Highly Efficient Amplitude Modulation of Terahertz Fano Resonance Based on Si Photoactive Substrate by Low Power Continuous Wave

Abstract

asymmetric feature, which is in direct contrast with the traditional Lorentz resonance. [2] In 1961, Ugo Fano discovered this resonance mode when he studied the auto-ionization state of atoms. [3] With the development of micro-nano science, sharp Fano resonances have been observed from the asymmetric split-ring resonators based on metasurface in recent years. [4,5] The asymmetry of the spectrum is mainly due to the symmetry breaking of the split ring structure, further causing the coupling of the fundamental mode (bright mode) and the high order mode (dark mode). The sharp Fano resonance can be widely used in notch filter, narrowband filter, photo-switching device, slow light device, etc. [6-9] In addition, Fano resonance of the metasurface is significantly sensitive to the characteristics of the external dielectric environment, thus effectively improving the sensitivity of the sensor, e.g., biochemical sensor. [10,11] Furthermore, Fano resonance based on metal metasurface provides a new approach for terahertz (THz) application, which can adjust the enhanced THz local electrical field by controlling the micro-nano material parameters and structural parameters. [12-14] Many types of researches relating to THz Fano resonance have been explored and implemented one after another, especially in terms of the control of THz Fano resonance intensity. [15] Those common control methods involve with the thermal, electrical, optical, hybrid opto-thermal, and hybrid opto-electrical routes. [16-20] In essence, these methods change the number of carriers in the resonance position in order to achieve the purpose of controlling the intensity. Optical control of Fano resonance intensity has attracted widespread attention. The common approach is to cover the thin photoactive materials on the surface of THz Fano meta devices, and the selected photoactive materials are germanium (Ge), molybdenum disulfide (MoS 2), perovskite, lead iodide (PbI 2), graphene, and so on. [18,21-25] And the modulation depth could be up to 90%. However, some disadvantages are still obvious. On the one hand, due to the inherent shortcomings of the thin photoactive materials (generally polycrystalline films), such as high lattice defect concentrations, it had to rely on high pump power in order to obtain 90% modulation depth, thus inducing enough carrier amounts. Therefore, expensive femtosecond pulse was generally needed as the optical pump source. [18,21-24] On the It is one of the effective methods to modulate Fano resonance intensity by adding the thin photoactive material in the Fano metadevice and then light pumping them. However, most photoactive materials not only depend on high power femtosecond pulse, but also violently destroy the initial Fano resonance, thus deteriorating the modulated amplitude range (less than 0.35 in previous reports). This study introduces a terahertz (THz) Fano resonator based on Si photoactive substrate (SPS) which is modulated by low power 1064 nm continuous wave without additional photoactive materials. 90% modulation dep...

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