Cw Laser Beam Research Articles

Adiabatic population transfer by acoustooptically modulated laser beams

We demonstrate coherent two-photon optical population transfer (STIRAP, J. Chem. Phys. 92, 5363 (1990)) between electronic states of atoms by use of two cw laser beams with high spectral resolution, which are modulated by an acoustooptical modulator (AOM) and provide a counterintuitive sequence of temporally delayed pulses of several hundred nanoseconds length. The efficiency of optical population transfer induced by this AOM-STIRAP method is shown in the lambda-type system of metastable Ne atoms, produced in a nozzle beam, where we compare directly spatially and phase separated pulses from cw laser beams (cw-STIRAP). As an application, efficient and selective transfer of population into a decaying electronically excited state of Na is demonstrated using the Doppler-free two-photon ladder-type transition 3S-3P-5S in a sodium cell. The experimental results are reproduced satisfactorily by use of a density matrix approach.

移動熱源によるレジノイド砥石のドレッシングに関する研究 (第3報)

The purpose of this study is to establish a new dressing technique of resinoid bond wheel by scanning the point of heat source on wheel surface. Dressing effect with CW YAG laser is testified in previous report. In this paper, Q-switched YAG laser is selected as a dressing tool for making more improvement of dressing effect with YAG laser, and dressing mechanism and grinding characteristics of dressed resinoid bond wheel with Q-switched YAG laser are experimentally discussed by observing the acting wheel surface and analyzing some grinding results with dressed wheel. Furthermore, dressing effect with Q-switched laser is verified by comparing with that with CW laser. Main conclusions obtained in this paper are as follows : (1) Scanning Q-switched YAG laser beam on resinoid bond wheel surface results in better dressing effect than that with CW laser beam. (2) Dressing with Q-switched laser of 2kHz in Q-switch frequency brings out best grinding characteristics of resinoid bond wheel in those of all Q-switch frequency. (3) In lower Q-switch frequency, heat affected structure generated on dressed wheel surface is completely removed as soon as laser beam is irradiated because of radical and frequent variation of heat energy supplied with laser beam.

Two-Photon near Infrared Excitation in Living Cells

Non-linear effects due to two-photon near infrared (NIR) excitation of endogenous and exogenous cellular chromophores allow novel techniques in tissue, cell and biomolecule diagnostics, as well as in intracellular micromanipulation (e.g. intracellular photochemistry). Two-photon NIR excitation may also result in cell damage effects. The high photon intensities (1024 photons cm−2 s−1) required for non-resonant two-photon excitation can be achieved by diffraction-limited focusing of continuous wave (cw) laser beams (cw microbeams) with powers in the mW range. For example, NIR traps (“laser tweezers”) used as force transducers and micromanipulation tools in cellular and molecular biology are sources of two-photon excitation. NIR traps can induce two-photon excited visible fluorescence and, in the case of <800 nm-traps, UVA-like cell damage. Multimode cw microbeams may enhance non-linear effects due to longitudinal mode-beating. To perform high scan rate two-photon fluorescence imaging, the application of ultrashort laser pulses of moderate peak power but low average power (pulsed microbeams) is required. In NIR femtosecond microscopes, non-destructive imaging of two-photon excited fluorophores in various human and culture cells was demonstrated for <2 mW average powers, <200 mW peak powers and 400 GW cm−2 intensities (700–800 nm, ∼150 fs, ∼100 MHz). However, higher average power levels may result in failed cell reproduction and cell death due to intracellular optical breakdown. In addition, destructive transient local heating and μN force generation may occur.

Optically driven Mie particles in an evanescent field along a channeled waveguide

We report a series of experiments in which Mie-scattering particles were optically driven in the evanescent field that is generated in a channeled waveguide. Polystyrene latex spheres with diameters of 1-5 microm were laterally trapped within the evanescent field, which was produced by a cw laser beam incident upon the waveguide and then longitudinally driven along the direction of the waveguide channel at speeds of as much as 14 microm/s. Metallic spheres of gold (0.5-microm diameter) and platinum (1-microm diameter) were also laterally trapped and longitudinally moved along the waveguide channel.

