWFA, when the laser-pulse duration is comparable with the plasma period.

Sensitive absorption measurements in the near-infrared region using off-axis integrated-cavity-output spectroscopy

Abstract: A novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of gas mixing ratios using near-infrared diode lasers and absorption-spectroscopy techniques. The instrument employs an off-axis trajectory of the laser beam through the cell to yield an effective optical path length of several kilometers without significant unwanted effects due to cavity resonances. As a result, a minimum detectable absorption of approximately 1.4×10-5 over an effective optical path of 4.2 km was obtained in a 1.1-Hz detection bandwidth to yield a detection sensitivity of approximately 3.1×10-11 cm-1 Hz-1/2. The instrument has been used for sensitive measurements of CO, CH4, C2H2 and NH3.

Reconstruction of attosecond harmonic beating by interference of two-photon transitions

Abstract: A method is proposed for detailed determination of the temporal structure of XUV pulses. The method is especially suited for diagnostics on attosecond pulses and pulse trains that originate from temporal beating of various harmonics of an ultrashort laser pulse. A recent experiment already showed the feasibility of this method when applied to long attosecond pulse trains, where it measured the average pulse characteristics. Here we argue that the same method is also suitable for determining differences between the individual attosecond pulses in a short train, or the properties of a single attosecond pulse.

Microchip traps and Bose-Einstein condensation

Abstract: The article gives an overview of the rapidly evolving field of magnetic microchip traps (also called ‘atom chips’) for neutral atoms Special attention is given to Bose–Einstein condensation in such traps, to the particular properties of microchip trap potentials, and to practical considerations in their design Scaling laws are developed, which lead to an estimate of the ultimate confinement that chip traps can provide Future applications such as integrated atom interferometers are discussed

Kerr-lens, mode-locked lasers as transfer oscillators for optical frequency measurements

Abstract: We introduce a novel concept for optical frequency measurement and division which employs a Kerr-lens, mode-locked laser as a transfer oscillator whose noise properties do not enter the measurement process. We experimentally demonstrate that this method opens up the route to phase-link signals with arbitrary frequencies in the optical or microwave range while their frequency stability is preserved.

Sub-part-per-billion detection of nitric oxide in air using a thermoelectrically cooled mid-infrared quantum cascade laser spectrometer

Abstract: Non-cryogenic, laser-absorption spectroscopy in the mid-infrared has wide applications for practical detection of trace gases in the atmosphere. We report measurements of nitric oxide in air with a detection limit less than 1 nmole/mole (<1 ppbv) using a thermoelectrically cooled quantum cascade laser operated in pulsed mode at 5.26 μm and coupled to a 210-m path length multiple-pass absorption cell at reduced pressure (50 Torr). The sensitivity of the system is enhanced by operating under pulsing conditions which reduce the laser line width to 0.010 cm-1 (300 MHz) HWHM, and by normalizing pulse-to-pulse intensity variations with temporal gating on a single HgCdTe detector. The system is demonstrated by detecting nitric oxide in outside air and comparing results to a conventional tunable diode laser spectrometer sampling from a common inlet. A detection precision of 0.12 ppb Hz-1/2 is achieved with a liquid-nitrogen-cooled detector. This detection precision corresponds to an absorbance precision of 1×10-5 Hz-1/2 or an absorbance precision per unit path length of 5×10-10 cm-1 Hz-1/2. A precision of 0.3 ppb Hz-1/2 is obtained using a thermoelectrically cooled detector, which allows continuous unattended operation over extended time periods with a totally cryogen-free instrument.

Artifacts in femtosecond transient absorption spectroscopy

Abstract: The paper discusses three different artifacts related to two-photon absorption (TPA), stimulated Raman amplification (SRA) and cross-phase modulation (XPM), all intrinsic to transient absorption measurements with femtosecond time resolution. Certain properties of these signals are analysed and shown to superimpose onto measured transient absorption spectra. Ways of reducing the influence of the artifacts discussed are suggested. A simple correcting procedure based on the linear intensity dependence of the artifacts discussed is proposed.

