Controlling nonlinear collapse of ellipticity and orientation of a co-variant vector optical field.

A vector optical field with inhomogeneous spatial polarization distribution offers what we believe to be a new paradigm to form controllable filaments. However, it is challenging to steer multiple performances (e.g. number, orientation, and interval) of filaments in transparent nonlinear media at one time. Herein, we theoretically self-design and generate a kind of believed to be novel ellipticity and orientation co-variant vector optical field to interact with Kerr medium to solve this issue. The collapsing behaviors of such a new hybrid vector optical field reveal that, by judiciously adjusting the inherent topological charge and initial phase of incident optical field, we are able to give access to stable collapsing filamentation with tunable numbers, orientations and interval. Additionally, the collapsing patterns presented are immune nearly to the extra random noise. The relevant mechanism behind the collapse of the vector optical field is elucidated as well. The findings in this work may have huge potential in optical signal processing, laser machining, and other related applications.

Resonance coupling between chiral quasi-BICs and achiral molecular excitons in dielectric metasurface J-aggregate heterostructures.

The realization of flexible tuning and enhanced chiral responses is vital for many applications in nanophotonics. This study proposes to manipulate the collective optical responses with heterostructures consisting of chiral dielectric metasurfaces and achiral J-aggregates. Owing to the resonance coupling between the chiral quasi-bound states in the continuum (QBICs) and the achiral exciton mode, large mode splitting and anticrossing are observed in both the transmission and circular dichroism (CD) spectra, which indicates the formation of hybrid chiral eigenmodes and the realization of the strong coupling regime. Considering that the radiative and dissipative damping of the hybrid eigenmodes depends on the coherent energy exchange, the chiral resonances can be flexibly tuned by adjusting the geometry and optical constants for the heterostructure, and the CD of the three hybrid eigenmodes approach the maximum (∼1) simultaneously when the critical coupling conditions are satisfied, which can be promising for enhanced chiral light-matter interactions.

Perfect absorption and phase singularities induced by surface lattice resonances for plasmonic nanoparticle array on a metallic film.

The formation of pairs of perfect absorption associated with phase singularities in the parameter space using the hybridized structure constructed with a metallic nanoparticle array and a metallic film is promising to enhance light-mater interactions. However, the localized plasmon resonances of the array possess strong radiative losses, which is an obstacle to improve the performances for many applications. On the contrary with the subwavelength array hybridized structure, this study shows that by enlarging the lattice spacing, the oscillator strength of the nanoparticles can be enhanced with the formation of surface lattice resonance, thereby leading to similar but much narrower pairs of perfect absorption due to the interactions with the Fabry-Pérot cavity modes. Furthermore, when the surface plasmon polariton mode shift to the same spectral range associated with the enlarged lattice spacing, the coupling and mode hybridization with the surface lattice resonance result in an anticrossing in the spectra. Although the resonance coupling does not enter the strong coupling regime, the quality factors (∼ 134) and near-field enhancements (∼ 44) are strongly enhanced for the hybridized resonance modes due to the effectively suppressed radiative losses compared with that of the localized plasmon resonances, which make the hybridized structure useful for the design of functional nanophotonic device such as biosensing, multi-model nanolasing, and high-quality imaging.

Strongly coupled evenly divided disks: a new compact and tunable platform for plasmonic Fano resonances.

Plasmonic artificial molecules are promising platforms for linear and nonlinear optical modulation at various regimes including the visible, infrared and terahertz bands. Fano resonances in plasmonic artificial structures are widely used for controlling spectral lineshapes and tailoring of near-field and far-field optical response. Generation of a strong Fano resonance usually relies on strong plasmon coupling in densely packed plasmonic structures. Challenges in reproducible fabrication using conventional lithography significantly hinders the exploration of novel plasmonic nanostructures for strong Fano resonance. In this work, we propose a new class of plasmonic molecules with symmetric structure for Fano resonances, named evenly divided disk, which shows a strong Fano resonance due to the interference between a subradiant anti-bonding mode and a superradiant bonding mode. We successfully fabricated evenly divided disk structures with high reproducibility and with sub-20 nm gaps, using our recently developed sketch and peel lithography technique. The experimental spectra agree well with the calculated response, indicating the robustness of the Fano resonance for the evenly divided disk geometry. Control experiments reveal that the strength of the Fano resonance gradually increases when increasing the number of split parts on the disk from three to eight individual segments. The Fano-resonant plasmonic molecules that can also be reliably defined by our unique fabrication approach open up new avenues for application and provide insight into the design of artificial molecules for controlling light-matter interactions.

