The development of a wide range of technologies based on terahertz (THz) electromagnetic radiation drives a strong demand for flexible optical elements. Metasurfaces based on metallic resonators offer a versatile toolkit that permits easy tuning over a wide spectral range by the geometric design. Herein, a mechanically tuned metasurface perfect absorber comprised of split‐ring resonators in combination with a metallic mirror in a microcavity arrangement, is demonstrated. By mechanically tuning the length of the microcavity in the range of 10 μm and above, precise control over the perfect absorption condition is exhibited. A maximum recorded extinction of 45.8 dB is obtained at the perfect absorption condition, corresponding to a suppression of the reflected radiation by almost five orders of magnitude. Experiments are performed in a reflection arrangement using a terahertz time‐domain spectrometer. Simulations of the experimental arrangement show that near‐field effects are weak and the enhancement of metamaterial perfect absorption is in agreement with purely interferometric effects.

Terahertz time-domain spectroscopy (TDS) is a powerful characterization technique which allows for the frequency-dependent complex refractive index of a sample to be determined. This is achieved by comparing the time-domain of a pulse transmitted through air to a pulse transmitted through a material sample; however, the requirement for an independent reference scan can introduce errors due to laser fluctuations, mechanical drift, and atmospheric absorption. In this paper, we present a method for determining complex refractive index without an air reference, in which the first pulse transmitted through the sample is compared against the “echo”, where the internal reflections delay the transmission of the echo pulse. We present a benchmarking experiment in which the echo reference method is compared to the traditional air method, and show that the echo method is able to reduce variation in real refractive index.

We demonstrate, experimentally, non-volatile optical control of terahertz metasurfaces comprising of a metallic split ring resonator array sandwiched between monolayer graphene and a photoconductive Fe:LiNbO 3 substrate. We demonstrate frequency selective tuning of THz transmission amplitude, and our results pave the way towards spatially resolved control of THZ metasurfaces for beam-steering, imaging, and sensing applications. The substrate (Fe:LiNbO 3 ) supports non-volatile yet reversible photoinduced charge distributions, which locally modify the electrostatic environment of the nano-thickness graphene monolayer, altering the graphene electrical conductivity and therefore changing the resonance spectra of the metamaterial array. We present lightinduced normalized transmittance changes up to 35% that are non-volatile and persist after the illumination source is removed, yet can be reversed by thermal annealing.

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