Characterization of localized circularly polarized THz electric fields using MAPbI3 thin films

Circularly polarized (CP) terahertz (THz) fields are promising for controlling angular momentum in solids. While resonator structures can be used to enhance THz electric fields, a highly localized enhancement makes it difficult to accurately characterize the amplitude and polarization of the local near-field maximum using thick electro-optic crystals. We demonstrate a local probing method based on a band edge modulation in methylammonium lead iodide (MAPbI3) perovskite thin films. The THz-induced transmittance change, ΔT, in the near-infrared (as well as the reflectance change, ΔR) can be used to determine the near-field amplitude, while the degree of the modulation of ΔT (or ΔR) due to the oscillating THz electric-field vector yields the ellipticity ε. The extracted ellipticity is close to the ideal value (within ±0.05) for ε > 0.4. A comparison with finite-difference time-domain simulations shows that this method can be used to characterize the amplitude and ellipticity of the enhanced CP field in the gap of a crosshair resonator with good accuracy. The simulations also reproduce the experimentally observed long-lived near-field waveform well and indicate that the observed difference between the theoretical and experimental ε mainly arises from a probe spot that is slightly displaced from the gap center by about 1 μm. Since this resonator structure has an enhancement factor of 11.8 ± 0.3 and a high preservation of circularity, the combination of this resonator with the proposed probing method should be useful for studies using strong THz fields.