P Braunlich

Recent Progress On Laser-Induced Modifications And Intrinsic Bulk Damage Of Wide-Gap Optical Materials

In this paper we provide a comprehensive review of our recent work on the nonlinear interaction between high intensity pulsed laser beams and transparent solids. New experimental techniques used to measure multiphoton absorption and energy deposition in wide-gap alkali halides in the prebreakdown regime have led to hard evidence refuting the avalanche model of laser-induced damage at visible laser wavelengths. These measurements, performed in specially purified materials, have allowed the discovery of the roles of laser-induced excitations in energy absorption, leading to the conclusion that virtually all lattice heating occurs via a nonlinear absorption of laser photons by multi-photon-excited free electrons. These results yield an experimentally confirmed theoretical definition of intrinsic, single pulse laser damage thresholds at 532 nm wavelength in three- and four-photon bandgap alkali halides. Extending this work to multipulse effects in the subthreshold intensity regime, we have formulated a new model of bulk damage based on thermomechanical stress induced by accumulation of multiphoton-generated lattice defects.

Laser‐stimulated thermoluminescence. II

The theory of heating a semi‐infinite two‐layer system with a localized CO2 laser beam of uniform circular intensity profile is applied to configurations consisting of thin thermoluminescent LiF:Mg,Ti phosphor layers on borosilicate glass substrates. We study thermoluminescence because it serves as a convenient monitor for the temperature distribution and because of its importance in developing the laser‐heating technique for solid‐state dosimetry. Experimental and calculated results are compared in an attempt to completely characterize the thermoluminescence response curves. The two‐layer system is heated in two different modes: the laser beam impinges onto (a) the phosphor layer, and (b) the glass substrate. We investigate in detail changes in the thermoluminescence response due to different laser powers and variations in the optical and thermal properties of the samples.

Theory of transient temperature response of a two‐layer system heated with a localized laser beam

A general solution of the heat diffusion equation is presented for the case of a semi‐infinite two‐layer system that is heated with a localized cw laser beam of Gaussian or uniform circular intensity profile. As an example, this theory is applied to thin layers of a thermoluminescent material on glass substrates. The transient thermoluminescence emission response is calculated and compared with experimental results, illustrating the validity of the solutions. Applications to transient laser heating of oxide‐semiconductor sandwiches are discussed.

F-center accumulation as a mechanism of multiple-pulse, laser-induced bulk damage in KBr and KI at 532 nm

Abstract Recent results suggest that laser-induced stable F-centers play a significant role in multishot, bulk laser damage in KBr and KI at 532 nm. The production of stable F-centers is a result of laser-induced electron-hole pair generation and is accompanied by an expansion of the crystal lattice. Thus, the nonuniform coloration produced by the focused laser beam leads to a buildup of local stresses resulting ultimately in catastrophic damage of the crystal. The model developed exhibits the observed dependence on photon flux density for multishot damage in KBr and KI for temperatures from 55 to 300 K.

Laser-induced modifications and the mechanism of intrinsic damage in wide gap optical materials

Abstract A brief outline of our previous investigations into the mechanism of intrinsic bulk laser induced damage and our main conclusions serve as the introduction to exciting new results obtained by Shen et al. [Phys. Rev. Lett. (June 5. 1989)]: who measured the free electron absorption of 1064 nm photons photoacoustically in NaCl and SiO2. The electrons were generated with a 266 nm pump pulse by two- or three-photon transitions from the valence band. For a given pump energy which is chosen sufficiently low to avoid heating of the interaction volume by itself, the dependence of the photoacoustic signal and temperature rise were observed as a function of the energy of the 1064 nm pulse and found to be in agreement with the theory of free carrier heating by Epifanov et al. Temperatures approaching the melting point were produced in the materials under prebreakdown conditions. This work provides direct experimental proof that free electron absorption is the primary mechanism for heating wide-gap optical materials by intense laser pulses in the visible and near infrared, confirming the indirect measurements reported earlier by us. No evidence of electron impact ionization and ensuing avalanche formation is found up to the intrinsic damage threshold, throwing doubt onto the commonly held belief that it is responsible for free electron generation and eventual breakdown at 1064 nm.

Transient solution of the diffusion equation for a composite system heated with a laser beam

The general solution of the thermal diffusion equation for the case of a semi‐infinite two‐layer system that is heated with a localized cw laser beam of circularly uniform intensity profile is extended to times after the pulse is switched off. An application of that solution in the field of solid‐state dosimetry is illustrated by simulating the experimental thermoluminescence response curves obtained from LiF:(Mg,Ti)‐phosphor‐coated borasilicate glass, for the special case of heating with a CO2‐laser beam that has a uniform and square intensity profile: In effect, we solve the diffusion equation experimentally.

Direct measurement of nonlinear energy deposition from an intense 532 nm photon field into alkali halides

Prebreakdown temperature increases exceeding 300 K in NaCl exposed to 80 psec pulses at 532 nm are reported along with four‐photon absorption cross sections in NaCl and KBr.

Non-Avalanche Dielectric Breakdown in Wide-Band-Gap Insulators at DC and Optical Frequencies

It is the purpose of this paper to put recent novel experimental efforts to understand dielectric breakdown in wide-band-gap materials in various fields into a common framework and outline significant changes in the understanding of dielectric breakdown at optical frequencies and free electron heating under DC conditions. New experimental techniques used to measure multiphoton absorption and energy deposition in wide-band-gap alkali halides in the prebreakdown regime have led to hard evidence refuting the avalanche model of laser-induced damage at visible laser wavelength. The experiments show that virtually all lattice heating occurs via nonlinear absorption of laser photons by multiphoton excited free electrons. Direct measurements of free electron heating by DC fields in thin SiO 2 -films and direct measurements of electron-phonon scattering rates of energetic free electrons impose a new understanding of carrier heating. The scattering of free electrons with non-polar acoustic phonons is found to be the dominant interaction in preventing the free carriers from reaching energies high enough to cause impact ionization and initiate avalanche breakdown. These results unambiguously show that the role of avalanche breakdown under DC conditions has been overestimated in the past.