Nonlinear Plasmonics Research Articles

Water-mediated polyol synthesis of pencil-like sharp silver nanowires suitable for nonlinear plasmonics.

We report a simple method to control the end shape of silver nanowires by adding pure water in the conventional polyol synthesis. The use of 0.2-0.4% (v/v) water in ethylene glycol as a solvent provides pencil-like silver nanowires with sharp ends in a high yield. We have demonstrated remote excitation of SHG on the sharp nanowires, promising a point light source for super resolution microscopy.

Parametric resonances in nonlinear plasmonics [Invited

In the context of nonlinear plasmonics, we review the recently introduced concept of plasmonic parametric resonance (PPR) and discuss potential applications of such phenomena. PPR arises from the temporal modulation of one or more of the parameters governing the dynamics of a plasmonic system and can lead to the amplification of high-order sub-radiant plasmonic modes. The theory of PPR is reviewed, possible schemes of implementation are proposed, and applications in optical limiting are discussed.

Nonlinear plasmonics of three-dimensional Dirac semimetals

The three-dimensional Dirac semimetal (3D DSM) is a new class of material with a slew of electronic and optical properties in common with graphene, while structurally having a bulk form like real metals. In particular, the Dirac band structure of 3D DSM conferred very high optical nonlinearities much like the case for graphene. Consequently, we found that 3D DSM has respectable nonlinear plasmonic performance in comparison with graphene, while retaining the structural benefits of bulk metals, having reduced passive plasmonic losses, and is much easier to handle in fabrication facilities. 3D DSM is expected to play a strong role in providing strong optical nonlinearities for all-optical switching and at the same time offering a superior platform for nanophotonic device integration.

Coulomb effects on the photoexcited quantum dynamics of electrons in a plasmonic nanosphere

With recent experiments investigating the optical properties of progressively smaller plasmonic particles, quantum effects become increasingly more relevant, requiring a microscopic description. Using the density matrix formalism we analyze the photo-excited few-electron dynamics of a small nanosphere. Following the standard derivation of the bulk plasmon we particularly aim on elucidating the role of the Coulomb interaction. Calculating the dielectric susceptibility spectrum in the linear optical response we find discrete resonances resulting from a collective response mediated by the Coulomb interaction between the electrons. In the nonlinear regime, the occupations of the system exhibit oscillations between the interacting eigenstates. Our approach provides an ideal platform to study and explain nonlinear and quantum plasmonics, revealing that the photo-excited dynamics of plasmonic nano spheres has similarities with and combines characteristics of both, the well-known two-level Rabi dynamics, and the collective many-electron behavior typical of plasmons.

Plasmonic Enhancement and Manipulation of Optical Nonlinearity in Monolayer Tungsten Disulfide

AbstractMonolayer transition metal dichalcogenides (TMDCs) have large second‐order optical nonlinearity owing to broken inversion symmetry in two‐dimensional (2D) crystals. However, despite the strong light–matter coupling in monolayer TMDCs, their nonlinear responses are ultimately limited by subnanometer thickness. Here, a dramatic enhancement of the second‐harmonic generation (SHG) is achieved from monolayer tungsten disulfide (WS2) incorporated onto a 2D silver (Ag) nanogroove grating with subwavelength pitch. By tuning surface plasmon mode and second‐harmonic frequency in resonance with the C exciton in WS2, a large SHG enhancement factor (≈400) and a large conversion efficiency (≈2.0 × 10–5) can be obtained. Furthermore, the azimuthal angle dependence of polarized SHG from monolayer WS2 can be manipulated by the nanogroove plasmonic mode. Based on this property, a polarization‐modulated optical encoding technique is demonstrated. The results suggest that 2D TMDC–plasmonic hybrid metasurface structures can provide an ideal integration platform for on‐chip nonlinear photonics and plasmonics.

Enhancement and control of the Goos—Hänchen shift by nonlinear surface plasmon resonance in graphene**Project supported by the National Natural Science Foundation of China (Grant No. 61505111).

