Showing papers in "Applied Physics B in 1995"
TL;DR: In this paper, a novel concept in far-field fluorescence microscopy, ground state depletion (GSD), was introduced to overcome the classical diffraction resolution limit. But the concept was not applied to the localization of the outer region of the focus of a scanning fluorescence microscope.
Abstract: We introduce and study a novel concept in farfield fluorescence microscopy fundamentally overcoming the classical diffraction resolution limit. This is accomlished by reducing the spatial extent of the effective focus of a scanning fluorescence microscope. The reduction is achieved by depleting the ground-state energy of the molecules located in the outer region of the focus. Our theoretical study shows that ground-state-depletion fluorescence microscopy has the potential of increasing the resolution of far-field fluorescence microscopy by an order of magnitude which is equivalent to a lateral resolution of 15 NM.
600 citations
324 citations
TL;DR: In this paper, the spectra of the excited-state absorption and the stimulated-emission cross-sections of Er3+ -doped Y3Al5O12, YAlO3, LiYF4, and KYF4 were analyzed in the infrared wavelength regions from 700 to 2100 nm.
Abstract: Detailed spectra of the excited-state absorption and the stimulated-emission cross-sections of Er3+ -doped Y3Al5O12, YAlO3, LiYF4, and KYF4 crystals are analyzed in the infrared wavelength regions from 700 to 2100 nm. The spectra were measured with a pump and probe technique employing a double modulation scheme. For Er (2%): Y3Al5O12 also the stimulated emission at 3 µm and the reabsorption due to excited-state absorption from the lower laser level are investigated.
183 citations
TL;DR: In this article, the authors compared laser-induced fluorescence in C2 from Laser-Vaporized Soot (LIF(C2)LVS), and laser-Induced Incandescence of soot (LII) signals.
Abstract: Strategies for spatially resolved soot volume-fraction measurements have been investigated in sooting laboratory flames with known soot characteristics. Two techniques were compared: Laser-Induced Fluorescence in C2 from Laser-Vaporized Soot (LIF(C2)LVS), and Laser-Induced Incandescence of soot (LII). The LII signal is the increased temperature radiation from soot particles which have been heated to temperatures of several thousand degrees as a consequence of absorption of laser radiation. The LIF(C2)LVS technique is based on the production of C2 radicals from laser-vaporized soot which occurs for laser intensities ≥107 W/cm2. A laser wavelength is chosen such that besides vaporizizng the soot, it also excites the C2 radicals, and the subsequent C2 fluorescence signal is detected. The signals from both techniques showed good correlation with soot volume fractions in the studied flame. The dependence of the signals on experimental parameters was studied, and the influence of interfering radiation, such as background flame luminosity and fluorescence from polyaromatic hydrocarbons, on studied signals was established. The potential of the two techniques for imaging of soot volume fractions in laboratory flames was demonstrated. Advantages and disadvantages of the studied techniques are discussed.
130 citations
TL;DR: Rhodamine 6G has been dissolved in copolymers of 2-HEMA and Methyl MethAcrylate (MMA) and the resulting solid-state solutions have been pumped at 337 nm and 532 nm.
Abstract: Rhodamine 6G has been dissolved in copolymers of 2-HydroxyEthyl MethAcrylate (HEMA) and Methyl MethAcrylate (MMA) and the resulting solid-state solutions have been pumped at 337 nm and 532 nm. Lasing efficiencies similar to those found in ethanol solution have been obtained with a 1:1 vol/vol HEMA: MMA copolymer matrix, and lifetimes of ca. 10 000 (337 nm pumping) and ca. 75 000 (532 nm pumping) pulses at repetition rates up to 15 Hz and 10 Hz, respectively, have been demonstrated.
109 citations
TL;DR: In this article, the authors compared the performance of seven different models for the same specified numerical experiment and found that the model solutions are similar in form but differ by up to a factor of 5 in the strength of the multiple-scattering contributions.
