Reconfigurable all-dielectric metalens with diffraction limited performance (1911.12970v2)

Abstract: Active metasurfaces, whose optical properties can be modulated post-fabrication, have emerged as an intensively explored field in recent years. The efforts to date, however, still face major performance limitations in tuning range, optical quality, and efficiency especially for non mechanical actuation mechanisms. In this paper, we introduce an active metasurface platform combining phase tuning covering the full 2$\pi$ range and diffraction-limited performance using an all-dielectric, low-loss architecture based on optical phase change materials (O-PCMs). We present a generic design principle enabling switching of metasurfaces between two arbitrary phase profiles and propose a new figure-of-merit (FOM) tailored for active meta-optics. We implement the approach to realize a high-performance varifocal metalens operating at 5.2 $\mu$m wavelength. The metalens is constructed using Ge2Sb2Se4Te1 (GSST), an O-PCM with a large refractive index contrast ($\Delta$ n > 1) and unique broadband low-loss characteristics in both amorphous and crystalline states. The reconfigurable metalens features focusing efficiencies above 20% at both states for linearly polarized light and a record large switching contrast ratio of 29.5 dB. We further validated aberration-free imaging using the metalens at both optical states, which represents the first experimental demonstration of a non-mechanical active metalens with diffraction-limited performance.

• The paper demonstrates a reconfigurable, non-mechanical metalens design using GSST that achieves diffraction-limited performance for varifocal imaging.
• It achieves a record 29.5 dB switching contrast and focusing efficiencies above 20% by discretizing continuous phase profiles into four levels.
• Experimental tests with a 5.2 μm laser validate high Strehl ratios and aberration-free imaging, highlighting its potential for adaptive optical systems.

Reconfigurable All-Dielectric Metalens with Diffraction-Limited Performance: An Expert Analysis

The paper presented in the paper "Reconfigurable all-dielectric metalens with diffraction-limited performance" explores the domain of active metasurfaces, focusing on their ability to dynamically tune optical responses without relying on mechanical actuation. The authors have introduced an innovative metasurface platform that utilizes optical phase change materials (O-PCMs) with full 2π phase tuning capabilities and diffraction-limited performance. The key contribution of this research is the realization of a high-performance varifocal metalens operating at a wavelength of 5.2 μm, fabricated using the optical material Ge2Sb2Se4Te1 (GSST) due to its large refractive index contrast and unique low-loss properties in both amorphous and crystalline states.

Major Contributions and Findings

The authors have proposed a generic design principle for enabling the switching of metasurfaces between two arbitrary phase profiles and introduced a novel figure-of-merit (FOM) tailored specifically for active meta-optics. The design achieves a high record switching contrast ratio of 29.5 dB, marking the highest reported value in active metasurface devices to date. The metalens demonstrated focusing efficiencies above 20% in both amorphous and crystalline states for linearly polarized light. This work presents the first experimental demonstration of a non-mechanical active metalens achieving diffraction-limited performance, validated through aberration-free imaging.

The authors utilized a systematic design approach whereby GSST Huygens meta-atoms were patterned on a CaF2 substrate. The bi-state varifocal metalens implemented two standard hyperbolic phase profiles that yield focal lengths of 1.5 mm and 2 mm, corresponding to numerical aperture (NA) values of 0.45 and 0.35, respectively. By discretizing the continuous phase profiles into four phase levels, a total of 16 distinct meta-atom designs were utilized, ensuring optimal phase error minimization and maximizing optical efficiency at both states.

Experimental Approach and Results

Fabrication employed electron beam lithography (EBL) and plasma etching, ensuring pattern fidelity and negligible surface roughness. Optical characterization was conducted using a 5.2 μm wavelength laser, revealing high Strehl ratios (>0.99) in the amorphous state, supporting the diffraction-limited claims. The focusing efficiencies, measured at 23.7% and 21.6% for the amorphous and crystalline states respectively, indicate significant advancements compared to prior varifocal metalens designs. Additionally, high-resolution imaging using USAF resolution charts confirmed diffraction-limited imaging performance with minimized crosstalk.

Implications and Future Prospects

This research illustrates the capacity of engineered metasurfaces to match traditional optical components in performance while offering benefits in terms of reduced size and complexity. The proposed metasurface design approach could revolutionize applications in imaging, beam steering, and adaptive optics, circumventing the need for bulky mechanical components. Future developments are expected to involve the integration of electrical switching mechanisms for O-PCMs, making practical deployment feasible for various optical applications.

Furthermore, the paper suggests potential for increasing the number of phase discretization levels to enhance efficiency and expand the design methodology to incorporate more arbitrary optical states. Such extension would benefit from employing advanced computational techniques, such as deep neural network-based design algorithms, highlighting the interdisciplinary nature of advancements in this domain.

In conclusion, the research presents a significant contribution to the field of metasurface optics, demonstrating the feasibility of high-performance, reconfigurable optical systems without reliance on mechanical actuation. The advancements pave the way for further exploration and potential commercialization of active metasurface technologies in various optical and photonic applications.