All-optical ultrafast polarization switching with nonlinear plasmonic metasurfaces

Abstract

Optical switching has important applications in optical information processing, optical computing, and optical communications. The long-term pursuit of optical switch is to achieve short switching time and large modulation depth. Among various mechanisms, all-optical switching based on Kerr effect represents a promising solution. However, it is usually difficult to compromise both switching time and modulation depth of a Kerr-type optical switch. To circumvent this constraint, symmetry selective polarization switching via second-harmonic generation (SHG) in nonlinear crystals has been attracting scientists' attention. Here, we demonstrate SHG-based all-optical ultrafast polarization switching by using geometric phase controlled nonlinear plasmonic metasurfaces. A switching time of hundreds of femtoseconds and a modulation depth of 97% were experimentally demonstrated. The function of dual-channel all-optical switching was also demonstrated on a metasurface, which consists of spatially variant meta-atoms. The nonlinear metasurface proposed here represents an important platform for developing all-optical ultrafast switches and would benefit the area of optical information processing.

Fig. 1.. All-optical polarization switching of the SH waves with a plasmonic metasurface.

(A) The geometrical parameters of a C_3h gold meta-atom. The arm length, width, and thickness of the meta-atom are 195, 80, and 30 nm, respectively. The meta-atoms are arranged in a hexagonal lattice with a period of 550 nm. (B) Different combinations of the two pump beams (red arrow) with H- (horizontal) and V- (vertical) polarization correspond to the generated SH waves (blue arrow) with H- or V-polarization. (C) Schematic diagram of all-optical polarization switching of SH waves with the plasmonic metasurface. When the time delay between two pump pulses is larger than the pulse duration, H-polarized SH waves are generated. When two pump pulses coincide in time domain, the V-polarized SH waves can be observed.

Fig. 2.. Nonlinear optical properties of the uniform plasmonic metasurface.

(A) SEM image of the plasmonic metasurface. Scale bar, 500 nm. (B) Measured wavelength-dependent SHG intensities for circularly polarized pump wave. Four kinds of circular polarization combinations of the pump waves and SH waves are recorded. (C) The polarization angle of the pump (α) and the SH wave (3θ − 2α), where θ is the orientation angle of the C₃ meta-atom. (D) Measured wavelength-dependent SHG intensities for linearly polarized pump wave. The results are normalized to the maximum SHG values in (B). H, horizontal polarization; V, vertical polarization; a.u., arbitrary units.

Fig. 3.. All-optical ultrafast polarization switching with the uniform plasmonic metasurface.

(A) Calculated delay-dependent intensities of the SH waves with H- and V-polarization, where τ₀ = 221.3 fs and the wavelength of 1220 nm of the pump wave are used for calculation. (B) Experimental setup for the ultrafast all-optical polarization switching. λ/2, half-wave plate; PBS, polarization beam splitter; M, mirror; DL, delay line; L, lens; S, sample; OL, objective lens; F, short-pass filter; P, linear polarizer; FM, flip mirror; SM, spectrometer. (C) Measured delay-dependent V-polarized SHG intensity from the uniform plasmonic metasurface. The delay-dependent SHG intensity is fitted by the Gaussian function (solid line) with a full-width at half-maximum (FWHM) of 521 fs. sCMOS, scientific complementary metal-oxide semiconductor.

Fig. 4.. All-optical ultrafast polarization switching with the dual-channel plasmonic metasurface.

(A) Schematic diagram of the dual-channel plasmonic metasurface with a τ pattern. In the region of τ (red), the meta-atoms are C_3v (one arm vertical), and the surrounding meta-atoms are C_3h (one arm horizontal). The SEM image of the metasurface is shown below (scale bar, 500 nm), in which the orange dashed line marks the boundary between the two kinds of meta-atoms. (B) Captured SHG images from the metasurface for the two pump waves with H- and V-polarization and different delay time (τ).