Abstract
Second-order nonlinear optical processes allow the conversion of electromagnetic signals to a higher frequency domain. Such so-called "upconversion" processes have been of interest for the optical detection of terahertz signals for decades. To enhance the nonlinear interaction, many investigations have focused on using high quality resonators, which improve the upconversion efficiency but limit the bandwidth. Here, we present a hybrid scheme that takes advantage of the high optical quality of a crystalline resonator while allowing the upconversion of signals at multiple frequency bands. We observed upconversion of six THz channels with MHz bandwidth, centered around 0.157 THz, and separated by about 10 GHz. The highest photon conversion efficiency at 0.157 THz was observed to be 3 × 10⁻⁶ per milliWatt of optical pump power. By numerically analyzing the landscape of nonlinear coupling in such resonators, we establish the conditions under which the demonstrated scheme can serve as an efficient multi-channel upconverter.