A sensitive, low noise, DC to 12 MHz, large area photodiode preamplifier for photothermal heterodyne imaging.

We report a compact, fast, and low-noise large area photodiode preamplifier designed for photothermal heterodyne imaging (PHI). The preamplifier exhibits a noise level of 7 nV/Hz1/2 and a bandwidth from DC to 12 MHz sufficient for PHI experiments. Simulations of the preamplifier bandwidth and noise agree with the observed experimental characterization and performance in our home-built PHI system. The observed noise is close to the inherent limitations of the SR844 lock-in amplifier used. The results indicate that the preamplifier is also useful for any other single frequency pump-probe detection technique, such as stimulated Raman scattering.