All-optical self-manipulation of light flow in on-chip topological waveguides based on synthetic dimension.

Topological photonic crystals provide a new platform for designing nanophotonic devices with robustness. Especially, all-optical devices, which use the light controlling light, based on nonlinear topological photonic crystals, have not been reported yet. In this article, we numerically investigate the robust self-manipulation of light flow in silicon topological photonic crystal waveguides based on the Kerr nonlinearity of silicon and topological edge states of photonic crystal waveguides. By adjusting the intensity of incident light at a communication wavelength of 1550 nm, the transmission path of the light flow in waveguides can be effectively controlled, and such manipulation is immune to some disturbances of nanostructures and thus shows the robustness. The results indicate that nonlinear topological photonic crystals have potential applications in on-chip integrated all-optical photonic devices.

Second harmonic generation in plasmonic metasurfaces enhanced by symmetry-protected dual bound states in the continuum.

We numerically investigate linear and nonlinear optical responses in metasurfaces consisting of Au double-gap split ring resonators (DSRRs). Symmetry-protected dual bound states in the continuum (BICs) in such plasmonic metasurfaces are observed at the near-infrared optical regime. Efficient second harmonic generation (SHG) is obtained at the quasi-BIC models due to the symmetry broken. The optimized SHG responses are obtained at the critical couplings between radiation and nonradiation processes at the linearly x- and y-polarized light, respectively. High conversion efficiency of SHG of a value 10-6 is arrived at the fundamental intensity of 10 GW/cm2 at the quasi-BIC wavelength under the y-polarized illumination. Large extrinsic and tunable chirality of linear and nonlinear optical responses empowered by quasi-BICs is acquired in asymmetry metasurfaces at oblique circularly polarized incidence. The results indicate that the plasmonic metasurfaces of symmetry-protected BICs at the near-infrared optical regime have great potential applications in the on-chip efficient frequency conversion, and the linear and nonlinear chiral manipulation.

Ultrahigh Modulation Enhancement in All-Optical Si-Based THz Modulators Integrated with Gold Nanobipyramids.

Optical regulation strategy with the aid of hybrid materials can significantly optimize the performance of terahertz devices. Gold nanobipyramids (AuNBPs) with synthetical tunability to the near-infrared band show strong local field enhancement, which improves optical coupling at the interface and benefits the modulation performance. We design AuNBPs-integrated terahertz modulators with multiple structured surfaces and demonstrate that introducing AuNBPs can effectively enhance their modulation depths. In particular, an ultrahigh modulation enhancement of 1 order of magnitude can be achieved in the AuNBPs hybrid metamaterials accompanied by the multifunctional modulation characteristics. By application of the coupled Lorentz oscillator model, the theoretical calculation suggests that the optical regulation with AuNBPs originates from increased damping rate and higher coupling coefficient under pump excitation. Additionally, a terahertz spatial light modulator is constructed to demonstrate multiple imaging display and consume extremely low power, which is promising for the potential application in spatial and frequency selective imaging.

Evolution of optical harmonic generation near bound-states in the continuum in hybrid plasmonic-photonic structures.

We investigate the nonlinear optical harmonic generation behaviors near the bound-states in the continuum (BICs) in hybrid plasmonic-photonic structures. The hybrid structures are designed to consist of a plasmonic grating covered with a nonlinear dielectric waveguide layer, which support two distinct groups of BICs, i.e. the symmetry-protected BICs and Friedrich-Wintgen BICs. The evolution of second- and third-harmonic generation (SHG and THG) near the two groups of BICs was studied. The high dependence of nonlinear response on the local field distribution and tensor components of susceptibility of nonlinear materials was determined. Especially, there exists optimized angles of incidence for efficient SHG and THG response due to the interaction of photonic and plasmonic modes. The results are important to understand the nonlinear response behaviors in hybrid plasmonic-photonic structures and to design the nonlinear photonic devices.

Highly-sensitive sensor based on toroidal dipole governed by bound state in the continuum in dielectric non-coaxial core-shell cylinder.

