Photoacoustic imaging with constant light intensity and time-modulated absorption

Photoacoustic imaging is generally based on photoacoustic generation induced by the absorption of a light beam with a time-modulated light intensity, whether in the time domain with nanosecond pulses or in the frequency domain with a modulation frequency in the MHz range. In this Letter, we introduce an optical-resolution photoacoustic microscopy technique based on time-modulated absorption, with a beam of constant geometry and constant intensity. More specifically, we consider the case of linear dichroic absorbing samples, which are shown theoretically and experimentally to generate a harmonic photoacoustic signal at angular frequency 2ω_rot, when illuminated with linearly polarized light whose polarization direction rotates with angular frequency ω_rot. The amplitude and phase of the photoacoustic signal are related to the amount of linear dichroism and to its orientation, respectively. As a unique feature of the proposed approach, photoacoustic signals are generated only from linear dichroic absorbers, with no background signals generated from non-dichroic absorbing structures.