Supplementary Information

This supplementary information file provides additional theoretical and experimental details supporting the findings in the main text. Figure S1 presents the calculated band structure of an infinite-period patch-antenna array and the electric field distribution within a unit cell. Figure S2 compares the field distributions and radiation patterns of the TM01 and TM00 modes in a four-period device with those of a conventional ridge waveguide. Figure S3 shows the calculated cavity quality factor as a function of the number of periods for different modes. Figure S4 displays continuous-wave emission spectra for devices with four, six, eight, and ten periods measured under various operating conditions. Figure S5 presents the light-current-voltage characteristics and emission spectra of a conventional Fabry-Pérot terahertz quantum cascade laser. Figure S6 provides light-current-voltage characteristics and emission spectra for additional few-period patch-antenna array devices. Section S1 describes the approach used to calculate the quality factor, confinement factor, and threshold gain in few-period patch-antenna array devices. Section S2 details the measurement procedure for continuous-wave output power for both the few-period patch-antenna array and Fabry-Pérot devices.