We introduce a single-laser-beam photothermal microscopy scheme for the detection of single absorbing nano-objects. Here, a modulated incident laser beam with a constant intensity offset serves as pump and probe beam at the same time. Using the out-of-phase scattering response of the retarded thermorefractive wave field, the method provides a selective contrast for absorbers over a possible background of scatterers. The use of a single wavelength and a single beam, considerably simplifies the setup and integration of photothermal detection in existing microscopy schemes.
Fluorescence intermittency is a random switching between emitting (on) and non-emitting (off) periods found for many single chromophores such as semiconductor quantum dots and organic molecules. The statistics of the duration of on- and off-periods are commonly determined by thresholding the emission time trace of a single chromophore and appear to be power law distributed. Here we test with the help of simulations if the experimentally determined power law distributions can actually reflect the underlying statistics. We find that with the experimentally limited time resolution real power law statistics with exponents α(on/off) ≳ 1.6, especially if α(on) ≠ α(off) would not be observed as such in the experimental data after binning and thresholding. Instead, a power law appearance could simply be obtained from the continuous distribution of intermediate intensity levels. This challenges much of the obtained data and the models describing the so-called power law blinking.
We present a simple scheme for the manipulation of light intensity by light mediated by a dissipative process. The implementation employs the heat released by an optically excited plasmonic metal nanoparticle to control the size of an isotropic bubble in a nematic liquid crystal film. The nematic film is designed as a zero-order half-wave plate that rotates an incident probe light polarization by π/2 and is blocked by an analyzing polarizer behind the structure. The growing isotropic bubble disturbs the half-wave plate and causes the probe to be transmitted through the modulator structure. Our results demonstrate that dissipative processes may be advantageously used to control light by light.
A method to measure thermal diffusivity around a single heated gold nanoparticle is presented. It is based on photothermal single particle microscopy and employs the phase delay of temperature modulation due to finite thermal diffusivity. The phase delay is detected optically averaging over the focal volume of a diffraction limited beam of light. Thermal diffusivity is extracted by comparison to electromagnetic scattering calculations of the photothermal signal. Measurements in the solid (polymer) and liquid (water) are presented and compare well with literature data. The method paves the way for extended measurements of non-diffusive and heterogeneous heat transport in complex media.
Using a fixed pump beam and a steerable probe beam, we image the thermal lens generated by a harmonically heated gold nanoparticle using photothermal deflection. In combination with a simple geometric optics model, we extract thermal diffusivities. The measurement technique is applied to reveal the anisotropic thermal conductivity of a nematic liquid crystal. As the measurement is highly local, it paves the way for applications in heterogeneous and anisotropic nanoscale thermal transport studies in complex biological systems.
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