The important role played by hot electrons in photocatalysis and light harvesting has attracted great interest in their dynamics and mechanisms of generation. Here, we theoretically study the temporal evolution of optically excited conduction electrons in small plasmon-supporting gold and silver nanoparticles. We describe the electron dynamics through a master equation incorporating transition rates for optical excitations and electron–electron collisions that are calculated using the screened interaction within an independent-electron picture. Upon optical excitation of the particle by a light pulse, a nonthermal electron distribution is produced, which takes 10s fs to thermalize at an elevated electron temperature due to electron–electron collisions and eventually relaxes back to ambient temperature via coupling to phonons and thermal diffusion. Phonons and diffusion are introduced through a phenomenological inelastic attenuation rate. We find the temporal evolution of the electron energy distribution to strongly depend on the total absorbed energy, which is in turn determined by particle size, pulse fluence, and photon energy. Our results provide detailed insight into hot-electron dynamics that can be beneficial for the design of improved photocatalysis and photodetection devices.

Monolayer transition metal dichalcogenides (TMDs) exhibit high nonlinear optical (NLO) susceptibilities. Experiments on MoS2 have indeed revealed very large second-order (χ (2) ) and third-order (χ (3) ) optical susceptibilities. However, third harmonic generation results of other layered TMDs has not been reported. Furthermore, the reported χ (2) and χ (3) of MoS2 vary by several orders of magnitude, and a reliable quantitative comparison of optical nonlinearities across different TMDs has remained elusive. Here, we investigate second-and third-harmonic generation, and three-photon photoluminescence in TMDs. Specifically, we present an experimental study of χ (2) , and χ (3) of four common TMD materials (MoS 2 , MoSe 2 , WS 2 and WSe 2 ) by placing different TMD flakes in close proximity to each other on a common substrate, allowing their NLO properties to be accurately obtained from a single measurement. χ (2) and χ (3) of the four monolayer TMDs have been compared, indicating that they exhibit distinct NLO responses. We further present theoretical simulations of these susceptibilities in qualitative agreement with the measurements. Our comparative studies of the NLO responses of different two-dimensional layered materials allow us to select the best candidates for atomic-scale nonlinear photonic applications, such as frequency conversion and all-optical signal processing.

The two-dimensionality of graphene and other layered materials can be exploited to simplify the theoretical description of their plasmonic and polaritonic modes. We present an analytical theory that allows us to simulate these excitations in terms of plasmon wave functions (PWFs). Closed-form expressions are offered for their associated extinction spectra, involving only two real parameters for each plasmon mode and graphene morphology, which we calculate and tabulate once and for all. Classical and quantum-mechanical formulations of this PWF formalism are introduced, in excellent mutual agreement for armchaired islands with > 10 nm characteristic size. Examples of application are presented to predict both plasmon-induced transparency in interacting nanoribbons and excellent sensing capabilities through the response to the dielectric environment. We argue that the PWF formalism has general applicability and allows us to analytically describe a wide range of 2D polaritonic behavior, thus facilitating their use for the design of actual devices. Plasmons are collective oscillations of conduction electrons found in different materials, where they interact strongly with light and can confine it down to nanoscale spatial regions to generate enormous optical field intensity enhancement [1]. These extraordinary properties are of paramount importance for a wide range of applications, such as optical sensing and modulation [2][3][4][5][6], the enhancement of nonlinear optical processes [7,8], photocatalysis [9][10][11][12][13][14], and photothermal therapies [15,16]. In these applications, precise spectral positioning of plasmon resonances is needed to achieve optimal performance. This is commonly achieved by fabricating noble metal nanostructures with specific sizes and morphologies. However, despite being the workhorse of plasmonics research, noble metals unfortunately present relatively large inelastic losses, thus limiting plasmon lifetimes in metallic nanostructures [17] and leading to a severe reduction in optical confinement. Additionally, the large number of electrons involved in the plasmons of metallic nanostructures limits the ways in which we can influence them in a dynamical fashion.Recently, highly-doped graphene has emerged as an outstanding plasmonic material [18][19][20][21][22][23][24][25][26][27][28][29][30][31] that simultaneously provides strong field confinement with relatively lower loss [32]. More importantly, plasmons in graphene are sustained by a small number of charge carriers compared to those of traditional noble metals, a property that makes them amenable to display new phenomena, including an unprecedented electro-optical response. Indeed, active tunability of the plasmon resonance frequency has been achieved via electrical gating [21][22][23][24][25][26][28][29][30]. Additionally, many of the aforementioned applications that were first realized using noble metal plasmons have now been realized using a tunable graphene platform [31][32][33][34]. However, the design of graphene-based plasmoni...

