Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications.

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Structural properties of self-organized semiconductor nanostructures

In this review we describe fundamentals of scanning near-field optical microscopy with aperture probes. After the discussion of instrumentation and probe fabrication, aspects of light propagation in metal-coated, tapered optical fibers are considered. This includes transmission properties and field distributions in the vicinity of subwavelength apertures. Furthermore, the near-field optical image formation mechanism is analyzed with special emphasis on potential sources of artifacts. To underline the prospects of the technique, selected applications including amplitude and phase contrast imaging, fluorescence imaging, and Raman spectroscopy, as well as near-field optical desorption, are presented. These examples demonstrate that scanning near-field optical microscopy is no longer an exotic method but has matured into a valuable tool.

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Scanning near-field optical microscopy with aperture probes: Fundamentals and applications

The femtosecond optical response of noble metal nanoparticles and its connection to the ultrafast electron dynamics are discussed in light of the results of high-sensitivity femtosecond pump−probe experiments. The physical origins of the nonlinear responses in the vicinity of the surface plasmon resonance and interband transition threshold are analyzed using extension of the theoretical models used in the bulk materials. These responses contain information on the electron interaction processes (electron−electron and electron−phonon scattering) that can thus be directly investigated in the time domain. Their size and environment dependences are discussed, and the results are compared to the ones in the bulk materials. Time-resolved techniques also permit direct study of the vibrational modes of metal nanoparticles and, in particular, the determination of their damping, which is a sensitive probe of the nature of the surrounding matrix and of the interface quality.

Ultrafast Electron Dynamics and Optical Nonlinearities in Metal Nanoparticles

In Forster resonance energy transfer (FRET), energy non-radiatively transfers from a blue-shifted emitter to a red-shifted absorber by dipole–dipole coupling. This study shows that plasmonics enables the opposite transfer direction, transferring the plasmonic energy towards the short-wavelength direction to induce charge separation in a semiconductor. Plasmon-induced resonance energy transfer (PIRET) differs from FRET because of the lack of a Stoke's shift, non-local absorption effects and a strong dependence on the plasmon's dephasing rate and dipole moment. PIRET non-radiatively transfers energy through an insulating spacer layer, which prevents interfacial charge recombination losses and dephasing of the plasmon from hot-electron transfer. The distance dependence of dipole–dipole coupling is mapped out for a range of detuning across the plasmon resonance. PIRET can efficiently harvest visible and near-infrared sunlight with energy below the semiconductor band edge to help overcome the constraints of band-edge energetics for single semiconductors in photoelectrochemical cells, photocatalysts and photovoltaics. Plasmon-induced resonance energy transfer is revealed and explored for solar energy harvesting from visible and near-infrared light.

Plasmon-induced resonance energy transfer for solar energy conversion

The authors review the physics of ultrafast dynamics in semiconductors and their heterostructures, including both the observed experimental phenomena and the theoretical description of the processes. These are probed by ultrafast optical excitation, generating nonequilibrium states that can be monitored by time-resolved spectroscopy. Light pulses create coherent superpositions of states, and the dynamics of the associated phase relationships can be directly investigated by means of many-pulse experiments. The commonly used experimental techniques are briefly reviewed. A variety of different phenomena can be described within a common theoretical framework based on the density-matrix formalism. The important interactions of the carriers included in the theoretical description are the phonon interactions, the interactions with classical and quantum light fields, and the Coulomb interaction among the carriers themselves. These interactions give rise to a strong interplay between phase coherence and relaxation, which strongly affects the non equilibrium dynamics. Based on the general theory, the authors review the physical phenomena in various semiconductor structures including superlattices, quantum wells, quantum wires, and bulk media. Particular results which have played a central role in understanding the microscopic origins of the relaxation processes are discussed in detail.

Theory of ultrafast phenomena in photoexcited semiconductors

The nonlinear response of single GaAs quantum dots is studied in femtosecond near-field pump-probe experiments At negative time delays, transient reflectivity spectra show pronounced oscillatory structure around the quantum dot exciton line, providing the first evidence for a perturbed free induction decay of the excitonic polarization Phase-disturbing Coulomb interactions between the excitonic polarization and continuum excitations dominate the optical nonlinearity on ultrafast time scales A theoretical analysis based on the semiconductor Bloch equations accounts for this behavior

Coherent nonlinear optical response of single quantum dots studied by ultrafast near-field spectroscopy.

Quasi-two-color 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 nonlinear change in reflectivity of a single quantum wire is mapped in real space and time. The experiments show that carrier relaxation in a single quantum wire occurs on a 100 fs time scale at room temperature.

Femtosecond near-field spectroscopy of a single gaas quantum wire

Quasi-one-dimensional carrier transport is studied in a single ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}/\mathrm{GaAs}$ quantum wire by combining two-color femtosecond pump and probe spectroscopy and near-field optical microscopy. Evidence is found for a transient nonambipolar electron transport along the wire axis on a picosecond time and 100 nanometer length scale.

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

Erratum: Coherent Nonlinear Optical Response of Single Quantum Dots Studied by Ultrafast Near-Field Spectroscopy [Phys. Rev. Lett.PRLTAO0031-9007 89 , 057401 (2002)]

Spatially resolved photoluminescence spectra of thin GaAs quantum wells are measured by near-field spectroscopy, and two-energy autocorrelation functions of the spectra are derived. We demonstrate distinctly different autocorrelation functions for a 3-nm-thick quantum well grown on a (311)A surface and for quantum wells grown on (100) substrates. The autocorrelation spectra of the (311)A GaAs quantum well are quantitatively described by a statistical disorder model with a single correlation length of 17 nm for the exciton center-of-mass motion. A shoulder in the autocorrelation spectrum at energies between 1 and 3 meV is identified as a signature of excitonic level repulsion in the disorder potential. In contrast, the characteristic feature in the autocorrelation spectra of the (100) quantum wells is an additional positive correlation peak at energies between 3 and 4 meV. This peak reflects the energy correlations between ground and excited exciton states in individual monolayer islands with a narrow size distribution. The results indicate that the disorder potential in these high-quality (100) GaAs quantum wells contains both contributions from monolayer islands and nanoroughness on a length scale of the exciton Bohr radius.

T. Guenther

Papers

Coherent nonlinear optical response of single quantum dots studied by ultrafast near-field spectroscopy.

Femtosecond near-field spectroscopy of a single gaas quantum wire

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

Erratum: Coherent Nonlinear Optical Response of Single Quantum Dots Studied by Ultrafast Near-Field Spectroscopy [Phys. Rev. Lett.PRLTAO0031-9007 89 , 057401 (2002)]

Near-field autocorrelation spectroscopy of disordered semiconductor quantum wells