Threshold Pulse Energy Research Articles

Efficient, kHz repetition rate, gain-switched Cr:forsterite laser

We have developed a gain-switched room-temperature Cr:forsterite laser operating at repetition rates of between 1 and 34 kHz, and pumped by a continuous wave, Q-switched Nd:YAG laser. With optimised output coupling, an output pulse energy of 52 μJ was measured at 1.5 kHz repetition rate, corresponding to 11% efficiency and 13% slope efficiency. Threshold pulse energy was 53 μJ. Output power of 370 mW was obtained at 10 kHz repetition rate and 4.4 W pump power. Water cooling was not required for repetition rates up to 10 kHz. In a tunable, folded resonator, the Cr:forsterite wavelength tuned between 1173 and 1338 nm. This laser operated with maximum pulse energy of 34 μJ, efficiency of 13%, and power of 307 mW. The laser output was close to diffraction-limited with M2 of 1.2.

Dynamics of passively Q-switched microchip lasers

Self-pulsing in passively Q-switched microchip lasers is investigated in detail. The typical range of values of the parameters motivates a new analysis of the laser rate equations. We determine basic properties of the laser intensity oscillations such as threshold conditions, repetition rate, pulsewidth, peak power, and pulse energy. We show that these oscillations appear through a quasi-vertical Hopf bifurcation located slightly above the lasing threshold. Our bifurcation results are verified numerically by modeling a microchip laser experiment with Nd:YAG as the gain medium and YAG:Cr as saturable absorber. Our results agree with the experiment to within 10%.

Superradiant lasing from J-aggregated molecules adsorbed onto colloidal silver

The picosecond time-resolved emission spectrum of the cyanine dye 1,1′-diethyl-3,3′bis-(3-sulfopropyl)-5,5′,6,6′-tetrachlorobenzimidazolocarbocyanine (also known as BIC) adsorbed onto colloidal silver was examined as a function of laser pulse energy at room temperature. BIC is found to aggregate on colloidal silver, and the number of coherently responding molecules involved in the one-exciton state (i.e., the coherence length) was estimated to involve 8–9 molecules. Lasing at a remarkably low incident pulse energy threshold was found for this system and explained in terms of a mechanism involving superradiant states created in coherently coupled adsorbed molecules that emit photons which stimulate emission from other spatially distributed superradiant states.

Laser-stimulated desorption and damage at polished CaF2 surfaces irradiated with 532 nm laser light

Laser-stimulated emission of Ca neutrals from polished CaF2 (111)-surfaces was studied with a quadrupole mass filter system under UHV conditions. For intensity dependent measurements performed at various spots of the crystal surface a threshold pulse energy could be determined where a sharp decrease in transmitted light intensity occurred. Only for a few measurements this threshold coincided with the ablation threshold (type 1 spots). In most cases, however, the transmission breakdown threshold was found to be distinctly lower than the ablation threshold (type 2 spots). For type 2 spots a weak emission of delayed calcium atoms was observed at the transmission breakdown threshold. Irradiated spots have been examined by optical and scanning electron microscopy. Damage phenomena observed included coloration, cracking along crystal axes, surface erosion and surface melting depending on incident laser intensity. Pronounced differences between type 1 and type 2 spots with respect to laser light entrance and exit surface damage morphology were found.

Sheet metal welding using a pulsed Nd: YAG laser-robot

This paper presents a pulsed Nd: YAG laser-robot system for spot and seam welding of mild steel sheets. The study evaluates the laser beams behaviour for welding, and then investigates pulsed Nd: YAG laser spot and seam welding processes. High pulse power intensity is needed to initiate the key-hole welding process and a threshold pulse energy to reach full penetration. In seam welding, a weld consists of successive overlapping spots. Both high pulse energy and high average power are needed to keep the key-hole welding going. A 70% overlap is used to define overlapping spot welding as seam welding and to optimize process parameters because a high tensile strength joint compatible with the strength of the base material can be obtained when the overlap is ≥ 70%; at the same time a smooth seam with full penetration is obtained. In these cases, the joints in pulsed Nd: YAG laser welding are comparable in strength to those obtained with CO 2 laser welding. Robot positioning and motion accuracies can meet the demands of Nd: YAG laser sheet metal welding, but its cornering accuracy affects the welding processes. The purpose of the study is to evaluate the YAG laser-robot system for production in the automotive industry.

