Sharp electromagnetic resonances play an essential role in physics in general and optics in particular. The last decades have witnessed the successful developments of high-quality (Q) resonances in microcavities operating below the light line, which however is fundamentally challenging to access from free space. Alternatively, metasurface-based bound states in the continuum (BICs) offer a complementary solution of creating high-Q resonances in devices operating above the light line, yet the experimentally demonstrated Q factors under normal excitations are still limited. Here, we present the realizations of quasi-BIC under normal excitation with a record Q factor up to 18 511 by engineering the symmetry properties and the number of the unit cells in all-dielectric metasurface platforms. The high-Q quasi-BICs exhibit exceptionally high conversion efficiency for the third harmonic generation and even enable the second harmonic generation in Si metasurfaces. Such ultrasharp resonances achieved in this work may immediately boost the performances of BICs in a plethora of fundamental research and device applications, e.g., cavity QED, biosensing, nanolasing, and quantum light generations.

We demonstrate experimentally that interaction between nonlocal solitons in nematic liquid crystals (NLC) can be controlled by the degree of nonlocality. For a given beam width, the degree of nonlocality can be modulated by changing the pretilt angle θ 0 of NLC molecules through bias voltage V . As V increases (so does θ 0 ), the degree of nonlocality decreases. When the degree of nonlocality is below a critical value, the solitons behave in the way like their local counterpart, i.e., in-phase solitons attract while out-of-phase solitons repulse each other. Such a voltage-controlled interaction between the solitons can be readily implemented in experiments.

Gold (Au) nanoparticles, particularly nanorods, are actively employed as imaging probes because of their special nonblinking and nonbleaching absorption, scattering, and emitting properties that arise from the excitation of surface plasmons. Herein, we report a novel sensing method that detects feature orientation at the nanoscale via the defocused imaging of individual Au nanorods (AuNRs) with an ordinary wide-field optical microscope. By simultaneously recording defocused images and two-photon luminescence intensities for a large number of individual AuNRs, we correlate their defocused images with their three-dimensional spatial orientations. The spatial orientation of many individual AuNRs can be monitored in situ and in real-time within a single frame, enabling its use as a technique for high-throughput sensing. The probe size can be as small as several nanometers, which is highly desirable for minimization of any potential interference from the probe itself. Furthermore, the sensing property is insensitive to the excitation polarization and the distribution of the probe aspect ratio, which allows AuNRs of any length within a proper regime to be used as orientation sensors without changing the laser frequency and polarization. These unique features make the orientation probes proposed here outstanding candidates for optical imaging and sensing in materials science and biological applications.

As one of the most important semiconductors, silicon has been used to fabricate electronic devices, waveguides, detectors, solar cells, etc. However, the indirect bandgap and low quantum efficiency (10−7) hinder the use of silicon for making good emitters. For integrated photonic circuits, silicon-based emitters with sizes in the range of 100−300 nm are highly desirable. Here, we show the use of the electric and magnetic resonances in silicon nanoparticles to enhance the quantum efficiency and demonstrate the white-light emission from silicon nanoparticles with feature sizes of ~200 nm. The magnetic and electric dipole resonances are employed to dramatically increase the relaxation time of hot carriers, while the magnetic and electric quadrupole resonances are utilized to reduce the radiative recombination lifetime of hot carriers. This strategy leads to an enhancement in the quantum efficiency of silicon nanoparticles by nearly five orders of magnitude as compared with bulk silicon, taking the three-photon-induced absorption into account.

No abstract