Metasurface spectrometers beyond resolution-sensitivity constraints

Abstract

Conventional spectrometer designs necessitate a compromise between their resolution and sensitivity, especially as device and detector dimensions are scaled down. Here, we report on a miniaturizable spectrometer platform where light throughput onto the detector is instead enhanced as the resolution is increased. This planar, CMOS-compatible platform is based around metasurface encoders designed to exhibit photonic bound states in the continuum, where operational range can be altered or extended simply through adjusting geometric parameters. This system can enhance photon collection efficiency by up to two orders of magnitude versus conventional designs; we demonstrate this sensitivity advantage through ultralow-intensity fluorescent and astrophotonic spectroscopy. This work represents a step forward for the practical utility of spectrometers, affording a route to integrated, chip-based devices that maintain high resolution and SNR without requiring prohibitively long integration times.

Fig. 1.. Bandstop strategy for BIC-inspired spectrometer design.

(A and B) Comparison of bandpass and bandstop strategies when designing a filter-array based spectrometer. (C) Simulated bandpass and qBIC bandstop features. (D) Light throughput as a function of the FWHM of transmission features under illumination of a broadband light (500 to 600 nm), as transmitted through simulated and commercial grating- and filter-based system with varying values of transmission FWHM.

Fig. 2.. Pixelated metasurfaces with polarization-independent qBICs.

(A) Schematic diagram of the as-designed diatomic metasurface, formed from cylindrical nanoholes, arranged in a square lattice, and etched into a TiO₂ slab with thickness H = 92 nm. Unit cells (left) spaced at a pitch P, compose of diagonally arranged holes with two different radii, r₀ and r₁. (B) Band structures and field distributions of such a metasurface when Δr = r₀ − r₁ = 0 nm. The middle panel shows a magnified region of the band structure near the doubly degenerate BICs. The right panel shows the field distributions of D-BIC1. (C) Band structure near D-qBIC, and corresponding field distributions of D-qBIC1 when Δr = 10 nm. (D) SEM images of a fabricated 1 × 5 qBIC-based metasurface filter array with Δr = 28, 38, 46, 52, and 58 nm, respectively. (E) Measured transmitted spectra at varying values of Δr. (F) Q factor of D-qBIC1 extracted from simulated and measured (E) transmitted spectra. (G) Measured transmission spectra under 0 and 90° polarizations, demonstrating a polarization-independent response. (H) Simulated metasurface transmission spectra for 100 filters with varying unit cell pitch, P. For all other demonstrations in this figure, P = 338 nm.

Fig. 3.. Microspectrometer based on qBIC metasurfaces.

(A) Schematic of the spectrometer featuring an individual 10 × 10 qBIC bandstop filter array integrated atop a CMOS imaging sensor. (B) SEM image of the fabricated metasurface filter array. (C) Bandstop transmission profiles t_i(λ) of the 100 filters. (D) Measured, reconstructed spectra of a series of monochromatic spectral lines (dark dotted lines) in the visible range. (E) Measured, reconstructed spectrum of two narrow spectral lines, demonstrating ability to resolve distinct monochromatic spectral features down to 1.7-nm separation. (F) Measured, reconstructed spectrum of a broadband light source, relative to the same incident signal measured by a conventional spectrometer. a.u., arbitrary units.

Fig. 4.. Demonstration of the metasurface spectrometer for low-light level detection.

(A) Resolution and lowest detectable intensity of five qBIC-based spectrometers with different values of Δr, each composed of 16 metasurface filter units, demonstrating a positive correlation between resolution and sensitivity. Inset shows SEM images of the five different spectrometers and a magnification of one filter unit (bottom right). (B) Fluorescence photographs of bacteria under different excitation intensities and (C) corresponding spectra measured by a qBIC-based metasurface spectrometer (red) and conventional mini-spectrometer (black), respectively. (D) Colorful photograph of the night sky, with magnified insets showing Venus in clear and cloudy conditions. (E) Visible range spectra of Venus corresponding to clear (top) and cloudy (bottom) conditions from (D), as measured by our BIC spectrometer versus a conventional mini-spectrometer. For both (C) and (E), the conventional mini-spectrometer model is an Avaspec-uls2048cl (600/mm grating, 25-μm slit, 37,5000 counts/μW per ms integration time, which represents high-end commercial mini/microspectrometers). (F) Light throughput efficiency (expressed as eventual intensity onto the detectors as a percentage of total light intensity before spectral dispersion/selection) and resolution of a selection of state-of-the-art (reconstructive and conventional) miniaturized spectrometers (, , –42).