Variation Of Laser Intensity Research Articles

Frequencies, times, and forces in the dynamics of Na clusters

Laser excitation of metal clusters depends on various parameters such as frequency, intensity, and pulse time. We give here an overview of relevant time scales and forces for the example of Na clusters. Variation of laser intensity is studied in connection with second harmonic generation. We see a rather sudden transition from a perturbative regime at low intensities to a field-dominated regime for larger intensities. We use the non-linearized time-dependent local density approximation (TDLDA), together with a local pseudopotential for the interaction of the valence electrons with the ions. Explicit ionic motion is taken into account in one test case where we demonstrate the importance of ionic effects for the detailed time evolution at times larger than 100 fs.

Influence of laser intensity on carrier generation in MOSFETs

Pulse laser-induced carrier transport in the p/n junction of MOSFETs was investigated. Measurements of dynamic voltage traces across p/n junction at the drain were performed with varying laser intensities under several reverse bias conditions. For low laser intensity, measured voltage signal shape is almost identical to that of the laser pulse intensity. However, at increased laser peak intensity, p/n junction voltages rise faster than laser pulse intensity and show substructures at their falling edges.

Importance of initial nucleation step on low temperature photoassisted MOVPE growth of ZnSe

In this work, low temperature photoassisted metal-organic vapor phase epitaxy (MOVPE) growth of ZnSe on GaAs was carried out using dimethylzinc (DMZn) as the Zn precursor and dimethylselnium (DMSe) or diethylselnium (DESe) as the Se precursor at atmospheric pressure under H2 carrier gas, using the 458 nm wavelength from an Ar+ laser. In situ laser interferometry showed that the growth rate for thin films is non-linear and was explained in terms of the efficiency of coupling of the photo-irradiation with the growing film. The growth rate (GR) can be described by GR = P η F (1 − exp (−α d)) where η is the photo-chemical quantum efficiency, d is the film thickness, F is a structural factor, P is the laser intensity and α is the absorption coefficient. In situ laser interferometry has been essential in making these measurements to observe the initial non-linearity and ensure that characteristic growth rates are taken from steady state growth. Growth using DMSe as the Se precursor was very photosensitive due to its higher photochemical quantum efficiency arising from the additional source of CH3. radicals. These layers were mostly rough due to the slight variation in laser intensity across a region giving different degree of enhancement which is magnified by the growth rate non-linearity leading to non-uniform, and roughened growth. In the case of growth using DESe, it was possible to grow an initial nucleation layer of ZnSe pyrolyticaly prior to photo-assisted growth which ensured that there was sufficient thickness, d, of ZnSe such that the exponential term in the above equation became negligible and the growth rate was dependent only on the linear terms. The slight variation in laser intensity across the surface does not then cause a very significant variation in growth rate, thus maintaining smooth morphology and uniform growth rate.

Structural and Mechanical Properties of Diamond-Like Carbon Films Prepared by Pulsed Laser Deposition With Varying Laser Intensity

ABSTRACTDiamond-like carbon (DLC) films have been prepared by pulsed laser deposition (PLD) (wavelength 248 nm), ablating highly oriented pyrolytic graphite (HOPG) at room temperature in a vacuum of 10−2Pa, at fluences between 0.5 and 35 Jcm−2. Films have been deposited on Si(100) with and without a SiC interlayer. Structural analysis, such as visible and UV Raman, Infrared and Electron Energy Loss (EEL) spectroscopies show that the films are hydrogen-free and undergo a transition, from mainly disordered graphitic to up to 80% tetrahedral amorphous carbon (ta-C), above a laser threshold fluence of 5 J cm−2. The measured residual stresses of as deposited ta-C films do not exceed 2 GPa. Scratch tests show excellent adhesion properties. Low friction coefficients (0.05-0.1) have been measured in ambient humidity. Nanoindentation indicates that film hardness is as high as 70 GPa

Modifying a Rodenstock scanning laser ophthalmoscope for imaging densitometry

The necessary modifications and technical requirements are described for using a commercially available scanning laser ophthalmoscope (Rodenstock Model 101 SLO) as an imaging densitometer to assess human photopigment distribution. The main requirements are a linear detector amplifier, fast shutters for the laser beams, and a trigger unit. Images must be compensated for varying laser intensity. Both rod and cone photopigments are measured with the 514-nm argon laser of the SLO. Discrimination is possible owing to the different spatial distribution. The cone pigment density peaks in the foveal center (D = 0.40) with a steep decrease with increasing eccentricity E (full width at half-maximum, 2.5 degrees ). Rod photopigment increases with increasing eccentricity (D = 0.23 for E = 11 degrees ). These values are in agreement with previous reported results obtained with scanning laser ophthalmoscopes specially designed for retinal densitometry and high stability.

