Electromagnetically induced transparency enabled by quasi-bound states in the continuum modulated by epsilon-near-zero materials.

Highly tunable electromagnetically induced transparency (EIT) with high-quality-factor (Q-factor) excited by combining with the quasi-bound states in the continuum (quasi-BIC) resonances is crucial for many applications. This paper describes all-dielectric metasurface composed of silicon cuboid etched with two rectangular holes into a unit cell and periodically arranged on a SiO2 substrate. By breaking the C2 rotational symmetry of the unit cell, a high-Q factor EIT and double quasi-BIC resonant modes are excited at 1224.3, 1251.9 and 1299.6 nm with quality factors of 7604, 10064 and 15503, respectively. We show that the EIT resonance is caused by destructive interference between magnetic dipole resonances and quasi-BIC dominated by electric quadrupole. Toroidal dipole (TD) and electric quadrupole (EQ) dominate the other two quasi-BICs. The EIT window can be successfully modulated with transmission intensity from 90% to 5% and modulation depths ranging from -17 to 24 dB at 1200-1250 nm by integrating the metasurface with an epsilon-near-zero (ENZ) material indium tin oxide (ITO) film. Our findings pave the way for the development of applications such as optical switches and modulators with many potential applications in nonlinear optics, filters, and multichannel biosensors.

Dual-Wavelength Forward-Enhanced Directional Scattering and Second Harmonic Enhancement in Open-Hole Silicon Nanoblock.

Nanostructures with appropriate sizes can limit light-matter interaction and support electromagnetic multipole resonance. The interaction between light and nanostructures is intimately related to manipulating the direction of scattered light in the far field as well as the electromagnetic field in the near field. In this paper, we demonstrate dual-wavelength directional forward-scattering enhancement in an individual open-hole silicon nanoblock (OH-SiNB) and simultaneously achieve bulk and surface electromagnetic field localization. The second harmonic generation is enhanced using electromagnetic field localization on the square hole surface. Numerical simulations reveal that the resonance modes, at λ1 = 800 nm and λ2 = 1190 nm, approximately satisfy the Kerker condition. In the near field, the magnetic dipole modes at dual wavelength all satisfy the boundary condition that the normal component of the electric displacement is continuous on the square holes surface, thus obtaining the surface electromagnetic field localization. Moreover, highly efficient second harmonic generation can be achieved at dual wavelengths using the surface electromagnetic field localization and the increased surface area of the square holes. Our results provide a new strategy for the integration of nanoantennas and nonlinear optoelectronic devices in optical chips.

Terahertz laser field manipulation on the electronic and nonlinear optical properties of laterally-coupled quantum well wires.

An intense terahertz laser field is shown to actively manipulate the electronic states, as well as the linear and nonlinear optical absorption coefficients, of the laterally-coupled quantum well wires (LCQWWs). The laser-dressed quantum states of the LCQWWs are achieved using the non-perturbative Floquet method and the two-dimensional diagonalization technique under the effective mass approximation. We have demonstrated that the intense terahertz laser field induces a strong deformation of the confinement potential configuration of the LCQWWs, thus pronouncedly dressing the energy levels and wave functions. An unambiguous picture is depicted for the evolution of the laser-dressed quantum states with the increase of the laser-dressed parameter characterizing the strength of the laser-dressed effect. On this basis, the resonant peak positions of the linear and nonlinear optical absorption coefficients feature a blue shift followed by a red shift with an increase of the laser-dressed parameter. Furthermore, the evolution of the peak values for the linear and third-order nonlinear optical absorption coefficients as a function of the laser-dressed parameter is comprehensively discussed. Moreover, in contrast to the case without intense terahertz laser field, the peak values of the linear, third-order nonlinear, and total optical absorption coefficients can be obviously enhanced at the same frequency position by manipulating the appropriate laser-dressed parameter. A similar feature can be found in the linear, third-order nonlinear, and total refractive index changes. Our findings are conducive to the implementation of the expected quantum states and nonlinear optical effects in the LCQWWs, paving the way for new designs in tunable optical switches, infrared photo-detectors and infrared modulators.

Controllable Formation of Luminescent Carbon Quantum Dots Mediated by the Fano Resonances Formed in Oligomers of Gold Nanoparticles.

Rapid and controllable formation of fluorescent carbon quantum dots (CQDs) is highly desirable in the fields of nanophotonics and biophotonics. Here, a novel strategy for creating CQDs, which emit white light efficiently under the excitation of either laser light or a mercury lamp, is proposed and demonstrated. The luminescent CQDs are generated by irradiating a poly(vinyl alcohol) (PVA) film doped with dense gold nanoparticles (AuNPs) with femtosecond laser pulses. The creation of CQDs from PVA is a two-step dehydration process mediated by AuNPs which act not only as heat sources but also as catalytic agents. The formation of CC, CC, and CO bonds is confirmed by infrared Fourier transformation spectroscopy and X-ray photoelectron spectroscopy. It is revealed both numerically and experimentally that a spatially localized temperature distribution at the deep subwavelength scale can be achieved in oligomers of AuNPs by resonantly exciting the Fano resonances formed in the oligomers of AuNPs, enabling the generation of CQDs with small diameters. As one of the potential applications, it is demonstrated that optical display and optical data storage with ultralow energy can be realized by selectively introducing luminescent CQDs in the AuNP/PVA film.

Hot luminescence from gold nanoflowers and its application in high-density optical data storage.

Gold nanoflowers with feature sizes ranging from several tenths to several hundred nanometers were synthesized by using the one-pot method. They were formed by the self-organization of gold nanoparticles of several nanometers and exhibited broad extinction spectra in the near infrared spectral range. Randomly distributed hot spots originating from the strongly localized modes were generated in gold nanoflowers and their appearances exhibited strong dependences on both the polarization and wavelength of the excitation light. Under the excitation of femtosecond laser pulses, such hot spots emitted efficient hot luminescence spanning the visible to near infrared spectral range. Distinct from the hot luminescence of single hot spots formed on rough gold and silver surfaces, the hot luminescence from gold nanoflowers composed of a large number of hot spots exhibited excitation-intensity dependence quite similar to the emission spectrum. It was demonstrated that the polarization- and wavelength-dependent hot luminescence of gold nanoflowers can be utilized to realize optical data storage with high density and low energy.

Bistable switching in the lossy side-coupled plasmonic waveguide-cavity structures.

We show numerically that the lossy side-coupled plasmonic resonators can be used as bistable switches without compensation. While the internal loss imposes on the bistable characteristics by reducing the transmission contrast and raising the input power requirement, it makes the switching more available by enlarging the width of the hysteresis loop. We also correct the nonlinear transmission formula of the resonators to adapt the lossy condition. Both the theoretical and simulation results are in good agreement.