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Thermal diffusivity is one of the main parameters to characterize the thermo-physical properties of materials, and advances in its measurement technique will have significant impact on materials science and related applications. Here a photoacoustic (PA) thermorelaxation microscopy is proposed as a new noncontact method to measure the thermal diffusivity. By delivering co-focused heating/probing laser pulse pairs with tunable time delays, the sample's in situ thermal relaxation behavior after the heating pulse excitation can be photoacoustically monitored based on the temperature-dependent property of the Grueneisen parameter. We theoretically deduced the dependence of the obtained PA thermorelaxation time on the thermal diffusivity, and the results coincided well with simulations. The feasibility of this method was validated by various industrial and biological samples. This method provides a new strategy for high-resolution thermal diffusivity measurement with flexible measurement conditions, prefiguring great potential for material and biological applications.

We proposed polarized photoacoustic microscopy (PPAM) for quantitative detection of a target's microscopic anisotropy based on the vectorial optical absorption by applying four linearly polarized laser beams as excitation sources. Compared to conventional photoacoustic microscopy that treats targets as isotropic absorbers, PPAM allows us to quantitatively detect the target's anisotropic features beyond optical absorption with a newly proposed parameter valued between 0 and 1. The feasibility of the method was validated by dichroic phantoms. The dichroic compound eyes of mantis shrimps were imaged in situ to demonstrate the method's capability for quantitative three-dimensional biological imaging. The PPAM method provides an effective and straightforward strategy for tissue polarimetry, prefiguring great potential for biological imaging and material inspection.

We have developed a second harmonic photoacoustic microscopy (SH-PAM) for subdiffraction-limited imaging based on nonlinear thermal diffusion. When a sine-modulated Gaussian temperature field is introduced by a laser beam, the temperature dependence of the thermal diffusivity induces a nonlinear photoacoustic (PA) effect and thus results in the production of second harmonic PA signals. We demonstrate through both simulation and experiment that the second harmonic PA images can be reconstructed with a lateral resolution exceeding that of conventional optical resolution PA microscopy. The feasibility of SH-PAM was verified on phantom samples. Amphioxus zygotes and germinated pollens have been studied by SH-PAM to demonstrate its biomedical imaging capability. This method expands the scope of conventional PA imaging and opens up new possibilities for super-resolution imaging, prefiguring great potential for biological imaging and material inspection.