2011/09/13 by Markus, Selmke, Marco Braun, Marco, Braun +2
Engineering · #FOS: Physical sciences #Near-Field Optical Microscopy #Optics (physics.optics) #Photoacoustic and Ultrasonic Imaging #Thermography and Photoacoustic Techniques
paper · pdf · doi:10.48550/arxiv.1109.2772
openalex publication_date 2011/09/13 · openalex created_date 2019/06/27 · openalex updated_date 2026/07/28
The action of a nanoscopic spherically symmetric refractive index profile on a focused Gaussian beam may easily be envisaged as the action of a phase-modifying element, i.e. a lens: Rays traversing the inhomogeneous refractive index field n(r) collect an additional phase along their trajectory which advances or retards their phase with respect to the unperturbed ray. This lens-like action has long been understood as being the mechanism behind the signal of thin sample photothermal absorption measurements [1, 2], where a cylindrical symmetry and a different lengthscale is present. In photothermal single (nano-)particle microscopy, however, a complicated, though prediction-wise limited, electrodynamic (EM) scattering treatment was established [3] during the emergence of this new technique. Our recent study extended [4] this EM-approach into a full ab-initio model describing the reality of the situation encountered and showed for the first time that the mechanism behind the signal, despite its nanoscopic origin, is also the lens-like action of the induced refractive index profile only hidden in the complicated guise of the theoretical Mie-like framework. The diffraction model proposed here yields succinct analytical expressions for the axial PT signal shape and magnitude and its angular distribution, all showing the clear lens-signature. It is further demonstrated, that the Gouy-phase of a Gaussian beam does not contribute to the relative photothermal signal in forward direction, a fact which is not easily evident from the more rigorous EM treatment. The model may thus be used to estimate the signal shape and magnitude in photothermal single particle microscopy.