Formation of optical resonances at the initial stage of radiation destruction of disperse medium made of a transparent dielectric by means of a powerful laser

Abstract

By means of numerical simulation, we study optical resonances that arise when radiation from an ytterbium fiber laser (λ = 1065 ± 3.25 nm), a blue diode laser (λ = 450±8 nm) or a CO₂ laser (10.6 μm) is scattered on a single particle of a transparent dielectric (an oxide, a fluoride, ZnSe), as well as in an ensemble of randomly packed particles of various sizes. If the particle diameter is comparable to the radiation wavelength (but is no less than 3λ/4) then, at a certain ratio between the particle diameter, the refractive index of its material and the radiation wavelength, the particle becomes a microresonator. The maximum intensity of scattered radiation inside or outside such particle depends on the combination of the above-said parameters and can exceed the intensity of the incident radiation dozens of times. In case of a powder, the interference pattern is formed by all particles at once and, occurrence of an optical resonance is more likely. Even with a slight change in the radiation wavelength of an ytterbium laser or a blue diode laser (within the lasing line width), some particles in the powder become microresonators, while others cease to be microcavities. Therefore, concentration of scattered radiation of the ytterbium laser in individual particles plays a key role at the initial stage of optical destruction of the powder. As the experiment shows, this allows to overcome the threshold of optical destruction of porous targets made of various transparent substances (CaF₂, SiO₂, BaF₂, YbF₃, Fe:MgAl₂O₄, Al₂O₃, 1mol.%Nd:Y₂O₃, YSZ, TiO₂, ZnSe) by means of a radiation pulse from an ytterbium laser having low intensity (0.46 MW/cm²). With the refractive index of the material increasing from 1.429 (CaF₂) to 2.479 (TiO₂), the average delay in the formation of a laser plume decreases from 46 ms to 25 μs, i.e. by three orders of magnitude. This correlates with the fact that in the calculations, the maximum intensity of this laser's radiation scattered in the powder increases with n increasing. In compliance with this are the results of obtaining nanopowders from transparent oxides and ZnSe by means of pulse-periodic radiation of an ytterbium laser. Output of the nanopowder Yb_0.05:Y_1.95O₃:(ZrO₂)_0.05 with n = 1.901 (for Y₂O₃) is 15 g/h, and the maximum output (100 g/h) is realized in case of ZnSe (n = 2.482). At the same time, it is not possible to obtain nanopowder from CaF₂ (n = 1.429), because the moving target does not evaporate.