Distinguishing Thermal from Nonthermal ("Hot") Carriers in Illuminated Molecular Junctions.

The search for the signature of nonthermal (so-called "hot") electrons in illuminated plasmonic nanostructures requires detailed understanding of the nonequilibrium electron distribution under illumination, as well as a careful design of the experimental system employed to distinguish nonthermal electrons from thermal ones. Here, we provide a theory for using plasmonic molecular junctions to achieve this goal. We show how nonthermal electrons can be measured directly and separately from the unavoidable thermal response and discuss the relevance of our theory to recent experiments.

Second-harmonic generation from subwavelength metal heterodimers.

We experimentally study the optical second-harmonic generation (SHG) from deep subwavelength gold-silver heterodimers, and silver-silver and gold-gold homodimers. Our results indicate a heterodimer SHG that is an order of magnitude more intense than that of the homodimers. In contrast, full-wave calculations that consider the surface and bulk contribution of individual particles, which is the conventional view on such processes, suggest that it is the silver-silver homodimer that should prevail. Based on the deep subwavelength dimension of our structure, we propose that the heterodimer nonlinearity results from a Coulomb interaction between lumped oscillating charges and not from the surface nonlinearity of each particle, as convention would have it. Our proposed model can explain the larger SHG emission observed in gold-silver heterodimers and reproduces its unique spectral lineshape.

Effective Electron Temperature Measurement Using Time-Resolved Anti-Stokes Photoluminescence.

Anti-Stokes photoluminescence of metal nanoparticles, in which emitted photons have a higher energy than the incident photons, is an indicator of the temperature prevalent within a nanoparticle. Previous work has shown how to extract the temperature from a gold nanoparticle under continuous-wave monochromatic illumination. We extend the technique to pulsed illumination and introduce pump-probe anti-Stokes spectroscopy. This new technique enables us not only to measure an effective electron temperature in a gold nanoparticle (∼103 K under our conditions), but also to measure ultrafast dynamics of a pulse-excited electron population, through its effect on the photoluminescence, with subpicosecond time resolution. We measure the heating and cooling, all within picoseconds, of the electrons and find that, with our subpicosecond pulses, the highest apparent temperature is reached 0.6 ps before the maximum change in magnitude of the extinction signal.

Assistance of metal nanoparticles in photocatalysis - nothing more than a classical heat source.

In a recent paper, we derived a self-consistent theory of the steady-state electron distribution of a metal under continuous wave illumination which treats thermal and non-thermal effects on the same footing. Here, we re-derive the main analytical results of that study from very simple arguments, and draw a series of conclusions which contradict claims made in previous studies of the steady-state distribution. In particular, we show that the faster chemical reactions reported in many previous papers are extremely unlikely to originate from high energy non-thermal electrons. Instead, the faster reactions very likely originate from a purely thermal effect.

Pulse propagation in the slow and stopped light regime.

Electromagnetic pulse propagation in the slow light regime and near a zero group velocity point is relevant to a plethora of potential applications, and has analogies in numerous other wave systems. Unfortunately, the standard frequency-based formulation for pulse propagation is unsuitable for describing the dynamics in such regimes, due to the divergence of the dispersion coefficients. Moreover, in the presence of absorption, it is not clear how to interpret the propagation dynamics due to the drastic change induced by absorption upon the dispersion curves. As a remedy, we present an alternative momentum-based formulation, which is rapidly converging in these regimes, and naturally suitable for lossy and nonlinear media. It is specialized to a waveguide geometry which provides a significant simplification with respect to existing momentum-based schemes. Doing so, we provide a somewhat alternative, yet intuitive picture of the seeming enhanced absorption and nonlinear response in these regimes, and show that light-matter interactions are not enhanced in the slow/stopped light regimes.

Ns-duration transient Bragg gratings in silica fibers.

