Patent ID: 8452360

Claim:
A method of detecting an analyte within a material, said method comprising the steps of: a) directing and overlapping first and second intensity modulated optical beams onto a surface of said material; wherein an absorption spectrum of said material comprises a spectral region substantially free of spectral features in the absence of said analyte, and wherein the presence of said analyte produces a spectral peak within said spectral region; wherein said first and second optical beams are provided with substantially equal intensity, and wherein said first and second optical beams are modulated at a substantially equal frequency with a phase difference of approximately 180 degrees; and wherein wavelengths of said first and second optical beams are selected to lie within said spectral region so that the presence of said analyte produces increased absorption of said first optical beam relative to absorption of said second optical beam; b) obtaining a signal by detecting photothermal emission radiated in response to differential absorption of said first and second optical beams, wherein said photothermal emission is detected with a phase-sensitive detection apparatus provided with a reference signal related to a phase of said modulated optical beams; and c) correlating said signal with an amount of said analyte in said material.