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.     

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.     

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.     

Self-mixing interference effects in tunable diode laser absorption spectroscopy

We report the effects of self-mixing interference on gas detection using tunable diode laser spectroscopy. For very weak feedback, the laser diode output intensity gains a sinusoidal modulation analogous to that caused by low finesse etalons in the optical path. Our experiments show that self-mixing interference can arise from both specular reflections (e.g. cell windows) and diffuse reflections (e.g. Spectralon^™ and retroreflective tape), potentially in a wider range of circumstances than etalon-induced interference. The form and magnitude of the modulation is shown to agree with theory. We have quantified the effect of these spurious signals on methane detection using wavelength modulation spectroscopy and discuss the implications for real gas detectors.     

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.     

Trace gas sensor based on quartz tuning fork enhanced laser photoacoustic spectroscopy

A compact photoacoustic gas sensor based on a quartz tuning fork and fiber-coupled distributed feedback (DFB) diode laser for detection of trace gas at atmospheric pressure has been developed. The sensor performance was evaluated by detection of water vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient of 1.68×10⁻⁸ cm⁻¹ W/Hz^1/2 was achieved. Influence of different acoustic microresonators and sample pressure on the sensor performance, and the characterization of the sensor response time were investigated. Approaches to improve the current sensor performance are discussed.     

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.     

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.     

Flexible laser diode trace gas sensor based on cavity ringdown spectroscopy with an optical fiber-coupled high-finesse external-cavity diode laser

A flexible and portable trace nitrogen dioxide sensor based on cavity ringdown spectroscopy using an optical fiber-coupled high-finesse cavity was successfully demonstrated. Tailoring the spatial mode matching condition of the core of an optical fiber and high-finesse external cavity allows for effective optical feedback into an antireflection-coated laser diode for stable resonant enhancement of the external cavity. The external cavity, which works as a ringdown cavity, could be remotely located from the light source and receiver section by only a single mode optical fiber. The sensitivity was found to be 1.0×10⁻⁷ cm⁻¹ in a compact 1-cm³ ringdown cavity volume.     

High repetition rate femtosecond laser nano-machining of thin films

Thin film laser micromachining has been utilized for repairing semiconductor masks, creating solar cells and fabricating MEMS devices. A unique high repetition rate femtosecond fiber laser system capable of variable repetition rates from 200 KHz to 25 MHz along with helium gas assist was used to study the effect of pulse repetition rate and pulse energy on femtosecond laser machining of gold-coated silicon wafer. It was seen that high repetition rates lead to smaller craters with uniform line width. Craters created at 13 MHz pulse repetition rate with 2.042 J/cm² beam energy fluence measured 110 nm in width and had a heat affected zone of 0.79 μm. It was found that pulse repetition rate only played a significant role in the size of the heat affected zone in the lower pulse energy ranges. In the future, a 1 W laser system will be acquired to find the optimal repetition rate that would create the minimal feature size with the least heat affected zone. Using this kind of setup along with techniques such as radial polarization and a different gas assist may enable us to create sub 100 nm feature size with good quality.     

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.     

Non-linear pressure dependence of A-state fluorescence lifetime of formaldehyde

Fluorescence lifetimes of formaldehyde excited at 352 nm ( A₂ – A₁ 4₀¹ band) were measured as a function of bath gas pressure. He, N₂, O₂, CO₂ and HCHO were investigated for the bath gas and the temperature dependence between 298 and 500 K for N₂ and O₂ bath gases was also examined. It was found that the non-linear pressure dependence of the lifetime is successfully reproduced by the model formula

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).     

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.     

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     

Prospects for precision measurements of atomic helium using direct frequency comb spectroscopy

We analyze several possibilities for precisely measuring electronic transitions in atomic helium by the direct use of phase-stabilized femtosecond frequency combs. Because the comb is self-calibrating and can be shifted into the ultraviolet spectral region via harmonic generation, it offers the prospect of greatly improved accuracy for UV and far-UV transitions. To take advantage of this accuracy an ultracold helium sample is needed. For measurements of the triplet spectrum a magneto-optical trap (MOT) can be used to cool and trap metastable 2³S state atoms. We analyze schemes for measuring the two-photon 2³S →4³S interval, and for resonant two-photon excitation to high Rydberg states, 2³S →3³P →n³S, D. We also analyze experiments on the singlet-state spectrum. To accomplish this we propose schemes for producing and trapping ultracold helium in the 1¹S or 2¹S state via intercombination transitions. A particularly intriguing scenario is the possibility of measuring the 1¹S →2¹S transition with extremely high accuracy by use of two-photon excitation in a magic wavelength trap that operates identically for both states. We predict a “triple magic wavelength” at 412 nm that could facilitate numerous experiments on trapped helium atoms, because here the polarizabilities of the 1¹S, 2¹S and 2³S states are all similar, small, and positive.     

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.     

Nanostructuring of thin gold films by femtosecond lasers

Novel experimental data on microstructuring of thin (60 nm) gold films by femtosecond laser pulses are presented and discussed. Material modifications are induced by different laser field distributions on the sample surface. Images of specially fabricated masks are transferred onto the gold surface with a 50× and 100× demagnifications. It is shown that, in the irradiated region of the gold film, the heated material tends to concentrate in the center. For example, a square-like field distribution on the target surface produces a cross with a jet in the middle. It is shown that this technique allows producing of a variety of microstructures with controllable nanorelief. Possible mechanisms leading to the observed material modifications as well as the resolution limits of this technique are discussed.     

The potential of CO2 laser photoacoustic spectrometry for?detection of methanol in alcoholic beverage

The first use of CO₂ laser photoacoustic measurements for detecting the methanol contents in alcohol-like solutions is presented. With an intracavity cell configuration, the minimum detectable concentration was ∼200 ppm for methanol and the linear range of the calibration curve for methanol was from 200 to 70000 ppm. For demonstrating the reliability of analysis in alcoholic beverages, a series of different concentrations of two-component samples was prepared and measured by the same procedures. The results showed the feasibility on determining methanol and ethanol contents accurately within a specific tolerance, limited mainly by background signal and laser stability. This potential method with no pre-treatment of samples takes only ∼10 min to finish one single measurement. It suggests that the PA detection is suitable for routine diagnosis of adulterated wines in commercial products.     

Continuous-wave cavity-ringdown detection of stimulated Raman gain spectra

Cavity ringdown (CRD) spectroscopy, with its high sensitivity, provides a novel way to perform continuous-wave (cw) stimulated Raman gain (SRG) spectroscopy, rather than by conventional optically detected coherent Raman techniques. Tunable cw laser light at ∼1544 nm is used to probe ringdown decay from a rapidly-swept, high-finesse optical cavity containing a gas-phase sample of interest and itself located inside the cavity of a cw single-longitudinal-mode Nd:YAG ring laser operating at ∼1064.4 nm. This approach is used to measure cw SRG spectra of the ν ₁ fundamental rovibrational Raman band of methane gas at ∼2916.5 cm⁻¹. The resulting SRG-CRD resonances have ringdown times longer than in the off-resonance case, in contrast to the usual shorter ringdown times arising from absorption and other loss processes. Previously reported noise-equivalent sensitivities have been substantially improved, by using a second ringdown cavity to facilitate subtraction of infrared-absorption background signals. Moreover, by employing a ringdown cavity in the form of a ring, the SRG-pump and CRD-detected Stokes beams can co-propagate uni-directionally, which significantly reduces Doppler broadening.