Final Report Summary - TRUEVIEW (Time-Resolved Ultrafast Electron Visualization of Evanescent Waves)
The main objective of the TRUEViEW project was to exploit the recent technological advances in time-resolved electron microscopy to study, with unprecedented space and time-resolution, the evolution of evanescent electromagnetic waves in nanophotonic and nanoplasmonic structures. In particular, the project focused on the use of the Photon-Induced Near-field Electron Microscopy (PINEM) technique, which uses very short synchronized electron pulses to sense photo-induced ultrafast electromagnetic fields confined to nanoscale structures. Since the PINEM methodology was discovered only in 2009, and the ultrafast electron microscope at EPFL was only the second of its kind in the world to become operational in 2013, the project started in a brand new scientific field, with many open-ended avenues to be explored. As a result, the project was designed to first probe and establish the potential of several research objectives, before focusing on the most promising directions in order to maximize the scientific relevance, impact and utility of the obtained results.
Main results and conclusions:
The first main result of the project came about when the researchers realized that with the unique versatility of the microscope, one could actually realize a new, hybrid form of electron microscopy that would detect the electron distribution mapped to two different degrees of freedom (such as space, energy and momentum) at the same time. Essentially this approach combines the two classic measurement methodologies in PINEM: imaging, which maps the spatial distribution of transmitted (and energy-filtered) electrons directly onto the two-dimensional detector, and spectroscopy, in which the electrons are dispersed according to their energy across the detector, yielding an electron energy spectrum. The scientific breakthrough came when the researchers realized that because these two modes highlight the wave and particle aspects of confined light, respectively, the hybrid approach would be capable of capturing both these aspects at the same time. By then designing an experiment where a plasmonic standing wave was photo-excited on an appropriately aligned nanoscale resonator, the researchers were able to capture an ultrafast snapshot of the confined light (i.e. plasmonic wave) mapping its distribution in energy and space along two orthogonal axes of the detector. The resulting image (Figure 1.pdf) represents a very intuitive, never-before-seen direct visualization of the dual nature of plasmons (confined light), showcasing their fundamental dual quantum nature simultaneously for the first time in a single experiment.
The second main result corresponds to the first time-resolved movie of a propagating plasmonic wave using the PINEM technique, which is very much in line with the project title. In the first part of the project, the researchers explored, both through experiments and extensive simulations, the different possible propagation media for this type of experiment in terms of materials and their form and shape. Taking into account various practical factors (e.g. plasmon characteristics, material stability, nanofabrication challenges), a nanopatterned metal-on-dielectric bilayer was found as the best compromise. The sample designs made use of nanoscale cavities perforating the metallic top layer (an ‘inverted’ resonator design), such that photo-excited plasmons would propagate along the buried interface between the metallic top layer and the dielectric substrate. The nanocavity designs were matched to the frequency of optical excitation and the corresponding layer thicknesses were used to balance the resulting plasmon dispersion characteristics with the electron transparency of the sample. In the main experiment, plasmons, or more precisely, surface plasmon polaritons (SPPs), were launched from an array of these nanocavities by an optical excitation pulse, and their propagation and interference was filmed by means of PINEM imaging using synchronized pulses of electrons at different time delays in a stroboscopic approach (the concept is sketched in Figure 2.pdf). Using a systematic analysis of the thus obtained movie frames based on spatial Fourier transforms, the researchers were able to quantitatively extract the corresponding experimental plasmon characteristics, which were found to match well with theoretical predictions. Moreover, in additional experiments on similar types of experiments, the researchers were able to demonstrate control of the plasmonic interference pattern through the combination of the nanopatterning design and the polarization of the excitation light. Together, these findings represent a scientific advance on many different levels. Not only does it demonstrate the first direct imaging of a plasmonic near-field confined to a buried interface, it also represents a breakthrough in ultrafast electron microscopy measurements as the first observation of the temporal evolution of a propagating plasmonic near-field using PINEM. As such this result fulfills the main project objective, by enabling the dynamic observation and control of plasmon-plasmon interference patterns with both nanometer and femtosecond scale precision, even for plasmonic near-fields confined to buried interfaces.
The project established the first ultrafast transmission electron microscope (TEM) system capable of PINEM measurements in Europe, which has significantly enhanced the presence, competitiveness and research excellence of the European Research Area in the field. Since the start of the project, several ultrafast TEM projects have been initiated, advanced or completed both in Europe and world-wide (e.g. in Strasbourg, Stockholm, Göttingen, Seoul, Michigan, etc.), and the research community is experiencing a surge of technological progress and scientific success. The work in the TRUEViEW project has been highly relevant in terms of both the fundamental understanding of plasmons and the technological advancement of optoelectronics research.
On the fundamental side, the hybrid PINEM experiment simultaneously visualized the dual nature of plasmons in a revolutionary new way. In our view, this experiment represents a new take on the concept of duality that connects to novices and experts in the fields alike in an intuitive manner. The impact of this work is underlined by its mass-media exposure after publication, which included highlights in the New York Times, CBS news and many other mainstream news outlets (currently, the corresponding article is ranked in the top 1% of the most impactful papers in all journals among tracked articles of a similar age).
In terms of applications, the project has delivered a new experimental tool that can extract extremely valuable quantitative information on the spatio-temporal evolution of plasmons in nanostructures, which are the intended information carriers of the future. The unique transmission geometry of PINEM and its now demonstrated ability to characterize and control plasmonic near-fields at their relevant spatial and time scales, even when confined to hidden or buried layers, enables a direct route toward the design of advanced heterostructured devices based on two-dimensional materials that are expected to have great impact in future optoelectronic technology.