High resolution nuclear magnetic resonance spectroscopy of highly-strained quantum dot nanostructures

Much new solid state technology for single-photon sources, detectors, photovoltaics and quantum computation relies on the fabrication of strained semiconductor nanostructures. Successful development of these devices depends strongly on techniques allowing structural analysis on the nanometer scale. However, commonly used microscopy methods are destructive, leading to the loss of the important link between the obtained structural information and the electronic and optical properties of the device. Alternative non-invasive techniques such as optically detected nuclear magnetic resonance (ODNMR) so far proved difficult in semiconductor nano-structures due to significant strain-induced quadrupole broadening of the NMR spectra. Here, we develop new high sensitivity techniques that move ODNMR to a new regime, allowing high resolution spectroscopy of as few as 100000 quadrupole nuclear spins. By applying these techniques to individual strained self-assembled quantum dots, we measure strain distribution and chemical composition in the volume occupied by the confined electron. Furthermore, strain-induced spectral broadening is found to lead to suppression of nuclear spin magnetization fluctuations thus extending spin coherence times. The new ODNMR methods have potential to be applied for non-invasive investigations of a wide range of materials beyond single nano-structures, as well as address the task of understanding and control of nuclear spins on the nanoscale, one of the central problems in quantum information processing

Measurement of the Knight field and local nuclear dipole-dipole field in an InGaAs/GaAs quantum dot ensemble

We present a comprehensive investigation of the electron-nuclear system of negatively charged InGaAs/GaAs self-assembled quantum dots under the influence of weak external magnetic fields (up to 2 mT). We demonstrate that, in contrast to conventional semiconductor systems, these small fields have a profound influence on the electron spin dynamics, via the hyperfine interaction. Quantum dots, with their comparatively limited number of nuclei, present electron-nuclear behavior that is unique to low-dimensional systems. We show that the conventional Hanle effect used to measure electron spin relaxation times, for example, cannot be used in these systems when the spin lifetimes are long. An individual nucleus in the QD is subject to milli-Tesla effective fields, arising from the interaction with its nearest-neighbors and with the electronic Knight field. The alignment of each nucleus is influenced by application of external fields of the same magnitude. A polarized nuclear system, which may have an effective field strength of several Tesla, may easily be influenced by these milli-Tesla fields. This in turn has a dramatic effect on the electron spin dynamics, and we use this technique to gain a measure of both the dipole-dipole field and the maximum Knight field in our system, thus allowing us to estimate the maximum Overhauser field that may be generated at zero external magnetic field. We also show that one may fine-tune the angle which the Overhauser field makes with the optical axis.Comment: 13 pages, 7 figure

Spin-valley dynamics in alloy-based transition metal dichalcogenide heterobilayers

Van der Waals heterobilayers based on 2D transition metal dichalcogenides have been recently shown to support robust and long-lived valley polarization for potential valleytronic applications. However, the roles of the chemical composition and geometric alignment of the constituent layers in the underlying dynamics remain largely unexplored. Here we study spin-valley relaxation dynamics in heterobilayers with different structures and optical properties engineered via the use of alloyed monolayer semiconductors. Through a combination of time-resolved Kerr rotation spectroscopic measurements and theoretical modeling for Mo1 − xWxSe2/WSe2 samples with different chemical compositions and stacking angles, we uncover the contributions of the interlayer exciton recombination and charge carrier spin depolarization to the overall valley dynamics. We show that the corresponding decay rates can be tuned in a wide range in transitions from a misaligned to an aligned structure, and from a hetero- to a homo-bilayer. Our results provide insights into the microscopic spin-valley polarization mechanisms in van der Waals heterostructures for the development of future 2D valleytronic devices

Manipulation of the nuclear spin ensemble in a quantum dot with chirped magnetic resonance pulses

The nuclear spins in nanostructured semiconductors play a central role in quantum applications(1-4). The nuclear spins represent a useful resource for generating local magnetic(5) fields but nuclear spin noise represents a major source of dephasing for spin qubits(2,3). Controlling the nuclear spins enhances the resource while suppressing the noise. NMR techniques are challenging: the group III and V isotopes have large spins with widely different gyromagnetic ratios; in strained material there are large atom-dependent quadrupole shifts(6); and nanoscale NMR is hard to detect(7,8). We report NMR on 100,000 nuclear spins of a quantum dot using chirped radiofrequency pulses. Following polarization, we demonstrate a reversal of the nuclear spin. We can flip the nuclear spin back and forth a hundred times. We demonstrate that chirped NMR is a powerful way of determining the chemical composition, the initial nuclear spin temperatures and quadrupole frequency distributions for all the main isotopes. The key observation is a plateau in the NMR signal as a function of sweep rate: we achieve inversion at the first quantum transition for all isotopes simultaneously. These experiments represent a generic technique for manipulating nanoscale inhomogeneous nuclear spin ensembles and open the way to probe the coherence of such mesoscopic systems