THz time-domain quasi-optical systems for characterization of frequency selective surfaces

At the expense of frequency resolution, time-domain spectrometers provide access to the frequency range 0.2 to 3 THz in a continuous fashion, unlike vector network analysers that enable a banded solution up to 1.1 THz [1]. This broadband response can be exploited to obtain the performance of frequency selective surfaces at the design frequency and harmonics in an effortless way. However, time-domain spectrometers rely on quasi-optical systems (usually based on a set of focusing and collimating lenses) that may induce artifacts in the measurements [2].
To illustrate the impact that the quasi-optical setup can have in the measured performance of frequency selective surfaces, we investigate with the TERA K15 all fibre-coupled THz spectrometer from Menlo Systems truncated subwavelength hole arrays displaying the so-called extraordinary transmission [3]. Three different setups are used to study the scattering from truncated subwavelength hole arrays under two different illumination conditions (collimated and focused beam illumination): (i) collimated beam illumination - setup 1 has no lenses before and after the sample; (ii) focused beam illumination - setup 2 has a focusing lens before the sample, whereas setup 3 has a focusing and a collimated lens before and after the sample, respectively. The distance between the detector unit and the sample is approximately 110 mm. For the measurements, the lock-in constant is set to 300 ms and the total temporal length of the recorded waveforms is at least 208 ps to have a spectral resolution of 4.8 GHz in the worst case. An efficient implementation of the Method of Moments [4] enable us to model the measurements in a fraction of the time that would take us with a full-wave electromagnetic simulation using the commercial software CST Microwave Studio.
Although the extraordinary transmission peak is observed with all setups, its amplitude depends strongly on the quasi-optics (i.e. setup) as well as on the number of holes. This strong dependence is linked to the optics and the leaky-wave mechanism mediating in the extraordinary transmission phenomenon. The scattering measurements demonstrate the appearance of a grating lobe above the extraordinary transmission frequency that is associated to the m = -2 space harmonics entering in the radiation region. Our results reveal that, although setup 3 is the preferred configuration for spectroscopy applications, it is the one with the most negative effect on the measurements of extraordinary transmission surfaces.

[1] M. Naftaly, Terahertz Metrology. Boston: Artech House, 2015.
[2] M. Navarro-Cía, V. Pacheco-Peña, S. A. Kuznetsov, and M. Beruete, “Extraordinary THz Transmission with a Small Beam Spot: The Leaky Wave Mechanism,” Advanced Optical Materials, vol. 6 (8), p. 1701312, Apr. 2018.
[3] T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arrays,” Nature, vol. 391(6668), 667–669, Feb. 1998.
[4] M. Camacho, R. R. Boix, and F. Medina, “Computationally efficient analysis of extraordinary optical transmission through infinite and truncated subwavelength hole arrays,” Physical Review E, vol. 93 (6), p. 063312, Jun. 2016.