Showing papers in "Applied Physics B in 1996"

Optical methods to characterise the composition and homogeneity of lithium niobate single crystals

Abstract: A number of the physical properties of lithium niobate strongly depend on sample composition. Although several procedures for the determination of the composition have already been published, a critical survey and an introduction to standard methods recommendable also for laboratories active in R&D of LiNbO3 devices is still missing. Within a detailed description of a series of methods, we summarise their capabilities and accuracy. The proposed optical characterisation methods, in particular those based on the generation of second harmonic light and those involving the measurement of birefringence and the UV absorption edge are found to be most convenient for an accurate and fast standard characterisation of LiNbO3 single crystals. An absolute accuracy of 0.1 mol% based on a comparison with the Curie temperature calibration method and a relative accuracy of up to 0.01 mol% are available. Some of these methods are also suited for the two or three dimensional homogeneity control of LiNbO3 single crystals.

Time-resolved laser optoacoustic tomography of inhomogeneous media

Abstract: The methods of time-resolved laser optoacoustic tomography of inhomogeneous media and related problems are reviewed. Time-resolved laser optoacoustic tomography allows one to measure the distribution of light absorption in turbid media with depth resolution up to several microns in real time. The theory of laser excitation of acoustic waves by absorbing of light in particles, dispersed in transparent, light-absorbing or scattering media, is developed. The distribution of light absorption can be obtained from the temporal course of acoustic pressure. Two schemes of acoustic wave detection — in the medium under testing (direct detection) and in transparent medium, coupled to the investigated one (indirect detection) — are discussed. In both cases the reconstruction of light absorption can be made by simple calculations. Test experiments with homogeneous and layered media confirm the proposed theoretical models and the possibility of using the proposed experimental schemes. Light absorption in homogeneous, inhomogeneous media and in absorbing particles dispersed in turbid media was investigated. The experimental setup allows one to measure the absorption coefficients over the range 1-500 cm−1 with the depth resolution 10–15 μm over the depth 1–1.5 mm.

Temperature and pressure dependences of the laser-induced fluorescence of gas-phase acetone and 3-pentanone

Abstract: Laser-Induced Fluorescence (LIF) from the S1 state of acetone and 3-pentanone was studied as a function of temperature and pressure using excitation at 248 nm. Additionally, LIF of 3-pentanone was investigated using 277 and 312 nm excitation. Added gases were synthetic air, O2, and N2 respectively, in the range 0–50 bar. At 383 K and for excitation at 248 nm, all the chosen collision partners gave an initial enhancement in fluorescence intensity with added gas pressure. Thereafter, the signal intensity remained constant for N2 but decreased markedly for O2. For synthetic air, only a small decrease occurred beyond 25 bar. At longer excitation wavelengths (277 and 312 nm), the corresponding initial rise in signal with synthetic air pressure was less than that for 248 nm. The temperature dependence of the fluorescence intensity was determined in the range 383–640 K at a constant pressure of 1 bar synthetic air. For 248 nm excitation, a marked fall in the fluorescence signal was observed, whereas for 277 nm excitation the corresponding decrease was only half as strong. By contrast, exciting 3-pentanone at 312 nm, the signal intensity increased markedly in the same temperature range. These results are consistent with the observation of a red shift of the absorption spectra (≈9 nm) over this temperature range. Essentially, the same temperature dependence was obtained at 10 and 20 bar pressure of synthetic air. It is demonstrated that temperatures can be determined from the relative fluorescence intensities following excitation of 3-pentanone at 248 and 312 nm, respectively. This new approach could be of interest as a non-intrusive thermometry method, e.g., for the compression phase in combustion engines.