Optical effects caused by high laser powers in ion-implanted Bi4Ge3O12waveguides

The 'optical writing' of permanent channel waveguides in the surface of He + ion-implanted bismuth germanate, Bi 4 Ge 3 O 12 , has been studied, using end-coupled argon and Ti:sapphire CW laser beams. Comparisons of the self-focusing effect in waveguides and bulk samples have been made for both Nd 3+ doped and undoped material. The permanent 'writing' effect has been compared with the structural change observed earlier in ion-implanted Bi 4 Ge 3 O 12 waveguides annealed at high temperature.

Laser-induced temperature rise in Gaussian-shaped holes

The temperature rise induced by a stationary cw laser beam on a substrate surface with an etched hole was investigated. The model was developed by considering a circular Gaussian laser beam that is irradiating a substrate of semi-infinite geometry. This model can be applied to any material but MnZn ferrite was selected as an example in our investigation. The laser-induced temperature rise on the surface with and without an etched hole was studied. It was found that the laser-induced temperature rise depends strongly on the depth of the etched hole but weakly on the radius of the etched hole. The results are expected to be useful in investigating the shapes of laser-etched patterns and the distributions of laser-induced thermal stresses and damages in substrate surfaces.

Dynamics of laser beam switching in sodium vapor

The interaction of left- and right-hand circularly polarized narrow-band cw laser beams that copropagate through sodium vapor and are tuned to the homogeneously broadened D1 transition can lead to the mutual extinction of both beams. Such beam switching is caused by an intensity- and polarization-dependent refractive index due to optical pumping even at low intensities. The dynamics of beam switching is investigated experimentally and explained qualitatively by the conventional J = 1/2 to J = 1/2 transition model. The accurate quantitative description, however, is only achieved by an extended model comprising hyperfine ground states in spite of homogeneous line broadening by argon buffer gas, which is larger than the hyperfine splitting.

Analysis of dynamic pattern formation in nonlinear Fabry-Perot resonators

We present a detailed analysis of transverse effects and pattern formation in bistable optical elements. The system we investigate consists of a Fabry-Perot resonator for the optical feedback element with a nematic liquid-crystal cell used as an optically nonlinear intracavity medium. On illumination with a cw-laser beam, the system causes the beam to break up into several individual spots, passing through several transitions before finally reaching a stationary state. We devise a theoretical model which is used as the basis for numerical simulations of the system. The simulation results are in good agreement with experiment. Finally, we characterize the principal instability of the system using a linear stability analysis of the theoretical model.

Determination of the absolute excited state density of a sodium target by means of beam deflection measurements

The average deflection of a laser excited, divergent sodium beam with a broad velocity distribution is measured by means of a Langmuir-Taylor detector and exploited for determining the absolute density of the excited state in the interaction area. Simulations of the excitation and deflection process point out that the knowledge of the velocity distribution of the sodium atoms plays a crucial part in obtaining reliable information from deflection measurements. From the comparison of simulated and experimental results we find that in our set-up typically 30% of the sodium atoms are in the excited state when both ground state levels are pumped with an intensity of 35 mW cm-2 from an electro-optically modulated CW laser beam; the corresponding excited state density is 1.5*109 cm-3.

Narrow coherence-induced peaks in Doppler-free spectra of thorium

We report the observation of narrow peaks in Doppler-free spectra of thorium. The peaks are superimposed upon saturation-broadened line profiles in thorium atomic spectra by use of counterpropagating cw laser beams with resonance ionization mass spectrometry detection. At low laser powers the spectra show a conventional Lamb dip. As the laser power is increased, a narrow peak obscures the dip (for two of four transitions studied). The narrow peak is attributed to coherence effects at high laser powers. To confirm this assignment, we calculated the saturated absorption line shapes by using the theory of Haroche and Hartmann [ Phys. Rev. A6, 1280 ( 1972)]. The spectral simulations reproduce the narrow peaks and show the same dependence with laser power and transition strength as do the data. There are some quantitative discrepancies, particularly in the overall saturation broadening, but given that there were no adjustable parameters in the calculations, the agreement between theory and experiment is considered quite good. The Einstein A coefficients used in the calculations were determined from an analysis of the laser-power dependence of the saturation broadening for each transition.

