Sensitive terahertz-wave detector using a quasi-phase-matched LiNbO3 at room temperature

Sensitive terahertz (THz)-wave sensor at room temperature is crucial for most applications such as 2-dimensional real-time imaging and nonlinear phenomena in semiconductors caused by multi-photon absorption, light-induced ionization, and saturated absorption. LiNbO3 is a promising material for frequency up- and down-conversion because of its high nonlinearity and high resistance to optical damage. In this report, we propose a slant-stripe-type periodically poled Mg doped LiNbO3 (PPMgLN) crystal for the construction of a practical THz detector. The PPMgLN solves compromised optical design and low coupling efficiency between THz and infrared (IR) pump beam due to imperfect dichroic coupler. The effective coupling of both pump beam and THz-wave into identical interaction region of up-conversion device promotes the THz detector in practical use. The phase-matched-condition in slant-stripe-type PPMgLN was designed to offer collinear propagation of two optical waves, the pump and up-conversion signal beams, because of efficient frequency conversion. The phase-mached-condition was calculated and a slant-stripe-type PPMgLN with an angle (alpha) of 20 degrees and a grating period (A) of 29.0 mu m was used in this experiment. A minimum detectable energy of 0.3 pJ/pulse at the frequency of 1.6 THz was achieved with the pump energy of 1.8 mJ/pulse in room temperature. The dynamic range of the incident THz-wave energy of 60 dB was demonstrated. Further improving for the sensitivity using longer interaction length in a PPMgLN crystal was also investigated.