Methods for controlling of the laser-induced absorption in a BTO crystal by using of cw-laser radiation

The iron-atom concentration distribution as well as the gas-phase temperature was measured via laser-induced fluorescence (LIF) during iron-oxide nanoparticle synthesis in a low-pressure hydrogen/oxygen/argon flame reactor using ironpentacarbonyl (Fe(CO)₅) as precursor. Temperature measurements based on multi-line NO-LIF imaging are used to correct for temperature-dependent ground-state populations. The concentration measurement is calibrated based on line-of-sight absorption measurements. The influence of the precursor on the flame is observed at precursor concentrations larger than 70 ppm as the flame front moves closer to the burner surface with increasing Fe(CO)₅ concentration.     

Rotational-coherence molecular laser isotope separation

We have proposed a laser isotope separation method, utilizing rotational coherence of a simple molecule. In the scheme, photoexcited molecules are isotopically separated by difference of rotational period between them. To illustrate this method, two-pulse photodissociation of mixed ⁷⁹Br₂/⁸¹Br₂ isotopes has been investigated theoretically. The photodissociation probabilities of ⁷⁹Br₂ and ⁸¹Br₂ have been calculated as functions of time delay between the photoexcitation and dissociation laser pulses. We have demonstrated that isotope enrichment factor of ⁷⁹Br relative to ⁸¹Br can be changed from 0.34 to 1.8, by simply changing the time delay only by 0.2 ns. Additionally, we have shown that this method is effective for heavy isotopes, based on mass dependence of the isotope enrichment factor.     

The effects of chemical etching of porous silicon on Raman spectra

We have investigated the effects of chemical etching on Raman spectra of porous silicon. The as-anodized porous silicon consisted mainly of crystalline silicon, as indicated by the Raman spectra. The background in the spectrum was strong, indicating that the porous silicon surface was rough due to the presence of pores. When chemical etching was performed five times, the Raman spectrum revealed the presence of spherically shaped nanocrystalline silicon whose diameter was around 3.5 nm. Further chemical etching, however, extinguished the nanocrystallites, in addition to smoothing the surface morphology.     

Development of an optical parametric generator with pulsed dye amplification for high-resolution laser spectroscopy

An injection-seeded optical parametric generator (OPG), coupled with three pulsed dye amplification (PDA) stages, was shown to produce tunable, narrow linewidth laser radiation. The OPG was composed of a pair of beta barium borate (β-BBO) crystals and pumped by the third harmonic (355 nm) output of a seeded Nd:YAG laser. The OPG was injection-seeded at the idler wavelength (824 nm) using an external cavity diode laser (ECDL) with a mode-hop-free tuning range of 20 GHz. Using the PDA stages, the OPG output signal (624 nm) was amplified to 19 mJ/pulse, while maintaining a spectral linewidth of approximately 160 MHz at full-width-half-maximum (FWHM) which was within a factor of 2 of the Fourier limit. A system of lenses and apertures was used to minimize amplified spontaneous emission (ASE) in the PDA stages. Using the OPG/PDA system, two-photon laser-induced fluorescence measurements of atomic oxygen were performed by sum-frequency-mixing the 624-nm beam with the third harmonic output of the seeded Nd:YAG laser to generate approximately 1 mJ/pulse of ultraviolet radiation near 226 nm. Voigt line shapes were found to be in good agreement with oxygen atom spectra in atmospheric-pressure, laminar, counter-flow flames; the magnitude of Doppler and collisional broadening was approximately the same. The measured O-atom concentration profile was found to compare well with that calculated using an opposed-flow flame code.     

A simple method to significantly increase filaments’ length and?ionization density

A simple method to produce longer filaments with higher ionization density in air by controlling the diameter of an aperture in the laser beam path is studied via an analysis of the backscattered N₂ fluorescence collected by LIDAR. Significant increase in the fluorescence signal (approximately by a factor of five depending on the conditions) and an increased filament length was observed at an optimum diameter. 3D + time stochastic numerical simulations have shown that the optimum aperture size corresponds to the case of multiple filament ‘squeezing’ around the propagation axis forming the regularized elongated structure with higher overall amount of plasma. The optimum range of aperture sizes is the same for the initial transverse perturbation scale variation at least within a factor of three.     

