Stability and optical quality of "windmill"-formed 8CB liquid crystal films for replenishable plasma mirrors

Abstract

Liquid crystal (LC) film plasma mirrors (PMs) based on 4-octyl-4'-cyanobiphenyl (8CB) are an enabling technology for reflecting high-fluence laser pulses. These freestanding LC films can achieve high optical quality and are well-suited for rep-rated applications, as motorized devices continuously replenish films over an aperture following each destructive laser shot. However, a systematic characterization of film quality as a function of seminal operating conditions had not yet been performed for the LC "windmill" version of the device, which aims to match the repetition rate of an existing "spinning disk" (SDI) version and the angular stability of the "linear slider" (LSTI) version. We determined the 8CB film quality using low-power wavefront measurements, and studied the film-to-film wavefront stability and formation reliability. The film-formation reliability of 8CB LC films demonstrated 97\% formation success at 2.7 mm/s film-forming speeds, but decreased to 45\% at 10.8 mm/s. These reliability numbers will inform future designs to reach Hz-level repetition rates and beyond. Depending on area-of-interest within the 10 mm diameter film, the added wavefront root-mean-squared (RMS) variation was as small as 10 nm for a 2 mm diameter region, and $<$50 nm for a 3 mm diameter region. Within the optimal 21-22${}^{\circ }$C operating regime, pointing fluctuations remained at or below 0.52 mrad. With a maximum effective film formation frequency of \textasciitilde0.25 Hz, these results establish windmill-formed 8CB films as promising candidates to pursue next-iteration improvements towards rep-rated plasma-mirror operation.

arXiv comment8 pages, 6 figures, 1 supplementary table