Marco Braun

Photothermal single-particle microscopy: detection of a nanolens.

Combining quantitative photothermal microscopy and light scattering microscopy as well as accurate MIE scattering calculations on single gold nanoparticles, we reveal that the mechanism of photothermal single-molecule/particle detection is quantitatively explained by a nanolensing effect. The lensing action is the result of the long-range character of the refractive index profile. It splits the focal detection volume into two regions. Our results lay the foundation for future developments and quantitative applications of single-molecule absorption microscopy.

Nano-lens diffraction around a single heated nano particle

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 [Appl. Opt. 34, 41-50 (1995)], [Jpn. J. Appl. Phys. 45, 7141-7151 (2006)], where a cylindrical symmetry and a different lengthscale is present. In photothermal single (nano-)particle microscopy, however, a complicated, though prediction-wise limited, electrodynamic scattering treatment was established [Phys. Rev. B 73, 045424 (2006)] during the emergence of this new technique. Our recent study [ACS Nano, DOI: 10.1021/nn300181h] extended this approach into a full ab-initio model 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 generalized Mie-like framework. The diffraction model proposed here yields succinct analytical expressions for the axial photothermal 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 presented model may thus be used to estimate the signal shape and magnitude in photothermal single particle microscopy.

Optically controlled thermophoretic trapping of single nano-objects.

Brownian motion is driven by thermal fluctuations and becoming more efficient for decreasing size and elevated temperatures. Here, we show that despite the increased fluctuations local temperature fields can be used to localize and control single nano-objects in solution. By creating strong local temperature gradients in a liquid using optically heated gold nanostructures, we are able to trap single colloidal particles. The trapping is thermophoretic in nature, and thus no restoring body force is involved. The simplicity of the setup allows for an easy integration and scalability to large arrays of traps.

Gaussian beam photothermal single particle microscopy

We explore the intuitive lensing picture of laser-heated nanoparticles occurring in single particle photothermal (PT) microscopy. The effective focal length of the thermal lens (TL) is derived from a ray-optics treatment and used to transform the probing focused Gaussian beam with ABCD Gaussian matrix optics. The relative PT signal is obtained from the relative beam-waist change far from the TL. The analytical expression is semiquantitative, capable of describing the entire phenomenology of single particle PT microscopy, and shows that the signal is the product of the point-spread functions of the involved lasers times a linear function of the axial coordinate. The presented particularly simple and intuitive Gaussian beam lensing picture compares favorably to the experimental results for 60 nm gold nanoparticles and provides the prescription for optimum setup calibration.

Single Molecules Trapped by Dynamic Inhomogeneous Temperature Fields.

We demonstrate a single molecule trapping concept that modulates the actual driving force of Brownian motion--the temperature. By spatially and temporally varying the temperature at a plasmonic nanostructure, thermodiffusive drifts are induced that are used to trap single nano-objects. A feedback controlled switching of local temperature fields allows us to confine the motion of a single DNA molecule for minutes and tailoring complex effective trapping potentials. This new type of thermophoretic microbeaker even provides control over a well-defined number of single molecules and is scalable to large arrays of trapping structures.

Photothermal single particle microscopy using a single laser beam

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.

Trapping of single nano-objects in dynamic temperature fields

In this article we explore the dynamics of a Brownian particle in a feedback-free dynamic thermophoretic trap. The trap contains a focused laser beam heating a circular gold structure locally and creating a repulsive thermal potential for a Brownian particle. In order to confine a particle the heating beam is steered along the circumference of the gold structure leading to a non-trivial motion of the particle. We theoretically find a stability condition by switching to a rotating frame, where the laser beam is at rest. Particle trajectories and stable points are calculated as a function of the laser rotation frequency and are experimentally confirmed. Additionally, the effect of Brownian motion is considered. The present study complements the dynamic thermophoretic trapping with a theoretical basis and will enhance the applicability in micro- and nanofluidic devices.

Twin-focus photothermal correlation spectroscopy

We introduce a novel concept for optical correlation spectroscopy of non-fluorescent absorbing nano-particles termed twin-focus photothermal correlation spectroscopy (Twin-PhoCS). This method exhibits a unique axial sensitivity, allowing for either the measurement of very slow axial dynamics or the determination of axial heterogeneities on sub-diffraction limited length-scales. To this end, the photothermal nano-lensing mechanism [Markus Selmke, Romy Schachoff, Marco Braun, and Frank Cichos, ACS Nano, 2012, 6(3), 2741] provides two sharply separated detection-subvolumes, comprising the twin-focus. The achieved axial sensitivity is superior to single detection-volume or dual-focus based fluorescence correlation spectroscopies. We demonstrate the sensitivity by measuring radiation-pressure induced flow velocities of diffusing 14 nm gold-colloids down to 10 nm ms−1 with the help of the provided analytical correlation functions.

Photothermal signal distribution analysis (PhoSDA).

Photothermal correlation spectroscopy (PhoCS) is a powerful counterpart to fluorescence correlation spectroscopy (FCS). Using PhoCS it is possible to probe the dynamics of non-fluorescent and non-bleaching ultra-stable metal-nanoparticles in solution and biological specimen, where they can be used as tracers and markers. This paper complements the absorption correlation method by a histogram analysis framework, the photothermal signal distribution analysis (PhoSDA). It is hereby possible to extract individual absorbent tracer concentrations, size dispersions, heterogeneous populations and focal geometry parameters which are otherwise inaccessible by correlation analysis.

Nano-lens diffraction around a single heated nano particle: errata

In our publication [Opt. Express, 20(7), 8055–8070 (2012)] a convergence issue resulted in a discrepancy between the relative photothermal signal of two models: the paraxial scalar diffraction model and the accurate vectorial generalized multilayer Lorenz-Mie scattering theory which served as a reference. The resolution yields the expected agreement.