[1] [1] Jepsen P U, Cooke D G, Koch M. Terahertz spectroscopy and imaging: modern techniques and applications[J]. Laser & Photonics Reviews, 2011, 5(1): 124-166.

[2] [2] CHIU C L, LIAO Y H. Analysis of a single ring resonator with 2×2 90-degree multimode waveguide turning couplers[J]. Optics Communications, 2016, 361: 124-129.

[5] [5] Hirata A, Yamaguchi R, Kosugi T, et al. 10-Gbit/s wireless link using InP HEMT MMICs for generating 120-GHz-band millimeter-wave signal[J]. IEEE Transactions on Microwave Theory & Techniques, 2009, 57(5): 1102-1109.

[7] [7] Gentili G G, Lucci L, Nesti R, et al. A novel design for a circular waveguide directional coupler[J]. IEEE Transactions on Microwave Theory & Techniques, 2009, 57(7): 1840-1849.

[9] [9] Gallot G, Jamison S P, Mcgowan R W, et al. Terahertz waveguides[J]. J. Opt. Soc. Am. B, 2000, 17(5): 851-863.

[13] [13] Bethe H A. Theory of diffraction by small holes[J]. Physical Review, 1944, 66(7-8): 163.