Bound states in the continuum (BICs) represent dark modes trapped in the radiation continuum. BICs received significant attention in optics and photonics as a simple tool to achieve giant quality factors by transforming them into quasi-BICs. Here, we report the observation of high-harmonic generation in dielectric metasurfaces hosting BICs. The metasurface is composed of a square lattice with parallel Si bars of a slightly different width placed on a transparent substrate. The structure is engineered to support a quasi-BIC in the mid-IR with a high quality factor. We tune the metasurface asymmetry to enable the optimal coupling condition that provide the highest high-harmonic generation efficiency. In the experiment, we demonstrate the generation of odd optical harmonics from the 3rd to the 11th order in the BIC regime and study their polarization dependence. We measure the dependence of the high-harmonic signal on the input intensity. The concept of metasurfaces with highly localized light boosted by BIC resonances provides a new degree of freedom to control experimentally strong nonlinear optical response.
The resonant high-index nanostructures open opportunities for control many optical effects via optically-induced electric and magnetic Mie resonances, mostly localized inside the structures. Especial interest such nanostructures represent for quantum emitters placed inside, that makes possible enhancement of quantum source emission through resonant coupling to localized modes. We have proposed the concept of active dielectric nanoantennas based on nanodiamonds with embedded NV-centers. The study of theoretically dependence of optical properties of this system on the spectral position of the resonant modes has demonstrated that that at some sizes of the diamond spherical particles and certain position of the dipole in the sphere the Purcell factor can achieve the value of 30. We have demonstrated experimentally that the photoluminescence properties of the NV-centers can be controlled via scattering resonances and observed a decrease of the NV-centers lifetime in the studied diamond particles, as compared to nonresonant nanodiamonds. These results are in a good agreement with our theoretical calculations for the average Purcell factor for multiple NV-centers within a nanoparticle. The simplicity of the proposed concept compared to existing photonic cavity systems and applicability for a wide range of color centers in diamond make active diamond nanoantenna an effective tool for creating controllable emitting elements in the visible range for future nanophotonic devices.
We demonstrate that a hybrid c-Si/Au nanocavity can serve as a multifunctional sensing platform for nanoscale (about 100 nm) thermometry with high accuracy (>0.4 K) and fast response (<0.1 second), controlled local optical heating up to 1200 K and also provide Raman scattering enhancement (>10^4 fold). The system has been tested in the experiment on thermally induced unfolding of BSA molecules, plased inside the hybrid nanocavity. Moreover, numerical modeling reveal, that two possible operation modes of the system: with and without considerable optical heating at the nanometer scale, while other functionalities (nanothermometry, RS enhancement, and tracing the events) are preserved. These regimes make the hybrid nanocavity more versatile sensing system than fully plasmonic counterparts. The simplicity and multifunctionality of the hybrid nanocavity make it a promising platform for photochemistry and photophysics applications.
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