Scaling narrowband THz generation to large apertures in LiNbO₃ and KTP

Millijoule-scale pulses of multicycle terahertz radiation (MC-THz) are increasingly being pursued as drivers for applications requiring high-fields and high spectral brightness. An attractive approach for generating high peak-power MC-THz pulses is nonlinear optical down-conversion of laser pulses in periodically-poled crystals. A principal limitation to the yield, however, is the small (sub-centimeter) apertures of commercially-available crystals which restrict the amount of laser energy that can be used. Here, we explore MC-THz generation by down conversion in two types of large-aperture media for which periodic poling has been achieved in different ways: (1) extension of traditional, voltage-based poling of bulk material to larger (centimeter) scales; and (2) manual poling by assembly of large aperture sub-millimeter thick wafers in alternating orientations. We explore the dependence of efficiency on laser peak fluence and crystal length for both types of media and extend upon previous work with the wafer approach by increasing the number of wafers in the stack, implementing cryogenic cooling and testing an alternate material: potassium titanyl phosphate (KTP). Driving with up to 0.2 J, half-picosecond laser pulses centered at 1,030 nm, we obtain conversion efficiencies of up to 0.14%, resulting in ∼1% bandwidth MC-THz pulses of up to 207 μJ.