Restoring the silenced surface second-harmonic generation in split-ring resonators by magnetic and electric mode matching.

Surface second-harmonic generation (SHG) in plasmonic metal nanostructures provides a promising approach to design compact and ultrafast nonlinear nanophotonics devices. However, typical plasmonic nanostructures, such as those with tiny gaps that provide strong near-field-amplified nonlinear sources, often suffer from the cancellation of nonlinear fields in the gaps, which results in the so-called silenced SHG and consequently attenuates the overall nonlinear conversion efficiency. In this study, we propose and demonstrate that the silenced SHG in a gold split-ring resonator can be effectively restored by carefully tailoring its gap geometry to avoid the cancellation of nonlinear fields in the gap and simultaneously achieve both spatial and frequency mode matching between the magnetic and the electric dipolar resonances. As a result, the effective nonlinear sources in the gap can be dramatically amplified and the surface second-harmonic emissions can be efficiently coupled out, leading to an SHG intensity enhancement of 7 times compared to a conventional split-ring resonator. The overall SHG conversion efficiency can thus be enlarged to about 1.49 × 10-8 in the near-infrared excitation region. Importantly, the restored surface second-harmonic emission exhibits the scattering characteristics of an ideal electric dipole, which can be very useful for nonlinear far-field manipulation such as beam steering and holograms.

Ideal magnetic dipole resonances with metal-dielectric-metal hybridized nanodisks.

Magnetic resonances generated with nonmagnetic nanostructures have been widely used to design various functional nanophotonic devices, and it is important to realize pure magnetic dipole scattering for the unambiguous study of magnetic light-matter interactions. However, the magnetic responses often spectrally overlapping with other multipoles, which is the main obstacle to achieve ideal magnetic dipole resonances. This study proposes and theoretically demonstrates that an ideal magnetic dipole resonance can be excited with metal-dielectric-metal hybridized nanodisks. It is shown that although the generated magnetic dipole scattering around the bonding resonance of the hybridized nanodisk is spectrally overlapping with strong electric dipole and electric quadrupole contributions, an almost perfect current loop can be generated by adjusting the geometry parameters and the refractive index of the dielectric layer, thereby leading to the suppressing of the overlapping multipoles and the formation of an ideal magnetic dipole scattering. What's more important is that both electric and magnetic near-fields are enhanced simultaneously with the increasing of the refractive index of the dielectric layer, which makes the hybridized nanodisk a promising platform for enhanced magnetic light-matter interactions.

High Q-factor with the excitation of anapole modes in dielectric split nanodisk arrays.

The simultaneous realization of high Q-factor resonances and strong near-field enhancements around and inside of dielectric nanostructures is important for many applications in nanophotonics. However, the incident fields are often confined within dielectric nanoparticles, which results in poor optical interactions with external environment. Near-field enhancements can be extended outside of dielectric nanostructures with proper design, but the Q-factor is often reduced caused by additional radiation losses. This paper shows that the obstacles to achieve high Q-factor, that is, the radiative losses can be effectively suppressed by using dielectric nanodisk arrays, where the Q-factor is about one order larger than that of the single disks associated with the nonradiating anapole modes and the collective oscillations of the arrays. When the resonance energies of the electric dipole mode and the subradiant mode are degenerate with each other, the destructive interference produces an effect analogous to electromagnetically induced transparency. Furthermore, the Q-factor can be extremely enlarged with dielectric split nanodisk arrays, where the present of the split gap does not induce additional losses. Instead, the coupling between the two interfering modes is modified by adjusting the gap width, which makes it possible to achieve high Q-factor and strong near-field enhancements around and inside of the split disks simultaneously. It is shown that the Q-factor is approaching to 106 when the gap width is about 110 nm, and the near-field enhancements around and inside of the split disks are about two orders stronger than that of the single disk.

Pronounced Fano Resonance in Single Gold Split Nanodisks with 15 nm Split Gaps for Intensive Second Harmonic Generation.

