Machine learning -- based diffractive imaging with subwavelength resolution

Far-field characterization of small objects is severely constrained by the diffraction limit. Existing tools achieving sub-diffraction resolution often utilize point-by-point image reconstruction via scanning or labelling. Here, we present a new imaging technique capable of fast and accurate characterization of two-dimensional structures with at least wavelength/25 resolution, based on a single far-field intensity measurement. Experimentally, we realized this technique resolving the smallest-available to us 180-nm-scale features with 532-nm laser light. A comprehensive analysis of machine learning algorithms was performed to gain insight into the learning process and to understand the flow of subwavelength information through the system. Image parameterization, suitable for diffractive configurations and highly tolerant to random noise was developed. The proposed technique can be applied to new characterization tools with high spatial resolution, fast data acquisition, and artificial intelligence, such as high-speed nanoscale metrology and quality control, and can be further developed to high-resolution spectroscop

Measuring Photonics in Photosynthesis: Combined Micro-Fourier Image Spectroscopy and Pulse Amplitude Modulated Chlorophyll Fluorimetry at the Micrometre-Scale

Natural photonic structures are common across the biological kingdoms, serving a diversity of functionalities. The study of implications of photonic structures in plants and other phototrophic organisms is still hampered by missing methodologies for determining in situ photonic properties, particularly in the context of constantly adapting photosynthetic systems controlled by acclimation mechanisms on the cellular scale. We describe an innovative approach to determining spatial and spectral photonic properties and photosynthesis activity, employing micro-Fourier Image Spectroscopy and Pulse Amplitude Modulated Chlorophyll Fluorimetry in a combined microscope setup. Using two examples from the photosynthetic realm, the dynamic Bragg-stack-like thylakoid structures of Begonia sp. and complex 2.5 D photonic crystal slabs from the diatom Coscinodiscus granii, we demonstrate how the setup can be used for measuring self-adapting photonic-photosynthetic systems and photonic properties on single-cell scales. We suggest that the setup is well-suited for the determination of photonic–photosynthetic systems in a diversity of organisms, facilitating the cellular, temporal, spectral and angular resolution of both light distribution and combined chlorophyll fluorescence determination. As the catalogue of photonic structure from photosynthetic organisms is rich and diverse in examples, a deepened study could inspire the design of novel optical- and light-harvesting technologies

Enhanced Light Absorption in All-Polymer Biomimetic Photonic Structures by Near-Zero-Index Organic Matter

11 pags., 6 figs.Natural photosynthetic photonic nanostructures can show sophisticated light–matter interactions including enhanced light absorption by slow light even for highly pigmented systems. Beyond fundamental biology aspects, these natural nanostructures are very attractive as blueprints for advanced photonic devices. But the soft-matter biomimetic implementations of such nanostructures is challenging due to the low refractive index contrast of most organic photonic structures. Excitonic organic materials with near-zero index (NZI) optical properties allow overcoming these bottlenecks. Here, it is demonstrated that the combination of NZI thin films with photonic multilayers like the ones found in nature enables broadband tunable strong reflectance as well as slow light absorption enhancement and tailored photoluminescence properties in the full VIS spectrum. Moreover, it is shown that this complex optical response is tunable, paving the way toward the development of active devices based on all-polymer and near-zero index materials photonic structures.The work by M.C.A., W.P.W., and M.L.-G. was supported by the “Towards Biomimetic Photosynthetic Photonics” project (POCI-01-0145-FEDER- 031739) co-funded by FCT and COMPETE2020. C.E.-V., S.N.-S, and I.P.-S. acknowledge financial support from MCIN/ AEI/10.13039/501100011033 (Grant No. PID2019-108954RB-I00) and the Xunta de Galicia/FEDER (grant GRC ED431C 2020/09). R.S. acknowledges Grants RTI 2018-096498-B-I00 and PID2021-123190OB-I00 funded by MCIN/AEI/ 10.13039/501100011033 and by “ERDF a way of making Europe.

Enhanced Light Absorption in All-Polymer Biomimetic Photonic Structures by Near-Zero-Index Organic Matter

11 pags., 6 figs.Natural photosynthetic photonic nanostructures can show sophisticated light–matter interactions including enhanced light absorption by slow light even for highly pigmented systems. Beyond fundamental biology aspects, these natural nanostructures are very attractive as blueprints for advanced photonic devices. But the soft-matter biomimetic implementations of such nanostructures is challenging due to the low refractive index contrast of most organic photonic structures. Excitonic organic materials with near-zero index (NZI) optical properties allow overcoming these bottlenecks. Here, it is demonstrated that the combination of NZI thin films with photonic multilayers like the ones found in nature enables broadband tunable strong reflectance as well as slow light absorption enhancement and tailored photoluminescence properties in the full VIS spectrum. Moreover, it is shown that this complex optical response is tunable, paving the way toward the development of active devices based on all-polymer and near-zero index materials photonic structures.The work by M.C.A., W.P.W., and M.L.-G. was supported by the “Towards Biomimetic Photosynthetic Photonics” project (POCI-01-0145-FEDER- 031739) co-funded by FCT and COMPETE2020. C.E.-V., S.N.-S, and I.P.-S. acknowledge financial support from MCIN/ AEI/10.13039/501100011033 (Grant No. PID2019-108954RB-I00) and the Xunta de Galicia/FEDER (grant GRC ED431C 2020/09). R.S. acknowledges Grants RTI 2018-096498-B-I00 and PID2021-123190OB-I00 funded by MCIN/AEI/ 10.13039/501100011033 and by “ERDF a way of making Europe.

Lattice modes and plasmonic linewidth engineering in gold and aluminum nanoparticle arrays

International audienceLattice modes have been proposed as a means to engineer and control the linewidth and spectral position of optical resonances in arrays of metallic nanoparticles sustaining localized surface plasmon (LSP) resonances. Lattice modes are produced by the interference of LSP-enhanced in-plane scattered light, leading to a Fano-like lineshape with reduced linewidth. In this paper, we study the lattice modes supported by gold and aluminium nanoparticle arrays in the visible and UV, both experimentally and theoretically. The measured and simulated dispersion curves allow us to comprehensively analyze the details of the LSP coupling in the array. We show that when the spectral position of the Rayleigh anomaly, which depends on the period of the array, is slightly blue-shifted with respect to the LSP resonance, the quality factor in the nanoparticle array is significantly increased. We also provide evidence that the formation for the lattice modes, i.e. the coupling between LSPs and the in-plane scattered light, critically depends on the incident light polarization, the coupling efficiency being maximum when the polarization direction is perpendicular to the propagation direction of the grazing wave. The results obtained provide design rules allowing high quality factor resonances throughout visible and ultraviolet spectral ranges, needed for sensing and active nanophotonic applications

Strong coupling in molecular systems: a simple predictor employing routine optical measurements

We provide a simple method that enables readily acquired experimental data to be used to predict whether or not a candidate molecular material may exhibit strong coupling. Specifically, we explore the relationship between the hybrid molecular/photonic (polaritonic) states and the bulk optical response of the molecular material. For a given material this approach enables a prediction of the maximum extent of strong coupling (vacuum Rabi splitting), irrespective of the nature of the confined light field. We provide formulae for the upper limit of the splitting in terms of the molar absorption coefficient, the attenuation coefficient, the extinction coefficient (imaginary part of the refractive index) and the absorbance. To illustrate this approach we provide a number of examples, we also discuss some of the limitations of our approach