The experimental equipment of the FIRM laboratory enables the investigation of low-energy phenomena in solid-state physics, such as cyclotron resonance in semiconductors, spin-wave excitation, magnetic phase transition, low-energy crystal field excitation, vortex dynamics in superconductors, superconducting energy gap, etc.
Experiment. The FIR/THz molecular gas laser FIRL-100 generates linearly polarized frequency lines ranging from 0.25 to 7.5 THz with an output power between 1 and 100 mW. The linear polarization can be converted to circular polarization using a broadband-tunable phase retarder. The THz laser beam is focused onto the sample in a magneto-optical cryostat SM4000 which provides horizontal magnetic fields up to ±11 T with optical access to the sample in Faraday and Voigt geometries. The temperature can be controlled from 2 to 300 K. Transmitted or reflected light is subsequently measured by a sensitive helium-cooled bolometer. Simultaneously with the optical properties, the electrical resistance can be monitored by employing four contacts in the corners of the sample.
Measurement techniques. In our typical measurement, the laser frequency is fixed, and the transmittance or reflectance signal is recorded while the magnetic field or sample temperature is swept. Fast switching between the left- and right-handed circular polarizations allows the precise measurement of circular dichroism.
Experimental results. We employed this setup to investigate: (i) effective electron mass in GaAs, (ii) magnon resonance in altermagnetic MnTe, (iii) new topological phase in Bi2O2Se, and (iv) vortex mass in superconductors.
