Theoretical models of laser-induced damage mechanisms in optical materials are reviewed: inclusion-initiated thermal explosion (extrinsic mechanism) and impact ionization (II) and photoionization (intrinsic mechanisms). Different approaches to II theory based on quantum kinetic equation, Boltzman equations, and rate equations are briefly described. A relative contribution of II and photoionization predicted by these models at different laser pulse durations, including femtosecond-range, are discussed and compared with available experimental data. Basing on an analysis of published theoretical and experimental results, a today’s state of understanding fundamental laser damage mechanisms is concluded.
Theoretical models of laser-induced damage mechanisms in optical materials are reviewed: inclusion-initiated thermal
explosion (extrinsic mechanism) and impact ionization and photoionization (intrinsic mechanisms). Different approaches to
impact ionization theory based on quantum kinetic equation, Boltzman equations, and rate equations are briefly described. A
relative contribution of impact ionization and photoionization predicted by these models at different laser pulse durations,
including fs-range, are discussed and compared with available experimental data. Basing on an analysis of published
theoretical and experimental results, a today’s state of understanding fundamental laser damage mechanisms is concluded.
We present the results of theoretical studies of the laser-induced damage in transparent solids containing absorbing inclusions. The investigation is based on the inclusion-initiated thermal explosion model. Key aspects of the model are considered: thermal instability initiated by the inclusions; a mechanism of photoionization of a surrounding layer of a host material by a thermal ultraviolet-radiation of laser-heated inclusions; the thermal instability kinetics, and an associated pulse-width dependence of the laser-induced damage threshold. Also, statistical features of the damage, caused by random spatial distribution of the inclusions in the materials, and a final stage of the damage process-a mechanical stress-produced crack formation-are analyzed. A comparison of the theoretical results, related to the pulse-width dependence of the damage threshold, with experimental data for some typical optical materials in a wide pulse-width range is presented.
The method of temporal laser pulses shaping based on stretching-compressing and profiling of a pulse by volumetric
chirped Bragg gratings is proposed and realized in ps-duration range. Experimental results of the study are presented.
Results of experimental and theoretical studies, carried out in the author's laboratory during past four decades, of
fundamental mechanisms of laser induced damage (LID) to transparent solids are reviewed. Major features of LID
experimentally observed in optical materials of different classes at various conditions (dependence of damage thresholds
on radiation frequency, pulse width, temperature, etc.) are discussed. Theoretical models of both extrinsic (absorbing
inclusion-initiated )and intrinsic (impact and multi-photon ionization) damage mechanisms are presented and their
predictions for damage features (frequency and pulse-width dependence) are discussed. Peculiarities of LID in an ultrashort
(ps-fs) laser pulse duration range are analysed . In this context a relative role of thermo-elastic stress-induced
crack formation and ablation processes is considered. Experimentally observed features of LID are compared with
theoretically predicted ones and conclusions are outlined on dominating LID mechanisms in real optical materials. Further directions in experimental and theoretical studies are discussed for investigating fundamental LID mechanisms
in the ultra-short time domain.
Structural and optical characteristics of polymer-filled nanoporous glass (PFNPG) composite was studied with holographic interferometry and atomic-force microscopy methods. A high homogeneity of the bulk PFNPG samples and their surfaces has been found. Light scattering in the samples was investigated. Rather low scattering losses have been observed in spite of heterogeneous structure of the composite. This result is discussed on a base of the Rayleigh scattering model. Lasing properties including a pump-to-laser conversion efficiency and a life time due to photodestruction of PFNPG samples activated with pyrromethene dyes are reported.
Properties of the polymer-filled nanoporous glass (PFNPG) doped with different family dyes are reviewed in this paper. They include mechanical, optical, and thermo-optical properties of PFNPG as a host material for lasing and nonlinear absorbing dyes. Data on lasing and nonlinear absorption characteristics of dye-doped PFNPG are presented. From an analysis of these characteristics it is concluded that PFNPG is a new optical material perspective for creation of highly efficient solid-state dye lasers and laser radiation control elements including Q-switchers, mode selectors, and laser radiation power limiters.
