Compact metalens-based integrated imaging devices for near-infrared microscopy

Scannable Dual-Focus Metalens with Hybrid Phase

Metalenses with two foci in the longitudinal or transverse direction, called bifocal or dual-focus metalenses, are promising building blocks in tomography techniques, data storage, and optical tweezers. For practical applications, relative movement between the beam and specimen is required, and beam scanning is highly desirable for high-speed operation without vibration. However, dual-focus metalenses employ a hyperbolic phase that experiences off-axis aberrations, which is not suitable for beam scanning. Here, we demonstrated a scannable dual-focus metalens by employing a new phase called "hybrid phase". The hybrid phase consists of a hyperbolic phase inside and a quadratic phase outside to reduce off-axis aberrations while maintaining a high numerical aperture. We show that the two foci of the scannable dual-focus metalens move together without severe distortion for incident angles of up to 2.5°. Our design easily extends to the case of multifocusing, which is essential for various applications ranging from imaging to manipulation.

Dielectric metalens for miniaturized imaging systems: progress and challenges

Lightweight, miniaturized optical imaging systems are vastly anticipated in these fields of aerospace exploration, industrial vision, consumer electronics, and medical imaging. However, conventional optical techniques are intricate to downscale as refractive lenses mostly rely on phase accumulation. Metalens, composed of subwavelength nanostructures that locally control light waves, offers a disruptive path for small-scale imaging systems. Recent advances in the design and nanofabrication of dielectric metalenses have led to some high-performance practical optical systems. This review outlines the exciting developments in the aforementioned area whilst highlighting the challenges of using dielectric metalenses to replace conventional optics in miniature optical systems. After a brief introduction to the fundamental physics of dielectric metalenses, the progress and challenges in terms of the typical performances are introduced. The supplementary discussion on the common challenges hindering further development is also presented, including the limitations of the conventional design methods, difficulties in scaling up, and device integration. Furthermore, the potential approaches to address the existing challenges are also deliberated.

Theoretical Comparison of Optothermal Absorption in Transmissive Metalenses Composed of Nanobricks and Nanoholes

Background: Optical components with high damage thresholds are very desirable in intense-light systems. Metalenses, being composed of phase-control nanostructures with peculiar properties, are one of the important component candidates in future optical systems. However, the optothermal mechanism in metalenses is still not investigated adequately. Methods: In this study, the optothermal absorption in transmissive metalenses made of silicon nanobricks and nanoholes is investigated comparatively to address this issue. Results: The geometrical dependencies of nanostructures’ transmittance, phase difference, and field distribution are calculated numerically via simulations. To demonstrate the optothermal mechanism in metalenses, the mean absorption efficiencies of the selected unit-cells, which would constitute metalenses, are analyzed. The results show that the electric field in the silicon zone would lead to an obvious thermal effect, and the enhancement of the localized electric field also results in the strong absorption of optical energy. Then, two typical metalenses are designed based on these nanobricks and nanoholes. The optothermal simulations show that the nanobrick-based metalens can handle a power density of 0.15 W/µm2, and the density of the nanohole-based design is 0.12 W/µm2. Conclusions: The study analyzes and compares the optothermal absorption in nanobricks and nanoholes, which shows that the electric-field distribution in absorbent materials and the localized-field enhancement are the two key effects that lead to optothermal absorption. This study provides an approach to improve the anti-damage potentials of transmissive metalenses for intense-light systems.