Photonic structures and metamaterials for the THz range


Photonic crystals and metamaterials are man-made structures which allow manipulating the radiation through carefully designed geometrical resonances or interference phenomena. The application of these structures to the THz range is quite promising for THz photonics namely with regard to the lack of natural materials with specific THz dielectric / magnetic properties (modulators, switches, filters, steerers, nonlinear devices, etc).  We particularly aim to design photonic structures controlled by external parameters such as electric field, magnetic field, illumination or temperature. [1]

THz metamaterials are artificial structures composed of patterns with a high contrast of electromagnetic properties of constituent materials and with unit cell dimensions much smaller than the THz wavelength. Since the propagating radiation cannot resolve the pattern, the metamaterials behave as materials with some effective optical properties – they can be assigned with an effective permittivity and permeability or effective refractive index. By the design of suitable pattern, one can prepare metamaterials with effective optical properties not found in nature. In particular, there is a substantial interest in achieving negative refraction metamaterials which are characterized by simultaneously negative permittivity and permeability. Since negative permeability is seldom found at frequencies exceeding a few GHz, tailoring metamaterials is one of the few possibilities how to achieve the negative permeability.

Negative effective permeability is frequently achieved by a sheet of metallic split-ring resonators. Another approach takes advantage of strong localization of magnetic field inside dielectric rods or spheres with high permittivity. This is due to a Mie resonance, which can have either electric of magnetic character. We used femtosecond laser machining applied to incipient ferroelectric SrTiO3 material to create an array of tunable permittivity dielectric rods. A magnetic Mie resonance with a range of negative effective permeability tunable by temperature was observed, Fig. 1. [2]

Dielectric spectra of SrTiO3
Figure 1: Top: dielectric spectra of bulk SrTiO3 show a high permittivity with a significant tunability versus temperature. Bottom: these properties are used to design metamaterial structures with tunable effective magnetic behaviour.

Similar work was carried out with TiO2 microspheres which can be embedded into various matrices and form a rigid metamaterial. [3] The Mie resonances in such structures were also investigated by near-field THz methods. [4]

Based on epitaxially strained thin films of SrTiO3 we demonstrated a planar metamaterial exhibiting a frequency-tunable response in the terahertz domain, controlled by a bias electric field. The active part of the metamaterial consists of a periodic metallic pattern deposited on SrTiO3 film. The role of the metallic structure is two-fold: it gives rise to the metamaterial resonance and it enables applying an electric bias to the strontium titanate layer. The strained film exhibits a pronounced dependence of its permittivity on the bias, which exerts a strong influence on the resonance. [5]

Tunable THz metamaterial structure
Figure 2: Schematic view of the tunable THz metamaterial structure based on epitaxial strained SrTiO3 films.
Left part: two SrTiO3/DyScO3 bilayers are deposited on a (1 1 0)-oriented DyScO3 substrate; the metallic metamaterial resonators on top of the multilayer serve also as electrodes.
Right part: scanning electron microscope image of the metamaterial structure.

[1] P. Kužel and F. Kadlec, Comptes Rendus Physique 9, 197 (2008).
[2] H. Němec, P. Kužel, F. Kadlec, C. Kadlec, R. Yahiaoui, and P. Mounaix, Phys. Rev. B 79, 241108 (2009); R. Yahiaoui, H. Němec, P. Kužel, F. Kadlec, C. Kadlec, and P. Mounaix, Opt. Lett. 34, 3541 (2009).
[3] M. Šindler, C. Kadlec, F. Dominec, P. Kužel, C. Elissalde, A. Kassas, J. Lesseur, D. Bernard, P. Mounaix, and H. Němec, Opt. Express 24, 18340 (2016).
[4] O. Mitrofanov, F. Dominec, P. Kužel, J. L. Reno, I. Brener, U-C. Chung, C. Elissalde, M. Maglione, and P. Mounaix, Opt. Express 22, 23034 (2014); I. Khromova, P. Kužel, I. Brener, J. L. Reno, U.-C. C. Seu, C. Elissalde, M. Maglione, P. Mounaix, and O. Mitrofanov, Laser Photon. Rev. 10, 681 (2016).
[5] C. Kadlec, V. Skoromets, F. Kadlec, H. Němec, H.-T. Chen, V. Jurka, K. Hruška, and P. Kužel, J. Phys. D: Appl. Phys. 51, 054001 (2018).


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