Detection and characterization of single molecules in aqueous solution

Abstract: Using a confocal microscope with a single-photon avalanche photodiode as detector, we studied photon bursts of single Rhodamine 6G (R6G) and Rhodamin B-zwitterion (RB) molecules in aqueous solution by excitation of the lowest excited singlet stateS 1 with a frequency-doubled titanium: sapphire laser. Multichannel scaler traces, the fluorescence autocorrelation function and fluorescence decay times determined by time-correlated single-photon counting have been measured simultaneously. The time-resolved fluorescence signals were analyzed with a maximum likelihood estimator. Fluorescence lifetime patterns in steps of 100 ps were generated by convolution with the excitation pulse. The lifetime of theS 1 state was derived from the Kullback-Leibler minimum discrimination information. We are able to demonstrate for the first time identification of two different single dye molecules via their characteristic fluorescence lifetimes of 1.79 ± 0.33 ns (RB) and 3.79 ± 0.38 ns (R6G) in aqueous solution.

Thin films by regular patterns of metal nanoparticles: Tailoring the optical properties by nanodesign

Abstract: Ultrathin metal films consisting of regular two-dimensional arrays of 44–95 nm sized silver nanoparticles were fabricated by electron-beam lithography. By independent variation of particle shape and interparticle distance, respectively, the film's optical extinction maximum wavelength can be tuned in a range from 460 nm to 520 nm and the bandwidth between 50 and 100 nm. The results suggests the potential of lithographically designed nanoparticle films as thin layer material with tailorable optical properties for applications in microoptics and optoelectronics.

Diode-pumped bulk erbium-ytterbium lasers

Abstract: A comprehensive review of the work done by our group, during the last few years, on diode-pumped cw operating bulk Er : Yb phosphate glass is presented. The high performance of this laser in terms of output power, laser slope efficiency, single longitudinal and transverse mode operation, laser tunability and frequency stability, amplitude noise, and active mode-locking operation, are considered. The combination of this high performance and potential low cost makes these laser devices attractive for a variety of applications including spectroscopy, metrology, optical radars, optical communications, and all optical switching.

Reduction of multi-photon ionization in dielectrics due to collisions

Abstract: The collisional effect due to the multi-photon ionization process in dielectric material has been studied. We found that the breakdown threshold of fused silica is the same for both linearly and circularly polarized light at 55 fs and 100 fs, which we believe is an indication of the suppression of multi-photon ionization in solids. By numerically solving the time-dependent Schrodinger equation with scattering, for the first time, we have observed substantial reduction of the multi-photon ionization rate in dielectrics due to collisions.

Trace pollutants analysis in soil by a time-resolved laser-induced breakdown spectroscopy technique

Abstract: The results of a joint experiment of IFAM-Pisa and ENEA-Frascati for the detection of traces of pollutants in soil by a time-resolved laser-induced spectroscopy technique are reported. Using samples of soil with known pollutants' concentration [Geochemical Exploration Reference (GXR) silicate from US Geological Survey], we were able to estimate the sensitivity of this Laser-Induced Breakdown Spectroscopy (LIBS) technique to be of the order of some parts per million for a vast class of metallic pollutants, including extremely dangerous soil pollutants such as copper, lead and chromium.

Optical properties and laser performance of neodymium doped scheelites cawo4 and nagd(wo4)2

Abstract: Nd-doped CaWO4 (CWO) and NaGd(WO4)2 (NGWO) single crystals with good optical quality have been grown by the Czochralski technique. The neodymium distribution coefficient in these matrices is about 0.4 for CWO and close to unity for NGWO. Polarized absorption and emission spectra have been recorded at room temperature and used to calculate the absorption and stimulated emission cross-sections.

Absolute CH concentrations in low-pressure flames measured with laser-induced fluorescence

Abstract: Laser-induced fluorescence in both theA−X andB−X band systems was used to measure absolute number densities of CH radicals in 40 Torr propane/air flames at temperatures near 1600 K. The fluorescence signal was calibrated against Rayleigh scattering in N2 and Raman scattering in H2. In a rich flame,Φ = 1.15, the concentration at the peak of the CH distribution was 5.8 ± 1.5 ppm, or (1.4 ± 0.4) × 1012 cm−3, with independent values obtained using both band systems and calibration methods in good agreement. This result compares well with a prediction of 8 ppm from a kinetic model of this flame.