Design and characterization of a near-diffraction-limited femtosecond 100-TW 10-Hz high-intensity laser system

Abstract: We present the design and characterization of a femtosecond high-intensity laser system emitting a near-diffraction-limited beam. This system was dimensioned in order to reach intensities in excess of 1020 W/cm2 at a high repetition rate for ultrahigh-field physics experiments. We describe the improvements that were added to a conventional chirp pulse amplification configuration in order to decrease the deleterious effects of gain narrowing, gain shifting, thermal focusing in the amplifier stages, and spatial degradation due to multipass amplification processes.

Wavelength-agile fiber laser using group-velocity dispersion of pulsed super-continua and application to broadband absorption spectroscopy

Abstract: A swept-wavelength source is created by connecting four elements in series: a femtosecond fiber laser at 1.56 μm, a non-linear fiber, a dispersive fiber and a tunable spectral bandpass filter. The 1.56-μm pulses are converted to super-continuum (1.1–2.2 μm) pulses by the non-linear fiber, and these broadband pulses are stretched and arranged into wavelength scans by the dispersive fiber. The tunable bandpass filter is used to select a portion of the super-continuum as a scan-wavelength output. A variety of scan characteristics are possible using this approach. As an example, an output with an effective linewidth of approximately 1 cm-1 is scanned from 1350–1550 nm every 20 ns. Compared to previous scanning benchmarks of approximately 1 nm/μs, such broad, rapid scans offer new capabilities: a gas sensing application is demonstrated by monitoring absorption bands of H2O, CO2, C2H2 and C2H6O at a pressure of 10 bar.

100-W average-power, high-energy nanosecond fiber amplifier

Abstract: We report on the fiber-based amplification of a Q-switched Nd:YAG thin-disk laser. At repetition rates between 3 and 50 kHz output powers up to 100 W are generated. Pulse energies up to 4 mJ, with diffraction-limited beam quality, are generated in a 30-μm Yb-doped large-mode-area fiber, furthermore pulse energies up to 8 mJ are achieved from a multimode fiber amplifier.

Time-resolved two photon photoemission electron microscopy

Abstract: Femtosecond, time-resolved two photon photoemission has been used to map the dynamics of photo-excited electrons at a structured metal/semiconductor surface. A photoemission microscope was employed as a spatially resolving electron detector. This novel setup has the potential to visualize variations of hot electron lifetimes in the femtosecond regime on heterogeneous sample surfaces and nanostructures.

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

Abstract: 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.

Material tensor parameters of LiNbO3 relevant for electro- and elasto-optics

Abstract: The complete set of self-consistent parameters of nominally undoped LiNbO3 crystals of congruent composition that describe the electro-optic, piezoelectric, elasto-optic, elastic, and dielectric response has been determined by numerically evaluating available measurements. The parameters were determined at room temperature and consist of the low-frequency clamped dielectric constants eS ij, elastic stiffness constants at constant electric field CE ijkl, piezoelectric stress coefficients eijk, elasto-optic constants at constant electric field pE ijkl, and clamped electro-optic coefficients rS ijk. It is shown that the complete set is required for calculating the effective electro-optic coefficients and dielectric constants in photorefractive applications of LiNbO3.

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

Abstract: 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.

High-energy ion generation in interaction. of short laser pulse with high-density plasma

Abstract: Multi-MeV ion production from the interaction of a short laser pulse with a high-density plasma, accompanied by an underdense preplasma, has been studied with a particle-in- cell simulation and good agreement is found with experiment. The mechanism primarily responsible for the acceleration of ions is identified. Comparison with experiments sheds light on the ion-energy dependence on laser intensity, preplasma scale length, and relative ion energies for a multi-species plasma. Two regimes of maximum ion-energy dependence on laser inten- sity, I, have been identified: subrelativistic, ∝ I; and relativistic, ∝ √ I. Simulations show that the energy of the accelerated ions versus the preplasma scale length increases linearly and then saturates. In contrast, the ion energy decreases with the thick- ness of the solid-density plasma.

