Understanding the conditions for the optimum nonlinear refraction of epsilon-near-zero films based on transparent conducting oxides.

Transparent Conducting Oxides (TCOs) exhibit a large and ultrafast intensity-dependent refractive index in their Epsilon-Near-Zero (ENZ) spectral region, which depends dramatically on the material properties and measurement arrangement conditions. Therefore, attempts to optimize the nonlinear response of ENZ TCOs usually involve extensive nonlinear optical measurements. In this work, we show that significant experimental work can be avoided by carrying out an analysis of the material's linear optical response. The analysis accounts for the impact of thickness-dependent material parameters on the absorption and field intensity enhancement under different measurement conditions and estimates the incidence angle required for achieving the maximum nonlinear response for a given TCO film. We perform measurements of angle-dependent and intensity-dependent nonlinear transmittance for Indium-Zirconium Oxide (IZrO) thin films with different thicknesses and demonstrate a good agreement between the experiment and theory. Our results also indicate that the film thickness and the excitation angle of incidence can be adjusted simultaneously to optimize the nonlinear optical response, allowing a flexible design of TCO-based highly nonlinear optical devices.

Composition-Dependent Cytotoxic and Antibacterial Activity of Biopolymer-Capped Ag/Au Bimetallic Nanoparticles against Melanoma and Multidrug-Resistant Pathogens.

Nanostructured silver (Ag) and gold (Au) are widely known to be potent biocidal and cytotoxic agents as well as biocompatible nanomaterials. It has been recently reported that combining both metals in a specific chemical composition causes a significant enhancement in their antibacterial activity against antibiotic-resistant bacterial strains, as well as in their anticancer effects, while preserving cytocompatibility properties. In this work, Ag/Au bimetallic nanoparticles over a complete atomic chemical composition range were prepared at 10 at% through a green, highly reproducible, and simple approach using starch as a unique reducing and capping agent. The noble metal nanosystems were thoroughly characterized by different analytical techniques, including UV-visible and FT-IR spectroscopies, XRD, TEM/EDS, XPS and ICP-MS. Moreover, absorption spectra simulations for representative colloidal Ag/Au-NP samples were conducted using FDTD modelling. The antibacterial properties of the bimetallic nanoparticles were determined against multidrug-resistant Escherichia coli and methicillin-resistant Staphylococcus aureus, showing a clear dose-dependent inhibition even at the lowest concentration tested (5 µg/mL). Cytocompatibility assays showed a medium range of toxicity at low and intermediate concentrations (5 and 10 µg/mL), while triggering an anticancer behavior, even at the lowest concentration tested, in a process involving reactive oxygen species production per the nanoparticle Au:Ag ratio. In this manner, this study provides promising evidence that the presently fabricated Ag/Au-NPs should be further studied for a wide range of antibacterial and anticancer applications.

Observation of the modification of quantum statistics of plasmonic systems.

For almost two decades, researchers have observed the preservation of the quantum statistical properties of bosons in a large variety of plasmonic systems. In addition, the possibility of preserving nonclassical correlations in light-matter interactions mediated by scattering among photons and plasmons stimulated the idea of the conservation of quantum statistics in plasmonic systems. It has also been assumed that similar dynamics underlie the conservation of the quantum fluctuations that define the nature of light sources. So far, plasmonic experiments have been performed in nanoscale systems in which complex multiparticle interactions are restrained. Here, we demonstrate that the quantum statistics of multiparticle systems are not always preserved in plasmonic platforms and report the observation of their modification. Moreover, we show that optical near fields provide additional scattering paths that can induce complex multiparticle interactions. Remarkably, the resulting multiparticle dynamics can, in turn, lead to the modification of the excitation mode of plasmonic systems. These observations are validated through the quantum theory of optical coherence for single- and multi-mode plasmonic systems. Our findings unveil the possibility of using multiparticle scattering to perform exquisite control of quantum plasmonic systems.

