Laser Damage Mechanism Research Articles

Effect of Laser Sources on the Structural Damage Mechanisms and Reverse-bias Leakages of Laser Lift-off GaN-based LEDs

The frequency-tripled Nd YAG laser (355nm) and the KrF pulsed excimer laser (248 nm) were used to separate GaN from sapphire substrates. The laser damage mechanism of these two laser sources and effects on the reverse-bias leakages of GaN-based light-emitting diodes (LEDs) were studied.

Study on Electrical Properties and 1064nm Pulse Laser Damage of Vanadium Oxide Thin Film

Vanadium oxide thin film which has a reversible semiconductor-metal phase transition has been obtained by reactive ion-beam sputtering and subsequent annealing in Ar gas. Micro-analysis shows that this thin film is homogeneous and compact, its spheric grain size is about 50nm and it is composed of VO2 and V2O5. Electrical properties testing indicate that its phase transition temperature is near 60°C, abrupt change of resistivity before and after phase transition is approximate to 3 orders of magnitude, temperature coefficient of resistance (TCR) is about -0.0393K-1 at 25°C and activation energy is about 0.3006eV at the range of low temperature. Using a pulse laser beam with wavelength of 1064nm and pulse width of 10ns, laser damage threshold was obtained to be 20.1mJ/cm2. Damage spot morphology of the film was also researched carefully to discover its laser damage mechanism. All results above prove that this thin film is a perfect thermo-sensitive material that can be used for uncooled infrared detector and laser protection.

Laser damage studies in nonlinear optical crystal sodium p-nitrophenolate dihydrate

We report the surface laser damage threshold in sodium p-nitrophenolate dihydrate, a nonlinear optical crystal. The experiment is performed with a pulsed Nd:YAG laser in TEM00 mode. The single shot damage thresholds are 11.16±0.28 GW cm-2 and 1.25±0.02 GW cm-2 for 1064 nm and 532 nm laser wavelengths respectively. A close correlation between the laser damage threshold and mechanical hardness is observed. A possible mechanism of laser damage is discussed.

Laser induced damage for diamond films

The outstanding optical and mechanical properties of diamond films are summarized. Laser damage for films irradiated with different wave lengths and pulse width are reviewed and the laser damage mechanism analyzed for different parameters. It is found that graphitization induced instability of the crystal lattice is the main reason for laser induced damage. There are two ways that lead to graphitized damage on the surface of diamond films under long and short laser pulses. For nanosecond or longer laser pulses, graphitization propagates vertically into the bulk of the diamond films. For femtosecond laser pulses, graphitization occurs on the surface of the diamond films layer by layer. Based on the physical properties and preparation technology, practical and promising applications of diamond films are proposed.

Damage thresholds for cultured retinal pigment epithelial cells exposed to lasers at 532 nm and 458 nm

The determination of safe exposure levels for lasers has come from damage assessment experiments in live animals, which typically involve correlating visually identifiable damage with laser dosimetry. Studying basic mechanisms of laser damage in animal retinal systems often requires tissue sampling (animal sacrifice), making justification and animal availability problematic. We determined laser damage thresholds in cultured monolayers of a human retinal pigment epithelial (RPE) cell line. By varying exposure duration and laser wavelength, we identified conditions leading to damage by presumed photochemical or thermal mechanisms. A comparison with literature values for ocular damage thresholds validates the in vitro model. The in vitro system described will facilitate molecular and cellular approaches for understanding laser-tissue interaction.

Optical breakdown of alkali halide crystals

It is shown that the metallization of a dielectric in the region of its interaction with laser radiation is a substantiated mechanism of laser damage of wide-gap dielectrics. Calculations of the radiation pressure produced by high-power laser radiation and the pressure of external bulk compression at which the dielectric energy-gap width becomes zero and which is calculated on the basis of the self-consistent statistical electron theory of ionic crystals have shown that these pressures are of the same order of magnitude.

Laser-induced damage studies in GaAs

Laser-induced damage studies have been carried out on monocrystal GaAs at 1.06 μm wavelength as a function of pulse repetition rate in the nanosecond regime. The single shot observed laser damage threshold is 0.9 J/cm 2 . It has been found that the damage threshold decreases when the sample is irradiated with large number of pulses. However, the above effect is observed only when the repetition rate is higher than 1 Hz . Various laser damage mechanism theories have been discussed to explain the results.

