Nanowaveguide Enhanced Photothermal Interferometry Spectroscopy

We report a new optical nanowaveguide enhanced photothermal (PT) interferometry spectroscopy method for trace molecule detection. Absorption of pump evanescent field of an optical nanowaveguide heats up the trace molecules surrounding the waveguide, causing the temperature of waveguide to rise via thermal conduction and modulating the refractive index and dimension of the nanowaveguide. The phase of a probe beam propagating through the same nanowaveguide is then modulated and can be detected with optic fiber interferometry. Numerical simulation with silica, cyclic transparent optical polymer, and silicon nanowaveguides with proper dimensions can achieve PT index modulation of 10 to over 8000 times that of the commercial HC-1550-02 photonic bandgap fiber. Experiments with 12-mm-long, 800-nm-diamter silica nanofiber demonstrated a lower detection limit of 600 parts per billion (ppb) acetylene at ambient conditions.