Large-area and transparent all-dielectric metasurfaces supporting photonic bound states in the continuum (BICs) offer several inherent advantages for highly sensitive biosensing applications. A BIC represents a unique mode within the energy spectrum of free-space waves that remains uncoupled with free-space radiation, resulting in a divergent radiative Q-factor and a topological singularity in reciprocal space. In this study, the synergistic combination of photonic crystal slabs (PhCS) supporting bound states in the continuum (BIC) with aptamers and molecularly imprinted polymers (MIPs) offers a groundbreaking approach to achieving ultrahigh sensitivity in detecting mycotoxins in wine and cytokines in artificial saliva. Mycotoxins, toxins produced by certain fungi, pose significant health risks when present in food and beverages like wine. Our research endeavors represent a significant step forward in the field of biosensing, offering a pathway toward the development of versatile, efficient, and reliable sensing platforms with broad applications across scientific, industrial, and societal domains.
A novel hyperspectral sensing imaging taking advantage of engineered all-dielectric metasurfaces supporting bound states in the continuum here is discussed. This approach combines surface-enhanced fluorescence and res- onant shift both based on high-Q resonances in proximity of bound states in the continuum. The amplification of the optical field on resonance allows increasing the fluorescence emission of a dye as a function of the spatial- variant dielectric environment in the near-field of the structure. We first demonstrate the fluorescence emission amplification by resonant pump matching in microscopy configuration. Then, we take advantage of Fano reso- nances in the fluorescence emission to map the spatially variant environment of biological cells. To demonstrate the real implementation of the proposed BIC-enhanced imaging as a platform for biosensing, hyperspectral maps of prostate cancer cells are experimentally reconstructed.
Moving the polarization of the incident wave along a meridian of the Poincaré sphere, experimentally we show that the coupling with the fundamental Bloch’s surface waves of the mode, provide a spatially coherent, macroscopic spinmomentum locked propagation along the symmetry axes of the PhCM. This novel mechanism of light-spin manipulation enables a versatile implementation of spin-optical structures that may pave the way to novel strategies for light spin technology and photonic multiplatform implementations.
The localization of the electromagnetic field at the nanoscale can play a key role in many applications, such as sensing, spectroscopy and energy conversion. In the last years, great efforts have been performed to study and realize all-dielectric loss-free nanostructures to confine the radiation without the limits imposed by plasmonic systems. It has been demonstrated that an all-dielectric photonic crystal (PhC) metasurface can support bound states in the continuum (BICs) - resonant states of infinite lifetime - due to the interaction between trapped electromagnetic modes. Experimentally, this involves very narrow coupled resonances, with a high Q-factor and a possible extremely large field intensity enhancement, up to 6 orders of magnitude. Here, we demonstrate that the field enhancement in proximity of the surface can be explored to boost lightmatter interaction in spectroscopic sensing. We design and realize an innovative sensing scheme for bulk and surface measurement with ultra-high figure of merit for the recognition of protein-protein interaction and the detection of low molecular weight molecules. In addition, we design a dielectric dual scheme based on metasurfaces supporting BICs to amplify fluorescence emission and Raman scattering of probe molecules dispersed on the surface of the PhC. Our results provide new solutions for light manipulation at the nanoscale, especially for sensing and nonlinear optics applications.
Metasurfaces are two-dimensional structures, arrays of scatterers with subwavelength separation or optically thin planar films, allowing light manipulation and enabling specific changes of optical properties, as for example beam-steering, anomalous refraction and optical-wavefront shaping. Due to the fabrication simplicity, the metasurfaces offer an alternative to 3-D metamaterials and providing a novel method for optical elements miniaturization. It has been demonstrated that a metasurface can support Bound States in Continuum (BIC), that are resonant states by zero width, due to the interaction between trapped electromagnetic. Experimentally, this involves very narrow coupled resonances, 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. Here, we demonstrate that the field enhancement in proximity of the surface can be applied to boost fluorescence emission of probe molecules dispersed on the surface of a photonic crystal membrane fabricated in silicon nitride. Our results provide new solutions for light manipulation at the nanoscale, especially for sensing and nonlinear optics applications.
The realization of miniaturized devices able to accumulate a higher number of information in a smallest volume is a challenge of the technological development. This trend increases the request of high sensitivity and selectivity sensors which can be integrated in microsystems. In this landscape, optical sensors based on photonic crystal technology can be an appealing solution. Here, a new refractive index sensor device, based on the bound states in the continuum (BIC) resonance shift excited in a photonic crystal membrane, is presented. A microfluidic cell was used to control the injection of fluids with different refractive indices over the photonic crystal surface. The shift of very high Q-factor resonances excited into the photonic crystal open cavity was monitored as a function of the refractive index n of the test liquid. The excellent stability we found and the minimal, loss-free optical equipment requirement, provide a new route for achieving high performance in sensing applications.
