Ultrafast relaxation of hot phonons in Graphene-hBN Heterostructures

Fast carrier cooling is important for high power graphene based devices. Strongly Coupled Optical Phonons (SCOPs) play a major role in the relaxation of photoexcited carriers in graphene. Heterostructures of graphene and hexagonal boron nitride (hBN) have shown exceptional mobility and high saturation current, which makes them ideal for applications, but the effect of the hBN substrate on carrier cooling mechanisms is not understood. We track the cooling of hot photo-excited carriers in graphene-hBN heterostructures using ultrafast pump-probe spectroscopy. We find that the carriers cool down four times faster in the case of graphene on hBN than on a silicon oxide substrate thus overcoming the hot phonon (HP) bottleneck that plagues cooling in graphene devices.Comment: Pages 1-12: Main manuscript. Pages 13-18: Supplementary materia

Ultrafast dynamics of neutral superexcited Oxygen: A direct measurement of the competition between autoionization and predissociation

Using ultrafast extreme ultraviolet pulses, we performed a direct measurement of the relaxation dynamics of neutral superexcited states corresponding to the nl\sigma_g(c^4\Sigma_u^-) Rydberg series of O_2. An XUV attosecond pulse train was used to create a temporally localized Rydberg wavepacket and the ensuing electronic and nuclear dynamics were probed using a time-delayed femtosecond near-infrared pulse. We investigated the competing predissociation and autoionization mechanisms for superexcited molecules and found that autoionization is dominant for the low n Rydberg states. We measured an autoionization lifetime of 92+/-6 fs and 180+/-10 fs for (5s,4d)\sigma_g and (6s,5d)\sigma_g Rydberg state groups respectively. We determine that the disputed neutral dissociation lifetime for the \nu=0 vibrational level of the Rydberg series is 1100+/-100fs.Comment: 5 pages, 4 figure

ULTRAFAST XUV SPECTROSCOPY TO PROBE CONICAL INTERSECTIONS AND EXCITED STATE DYNAMICS

\begin{wrapfigure}{l}{0pt} \includegraphics[scale=0.3]{Conical.eps} \end{wrapfigure} Femtosecond and attosecond XUV spectroscopy was used to study of electron dynamics stemming from many-body interactions, including the coupling between the electronic and nuclear degrees of freedom, electronic correlations, external light fields, or a combination thereof. \footnote{This work was supported by the U. S. Army Research Laboratory and the U. S. Army Research Office under grant number W911NF-14-1-0383 and the National Science Foundation (NSF) award number PHY-1505556.} Conical intersections are an important topic of investigation because they serve as nature’s energy funnels in many biochemical processes, e.g. vision, light harvesting, etc. We focused on nuclear motion mediated evolution of an electron hole near a conical intersection in a CO$_2$ ion. Using pump-probe photodissociation spectroscopy, we made quantitative measurements of electronic couplings and monitored the role of decoherence in such dynamics, thereby probing the fundamental mechanisms responsible for the charge and energy redistribution in molecules. In another study, time resolved XUV photoelectron spectroscopy was applied to identify the role of multi electron excitations in the ultrafast Rydberg state dissociation of highly excited states in O$_2$. The talk will also discuss new opportunities arising the application of attosecond soft-x-ray sources

Evolution of the electronic band structure of twisted bilayer graphene upon doping

The electronic band structure of twisted bilayer graphene develops van Hove singularities whose energy depends on the twist angle between the two layers. Using Raman spectroscopy, we monitor the evolution of the electronic band structure upon doping using the G peak area which is enhanced when the laser photon energy is resonant with the energy separation of the van Hove singularities. Upon charge doping, the Raman G peak area initially increases for twist angles larger than a critical angle and decreases for smaller angles. To explain this behavior with twist angle, the energy of separation of the van Hove singularities must decrease with increasing charge density demonstrating the ability to modify the electronic and optical properties of twisted bilayer graphene with doping.Comment: 10 pages, 4 figure

Beyond the single-atom response in absorption lineshapes: Probing a dense, laser-dressed helium gas with attosecond pulse trains

We investigate the absorption line shapes of laser-dressed atoms beyond the single-atom response, by using extreme ultraviolet (XUV) attosecond pulse trains to probe an optically thick helium target under the influence of a strong infrared (IR) field. We study the interplay between the IR-induced phase shift of the microscopic time-dependent dipole moment and the resonant-propagation-induced reshaping of the macroscopic XUV pulse. Our experimental and theoretical results show that as the optical depth increases, this interplay leads initially to a broadening of the IR-modified line shape, and subsequently to the appearance of new, narrow features in the absorption line.Comment: 5 pages, 5 figure

Time resolved evolution of structural, electrical, and thermal properties of copper irradiated by an intense ultrashort laser pulse

The dynamical properties of copper metal are obtained on a picosecond time scale using 100 fs laser pulse at 1015 Wcm-2-an intensity regime relevant to femtosecond micromachining. The dissipation mechanisms and scaling laws spanning a wide temperature range are obtained from femtosecond pump-probe reflectivity. We observe obliteration of the crystalline structure in copper within 400 fs due to lattice disorder caused by the intense laser pulse. The electrical resistivity is obtained by studying the probe reflectivity evolution from 0 to 30 ps. The "resistivity saturation" effect in an unexplored regime intermediate to hot plasma and cold solid is studied in detail. The temperature evolution and thermal conductivity values are also obtained