The pixels on the new metasurface consist of three types of silicon nanoblocks whose precise dimensions correspond to the wavelengths of three different colors: red, green, and blue. To enhance the efficiency for the blue light, two identical nanoblocks corresponding to the blue light are arranged in each pixel, along with one nanoblock for red light and one for green light.
The researchers explain that each pixel can be thought of as a "meta-molecule" because it is the basic repeating, subwavelength unit of the larger metasurface that constitutes the entire hologram. The meta-molecules enable the metasurface to control light in ways that are not possible without modern nanoscale design.
When red, green, and blue lasers illuminate the hologram, each nanoblock manipulates the phase of its corresponding color. The researchers explain that a key achievement of the study was to minimize the interactions between nanoblocks so that the nanoblocks function almost independently of each other. Then by orienting the nanoblocks in different ways, the researchers could change the light's phase manipulation, resulting in different holographic images.
Nanoletters - Visible-Frequency Dielectric Metasurfaces for Multiwavelength Achromatic and Highly Dispersive Holograms
Dielectric metasurfaces built up with nanostructures of high refractive index represent a powerful platform for highly efficient flat optical devices due to their easy-tuning electromagnetic scattering properties and relatively high transmission efficiencies. Here we show visible-frequency silicon metasurfaces formed by three kinds of nanoblocks multiplexed in a subwavelength unit to constitute a metamolecule, which are capable of wavefront manipulation for red, green, and blue light simultaneously. Full phase control is achieved for each wavelength by independently changing the in-plane orientations of the corresponding nanoblocks to induce the required geometric phases. Achromatic and highly dispersive meta-holograms are fabricated to demonstrate the wavefront manipulation with high resolution. This technique could be viable for various practical holographic applications and flat achromatic devices.