Jesse C. Petersen

Snapshots of non-equilibrium Dirac carrier distributions in graphene

The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all of these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy- and momentum-dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme-ultraviolet pulses to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. Whereas excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find any indication of carrier multiplication, questioning the applicability of graphene for light harvesting.

Clocking the Melting Transition of Charge and Lattice Order in 1T−TaS2 with Ultrafast Extreme-Ultraviolet Angle-Resolved Photoemission Spectroscopy

Charge density waves (CDWs) underpin the electronic properties of many complex materials. Near-equilibrium CDW order is linearly coupled to a periodic, atomic-structural distortion, and the dynamics is understood in terms of amplitude and phase modes. However, at the shortest timescales lattice and charge order may become de-coupled, highlighting the electronic nature of this many-body broken symmetry ground state. Using time and angle resolved photoemission spectroscopy with sub-30-fs XUV pulses, we have mapped the time- and momentum-dependent electronic structure in photo-stimulated 1T-TaS2, a prototypical two-dimensional charge density wave compound. We find that CDW order, observed as a splitting of the uppermost electronic bands at the Brillouin zone boundary, melts well before relaxation of the underlying structural distortion. Decoupled charge and lattice modulations challenge the view of Fermi Surface nesting as a driving force for charge density wave formation in 1T-TaS2.

Bi-directional ultrafast electric-field gating of interlayer charge transport in a cuprate superconductor

Scientists demonstrate that strong single-cycle terahertz pulses can switch off interlayer superconductivity in a cuprate superconductor while leaving in-plane superconductivity unaltered. The effect may prove useful for studying and controlling the behaviour of future ultrafast nanoelectronics.

Polaronic Conductivity in the Photoinduced Phase of 1T−TaS2

The transient optical conductivity of photoexcited 1T-TaS2 is determined over a three-order-of-magnitude frequency range. Prompt collapse and recovery of the Mott gap is observed. However, we find important differences between this transient metallic state and that seen across the thermally driven insulator-metal transition. Suppressed low-frequency conductivity, Fano phonon line shapes, and a midinfrared absorption band point to polaronic transport. This is explained by noting that the photoinduced metallic state of 1T-TaS2 is one in which the Mott gap is melted but the lattice retains its low-temperature symmetry, a regime only accessible by photodoping.

Population inversion in monolayer and bilayer graphene

The recent demonstration of saturable absorption and negative optical conductivity in the Terahertz range in graphene has opened up new opportunities for optoelectronic applications based on this and other low dimensional materials. Recently, population inversion across the Dirac point has been observed directly by time- and angle-resolved photoemission spectroscopy (tr-ARPES), revealing a relaxation time of only ∼130 femtoseconds. This severely limits the applicability of single layer graphene to, for example, Terahertz light amplification. Here we use tr-ARPES to demonstrate long-lived population inversion in bilayer graphene. The effect is attributed to the small band gap found in this compound. We propose a microscopic model for these observations and speculate that an enhancement of both the pump photon energy and the pump fluence may further increase this lifetime.

Possible observation of parametrically amplified coherent phasons in K0.3MoO3 using time-resolved extreme-ultraviolet angle-resolved photoemission spectroscopy

. Prompt depletion of the charge-densitywavecondensatelaunchescoherentoscillationsoftheamplitudemode,observedasa1.7-THz-frequencymodulationofthebondingbandposition.Incontrast,theantibondingbandoscillatesatabouthalfthisfrequency.Weattributetheseoscillationstocoherentexcitationofphasonsviaparametricamplificationofphasefluctuations.DOI: 10.1103/PhysRevB.88.045104 PACS number(s): 78

Ultrafast science and development at the Artemis facility

The Artemis facility for ultrafast XUV science is constructed around a high average power carrier-envelope phasestabilised system, which is used to generate tuneable pulses across a wavelength range spanning the UV to the far infrared, few-cycle pulses at 800nm and short pulses of XUV radiation produced through high harmonic generation. The XUV pulses can be delivered to interaction stations for materials science and atomic and molecular physics and chemistry through two vacuum beamlines for broadband XUV or narrow-band tuneable XUV pulses. The novel XUV monochromator provides bandwidth selection and tunability while preserving the pulse duration to within 10 fs. Measurements of the XUV pulse duration using an XUV-pump IR-probe technique demonstrate that the XUV pulselength is below 30 fs for a 28 fs drive laser pulse. The materials science station, which contains a hemispherical electron analyser and five-axis manipulator cooled to 14K, is optimised for photoemission experiments with the XUV. The end-station for atomic and molecular physics and chemistry includes a velocity-map imaging detector and molecular beam source for gas-phase experiments. The facility is now fully operational and open to UK and European users for twenty weeks per year. Some of the key new scientific results obtained on the facility include: the extension of HHG imaging spectroscopy to the mid-infrared; a technique for enhancing the conversion efficiency of the XUV by combining two laser fields with non-harmonically related wavelengths; and observation of D3+ photodissociation in intense laser fields.

Ultrafast Tr-ARPES with Artemis XUV Beamline

A new HHG XUV beamline at Artemis, user open-access facility at CLF, offers unique capabilities optimised for Tr-ARPES. Current result on ultrafast melting of Mott and charge order in TaS2 will be presented.

Time-and-Angle-Resolved Photoemission Spectroscopy with Mid-IR Pump and 30 fs EUV Probe

Time- and angle-resolved photoemission spectroscopy directly enables observation of electron dynamics in condensed matter. Using EUV high harmonic probe extends the observation window in energy and momentum. Tuneable mid-infrared pumping allows control of excitation mechanisms.

Monochromatised XUV Pulses for Ultrafast Science at the Artemis Facility

XUV pulses produced through high harmonic generation can probe electron dynamics in complex solid materials and in gas-phase atoms and molecules. This is demonstrated in gas-phase and condensed matter experiments at the Artemis facility.