Switchable multi-channel electromagnetic induced transparency (EIT) is achieved in designed terahertz metasurfaces. The underlying physics of the EIT effect is analyzed. The influence of structure parameters on the EIT effect is investigated. A metasurface with a symmetrical unit cell is proposed, by which polarization-independent EIT is achieved. In addition, photosensitive silicon is integrated into the metal layer aiming for active modulation. Tunable EIT with adjustable peak frequency and amplitude is obtained. Moreover, the dynamic control is not limited to a single EIT transparency window. We show that the number of EIT transparency windows can be tailored, and switchable multi-channel EIT are successively realized. The results would provide significant guidance in multifunctional active devices, such as modulators and switches in nanophotonics optics.
A symmetry-breaking nanostructure is proposed to achieve multiple Fano resonances. The nanostructure consists of an asymmetric ring resonator coupled to a plasmonic waveguide. The broken symmetry is introduced by deviating the centers of regular ring. New resonant modes that are not accessible through a regular symmetric ring cavity are excited. Thus, one asymmetric cavity can provide more than one resonant mode with the same mode order. As a result, the interval of Fano resonances is greatly reduced. By combining different rings with different degrees of asymmetry, multiple Fano resonances are generated. Those Fano resonances have different dependences on structural parameters due to their different physical origin. The resonance frequency and resonance peak number can be arbitrarily adjusted by changing the degree of asymmetry. This research may provide new opportunities to design on-chip optical devices with great tuning performance.
We develop a rigorous theory to describe coupled multiple nonlinear processes in a single nonlinear multilayer structure using iteration technique and transfer-matrix method. Pump depletion is taken into account. The validity of the theory is confirmed by analyzing coupled third-harmonic generation (CTHG). The CTHG process consists of two processes: second-harmonic generation and sum-frequency generation. These two processes are coupled together when their phase-matching conditions are satisfied simultaneously. The conversion efficiencies of second-harmonic and third-harmonic under various phase-matching conditions are studied numerically. The results demonstrate that our theory can efficiently and accurately deal with the coupled nonlinearity problem. The model can deal with periodic or aperiodic nonlinear structures. Our approach may be helpful in designing nonlinear photonic crystals and quasi-phase-matching structures.
Quadratic parametric processes in a one-dimensional nonlinear structure were investigated by using a nonlinear transfer matrix and iterative algorithm. Our method can provide an accurate description in the regime of pump depletion. Detailed analysis shows iteration is important for achieving a precise result. In order to simplify the complex iterative derivation, we develop a simplified model for quasi-phase-matched (QPM) second harmonic generation. We extend the simplification strategy to treat other quadratic nonlinear interactions in a QPM structure. The validity of simplification is demonstrated by numerically comparing conversion efficiencies. Our method provides a simple and efficient way to investigate secondorder parametric processes in QPM structures.
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