We demonstrate, for the first time to our knowledge, fast all-optical switching in standard silica fibers, based on a transient Bragg grating. The grating is implemented in the fiber using an immunization photo-pretreatment process, followed by side illumination with femtosecond laser pulses through a suitable phase mask. Each pulse is nonlinearly absorbed, creating a thermal grating that is washed out by thermal diffusion. Reflections measured from such gratings are characterized by a very fast rise time, nanoseconds duration, and a high extinction ratio.

Temperature- and roughness- dependent permittivity of annealed/unannealed gold films.

Intrinsic absorption and subsequent heat generation have long been issues for metal-based plasmonics. Recently, thermo-plasmonics, which takes the advantage of such a thermal effect, is emerging as an important branch of plasmonics. However, although significant temperature increase is involved, characterization of metal permittivity at different temperatures and corresponding thermo-derivative are lacking. Here we measure gold permittivity from 300K to 570K, which the latter is enough for gold annealing. More than one order difference in thermo-derivative is revealed between annealed and unannealed films, resulting in a large variation of plasmonic properties. In addition, an unusual increase of imaginary permittivity after annealing is found. Both these effects can be attributed to the increased surface roughness incurred by annealing. Our results are valuable for characterizing extensively used unannealed nanoparticles, or annealed nanostructures, as building blocks in future thermo-nano-plasmonic systems.

Experimental proof of concept of nanoparticle-assisted STED.

We imaged core-shell nanoparticles, consisting of a dye-doped silica core covered with a layer of gold, with a stimulated emission depletion, fluorescence lifetime imaging (STED-FLIM) microscope. Because of the field enhancement provided by the localized surface plasmon resonance of the gold shell, we demonstrate a reduction of the STED depletion power required to obtain resolution improvement by a factor of 4. This validates the concept of nanoparticle-assisted STED (NP-STED), where hybrid dye-plasmonic nanoparticles are used as labels for STED in order to decrease the depletion powers required for subwavelength imaging.

Crossover from Nonthermal to Thermal Photoluminescence from Metals Excited by Ultrashort Light Pulses.

Photoluminescence from metal nanostructures following intense ultrashort illumination is a fundamental aspect of light-matter interactions. Surprisingly, many of its basic characteristics are under ongoing debate. Here, we resolve many of these debates by providing a comprehensive theoretical framework that describes this phenomenon and support it by an experimental confirmation. Specifically, we identify aspects of the emission that are characteristic to either nonthermal or thermal emission, in particular, differences in the spectral and electric field dependence of these two contributions to the emission. Overall, nonthermal emission is characteristic of the early stages of light emission, while the later stages show thermal characteristics. The former dominate only for moderately high illumination intensities for which the electron temperature reached after thermalization remains close to room temperature.

Photothermal nonlinearity in plasmon-assisted photocatalysis.

Understanding the intricate relationship between illumination and temperature in metallic nano-particles is crucial for elucidating the role of illumination in various physical processes which rely on plasmonic enhancement but are also sensitive to temperature. Recent studies have shown that the temperature rise in optically thick ensembles of metal nanoparticles under intense illumination is dominated by the thermal conductivity of the host, rather than by the optical properties of the metal or the host. Here, we show that the temperature dependence of the thermal conductivity of the host dominates the nonlinear photothermal response of these systems. In particular, this dependence typically causes the temperature rise to become strongly sublinear, reaching even several tens of percent. We then show that this effect can explain experimental observations in several recent plasmon-assisted photocatalysis experiments. Under certain conditions, we show that thermal emission may also contribute to photothermal nonlinearity. This shows that any claim for the dominance of non-thermal electrons in plasmon-assisted photocatalysis must account first for this photothermal nonlinear mechanism.

Broadband time-reversal of optical pulses using a switchable photonic-crystal mirror.

Recently, Chumak et al. have demonstrated experimentally the time-reversal of microwave spin pulses based on non-adiabatically tuning the wave speed in a spatially-periodic manner [Nat. Comm. 1, 141 (2010)]. Here, we solve the associated wave equations analytically, and give an explicit formula for the reversal efficiency. We discuss the implementation for short optical electromagnetic pulses and show that the new scheme may lead to their accurate time-reversal with efficiency higher than before.

