Theoretical study of dynamic focal field modulation via stretchable diatomic metalens.

Planar metalenses have distinct advantages over their traditional bulky refractive lens in terms of being lightweight and integrable. Their remarkable ability to modulate the phase and amplitudes of incident light without restrictions offers a revolutionary approach for a multitude of frontier applications. In recent years, tunability has become a prominent direction to pursue for the investigation of planar metalens. However, existing studies on tunable metalenses predominantly concentrate on adjusting the focal length to achieve zooming effects in optical imaging, while less attention has been dedicated to the tunability of the focal field itself. This aspect, if explored, could significantly broaden the scope and flexibility of their applications, particularly in multi-mode optical imaging. In this work, a flexible and stretchable metalens is proposed and theoretically demonstrated for the dynamic tuning of the focal field morphology. To fulfill the phase requirement during dynamic modulation, the diatomic coupled resonator is applied as the basic element, which possesses higher-order freedom of phase modulation capability. Through the symmetry reforming process by transverse stretching along the horizontal direction, the focal field of the metalens can be converted from a diffraction-limited airy spot into a uniform transverse optical needle. The length of the transverse optical needle can be precisely tailored according to the degree of deformation of the metalens. This research presents a method for light field modulation and holds extensive potential for applications in dual-mode laser-scanning confocal microscopy, laser processing, optical manipulation, etc.