T. Demel

Spectroscopy of quantum dots and antidots

Abstract The remarkable progress of submicron technology has made it possible to realize man-made low-dimensional electronic systems. Starting from two-dimensional electronic systems (2DES) in semiconductor heterostructures the electrons are further confined by lateral potentials acting on a submicron scale. This induces quantum confined energy states such that, for wires, a set of one-dimensional subbands with free dispersion in only one direction is formed or, for dots, artificial “atoms” with a totally discrete energy spectrum are obtained. A reversed structure with respect to quantum dots are “antidots” where geometrical holes are “punched” into an originally 2DES. These low-dimensional systems exhibit unique properties. In this review we would like to discuss some recent results on far-infrared excitations in quantum dots and antidots.

Plasma and single particle excitations in quasi-one-dimensional electron systems

Abstract The equilibrium properties and dynamic response of quasi-one-dimensional electron systems (1DES) have been calculated for a confinement modelled by a harmonic oscillator potential. These calculations are compared with self-consistently calculated 1D subband separations in a split-gate configuration. It is shown that in the limit of a vanishing occupation the classical dynamic response frequency coincides with the subband separation. With increasing number of occupied 1D subbands the dynamic response frequency decreases slightly whereas the subband separation decreases drastically. Thus, for a large number of occupied 1D subbands, the dynamic response frequency is significantly higher than the 1D subband separation as is observed in experiments.

One-dimensional electronic systems in ultra-fine mesa etched InGaAs-InAlAs-InP quantum wires

Abstract Quantum wire structures have been prepared by deep mesa etching of modulation doped InGaAs-InAlAs-InP heterostructures. In very narrow wires (width t ≈ 300 nm) it was possible to realize one-dimensional electronic systems (IDES) with quantum confined energy levels. The separation of the ID subbands was, as determined from magnetic depopulation, about 2.5 meV.

Far-infrared spectroscopy of one- and zero-dimensional electronic systems

Abstract We review the far infrared (FIR) response of one- and zero-dimensional electronic systems (0DES and 1DES), quantum wires and quantum dots, respectively, which have been realized by ultrafine mesa etching of modulation-doped AlGaAs/GaAs heterostructures. From dc magnetotransport measurements on quantum wires the formation of 1D subbands with typical energy separations of 1 to 3 meV was found in electron channels of 400 to 150nm width. However, in the far infrared (FIR) response resonances at significantly higher frequencies were observed as compared to the single particle energy level separation. We will discuss that the optical response exhibits a very complex behaviour which is dominated by collective effects. In the case of a purely parabolic external potential the FIR response can be easily understood as the excitation of a rigid oscillation of the whole electron distribution. Especially in our quantum dot structures we are able to resolve additional excitation modes and a resonant anticrossing, probing the internal motion of the electrons within a single dot. We will discuss these excitations starting from two different models, namely from a classical one, which is based on plasmonic excitations in 2DES of finite size, and, in more details, from a quantum mechanical model, which treats atom-like systems with discrete energy levels including collective corrections.

Grating coupler effects on inelastic light scattering by plasmons in micro structured GaAs MQW systems

Abstract Plasmons in modulation doped MQW systems are studied by micro Raman spectroscopy on unstructured samples and after preparation of a grating in the upper part of the sample. The structured samples show a series of additional peaks in the energy range of the plasmon dispersion measured on the unstructured sample using different scattering geometries. They are attributed to plasmons excited by the grating coupler effect of the wire system.

Spectroscopy on One-Dimensional Electronic Systems

Single and multi-layered quantum wire structures have been prepared starting from modulation doped AlGaAs/GaAs heterostructures and multi-quantum well systems. The energy spectrum of these one-dimensional electronic systems (1DES) can be characterized by do magnetotransport measurements. Quantum wires with 400 to 150nm wide electron channels exhibit typical energy separations of 1 to 3meV for the 1D subbands. The far infrared (FIR) response of these systems is strongly governed by collective effects, which gives the FIR resonances the character of local plasmon modes. In the multi-layered quantum wire structures optical and acoustical type of layer-coupled plasmon modes are observed.

DYNAMIC EXCITATIONS IN QUANTUM WIRES, QUANTUM DOTS AND ANTIDOTS

With the remarkable advances in submicron technology it is now possible to realize man-made low-dimensional electronic systems, i.e., quantum wires, quantum dots and antidots. Far-infrared spectroscopy should give a most direct access to the quantum confined energy levels in these systems. It turns out that the dynamic response of these systems exhibits a very interesting complex interplay of atom-like single-particle and manybody effects which will be reviewed in this paper.

Self-Consistent Screening, Single Particle Energy and Plasmon Excitation in a Quasi-One-Dimensional Electronic System

A consistent theory is discussed for the self-consistent screening, the single-particle energy and the dipole plasmon frequency in a one-dimensional electronic system (IDES) with a parabolic confining potential. Comparison with experiments allows a consistent characterisation of the parameters of the IDES.

Quantum dots in In.47Ga.53AsInAlAsInP heterostructures

Abstract Quantum dots with geometrical diameter of about 400 nm have been prepared by deep mesa etching starting from modulation doped InGaAsInAlAsInP heterostructures. From microwave and far infrared (FIR) transmission experiments we find that the dots contain about 600 electrons per dot on discrete quantum confined energy levels with 1 meV separation. The FIR response is strongly governed by collective effects and shows in a magnetic field a complex mode structure.

Spectroscopic Investigations of Quantum Wires and Quantum Dots

The remarkable progress of submicron technology in the eighties has made it possible to realize quantum wires and quantum dots. Starting from two-dimensional electronic Systems (2DES) in semiconductor heterostructures electrons are further confined by lateral Potentials acting on a submicron scale, which induce quantum confined energy states such that, for wires, a set of 1D subbands with free dispersion in only one direction is formed or, for dots, artificial “atoms” with a totally discrete energy spectrum are obtained. These low-dimensional Systems exhibit unique properties. We give an introduction to this field of 1DES and ODES by reviewing far-infrared excitations in quantum wires and dots and photoluminescence studies of quasi 1D excitons in quantum wires.