Six billion-fold amplification via a two-stage terahertz parametric amplifier

We have been using the principles of injection-seeded terahertz parametric generation (TPG) to develop the terahertz parametric amplifier (TPA) since 2014. Although we previously achieved amplification of 55 dB at 1.6 THz, amplification of low-frequency terahertz waves (≤1 THz) or ultraweak inputs was difficult, because broadband terahertz waves (spontaneous TPG) were generated and interfered with amplification. Thus, we herein present a new technique that divides terahertz-wave amplification into two steps: preamplification and main amplification. In this configuration, the terahertz wave is upconverted to a near-infrared (NIR) beam. Then, the noise is removed under the NIR condition and converted again to terahertz waves by the main amplifier. This two-stage TPA affords an extremely high amplification factor (up to six-billion-fold; 98 dB) for an ultraweak input of 500 zJ (zJ = 10−21 J). This is the first amplifier that has such a high gain for sub-aJ order input in the terahertz region.