Nonlinear metasurface engineering with disordered gold nanorods on ITO: a cost-effective approach to broadband response, polarization-independence, and weak nonlinear index dispersion.

The strong coupling of epsilon-near-zero materials with nanoantennas has demonstrated enhanced nonlinear optical responses, yet practical challenges persist. Here, we propose an alternative: an ultrathin metasurface featuring broadband response with a weakly dispersive nonlinear index, achieved through a simple implementation. Our metasurface, comprising a disordered gold nanorod array on indium tin oxide, exhibits polarization-independent behavior and a large average nonlinear refractive index of 5 cm2/GW across a broad wavelength range (1000-1300 nm). Enhanced performance is attributed to the weak coupling between gold nanorods and indium tin oxide, offering a cost-effective method for nonlinear optical metasurfaces and a flexible design in nanophotonic applications.

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.

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.

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.

Reply to the 'Comment on "Thermal effects - an alternative mechanism for plasmon-assisted photocatalysis"' by P. Jain, Chem. Sci., 2020, 11, DOI: 10.1039/D0SC02914A.

In his Comment to our paper "Thermal effects - an alternative mechanism for plasmon-assisted photocatalysis", Jain correctly points out that using an Arrhenius fit to the reaction rate is not enough to distinguish thermal from non-thermal effects.

Comment on "Quantifying hot carrier and thermal contributions in plasmonic photocatalysis".

Zhou et al (Reports, 5 October 2018, p. 69) claim to have proven dominance of "hot" electrons over thermal effects in plasmonic photocatalysis. We identify experimental flaws that caused overestimation of the hot carrier contribution. As an alternative interpretation, we fully reproduce their data using a purely thermal Arrhenius law with a fixed activation energy and intensity-dependent heating.

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.

Interface States and Interface-Bulk Correspondence of One-dimensional Hyperbolic Metamaterials.

We investigate the interface state on one-dimensional hyperbolic metamaterial (1DHMM). Initially, we analyze the plasmonic band structure of binary 1DHMM and analytically determine its band crossing condition. Then, we scrutinize the existence of an interface state in the plasmonic band gap of 1DHMM on three types of interfaces: dielectric/1DHMM, metal/1DHMM, and 1DHMM/1DHMM. We find that the band crossing dramatically influences the existence of an interface state. We also show a rigorous relation between the existence of the interface state of 1DHMM in the plasmonic band gap and the wave admittance in the plasmonic band region. More importantly, this relation not only holds for binary 1DHMM but also can be generalized to any 1DHMM with inversion symmetry. We also characterize the interface state by the transverse spin angular momentum and reveal the transverse spin flipping of the interface state.

Study of optical phase transduction on localized surface plasmon resonance for ultrasensitive detection.

Localized surface plasmon resonance (LSPR) has shown its remarkable applications in biosensing, bioimaging, and nanophotonics. Unlike surface plasmon polariton (SPP), the current studies regarding LSPR as biosensor were restricted in probing the extinction spectra, and thus limit the performance in biosensing and bioimaging. Here, we reveal that optical phase of LSPR provides an acute change at resonance beyond extinction spectra, which permits an ultra-high sensitivity in phase interrogation. We found that optical phases of LSPR show two orders of magnitude higher sensing resolution than extinction spectra among the same nanostructures. For the first time, we demonstrated the feasibility of probing optical phase transduction in LSPR for biosensing, and the sensitivity is superior to not only the extinction spectra among the same metallic nanostructures, but also the LSPR sensors among the current literatures. In summary, the exploitation of LSPR by phase interrogation essentially complements the sensitivity insufficiency of LSPR, and provides new access to understanding and using the rich physics of LSPR.

Asymmetric coupling between subradiant and superradiant plasmonic resonances and its enhanced sensing performance.

We present symmetric and asymmetric couplings within a pair of split-ring resonators (SRRs). The former shows a single transmittance dip, following the equivalent circuit model; yet, the latter introduces an additional transmittance peak, stemming from an asymmetrically coupled resonance (ACR) between the subradiant and superradiant modes. The mechanism of such induced transparency is elucidated well by the suppression of induced currents within the SRR element with a lower quality factor. Finally, the excitation of ACR is further associated with remarkable confinement of electromagnetic field, providing a compelling sensing performance based on its excellent sensitivity and figure of merit.