The Goos–Hänchen (GH) shift of graphene in the terahertz frequency range is investigated, and an extremely high GH shift is obtained owing to the excitation of surface plasmon resonance in graphene in the modified Otto configuration. It is shown that the GH shift can be positive or negative, and can be enhanced by introducing a nonlinearity in the substrate. Large and bistable GH shifts are demonstrated to be due to the hysteretic behavior of the reflectance phase. The bistable GH shift can be manipulated by changing the thickness of the air gap and the Fermi level or relaxation time of graphene.

Nonlocal and Nonlinear Surface Plasmon Polaritons and Optical Spatial Solitons Induced by the Thermocapillary Effect.

We study the propagation of surface plasmon polaritons (SPPs) on a metal surface which hosts a thin film of a liquid dielectric. The Ohmic losses that are inherently present due to the coupling of SPPs to conductors' electron plasma, induce temperature gradients and fluid deformation driven by the thermocapillary effect, which lead to a nonlinear and nonlocal change of the effective dielectric constant. The latter extends beyond the regions of highest optical intensity and constitutes a novel thermally self-induced mechanism that affects the propagation of the SPPs. We derive the nonlinear and nonlocal Schrödinger equation that describes propagation of low intensity SPP beams, and show analytically and numerically that it supports a novel optical spatial soliton excitation.

Understanding the Second-Harmonic Generation Enhancement and Behavior in Metal Core–Dielectric Shell Nanoparticles

To explain recently observed up to 50-fold enhancement of the second harmonic (SH) signal from gold nanoparticles after depositing of a thin dielectric layer, we perform an analytical study of the SH generation by metal core–dielectric shell spherical nanoparticles (NPs). We derive analytical expressions for the SH intensity accounting for electric dipole and quadrupole impacts. On this basis, we discuss the SH response spectrum of coated gold and silver nanoparticles. The possibility of adjusting the SH scattering regime to dipole or quadrupole resonant mode by varying the shell thickness is shown. We demonstrate that the thin, up to several NPs radii, dielectric cover of the NPs gives a significant advantage in the SH intensity in a line with the resonant enhancement. This effect is most pronounced in gold nanoparticles and even dominates resonant enhancement of the SH generation. This new result in nonlinear plasmonics provides a simple recipe for gold nanoparticles: thicker dielectric coating–higher t...

Dynamics of Second-Harmonic Generation in a Plasmonic Silver Nanorod

Second-harmonic generation in plasmonic nanostructures is known to enable the observation of modes with vanishing dipolar moments, i.e., having small radiation losses and thus long lifetimes. With the aid of a full wave numerical method, we study the far-field temporal dynamics of the linear and nonlinear responses of a silver nanorod driven by femtosecond pulses. The results show that the plasmons lifetime is observable in the decaying field oscillations surviving after the exciting pulse, for both processes, and fits with the damped harmonic oscillator model. In addition, using a detailed mode analysis, we find that the multipolar characteristic of the nonlinear radiation is strongly influenced by both the pulse central frequency and width. Implications for the accurate measurement of plasmon lifetime with the help of nonlinear optics are discussed, especially the need to carefully disentangle the linear and nonlinear plasmon dynamics.

Transient nonlinear plasmonics in nanostructured graphene

Plasmons in highly doped graphene offer the means to dramatically enhance light absorption in the atomically thin material. Ultimately the absorbed light energy induces an increase in electron temperature, accompanied by large shifts in the chemical potential. This intrinsically incoherent effect leads to strong intensity-dependent modifications of the optical response, complementing the remarkable coherent nonlinearities arising in graphene due to interband transitions and anharmonic intraband electron motion. Through rigorous time-domain quantum-mechanical simulations of graphene nanoribbons, we show that the incoherent mechanism dominates over the coherent response for the high levels of intensity required to trigger nonperturbative optical phenomena such as saturable absorption. We anticipate that these findings will elucidate the role of coherent and incoherent nonlinearities for future studies and applications of plasmon-assisted nonlinear optics.

Nondestructive Approach for Additive Nanomanufacturing of Metallic Nanostructures in the Air.