The electromagnetic fields distributed on the surface region of the nanostructure is very important to improve the performance of the sensor. Here, we proposed a highly sensitive sensor based on toroidal dipole (TD) governed by bound state in the continuum (BIC) in all-dielectric metasurface consisting of single non-coaxial core-shell cylinder nanostructure array. The excitation of TD resonance in a single nanostructure is still challenging. The designed nanostructure not only supports TD resonance in a single nanostructure but also has very high Q-factor. More importantly, its electric field distributes at the surface of outer cylinder-shell, which is very suitable for biosensing. To evaluate the sensing performance of our proposed structure, we investigated the sensitivity and the figure of merit (FOM) of nanostructure with different structural parameters. Maximum sensitivity and FOM can reach up to 342 nm/RIU and 1295 when the asymmetric parameter d =10 nm. These results are of great significance to the research of TD resonance and the development of ultrasensitive sensor.

LSPR optical fiber sensor based on 3D gold nanoparticles with monolayer graphene as a spacer.

Localized surface plasmon resonance (LSPR) optical fiber biosensing is an advanced and powerful label-free technique which gets great attention for its high sensitivity to refractive index change in surroundings. However, the pursuit of a higher sensitivity is still challenging and should be further investigated. In this paper, based on a monolayer graphene/gold nanoparticles (Grm/Au NPs) three-dimensional (3D) hybrid structure, we fabricated a D-shaped plastic optical fiber (D-POF) LSPR sensor using a facile two-step method. The coupling enhancement of the resonance of this multilayer structure was extremely excited by the surface plasmon property of the stacked Au NPs/Grm layer. We found that the number of plasmonic structure layers was of high importance to the performance of the sensor. Moreover, the optimal electromagnetic field enhancement effect was found in three-layer plasmonic structure. Besides, the n*(Grm/Au NPs)/D-POF sensor exhibited outstanding performance in sensitivity (2160 nm/RIU), linearity (linear fitting coefficient R2 = 0.996) and reproducibility. Moreover, the sensor successfully detected the concentration of glucose, achieving a sensitivity of 1317.61 nm/RIU, which suggested a promising prospect for the application in medicine and biotechnology.

Ultralow-level all-optical self-switching in a nanostructured moiré superlattice.

We report an all-optical self-switching performed at an ultralow-level of intensity in a nanostructured moiré superlattice on a silicon platform. The moiré superlattice was formed by twisting two sets of triangular lattices in a silicon membrane in the same layer with a twist angle of 9.43°. The near flatband was formed, and the electric field was well confined in the center of the superlattice, which enabled all-optical switching under an ultralow intensity when the Kerr nonlinearity of silicon was considered. The intensity, which was reduced to 300 W/m2 and even 20 W/m2, can cause the transmittance of the nanostructure to change from 0 to 80% under x- and y-polarized pump light, respectively, and could be further decreased by optimizing the nanostructure or nonlinear materials. The results indicate that moiré superlattices fabricated from nonlinear materials are promising for integrated all-optical devices.

Low-threshold and controllable nanolaser based on quasi-BIC supported by an all-dielectric eccentric nanoring structure.

High-Q factor can enhance the interaction between light and matter, which is an important parameter to decrease the threshold of nanolasers. Here, we theoretically propose an eccentric nanoring structure with a high and controllable Q factor to realize a low-threshold and controllable nanolaser by amplifying the quasi-bound states in the continuum (quasi-BIC). The designed nanostructure supports a quasi-BIC because of the symmetry protection-breaking of the nanostructure. The quasi-BIC has a very high Q factor of about 9.6×104 and can also be adjusted by changing structural parameters. We use the energy level diagram of the four-level two-electron system to study the lasing action of the eccentric nanoring structure. The results show that the nanolaser has a relatively low threshold of about 6.46 µJ/cm2. Furthermore, the lasing behavior can be tuned by controlling the structural parameters of the eccentric circular ring structure.

Ultimate conversion efficiency of second harmonic generation in all-dielectric resonators of quasi-BICs in consideration of nonlinear refraction of dielectrics.