Frequency stabilization of CO laser using RF optogalvanic Lamb-dip

The Lamb dip of CO rovibrational transition is detected by a room temperature extracavity RF optogalvanic cell and employed to stabilize the frequency of a CO laser. The S/N ratio of optogalvanic signal is about 2000  at optical power < 1 W. The relative depth of Lamb dip is 2.3%. The S/N ratios of first and third harmonic demodulated saturation signals are about 40  and 10  , respectively. The CO laser is stabilized using the first harmonic demodulated signal, and the frequency stability is better than 300 kHz. Concurrently, the influences of operational parameters, which include the coil current, partial pressures of gas mixture, are investigated. A simple model for the influence of coil current is presented, and further improvements are addressed as well.     

A KTiOAsO4 Raman laser

Efficient nanosecond stimulated Raman scattering is observed in KTiOAsO₄ within an intracavity Raman laser configuration. A diode-end-pumped acousto-optically Q-switched Nd:YAG laser emitting at 1064.2 nm is employed as the pumping source. And efficient generation of the first-Stokes line at 1091.5 nm is observed. With an incident diode power of 8.11 W, a first-Stokes power of 1.38 W is obtained at a pulse repetition rate of 25 kHz, corresponding to a diode-to-Stokes conversion efficiency of 17%. The pulse width is 6.5 ns and the peak power is 8.5 kW. The performance characteristics of the device demonstrate that KTiOAsO₄ is competent and reliable for nanosecond Raman lasers.     

Investigation on the output energy characteristic of optical limiting based on stimulated Brillouin scattering

Stimulated Brillouin scattering (SBS) featuring sub-nanosecond response time takes place at a high power threshold, which enables its application at a high power density. When the intensity of input light excesses the SBS threshold, strong SBS process takes place through SBS medium, leading to a quick energy transfer from pump to the Stokes and thereby an optical limiting characteristic in the output energy. In this paper, the correlation between SBS output energy and input power density is numerically simulated and validated in the Nd: YAG Q-switch laser system. The results indicate that not only the output energy exhibits an optical limiting characteristic, but also the clamped value of output energy can be controlled by changing the medium or the focal length.     

Propagation of terawatt laser pulses in the air

One of the problems when increasing the intensity of a femtosecond laser pulse is the propagation of the beam. As the intensity increases nonlinear effects begin to play a significant role. When arriving to the terawatt domain, nonlinear effects and filamentation give rise to a new phenomenology in the propagation. The aim of this paper is to analyze new possibilities to control the beam shape to Taylor the interaction of the beam with the target at large distances.     

High-resolution infrared polarization spectroscopy and?degenerate four wave mixing spectroscopy of methane

High-resolution infrared polarization spectroscopy (IR-PS) and degenerate four wave mixing (IR-DFWM) spectroscopy of methane using a diode-seeded modeless laser (DSML) system are reported. Mid-infrared radiation around 3.3 μm is generated by difference frequency mixing of the single-mode output of the DSML around 0.634 μm with the frequency-doubled output of a single-mode Nd:YAG pump laser at 0.532 μm. Polarization spectroscopy signals in the forward geometry were generated in methane at around 5 Torr pressure. IR-PS spectra were recorded with a typical signal-to-noise ratio of 150:1 with methane pressures of at least 1 Torr. The line shape of the IR-PS signals was analysed to measure pressure broadening induced by nitrogen buffer gas yielding a value of 6.3±1.5 MHz Torr⁻¹. IR-DFWM spectra of methane were generated in the counter-propagating pump geometry yielding Doppler-free signals with signal-to-noise ratios of typically 650:1. Signals were obtained at methane pressures down to less than 10 mTorr. A comparison of IR-PS and IR-DFWM is made indicating that IR-DFWM has some advantages over IR-PS in this spectral region in terms of sensitivity, signal-to-noise ratio and ease of use. The results illustrate the utility of the DSML for high-resolution nonlinear spectroscopy in the mid infrared.     

