Tunable Pulsed Laser Research Articles

Isobar suppression in AMS using laser photodetachment

We are investigating the possibility of using laser photodetachment of negative atomic and molecular ions as an additional isobaric selection filter in accelerator mass spectrometry. The aim of this study is to find a possibility to further improve the detection limit for long-lived heavy radionuclides at AMS facilities. We will focus on the astrophysical relevant radionuclide 182Hf, which is one of the isotopes measured with the 3MV tandem AMS facility VERA (Vienna Environmental Research Accelerator) at the University of Vienna. Laser-induced isobar suppression is also of importance for radioactive-beam facilities.The present detection limit for measuring the isotope ratio 182Hf/Hf at VERA is 1×10-11. The limiting factor is the strong background of the stable isobar 182W. Currently this background is suppressed using suitable molecular ions in the injection stage. Selective laser photodetachment of the negative ions at the injector can lead to an additional suppression of the interfering isobar. Test experiments have been carried out at the negative ion laser spectroscopy setup at Göteborg University. In a small ion beam apparatus pulsed tunable laser radiation is used to measure the photodetachment cross-section of different atomic and molecular negative ions. We will present studies of the photodetachment process for various tungsten and hafnium molecules with the aim to find a selective isobaric suppression scheme using laser photodetachment spectroscopy in combination with AMS.

Generation of defects in amorphousSiO2 assisted by two-step absorption on impurity sites

Generation of the Si dangling bond defect in amorphousSiO2 (E′ centre) induced by tunable pulsed UV laser radiation was investigated by in situ opticalabsorption measurements. The defect generation efficiency peaks when the photon energy equals∼5.1 eV, it depends quadratically on laser intensity and is correlated with the native linearabsorption due to Ge impurities. We propose a model in which the generation ofE′ is assisted by a two-step absorption process occurring on Ge impurity sites.

Instrumental factors in resonance enhanced multi-photon ionization of FIB-sputtered atoms

Resonance enhanced multi-photon ionization (REMPI) is a promising ionization technique for realizing both the high sensitivity and selectivity. The authors developed an apparatus equipped with a gallium focused ion beam (FIB) and a tunable pulsed dye laser system for REMPI for micro- and nano-scale analysis of solid surfaces. For the practical application of REMPI method to FIB-sputtered atoms, many factors affecting REMPI intensity should be considered and optimized. This paper will discuss sensitivity and stability in relation to the laser beam stability, and the optimization of the extraction of laser-ionized species into time-of-flight mass spectrometry (TOF-MS).

Selective ultrasonic focusing towards an optical contrast agent by use of photoacoustic‐guided time‐reversal

We recently demonstrated experimentally that optically absorbing regions embedded at centimetric depths in an optically diffusive medium can be used, through the photoacoustic effect, as ultrasound sources for time‐reversal techniques [Bossy et al., APL 89(18), 2006]. If the optically absorbing region is spatially localized, the emission of the time‐reversed photoacoustic waves provides a method to automatically focus ultrasound towards the absorbing region. In biological tissue, this approach has the potential to automatically guide high‐intensity focused ultrasound (HIFU) beams towards a targeted region labelled with an optically absorbing contrast agent. However, such an approach is conceivable only if the HIFU beam is guided towards the sole targeted region, and not towards other potentially strong endogenous optical absorbers (such as blood vessels). In this work, we present a method to selectively focus time‐reversed photoacoustic waves towards an exogenous wavelength‐dependent optical contrast agent. Experiments were performed in vitro on tissue‐mimicking phantoms, with nano‐particles as contrast agents. A tunable nanosecond pulsed laser was used with different optical wavelengths to generate photoacoustic waves. A multi‐channel time‐reversal ultrasound system was used to emit ultrasound signals derived from time‐reversed photoacoustic signals obtained with different optical wavelengths.

