Very-large-scale-integrated high quality factor nanoantenna pixels

Abstract

Metasurfaces precisely control the amplitude, polarization and phase of light, with applications spanning imaging, sensing, modulation and computing. Three crucial performance metrics of metasurfaces and their constituent resonators are the quality factor (Q factor), mode volume (V_m) and ability to control far-field radiation. Often, resonators face a trade-off between these parameters: a reduction in V_m leads to an equivalent reduction in Q, albeit with more control over radiation. Here we demonstrate that this perceived compromise is not inevitable: high quality factor, subwavelength V_m and controlled dipole-like radiation can be achieved simultaneously. We design high quality factor, very-large-scale-integrated silicon nanoantenna pixels (VINPix) that combine guided mode resonance waveguides with photonic crystal cavities. With optimized nanoantennas, we achieve Q factors exceeding 1,500 with V_m less than 0.1 ${\left(\lambda /n_{\mathrm{air}}\right)}^3$ . Each nanoantenna is individually addressable by free-space light and exhibits dipole-like scattering to the far-field. Resonator densities exceeding a million nanoantennas per cm² can be achieved. As a proof-of-concept application, we show spectrometer-free, spatially localized, refractive-index sensing, and fabrication of an 8 mm × 8 mm VINPix array. Our platform provides a foundation for compact, densely multiplexed devices such as spatial light modulators, computational spectrometers and in situ environmental sensors.

Fig. 1 |. VINPix resonators.

a, Schematic of an array of individually addressable 15-μm-long high-$Q$ photonic antennas (VINPix) made of Si nanoblocks on a sapphire substrate. The resonators are excited using a normally incident NIR laser source, and the scattered light is recorded using a camera or an imaging spectrometer. b, Representation of a VINPix’s structural design, broken into three sections: a photonic cavity section, a tapered mirrors section and a padding mirrors section. c, Top view (SEM image) of VINPix resonators with different tapering functions—polynomials of order, p = 0–6 from top to bottom—without any padding sections. d, Angled SEM images of VINPix without padding sections, and p = 1, 2, 4 and 6, as labelled. e, Angled SEM image, with enlarged inset of the cavity section, of a representative 15-μm-long VINPix consisting of a 7-μm-long cavity section, 3-μm-long tapered mirror sections and 1-μm-long padding sections. f, Angled SEM image, with enlarged inset of the cavity section, of a slotted VINPix with a 70-nm-wide slot. g, A large-scale VINPix array patterned with VINPix resonators spanning an area of 8 mm × 8 mm on a 10 mm × 10 mm chip. h, Dark-field optical microscopy image of a small section of a VINPix array.

Fig. 2 |. Photonic mirrors confine GMRs.

a, Simplified TE band diagram of infinitely long photonic cavity with average width, $d=600\text{nm}$, and $\text{Δ}d=50\text{nm}$, the unit cell size $\left(b_y\right)=660\text{nm}$. Bands at 207 THz and 262 THz (for $k_{\Vert }=0$) are the bonding and anti-bonding GMRs of interest. Refer to Supplementary Fig. 1 for a schematic of our waveguide cavity and band diagram calculations. b, Simulated normalized electric field enhancements at the cross-section of the unit cell of an infinitely long cavity with $\text{Δ}d=50\text{nm}$, of the bonding GMR. Geometrical parameters of the unit cell are: height of 600 nm, average width $(d)=600\text{nm}$ nm, thickness $(t)=160\text{nm}$ and block spacing $\left(a_y\right)=330\text{nm}$. Scale bar, 200 nm. The colour bar is linearly scaled. c, Simulated $Q$ factors of the GMR for waveguide cavities of different lengths. The stars correspond to waveguide cavities of infinite length. d, Simplified TE band diagram for a mirror segment with $d=600\text{nm}$ with labelled radiation zone, the bonding and anti-bonding modes, and the corresponding mode gap. Simulated mode profiles of the bonding and anti-bonding modes are shown in Supplementary Fig. 1. e, Mirror strength calculated using band positions. Refer to Supplementary Fig. 2 for band positions of the bonding mode (dielectric band edge), anti-bonding mode (air band edge) and the mid-gap frequency for different mirror segments. f,g, (left) Simulated cross-sectional field profiles for the x component of the electric field (colour bar is linearly scaled) and (right) corresponding FT spectra (colour bar is logarithmically scaled) to visualize the out-of-plane scattering for a VINPix with $\text{Δ}d=0$, with a tapered mirrors section of p = 0 (f) and p = 4 (g). The region inside the circle is the radiation zone. Nanoblocks are marked with black borders to aid visualization. Scale bar, 1 μm. NF stands for near-field.

