Broadband Achromatic Optical Needle by Axially Overlapping Dispersive Focal Spots of Different Wavelengths

ABSTRACT By axially overlapping dispersive focal spots of different wavelengths, this paper proposes broadband achromatic optical needles with controllable depth of focus generated based on reflective metalenses with binary or 4‐level geometric phase modulation. Phase profiles of the metalenses are optimized by using a genetic algorithm in combination with fast Hankel transform to efficiently achieve the desired optical needles. Only a single wavelength optimization is needed within this framework, and we devise several samples with depth of focus of 2, 4, and 6 µm under binary and 4‐level phase modulation, respectively. The results indicate that the proposed method yields the required achromatic performance of the optical needle in the wide waveband ranging from 572.8 to 732.8 nm. It is believed that this method offers a promising approach for applications in high‐resolution imaging, integrated spectral detection, and advanced display. Our design strategy can provide a new physical carrier and device solution for precise, parallel, and multicolor control of the light field in 3D space.