Development of a Tunable Ultrashort Dye Laser System by Using Molecular Photonic Processes and a Nanosecond Pumping Laser

The paper presents the physical processes and operational characteristics of an original subpicosecond dye laser system which is designed and successfully developed at our laboratory basing on the combination use of three photonic processes of dye molecules: 1) Fast spectrotemporal evolutions in the broadband dye laser emissions; 2) Generation of a chain of ultrashort pulses (spikes) from low-Q micro-cavity dye lasers; and 3) Non-linear resonant interaction between ultrashort laser pulses and highly-saturated dye media. Such a laser system provides a stable generation of single 500 fs pulses with a peak power of 300 MW at 606 nm. Particularly, the whole sub-picosecond laser system is pumped by a single nanosecond laser (Q-switched Nd:YAG laser). The spectral and time processes involved in these pulse-shortening methods are analyzed with a rate-equation model extended to wavelengths. A white light continuum generation was obtained by focusing the 500 fs pulses into 2 cm water cell. As a result, it enables us to produce widely tunable ultrashort laser pulses with a spectral selection and amplification of the supercontinuum spectrum.

Spectroscopic investigation of homoepitaxial CVD diamond for detection applications

Three-layer structures consisting of intrinsic/B-doped homoepitaxial CVD diamond grown onto commercial HPHT Ib substrates have been studied by means of Raman spectroscopy and photoluminescence (PL). The intrinsic layers have been deposited, at fixed methane to hydrogen ratio (1%), by systematically changing the substrate temperature (620–820 °C). Raman measurements point out the excellent crystalline quality and phase purity of the samples. Moreover, flat PL spectra in a wide energy range (1.7 eV–2.7 eV) indicate also their great purity. As the free-exciton recombination can be used to further probe the quality of synthetic diamond, measurements of free-exciton emission at room temperature have been also performed. The excitation was produced by a 5 ns pulsed tunable laser irradiation. The results have been compared with the detection characteristics of simple alpha-particle detector prototypes based on the analyzed samples. A clear correlation between excitonic emission and detector sensitivity is demonstrated. On the basis of these results, low methane concentrations (approx. 1% CH 4/H 2) in the deposition gas mixture and intermediate substrate temperatures (approx. 720 °C–770 °C) have been identified as the best working conditions of our growth reactor.

Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Using a Mid-Infrared Tunable Laser for Direct Protein Analysis

Matrix-assisted laser desorption/ionization (MALDI) using a mid-infrared (MIR) laser is a promising technique for the study of biomolecules. We achieved the measurement of an insoluble protein under strong denaturing conditions using two lasers, a UV laser and a free electron laser (FEL). The FEL is a powerful tool for the IR-MALDI; however, it is expensive and difficult to operate. We developed a MIR-MALDI time-of-flight mass spectrometer combined with a tunable MIR nanosecond pulsed laser, which was developed by the Institute of Physical and Chemical Research (RIKEN; Wako, Japan) and Kawasaki Heavy Industries, Ltd. (Tokyo, Japan). We evaluated the advantages of MIR-MALDI using this MIR nanosecond pulsed laser with a urea matrix and compared the results with that from previous study using the FEL. The molecular mass of insulin with urea was obtained at wavelength between 5.8 and 6.2 mum, which corresponds to the >C=O stretching vibration mode. In particular, a high SNR was observed at a wavelength of 5.9 mum. This technique produced a better SNR than that of a previous study that used the FEL.

High-energy pulsed tunable mid-infrared laser aids biomedical applications

High-energy pulsed tunable mid-infrared laser aids biomedical applications

Generation of Fiber Tunable High-power Picosecond Laser Pulse

A fiber tunable high-power picosecond laser system has been for the first time demonstrated. A gain switch semiconductor laser diode was introduced as seed source, and a multi-stage single mode Yb-doped fiber amplifier was combined with a single mode double-clad Yb-doped fiber amplifier to construct the amplification system. High stability and good beam quality pulses with 1MHz tunable repetition rate, 3W average power, 90ps pulse duration, and central wavelength tunable from 1053 nm to 1073 nm were generated. The completely fiber integrated approach has the potential to be scaled to significantly higher average powers.

Picosecond ultrasonic investigations of phonons in 2D nano-scaled lattices

We time-resolved the acoustical response of lattices of aluminum nano-dots with a step of a few hundreds nanometers using tunable femtosecond laser pulses in a pump and probe scheme. We detected two kinds of modes, the first being the individual modes of the dots. The other modes are shown to be both dependent on the dot size and on the lattice and are thus interpreted as collective modes. Using several step sizes we show that we can plot the phonon dispersion relation. A simple analytical model very well reproduces the data from which we can describe completely the dependence of the lattice modes on the sample parameters.

