Time-domain observation of interlayer exciton formation and thermalization in a MoSe2/WSe2 heterostructure

: Vertical heterostructures of transition metal dichalcogenides (TMDs) host interlayer excitons with electrons and holes residing in different layers. With respect to their intralayer counterparts, interlayer excitons feature longer lifetimes and diffusion lengths, paving the way for room temperature excitonic optoelectronic devices. The interlayer exciton formation process and its underlying physical mechanisms are largely unexplored. Here we use ultrafast transient absorption spectroscopy with a broadband white-light probe to simultaneously resolve interlayer charge transfer and interlayer exciton formation dynamics in a MoSe2/WSe2 heterostructure. We observe an interlayer exciton formation timescale nearly an order of magnitude (~1 ps) longer than the interlayer charge transfer time (~100 fs). Microscopic calculations attribute this relative delay to an interplay of a phonon-assisted interlayer exciton cascade and thermalization, and excitonic wave-function overlap. Our results may explain the efficient photocurrent generation observed in optoelectronic devices based on TMD heterostructures, as the interlayer excitons are able to dissociate during thermalization

Unconventional double-bended saturation of carrier occupation in optically excited graphene due to many-particle interactions

Saturation of carrier occupation in optically excited materials is a well-established phenomenon. However, so far, the observed saturation effects have always occurred in the strong-excitation regime and have been explained by Pauli blocking of the optically filled quantum states. On the basis of microscopic theory combined with ultrafast pump-probe experiments, we reveal a new low-intensity saturation regime in graphene that is purely based on many-particle scattering and not Pauli blocking. This results in an unconventional double-bended saturation behaviour: Both bendings separately follow the standard saturation model exhibiting two saturation fluences; however, the corresponding fluences differ by three orders of magnitude and have different physical origin. Our results demonstrate that this new and unexpected behaviour can be ascribed to an interplay between time-dependent many-particle scattering and phase-space filling effects

Status of sFLASH, the Seeding Experiment at FLASH

Recently, the Free electron LASer in Hamburg FLASH at DESY has been upgraded considerably [1]. Besides increasing the maximum energy to about 1.2 GeV and installation of a third harmonic rf cavity linearizing the longitudinal phase space distribution of the electron bunch, an FEL seeding experiment at wavelengths of about 35 nm has been installed. The goal is to establish direct FEL seeding employing coherent VUV pulses produced from a powerful drive laser by high harmonic generation HHG in a gas cell. The project, called sFLASH, includes generation of the required HHG pulses, transporting them to the undulator entrance of a newly installed FEL amplifier, controlling spatial, temporal and energy overlap with the electron bunches and setting up a pump probe pilot experiment. Sophisticated diagnostics is installed to characterize both HHG and seeded FEL pulses, both in time and frequency domain. Compared to SASE FEL pulses, almost perfect longitudinal coherence and improved synchronization possibilities for the user experiments are expected. In this paper the status of the experiment is presente

Technical Design of the XUV Seeding Experiment at FLASH

The Free electron laser at Hamburg FLASH operates in the Self Amplified Spontaneous Emission SASE mode, delivering to users photons in the XUV wavelength range. The FEL seeding schemes promise to improve the properties of the generated radiation in terms of stability in intensity and time. Such an experiment using higher harmonics of an optical laser as a seed is currently under construction at FLASH. The installation of the XUV seeding experiment sFLASH is going to take place in fall 2009. This includes mounting of new variable gap undulators upstream of the existing SASE undulators, building the XUV seed source as well as installation of additional photon diagnostics and electron beam instrumentation. In this contribution the layout of sFLASH will be discussed together with the technical design of its major component

Photon Diagnostics for the Seeding Experiment at FLASH

Starting from next year the technical feasibility of a direct seeding scheme at 30 and 13 nm will be studied at the Free electron LASer in Hamburg FLASH . During a major shutdown the SASE FEL facility will be upgraded and it is planned to install in addition a high harmonic generation HHG seed laser, a new chain of 10m variable gap undulators and a dedicated commissioning beamline for photon diagnostics and pilot time resolved pump probe experiments. Besides demonstrating successful seeding at short wavelength, the project aims for time resolution in the 10 fs range to study ultrafast processes by combining the naturally synchronized FEL and optical seed laser pulses. After the extraction of the radiation in a magnetic chicane, a short branch will accommodate intensity and beam monitors and a high resolution spectrometer. The intensity monitor detects scattered photons from a gold mesh on a shotto shot basis using micro channel plates MCP and XUV diodes. It is designed to detect photons several orders of magnitude apart in flux, i.e. spanning the wide range from spontaneous emission up to the seeded FEL radiation at gigawatt power level. Simulations of this device are presented as well as calibration measurements carried out at FLAS