Second harmonic generation (SHG) is a coherent nonlinear phenomenon that plays an important role in laser color conversion. Lithium niobate (LN), which features both a large band gap and outstanding second-order nonlinearities, acts as an important optical material for nonlinear frequency conversion covering a wide spectral range from ultraviolet to mid-infrared. Here we experimentally demonstrate LN metasurfaces with controllable SHG properties. Distinct enhancements for the SHG efficiency are observed at Mie-resonances. And by changing the geometric parameters thus the resonances of the metasurfaces, we manage to selectively boost the SHG efficiency at different wavelengths. Our results would pave a way for developing with high flexibility the novel compact nonlinear light sources for applications, such as biosensing and optical communications.
Many applications of metasurfaces require an ability to dynamically change their properties in time domain. Electrical tuning techniques are of particular interest, since they pave a way to on-chip integration of metasurfaces with optoelectronic devices. In this work, we propose and experimentally demonstrate an electro-optic lithium niobate (EO-LN) metasurface that shows dynamic modulations to phase retardation of transmitted light. Quasi-bound states in the continuum (QBIC) are observed from our metasurface. And by applying external electric voltages, the refractive index of the LN is change…
Lithium niobate is a multi-functional material, which has been regarded as one of the most promising platform for the multi-purpose optical components and photonic circuits. Targeting at the miniature optical components and systems, lithium niobate microstructures with feature sizes of several to hundreds of micrometers have been demonstrated, such as waveguides, photonic crystals, micro-cavities, and modulators, et al. In this paper, we presented subwavelength nanograting metasurfaces fabricated in a crystalline lithium niobate film, which hold the possibilities towards further shrinking the …
The phase-matching condition is a key aspect in nonlinear wavelength conversion processes, which requires the momenta of the photons involved in the processes to be conserved. Conventionally, nonlinear phase matching is achieved using either birefringent or periodically poled nonlinear crystals, which requires careful dispersion engineering and are usually narrowband. In recent years, metasurfaces consisting of densely packed arrays of optical antennas have been demonstrated to provide an effective optical momentum to bend light in arbitrary ways. Here, we demonstrate that gradient metasurface…
Electro-optical modulation of light polarization in a nonlocal lithium niobate metasurface
Politecnico di Milano（米兰理工大学）；EPFL, Insitute of Mechanical Engineering, Laboratory of Nanoscience for Energy Technologies（洛桑联邦理工学院机械工程学院纳米能源技术实验室）；ETH Zürich, Department of Physics, Institute for Quantum Electronics（苏黎世联邦理工学院物理系量子电子研究所）；Università di Brescia（布雷西亚大学）
Electro-optically tunable second-harmonic generation in lithium niobate metasurfaces boosted by Brillouin zone folding induced bound states in the continuum
In display technologies or data processing, planar and subwavelength free-space components suited for flat photonic devices are needed. Metasurfaces, which shape the optical wavefront within hundreds of nanometers, can provide a solution for thin and portable photonic devices, e.g. as CMOS-compatible modules. While conventional electro-optic modulators are inconvenient to operate in free space configurations, its principle can largely be applied to the development of active metasurfaces with the prospect of modulation speeds up to the GHz region. Here, we use this concept to realize fast and c…
Nonlinear optical phenomena in nanostructured materials have been challenging our perceptions of nonlinear optical processes that have been explored since the invention of lasers. For example, the ability to control optical field confinement, enhancement, and scattering almost independently, allows nonlinear frequency conversion efficiencies to be enhanced by many orders of magnitude compared to bulk materials. Also, the subwavelength length scale renders phase matching issues irrelevant. Compared with plasmonic nanostructures, dielectric resonator metamaterials show great promise for enhanced…
Dynamically reconfigurable metasurfaces promise compact and lightweight spatial light modulation for many applications, including LiDAR, AR/VR, and LiFi systems. Here, we design and computationally investigate high quality factor silicon-on-lithium niobate metasurfaces with electrically-driven, independent control of its constituent nanobars for full phase tunability with high tuning efficiency. Free-space light couples to guided modes within each nanobar via periodic perturbations, generating quality factors exceeding 30,000, while maintaining bar spacing <$λ$/1.5. We achieve nearly 2$π$ phas…
