Fig. 3
Reconfigurable optical metamaterials triggered by different stimuli. a Illustration of an ethyl red based optically reconfigured metamaterial. Upon green laser excitation (control beam), the isomeric state of the ethyl red layer is changed, which results in a refractive index (RI) change and thus switching the optical properties of the soft metamaterial. b (Upper panel) Under a 4 mW green light excitation, both polarization azimuth angle ϕ and ellipticity angle χ witness blue shifts. (Lower panel) By increasing the control beam intensity, the transmitted ellipticity angle increases at a peak wavelength range of 760–820 nm. c Schematic of an electrically reconfigurable polyaniline (PANI) nanoparticle-on-mirror (NPoM) soft metamaterial unit cell. When voltage is applied, the thin PANI layer surrounding the AuNP will undergo a redox process where electrons transfer from PANI to the Au mirror underneath. This leads to a change in RI and the associated optical characteristics. d Measured scattering spectra of the scalable PANI NPoM metamaterial. By increasing the voltage from -0.3 V to 0.8 V, the resonance wavelength blue-shifted 79 nm (Δλ = c0 − c2+) due to the oxidization of PANI coating to PANI2. e LC-incorporated dielectric metamaterial for thermally tunable spontaneous emission. The device consists of silicon nanodisks embedded in LCs on a fluorescent glass substrate. Below a critical temperature of 58 °C, the LC remains in a nematic phase (left). Upon heating over the critical temperature, the LC changes to an isotropic phase (right) and the RI of LCs varies, enabling thermally tunable spontaneous emission. f Spontaneous emission spectra of the metamaterial in (e) at different temperatures. Heating above the critical temperature (T > Tc) of 58 °C leads to a pronounced red shift of the emission peak. g A mechanically reconfigurable metasurface consists of AuNRs on a PDMS substrate with a tunable focal length. h Measured longitudinal beam profiles of the AuNR/PDMS metasurface with different stretch ratios. The focal length can be continuously tuned by stretching. a, b Reprinted with permission from [97], Copyright (2017) Nature Publishing Group. c, d Reprinted with permission from [110], Copyright (2019) American Association for the Advancement of Science. e, f Reprinted with permission from [119], Copyright (2018) ACS Publications. g, h Reprinted with permission from [130], Copyright (2016) ACS Publications