Resonance-Enhanced 2 + 2 Photon Ionization of Nitrogen: The Lyman-Birge-Hopfield Band System

High-resolution resonance-enhanced multiphoton ionization (REMPI) spectroscopy of a pulsed nitrogen beam is applied to determine the (1, 0) to (4, 0) line positions (J″ ≤ 2) of the a1Πg ← X1Σ+g transition. A strong a.c. Stark effect is observed even next to the REMPI threshold pulse energies and is tentatively correlated with certain energy levels near the three-photon energy. At the same time, the REMPI signal, which is not exactly proportional to the a ← X Franck-Condon factors, is dominated by the near-resonant enhancement due to these levels. This is particularly pronounced in the case of the (3, 0) band. In the three other cases, precise zero pulse energy values of the line positionscan be determined by linear extrapolation. The signs of the slopes for these extrapolations are opposite to those expected from the theoretical expression for the dynamical Stark shift. The obtained band origins are used to recalibrate the literature data which, so far, have exhibited major mutual disagreement. A Dunham-type least-squares fit of more than 3100 available tabulated spectral line positions yields a greatly improved mathematical description of this transition for v′ ≤ 15 and v″ ≤ 27, as well as ofa few A3Σ+u and b′1Σ+u levels. The X-state vibrational levels cannot be adequately described by a simple v″ + 12 polynomial. It is, however, found that the application of two separate polynomials for v″ ≤ 5 and v″ > 5 can remove all discrepancies. There are two conclusions from these calculations. The first is that there is an obvious onset of enhanced anharmonicity of the N2 ground state for v″ = 5 which is not observed for the isoelectronic CO molecule. This effect has already been found in earlier, shorter, and less accurate Dunham expansions. These earlier polynomials, however, do not take into account the sudden change of the vibrational frequency at v″ = 5 which must be concluded from two independent experiments by R. E. Miller. This small but distinct change suggests the mathematical treatment of the vibrational levels in the two ranges specified above. An unambiguous interpretation of the effect is not possible at present. No nearby 1Σ+g state is known which might act as a perturber, and, alternatively, a change of the electronic configuration needs to be quantified by a detailed theoretical study. Accurate measurements of the X-state vibrational levels are still rather sparse. Our work strongly suggests further experiments need to be done at a ≤0.001 cm−1 accuracy level that cover the full level range from v″ = 0 to at least v″ = 15 in order to obtain an even more unambiguous representation of this state and to reveal more details of the rotational structure.

Laser ablation of ZnSe using pulsed 193-nm irradiation

Plasma jets have been produced by the pulsed laser ablation of bulk ZnSe targets. Electrical measurements show that a sudden increase in ion yield occurs at a threshold pulse energy density of 210 mJ cm−2 pulse−1. The plasmas obtained do not exhibit overall electrical neutrality, but have a net positive charge, while the ZnSe target develops a corresponding negative charge. The initial ion translational kinetic energies are also found to be very low, <1 eV. These observations suggest that the ablation process is the result of a buildup of photogenerated holes at the semiconductor surface sufficient to result in disintegration of the lattice and rapid vaporization at low thermal energies.

Effects of inverse bremsstrahlung in laser-induced plasmas from a graphite surface

The kinetic energy of electrons emitted due to laser interaction with a graphite surface was studied with a time-of-flight spectrometer. In addition the yields of carbon atomic and molecular ions were measured as a function of laser pulse energy. Pulse energy thresholds for ion emission are observed to correlate with the observed maximum electron energies. Furthermore, the data suggest that ionic carbon clusters can be dissociated by energetic electrons or photons created in the plasma. We believe that initially photoemitted electrons are accelerated by inverse bremsstrahlung to the energies required for electron impact ionization and dissociation

The interaction between excimer laser energy and vascular tissue

The effects of XeF1 excimer laser on isolated normal and atherosclerotic aorta were studied. Experiments were performed in flowing water at constant temperature, flow rate, water depth, pulse width (10 nsec), wavelength (351 nm), beam size (1 mm 2) and focal length (50 cm). The number of pulses, the pulse energy, and the pulse frequency were varied, and the vascular tissue was studied histologically. The following observations were made: (1) tissue ablation required a minimum threshold pulse energy and was nonlinearly proportional to the number of pulses and the pulse energy delivered; (2) precise tissue ablation occurred at low pulse frequencies, but changes resembling a thermal process were seen as pulse frequency increased; (3) calcified plaque was more photoresistant than atheroma or normal vessel; (4) excimer laser energy was markedly attenuated by blood; and (5) the time interval between pulses and high peak power are related to the precision of ablation by pulsed excimer laser. It is concluded that excimer laser can rapidly and precisely ablate vascular tissue by a photothermal process.

Pulsed laser heating of silicon: The coupling of optical absorption and thermal conduction during irradiation

Transient laser annealing of semiconductors is examined extensively and the rise in lattice temperature specifically attained in silicon under the influence of a high-power laser beam irradiation is characterized analytically over a wide range of laser wavelengths, intensities, and durations. For the case of silicon, with increasing lattice temperature the optical absorption coefficient is drastically enhanced, while the thermal conductivity is considerably reduced. These two temperature-dependent material parameters are, therefore, strongly coupled during the laser beam irradiation, and the effects of this coupling on the ensuing lattice-temperature rise are examined comprehensively. Specifically, the threshold pulse energy for the onset of surface melting is analytically calculated as a function of both material parameters and operating laser beam characteristics.

