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Optical field amplification at the nanoscale can be used to increase light matter interaction enhancing a plethora of physical processes. More recently it has been demonstrated that a dielectric metasurface can support Bound States in the Continuum (BICs), i.e. resonant states of ideally zero width due to the interaction between trapped electromagnetic modes. Experimentally, this involves very narrow coupled resonances in a photonic crystal metasurface (PhCM), with a high Q-factor and an extremely large field intensity enhancement, up to 6 orders of magnitude larger than the intensity of the incident beam [1]. The field enhancement in proximity of the surface can be used for real world applications in standard optical microscopy. Specifically, we experimentally demonstrated that the BIC optical field can boost the fluorescence emission of probe molecules of 103-fold and also observed a significant amplification of the Raman scattering, with a possible large impact for label-free spectroscopic imaging in bio-medicine and cancer research. In addition, we demonstrated a novel approach based on the BIC pre-amplification of a model plasmonic system placed on the PhCM. All the results are obtained with large-area PhCM resonators made of silicon nitride, thus free from absorption losses in all the visible and infrared range.


a. Schematic layout of the two-dimensional square PhCM. b. Scanning electron micrographs of a PhCM sample, top-view, side-view and tilted view

Detecting  low-molecular weight molecules is extremely difficult, but the very intense optical field at the surface and the minimal loss of this transparent device make these PhCM structures extremely sensitive to the external environment perturbation. Specifically, the realization of an efficient optical sensor based on a photonic crystal supporting BIC is explored [2]. 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. Furthermore, we applied this new BIC-based approach to the label-free recognition of a specific protein-protein interaction, detecting the association between the interacting domain of p53 and its protein regulatory partner murine double minute 2 (MDM2) [3]. The method can be extended to the study of the dissociation effect exerted by other molecules on p53•MDM2 complex, even at the nM range, therefore being of great importance in the first approach towards the discovery of substances with pharmacological activity against cancer



a. BIC amplitude over the PhC with superimposed arrow maps of the electric field: as clearly visible, the electric field when a resonance trapped-BIC is coupled forms a lattice of vortices and antivortices that cannot couple to radiating waves since it is evanescent with no out-of-plane components of Poynting vector. b. Intensity profile of the electric field in the side view of one unit cell. The field is evanescent in both of z-directions.

The experimental observation of a strong spin-orbit asymmetry (spin-directive coupling) in the transport of the electromagnetic field resonantly amplified in correspondence of the bound state in the continuum is also studied. Spin-polarized directive coupling of light can be achieved with helical optical modes of photonic topological insulators and basically with inhomogeneous optical fields like surface waves manifesting spin-momentum locking, a phenomenon that features spin-based integrated quantum technologies. We demonstrated that BICs excited in a two-dimensional PhCM of silicon nitride are characterized by a transverse spin angular momentum (SAM) density [4]. We observed emergent surface waves propagating along the symmetry axes of the PhCM at the BIC, whose intensity depends on the input polarization. Specifically, the helicity of the input circular polarization determines the direction of propagation.  Our experimental results point out the possibility of a BIC-enhanced macroscopic spin-directive coupling. The phenomenon is easily scalable at any wavelength. This paves the way to novel implementation of PhCM resonators with potential multiplatform implementations.

Collaborations:

G. Zito, S. Managò, A. C. De Luca, CNR-IBP - Naples (IT)

E. Penzo, S. Cabrini, Molecular Foundry - Berkeley (USA)

A. Lamberti - University Federico II of Naples (IT)

References:

[1] Giant field enhancement in photonic resonant lattices, V. Mocella and S. Romano | Phys. Rev. B 92, 155117 (2015)

[2] Label-Free Sensing of Ultralow-weight Molecules with All-Dielectric Metasurfaces Supporting Bound States in the Continuum, S. Romano et al., Photonics Research , Vol. 6, No. 7  (2018)

[3] Optical biosensors based on photonic crystals supporting bound states in the continuum, S. Romano et al | Materials 2018, 11(4)

[4] Quantum Spin-Hall Effect of Light at Bound States in the Continuum, G. Zito et al |    arXiv:1710.10862

 

Contacts:

Dr. Silvia Romano silvia.romano@na.imm.cnr.it

Dr. Vito Mocella vito.mocella@na.imm.cnr.it

 

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