All-optical NMR in semiconductors provided by resonant cooling of nuclear spins interacting with electrons in the resonant spin amplification regime

Resonant cooling of different nuclear isotopes manifested in optically induced nuclear magnetic resonances (NMR) is observed in $n$-doped CdTe/(Cd,Mg)Te and ZnSe/(Zn,Mg)Se quantum wells and for donor-bound electrons in ZnSe:F and GaAs epilayers. By time-resolved Kerr rotation used in the regime of resonant spin amplification, we can expand the range of magnetic fields where the effect can be observed up to nuclear Larmor frequencies of 170 kHz. The mechanism of the resonant cooling of the nuclear spin system is analyzed theoretically. The developed approach allows us to model the resonant spin amplification signals with NMR features.

Figure 1

Photoluminescence spectra of studied structures: (a) 20-nm-thick CdTe/(Cd,Mg)Te QW; (b) ZnSe:F epilayer and 20-nm-thick ZnSe/(Zn,Mg)Se QW; (c) GaAs epilayer. Exciton (X), trion (T), donor-bound exciton ${\mathrm{D}}^0\mathrm{X}$, free exciton (FX), and donor-acceptor recombination (DA) lines are marked by arrows. $B=0$ T and $T=1.8$ K.

Figure 2

(a) Kerr rotation pump-probe signal of 20-nm-thick CdTe/(Cd,Mg)Te QW measured at trion resonance. $B=0.25$ T and $T=1.8$ K. Red arrow shows the time delay of $\Delta t=-80$ ps at which the RSA signals were detected. (b) RSA signals measured at the trion (blue line) and exciton (black line) resonances for $f_{\mathrm{m}}=50$ kHz. The red line shows the trion signal measured with an additional amplitude modulation of the pump beam at 10 Hz. $P_{\mathrm{pump}}=1$ W/${\mathrm{cm}}^2$ and $P_{\mathrm{probe}}=0.2$ W/${\mathrm{cm}}^2$. Inset (c) zooms the NMR features. Calculated $B_{\mathrm{NMR}}$ from Table 1 for different isotopes are marked by arrows.

Figure 3

RSA signals for ZnSe:F epilayer (a) and 20-nm-thick ZnSe/(Zn,Mg)Se QW (b), (c) measured for different modulation frequencies at $T=1.8$ K. $\Delta t=-35$ ps, $P_{\mathrm{pump}}=5$ W/${\mathrm{cm}}^2$, and $P_{\mathrm{probe}}=0.2$ W/${\mathrm{cm}}^2$. Calculated $B_{\mathrm{NMR}}$ from Table 1 are shown by arrows.

Figure 4

RSA signal for GaAs epilayer measured at resonant pumping of the ${\mathrm{D}}^0\mathrm{X}$ transition at $f_{\mathrm{m}}=42$ kHz. $T=1.8$ K. $\Delta t=-50$ ps, $P_{\mathrm{pump}}=5$ W/${\mathrm{cm}}^2$, and $P_{\mathrm{probe}}=0.2$ W/${\mathrm{cm}}^2$. Calculated $B_{\mathrm{NMR}}$ from Table 1 are shown by arrows.

Figure 5

Scheme of the electron-nuclear spin interaction process in the rotating coordinate system.

Figure 6

Calculated RSA spectra for CdTe-, ZnSe-, and GaAs-based QWs. Dashed (red) lines show RSA spectra without accounting for the nuclear effects. Solid (blue) lines show RSA spectra with the nuclear spin effects taken into account.