High‐Q Nanophotonics over the Full Visible Spectrum Enabled by Hexagonal Boron Nitride Metasurfaces

Abstract

amplitude, [3,4] directionality of light scattering, [5,6] spin, [7,8] and orbital angular momentum [9,10] without the limitations of intrinsic material losses as for metal-based approaches. In particular, applications driven by near-field enhancement, such as biomolecular sensing, rely on high resonance quality (Q) factors (defined as resonance wavelength divided by line width), and hence high electromagnetic near-field intensities to achieve maximum specimen sensitivity. [11,12] The inherent correlation between the resonance quality factor and the resonator refractive index [13] known from, for example, Mie theory, thus led to the advance of all-dielectric nanophotonics based on high-refractive-index material systems, such as silicon, [14,15] germanium, [16,17] or gallium phosphide. [18,19] Although these materials offer great properties for high-Q resonances in the near-infrared (NIR) and infrared (IR) spectral region, they are accompanied by high material-intrinsic interband absorption losses throughout the visible spectral range due to their intermediate bandgap energies. Owing to these fundamental material limitations, lossless high-index materials throughout the complete visible spectral range are lacking. [20][21][22][23] In particular, for the visible wavelength range, there exists a competition between a large bandgap for lossless All-dielectric optical metasurfaces with high quality (Q) factors have been hampered by the lack of simultaneously lossless and high-refractive-index materials over the full visible spectrum. In fact, the use of low-refractiveindex materials is unavoidable for extending the spectral coverage due to the inverse correlation between the bandgap energy (and therefore the optical losses) and the refractive index (n). However, for Mie resonant photonics, smaller refractive indices are associated with reduced Q factors and low mode volume confinement. Here, symmetry-broken quasi bound states in the continuum (qBICs) are leveraged to efficiently suppress radiation losses from the low-index (n ≈ 2) van der Waals material hexagonal boron nitride (hBN), realizing metasurfaces with high-Q resonances over the complete visible spectrum. The rational use of low-and high-refractive-index materials as resonator components is analyzed and the insights are harnessed to experimentally demonstrate sharp qBIC resonances with Q factors above 300, spanning wavelengths between 400 and 1000 nm from a single hBN flake. Moreover, the enhanced electric near fields are utilized to demonstrate second-harmonic generation with enhancement factors above 10 2 . These results provide a theoretical and experimental framework for the implementation of lowrefractive-index materials as photonic media for metaoptics.

All-2D material photonic devices

“…hBN metasurfaces were also fabricated demonstrating a sharp BIC resonance with Q-factors above 300. 31…”

Engineering Quantum Nanophotonic Components from Hexagonal Boron Nitride

“…Froch et al [28] Resist 1 mT 50/100 W 10% -1D PCC ≈2500 Kuhner et al [33] Metal 6 mT 150/300 W -4 nm s −1 BIC a) cavity ≈200…”

Dual‐Mode Polarization Control with Quasi‐Bound States in the Continuum

“…By simply scaling the size of the nanostructure's unit cell, q‐BIC modes can be tuned almost at will across the electromagnetic spectrum. [ 13 ] As an example, here we investigate a device with five q‐BIC modes in the near‐IR frequency range (Figure 1b). The meta‐polarizer is therefore composed of a set of silicon nano‐bars with periodicity p = 500 nm, widths D 1 = 175 nm and D 2 = D 1 (1‐ α) = 87.5 nm [where we assume the asymmetry factor α = 0.5 and the term (1‐ α) indicates the ratio of the nanobars’ widths D 2 / D 1 ] and height h = 145 nm, placed on top of 2 µm‐thick SiO 2 layer and a Si substrate, where the real and imaginary part of silicon dielectric permittivity are taken from Reference.…”