We demonstrate that a periodic lattice of detuned resonators can suppress the THz extinction at the central resonant frequency, leading to an enhanced transparency due to diffraction. The system consists of metallic rods of two different sizes, each of them supporting a strong half-wavelength (λ/2) resonance, which are spatially displaced 0 M. C. Schaafsma and A. Bhattacharya contributed equally to this work. 1 within the unit cell of the lattice. Using a coupled dipole model we show that the Diffraction Enhanced Transparency (DET) window has its origin in the interference between two surface lattice resonances, arising from the diffractively enhanced radiative coupling of the λ/2 resonances in the lattice. Group-index measurements show that the THz field is strongly delayed by more than four orders of magnitude at the transparency window. Since DET does not involve the near-field coupling of resonators, the fabrication tolerance to imperfections is expected to be very high. This remarkable response renders these systems as very interesting components for THz communication.
Photoconductive antennas (PCAs) are among the most conventional devices used for emission as well as detection of terahertz (THz) radiation. However, due to their low optical-to-THz conversion efficiencies, applications of these devices in out-of-laboratory conditions are limited. In this paper, we report several factors of enhancement in THz emission efficiency from conventional PCAs by coating a nano-layer of dielectric (TiO2) on the active area between the electrodes of a semi-insulating GaAs-based device. Extensive experiments were done to show the effect of thicknesses of the TiO2 layer on the THz power enhancement with different applied optical power and bias voltages. Multiphysics simulations were performed to elucidate the underlying physics behind the enhancement of efficiency of the PCA. Additionally, this layer increases the robustness of the electrode gaps of the PCAs with high electrical insulation as well as protect it from external dust particles.
Using micro-structured photo-conducting probes, we demonstrate full vectorial mapping of the complex electric fields in the near-field region of a resonant structure at THz frequencies. The investigated structure represents the simplest case of a resonator: a metallic rod. We show field amplitude as well as phase maps for the three field components at the half wavelength (λ/2) resonance of the rod. The field as well as the phase distributions are in excellent agreement with our physical understanding of local electric-field distributions in the vicinity of λ/2 resonant structures and are validated by numerical simulations. These measurements can be a platform for performance optimization of the emerging field of THz photonic and plasmonic devices with complex sub-wavelength structures.
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