THE NATURE OF THE FRACTURE OF OPTICAL SINGLE CRYSTALS BY PULSES OF LASER RADIATION

E. K. ZAVADOVSKAYA, I. S. SHISHKIN, M. I. RUDENKO, V. M. TKACHENKO

The study of questions concerning the interaction of powerful light pulses with materials is of great interest for solid-state physics, as well as for the solution of certain scientific and technical problems.

There are many publications devoted to the investigation of the processes of interaction of laser radiation with various materials. The range of objects subjected to laser irradiation is very broad; in this connection, the main attention has been devoted to the study of the behavior of structural \([1 \div 6]\) and inorganic \([7 \div 13]\) materials.

A special place in such studies is assigned to optical single crystals and other transparent dielectrics used in optical systems and other devices of modern technology and belonging to the class of materials highly vulnerable to laser radiation \([14 \div 17]\).

It should be noted that, despite the numerous attempts to establish a relationship between the characteristics of the objects under study and the irradiation conditions, in most cases the observations are descriptive in character and concern only the external picture of the visible damage in the materials. Apparently, the reason for this is that, among several assumed mechanisms of the processes of interaction of powerful laser-radiation pulses with materials, not one is at present preferred \([18]\).

The interest in the applied aspects of the problem of interaction of powerful light pulses with materials led to the formulation of the present work, devoted to the investigation of the regularities of the process of fracture of optical single crystals by modulated and unmodulated radiation of a laser on glass with \(Nd^{3+}\).

The objects of investigation were specimens of NaCl and LiF single crystals grown by the Kyropoulos method in air and then annealed for 24 h at a temperature \(T_{\text{ann}} = T_{\text{melt}} - 100^\circ\text{C}\), followed by its reduction at a rate of \(10^\circ/\text{min}\).

In the experiments, specimens with dimensions \(30 \times 30 \times 7\ \text{mm}^3\), cleaved from the central part of massive ingots, were irradiated; moreover, when pulses of modulated radiation acted on the specimens, the latter was focused in the volume, whereas when pulses of unmodulated radiation acted, it was focused on the surface of the irradiated specimens.

The macroscopic picture of the structural damage to the crystals accompanying the interaction of radiation pulses with them represe-

is shown in Fig. 1. In the first case, damage of the “rosette” type is observed in the bulk of the specimens, the petals of which are arranged along the crystallographic directions {100}, {010}, and {001} (Fig. 1a). The presence of the above-mentioned petals in the “rosettes” indicates that structural damage in the irradiated crystals occurs predominantly along the principal crystallographic axes.

In this case the dependence of the length \(l_i\) of the “rosette” petals on the intensity \(I\) of the laser radiation for single crystals of the types studied is linear (Fig. 2). However, the magnitude of the change in petal length under the same irradiation conditions in NaCl crystals is considerably greater than in LiF; i.e., crystals with a higher lattice energy are more resistant to laser radiation. This confirms the idea of a relationship between physical properties and lattice energy developed in [19, 20].

In the second case (Fig. 1b), on the surface of the irradiated specimens there is formed a zone of continuous destruction of regular shape and of a definite depth \(h_i\), as well as a spallation zone characterized by the thickness \(\delta_i\) of the layer being separated.

Cleavage of specimens irradiated by pulses of modulated radiation, carried out in such a way that the cleavage plane passed through the center of the focal spot, as well as visual observation of the dislocation structure of the irradiated crystals by chemical etching [20], made it possible to establish that the character of the structural damage corresponds to that described earlier [21—22] (see also Fig. 3a).

Fig. 1. Pattern of destruction of specimens of optical single crystals by laser-radiation pulses:
a) modulated radiation (\(1\)—direction of pulse propagation, \(2\)—surface of the irradiated specimen, \(3\)—“rosette”);
b) in the free-generation regime (\(S_i\)—area of the zone of continuous destruction on the crystal surface, \(h_i\)—depth of the zone of continuous destruction on the crystal surface, \(\delta_i\)—thickness of the spallation zone).