Abstract: Multiple-scattering LIDAR return calculations obtained by seven different models for the same specified numerical experiment are compared. This work results from an international joint effort stimulated by the workshop group called MUSCLE for MUltiple SCattering Lidar Experiments. The models include approximations to the radiative-transfer theory, Monte-Carlo calculations, a stochastic model of the process of multiple scattering, and an extension of Mie theory for particles illuminated by direct and scattered light. The model solutions are similar in form but differ by up to a factor of 5 in the strength of the multiple-scattering contributions. Various reasons for the observed differences are explored and their practical significance is discussed.
107 citations
TL;DR: In this article, a truncation of the high-energy tail of the thermal distribution followed by collisional relaxation is proposed for the evaporation of trapped atoms, which is solved analytically for arbitrary power-law potentials.
Abstract: Evaporative cooling of trapped atoms is described as a sequence of truncation of the high-energy tail of the thermal distribution followed by collisional relaxation This model is solved analytically for arbitrary power-law potentials The threshold density for accelerated evaporation is found to be lowest in a three-dimensional linear potential
93 citations
TL;DR: In this paper, the difference between both actively and passively mode-locked lasers with Gain-at-the-End (GE) and Gain-in-the Middle (GM) at the example of Nd:YLF was systematically investigated.
Abstract: We systematically investigate the difference between both actively and passively mode-locked lasers with Gain-at-the-End (GE) and Gain-in-the-Middle (GM) at the example of Nd:YLF lasers. The GE laser generates pulse widths approximately three times shorter than a comparable GM cavity. This is due to enhanced Spatial Hole Burning (SHB) which effectively flattens the saturated gain and allows for a larger lasing bandwidth compared to a GM cavity. We first investigate enhanced SHB by measuring the cw mode spectrum, where we have observed that the mode spacing in GE cavities depends primarily on the crystal length. This was also confirmed for a Nd:LSB crystal, where the pump absorption length was significantly shorter than the crystal length. In mode-locked operation, pulse widths of 4 ps for passive mode locking and 5 ps for active mode locking are demonstrated with GE cavities, compared to 11 ps for passive and 17 ps for active mode locking with GM cavities. Additionally, the time-bandwidth product for the GE cavity is approximately twice the ideal product for a sech2 pulse shape and cannot be improved by dispersion compensation alone, while the GM cavity has nearly ideal time-bandwidth-limited performance. The results for the GM cavity compare well to existing theories taking into account the added effect of pump-power-dependent gain bandwidth which increases the bandwidth of Nd: YLF from 360 to > 500 GHz. In a following paper [1] (called Part II) a rigorous theoretical treatment of the effects due to SHB will be presented.
88 citations
TL;DR: In this article, the instantaneous local temperature is measured in a Rapid-Compression Machine (RCM) after the compression, and the temperature history is sampled at the laser pulse rate, and it exhibits large temperature fluctuations just after the end of compression.
Abstract: The instantaneous local temperature is measured in a Rapid-Compression Machine (RCM) after the compression. The technique that we have used is the laser Rayleigh scattering at 532 nm. Despite the important background noise due to the very confined RCM chamber, optimum optical conditions lead to a single-shot temperature accuracy of 30 K at the end of compression. The temperature history is sampled at the laser pulse rate, and it exhibits large temperature fluctuations just after the end of compression. Comparison with the extensively used calculated adiabatic core gas temperature shows excellent agreement, at least in the time interval corresponding to ignition delays ( < 100 ms). This first experimental assessment of core-gas assumption is important for chemical-kinetics numerical predictions in RCMs.
72 citations
TL;DR: In this article, a charge-transport model assuming one center in three different valence states is discussed for explanation of the photorefractive effect of light-induced charge transport.
Abstract: A new charge-transport model assuming one center in three different valence states is discussed for explanation of the photorefractive effect. Quantitative description of experimental results in KNb03:Fe by the model is demonstrated. Many similarities with the so-called “two-center” model are found although the microscopic explanation of the light-induced charge transport is rather different.
70 citations
TL;DR: In this article, the expansion of a small cloud of85Rb atoms in three-dimensional optical molasses (lin ⊥ lin and σ+ − σ− configurations) and observed diffusive motion were studied.
