Tobias Guenther

Femtosecond near-field spectroscopy: carrier relaxation and transport in single quantum wires.

Quasi‐two‐colour femtosecond pump and probe spectroscopy and near‐field scanning optical microscopy are combined to study the carrier dynamics in single semiconductor nanostructures. In temporally, spectrally and spatially resolved measurements with a time resolution of 200 fs and a spatial resolution of 200 nm, the non‐linear change in reflectivity of a single quantum wire is mapped in real space and time. The experiments show that carrier relaxation into a single quantum wire occurs on a 100 fs time scale at room temperature. Evidence is given for a transient unipolar electron transport along the wire axis on a picosecond time and 100 nm length scale.

Near-field photocurrent imaging of the optical mode profiles of semiconductor laser diodes

The potential of near-field photocurrent spectroscopy for direct imaging of mode profiles of submicron-sized waveguides in optoelectronic devices is demonstrated. The technique combines the submicron spatial resolution of near-field optics with tunable laser excitation, allowing for selective investigation of the waveguide properties of the device structure. Experiments on InGaAs/AlGaAs high-power laser diodes with different waveguide designs provide direct visualization of the effect of the waveguide design on (i) the number of guided modes and (ii) the spatial profile of both fundamental and higher-order modes. The technique thus provides a sensitive tool for nondestructive in situ analysis of waveguide properties in optoelectronic devices.

Femtosecond near-field spectroscopy of single quantum dots

Excitonic and spin excitations of single semiconductor quantum dots currently attract attention as possible candidates for solid state based implementations of quantum logic devices. Due to their rather short decoherence times in the picosecond to nanosecond range, such implementations rely on using ultrafast optical pulses to probe and control coherent polarizations. We combine ultrafast spectroscopy and near-field microscopy to probe the nonlinear optical response of a single quantum dot on a femtosecond time scale. Transient reflectivity spectra show pronounced oscillations around the quantum dot exciton line. These oscillations reflect phase-disturbing Coulomb interactions between the exitonic quantum dot polarization and continuum excitations. The results show that although semiconductor quantum dots resemble in many respects atomic systems, Coulomb many-body interactions can contribute significantly to their optical nonlinearities on ultrashort time scales.

Time-resolved near-field optical spectroscopy of single semiconductor quantum wires

Carrier dynamics in single quasi-1D GaAs quantum wires are studied in a wide temperature range by near-field scanning optical microscopy using pico- and femtosecond pulses. Luminescence and pump-probe experiments with a spatial resolution of 250 nm and a time resolution of up to 200 fs allow for a separation of carrier transport along the quantum wire and in the embedding GaAs quantum well from local carrier relaxation phenomena. We demonstrate that local potential barriers close to the quantum wire strongly affect the lateral carrier transport into the quantum wire. This drift-diffusive motion occurs in the pico- to nanosecond regime with diffusion lengths of up to several microns. Diffusive transport along the quantum wire is characterized by carrier motion on a somewhat shorter picosecond time scale. In contrast, sub-picosecond relaxation times are found for the redistribution of carriers from high-lying to low-lying quantum wire states. This relaxation is governed by electron-electron and electron-phonon scattering.

Ultrafast near-field pump-probe spectroscopy of quasi-one-dimensional transport in a single quantum wire

Carrier-induced reflectivity changes from a single quantum wire are mapped with femtosecond temporal and sub wavelength spatial resolution. Carrier relaxation in a single wire on a 100 fs time scale and quasi-one-dimensional carrier transport are resolved.