# Self-Referencing Photothermal Common-Path Interferometry to Measure Absorption of Si3N4 Membranes for Laser-Light Sails

## TL;DR

Researchers measured the light absorption of silicon nitride membranes to evaluate their suitability as laser-propelled spacecraft sails.

## Contribution

A self-referencing photothermal interferometry method was developed to measure low optical absorption in thin membranes.

## Key findings

- Si3N4 has an absorption coefficient of (1.5–3) × 10–2 cm–1 at 1064 nm.
- Si3N4 can withstand laser intensities up to ∼10 GW/m2.
- Silicon-rich SiNx has much higher absorption (8 cm–1) and is unsuitable for high-intensity lasers.

## Abstract

Laser-light sails
are a spacecraft concept, wherein lightweight
“sails” are propelled by high-intensity lasers. We investigated
the near-infrared absorption of free-standing membranes of stoichiometric
silicon nitride (Si3N4), a candidate sail material.
To resolve the small but nonzero optical loss, we used photothermal
common-path interferometry (PCI), for which we developed a self-referencing
modality where a PCI measurement is performed twice: once on a bare
membrane, and a second time with monolayer graphene deposited on the
membrane. The graphene increases the absorption of the sample by orders
of magnitude, such that it can be measured by ellipsometry without
significantly affecting the thermal properties. We measured the absorption
coefficient of Si3N4 to be (1.5–3) ×
10–2 cm–1 at 1064 nm, making it
a suitable sail material for laser intensities as high as ∼10
GW/m2. By comparison, silicon-rich “low stress”
SiN
x
(x ∼ 1),
with a measured absorption coefficient of approximately 8 cm–1, is unlikely to survive such high laser intensities. Our self-referencing
technique enables the testing of low-loss membranes of various materials
for laser sails and other applications.