Ultra-fast dynamics in the nonlinear optical response of silver nanoprism ordered arrays

Interfacially confined preparation of fumaronitrile-based nanofilms exhibiting broadband saturable absorption properties

Interfacial nanofilms with nonlinear optical (NLO) properties were prepared via confined dynamic condensation of 4,4'-methylenedianiline (MDA) with the synthesized 2,3-bis(4-(bis(4-formylphenyl)amino)phenyl)fumaronitrile (BTFA). Investigated using the open-aperture Z-scan technique, BTFA showed reverse saturable absorption ascribed to the synergetic mechanisms of two-photon and excited-state absorption. In contrast, the as-prepared nanofilms demonstrated broadband saturable absorption within the spectral range of 720∼1700 nm. The characteristics of nonlinear absorption coefficient (β) decreased along with increasing the incident pulse intensity. Taking advantage of the flexibility and post-machinability properties, the folding layers of the nanofilms offered the feasibility to fine-tune the specific NLO responses. The optimal β value was found to be -10.1 cm/MW for eight-layer nanofilm as well as the normalized transmittance increased up to 35-fold at 800 nm. Utilized as a conceptual saturable absorber, the representative modulation depth and saturation intensity were observed to be around 2.4% and 7.37 GW/cm² at 800 nm, respectively, comparable to traditional two-dimensional (2D) materials. Aiming to clarify the possible underlying physical processes, a four-level model was employed to illustrate the fast relaxation of the excited states. Present work demonstrates that proper design of building blocks combined with interfacially confined dynamic condensation enables rational development of high-performance NLO materials.

Surface Plasmon Resonance of Large-Size Ag Nanobars

Silver nanobars have attracted much attention due to their distinctive localized surface plasmon resonance (LSPR) in the visible and near-infrared regions. In this work, large-size Ag nanobars (length: 400~1360 nm) working at a longer-wavelength near-infrared range (>1000 nm) have been synthesized. By using the finite-difference time-domain (FDTD) simulation, the LSPR properties of a single large-size Ag nanobar are systematically investigated. The LSPR in Ag nanobar can be flexibly tuned in a wide wavelength range (400~2000 nm) by changing the bar length or etching the bar in the length direction. Our work provides a flexible way to fabricate nanoparticle arrays using large-size nanobars and throws light on the applications of large-size nanomaterials on wide spectral absorbers, LSPR-based sensors and nanofilters.

Optimal geometry for plasmonic sensing with non-interacting Au nanodisk arrays

Combining finite elements method electrodynamic simulations and cost-effective and scalable nanofabrication techniques, we carried out a systematic investigation and optimization of the sensing properties of non-interacting gold nanodisk arrays. Such plasmonic nanoarchitectures offer a very effective platform for fast and simple, label-free, optical bio- and chemical-sensing. We varied their main geometrical parameters (diameter and height) to monitor the plasmonic resonance position and to find the configurations that maximize the sensitivity to small layers of an analyte (local sensitivity) or to the variation of the refractive index of an embedding medium (bulk sensitivity). The spectral position of the plasmonic resonance can be tuned over a wide range from the visible to the near-IR region (500-1300 nm) and state-of-the-art performances can be obtained using the optimized nanodisks; we obtained local and bulk sensitivities of S 0 = 11.9 RIU-1 and S bulk = 662 nm RIU-1, respectively. Moreover, the results of the simulations are compared with the performances of experimentally synthesized non-interacting Au nanodisk arrays fabricated by combining sparse colloidal lithography and hollow mask lithography, with the parameters obtained by the sensitivity numerical optimization. An excellent agreement between the experimental and the simulated results is demonstrated, confirming that the optimization performed with the simulations is directly applicable to nanosensors realized with cost-effective methods, due to the quite large stability basin around the maximum sensitivities.

