Inverse design of near-infrared narrow-spacing achromatic metalenses based on an improved adaptive genetic algorithm

Nano-optical imaging technology is one of the key enabling approaches in ultrafast imaging and precision optical measurement systems. Ultrafast imaging typically involves the simultaneous acquisition of multiwavelength information within a narrow spectral interval, imposing stringent requirements on the specificity and precision of optical field manipulation at the subwavelength scale. However, conventional narrowband-achromatic metalens design strategies struggle to concurrently achieve high focusing efficiency and spatial resolution under dense-wavelength operation, which constrains their use in high-performance ultrafast imaging systems. To solve this problem, we propose an inverse design method for a near-infrared narrow-spacing multiwavelength TiO2 metalens based on an adaptive genetic algorithm, realizing multiwavelength achromatic focusing with equal focal length and equal intensity. By constructing a targeted multi-objective optimization function and introducing an adaptive adjustment strategy, the structural parameters of the metalens are efficiently optimized, enabling precise multiwavelength dispersion control capability. Numerical simulations demonstrate that under the incidence of four nonuniformly spaced near-infrared wavelengths (760, 772.5, 780, and 790 nm), the designed TiO2 metalens forms highly overlapping focal spots at the designed focal point, exhibiting good axial tolerance characteristics. The focusing efficiencies at the corresponding wavelengths reach 40.2%, 40.28%, 45%, and 45.2%, respectively. The full widths at half-maximum of the focal spots at different wavelengths measures 1.06, 1.06, 1.04, and 1.05 times their corresponding wavelengths, all of which surpass the diffraction limit defined by the Rayleigh criterion, indicating that the metalens has achieved achromatic focusing with subdiffraction resolution. This method provides an efficient design strategy for optical devices in large-scale, multiwavelength synchronously modulated ultrafast imaging systems, with promising applications in ultrafast dynamic process observation and precision imaging.