arXiv subjects

Luca Nils Philipp

Publications and source records attributed to Luca Nils Philipp.

9 recordsLinked to original sources

arXiv2026

High-Order Response Functions with Duschinsky Coupling and Finite Temperature: Application to Two-Dimensional Resonance Raman Spectroscopy

In this work, we derive closed-form expressions for multi-time correlation functions underlying higher-order nonlinear spectroscopic response functions within the harmonic approximation. Our method includes displacements, frequency changes, and Duschinsky rotation between the vibrational modes of different electronic states while retaining polynomial scaling with the number of degrees of freedom. Finite temperature is accounted for without summation over vibrational eigenstates. To show the capabilities of our method, we apply it to calculate fifth-order two-dimensional resonance Raman (2DRR) spectra of two-mode model systems and naphthalene. Comparison with the displaced harmonic oscillator model shows that Duschinsky rotation modifies the positions and relative intensities of diagonal and cross peaks. Overall, our results demonstrate that 2DRR spectroscopy is particularly sensitive to changes in normal coordinates between electronic states and can provide distinct signatures of Duschinsky coupling in molecular systems.

physics.chem-ph↗

arXiv2026

Disorder-induced Dark States Line Shape in Pump-Probe Spectroscopy of Polaritons

The formation of hybrid light--matter states called polaritons provides a route to shape the photophysics and photochemistry of molecules. Accordingly, the dynamics of polaritons following photoexcitation is extensively studied. In particular, the role of the dark state manifold in such processes remains unclear. Here, we investigate the line shape of pump-probe spectra of polaritons emerging at the dark states energy under the influence of disorder. Previously, we already investigated the pump-probe line shapes of polaritons in a disorder-free model and identified distinct signatures of relaxation into dark states, thus providing an indirect probe of this relaxation. Since the transition dipole moment of dark states vanishes in a disorder-free model, they cannot be directly probed. However, dark states acquire small transition dipole moments as soon as disorder is explicitly included in the model enabling them to be directly probed. In this work, we demonstrate that the inclusion of the relaxation to dark states and disorder leads to the evolution of the spectral shape at the DS energy from a derivative-like into an absorptive line shape. Furthermore, we investigate the dependence of the line shape on the disorder strength and its asymptotic scaling for a large number of coupled molecules. Our results demonstrate that probing dark states can help to single out the polaritonic response from the total signal.

physics.chem-ph↗

arXiv2026

Exciton-Exciton and Exciton-Photon Annihilation in Polaritonic Systems

Strong light--matter interactions forming hybrid quasiparticles termed polaritons can specifically tailor molecular photophysics. In this spirit, enhancing energy transport has recently been of special interest. Exciton--exciton annihilation is commonly used to quantify energy transfer in excitonic systems, and has been recently applied to investigate transport dynamics in polaritonic systems. However, the interpretation of experimental findings is challenging without a microscopic theory that accounts for the various nonradiative relaxation channels determining the quasiparticle diffusion length. In this work, we develop a microscopic model for polariton annihilation based on exciton--exciton annihilation and propose an exciton--photon annihilation as the decisive process that competes with exciton--exciton annihilation. The interplay between exciton--exciton and exciton--photon annihilation ultimately governs the annihilation dynamics and sets the fundamental limit to the transport efficiency. Our model explains recent experimental results and demonstrates that increased annihilation rates might serve as an explicit fingerprint to differentiate between the response of polaritons and other untargeted effects.

physics.chem-ph↗

arXiv2025

Competing excitonic couplings as origin of mimicked phase transitions in zinc-phthalocyanine single crystals

The optical properties of molecular crystals are largely determined by the excitonic coupling of neighboring molecules. This coupling is extremely sensitive to the arrangement of adjacent molecular units, as their electronic interaction is defined by the relative orientation of the individual transition dipole moments and their wave function overlap. Hence, the optical properties, such as fluorescence, are usually highly anisotropic and good indicators of structural changes during the variation of intensive thermodynamic parameters like temperature or pressure. Here, we discuss the peculiar though archetypical case of $β$-phase zinc-phthalocyanine: In single crystals, we report a sudden change of spectral emission with temperature from a broad, unpolarized Frenkel-exciton type luminescence to a narrow, highly polarized superradiance-like fluorescence below 80 K. Surprisingly, we find that there is no sign of a discrete structural phase transition in this temperature regime. To understand this apparent contradiction, we perform polarization-, temperature- and time-dependent photoluminescence measurements along different crystallographic directions to fully map the emission characteristics of the crystal-exciton. By means of ab-initio calculations on a density functional theory level we conclude that our observations are consistent with a dimer exciton model when considering thermalized electronic states. As such, our study presents a representative case study on a well-established molecular material class demonstrating that caution is advised when attributing discrete changes in electronic observables to a structural phase transition. As we show for zinc-phthalocyanine in its $β$-phase modification, slowly varying excitonic couplings and thermal redistribution of excitations can mimic the same signatures attributed to a structural phase transition.

cond-mat.mtrl-sci↗

arXiv2025

Probing plexciton dynamics with higher-order spectroscopy

Coupling molecular transition dipole moments to surface-plasmon polaritons (SPPs) results in the formation of new optical quasiparticles, i.e., plexcitons. Mixing the specific properties of matter excitations and light modes has proven to be an efficient strategy to alter a variety of molecular processes ranging from chemical reactions to exciton transport. Here, we investigate energy transfer in a plexcitonic system of zinc phthalocyanine (ZnPc) molecules aggregated in the crystalline α-phase and an SPP on a planar gold surface. By tuning the angle of incidence, we vary the degree of mixing between excitonic and SPP character of the excited state. We apply our recently developed higher-order pump-probe spectroscopy to separate the system's fifth-order signal describing the dynamics of two-particle interactions. The time it takes for two quasiparticles to meet and annihilate is a measure of their movement and thus the transport of excitation energy in the system. We find that the transport extracted from the fifth-order signal is surprisingly unaffected by the mixing ratio of exciton and SPP contributions of the plexciton. Using a rate equation model, we explain this behavior by fast transition from the plexcitonic states to many localized excitonic dark states that do not have an SPP contribution. Our results give an indication of how hybrid exciton-plasmon systems should be designed to exploit the delocalization of the involved plasmon modes for improved transport.

