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Search Results: 1 - 10 of 1931 matches for " Emma Springate "
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Population Inversion in Monolayer and Bilayer Graphene
Isabella Gierz,Matteo Mitrano,Jesse C. Petersen,Cephise Cacho,I. C. Edmond Turcu,Emma Springate,Alexander St?hr,Axel K?hler,Ulrich Starke,Andrea Cavalleri
Physics , 2014, DOI: 10.1088/0953-8984/27/16/164204
Abstract: 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.
Snapshots of non-equilibrium Dirac carrier distributions in graphene
Isabella Gierz,Jesse C. Petersen,Matteo Mitrano,Cephise Cacho,Edmond Turcu,Emma Springate,Alexander St?hr,Axel K?hler,Ulrich Starke,Andrea Cavalleri
Physics , 2013, DOI: 10.1038/nmat3757
Abstract: 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 semiconducting 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 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 ultra-violet (EUV) pulses at 31.5 eV photon energy 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. While 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 indication for carrier multiplication, questioning the applicability of graphene for light harvesting. Time-resolved photoemission spectroscopy in the EUV emerges as the technique of choice to assess the suitability of new materials for optoelectronics, providing quantitatively accurate measurements of non-equilibrium carriers at all energies and wavevectors.
Probing the structure and dynamics of molecular clusters using rotational wavepackets
Gediminas Galinis,Cephise Cacho,Richard T. Chapman,Andrew M. Ellis,Marius Lewerenz,Luis G. Mendoza Luna,Russell S. Minns,Mirjana Mladenovic,Arnaud Rouzée,Emma Springate,I. C. Edmond Turcu,Mark J. Watkins,Klaus von Haeften
Physics , 2014, DOI: 10.1103/PhysRevLett.113.043004
Abstract: The chemical and physical properties of molecular clusters can heavily depend on their size, which makes them very attractive for the design of new materials with tailored properties. Deriving the structure and dynamics of clusters is therefore of major interest in science. Weakly bound clusters can be studied using conventional spectroscopic techniques, but the number of lines observed is often too small for a comprehensive structural analysis. Impulsive alignment generates rotational wavepackets, which provides simultaneous information on structure and dynamics, as has been demonstrated successfully for isolated molecules. Here, we apply this technique for the firsttime to clusters comprising of a molecule and a single helium atom. By forcing the population of high rotational levels in intense laser fields we demonstrate the generation of rich rotational line spectra for this system, establishing the highly delocalised structure and the coherence of rotational wavepacket propagation. Our findings enable studies of clusters of different sizes and complexity as well as incipient superfluidity effects using wavepacket methods.
Ramifications of Optical Pumping on the Interpretation of Time-Resolved Photoemission Experiments on Graphene
S?ren Ulstrup,Jens Christian Johannsen,Federico Cilento,Alberto Crepaldi,Jill A. Miwa,Michele Zacchigna,Cephise Cacho,Richard T. Chapman,Emma Springate,Felix Fromm,Christian Raidel,Thomas Seyller,Phil D. C. King,Fulvio Parmigiani,Marco Grioni,Philip Hofmann
Physics , 2015,
Abstract: In pump-probe time and angle-resolved photoemission spectroscopy (TR-ARPES) experiments the presence of the pump pulse adds a new level of complexity to the photoemission process in comparison to conventional ARPES. This is evidenced by pump-induced vacuum space-charge effects and surface photovoltages, as well as multiple pump excitations due to internal reflections in the sample-substrate system. These processes can severely affect a correct interpretation of the data by masking the out-of-equilibrium electron dynamics intrinsic to the sample. In this study, we show that such effects indeed influence TR-ARPES data of graphene on a silicon carbide (SiC) substrate. In particular, we find a time- and laser fluence-dependent spectral shift and broadening of the acquired spectra, and unambiguously show the presence of a double pump excitation. The dynamics of these effects is slower than the electron dynamics in the graphene sample, thereby permitting us to deconvolve the signals in the time domain. Our results demonstrate that complex pump-related processes should always be considered in the experimental setup and data analysis.
Ultrafast Dynamics of Massive Dirac Fermions in Bilayer Graphene
S?ren Ulstrup,Jens Christian Johannsen,Federico Cilento,Jill A. Miwa,Alberto Crepaldi,Michele Zacchigna,Cephise Cacho,Richard Chapman,Emma Springate,Samir Mammadov,Felix Fromm,Christian Raidel,Thomas Seyller,Fulvio Parmigiani,Marco Grioni,Phil D. C. King,Philip Hofmann
Physics , 2014, DOI: 10.1103/PhysRevLett.112.257401
Abstract: Bilayer graphene is a highly promising material for electronic and optoelectronic applications since it is supporting massive Dirac fermions with a tuneable band gap. However, no consistent picture of the gap's effect on the optical and transport behavior has emerged so far, and it has been proposed that the insulating nature of the gap could be compromised by unavoidable structural defects, by topological in-gap states, or that the electronic structure could be altogether changed by many-body effects. Here we directly follow the excited carriers in bilayer graphene on a femtosecond time scale, using ultrafast time- and angle-resolved photoemission. We find a behavior consistent with a single-particle band gap. Compared to monolayer graphene, the existence of this band gap leads to an increased carrier lifetime in the minimum of the lowest conduction band. This is in sharp contrast to the second sub-state of the conduction band, in which the excited electrons decay through fast, phonon-assisted inter-band transitions.