In this article, a mechanism for quick release and transfer of gold nanoparticles (GNPs) from a soft substrate to another substrate under laser illumination is investigated. The heating of GNPs on a soft substrate with a continuous-wave laser causes a rapid thermal expansion of the substrate, which can be used to selectively release and place GNPs onto another surface. In-plane and out-of-plane nanostructures are successfully fabricated using this method. This rapid release-and-place process can be used for additive nonmanufacturing of metallic nanostructures under ambient conditions, which paves a way for affordable nanomanufacturing and enables a wide variety of applications in nanophotonics, ultrasensitive sensing, and nonlinear plasmonics.

Semiclassical fluid model of nonlinear plasmons in doped graphene

A nonlinear fluid model of high-frequency plasmons in doped graphene is derived by taking fluid moments of the semi-classical kinetic equation for the electron gas. As a closure of the fluid moments, adiabatic compression is assumed with a given form of the distribution function, combined with an exact linear response based on the linearized Vlasov-Poisson system. In the linear regime, the model is in the long wavelength limit consistent with previous results using the random phase approximation for a two-dimensional electron gas, while it neglects the short-range interactions between massless Dirac fermions. The fluid model may be used to study the non-linear plasmonic wave mixing and optical coupling to lasers in graphene.

Massive ordering and alignment of cylindrical micro-objects by photovoltaic optoelectronic tweezers.

Optical tools for manipulation and trapping of micro- and nano-objects are a fundamental issue for many applications in nano- and biotechnology. This work reports on the use of one such method, known as photovoltaic optoelectronics tweezers, to orientate and organize cylindrical microcrystals, specifically elongated zeolite L, on the surface of Fe-doped LiNbO3 crystal plates. Patterns of aligned zeolites have been achieved through the forces and torques generated by the bulk photovoltaic effect. The alignment patterns with zeolites parallel or perpendicular to the substrate surface are highly dependent on the features of light distribution and crystal configuration. Moreover, dielectrophoretic chains of zeolites with lengths up to 100μm have often been observed. The experimental results of zeolite trapping and alignment have been discussed and compared together with theoretical simulations of the evanescent photovoltaic electric field and the dielectrophoretic potential. They demonstrate the remarkable capabilities of the optoelectronic photovoltaic method to orientate and pattern anisotropic microcrystals. The combined action of patterning and alignment offers a unique tool to prepare functional nanostructures with potential applications in a variety of fields such as nonlinear optics or plasmonics.

Nonlinear graphene plasmonics.

The rapid development of graphene has opened up exciting new fields in graphene plasmonics and nonlinear optics. Graphene's unique two-dimensional band structure provides extraordinary linear and nonlinear optical properties, which have led to extreme optical confinement in graphene plasmonics and ultrahigh nonlinear optical coefficients, respectively. The synergy between graphene's linear and nonlinear optical properties gave rise to nonlinear graphene plasmonics, which greatly augments graphene-based nonlinear device performance beyond a billion-fold. This nascent field of research will eventually find far-reaching revolutionary technological applications that require device miniaturization, low power consumption and a broad range of operating wavelengths approaching the far-infrared, such as optical computing, medical instrumentation and security applications.

Cherenkov Radiation Control via Self-accelerating Wave-packets

Cherenkov radiation is a ubiquitous phenomenon in nature. It describes electromagnetic radiation from a charged particle moving in a medium with a uniform velocity larger than the phase velocity of light in the same medium. Such a picture is typically adopted in the investigation of traditional Cherenkov radiation as well as its counterparts in different branches of physics, including nonlinear optics, spintronics and plasmonics. In these cases, the radiation emitted spreads along a “cone”, making it impractical for most applications. Here, we employ a self-accelerating optical pump wave-packet to demonstrate controlled shaping of one type of generalized Cherenkov radiation - dispersive waves in optical fibers. We show that, by tuning the parameters of the wave-packet, the emitted waves can be judiciously compressed and focused at desired locations, paving the way to such control in any physical system.

