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Single-layer transition metal dichalcogenides are at the center of an ever increasing research effort both in terms of fundamental physics and applications. Exciton–phonon coupling plays a key role in determining the (opto)electronic properties of these materials. However, the exciton–phonon coupling strength has not been measured at room temperature. Here, we use two-dimensional micro-spectroscopy to determine exciton–phonon coupling of single-layer MoSe2. We detect beating signals as a function of waiting time induced by the coupling between A excitons and A′1 optical phonons. Analysis of beating maps combined with simulations provides the exciton–phonon coupling. We get a Huang–Rhys factor ~1, larger than in most other inorganic semiconductor nanostructures. Our technique offers a unique tool to measure exciton–phonon coupling also in other heterogeneous semiconducting systems, with a spatial resolution ~260 nm, and provides design-relevant parameters for the development of optoelectronic devices. The exciton–phonon coupling (EXPC) affects the opto-electronic properties of atomically thin semiconductors. Here, the authors develop two-dimensional micro-spectroscopy to determine the EXPC of monolayer MoSe2.

Optical parametric amplification is a second-order nonlinear process whereby an optical signal is amplified by a pump via the generation of an idler field1. This mechanism is inherently related to spontaneous parametric down-conversion, which currently constitutes the building block for entangled photon pair generation2, a process that is exploited in modern quantum technologies. Here we demonstrate single-pass optical parametric amplification at the ultimate thickness limit; using semiconducting transition metal dichalcogenides3,4, we show that amplification can be attained over propagation through a single atomic layer. Such a second-order nonlinear interaction at the two-dimensional limit bypasses phase-matching requirements5 and achieves ultrabroad amplification bandwidths. In agreement with first-principle calculations, we observe that the amplification process is independent of the in-plane polarization of signal and pump fields. By the use of AA-stacked multilayers, we present a clear pathway towards the scaling of conversion efficiency. Our results pave the way for the development of atom-sized tunable sources of radiation with potential applications in nanophotonics and quantum information technology. Single-pass optical parametric amplification is demonstrated following propagation though an atomically thin semiconducting transition metal dichalcogenide. The demonstration may lead to atom-sized tunable light sources.

The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS2 on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process., The formation dynamics of excitons in 2D transition metal dichalcogenides are challenging to probe directly because of their inherently fast timescales. Here, the authors use extremely short optical pulses to excite an electron-hole plasma, and show the formation of 2D excitons in MoS2 on the timescale of 30 fs.

Optical parametric amplification is a coherent mechanism whereby an optical signal is amplified by a pump via the generation of an idler field and it is the key ingredient of parametric oscillators. Here we demonstrate optical parametric amplification by monolayer transition-metal dichalcogenides, showing that amplification can be attained over a propagation through an atomic layer. The surface-like second-order nonlinear interaction bypasses phase-matching constraints, enabling ultrabroadband collinear amplification, generally unattainable due to material dispersion. Moreover, the amplification process is invariant over signal and pump in-plane polarizations. Our experimental findings pave the way for innumerable applications in nanophotonics and quantum information technology.

We use ultrafast spectroscopy to resolve the interlayer charge scattering processes in heterostructures (HS) of two-dimensional materials. In a WSe2/MoSe2 HS we photogenerate intralayer excitons in MoSe2 and observe hole injection in WSe2 on the sub-picosecond timescale, leading to the formation of interlayer excitons. The temperature dependence of the build-up and decay of interlayer excitons provide insights into the coupling mechanisms. In a graphene/WS2 HS we observe ultrafast charge transfer from graphene to the semiconductor. The data are consistent with hot electron/hole transfer, whereby a tail the hot Fermi-Dirac carrier distribution in graphene tunnels through the Schottky barrier into WS2.

Excitonic interactions determine the photoelectric properties of two-dimensional semiconductors. By using spatially resolved coherent two-dimensional micro-spectroscopy, we are able to determine the strength of exciton–phonon coupling in single-layer MoSe2 at room temperature.

Atomically thin transition-metal dichalcogenides (TMDs) exhibit strong light–matter interaction and pronounced excitonic behavior. Understanding the nonequilibrium dynamical processes of photoexcited carriers in such materials is a key step in view of their technological applications. Here, we review the ultrafast photophysics of 2D TMDs and related heterostructures (HSs) measured by femtosecond optical spectroscopy. First, we provide a general introduction on the physics of the materials and a brief explanation of the ultrafast optical techniques. Then we discuss the physical processes governing their nonequilibrium optical response, with a particular emphasis on the intervalley scattering dynamics, and the charge-transfer processes in 2D HSs. We conclude by discussing open issues and perspectives for future experiments.