A polymer-filled nanoporous glass composite (PFNPG) has been shown recently to be a very attractive host material for solid-state dye lasers. In particular, a high-conversion (lasing) efficiency, up to 70%, has been demonstrated for pyrromethene-family dyes impregnated to PFNPG, in a visible spectral range at 2-nd harmonic nanosecond pulse duration Nd:YAG laser excitation. For development of PFNPG-based dye lasers it is important to investigate lasing characteristics of this type matrices at various excitation (pump) wavelengths. In this context we have investigated the conversion efficiency, η, and emission spectra, fem, of phenolemine 512 (Ph 512) and pyromethene 580 (PM 580) dyes impregnated into PFNPG, at excitation wavelengths, λp, varied in a range of their main absorption bands. A microsecond pulse duration liquid solution dye laser was used in our experiments as a pump source. The conversion efficiency, η=32% and 47%, has been observed at λ=525 nm for Ph 512 and PM 580 respectively with output energy as high as 250 mJ. "Anomalous" dependencies of η and fem upon λp have been revealed for both dyes. In particular, two-frequency lasing has been observed for Ph 5 12 at λ=535 nm. Mechanism of the lasing anomalies is supposed to be related to photoinduced absorption in lasing dyes.
We report on a systematic study of the laser threshold and slope efficiency of solid-state dyes lasers operating in the near-IR under microsecond(s) pumping. The excitation source for this work is a flashlamp-pumped dye laser operating at a wavelength of approximately equal to 660 nm with a pulse duration of approximately equal to 1.25 microsecond(s) . A major objective of this work is to demonstrate laser characteristics traceable to direct diode-pumping of a dye- doped solid matrix. Detailed measurements were made for three laser dyes and three different host materials. Dyes evaluated include Rhodamine 700, Rhodamine 800 and Oxazine 725 at various concentrations. The host matrices studied were modified PMMA, polymer-filled nanoporous glass, and organically modified silicate. Laser measurements included output energy and wavelength as functions of input energy and resonator feedback. Findlay-Clay analyses were performed to extract information on the round-trip cavity losses in each case. Temporal waveforms were obtained for the pump and output pulses in order to evaluate the extent of tracking, and to study the mechanisms for loss of tracking. The highest laser efficiency observed was 43 percent for Oxazine 725 in MPMMA for a 2-mm thick sample. This material had a laser wavelength of approximately 690 nm for Roc < 0.70. The value increased to approximately 735 nm, however, for R > 0.80. The longest output wavelength observed in this study was approximately 797 nm for Rhodamine 800 in PFNPG. The highest efficiency observed for this particular sample was 21 percent. Its emission wavelength was relatively insensitive to feedback over the range studied. Measurements of laser threshold are presented and discussed for the materials investigated.
This paper reports on a systematic study ofthe beam quality ofsolid-state dye lasers for different resonator designs. The resonators investigated include: multimode stable, hard-edge unstable, and unstable with a graded-reflectivity mirror (GRM) output coupler. The purpose ofthe work was to investigate the conditions under which good beam quality can be achieved, and to examine the possible trade-off between beam quality and energy extraction efficiency. Beam quality was quantified through conventional M2 measurements. A hard-edge unstable resonator of magnification ?2.5 produced a beam ofM2 ?11 - 12 at a PRF of 2Hz and a pump flux of 5 MW/cm2. The value increased to ? 18 — 19 at a pump flux of2O MW/cm2. This can be compared with an M2 of ? 140 - 150 with the multimode stable resonator. Preliminary measurements with the GRM resulted in M2 values of ? 5 to 6 at 5 MW/cm2.
This paper reports on the laser emission properties of Pyrromethene 580, Pyrromethene 597, Pyrromethene 650 and Rhodamine 11B in the novel matrix polymer-filled microporous glass (PFMPG). This host material combines the advantages of an organic environment for the dye with the superior thermooptical and mechanical properties of an inorganic glass. Laser efficiency was measured as a function of pump flux for different dye concentrations, resonator feedback, and locations on the sample. Service life, defined as the number of pulses for the output to drop to the 70% point, was recorded at 5 Hz for the higher dye concentrations. The highest efficiencies were observed for Pyrromethene 597 (is congruent to 70%), which had a service life of is congruent to 60,000 shots at 25 MW/cm2 and is congruent to 45,000 shots at 50 MW/cm2. The longest service life was measured for Rhodamine 11B (is congruent to 110,000 pulses at 25 MW/cm2), but this dye had somewhat lower efficiency (is congruent to 50 - 55%). Thermal lensing measurements were made for dye-doped PFMPG and MPMMA, and showed that the lensing is much lower in the hybrid matrix. The agreement with the theoretical modeling is very good.
