Compact Folded Metasurface Spectrometer

Abstract: Recent advances in optical metasurfaces enable control of the wavefront, polarization and dispersion of optical waves beyond the capabilities of conventional diffractive optics. An optical design space that is poised to highly benefit from these developments is the folded optics architecture where light is confined between reflective surfaces and the wavefront is controlled at the reflective interfaces. In this manuscript we introduce the concept of folded metasurface optics by demonstrating a compact high resolution optical spectrometer made from a 1-mm-thick glass slab with a volume of 7 cubic millimeters. The spectrometer has a resolution of 1.2 nm, resolving more than 80 spectral points in a 100-nm bandwidth centered around 810 nm. The device is composed of three different reflective dielectric metasurfaces, all fabricated in a single lithographic step on one side of a transparent optical substrate, which simultaneously acts as the propagation space for light. An image sensor, parallel to the spectrometer substrate, can be directly integrated on top of it to achieve a compact mono- lithic device including all the active and passive components. Multiple spectrometers, with similar or different characteristics and operation bandwidths may also be integrated on the same chip and fabricated in a batch process, significantly reducing their costs and increas- ing their functionalities and integration potential. In addition, the folded metasystems design can be applied to many optical systems, such as optical signal processors, interferometers, hyperspectral imagers and computational optical systems, significantly reducing their sizes and increasing their mechanical robustness and potential for integration.

• The paper introduces a compact folded metasurface spectrometer that integrates multiple reflective dielectric metasurfaces on a single substrate.
• The device achieves a spectral resolution of approximately 1.2 nm over a 100 nm bandwidth, validated by close agreement between experimental and simulated results.
• The streamlined design facilitates scalable fabrication and potential applications in hyperspectral imaging and consumer electronics.

Compact Folded Metasurface Spectrometer: An Overview

The study introduces the concept of folded optical metasystems through the development of a compact folded metasurface spectrometer. This novel device integrates multiple reflective dielectric metasurfaces on a single substrate, leading to a highly miniaturized optical spectrometer measuring only 1 mm in thickness and occupying a volume of 7 mm³. The research demonstrates a spectrometer with an impressive spectral resolution of approximately 1.2 nm over a bandwidth of 100 nm, emphasizing the potential for integrating such devices into compact systems and various consumer electronic applications due to their reduced size and high performance.

Key Innovations and Findings

The introduction of folded metasurface optics marks a significant departure from traditional optical spectrometer configurations, which typically rely on bulky free-space optical elements such as gratings and lenses. This research leverages dielectric metasurfaces—highly efficient diffractive optical elements—to control phase, polarization, and dispersion with high precision. The integration of these metasurfaces into a single reflective substrate allows for the confinement and manipulation of light within the device, enhancing compactness and mechanical robustness.

The research outcomes include:

• Design and Fabrication: The spectrometer is composed of three distinct metasurfaces fabricated using a single lithographic step, illustrating the capability for high-efficiency mass production. The metasurfaces perform the roles of dispersion and focusing, with carefully optimized phase profiles aiding in minimizing geometric aberrations.
• Experimental Validation: The device demonstrates excellent agreement between measured and simulated spectral resolutions across the designated bandwidth, confirming its capability to resolve wavelengths separated by approximately 1.2 nm. Moreover, the measured focusing efficiency suggests the design's practical feasibility despite minor discrepancies attributed to fabrication variations.
• Practical Implications: The compact spectrometer's design facilitates direct integration with image sensors, leading to potential applications in hyperspectral imaging and consumer electronics. The simplified form factor positions it as a contender for replacing traditional spectrometers in space-constrained devices like smartphones.

Theoretical and Practical Implications

The development of the folded metasurface spectrometer signifies an important stride in the field of nanophotonics and metasurface optics. The implications are manifold:

• Theoretical Impact: The research underscores the efficacy of metasurfaces in achieving complex optical functionalities that were previously unattainable with single-layer, local metasurfaces. This breakthrough opens up avenues for using metasurfaces in more complex optical systems like computational optical systems and interferometers.
• Practical Applications: The compactness and integration capability suggest a broad range of applications from medical diagnostics to optical communications, where size and resolution are critical.
• Potential for Commercialization: The scalable fabrication process and reduced production costs due to batch processing, combined with the substrate's role as part of the optical system, render these devices suitable for mass-market applications. Enhanced functionalities such as multi-wavelength operation can further extend their versatility, fulfilling diverse application requirements.

Future Directions

The study provides groundwork for ongoing advancements in metasurface technologies, particularly focusing on improving efficiency through the use of enhanced reflective coatings and advanced design methodologies. Additionally, exploring combinations with state-of-the-art techniques like high-contrast gratings promises further performance optimization.

As research in optical metasurfaces progresses, it's anticipated that these developments will catalyze innovations across various optical systems, including on-chip interferometers and advanced imaging systems, by leveraging the compactness and integrative strengths of folded metasystems.