Ultrasensitive nanoplasmonic biosensor based on interferometric excitation of multipolar plasmonic modes.

We propose a nanoplasmonic interferometric biosensor, which exploits the selective excitation of multipolar plasmonic modes in a nanoslit to provide a novel scheme for highly-sensitive biosensing. In this design, two counter-propagating surface plasmon polaritons interfere at the location of the nanoslit, selectively exciting the dipolar and quadrupolar modes of the structure depending on the phase relationship induced by the analyte. The contrasting radiation patterns produced by these modes result in large changes in the angular distribution of the transmitted light that depends on the analyte concentration. The resultant far-field is numerically modeled and the sensing performance of the structure is assessed, resulting in maximum bulk and surface sensitivities of SB = 1.12 × 105 deg/RIU and SS = 302 deg/RIU, respectively, and a bulk-sensing resolution of the order of 10-8 RIU. The design allows ample control over the trade-off between operating range and resolution through the slit's width, making this platform suitable for a broad range of sensing requirements.

Measurement of the Photon-Plasmon Coupling Phase Shift.

Scattering processes have played a crucial role in the development of quantum theory. In the field of optics, scattering phase shifts have been utilized to unveil interesting forms of light-matter interactions. Here we investigate the phase shift experienced by a single photon as it scatters into a surface plasmon polariton and vice versa. This coupling phase shift is of particular relevance for quantum plasmonic experiments. Therefore, we demonstrate that the photon-plasmon interaction at a plasmonic slit can be modeled through a quantum-mechanical tritter, a six-port scattering element. We show that the visibilities of a double-slit and a triple-slit interference patterns are convenient observables to characterize the interaction at a slit and determine the coupling phase. Our accurate and simple model of the interaction, validated by simulations and experiments, has important implications not only for quantum plasmonic interference effects, but is also advantageous to classical applications.

Chiroptical response of a single plasmonic nanohelix.

We investigate the chiroptical response of a single plasmonic nanohelix interacting with a weakly focused circularly polarized Gaussian beam. The optical scattering at the fundamental resonance is characterized experimentally and numerically. The angularly resolved scattering of the excited nanohelix is verified experimentally and it validates the numerical results. We employ a multipole decomposition analysis to study the fundamental and first higher-order resonance of the nanohelix, explaining their chiral properties in terms of the formation of chiral dipoles.

Beyond the perturbative description of the nonlinear optical response of low-index materials.

We show that standard approximations in nonlinear optics are violated for situations involving a small value of the linear refractive index. Consequently, the conventional equation for the intensity-dependent refractive index, n(I)=n0+n2I, becomes inapplicable in epsilon-near-zero and low-index media, even in the presence of only third-order effects. For the particular case of indium tin oxide, we find that the χ(3), χ(5), and χ(7) contributions to refraction eclipse the linear term; thus, the nonlinear response can no longer be interpreted as a perturbation in these materials. Although the response is non-perturbative, we find no evidence that the power series expansion of the material polarization diverges.

Exotic looped trajectories of photons in three-slit interference.

The validity of the superposition principle and of Born's rule are well-accepted tenants of quantum mechanics. Surprisingly, it has been predicted that the intensity pattern formed in a three-slit experiment is seemingly in contradiction with the most conventional form of the superposition principle when exotic looped trajectories are taken into account. However, the probability of observing such paths is typically very small, thus rendering them extremely difficult to measure. Here we confirm the validity of Born's rule and present the first experimental observation of exotic trajectories as additional paths for the light by directly measuring their contribution to the formation of optical interference fringes. We accomplish this by enhancing the electromagnetic near-fields in the vicinity of the slits through the excitation of surface plasmons. This process increases the probability of occurrence of these exotic trajectories, demonstrating that they are related to the near-field component of the photon's wavefunction.

Polarization Shaping for Control of Nonlinear Propagation.

