Spin-flip Raman scattering (SFRS) is an example of electronic process in a much broader field of inelastic light scattering. Unlike conventional Raman scattering by optical and acoustic phonons, where the initial and final electronic states are the same but the number of elementary excitations (phonons) changes, the initial and final electronic states in SFRS are the different spin-states of electrons and holes. The magnitude of the Raman shift in the SFRS process corresponds to the Zeeman energies of electronic states, which are split by magnetic field. For example, the Raman peak, which corresponds to the electronic Spin-Flip process, has the Raman shift with respect to the laser line equal to ge×mB×B, where ge is the electron g-factor, mB is the Bohr magneton, and B is the external magnetic field. In undoped nanostructures, SFRS can be described as the double resonant process of electronic spin-flip assisted by emission or absorption of acoustic phonons.
SFRS is a very powerful approach for investigation of new materials and nanostructures. This technique has several unique features: SFRS allows one to measure the g-factors of intrinsic excitons, as well as g-factors of electrons and holes, and thus band parameters and their modification under the influence of the quantum confinement conditions can be determined. Even in the presence of large inhomogeneous broadenings, SFRS yields sharp Raman lines that can be analyzed easily. In addition, by tuning the angle between the quantum confinement planes and the external magnetic field, the symmetry of electron and hole g factor tensors can be determined separately, unlike, e.g., in electron-spin resonance or photoluminescence measurements. SFRS can also be applied to study excitons localized at quantum well defects and interface roughness or excitons confined in quantum dots formed by self-organized growth. The SFRS efficiency resonates strongly at electronic interband transitions. Thus, for appropriate excitation energies, SFRS has unique sensitivity to intrinsic or extrinsic (defect-related) material properties.
In recent years, SFRS has been applied intensively to semiconductor nanostructures. The first observation of the SFRS in semiconductor Quantum Dots was reported by A.A. Sirenko et. al in 1996. Two years after, A.A. Sirenko and co-workers reported the first observation of the Spin-Flip process in CdS quantum dots at room temperature. Among other topics studied by Andrei Sirenko using SFRS are acceptor-bound and intrinsic excitons in GaAs/AlGaAs quantum wells and GaAs/AlAs superlattices, quantum size effects and anisotropies of g factors in CdTe/CdMgTe quantum wells, localized excitons in quantum dots and nanostructures formed by self-organized growth (InAs/GaAs, InP/InGaP) or by diffusion (CdS) in a glass matrix.
Figure 1. (a) Spin-Flip Raman Scattering (SFRS) spectra for CdS quantum dots with the radius of 87 Å measured at different magnetic field values B = 4 T, 6 T, 8 T, 10 T, 12 T, and 14 T .
(b) SFRS spectra in bulk CdS crystal measured in Faraday and Voigt configurations for B =12 T.
The sharp peaks correspond to the flip of electron spin assisted by emission (Stokes part of the spectrum) or absorption (anti-Stokes part of the spectrum) of acoustic phonon with the energy equal to the Zeeman splitting between electron spin states. The neutral density filter has been used to attenuate the intensity of the laser line, which is labeled with L at zero Raman shift.
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