Soft optical metamaterials

Abstract

Optical metamaterials consist of artificially engineered structures exhibiting unprecedented optical properties beyond natural materials. Optical metamaterials offer many novel functionalities, such as super-resolution imaging, negative refraction and invisibility cloaking. However, most optical metamaterials are comprised of rigid materials that lack tunability and flexibility, which hinder their practical applications. This limitation can be overcome by integrating soft matters within the metamaterials or designing responsive metamaterial structures. In addition, soft metamaterials can be reconfigured via optical, electrical, thermal and mechanical stimuli, thus enabling new optical properties and functionalities. This paper reviews different types of soft and reconfigurable optical metamaterials and their fabrication methods, highlighting their exotic properties. Future directions to employ soft optical metamaterials in next-generation metamaterial devices are identified.

Keywords: Metamaterials; Metasurfaces; Nanofabrication; Nanophotonics; Reconfigurable metamaterials; Soft matter.

Fig. 1

Liquid-crystal (LC) and biomaterials-based metamaterials and metafluids. a An illustration of a typical LC-based metamaterial, where metallic structures are dispersed in a LC solution. LC-based metamaterials can achieve optical properties tuning by changing the alignment and orientation of LC molecules via external voltage. b The LC-infiltrated metamaterial shows a refractive index ranging from negative to zero and positive values by adjusting the permittivity of LCs. c An illustration of metafluid, where metal-dielectric metamolecules are dispersed in a fluid. Metafluid allows the dispersion of different materials or metamolecules to integrate different optical properties in order to attain exotic effective material properties. d The transmission spectrum of a metafluid containing boron-doped silicon nanocrystals and five types of Au nanorods (AuNRs) with different geometries. This metafluid possesses a transparency window at 890 nm, with a narrow window width of 0.22 eV. The transparency window is guided by the superposition of the five types of AuNRs whose transmission spectra are shown in the inset. e An illustration of a silk protein-based biomaterial metamaterial, where metal nanoparticles are doped in the silk protein structure. f The reflection spectra of the bio-compatible silk plasmonic absorber sensor (SPAS) immersed in air (refractive index n = 1), isopropyl alcohol (IPA, n = 1.37), and water (n = 1.32). Larger refractive index can result in reflection resonance redshift in SPAS, facilitating its environment sensing application. b Reprinted with permission from [52], Copyright (2007) OSA Publishing. d Reprinted with permission from [66], Copyright (2016) ACS Publications. f Reprinted with permission from [72], Copyright (2015) ACS Publications

Fig. 2

Polymer-based optical metamaterials. a A large-area (8.7 cm × 8.7 cm) flexible PDMS based Ag negative index metamaterial (NIM) that shows negative index of refraction at optical frequencies. PDMS has the advantage of enabling direct pattern transfer, benefiting from PDMS’s low surface energy. b The transmission (T) and reflection (R) spectra of the large-area PDMS-based Ag NIM metamaterial. The metamaterial shows negative RI at a wavelength range of 1.7 µm to 2.4 µm. c An illustration of a flexible metamaterial consists of Au nanodisk array on top of an SU-8 substrate. SU-8 has high chemical and thermal resistance and good mechanical properties, rendering it useful as a flexible metamaterial substrate. d The reflection and transmission spectra of a flat (left) and a bent (right) SU-8-based flexible gold nanodisk metamaterial. The reflection is dependent on bending, but transmission of this flexible metamaterial is invariant of bending. e A scanning electron microscopy (SEM) image of a PEN-based Au split ring resonator (SRR) metamaterial. PEN has a high glass transition temperature and is transparent in visible and near-infrared (NIR) wavelength ranges. f The transmission spectra across visible and NIR wavelength ranges of a PEN-based Au SRR metamaterial with and without an applied out-of-plane strain. Given a strain of 1232 Pa, the electric peak shifts from 894 to 973 nm, showing a sensitivity of 0.06 nm/Pa. a, b Reprinted with permission from [81], Copyright (2011) Nature Publishing Group. c, d Reprinted with permission from [89], Copyright (2011) AIP Publishing LLC. e, f Reprinted with permission from [91], Copyright (2011) ACS Publications

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 > T_c) 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

Fig. 4

Fabrication methods of soft optical metamaterials. a An illustration of the direct top-down patterning of an SU-8 based metamaterial with a sacrificial layer. After the patterning, SU-8-based Au soft metamaterial can be realized by releasing the sacrificial layer. b A PDMS-based flexible metamaterial fabricated by stencil lithography. First, a stencil is made and placed above the PDMS substrate with a precise control of the gap distance. Then, gold is deposited through the stencil onto the substrate, forming bowtie nanostructures. c Nanoimprint lithography for fabricating a large-area PDMS-based flexible metamaterial. First, a stamp consists of silicon wafers is ‘inked’. Then, the stamp is contacted against a target PDMS substrate. Finally, residual material on the stamp is removed to prepare for re-usage. d A protein-assisted self-assembly process to fabricate an AgNP-polystyrene metafluid. AgNPs are first functionalized with biotin terminated ligands (i), and then mixed with a high ionic solution of streptavidin-coated polystyrene NPs (ii) to form a metafluid consisting AgNP-polystyrene metamolecules (iii). Here, the highly specific chemical recognition between protein streptavidin and biotin ligands ensures the AgNPs to be closed packed around the polystyrene NPs. e Schematic illustration of DNA-programmable nanoparticle crystallization where the length of the linker strands can be tuned by increasing the value of n. DSP refers to the cyclic dithiol. a Reprinted with permission from [89], Copyright (2011) AIP Publishing LLC. b Reprinted with permission from [94], Copyright (2011) Wiley–VCH. c Reprinted with permission from [81], Copyright (2011) Nature Publishing Group. d Reprinted with permission from [65], Copyright (2013) ACS Publications. e Reprinted with permission from [143], Copyright (2013) Wiley–VCH