We present an overview of our theoretical studies on laser- induced damage (LID) in transparent optical materials including recent results on a pulsewidth dependence of the LID threshold, thermoelastic stress-induced damage criteria, damage morphology features, peculiarities of the LID at ultrashort pulse irradiation. Main attention is focused on inclusion-initiated damage, but some results are related also to intrinsic damage mechanisms. Theoretical results are compared with available experimental data on pulsewidth dependence of the LID threshold in bulk optical materials, on damage feature in thin films and on damage morphology studies. A good agreement of theoretically predicted and observed features is demonstrated, in particular, for the pulsewidth dependence of the LID threshold in a wide pulsewidth range from nanoseconds to femtoseconds.
Generation of ultrashort pulses (picosecond range duration) in pyrromethene-580 (PM-580) and xanthene 11B dyes doped modified PMMA polymer lasers is reported. Active polymer elements were pumped by second harmonic radiation of the 50 picosecond Nd:YAG laser. A single-pulse and train-pulse laser generation was observed at different conditions (variable optical resonator length and pump energy) with a pulsewidth ranging from 140 ps to 30 ps. The train-pulse generation is attributed to mode-locking of a laser resonator. The lifetime (tau) e, of an excited quantum state of a laser transition of the PM- 580 dye molecules has been estimated ((tau) e equals 200 divided by 250 ps) from the pulse-train duration.
In this paper we analyze features of absorbing inclusion-initiated laser-induced failure of a transparent dielectric surface in a multishop irradiation regime. Ablation and thermoelastic stress-induced damage mechanisms are investigated. The ablation is shown to be the dominant mechanism in the ultrashort pulsewidth domain, whereas the latter is realized at longer pulses. The alteration of damage mechanisms at pulsewidth variation is also characteristic for intrinsic damage mechanisms. Resent experimental results obtained for the ultrashort pulse damage are critically discussed in a context of a theory developed in this paper.
An adequate choice of a damage criterion has a key significance for development of a laser induced damage (LID) theory in transparent dielectrics. In this connection a comparative analysis of damage criteria applicable to different damage mechanisms is done in this paper. A consistent formulation of a thermoelastic damage criterion is given and one of the most important LID characteristics - the pulsewidth dependence of the LID threshold is obtained. An applicability range of the damage criterion and the pulsewidth dependence are discussed. Theoretical results for the pulsewidth dependence are compared with available experimental data and a good agreement is found in a very wide pulsewidth range from nanoseconds to femtoseconds.
Multiphoton absorption is able to produce primary free electron concentration, launching avalanche ionization mechanism in the transparent solids. We applied transient photoconductivity technique to investigate free carrier generation, caused by two-photon absorption in the alkali halide undoped crystals. Using radiation of tunable optical parametric oscillator-amplifier, the excitation spectra and temperature dependencies for conduction band electron concentration were measured. Two variations of experiment included optical excitation by 'equal' quanta and by combination of tunable radiation with a fixed wavelength laser source. As it was shown, an efficient free electron producing was observed not only for interband optical transitions, but also for two-photon exciton absorption at room temperature. An analysis of the data obtained and comparison to conventional photoconductivity spectra indicated thermal ionization of the exciton states in the temperature range above 300 K. The corresponding exciton activation energy in KI was estimated and found to be consistent with the location of energy levels, allowed for two-photon exciton transitions. The results obtained qualify transient photoconductivity and as an effective tool for investigation of multiphoton excitation and following electron transitions in the wide-band gap crystals.
A model of laser-induced damage in optical coatings due to photoionization thermal explosion of absorbing inclusions proposed and analyzed in our previous work reasonably explained dependencies of the laser induced damage threshold (LIDT) on a material band-gap energy and a coating thickness. Continuing the analysis of the model in this paper we present results on a laser radiation wavelength dependence of LIDT. It is shown hat his dependence is determined mainly by two factors: wavelength dependencies of inclusion absorption cross section and absorption coefficient of conduction band electrons. A computer analysis done taking into account both factors. Effects of coating thickness and damage statistics are also investigated. Theoretical results are compared with experimental data.
Laser-related properties of pyrromethene-580 doped modified polymethyl methacrylate, including conversion efficiency, operational lifetime, laser induced damage resistance of the polymer matrix and photodestruction of the impregnated dye, were studied under short-pulse and long-pulse laser pumping. High efficiency and operational lifetime have been achieved in the short-pulse mode. Effect of a laser resonator feedback on both the conversion efficiency and lifetime was revealed and studied in the long-pulse operation mode.
