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Certain porous-silicon (PS) films exhibit well-resolved Fabry–Pérot fringes in their optical reflection spectra due to thin-film interference. The fringes shift to higher wavelengths when the PS is exposed to vapors from organic solvents, as a result of an increase in the average refractive index of the PS layer. If a small diode laser is used as the light source, the shift of the Fabry–Pérot fringes upon analyte adsorption results in a change in the reflected light intensity, which correlates with the concentration of the analyte (ethanol) in an air stream. Based on this principle, a PS vapor sensor has been demonstrated with a detection limit of 500 ppb and a dynamic range of nearly five orders of magnitude. © 2000 American Institute of Physics.