Structural information on electronically excited neutral molecules can be indirectly retrieved, largely through pump-probe and rotational spectroscopy measurements with the aid of calculations. Here, we demonstrate the direct structural retrieval of neutral carbonyl disulfide (CS2) in the " excited electronic state using laser-induced electron diffraction (LIED). We unambiguously identify the ultrafast symmetric stretching and bending of the field-dressed neutral CS2 molecule with combined picometre and attosecond resolution using intra-pulse pump-probe excitation and measurement. We invoke the Renner-Teller effect to populate the " excited state in neutral CS2, leading to bending and stretching of the molecule. Our results demonstrate the sensitivity of LIED in retrieving the geometric structure of CS2, which is known to appear as a two-centre scatterer. SignificanceLaser-induced electron diffraction is a molecular-scale electron microscope that captures clean snapshots of a molecule's geometry with sub-atomic picometre and attosecond spatio-temporal resolution. We induce and unambiguously identify the stretching and bending of a linear triatomic molecule following the excitation of the molecule to an excited electronic state with a bent and stretched geometry. We show that we can directly retrieve the structure of electronically excited molecules that is otherwise possible through indirect retrieval methods such as pump-probe and rotational spectroscopy measurements.2 Many important phenomena in biology, chemistry and physics can only be described beyond the Born-Oppenheimer (BO) approximation, giving rise to nonadiabatic dynamics and the coupling of nuclear (vibrational and rotational) and electronic motion in molecules (1-7). One prominent example where the BO approximation breaks down is the Renner-Teller effect (8,9): in any highly symmetric linear molecule with symmetry-induced degeneracy of electronic states, non-adiabatic coupling of (vibrational) nuclear and electronic degrees of freedom can lead to the distortion of the nuclear framework on a timescale comparable with electronic motion. The system's symmetry is then reduced by the bending of the molecule to split the degenerate electronic state into two distinct potential energy surfaces (PESs), leading to a more stable, bent conformer.Here, we demonstrate the direct imaging of Renner-Teller non-adiabatic vibronic dynamics in neutral CS2 with combined picometre and attosecond resolution through intra-pulse pump-probe excitation and measurement with laser-induced electron diffraction (LIED) (10-16). Our results shed light on the vibronic excitation of a neutral linear molecule in the rising edge of our laser field that causes bending and stretching of the molecule. High momentum transfers experienced by the electron wave packet (EWP; Up = 85 eV) with large scattering angles enable the electron to penetrate deep into the atomic cores, allowing us to resolve a strongly symmetrically stretched and bent CS2 molecule most likely in the B " & B ' excited elec...

As a two-dimensional semimetal, graphene offers clear advantages for plasmonic applications over conventional metals, such as stronger optical field confinement, in situ tunability, and relatively low intrinsic losses. However, the operational frequencies at which plasmons can be excited in graphene are limited by the Fermi energy E, which in practice can be controlled electrostatically only up to a few tenths of an electronvolt. Higher Fermi energies open the door to novel plasmonic devices with unprecedented capabilities, particularly at mid-infrared and shorter-wave infrared frequencies. In addition, this grants us a better understanding of the interaction physics of intrinsic graphene phonons with graphene plasmons. Here, we present FeCl-intercalated graphene as a new plasmonic material with high stability under environmental conditions and carrier concentrations corresponding to E > 1 eV. Near-field imaging of this highly doped form of graphene allows us to characterize plasmons, including their corresponding lifetimes, over a broad frequency range. For bilayer graphene, in contrast to the monolayer system, a phonon-induced dipole moment results in increased plasmon damping around the intrinsic phonon frequency. Strong coupling between intrinsic graphene phonons and plasmons is found, supported by ab initio calculations of the coupling strength, which are in good agreement with the experimental data.