Terahertz (THz) waves hold significant application potential in fields such as spectroscopy, integrated optics, and imaging. With the advancement of high-power femtosecond laser technology, lithium niobate crystals and waveguides have emerged as efficient, high-beam quality, and stable THz sources for generating THz radiation. Notably, the tilted-pulse-front method has led to significant progress in generating strong THz radiation from lithium niobate crystals. However, the spectral center frequency of strong THz sources generated by lithium niobate prisms is typically challenging to modulate, limiting their use in tunable multi-wavelength applications. We present a tunable THz source based on a lithium niobate waveguide, wherein the waveguide’s thickness is controlled to modulate the central frequency of the emitted THz spectrum, achieving tunable single-frequency THz wave output. In addition, we utilize an interdigital photoconductive antenna to detect the THz signals generated by the lithium niobate waveguide, enhancing the system’s signal-to-noise ratio.

The experimental setup involves a laser with a wavelength of 800 nm, a pulse width of 35 fs, and a repetition rate of 500 Hz. A cylindrical lens with a focal length of 70 mm is used to modulate the laser beam into a line laser mode suitable for transverse excitation. Two off-axis mirrors with a focal length of 101.1 mm are used to collect the THz waves generated by the lithium niobate waveguide. An interdigital photoconductive antenna is used to collect the far-field THz signals, with a wide-angle silicon lens that efficiently collects elliptical THz spots, ensuring high detection efficiency without the need for additional optical components.

In the transverse excitation mode, by comparing the time-domain and spectral characteristics of THz waves generated by lithium niobate waveguides of varying thicknesses and lengths, the following conclusions are drawn. 1) The center frequency of THz waves generated by the lithium niobate waveguide is affected by the waveguide’s thickness, but not by its length. 2) The physical principle underlying narrowband THz wave generation in lithium niobate waveguides is that only THz frequencies that confirm to the waveguide phase matching mode can be transmitted over long distances within the waveguide. 3) Measurement of THz waves generated by lithium niobate waveguides of identical thickness but different lengths shows that as the waveguide length increases, the center frequency of the THz waves gradually approaches the phase-matching frequency, consistent with the waveguide mode dispersion frequency. Only THz frequencies with a phase velocity matching the group velocity of the femtosecond laser can be effectively transmitted over long distances and out of the waveguide. 4) The high signal-to-noise ratio of the interdigital photoconductive antenna enables precise measurement of the THz waves reflected from the waveguide’s end face, further confirming that the THz wave transmission in the lithium niobate waveguide is consistent with the phase matching of the waveguide mode.

During femtosecond laser transverse excitation of lithium niobate waveguides, the pump laser energy is continuously converted into THz waves. By adjusting the thickness and length of the waveguide, the center frequency and intensity of the generated THz waves can be modulated. Compared to traditional forward excitation modes, the transverse excitation mode allows for the generation of higher energy narrowband THz pulses. It is expected that by cascading lithium niobate waveguides of varying thicknesses and lengths, a continuously tunable narrowband pulsed THz radiation source can be realized under the same pump laser conditions.