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1. Broadband achromatic dielectric metalenses

   https://doi.org/10.1038/s41377-018-0078-x  

   Shrestha, Sajan ; Overvig, Adam C. ; Lu, Ming ; Stein, Aaron ; Yu, Nanfang ( November 2018 , Light: Science & Applications)

   Metasurfaces offer a unique platform to precisely control optical wavefronts and enable the realization of flat lenses, or metalenses, which have the potential to substantially reduce the size and complexity of imaging systems and to realize new imaging modalities. However, it is a major challenge to create achromatic metalenses that produce a single focal length over a broad wavelength range because of the difficulty in simultaneously engineering phase profiles at distinct wavelengths on a single metasurface. For practical applications, there is a further challenge to create broadband achromatic metalenses that work in the transmission mode for incident light waves with any arbitrary polarization state. We developed a design methodology and created libraries of meta-units—building blocks of metasurfaces—with complex cross-sectional geometries to provide diverse phase dispersions (phase as a function of wavelength), which is crucial for creating broadband achromatic metalenses. We elucidated the fundamental limitations of achromatic metalens performance by deriving mathematical equations that govern the tradeoffs between phase dispersion and achievable lens parameters, including the lens diameter, numerical aperture (NA), and bandwidth of achromatic operation. We experimentally demonstrated several dielectric achromatic metalenses reaching the fundamental limitations. These metalenses work in the transmission mode with polarization-independent focusing efficiencies up to 50% and continuously provide a near-constant focal length overλ = 1200–1650 nm. These unprecedented properties represent a major advance compared to the state of the art and a major step toward practical implementations of metalenses.

    

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2. Focusing on bandwidth: achromatic metalens limits

   https://doi.org/10.1364/OPTICA.389404  

   Presutti, Federico ; Monticone, Francesco ( June 2020 , Optica)

   Metalenses have shown great promise in their ability to function as ultracompact optical systems for focusing and imaging. Remarkably, several designs have been recently demonstrated that operate over a large range of frequencies with minimized chromatic aberrations, potentially paving the way for ultrathin achromatic optics. Here, we derive fundamental bandwidth limits that apply to broadband optical metalenses regardless of their implementation. Specifically, we discuss how the product between achievable time delay and bandwidth is limited in any time-invariant system, and we apply well-established bounds on this product to a general focusing system. We then show that all metalenses designed thus far obey the appropriate bandwidth limit. The derived physical bounds provide a useful metric to compare and assess the performance of different devices, and they offer fundamental insight into how to design better broadband metalenses.

    

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3. Photonic (computational) memories: tunable nanophotonics for data storage and computing

   https://doi.org/10.1515/nanoph-2022-0089  

   Lian, Chuanyu ; Vagionas, Christos ; Alexoudi, Theonitsa ; Pleros, Nikos ; Youngblood, Nathan ; Ríos, Carlos ( May 2022 , Nanophotonics)

   Abstract The exponential growth of information stored in data centers and computational power required for various data-intensive applications, such as deep learning and AI, call for new strategies to improve or move beyond the traditional von Neumann architecture. Recent achievements in information storage and computation in the optical domain, enabling energy-efficient, fast, and high-bandwidth data processing, show great potential for photonics to overcome the von Neumann bottleneck and reduce the energy wasted to Joule heating. Optically readable memories are fundamental in this process, and while light-based storage has traditionally (and commercially) employed free-space optics, recent developments in photonic integrated circuits (PICs) and optical nano-materials have opened the doors to new opportunities on-chip. Photonic memories have yet to rival their electronic digital counterparts in storage density; however, their inherent analog nature and ultrahigh bandwidth make them ideal for unconventional computing strategies. Here, we review emerging nanophotonic devices that possess memory capabilities by elaborating on their tunable mechanisms and evaluating them in terms of scalability and device performance. Moreover, we discuss the progress on large-scale architectures for photonic memory arrays and optical computing primarily based on memory performance. 