High Sensitivity Integrated Visible to Mid-Infrared Nonlinear Plasmonic Sensor

We propose a Kretschmann-based nonlinear plasmonic sensor with a gold thin film deposited on a glass prism. Visible and mid-infrared signals are generated in this configuration through the nonlinear processes of sum- and difference-frequency generation, respectively. The calculated maximum sensitivity and figure of merit of our sum-frequency-based sensor is an order of magnitude higher than that of a traditional Kretschmann-based sensor in the visible range. Our difference-frequency-based sensor has a maximum sensitivity of $1.0\times 10^{6}$ $\text{nm/RIU}$ in air at 4.29 $\mu\text{m}$ , which is three orders of magnitude higher than that of existing devices in the mid-infrared range, with its maximum figure of merit almost two orders of magnitude higher than the alternatives. By comparison, the calculated sensitivity for operation in water for both sum- and difference-frequency is about half that in air. We, thus, demonstrate significant gains in the sensitivity of the well-known Kretschmann-based plasmonic sensor over a wide wavelength range, without modifying the physical sensor, but by exploiting and simply taping the nonlinear optical properties of the system.

Nonlocality-Broaden Optical Bistability in a Nonlinear Plasmonic Core–Shell Cylinder

With the self-consistent mean-field method in the framework of full-wave nonlocal scattering theory, we theoretically investigate the optical bistability in a nonlocal metallic nanocylinder coated with Kerr-type nonlinear shell. A nonlocality enhanced Fano profile is found for this coated cylinder in the linear limit. We illustrate the relation between the linear plasmonic resonant wavelength and the viable parameters for optical bistability in parameter space. It is found that nonlocality will lead to impressive blue shift of the resonant wavelength, and hence dramatically increase the bistable region in the parameter space of incident wavelength and geometrical factor. We demonstrate the input-field-controllable and input-wavelength-controllable scatterings in the nonlinear case, respectively. It indicates that nonlocal effects show opposite influences on these two nonlinear scattering processes, and the bistability in the scattering spectrum is weaken by nonlocality. Our study reveals that these self-t...

Nonlinear plasmonics in eutectic composites: Second harmonic generation and two-photon luminescence in a volumetric Bi2O3-Ag metamaterial

The nonlinear optical effect of second harmonic generation can be very strong when originating from nanoplasmonic structures, due to enhancement of the surrounding material's intrinsic non-linear optical properties or due to its occurrence as a result of the plasmonic structure. However, manufacturing of large-scale three dimensional nanoplasmonic structures is still a challenge. Here, we demonstrate the two-photon luminescence and second-harmonic generation in a Bi2O3-Ag eutectic-based metamaterial exhibiting a hierarchic structure of nano- and micro-sized silver precipitates. The investigations employed a microscope system combined with polarimetric analysis. It appears that the second-harmonic-generation arises from the silver plasmonic structure rather than from the nonlinear effects of the bismuth oxide matrix. Both quadrupolar and dipolar modes of polarization are observed.

A Brief Survey on Nonlinear Surface Plasmonics

With the rapid development of optical nonlinearities, the study of nonlinear surface plasmonics becomes more popular. Due to the huge potential application value of surface plasmonics, lots of researches in this field become the hotspot. This paper reviewed the history and basic researches of nonlinear plasmonics and focused on several hot topics attracting global attentions. In addition, we introduced the application status of nonlinear surface plasmonics and draw a conclusion. Meanwhile we outlooked the future developments.

Bistable scattering in graphene-coated dielectric nanowires.

In nonlinear plasmonics, the switching threshold of optical bistability is limited by the weak nonlinear responses from the conventional Kerr dielectric media. Considering the giant nonlinear susceptibility of graphene, here we develop a nonlinear scattering model under the mean field approximation and study the bistable scattering in graphene-coated dielectric nanowires based on the semi-analytical solutions. We find that the switching intensities of bistable scattering can be smaller than 1 MW cm-2 at the working frequency. To further decrease the switching intensities, we show that the most important factor that restricts the bistable scattering is the relaxation time of graphene. Our work not only reveals some general characteristics of graphene-based bistable scattering, but also provides a guidance to further applications of optical bistability in the high speed all-optical signal processing.