Figure 2 Experimental setups for performance test of THz counter
Figure 3 Measurement precision and instability of THz counter developed at NICT
The frequency of a THz source must be down-converted to accessible radio frequency by mixing with a THz frequency reference, since modern electronics is not fast yet to directly count the THz frequency (more than a hundred billion cycle per second). A set of regularly-spaced and spectrally broadening THz frequency references, or a THz frequency comb, suits for the beat-down method against an arbitrary frequency from various THz sources. A semiconductor-superlattice harmonic mixer (SLHM) could be superior to other semiconductor-based devices or nonlinear optical crystals for creating the THz comb in terms of compact application and usability. Nevertheless, the thorough performance test of a frequency counter based on the SLHM has not been carried out to the best of our knowledge.
A semiconductor-superlattice electron device with the nonlinear negative differential resistance, which was proposed by Nobel prize winner Esaki and Tsu in the early 1970s, serves as a frequency mixer for an input THz wave and harmonics of a microwave local oscillator, that lie on the THz band and form a comb structure. This SLHM is simple and operational at room temperature, which facilitate the practical applications. Thanks to the SLHM, NICT developed a new THz frequency counter, making the footprint shrinkage by 97% and no ultrashort-pulse-laser expertise.
The measurement precision of the THz counter developed was tested by two methods: comparison of two THz frequencies determined using two identical counters (Figure 2 (a)) and direct measurement of a known frequency from a THz-quantum cascade laser (QCL) highly-stabilized to the THz comb by a single counter (Figure 2 (b)). The former is likely suitable for less than 1 THz, where THz sources with the low phase noise become available. Whereas the latter allows the test at typically above 1 THz, where powerful THz-QCLs exist with the established phase stabilization technology. Using these complementary methods, we successfully revealed a measurement uncertainty of 9.6 x 10-17 (Figure 3 (a)) and a instability of less than 3.7 x 10-13 at 1 s averaging time (Figure 3 (b)) over a four-octave range from 0.1 THz to 2.8 THz. Besides, detailed research study regarding the THz-comb generation in the SLHM indicated the potential measurable frequency up to 3.7 THz.
The high-precision and wide-measurable band THz counter could become an essential metrological tool for high-speed wireless communications on the Beyond 5G / 6G, sensing and imaging, biology and medicine, and radio astronomy etc. Among them, an ultra-accurate THz frequency standard referencing ultracold molecules, or a THz molecular clock, needs the counter in order to determine the absolute frequency with more than 10-15 precision as well as compare the THz molecular clock with an optical atomic one for searching time variations of fundamental physics constant.