High-order optical harmonic generation from solid surfaces

Abstract: During the interaction of an intense ultrashort laser pulse with solid targets, a thin layer of surface plasma is generated in which the density drops to the vacuum level in a distance much shorter than the wavelength. This sharp plasma-vacuum boundary performs an oscillatory motion in response to the electromagnetic forces of the intense laser light. It is shown that the generation of reflected harmonics can be interpreted as a phase modulation experienced by the light upon reflection from the oscillating boundary. The modulation side-bands of the reflected frequency spectrum correspond to odd and even harmonics of the laser frequency. Retardation effects lead to a strong anharmonicity for high velocities of the plasma-vacuum boundary. As a result, harmonic generation is strongly enhanced in the relativistic regime of laser intensities.

Phase measurements in nonlinear optics

Abstract: Measurement of the phase, and not only the intensity, of the nonlinear optical response of a system is important in many applications which are reviewed here. We demonstrate and compare several techniques for direct measurement of the phase of the nonlinear susceptibilities through nonlinear interferometry. Different ways of imparting a controllable phase shift are discussed. Unless the coherence length is impractically large, the preferred method is to use a variable-pressure gas cell to control the phase difference between the input laser pulse and the nonlinear optical signal.

Polarization effects on the line-strength calculations of Er 3+ -doped LiNbO 3

Abstract: The polarized optical absorption of Er3+-doped LiNbO3 has been obtained and analyzed within the framework of the Judd-Ofelt theory including polarization dependences. These results are compared with those obtained under unpolarized conditions. Fluorescence lifetimes for different Er3+ concentrations have also been measured in the temperature range 10–300 K.

Counting of particles in aqueous solutions by laser-induced photoacoustic breakdown detection

Abstract: Laser-induced photoacoustic breakdown detection is applied for the determination of particles in aqueous dispersion. Polystyrene (latex), alumina and thoriasol particles of different sizes are investigated for demonstration. Laser pulses at 500 nm with 28 ns pulse width (FWHM), generated by an excimer (λ = 308 nm) pumped dye laser, are focused into aqueous particle dispersions to produce breakdown. The dye laser pulse energy is fixed at a value lower than the breakdown threshold of pure water to initiate a dielectric breakdown only when particles are present in a given focus volume. Focus volume parameters and particle size dependent breakdown thresholds are calculated and the results are compared with data from literature. Dependence of breakdown threshold values on the chemical composition of the particles is determined for polystyrene, alumina and thoriasol particles.

Simultaneous Raman/LIF measurements of major species and NO in turbulent H2/air diffusion flames

Abstract: A single-pulse spontaneous Raman scattering apparatus, based on a flashlamp-pumped dye laser, was used to determine the concentrations of the major species and the temperature in turbulent H2/N2/air jet diffusion flames. The concentrations of nitric oxide were simultaneously measured by Laser-Induced Fluorescence (LIF) after excitation of theA2Σ+−X2Π transition with a Nd: YAG-pumped dye laser. Some fundamentals of the employed methods, including the calibration procedure, quenching corrections, and accuracy are discussed. Besides a detailed study of the experimental technique, a main goal of the presented investigations was the generation of comprehensive data sets of high accuracy from well-defined turbulent flames which allow for a quantitative comparison with model calculations. Two flames with different fuel dilution and Reynolds numbers were investigated in a pattern of typically 100 measuring locations each comprising 300 single shots. In addition, four flames with different flow velocities but same fuel composition were compared with respect to their temperature and NO concentration profiles. The results show that differential diffusion plays an important role in these flames, especially near the flame base, where the temperature is increased above the adiabatic flame temperature and deviations from adiabatic equilibrium are large. The correlations between NO and mixture fraction and NO and temperature reveal characteristic features of the different flames.

