Polarization resolved Electron Spin Resonance in two-dimensional electron systems

Electron spin resonance (ESR) has long been used in two-dimensional electron systems (2DESs) to probe g-factor anisotropy, spin–orbit interactions, hyperfine coupling and various collective many-body spin phenomena. Most studies measure ESR as a peak in photoresistance of the Hall bars, where the excitation-field distribution and polarization are distorted, complicating the analyzis of the excitation conditions. On the contrary, we report polarization-resolved ESR in sub-THz transmission using a quasi-optical setup and large samples for high polarization purity. The 2DES is in a 4.5 nm AlAs quantum well with a single isotropic valley. In Faraday geometry the resonance appears only for circular polarization whose helicity is opposite to that of cyclotron resonance; in Voigt geometry the ESR amplitude shows a twofold dependence on the azimuthal angle of an in-plane field. These selection rules indicate Dresselhaus (bulk-inversion-asymmetry) spin–orbit coupling mediates electric-dipole absorption. We also study 15 nm AlAs wells with an active pseudospin; polarization resolution reveals rich pseudospin-dependent ESR features, identifying a microscopic pathway for electrically driven spin absorption in 2DEGs with implications for spintronics.