arXiv subjects

Joel Yuen-Zhou

Publications and source records attributed to Joel Yuen-Zhou.

At least 37 records · Page 2Linked to original sources

arXiv2024

Blackbody radiation and thermal effects on chemical reactions and phase transitions in cavities

An important question in polariton chemistry is whether reacting molecules are in thermal equilibrium with their surroundings. If not, can experimental changes observed in reaction rates of molecules in a cavity (even without optical pumping) be attributed to a higher/lower temperature inside the cavity? In this work, we address this question by computing temperature differences between reacting molecules inside a cavity and the air outside. We find this temperature difference to be negligible for most reactions. On the other hand, for phase transitions inside cavities, as the temperature of the material is actively maintained by a heating/cooling source in experiments, we show cavities can modify observed transition temperatures when mirrors and cavity windows are ideal (non-absorbing); however, this modification vanishes when real mirrors and windows are used. Finally, we find substantial differences in blackbody spectral energy density between free space and infrared cavities, which reveal resonance effects and could potentially play a role in explaining changes in chemical reactivity in the dark.

physics.chem-ph↗

arXiv2024

Extracting accurate light-matter couplings from disordered polaritons

The vacuum Rabi splitting (VRS) in molecular polaritons stands as a fundamental measure of collective light-matter coupling. Despite its significance, the impact of molecular disorder on VRS is not fully understood yet. This study delves into the complexities of VRS amidst various distributions and degrees of disorder. Our analysis provides precise analytical expressions for linear absorption, transmission, and reflection spectra, along with a "sum" rule, offering a straightforward protocol for extracting accurate collective light-matter coupling values from experimental data. Importantly, our study cautions against directly translating large VRS to the onset of ultrastrong coupling regime. Furthermore, for rectangular disorder, we witness the emergence of narrow side bands alongside a broad central peak, indicating an extended coherence lifetime even in the presence of substantial disorder. These findings not only enhance our understanding of VRS in disordered molecular systems but also open avenues for achieving prolonged coherence lifetimes between the cavity and molecules via the interplay of collective coupling and disorder.

physics.chem-ph↗

arXiv2024

Precision Franck-Condon spectroscopy from highly-excited vibrational states

As per the Franck-Condon principle, absorption spectroscopy reveals changes in nuclear geometry in molecules or solids upon electronic excitation. It is often assumed these changes cannot be resolved beyond the ground vibrational wavefunction width ($\sqrt{\hbar/mω}$). Here, we show this resolution dramatically improves with highly-excited vibrational initial states (with occupation number $\langle n\rangle$). These states magnify changes in geometry by $2\langle n\rangle +1$, a possibly counterintuitive result given the spatial uncertainty of Fock states grows with $n$. We also discuss generalizations of this result to multimode systems. Our result is relevant to optical spectroscopy, polariton condensates, and quantum simulators ($\textit{e.g.}$, boson samplers).

physics.chem-ph↗

arXiv2023

Linear response of molecular polaritons

In this article, we show that the collective light-matter strong coupling regime, where $N$ molecular emitters couple to the photon mode of an optical cavity, can be mapped to a quantum impurity model where the photon is the impurity that is coupled to a bath of anharmonic transitions. In the thermodynamic limit where $N\gg1$, we argue that the bath can be replaced with an effective harmonic bath, leading to a dramatic simplification of the problem into one of coupled harmonic oscillators. We derive simple analytical expressions for linear optical spectra (transmission, reflection, and absorption) where the only molecular input required is the molecular linear susceptibility. This formalism is applied to a series of illustrative examples showcasing the role of temperature, disorder, vibronic coupling, and optical saturation of the molecular ensemble, explaining that it is useful even when describing an important class of nonlinear optical experiments. For completeness, we provide a comprehensive Appendix that includes a self-contained derivation of the relevant spectroscopic observables for arbitrary anharmonic systems (for both large and small $N$) within the rotating-wave approximation. While some of the presented results herein have already been reported in the literature, we provide a unified presentation of the results as well as new interpretations that connect powerful concepts in open quantum systems and linear response theory with molecular polaritonics.

