Coherent THz-wave phase detection with backward nonlinear optical interferometer

Coherent phase detection of terahertz (THz) waves is essential for quantitative THz-wave metrology, and nonlinear optical conversion provides a promising route toward sensitive optical readout of THz wave. Here, we demonstrate coherent phase detection of a 0.305-THz wave using backward THz-wave parametric oscillation (BW-TPO) in periodically poled lithium niobate (PPLN). In the BW-TPO process, the incident THz-wave phase is transferred to a near-infrared optical signal and is read out by optical interference. By independently modulating the phases of the pump, idler, seed, and THz waves, we experimentally identify the phase-transfer relations and show cancelation of pump-phase fluctuations. We further implement an in-crystal SU(1,1)-type nonlinear interferometer yielding high-contrast nonlinear interference with a fringe visibility of 0.906 ± 0.027 and a phase precision of 53.0 ± 26.5 mrad. Coherent oscillations with the 0.983-mm THz wavelength are also observed for THz-wave incidence rotated by 90° and 180° from the nominal phase-matching direction, supporting phase transfer through phonon-polariton excitation. These results establish BW-TPO as a narrowband, phase-preserving THz-to-optical interface for phase-sensitive spectroscopy, imaging, and metrology.