Abstract: We have studied the expansion of a small cloud of85Rb atoms in three-dimensional optical molasses (lin ⊥ lin and σ+ − σ− configurations) and observed diffusive motion. We determined the spatial-diffusion coefficients for various laser intensities and detunings, and compared them (in the case of lin ⊥ lin molasses) to values calculated from friction and momentum-diffusion coefficients of a one-dimensional (1D) theory of laser cooling. The predicted variations of the spatial-diffusion coefficient with laser intensity and detuning are in good qualitative agreement with the experimental data. We found that the minimal value observed experimentally, ≈ 6 × 10−4 cm2/s, lies within a factor of 3 of the 1D theoretical minimum, ≈, 26ħ/M, whereM is the atomic mass.
TL;DR: In this article, a Monte-Carlo model was developed for calculating multiply scattered LIDAR returns, where the magnitude of multiple-scattering effects is largely determined by the optical depth across the receiver field of view at the cloud.
Abstract: A Monte-Carlo model is described which has been developed for calculation of multiply scattered LIDAR returns. Results are shown for the common problem selected by the MUSCLE (MUltiple SCattering LIDAR Experiments) group for intercomparison, which represents a typical ground-based cloud-sensing scenario. This is contrasted with returns from the same cloud sensed by a space-based LIDAR, where multiple-scattering effects are much greater. The magnitude of multiple-scattering effects is seen to be largely determined by the optical depth across the receiver field of view at the cloud.
TL;DR: In this paper, one-dimensional single-pulse measurements of temperature and major species concentration (O2, N2, H2O and H2) in a turbulent H2/air jet diffusion flame using Raman and Rayleigh scattering of KrF* excimer-laser radiation are reported.
Abstract: In this article, we report on one-dimensional single-pulse measurements of temperature and major-species concentration (O2, N2, H2O and H2) in a turbulent H2/air jet diffusion flame using Raman and Rayleigh scattering of KrF* excimer-laser radiation. Spatial resolution of ≈ 0.5 mm along a 6mm long line has been obtained, with reasonable error limits for mole fraction (Δχ = 5 % for N2 detection) and temperature (ΔT = 8 %) determination at flame temperatures. We present various profiles showing the composition and temperature along a line at different heights in the flame with particular emphasis on the lift-off region (i.e. lowx/D). In this zone, temperature and mixture fraction can be determined simultaneously — from a single laser pulse — in a spatial region extending from unburnt gas in the center of the jet across the flame front into the cool air of the surrounding atmosphere. This allows for the first time the systematic study of the shape and width of the high-temperature region and the corresponding concentration and temperature gradients. The comparison of averaged data and scatter plots with previous pointwise measurements shows good agreement.
TL;DR: In this article, the authors present experimental confirmation of greatly enhanced Schawlow-Townes fluctuations in an unstable-resonator laser with a Petermann-noise enhancement factor of several hundred times.
Abstract: Expanding the fields of a laser cavity in a set of orthonormal modes is a standard technique in laser theory. Expansion in a normal mode set is also the basis of the concept of “photons”. A substantial number of practical lasers do not, however, support any kind of normal or orthogonal cavity modes, and thus, their fields cannot be represented (at least not easily) in terms of normal modes, or photons. This leads to a number of unusual results, including situations in which the lowest-order mode of a cavity can contain substantially more energy than the total energy in the cavity, as well as enhanced quantum spontaneous emission far stronger than the “single extra photon” level characteristic of an ordinary laser oscillator. We review the theoretical origins of these unusual effects and present experimental confirmation of greatly enhanced Schawlow-Townes fluctuations in an unstable-resonator laser with a Petermann-noise enhancement factor of several hundred times.
TL;DR: In this paper, the relative ion number stored in a Paul trap within the stability diagram given by the solution of the equation of motion reveal many lines, where only few or no ions can be confined.
Abstract: Systematic measurements of the relative ion number stored in a Paul trap within the stability diagram given by the solution of the equation of motion reveal many lines, where only few or no ions can be confined. The observations can be explained by the presence of perturbations from higher-order components in the trapping potential, which is a quadrupole potential in the ideal case. The resonances follow the equation (nr/2)βr + (nr/2)βz = 1,nr +nz =N, where 2N is the order of the perturbation,nr,nz are integer andβr,βz are stability parameters of the trap. The experiments were performed on H+ and H2+ ions, which are detected after a storage time of 0.3 s by ejection from the trap.