We study the nonlinear optical propagation of two different classes of light beams with space-varying polarization-radially symmetric vector beams and Poincaré beams with lemon and star topologies-in a rubidium vapor cell. Unlike Laguerre-Gauss and other types of beams that quickly experience instabilities, we observe that their propagation is not marked by beam breakup while still exhibiting traits such as nonlinear confinement and self-focusing. Our results suggest that, by tailoring the spatial structure of the polarization, the effects of nonlinear propagation can be effectively controlled. These findings provide a novel approach to transport high-power light beams in nonlinear media with controllable distortions to their spatial structure and polarization properties.

Chiral optical response of planar and symmetric nanotrimers enabled by heteromaterial selection.

Chirality is an intriguing property of certain molecules, materials or artificial nanostructures, which allows them to interact with the spin angular momentum of the impinging light field. Due to their chiral geometry, they can distinguish between left- and right-hand circular polarization states or convert them into each other. Here we introduce an approach towards optical chirality, which is observed in individual two-dimensional and geometrically mirror-symmetric nanostructures. In this scheme, the chiral optical response is induced by the chosen heterogeneous material composition of a particle assembly and the corresponding resonance behaviour of the constituents it is built from, which breaks the symmetry of the system. As a proof of principle, we investigate such a structure composed of individual silicon and gold nanoparticles both experimentally, as well as numerically. Our proposed concept constitutes an approach for designing two-dimensional chiral media tailored at the nanoscale, allowing for high tunability of their optical response.

Large optical nonlinearity of indium tin oxide in its epsilon-near-zero region.

Nonlinear optical phenomena are crucial for a broad range of applications, such as microscopy, all-optical data processing, and quantum information. However, materials usually exhibit a weak optical nonlinearity even under intense coherent illumination. We report that indium tin oxide can acquire an ultrafast and large intensity-dependent refractive index in the region of the spectrum where the real part of its permittivity vanishes. We observe a change in the real part of the refractive index of 0.72 ± 0.025, corresponding to 170% of the linear refractive index. This change in refractive index is reversible with a recovery time of about 360 femtoseconds. Our results offer the possibility of designing material structures with large ultrafast nonlinearity for applications in nanophotonics.

Light-Drag Enhancement by a Highly Dispersive Rubidium Vapor.

The change in the speed of light as it propagates through a moving material has been a subject of study for almost two centuries. This phenomenon, known as the Fresnel light-drag effect, is quite small and usually requires a large interaction path length and/or a large velocity of the moving medium to be observed. Here, we show experimentally that the observed drag effect can be enhanced by over 2 orders of magnitude when the light beam propagates through a moving slow-light medium. Our results are in good agreement with the theoretical prediction, which indicates that, in the limit of large group indices, the strength of the light-drag effect is proportional to the group index of the moving medium.

Strong, spectrally-tunable chirality in diffractive metasurfaces.

Metamaterials and metasurfaces provide a paradigm-changing approach for manipulating light. Their potential has been evinced by recent demonstrations of chiral responses much greater than those of natural materials. Here, we demonstrate theoretically and experimentally that the extrinsic chiral response of a metasurface can be dramatically enhanced by near-field diffraction effects. At the core of this phenomenon are lattice plasmon modes that respond selectively to the illumination's polarization handedness. The metasurface exhibits sharp features in its circular dichroism spectra, which are tunable over a broad bandwidth by changing the illumination angle over a few degrees. Using this property, we demonstrate an ultra-thin circular-polarization sensitive spectral filter with a linewidth of ~10 nm, which can be dynamically tuned over a spectral range of 200 nm. Chiral diffractive metasurfaces, such as the one proposed here, open exciting possibilities for ultra-thin photonic devices with tunable, spin-controlled functionality.

Measurement of the complex nonlinear optical response of a surface plasmon-polariton.