Monitoring laser cleaning of titanium alloys by probe beam reflection and emission spectroscopy

The energy relaxation of electrons in InN epilayers is investigated by excitation- and electric field-dependent photoluminescence (PL). From the high-energy tail of PL, we determine the electron temperature of the hot carriers. It was found that the electron temperature variation can be explained by a model in which the longitudinal optical (LO)-phonon emission is the dominant energy relaxation process. The LO-phonon lifetime is fitted to be 0.89 ps, which is higher than the theoretical phonon lifetime. This deviation is attributed to the presence of the non-equilibrium hot-phonon effects. PACS 78.55.Cr; 78.66.Fd; 61.66.Fn; 78.20.Jq; 63.20.kd     

Investigation of heavy-metal accumulation in selected plant samples using laser induced breakdown spectroscopy and laser ablation inductively coupled plasma mass spectrometry

Single-pulse Laser-Induced Breakdown Spectroscopy (LIBS) and Laser-Ablation Inductively Coupled Plasma Mass-Spectrometry (LA-ICP-MS) were applied for mapping the silver and copper distribution in Helianthus Annuus L. samples treated with contaminant in controlled conditions. For Ag and Cu detection the 328.07 nm Ag(I) and 324.75 nm Cu(I) lines were used, respectively. The LIBS experimental conditions (mainly the laser energy and the observation window) were optimized in order to avoid self-absorption effect in the measured spectra. In the LA-ICP-MS analysis the Ag 107 and Cu 63 isotopes were detected. The capability of these two analytical techniques for high-resolution mapping of selected trace chemical elements was demonstrated.     

Display glass cutting by femtosecond laser induced single shot periodic void array

We propose an idea of fast cutting a display glass plate where the sample is pre-processed micromachining single shot rear-surface and internal void arrays aligned on working plane prior to glass cleaving. Single shot void morphology is investigated varying input pulse energy, focusing depth, and scanning speed. A femtosecond laser with pulse duration of 172 fs, central wavelength of 780 nm, and repetition rate of 1 kHz is used to fabricate voids.     

Infrared laser-spectroscopic analysis of <Superscript>14</Superscript>NO and <Superscript>15</Superscript>NO in human breath

We report on monitoring of nitric oxide (NO) traces in human breath via infrared cavity leak-out spectroscopy. Using a CO sideband laser near 5 μm wavelength and an optical cavity with two high-reflectivity mirrors (R=99.98%), the minimum detectable absorption is 2×10⁻¹⁰ cm⁻¹ Hz^1/2. This allows for spectroscopic analysis of rare NO isotopologues with unprecedented sensitivity. Application to simultaneous online detection of ¹⁴NO and ¹⁵NO in breath samples collected in the nasal cavity is described for the first time. We achieved a noise-equivalent detection limit of 7 parts per trillion for nasal ¹⁵NO (integration time: 70 s).     

Sensitive detection of ammonia and ethylene with a pulsed quantum cascade laser using intra and interpulse spectroscopic techniques

Spectroscopic concentration measurements of ammonia and ethylene were done with a pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm⁻¹. An astigmatic Herriot cell with 150 m path length was employed, and we compare the results from experiments using inter- and intrapulse techniques, respectively. The measurements include the detection of ammonia in breath with these methodologies. In the interpulse technique, the laser was excited with short current pulses (5–10 ns), and the pulse amplitude was modulated with an external current ramp resulting in a ∼0.3 cm⁻¹ frequency scan. A standard amplitude demodulation technique was implemented for extracting the absorption line, thus avoiding the need for a fast digitizer or a gated integrator. In the intrapulse technique, a linear frequency down-chirp is used for sweeping across the absorption line. A 200 ns long current pulse was used for these measurements which resulted in a spectral window of ∼1.74 cm⁻¹ during the down-chirp. The use of a room temperature mercury-cadmium-telluride detector resulted in a completely cryogen free spectrometer. We demonstrate detection limits of ∼3 ppb for ammonia and ∼5 ppb for ethylene with less than 10 s averaging time with the intrapulse method and ∼4 ppb for ammonia and ∼7 ppb for ethylene with the interpulse technique with an integration time of ∼5 s.     