TL;DR: In this paper, the authors investigated the dynamics of end-pumped solid-state laser due to enhanced spatial hole burning and showed that the mode spacing of the cw running modes is essentially determined by the length of the gain medium and only weakly depends on the absorption depth of the pump transition.
Abstract: In Part I of this paper [1] experimental results were presented and discussed. In this part, we investigate theoretically the dynamics of end-pumped solid-state lasers due to enhanced spatial hole burning. This becomes possible by a fast numerical implementation of the saturated gain in the presence of strong spatial hole burning that allows to treat the multimode case for an arbitrary pumping level. We find for a wide range of laser parameters that the mode spacing of the cw running modes is essentially determined by the length of the gain medium and only weakly depends on the absorption depth of the pump transition. It is shown that spatial hole burning can lead to a completely flat saturated gain profile over half of the gain bandwidth. In mode-locked lasers, the flat gain due to spatial hole burning results in shorter pulses. But the pulses are neither Gaussian-nor sech-shaped as they are in actively or passively mode-locked lasers without spatial hole burning. Further, we show that soliton-like pulse shaping can be used to restore a transform-limited sech-shaped pulse in an end-pumped solid-state laser while exploiting the full gain bandwidth of the laser material.
TL;DR: An analytical approach to LIDAR return signal calculation with regard to multiple scattering is suggested and appears to be time saving in comparison with known methods.
Abstract: An analytical approach to LIDAR return signal calculation with regard to multiple scattering is suggested. Two versions of the method are developed. The first one is completely analytical and undispensible for qualitative studies. The second semianalytical approach provides a sufficient accuracy up to a sounding optical depth about 5. Being somewhat more tedious than the proposed analytical solution, the second approach appears to be time saving in comparison with known methods.
TL;DR: The position of a slow atom passing through a standing-wave light field in an ultrahigh-finesse optical resonator can be measured by observing either the intensity of the light transmitted through the cavity or its phase as mentioned in this paper.
Abstract: The position of a slow atom passing through a standing-wave light field in an ultrahigh-finesse optical resonator can be measured by observing either the intensity of the light transmitted through the cavity or its phase. Apart from the periodicity of the standing wave, both techniques allow to determine the position of the particle with a resolution much better than the standard classical diffraction limitΔϰ ≥ λ/2. Position measurements with uncertainty <λ/20 seem to be possible with all-optical techniques.
TL;DR: In this article, it was demonstrated that precise spontaneous Raman scattering measurements with deep-UV lasers are in principle possible in hydrocarbon flames, even with liquid fuels, by the use of polarization properties.
Abstract: It is demonstrated that precise spontaneous Raman scattering measurements with deep-UV lasers are in principle possible in hydrocarbon flames - even with liquid fuels - by the use of polarization properties. In contrast to the highly polarized Raman emission, the strong interfering broadband emission from combustion intermediates is unpolarized. Subtracting the emission intensities for the electric vector of the laser light perpendicular and parallel to the direction of observation allows to extract the pure Raman signals. Different applications of this technique are discussed.
TL;DR: In this paper, the authors used hard X-rays from a laser-produced plasma source for imaging of technical and biological objects using single-shot imaging of biological samples, which utilised focused radiation from a short-pulse terawatt laser.
Abstract: Imaging of technical and biological objects using hard X-rays from a laser-produced plasma source is demonstrated. Magnification radiography and single-shot imaging of biological samples are feasible with the source, which utilised focused radiation from a short-pulse terawatt laser. Differential imaging with element specificity and a new projection geometry for X-ray radiography are proposed.
TL;DR: In this paper, the authors describe the accurate measurement of ultralow loss in a high-finesse Fabry-Perot interferometer using a diode-pumped Nd:YAG laser locked to the longitudinal mode with an active frequency-stabilization technique.
Abstract: We describe the accurate measurement of ultralow loss in a high-finesse Fabry-Perot interferometer using a diode-pumped Nd:YAG laser locked to the longitudinal mode with an active frequency-stabilization technique. By measuring the resonance full width and the free spectral range with the frequency response functions, and by measuring the transmission efficiency on resonance, the finesse and the loss at 1064 nm are accurately measured to be 78100 ± 1200 (reflectance of 99.99598 ± 0.00006%) and 1 5.9 ±2.0 × 10−6 (15.9 ± 2.0 ppm), respectively.
