J. A. H. Stotz

Coherent spin transport through dynamic quantum dots

Long coherence lifetimes of electron spins transported using moving potential dots are shown to result from the mesoscopic confinement of the spin vector. The confinement dimensions required for spin control are governed by the characteristic spin-orbit length of the electron spins, which must be larger than the dimensions of the dot potential. We show that the coherence lifetime of the electron spins is independent of the local carrier densities within each potential dot and that the precession frequency, which is determined by the Dresselhaus contribution to the spin-orbit coupling, can be modified by varying the sample dimensions resulting in predictable changes in the spin-orbit length and, consequently, in the spin coherence lifetime.

Calculation and experimental verification of the acoustic stress at GHz frequencies in SAW resonators

High power applications of surface acoustic wave (SAW) devices may lead to acoustomigration in their thin metal electrodes, which deteriorates the performance or may even destroy the SAW device. It is confirmed in this paper that the mechanism of acoustomigration is caused by the SAW-induced stress in the metal. The quantitative calculation of this stress are shown in detail, starting from the widely used P-Matrix model as a standard analysis tool. The combination with the partial wave method (PWM) yields the stress distribution inside the metal. This approach provides the flexibility to determine the stresses for any given point in a SAW device, for any input power, frequency, wavetype, device geometry, or metal layer. In order to confirm the absolute values of the stress components, we calculated and measured displacements as a function of input power and frequency.

Enhanced spin coherence via mesoscopic confinement during acoustically induced transport

Long coherence lifetimes of electron spins transported using moving potential dots are shown to result from the mesoscopic confinement of the spin vector. The confinement condition to control electron spin dephasing is governed by the relation between the characteristic spin–orbit length of the electron spins and the dimensions of the dot potential, which governs the electron spin coherence lifetime. The spin–orbit length is a sample-dependent parameter determined by the Dresselhaus contribution to the spin–orbit coupling and can be predictably controlled by varying the sample geometry. We further show that the coherence lifetime of the electron spins is independent of the local carrier densities within each potential dot, which suggests the possibility of coherent, long-range transport of single electron spins.

A gaas phononic crystal with shallow noncylindrical holes

A square lattice of shallow, noncylindrical holes in GaAs is shown to act as a phononic crystal (PnC) reflector. The holes are produced by wet-etching a GaAs substrate using a citric acid:H2O2 etching procedure and a photolithographed array pattern. Although nonuniform and asymmetric etch rates limit the depth and shape of the phononic crystal holes, the matrix acts as a PnC, as demonstrated by insertion loss measurements together with interferometric imaging of surface acoustic waves propagating on the GaAs surface. The measured vertical displacement induced by surface phonons compares favorably with finite-difference time-domain simulations of a PnC with rounded-square holes.

InAsP Quantum Dots in InP Nanowire Waveguides as Sources of Quantum Light

Quantum dots, a 3D analogue of the quantum potential well, have recently become a commonplace tool for production of non-classical light (Buckley S, Rivoire K, Vu J: Rep Prog Phys 75:126503, 2012; Lodahl P, Mahmoodian S, Stobbe S: Rev Mod Phys 87:347–400, 2015)

Acoustically‐driven Single Photon Sources on (311)A GaAs

We employ surface acoustic waves to control the transfer of photo‐generated carriers between interconnected quantum wells and wires grown on pre‐patterned (311)A GaAs substrates. The wires are embedded at photo‐lithographically defined positions within (Al,Ga)As/GaAs quantum well. Optical studies on these structures have shown sharp PL lines and antibunched photons with tunable emission energy, revealing the presence of several recombination centers within the wire. The spatial separation of these recombination centers emitting single photons is determined from time‐resolved measurements.

Electron spin dynamics during transport using moving quantum dots

We present measurements showing that the long coherence lengths achieved during the transport of electron spins using surface acoustic waves result from the confinement of the spins within moving quantum dots. This confinement reduces the D’yakonov‐Perel’ spin dephasing effects that are prominent in GaAs quantum wells. As a result, the spin coherence lengths are independent of the number of electrons in each dot thus providing the ability to transport a single electron spin in a well‐defined moving potential.

Mobile potential dots in GaAs quantum wells

Confined and mobile potential dots (dynamic dots, DDs) are created using two orthogonally propagating surface‐acoustic‐wave beams. Using spatially and time‐resolved photoluminescence measurements, the compressive and tensile strain fields at the DD centers have been imaged by analyzing the polarization‐dependent luminescence from charge carriers transported by the DDs.