Effects of Photon-Momentum and Magnetic-Field Reversal on the Far-Infrared Electric-Dipole Spin Resonance in InSb

Far-infrared magnetotransmission experiments on $n$-type InSb in the parallel Voigt geometry ($\overrightarrow{\mathrm{E}}\parallel \overrightarrow{\mathrm{B}}\perp \overrightarrow{\mathrm{k}}$) conclusively demonstrate that the electric-dipole-excited spin resonance (EDSR) of conduction and donor-bound electrons in this material is allowed through lack of inversion symmetry. Quite unexpectedly for this geometry, the EDSR spectra also reveal an anomalous dependence on the sign of either $\overrightarrow{\mathrm{B}}$ or $\overrightarrow{\mathrm{k}}$. It is shown, through time-reversal-symmetry arguments, that the above behavior represents an explicit contribution of the photon momentum to the dielectric response function.