Photochemically-induced acousto-optics in gases
arxiv(2024)
摘要
Acousto-optics consists of launching acoustic waves in a medium (usually a
crystal) in order to modulate its refractive index and create a tunable optical
grating. In this article, we present the theoretical basis of a new scheme to
generate acousto-optics in a gas, where the acoustic waves are initiated by the
localized absorption (and thus gas heating) of spatially-modulated UV light, as
was demonstrated in Y. Michine and H. Yoneda, Commun. Phys. 3, 24 (2020). We
identify the chemical reactions initiated by the absorption of UV light via the
photodissociation of ozone molecules present in the gas, and calculate the
resulting temperature increase in the gas as a function of space and time.
Solving the Euler fluid equations shows that the modulated, isochoric heating
initiates a mixed acoustic/entropy wave in the gas, whose high-amplitude
density (and thus refractive index) modulation can be used to manipulate a
high-power laser. We calculate that diffraction efficiencies near 100 percent
can be obtained using only a few millimeters of gas containing a few percent
ozone fraction at room temperature, with UV fluences of less than 100 mJ/cm2,
consistent with the experimental measurements by Michine and Yoneda. Gases have
optics damage thresholds two to three times beyond those of solids; these
optical elements should therefore be able to manipulate kJ-class lasers. Our
analysis suggest possible ways to optimize the diffraction efficiency by
changing the buffer gas composition.
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