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.

Multiple surface lattice resonances of overlapping nanoparticle arrays with different lattice spacing.

Multiple surface lattice resonances generated with nanoparticle arrays are promising to enhance light-matter interactions at different spectral positions simultaneously, and it is important to tailor these resonances to desired frequencies for practical applications such as multi-modal nanolasing. To this end, this study proposes to generate multiple surface lattice resonances using overlapping nanoparticle arrays with different lattice spacing. Both full-wave numerical simulations and analytical coupled dipole approximation calculations reveal that for the overlapping structures composed with two different gold nanosphere arrays, both surface lattice resonances for the element structures are effectively excited. Considering that the optical responses are governed by the dipole-dipole interactions between the nanoparticles, it is interesting to find that the multiple surface lattice resonances are almost invariant by adjusting the relative shifts between the two arrays, which can be useful to tailor the high-quality factor resonances to desired spectral positions. In addition, due to the same reason, it is also shown that the multiple surface lattice resonances can be further finely tuned by selectively removing specific nanoparticles in the array. We anticipate that the tolerance to generate multiple surface lattice resonances and the flexible tunability make the overlapping nanoparticle arrays useful to design high performance linear and nonlinear nanophotonic devices.

Multi-mode strong coupling in Fabry-Pérot cavity-WS2 photonic crystal hybrid structures.

Optical microcavities embedded with transition metal dichalcogenide (TMDC) membranes have been demonstrated as excellent platforms to explore strong light-matter interactions. Most of the previous studies focus on strong coupling between excitons of unpatterned TMDC membranes and optical resonances of various microcavities. It is recently found that TMDC membranes patterned into photonic crystal (PhC) slabs can sustain guided-mode resonances that can be excited and probed by far-fields. Here, we present a comprehensive theoretical and numerical study on optical responses of Fabry-Pérot (F-P) cavity-WS2 PhC hybrid structures to investigate the multi-mode coupling effects between excitons, guided-mode resonances and F-P modes. We show that both the exciton resonance and the guide-mode resonance of the WS2 PhC can strongly interact with F-P modes of the cavity to reach strong coupling regime. Moreover, a Rabi splitting as large as 63 meV is observed for the strong coupling between the guided-mode resonance and the F-P mode, which is much larger than their average dissipation rate. We further demonstrate that it is even possible to realize a triple mode strong coupling by tuning the guide-mode resonances spectrally overlapped with the exciton resonance and the F-P modes. The hybrid polariton states generated from the triple mode coupling exhibit a Rabi splitting of 120 meV that greatly exceeds the criterion of a triple mode strong coupling (∼29.3 meV). Our results provide that optical microcavities embedded with TMDC PhCs can serve as promising candidates for polariton devices based on multi-mode strong coupling.

Competition mechanism of exciton decay channels in the stacked multilayer tungsten sulfide.

The competition mechanism of exciton decay channels in the multilayer TMDs remains poorly understood. Here, the exciton dynamics in the stacked WS2 was studied. The exciton decay processes are divided into the fast and slow decay processes, which are dominated by the exciton-exciton annihilation (EEA) and defect-assisted recombination (DAR), respectively. The lifetime of EEA is on the order of hundreds of femtoseconds (400∼1100 fs). It is decreased initially, followed by an increase with adding layer thickness, which can be attributed to the competition between phonon-assisted effect and defect effect. The lifetime of DAR is on the timescale of hundreds of picoseconds (200∼800 ps), which is determined by the defect density especially in a high injected carrier density.

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.

The Geometry of Nanoparticle-on-Mirror Plasmonic Nanocavities Impacts Surface-Enhanced Raman Scattering Backgrounds.

Surface-enhanced Raman scattering (SERS) has garnered substantial attention due to its ability to achieve single-molecule sensitivity by utilizing metallic nanostructures to amplify the exceedingly weak Raman scattering process. However, the introduction of metal nanostructures can induce a background continuum which can reduce the ultimate sensitivity of SERS in ways that are not yet well understood. Here, we investigate the impact of laser irradiation on both Raman scattering and backgrounds from self-assembled monolayers within nanoparticle-on-mirror plasmonic nanocavities with variable geometry. We find that laser irradiation can reduce the height of the monolayer by inducing an irreversible change in molecular conformation. The resulting increased plasmon confinement in the nanocavities not only enhances the SERS signal, but also provides momentum conservation in the inelastic light scattering of electrons, contributing to the enhancement of the background continuum. The plasmon confinement can be modified by changing the size and the geometry of nanoparticles, resulting in a nanoparticle geometry-dependent background continuum in SERS. Our work provides new routes for further modifying the geometry of plasmonic nanostructures to improve SERS sensitivity.

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.

Sensitive vapor detection with hollow thin film arrays.