We observe experimentally the self-phase modulation of a surface plasmon-polariton (SPP) propagating along a gold film bounded by air in a Kretschmann-Raether configuration. Through analyzing the power dependence of the reflectance curve as a function of the incidence angle, we characterize the complex-valued nonlinear propagation coefficient of the SPP. Moreover, we present a procedure that can further extract the complex value of the third-order nonlinear susceptibility of gold from our experimental data. Our work provides direct insights into nonlinear control of SPPs utilizing the nonlinearity of metals, and serves as a practical method to measure the complex-valued third-order nonlinear susceptibility of metallic materials.

Enhancing optical field intensities in Gaussian-profile fiber Bragg gratings.

Gaussian profile fiber Bragg gratings exhibit narrow-bandwidth transmission peaks with significant group delay at the edge of their photonic bandgap. We demonstrate group delays ranging from 0.2 to 5.6 ns from a 1.2 cm structure. Simulations suggest such a device would be capable of enhancing the field intensity of incoming light by a factor of 800. Enhancement is confirmed by photothermally induced bistability of these peaks even at sub-milliwatt input powers with as much as a four-fold difference in the magnitude of their responses. The strong field intensities of these modes could significantly enhance desired nonlinear optical responses in fiber, provided the impact of absorption is addressed.

Theory of noise in high-gain surface plasmon-polariton amplifiers incorporating dipolar gain media.

A theoretical analysis of noise in high-gain surface plasmon-polariton amplifiers incorporating dipolar gain media is presented. An expression for the noise figure is obtained in terms of the spontaneous emission rate into the amplified surface plasmon-polariton taking into account the different energy decay channels experienced by dipoles in close proximity to the metallic surface. Two amplifier structures are examined: a single-interface between a metal and a gain medium and a thin metal film bounded by identical gain media on both sides. A realistic configuration is considered where the surface plasmon-polariton undergoing amplification has a Gaussian field profile in the plane of the metal and paraxial propagation along the amplifier's length. The noise figure of these plasmonic amplifiers is studied considering three prototypical gain media with different permittivities. It is shown that the noise figure exhibits a strong dependance on the real part of the permittivities of the metal and gain medium, and that its minimum value is 4/π(∼3.53 dB). The origin of this minimum value is discussed. It is also shown that amplifier configurations supporting strongly confined surface plasmon-polaritons suffer from a large noise figure, which follows from an enhanced spontaneous emission rate due to the Purcell effect.

Measuring gain and noise in active long-range surface plasmon-polariton waveguides.

We describe techniques and an experimental setup to measure the gain and noise characteristics of a long-range surface plasmon-polariton amplifier consisting of a symmetric metallic stripe waveguide incorporating optically pumped dye molecules in the solution as the gain medium. The setup is capable of acquiring absolute power measurements at the amplifier's output over a narrow optical bandwidth. This allows independent characterization of the amplifier's gain via measurements of stimulated emission and via measurements of amplified spontaneous emission (ASE) over a narrow optical bandwidth, both obtained during the same experimental run. In addition, the absolute power measurements of ASE quantify directly the amplifier's noise.

Modeling surface plasmon-polariton gain in planar metallic structures.

Amplification of the single-interface and long-range surface plasmon-polariton modes is studied in planar metallic structures incorporating gain media formed by Rhodamine 6G dye molecules in solution. We employ a theoretical model that accounts for the nonuniformity of the gain medium close to the metal surface due to position-dependent dipole lifetime and pump irradiance. The results of this model are used as a baseline for a comparative study against two simplified models: one neglects the position-dependent dipole lifetime while the other assumes a uniform gain medium. The discrepancies between the models are explained in terms of the mode overlap with the gain distribution near the metal. For the cases under analysis, the simplified models estimate the required pump irradiance with deviation factors that vary from 1.45 at the lossless conditions to 8 for gains near saturation. The relevance of describing properly the amount o gain interacting with the SPP mode and the role played by the dipole quantum efficiency are discussed.