Time-resolved Kerr rotation and photoluminescence measurements are performed on MOCVD-grown monolayer tungsten diselenide (WSe2). We observe a surprisingly long-lived Kerr rotation signal (∼80 ns) at 10 K, which is attributed to spin/valley polarization of the resident holes. This polarization is robust to transverse magnetic field (up to 0.3 T). Wavelength-dependent measurements reveal that only excitation near the free exciton energy generates this long-lived spin/valley polarization.

We demonstrate a method to extend the range of pulsed laser spin noise measurements to long spin lifetimes. We use an analog time-domain detection scheme with a bandwidth limited only by laser pulse duration. Our model uses statistics and Bloch-Torrey equations to extract the Lande g-factor, Faraday cross-section σ F , and spin lifetime τ S , while accounting for finite detector response. Varying the magnetic field with a fixed probe-probe delay yields τ S when it is longer than the laser repetition period. Varying the probe-probe delay with a fixed field produces a time-domain measurement of the correlation function.Noise is an expression of a system's fundamental behavior. The fluctuation-dissipation theorem, 1,2 when combined with the Wiener-Khinchin theorem, 3 implies that a system's dynamic response can be extracted from correlation measurements. Spin noise was theoretically postulated in 1946 4 and then demonstrated on atomic gases, 5-7 nuclear spins, 8 and with scanning tunneling microscopy.9 More recently, spin noise spectroscopy on semiconductors 10,11 has revealed its potential for performing contactless and non-perturbative measurements that can also more directly access the Faraday crosssection.12 Experiments that utilize spin noise as a level of contrast have performed non-destructive spatial resolution of a semiconductor's dopant density 13 and homogeneous absorption linewidth measurements of quantum dot ensembles.14 Conventional spin noise spectroscopy utilizes a continuous wave laser transmitted through a sample to measure the Faraday rotation produced by stochastic spin fluctuations. A magnetic field is applied orthogonal to the optical path to induce precession and measure the Lande g-factor. A power spectrum is produced either by using electronic sampling and Fourier transforming the data set 15,16 or using a spectrum analyzer. 10,17 Isolation of the spin noise contribution from a typically much larger background is accomplished by taking the difference of the finite power spectra for two system states. In these measurements, the bandwidth is limited by the sampling rate.In order to access high-frequency dynamics, theoretical 18 and experimental 19-21 efforts have included the use of ultrafast pulsed lasers. Ultrafast spin noise spectroscopy allows for a direct time-domain measurement of the spin correlation function, with a bandwidth determined by the laser pulse duration. 19,20However, the temporal range is limited by the optical delay time between probe pulses and, ultimately, the laser repetition period t rep , which makes it difficult to accurately determine the spin lifetime τ S when τ S > ∼ t rep . In this Letter, we demonstrate a Resonant Spin Noise (RSN) technique that enables accurate measurements a) Electronic mail: bpursley@umich.edu of the spin lifetime when τ S > ∼ t rep and clarify how to theoretically model the ultrafast spin noise signal and extract the Faraday cross-section. We compare our data and model with the model presented in Ref. 21 which predicts a Lorentzian peak at ...

Modern electronic devices utilize charge to transmit and store information. This leaves the information susceptible to external influences, such as radiation, that can introduce short timescale charge fluctuations and, long term, degrade electronic properties. Encoding information as spin polarizations offers an attractive alternative to electronic logic that should be robust to randomly polarized transient radiation effects. As a preliminary step towards radiation-resistant spintronic devices, we measure the spin properties of n-GaAs as a function of radiation fluence using time-resolved Kerr rotation and photoluminescence spectroscopy. Our results show a modest to negligible change in the long-term electron spin properties up to a fluence of 1x10 14 (5 MeV protons)/cm 2 , even as the luminescence decreases by two orders of magnitude.The vast majority of modern technology relies on controlling electronic charge within a circuit. Timing, location, and quantity of the charge are the fundamental parameters for logic operations. Anything that can disrupt control of these parameters is an information processing hazard. Radiation filled environments are a difficult challenge for electronic logic as particle collisions can randomly introduce large quantities of charge in the short term and degrade circuit properties in the long term. Spin based logic has been proposed as an alternative that would offer novel functionality 1-9 and the transmitted information should be inherently robust to short term charge effects. A radiation-resistant spintronic device should account for bursts of induced electrical current so that it would not be damaged, while accurately measuring the quantity of spin current. An ideal device would use a pure spin current such that charge and spin behavior are completely decoupled.In order to fabricate a radiation-resistant spintronic device, a material must be chosen that is relevant for spintronics applications and largely maintain its spin dependent properties after irradiation. In this paper, we explore the effects of irradiation on the spin properties of bulk Si-doped n-GaAs samples cleaved from an offthe-shelf wafer. We expose several samples to proton irradiation, and then characterize them using photoluminescence (PL) and gamma spectroscopy. We then perform resonant spin amplification (RSA) using pumpprobe Kerr rotation to extract the spin dependent parameters. Our results show that the spin lifetime and g-factor of bulk n-GaAs, doped near the metal-to-insulator transition, is largely unaffected by proton irradiation. We recommend n-GaAs for further study as a candidate for radiation-resistant spintronic devices.All samples were cleaved into 4 mm x 4 mm x 0.5 mm