In this manuscript, we explored the performance of a hollow thin film array (HTFA) for the detection of HCl vapor based on fluorescence quenching. The HTFA structure was fabricated by manually stacking layers of an active thin film and a supporting film, alternately, with a hollow structure in each supporting film. The total penetration depth of vapor molecules in the HTFA sample is 2n times increased, where n is the layer number of the active thin film. We tested the sensing performance of the HTFA sample using fluorescence emission and laser emission in a Fabry-Pérot (FP) microcavity. In the fluorescence sensing, the sensing efficiency increases with the vapor concentration, and can be as high as 80% with a vapor concentration of 400 ppm. While in the laser sensing, the efficiency can achieve 100% with an external pump intensity three times of the lasing threshold at a vapor concentration of 85 ppm. The HTFA sample is not only suitable for vapor detection but also suitable for molecule detection in liquid.

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.

DNA Melting Analysis with Optofluidic Lasers Based on Fabry-Pérot Microcavity.

We conduct DNA high-resolution melting (HRM) analysis using optofluidic lasers based on a Fabry-Pérot microcavity. Compared to the fluorescence-based HRM, the laser-based HRM has advantages of higher emission intensity for better signal-to-noise ratio and sharper transition for better temperature resolution. In addition, the melting temperature can be lowered by optimizing the laser conditions such as external pump and cavity Q-factor. In this work, we first theoretically analyze the laser-based HRM. Then experiments are performed on three long DNA sequences as model systems, one being 99 bases and the other two being 130 bases long but with different GC contents. We show that the laser-based HRM is able to distinguish the target and the single-base mismatched DNA as long as 130 bases and with nearly 50% GC content. The dependence of laser threshold on the temperature for each DNA sample is first experimentally investigated and by optimizing the external pump, the melting temperature is reduced by more than 10 °C, compared to the fluorescence-based HRM for long DNA sequences up to 130 bases. Finally, we demonstrate an alternative method of using the laser-based HRM for rapid DNA screening that does not exist for the fluorescence-based HRM, in which laser excitation is scanned at a fixed temperature to distinguish the target and the base-mismatched DNA sequences. It is shown that the 130-bases-long DNA with nearly 50% GC content can have as much as 20% difference in the laser threshold and 40% difference in the laser output slope between the target and the single-base mismatched sequences, despite only 0.5 °C difference in their melting temperature, indicating that the laser-excitation-scanning method can also be suitable for long DNA sequences with higher GC content.

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.

Sharp convex gold grooves for fluorescence enhancement in micro/nano fluidic biosensing.

The enhancement of biosensing sensitivity based on a quantum dot (QD) is limited by the long distance between the QD and the substrate in in vitro detection, which prevents the development of biosensors. Here an individual sharp convex gold groove is proposed to enhance remote fluorescence by exciting and collecting fluorescence efficiently. The structure shows a higher emission power than other wider gold groove structures when the QD is individually placed at five random positions inside the groove. Compared with bare glass, the total power enhancement factor of our structure is up to 17.0 times, 6.6 times and 6.4 times when the QD is 3.5 µm, 7.6 µm and 9.0 µm away from the bottom of the groove, respectively, due to the scattered emission of the QD and guided resonance modes inside the groove. In addition, the structure is easy to fabricate. The individual sharp convex gold groove is expected to be used as one unit of multi-channels in micro/nano fluidic biosensing. The sample volume could be very small or large according to real applications due to the particular geometric features of our structure.

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.

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.

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.

Enhanced Broadband Electromagnetic Absorption in Silicon Film with Photonic Crystal Surface and Random Gold Grooves Reflector.

We show a hybrid structure consisting of Si film with photonic crystal surface and random triangular gold grooves reflector at the bottom, which is capable of realizing efficient, broad-band, wide-angle optical absorption. It is numerically demonstrated that the enhanced absorption in a broad wavelength range (0.3-9.9 µm) due to the scattering effect of both sides of the structure and the created resonance modes. Larger thickness and period are favored to enhance the absorption in broader wavelength range. Substantial electric field concentrates in the grooves of surface photonic crystal and in the Si film. Our structure is versatile for solar cells, broadband photodetection and stealth coating.

Optofluidic laser array based on stable high-Q Fabry-Pérot microcavities.

We report the development of an optofluidic laser array fabricated on a chip using stable plano-concave Fabry-Pérot (FP) microcavities, which are far less susceptible to optical misalignment during device assembly than the commonly used plano-plano FP microcavities. The concave mirrors in our FP microcavities were created by first generating an array of microwells of a few micrometers in depth and a few tens of micrometers in diameter on a fused silica chip using a CO2 laser, followed by coating of distributed Bragg reflection (DBR) layers. The plano-concave FP microcavity had a Q-factor of 5.6 × 10(5) and finesse of 4 × 10(3), over 100 times higher than those for the FP microcavities in existing optofluidic lasers. 1 mM R6G dye in ethanol was used to test the plano-concave FP microcavities, showing an ultralow lasing threshold of only 90 nJ mm(-2), over 10 times lower than that in the corresponding unstable plano-plano FP microcavities formed by the same DBR coatings on the same chip. Simultaneous laser emission from the optofluidic laser array on the chip and single-mode lasing operation were also demonstrated. Our work will lead to the development of optofluidic laser-based biochemical sensors and novel on-chip photonic devices with extremely low lasing thresholds (nJ mm(-2)) and mode volumes (fL).