We investigate second harmonic generation (SHG) in all-dielectric resonance nanostructures of high-Q factors assisted by quasi-bound states in the continuum (quasi-BICs). The typical resonators, e.g., guided-mode resonance gratings and asymmetric metasurfaces, fabricated by AlGaAs were numerically studied with the consideration of nonlinear refraction of AlGaAs. The resonance peak and line-shape of linear transmission and SHG spectra in the resonators can be dramatically changed under intense pump intensities. The SHG conversion efficiency in the nanostructures working at quasi-BICs is much lower than the traditionally expected values without considering the nonlinear refraction of dielectrics. The ultimate SHG conversion efficiency is finally obtained. The investigation has the significance for the design and understanding of efficient nonlinear metasurfaces of high-Q factors.

Bistability of optical harmonic generation in monolayer graphene plasmonics.

We report the bistability of second- and third-harmonic generation in monolayer graphene plasmonics supported by graphene nanoribbon arrays. The nonlinear optical bistability of harmonic generation at the ultra-low threshold intensity ∼100kW/cm2, along with the traditional linear optical bistability of transmittance, is observed due to the different local fundamental fields at the lower and higher state when the Kerr effect of graphene is considered. Importantly, the working fundamental wavelength can be tuned by the Fermi level of graphene and geometrical structure, which leads to the linear and nonlinear optical bistability available in a broadband for potential applications in advanced all-optical devices.

Optical bistability in gap-plasmon metasurfaces in consideration of classical nonlocal effects.

Optical bistability of linear reflectance and third-harmonic generation is investigated in a metasurface consisting of metallic grating coupled with metallic film spaced with nonlinear dielectric material. Linear optical reflectance and electric field enhancement are achieved for gaps <20 nm in the presence of classical nonlocality in metallic nanostructures. Enlarged thresholds from the higher to lower reflectance states are observed from 140 kW/cm2 for the local model to 300 kW/cm2 for the nonlocal model for 0.5-nm gaps. Though the linear reflectance almost overlaps for local and nonlocal models for 20-nm gaps, the optical bistability hysteresis loops retain large differences because local field differences are amplified owing to the relation of nonlinear refraction with square of local field and historical evolution of the optical bistability.

Giant enhancement of harmonic generation in all-dielectric resonant waveguide gratings of quasi-bound states in the continuum.

We report the giant enhanced optical harmonic generation in all-dielectric silicon nitride (SiN) based resonant waveguide gratings (RWGs) of quasi-bound states in the continuum (BICs) of ultra-high Q factor and localized field. The BICs are realized by tuning the excitation of the guided modes modulated by geometry parameters of four-part grating layer. At a feasible structure of quasi-BIC for nanofabrication, the SHG and THG are enhanced by 103 and 106, compared with those from the RWGs of traditional two-part grating layer, respectively, and even up to 108 and 1010 compared with those from the planar SiN film, respectively. The resonance wavelength of quasi-BICs can be effectively tuned by the angle of incidence, while almost not affect the enhancement of SHG and THG response. Our results show that the efficiency harmonic generation from all-nonlinear-dielectric RWGs of quasi-BICs has potential applications for the integrated nonlinear photonic devices.

Broadband indium tin oxide nanowire arrays as saturable absorbers for solid-state lasers.

Indium Tin Oxide nanowire arrays (ITO-NWAs), as epsilon-near-zero (ENZ) materials, exhibit a fast response time and a low saturable absorption intensity, which make them promising photoelectric materials. In this study, ITO-NWAs were successfully fabricated using a chemical vapor deposition (CVD) method, and the saturable absorption properties of this material were characterized in the near-infrared region. Further, passively Q-switched all-solid-state lasers were realized at wavelengths of 1.0, 1.3, and 2.0 µm using the as-prepared saturable absorber (SA). To the best of our knowledge, we present the first application of ITO-NWAs in all-solid-state lasers. The results reveal that ITO-NWAs may be applied as an SA while developing Q-switched lasers and that they exhibit a broad application prospect as broadband saturable absorption materials.

Watt-level ultrafast bulk laser with a graphdiyne saturable absorber mirror.

Few-layered graphdiyne (GDY) was successfully fabricated and applied as a saturable absorber to generate a watt-level ultrafast solid-state bulk laser. The maximum output power of up to 1.27 W was obtained with a pulse width of 23 ps and a repetition rate of 92.9 MHz, using Nd:YVO4 crystal as a gain medium. To the best of our knowledge, this is the first application of GDY as a mode locker in all-solid-state bulk lasers. These results indicate the promising potential of GDY for producing high-power ultrafast lasers.

