IEEE Transactions on Terahertz Science and Technology | 2019
Chip-Scale Terahertz Carbonyl Sulfide Clock: An Overview and Recent Studies on Long-Term Frequency Stability of OCS Transitions
Abstract
This paper reviews the recent innovations of a miniature time-keeping device with high affordability: chip-scale molecular clock. It is based on the recent research progress of high-precision terahertz (THz) rotational spectrometers, especially those using CMOS integrated circuit technologies. The clock probes the rotational modes of carbonyl sulfide (<inline-formula><tex-math notation= LaTeX >$^{16}$</tex-math></inline-formula> O<inline-formula><tex-math notation= LaTeX >$^{12}$</tex-math></inline-formula> C<inline-formula><tex-math notation= LaTeX >$^{32}$</tex-math></inline-formula> S) molecule gas and then calibrates its MHz output according to the measured terahertz transition frequency of OCS. In contrast to cesium/rubidium atomic clocks, the THz OCS clock has fully electronic operations and, hence, a significantly simplified implementation. In particular, it is realizable on a waveguide-attached CMOS chip, which minimizes the form factor and cost. Based on a lab scale prototype with an Allan deviation in the 10<inline-formula><tex-math notation= LaTeX >$^{-11}$</tex-math></inline-formula> range, this paper for the first time studies two critical metrics related to the long-term stability of THz OCS clocks. These metrics are the clock sensitivities to temperature change of the OCS gas and external magnetic field. The measured average temperature coefficient of the clock, without ovenized temperature stabilization and temperature compensation, is 9.5 × 10<inline-formula><tex-math notation= LaTeX >$^{-11}$</tex-math></inline-formula>/<inline-formula><tex-math notation= LaTeX >$^\\circ$</tex-math></inline-formula>C in the range of 28<inline-formula><tex-math notation= LaTeX >$\\ ^\\circ$</tex-math></inline-formula>C–70<inline-formula><tex-math notation= LaTeX >$\\ ^\\circ$</tex-math></inline-formula>C. Next, the measured clock shift in response to a 75\xa0G external magnetic field is <inline-formula><tex-math notation= LaTeX >$< $</tex-math></inline-formula>4 × 10<inline-formula><tex-math notation= LaTeX >$^{-11}$</tex-math></inline-formula>, with a theoretical value near 10<inline-formula><tex-math notation= LaTeX >$^{-13}$</tex-math></inline-formula>. Lastly, this paper also reviews the first reported CMOS prototype of the clock, which consumes only 66\xa0mW dc power and achieves an Allan deviation of 3.8 × 10<inline-formula><tex-math notation= LaTeX >$^{-10}$</tex-math></inline-formula> (<inline-formula><tex-math notation= LaTeX >$\\tau$</tex-math></inline-formula> = 1000\xa0s). Approaches for performance improvements and potential monolithic chip implementation are discussed. These studies present the feasibility of a CMOS-based magnetic-shield/heater-free clock with high energy efficiency and sub-<italic>ppb</italic> stability over a wide range of operation condition.