Polarized spectral properties of Yb3+ : Li2Gd4(MoO4)7 crystal: a candidate for tunable and ultrashort pulse lasers

Detailed polarized spectral properties of a 3.2 at.% Yb3+ : Li2Gd4(MoO4)7 crystal, including absorption cross-section, emission cross-section, up-conversion spectrum and intrinsic fluorescence lifetime, were investigated. The laser potentiality was also evaluated and the results show that this crystal is a good candidate for tunable and ultrashort pulse lasers.

Polarized spectral characterization and laser demonstration of Nd3+:Li2Gd4(MoO4)7 crystal

Detailed polarized spectral properties of Nd3+:Li2Gd4(MoO4)7 crystal grown by the Czochralski method have been investigated. The Judd-Ofelt intense parameters Ω2,4,6, the absorption and emission cross sections, and the branching ratios were calculated for each polarization of the main transitions. The large absorption cross section (16.6×10−20cm2 of π polarization) and broad absorption band (7nm of π polarization) around 804nm indicate that this crystal can be pumped efficiently by diode lasers. The broad emission bands from the F3∕24 multiplet show that the crystal is a promising medium for tunable and short pulse lasers. Pumped by a Ti:sapphire laser, the maximum 514mW quasi-cw laser output around 1060nm has been obtained with a 1.5% output couple, the threshold is 35mW, and the slope efficiency is 32%.

Dynamical ultrafast all-optical switching of planar GaAs∕AlAs photonic microcavities

The authors study the ultrafast switching-on and -off of planar GaAs∕AlAs microcavities. Up to 0.8% refractive index changes are achieved by optically exciting free carriers at λ=1720nm and pulse energy Epump=1.8±0.18μJ. The cavity resonance is dynamically tracked by measuring reflectivity versus time delay with tunable laser pulses, and is found to shift by as much as 3.3 linewidths within a few picoseconds. The switching-off occurs with a decay time of ∼50ps. They derive the dynamic behavior of the carrier density and of the complex refractive index. They propose that the inferred 10GHz switching rate may be tenfold improved by optimized sample growth.

COHERENCE AND SPIN RELAXATION OF INTERWELL EXCITONS IN GaAs/AlGaAs COUPLED QUANTUM WELLS

The kinetics of spin orientated interwell excitons gas has been investigated in GaAs / AlGaAs coupled quantum wells (n–i–n heterostructures). In these structures, the electrons and the holes are spatially separated between neighboring quantum wells by a narrow AlAs barrier. The time evolution and kinetics of photoluminescence spectra of interwell excitons have been measured under the pulse resonance excitation of intrawell 1sHH excitons using a pulsed tunable laser. The temporal formation of collective excitons phase and the temperature dependence of its spin relaxation rate have been studied. It was observed that spin relaxation rate of interwell excitons in the collective phase is strongly reduced. The observed effect is an evidence of the coherence of indirect excitons' collective phase at the temperatures below the critical one.

Spectroscopic properties of Er3+ ions in Li6Y(BO3)3 crystal

Pure and Er3+-doped Li6Y(BO3)3 single crystals have been grown by the Czochralski method. The angular relationship between the crystallographic axes and the principal axes of the optical indicatrix has been determined. The principal refractive indices of the pure Li6Y(BO3)3 crystal have been measured. The polarized absorption and fluorescence spectra of the Er3+-doped Li6Y(BO3)3 crystal have been recorded. Spectroscopic parameters of the Er3+-doped Li6Y(BO3)3 crystal, including the intensity parameters Ωt (t=2,4,6), spontaneous emission probabilities, fluorescence branching ratios, radiative lifetimes, fluorescence quantum efficiency, and stimulated emission cross sections have been calculated and analyzed. The results show that the Er3+-doped Li6Y(BO3)3 crystal may be a gain medium for tunable and ultra short pulse lasers around 1.55 μm.