Photoacoustic soot sensor for in-situ black carbon monitoring

Abstract: The PhotoAcoustic Soot Sensor (PASS) for in situ black carbon mass monitoring is presented. The sensor combines a high-power laser diode (λ = 802 nm;P = 450 mW) and a novel spectrophone setup to achieve a portable sensor system for black carbon measurements. The acoustic resonator with aQ-factor of ≈ 300 is operated in its 2nd azimuthal mode at 6670 Hz. To estimate the effects of the window position, laser beam collimation, and different loss mechanisms on spectrophone sensitivity a model treating these effects with respect to the signal strength of the azimuthal modes is described. It gives a cell constant of 5.8 V/(W m−1) in good agreement with 5.9 V/(W m−1) obtained from measurements with particulate carbon. Additionally, this model permits a method for an absolute calibration of the spectrophone. To improve the signal-to-noise ratio, the photoacoustic signal is estimated by a weighted least-squares fit to an averaged line profile of the excited normal mode instead of a direct measurement of peak height. Finally, the application of this data processing algorithm yields a detection limit of 1.5 × 10−6 m−1 or 0.5 μg black carbon per m3

Ultrafast dynamics of carrier-induced absorption changes in highly-excited CdSe nanocrystals

Abstract: We use femtosecond time-resolved transmission spectroscopy to study the density and time dependence of transient absorption changes of CdSe nanocrystals. Our data show pronounced absorption saturation up to complete bleaching of the lowest optical transition. At high carrier density the nonlinear spectra show several peaks that can be related to the two lowest quantized electron states. Thetime dependence of the carrier-induced absorption changes indicates an ultrafast relaxation process within the strongly broadened absorption lines.

A cylindrical liquid-core waveguide

Abstract: A novel concept for a liquid-core lightguide in cylindrical symmetry is presented. For a concrete 17 cm long device using water as a core and the transparent fluorinated polymer Teflon as a cladding layer, the coupling efficiency and guiding properties are investigated theoretically and compared with experimental results. The guide is found to exhibit low losses. A possible application of this type of guide is investigated by replacing the optical cell of a standard UV/VIS spectrometer by the cylindrical liquid-core waveguide. An increasing sensitivity for especially low concentrations of a polluting dye by a factor of about 15 is observed. Concentrations as low as a few parts per billion of the pollutant can be detected with a good signal-to-noise ratio.

Single fluorescence centers on the tips of crystal needles: First observation and prospects for application in scanning one-atom fluorescence microscopy

Abstract: A scanning optical fluorescence microscope is suggested, whose active element (“needle”) is made of a crystal containing impurity ions or color centers subject to excitation by laser radiation. The excitation energy from a single impurity center at the very tip of the needle is transferred by the resonance dipole-dipole exchange mechanism to fluorescence centers on the surface of the sample under study. It is demonstrated that this approach can help attain a nanometer-high spatial resolution at a high sensitivity substantially exceeding in some cases the sensitivity of the “standard” near-field fluorescence microscopy technique. Various crystals and impurity centers, potentially most suitable for the implementation of the method under consideration, are briefly analyzed. Information is presented on the manufacture of sharp-pointed needles from LiF crystals containing F2-centers and the first observation of single F2-centers on their tips by the laser-photoelectron projection microscopy technique, which allows one to speak of the practical creation of the first active elements for the microscope suggested.

