We present the first quantitative measurements of the torque exerted on a single gold nanorod in a polarized three-dimensional optical trap. We determined the torque both by observing the time-averaged orientation distribution and by measuring the dynamics of the rotational brownian fluctuations. The measurements are in good agreement with calculations, where the temperature profile around the hot nanorod gives rise to a reduced, effective viscosity. The maximum torque on a 60  nm×25  nm nanorod was 100  pN·nm, large enough to address single-molecule processes in soft and biological matter.

We form sub-micrometer-sized vapor bubbles around a single laser-heated gold nanoparticle in a liquid and monitor them through optical scattering of a probe laser. Bubble formation is explosive even under continuous-wave heating. The fast, inertia-governed expansion is followed by a slower contraction and disappearance after some tens of nanoseconds. In a narrow range of illumination powers, bubble time traces show a clear echo signature. We attribute it to sound waves released upon the initial explosion and reflected by flat interfaces, hundreds of microns away from the particle. Echoes can trigger new explosions. A nanobubble's steady state (with a vapor shell surrounding the heated nanoparticle) can be reached by a proper time profile of the heating intensity. Stable nanobubbles could have original applications for light modulation and for enhanced optical-acoustic coupling in photoacoustic microscopy.

Triplet states can be interesting for optical switching of molecular fluorescence as well as quantum experiments relying on the manipulation of spin states. However, the ground state of molecules is usually a singlet state. It is therefore interesting to study the intersystem crossing (ISC) rates between singlet and triplet states. We have measured the autocorrelation function of the fluorescence from single perylene molecules in an ortho-dichlorobenzene host matrix at cryogenic temperatures (1.3 K). We observed two time scales in the autocorrelation function corresponding to intersystem crossing to two indistinguishable triplet states (TX and TY) and a third triplet state (TZ). By studying the power dependence of the correlation times and contrasts in the autocorrelation functions of single molecules, we determine the ISC rates of perylene for the first time.

We study single dibenzoterrylene (DBT) molecules embedded in 1,4-dichlorobenzene (para-dichlorobenzene, pDCB) at 1.2 K. Due to the relatively low melting point of pDCB (53 °C), this host-guest system can be easily prepared from the molten phase. Narrow linewidths, stable molecular lines and high saturation count rates of single DBT molecules were observed. For this reason, we consider this host-guest system a promising candidate for the study of interactions of single molecules with other small objects such as waveguides or nanoparticles.

We present measurements of the polarized extinction of gold nanorod suspensions exposed to an external electric field. By employing an amplitude modulated field in combination with lock-in detection we resolve changes in the optical density as low as 10(-6) in an integration time of 10 s. This sensitivity allows us to probe the partial alignment of small gold nanorods with an aspect ratio of 2.5 and a width ranging from 13 nm to 28 nm. The degree of orientation scales as the square of the electric field strength, as expected for an induced dipole moment in an external field. By varying the modulation frequency we measure the rotation diffusion constant of different samples, which are in excellent agreement with the calculated values for a short cylinder.

We consider a single dipole evanescently coupled to a cylindrical multimode waveguide. The emission rate into the waveguide is calculated as a function of the waveguide diameter and the dipole orientations, and the result is confirmed by finite-difference-time-domain simulations. We show that as the guide radius increases, the coupling to a given mode decreases but new decay channels to higher order modes open up to increase the density of states. This study gives insight for designing waveguide-based single photon sources that exploit superposition of transverse modes.

We propose using halogenated organic dyes as nanoprobes for electric fields and show their greatly enhanced Stark coefficients using density functional theory (DFT) calculations. We analyse halogenated variants of three molecules that have been of interest for cryogenic single molecule spectroscopy: perylene, terrylene, and dibenzoterrylene, with the zero-phonon optical transitions at blue, red, and near-infrared. Out of all the combinations of halides and binding sites that are calculated, we have found that fluorination of the optimum binding site induces a dipole difference between the ground and excited states larger than 0.5 D for all three molecules with the highest value of 0.69 D for fluoroperylene. We also report on the synthesis of 3-fluoroterrylene and the bulk spectroscopy of this compound in liquid and solid organic environments.