Momentum-dependent snapshots of a melting charge density wave
Jesse C. Petersen,Stefan Kaiser,Nicky Dean,Alberto Simoncig,Haiyun Liu,Adrian L. Cavalieri,Cephise Cacho,I. C. Edmond Turcu,Emma Springate,Fabio Frassetto,Luca Poletto,Sarnjeet S. Dhesi,Helmuth Berger,Andrea Cavalleri
Physics , 2010, DOI: 10.1103/PhysRevLett.107.177402
Abstract: 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.
Phonon-pump XUV-photoemission-probe in graphene: evidence for non-adiabatic heating of Dirac carriers by lattice deformation
Isabella Gierz,Matteo Mitrano,Hubertus Bromberger,Cephise Cacho,Richard Chapman,Emma Springate,Stefan Link,Ulrich Starke,Burkhard Sachs,Martin Eckstein,Tim O. Wehling,Mikhail I. Katsnelson,Alexander Lichtenstein,Andrea Cavalleri
Physics , 2014, DOI: 10.1103/PhysRevLett.114.125503
Abstract: We modulate the atomic structure of bilayer graphene by driving its lattice at resonance with the in-plane E1u lattice vibration at 6.3um. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES) with extreme ultra-violet (XUV) pulses, we measure the response of the Dirac electrons near the K-point. We observe that lattice modulation causes anomalous carrier dynamics, with the Dirac electrons reaching lower peak temperatures and relaxing at faster rate compared to when the excitation is applied away from the phonon resonance or in monolayer samples. Frozen phonon calculations predict dramatic band structure changes when the E1u vibration is driven, which we use to explain the anomalous dynamics observed in the experiment.
Direct view on the ultrafast carrier dynamics in graphene
Jens Christian Johannsen,S?ren Ulstrup,Federico Cilento,Alberto Crepaldi,Michele Zacchigna,Cephise Cacho,I. C. Edmond Turcu,Emma Springate,Felix Fromm,Christian Raidel,Thomas Seyller,Fulvio Parmigiani,Marco Grioni,Philip Hofmann
Physics , 2013, DOI: 10.1103/PhysRevLett.111.027403
Abstract: The ultrafast dynamics of excited carriers in graphene is closely linked to the Dirac spectrum and plays a central role for many electronic and optoelectronic applications. Harvesting energy from excited electron-hole pairs, for instance, is only possible if these pairs can be separated before they lose energy to vibrations, merely heating the lattice. While the hot carrier dynamics in graphene could so far only be accessed indirectly, we here present a direct time-resolved view on the Dirac cone by angle-resolved photoemission (ARPES). This allows us to show the quasi-instant thermalisation of the electron gas to a temperature of more than 2000 K; to determine the time-resolved carrier density; to disentangle the subsequent decay into excitations of optical phonons and acoustic phonons (directly and via supercollisions); and to show how the presence of the hot carrier distribution affects the lifetime of the states far below the Fermi energy.
Observation of Ultrafast Free Carrier Dynamics in Single Layer MoS$_2$
Antonija Grubi?i? ?abo,Jill A. Miwa,Signe S. Gr?nborg,Jonathon M. Riley,Jens C. Johannsen,Cephise Cacho,Oliver Alexander,Richard T. Chapman,Emma Springate,Marco Grioni,Jeppe V. Lauritsen,Phil D. C. King,Philip Hofmann,S?ren Ulstrup
Physics , 2015, DOI: 10.1021/acs.nanolett.5b01967
Abstract: The dynamics of excited electrons and holes in single layer (SL) MoS$_2$ have so far been difficult to disentangle from the excitons that dominate the optical response of this material. Here, we use time- and angle-resolved photoemission spectroscopy for a SL of MoS$_2$ on a metallic substrate to directly measure the excited free carriers. This allows us to ascertain a direct quasiparticle band gap of 1.95 eV and determine an ultrafast (50 fs) extraction of excited free carriers via the metal in contact with the SL MoS$_2$. This process is of key importance for optoelectronic applications that rely on separated free carriers rather than excitons.
Ethics without Morality, Morality without Ethics—Politics, Identity, Responsibility in Our Contemporary World  [PDF]
Emma Palese
Open Journal of Philosophy (OJPP) , 2013, DOI: 10.4236/ojpp.2013.33055
Abstract: Ethics without morality and morality without ethics are the characteristics of two distinct eras: modernity and post-modernity. The duty to obey the law is an ethical act, but not always moral. Morality in fact is something more: a principle of responsibility and an index of humanity. This paper aims to explain the historical relationship between morality, ethics and politics up to the present day. The erosion of the nation-state, global capitalism, bio-economy leads us to rethink the meaning of ethics, morality and politics. A utilitarian ethics and a necessary morality may be the new frontiers of our contemporary world.
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