AI-based Metasurface Lens Design

Introduction

Conventional optical imaging systems are bulky and complex, requiring multiple elements to correct aberrations. Optical metasurfaces, planar structures that are capable of manipulating light at subwavelength scales, are compact alternatives to conventional refractive optical elements. Their miniature volume is suitable for technologies like AR/VR displays and wearables. However, existing metalenses face significant challenges from monochromatic (e.g., coma) and chromatic aberrations, limiting their applicability. Here we present an end-to-end AI-based computational method that parametrizes the profile of metalenses and optimizes it based on customized loss functions. This innovation enables wide-angle imaging with corrected aberrations while retaining a single-layer form factor, overcoming the key limitations of existing metalenses and advancing their potential for miniaturized imaging systems.

Background

What Is Metasurface?

A metasurface is an two-dimensional artificially engineered material composed of subwavelength-scaled patterns. Metasurfaces manipulate electromagnetic waves, e.g. light waves, through specific boundary conditions imposed at their interfaces, unlike conventional materials which manipulate EM waves through its bulk properties, such as refractive index. Metasurface's unique approach enables precise control over wavefronts, allowing for innovative applications such as planar lenses (metalenses) and holograms. Their thin profile and design flexibility are important in integrated photonics and advanced imaging systems.

File:Fig-metasurface.png

Figure 1: Metasurfaces with various nanostructures. (Source: Meinzer 2014)

Metasurface Optic

Transfer function of conventional thin refractive lenses has amplitude and phase. Phase change, of an incident plane wave propagating through the lens, will leads to a change in the wave vector. That is, the incident plane wave will change its propagating direction. In a metalens, the phase is induced via the response of nanostructures. By applying local, gradient phase shifts to incoming waves, metasurfaces generalize the conventional laws of reflection and refraction, called generalized Snell's law. In the equation below, ${\theta }_t$, ${\theta }_i$ are the reflection/refraction and incident angle, respectively; $n_t$, $n_i$ are the refractive index of the corresponding material; $\lambda$ is the wavelength of the light; and $\Phi$ is the phase profile of the metalens.

$$n_t\sin {\theta }_t-n_i\sin {\theta }_i=\frac{\lambda }{2\pi }\frac{\text{d}\Phi }{\text{d}x}$$

For conventional laws of reflection and refraction, the term on the right hand side of the equation is 0. The generalized Snell's law implies that we can control the refraction angle by designing the phase profile of the metalens.

File:Gsl.png

Figure 2: Generalized Snell's law. (Source: Yu 2011)

Hyperbolic Phase Profile

Hyperbolic phase profile is commonly used for metalens, given by the equation below, where $\rho =\sqrt{x^2+y^2}$ and $(x,y)$ represents the coordinate at the aperture plane; $f$ is the designed focal length; and $\lambda$ is the wavelength of light that the metalens designed for.

$$\Phi (\rho )=-\frac{2\pi }{\lambda }(\sqrt{{\rho }^2+f^2}-f)$$

In Fig. 3, the amplitude, which is fixed to 1, and the hyperbolic phase is shown. This metalens has the following specs:

• Radius $R=100\mu m$,
• Focal length $f=200\mu m$,
• Numerical aperture $NA=0.89$,
• Pixel size $330nm$.

File:Hyperbolic phase.png

Figure 3: The constant amplitude profile and the hyperbolic phase profile of metalens.

Appendix

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