Electromagnetic surface waves, analogous to the classic surface plasmons can be supported to any interface, providing that the effective permittivity have an opposite sign. Localized plasmonlike modes and guided mode resonances are established in a photonic crystal slab irradiated with out-of-plane incident radiation, making photonic crystals a very appealing alternative to plasmonic substrates, avoiding the limits of absorption losses in metals.
The existence of a special type of resonances in the visible transmission spectrum of a very thin two-dimensional photonic crystal slab is demonstrated. We illustrate a controlling mechanism that allows the stabilization of the field amplification in a thin layer lattice with low contrast dielectric. Numerical simulations show that an extremely large field enhancement, as large as 700 times the amplitude of the incident wave, connected with high Q-factor resonances can be axcited. The connection with the bound states in continuum phenomenon is highlighted.
A giant field enhancement, respect to the amplitude of the incident wave is achieved in a thin layer lattice with low contrast dielectric, is demonstrated. The key mechanism is a careful control of the parameters, which allows a stabilization of the coupling resonances.
In this work, resonance phenomena in a negative photonic crystal are experimentally detected and discussed. Localized surface modes and guided mode resonances appear in the reflection spectrum of a photonic crystal slab interacting with external infrared radiation and can be connected with the negative refractive index of the sample . These phenomena can provide an efficient way to confine the radiation into the structure, with an high field enhancement and a strong sensibility of the resonance position to the refractive index variations.
In this paper we report a new set of accurate measurements of guided mode resonances coupled in a negative photonic
crystal slab. Narrow peaks are visible in the reflection spectrum with a full-width at half maximum (FWHM) of less than
2 nm. In addition to the traditional measurements of the reflected signal, we present the imaging of the coupled radiation
propagating into the slab. Finally, by comparison with the already known phenomenological analysis [1] we propose a
new physical model of the phenomenon. The experimental data shows an excellent agreement with mentioned theory.
High frequencies (visible and near infrared) applications of metamaterials and plasmonic structures are strongly limited by dissipative losses in structures, due to poor conductivity of most used metals in this frequency range. The use of high temperature superconductors (HTSC) is a possible approach to this problem, being HTSC plasmonic materials at nonzero temperature. Negative dielectic constant and variety of charge carriers (electrons or holes) are further very attractive features for plasmonic applications. Characterization of the high frequency response of these materials is then necessary in order to correctly understand the optical parameters of HTSC. We report on FTIR and ellipsometry measurements on NdBa2Cu3O7-δ (Nd123) and the ruthenocuprate superconductor GdSr2RuCu2O8-δ (Gd1212) in optical and near infrared regime. Among YBCO-like cuprate superconductors, Nd123 presents the highest Tc (96K), and the most interesting magnetic response properties. Even more interesting, in view of use for metamaterial, is Gd1212, whose main characteristic is the coexistence, in the same cell, of superconductivity and magnetic order below Tc: Ru ions intrinsic magnetic moments order themselves below 135K, whereas superconductivity onset is at about 40K, depending on fabrication details. We performed measurements on Melt-Textured bulk samples, which present the best superconducting properties. Results confirm the promising feature of the considered materials; further analyses, also on powders and films, are in progress.
Photonic crystal metamaterial can exhibit negative index properties and this behaviour is well described by a resonator model. In this work, we present the experimental evidence that a Lorentz resonator correctly reconstruct data obtained with a negative refracting Photonic Crystal (PhC) by using a standard optical technique, such as ellipsometry. In particular we show that, in the frequency range in which the effective refractive index, neff, is equal to -1, the incident light couples efficiently to the guided modes in the top surface layer of the PhC metamaterial. These modes resemble surface plasmon polariton resonances. In add we present measurements by using standard technique of prism coupling evanescent wave. Once again the presence of localized plasmon-like modes at the surface of a silicon two-dimensional photonic crystal slab is demonstrated. Also in this case, in analogy with surface plasmons supported in metals in a photonic crystal metamaterial, the electromagnetic surface waves arise from a negative effective permittivity. These results opens new strategies in light control at the nanoscale, allowing on chip light manipulation in a wide frequency range and avoiding the intrinsic limits of plasmonic structures due to absorption losses in metals. Such negative index PhC materials may be of use in biosensing applications.
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