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4. High-resolution and compact serpentine integrated grating spectrometer

   https://doi.org/10.1364/JOSAB.423968  

   Brand, Michael ; Zhang, Bohan ; Onural, Deniz ; Al Qubaisi, Kenaish ; Popović, Miloš ; Dostart, Nathan ; Wagner, Kelvin ( June 2021 , Journal of the Optical Society of America B)

   Integrated astrophotonic spectrometers are integrated variants of conventional free-space spectrometers that offer significantly reduced size, weight, and cost and immunity to alignment errors, and can be readily integrated with other astrophotonic instruments such as nulling interferometers. Current integrated dispersive astrophotonic spectrometers are one-dimensional devices such as arrayed waveguide gratings or planar echelle gratings. These devices have been limited to${10}^4$resolving powers and$<<\#comment/>1000$spectral bins due to having limited total optical delay paths and 1D detector array pixel densities. In this paper, we propose and demonstrate a high-resolution and compact astrophotonic serpentine integrated grating (SIG) spectrometer design based on a 2D dispersive serpentine optical phased array. The SIG device combines a 5.2 cm long folded delay line with grating couplers to create a large optical delay path along two dimensions in a compact integrated device footprint. Analogous to free-space crossed-dispersion high-resolution spectrometers, the SIG spectrometer maps spectral content to a 2D wavelength-beam-steered folded-raster emission pattern focused onto a 2D detector array. We demonstrate a SIG spectrometer with$\sim <\#comment/>100\mathrm{k}$resolving power and$\sim <\#comment/>6750$spectral bins, which are approximately an order of magnitude higher than previous integrated photonic designs that operate over a wide bandwidth, in a$0.4{\mathrm{m}\mathrm{m}}^2$footprint. We measure a Rayleigh resolution of$1.93\pm <\#comment/>0.07\mathrm{G}\mathrm{H}\mathrm{z}$and an operational bandwidth from 1540 nm to 1650 nm. Finally, we discuss refinements of the SIG spectrometer that improve its resolution, bandwidth, and throughput. These results show that SIG spectrometer technology provides a path towards miniaturized, high-resolution spectrometers for applications in astronomy and beyond.

    

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5. Hard convex lens-shaped particles: metastable, glassy and jammed states

   https://doi.org/10.1039/c8sm01519h  

   Cinacchi, Giorgio ; Torquato, Salvatore ( October 2018 , Soft Matter)

   We generate and study dense positionally and/or orientationally disordered, including jammed, monodisperse packings of hard convex lens-shaped particles (lenses). Relatively dense isotropic fluid configurations of lenses of various aspect ratios are slowly compressed via a Monte Carlo method based procedure. Under this compression protocol, while ‘flat’ lenses form a nematic fluid phase (where particles are positionally disordered but orientationally ordered) and ‘globular’ lenses form a plastic solid phase (where particles are positionally ordered but orientationally disordered), ‘intermediate’, neither ‘flat’ nor ‘globular’, lenses do not form either mesophase. In general, a crystal solid phase (where particles are both positionally and orientationally ordered) does not spontaneously form during lengthy numerical simulation runs. In correspondence to those volume fractions at which a transition to the crystal solid phase would occur in equilibrium, a ‘downturn’ is observed in the inverse compressibility factor versus volume fraction curve beyond which this curve behaves essentially linearly. This allows us to estimate the volume fraction at jamming of the dense non-crystalline packings so generated. These packings are nematic for ‘flat’ lenses and plastic for ‘globular’ lenses, while they are robustly isotropic for ‘intermediate’ lenses, as confirmed by the calculation of the τ order metric, among other quantities. The structure factors S ( k ) of the corresponding jammed states tend to zero as the wavenumber k goes to zero, indicating they are effectively hyperuniform ( i.e. , their infinite-wavelength density fluctuations are anomalously suppressed). Among all possible lens shapes, ‘intermediate’ lenses with aspect ratio around 2/3 are special because they are those that reach the highest volume fractions at jamming while being positionally and orientationally disordered and these volume fractions are as high as those reached by nematic jammed states of ‘flat’ lenses and plastic jammed states of ‘globular’ lenses. All of their attributes, taken together, make such ‘intermediate’ lens packings particularly good glass-forming materials. 

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