Temperature Characterization of Pulsed Laser Annealing of Semiconductors

ABSTRACTThe dynamic characteristics of lattice temperature, T attained in Si under the influence of a high power laser beam irradiation are examined analytically over a wide range of laser wavelengths, pulse intensities and durations. The strongly temperature-dependent material parameters, such as optical absorption coefficient α(T) and thermal diffusivity D(T) are incorporated in heat diffusion equation, and their nonlinear coupling effect on the ensuing temperature in laser-processed semiconductors are examined. Specifically, the threshold pulse energy for surface melting is characterized as a function of both material and annealing laser beam parameters. This analytic description of pulsed laser heating of semiconductors should provide considerable insight into the transient heating phenomena and localized material modifications.

Thermal Analysis and Experimental Evaluation of Melting Threshold Energy of Si Thin Film Structure in Laser Annealing

Simultaneous heat-diffusion equations including laser light absorption in an Si thin film structure are solved analytically to give the time evolution of temperature. In this model, it can be concluded that the threshold pulse energy needed to melt the surface of amorphous Si film depends on the pulse duration, the absorption coefficient, the thickness of the Si film, and the thermal properties of the substrates. Experiments were performed on amorphous Si films of various thicknesses on fused quartz and crystalline Si substrates to verify the analytical results. The samples were irradiated with a Q-switched Nd-YAG laser with time durations of 80 ns at λ=1.06 µm and 50 ns at λ=0.53 µm. The experimental results on the dependence of the melting threshold energy on the thickness of the amorphous Si are in good agreement with the calculated results.

Pulsed laser annealing of single-crystal and ion-implanted semiconductors

The dynamic characteristics of both single-crystal and ion-implanted semiconductor layers annealed by a pulsed high-power laser beam is examined analytically for the first time by means of a parametrized perturbation method. The laser-induced lattice temperature rise is explicitly related to the laser beam parameters as well as the semiconductor properties. Specifically, the temperature in the annealed semiconductor is characterized in terms of the ambipolar diffusion length of hot, excess charge carriers, the optical attenuation coefficient of the medium, and the operating laser power, and the threshold pulse energy for surface melting is calculated for the case of silicon devices. It is shown that the pulse energy required for the onset of surface melting is sensitively dependent on the optical absorption coefficient, decreases significantly with increasing pulse intensity, and increases remarkably with increasing diffusion length of excess charge carriers.

Pulsed clearing of a synthetic aqueous aerosol by CO2laser radiation

An experimental investigation was made of the action of a microsecond pulse in the 10.6 µ range on the dispersive properties of a fine-droplet synthetic aqueous fog. It was found that when a certain threshold pulse energy was reached, a state of total clearing occurred and remained stable for a time governed by the wind-induced drift of the fog. At subthreshold pulse energies, secondary condensation occurred causing clouding of the medium at the 0.63 µ wavelength. An optothermodynamic analysis was made of the clearing process and the recondensation kinetics was analyzed.

Laser heating of semiconductors—effect of carrier diffusion in nonlinear dynamic heat transport process

An analytic theory is presented to describe the nonlinear dynamic heat transport process in a semiconductor irradiated by a pulsed laser beam. The input rate of laser energy to the lattice is sensitively influenced by the ambipolar diffusion of the dense, laser-produced excess charge carriers. Additionally, the high heating rate of the laser beam significantly changes the material transport coefficients during the pulse. This nonlinear laser beam-solid interaction is examined from the viewpoint of the transport process, using a parametrized perturbation technique in Green′s function formulation. An explicit, analytical solution of the lattice temperature rise is presented and the threshold pulse energy for the onset of surface melting for the case of amorphous Si is calculated as a function of laser beam intensity as well as the carrier diffusion length. Our results are compared with both the melting and nonthermal models for laser annealing.

The depth of defect annihilation in silicon by pulse laser annealing: Experiment and theory

Defect annihilation has been observed in silicon as a result of laser annealing using a Q-switched ruby laser. The threshold pulse energy density for defect annihilation was found to be 1 J/cm2 (pulse length ∼25 ns), and the depth of the defect-free region was found to increase with increasing pulse energy. A thermal model was developed which predicts the depth of melting and the recrystallization velocity. The calculated depth of melting was found to be in good agreement with the experimentally determined depth of defect annihilation. The defect annihilation occurs as a direct result of the extremely high recrystallization velocities which are associated with laser annealing with short pulses.

Method for accurate determination of threshold pulse energies for laser-induced gas breakdown

A system for accurate determination of breakdown thresholds is described. Automatic recording of individual laser pulse energies allows statistical analysis of the outcomes of many laser pulses and a consequent determination of relative threshold pulse energies with better than 1% accuracy, in spite of much larger variations in laser energy from pulse to pulse. Other statistical information such as the probability of breakdown as a function of pulse energy and pulse-to-pulse correlations can be measured with comparable accuracy.

Electric stress and laser surface damage

Small craters are formed when a self-focused laser filament reaches an end surface of a glass sample. These may be formed by the electric surface stress. The normal pressure is typically 107N.m-2(1500 lbf/in2) at threshold pulse energies for a track radius of 2.5 μm.