Metasurfaces—structured arrays of nanoresonators—are becoming more and more essential in our every-day life. They are already employed in smartphone cameras, are being developed for LiDAR and 3D sensing, and will constitute even more devices in the near future.

One of the most intriguing applications of metasurfaces is nonlinear frequency conversion, in particular second-harmonic generation: two photons of incoming light are converted into a single photon with twice the frequency (half the wavelength). Metasurfaces can enhance and control this process despite being very thin. Their performance in nonlinear frequency conversion mainly depends on two parameters: geometry, i.e., the shape of nanoresonators and their arrangement, and the material. After fabrication metasurface properties can still be tuned, e.g., by changing its environment with added liquid crystals, by applying voltage, or by heating. However, these methods will not only affect nonlinear frequency conversion, but also linear properties like transmittance or reflectance. A method to independently control the linear and nonlinear response of metasurfaces did not exist so far.

In the work done by a research team from the Institute of Applied Physics and the Institute of Solid State Physics at Friedrich Schiller University Jena, Germany. Researchers harvest this effect and modify second-harmonic generation while keeping the linear properties of metasurfaces constant. They realize metasurfaces from lithium niobate with periodically inverted nonlinearity. This inversion creates a diffraction grating affecting only the second harmonic. The relevant research results were published in Photonics Research, Volume 11, No. 2, 2023 (Anna Fedotova, Mohammadreza Younesi, Maximilian Weissflog, Dennis Arslan, Thomas Pertsch, Isabelle Staude, Frank Setzpfandt. Spatially engineered nonlinearity in resonant metasurfaces[J]. Photonics Research, 2023, 11(2): 252).

Researchers explore metasurfaces from lithium niobate, a dielectric widely used in integrated optics and photonics. Besides having many other advantageous properties, lithium niobate is ferroelectric: in its crystal structure, negative and positive charges are separated creating a spontaneous polarization. When applying a strong electric field, the ion positions in the crystal lattice can be switched. This correspondingly changes the sign of the nonlinearity, e.g. the second-order nonlinear susceptibility χ⁽²⁾ of the material as schematically shown by the blue and red columns in Fig. 1a. This switching process is called electric-field poling. Interestingly, linear properties of a poled metasurface such as transmittance or reflectance remain unchanged, as shown in Fig. 1b, while the second-harmonic response based on the second-order nonlinear susceptibility is altered.

When exciting these metasurfaces with a pulsed femtosecond laser, second harmonic generated in them is diffracted by two diffraction gratings: one formed by the metasurface periodic geometry itself and one in the nonlinearity induced by poling.

By varying the size of domains where the nonlinearity is inverted from 0.5 µm to 2 µm (poling period from 1 µm to 4 µm) but keeping the other geometry parameters of the metasurface the same, researchers show how the diffraction pattern changes. An example for this is depicted in Figs. 1c and d. The poled metasurface in Fig. 1c has six additional diffraction orders compared to a non-poled one.

Figure 1. (a) Scanning electron micrograph of a metasurface, false red color highlights poled nanoresonators and blue color non-poled ones. (b) Experimental (solid) and simulated (dotted) transmittance spectra of poled (red) and non-poled (blue) metasurfaces. Diffraction patterns of second harmonic generated in (c) poled and (d) non-poled metasurfaces.

In this work, researchers demonstrated that electric-field poling adds another degree of freedom for designing nonlinear metasurfaces and enhances their capabilities. The next step in this journey can be poling metasurfaces aperiodically and creating patterns to shape the second-harmonic emission, which opens up another route to manipulate its directionality and can be used for nonlinear holograms.