COMMENTS

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Did you speculate as to the lifetime of the F centers?

These are fairly stable F centers. If we can color the crystal using for example a UV pulse so that the F centers are visible to the eye, they will persist for days. We are only concerned with the F centers which are long lived. Some of them, in fact, most likely the majority of them, do recombine relatively quickly, but there will be some where the displaced atom is removed far enough away from the vacancy that they will persist in the crystal.

I'd like to ask a question about those points you threw away. Were there any that you threw away at 55° K?

There were.

There was almost a vertical transition in the data at 55° K. At a certain peak flux I never saw any sites which survived more than 2 to 3 shots. Below that flux I could get sites that would survive a couple of thousand shots. However, there were some below that flux which would succumb at a lower number of shots. They are probably due to some sort of a defect.

In your final plot, where you showed a plot of the number of pulses to damage versus the incident flux, was there, on your theoretical curve, an adjustable fit parameter?

There are 2 parameters which can be adjusted, I don't have any limits on at the moment as to their values. One is in material dependent parameter, the gamma that was shown in the expression. There is a little leeway on the relative contributions between the F center induced stress and thermal stress. They are certainly not exactly accurate and I believe that there is going to be some problem with that number. As a result there is a little bit of leeway in the coefficient of that variable.

I would like to ask you a question on your experimental setup. What fiber diameter did you use, what is the numerical aperture or acceptance angle and what is the length of the fiber?

The fiber diameter was 1 mm and the numerical aperture 0.2.

In your last slide you indicated that you might want to use Raman scattering to look at stress. Could you tell us a little bit about how you want to do that?

The idea is to use a second dialyzer beam. At various points in the experiment we would actually interrupt the firing and use a dialyzer tuned to achieve a resonant Raman scattering.

This is a cubic crystal. Would you expect Raman scattering in KBR?

Once you produce the F centers you will have first order Raman scattering. The interaction volumes are fairly small, and so the total number of them is not that great, so it may involve integrating. We don't know how long we'll have to integrate the signal and how difficult it will actually be to detect, but that's our hope.

I just want to comment that there's an increasing body of knowledge that indicates that the F center generation is intimately connected with the dislocation content and I would like to see you make some effort to include that into your mechanism.

I am aware of this. At this point, it is not in there, but I hope to look into that, I have not done it at this point.

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Is there a temperature at which you would expect the healing to offset the temperature dependence you've seen in the generation? Would you get the threshold starting to go up with temperature?

I am not certain. We have tried some experiments at temperatures above room temperature, but we have the problem that with increased temperature the material actually comes off the crystal itself, so our optics have been impaired when we tried the experiment. It is conceivable that at a high enough temperature the mobility of the interstitials would increase, but then they would be likely to move away from the vacancies, so I don't know that you would get a complete healing.

Manuscript Received
1-25-89

Laser Heating of Free Electrons and the Mechanism of Intrinsic Laser Breakdown in Wide-Gap Optical Materials at 1064 nm

X. A. Shen, Scott C. Jones, and Peter Braunlich

Department of Physics

Washington State University
Pullman, WA 99164-2814

Free electron absorption of 1064 nm photons is measured photoacoustically in NaCl and SiO2. The electrons are generated with a 266 nm pump pulse by two- or three-photon transitions from the valence band. For a given pump pulse energy which is chosen sufficiently low to avoid heating of the interaction volume by itself, the dependence of the photoacoustic signal and temperature rise are 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 are 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 Shen et al. 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.

Keywords: Avalanche ionization; free electron absorption; fused silica; NaCl

1. Introduction

Generation of free electrons and subsequent lattice heating via electron-photon-phonon interactions are the two major processes leading to laser-induced intrinsic damage in transparent optical materials. 14 We have shown recently that in KBr and NaCl, exposed to intense laser pulses at 532 nm, these two processes are free electron formation by four-photon transitions across the band gap and free-electron photon absorption,5,6 resulting in a significant rise of lattice temperature. Damage occurs when the peak temperature in the interaction volume approaches the melting point. The experiments provide conclusive proof that under these conditions impact ionization does not measurably contribute to free carrier formation.

In previous investigations of laser damage, a single wavelength is used to achieve the above two processes. Each pulse serves two purposes: (a) generating free-electrons and (b) subsequently heating them via free-electronphoton-phonon interactions. But these studies can easily be carried further by employing simultaneously two laser pulses of different wavelengths. One can efficiently generate free electrons with a short wavelength and heat them with a longer one, thus separating processes (a) and (b) for independent study. The wavelengths for the two pulses have to be chosen so that the free carrier generating pump pulse does not contribute significantly to lattice heating, while the heating pulse does not create a significant number of carriers via multiphoton absorption. By keeping the energy of the pump pulse, and, thus, the total number of free electrons constant and varying that of the heating pulse, we can then measure with a photoacoustic method the total energy absorbed by the lattice as a function of the laser flux and determine its power dependence.

The materials chosen in our investigation are NaCl and SiO2 (fused silica). As pump pulses, we use the fourth harmonic (266 nm) of a Nd:YAG laser and its fundamental wavelength (1064 nm) to heat the electrons. The carrier generation process here is two- or three-photon absorption and its efficiency does not require pulses of very high flux to generate a sufficient density (≈ 1016 cm-3) of electron-hole pairs for the measurement of free electron heating. Furthermore, photon absorption by free electrons is less efficient at 266 nm than it is at 1064 nm (approximately by a factor of 8, e.g., in KBR). Thus, we can neglect the contribution from the pump pulse to lattice heating. Similarly, we do not have to consider carrier generation by 1064 nm pulses because it would require valence electrons to absorb at least seven photons simultaneously to cross the band gap. The experiments were also performed with the intention to verify the validity of so-called multiphoton-assisted avalanche mechanism of laser