A team of researchers from Tel Aviv University has developed a novel metasurface that can change its optical properties in response to electric and light signals. The metasurface, which consists of a layer of liquid crystals on top of a gold nanorod array, can exhibit nonlocal effects that enhance the generation of second-harmonic light, a nonlinear optical phenomenon that doubles the frequency of the incoming light.
Nonlocal Effects on Metasurfaces
Metasurfaces are thin layers of nanostructures that can manipulate light in various ways, such as focusing, steering, polarizing, or filtering. They have applications in holography, imaging, sensing, communication, and information processing. However, most metasurfaces are passive and fixed, meaning that they cannot be tuned or switched to perform different functions.
Nonlocal effects on metasurfaces occur when the nanostructures interact with each other collectively rather than individually, resulting in a hybrid mode that spans over the entire metasurface. This mode can have a high quality factor and a narrow spectral linewidth, which can enhance the light-matter interaction and promote nonlinear optical effects, such as frequency conversion and stimulated scattering.
Nonlinear optical effects are desirable for many applications, as they can generate new frequencies of light, modulate the intensity or phase of light, or produce entangled photon pairs. However, they are usually weak and require high-intensity light sources or large optical devices. Nonlocal metasurfaces can overcome these limitations by confining and enhancing the light field at the nanoscale.
Electric and All-Optical Tunability of Nonlocal Second-Harmonic Generation
The researchers from Tel Aviv University have demonstrated a new way to achieve electric and all-optical tunability of nonlocal second-harmonic generation (SHG) from a nonlinear metasurface. SHG is a process that converts two photons of the same frequency into one photon of twice the frequency, resulting in a color change of the light.
The metasurface they designed consists of a layer of twisted nematic liquid crystals (LCs) on top of a gold nanorod array. The LCs are molecules that can change their orientation and refractive index when subjected to an electric field or light. The gold nanorods are metallic nanostructures that support localized surface plasmon resonances (LSPRs), which are collective oscillations of electrons at the surface of the metal.
When the metasurface is illuminated by a laser beam at a certain angle, it can excite a nonlocal mode that couples the LSPRs of the nanorods with the diffraction orders of the array. This mode is called a surface lattice resonance (SLR) and it has a high quality factor and a narrow spectral linewidth. The SLR can enhance the SHG from the metasurface by several orders of magnitude.
The researchers found that they can tune or switch the SHG from the metasurface by applying an electric voltage or another laser beam to the LC layer. The electric voltage can change the orientation and refractive index of the LCs, which in turn can modify the SLR condition and affect the SHG efficiency. The researchers observed more than 25 dB electrical switching amplitude of the SHG from the metasurface.
The second laser beam can also change the orientation and refractive index of the LCs by inducing a phase transition from nematic to isotropic state. This phase transition can be imprinted on the SHG signal, creating an all-optical modulation effect. The researchers demonstrated that they can use this effect to encode information on the SHG signal by modulating the second laser beam with different patterns.
A Promising Route for Active Nonlinear Optical Metadevices
The researchers claim that their work introduces a promising route for active nonlinear optical metadevices at the nanoscale. They envision that their metasurface can be used for sensing, signal processing, encryption, and communication applications. They also suggest that their approach can be extended to other nonlinear optical effects and other types of LCs or phase-change materials.
The research was published in Science Advances 1 and was supported by grants from the Israel Science Foundation, the European Research Council, and the German-Israeli Foundation for Scientific Research and Development.