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

Probing Temperature-Induced Plasmonic Nonlinearity: Unveiling Opto-Thermal Effects on Light Absorption and Near-Field Enhancement

Precise measurement and control of local heating in plasmonic nanostructures are vital for diverse nanophotonic devices. Despite significant efforts, challenges in understanding temperature-induced plasmonic nonlinearity persist, particularly in light absorption and near-field enhancement due to the absence of suitable measurement techniques. This study presents an approach allowing simultaneous measurements of light absorption and near-field enhancement through angle-resolved near-field scanning optical microscopy with iterative opto-thermal analysis. We revealed gold thin films exhibit sublinear nonlinearity in near-field enhancement due to nonlinear opto-thermal effects, while light absorption shows both sublinear and superlinear behaviors at varying thicknesses. These observations align with predictions from a simple harmonic oscillation model, in which changes in damping parameters affect light absorption and field enhancement differently. The sensitivity of our method was experimentally examined by measuring the opto-thermal responses of three-dimensional nanostructure arrays. Our findings have direct implications for advancing plasmonic applications, including photocatalysis, photovoltaics, photothermal effects, and surface-enhanced Raman spectroscopy.

Engineering the temporal dynamics of all-optical switching with fast and slow materials

All-optical switches control the amplitude, phase, and polarization of light using optical control pulses. They can operate at ultrafast timescales - essential for technology-driven applications like optical computing, and fundamental studies like time-reflection. Conventional all-optical switches have a fixed switching time, but this work demonstrates that the response-time can be controlled by selectively controlling the light-matter-interaction in so-called fast and slow materials. The bi-material switch has a nanosecond response when the probe interacts strongly with titanium nitride near its epsilon-near-zero (ENZ) wavelength. The response-time speeds up over two orders of magnitude with increasing probe-wavelength, as light's interaction with the faster Aluminum-doped zinc oxide (AZO) increases, eventually reaching the picosecond-scale near AZO's ENZ-regime. This scheme provides several additional degrees of freedom for switching time control, such as probe-polarization and incident angle, and the pump-wavelength. This approach could lead to new functionalities within key applications in multiband transmission, optical computing, and nonlinear optics.

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.

Water-Immersion Laser-Scanning Annealing for Improving Polycrystalline Au Films

A water-immersion laser-scanning annealing (WILSA) method was developed for the heat treatment of a deposited polycrystalline Au film on a glass. The material characterization using X-ray diffraction, field-emission scanning electron microscopy, and electron backscatter diffraction shows improved crystallinity with a more uniform crystallographic orientation of (111) and the grain growth of the annealed Au film. Additionally, the optical constants of the Au film before and after annealing were characterized by spectroscopic ellipsometry in the visible to near-infrared (NIR) regime, and the corresponding optical densities (ODs) were measured by transmittance spectroscopy. Our results show that the extinction coefficient and the OD of the annealed film are significantly reduced, particularly in the NIR regime. This is because the grain growth caused by the annealing reduces the density of grain boundaries, leading to the decrease of the loss of free electrons' scattering at grain boundaries. Hence, the damping effect of the surface plasmon is reduced. Additionally, the integrity of the WILSA-treated thin film is kept intact without pinholes, usually produced by the conventional thermal annealing. Based on the improved optical property of the WILSA-treated Au film, two performances of an insulator-metal-insulator (IMI) layered structure of biosensors are theoretically analyzed. Numerical results show that the propagation length of a long-range surface plasmon polariton along an IMI structure with an annealed Au film is significantly increased, compared to an unannealed film, particular in the NIR region. For the other application of using an IMI sensor to detect the shift of the surface-plasmon-resonance dip in the total internal reflection spectrum for the measurement of a change of the medium's refractive index, the sensitivity is also profoundly improved by the WILSA method. It is worth mentioning that the optimal heating conditions (laser wavelength, fluence, exposure time, and scanning step) depend on the thickness of the Au film. Our study provides a postprocess of WILSA to improve the optical properties of a deposited polycrystalline Au film for raising the sensitivity of the related biosensors.

