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Nonlinear optical interactions are of fundamental significance for advanced photonic applications, but usually the nonlinearity magnitude is insufficient. Here we review recent progresses to boost the optical nonlinearity of metal or semiconductor nanostructures via the combination of Mie resonance and coupled photothermal/thermo-optical effects. In plasmonic and silicon nanoparticles, the effective photothermal nonlinear index n2 is enhanced by 103 and 105 times over that of bulk, respectively. The large nonlinearities enable applications of not only all-optical switch, but also super-resolution imaging based on suppression of scattering, saturation (sub-linearity) and reverse saturation (super-linearity).

Light manipulation at the nanoscale is the vanguard of plasmonics. Controlling light radiation into a desired direction in parallel with high optical signal enhancement is still a challenge for designing ultracompact nanoantennas far below subwavelength dimensions. Here, we theoretically demonstrate the unidirectional emissions from a local nanoemitter coupled to a hybrid nanoantenna consisting of a plasmonic dipole antenna and an individual silicon nanorod. The emitter near-field was coupled to the dipolar antenna plasmon resonance to achieve a strong radiative decay rate modification, and the emitting plasmon pumped the multipoles within the silicon nanorod for efficient emission redirection. The hybrid antenna sustained a high forward directivity (i.e., a front-to-back ratio of 30 dB) with broadband operating wavelengths in the visible range (i.e., a spectral bandwidth of 240 nm). This facilitated a large library of plasmonic nanostructures to be incorporated, from single element dipole antennas to gap antennas. The proposed hybrid optical nanorouter with ultracompact structural dimensions of 0.08 &lambda, 2 was capable of spectrally sorting the emission from the local point source into distinct far-field directions, as well as possessing large emission gains introduced by the nanogap. The distinct features of antenna designs hold potential in the areas of novel nanoscale light sources, biosensing, and optical routing.

We systematically investigate the influence of photothermal effects on the scattering property of plasmonic nanoparticles. Temperature rising and scattering-intensity map are studied both theoretically and experimentally. Applying these features, we demonstrate optical imaging based on photothermal effects of plasmonic nanoparticles.

Photoluminescence of single gold bipyramid due to the radiative decay of the longitudinal surface plasmon is used to determine its 3D orientation using a direct emission pattern imaging technique. The photoluminescence (PL) intensity from individual gold bipyramids is recorded at the objective's back focal plane, which directly maps the angular emission distribution. By correlating the PL spectra, dark-field scattering, emission pattern, and atomic force microscopy data, the spatial orientation and the radiation characteristics of an individual nanoantenna can be unambiguously determined. The PL emission pattern imaging can be used as a stand-alone technique for the fast and facile determination of nanoantenna orientations. The experimental data also agree well with a simple analytical model based on a far-field angular distribution of an isolated dipole emitter on the interface. In situ measurements are also performed on the PL pattern of a single gold bipyramid during optical heating to monitor the photothermal reshaping of the nanoantenna. The results show that the PL emission pattern imaging method is highly sensitive to changes of the geometry and orientation of the nanoparticle. The PL direct imaging technique offers great advantages, including a facile full-3D angle-resolved capability, free of photobleaching and photoblinking. Additionally, the results reveal that the gold bipyramids can generate efficient PL emission, and the unique features of gold bipyramids would be outstanding candidates for orientation sensing. These findings have great potential for characterizing optical nanoantennas, for optical imaging and sensing in materials science and biological applications.

In this study, we theoretically investigated the single-molecule fluorescence enhancement in a slit–groove structure. The excitation field enhancement, modified quantum efficiency, collection efficiency, and position dependence of a single-molecule spontaneous emission coupled to a plasmonic antenna are explored together. Simulation results revealed that the metal gratings play a crucial role in strengthening the local electromagnetic field enhancement in the excitation process and beaming the radiation light into a tiny angular volume in the emission process, whereas the total radiative decay rate and quantum efficiency are mainly determined by the slit cavity. Our findings provide an intuitive guideline to further optimize the plasmonic antenna for single-molecule detection and the results make a promising route to the development of photonic devices for the manipulation of single-molecule spontaneous emission.

