Nonlinear photothermal processes are interesting and important at both fundamental and applications levels. Thermal-wave fields, limited by the physics of parabolic diffusion, can only produce thermal gradient driven energy transport and are, therefore, lossy or diffuse. As a result, photothermal techniques have been unable to generate three-dimensional subsurface imaging, resulting in depth-integrated planar images severely limited in axial and lateral resolution to within the modulation-frequency-dependent thermal diffusion length (hundreds of μm to ~ 1 mm). In media which exhibit temperature dependence of their thermal conductivity and specific heat capacity the conventional frequency-domain Fourier heat conduction equation (the thermal-wave equation) becomes non-linear in the oscillating temperature field. The medium generates a thermal-wave frequency multiplexing response in the fundamental as well as all harmonics of the excitation frequency. This behavior has advantages in thermal-wave imaging of materials, including biological tissues. Given the fact that increasing frequency is associated with a shorter thermal wavelength and thus increased spatial lateral and axial resolution, driving the thermal-wave response at higher harmonics improves the quality of photothermal images at the possible expense of signal-to-noise ratio as the harmonic amplitudes are significantly lower than the fundamental. This can be counterbalanced by a decrease in signal noise with increasing frequency. Additional advantages can accrue if the imaged field is non-uniform not only optically but also thermally: The non-linear signal generated at regions of interest (ROI) with high optical absorption properties may exhibit enhancement over that originating in regions of lower absorptivity because the harmonic response of stronger absorbers is proportionately higher compared to weaker absorbers, such as background absorption. As a result, non-linear photothermal imaging in the second harmonic is likely to exhibit higher contrast than imaging in the fundamental.
The article by He et al. nicely incorporates and displays all the aforementioned features of non-linear photothermal imaging in a first application of this technique to biomedical imaging. The non-linear photothermal process is used in a pump-probe thermoreflectance modality to generate signals from mouse skin melanoma ROI at high (~ 30 kHz) frequencies. Contrast is generated by the melanin in the melanoma. The spatial resolution enhancement of the second harmonic image is found to be ~ 42%, and the contrast enhancement is around 15-20%. The work reported in this paper can be of significance to biomedical microscopy which is more sensitive than fluorescence microscopy and labelfree, yet, it traditionally suffers from poor spatial resolution under linear imaging conditions.
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