quant-ph↗

arXiv2023

Molecular and solid-state topological polaritons induced by population imbalance

Strong coupling between electronic excitations in materials and photon modes results in the formation of polaritons, which display larger nonlinearities than their photonic counterparts due to their material component. We theoretically investigate how to optically control the topological properties of molecular and solid-state exciton-polariton systems by exploiting one such nonlinearity: saturation of electronic transitions. We demonstrate modification of the Berry curvature of three different materials when placed within a Fabry-Perot cavity and pumped with circularly polarized light, illustrating the broad applicability of our scheme. Importantly, while optical pumping leads to non-zero Chern invariants, unidirectional edge states do not emerge in our system as the bulk-boundary correspondence is not applicable. This work demonstrates a versatile approach to control topological properties of novel optoelectronic materials.

physics.chem-ph↗

arXiv2023

Direct observation of geometric phase in dynamics around a conical intersection

Conical intersections are ubiquitous in chemistry and physics, often governing processes such as light harvesting, vision, photocatalysis, and chemical reactivity. They act as funnels between electronic states of molecules, allowing rapid and efficient relaxation during chemical dynamics. In addition, when a reaction path encircles a conical intersection, the molecular wavefunction experiences a geometric phase, which can affect the outcome of the reaction through quantum-mechanical interference. Past experiments have measured indirect signatures of geometric phases in scattering patterns and spectroscopic observables, but there has been no direct observation of the underlying wavepacket interference. Here, we experimentally observe geometric-phase interference in the dynamics of a wavepacket travelling around an engineered conical intersection in a programmable trapped-ion quantum simulator. To achieve this, we develop a technique to reconstruct the two-dimensional wavepacket densities of a trapped ion. Experiments agree with the theoretical model, demonstrating the ability of analog quantum simulators -- such as those realised using trapped ions -- to accurately describe nuclear quantum effects.

quant-ph↗

arXiv2023

Thermodynamic coupling of reactions via few-molecule vibrational polaritons

Interaction between light and molecular vibrations leads to hybrid light-matter states called vibrational polaritons. Even though many intriguing phenomena have been predicted for single-molecule vibrational strong coupling (VSC), several studies suggest that these effects tend to be diminished in the many-molecule regime due to the presence of dark states. Achieving single or few-molecule vibrational polaritons has been constrained by the need for fabricating extremely small mode volume infrared cavities. In this work, we propose an alternative strategy to achieve single-molecule VSC in a cavity-enhanced Raman spectroscopy (CERS) setup, based on the physics of cavity optomechanics. We then present a scheme harnessing few-molecule VSC to thermodynamically couple two reactions, such that a spontaneous electron transfer can now fuel a thermodynamically uphill reaction that was non-spontaneous outside the cavity.

physics.chem-ph↗

arXiv2023

Swinging between shine and shadow: Theoretical advances on thermally-activated vibropolaritonic chemistry (a perspective)

Polariton chemistry has emerged as an appealing branch of synthetic chemistry that promises mode selectivity and a cleaner approach to kinetic control. Of particular interest are the numerous experiments in which reactivity has been modified by virtue of performing the reaction inside infrared optical microcavities in the absence of optical pumping; this effort is known as "vibropolaritonic chemistry." The optimal conditions for these observations are (1) resonance between cavity and reactive modes at normal incidence ($k=0$), and (2) monotonic increase of the effect with the concentration of emitters in the sample. Importantly, vibropolaritonic chemistry has only been experimentally demonstrated in the so-called "collective" strong coupling regime, where there is a macroscopic number of molecules (rather than a single molecule) coupled to each photon mode of the microcavity. Strikingly, efforts to understand this phenomenon from a conceptual standpoint have encountered several roadblocks and no single, unifying theory has surfaced thus far. This perspective documents the most relevant approaches taken by theorists, laying out the contributions and unresolved challenges from each work. We expect this manuscript to not only serve as a primer for experimentalists and theorists alike, but also inform future endeavors in the quest for the ultimate formalism of vibropolaritonic chemical kinetics.

