This talk will report on the use of two different photoswitch-containing polymers for metasurface resonance tuning. Each photoswitch can be independently coated on a metasurface, and induce either red- or blueshifts in the metasurface resonance. However, the disparate wavelength tuning of the two photoswitches means they can also be used in combination. We demonstrate that light-responsive photoswitches can be employed alone, or in combination with other stimuli, to add advanced functionality to metasurface resonance tuning and enable potential applications in logic processing or optical neural networks.
Integrating metasurfaces into Liquid Crystal (LCs) cells is a suitable pathway for the realization of tunable optical devices. In such cells, the initial alignment of the LC molecules can be controlled by photoalignment layers. Here, we study the integration of a homogeneous silicon nanocylinder metasurface into an inhomogeneously aligned LC. To locally induce a change in the alignment direction of the LC starting from homogeneous exposure with x-polarized blue light, the photoalignment layers are re-exposed with structured y-polarized blue light. In the spatially-resolved transmittance spectra of the LC integrated metasurface, the double-exposed region can be identified by wavelength-dependent transmittance changes induced by the reorientation of the LC molecules and corresponding spectral shifts of the metasurface Mie resonances. Our results demonstrate that metasurfaces embedded into inhomogeneously aligned LCs allow for the controlled implementation of arbitrary spatial patterns. Possible applications include reconfigurable images, holograms, gratings and Fresnel zone plates.
Tunable and light-emitting metasurfaces have attracted increasing research interest in recent years. Here by combining liquid-crystal-integrated metasurfaces with a fluorescent substrate, we demonstrate active tuning of the emission spectrum and pattern in the red wavelength range. The measurements are performed with the techniques of momentum-space resolved spectroscopy and back-focal-plane imaging, showing a maximum of 16 nm shift of the emission wavelengths from 677 nm to 693 nm, and significant changes in the emission pattern at 660 nm. The results are further verified with numerical simulations. Our work paves the way towards actively controllable metasurface-based sources of complex light fields.
Ultrathin metasurfaces have shown the capability to influence all aspects of light propagation. This has made them promising options for replacing conventional bulky imaging optics while adding advantageous optical properties or functionalities. We demonstrate that such metasurfaces can also be applied for single-lens three-dimensional (3-D) imaging based on a specifically engineered point-spread function (PSF). Using Huygens’ metasurfaces with high transmission, we design and realize a phase mask that implements a rotating PSF for 3-D imaging. We experimentally characterize the properties of the realized double-helix PSF, finding that it can uniquely encode object distances within a wide range. Furthermore, we experimentally demonstrate wide-field depth retrieval within a 3-D scene, showing the suitability of metasurfaces to realize optics for 3-D imaging, using just a single camera and lens system.
Optical metasurfaces based on dielectric Mie-resonators were established as an efficient platform for realizing a multitude of optical functionalities. Recently, tunable optical dielectric metasurfaces have attracted increased research interest, and various tunable dielectric metadevices have been demonstrated. Infiltrating dielectric metasurfaces with nematic liquid crystals (LCs) represents an efficient and convenient tuning approach [1], which is compatible with established LC industrial technologies. Here we demonstrate two electrically tunable LC-infiltrated dielectric metasurfaces working at near-infrared and visible wavelengths, respectively. We demonstrate that the metasurfaces can be electrically tuned into and out of the so-called Huygens’ regime of spectrally overlapping electric and magnetic dipolar resonances by application of an external voltage. For the first time to our knowledge, we have utilized a LC photoalignment material [2] to realize LC-tunable metasurface devices with drastic improvement of their tuning performance and reproducibility. In particular, we demonstrate tuning of the metasurface transmission from nearly opaque to nearly transparent at 1070 nm. Furthermore, we demonstrate a switchable silicon transmissive display with 53% contrast, operating in the visible spectral range. Finally, we propose a novel route toward phase-only tuning by applying simultaneous electrical and thermal stimuli to the LC-infiltrated dielectric Huygens’ metasurfaces. In our numerical simulations, we observe 178° phase modulation with a transmittance exceeding 64% over the entire tuning range at 1078 nm wavelength.
[1] A. Komar et al., Appl. Phys. Lett. 110(7), 071109 (2017).
[2] I. I. Rushnova et al., Opt. Commun. 413, 179-183 (2018).
Nanoscale dielectric resonator antennas (DRAs) are promising elements for constructing the next generation of efficient and compact optical devices. Their efficient light manipulation capability underpinned by electric and magnetic resonances at visible frequencies is appealing for optical metasurfaces with various functions such as anomalous re ection, polarization conversion and surface plasmon coupling. To realize these functions, the resonance properties of the individual DRA elements are of critical importance. In this paper, we study the resonance breakdown of nanoscale cylindrical DRAs on metallic substrates. By gradually increasing the relative permittivity of DRAs on a metallic ground plane from low to high values, we observe two types of resonance breakdown and on that basis we can define a permittivity range for efficient resonance. More specifically, the resonance breakdown occuring at low DRA permittivities is a result of weak confinement and excessive radiation loss. The resonance breakdown at high DRA permittivities is a result of an elevated plasmonic loss at the metal- dielectric interface when the negative permittivity of the metal and the positive permittivity of the dielectric material have matched in their absolute values. The latter breakdown can be avoided by inserting a thin dielectric spacer with a low permittivity between the metal and dielectric. This study suggests important considerations for designing dielectric resonator metasurfaces at the visible frequencies.
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