Single metallic nanostructures supporting strong Fano resonances allow more compact nanophotonics integration and easier geometrical control in practical applications such as enhanced spectroscopy and sensing. In this work, we designed a class of plasmonic split nanodisks that show pronounced Fano resonance comparable to that observed in widely studied plasmonic oligomer clusters. Using our recently developed "sketch and peel" electron-beam lithography, split nanodisks with varied diameter and split length were fabricated over a large area with high uniformity. Transmission spectroscopy measurements demonstrated that the fabricated structures with 15 nm split gap exhibit disk diameter and split length controlled Fano resonances in the near-infrared region, showing excellent agreement with simulation results. Together with the plasmon hybridization theory, in-depth full-wave analyses elucidated that the Fano resonances observed in the split nanodisks were induced by mode interference between the bright antibonding dipole mode of split disks and the subradiant mode supported by the narrow split gap. With the giant near-field enhancement enabled by the intensive Fano resonance at the tiny split gap, strong wavelength-dependent second harmonic generation was observed under near-infrared excitation. Our work demonstrated that single split nanodisks could serve as important building blocks for plasmonic and nanophotonic applications including sensing and nonlinear optics.

Anticrossing double Fano resonances generated in metallic/dielectric hybrid nanostructures using nonradiative anapole modes for enhanced nonlinear optical effects.

Third-harmonic generation with metallic or dielectric nanoparticles often suffer from, respectively, small modal volumes and weak near-field enhancements. This study propose and demonstrate that a metallic/dielectric hybrid nanostructure composed of a silver double rectangular nanoring and a silicon square nanoplate can be used to overcome these obstacles for enhanced third-harmonic generation. It is shown that the nonradiative anapole mode of the Si plate can be used as a localized source to excite the dark subradiant octupole mode of the Ag ring, and the mode hybridization leads to the formation of an antibonding and a bonding subradiant collective mode, thereby forming anticrossing double Fano resonances. With the strong coupling between individual particles and the effectively suppressed radiative losses of the Fano resonances, several strong hot spots are generated around the Ag ring due to the excitation of the octupole mode, and electromagnetic fields within the Si plate are also strongly amplified, making it possible to confine more incident energy inside the dielectric nanoparticle. Calculation results reveal that the confined energy inside the Si plate and the Ag ring for the hybrid structures can be about, respectively, more than three times and four orders stronger than that of the corresponding isolated nanoparticles, which makes the designed hybrid nanostructure a promising platform for enhanced third-harmonic generation.

Polarization-Independent Multiple Fano Resonances in Plasmonic Nonamers for Multimode-Matching Enhanced Multiband Second-Harmonic Generation.

Plasmonic oligomers composed of metallic nanoparticles are one class of the most promising platforms for generating Fano resonances with unprecedented optical properties for enhancing various linear and nonlinear optical processes. For efficient generation of second-harmonic emissions at multiple wavelength bands, it is critical to design a plasmonic oligomer concurrently having multiple Fano resonances spectrally matching the fundamental excitation wavelengths and multiple plasmon resonance modes coinciding with the harmonic wavelengths. Thus far, the realization of such a plasmonic oligomer remains a challenge. This study demonstrates both theoretically and experimentally that a plasmonic nonamer consisting of a gold nanocross surrounded by eight nanorods simultaneously sustains multiple polarization-independent Fano resonances in the near-infrared region and several higher-order plasmon resonances in the visible spectrum. Due to coherent amplification of the nonlinear excitation sources by the Fano resonances and efficient scattering-enhanced outcoupling by the higher-order modes, the second-harmonic emission of the nonamer is significantly increased at multiple spectral bands, and their spectral positions and radiation patterns can be flexibly manipulated by easily tuning the length of the surrounding nanorods in the nonamer. These results provide us with important implications for realizing ultrafast multichannel nonlinear optoelectronic devices.

Polarization state-based refractive index sensing with plasmonic nanostructures.

