We report a one-step additive manufacturing process to fabricate metalenses for visible wavelengths. Nanostructures with aspect ratios larger than eight and critical dimensions smaller than 60 nm were produced using nanoimprint lithography and a titanium dioxide nanocrystal-based imprint material, resulting in inorganic structures exhibiting a refractive index of n = 1.9. As a demonstration, we fabricate metalenses with numerical apertures (NAs) of 0.2 and focusing efficiencies over 50%. Manufacturability was assessed by performing 15 manual imprints in 30 min (2 min of process time per imprint) with a single stamp. All imprinted lenses exhibit comparable performance, paving the way for high-throughput and low-cost manufacturing of flat optical devices. Metalenses with a diameter of 4 mm were also fabricated to investigate the success of large area replication using this process, showing efficiencies of 43%, indicating good macroscopic imprinting.

Spectral imagers divide scenes into quantitative and narrowband spectral channels. They have become important metrological tools in many areas of science, especially remote sensing. Here, we propose and experimentally demonstrate a snapshot spectral imager using a parallel optical processing paradigm based on arrays of metasystems. Our multi-aperture spectral imager weighs less than 20 mg and simultaneously acquires 20 image channels across the 795- to 980-nm spectral region. Each channel is formed by a metasurface-tuned filter and a metalens doublet. The doublets incorporate absorptive field stops, reducing cross-talk between image channels. We demonstrate our instrument’s capabilities with both still images and video. Narrowband filtering, necessary for the device’s operation, also mitigates chromatic aberration, a common problem in metasurface imagers. Similar instruments operating at visible wavelengths hold promise as compact, aberration-free color cameras. Parallel optical processing using metasystem arrays enables novel, compact instruments for scientific studies and consumer electronics.

Due to its absorption properties in atmosphere, the mid-infrared (mid-IR) region has gained interest for its potential to provide high data capacity in free-space optical (FSO) communications. Here, we experimentally demonstrate wavelength-division-multiplexing (WDM) and mode-division-multiplexing (MDM) in a ~0.5 m mid-IR FSO link. We multiplex three ~3.4 μm wavelengths (3.396 μm, 3.397 μm, and 3.398 μm) on a single polarization, with each wavelength carrying two orbital-angular-momentum (OAM) beams. As each beam carries 50-Gbit/s quadrature-phase-shift-keying data, a total capacity of 300 Gbit/s is achieved. The WDM channels are generated and detected in the near-IR (C-band). They are converted to mid-IR and converted back to C-band through the difference frequency generation nonlinear processes. We estimate that the system penalties at a bit error rate near the forward error correction threshold include the following: (i) the wavelength conversions induce ~2 dB optical signal-to-noise ratio (OSNR) penalty, (ii) WDM induces ~1 dB OSNR penalty, and (iii) MDM induces ~0.5 dB OSNR penalty. These results show the potential of using multiplexing to achieve a ~30X increase in data capacity for a mid-IR FSO link.

Metasurfaces are arrays of subwavelength meta-atoms that shape waves in a compact and planar form factor. Analysis and design of metasurfaces require methods for modeling their interactions with waves. Conventional modeling techniques assume that metasurfaces are locally periodic structures excited by plane waves, restricting their applicability to gradually varying metasurfaces that are illuminated with plane waves. Here we introduce the discrete-space impulse response concept that enables the development of accurate and general models for metasurfaces. According to the proposed model, discrete impulse responses are assigned to metasurface unit cells and are used to determine the metasurface response to any arbitrary incident waves. We verify the accuracy of the model by comparing its results with full-wave simulations. The proposed concept and modeling technique are applicable to linear metasurfaces with arbitrary meta-atoms, and the resulting system-level models can be used to accurately incorporate metasurfaces into simulation and design tools that use wave or ray optics.