Not Just Better Resolution: A Detailed Study of the Signal Distribution in Mid-Infrared Optical Photothermal Imaging

The spatial resolution of an imaging system is generally evaluated by analysis of the point spread function (PSF), that is, the system response to a point source in the object plane. The PSF, albeit a fundamental concept in light microscopy, has been sparsely studied in optical photothermal infrared (O-PTIR) imaging. Although there is only a minor difference between the photothermal and optical intensity distributions in the radial direction, the axial photothermal signal is markedly different from the imaging response of conventional light microscopes. Previous studies of particles beyond the visible diffraction limit have shown that the O-PTIR signal has two lobes and strongly varies with the offset between the foci of the pump and the probe laser. We will therefore present theoretical and experimental data to investigate the radial and axial intensity distributions of a commercial Raman microscope and a custom O-PTIR transmission instrument, with particular emphasis on the axial PSF of the O-PTIR system. Our data suggest that the two-lobed signal shape is also present in objects on the order of the probe beam wavelength.