biostudies-literatureZhao PHong Kong Polytechnic UniversityGovernment of the Hong Kong Special Administrative Region2249-2255https://www.ebi.ac.uk/biostudies/studies/S-EPMC12922175biostudies-literatureUnknown26(6)Optical nanofibers (ONF) have emerged as versatile platforms for studying light-gas interactions at the micro/nanoscale, yet existing ONF gas sensors remain limited in detection sensitivity. Here, we report a polarization-mode photothermal interferometry technique that precisely measures the gas absorption-induced phase difference between two polarization states of the symmetric supermode of a single-mode ONF coupler. The high power density and large evanescent field associated with the ONF coupler enhance the efficiency of photothermal phase modulation, while the strong waveguide birefringence and noise-immune differential phase detection confer environmental immunity, jointly yielding an order-of-magnitude enhancement in the signal-to-noise ratio. With a 2 cm-long overcoupled ONF couplerNano lettersUltrasensitive Gas Detection via Polarization-Mode Photothermal Interferometry in a Single-Mode Nanofiber Coupler.PMC129221751-ZVY41-W23D152234211-CDJ6Jin WHo HLZhao SZhao PBao HfalseUltrasensitive Gas Detection via Polarization-Mode Photothermal Interferometry in a Single-Mode Nanofiber Coupler.Optical nanofibers (ONF) have emerged as versatile platforms for studying light-gas interactions at the micro/nanoscale, yet existing ONF gas sensors remain limited in detection sensitivity. Here, we report a polarization-mode photothermal interferometry technique that precisely measures the gas absorption-induced phase difference between two polarization states of the symmetric supermode of a single-mode ONF coupler. The high power density and large evanescent field associated with the ONF coupler enhance the efficiency of photothermal phase modulation, while the strong waveguide birefringence and noise-immune differential phase detection confer environmental immunity, jointly yielding an order-of-magnitude enhancement in the signal-to-noise ratio. With a 2 cm-long overcoupled ONF coupler2026-01-01T00:00:00Z2026 Feb2026-07-16T12:33:08.519Z2026-07-09T10:55:03.754ZS-EPMC129221754162906710.1021/acs.nanolett.5c06094