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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - BATH (A Probe for Environment Properties in Open Quantum Systems: Accessing Spectral Densities with Multi-Dimensional Coherent Spectroscopy)

Teaser

Summary

The excitation of matter by light underlies some of the most important human-made technologies, and processes in nature. The dynamics of the photoexcited state is modulated by the motion of the environment which can successfully steer it to a desired state, or alternatively destroy it before it can be converted to useful energy. The BATH project strives to recover the information on the system-bath interaction using two-dimensional electronic spectroscopy (2DES). This technique is the most sophisticated third-order time resolved technique providing detailed information on the dynamics of electronic excitation on a sub-20fs timescale.

The work preferentially focuses on materials incorporating both plasmonic nanoparticles and molecular aggregates. From a fundamental standpoint they can exhibit quantum interferences, and have a complex dissipative environmental bath consisting of molecular vibrations, solvent motion and phonon modes of the lattice. In addition, many-body effects (e-e scattering) influence the early dynamics. From a practical standpoint, plasmon-based materials are candidates for quantum optics manipulations (cavity quantum electrodynamics, adiabatic passage methods, coherent population trapping) as well as photocatalysis (plasmon enhanced photochemistry). They are thus an ideal problem to answer fundamentally important questions with very realistic applications.

The work is arranged into two mutually supporting directions. We carry out spectroscopy experiments (pump-probe, 2DES) on plasmon-based materials to reveal their dissipative mechanisms and identify the features of the 2DES spectrum which can be directly associated to system-bath couplings. We also develop the theory to understand the photoexcited dynamics using simple models that can be solved analytically, and carry out simulations of the 2DES experiment.

Work performed

The project concluded successfully as we made important strides in our understanding and description of dissipative evolution of plasmon-based materials with multidimensional spectroscopy. The directions initiated by the fellowship have turned into fully-fledged efforts that are continuing beyond its end-date. We separate the results of the project into a theoretical and an experimental component. The most salient milestones of the project are:

Part 1: Formal theory development and 2DES simulation

1) We have developed a formalism of non-norm conserving evolution that is applicable to molecules coupled to plasmons, and which goes beyond the state-of-the-art description of adiabatic elimination used for dissipative qubit state preparation.
2) We have provided a general perspective of Fano interferences and coherent population trapping in unitary as well as dissipative evolution.
3) We have derived the equations for the third order nonlinear response of systems with discrete-extended energy spectrum (Fano Hamiltonians) which can successfully describe the experiments.

Part 2: 2DES experiments of plasmon-molecule hybrids

4) We have carried out a comprehensive study of plexciton systems consisting of Ag nanoprisms decorated by molecular aggregates. We have assigned the dissipative rates to physical mechanisms of molecular vibrations, lattice phonons and e-e scattering processes.
5) We have described the coupling of the excitation to acoustic breathing modes of the nanoparticle, and their signatures in 2DES spectra.

Final results

The scientific impact of the work is as follows:

1) The theory of non-norm conserving evolution can describe the density matrix with higher fidelity than the current best formalism and is applicable to a large number of current quantum information experiments (for example, ion traps).
2) The description of the 2DES spectrum of systems with Fano interferences was the first time this model was solved. It has wide applicability, being relevant in ionization experiments and for molecules adsorbed on surfaces.
3) We have been the first ones to assign the dissipative pathways in plexciton materials, and one of a few groups to analyze the 2D signatures of plasmon-based materials.

The theoretical work developed will be useful for many types of experiments and we have made an effort to make it easily accessible. The conclusions derived from our experiments help design better plasmon-based materials so that they will contribute to the effort of making better solar energy technologies, and bring to a reality the possibility of doing quantum optics in a beaker.