Session W17: Chemical Physics of Plasmonic Nanostructures: Hot Carriers and Strong Coupling

Spectroscopic probes of plasmon-driven chemical reactions

Plasmonic materials are highly promising catalysts for driving energetically unfavorable chemical reactions with sunlight, due to their large optical cross sections and ability to generate a number of hot holes and electrons. However, the efficiencies of most plasmon-driven processes are quite low, likely due to the lack of mechanistic understanding of the underlying physical processes. Plasmons can concentrate electromagnetic fields, can generate highly energetic electrons and holes, and can heat up local environments. An understanding of the energy partitioning into each of these processes is crucial to the design of plasmonic photocatalysts which are optimized for chemical selectivity. Here I'll discuss our development of ultrafast surface-enhanced Raman spectroscopy (SERS) to probe the contributions of plasmon-generated hot electron transfer, heating, and vibrational energy transfer on timescales relevant to photocatalysis. Specifically, we are able to quantify plasmon-driven charge transfer processes by monitoring the rate and yield of reduced molecular adsorbates, as well as monitoring energy transfer and heating processes through ultrafast Raman thermometry. These efforts in developing a fundamental understanding of plasmon-mediated processes in molecules will ultimately aid in the rational design of cost-effective plasmonic materials capable of driving industrially relevant chemistries using solar radiation.   

Cavity-Enabled Enhancement of Ultrafast Intramolecular Vibrational Redistribution over Pseudorotation

Vibrational Strong Coupling (VSC) between molecular vibrations and microcavity photons yields a few polaritons (light-matter modes) and many dark modes (with negligible photonic character). Although VSC is reported to alter thermally-activated chemical reactions, its mechanisms remain opaque. To shed light on this problem, we followed ultrafast dynamics of a simple unimolecular vibrational energy exchange in Fe(CO)₅ under VSC, which showed two competing channels: pseudorotation and intramolecular vibrational-energy redistribution (IVR). We found that, under polariton excitation, energy exchange was overall accelerated, with IVR becoming faster and pseudorotation being slowed down. However, dark mode excitation revealed unchanged dynamics compared to outside of cavity, with pseudorotation dominating. Thus, despite controversies of thermally-activated VSC modified chemistry, our work showed VSC can indeed alter chemistry upon non-equilibrium preparation of polaritons.

One-Sentence Summary: Vibrational polariton alters ultrafast molecular dynamics in liquid phase, and dark modes show no influences.   

Optical Control over Thermal Distributions in Topologically Trivial and Non-Trivial Plasmon Lattices

A consequence of thermal diffusion is that heat, even when applied to a localized region of space, has the tendency to produce a temperature change that is spatially uniform throughout a material. The degree of spatial correlation between the heat power supplied and the temperature change that it induces is therefore likely to be small. Yet, the ability to control heat flow and thus temperature at both nanoscale (   

Room-temperature strong coupling in plasmonic nanocavities

Coupling optical and vibrational transitions in molecular or solid-state systems to a single mode of an optical cavity has the potential to enable nonlinear-optical applications and control of chemical pathways. These applications arise both in the strong-coupling regime and in the high-cooperativity regimes, which both require coupling strengths to be large compared to decoherence rates of the emitter and of the cavity photons. In photonic cavities, the diffraction limit places a minimum on the mode volume and thus a maximum on the coupling strength; strong coupling in these cavities therefore generally requires operation at cryogenic temperatures when small numbers of emitters are involved. Using plasmonic nanocavities overcomes this restriction, enabling high cooperativity and strong coupling at room temperature with a single quantum dot, ultrastrong coupling in the infrared with a microscopic volume of material, and second-harmonic generation from strongly-coupled states.   

Exploring Hot Electron Behavior in the Steady State

Plasmonic nanostructures could provide new pathways for photocatalysis. Upon illumination, non-thermal carriers are generated, which later thermalize to generate “hot” carriers. These carriers relax to elevate the lattice temperature of the metal. Hot electrons are often envisioned to be used under steady state (SS) conditions which may impart distinct electronic behavior compared to under pulsed conditions which are used in hot electron ultra-fast studies.

To quantify hot electron dynamics under SS conditions, our lab has developed a continuous wave Raman spectroscopy method that probes non-equilibrium electron behavior under SS optical excitation conditions using two Fermi-Dirac distributions with distinct temperatures: the hot electron temperature and the lattice temperature.

In this talk, I will discuss our studies of interband versus intraband excitation of a gold nanostructures. Although interband (532 nm) and intraband excitation (658 nm) have different carrier excitation mechanisms, theoretically there is no expected difference in carrier populations after carriers have thermalized. Our studies, however, show clear differences in hot electron temperature and population size in the SS. We observe increases of hot electron lifetime up to 217% for intra- versus interband excitation and increases of hot electron temperature up to 145%. We discuss possible interpretations for this observation, which exemplifies the importance of probing hot electrons in the steady state.

    

Strong Coupling, Disordered Environments, and Chemistry Under Illumination

In this talk, I will highlight our recent work in the area of polaritonics and molecules in cavities, whereby we have sought to understand how molecules collectively subjected to strong light-matter coupling can change their behavior given the weak coupling of one individual molecule to a field. I will focus on molecules under illumination, with and without disorder, and I will show what interesting features emerge and what interesting features do not emerge under our simulations. In particular, I will highlight the possibility that one can use strong coupling to achieve large localization of vibrational excitation. Finally, I will propose a few new directions for modeling nonadiabatic motion in the presence of strong light-matter coupling.   