Fig. 3 |. Optimization and characterization of VINPix resonators.

a, Simulated $Q$ factors of 15-μm-long VINPix with a 5-μm-long cavity of different $\left(\text{Δ}d\right)$ and 5-μm-long mirror sections of different $p$. b, Simulated $Q$ factors of 15-μm-long VINPix with varying fractional configurations of the cavity sections’, tapered mirror sections’ ($p=4$) and padding mirror sections’ lengths. The arrows point towards the respective axes for one representative configuration. c, Simulated normalized electric near-field (NF) enhancements at the cross-section of a VINPix with 7-μm-long cavity section of $\text{Δ}d=50\text{nm}$, 3-μm-long mirror sections of p = 4 and 1-μm-long padding sections. The colour bar is logarithmically scaled. d, Far-field (FF) simulation plot of the optimized VINPix. The concentric circles represent 10°, 30°, 60° and 90° from the centre. The colour bar is linearly scaled. e, Simulated electric near-field (NF) profile through the cavity of the optimized VINPix. Colour bar is linearly scaled. f, Representative SEM image of an array of 15-μm-long VINPix with $p=4$ and $\text{Δ}d=50\text{nm}$. g, Spectral image from five individual VINPix as marked in f (left) and normalized row-averaged reflected intensities corresponding to each VINPix (right). The colour bar is linearly scaled. h, Experimentally characterized Q factors of 15-μm-long VINPix with 7-μm-long cavity sections of $\text{Δ}d=50\text{nm}$ and 100 nm, and 4-μm-long tapered mirror sections of different polynomial orders. Average values and standard deviations (represented with error bars) correspond to 30 VINPix resonators measured for each set. i, Schematic of a VINPix array patterned with VINPix resonators $(\text{Δ}d=100\text{nm})$ spanning an area of 8 mm × 8 mm on a 10 mm × 10 mm sapphire substrate. Resonators are spaced by 30 μm. j, Representative reflection spectra from individual resonators selected from the three regions of the chip—S1, S2 and S3 as marked and colour coded in i. k, Top: averaged $Q$ factor values and standard deviations (represented with error bars) recorded across 30 resonators for each section. Bottom: averaged resonance wavelengths and standard deviations (represented with error bars) recorded across 30 resonators for each section.

Fig. 4 |. Sensing changes in the local refractive index using high-resolution hyperspectral imaging.

a, Schematic of our hyperspectral imaging setup. A VINPix array patterned with a PMMA layer on top in the shape of an ‘S’ is illuminated using a normally incident narrow band tunable NIR light source and the reflected images are recorded on a camera. b, Extracted spectral data corresponding to $R_{\text{PMMA}}$ and $R_{\text{water}}$ is shown with a VINPix resonator outside and inside the $‘\text{S}’$, respectively. c, Schematic of the PMMA patterned array where certain VINPix structures are covered under PMMA. d, Optical microscopic image of the VINPix array after patterning a PMMA layer in the shape of the ‘S’ where the resonators inside the $‘\text{S}’$ are exposed to the top medium and the rest are covered under PMMA resist. e, Image frames recorded on the camera at the two resonance wavelengths where ${\lambda }_{\text{water}}$ corresponds to ~1,570 nm for GMR wavelengths of VINPix inside the $‘\text{S}’$ and ${\lambda }_{\text{PMMA}}$ corresponds to ~1,610 nm for GMR wavelengths of VINPix outside the $‘\text{S}’$. Colour bar is linearly scaled. f, Top: spatial resonance-shift map generated by extracting spectral information for all the 126 VINPix resonators recorded in the data cube. Bottom: histogram displaying the GMR wavelengths of all the recorded VINPix resonators.

Fig. 5 |. Slots boost light confinement within our VINPix resonators.

a, Schematic of a slotted VINPix. b, A comparison of simulated effective mode volumes for our perturbed waveguide cavity, optimized VINPix, and a slotted VINPix with a 30-nm-wide slot. c, Top-down SEM images of a slotted VINPix with insets showing the cavity and mirror sections. d, Simulated normalized electric field enhancements at the cross-section of a 15-μm-long slotted VINPix with a 30-nm-wide slot. The colour bar is logarithmically scaled. NF, near field. e, Zoomed-in normalized electric field enhancements of a small region at the centre of the VINPix. The colour bar is logarithmically scaled. f, Simulated (stars) and experimentally characterized (circles) $Q$ factors of slotted VINPix with 70-nm-wide and 100-nm-wide slots. Average values and standard deviations (represented with error bars) correspond to 30 slotted VINPix resonators measured for each set. g, Resonant wavelength measurements as a function of background medium refractive index with slotted VINPix. Average values and standard deviations (represented with error bars) correspond to 30 resonators measured for each set. The lines represent linear fits to the data. h, Spectra of characterized representative VINPix with and without slots at 0 and 15% NaCl concentrations.