Self-referencing photothermal common-path interferometry to measure absorption of Si3N4 membranes for laser-light sails
arxiv(2024)
摘要
Laser-light sails are a spacecraft concept wherein lightweight "sails" are
propelled to high speeds by lasers with high intensities. The sails must
comprise materials with low optical loss, to minimize the risk of laser damage.
Stoichiometric silicon nitride (Si_3N_4) is a candidate material with low
loss in the near infrared, but the precise absorption coefficient has not been
characterized in the membrane form-factor needed for sails. We use photothermal
common-path interferometry (PCI), a sensitive pump-probe technique, to measure
the absorption coefficient of stoichiometric and nonstoichiometric silicon
nitride. To calibrate PCI measurements of membranes, we developed a
self-referencing technique where a measurement is performed twice: once on a
bare membrane, and a second time with a monolayer of graphene deposited on the
membrane. The absorption of the sample with graphene can be measured by both
PCI and more-conventional spectroscopic techniques, enabling the calibration of
the PCI measurement. We find that with an absorption coefficient of (2.09 ±
0.76) × 10^-2 cm^-1 at 1064 nm, Si_3N_4 is a suitable
laser-sail material for laser intensities as high as 10 GW/m^2, which have
been proposed for some laser-sail missions, while silicon-rich SiN_x (x 1),
with an absorption coefficient of 7.94 ± 0.50 cm^-1, is unlikely to
survive such high laser intensities.
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