Nonlinear effects in single-particle photothermal imaging

Although photothermal imaging was originally designed to detect individual molecules that do not emit or small nanoparticles that do not scatter, the technique is now being applied to image and spectroscopically characterize larger and more sophisticated nanoparticle structures that scatter light strongly. Extending photothermal measurements into this regime, however, requires revisiting fundamental assumptions made in the interpretation of the signal. Herein, we present a theoretical analysis of the wavelength-resolved photothermal image and its extension to the large particle scattering regime, where we find the photothermal signal to inherit a nonlinear dependence upon pump intensity, together with a contraction of the full-width-at-half-maximum of its point spread function. We further analyze theoretically the extent to which photothermal spectra can be interpreted as an absorption spectrum measure, with deviations between the two becoming more prominent with increasing pump intensities. Companion experiments on individual 10, 20, and 100 nm radius gold nanoparticles evidence the predicted nonlinear pump power dependence and image contraction, verifying the theory and demonstrating new aspects of photothermal imaging relevant to a broader class of targets.

Automated summary

Although originally designed for detecting individual molecules or small nanoparticles, photothermal imaging is now used to characterize larger structures that scatter light strongly. The interpretation of the photothermal signal in this regime requires revisiting fundamental assumptions. Theoretical analyses reveal nonlinearity in the photothermal signal with pump intensity and contraction of the point spread function, and show deviations between photothermal spectra and absorption spectra at higher pump intensities. Companion experiments on gold nanoparticles confirm the theoretical predictions and demonstrate new aspects of photothermal imaging.