High-Resolution and Wide-Span Terahertz Frequency Measurement Using Nonlinear Up-Conversion

Terahertz (THz) frequency metrology has become a topic of increasing interest due to the potential applications for THz radiation. While a diversity of THz radiation sources has been demonstrated, there are arguably fewer methods that can be used for the detection of THz radiation, and this has limited the ability to perform spectroscopic measurements in the information-rich THz frequency range. A technique that may facilitate the wider application and accessibility of THz spectroscopy is the nonlinear up-conversion of THz fields to the near-infrared (NIR) wavelength range, where there is a plethora of well-developed detection techniques. In this work, we demonstrate the measurement of THz frequency, using up-conversion in an organic crystal, in effect, mapping the target THz spectrum to a spectrum in the NIR wavelength range. The demonstrated measurement range covers a band of up to 29.15 THz (3.45 octaves), which complements the capabilities of existing heterodyne techniques. The ability of this technique to detect slight frequency shifts is examined and methods by which this can be further improved are proposed. The techniques presented in this work have the potential to enable frequency metrology of electromagnetic radiation across the optical-to-THz regions.