Attosecond screening dynamics mediated by electron-localization

Transition metals with their densely confined and strongly coupled valence electrons are key constituents of many materials with unconventional properties, such as high-Tc superconductors, Mott insulators and transition-metal dichalcogenides. Strong electron interaction offers a fast and efficient lever to manipulate their properties with light, creating promising potential for next-generation electronics. However, the underlying dynamics is a fast and intricate interplay of polarization and screening effects, which is poorly understood. It is hidden below the femtosecond timescale of electronic thermalization, which follows the light-induced excitation. Here, we investigate the many-body electron dynamics in transition metals before thermalization sets in. We combine the sensitivity of intra-shell transitions to screening effects with attosecond time resolution to uncover the interplay of photo-absorption and screening. First-principles time-dependent calculations allow us to assign our experimental observations to ultrafast electronic localization on d-orbitals. The latter modifies the whole electronic structure as well as the collective dynamic response of the system on a timescale much faster than the light-field cycle. Our results demonstrate a possibility for steering the electronic properties of solids prior to electron thermalization, suggesting that the ultimate speed of electronic phase transitions is limited only by the duration of the controlling laser pulse. Furthermore, external control of the local electronic density serves as a fine tool for testing state-of-the art models of electron-electron interactions. We anticipate our study to facilitate further investigations of electronic phase transitions, laser-metal interactions and photo-absorption in correlated electron systems on its natural timescale

Vibronic resonances facilitate excited state coherence in light harvesting proteins at room temperature

Until recently it was believed that photosynthesis, a fundamental process for life on earth, could be fully understood with semi-classical models. However, puzzling quantum phenomena have been observed in several photosynthetic pigment-protein complexes, prompting questions regarding the nature and role of these effects. Recent attention has focused on discrete vibrational modes that are resonant or quasi-resonant with excitonic energy splittings and strongly coupled to these excitonic states. Here we unambiguously identify excited state coherent superpositions in photosynthetic light-harvesting complexes using a new experimental approach. Decoherence on the timescale of the excited state lifetime allows low energy (56 cm-1) oscillations on the signal intensity to be observed. In conjunction with an appropriate model, these oscillations provide clear and direct experimental evidence that the persistent coherences observed require strong vibronic mixing among excited states

Transient Optical Picocavities Within Coupled Plasmonic Nanostructures

Plasmonic nanocavities, such as a nanometre scale gap between a gold nanoparticle and gold mirror (NPoM), confine light beyond the free space diffraction limit. While these enhanced field intensities allow resolvable measurements of vibrational scattering from only a few hundred molecules, ensemble averaging destroys all information on individual molecular local environments. In this thesis, I first investigate the single molecule vibrational scattering from a molecule placed into NPoM using a DNA structure. The DNA complicates the response, which is time variant with transient features suggestive of possible picocavity formation. Picocavities are transient atomic scale features on the metal surfaces which further confine fields (effective volume <1 nm3) with strong field gradients that locally alter the rules for vibrational scattering efficiency. These can alter the spectral response of a single nearby molecule (isolating it spectrally) and were previously noted in NPoM at cryogenic temperatures. I change the gap material to a molecular monolayer to simplify the system and explore room temperature picocavities. I use automated analysis of large experimental datasets to detect and isolate transient vibrational scattering. Picocavity generation is found to depend on the local chemical environment near the gold surface. Picocavities are observed to chemically interact with the molecule being optically probed. This perturbs bond strengths across the molecule with the strength and direction of this perturbation being highly sensitive to the relative picocavity location on a < 0.1Å scale. This single molecule – metal atom system is explored by comparing experimental data to a theoretical Density Function Theory model. Next, I extract the spatial distribution of picocavity formation in the gap by comparing transient scattering at two simultaneous wavelengths of light. Picocavities are found to more likely form at regions of higher optical intensity within the NPoM gap. This suggests that light plays a direct role in the yet undetermined picocavity generation mechanism

Roadmap on semiconductor-cell biointerfaces.

This roadmap outlines the role semiconductor-based materials play in understanding the complex biophysical dynamics at multiple length scales, as well as the design and implementation of next-generation electronic, optoelectronic, and mechanical devices for biointerfaces. The roadmap emphasizes the advantages of semiconductor building blocks in interfacing, monitoring, and manipulating the activity of biological components, and discusses the possibility of using active semiconductor-cell interfaces for discovering new signaling processes in the biological world

Harnessing optical micro-combs for microwave photonics

In the past decade, optical frequency combs generated by high-Q micro-resonators, or micro-combs, which feature compact device footprints, high energy efficiency, and high-repetition-rates in broad optical bandwidths, have led to a revolution in a wide range of fields including metrology, mode-locked lasers, telecommunications, RF photonics, spectroscopy, sensing, and quantum optics. Among these, an application that has attracted great interest is the use of micro-combs for RF photonics, where they offer enhanced functionalities as well as reduced size and power consumption over other approaches. This article reviews the recent advances in this emerging field. We provide an overview of the main achievements that have been obtained to date, and highlight the strong potential of micro-combs for RF photonics applications. We also discuss some of the open challenges and limitations that need to be met for practical applications.Comment: 32 Pages, 13 Figures, 172 Reference

Research and technology highlights of the Lewis Research Center

Highlights of research accomplishments of the Lewis Research Center for fiscal year 1984 are presented. The report is divided into four major sections covering aeronautics, space communications, space technology, and materials and structures. Six articles on energy are included in the space technology section