We study the hydration of protons in liquid water using terahertz time-domain spectroscopy and polarization-resolved femtosecond midinfrared pump-probe spectroscopy. We observe that the addition of ...protons leads to a very strong decrease of the dielectric response of liquid water that corresponds to 19+/-2 water molecules per dissolved proton. This depolarization results from water molecules ( approximately 4) that are irrotationally bound to the proton and from the motion of water (corresponding to the response of approximately 15 water molecules) involved in the transfer of the proton charge.
We have studied the influence of excess protons on the vibrational energy relaxation of the O-H and O-D stretching modes in water using femtosecond pump-probe spectroscopy. Without excess protons, we ...observe exponential decays with time constants of 1.7 and 4.3 ps for the bulk and anion bound O-D stretch vibrations. The addition of protons introduces a new energy relaxation pathway, which leads to an increasingly nonexponential decay of the O-D stretch vibration. This new pathway is attributed to a distance-dependent long range dipole-dipole (Forster) interaction between the O-D stretching vibration and modes associated with dissolved protons. The high efficiency of hydrated protons as receptors of vibrational energy follows from the very large absorption cross section and broad bandwidth of protons in water. For a proton concentration of 1M we find that Forster energy transfer occurs over an average distance of 4.5 A, which corresponds to a separation of about two water molecules.
We report on highly efficient THz high harmonic generation in graphene up to 7th order, at room temperature and under ambient conditions. The process is facilitated by dynamical electron heating and ...cooling in THz fields.
Topologically-protected surface states present rich physics and promising spintronic, optoelectronic and photonic applications that require a proper understanding of their ultrafast carrier dynamics. ...Here, we investigate these dynamics in topological insulators (TIs) of the bismuth and antimony chalcogenide family, where we isolate the response of Dirac fermions at the surface from the response of bulk carriers by combining photoexcitation with below-bandgap terahertz (THz) photons with TI samples with varying Fermi level, including one sample with the Fermi level located within the bandgap. We identify distinctly faster relaxation of charge carriers in the topologically-protected Dirac surface states (few hundred femtoseconds), compared to bulk carriers (few picoseconds). In agreement with such fast cooling dynamics, we observe THz harmonic generation without any saturation effects for increasing incident fields, unlike graphene which exhibits strong saturation. This opens up promising avenues for increased THz nonlinear conversion efficiencies, and high-bandwidth optoelectronic and spintronic information and communication applications.
For many of the envisioned optoelectronic applications of graphene it is crucial to understand the sub-picosecond carrier dynamics immediately following photoexcitation, as well as the effect on the ...electrical conductivity - the photoconductivity. Whereas these topics have been studied using various ultrafast experiments and theoretical approaches, controversial and incomplete explanations have been put forward concerning the sign of the photoconductivity, the occurrence and significance of the creation of additional electron-hole pairs, and, in particular, how the relevant processes depend on Fermi energy. Here, we present a unified and intuitive physical picture of the ultrafast carrier dynamics and the photoconductivity, combining optical pump - terahertz probe measurements on a gate-tunable graphene device, with numerical calculations using the Boltzmann equation. We distinguish two types of ultrafast photo-induced carrier heating processes: At low (equilibrium) Fermi energy (\(E_{\rm F} \lesssim\) 0.1 eV for our experiments) broadening of the carrier distribution involves interband transitions - interband heating. At higher Fermi energy (\(E_{\rm F} \gtrsim\) 0.15 eV) broadening of the carrier distribution involves intraband transitions - intraband heating. Under certain conditions, additional electron-hole pairs can be created (carrier multiplication) for low \(E_{\rm F}\), and hot carriers (hot-carrier multiplication) for higher \(E_{\rm F}\). The resultant photoconductivity is positive (negative) for low (high) \(E_{\rm F}\), which originates from the effect of the heated carrier distributions on the screening of impurities, consistent with the DC conductivity being mostly due to impurity scattering. The importance of these insights is highlighted by a discussion of the implications for graphene photodetector applications.
Van der Waals heterostructures have emerged as promising building blocks that offer access to new physics, novel device functionalities, and superior electrical and optoelectronic properties. ...Applications such as thermal management, photodetection, light emission, data communication, high-speed electronics and light harvesting require a thorough understanding of (nanoscale) heat flow. Here, using time-resolved photocurrent measurements we identify an efficient out-of-plane energy transfer channel, where charge carriers in graphene couple to hyperbolic phonon polaritons in the encapsulating layered material. This hyperbolic cooling is particularly efficient, giving picosecond cooling times, for hexagonal BN, where the high-momentum hyperbolic phonon polaritons enable efficient near-field energy transfer. We study this heat transfer mechanism through distinct control knobs to vary carrier density and lattice temperature, and find excellent agreement with theory without any adjustable parameters. These insights may lead to the ability to control heat flow in van der Waals heterostructures.
Understanding thermal transport in layered transition metal dichalcogenide
(TMD) crystals is crucial for a myriad of applications exploiting these
materials. Despite significant efforts, several ...basic thermal transport
properties of TMDs are currently not well understood. Here, we present a
combined experimental-theoretical study of the intrinsic lattice thermal
conductivity of the representative TMD MoSe$_2$, focusing on the effect of
material thickness and the material's environment. We use Raman thermometry
measurements on suspended crystals, where we identify and eliminate crucial
artefacts, and perform $ab$ $initio$ simulations with phonons at finite, rather
than zero, temperature. We find that phonon dispersions and lifetimes change
strongly with thickness, yet (sub)nanometer thin TMD films exhibit a similar
in-plane thermal conductivity ($\sim$20~Wm$^{-1}$K$^{-1}$) as bulk crystals
($\sim$40~Wm$^{-1}$K$^{-1}$). This is the result of compensating phonon
contributions, in particular low-frequency modes with a surprisingly long mean
free path of several micrometers that contribute significantly to thermal
transport for monolayers. We furthermore demonstrate that out-of-plane heat
dissipation to air is remarkably efficient, in particular for the thinnest
crystals. These results are crucial for the design of TMD-based applications in
thermal management, thermoelectrics and (opto)electronics.
Photoexcitation of graphene leads to an interesting sequence of phenomena, some of which can be exploited in optoelectronic devices based on graphene. In particular, the efficient and ultrafast ...generation of an electron distribution with an elevated electron temperature and the concomitant generation of a photo-thermoelectric voltage at symmetry-breaking interfaces is of interest for photosensing and light harvesting. Here, we experimentally study the generated photocurrent at the graphene-metal interface, focusing on the time-resolved photocurrent, the effects of photon energy, Fermi energy and light polarization. We show that a single framework based on photo-thermoelectric photocurrent generation explains all experimental results.
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