physics.chem-ph↗

arXiv2023

Chiral edge waves in a dance-based human topological insulator

Topological insulators are insulators in the bulk but feature chiral energy propagation along the boundary. This property is topological in nature and therefore robust to disorder. Originally discovered in electronic materials, topologically protected boundary transport has since been observed in many other physical systems. Thus, it is natural to ask whether this phenomenon finds relevance in a broader context. We choreograph a dance in which a group of humans, arranged on a square grid, behave as a topological insulator. The dance features unidirectional flow of movement through dancers on the lattice edge. This effect persists when people are removed from the dance floor. Our work extends the applicability of wave physics to the performance arts.

cond-mat.mes-hall↗

arXiv2023

Vibropolaritonic Reaction Rates in the Collective Strong Coupling Regime: Pollak-Grabert-Hänggi Theory

Following experimental evidence that vibrational polaritons, formed from collective vibrational strong coupling (VSC) in optical microcavities, can modify ground-state reaction rates, a spate of theoretical explanations relying on cavity-induced frictions has been proposed through the Pollak-Grabert-Hänggi (PGH) theory, which goes beyond transition state theory (TST). However, by considering only a single reacting molecule coupled to light, these works do not capture the ensemble effects present in experiments. Moreover, the relevant light-matter coupling should have been $\sqrt{N}$ times smaller than those used by preceding works, where $N\approx10^{6}-10^{12}$ is the ensemble size. In this work, we explain why this distinction is significant and can nullify effects from these cavity-induced frictions. By analytically extending the cavity PGH model to realistic values of $N$, we show how this model succumbs to the polariton "large $N$ problem", that is, the situation whereby the single reacting molecule feels only a tiny $1/N$ part of the collective light-matter interaction intensity, where $N$ is large.

physics.chem-ph↗

arXiv2023

Understanding the Energy Gap Law under Vibrational Strong Coupling

The rate of non-radiative decay between two molecular electronic states is succinctly described by the energy gap law, which suggests an approximately-exponential dependence of the rate on the electronic energy gap. Here, we inquire whether this rate is modified under vibrational strong coupling, a regime whereby the molecular vibrations are strongly coupled to an infrared cavity. We show that, under most conditions, the collective light-matter coupling strength is not large enough to counter the entropic penalty involved with using the polariton modes, so the energy gap law remains unchanged. This effect (or the lack thereof) may be reversed with deep strong light-matter couplings or large detunings, both of which increase the upper polariton frequency. Finally, we demonstrate how vibrational polariton condensates mitigate the entropy problem by providing large occupation numbers in the polariton modes.

physics.chem-ph↗

arXiv2022

Collective dynamics Using Truncated Equations (CUT-E): simulating the collective strong coupling regime with few-molecule models

The study of molecular polaritons beyond simple quantum emitter ensemble models (e.g., Tavis-Cummings) is challenging due to the large dimensionality of these systems (the number of molecular emitters is $N\approx 10^{6}-10^{10}$) and the complex interplay of molecular electronic and nuclear degrees of freedom. This complexity constraints existing models to either coarse-grain the rich physics and chemistry of the molecular degrees of freedom or artificially limit the description to a small number of molecules. In this work, we exploit permutational symmetries to drastically reduce the computational cost of \textit{ab-initio} quantum dynamics simulations for large $N$. Furthermore, we discover an emergent hierarchy of timescales present in these systems, that justifies the use of an \textit{effective} single molecule to approximately capture the dynamics of the entire ensemble, an approximation that becomes exact as $N\rightarrow \infty$. We also systematically derive finite $N$ corrections to the dynamics, and show that addition of $k$ extra effective molecules is enough to account for phenomena whose rates scale as $\mathcal{O}(N^{-k})$. Based on this result, we discuss how to seamlessly modify existing single-molecule strong coupling models to describe the dynamics of the corresponding ensemble, as well as the crucial differences in phenomena predicted by each model. We call this approach Collective dynamics Using Truncated Equations (CUT-E), benchmark it against well-known results of polariton relaxation rates, and apply it to describe a universal cavity-assisted energy funneling mechanism between different molecular species. Beyond being a computationally efficient tool, this formalism provides an intuitive picture for understanding the role of bright and dark states in chemical reactivity, necessary to generate robust strategies for polariton chemistry.

