Optical cooling of nuclear spins in GaAs/(Al,Ga)As quantum wells at subkelvin temperatures: Evidence of the dynamic self-polarization of nuclear spins

TL;DR

This paper tackles achieving deep cooling and potential ordering of the nuclear spin system in GaAs-based quantum wells by optical pumping. It combines time-resolved Kerr rotation and optical orientation of photoluminescence to quantify dynamic nuclear polarization, Overhauser fields, and nuclear spin temperatures down to the subkelvin regime. Key findings include Overhauser fields up to about $B_N \approx 3.1$ T at 1.6 K and signatures consistent with dynamic self-polarization at 300 mK, with nuclear spin temperatures potentially as low as $\Theta_N \sim 200$ nK in the DSPNS regime. The results demonstrate efficient nuclear spin cooling in a wide GaAs QW and open avenues toward nuclear-spin ordering and long-lived quantum memory in semiconductor nanostructures.

Abstract

We investigate the dynamic polarization of nuclear spins in a nominally undoped GaAs/Al$_{0.35}$Ga$_{0.65}$As quantum well using two complementary experimental approaches: time-resolved Kerr rotation and optical orientation measurements of photoluminescence. Using the first technique, we measure a remarkably large Overhauser field of 3.1 T in a geometry close to the Faraday configuration for a 19.7 nm wide quantum well at a temperature of 1.6 K. A nuclear spin temperature of 6.4 $μ$K is measured at an external magnetic field of 0.006 T following an adiabatic sweep from 0.6 T. Despite the quadrupole-induced nuclear spin splitting inherent to nanostructures, the nuclear spin system is found to follow the predictions of spin temperature theory. Using the optical orientation of the photoluminescence, we investigated nuclear spin dynamics at millikelvin temperatures down to 300 mK. At a temperature of 500 mK, an Overhauser field of 160 mT is generated in an oblique but nearly Voigt magnetic field using low optical power to avoid heating. The nuclear polarization build-up time is of 150 s, consistent with earlier reports at higher temperatures, where hyperfine scattering on free photoexcited electrons governs relaxation. At 500 mK, the onset of dynamic self-polarization of nuclear spins is observed, which becomes more pronounced as the lattice temperature is further reduced to 300 mK. The estimated nuclear spin temperature in the dynamic self-polarization regime can be as low as 200 nK.

Figures (11)

• Figure 1: Time-resolved Kerr rotation setup. Magnetic field components $B_x$, $B_y$ and $B_z$ indicate field orientations corresponding to Voigt and Faraday geometries. For a circularly polarized pump with constant $\sigma^+$ helicity, a $\lambda/4$ wave plate was used, which was replaced by a photoelastic modulator (PEM) for a modulated $\sigma^+/\sigma^-$ pump.
• Figure 2: Setup for DNP and optical nuclear cooling. NPBS is non-polarizing beam splitter. The magnetic field components $B_x$, $B_y$ and $B_z$ indicate field orientations corresponding to Voigt and Faraday geometries.
• Figure 3: Photoluminescence spectrum of the three widest out of thirteen GaAs/Al$_{0.35}$Ga$_{0.65}$As quantum wells measured at $T = 1.6$ K. The widths of the quantum wells are shown near their respective spectral lines, while arrows mark the position of the exciton (X) and trion (X$^-$) lines for the 19.7-nm-wide quantum well. Excitation energy $E_{exc} = 1.823$ eV and power $P = 1$ mW.
• Figure 4: Time-resolved Kerr rotation signals measured at $T = 1.6$ K in a magnetic field of $B = 3$ T tilted by $\theta = 75^\circ$. The pump was modulated between $\sigma^+$ and $\sigma^-$ polarization at 50 kHz, with energy $E_{pump} = 1.527$ eV and power $P_{pump} = 10$ mW. The red line is a fit to Eq. \ref{['eq:TRKR_fit']}.
• Figure 5: Comparison of TRKR signals measured at $T = 1.6$ K in tilted magnetic field of $B = 3$ T at: (a) $\theta = 75^\circ$, (b) $\theta = 45^\circ$ and (c) $\theta = 15^\circ$. The pump has constant $\sigma^+$ circular polarization, $E_{pump} = 1.527$ eV, and $P_{pump} = 10$ mW. The red lines are fits using Eq. \ref{['eq:TRKR_fit']} with parameters given in Table \ref{['tab:nuclear_fields']}.