Theory of pulsewidth dependence of laser induced damage threshold (LIDT) in transparent solids is presented. The damage is supposed to be initiated by thermal explosion of absorbing inclusions. The investigation of thermal explosion is based on an analysis of the heat transfer equation and a new approach to solving this equation is developed allowing to study kinetics of thermal explosion without any modeling presentation of an absorption mechanism. It is shown that the key parameter determining a dependence of LIDT upon a laser pulsewidth, (tau) p, is the heat transfer time, (tau) , from an inclusion to a surrounding medium. At (tau) p >> (tau) a damage threshold is characterized by a laser radiation intensity, whereas at (tau) p << (tau) --by an energy density. The pulsewidth dependence of the LIDT has been investigated for rectangular and gaussian shapes of laser pulses and it has been established that the dependencies considerably differ in these two cases in a range of (tau) p approximately (tau) . An effect of damage statistics, connected with a random spatial distribution of inclusions in a material, is also investigated. For the case of one-type inclusions (single-(tau) inclusions) it is shown: the statistics does not change a functional form of the pulsewidth dependence of the LIDT and correct only the LIDT values by a spot-size factor. Theoretical results are compared with experimental data published by different research groups for the laser damage in a nanosecond-picosecond region.
A problem of a laser induced damage to dielectric coatings due to thermal explosion of the absorbing inclusions is investigated. It is shown that major parameters governing damage characteristics of the coating are band gaps of coating and substrate materials. In particular, damage threshold dependences on a coating thickness and a position of the inclusion in the coating or on the coating-substrate interface are connected with an effect of this fundamental parameter. Experimental data are in qualitative agreement with theoretical predictions.
A study is reported of the influence of temporal fluctuations of laser radiation on the development of thermal explosion of absorbing inclusions and on the statistical properties of the laser induced damage in tranparent dielectrics. A fluctuation time scale in which the fluctuations effect the thermal explosion of inclusions is established. An analysis is made of the conditions ensuring control of temporal fluctuations of laser radiation so as to eliminate their influence on the experimental statistical relationships of absorbing inclusions in the bulk and on the surface of a sample.
A phenomenological model for accumulation of irreversible changes initiated by absorbing inclusion in transparent dielectric material under laser radiation is proposed. Kinetics of the accumulation process is investigated for both highly absorbing and low absorbing inclusions. Main regularities of the accumulation effect are derived, particularly the dependence of multishot damage threshold upon a number of pulses. Results of the theoretical analysis are compared with experimental data.
The potentials of laser-induced microwave photoconductivity (LMP) as a diagnostic method for semiconductor surface studies are considered in this paper. The principals and practical realization aspects including a block-diagram and technical characteristics of the LMP apparatus, and some experimental data on electron-recombination properties of Si and GaAs surfaces are presented. The data include investigation of non equilibrium carrier relaxation channels and rates, influence of various defects (traps, adhesion centers, dislocations etc.), and spatial variations of these relaxation characteristics in surface layers modified with different procedures (ion implantation, laser annealing, mechanical treatment). Among the remote action of surface modification and interaction of locally modified zones. These effects are shown to be very important in the physics and technology of semiconductor surface modification. It is concluded that the LMP method is very informative diagnostic tool for testing and studying of semiconductor surfaces and device structures used in microelectronics, power electronics and optoelectronics.
The theory of laser induced damage to an optical single-layer dielectric film (coating) initiated by an absorbing inclusion is presented. The damage model based on the photoionization mechanism due to thermal radiation of the laser heated inclusion is analyzed. Dependences of the laser induced damage threshold upon coating/substrate band-gap ratio and absorption cross-section ratio, and upon coating thickness are derived from a solution of heat and electron diffusion equations. The coating thickness dependence predicted is compared with experimental data.
Polymer-filled microporous glass (PFMG) composite materials have been recently proposed as a proper
host for dyes to create solid-state dye lasers and laser beam control elements (Q-switchers, etc.) [1,2]. In
this paper we report investigation of some laser-related properties of Polymethilmethacrylate (PMAA) -
filled porous glass doped with Rhodamine 6G perchiorate (active lasing dye) and 1055 dye (passive
bleachable dye): laser induced damage threshold, lasmg efficiency, bleaching efficiency, and
microhardness have been measured. All these characteristics have been found to be rather high
indicating that PFMG composite materials are perspective hosts for dye impregnation and fabrication
highly effective solid-state dye lasers and other laser related elements (Q-switchers, mode-lockers, modeselectors,
spatial filters).
