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Structured nonlinear Metamaterials for efficient generation and Active functional control of Radiation of THz light

Periodic Reporting for period 4 - SMART (Structured nonlinear Metamaterials for efficient generation and Active functional control of Radiation of THz light)

Periodo di rendicontazione: 2021-07-01 al 2022-12-31

The terahertz optical regime, covering the long wavelength end of the optical spectrum, has been for many years the least explored spectral regime. Recent interest in this regime has led to important emerging applications spanning many disciplines including medical, biological, materials sciences, communications, security, and basic sciences. However, advances in these emerging applications are held back by the lack of good and controllable terahertz light sources. This project is aimed at developing a new family of THz sources with unmatched functionality. The developed sources are based on nano-engineered nonlinear heterostructured metamaterials, man-made materials with artificial optical properties. In the project we work to study and design novel active metamaterials that efficiently emit THz light at broad range of frequencies, designed shapes and desired polarization, focus it directly from the emitter to a desired sample location and even actively steer and modify its radiation properties all-optically. Overall the aim of this research is to develop a unique family of THz light emitters that will lead to the, long sought for, leap in THz technology and will open the door to new applications and to new tools for advancing fundamental science.
The first part of the project was dedicated to build the experimental setup that allows to generate THz radiation from nonlinear metasurfaces and to characterize it. In addition, we developed the fabrication capabilities to allow us to fabricate mm-cm scale metasurfaces, which scale is essential to pursue the aims of SMART. We also developed the numerical skills to model the nonlinear interaction with the metasurfaces. We were able to experimentally demonstrate for the first time and study the THz emission from SMART nonlinear photonic crystals. This led to emission of few cycle THz pulses with engineered angular dispersion, which opens the door for development of new THz spectroscopy schemes, among other applications. We also showed that we can use the SMART concept for THz beam shaping and active control. We demonstrated generation of THz beam that cannot be demonstrated conventional mode conversion schemes. In addition, we introduced novel schemes for THz pulse shaping relying on direct space to time mapping by SMART metasurfaces. We also demonstrated new type of active nonlinear THz emitting lenses. These lenses generate broadband THz radiation and focus each generated frequency on a different focal point along the optical axis. We also studied the interaction in nonlinear THz waveguide configuration and showed the generated waveguides modes, and their spectral properties, can be controlled by the geometrical design of the nonlinear metasurface and waveguide configuration. This provided another novel promising platform for THz generation and manipulation. In addition, we also studied new ways to enhance, manipulate and control the nonlinear interaction on the metasurfaces. We found that collective interactions on the metasurface provide a promising path to significantly enhance the total nonlinear interaction, while showing interesting effects of slow light, induced transparencies and elimination of some nonlinear pathways. An important discovery in the project was related to the understanding of the underlying mechanisms that are responsible for the relatively strong THz emission from the metasurfaces. We realized that a very thin conductive layer that is used for the fabrication process actually enhanced the THz emission by two orders of magnitude. We then showed that by nanostructuring also this layer we can have another order of magnitude improvement in the THz radiation strength. Another important achievement was the ability to use a geometric phase concept in the THz generation process. This ability allowed us to obtain perfect control over the spatial polarization phase and amplitude of the emitted THz waves. By that we could demonstrate a new family of functional nonlinear THz emitters for various applications.
The developed SMART emitters provide new functionalities and abilities for control over THz waves that are significantly beyond the state of the art in the field. These new functionalities may find way into improving current and future applications of THz waves for promoting materials sciences, physics, chemistry and biomedicine, and related technologies.