Absorber-Specific Dynamics in Vibration-Cavity Polaritons

Strong coupling between an optical mode and an ensemble of molecular vibrations creates new vibrationcavity polariton modes, whose very presence can strongly modify reaction dynamics even without excitation. The ultrafast nonlinear spectroscopy of the polaritonic system, comprising the polariton modes and a reservoir of dark states, might serve to elucidate the mechanism of the reaction modification as well as shed light on novel applications in photonics. Thus far, studies of ultrafast dynamics under strong coupling have largely been limited to hexacarbonyls in solution. Here, we present our recent results from nitroprusside and thiocyanate ions, which have strong infrared transitions but are distinct from hexacarbonyls in several important respects. We discuss the similarities and differences between the vibration-cavity polariton dynamics in these systems and how the results solidify our understanding of polariton dynamics in general.   

Energy transport in disordered polaritonic materials

Despite several observations of modified photochemistry, thermal reactivity and ultrafast intermolecular energy transfer under strong light-matter interactions, fundamental questions remain on the theoretical limits and optimal conditions for achieving control of molecular materials with optical microcavities. In this presentation, we will report our recent progress towards the identification of parameters leading to a maximization of the rate and efficiency of energy transport in disordered polaritonic materials. We will describe our theoretical models and key results including quantitative estimates of (polariton-mediated) intermolecular energy diffusion, polariton wave packet propagation and localization, and cavity-induced intermolecular correlations in binary mixtures. Connections to recent experiments and implications for polariton chemistry model building will be highlighted.   

Entangling molecules in the strong coupling regime

The control of atoms has enabled the creation of high-fidelity entangling gates for quantum computation [1] and quantum simulation [2]. Being able to control molecules at a quantum level, would give access to further degrees of freedom such as the vibrational or rotational degrees to the internal state structure. Entangling those degrees of freedom offers unique opportunities in quantum information processing, especially in the construction of quantum memories. A typical way to control physical and chemical processes is by coupling the material system to the quantized electromagnetic field within photonic environments [3]. In this work, we leverage this scheme to achieve molecule-molecule entanglement [4]. In particular, we consider two identical molecules spatially separated by a variable distance within a photonic environment in a high-Q infrared cavity. By resonantly coupling the effective cavity mode to a specific vibrational frequency of both molecules, we investigate how strong light-matter coupling can be used to control entanglement between vibrational quantum states of both molecules.

[1] D. Kienzler et al., Phys. Rev. Lett. 116, 140402 (2016).

[2] B. Vlastakis et al., Science 342, 607 (2013).

[3] J. A. Hutchison et al., Angewandte Chemie International Edition 51, 1592 (2012).

[4] R. Horodecki et al., Rev. Mod. Phys. 81, 865 (2009).   

Multimode ultra-strong and strong coupling to Mid-IR vibrational transitions in direct laser written plasmonic nano-patch antennas

Plasmonic resonators have become a key system for enhancing light-matter interactions due to its deep subwavelength confinement and enhanced field strengths; with metal-insulator-metal (MIM) plasmonic nano-patch antennas or cavities achieving some of the largest field confinements.  Applying these systems to the field of vibrational strong coupling (VSC) is of particular importance as the relatively large volumes of the mid-IR photonic cavities and weak oscillator strengths of vibrational transitions restricts VSC studies and devices to large ensembles of molecules, thus representing a potential limitation of this promising field.  In this work, we employ a novel multimode L-shaped MIM cavity to enable multimode strong coupling between two plasmonic modes, resonances associated with the length each cavity arm, and a vibrational transition in the insulator material. Strong and ultra-strong splittings are observed in the three-branched system. In addition to the deeply wavelength mode volumes <<(λ/2n)³ realized, the 3D-printed nature of these attoliter-sized nano-cavities offer the capability to explore intricate plasmonic cavity designs that would be required for VSC applications such as controlling chemical reactions and creating chemical sensors.   

Plasmonic substrates modify phase transitions via vibrational strong coupling

Vibrational strong coupling (VSC) has been explored recently as a means to alter chemical reactions. When molecules exchange energy with an optical cavity faster than losses to the environment, the strong coupling regime is reached, primarily characterized by Rabi splitting of the original eigenstates into upper and lower polariton modes, which are hybridized light-matter states. Usually, Fabry–Pérot (F.P.) cavities are used for coupling due to their high Q-factors; however, the chemical effects that can be expected are intrinsically limited by diploe misorientation, relatively small coupling per molecule, and the large number of dark modes. To better understand the limitations this imposes for modifying chemical properties, we have developed plasmonic substrates as an alternative optical platform for achieving VSC with surface-deposited molecules. Plasmonic substrates were designed to provide angle-independent coupling to ensembles of molecules, which may help decrease the number of unperturbed molecules during VSC. We are currently monitoring how the plasmonic substrates change the dehydration temperature Copper sulfate pentahydrate. This analyte is advantageous because it undergoes four dehydration states that can be monitored as a function of temperature using Raman spectroscopy. To do date, we have observed the first direct evidence of modified phase transition temperatures due to VSC. Furthermore, confocal Raman mapping of the “open cavity” platform allows us to probe where in space the highest degree of modification takes place. It appears that the dehydration occurs at lower temperatures in the optical “hotspots” of the coupled plasmonic geometry. This work has the potential to be generalized to broad range of molecular and plasmonic systems, opening new pathways in plasmon-mediated chemical reactions and how they can be impacted by strong coupling.