TL;DR: The Nd-doped phosphate glass described in this article can withstand 2.3 times greater thermal loading without fracture, compared to APG-1 (commercially available Average-Power Glass from Schott Glass Technologies).
Abstract: The Nd-doped phosphate laser glass described herein can withstand 2.3 times greater thermal loading without fracture, compared to APG-1 (commercially available Average-Power Glass from Schott Glass Technologies). The enhanced thermal loading capability is established on the basis of the intrinsic thermomechanical properties (expansion, conduction, fracture toughness, and Young's modulus), and by direct thermally induced fracture experiments using Ar-ion laser heating of the samples. This Nd-doped phosphate glass (referred to as APG-t) is found to be characterized by a 29% lower gain cross section and a 25% longer low-concentration emission lifetime. Other measurements pertaining to the concentration quenching, thermal lensing, and saturation of the extraction are also described in this article. It is note-worthy that APG-t offers increased bandwidth near the peak of the 1054 nm gain spectrum, suggesting that this material may have special utility as a means of generating and amplifying ultrashort pulses of light.
TL;DR: By combined studies of electron spin resonance and optical absorption at low temperatures, the charge transfer bands of BaTiO3:Rh were identified to be peaked near 16 and 19 eV, respectively.
Abstract: By combined studies of electron spin resonance and optical absorption at low temperatures, the charge-transfer bands of Rh5+ and Rh4+ are identified to be peaked near 16 and 19 eV, respectively On this basis, the light-induced charge-transfer processes in BaTiO3:Rh are unraveled at room temperature It is shown that three charge states of Rh are involved, leading to two levels: the shallow Rh4+/5+ and the deeper Rh3+/4+ level The optical behaviour of these two levels corresponds to those expected from a ‘two-center’ model The present paper represents the first atomic-scale identification of three charge states of one element leading to optical ‘two-level’ response
TL;DR: In this article, a computational method is described in order to correct OH LIF temperature measurements for absorption of laser energy and trapping of fluorescence, which can be used as a correction data base both in case of (0-0) and (1−0) excitations.
Abstract: A computational method is described in order to correct OH LIF temperature measurements for absorption of laser energy and trapping of fluorescence. Calculations are performed in a large range of flame conditions and can be used as a correction data base both in case of (0-0) and (1−0) excitations. Comparison of corrected temperatures profiles obtained in a 40 Torr methanol/air flame, for both kinds of Laser-Induced Fluorescence (LIF) excitations shows a very good agreement. This method is applied to measure the temperature profile of a methanol flame perturbed by a sampling probe. The LIF collection volume is located at the actual probe sampled volume using an experimental procedure already described. Spatial resolution and sensitivity of temperature measurements are sufficiently efficient to highlight, for the first time by LIF, an indubitable cooling effect due to the probe presence that induces important OH profile change. According to flame chemical modelling, it is shown that both effects are strongly correlated.
TL;DR: In this article, the interaction of a polarizable atom with the electric field of a charged wire alone is not possible because of the 1/r2 form of the interaction potential, however, one can build a microscopic trap with a combination of a magnetic field generated by a straight wire and the static electric field generated from a concentric charged ring which provides the longitudinal confinement.
Abstract: We present new ways of trapping a neutral atom with static electric and magnetic fields. We discuss the interaction of a neutral atom with the magnetic field of a current carrying wire and the electric field of a charged wire. Atoms can be trapped by the 1/r magnetic field of a current-carrying wire in a two-dimensional trap. The atoms move in Kepler-like orbits around the wire and angular momentum prevents them from being absorbed at the wire. Trapping was demonstrated in an experiment by guiding atoms along a 1 m long current-carrying wire. Stable traps using the interaction of a polarizable atom with the electric field of a charged wire alone are not possible because of the 1/r2 form of the interaction potential. Nevertheless, we show that one can build a microscopic trap with a combination of a magnetic field generated by a current in a straight wire and the static electric field generated by a concentric charged ring which provides the longitudinal confinement. In all of these traps, the neutral atoms are trapped in a region of maximal field, in theirhigh-field seeking state.