Here we exploit transient reflectivity with sub-20fs temporal resolution to extract the exciton formation timescale from the build up of A/B exciton dynamics of 1L-MoS2. The timescale for this process ranges from ~15fs to ~35fs. © 2020 The Author(s)

We investigate charge transfer dynamics in graphene/WS2 heterostructures via transient reflection spectroscopy. By selectively exciting graphene with NIR sub-20fs pulses we resolve the build-up of the WS2 excitonic transitions due to the hot-carriers tunneling.

Single layer (1L) transition metal dichalcogenides (TMDs) have attracted huge interest because of their peculiar optoelectronic properties and strong excitonic effects[1]. Van der Waals heterostructures, obtained by stacking two 1L-TMDs on top of each other, have been intensively investigated because of their possible applications in electronics and optoelectronics[2]. Of particular interest are type II TMDs heterostructures where the peculiar band alignment leads to a staggered gap (the valence band maximum and the conduction band minumum are in the same layer). The main reason is due to the fact that this kind of band alignment allows an efficient charge separation which is particularly beneficial for photovoltaics and solar cell technology based on atomically thick materials.

Graphene is an ideal material for integrated nonlinear optics thanks to its strong light-matter interaction and large nonlinear optical susceptibility. Graphene has been used in optical modulators, saturable absorbers, nonlinear frequency converters, and broadband light emitters. For the latter application, a key requirement is the ability to control and engineer the emission wavelength and bandwidth, as well as the electronic temperature of graphene. Here, we demonstrate that the emission wavelength of graphene$'$ s broadband hot carrier photoluminescence can be tuned by integration on photonic cavities, while thermal management can be achieved by out-of-plane heat transfer to hexagonal boron nitride. Our results pave the way to graphene-based ultrafast broadband light emitters with tunable emission.

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

Due to their strong light-matter interaction, monolayer transition metal dichalcogenides (TMDs) have proven to be promising candidates for nonlinear optics and optoelectronics. Here, we characterize the nonlinear absorption of chemical vapour deposition (CVD)-grown monolayer MoSe2 in the 720–810 nm wavelength range. Surprisingly, despite the presence of strong exciton resonances, monolayer MoSe2 exhibits a uniform modulation depth of ∼80 ± 3% and a saturation intensity of ∼2.5 ± 0.4 MW/cm2. In addition, pump-probe spectroscopy is performed to confirm the saturable absorption and reveal the photocarrier relaxation dynamics over hundreds of picoseconds. Our results unravel the unique broadband nonlinear absorptive behavior of monolayer MoSe2 under ultrafast excitation and highlight the potential of using monolayer TMDs as broadband ultrafast optical switches with customizable saturable absorption characteristics.

The absorption spectra of 2D semiconductors are dominated by excitons with binding energy of several hundreds of meV. Nevertheless, even single layers show an appreciable photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating some degree of free charge carrier photogeneration. Here, we perform ultrafast transient absorption spectroscopy on monolayer MoS2 in a field-effect transistor configuration. We show that even a moderate in-plane electric field of a few kV cm(-1) can significantly increase the yield of charge carriers from photogenerated hot electron-hole pairs.

A full exploitation of the unique features of graphene and related 2D materials in micro and nano-scaled optoelectronic devices for high-data rate photonic systems calls for a deep understanding of their fundamental structural, electronic, photophysical properties and fast dynamics. We introduce some advanced spectroscopic techniques developed to this aim, while reviewing their specific potentialities and highlighting recent results. To provide absolute local nanoscale thickness metrology of as-grown 2D materials, Scanning Auger Electron Microspectroscopy has been calibrated. Results obtained for graphene and graphene oxide (GO) are reported, showing sub-monolayer resolution. Ultrafast optical transient absorption spectroscopy, with temporal resolution down to 10fs, unveils the relaxation dynamics of hot electrons in single layer graphene and the formation and radiative recombination via stimulated emission of biexcitons in ultra-narrow, structurally well-defined nanoribbons (GNRs). By recently developed ultrafast Time- and Angle-resolved Photo-Electron Spectroscopy (TR-ARPES) we investigate electronic states and spin dynamics close to the Dirac cone, in graphene and metal chalcogenide topological insulators (TI) under fs strong laser excitation, in view of spintronic applications.

We study the ultrafast valley relaxation dynamics in monolayer MoS2 by time resolved Faraday rotation and circular dichroism. We find that the intervalley scattering process is ultrafast and display a peculiar bi-exponential behavior.

