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

Area of Science:

• Materials Science
• Optics
• Aerospace Engineering

Background:

• Laser-light sails represent a novel spacecraft propulsion concept.
• Stoichiometric silicon nitride (Si3N4) is a potential material for these sails.
• Accurate measurement of low optical absorption is crucial for sail material selection.

Purpose of the Study:

• To investigate the near-infrared absorption of stoichiometric silicon nitride (Si3N4) membranes.
• To develop and validate a self-referencing photothermal common-path interferometry (PCI) technique for measuring low optical loss.
• To assess the suitability of Si3N4 as a laser sail material.

Main Methods:

• Developed a self-referencing photothermal common-path interferometry (PCI) method.
• Measured optical absorption of Si3N4 membranes before and after graphene deposition.
• Utilized ellipsometry to quantify absorption changes induced by graphene.

Main Results:

• Measured the absorption coefficient of Si3N4 at 1064 nm to be (1.5-3) × 10^-2 cm^-1.
• Determined Si3N4 is suitable for laser intensities up to ~10 GW/m^2.
• Found silicon-rich SiNx (x ~ 1) has a higher absorption (~8 cm^-1) making it unsuitable.

Conclusions:

• Stoichiometric Si3N4 is a promising low-loss material for laser light sails.
• The developed self-referencing PCI technique is effective for characterizing low-loss membranes.
• This technique can be applied to evaluate various materials for laser sail applications and beyond.