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In the past twenty years, we have reached a broad understanding of many light-driven phenomena in nanoscale systems. The temporal dynamics of the excited states are instead quite challenging to explore, and, at the same time, crucial to study for understanding the origin of fundamental physical and chemical processes. In this review we examine the current state and prospects of ultrafast phenomena driven by plasmons both from a fundamental and applied point of view. This research area is referred to as ultrafast plasmonics and represents an outstanding playground to tailor and control fast optical and electronic processes at the nanoscale, such as ultrafast optical switching, single photon emission and strong coupling interactions to tailor photochemical reactions. Here, we provide an overview of the field, and describe the methodologies to monitor and control nanoscale phenomena with plasmons at ultrafast timescales in terms of both modeling and experimental characterization. Various directions are showcased, among others recent advances in ultrafast plasmon-driven chemistry and multi-functional plasmonics, in which charge, spin, and lattice degrees of freedom are exploited to provide active control of the optical and electronic properties of nanoscale materials. As the focus shifts to the development of practical devices, such as all-optical transistors, we also emphasize new materials and applications in ultrafast plasmonics and highlight recent development in the relativistic realm. The latter is a promising research field with potential applications in fusion research or particle and light sources providing properties such as attosecond duration.

In many paradigmatic materials, like Transition Metal Dichalcogenides, the role played by the spin degrees of freedom is as important as the one played by the electron-electron interaction. Thus an accurate treatment of the two effects and of their interaction is necessary for an accurate and predictive study of the optical and electronic properties of these materials. Despite the GW-BSE approach correctly accounts for electronic correlations the spin-orbit coupling effect is often neglected or treated perturbatively. Recently spinorial formulations of GW-BSE have become available in different flavours in material-science codes. Still an accurate validation and comparison of different approaches is missing. In this work we go through the derivation of non collinear GW-BSE approach. The scheme is applied to transition metal dichalcogenides comparing perturbative and full spinorial approach. Our calculations reveal that dark-bright exciton splittings are generally improved when the spin orbit coupling is included non perturbatively. The exchange-driven intravalley mixing between the A and B exciton is found to be extremely important in the case of MoSe$_2$. We finally define the excitonic spin and use it to sharply analyze the spinorial properties of Transition Metal Dichalcogenides excitonic states., Comment: 22 pages, 4 figures

The Bethe-Salpeter Equation (BSE) can be applied to compute from first-principles optical spectra that include the effects of screened electron-hole interactions. As input, BSE calculations require single-particle states, quasiparticle energy levels and the screened Coulomb interaction, which are typically obtained with many-body perturbation theory, whose cost limits the scope of possible applications. This work tries to address this practical limitation, instead deriving spectral energies from Koopmans-compliant functionals and introducing a new methodology for handling the screened Coulomb interaction. The explicit calculation of the $W$ matrix is bypassed via a direct minimization scheme applied on top of a maximally localised Wannier function basis. We validate and benchmark this approach by computing the low-lying excited states of the molecules in Thiel's set, and the optical absorption spectrum of a $\text{C}_{60}$ fullerene. The results show the same trends as quantum chemical methods and are in excellent agreement with previous simulations carried out at the TD-DFT or $G_{0}W_{0}$-\text{BSE} level. Conveniently, the new framework reduces the parameter space controlling the accuracy of the calculation, thereby simplifying the simulation of charge-neutral excitations, offering the potential to expand the applicability of first-principles spectroscopies to larger systems of applied interest.

Transition metal dichalcogenides (TMDs) are emerging as promising two-dimensional (2d) semiconductors for optoelectronic and flexible devices. However, a microscopic explanation of their photophysics -- of pivotal importance for the understanding and optimization of device operation -- is still lacking. Here we use femtosecond transient absorption spectroscopy, with pump pulse tunability and broadband probing, to monitor the relaxation dynamics of single-layer MoS2 over the entire visible range, upon photoexcitation of different excitonic transitions. We find that, irrespective of excitation photon energy, the transient absorption spectrum shows the simultaneous bleaching of all excitonic transitions and corresponding red-shifted photoinduced absorption bands. First-principle modeling of the ultrafast optical response reveals that a transient bandgap renormalization, caused by the presence of photo-excited carriers, is primarily responsible for the observed features. Our results demonstrate the strong impact of many-body effects in the transient optical response of TMDs even in the low-excitation-density regime., Comment: This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Nano after peer review and technical editing by the publisher. To access the final edited and published work see: http://pubs.acs.org/articlesonrequest/AOR-fcCuTYn5m6wiDTIg98Kx

Excitonic effects due to electron-hole coupling play a fundamental role in renormalising energy levels in dye sensitised and organic solar cells determining the driving force for electron extraction. We show that first-principles calculations based on many-body perturbation theory within the GW-BSE approach provide a quantitative picture of interfacial excited state energetics in organic dye-sensitized TiO2 , delivering a general rule for evaluating relevant energy levels.To perform GW-BSE calculations in such large systems we introduce a new scheme based on maximally localized Wannier functions. With this method the overall scaling of GW-BSE calculations is reduced from O(N4) to O(N3).

The effect of the local fields on the absorption spectra of silicon nanoclusters (NCs), freestanding or embedded in SiO2, is investigated in the DFT-RPA framework for different size and amorphization of the samples. We show that local field effects have a great influence on the optical absorption of the NCs. Their effect can be described by two separate contributions, both arising from polarization effects at the NC interface. First, local fields produce a reduction of the absorption that is stronger in the low energy limit. This contribution is a direct consequence of the screening induced by polarization effects on the incoming field. Secondly, local fields cause a blue shift on the main absorption peak that has been explained in terms of perturbation of the absorption resonance conditions. Both contributions do not depend either on the NC diameter nor on its amorphization degree, while showing a high sensitivity to the environment enclosing the NCs.

The case study of a 32-atoms Si nanocrystallite (NC) embedded in a SiO2 matrix, both crystalline and amorphous, or free-standing with different conditions of passivation and strain is analyzed through ab-initio approaches. The Si32/SiO2 heterojunction shows a type I band offset highlighting a separation between the NC plus the interface and the matrix around. The consequence of this separation is the possibility to correctly reproduce the low energy electronic and optical properties of the composed system simply studying the suspended NC plus interface oxygens with the appropriate strain. Moreover, through the definition of an optical absorption threshold we found that, beside the quantum confinement trend, the amorphization introduces an additional redshift that increases with increasing NC size: i.e. the gap tends faster to the bulk limit. Finally, the important changes in the calculated DFT-RPA optical spectra upon inclusion of local fields point towards the need of a proper treatment of the optical response of the interface region.