Ultrafast transient reflectivity measurements were performed in epitaxial 4x4 silicene grown on Ag(111). Comparison with bulk silicon and silver response highlighted the occurrence of peculiar photo-physical mechanisms, suggesting a metallic-like behavior in silicene. © 2014 OSA.

We temporally compress highly charged electron bunches (100 fC). The linear chirp of a waterbag bunch is inverted by using a synchronized 3 GHz cavity leading to short (< 100 fs) and high-density electron pulses.

In this letter we demonstrate the possibility to determine the temporal and spectral structure (spectrogram) of a complex light pulse exploiting the ultrafast switching character of a non-thermal photo-induced phase transition. As a proof, we use a VO2 multi-film, undergoing an ultrafast insulator-to-metal phase transition when excited by femtosecond near-infrared laser pulses. The abrupt variation of the multi-film optical properties, over a broad infrared/visible frequency range, is exploited to determine, in-situ and in a simple way, the spectrogram of a supercontinuum pulse produced by a photonic crystal fiber. The determination of the structure of the pulse is mandatory to develop new pump-probe experiments with frequency resolution over a broad spectral range (700-1100 nm)., Comment: 4 pages, 3 figures, letter

The temperature dependence of the thermal boundary resistivity is investigated in glass-embedded Ag particles of radius 4.5 nm, in the temperature range from 300 to 70 K, using all-optical time-resolved nanocalorimetry. The present results provide a benchmark for theories aiming at explaining the thermal boundary resistivity at the interface between metal nanoparticles and their environment, a topic of great relevance when tailoring thermal energy delivery from nanoparticles as for applications in nanomedicine and thermal management at the nanoscale, Comment: 4 pages, 3 figures

Optical harmonic generation occurs when high intensity light (>1010 W m–2) interacts with a nonlinear material. Electrical control of the nonlinear optical response enables applications such as gate-tunable switches and frequency converters. Graphene displays exceptionally strong light–matter interaction and electrically and broadband tunable third-order nonlinear susceptibility. Here, we show that the third-harmonic generation efficiency in graphene can be increased by almost two orders of magnitude by controlling the Fermi energy and the incident photon energy. This enhancement is due to logarithmic resonances in the imaginary part of the nonlinear conductivity arising from resonant multiphoton transitions. Thanks to the linear dispersion of the massless Dirac fermions, gate controllable third-harmonic enhancement can be achieved over an ultrabroad bandwidth, paving the way for electrically tunable broadband frequency converters for applications in optical communications and signal processing. Gate tunable and ultrabroadband third-harmonic generation can be achieved in graphene, paving the way for electrically tunable broadband frequency converters for applications in optical communications and signal processing.

Optical parametric amplification (OPA) is a second-order nonlinear optical process whereby a signal field is amplified by a pump while generating an idler field, and allows the generation of broadly tunable coherent radiation for many photonic applications, e.g. spontaneous parametric down-conversion [1] for entangled photon pair generation. Here we demonstrate single-pass OPA at the ultimate thickness limit in monolayer transition-metal dichalcogenides [2] . The absence of dispersion-induced phase mismatch, thanks to the vanishing thickness of the nonlinear medium, bypasses phase-matching requirements and achieves ultrabroad amplification bandwidths ( Fig. 1a-b ), with a linear dependence of the amplification gain with pump and signal powers ( Fig. 1c ).

We demonstrate single-pass optical parametric amplification (OPA) in monolayer semiconducting transition-metal dichalcogenides. Our experimental findings of OPA efficiency and polarization dependence are fully supported by first-principle calculations of the nonlinear response within a tight-binding model.

The calculation of the equilibrium optical properties of bulk silicon by using the Bethe--Salpeter equation solved in the Kohn--Sham basis represents a cornerstone in the development of an ab--initio approach to the optical and electronic properties of materials. Nevertheless calculations of the {\em transient} optical spectrum using the same efficient and successful scheme are scarce. We report, here, a joint theoretical and experimental study of the transient reflectivity spectrum of bulk silicon. Femtosecond transient reflectivity is compared to a parameter--free calculation based on the non--equilibrium Bethe--Salpeter equation. By providing an accurate description of the experimental results we disclose the different phenomena that determine the transient optical response of a semiconductor. We give a parameter--free interpretation of concepts like bleaching, photo--induced absorption and stimulated emission, beyond the Fermi golden rule. We also introduce the concept of optical gap renormalization, as a generalization of the known mechanism of band gap renormalization. The present scheme successfully describes the case of bulk silicon, showing its universality and accuracy., Comment: 14 pages, 13 figures