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A novel optical label-free bio-sensing platform based on a new class of resonances supported in a photonic crystal metasurface is reported herein. Molecular binding is detected as a shift in the resonant wavelength of the bound states in the continuum of radiation modes. The new configuration is applied to the recognition of the interaction between protein p53 and its protein regulatory partner murine double minute 2 (MDM2). A detection limit of 66 nM for the protein p53 is found. The device provides an excellent interrogation stability and loss-free operation, requires minimal optical interrogation equipment and can be easily optimized to work in a wide wavelength range.

Plasmonic substrates play a crucial role in the confinement and manipulation of localized electromagnetic fields at the nanoscale. The large electromagnetic field enhancement at metal/dielectric interfaces is widely exploited in surface-enhanced fluorescence (SEF) and surface-enhanced Raman scattering (SERS) spectroscopies. Despite the advantage of near-field enhancement, unfortunately, in metals, the large absorption at optical frequencies induces local heating of the analyte fluid with possible damage of the biological material. In addition, in SEF plasmonic substrates, spacer layers are necessary to minimize undesired fluorescence quenching due to nonradiative decay, which strongly depends on the distance between molecules and metallic substrates. Therefore, the possibility of managing surface electromagnetic states mimicking surface-plasmon resonances in terms of spatial localization, high-field intensity, and dispersion characteristics, while avoiding metallic losses is of great interest. However, dielectric nanoantennas can currently provide limited possibilities in the visible range of optical frequencies. We present the realization of all-dielectric metasurfaces made of nanostructured transparent silicon nitride supporting bound states in the continuum (BICs). We show that this special kind of Fano resonances can be effectively used in standard microscopy for practical applications. We achieved concurrent enhancements of ∼10 3 fold of fluorescence emission and Raman scattering farfield intensities of molecules dispersed on these metasurfaces. In addition, we demonstrate that the gain of conventional SERS signals can be increased by more than one order of magnitude by resonant matching of the localized surface plasmon resonance with the BIC field. Our results can find significant applications for enhanced sensing, Raman imaging, and nonlinear processes.

Inspired by the concept of complementary media, we experimentally demonstrate that an engineered metamaterial made of alternating, stripe layers of negatively refracting (photonic crystals) and positively refracting (air) materials strongly collimates a beam of near-infrared light. This quasi-zero-average-index metamaterial fully preserves the beam spot size throughout the sample for a light beam traveling through the metamaterial a distance of 2 mm-more than 1000 times the input wavelength lambda=1.55 microm. These results demonstrate the first explicit experimental verification of optical antimatter as proposed by Pendry and Ramakrishna [J. Pendry and S. Ramakrishna, J. Phys. Condens. Matter 15, 6345 (2003)10.1088/0953-8984/15/37/004], using two complementary media in which each n(eff)=-1 layer appears to annihilate an equal thickness layer of air.

The characterization of partially coherent light transmission by micrometer sized valves of marine diatoms is an interesting optical challenge and, from the biological point of view, is of outmost relevance in order to understand evolution mechanisms of such organisms. In the present work, we have studied the transmission of light coming from a monochromator through single valves of Coscinodiscus wailesii diatoms. Incoming light is confined by the regular pore pattern of the diatom surface into a spot of few microns, its dimensions depending on wavelength. The effect is ascribed to the superposition of wavefronts diffracted by the pores' edges. Numerical simulations help to demonstrate how this effect is not present in the ultraviolet region of the light spectrum, showing one of the possible evolutionary advantages represented by the regular pores patterns of the valves.

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Herein, we demonstrate a cavity-enhanced hyperspectral refractometric imaging using an all-dielectric photonic crystal slab (PhCS). Our approach takes advantage of the synergy between two mechanisms, surfaceenhanced fluorescence (SEF) and refractometric sensing, both based on high-Q resonances in proximity of bound states in the continuum (BICs). The enhanced local optical field of the first resonance amplifies of 2 orders of magnitude the SEF emission of a probe dye. Simultaneously, hyperspectral refractometric sensing, based on Fano interference between second mode and fluorescence emission, is used for mapping the spatially variant refractive index produced by the specimen on the PhCS. The spectral matching between first resonance and input laser is modulated by the specimen local refractive index, and thanks to the calibrated dependence with the spectral shift of the Fano resonance, the cavity tuning is used to achieve an enhanced correlative refractometric map with a resolution of 10 −5 RIU within femtoliter-scale sampling volumes. This is experimentally applied also on live prostate cancer cells grown on the PhCS, reconstructing enhanced surface refractive index images at the single-cell level. This dual mechanism of quasi-BIC spatially variant gain tracked by quasi-BIC refractometric sensing provides a correlative imaging platform that can find application in many fields for monitoring physical and biochemical processes, such as molecular interactions, chemical reactions, or surface cell analysis.

Hundreds of papyrus rolls, buried by the eruption of Mount Vesuvius in 79 AD and belonging to the only library passed on from Antiquity, were discovered 260 years ago at Herculaneum. These carbonized papyri are extremely fragile and are inevitably damaged or destroyed in the process of trying to open them to read their contents. In recent years, new imaging techniques have been developed to read the texts without unwrapping the rolls. Until now, specialists have been unable to view the carbon-based ink of these papyri, even when they could penetrate the different layers of their spiral structure. Here for the first time, we show that X-ray phase-contrast tomography can reveal various letters hidden inside the precious papyri without unrolling them. This attempt opens up new opportunities to read many Herculaneum papyri, which are still rolled up, thus enhancing our knowledge of ancient Greek literature and philosophy.