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Logo: Institute for Solid State Physics – Nanostructures Group
Logo Leibniz Universität Hannover
Logo: Institute for Solid State Physics – Nanostructures Group
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Research Projects

Optical Investigations on Ultra Pure 28Si:P

Donor bound electron spins in an isotopically enriched 28Si:P matrix exhibit ultra-long spin coherence at low temperatures, due to the non-existing nuclear spin of the host material, resulting in a suppression of the hyperfine interaction.

We investigate optically the addressability, optical selection rules and coherence time of the donor bound electrons in this material system.

Beside standard methods such as photoluminescence and Hanle-depolarization, we will employ semiconductor spin noise spectroscopy. As a long-term goal, the possibility of entanglement of two solid state systems as a potential quantum-repeater based on 28Si:P will be inspected. 

Spin Dynamic of Single Spins in Quantum Dots

We adress all optically the hole spin of individual (InGa)As quantum dots via spin noise spectroscopy.

The measurements are performed in an ultra-stable home-built sample inset. The quantum dot can be selected by moving the sample with piezo positioners. Additionally, there is a 2D vector magnet up to 30 mT.

The experiments are worked out at 4 K with a typical spin noise setup in reflection geometry.

The experiments yield a very long T1 spin lifetime. This time strongly depends on the external magnetic field. By the direction of the magnetic field we can vary the measured spin lifetime - either the longitudinal T1 or the transverse T2 spin lifetime. In case of an external magnetic field parallel to the probe laser direction, we also enlarge the lifetime of the hole spins due to effects that are unknown yet. The system under investigation is an eminent model system of the central spin problem.

Ongoing experiments will prospectively investigate the spin dynamics at higher external magnetic fields up to a few Tesla. Here, phonon assisted spin relaxation will be dominant and decrease the spin lifetime again.

Additionally, we aim for a deeper understanding of charge fluctuation processes in the closer surrounding of the quantum dot leading to a significant shift of the quantum dot resonance energy in time via the Stark shift.

Further, there is still an influence of the probe laser intensity on the measured quantum dot linewidth and spin lifetime, which is under investigation.

Interplay of Electron and Nuclear Spin Noise

The neutral exciton transition (D0X) of Si-donor bound electrons is probed resonantly and the sample represents an ensemble of central spins.

We measure the electron spin dynamic at 4.2K by spin noise spectroscopy. The difference spectrum shows unambiguously the hyperfine coupling. The theoretical model is based on finite correlation time and explains the lineshape of longitundal and transverse spin noise powers.
Further, the longitudinal spin lifetime is dependent on the excitation density and limited by the reorientation of nuclear spins.

Experimental spin noise difference spectrum of donor-bound electrons with the individuell contributions of transverse and longitudinal spin lifetime.

The experiments yield for low frequencies the nuclear spin noise of the different isotopes. By that, we perform all optical NMR at low magnetic fields and thermal equilibrium and detect signatures of small quadrupolar interactions.

Nuclear spin noise spectra as a function of the transverse magnetic field.

Time Resolved Photoluminescence of InGaN/GaN Quantum Wells

Group-III nitride based heterostructures contain an internal electric field that results in a reduced overlap of the wavefunctions of electron and hole and is the main reason for a low quantum efficency. We examine GaN/InGaN quantum wells (QW) and apply external stress in order to vary the internal piezoelectric fields and thereby influence the optical properties of the QWs. We measure time resolved photoluminescence (TRPL) and perform pump probe (PP) experiements on single and multiple GaN/InGaN QW under varied strain that is applied uniaxial, along the growth direction with a pressure cell. Additionally we measure the spin dynamics via Kerr rotation to gain information on the internal electric fields since the spin dephasing rate is directly related to this fields via the Rashba effect.

Streak camera image of temporal decay of the quantum well photoluminescence together with a single photoluminesce spectrum at fixed time and time trace at fixed energy.

Ultrafast Spin Noise Spectroscopy

The applicability of the experimental technique of spin noise spectroscopy is excented to the regime of very short timescales. The continous wave laser source is replaced by ultrafast laser oscillators and the time resolution is only limited by the pulse width of the laser system. By that, we can detect bandwidths of several hunderd gigahertz.

Experimental setup with two syncronized picosecond laser oscillators (a) and a schematic measurement sequence (b).

This technique of ultrafast spin noise spectroscopy is applied to highly-n-doped bulk GaAs. These measurements yield a large g-factor fluctuation possibly due to the stochastic nature of dopant distribution.

Typical spin correlation derivative of free electron spins precessing in a transverse magnetic field (top). Dependence of the spin dephasing time on the transverse magnetic field (bottom).