The continuum absorption of argon, krypton and xenon dense plasmas produced in flashlamps

The optical thickness of dense plasma, created in linear flashtubes filled with noble gases, is measured over the wavelength range 466-780 nm using a CW laser beam. The electron density and temperature radial profiles of the plasma are determined by spectroscopic methods, infrared laser interferometry and local thermodynamic equilibrium equations. Knowing those profiles, the absorption coefficients and consequently the xi fb factors are deduced from optical thickness measurements. These factors are compared with theoretical values of Hofsaess (1978) and with previously published experimental results. The dependence of xi fb on the wavelength and the plasma parameters (electron density in the range 7*1017-1.7*1018 cm-3, temperature between 10900 and 17000 K) is discussed. The effect of the interparticle interaction on the absorption in the dense plasmas is only sensitive to the shift of the photoionization thresholds of atoms.

Transient grating optical nonlinearities of dye-doped cholesteric liquid crystals

Abstract Transient optical nonlinearities in dye-doped cholesteric liquid crystals are investigated with laser-induced dynamic grating experiments using nanosecond and picosecond excitation pulses. Selfdiffraction efficiencies of the induced optical gratings up to more than 10% have been observed. Grating buildup and decay are investigated by diffraction of a cw laser beam. The observed nonlinearities are explained in terms of fast electronic effects occuring in the dye-molecules as a primary process, and slower secondary processes connected with thermal gratings arising from radiationless recombination of excited molecules, which change the optical properties of the liquid crystalline host.

Investigation of the soft-mode relaxation in a ferroelectric liquid crystal with picosecond laser-induced dynamic gratings.

The relaxation of light-induced tilt-angle changes in a ferroelectric smectic-C * liquid crystal is investigated with picosecond laser-induced dynamic gratings. Grating buildup and decay is monitored by diffraction of a cw laser beam. It is shown that reorientation of the liquid-crystal molecules is achieved by rapid optical heating and follows the temperature rise with a characteristic time constant, depending on temperature. The observed rise times correspond to the soft-mode relaxation of the order parameter in a Landau theory describing the smectic-C * -smectic-A phase transition

The Protective Characteristics of Polycarbonate Lenses Against CO2 Laser Radiation

The vulnerability of plastic laser eye protection used with Class 4 CW lasers was studied using a high-power industrial laser to produce 4 by 4 cm burn zones. Since a CW (or repetitively pulsed) laser beam can burn through such eye protectors, this potential is of concern to Laser Safety Officers (LSOs) prescribing laser eye protectors and transparent barriers for use around both the 10 to 80-W CO2 lasers used in surgery as well as multi-kilowatt industrial CO2 lasers. Polycarbonate filters are shown to have superior resistance to burn through, and thresholds for protective lens burn-through in 3–10 s are provided. It is recommended that LSOs consider polycarbonate transparent impact safety lenses to have a protective optical density (OD) of 4.

Laser-induced bubble trapping in liquids and its effect on light thermal blooming

Bubble formation in a weak absorbing liquid and its subsequent trapping under the action of a cw laser beam of relatively low power are reported. The effect of the bubble trapping on the light thermal blooming is also presented. A new phenomenon of hysteresis of the thermal blooming is described. The physical origin of the observed effects is discussed.

Monitoring real-time CBE growth of GaAs and AlGaAs using dynamic optical reflectivity

Dynamic optical reflectivity (DOR) uses the interference oscillations arising from the multiple reflections, of a normally incident CW laser beam, between the surface of a growing film and the film-substrate interface. The oscillations have a period determined by the refractive index of the film and the laser wavelength. DOR measurements have been made, in real time, during the CBE growth of Al x Ga 1− x As layers on a GaAs(100) substrate. The results show that the growth rate and the aluminum composition x can be monitored.