Oxygen concentration and temperature measurements in N 2 -O 2 mixtures using rotational coherent anti-Stokes Raman spectroscopy

Abstract: The accuracy and precision of oxygen concentration and temperature measured by dual-broadband rotational Coherent Anti-Stokes Raman Spectroscopy (CARS) were investigated in nitrogen-oxygen mixtures at atmospheric pressure and temperatures between 290 and 1410 K. The relative standard deviation of temperatures evaluated from pure oxygen rotational CARS spectra was found to be around 5%, and the mean temperature was the same as for nitrogen CARS spectra, except for temperatures above 1000 K, where the temperature was 120 K below the correct value. The in situ calibrated oxygen concentrations were within 10% of the correct value, with a standard deviation of around 1.2% for the mixtures of 12 and 20% oxygen in nitrogen. For the lowest oxygen concentrations considered in this study (2 and 4%), the systematic errors in the evaluated concentrations were very large, and the standard deviation of repeated single-shot measurements was above 2%. However, employing weighting in the spectral fitting routine reduced the errors in the concentration and the single-shot standard deviation was lowered to 0.5%. Finally, it was shown that spectral interference (from oxygen) in a rotational CARS spectrum of nitrogen generally had little impact on the temperature evaluated from fitting the spectra to theoretical nitrogen spectra.

A detailed rate equation model for the simulation of energy transfer in OH laser-induced fluorescence

Abstract: A rate equation model (the LASKIN program packet) has been developed for the detailed computation of energy transfer in Laser-Induced Fluorescence (LIF). Calculations of this type are necessary for analysis of the influence of energy transfer processes [e.g., electronic quenching and Rotational Energy Transfer (RET)] on the fluorescence signal. The model has been utilized to examine linear LIF in the OHA 2Σ+-X 2 Π (0, 0) band. Available data on quenching, RET and spontaneous emission rates for the (A, v′ = 0) state have been reviewed, and models for the state-specific RET and quenching rates have been developed. The accuracy of the calculations has been confirmed by comparison with experimental data, and the LASKIN program has been applied to the analysis of potential error sources in the widely applied two-line LIF temperature-measurement technique. Extensions of the model to the examination of saturated LIF, OHA–X (1, 0) and (3, 0) excitation and LIF of other species (e.g., O2 and NO) are discussed.

Minimization of cavitation effects in pulsed laser ablation illustrated on laser angioplasty

Abstract: Cavitation effects in pulsed laser ablation can cause severe deformation of tissue near the ablation site. In angioplasty, they result in a harmful dilatation and invagination of the vessel walls. We suggest to reduce cavitation effects by dividing the laser pulse energy into a pre-pulse with low and an ablation pulse with high energy. The pre-pulse creates a small cavitation bubble which can be filled by the ablation products of the main pulse. For suitable energy ratios between the pulses, this bubble will not be enlarged by the ablation products, and the maximal bubble size remains much smaller than after a single ablation pulse. The concept was analyzed by numerical calculations based on the Gilmore model of cavitation dynamics and by high-speed photography of the effects of single and double pulses performed with a silicone tube as vessel model. The use of double pulses prevents the deformation of the vessel walls. The concept works with an energy ratio of up to about 1:30 between the pulses. For the calculated optimal ratio of 1:14.6, the bubble volume is reduced by a factor of 17.7. The ablation pulse is best applied when the pre-pulse bubble is maximally expanded, but the timing is not very critical.

Frequency locking and unlocking in a femtosecond ring laser with application to intracavity phase measurements

Abstract: Frequency coupling between the counter-propagating pulses inside a femtosecond ring laser gyroscope is studied. It is shown that frequency lock-in results from the counter-propagating pulses overlapping at or near a scattering surface. Nonetheless, the absence of lock-in with a scatterer at the pulse crossing point is demonstrated for the special case of symmetric scattering. The decoupling of the counter-propagating pulses makes possible the measurement of non-reciprocal phase differences of 10−5 and index differences of 10−10 over 1 cm. Besides measuring rotations, applications are found in the measurement of small changes in the index of refraction, and electrical transients on the 100 fs time scale.

Laser induced breakdown of Ar, N2 and O2 gases using 1.064, 0.532, 0.355 and 0.266 μm radiation

Abstract: An experimental investigation of laser-induced breakdown using Nd:YAG laser harmonics for argon, nitrogen and oxygen gases is reported. Pressure dependence as well as wavelength dependence of the breakdown threshold irradianceIth is investigated. The experimental observations for 1.064 and 0.532 μm laser wavelengths are in agreement with theoretical calculations which include the effects of multiphoton ionization and cascade ionization.

