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. 2023 Oct 25;15(42):49468-49477.
doi: 10.1021/acsami.3c09896. Epub 2023 Oct 10.

Thermoplasmonic Controlled Optical Absorber Based on a Liquid Crystal Metasurface

Affiliations

Thermoplasmonic Controlled Optical Absorber Based on a Liquid Crystal Metasurface

Francesca Petronella et al. ACS Appl Mater Interfaces. .

Abstract

Metasurfaces can be realized by organizing subwavelength elements (e.g., plasmonic nanoparticles) on a reflective surface covered with a dielectric layer. Such an array of resonators, acting collectively, can completely absorb the resulting resonant wavelength. Unfortunately, despite the excellent optical properties of metasurfaces, they lack the tunability to perform as adaptive optical components. To boost the utilization of metasurfaces and realize a new generation of dynamically controlled optical components, we report our recent finding based on the powerful combination of an innovative metasurface-optical absorber and nematic liquid crystals (NLCs). The metasurface consists of self-assembled silver nanocubes (AgNCs) immobilized on a 50 nm thick gold layer by using a polyelectrolyte multilayer as a dielectric spacer. The resulting optical absorbers show a well-defined reflection band centered in the near-infrared of the electromagnetic spectrum (750-770 nm), a very high absorption efficiency (∼60%) at the resonant wavelength, and an elevated photothermal efficiency estimated from the time constant value (34 s). Such a metasurface-based optical absorber, combined with an NLC layer, planarly aligned via a photoaligned top cover glass substrate, shows homogeneous NLC alignment and an absorption band photothermally tunable over approximately 46 nm. Detailed thermographic studies and spectroscopic investigations highlight the extraordinary capability of the active metasurface to be used as a light-controllable optical absorber.

Keywords: active control; colloidal nanoparticles; liquid crystals; lithography-free; metasurface; thermoplasmonics.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Schematic of the procedure adopted for preparing a colloidal metasurface by a bottom-up approach. The protocol involves functionalizing a 50 nm thick Au substrate by electrostatic layer by layer (eLbL) assembly with the PEM sequence PAH/PSS/PAH/PSS/PAH as a dielectric layer. Finally, after the final rinsing step with Millipore water, AgNCs were incorporated by drop-casting, and the system was incubated for 24 h at 4 °C in the dark and rinsed again with Millipore water before characterizations.
Figure 2
Figure 2
Metasurface morphology and optical characterization. Morphological analyses were performed by optical microscopy (a) and SEM microscopy (b). The inset of (b) shows the AgNCs monodispersed on the dielectric layer. Reflectance spectroscopy characterization of the metasurface. The inset reports a photograph of the metasurface (c). Schematic of the customized reflective fiber-coupled spectrophotometer equipped with an NIR laser used for the optical and thermospectrophotometric characterizations (d).
Figure 3
Figure 3
Reflectance spectroscopy characterization (a) of the metasurface cell before (blue, solid curve) and after (red, solid curve) infiltration with NOA 61. The dashed lines of the corresponding color show the results from numerical simulations for the empty cell (dashed blue line) and the cell filled with NOA 61 (dashed red line). The vertical red line indicates the reflectance of the metasurface sample at 808 nm. The optical shift of the metasurface peak accounts for the sensitivity to the n change. Photos of the as-prepared metasurface (b) and during (c) and at the end (d) of the infiltration process with NOA 61.
Figure 4
Figure 4
Photothermal investigation of the NOA 61-metasurface sample. Schematic of the thermo-optical setup designed to analyze the photothermal properties of the metasurface: the laser spot is focused on the center of the sample, which is monitored with a thermal camera to record the thermoplasmonic heating upon laser illumination. A power meter is placed behind the metasurface cell to measure the laser intensity (a). Plot reporting the maximum temperature (Tmax) values as a function of irradiation time for the metasurface cell infiltrated with air (blue curve) or NOA 61 (red curve) obtained by setting the laser intensity at 8.4 W/cm2. The inset reports a thermographic image of the cell infiltrated with NOA 61 acquired before turning off the laser (b).
Figure 5
Figure 5
Schematic of the NLC metasurface cell preparation. The NLC is sandwiched between the metasurface and a planarly photoaligned top cover glass substrate.
Figure 6
Figure 6
POM view of the NLC metasurface sample between crossed polarizers and the corresponding sample photos (a, b). The molecular director was aligned at 45° (a) and 0° (b) with respect to the polarizer/analyzer axis. Mueller matrix polarimeter characterization of the NLC metasurface sample (c).
Figure 7
Figure 7
Reflectance spectrum of the NLC metasurface cell (green line) showing a 74 nm red shift with respect to the reflectance spectrum of an empty metasurface cell (blue line) (a). Time–temperature profile of the NLC metasurface cell irradiated with a NIR laser (b). The inset shows a thermographic image of the NLC metasurface, acquired before shutting down the laser. Reflectance spectra of the NLC metasurface cell were measured at different NIR laser power densities. The spectra show the possibility of actively modulating the position of the NLC metasurface absorption band as a function of light intensity. The LC phase can be controlled from a nematic (green line, laser switched off) to an isotropic state (orange line, laser intensity at 8.4 W/cm2) induced by the photothermal heating of the metasurface (c). Absorption peak positions are a function of the laser intensity. A calibration curve is extrapolated by fitting the experimental points with a parabolic function. The inset reports numerical simulations of the reflectance spectra for the NLC metasurface cell in the nematic (green dashed line) and isotropic (orange dashed line) states (d).

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