Diffractive dipolar coupling in non-Bravais plasmonic lattices

Honeycomb plasmonic lattices are paradigmatic examples of non-Bravais lattices. We experimentally measure surface lattice resonances in effectively free-standing honeycomb lattices composed of silver nanospheres. By combining numerical simulations with analytical methods, we analyze the dispersion relation and the near-field properties of these modes along high symmetry trajectories. We find that our results can be interpreted in terms of dipole-only interactions between the two non-equivalent triangular sublattices, which naturally lead to an asymmetric near-field distribution around the nanospheres. We generalize the interaction between the two sublattices to the case of variable adjacent interparticle distance within the unit cell, highlighting symmetry changes and diffraction degeneracy lifting associated to the transition between Bravais and non-Bravais lattices.

Circular Dichroism in Low-Cost Plasmonics: 2D Arrays of Nanoholes in Silver

Arrays of nanoholes in metal are important plasmonic devices, proposed for applications spanning from biosensing to communications. In this work, we show that in such arrays the symmetry can be broken by means of the elliptical shape of the nanoholes, combined with the in-plane tilt of the ellipse axes away from the array symmetry lines. The array then differently interacts with circular polarizations of opposite handedness at normal incidence, i.e., it becomes intrinsically chiral. The measure of this difference is called circular dichroism (CD). The nanosphere lithography combined with tilted silver evaporation was employed as a low-cost fabrication technique. In this paper, we demonstrate intrinsic chirality and CD by measuring the extinction in the near-infrared range. We further employ numerical analysis to visualize the circular polarization coupling with the nanostructure. We find a good agreement between simulations and the experiment, meaning that the optimization can be used to further increase CD.

Tailorable Dynamics in Nonlinear Optical Metasurfaces

Controlling light with light is essential for all-optical switching, data processing in optical communications and computing. Until now, all-optical control of light has relied almost exclusively on nonlinear optical interactions in materials. Achieving giant nonlinearities under low light intensity is essential for weak-light nonlinear optics. In the past decades, such weak-light nonlinear phenomena have been demonstrated in photorefractive and photochromic materials. However, their bulky size and slow speed have hindered practical applications. Metasurfaces, which enhance light-matter interactions at the nanoscale, provide a new framework with tailorable nonlinearities for weak-light nonlinear dynamics. Current advances in nonlinear metasurfaces are introduced, with a special emphasis on all-optical light controls. The tuning of the nonlinearity values using metasurfaces, including enhancement and sign reversal is presented. The tailoring of the transient behaviors of nonlinearities in metasurfaces to achieve femtosecond switching speed is also discussed. Furthermore, the impact of quantum effects from the metasurface on the nonlinearities is introduced. Finally, an outlook on the future development of this energetic field is offered.

Broadband tunable nonlinear optical response in plasmonic metamaterials -INVITED

Nanostructured materials with tunable nonlinear optical response are of great interest for different applications in nanophotonics. In this work we report the results of a comprehensive study on the nonlinear optical properties of two kinds of plasmonic metamaterials, i.e., honeycomb nanoprism arrays and multilayer hyperbolic metamaterials, which proved to have a very rich spectrum of parameters (as metamaterials morphology and composition, wavelength and polarization of an input beam) to exploit for controlling their nonlinear response over a broad spectral range.

Polarization dependence of second harmonic generation from plasmonic nanoprism arrays

The second order nonlinear optical response of gold nanoprisms arrays is investigated by means of second harmonic generation (SHG) experiments and simulations. The polarization dependence of the nonlinear response exhibits a 6-fold symmetry, attributed to the local field enhancement through the excitation of the surface plasmon resonances in bow-tie nanoantennas forming the arrays. Experiments show that for polarization of the input light producing excitation of the plasmonic resonances in the bow-tie nanoantennas, the SHG signal is enhanced; this despite the fact that the linear absorption spectrum is not dependent on polarization. The results are confirmed by electrodynamic simulations which demonstrate that SHG is also determined by the local field distribution in the nanoarrays. Moreover, the maximum of SHG intensity is observed at slightly off-resonance excitation, as implemented in the experiments, showing a close relation between the polarization dependence and the structure of the material, additionally revealing the importance of the presence of non-normal electric field components as under focused beam and oblique illumination.