Efficiency of various mechanisms of the laser damage in transparent solids

Criteria for the laser damage in transparent solids are compared for different mechanisms of radiation absorption. It is shown that the most efficient mechanism of laser damage in a broad range of pulse durations (from millisecond to femtosecond laser pulses) is the photoionisation thermal explosion of absorbing inclusions. It is found that upon irradiation by long laser pulses, the damage proceeds through a melting stage. A fundamental feature of the damage is that, to produce a crack in a transparent solid, the size of an absorbing region should exceed a critical size.

Mechanism of laser damage of transparent semiconductors

Damage of transparent semiconductors and dielectrics under the action of high-intensity laser radiation fluxes is mainly attributed to absorptive inclusions, which being heated up to the melting threshold, result in thermal stresses due to the occurrence of microcracks and pores in materials. In this connection a local spherical area molten by laser radiation and then solidifying in a transparent solid in conditions of external cooling is considered and the temperature fields in the solidified region are calculated. The temperature fields and temporary stress dependences on thickness of a solidified region at different speeds of a solidified front are graphically analyzed. The conditions of occurrence of caverns in irradiated crystals are discussed. The present results can be used in the analysis of the damage processes of optical components of power laser devices, in particular made from ZnSe semiconductors.

Chitosan-based hydrogels: A new polymer-based system with excellent laser-damage threshold properties

Two types of chitosan hydrogel systems have been prepared that have a laser damage threshold (LDT) up to 35 times higher than commercial PMMA bulk materials. For these samples, the LDT increases with increasing water content. The mechanism of laser damage and the contribution of water to their high laser damage resistance have been examined. DSC measurements indicate water within the hydrogels exist in various states, each with different laser damage resistance properties. These various states play a key role in determining the LDT by controlling the dissipation of laser energy and providing a mechanism for self-healing. This preliminary research shows that polymer hydrogels have potential for high power laser applications because they combine good mechanical integrity due to the polymer frame and good energy dissipation and healing characteristics due to the molecular mobility of water. These two traits allow for bulk shape-retaining films with high laser damage thresholds and potential reversibility in damage processes. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 769–778, 1999

Thermoelastic and ablation mechanisms of laser damage to the surfaces of transparent solids

An investigation is reported of the relationships governing laser damage to the surfaces of transparent dielectrics, initiated by absorbing inclusions, under repeated irradiation conditions. It is shown that ultrashort pulses produce damage by the ablation mechanism, whereas 'long' laser pulses cause damage by the thermoelastic mechanism. The relationships established for the change in the damage mechanism with change in the pulse duration apply also to intrinsic (collisional and multiphoton ionisation) damage mechanisms.

Impact of thermal boundary resistance on laser inclusion damage

The present work rigorously analyzes the effect of thermal boundary resistance (TBR) on laser heating of an inclusion embedded in a nonabsorbing optical medium. The TBR impedes the flow of heat across the inclusion/ medium interface, enhances the inclusion heating rate, and reduces the laser damage threshold. The results indicate that in some cases the damage threshold is only half of that predicted when the boundary resistance is neglected. Both an exact solution and approximate scaling relations are developed that incorporate the TBR. The scaling relations show that when the TBR is considered, the damage threshold can have a stronger dependence on the pulse width than previously predicted. l Many novel applications exist for these lasers including laser material pro- cessing, high temporal and spatial resolution measurements, and laser fusion. One of the biggest obstacles in the appli- cation of these lasers is radiation damage incurred by optical components (windows, prisms, etc.) in the laser system. One laser damage mechanism is the so called inclusion-induced damage model. Essentially, inclusions—physical impurities in an otherwise pure optical component material—absorb high-intensity electromagnetic radiation and convert this ra- diation to thermal energy, causing the inclusion to heat up. As the inclusion temperature increases, the surrounding (host) material attempts to conduct the thermal energy away. If the host material cannot adequately conduct away this thermal energy, and the host or inclusion temperature exceeds some threshold temperature—e.g., the melting temperature of the host material, the vaporization temperature of the particle, or a particle temperature that causes fracture in the host— then permanent damage to the optical component can re- sult.24 Inclusion-dominated damage is particularly prevalent in thin film applications where the inclusion density is much higher than that of the bulk film material.3 In the following, particle refers to the absorbing inclusion embedded within the nonabsorbing (i.e., dielectric) host optical material, which is called the medium. To quantify the damage process, damage thresholds are assigned to optical components that characterize the laser radiation limits the component can safely tolerate without sustaining damage. This work is unique in that the thermal boundary resistance (TBR) between the particle and host medium is incorporated in a laser damage model that predicts the damage threshold of optical media. The TBR increases the heating rate of the inclusion and reduces the damage threshold of the optical material. Also, the TBR effect in-