Photothermal flow cytometry in vitro for detection and imaging of individual moving cells

Photothermal (PT) cytometry has recently demonstrated great potential for the label-free detection of nonfluorescent cells under static conditions. The goal of our investigation was to expand this technique to the detection of flowing cells in vitro. Cells in flow were irradiated with short, tunable laser pulses (420-2,300 nm, 8 ns), and the absorbed energy was detected by monitoring of the temperature-dependent variations in the refractive index in the cells with a second, collinear probe beam in two modes: (a) PT imaging of single cells with a pulsed probe beam (639 nm, 13 ns) and (b) thermolens monitoring of the integral PT responses from individual cells as whole with a continuous-wave probe beam (633 nm, 2 mW). PT flow cytometry at the current speed of analysis of 10 cell/s, with the capability to image selected cells of interest flowing at velocities up to 2 m/s, demonstrated the capability for (a) label-free detection of flowing single cells (e.g., blood and cancer cells) on the basis of the differences in their endogenous absorption properties, (b) identification of cells labeled with gold nanoparticles, (c) rapid cell viability testing, (d) aggregation immunoassay, and (e) optimization of selective nanophotothermolysis. PT cytometry can be extended to the study of cells in flow. This new technique increases the speed of cell analysis approximately 10(2) times over that of conventional PT technique, with the potential to achieve a rate of 10(4)-10(5) cells/s in specific PT applications, which has previously been realized only with cells under static conditions.

Combined time- and wavelength-division-multiplexing demodulation technique of fiber grating sensor arrays using a tunable pulsed laser

Combined time- and wavelength-division-multiplexing demodulation technique of fiber Bragg grating (FBG) sensor arrays using a tunable pulsed laser is proposed and demonstrated. A tunable fiber laser based on a matched FBG is applied. The wavelengths of the sensing FBGs are obtained by detecting the maximum voltages with a photodiode that avoids the complex demodulation process. The advantages of this scheme are simple structure, high signal-to-noise ratio, and the sensing signals obtained by detecting the maximum voltages.

Tunable subnanosecond laser pulse generation using an active mirror concept

We report (theory, experimental check) an improved approach for generation of a tunable, subnanosecond pulse (0.1–0.4 ns), based on a single pulsation (“spike”) separation from the transient oscillations in a dye laser with active mirror (AMIR). A pumping by 20–50 ns pulses from Q-switched Nd:YAG laser is considered. The separation is in original, two-spectral selective channels cavity, where the forced by AMIR quenched generation at one of the wavelength stops initially started spiking generation at the other wavelength after the first spike development. The AMIR quickly starts the quenching generation at a precisely controlled moment and with necessary intensity thus assuring the desired separation. An advantage is a high reproducibility of the separation for high (~250%) pump power fluctuations combined with tuning in large range (~20 nm). To obtain such an operation we form ~1 ns leading front pump pulse by electrooptical temporal cutting of the input pump pulse and use an optical delay line. This increases also a few times the power in the separated spike (to be ~100 kW). Our approach widens the combinations of lasers for effective applications of spike separation technique (dye lasers excited by Q-switched solid-state or Cu-vapor lasers).

Doppler-free Stark spectroscopy of the second excited level of atomic hydrogen for measurements of electric fields

The Stark splitting of the second excited level of atomic hydrogen (with principal quantum number $n=3$) has been measured by pulsed laser spectroscopy in order to determine small electric fields by optical means. Doppler-free excitation with two counterpropagating laser beams at $205\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ allows us to measure the $n=3$ Stark spectra either by an optogalvanic signal after ionization by a third laser photon or by detection of the Balmer-$\ensuremath{\alpha}$ fluorescence photons emitted by the excited atoms. Three different cases of laser polarization have been investigated in order to evaluate the possibilities and limitations of this one-step laser excitation method for electric field determination, especially at field strengths below $200\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{cm}$. The method provides the best field sensitivity achieved with lower excited levels that can be reached directly by two-photon absorption from the ground state (in contrast to the more sensitive Rydberg levels). For this purpose, an advanced tunable pulsed uv laser with narrow bandwidth almost at the Fourier limit has been used. The high spectral resolution allowed us to deduce electric field strengths as small as $50\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{cm}$ from the frequency shift between two selected isolated Stark components. As a main advantage, this simple analysis of the measured spectra does not rely on knowledge of the line intensities which may be strongly affected by the measuring conditions.

Time-Division Multiplexing Fiber Grating Sensor With a Tunable Pulsed Laser

A novel time-division-multiplexing fiber Bragg grating (FBG) sensor with a tunable pulsed laser is proposed and demonstrated. A tunable pulsed fiber laser based on a matched FBG is applied. The wavelengths of the sensing FBGs are obtained by detecting maximum voltages with a photodiode, which avoids a complex demodulation process. The advantages of the sensor include simple structure, high signal-to-noise ratio, and the sensing signals obtained by detecting the maximum voltages