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Optical metasurfaces composed of designed Mie-resonant semiconductor nanoparticles arranged in a plane offer unique opportunities for controlling the properties of light fields [1]. Such metasurfaces can impose a spatially variant phase shift onto an incident light field, thereby providing control over its wave front with high transmittance efficiency. They can also e.g. act as polarizing optical elements, exhibit tailored nonlinear optical properties, or manipulate spontaneous emission processes of nanoscale emitters integrated in the metasurface architecture. However, the optical response of most semiconductor metasurfaces realized so far was permanently encoded into the metasurface structure during fabrication. Recently, a growing amount of research is concentrating on obtaining dynamic control of their optical response, with the aim of creating metasurfaces with functionalities that can be tuned, switched or programmed on demand.
This talk will provide an overview of our recent advances in actively tunable Mie-resonant semiconductor metasurfaces. In particular, by integrating silicon metasurfaces into a liquid-crystal (LC) cell, we can tune their linear-optical transmittance and reflectance spectra by application of a voltage [2]. In our work, we utilize a LC photoalignment material [3] during the assembly of the LC metasurfaces, leading to a drastic improvement of the tuning performance and reproducibility. Based on this method, we demonstrate electrical tuning of LC-infiltrated dielectric metasurfaces at near-infrared and visible wavelengths. We show that these metasurfaces can be tuned into and out of the so-called Huygens’ regime of spectrally overlapping electric and magnetic dipolar resonances, which is characterized by near-unity resonant transmission, by application of an external voltage. In particular, we demonstrate tuning of the metasurface transmission from nearly opaque to nearly transparent at 1070 nm. Furthermore, making use of the strong modulation of the metasurface response in combination with patterned electrodes, we experimentally demonstrate a transparent metasurface display device operating in the visible spectral range.
However, while the integration of silicon metasurfaces into nematic LC cells represents an efficient and versatile tuning approach showing large resonance shifts and strong tuning contrast, the switching times that can be achieved based on this approach are limited. Thus, as an alternative tuning mechanism allowing for ultrafast operation, we consider the transient changes of the optical properties of semiconductor materials when optically pumped by femtosecond laser pulses. These changes can lead to pronounced changes of the resonance condition for semiconductor metasurfaces at an ultrafast time scale. Our recent progress in ultrafast switching and tuning of semiconductor metasurfaces based on different material platforms and different physical mechanisms occurring at an ultrafast time scale will be discussed [4,5]. Furthermore, strategies to translate ultrafast tuning of metasurface resonances to ultrafast control of more complex metasurface functionalities such as wavefront shaping will be outlined.
[1] I. Staude & J. Schilling, Nature Photon. 11, 274 (2017).
[2] A. Komar et al., Appl. Phys. Lett. 110(7), 071109 (2017).
[3] I. I. Rushnova et al., Opt. Commun. 413, 179 (2018).
[4] M. R. Shcherbakov et al., Nano Lett. 15, 6985 (2015).
[5] M. R. Shcherbakov et al., Nat. Commun. 8, 17 (2017).
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