We report experimental behaviors of polarization-dependent, plasmonic-enhanced molecular fluorescence within isolated bowtie nano-apertures (BNAs) milled in aluminum films. BNAs provide efficient control of the fluorescent count rate per molecule and the decay lifetime of the molecules and provide an effective detection volume at the nanometer scale by tuning either the excitation light polarization or the BNA size. Interestingly, large BNAs (>300 nm) present high plasmonic-enhanced fluorescence efficiency and can simultaneously confine the detection volume below the subdiffraction limit. Numerical simulations were performed that agreed qualitatively with the experimental results. The BNAs have potential applications, especially for single-molecule biological analysis.

cited By 26; International audience; We perform a theoretical investigation of individual and coupled gold nanorods as plasmonic nanosensors using the finite-difference time-domain method. Key features of single-nanorod sensors are discussed. The sensitivity distribution of an individual nanorod is anisotropic. The characteristic sensitivity decay length of a single-nanorod sensor is comparable to its diameter. Plasmonic sensing abilities are additive, so analyte-detection sensitivity is not affected by substrates or surface treatments; shifts caused by analytes are only determined by their positions relative to the sensor. Coupled nanorods enhance and concentrate plasmonic sensitivities, and the sensitivity within the gap can be over an order of magnitude higher than that at the nanorod cylinder. The sensitivities of coupled nanorods are only higher than those of individual nanorods when the analytes are anchored within the gaps between nanorods. The calculations show that a single biological molecule can be detected by optimizing nanostructure design and surface treatments to anchor analytes locally on high-sensitivity areas of the sensor surface. Our simulation results assist the design and optimization of plasmonic nanosensors, using single or coupled nanorods. © 2011 American Chemical Society.

Stimulated emission depletion nanoscopy and its derivatives based on saturation induced competition effects have become an indispensable tool for studying cellular events and their dynamics in living conditions. The successful implementation of these techniques heavily relies on the competition between excitation induced spontaneous emission and stimulated emission from fluorescent dyes. The use of two laser beams at different wavelengths perplexes the optical system and the high intensity saturation beam inevitably introduces detrimental photobleaching effects. Harnessing the emerging saturation scattering of plasmonic nanoparticles, here, we demonstrate a novel fluorescence-free single-wavelength super-resolution imaging technique using gold nanoparticles. A lateral resolution of 101.2 nm (

We report detailed investigations of the photoluminescence (PL) generated from an individual gold nanoflower, a highly branched plasmonic nanoparticle. Compared to nanostructures with simple shapes, such as spheres, nanorods, and bipyramids, nanoflowers exhibit more distinct features, i.e., the PL spectra and far-field emission patterns are strongly dependent on the wavelength and polarization of the excitation light. The experimental results are qualitatively explained using theoretical calculations. In addition, the intrinsic PL signal is highly dominated by localized surface plasmon resonances. The crucial role of plasmonic coupling in complex nanostructures during the plasmon-enhanced PL process is highlighted. The findings contribute to a deeper understanding of the PL properties of metallic nanoparticles. This study will be beneficial for several potential applications, including optical imaging and sensing in the fields of materials science and biology.

cited By 4; International audience; Single gold nanoshell with mutilpolar plasmon resonances is proposed to enhance two-photon fluorescence efficiently. The single emitter single nanoshell configuration is studied systematically by employing the finite-difference time-domain method. The emitter located inside or outside the nanoshell at various positions leads to a significantly different enhancement effect. The fluorescent emitter placed outside the nanoshell can achieve large fluorescence intensity given that both the position and orientation of the emission dipole are optimally controlled. In contrast, for the case of the emitter placed inside the nanoshell, it can experience substantial two-photon fluorescence enhancement without strict requirements upon the position and dipole orientations. Metallic nanoshell encapsulating many fluorescent emitters should be a promising nanocomposite configuration for bright two-photon fluorescence label. The results provide a comprehensive understanding about the plasmonic-enhanced two-photon fluorescence behaviors, and the nanocomposite configuration has great potential for optical detecting, imaging and sensing in biological applications. © 2014 Science China Press and Springer-Verlag Berlin Heidelberg.

In this article, we present a refined nanostructuring method, lift-off nanoimprint lithography (LO-NIL), which allows the deposition of high-quality metal nanostructures due to a bilayer resist process and compare it to nano-transfer printing (nTP), a purely additive metal printing technique. LO-NIL and nTP are used as accurate methods for the fabrication of ordered plasmonic metal nanostructure arrays on semiconducting substrates over large areas using the example of gold nanodisks on silicon. The possibility of feature size adjustment in LO-NIL during the fabrication process is especially useful for tuning plasmonic resonance peaks between the visible and the mid-infrared range as well as fine-tuning of these resonances. In UV-VIS-NIR spectroscopic measurements, a significant blueshift in the plasmonic resonance was found for nTP samples compared to the ones fabricated with the lift-off technique. It was concluded that this shift originates from a metal/substrate interface roughness resulting in a chang...

