We report on the widely tunable monochromatic terahertz (THz)-wave generation using a GaP planar waveguide under collinear phase-matched difference frequency mixing, which is based on electrically synchronized optical parametric oscillators (OPOs). By adjusting the fundamental wavelength of OPOs in the range from 920 to 1500 nm, THz waves are generated in the range from 0.48 to 4.3 THz using a 290-μm-thick planar GaP waveguide. The collinear phase-matching condition is satisfied in this range, which is consistent with the theoretical estimation. The wide tuning characteristics of THz waves obtained from the GaP planar waveguide structure are suitable for efficient and compact THz sources.
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