The photonic carrier frequency downconverter is an active device that directly converts digital coherent optical data signals to the digital coherent wireless data signals and furthermore to the intermediate frequency (IF) signals, too. Its key functionality is called photonic double-mixing, consisting of a photomixing of an optical data signal with an optical subcarrier signal to generate a down-converted terahertz (THz) wireless data signal and an rf-mixing of the generated THz data signal and a local (LO) signal to generate an IF signal. It also works to mix the THz wireless data signal with a LO signal generated by photomixing two optical subcarrier signals to generate an IF signal. At the present, various photomixers as well as several photonic carrier frequency downconverters have been developed  - . However, their operation frequency range is limited by the electron transit time effect. In this study, we demonstrate the 1-THz band photonic double-mixing operation by using plasmonic technology. Plasmons, the quanta of the collective oscillation of electrons, are expected to overcome the electron transit time limitation .