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Multidimensional Ultrafast Time-Interferometric Spectroscopy of Coherent Phenomena in all Environments

Final Report Summary - MULTISCOPE (Multidimensional Ultrafast Time-Interferometric Spectroscopy of Coherent Phenomena in all Environments)

In project MULTISCOPE we developed and applied novel methods of femtosecond time-resolved nonlinear optical spectroscopy to investigate the significance and consequences of coherent effects for a variety of photophysical and photochemical processes. We used coherent two-dimensional (2D) spectroscopy as an ideal tool to study electronic coherences. In contrast to most conventional schemes, the main focus of the project was not on measuring the coherently emitted light field within a four-wave-mixing process but rather to implement a range of incoherent observables (ion mass spectra, fluorescence, and photoelectrons). We were able to demonstrate that we could extract all desired coherent information using the technique of “phase cycling” with collinear pulse sequences from a femtosecond pulse shaper. In this approach, one varies systematically the phases of individual excitation pulses and can then reconstruct various desired nonlinear response-function contributions by suitable linear combinations of the raw-data observables. This approach made possible a range of interdisciplinary experiments on quantum systems in all states of matter, i.e. in the liquid phase, the gas phase, and on surfaces. In particular, we developed a range of new 2D spectroscopy method variants.

For the case of liquid-phase investigations, we introduced rapid-scan 2D fluorescence spectroscopy. This setup contains no movable parts and is thus compact and robust. All spectroscopic parameters are varied electronically on a 1-kHz shot-to-shot basis using a customized femtosecond pulse shaper. The approach was extended to encompass correlations between zero-, one-, two-, and three-quantum electronic coherences in three frequency dimensions. We showed that one can obtain fifteen different fourth-order and sixth-order three-dimensional spectra simultaneously within just 8 min of measurement time. In another approach we developed exciton–exciton-interaction two-dimensional (EEI2D) spectroscopy that is sensitive to interactions between electronic excitations. We have used EEI2D signals to study exciton diffusion in various self-assembled aggregates and polymers and found that, in general, the conventionally assumed normal diffusion equation is not applicable, but that propagation may rather occur in a sub-diffusion regime due to the presence of local traps. Such deviations, having become accessible with our new EEI2D method, are relevant for understanding exciton diffusion and optimizing materials for energy transport efficiency.

For gas-phase samples, we developed 2D spectroscopy of molecular beams using time-of-flight mass spectrometry. Thus it became possible to analyze interaction-free molecules. Recording a mass spectrum for each parameter setting of the pulse shaper and Fourier-transforming the ion signal over the time delays, we obtained 2D spectra not only for the parent molecule but also for all fragments. This method makes it possible to investigate the evolution of coherences during bond breakage and study the influence of the environment on molecular coherences.

In the case of surface-science studies, we designed and implemented in our laboratory a new setup for time-resolved photoemission electron microscopy (TR-PEEM) including aberration correction that offers high spatial resolution (~3 nm) in combination with tunable (200–970 nm) short-pulse excitation (sub-20 fs) at high repetition rate (1 MHz). This was realized with a customized noncollinear optical parametric amplifier pumped by a fiber laser amplifier. In combination with pulse shaping, we have used this setup for 2D nanoscopy, i.e. acquiring 2D spectra with nanoscopic spatial resolution based on photoelectron detection. On the example of plasmonic nanoslit resonators, we detected apparent spatial variations of the Q-factor and resonance frequency that are commonly considered to be global properties for a single mode. By using the concept of quasinormal modes we explained these local differences by crosstalk of adjacent resonator modes. Our findings are important in view of time-domain studies of plasmon-mediated strong light−matter coupling. First experiments on molecular layers using PEEM detection were also performed, demonstrating the applicability of 2D nanoscopy to organic and hybrid systems. Furthermore, we used PEEM to prove experimentally our suggested concept for coherent periodic energy transfer between widely separated nanoemitters. Strong coupling over a distance of twice the wavelength was achieved in a combination of localized and delocalized plasmonic modes. The resulting energy transfer efficiency was two orders of magnitude larger than with simpler plasmonic devices. First experiments using 2D photoelectron spectroscopy on molecular films were also conducted.

Lastly, we developed coherent 2D micro-spectroscopy, a combination of 2D spectroscopy and optical microscopy. Using a high-numerical-aperture objective (NA = 1.4) we reached ~300 nm spatial resolution and ~10 fs temporal resolution for fluorescence-detected 2D spectra. The method was applied to resolve the phonon sideband structure in single-layer MoSe2, a two-dimensional material of the class of transition metal dichalcogenides that is of interest for optoelectronic applications. We were able to measure and assign the energy structure due to exciton–phonon coupling that is hidden in conventional linear absorption or photoluminescence experiments because of inhomogeneous broadening. As an emerging technique, combining 2D micro-spectroscopy with single-molecule approaches, we succeeded in obtaining a 2D spectrum from an individual single-layer carbon nanotube. This merging of two separated research directions, single-molecule and 2D spectroscopy, opens the door for future 2D studies of organic or hybrid systems in the few- or even single-particle limit.