Polarization Raman microprobe analysis of laser melting and etching in silicon

Polarization Raman microprobe spectroscopy is used to study crystalline silicon heated to the melting point by a tightly focused cw laser beam, which is either fixed or scanned across the surface. By examining optical phonons in solid silicon, the real-time Raman spectrum monitors the progress of silicon flow during melting and the trench depth during melt-assisted etching. Raman peaks lie between 482 cm−1, which is the Raman shift for silicon uniformly heated to the melting point (1690 K), and ∼510 cm−1, which is the Raman shift for c-Si heated just to the melting point and probed by the same beam. During laser melting with a static laser, the Raman spectrum of scattered light with z(x,y)z̄ polarization has two peaks, while the z(x,x)z̄ spectrum has one peak. This shows that at the beginning of melting in vacuum by a static laser there is a central region with solid silicon floating in the melt, which is surrounded by hot solid material. Because of the flow of the molten semiconductor, the temperature profile changes, causing the Raman spectrum to change rapidly. Laser melting of c-Ge and thin films of silicon in vacuum is also studied, as is the melting of c-Si by a static laser in the presence of an inert buffer gas. The presence of an inert buffer is shown to affect the temperature profile very strongly during melting and also during laser heating at lower laser powers when no melting occurs. During scanned laser melting and etching, the Raman spectrum has a single peak using either polarization configuration. Raman analysis during melting of silicon by a scanning laser shows that the average temperature in the probed region is much higher when there is a gas-phase argon buffer present (and no etching) than when there is an etching mixture of argon/chlorine gas (and etching). Along with these real-time Raman measurements, the reflection of the incident laser was monitored, and post-process Raman analysis and profilometry were also performed to characterize surface changes due to melting and etching.

Measurement of triplet optical densities of organic compounds in the presence of photodecomposition

We have developed a method to measure triplet optical densities OD(r)(lambda) when considerable photodecomposition is present. The method is demonstrated on Coumarin 120. By focusing an UV cw laser beam on a sample with the aid of a lens, high triplet optical densities OD(T)(lambda) can be recorded. However, the excitation of organic compounds with light causes varying degrees of photodecomposition. In order to take photodecomposition into account, one records triplet optical-density values as well as the accumulation of absorbing photoproducts as a function of time. Turning off the cw laser excitation, one records the accumulation of photoproducts only. Separating the two processes, one can determine how the triplet optical density OD(T) declines as a function of time t. The obtained curve can be expressed with an equation that appears to decline exponentially with time. This allows one to extrapolate back to t = 0 and recover OD(T) when there was no photodecomposition. The extrapolated OD(T) values are used to obtain triplet-extinction coefficients (T)(lambda) by McClure's method of Coumarin 120.

A 5 kW CW CO/sub 2/ laser using a novel negative-branch unstable resonator with a phase-unifying output coupler

A 5 kW CW CO/sub 2/ laser using a negative-branch unstable resonator (M=-1.5) is proposed and characterized experimentally. The resonator consists of a stepwise variable reflecting output coupler, called the phase-unifying output coupler, and a total reflector. A 5 kW CW laser beam with diffraction limited quality (2 theta =0.45 mrad) is obtained and affected by the focal point in the resonator. The misalignment angle to reduce the laser power to 95% is improved by a factor of 19 compared with a positive-branch unstable resonator (M=1.5) with a phase-unifying output coupler at the same resonator length. >

Evidence for a defect-assisted low resistive conductivity in cw laser beam mixed Au/Te/Au/GaAs contacts.

ABSTRACTCW laser beam mixed Au/Te/Au/n-GaAs contacts have been studied by RBS, Möss-bauer and Raman spectroscopy. For both low and high laser power mixing, resulting in respectively Schottky-type and ohmic contacts, the formation of non-uniformly dispersed Ga2Te3 crystallites was observed. However, for the ohmic contacts the formation of a high density of defect complexes in the GaAs surface layers was revealed. The experimental results suggest that the ohmic conduction mechanism is based on a hopping or a defect assisted tunneling process through the contact zone, consistent with the amorphous/highly defective heterojunction model.