Generalized photothermal correlation spectroscopy

Nano-lens diffraction around a single heated nano particle

“…On the other hand, the profile extends infinitely as 1/r and thus misses a characteristic length scale. This is the reason, why the description presented in this paper and within the recent EM-study [4] provide no evidence for a role-play of the Gouy-phase which is otherwise important for the probing of small scatterers as shown by Hwang and Moerner [18]. While models for spherical absorbers have been put forward [19,20], these were numerical in nature and targeted for large µm-sized absorbers.…”

“…The lens to be considered in single particle generated nano-lens experiments such as photothermal microscopy originates from the absorption of optical power provided by a focused laser beam by a single nano-particle. The absorbing particle can be treated as a point-like heat-source, yielding the steady-state temperature profile T (r) = T 0 + ∆T 0 R/r which decays with the inverse distance r from the particle of radius R. In the case of modulated heating, as utilized in the lock-in approach common to photothermal single particle microscopy, the assumption remains valid as long as the modulation frequencies used remain below 1 MHz, typically [3,4]. By the temperature dependence of the surrounding mediums' refractive index n, a corresponding refractive index profile n (r) is established:…”

“…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.…”

Photothermal Single-Particle Microscopy: Detection of a Nanolens

“…1, right). Recording these scattering intensities at different detection numerical apertures emphasizes the importance of the phase relation of scattered and transmitted electric fields [13]. The interference patterns largely vanish when considering the photothermal signal, which is the difference in the probe laser scattering signal of a heated nanoparticle including the long range refractive index profile and a nonheated particle without the refractive index change.…”

Brownian Thermometry Beyond Equilibrium

“…So far, a number of predictions of the theory of hot Brownian motion, as sketched above, could be validated with great precision by experiments [73,79,70,77,36] and non-equilibrium molecular dynamics simulations [16,36]. Conversely, after the accuracy of the theory has been established, it can help to develop practically useful innovative experimental applications, such as photothermal microscopy/spectroscopy setups, which employ nano-particles suspended in water as dynamic tracers [79,70,11].…”