Browsing by Author "Antosiewicz, Tomasz J."

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Computational Design of Alloy Nanostructures for Optical Sensing of Hydrogen
(2022-08-26) Ekborg-Tanner, Pernilla; Rahm, J. Magnus; Rosendal, Victor; Bancerek, Maria; Rossi, Tuomas P.; Antosiewicz, Tomasz J.; Erhart, Paul
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Pd nanoalloys show great potential as hysteresis-free, reliable hydrogen sensors. Here, a multiscale modeling approach is employed to determine optimal conditions for optical hydrogen sensing using the Pd-Au-H system. Changes in hydrogen pressure translate to changes in hydrogen content and eventually the optical spectrum. At the single particle level, the shift of the plasmon peak position with hydrogen concentration (i.e., the "optical"sensitivity) is approximately constant at 180 nm/cH for nanodisk diameters of 100 nm. For smaller particles, the optical sensitivity is negative and increases with decreasing diameter, due to the emergence of a second peak originating from coupling between a localized surface plasmon and interband transitions. In addition to tracking peak position, the onset of extinction as well as extinction at fixed wavelengths is considered. We carefully compare the simulation results with experimental data and assess the potential sources for discrepancies. Invariably, the results suggest that there is an upper bound for the optical sensitivity that cannot be overcome by engineering composition and/or geometry. While the alloy composition has a limited impact on optical sensitivity, it can strongly affect H uptake and consequently the "thermodynamic"sensitivity and the detection limit. Here, it is shown how the latter can be improved by compositional engineering and even substantially enhanced via the formation of an ordered phase that can be synthesized at higher hydrogen partial pressures.
Dipolar coupling of nanoparticle-molecule assemblies
(2021-03-07) Fojt, Jakub; Rossi, Tuomas P.; Antosiewicz, Tomasz J.; Kuisma, Mikael; Erhart, Paul
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Strong light-matter interactions facilitate not only emerging applications in quantum and non-linear optics but also modifications of properties of materials. In particular, the latter possibility has spurred the development of advanced theoretical techniques that can accurately capture both quantum optical and quantum chemical degrees of freedom. These methods are, however, computationally very demanding, which limits their application range. Here, we demonstrate that the optical spectra of nanoparticle-molecule assemblies, including strong coupling effects, can be predicted with good accuracy using a subsystem approach, in which the response functions of different units are coupled only at the dipolar level. We demonstrate this approach by comparison with previous time-dependent density functional theory calculations for fully coupled systems of Al nanoparticles and benzene molecules. While the present study only considers few-particle systems, the approach can be readily extended to much larger systems andto include explicit optical-cavity modes.
Ultrastrong Coupling of a Single Molecule to a Plasmonic Nanocavity: A First-Principles Study
(2022-03-16) Kuisma, Mikael; Rousseaux, Benjamin; Czajkowski, Krzysztof M.; Rossi, Tuomas P.; Shegai, Timur; Erhart, Paul; Antosiewicz, Tomasz J.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Ultrastrong coupling (USC) is a distinct regime of light-matter interaction in which the coupling strength is comparable to the resonance energy of the cavity or emitter. In the USC regime, common approximations to quantum optical Hamiltonians, such as the rotating wave approximation, break down as the ground state of the coupled system gains photonic character due to admixing of vacuum states with higher excited states, leading to ground-state energy changes. USC is usually achieved by collective coherent coupling of many quantum emitters to a single mode cavity, whereas USC with a single molecule remains challenging. Here, we show by time-dependent density functional theory (TDDFT) calculations that a single organic molecule can reach USC with a plasmonic dimer, consisting of a few hundred atoms. In this context, we discuss the capacity of TDDFT to represent strong coupling and its connection to the quantum optical Hamiltonian. We find that USC leads to appreciable ground-state energy modifications accounting for a non-negligible part of the total interaction energy, comparable to kBT at room temperature.