Achim Hartschuh

Near-field optical spectroscopy with 20 nm spatial resolution

A near-field optical method is introduced that makes use of the strongly enhanced electric field close to a sharply pointed metal tip under laser illumination. The tip is held a few nanometers above the sample surface so that a highly localized interaction between the enhanced field and the sample is achieved. The method has been successfully combined with vibrational spectroscopy by making use of the well-known effect of surface enhanced Raman scattering (SERS). We mapped out the vibrational modes of individual single-walled carbon nanotubes (SWNT) with a resolution better than 20 nm. The technique has great potential for becoming a routine tool for the chemical analysis of surfaces at high spatial resolution.

Nanoscale optical spectroscopy and detection

We use a laser-irradiated metal tip to create a locally enhanced field at the tip apex. The tip acts as an optical antenna and is held a few nanometers above the sample surface so that a highly localized interaction between the enhanced field and the sample is achieved. The method has been successfully combined with vibrational spectroscopy by making use of the well-known effect of surface enhanced Raman scattering (SERS). We mapped out the vibrational modes of individual single-walled carbon nanotubes (SWNT) with a resolution down to l0nm.

Individual single wall carbon nanotube photonics

Single carbon nanotube fluorescence spectroscopy was used to determine the emission lineshape. Nanotube fluorescence unexpectedly lacks intensity or spectral fluctuations resulting in a stable, single infrared photon source for exciting applications in quantum optics.

Individual single-wall carbon nanotube photonics

The electronic structure of SWNTs was investigated using the complementary techniques of single molecule photoluminescence spectroscopy and ultrafast optical spectroscopy. We found that photoexcited electrons in SWNTs isolated in surfactant micelles decay through many channels exhibiting a range of decay times (~200 fs to ~ 120 ps). The magnitude of the longest-lived component in the ultrafast signal specifically depends on resonant excitation, thus suggesting that this lifetime corresponds to the band-edge relaxation time. Fluorescence spectra from single SWNTs are well described by a single, Lorentzian lineshape. However, nanotubes with identical structure fluoresce over a distribution of peak positions and line widths not observed in ensemble studies, caused by localized defects and electrostatic perturbations. Unlike for most other single molecules, for SWNTs the photoluminescence unexpectedly does not show any intensity or spectral fluctuations at 300K. This lack of photoluminescence intensity blinking or bleaching demonstrates that SWNTs have the potential to provide a stable, single molecule infrared photon source, allowing for the exciting possibility of single nanotube integrated photonic devices and biophotonic sensors.

Nanoparticle-sensor based on optical gradient force

We present a detection scheme for nanoscale particles based on the optical gradient force and torque near a tightly focused laser beam. The focus affects the path of nanoparticles passing by and a quadrant detector records the particle trajectory. A feedback system continuously adjusts the laser power thereby preventing the particles from being trapped. Particle size and shape can be assessed by evaluating the time-trace of the quadrant detector signal.