X-ray yields from Xe clusters heated by short pulse high intensity lasers

We have examined the absolute yield of keV x-rays emitted from gaseous plasmas created by the intense irradiation of large Xe clusters. We find that >10 μJ of x rays with photon energies above 1 keV are produced from clustering Xe gas targets when heated by 250 mJ, 2 ps laser pulses at an intensity of ∼1017 W/cm2. The yields show strong laser intensity dependence and variation with Xe cluster size.

Stark broadening using crossed Gaussian beams in stimulated Raman spectroscopy of N 2

We have studied the effect of crossing two Gaussian laser beams on Stark-broadened line shapes in stimulated Raman gain spectroscopy of N 2 Q-branch transitions. At about 350 GW/cm 2 of pump laser peak intensity in a 0.11 atm, 298 K gas sample, the Stark-broadened crossed-beam linewidth is about twice the linewidth of collinear beams. For the crossed condition, the two focussed (f/93) beams are crossed at 0.9 deg. The line shape measurements are in qualitative agreement with a numerical model for Stark-broadened line shapes that spatially resolves the laser intensity variations over the sample volume.

Entwicklung einer Laserstrecke zur Erweiterung der Scanning-Laser-Ophthalmoskopie

Scanning-laser ophthalmoscopy is one of the most efficient imaging techniques for investigating the human fundus. The fundus can be scanned with visible and infrared laser light. Using visible lasers, stimuli (e. g. fixation targets, text or perimetric targets) can be directly projected on the retina by modulating the intensity of the scanning laser beam. The distribution of the photopigments can be measured in vivo with high spatial resolution (scanning-laser densitometry). To improve the density measurements and other methods (perimetry, electrophysiology) an external laser source with the following features has been developed: the wavelength is 594 nm (HeNe), and due to an external stabilisation the variation of laser intensity is less than 1 %. An acousto-optic modulator (AOM) allows the modulation of the beam intensity with a rise time less than 90 ns and a dynamic range of 40 dB. Using a TTL triggered mechanical shutter, the illumination time can be restricted to single video frames (40 ms). This external laser beam can now be coupled with the SLO, thus improving the instrument for clinical investigation.

Effect of laser pulse shape on multiphoton excitation of a polyatomic molecule

The influence of the laser pulse shape on the multiple photon excitation dynamics of a large molecule like has been studied numerically in the pico- and nanosecond scale. In the theoretical model used multiphoton excitations are described in terms of the optical Bloch equations for the density matrix of four anharmonically shifted discrete vibrational levels coupled by the laser field and undergoing irreversible loss from the topmost level to the quasicontinuum (QC). The excitation in the QC is described by rate equations with loss from the discrete region as input. These equations are solved using the Monte Carlo technique suitably modified for a sharply varying laser intensity profile. The results show that the excitation of the QC and absorption due to a shaped laser pulse is greater than the corresponding quantities for a constant-intensity pulse of the same frequency, fluence and duration around the fundamental. The differences are small when the discrete-level bottleneck is very low (e.g. at the three-photon resonance frequency) or very high (e.g. when the frequency is blue shifted from the fundamental). The time dependence of multiphoton absorption (MPA) and temporal distribution of excited populations, is modified greatly by the pulse shape. The energy distributions of populations in the QC at different times, however, are not affected very much.

Time-resolved Fourier spectroscopy for activated optical materials

A low-cost add-on to commercial Fourier-transform spectrometers that have a continuously scanning Michelson interferometer has been developed for high-resolution, broadband, time-resolved spectroscopy. A number of innovations have been implemented to enable near-IR, visible, and UV photolumi-nescence studies. These include error correction and normalization of interferogram points to correct for laser intensity variations and missed shots, reduction of mirror-speed variations with recognition and avoidance of the timing mistakes they cause, and simple white-light-interferogram advancement optics that leave high-frequency modulation efficiency for the signal of interest unchanged in dynamically aligned systems. Application to energy-transfer phenomena in solid-state-laser media is described.