Pulsed CO 2 laser damage of diamond-like carbon films

Results on pulsed CO 2 laser damage of amorphous hydrogenated carbon coatings deposited onto silicon substrates are presented. The structural and optical changes induced by laser radiation were studied by scanning electron microscopy, Raman spectroscopy and other methods. Single and multiple irradiations of the films were performed. The mechanism of laser damage is discussed.

Interaction of laser radiation with structures of the eye

An overview of laser-induced ocular injury and strategies for their prevention are presented based on an understanding of the general principles of laser interaction with biological tissue. These principles are outlined and subsequently applied in a discussion of specific laser damage mechanisms occurring within the eye over the visible, ultraviolet, and infrared bands. The ocular structures primarily at risk in each of these spectra are identified, and the international standards of maximum permissible exposures are presented and discussed. A survey of 58 case histories of laser exposure accidents resulting in permanent injury is also provided to characterize and give insight into actual accident scenarios. Basic control measures designed to minimize the incidence of laser accidents in the laboratory are discussed, including the selection of proper eye protection and the control of facility access.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Thermal explosion of absorbing inclusions as the mechanism of laser damage to insulator surfaces

An analysis is made of the problem of thermal explosions of inclusions in the surface layer of an insulator. The relationships are obtained for the description of the threshold intensities needed for thermal explosion of an inclusion located in the bulk of an insulator and on its surface. A study is made of the kinetics of thermal explosions. The statistics of laser damage to surfaces and the dependence of the surface damage threshold on the pulse duration and the size of the irradiated region are considered. The results are compared with experimental data.

Structural aspects of laser-induced damage and their functional implications.

The mechanisms of laser damage to the retina are described and observations from laboratory studies are correlated with those from clinical exposure of patients. The sight-threatening consequences of laser lesions are listed, and the further implications of haemorrhages at various levels and locations in the retina are reported. This multifaceted and extensive database is used in developing a system for scoring the sight-threatening potential of a given type of laser lesion in a given retinal location.

Mechanism of laser damage to polymer materials

Numerical estimates are obtained of stresses which appear in polymethylmethacrylate (PMMA) as a result of heating of microinclusions. The results of model experiments on pulsed electrical heating of microscopic wires in PMMA agree with these estimates and they demonstrate that the cracking of this polymer does not occur at least up to inclusion temperatures 2×103°C. An analysis is made of the gaseous products of laser pyrolysis of PMMA. The results obtained are in good agreement with the thermochemical model of laser damage to polymers.

Use of a plasma temporal attenuator to investigate the pulse length dependence of CO2 laser induced damage

A surface initiated plasma temporal attenuator enabling production of CO2 laser pulses with tunable durations (20–90 ns), similar temporal profiles, and near Gaussian spatial profiles, is described. Use of the technique to investigate the pulse length dependence and thereby elucidate the mechanism of CO2 laser damage is demonstrated for uncoated and diamondlike carbon antireflection coated n-type Ge.

Microstructure of vapor-deposited optical coatings

The microstructure of thin films applied by conventional physical vapor deposition for use as optical coatings is columnar for most of the materials commonly used. This has been established for about a decade through numerous experimental observations employing microfractographical replication for use with high resolution transmission electron microscopes. Scanning electron microscopes are more useful investigating coating defects, the most remarkable of these defects being known as nodules. From fundamental considerations of nucleation and growth of thin films, the origin of both columns and nodules and the dependence of their appearance on the deposition conditions are discussed in some detail. A simple 2-D simulation model assuming limited surface mobility of adatoms or admolecules shows striking similarities to peculiar properties of both columnar and nodular growth seen in actual investigations. Conclusions are drawn as to how the two types of microstructure influence general film properties and, in particular, how they influence possible laser damage mechanisms.

Laser radiation hazards

Lasers are being employed in an increasingly diverse range of applications, but laser safety is rightly a topic of current concern. This paper considers the radiometry of laser exposure, briefly reviews the biological mechanisms of laser damage, and discusses the ways in which laser safety standards are applied to ensure safe operation of laser equipment.