Landé g Factor of Excitons from Circularly Polarized Low-Field Magnetomodulated Reflectance Spectroscopy: Application to Bulk 2H -

Measurement of the Land\'e $g$ factor (LGF) normally involves determining the energy splitting of spectral features under a dc magnetic field of several teslas. We show that circular-polarization-resolved magnetomodulation spectroscopy, with a small ac field of the order of 50 mT rms, can be used to measure the LGF of excitons in semiconductors. The technique is easy to implement with a microscope arrangement, enabling measurements with high spatial resolution at cryogenic temperatures. We validate the technique and determine its limits through measurements on excitons in a ${\text{GaAs/Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum well. We then apply it to microscopic flakes of bulk $2H$-${\mathrm{Mo}\mathrm{S}}_{2}$ and determine the LGF of the ground-state ${A}_{1s}$ exciton and the next-higher-energy exciton labeled $IL$, at energies around the direct gap of $2H$-${\mathrm{Mo}\mathrm{S}}_{2}$. The LGF of $IL$ is found to be much larger in magnitude and of opposite sign compared to that of ${A}_{1s}$. This suggests that it cannot be an excited state of the $A$ exciton as was previously assigned. Using an approximate model, the LGF value of the $IL$ exciton is shown to be consistent with an interlayer exciton that was earlier identified in few-layer $2H$-${\mathrm{Mo}\mathrm{S}}_{2}$.