Digital, phase-sensitive detection for in situ diode-laser spectroscopy under rapidly changing transmission conditions

Abstract: A new harmonic detection scheme for fully digital, fast-scanning wavelength-modulation spectroscopy (DFS-WMS) is presented. DFS-WMS is specially suited for in situ absorption measurements in combustion environments under fast fluctuating transmission conditions and is demonstrated for the first time by open-path monitoring of ambient oxygen using a distributed-feedback diode laser, which is doubly modulated with a fast linear 1 kHz-scan and a sinusoidal 300 kHz-modulation. After an analog high-pass filter, the detector signal is digitized with a 5 megasample/s 12-bit AD-converter card plugged into a PC and subsequently – unlike standard lock-ins – filtered further by co-adding 100 scans, to generate a narrowband comb filter. All further filtering and the demodulation are performed completely digitally on a PC with the help of discrete Fourier transforms (DFT). Both 1f- and 2f-signals, are simultaneously extracted from the detector signal using one ADC input channel. For the 2f-signal, a linearity of 2% and a minimum detectable absorption of 10-4 could be verified experimentally, with the sensitivity to date being limited only by insufficient gain on the 2f-frequency channel. Using the offset in the 1f signal as a transmission ‘probe’, we could show that the 2f-signal can be transmission-corrected by a simple division by the 1f-background, proving that DFS-WMS provides the possibility of compensating for transmission fluctuations. With the inherent suppression of additive noise, DFS-WMS seems well suited for quantitative in situ absorption spectroscopy in large combustion systems. This assumption is supported by the first measurements of oxygen in a high-pressure combustor at 12 bar.

Characterization of laser-ablated and chemically reduced silver colloids in aqueous solution by UV/VIS spectroscopy and STM/SEM microscopy

Abstract: Silver colloids in aqueous solution were studied by different scanning microscopy techniques and UV/VIS spectroscopy. The silver colloids were produced either by chemical reduction or by nanosecond laser ablation from a solid silver foil in water. Variation of laser power and ablation time leads to solutions of metal clusters of different sizes in water. We characterized the electronic absorption of the clusters by UV/VIS spectroscopy. STM (scanning tunneling microscope) imaging of the metal colloids shows atomic resolution of rod- or tenon-like silver clusters up to 10-nm length formed by laser ablation. Our scanning electron microscope measurements, however, show that much larger silver colloids up to 5-μm length are also formed, which are not visible in the STM due to their roughness. We correlate them with the long-wavelength tail of the multimodal UV/VIS spectrum. The silver colloids obtained by chemical reduction are generally larger and their electronic spectra are red-shifted compared to the laser-ablated clusters. Irradiation of the colloid solution with nanosecond laser pulses of appropriate fluence at 532 nm and 355 nm initially reduced the colloid size. Longer irradiation at 355 nm, however, leads to the formation of larger colloids again. There seems to be a critical lower particle size, where silver clusters in aqueous solution become unstable and start to coagulate.

Laser frequency stabilization by Doppler-free magnetic dichroism

Abstract: We report a method of laser stabilization to an atomic transition frequency, which is based on a magnetically induced nonlinear circular dichroism in the counter-propagating pump-probe configuration. The proposed method does not require any modulation of the laser frequency. In comparison to the linear, Doppler-broadened dichroic lock, the proposed version provides a precise locking to a well-defined atomic resonance at a cost of a smaller capture range. A simple model description of the locking signal is presented and two experimental realizations are shown: for a dye laser at λ=589.6 nm (Na D1 line) and for a diode laser at 795 nm (Rb D1).

Development and characterization of Yb-Er laser glass for high average power laser diode pumping

Abstract: A new strong erbium laser glass (SELG) based on a boro-alumo-phosphate composition is reported. We discuss the synthesis and chemical properties together with spectroscopic and thermo-mechanical da ...

Acceleration of ultra-relativistic electrons using high-intensity TM01 laser beams

Abstract: It is well known that very high axial – or longitudinal – electric fields can be obtained in vacuo using very intense focused laser beams. If these fields are in the transverse magnetic TM01 mode, very high gains in energy (exceeding 1 GeV) can be obtained if sufficiently energetic ultra-relativistic electrons (say 1 GeV) are injected on-axis and near the optimum phase. This energy gain is obtained even though the phase velocity of the axial component of the TM01 laser beam is greater than the vacuum velocity of light. The result is apparently related to the Gouy phase shift at focus.

