## 2 Dynamic Nuclear Polarization in Semiconductor Nano-and Microstructures
### Optically Induced DNP in Semiconductors
#### 2.1.4 Optical Spin Orientation in Various Semiconductors
By increasing the depth of the donor levels (Figure 9), a very significant nuclear polarization was achieved because of the increase of the electron spin relaxation time. Figure 7: Limiting relative polarization \(\left(\frac{\langle I_{2}\rangle}{T_{0}}\right)_{m}\) of the \({}^{29}\)Si nuclei as a function of the concentration \(n_{\text{ph}}\) of photoexcited electrons in \(n\)-type Si irradiated with unpolarized light. (Reproduced from Reference [27], with the permission of American Institute of Physics). The mechanism of optical pumping can be considerably more complicated and less well understood. Both contact hyperfine and dipolar hyperfine interactions can contribute appreciably to cross-relaxation. The spin polarization of the photoexcited electrons may depend on the pumping wavelength and the magnetic field. The precise nature of the photoexcitation that dominate optical pumping, which are expected to be localized excitations, may be unknown. The roles of photogenerated holes, doped carriers and radiationless transitions to a metastable excited state after the initial absorption of a photon are to be clarified [31]. A theory of the optical spin orientation under indirect optical injection of carriers and spins in bulk Si, proposed in Reference [33], predicted the maximum degree of spin polarization as high as 25% at low temperatures and 15% at high temperatures. These values of the degree of the spin polarization implied the term _\"optically induced spin hyperpolarization\"_ for the phenomenon, which is  
Figure 8: Dependence of the \({}^{31}\)P optically-pumped nuclear magnetic resonance (NMR) lineshape on the period during which the shutter is closed \(\tau_{D}\), with the period during which the shutter is open \(\tau_{L}\) = 120 s. At short \(\tau_{D}\), the lineshape is dominated by signal from the optically-pumped surface-layer. At long \(\tau_{D}\), the lineshape is dominated by signal from the bulk of the InP sample which increases due to spin-lattice relaxation. (Reprinted from Reference [31], © (1998), with permission from Elsevier). Figure 9: \({}^{113}\)Cd NMR spectra of the high-resistivity CdS single crystal recorded at 2 K. The Boltzmann signal is shown in (**a**), the _optically enhanced_ signal after an irradiation time \(\tau_{L}\) = 5600 s at 2.60 eV with circularly polarized light and 520 mW laser power (**c**). The control measurement (**b**) was obtained under the same conditions as (**c**) but with the shutter closed during \(\tau_{L}\). (Reprinted figure with permission from Reference [32] © (1999) by the American Physical Society). the main objective of the present review. The theory of Reference [33] was immediately applied to analyze two-photon indirect optical injection of carriers and spins and two-color coherent control in bulk Si [34]. Measurements of the electron spin lifetime and optical orientation efficiency were also carried out in Ge by using electrical detection of radio frequency modulated spin polarization [35]. The optical spin injection and spin lifetime in Ge heterostructures were experimentally investigated in Reference [36]. Th valley-dependent spin polarization and long-lived electron spins were detected in Ge [37]. Spin-dependent optical properties were investigated as a function of strain in Si and Ge [38]. Theory of spin-dependent intravalley and intervalley electron-phonon scatterings was developed for Ge [39]. The optical spin-selection rules in bulk Si and Ge/Si quantum dots were experimentally unveiled in Reference [40]. Two optical routes of cold carrier injection in Si were revealed by time-resolved excitation spectroscopy [41]. The optical generation of pure spin currents was detected at the indirect gap of bulk Si [42]. Hot phonon and carrier relaxation in Si single crystalline wafers with the (100) crystallographic orientation were determined by transient extreme ultraviolet spectroscopy [43]. The spin-to-charge conversion for hot photoexcited electrons was experimentally detected in Ge [44].