For application of optical parametric oscillator (OPO) to investigation of nonlinear interaction of laser radiation with matter the factor of importance is stability of light spatial distribution and spotsize position on a target through a tuning range. Collinear temperature tunable schemes show an advantage for these purposes, in particular, for small pump beam diameters. A simple and efficient visible range parametric converter can be realized using two-pass configuration, where parametric luminescence is excited on the first pass through a nonlinear crystal and amplified on the second pass after spatial filtering. Lack of resonator simplifies high power UV pumping and getting of relatively narrow emission spectrum. Using such an approach, we have made the oscillator-amplifier system temperature tunable in the range of 440 to 670 nm employing 4-cm-length ADP crystal pumped by 266 nm radiation from the single-mode YAG:Nd laser. The output energy of 3 mJ in about 1-ns pulsewidth has been achieved with total conversion efficiency of 10%. A spatial profile of the output beam kept its shape within the branch of the tuning curve. This allowed us to use the device as a proper tool for investigation of two-photon excitation in undoped CsI and KI single crystals. The OPO signal output was used to record photoconductivity spectra in these materials.
In many respects solid polymers are attractive hosts for dyes to produce on their base various optical elements for laser applications including tunable lasers and laser beam control saturable filters. For this reason various dye-impregnated polymer materials were extensively studied in many laboratories during last 25 years. A critical review of these studies is presented in this paper with main attention to analysis of processes responsible for major laser characteristics of dye - impregnated polymer materials: lasing efficiency, bleaching efficiency, laser damage resistance of polymer matrix, photostability of dyes under high power laser radiation and dye deterioration during long-term use and storage. Results achieved in General Physics Institute of Russian Academy of Sciences and collaborating industrial research laboratories on creation of highly effective dye-impregnated modified polymer materials for visible and near IR laser application possesing high laser damage resistance, dye photostability, laser oscillation efficiency and laser induced bleaching efficiency are presented. Limitations for polymer-host dye lasers, due to rather low thermal conductivity of polymer materials, arising, in particular, at flash la~p pumping and high repetition rates, are discussed. Some practical methods allowing to overcome these limitations are also discussed.
A critical analysis of available theoretical and experimental data for a pulse-width dependence of laser induced damage thresholds (LIDT) in various optical materials is given along with new results of our recent measurements obtained in nano-picosecond range at 1064 nm wavelength for alkali halide crystals (NaCL, KCl, KBr). Three YAG:Nd laser oscillator- amplifier systems operating in Q-switch and mode-locking modes provided highly stable light pulses at 2 ns, 15 ns, and 50 ps with Gaussian spatial profile of beams were used in the experiments. Special attention was paid to the comparability of the LIDT test conditions for various pulse widths, implying the carefully controlled similarity of beam intensity spatial distributions and temporal profiles. The experimental data are analyzed on the basis of theoretical predictions for different damage mechanisms. It is concluded that pulse-width scaling of LIDT is still a problem, and an adequate approach to its solution is formulated.
A novel diagnostic technique for the study of semiconductor surfaces based on laser induced microwave photoconductivity detection with high spatial and temporal resolution is described. Experimental data on nonlocal modification effects in laser produced scribing, doping, and annealing of ion implanted GaAs and Si are presented. Mechanisms of nonlocal structural and electric properties changers under modification are analyzed. Nonlocality phenomena are concluded to be universal for any strong modification procedure and very important for microelectronics technology.
Fundamental aspects of multishot laser damage are considered: thermal instability in an optical medium due to pulseto--pulse accumulation o absorbing derects mechanisms o laser induced detect generation. Eeotive methods or suppressing a deeot rormation to increase laser damage resistance are discussed and illustrated by experimental data or various type optical materials. Key words: multishot and singleshot laser damage instability inclusion accumulation eect 5 thermochemical mechanochemicalprocesses thermoelastic stresses leser damage resistance of crystals and polymers laser conditioning. 1 .
1. Central Clinical Hospital No 1, the USSR Ministry of Communications,
Moscow
2. II Surgery Department, Central Institute for Advanced Medical Training,
Moscow
3. Central Clinical Hospital No 4, the USSR Ministry of Communications,
Moscow
4. Central Clinical Hospital of the RSFSR Ministry of Health, Moscow
5. General Physics Institute of the USSR Academy of Sciences, Moscow
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