Optical Absorption-Based Studies of Single Nanostructures

Spatial modulation spectroscopy (SMS) has been used to image single organic nanoparticles doped with non-fluorescent, near-IR croconaine dyes. Based on the measured extinction cross-section of the nanoparticles, the number of dye molecules per particle has been determined. SMS images were recorded for particles within EMT-6 breast cancer cells which allowed mapping of the nanoparticle location and the amount of dye in a single cell. This demonstrates how SMS can facilitate efforts to optimize dye-doped nanoparticles for effective photothermal treatment of cancer. SMS imaging has been extended to particles with sizes comparable to or larger than the laser spot, where the shape of the particle matters.

The mechanical resonances of metal nanostructures are strongly affected by their environment. Ultrafast pump-probe microscopy experiments has been used to study damping of the breathing mode vibrations of single gold nanowires by liquids with different viscosities. The measured quality factors for liquid damping are in good agreement with continuum mechanics calculations for an inviscid fluid showing that liquid damping is controlled by radiation of sound waves into the medium.

Transient absorption microscopy (TAM) measurements have been used to study the optical properties of surface plasmon polariton (SPP) modes in gold nanoplates on a glass. The TAM images show an oscillation in the signal across the nanoplate due to interference between the bound and leaky SPP modes. Back focal plane (BFP) imaging was used to measure the wavevector of the leaky mode. By combining the results from two techniques, the wavevector of the bound mode has been determined. These experiments represent the first far-field optical measurement of the wavevector for the bound mode in metal nanostructures.

Femtosecond laser excitation of strongly absorbing thin films generates picosecond acoustic waves in the surrounding medium. In time-domain transient absorption experiments these waves can give rise to Brillouin oscillations. The attenuation of the oscillations has been investigated for different excitation and detection conditions. The results show that the measured attenuation constants strongly depend on the numerical aperture of the microscope objective used. These results are important for understanding the spectral resolution limits in imaging applications of the Brillouin scattering effect.