Intensity-dependent modulation of optically active signals in a chiral metamaterial

Sean P Rodrigues,Shengxiang Wang,Wenshan Cai,Augustine M Urbas,Shoufeng Lan,Yonghao Cui,Patrick W Panuski,Lei Kang

doi:10.1038/ncomms14602

Abstract

Chiral media exhibit optical phenomena that provide distinctive responses from opposite circular polarizations. The disparity between these responses can be optimized by structurally engineering absorptive materials into chiral nanopatterns to form metamaterials that provide gigantic chiroptical resonances. To fully leverage the innate duality of chiral metamaterials for future optical technologies, it is essential to make such chiroptical responses tunable via external means. Here we report an optical metamaterial with tailored chiroptical effects in the nonlinear regime, which exhibits a pronounced shift in its circular dichroism spectrum under a modest level of excitation power. Strong nonlinear optical rotation is observed at key spectral locations, with an intensity-induced change of 14° in the polarization rotation from a metamaterial thickness of less than λ/7. The modulation of chiroptical responses by manipulation of input powers incident on chiral metamaterials offers potential for active optics such as all-optical switching and light modulation.

Highlights

Chiral media exhibit optical phenomena that provide distinctive responses from opposite circular polarizations
The difference in absorption between the two circular polarizations is commonly known as circular dichroism and is one of the quantitative measurements of optical activity
The other, optical rotatory dispersion, describes the rotation of a plane of polarization of a linearly polarized beam as it passes through the chiral medium, and can be calculated from the circular dichroism using Kramers–Kronig relations[1]

Summary

Introduction

Chiral media exhibit optical phenomena that provide distinctive responses from opposite circular polarizations. Chiral metamaterials typically have a lattice-constant factors smaller than the operating wavelength of interest, to instigate plasmonic resonances for chiral selective absorption The strength of these chiral metamaterials has been demonstrated in their applications as broadband circular polarizers[13,14], chiral mirrors[15], negative refractive materials[16] and large-scale chiroptical patterns[17]. The net effect of these nonlinear interactions influences the electric susceptibility, where the diagonal and non-diagonal terms of the tensor describe the manipulation of the optically induced birefringence and the nonlinear change in rotation angle respectively[19] It was not until 1979 that the electronic and thermal effects had been fully separated in crystals with chiroptical responses[23]. In measuring the nonlinear response, we demonstrate a blue-shift of 10 nm of the spectral resonance at 15 mW intensities and a nonlinear optical rotation coefficient of (1.76 Â 102)° cm W À 1

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Results

Conclusion