A continuously tunable long-wavelength cw IR source for high-resolution spectroscopy and trace-gas detection

Abstract: A new widely tunable source in the infrared for use in high-resolution spectroscopy and trace-gas detection is described. This spectroscopic source is based on Difference Frequency Generation (DFG) in gallium selenide (GaSe) and is continuously tunable in the 8.8–15.0 μm wavelength region. Such a DFG source operates at room temperature which makes it a useful alternative to a lead-salt diode-laser- based detection system that requires cryogenic temperatures and numerous individual diode lasers.

Birth and evolution of wave-front dislocations in a laser beam passed through a photorefractive LiNbO3: Fe crystal

Abstract: We report on an experimental and numerical investigation of the process of spontaneous optical vortices nucleation in a wave front of a laser beam passed through a photorefractive LiNbO3 : Fe crystal with self-induced nonlinear lens. The complex lens structure produces mainly defocusing of the beam passing through the crystal due to a negative variation of the refractive index, whereas side parts of the lens have a positive sign of refractive-index variation and partially focus the beam. The resulting wave-front distortions lead to a phase bifurcation occurring at a certain distance after the crystal when the amplitude of the light wave becomes zero. We study in detail the process of edge dislocation nucleation and its decay in the near field producing a pair of unity-charged opposite-sign screw dislocations. After birth, they spread along dislocation axes as stable objects.

Nanolithography with metastabile helium

Abstract: We have used a self-assembling monolayer of dodecanethiole molecules as the resist for a lithography technique based on a beam of metastable helium atoms Doses as low as 3 metastable helium atoms per 10 molecules are enough to write patterns into this resist An edge resolution of 30 nanometers is demonstrated The writing mechanism is based on the damage of the resist due to Penning ionization

Reverse saturable absorption and optical limiting in indanthrone dyes

Abstract: Fluence dependence of transmission in three dyes, viz. indanthrone, dichloroindanthrone and violanthrone, is reported at 532 nm as well as at 1.06 µm. These dyes show optical limiting at both these wavelengths for nanosecond pulses. The limiting is due to the combined effect of reverse saturable absorption from the triplet state and thermal defocussing in the dye solutions.

A scaling formalism for the representation of rotational energy transfer in OH (A2Sigma+) in combustion experiments

Abstract: In order to predict spectra and temporal decays of the OH radical measured by Laser-Induced Fluorescence (LIF), the collisional energy transfer between different quantum states must be taken into account. At the elevated temperatures relevant for combustion studies, the large number of interacting quantum states and corresponding state-to-state energy transfer coefficients precludes the experimental measurement of all necessary information for the appropriate mixtures of colliders. Therefore, a scaling procedure has been devised which allows the representation of the matrix of Rotational Energy Transfer (RET) coefficients on the basis of measured data with four scaling coefficients. The scaling coefficients have been determined by comparison of the calculated RET rates with available measured data. The mathematical formalism for the scaling law - the ECS-EP law - is based on the Energy Corrected Sudden (ECS) law and includes an Exponential Power law (EP) for the representation of the basis coefficients.

High nuclear polarization in 3 He and 3 He- 4 He gas mixtures by optical pumping with a laser diode

Abstract: New diode lasers delivering 50 mW output power at 1083 nm are shown to be efficient sources for optical pumping of helium. They can polarize nuclei in a3He gas up toM = 50% over the pressure range 0.4–1.6 torr. Larger nuclear polarizationsM of3He nuclei, of order 80%, can be obtained in3He-4He mixtures when the laser frequency is tuned to a4He line. A standard optical measurement of nuclear polarizationM has been extended to the case of3He-4He mixtures. The effect of various parameters on the steady-state polarizationM and on the pumping timeT p is discussed.