Although a relatively new area of nanoscience, nonlinear plasmonics has become a fertile ground for the development and testing of new ideas pertaining to light–matter interaction under intense field conditions, ideas that have found a multitude of applications in surface science, active photonic nanodevices, near-field optical microscopy, and nonlinear integrated photonics. In this review, we survey the latest developments in nonlinear plasmonics in three-dimensional (metallic) and two-dimensional (graphene) nanostructures and offer an outlook on future developments in this field of research. In particular, we discuss the main theoretical concepts, experimental methods, and computational tools that are used together in modern nonlinear plasmonics to explore in an integrated manner nonlinear optical properties of metallic and graphene based nanostructures.

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Nonlinear optics in plasmonic nanostructures

In this paper, we develop a classical electrodynamic theory to study the optical nonlinearities of metallic nanoparticles. The quasi free electrons inside the metal are approximated as a classical Coulomb-interacting electron gas, and their motion under the excitation of an external electromagnetic field is described by the plasma equations. This theory is further tailored to study second-harmonic generation. Through detailed experiment-theory comparisons, we validate this classical theory as well as the associated numerical algorithm. It is demonstrated that our theory not only provides qualitative agreement with experiments but it also reproduces the overall strength of the experimentally observed second-harmonic signals.

Classical theory for second-harmonic generation from metallic nanoparticles. Phys Rev B 79:235109

Optical metamaterials have redefined how we understand light in notable ways: from strong response to optical magnetic fields, negative refraction, fast and slow light propagation in zero index and trapping structures, to flat, thin and perfect lenses. Many rules of thumb regarding optics, such as mu = 1, now have an exception, and basic formulas, such as the Fresnel equations, have been expanded. The field of metamaterials has developed strongly over the past two decades. Leveraging structured materials systems to generate tailored response to a stimulus, it has grown to encompass research in optics, electromagnetics, acoustics and, increasingly, novel hybrid materials responses. This roadmap is an effort to present emerging fronts in areas of optical metamaterials that could contribute and apply to other research communities. By anchoring each contribution in current work and prospectively discussing future potential and directions, the authors are translating the work of the field in selected areas to a wider community and offering an incentive for outside researchers to engage our community where solid links do not already exist.

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Roadmap on optical metamaterials

This work presents molecular-level investigations of how well-characterized silica-supported phospholipid bilayers formed from either pure DOPC or a 9:1 mixture of DOPC:DOTAP interact with positively and negatively charged 4 nm gold metal nanoparticles at pH 7.4 and NaCl concentrations ranging from 0.001 to 0.1 M. Second harmonic generation (SHG) charge screening measurements indicate the supported bilayers carry a negative interfacial potential. Resonantly enhanced SHG measurements probing electronic transitions within the gold core of the nanoparticles show the particles interact irreversibly with the supported bilayers at a range of concentrations. At 0.1 M NaCl, surface coverages for the particles functionalized with the negatively charged ligand mercaptopropionic acid (MPA) or wrapped in the cationic polyelectrolyte poly(allylamine) hydrochloride (PAH) are estimated from a joint analysis of QCM-D, XPS, AFM, and ToF-SIMS to be roughly 1 × 107 and 1 × 1011 particles cm–2, respectively. Results from com...

Direct Probes of 4 nm Diameter Gold Nanoparticles Interacting with Supported Lipid Bilayers

We have experimentally observed around 2 orders of magnitude circular dichroism (CD) enhancement in the visible region for cysteine molecules located in the hotspots of gold nanosphere clusters. The observed plasmon-induced CD responses show a significant correlation with the chiral nature of molecules at the hotspots. These results provide a concrete experimental demonstration on the predicted chiroptical transfer and amplification effect that arises from hotspot-mediated exciton–plasmon interactions in a strongly coupled metallic nanostructure, even though the exciton–plasmon coupling works at a far off-resonant regime. Our findings suggest here that plasmonic hotspot-based CD amplifier may provide a new strategy for ultrasensitive detection and quantification of molecular chirality—a key aspect for various bioscience and biomedicine applications.

Experimental Observation of Giant Chiroptical Amplification of Small Chiral Molecules by Gold Nanosphere Clusters

The theory of second harmonic generation (SHG) in three-dimensional structures consisting of arbitrary distributions of metallic spheres made of centrosymmetric materials is developed by means of multiple scattering of electromagnetic multipole fields. The electromagnetic field at both the fundamental frequency and second harmonic, as well as the scattering cross section, are calculated in a series of particular cases such as a single metallic sphere, two metallic spheres, chains of metallic spheres, and other distributions of the metallic spheres. It is shown that the linear and nonlinear optical response of all ensembles of metallic spheres is strongly influenced by the excitation of localized surface plasmon-polariton resonances. The physical origin for such a phenomenon has also been analyzed.

Second harmonic generation in three-dimensional structures based on homogeneous centrosymmetric metallic spheres.

A fast and general technique to investigation the interaction between a fast electron and nonlinear materials consisting of centrosymmetric spheres is presented by means of multiple scattering of electromagnetic multipole fields. Two kinds of new effect, the negative electron energy loss caused by the second-harmonic field and the second-harmonic Smith-Purcell radiation using finite chain of nonlinear spheres, are predicted for the first time. It is shown that these new effects can be probed by the electron energy loss spectrum, suggesting their possible applications in tunable light sources for the second-harmonic generation.

Negative electron energy loss and second-harmonic emission of nonlinear nanoparticles

In this work, we proposed a black phosphorus (BP) achiral metasurface and theoretically study the chiroptical response arising from extrinsic 2D-chirality in the mid-infrared regime. The achiral metasurface is composed of a monolayer BP sheet sandwiched by a silver ring array and dielectric spacer stacking on a silver substrate. The giant circular conversion dichroism (CCD) of the achiral metasurface is allowed at oblique incidence for the cooperative interaction of BP anisotropic surface plasmon modes and localized surface plasmons in metal rings, and the integrated BP can dynamically modulate the chiroptical response by controlling the doping concentration of BP. Furthermore, we found that a multiband phenomenon for CCD response occurs when tuning the thickness of the spacer. The proposed hybrid achiral metasurface provides more flexible opportunities to realize active polarization modulator, biosensor and chiral detection.

Jinying Xu

Papers

Second harmonic generation in three-dimensional structures based on homogeneous centrosymmetric metallic spheres.

Negative electron energy loss and second-harmonic emission of nonlinear nanoparticles

Giant 2D-chiroptical response in an achiral metasurface integrated with black phosphorus.

Tunable circular conversion dichroism of single-layer twisted graphene-patterned metasurface