Radu Malureanu

Terahertz-induced Kerr effect in amorphous chalcogenide glasses

We have investigated the terahertz-induced third-order (Kerr) nonlinear optical properties of the amorphous chalcogenide glasses As2S3 and As2Se3. Chalcogenide glasses are known for their high optical Kerr nonlinearities which can be several hundred times greater than those of fused silica. We use high-intensity, single-cycle terahertz pulses with a maximum electrical field strength exceeding 400 kV/cm and frequency content from 0.2 to 3.0 THz. By optical Kerr-gate sampling, we measured the terahertz-induced nonlinear refractive indices at 800 nm to be n2=1.746×10−14cm2/W for As2S3 and n2=3.440×10−14 cm2/W for As2Se3.

Plasmonic dimer metamaterials and metasurfaces for polarization control of terahertz and optical waves

We explore the capabilities of planar metamaterials and metasurfaces to control and transform the polarization of electromagnetic radiation, and present a detailed covariant multipole theory of dimer-based metamaterials. We show that various optical properties, such as optical activity, elliptical dichroism or polarization conversion can be achieved in metamaterials made of simple shapes, such as nanorods, just by varying their geometrical arrangement. By virtue of the Babinet principle, the proposed theory is extended to inverted structures (membranes) where rods are replaced by slots. Such free-standing “metasurface membranes” can act as thin-film spectrally sensitive polarization shapers for THz radiation. Proof-of-principle devices (a linear polarizer and a structure with giant optical activity) are fabricated and characterized. Experimental results coincide with those of full-wave numerical simulations, and are in good agreement with analytical predictions.

Linear and nonlinear properties of chalcogenide glasses in the terahertz frequency

Terahertz (THz) waves have the potential to improve a wide range of devices in the space, defense and semiconductor industries as well as offering the possibility of investigating various molecules of interest in biology, medicine, art etc. For this reason, THz sources, detectors and passive linear components have greatly been developed in recent years. However there is still no any non-linear device available. This is mainly due to the lack of the rigorous investigation of non-linear characteristics of materials in the THz range. We chose to investigate chalcogenide glasses due to their high nonlinear coefficients in the optical range, which might suggest pronounced nonlinear behavior also at THz frequencies. Here we present measurements, both linear and nonlinear, of As2S3 and As2Se3. For the linear measurements we employed a combination of THz time-domain spectroscopy setups. In the low frequency range we used a standard THz-TDS setup based on photoconductive switches while in the higher frequency domain we used an air biased coherent detection (ABCD) setup. This allowed for a wide frequency range (from 0.2 to 18 THz) investigation of the refractive index of the glasses. The nonlinear coefficient in the THz range was investigated using a lithium niobate based setup that allows us to reach electrical field strength exceeding 400 kV/cm. Results from both investigations will be presented during the talk.

A new mechanism to design transparent electrodes: THz realizations

We proposed a simple scheme to make a continuous metallic film on a semi-infinite substrate optically transparent, thus obtaining a completely transparent electrode in a desired frequency range. By placing a sub-wavelength composite layer consisting of dielectric and metallic stripes on top of the metallic one, we found that the back-scattering from the metallic film can be almost perfectly canceled by the composite layer under certain conditions, leading to transparency of the whole structure. Since our mechanism does not require any openings on the opaque metallic plate, the proposed structure retains the full electric and mechanical properties of a natural metal. The present mechanism is insensitive to structural disorders and broad variation of incidence angles. Meanwhile, we performed proof-of-concept experiments in the terahertz domain to verify our theoretical predictions, using carefully designed metamaterials to mimic plasmonic metals in optical regime. Experiments are in excellent agreements with full-wave simulations.