Photothermal spectroscopy of gases is explored using optical detection of the thermal expansion of the gas through the use of photorefractive dynamic holography. The photorefractive effect in bismuth silicon oxide is exploited to demodulate the optical phase shift of a signal beam traversing a gaseous environment and coincident with a tunable chopped excitation beam. Molecular absorption at the excitation wavelength produces heat that causes local expansion and subsequent acoustic wave radiation. A test cell, although unnecessary for this technique that can perform measurements in a containerless or open environment, was used to provide known gas mixtures to be tested. A model of the photoacoustic absorption and the optical phase detection process has been developed at low frequencies, where the process can be approximated as photothermal expansion. Measurement and modeling results are presented that illustrate the ability of the method to detect water vapor and hydrogen fluoride concentrations in nitrogen atmosphere backgrounds near 800 nm, currently producing sensitivities in the 20 to 1000 ppm range. The effects of buffer gas concentrations, excitation frequencies, and the ability to measure temporal changes of trace gas concentrations are illustrated. Limitations of the technique and methods for extending to ppb sensitivities with infrared excitation are also discussed.