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Controllable circular patterns on silicon induced by bubble-diffracted femtosecond laser pulses in liquid

A new technique of silicon nanostructuring in liquid with a pair of Gaussian-shaped femtosecond laser pulses is reported. It enables producing circular ripple patterns characterized by high surface-relief modulation, undersurface annular nanocavities, and interfacial smoothness. The mechanism of high-frequency spatial modulation of the laser radiation via scattering of the second pulse by the bubble, induced by the first pulse, is proposed. The control over the specific pattern through the laser intensity variation is shown.