Metasurfaces consisting of nano-scale structures are underpinning new physical principles for the creation and shaping of quantum states of light. Multi-photon states that are entangled in spatial or angular domains are an essential resource for quantum imaging and sensing applications, however their production traditionally relies on bulky nonlinear crystals. We predict and demonstrate experimentally the generation of spatially entangled photon pairs through spontaneous parametric down-conversion from a metasurface incorporating a nonlinear thin film of lithium niobate. This is achieved throu…
Electric field detection has been widely utilized in many fields such as scientific research and integrated circuits. To enhance the tuning sensitivity of E-field sensor, in this paper, we theoretically proposed a highly sensitive E-field sensor composed of etchless lithium niobate (LN) material and hybrid coupling-grating systems in the visible near-infrared regime. Such configuration supports high-quality factor quasi-BIC resonance, which generates strong localized field confinement. Due to the large electro-optic coefficient of LN material, one can shift the wavelength and reflection ratio …
Enhancing broadband second harmonic generation in a thin film lithium niobate racetrack resonator with tunable-coupling
Centre suisse d’électronique et de microtechnique (CSEM)（瑞士电子和微技术中心）；Photonic Systems Laboratory, École Polytechnique Fédérale de Lausanne（洛桑联邦理工学院光子系统实验室）
High-index nanoparticle lattices have attracted a lot of interest recently as they support both optical electric and magnetic resonances and can serve as functional metasurfaces. Here we demonstrate that under particular conditions, the all-dielectric nanoparticle metasurfaces can resonantly suppress transmission. Electric and magnetic dipole resonances of silicon nanoparticle arrays are studied in the air and in the dielectric matrix in visible and near-infrared spectral ranges. We show that the wave resonantly scattered forward by the one or both electric and magnetic dipole moments of nanop…
The rapid development of metasurfaces - 2D ensembles of engineered nanostructures - is presently fostering a steady drive towards the miniaturization of many optical functionalities and devices to a subwavelength size. The material platforms for optical metasurfaces are rapidly expanding and for the past few years, we are seeing a surge in establishing meta-optical elements from high-index, highly transparent materials with strong nonlinear and electro-optic properties. Crystalline lithium niobate (LN), a prime material of choice in integrated photonics, has shown great promise for future meta…
Silicon-based nonlinear metasurfaces were implemented only with third-order nonlinearity due to the crystal centrosymmetry and the efficiencies are considerably low, which hinders their practical applications with low-power lasers. Here, we propose to integrate a two-dimensional GaSe flake onto a silicon metasurface to assist high-efficiency second-order nonlinear processes, including second-harmonic generation (SHG) and sum-frequency generation (SFG). By resonantly pumping the integrated GaSe-metasurface, which supports a Fano resonance, the obtained SHG is about two orders of magnitude stron…
The emerging all-dielectric platform exhibits high-quality ($Q$) resonances governed by the physics of bound states in the continuum (BIC) that drives highly efficient nonlinear optical processes. Here we demonstrate the robust enhancement of third-(THG) and fifth-harmonic generation (FHG) from all-dielectric metasurfaces composed of four silicon nanodisks. Through the symmetry breaking, the genuine BIC transforms into the high-$Q$ quasi-BIC resonance with tight field confinement for record high THG efficiency of $3.9\times10^{-4}$ W$^{-2}$ and FHG efficiency of $4.8\times10^{-10}$ W$^{-4}$ us…
We present a double-layer dielectric metasurface obtained by stacking a silicon nanodisc array and a silicon photonic crystal slab with equal periodicity on top of each other. We focus on the investigation of electric near-field enhancement effects occurring at resonant excitation of the metasurface and study its optical properties numerically and experimentally. We find that the major difference in multi-layer metasurfaces when compared to conventional single-layer structures appears to be in Rayleigh-Wood anomalies: they are split into multiple different modes which are themselves spectrally…
Parametric oscillation is a fundamental concept that underlies nonlinear wave-matter interactions, leading to generation or amplification of new frequency components. Using a temporal modulation generated by the heterodyne interference of a control direct current (dc) electric field and an optical field, we resonantly drive large-amplitude oscillations of amorphous silicon meta-atoms - nanoscale building blocks of an optical metasurface. We observed gigantically enhanced second harmonic generation (SHG) with an electric-field-controlled on off ratio over 10000 and an ultra-high modulation dept…
