We report on the observation of terahertz ͑THz͒ radiation emitted from antiferromagnetic ͑AFM͒ single-crystal nickel oxide irradiated with femtosecond laser pulses. Periodic oscillations observed in the THz waveforms are assigned to the radiation from coherent AFM magnons excited by the laser pulses. Impulsive stimulated Raman scattering process is a possible mechanism of the coherent AFM magnon excitation by the laser pulses. The excited magnons in NiO generate THz waves by magnetic dipole radiation, which is the inverse process of AFM resonance absorption of THz waves.

We present a frequency-domain electron spin resonance (ESR) measurement system using terahertz time-domain spectroscopy. A crossed polarizer technique is utilized to increase the sensitivity in detecting weak ESR signals of paramagnets caused by magnetic dipole transitions between magnetic sublevels. We demonstrate the measurements of ESR signal of paramagnetic copper(II) sulfate pentahydrate with uniaxial anisotropy of the g-factor under magnetic fields up to 10 T. The lineshape of the obtained ESR signals agrees well with the theoretical predictions for a powder sample with the uniaxial anisotropy.

We report the observation of terahertz emission from coherent antiferromagnetic (AFM) magnons excited by femtosecond laser pulses. Impulsive stimulated Raman scattering (ISRS) process is a possible mechanism of the coherent AFM magnon excitation by the laser pulses. The excited coherent AFM magnons in NiO produce the terahertz emission via magnetic dipole radiation that is an inverse process of AFM resonance absorption of the terahertz wave.

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