A Model for Laser Hole Drilling in Metals

A direct computer simulation technique is developed to analyze quantitatively the influence of the fluid flow and heat transfer in the transient development of a laser drilled hole in a turbine airfoil material, where the material removal is effected by vaporization and melt ejection. The coupled conduction heat transfer in the solid and the advection-diffusion heat transfer in the liquid metal, the fluid dynamics of melt expulsion and the tracking of solid–liquid and liquid–vapor interfaces have been mathematically modeled for the 2D axisymmetric case. The donor–acceptor cell method using the volume of fluid approach is used to solve the complex problem and a versatile numerical code has been developed. It takes into account all thermophysical properties including latent heat of vaporization, gravity, and surface tension driving forces. The novelty of this model is to treat the melted pool surface as a deformable free surface. The impressed pressure and temperature on the melt surface is provided by an 1D gas dynamics model whose vaporization kinetics are also discussed. The model is used to simulate drilling for a number of spatially and temporally varying laser intensity profiles. It is found that resolidification of melt (recast formation) occurred throughout the pulse interval and had significant effect on the developing hole geometry, while the effect of vaporization material removal on the hole geometry is found to be small. Comparison of the simulated results indicates the material removed per joule of energy absorbed appears to be inversely proportional to the square root of the peak beam intensity and the drilling rate appears to be proportional to the square root of the surface pressure.

Room-temperature inverse Faraday effect in terbium gallium garnet

The inverse Faraday effect (IFE) in terbium gallium garnet (TGG) is observed at room temperature. A circularly polarized 532 nm optical pulse from a frequency doubled Nd:YAG laser traverses a TGG crystal, resulting in an induced voltage in a surrounding detection coil. Variation of laser intensity and polarization direction leads to changes in the induced voltage in accordance with Faraday’s law of induction. The IFE is attributed to the optical Stark effect.

Limitations in acousto-optic FM demodulators

The ability of an acousto-optic cell to deflect a light beam through an angle proportional to the frequency applied to its transducer has been exploited to construct a simple yet effective FM demodulator. By measuring the position of the deflected beam with some form of detector assembly, the deviation from a given centre frequency can be obtained. The performance of the system is analysed and expressions for the system linearity, bandwidth and minimum frequency resolution are obtained. Two different types of detector assembly are considered: a single detector with a knife-edge spatial filter and a bicell detector. The performance of the bicell system is found to be superior by virtue of its ability to reduce/cancel the effects of amplitude modulation on the signal and variations in laser intensity. It is shown that systems can be designed to operate at centre frequencies from tens of MHz to a few GHz, with maximum frequency deviations and modulation frequencies from a few kHz to tens of MHz and with the ability to detect minimum frequency deviations down to tens of Hz. Experimental results are presented to demonstrate the validity of these expressions. It is concluded that an acousto-optic system may be used to measure the FM characteristics of a signal. There is also potential for applications in other areas where fine measurement of frequency deviation is required, such as Doppler analysis.

Phenomenological description of the optical field chaos in storage ring free-electron lasers.

The problem of optical field chaos in a storage ring free-electron laser oscillator has been discussed by using a phenomenological model. The result of theoretical analysis and numerical simulation shows that the variation of laser intensity versus time can be both periodic and chaotic when there is a weak gain modulation in the optical cavity. As time increases, the leading Lyapunov characteristic exponent of the system goes to a negative and a positive real number, respectively. Further research is carried out, and a chaotic transition via period-doubling bifurcation has been found when there are variations in the modulation parameters. The behavior of the system with two different kinds of gain modulation is also discussed.

Mode-structure influences on the laser-induced optogalvanic effect in pre-breakdown discharges

The technique of quantitative laser-induced optogalvanic perturbation of pre-breakdown discharges, established in previous work, is developed through an investigation of the effect of the mode structure of the laser radiation. Results are presented using three different mode structures: single mode and two qualitatively different multimode structures. These show that one of the multimode structures, readily generated by a ring dye laser, is unsuitable for quantitative optogalvanic investigations. The other controlled multimode structure gives results broadly similar to those from single-mode perturbation. However, optogalvanic signals are harder to locate with single-mode perturbation and metastable-particle-density decay rates are higher, for a given laser intensity, thus making analysis more difficult. Also, effects possibly due to saturation become apparent at relatively low laser intensities with single-mode perturbation. Both multimode and single-mode perturbation show effects with variation of laser intensity that are not in accordance with existing theoretical expectations. These effects might be due to a subset of the 1s5 metastable population being unable to absorb the laser radiation, because of its polarization state. This implies that the magnetic substates of the 1s5 state are surprisingly resilient to collisional redistribution of population among them.