A Low-Temperature Annealing Method for Alloy Nanostructures and Metasurfaces: Unlocking a Novel Degree of Freedom

The material and exact shape of a nanostructure determine its optical response, which is especially strong for plasmonic metals. Unfortunately, only a few plasmonic metals are available, which limits the spectral range where these strong optical effects can be utilized. Alloying different plasmonic metals can overcome this limitation, at the expense of using a high-temperature alloying process, which adversely destroys the nanostructure shape. Here, a low-temperature alloying process is developed where the sample is heated at only 300 °C for 8 h followed by 30 min at 450 °C and Au-Ag nanostructures with a broad diversity of shapes, aspect ratios, and stoichiometries are fabricated. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses confirm the homogeneous alloying through the entire sample. Varying the alloy stoichiometry tunes the optical response and controls spectral features, such as Fano resonances. Binary metasurfaces that combine nanostructures with different stoichiometries are fabricated using multiple-step electron-beam lithography, and their optical function as a hologram or a Fresnel zone plate is demonstrated at the visible wavelength of λ = 532 nm. This low-temperature annealing technique provides a versatile and cost-effective way of fabricating complex Au-Ag nanostructures with arbitrary stoichiometry.

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.

Tailorable multifunctionalities in ultrathin 2D Bi-based layered supercell structures

Two-dimensional (2D) materials with robust ferromagnetic behavior have attracted great interest because of their potential applications in next-generation nanoelectronic devices. Aside from graphene and transition metal dichalcogenides, Bi-based layered oxide materials are a group of prospective candidates due to their superior room-temperature multiferroic response. Here, an ultrathin Bi₃Fe₂Mn₂O_10+δ layered supercell (BFMO322 LS) structure was deposited on an LaAlO₃ (LAO) (001) substrate using pulsed laser deposition. Microstructural analysis suggests that a layered supercell (LS) structure consisting of two-layer-thick Bi-O slabs and two-layer-thick Mn/Fe-O octahedra slabs was formed on top of the pseudo-perovskite interlayer (IL). A robust saturation magnetization value of 129 and 96 emu cm^-3 is achieved in a 12.3 nm thick film in the in-plane (IP) and out-of-plane (OP) directions, respectively. The ferromagnetism, dielectric permittivity, and optical bandgap of the ultrathin BFMO films can be effectively tuned by thickness and morphology variation. In addition, the anisotropy of all ultrathin BFMO films switches from OP dominating to IP dominating as the thickness increases. This study demonstrates the ultrathin BFMO film with tunable multifunctionalities as a promising candidate for novel integrated spintronic devices.

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.

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.

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.

Tracking ultrafast hot-electron diffusion in space and time by ultrafast thermomodulation microscopy

The ultrafast response of metals to light is governed by intriguing nonequilibrium dynamics involving the interplay of excited electrons and phonons. The coupling between them leads to nonlinear diffusion behavior on ultrashort time scales. Here, we use scanning ultrafast thermomodulation microscopy to image the spatiotemporal hot-electron diffusion in thin gold films. By tracking local transient reflectivity with 20-nm spatial precision and 0.25-ps temporal resolution, we reveal two distinct diffusion regimes: an initial rapid diffusion during the first few picoseconds, followed by about 100-fold slower diffusion at longer times. We find a slower initial diffusion than previously predicted for purely electronic diffusion. We develop a comprehensive three-dimensional model based on a two-temperature model and evaluation of the thermo-optical response, taking into account the delaying effect of electron-phonon coupling. Our simulations describe well the observed diffusion dynamics and let us identify the two diffusion regimes as hot-electron and phonon-limited thermal diffusion, respectively.

Metal nanospheres under intense continuous-wave illumination: A unique case of nonperturbative nonlinear nanophotonics

We show that the standard perturbative (i.e., cubic) description of the thermal nonlinear response of a single metal nanosphere to intense continuous-wave (CW) illumination is sufficient only for a temperature rise of up to 100 degrees above room temperature. Beyond this regime, the slowing down of the temperature rise requires a nonperturbative description of the nonlinear response, even though the permittivity is linearly dependent on the temperature and despite the deep subwavelength effective propagation distances involved. Using experimental data, we show that, generically, the increase of the imaginary part of the metal permittivity dominates the increase of the host permittivity as well as the resonance shift due to the joint changes to the real parts of the metal and host. Thus, the main nonlinear effect is a decrease of the quality factor of the resonance. We further analyze the relative importance of the various contributions to the temperature rise and thermal nonlinearity, compare the nonlinearity of Au and Ag, demonstrate the potential effect of the nanoparticle morphology, and show that although the thermo-optical nonlinearity of the host typically plays a minor role, its thermal conductivity and its temperature dependence is important. Finally, we discuss the differences between CW and ultrafast thermal nonlinearities.