Highly tunable refractive index visible-light metasurface from block copolymer self-assembly

Abstract

The refractive index of natural transparent materials is limited to 2-3 throughout the visible wavelength range. Wider controllability of the refractive index is desired for novel optical applications such as nanoimaging and integrated photonics. We report that metamaterials consisting of period and symmetry-tunable self-assembled nanopatterns can provide a controllable refractive index medium for a broad wavelength range, including the visible region. Our approach exploits the independent control of permeability and permittivity with nanoscale objects smaller than the skin depth. The precise manipulation of the interobject distance in block copolymer nanopatterns via pattern shrinkage increased the effective refractive index up to 5.10. The effective refractive index remains above 3.0 over more than 1,000 nm wavelength bandwidth. Spatially graded and anisotropic refractive indices are also obtained with the design of transitional and rotational symmetry modification.

Figure 1. Formation of nanoparticle ensemble visible metamaterials.

(a) Schematic for metal nanoparticle ensemble preparation by (i) BCP self-assembly and substrate transfer and (ii) pattern shrinkage. (b) SEM images of hexagonal Au nanoparticle arrays as-prepared from BCP self-assembly (upside: plane view, downside: 60° tilted view). (c) SEM and AFM images of intermediate corrugated nanopattern structure observed during lateral pattern shrinkage. (d) SEM images of Au nanoparticle ensembles after complete pattern shrinkage (upside: plane view, downside: 60° tilted view). (e,f) Distribution of interparticle distance (e) and particle diameter before (red) and after (blue) shrinkage (f). (The statistical errors in e,f are presented based on Poisson distribution.)

Figure 2. Optical properties of metallic nanoparticle ensemble before and after shrinkage.

(a,b) Ultraviolet–vis spectroscopy and photographs of Au, Ag and Au–Ag alloy nanoparticle ensemble before (a) and after shrinkage (b). (c,d) Corresponding FDTD transmittance simulations of Au, Ag nanoparticle ensembles. (e) Simulated intensity profile of electric field between neighbouring Au nanoparticles. (f) Surface-enhanced Raman spectroscopy of Au nanoparticle ensemble for R6G.

Figure 3. Refractive index of metallic nanoparticle ensemble.

Ellipsometry measurements of (a) refractive indices and (b) extinction coefficients of Au, Ag and Au–Ag alloy nanoparticle ensembles before (dashed line) and after (solid line) pattern shrinkage.

Figure 4. Tunable refractive index metamaterials via symmetry modification.

(a) Schematic and photograph of nanoparticle ensemble with gradually varying interparticle distance by inhomogeneous thermal shrinkage. (b–d) Corresponding SEM images of unshrunken region (b), intermediate region (c) and fully shrunken region (d). (e,f) Nanoparticle ensembles with fourfold symmetry (square array; e) and twofold symmetry obtained from anisotropic shrinkage of hexagonal array in different directions (f). (g,h) SEM image anisotropic nanoparticle ensemble obtained from unidirectional stretching (g) and its birefringent UV-vis spectroscopy in response to polarized light (h). (i) Photograph of Au nanoparticle ensemble transferred on a conventional PDMS film surface.