Plasmon mediated interactions between metal nanoparticles and molecules

Fig. 1: A single silver nanorod rotates about 20° anti-clockwise. The clip consists of a series of scattering images during which the sample was manually rotated.

Optical properties of metal nanorods

The plasmon resonances of metal nanorods can be tuned throughout the visible by varying their aspect ratio R=a/b (i.e. the ratio of their length to width). For gold particles, the first plasmon band around 520 nm corresponds to the short axis b, the second to the long axis a. The longer the nanorods, the more red shifted the second plasmon band is (see Fig. 2).

Fig. 2: UV/Vis spectra of gold nanorods with different aspect ratio R in water.

Fig. 3: SEM image of gold nanorods.

Confocal interference scattering microscopy

We study the scattering patterns of single metal nanoparticles by confocal microscopy in combination with higher order laser modes. This technique allows to image the orientation of nanorods being much smaller than the wavelength (see Fig. 3) with a precision of less than 1° (see Failla et al., Opt. Expr. 15, 8532-8542, 2007).

Fig. 4: Two gold nanorods of different orientations. From left to right: in situ AFM-image, experimental and theoretically simulated pattern. (a)/(b): excitation with azimuthally/radially polarized light. The particles? orientation is directly imaged (from Failla et al., Nano Letters 6, 1374-1378, 2006).

In addition, we are able to distinguish between particles of different shapes, e.g. gold nanospheres, rods and triangles (see Fig. 4).

Fig. 5: Particles of different shape can be distinguished by their scattering patterns. From left to right: single gold nanosphere, -rod and triangle after excitation with azimuthally/radially (top/bottom) polarized light (from Züchner et al., J. Micr. 229, 337-343, 2008).