Control of relative electron densities and spacing of two laser induced plasmas by spatial light modulation of femtosecond laser

Abstract

This study demonstrates a novel technique for independently controlling the relative electron densities and axial separation of two laser induced plasmas (LIPs) formed in air by an ultrashort pulse laser (USPL). A spatial light modulator (SLM) provides the means of control by altering the wavefront of a 35 fs, 1.6–2.5 mJ pulse from a Ti:Sapphire USPL with a 790 nm center wavelength. After modification by the SLM, a lens focuses the pulse, which increases its intensity and generates plasma by ionizing air. Holograms displayed on the SLM diffract a controlled amount of laser energy to a first-order focal spot some distance from the lens focus, and sufficient laser power generates two LIPs spaced from one another along the laser propagation direction. The hologram contrast dictates the relative intensity of the light arriving at each focus, and the spatial gradient of the phase shift applied by the hologram determines the distance between the two foci. Analytic solutions of linear light propagation and scalar diffraction theory computed with the convolution method are used to determine initial hologram designs meant to deliver various foci spacings and relative intensities. Ultrashort pulse lasers (USPLs) supply pulse powers sufficient to induce filamentation in air, a result of non-linear optical phenomena that extends the range of intense laser propagation and generates plasma. Comparing images of linear propagation at low laser powers with relative line-integrated electron density measurements taken during plasma formation indicate any extant non-linear processes do not prevent predictable control of plasma geometry for 0.3 m and 0.5 m focal lengths.