Fig. 1. Schematic diagram of electric quadrupole antenna device： （a）antenna array； （b）single antenna

Fig. 2. Calculated result using full wave finite element method under periodic boundary conditions: (a) E_z distribution; (b) E_x distribution; (c) H_y distribution

Fig. 3. Full structure finite element simulation results of N=6，P=90 μm，W=2 μm device: (a) Comparison of threshold gains when materials are transparent between the in-phase electric quadrupole mode and other modes near the frequencies; (b) Image of the far-field electric field distribution

Fig. 4. （a）Thermal simulation results of FP cavity； （b）EQA thermal simulation results； （c）Injection power density of active region in FP cavity and EQA vs. highest temperature； （d）Temperature distribution vs. active area depth curve along the green dashed line （right side）of the EQA section

Fig. 5. Scanning electron microscope （SEM）image of the Electric Quadrupole Antenna THz-QCL with structure of L=24.3 μm， P=90 μm， N=6， W=4 μm

Fig. 6. Test results in pulse mode of device A: (a) Spectrum at 20 K with different pumping current densities; (b) Light power-current-voltage (L-I-V) curves of device A at different temperatures, the spectra in the inset demonstrates a SMSR of 20 dB; (c) Far-field distribution

Fig. 7. CW test results of EQA: (a) spectrum at 20 K with different current densities; (b) Light power-current-voltage (L-I-V) curves at different temperatures, with the spectra in the inset demonstrating a SMSR of 19.5 dB

Fig. 8. Variable-temperature L-I-V curves for FP cavity: (a) pulse mode; (b) CW mode; experimental data and thermal resistance fitting data for pulse mode and CW mode of (c) FP cavity and (d) EQA array