physics.chem-ph↗

arXiv2025

Lineshapes in Pump-Probe Spectroscopy of Polaritons

Forming new hybrid quasiparticles by strong light-matter coupling is a promising tool for tailoring photophysics and photochemistry of molecules. Thus, the ultrafast dynamics of polaritons formed upon strong light-matter coupling has been extensively studied by pump-probe spectroscopy. Although it was predicted that the partial photonic character of polaritons should shorten their lifetime compared to purely molecular excited states, many studies do not observe this effect. So far, the unexpected longevity of the spectral signatures was either explained by relaxation into a manifold of so-called dark states or by other uncontrolled effects that change the properties of cavity materials. In order to resolve these issues, we investigate here the dependence of the lineshape of pump-probe spectra of polaritons on the ratio of photonic and molecular character. Furthermore, by phenomenologically including relaxation to dark states, we find that it is possible to spectrally resolve this relaxation process by observing a characteristic phase flip in the pump-probe signal. Our results show that the signatures of various effects and their contributions to the polariton dynamics can be disentangled from the spectral lineshapes.

physics.chem-ph↗

arXiv2025

DIALECT, a software package for exciton spectra and dynamics in large molecular assemblies from weak to strong light-matter coupling regimes

The software package DIALECT is introduced, which provides the capability of calculating excited-state properties and nonadiabatic dynamics of large molecular systems and can be applied to simulate energy and charge-transfer processes in molecular materials. To this end, we employ the FMO-LC-TDDFTB methodology, which combines the use of the fragment molecular orbital approach with the density-functional tight-binding method and an excitonic Hamiltonian including local and charge-transfer excitations. In this work, we present the features and capabilities of the DIALECT software package in simulating the excited state dynamics of molecules and molecular aggregates using exemplary trajectory surface hopping as well as decoherence corrected Ehrenfest dynamics calculations in the framework of LC-TDDFTB and FMO-LC-TDDFTB. In addition, the capability of simulating the polaritonic excited state properties is highlighted by the calculation of the polariton dispersion of an aggregate of naphthalene molecules. The development of the DIALECT program will facilitate the investigation of exciton and charge transport in large and complex molecular systems, such as biological aggregates, nanomaterials and other complex organic molecular systems.

physics.chem-ph↗

arXiv2024

Anisotropic Photo-Physical Properties of Plexcitons in Strongly Coupled Metal-Organic Thin Films

Exciton plasmon polaritons have gained increasing interests over recent years due to their versatile properties emerging by the underlying light-matter coupling and making them potential candidates for new photonic applications. We have advanced this concept by studying thin films of laterally aligned J-type aggregates of self-assembled tetra-bay phenoxy-dendronized perylene bisimide (PBI) molecules, arranged in a helical manner of three strains on a silver surface. As a result of the interaction between the uniformly aligned dipole moments and the surface plasmons of a thin silver layer underneath, the excitonic state at 1.94 eV evolves into dispersions in absorption and emission, both characterized by a distinct anisotropy. The coupling constant defined by the scalar product of the transition dipole moment $\vecμ$ and the surface plasmon wavevector $\vec{k}_x$ shows a pronounced two-fold rotational symmetry with values between almost 0 to 28 meV. Complementary TD-DFT calculations of the angular dependent absorption and photoluminescence provide insights in the coherent energy exchange between the excitonic and plasmonic sub-systems. Additionally, power dependent PL studies yield first evidence that the diffusion length of the coupled exciton-plasmon polaritons exceeds that of the mere Frenkel state in neat PBI by at least one order of magnitude. Our results not only demonstrate the possibility to control the photo-physical properties of strongly coupled states by their spatially anisotropic light-matter interaction but also reveal innovative strategies to influence opto-electronic device operation by the directional transport of hybrid state energy.

physics.optics↗

arXiv2024

FMO-LC-TDDFTB method for excited states of large molecular assemblies in the strong light-matter coupling regime

We present a new methodology to calculate the strong light-matter coupling between photonic modes in microcavities and large molecular aggregates that consist of hundreds of molecular fragments. To this end, we combine our fragment molecular orbital long-range corrected time-dependent density functional tight-binding methodology with a generalized Tavis-Cummings Hamiltonian. We employ an excitonic Hamiltonian, which is built from a quasi-diabatic basis that is constructed from locally excited and charge-transfer states of all molecular fragments. In order to calculate polaritonic states, we extend our quasi-diabatic basis to include photonic states of a microcavity and derive and implement the couplings between the locally excited states and the cavity states and built a Tavis-Cummings Hamiltonian that incorporates the intermolecular excitonic couplings. Subsequently, we demonstrate the capability of our methodology by simulating the influence of the electric field polarization on the polaritonic spectra for a tetracene aggregate of 125 monomers. Furthermore, we investigate the dependence of the splitting of the upper and lower polaritonic branches on the system size by comparing the spectra of three different tetracene clusters. In addition, we investigate the polariton dispersion of a large tetracene aggregate for electric field polarizations in the x, y and z direction. Our new methodology can facilitate the future study of exciton dynamics in complex molecular systems, which consist of up to hundreds of molecules, that are influenced by strong light-matter coupling to microcavities.

physics.chem-ph↗