Tunable photonic-like modes in graphene-coated nanowires.

In this study, the dispersion equations of a graphene-coated nanowire (GN) are solved. It is found that in this waveguide, besides the surface plasmon polaritons (SPPs), there is another branch of guided modes, called photonic-like modes. The propagation distances of the photonic-like modes can be five orders of magnitude longer than those of the SPPs. Moreover, they can be modulated in the range of 10-4 to 1 m by changing the chemical potential of graphene. In particular, the mode field distributions remain nearly unchanged during the modulation. Based on the analysis performed using COMSOL Multiphysics, we further demonstrated that the propagation losses of the photonic-like modes are dependent on not only the chemical potential of graphene, but also the mode power proportion inside graphene. The photonic-like modes have tremendous potential to be used in optical switches, modulators, and switches in magnetic fields at the nanoscale.

Continuous-wave and Q-switched Nd:BGSO lasers based on bismuth nanosheets absorber.

This Nd:BG1-xSxO (Nd:BGSO) crystal was grown using the micro-pulling-down method, and the continuous-wave laser operation of this crystal was demonstrated for the first time, to the best of our knowledge. The maximum output power of 1.038 W was obtained under the absorbed pump power of 3.01 W, which corresponds to a slope efficiency of 31.3%. Bismuth nanosheets were first employed as saturable absorbers to generate a passively Q-switched Nd:BGSO laser. Stable Q-switched pulses with the shortest pulse width of 376.5 ns and the maximum repetition rate of 136.6 kHz were achieved at the absorbed pump power of 3.01 W. The largest pulse energy and highest peak power achieved were 0.94 µJ and 2.48 W, respectively.

Generation of high-energy rectangular pulses in a nonlinear polarization rotation mode-locked ring fiber laser.

In this paper, a novel high-energy mode-locked fiber laser based on the nonlinear polarization rotation (NPR) technique is presented to generate 331 nJ rectangular pulses. When pump power was 2659 mW, the maximum output power would be 102.3 mW; the maximum peak power was 41.74 W under the pump power of 1766 mW. In this study, the use of two homemade laser diodes and other common fiber devices was a vital step to achieve the low-cost and high-efficiency NPR mode-locked fiber laser. Based on these results, a novel approach could be developed to realize a high-energy rectangular pulse and promote the practical applications of the NPR mode-locked fiber laser in the field of ultrafast photonics.

Third-harmonic generation from gold nanowires of rough surface considering classical nonlocal effect.

We demonstrate that the nonlocal dielectric response of metal, comparing with the traditional local model, will significantly boost the third-order harmonic generation (THG) from gold nanowires of rough surface by a factor of several orders of magnitude. The enhancement is probably due to the penetrated field into the fine nanostructures on the metal surface in nonlocal model. The anisotropy THG efficiency versus the angle of incidence is also demonstrated due to the inhomogeneous surface morphology. The possible ways to verify the nonlocal effect to the THG are demonstrated. The results have a general significance in explaining the experimental observations.

Enhancement of second-harmonic generation from silicon nitride with gold gratings.

We report strong enhancement of second-harmonic generation in a hybrid nanostructure with gold gratings embedded in a silicon nitride film. Compared to a flat silicon nitride film, the enhancement factor can be as large as 102 to 103 for transverse magnetic and electric polarizations, respectively in good agreement with numerical results calculated using finite element method. For both polarizations, the enhancement arises from a resonance between the waveguide modes and grating.

Third-harmonic UV generation in silicon nitride nanostructures.

We report on strong UV third-harmonic generation from silicon nitride films and resonant waveguide gratings. We determine the absolute value of third-order susceptibility of silicon nitride at wavelength of 1064 nm to be χ(³) (-3ω,ω,ω,ω) = (2.8 ± 0.6) × 10⁻²°m²/V², which is two orders of magnitude larger than that of fused silica. The third-harmonic generation is further enhanced by a factor of 2000 by fabricating a resonant waveguide grating onto a silicon nitride film. Our results extend the operating range of CMOS-compatible nonlinear materials to the UV spectral regime.