Influence of irradiation conditions on plasma evolution in laser-surface interaction

The plasma plume induced by pulsed CO2 laser irradiation of a Ti target at power densities up to 4×108 W cm−2 was studied by emission spectroscopy. Time- and space-resolved measurements were performed by varying laser intensity, laser temporal pulse shape, ambient gas pressure, and the nature of the ambient gas. Experimental results are discussed by comparison with usual models. We show that shock wave and plasma propagation depend critically on the ratio Ivap/Ii, Ivap being the intensity threshold for surface vaporization and Ii the plasma ignition threshold of the ambient gas. Spectroscopic diagnostics of the helium breakdown plasma show maximum values of electron temperature and electron density in the order of kTe∼10 eV and ne=1018 cm−3, respectively. The plasma cannot be described by local thermodynamic equilibrium modeling. Nevertheless, excited metal atoms appear to be in equilibrium with electrons, hence, they can be used like a probe to measure the electron temperature. In order to get information on the role of the plasma in the laser-surface interaction, Ti surfaces were investigated by microscopy after irradiation. Thus an enhanced momentum transfer from the plasma to the target due to the recoil pressure of the breakdown plasma could be evidenced.

Measurement of temperature PDFS in turbulent flames by the CARS technique

Accurate temperature measurement with the CARS technique is difficult when appreciable temperature gradients occur within the measurement volume. The problem is aggravated by the non-linearity of the spectral intensity with temperature. The paper reports on the results of steps taken partially to ameliorate the problem, in the context of attempts to measure temperature pdfs in turbulent combustion. These involve a two beam system for the emitted nitrogen spectra, one beam of which is attenuated to reduce the intensity. By this means the full range of intensities that arise in combustion can be accommodated by the diode array camera and accuracy is preserved over a wide range. Furthermore, the intensity of the spectrum is measured and a calibration relates this to the proportion of hot gas in the control volume, although this strategy is only partially successful because of the variation of laser intensity from shot to shot. Within the spectrum a ‘hot’ band and a ‘cold’ band are used separately for theoretical spectral matching and consequent temperature measurement. This is relevant to the case where a thin flame lies across the measurement volume. Further refinements make some allowance for flame thickness, but even with these measures some ambiguities remain. The refinements are discussed in relation to measurements of temperature probability density functions through a premixed methane-air turbulent flame of relatively low Karlovitz number on a V burner. Expected bimodal distributions are confirmed.

Theoretical analysis of power generation in gasdynamic lasers

The power generation in a gasdynamic laser (GDL) has been theoretically studied. Quasi-one-dimensional unsteady flow equations are numerically solved simultaneously with the rate equation for the laser intensity inside the laser cavity. Steady-state coupled solutions for the field variables and the laser intensity inside the GDL cavity are obtained by a time-dependent numerical technique. The laser intensity variations along the axis of the flow (across the laser cavity) are computed for different laser gas mixture compositions and reservoir conditions. The analysis predicts a sharp spike in the laser intensity at the leading edge of the laser cavity followed by a sharp fall from which it recovers to a uniform constant value for the lower H2O concentrations. It is shown that the H2O concentration above 2% in the CO2-N2-H2O gas mixture is detrimental to the power generation.

Effects of irradiation non-uniformity on X-ray laser physics

Time resolved monochromatic imaging diagnostic is used to study in lasing plasma the effects of laser irradiation non-uniformity on line focused plasma conditions. Significant large scale non-uniformities are observed with a conventional focusing system. A new focusing arrangement (SWA) is used to produce a more uniform linear plasma. Quantitative measurements of the influence of the local laser intensity variations on the ionization balance and on the recombination time history are presented and discussed.

Effect of Laser Intensity Variation and Coherence on the Resonant Excitation of the 61P1–61D2Transition of Atomic Mercury

Abstract The effects of resonant laser excitation of an atomic transition whose lower state is not a ground state are reported. The system studied is the optical excitation to the 61D2 state from the electron-excited 61P1 state of Hg. Both the total fluorescence intensity and the time-resolved electron-photon coincidence fluorescence signal from the 61D2–63P1 relaxation channel are observed as the laser intensity is varied. In the coincidence signals, a manifestation of Rabi cycling in the optically excited transition is evident. The experimental data are in generally good agreement with predictions from a quantum-electrodynamic model.