Terahertz Phase Shift and Its Modulation in NdGaO₃ Single Crystals

Finding appropriate materials to shape the terahertz (THz) wave is highly desired due to the increasing development of practical devices and application systems. In this work, the THz phase shifts of four typical oxide crystals, i.e., NdGaO3 (NGO), quartz, (LaAlO3)0.3(Sr2AlTaO6)0.35, and sapphire, have been investigated by using THz time-domain spectroscopy. It is interesting to find that the NGO crystal is the only one that shows a distinct phase shift. The phase shift (Δφ) of a ∼500 μm-thick NGO crystal is almost linearly dependent on the THz frequency and it reaches as large as ∼94° at 1.5 THz with a temperature variation from 100 to 400 K. Moreover, the THz phase shift of NGO possesses crystal anisotropy and Δφ(100) > Δφ(001) > Δφ(110). In addition, the effects of both electric field and laser illumination on the THz phase shift in the NGO crystal have also been explored. It is found that the electric field (∼260 V/cm) has negligible effect, while the laser illumination can efficiently cause a noticeable THz shift and Δφ ∼ 78° with good manipulation stability can be achieved with 20 J/cm2 light fluence. These findings suggest that NGO crystals are an appropriate candidate for THz phase modulator and their sensitivity and stability are expected to have a great technological impact and offer prospects for their applications in THz optics.