quant-ph↗

arXiv2022

Driving chemical reactions with polariton condensates

When molecular transitions strongly couple to photon modes, they form hybrid light-matter modes called polaritons. Collective vibrational strong coupling is a promising avenue for control of chemistry, but this can be deterred by the large number of quasi-degenerate dark modes. The macroscopic occupation of a single polariton mode by excitations, as observed in Bose-Einstein condensation, offers promise for overcoming this issue. Here we theoretically investigate the effect of vibrational polariton condensation on the kinetics of electron transfer processes. Compared with excitation with infrared laser sources, the condensate changes the reaction yield significantly due to additional channels with reduced activation barriers resulting from the large accumulation of energy in the lower polariton, and the many modes available for energy redistribution during the reaction. Our results offer tantalizing opportunities to use condensates for driving chemical reactions, kinetically bypassing usual constraints of fast intramolecular vibrational redistribution in condensed phase.

physics.chem-ph↗

arXiv2022

Enantioselective Topological Frequency Conversion

Two molecules are enantiomers if they are non-superimposable mirror images of each other. Electric dipole-allowed cyclic transitions $|1\rangle\to|2\rangle\to|3\rangle\to|1\rangle$ obey the symmetry relation $\mathcal{O}^{R}=-\mathcal{O}^{S}$, where $\mathcal{O}^{R,S}=(μ_{21}^{R,S}E_{21})(μ_{13}^{R,S}E_{13})(μ_{32}^{R,S}E_{32})$, and $R,S$ label the two enantiomers. Here we generalize the concept of topological frequency conversion to an ensemble of enantiomers. We show that, within a rotating-frame, the pumping power between fields of frequency $ω_{1}$ and $ω_{2}$ is sensitive to enantiomeric excess, $\mathcal{P}_{2\to1}=\hbar\frac{ω_{1}ω_{2}C_{L}^{R}}{2π}(N_{R}-N_{S})$, where $N_{i}$ is the number of enantiomers $i$ and $C_{L}^{R}$ is an enantiomer-dependent Chern number. Connections with chiroptical microwave spectroscopy are made. Our work provides an underexplored and fertile connection between topological physics and molecular chirality.

quant-ph↗

arXiv2022

Generalization of the Tavis-Cummings model for multi-level anharmonic systems: insights on the second excitation manifold

Confined electromagnetic modes strongly couple to collective excitations in ensembles of quantum emitters, producing light-matter hybrid states known as polaritons. Under such conditions, the discrete multilevel spectrum of molecular systems offers an appealing playground for exploring multiphoton processes. This work contrasts predictions from the Tavis-Cummings (TC) model, in which the material is a collection of two-level systems, with the implications of considering additional energy levels with harmonic and anharmonic structures. We discuss the exact eigenspectrum, up to the second excitation manifold, of an arbitrary number $N$ of oscillators collectively coupled to a single cavity mode in the rotating-wave approximation. Elaborating on our group-theoretic approach [New J. Phys. 23, 063081 (2021)], we simplify the brute-force diagonalization of a gigantic $N^2\times N^2$ Hamiltonian (where $N=10^6-10^{10}$, as experiments suggest) to the diagonalization of, at most, $4\times4$ matrices. We thoroughly discuss the eigenstates and the consequences of weak and strong anharmonicities. Furthermore, we find resonant conditions between bipolaritons and anharmonic transitions where two-photon absorption can be enhanced. Finally, we conclude that energy shifts in the polaritonic states induced by anharmonicities become negligible for large $N$. Thus, calculations with a single or few emitters qualitatively fail to represent the nonlinear optical response of the collective strong coupling regime. Our work highlights the rich physics of multilevel anharmonic systems coupled to cavities absent in standard models of quantum optics. We also provide concise tabulated expressions for eigenfrequencies and transition amplitudes, which should serve as a reference for future spectroscopic studies of molecular polaritons.