Filamentation of femtosecond laser pulses in turbulent air

Abstract: Formation and wandering of filaments in air are studied both experimentally and numerically. Filament-center deflections are collected from 1100 shots of 190-fs and 800-nm pulses in the plane perpendicular to the propagation direction. To calculate the filament wandering in air we have developed a model of powerful femtosecond laser pulse filamentation in the Kolmogorov atmospheric turbulence and employed the Monte Carlo method to model the propagation of several hundred laser pulses. Statistical processing of experimental and numerical data shows that filament-center displacements in the transverse plane obey the Rayleigh-distribution law. Parameters of the Rayleigh distribution obtained for numerical and experimental data are close to each other.

Adaptive shaping of two-cycle visible pulses using a flexible mirror

Abstract: We present a double-pass non-collinear optical parametric amplifier (NOPA) that delivers 4-fs visible–near-infrared pulses. The paper discusses geometrical and temporal properties of the NOPA configuration, which allow us to broaden the gain band of simultaneous parametric amplification in excess of 250 THz. The key elements of the bandwidth enhancement include (i) the use of a broadband second-harmonic pump, (ii) optimization of the incidence angle of all monochromatic spectral components of the pump in order to produce a favorable offset of the corresponding phase-matching curves, and (iii) selection of the chirp rate and time delay between the stretched pump and seed pulses. We next devise a two-stage compressor that incorporates a flexible mirror for adaptive pulse shaping and develop a simple and trustworthy feedback loop based on a one-dimensional spectral measurement. Our rapid numerical algorithm for adaptive control of the flexible membrane is found superior to more complex search routines that are less resistant to the fluctuations of the laser intensity. The automated optimization procedure results in the generation of two-cycle pulses with a carrier wavelength at ∼600 nm. The absence of deep modulation on the parametrically amplified spectrum in combination with the adaptive phase correction lead to a high quality of the temporal profile and allow concentrating 90% of the pulse energy within only a 7.5-fs time window.

Reverse-symmetry waveguides: theory and fabrication

Abstract: We present an extensive theoretical analysis of reverse-symmetry waveguides with special focus on their potential application as sensor components in aqueous media and demonstrate a novel method for fabrication of such waveguides. The principle of reverse symmetry is based on making the refractive index of the waveguide substrate less than the refractive index of the medium covering the waveguiding film (nwater=1.33). This is opposed to the conventional waveguide geometry, where the substrate is usually glass or polymers with refractive indices of ≈1.5. The reverse configuration has the advantage of deeper penetration of the evanescent electromagnetic field into the cover medium, theoretically permitting higher sensitivity to analytes compared to traditional waveguide designs. We present calculated sensitivities and probing depths of conventional and reverse-symmetry waveguides and describe schemes for easy implementation of reverse symmetry. Polymer waveguides are demonstrated to be candidates for cheap, mass-producible reverse-symmetry sensor modules. The grating-coupled waveguiding films of controlled thickness are produced by soft lithography. The resulting films are combined with air-grooved polymer supports to form freestanding single-material polymer waveguides of reverse symmetry capable of guiding light.

Generation and characterization of polarization-shaped femtosecond laser pulses

Abstract: We demonstrate the generation and complete characterization of femtosecond laser pulses which change their intensity, frequency, and light polarization almost arbitrarily within a single pulse employing the new technique of femtosecond polarization pulse shaping. Specifically, the degree of polarization ellipticity as well as the orientation of the elliptical principal axes can be varied as a function of time in a completely controllable manner, using a 128-pixel, two-layer, liquid-crystal display (LCD) inside a zero-dispersion compressor. A mathematical formalism is presented with which polarization-shaped pulse parameters can be calculated and used to generate intuitive quasi-three-dimensional electric field representations. However, laboratory realization requires accurate and complete experimental pulse characterization methods. For this purpose, the technique of dual-channel spectral interferometry is employed. Furthermore, Jones calculus for polarization-shaped pulses with experimentally determined Jones matrices is developed. It can be used to predict and account for all pulse-shape modifications occurring in various optical elements of the pulse shaper and the experimental setup.