Spectral-based methods are often used for label-free biosensing. However, practical implementations with plasmonic nanostructures suffer from a broad line width caused by strong radiative and nonradiative losses, and the sensing performance characterized by figure of merit is poor for these spectral-based methods. This study provides a polarization state-based method using plasmonic nanostructures to improve the sensing performance. Instead of the intensity spectrum, the polarization state of the transmitted field is monitored to analyze variations of the surrounding medium. The polarization state of incidence is strongly modified due to the excitation of surface plasmons, and the ellipticity of the transmitted field changes dramatically around plasmon resonances. Sharp resonances with line widths down to sub-nanometer are achieved by plotting the spectra of the reciprocal of ellipticity. Therefore, the sensing performance can be significantly improved, and a theoretical value of the figure of merit exceeding 1700 is achieved by using the polarization state-based sensing approach.

Multiple Fano resonances in plasmonic heptamer clusters composed of split nanorings.

Fano resonances in plasmonic nanostructures are important for plasmon line shaping. Compared to a single Fano resonance, multiple Fano resonances can modify plasmon lines at several spectral positions simultaneously, but they often suffer from weak modulation depths. In this paper, plasmonic heptamer clusters comprising split nanorings are designed to form multiple Fano resonances. Three prominent Fano resonances are observed in the spectra due to the formation of multiple narrow subradiant resonances, and the multiple Fano resonances can be switched on and off by adjusting the polarization direction. Particularly, by modifying the geometry parameters, there is a large tunability of the modulation depth of each Fano resonance. Heptamer clusters comprising split nanorings are highly suitable for plasmon line shaping, and it is expected that they are useful for multiwavelength biosensing and surface-enhanced Raman scattering.

Plasmonic-induced optical transparency in the near-infrared and visible range with double split nanoring cavity.

Plasmonic-induced optical transparency with double split nanoring cavity is investigated with finite difference time domain method. The coupling between the bright third-order mode of split nanoring with one gap and the dark quadrupole mode of split nanoring with two gaps leads to plasmonic analogue of electromagnetically induced transparency. The transparence window is easily modified to the near-infrared and visible range. Numerical results show a group index of 16 with transmission exceeding 0.76 is achieved for double split nanoring cavity. There is large cavity volume of double split nanoring, and the field enhancement inside the cavity is homogenous. Double split nanoring cavity could be a good platform for slow light and sensing applications.

Surface plasmons amplifications in single Ag nanoring.

The stimulated amplifications of surface plasmons (SPs) propagating along a single silver nanoring is theoretically investigated by considering the interactions between SPs and activated semiconductor quantum dots (SQDs). Threshold condition for the stimulated amplifications, the SP density as a function of propagation length and the maximum SP density are obtained. The SPs can be nonlinearly amplified when the pumping rate of SQDs is larger than the threshold, and the maximum value of SP density increases linearly with the pumping rate of SQDs.

Modified effective dielectric function for metallic granular composites with high percolation threshold.

We propose the effective dielectric function theory of metal granular composites modified with the metal particle size. The modified theory is used to explain the electrical conductivity, resonant plasmon absorption, and large nonlinear absorption of Au-TiO2 granular composite films with high-density metallic particles and a high electric percolation threshold. It is revealed that the decreasing metal particle size leads to an increasing percolation threshold and large enhancement of optical nonlinearity of the composites.

Surface plasmon propagation in a pair of metal nanowires coupled to a nanosized optical emitter.

The coupling, propagations, and far-field emissions of surface plasmons in a pair of Au nanowires with a dipole emitter have been investigated using the finite-difference time domain method. The surface plasmon wavelength is tunable from 650 to 380 nm by adjusting the distance between the two wires, which leads to an enhancement of coupling constant and density of states of the surface plasmon. The converted energy from the dipole emitter to the propagating surface plasmon as well as the far-field emission intensity of a pair of Au nanowires increase to approximately four times as large as those of a single nanowire.

Coherent exciton-plasmon interaction in the hybrid semiconductor quantum dot and metal nanoparticle complex.

We studied theoretically the exciton coherent dynamics in the hybrid complex composed of CdTe quantum dot (QDs) and rodlike Au nanoparticles (NPs) by the self-consistent approach. Through adjusting the aspect ratio of the rodlike Au NPs, the radiative rate of the exciton and the nonradiative energy transfer rate from the QD to the Au NP are tunable in the wide range 0.05-4 ns(-1) and 4.4 x 10(-4) to 2.6 ns(-1), respectively; consequently, the period of Rabi oscillations of exciton population is tunable in the range 0.6 pi-9 pi.