A narrow linewidth and frequency-stable probe laser source for the <Superscript>88</Superscript>Sr<Superscript>+</Superscript> single ion optical frequency standard

In this paper, we describe in detail a narrow linewidth and frequency-stable laser source used to probe the 5s ² S _1/2–4d ² D _5/2 clock transition of the ⁸⁸Sr⁺ optical frequency standard. The performance of the laser system is investigated with studies of its frequency drift rates and with high resolution spectra of the ⁸⁸Sr⁺ clock transition. The observed short-term drift rates are typically in the range of 10 to 23 mHz/s, and the current long-term drift rate is 13.9(3) mHz/s. The laser stability, after subtraction of linear drifts, reaches 5×10⁻¹⁶ at an averaging time of 3000 s. This high level of stability is attributed for the most part to stabilization of the reference cavity at the temperature where the coefficient of linear thermal expansion crosses zero. An upper bound for the laser linewidth is given by the observation of a Fourier-transform limited resonance of 4.3 Hz (Δν/ν=1×10⁻¹⁴) on the ⁸⁸Sr⁺ clock transition. The effective averaging time during the linewidth measurements was about 100 s.     

Wigner Kirkwood ℏ-expansion of the density matrix in inhomogeneous superfluid Fermi systems

The generalized density matrix of superfluid inhomogeneous Fermi systems is expanded in powers of up to order ². This constitutes the generalisation of the Wigner Kirkwood -expansion of the density matrix of normal fluid systems to the pairing case.One of the authors (P.S.) is very grateful to D. Gogny for contributions in an early stage of this work several years back. He also acknowledges fruitful discussions with M. Centelles and X. Vinas.     

Influence of the pump threshold on the single-frequency output power of singly resonant optical parametric oscillators

We demonstrate that for a given pump source, there is an optimum pump threshold to achieve the maximum single-frequency output power in singly resonant optical parametric oscillators. Therefore, cavity losses and parametric amplification have to be adjusted. In particular, continuous-wave output powers of 1.5 W were achieved with a 2.5 cm lithium niobate crystal in comparison with 0.5 W by a 5 cm long crystal within the same cavity design. This counter-intuitive result of weaker amplification leading to larger powers can be explained using a model from L.B. Kreuzer (Proc. Joint Conf. Lasers and Opt.-Elect., p. 52, 1969). Kreuzer also states that single-mode operation is possible only up to pump powers which are 4.6 times the threshold value. Additionally, implementing an outcoupling mirror to increase losses, single-frequency waves with powers of 3 W at 3.2 μm and 7 W at 1.5 μm could be generated simultaneously.     

Theoretical analysis of a quartz-enhanced photoacoustic spectroscopy sensor

Quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors are based on a recent approach to photoacoustic detection which employs a quartz tuning fork as an acoustic transducer. These sensors enable detection of trace gases for air quality monitoring, industrial process control, and medical diagnostics. To detect a trace gas, modulated laser radiation is directed between the tines of a tuning fork. The optical energy absorbed by the gas results in a periodic thermal expansion which gives rise to a weak acoustic pressure wave. This pressure wave excites a resonant vibration of the tuning fork thereby generating an electrical signal via the piezoelectric effect. This paper describes a theoretical model of a QEPAS sensor. By deriving analytical solutions for the partial differential equations in the model, we obtain a formula for the piezoelectric current in terms of the optical, mechanical, and electrical parameters of the system. We use the model to calculate the optimal position of the laser beam with respect to the tuning fork and the phase of the piezoelectric current. We also show that a QEPAS transducer with a particular 32.8 kHz tuning fork is 2–3 times as sensitive as one with a 4.25 kHz tuning fork. These simulation results closely match experimental data.     

Substrate dependence of ion motion in femtosecond laser ablation cloud observed by planar laser-induced fluorescence

We have observed the motion of Sm⁺ ions as well as Sm atoms produced by femtosecond laser ablation of a solidified samarium solution sample on substrates by using a planar laser-induced fluorescence method. Kinetic energies of both Sm⁺ ions and Sm atoms increase as the electrical conductivity of the substrate decreases, which suggests the effect of surface charging. The kinetic energy of Sm⁺ ions is larger than that of Sm atoms for a variety of substrates due to the further electrical acceleration by the surface charge. The knowledge of ion motion will be the key information for the optimization of femtosecond laser simultaneous atomization and ionization of organic and inorganic samples on substrates.