quant-ph↗

arXiv2022

Catalysis by Dark States in Vibropolaritonic Chemistry

Collective strong coupling between a disordered ensemble of $N$ localized molecular vibrations and a resonant optical cavity mode gives rise to 2 polariton and $N-1\gg2$ dark modes. Thus, experimental changes in thermally-activated reaction kinetics due to polariton formation appear entropically unlikely and remain a puzzle. Here we show that the overlooked dark modes, while parked at the same energy as bare molecular vibrations, are robustly delocalized across $\sim$2-3 molecules, yielding enhanced channels of vibrational cooling, concomitantly catalyzing a chemical reaction. As an illustration, we theoretically show a $\approx$50\% increase in an electron transfer rate due to enhanced product stabilization. The reported effects can arise when the homogeneous linewidths of the dark modes are smaller than their energy spacings.

quant-ph↗

arXiv2021

Nonequilibrium effects of cavity leakage and vibrational dissipation in thermally-activated polariton chemistry

In vibrational strong coupling (VSC), molecular vibrations strongly interact with the modes of an optical cavity to form hybrid light-matter states known as vibrational polaritons. Experiments show that the kinetics of thermally activated chemical reactions can be modified by VSC. Transition-state theory, which assumes that internal thermalization is fast compared to reactive transitions, has been unable to explain the observed findings. Here, we carry out kinetic simulations to understand how dissipative processes, namely those that VSC introduces to the chemical system, affect reactions where internal thermalization and reactive transitions occur on similar timescales. Using the Marcus-Levich-Jortner type of electron transfer as a model reaction, we show that such dissipation can change reactivity by accelerating internal thermalization, thereby suppressing nonequilibrium effects that occur in the reaction outside the cavity. This phenomenon is attributed mainly to cavity decay (i.e., photon leakage), but a supporting role is played by the relaxation between polaritons and dark states. When nonequilibrium effects are already suppressed in the bare reaction (the reactive species are essentially at internal thermal equilibrium throughout the reaction), we find that reactivity does not change significantly under VSC. Connections are made between our results and experimental observations.

quant-ph↗

arXiv2021

Generalization of the Tavis-Cummings model for multi-level anharmonic systems

The interaction between anharmonic quantum emitters (e.g., molecular vibrations) and confined electromagnetic fields gives rise to quantum states with optical and chemical properties that are different from those of their precursors. The exploration of these properties has been typically constrained to the first excitation manifold, the harmonic approximation, ensembles of two-level systems [Tavis-Cummings (TC) model], or the anharmonic single-molecule case. The present work studies, for the first time, a collective ensemble of identical multi-level anharmonic emitters and their dipolar interaction with a photonic cavity mode. The permutational properties of the system allow identifying symmetry classified submanifolds in the energy spectrum. Notably, in this approach, the number of particles, typically in the order of several millions, becomes only a parameter from the operational standpoint, and the size of the dimension of the matrices to diagonalize is independent of it. The formalism capabilities are illustrated by showing the energy spectrum structure, up to the third excitation manifold, and the calculation of the photon contents as a permutationally invariant quantity. Emphasis is placed on (a) the collective (superradiant) scalings of light-matter couplings and the various submanifolds of dark (subradiant) states with no counterpart in the single-molecule case, as well as (b) the delocalized modes containing more than one excitation per molecule with no equivalent in the TC model. We expect these findings to be applicable in the study of non-linear spectroscopy and chemistry of polaritons.

physics.optics↗