We review the recent progress made by us on power scaling of terahertz (THz) waves and development of
compact and portable THz sources. By reversely stacking GaP plates, we were able to improve the photon
conversion efficiency from 25% to 40%, which is the record-high value. As the number of the stacked GaP
plates was increased from 4 to 5, the output power was decreased. This is the evidence on back conversion.
In order to make our THz source truly compact and portable, we investigated a new route to THz generation
by mixing two frequencies generated by a single Nd:YLF solid-state laser. After two Nd:YLF crystals were
introduced in the laser cavity, the output power was scaled up to 4.5 μW. Such a configuration exhibits
versatile characteristics such as the generation of different THz frequencies by combining two different laser
crystals. Our recent investigation of THz generation based on passively Q-switched dual-frequency pulses
may help us with further reducing the dimension of our compact and portable THz source.