Final Report Summary - THZ-PLASMON (Ultra-fast control of THz plasmon polariton resonances) The goal of the ERC project “Ultra-fast control of THz plasmon polariton resonances (THz Plasmon)” was to demonstrate the excitation and control of localized surface plasmon polaritons (LSPPs) in semiconductor microparticles known as THz-antennas. Surface plasmon polaritons are the coherent oscillation of free charge carriers driven by electromagnetic fields and are characterized by the resonant enhancement of optical extinction and local electromagnetic fields. To investigate surface plasmon polaritons localized to particles, we have fabricated resonant structures on silicon, GaAs and gold layers. A unique advantage of semiconductors over metals for plasmonics is that the carrier density in semiconductors can be easily controlled with impurities (doping), and it can be actively tuned by optical pumping. THz antennas have been investigated by measuring the THz extinction of arrays and of individual structures. For the last study, we have introduced conical waveguides that allows the concentration of THz radiation in small volumes, increasing the extinction signals of single antennas. In order to achieve a full all-optical control of the resonant response and the local field of THz antennas we have introduced a novel approach to generate them. This approach consists on the illumination of flat semiconductor layers with the structured beam that is generated by its reflection on a spatial light modulator (SLM). In this way, photo-excited charge carriers are only generated on the regions of the semiconductor layer that are illuminated. These illuminated areas can have designed dimensions to allow the excitation of THz-LSPPs. By using this approach we have demonstrated the optical generation of linear THz antennas and THz dimers. The resonant response of these antennas and their associated near fields can be controlled by changing the pattern on the SLM. We have also demonstrated THz beam steering with photo-generated metasurfaces on, otherwise, flat semiconductor layers. These results open a wide range of possibilities for the all-optical spatial control of resonances on flat surfaces and the concomitant control of THz extinction and local fields. In order to investigate the local fields associated to THz antennas, we have developed a new THz near-field microscope, which can measure the 3 electric near-field components. These measurements are done in the time domain, retrieving the amplitude and the phase of the near field. Using this instrument, we have found that the resonant response of THz antennas can be different in the near- and in the far-field due to interference of the scattered waves. These results stress the relevance of directly measuring the near-field of structures that are designed for enhancing this near-field, e.g. for sensing applications. We have also combined THz far-field with near-field spectroscopy to unravel the mechanisms leading to electromagnetic induced transparency (EIT). This phenomenon consists of a frequency window in which the transmission through a sample increases as a result of the coupling and interference of two or more resonances. The interest in EIT resides in the fact that local electromagnetic fields are resonantly enhanced, leading also to a strong dispersion and a reduction of the wave group velocity at frequencies in which the sample is transparent. EIT is of special interest for the design of components for THz communication.
