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Metasurfaces composed of designed Mie-resonant semiconductor nanoparticles offer unique opportunities for controlling the properties of light fields transmitted through them or reflected from them [1]. Such metasurfaces can e.g. impose a spatially variant phase shift onto an incident light field, thereby providing control over its wave front with high transmittance efficiency [2]. However, most semiconductor metasurfaces realized so far were passive and linear, and their optical response was permanently encoded into the structure during fabrication. Recently, a growing amount of research is concentrated on the integration of emitters and optical nonlinearities into semiconductor metasurfaces and on obtaining dynamic control of their optical response.
This talk will provide an overview of our recent advances in active, nonlinear, and tunable Mie-resonant semiconductor metasurfaces. In particular, we have studied spontaneous emission from metasurfaces incorporating various types of emitters, including semiconductor quantum dots, monolayers of transition metal dichalcogenides, and fluorescent centers in the substrate. For emission frequencies below or near the fundamental electronic bandgap of the respective semiconductor material, the individual Mie-resonant metasurface building blocks can provide the functionality of dielectric nanoantennas, and exhibit high directivity, Purcell enhancement, and near-unity radiation efficiencies [3,4]. Consequently, we may understand an active metasurface as a two-dimensional array of resonant dielectric nanoantennas, which are excited by localized emitters that are located in their vicinity. In order to characterize the emission properties of our active metasurfaces, we perform micro-photoluminescence imaging, spectroscopy, and Fourier imaging for a variety of different active metasurface architectures. Our results show that the directional and spectral properties of the emitted light can be tailored by the metasurface design.
Using nonlinear Fourier imaging, we have also investigated second harmonic generation in metasurfaces composed of III-V semiconductors. We show that the generated second harmonic signal depends sensitively on the polarization of the pump field.
For dynamic tuning of the metasurface response, we make use of the strong spectral dispersion associated with the resonant optical response of the metasurfaces in combination with the sensitivity of the resonance properties on the dielectric environment of the individual nanoresonators. Specifically, by integrating a silicon metasurface into a nematic-liquid-crystal cell, we have demonstrated dynamic tuning of the metasurface linear-optical response using an applied voltage as control parameter [5]. Finally, by combining resonance tuning based on liquid crystals with spontaneous emission enhancement by Mie-type resonances, we have achieved dynamic control of the emission from an active metasurface consisting of silicon nanoresonators coupled to a fluorescent substrate.
[1] I. Staude & J. Schilling, Nature Photon. 11, 274–284 (2017).
[2] K. E. Chong et al., Nano Lett. 15, 5369–5374 (2015).
[3] A. E. Krasnok et al., Opt. Exp. 20, 20599 (2012).
[4] X. Zambrana-Puyalto & N. Bonod, Phys. Rev. B 91, 195422 (2015).
[5] A. Komar et al., Appl. Phys. Lett. 110, 071109 (2017).
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