Time-Domain Magnetic Field-Difference Spectroscopy for Semiconductors Using Circularly Polarized Terahertz Pulses

We propose a simple method for sensitive magnetooptic spectroscopy in the terahertz (THz) frequency regime without the need for detecting two orthogonally polarized field components. This simple method is based on the time-domain reflection spectroscopy and employs a single circular polarization state for probing free carriers under a magnetic field that is modulated by a rotating disk with permanent magnets. The evaluated modulation signal directly reflects the Faraday/Kerr ellipticity and optical rotation, which are sensitive to the characteristics of free carriers in semiconductors. We present results of the magneto-optic Kerr spectroscopy on nand p-Ge substrates with different carrier concentrations and InSb. The experimental results indicate larger Kerr ellipticity and optical rotation near the plasma frequency. Due to the Faraday effect, we also observe an enhanced modulation signal of the THz pulse reflected from the backside surface of the low-carrier-concentration sample. Numerical simulations based on a simple free-carrier model reproduce our experimental results well. Our demonstration will help to advance multipurpose characterization techniques for various semiconductors.