TL;DR: The procedure of the Florence group for calculation of LIDAR return from clouds is briefly outlined in this paper, and the results of the particular case chosen for a comparison with other groups are presented.
Abstract: The procedure of the Florence group for calculation of LIDAR return from clouds is briefly outlined. The results of the particular case chosen for a comparison with other groups are presented.
TL;DR: In this article, the motion of a single charged particle in a Paul trap in the presence of the damping force is investigated theoretically and the modified stability diagrams in the parameter space are calculated.
Abstract: The motion of a single charged particle in a Paul trap in the presence of the damping force is investigated theoretically and the modified stability diagrams in the parameter space are calculated. The results show that the stable regions in thea–q parameter plane are not only enlarged but also shifted. Consequently, the damping force causes instability in some cases, contrary to intuition. As a by-product of the calculation, we derive new theoretical approximate expressions for the secular-oscillation frequency. In the limiting case of no damping, these formulas are in good agreement with early measurements done by Wuerker et al.
TL;DR: In this paper, the authors demonstrate that high-frequency excess-noise contributions above several MHz can be attributed to mode hopping and mode partition noise during multimode laser operation, and they also discuss how a FM-TDLAS spectrometer can be interpreted as an optimized Michelson interferometer for absolute distance measurements and therefore is extremely sensitive towards drift effects.
Abstract: Tunable Diode-Laser Absorption Spectroscopy (TDLAS) is increasingly being used to measure trace-gas concentrations down to low part-per-billion levels (1 ppbv = 10−9 volume mixing ratio). Semiconductor lead-salt diode lasers give access to the mid-infrared spectral region and the application of high-Frequency Modulation (FM) schemes can further improve the sensitivity and detection speed of modern instrumentation. Several factors influence or even limit spectrometer performance. The central elements in such spectrometers are lead-salt diode lasers. Experimental data will be presented, which demonstrate that high-frequency excess-noise contributions above several MHz can be attributed to mode hopping and mode partition noise during multimode laser operation. Additionally it will be discussed how a FM-TDLAS spectrometer can be interpreted as an optimized Michelson interferometer for absolute distance measurements and, therefore, is extremely sensitive towards drift effects. The higher the modulation frequency, the higher is the drift sensitivity of the spectrometer due to interferometric effects. These drift effects are a second factor affecting ultrasensitive measurements. While wideband-laser noise characteristics call for high modulation frequencies, the aforementioned interferometric effects in the spectrometer require low modulation frequencies.
TL;DR: In this paper, the authors measured the topological phase shift due to the interaction of a static electric field with the magnetic dipole moment of a moving atom (Aharonov-Casher effect).
Abstract: Consecutive, phase-coherent, near-resonant optical excitations of atoms have been used to realize an atom interferometer with a beam of thermal calcium atoms. We have measured the topological phase shift due to the interaction of a static electric field with the magnetic dipole moment of a moving atom (Aharonov-Casher effect). The observed phase shift was proportional to the electric field and, within our experimental uncertainty, independent of the particle's velocity. The measured value of the phase shift has been found to agree with the predicted one within a relative uncertainty of 2.2%.
TL;DR: In this article, the Energy-Corrected Sudden scaling law was employed to set up an empirical model for the calcu- lation of H2 linewidths in high-pressure hydrocarbon flames with H2 as a minority species.
Abstract: To establish Ha CARS thermometry at high pressure, accumulated H2Q-branch CARS spectra were recorded in the exhaust of a fuel-rich CH4/air flame at pressures between 5 and 40 bar. Temperatures were de- duced by fitting theoretical spectra to experimental data points. The Energy-Corrected Sudden (ECS) scaling law was employed to set up an empirical model for the calcu- lation of H2 linewidths in high-pressure hydrocarbon flames with H2 as a minority species. Experimental H2 CARS spectra could be simulated very accurately with this model. The evaluated temperatures agreed well with reference temperatures obtained by spontaneous rota- tional Raman scattering of Nz.