International audience; Plasmonic refractometric nanosensors based on single nanostructures, i.e. spherical, nanorod- and bipyramid-shaped gold nanoparticles, are investigated and compared numerically by employing the finite-difference time-domain method. The results show that the plasmonic sensing ability is distributed anisotropically around the nanorod and bipyramid, even for spherical nanoparticles when the illumination light is linearly polarized. To optimize nanosensor performance, some anisotropy in the shape of nanoparticles is required, this latter serving as an intrinsic light polarization filter to suppress the disturbance from localized surface plasmon resonance in other directions. The plasmonic near-field can be engineered by controlling the shape to achieve a concentrated and localized electromagnetic field, in direct relation with the sensing ability. Taking these factors into account, the gold bipyramid nanoconstruct which is easily available in experiment is proposed as an efficient plasmonic sensing platform. The bipyramid presents both highly localized sensitivity and high scattering cross-section, thus avoiding the trade-off during the selection of the widely used nanorod-shaped sensors. The parameters of the bipyramid structure can be optimized by numerical simulation to improve the plasmonic sensing. Our findings permit a deeper understanding of single-nanoparticle-sensor behavior, and the study provides an opportunity to optimize the plasmonic sensor.

International audience; We propose and justify by the finite-difference time-domain method an efficient strategy to enhance the spontaneous emission of a fluorophore with a multi-resonance plasmonic antenna. The custom-designed asymmetrical antenna consists of two plasmonic nanoparticles with different sizes and is able to couple efficiently to free space light through multiple localized surface plasmon resonances. This design simultaneously permits a large near-field excitation near the antenna as well as a high quantum efficiency, which results in an unusual and significant enhancement of the fluorescence of a single emitter. Such an asymmetrical antenna presents intrinsic advantages over single particle or dimer based antennas made using two identical nanostructures. This promising concept can be exploited in the large domain of light-matter interaction processes involving multiple frequencies.

cited By 6; International audience; A single gold nano-cylinder presenting multipolar plasmon resonances to enhance two-photon fluorescence is investigated employing three dimensional finite-difference time-domain method. Cylinders of large dimension usually display dipolar and quadrupolar plasmonic resonances. We demonstrate that the dipolar resonance can couple with the incident light resulting in a large localized field enhancement which increases the molecular excitation rate. At the same time, the radiative quadrupolar mode overlaps with the emission band of excited fluorophores to assist the fluorescence emission due to an enhancement in the quantum efficiency. Such dipole-quadrupole jointly enhanced two-photon fluorescence exhibits exceptionally promise in brighter label design. © 2012 American Institute of Physics.

cited By 24; International audience; Nanoshell and Matryoshka-nanoshell constructs are rationally optimized utilizing the finite-difference time-domain method to design probes with enhanced fluorescence. Through the systematical investigation of interactions between the spontaneous emission of a single emitter and a metal-based nanostructure, the plasmonic-enhanced fluorescence is found maximal for certain morphologies that balance two competitive factors: field enhancement and quantum efficiency. For instance, key parameters such as the emitters position, its orientation, and the spectral overlaps between molecular bands and plasmon resonance are investigated to fully understand the complete behavior of the system. In the case of metal nanoshells, it is shown that the molecular fluorescence is differently enhanced inside or outside the shell. In that of Matryoshka-nanoshells that consist of concentric gold core and shell, the construct appears as an exceptionally promising probe especially when the fluorophore lies within the gap layer. Indeed, the strong coupling between the adjacent core and shell allows electromagnetic excitation to be squeezed within the gap, so resulting in giant fluorescence and photostability. Such a construct opens a way for still increasing the sensitivity of fluorescence detection, which is promising for almost all biological imaging applications. © 2012 American Chemical Society.

In this study, molecule fluorescence modified by slit-based nanoantennas surrounded with metal gratings was investigated by employing the finite-difference time-domain method. We quantified the relative contribution of excitation and emission gains to the total fluorescence enhancement. The simulation results show that the asymmetric dual-grating (DG) antenna provides an efficient way to control the local excitation enhancement, the collection efficiency, and the quantum efficiency separately for bright emission and beaming light. We also investigated the dependence of fluorescence enhancement on the geometric parameters of the antenna, such as the nano-slit width and number of grooves. The asymmetric DG structure greatly improves the flexibility of the nanostructure design to further optimize the plasmonic enhancement effect and provides a promising route to manipulate single-molecule fluorescence emission.

We proposed the estimation of the plasmonic near-field volume in the vicinity of a single gold nanoparticle, and observed experimentally the near-field variation due to a change in the polarization of the illuminating light. Under total-internal-reflection illumination, the plasmonic near-field volume is varied by tuning the polarization of the excitation light. The variation in the optical near-field around a single gold nanoparticle was simulated theoretically with a finite-difference time domain method, and was characterized experimentally employing a fluorescence correlation spectroscopy technique. The experimental results are in agreement quantitatively with the theoretical analysis. These results are highly relevant to important efforts to clarify the interaction between the emitter and the plasmonic antenna, and should be helpful in developing a plasmonic-enhanced total-internal-reflection fluorescence imaging microscope.