A high-quality smart filter for terahertz range with relative tunability reaching 20% has been demonstrated. The filter is based on a narrow transmission band, which originates from a defect mode that appears due to insertion of a single crystal of KTaO 3 into otherwise periodic one-dimensional photonic crystal. Frequencies of defect modes are controlled by the refractive index of the defect: their high tunability is achieved by the strong temperature dependence of the dielectric properties of KTaO 3 . The low losses of KTaO 3 lead to a high peak transmission of the filter. Influence of the defect losses on the properties of the filter is also discussed.

Using finite-difference time-domain simulations, we study the interactions of electromagnetic radiation with a square array of dielectric rods parallel to the electric vector. We observe the electric and magnetic Mie resonances which induce intervals of negative effective permittivity and permeability and which contribute to the formation of the photonic band gaps. Owing to the interplay of these phenomena, a narrow spectral range with a negative refractive index can occur. However, this requires the filling fraction of the dielectric to fall into a well defined interval of values and its permittivity to exceed a minimum of about 50. We discuss these phenomena from the perspective of both photonic crystal and metamaterial concepts.

We investigate time-resolved terahertz conductivity of thin superconducting NbN films with various thicknesses upon their excitation by intense femtosecond laser pulses. The recovery dynamics following a complete destruction of the superconducting state occurs via a growth of superconducting islands in the normal-state environment. This is in contrast with previous observations of the recovery upon strong-field terahertz excitation [R. Matsunaga and R. Shimano, Phys. Rev. Lett. 109, 187002 (2012)]. We observe that the density of electronic states in the superconducting islands deviates from the BCS theory predictions on a subnanosecond time scale, while equilibrium terahertz conductivity spectra confirm the standard BCS behavior in the ground state.

A one-dimensional photonic crystal possessing an electric-field-tunable defect mode in the lowest forbidden band is demonstrated. The compact photonic structure consists of two symmetric Bragg mirrors made of alternate quarter-wave layers of SiO2 and CeO2 separated by a defect layer of an incipient ferroelectric SrTiO3 with electrodes transparent for terahertz radiation on its both sides. The applied bias is then perpendicular to the layer and modifies the in-plane dielectric function, which is probed by the transverse terahertz wave. The observed tunable behavior is in agreement with the model of the ferroelectric soft mode behavior in SrTiO3 single crystals. The defect-mode frequency tunability is proportional to that of the soft mode: we achieved a relative tunability of 6.5% at 105 K under an electric bias of 60 kV/cm.