Nonlinear side effects of fs pulses inside corneal tissue during photodisruption

Abstract: In order to evaluate the potential for refractive surgery, fs laser pulses of 150-fs pulse duration were used to process corneal tissue of dead and living animal eyes. By focusing the laser radiation down to spot sizes of several microns, very precise cuts could be achieved inside the treated cornea, accompanied with minimum collateral damage to the tissue by thermal or mechanical effects. During histo-pathological analysis by light and transmission electron microscopy considerable side effects of fs photodisruption were found. Due to the high intensities at the focal region several nonlinear effects occurred. Self-focusing, photodissociation, UV-light production were observed, leading to streak formation inside the cornea.

Comparison between c-cut and a-cut Nd:YVO4 lasers passively Q-switched with a Cr4+:YAG saturable absorber

Abstract: Comparison between c-cut and a-cut Nd:YVO4 microchip lasers passively Q-switched with a Cr4+:YAG saturable absorber is experimentally made. The lower emission cross section of the c-cut Nd:YVO4 crystal can enhance the passive Q-switching effect to produce a peak power 10 times higher than that obtained with the a-cut crystal. The experimental result further reveals that a c-cut Nd:YVO4 crystal is a very convenient material for short-pulse (sub-nanosecond) and high-peak-power (>10 kW) lasers.

Spatio-temporal characterization of the electric field of ultrashort optical pulses using two-dimensional shearing interferometry

Abstract: We report the complete spatio-temporal characterization of ultrashort light pulses using a self-referencing device based on shearing interferometry in the space and frequency domains The apparatus combines spectral phase interferometry for direct electric field reconstruction with a spectrally resolved spatial shearing interferometer The electric field as a function of one transverse spatial coordinate and time is obtained by means of a direct algebraic phase-reconstruction algorithm The method has been tested on several common laboratory situations in which space–time coupling is quantified

Applications of Kalman filtering to real-time trace gas concentration measurements

Abstract: A Kalman filtering technique is applied to the simultaneous detection of NH3 and CO2 with a diode-laser-based sensor operating at 1.53 micrometers. This technique is developed for improving the sensitivity and precision of trace gas concentration levels based on direct overtone laser absorption spectroscopy in the presence of various sensor noise sources. Filter performance is demonstrated to be adaptive to real-time noise and data statistics. Additionally, filter operation is successfully performed with dynamic ranges differing by three orders of magnitude. Details of Kalman filter theory applied to the acquired spectroscopic data are discussed. The effectiveness of this technique is evaluated by performing NH3 and CO2 concentration measurements and utilizing it to monitor varying ammonia and carbon dioxide levels in a bioreactor for water reprocessing, located at the NASA-Johnson Space Center. Results indicate a sensitivity enhancement of six times, in terms of improved minimum detectable absorption by the gas sensor.

Laser operation of the new stoichiometric crystal KYb(WO4)2

Abstract: We demonstrate for the first time laser operation with the monoclinic stoichiometric crystal KYb(WO4)2 (hereafter KYbW). Lasing on the 2 F 5/2 ?2 F 7/2 transition of Yb3+ at room temperature has been achieved near 1074 nm with >41% slope efficiency (>26% maximum conversion efficiency) using a 0.5-mm-thick plate of KYbW. This new laser material holds great promise for diode-pumped high-power lasers, thin-disk and waveguide designs as well as for ultrashort (ps/fs) pulse laser systems.

Detection of small forest fires by lidar

Abstract: The possibility of detecting small forest fires with the help of a simple and cheap lidar operating at 0.532-μm wavelength up to distances of about 6.5 km is demonstrated. The values of the signal-to-noise ratio (SNR) achieved in the experiments are consistent with theoretical estimations obtained by computational modeling of the lidar detection process, including simulation of the smoke-plume shape and of the laser beam–plume interaction. This model was used to assess the potential of the lidar technique for fire surveillance in large forest areas. In particular, the upper limiting range for effective detection (SNR>5) of small localized fires in dry- and clear-weather conditions is estimated at 7–15 km depending on operation mode, burning rate, and observation geometry.