All-dielectric optical metasurfaces are a workhorse in nano-optics due to both their ability to manipulate light in different degrees of freedom and their excellent performance at light frequency conversion. Here, we demonstrate first-time generation of photon pairs via spontaneous parametric-down conversion in lithium niobate quantum optical metasurfaces with electric and magnetic Mie-like resonances at various wavelengths. By engineering the quantum optical metasurface, we tailor the photon-pair spectrum in a controlled way. Within a narrow bandwidth around the resonance, the rate of pair pr…
Tailoring optical nonlinear effects (e.g. harmonic generation, sum-frequency mixing, etc.) in the recently emerging all-dielectric platform is important for both the fundamental science and industrial development of high-efficiency, ultrafast, and miniaturized photonic devices. In this work, we propose a novel paradigm for dynamically switchable high-harmonic generation in Silicon nanodimer metasurfaces by exploiting polarization-controlled dual bound states in the continuum (BIC). Owing to the high-quality factor of BIC resonances, efficient harmonic signals including the third-harmonic gener…
Enhanced third-harmonic generation and degenerate four-wave mixing in an all-dielectric metasurfaces via Brillouin zone folding-induced bound states in the continuum
Dynamic optical metasurfaces with ultrafast temporal response, i.e., spatiotemporal optical metasurfaces, provide attractive solutions and open fascinating perspectives for modern highly integrated optics and photonics. In this work, electro-optically controlled optical metasurfaces operating in reflection and utilizing resonant waveguide mode excitation are demonstrated from the viewpoint of free-space propagating light modulation. The modulation of reflected light power with superior characteristics in comparison with prior research is achieved by identifying a suitable low-loss waveguide mo…
Optical metasurfaces have been extensively investigated, demonstrating diverse and multiple functionalities with complete control over the transmitted and reflected fields. Most optical metasurfaces are however static, with only a few configurations offering (rather limited) electrical control, thereby jeopardizing their application prospects in emerging flat optics technologies. Here, we suggest an approach to realize electrically tunable optical metasurfaces, demonstrating dynamic Fresnel lens focusing. The active Fresnel lens (AFL) exploits the electro-optic Pockels effect in a 300-nm-thick…
Spontaneous parametric down-conversion (SPDC) is one of the most versatile nonlinear optical techniques for the generation of entangled and correlated single-photon pairs. However, it suffers from very poor efficiency leading to extremely weak photon generation rates. Here we propose a plasmonic metasurface design based on silver nanostripes combined with a bulk lithium niobate (LiNbO3) crystal to realize a new scalable, ultrathin, and efficient SPDC source. By coinciding fundamental and higher order resonances of the metasurface with the generated signal and idler frequencies, respectively, t…
Lattice tuning of optical and mechanical interactions in gold nanodisk metasurfaces
Universidad de Buenos Aires（布宜诺斯艾利斯大学）；CONICET-Universidad de Buenos Aires（阿根廷国家科学与技术研究理事会-布宜诺斯艾利斯大学）；Ludwig-Maximilians-Universität München（慕尼黑路德维希-马克西米利安大学）
Nonlinear Self-Action across Temporal Regimes in Resonant Dielectric Metasurfaces
Politecnico di Milano（米兰理工大学）；Institute of Photonic and Nanotechnology (IFN) - Consiglio Nazionale delle Ricerche (CNR)（光子与纳米技术研究所（IFN）- 意大利国家研究委员会（CNR））；Institute of Photonics and Quantum Sciences, Heriot-Watt University（赫瑞-瓦特大学光子与量子科学研究所）；ICFO - Institut de Ciències Fotòniques, the Barcelona Institute of Science and Technology（ICFO - 巴塞罗那科学和技术理工学院光子科学研究所）；Università degli Studi di Brescia（布雷西亚大学）
Miniaturised entangled photon sources are highly demanded for the development of integrated quantum photonics. Since the invention of subwavelength optical metasurfaces and their successes at replacing bulky optical components, the possibility of implementing entangled photon sources on such devices is actively investigated. Here, as a first step towards the development of quantum optical metasurfaces (QOM), we demonstrate photon pair generation via spontaneous parametric down-conversion (SPDC) from subwavelength films. We achieved photon pair generation with a high coincidence-to-accidental r…
Resonant structures in modern nanophotonics are non-Hermitian (leaky and lossy), and support quasinormal modes. Moreover, contemporary cavities frequently include 2D materials to exploit and resonantly enhance their nonlinear properties or provide tunability. Such materials add further modeling complexity due to their infinitesimally thin nature and strong dispersion. Here, a formalism for efficiently analyzing third harmonic generation (THG) in nanoparticles and metasurfaces incorporating 2D materials is proposed. It is based on numerically calculating the quasinormal modes in the nanostructu…