The change in the dislocation structure of the irradiated specimens around the focal spots (Fig. 3) indicates that the centers of the focal spots are zones of intense heat release, the consequence of which is the generation of a mass of chaotically arranged dislocations. Their density increases by 2–3 orders of magnitude compared with that present in unirradiated specimens (and is the same for all types of crystals studied, \(\simeq 5 \cdot 10^4\ \text{cm}^{-2}\)).

The aforementioned local release of heat of high density, occurring almost instantaneously (the duration of the radiation pulses did not exceed 20 ns), leads to the generation of thermomechanical stresses. In turn, a manifestation of the latter is the observed

the experimentally observed sharp increase in dislocation density in the region of the focal spots and the appearance of microcracks (Fig. 3a).

Examination of the microscopic picture of structural disturbances in irradiated samples of optical single crystals made it possible to reveal the existence of a central zone in the region of the focal spots, which is at high temperature in a quasi-equilibrium state for time intervals much longer than the duration of the radiation pulses. The presence of this zone at all laser-radiation energies used in the experiments confirms that absorption of the energy

Fig. 2. Dependence of the length \(l_i\) of the “rosette” petals on the intensity \(I\) of the laser radiation: 1 — for NaCl, 2 — for LiF

of radiation and, consequently, destruction of the crystals occurs within the limits of the focal spots.

Fig. 3. Microstructure of the sample: a) in the region of the focal spot; b) at the end of a “rosette” petal (magnification 100*)

A sign of the presence in the central zone of the focal spots of the aforementioned thermal “peaks” was complete melting and even partial evaporation of the substance in these zones at laser-radiation intensities exceeding the systematically investigated range \(10^{10} \div 10^{12}\ \mathrm{W \cdot cm^2}\).

Substantially different from that described is the nature of the structural disturbances in single crystals that occurs when they are acted upon by laser-radiation pulses in the free-running regime. The scale of the structural disturbances in the latter case sharply increases-

…; moreover, their parameters \(h_i\), \(S_i\), and \(\delta_i\) (Fig. 1,b) become, in order of magnitude, comparable with the dimensions of the irradiated specimens.

The presence in the irradiated crystals of a zone of continuous destruction confirms that, in this case as well, the interaction of the material of optical single crystals with laser radiation is accompanied by their high-temperature heating and by evaporation of the products of destruction, with the direction of ejection of material being perpendicular to the surface of the irradiated crystal specimens.

The parameters that quantitatively describe the pattern of destruction of single crystals are in a definite relation to the energy of the laser radiation in the pulse (Fig. 4). The existence of such a dependence makes it possible,

Fig. 4. Dependence of the parameters describing the pattern of destruction of NaCl single crystals on the energy density of laser radiation \(w_0\):
a) depth \(h_i\) and area \(S_i\) of the zone of continuous destruction;
b) thickness \(\delta_i\) of the delamination zone.

in particular, to predict the scale of microscopic structural disturbances in crystal specimens over a wider range of laser-radiation energy densities than those used in the present work.

The experimental results obtained make it possible to draw the following conclusions:

1. When specimens of NaCl and LiF are exposed to pulses of modulated laser radiation, disturbances of the “rosette” type are observed, the petals of which are arranged along the crystallographic directions \(\{100\}\), \(\{010\}\), \(\{001\}\).

Pulses of unmodulated laser radiation create zones of continuous destruction and delamination on the surface of the irradiated specimens.

1. The dependence of the length \(l_i\) of the “rosette” petals on the intensity of the modulated laser radiation is linear for NaCl and LiF.

1. The focal spots that arise upon irradiation by pulses of modulated laser radiation are a zone of intense heat release.

2. The variation of the parameters \(S_i\), \(\delta_i\), and \(h_i\), which describe the pattern of destruction of single crystals, as a function of the logarithm of the energy in a pulse of unmodulated laser radiation is linear in character.

3. Crystals with higher lattice energy are more resistant to laser radiation.

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