A quantitative approach to soliton instability.

We present an approach for instabilities of solitons that is based on the spectrum of a fourth-order linearized operator. Unlike the standard approach which is based on the slope (Vakhitov-Kolokolov) condition, this approach provides the quantitative value of the instability rate and the qualitative nature of the instability dynamics.

Time reversal in dynamically tuned zero-gap periodic systems.

We show that short pulses propagating in zero-gap periodic systems can be reversed with 100% efficiency by using weak nonadiabatic tuning of the wave velocity at time scales that can be much slower than the period. Unlike previous schemes, we demonstrate reversal of broadband (few cycle) pulses with simple structures. Our scheme may thus open the way to time reversal in a variety of systems for which it was not accessible before.

Theory of "Hot" Photoluminescence from Drude Metals.

We provide a complete quantitative theory for light emission from Drude metals under continuous wave illumination, based on our recently derived steady-state nonequilibrium electron distribution. We show that the electronic contribution to the emission exhibits a dependence on the emission frequency which is very similar to the energy dependence of the nonequilibrium distribution, and characterize different scenarios determining the measurable emission line shape. This enables the identification of experimentally relevant situations, where the emission lineshapes deviate significantly from predictions based on the standard theory (namely, on the photonic density of states), and enables the differentiation between cases where the emission scales with the metal object surface or with its volume. We also provide an analytic description (which is absent from the literature) of the (polynomial) dependence of the metal emission on the electric field, its dependence on the pump laser frequency, and its nontrivial exponential dependence on the electron temperature, both for the Stokes and anti-Stokes regimes. Our results imply that the emission does not originate from either Fermion statistics (due to e-e interactions), and even though one could have expected the emission to follow boson statistics due to involvement of photons (as in Planck's Black Body emission), it turns out that it deviates from that form as well. Finally, we resolve the arguments associated with the effects of electron and lattice temperatures on the emission, and which of them can be extracted from the anti-Stokes emission.

Parametric study of temperature distribution in plasmon-assisted photocatalysis.

Recently, there has been a growing interest in the usage of mm-scale composites of plasmonic nanoparticles for enhancing the rates of chemical reactions; the effect was shown recently to be predominantly associated with the elevated temperature caused by illumination. Here, we study the dependence of the temperature distribution on the various parameters of these samples, and provide analytic expressions for simple cases. We show that since these systems are usually designed to absorb all the incoming light, the temperature distribution in them is weakly-dependent on the illumination spectrum, pulse duration, particle shape, size and density. Thus, changes in these parameters yield at most modest quantitative changes. We also show that the temperature distribution is linearly dependent on the beam radius and the thermal conductivity of the host. Finally, we study the sensitivity of the reaction rate to these parameters as a function of the activation energy and show how it manifests itself in various previous experimental reports. These results would simplify the optimization of photocatalysis experiments, as well as of other energy-related applications based on light harvesting for heat generation.

Thermal effects - an alternative mechanism for plasmon-assisted photocatalysis.

Recent experiments claimed that the catalysis of reaction rates in numerous bond-dissociation reactions occurs via the decrease of activation barriers driven by non-equilibrium ("hot") electrons in illuminated plasmonic metal nanoparticles. Thus, these experiments identify plasmon-assisted photocatalysis as a promising path for enhancing the efficiency of various chemical reactions. Here, we argue that what appears to be photocatalysis is much more likely thermo-catalysis, driven by the well-known plasmon-enhanced ability of illuminated metallic nanoparticles to serve as heat sources. Specifically, we point to some of the most important papers in the field, and show that a simple theory of illumination-induced heating can explain the extracted experimental data to remarkable agreement, with minimal to no fit parameters. We further show that any small temperature difference between the photocatalysis experiment and a control experiment performed under external heating is effectively amplified by the exponential sensitivity of the reaction, and is very likely to be interpreted incorrectly as "hot" electron effects.