J. Seufert

Stark effect and polarizability in a single CdSe/ZnSe quantum dot

The quantum-confined Stark effect in a single self-assembled CdSe/ZnSe quantum dot was studied by means of highly spatially resolved photoluminescence spectroscopy. A nanotechnological approach making use of a capacitor-like geometry enabled us to apply a well-defined lateral electric field on the quantum dots. Stark shifts of up to 1.1 meV were obtained, which can be well fitted by a purely quadratic dependence on an electric field. In quite good agreement with theoretical calculations, an exciton polarizability of 4.9×10−3 meV/(kV/cm)2 can be extracted, while the permanent dipole moment in the lateral direction is found to be negligible.

Spectral diffusion of the exciton transition in a single self-organized quantum dot

We report on reversible spectral shifts in the emission spectra of self-organized CdSe single quantum dots on a time scale of seconds. Energy shifts of up to 3.5 meV have been observed and can be attributed to the Stark effect caused by fluctuating local electric fields. Most surprisingly, the energy shift turns out to be quasi-periodic with time constants between 70 and 190 s.

Quantum dot formation by segregation enhanced CdSe reorganization

The influence of the growth conditions during capping of CdSe/ZnSe quantum structures grown on GaAs(001) by molecular-beam epitaxy (MBE) were systematically investigated by high-resolution x-ray diffraction, transmission electron microscopy, and temperature dependent, partly time-resolved photoluminescence spectroscopy. The results clearly indicate formation of quantum wells with potential fluctuations if conventional MBE is used for capping the CdSe by ZnSe. In contrast, quantum dot formation occurs using migration enhanced epitaxy for this growth step. In the latter case, quantum dots can be obtained without formation of stacking faults.

Widely tunable quantum cascade lasers with coupled cavities for gas detection

The authors report the fabrication of widely tunable monolithic quantum cascade lasers (QCLs) with coupled Fabry–Perot (FP) cavities on indium phosphide. Quasicontinuous tuning of the single mode emission over a total spectral range of 242 nm was realized at two regions between 8.394 and 8.785 μm. An absorption experiment with ammonia shows principle feasibility of gas detection with multisegment QCL devices. Good agreement of the experimentally observed tuning behavior with the one expected from calculated FP mode-combs indicates that the change in the refractive index is mainly due to thermal heating as a result of current injection.

Single-electron charging of a self-assembled II-VI quantum dot

We have studied single-electron injection into individual self-assembled CdSe/ZnSe quantum dots. Using nanostructured contacts to apply a vertical electric field, excess electrons are promoted to the single-quantum-dot ground state in a controlled fashion. Spatially-resolved photoluminescence spectroscopy is applied to demonstrate single-quantum-dot charging via the formation of single zero-dimensional charged excitons with a binding energy on the order of 10 meV.

Widely tunable single-mode quantum cascade lasers with two monolithically coupled Fabry-Pérot cavities

The authors report on tunable, miniaturized, two-segment coupled cavity quantum cascade lasers fabricated in a process with a single dry etch step. They observed controllable vernier-effect-based mode switching, induced by the spectral shift of the short cavity’s Fabry-Perot mode comb due to Joule heating. With the help of this discontinuous tuning mechanism, single-mode operation with routinely ∼20dB side mode suppression ratio was achieved over a broad spectral range. The authors measured a quasicontinuous tuning range of 12cm−1 (138nm) and an entire single-mode tuning range of 16.5cm−1 (190nm), centered around 933cm−1 (10.7μm).

Manipulating single quantum dot states in a lateral electric field

Abstract We demonstrate measurements of the quantum confined Stark effect on single self-assembled CdSe/ZnSe quantum dots. For this purpose, a nano-scaled capacitor was developed being capable of exerting lateral electric fields up to 15 kV / cm on single quantum dots. Stark shifts of up to 1.1 meV have been obtained for the single exciton emission accompanied by a line width broadening due to field-induced carrier tunneling into the barrier. Evaluating the redshift of the luminescence signal as a function of the electric field enables us to extract the polarizability as well as information on the permanent dipole of a single quasi-zero-dimensional exciton.

Widely tunable laterally coupled distributed feedback laser diodes for multispecies gas analysis based on InAs/InGaAs quantum-dash material

Applying the concept of binary superimposed gratings, widely tunable single-mode laser diodes suitable for multispecies gas detection in the 1.8 microm wavelength range could be manufactured on InAs/InGaAs quantum dash-in-a-well material. A discrete wavelength tuning range of 21 nm as well as continuous tuning over 0.8 nm are demonstrated. Water and hydrogen chloride could be detected at absorption lines 13 nm apart.

Photonic crystal quantum cascade lasers with improved threshold characteristics operating at room temperature

We fabricated two-dimensional photonic crystal GaAs/AlGaAs quantum cascade lasers operating at room temperature. Compared to reference devices with cleaved facets, the high reflectivity photonic crystal mirrors lead to a ~34% reduction of the threshold currents

Optical spectroscopy on non-magnetic and semimagnetic single quantum dots in external fields

We demonstrate the ability to control the eigenstates in single quantum dots by applying well-defined external fields. Electric fields oriented in-plane as well as perpendicular to the disc-shaped dots allow a modification of the spatial part of the excitonic wavefunction, giving access to the charge distribution in the dot. In contrast, magnetic fields modify the spin part of the wavefunction, resulting in a Zeeman splitting and a diamagnetic shift of the photoluminescence emission. We used the unique property of semimagnetic quantum dots to tailor the effective g-factor, i.e. the sensitivity of the eigenstates to external magnetic fields, by about two orders of magnitude simply by varying the Mn concentration in the dots.