The mechanism of the fine ripples, perpendicular to laser polarization, on the surface of (semi)transparent materials with period smaller than the vacuum wavelength, λ, of the incident radiation is proposed and experimentally validated. The sphere-to-plane transformation of nanoplasma bubbles responsible for the in-bulk ripples accounts for the fine ripples on the surface of dielectrics and semiconductors. The mechanism is demonstrated for 4H:SiC and sapphire surfaces using 800 nm/150 fs and 1030 nm/300 fs laser pulses. The ripples are pinned to the smallest possible standing wave cavity inside material of refractive index n. This defines the corresponding period, Λ = (λ/n)/2, of a light standing wave with intensity, E(2), at the maxima of which surface ablation occurs. The mechanism accounts for the fine ripples at the breakdown conditions. Comparison with ripples recorded on different materials and via other mechanisms using femtosecond pulses is presented and application potential is discussed.

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A kinetic model of the working medium of a discharge excilamp on the B-X transition of mercury bromide, HgBr, excimer molecules has been proposed. The model explains the nonmonotonic dependence of the excilamp radiation intensity on the partial pressure of mercury dibromide molecules by the attachment of electrons to these molecules. HgBr(X) molecules were found to transit into the HgBr(B) excited state due to their collisions with high-energy electrons, thereby improving the excilamp characteristics. K e y w o r d s: excilamp, gas discharge plasma, kinetics, excimer radiation, mercury halides.