Nonlinear optical harmonic generation is of great significance in a broad range of technologies and has been attracting much attention in photonics, chemistry, and biosensing. Recently, with the advent of artificial electromagnetic materials, such as metamaterials and metasurfaces, that go beyond natural materials in many aspects, significant attention has been devoted to the nonlinear optical processes in artificial electromagnetic materials. In particular, epsilon-near-zero (ENZ) media with a vanishing permittivity are found to exhibit pronounced nonlinear optical properties. The ENZ media can provide large field enhancement due to the continuity of normal component of electric displacement field across the interface. Consequently, the ENZ media can give rise to unprecedented strong second-harmonic generation (SHG). Up to now, many theoretical and experimental works have demonstrated the SHG enhancement effect of the ENZ media.
However, very recently, the research group led by Prof. Jie Luo from Soochow University and Prof. Lei Gao from Suzhou City University, unveiled the SHG quenching effect of the ENZ media, which originates from the extraordinary evanescent scattering waves nearby the ENZ media. Based on this effect, a kind of dynamically controllable optical metasurfaces exhibiting switchable SHG quenching effect was proposed. The relevant research results are published in Photonics Research, Volume. 11, Issue 8, 2023 (Chenglin Wang, Ran Shi, Lei Gao, Alexander S. Shalin and Jie Luo. Quenching of second-harmonic generation by epsilon-near-zero media [J]. Photonics Research, 2023, 11 (8): 1437-1448).
The team showed that the ENZ media can be exploited to "turn off" the SHG from a nonlinear particle. When a tiny nonlinear particle is placed very close to a subwavelength ENZ particle, the SHG conversion efficiency is reduced by more than two orders of magnitude as compared to that from the nonlinear particle alone. The SHG quenching effect attributes to the extraordinary local evanescent fields occurring near the ENZ particle due to evanescent scattering waves. Remarkably, the prohibition of electric fields appears besides the ENZ particle, and therefore, the SHG from the tiny nonlinear particle placed there would be suppressed. The extraordinary evanescent scattering waves were found to exist for both isotropic and anisotropic ENZ particles, irrespective of their shapes (Figs. 1 and 2).
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Fig. 1. (a) Schematic illustration of SHG from a tiny nonlinear particle. (b) The SHG is quenched when a linear ENZ particle is placed very close to the tiny nonlinear particle
Fig. 2. (a) Schematic layout of the configuration for exploring the SHG quenching effect. It is composed of a linear ENZ spherical particle accompanied with a tiny nonlinear spherical particle in the deep-subwavelength scale. (b) The blue solid lines denote the normalized scattering cross section of SHG SCS2ω as a function of the position of the nonlinear particle along the trajectory in (a). The red dots show the SCS2ω from the nonlinear particle alone, which is normalized to 1.
Based on the principle of SHG quenching effect, the team proposed a kind of dynamically controllable optical metasurface integrated with anisotropic ENZ media, which consist of alternative layers of semiconductor material cadmium oxide (CdO) and phase-change material germanium telluride (GeTe). It was demonstrated that through changing the phase states of GeTe, the SHG from the metasurface can be switched on or off. This demonstrated a feasible approach for controlling nonlinear responses with ENZ media, which may find applications in optical switches and modulators (Fig. 3).
Fig. 3. [(a) and (b)] Schematic graphs of an optical metasurface which can be dynamically switched to exhibit (a) high, (b) low SHG conversion efficiency through controlling the phase states of its constituent GeTe. (c) The real part of εy,eff, i.e., Re(εy,eff), of the central CdO-GeTe multilayer with GeTe in crystalline (blue) or amorphous (red) phase as the function of wavelength. The inset shows the imaginary part of εy,eff, i.e., Im(εy,eff). (d) The SHG energy from the metasurface with its constituent GeTe in crystalline (green) or amorphous (yellow) phase.
The authors highlighted their finding: this work breaks the traditional conception that the ENZ media are always responsible for SHG enhancement and enriches the understanding of the experimental observation of optical nonlinearity. In addition, this work paves a new way for SHG signal manipulation.
