S. Schön

Measurement of Rashba and Dresselhaus spin–orbit magnetic fields

Spin–orbit coupling is a manifestation of special relativity. In the reference frame of a moving electron, electric fields transform into magnetic fields, which interact with the electron spin and lift the degeneracy of spin-up and spin-down states. In solid-state systems, the resulting spin–orbit fields are referred to as Dresselhaus and Rashba fields, depending on whether the electric fields originate from bulk or structure inversion asymmetry, respectively. Yet, it remains a challenge to determine the absolute value of both contributions in a single sample. Here, we show that both fields can be measured by optically monitoring the angular dependence of the electrons’ spin precession on their direction of motion with respect to the crystal lattice. Furthermore, we demonstrate spin resonance induced by the spin–orbit fields. We apply our method to GaAs/InGaAs quantum-well electrons, but it should be universally useful to characterize spin–orbit interactions in semiconductors, and therefore could facilitate the design of spintronic devices.

Low-loss GaInNAs saturable absorber mode locking a 1.3-μm solid-state laser

We have demonstrated stable self-starting passive cw mode locking of a solid-state laser at about 1.3 μm using a GaInNAs semiconductor saturable absorber mirror (SESAM). GaInNAs SESAMs show negligible nonsaturable losses, low saturation fluences (11 μJ/cm2) and picosecond decay times which make them well-suited for self-starting and stable cw mode locking. Sub-10-ps pulses were produced with a Nd:YLF laser at 1314 nm. The incorporation of about 2% nitrogen into InGaAs redshifts the absorption edge above 1330 nm and reduces the strain in the saturable absorber grown on a GaAs/AlAs Bragg mirror. Final absorption edge adjustments have been made with thermal annealing which blueshifts the absorption edge.

Piezoresistive InGaAs/GaAs Nanosprings with Metal Connectors

This paper presents the fabrication, assembly, and characterization of piezoresistive nanosprings for creating nanoelectromechanical systems. The fabrication process is based on conventional microfabrication techniques to create a planar pattern in a 27nm thick, n-type InGaAs/GaAs bilayer that self-forms into three-dimensional structures during a wet etch release. As the nanosprings have lower doped thin and flexible layers, small metal pads have been attached to both sides for achieving stable ohmic contact with electrodes. Nanorobotic manipulation is applied to assemble the nanosprings between electrodes using electron-beam-induced deposition inside a scanning electron microscope, and the bridged nanosprings were then characterized for electromechanical properties. With their strong piezoresistive response, low stiffness, large-displacement capability, and excellent fatigue resistance, they are well-suited to function as sensing elements in high-resolution, large-range electromechanical sensors.

Self-Aligned Charge Read-Out for InAs Nanowire Quantum Dots

A highly sensitive charge detector is realized for a quantum dot in an InAs nanowire. We have developed a self-aligned etching process to fabricate in a single step a quantum point contact in a two-dimensional electron gas and a quantum dot in an InAs nanowire. The quantum dot is strongly coupled to the underlying point contact that is used as a charge detector. The addition of one electron to the quantum dot leads to a change of the conductance of the charge detector by typically 20%. The charge sensitivity of the detector is used to measure Coulomb diamonds as well as charging events outside the dot. Charge stability diagrams measured by transport through the quantum dot and charge detection merge perfectly.

Ultrabroadband AIGaAs/CaF2 semiconductor saturable absorber mirrors

Ultrabroadband semiconductor saturable absorber mirrors (SESAMs) are required to support self-starting sub-10-fs-pulse generation with Ti:sapphire lasers. Conventional AlxGa1−xAs/AlAs SESAMs are limited by the reflection bandwidth of about 60 nm of the bottom Bragg mirror. In this letter, we demonstrate a GaAs saturable absorber which is epitaxially grown on CaF2 using molecular-beam epitaxy. Even though the difference of the thermal expansion coefficient is very large, we were able to demonstrate good modulation depth with small nonsaturable losses. This is interesting for ultrabroadband SESAMs because the large refractive-index difference between CaF2 and AlxGa1−xAs results in very broadband AlxGa1−xAs/CaF2 Bragg mirrors extending over about a 400-nm-wide reflection bandwidth for a center wavelength of 850 nm.

Luminescence decay kinetics in homogeneously and delta-doped ZnS:Mn

The luminescence decay kinetics of homogeneously and delta-doped ZnS:Mn thin film phosphors was investigated. A quantitative model based on the hopping model of energy transfer theory was developed to described the concentration quenching phenomenon in ZnS:Mn. The model predicted the dependence of the energy transfer rate on material parameters such as the Mn and defect concentrations. The luminescence decay of homogeneously doped ZnS:Mn consisted of two exponential components at 10 K. The fast component of 120 μs was attributed to exchange-coupled pair emission and the slow component of 1.6 ms to isolated Mn ions. As the temperature was increased, the exchange-coupled pair emission disappeared and the decay became strongly nonexponential. The nonexponentiality was attributed to nonradiative energy transfer processes. The concentration dependence of the effective lifetime was also found to change with temperature. The investigation on the temperature dependence revealed two regimes of concentration which ...

Homogeneous and δ-doped ZnS:Mn grown by MBE

Abstract An investigation is reported on the growth of homogeneously and δ-doped ZnS:Mn layers. Both undoped and Mn-doped ZnS thin films were grown by MBE and GSMBE using a sulfur overpressure. Photoluminescence measurements on undoped material indicated that high crystal quality was obtained, with band edge emission intensity several orders of magnitude higher than that from the self-activation peak, for a growth temperature of 180°C. Homogeneously doped ZnS:Mn films showed a maximum brightness at a Mn concentration of 3 mol% Mn before luminescence quenching was observed. The δ-doping technique was shown to quadruple the photoluminescence intensity compared to equivalently doped homogeneous material grown under the same conditions. The optimum distance between doping planes for a constant in-plane concentration was found to be 9–15 nm. SIMS studies of the diffusion behavior of Mn into the undoped ZnS layers revealed broad Mn peaks with a FWHM of about 10 nm.

Detecting terahertz current fluctuations in a quantum point contact using a nanowire quantum dot

We use a nanowire quantum dot to probe high-frequency current fluctuations in a nearby quantum point contact. The fluctuations drive charge transitions in the quantum dot, which are measured in real-time with single-electron detection techniques. The quantum point contact (GaAs) and the quantum dot (InAs) are fabricated in different material systems, which indicates that the interactions are mediated by photons rather than phonons. The large energy scales of the nanowire quantum dot allow radiation detection in the long-wavelength infrared regime.

Counting statistics in an InAs nanowire quantum dot with a vertically coupled charge detector

A gate-defined quantum dot (QD) in an InAs nanowire is fabricated on top of a quantum point contact realized in a two-dimensional electron gas. The strong coupling between these two quantum devices is used to perform time-averaged as well as time-resolved charge detection experiments for electron flow through the quantum dot. We demonstrate that the Fano factor describing shot noise or time-correlations in single-electron transport depends in the theoretically expected way on the asymmetry of the tunneling barriers even in a regime where the thermal energy kBT is comparable to the single-particle level spacing in the dot.

Passively mode-locked 1.3-/spl mu/m multi-GHz Nd:YVO/sub 4/ lasers with low timing jitter

We demonstrate diode-pumped passively mode-locked 1.34-/spl mu/m Nd:YVO/sub 4/ lasers with repetition rates of 5 and 10GHz. Passive mode locking is achieved by using a novel GaInNAs-based saturable absorber mirror. Phase noise measurements prove the low timing jitter that can be obtained with this kind of lasers.