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.

We use time- and angle-resolved photoemission spectroscopy with sub-30-fs extreme-ultraviolet pulses to map the time- and momentum-dependent electronic structure of photoexcited 1T-TaS(2). This compound is a two-dimensional Mott insulator with charge-density wave ordering. Charge order, evidenced by splitting between occupied subbands at the Brillouin zone boundary, melts well before the lattice responds. This challenges the view of a charge-density wave caused by electron-phonon coupling and Fermi-surface nesting alone, and suggests that electronic correlations play a key role in driving charge order.

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.

In cuprate superconductors, tunnelling between planes makes three-dimensional superconductive transport possible. However, the interlayer tunnelling amplitude is reduced when an order-parameter-phase gradient between planes is established. As such, interlayer superconductivity along the c-axis can be weakened if a strong electric field is applied along the c-axis. In this Letter, we use high-field single-cycle terahertz pulses to gate interlayer coupling in La_1.84Sr_0.16CuO₄. We induce ultrafast oscillations between superconducting and resistive states and switch the plasmon response on and off, without reducing the density of Cooper pairs. In-plane superconductivity remains unperturbed, revealing a non-equilibrium state in which the dimensionality of the superconductivity is time-dependent. The gating frequency is determined by the electric field strength. Non-dissipative, bi-directional gating of superconductivity is of interest for device applications in ultrafast nanoelectronics and represents an example of how nonlinear terahertz physics can benefit nanoplasmonics and active metamaterials

G rowing evidence indicates that the superconducting pyrochlore Cd 2 Re 2 O 7 exhibits a structural phase transition at T c = 200 K with an unusual tensor character 1-3 . The structural order parameter for this state is two-dimensional, and spanned by distinct but nearly degenerate crystallographic structures I4 1 22 and I4m2 (ref. 1). Symmetry rules imply that the low-energy excitations of the ordered state are Goldstone phonons, or long wavelength fluctuations between the two crystal structures. These are the structural equivalents of magnons in an XY antiferromagnet, with the two crystal structures analogous to orthogonal spin directions in the xy-plane. Goldstone phonons have been observed in Raman spectroscopy 3 , but high-resolution X-ray and neutron scattering experiments have produced conflicting assignments of the static low-temperature structure 4-6 . Here, we use optical secondharmonic generation with polarization sensitivity to assign the I4m2 structure unambiguously and verify an auxiliary condition on the structure that is implied by the order parameter symmetry. We also show that the temperature dependence of the order parameter is consistent with thermal occupation of the Goldstone mode. The methodology may be applied more widely in characterizing ordered states in matter.Various experimental probes show a continuous cubic-totetragonal transition in Cd 2 Re 2 O 7 at T c (refs 1,2,4-10). However, as Anderson and Blount pointed out over 40 years ago 11 , a less conventional order parameter, possibly with ferroelectric character, must accompany strain to make any cubic-to-tetragonal transition continuous. Experiments have indeed ruled out strain as the primary order parameter for Cd 2 Re 2 O 7 (ref. 2), and both X-ray and neutron diffraction reveal broken inversion symmetry below T c (refs 4-6). Theoretical analysis indicates that the true order parameter is a second-rank pseudotensor, corresponding to the E u representation of the cubic point group 1,2 , shown in Fig. 1. The more familiar types of order all have lower rank: vectors for ferroelectricity, inversion-symmetric pseudovectors for ferromagnetism and second-rank tensors for ferroelasticity. Like the vector order of ferroelectricity, E u tensor order in In 0 Out

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.

We use time-and angle-resolved photoemission spectroscopy in the extreme ultraviolet to measure the timeand momentum-dependent electronic structures of photoexcited K 0.3 MoO 3 . Prompt depletion of the chargedensity wave condensate launches coherent oscillations of the amplitude mode, observed as a 1.7-THz-frequency modulation of the bonding band position. In contrast, the antibonding band oscillates at about half this frequency. We attribute these oscillations to coherent excitation of phasons via parametric amplification of phase fluctuations. The total energy of a low-dimensional metal can be lowered by a periodic distortion of the crystal lattice. Such a Peierls transition enlarges the unit cell and redistributes charge density, opening band gaps at the Fermi level.1 The resulting charge-density wave (CDW) ground state exhibits new low-energy collective excitations, the amplitudon and phason, which correspond to distortions and translations of the modulated charge density. 2The amplitude mode is weakly momentum dependent with a finite frequency at q = 0. In contrast, the phase-mode energy increases with momentum q, dispersing linearly near the Brillouin-zone center. In the case of noncommensurate CDWs, the phase mode ideally has zero energy at q = 0, corresponding to zero dc resistance. However, pinning to defects produces a gap in the phase-mode spectrum of most materials, and the phase-mode frequency is nonzero at q = 0. 3A widely studied quasi-one-dimensional (1D) CDW material is the linear chain compound K 0.3 MoO 3 , known as blue bronze. 4,5 Below T CDW = 180 K, a CDW forms, and a gap opens at the Fermi level.6,7 New collective excitations appear in the infrared, Raman, and neutron spectra. [8][9][10][11] The amplitude mode lies at 1.7 THz and softens with increasing temperature.11,12 The q = 0 phase mode is pinned, and its frequency is 0.1-0.2 THz. 11,13Femtosecond optical pulses can be used to trigger rearrangements in the collective properties of this and other complex solids.14 The photoinduced dynamics driven by ultrashort optical pulses result from strong perturbations in the ground state and proceed along physical pathways that are not easily predicted by the linear-response theory used to describe fluctuations of the ground state. For example, intense photoexcitation can readily destroy charge gaps, 15,16 and the underlying coherent pathways can only be partially understood by considering the near-equilibrium normal modes of the solid. 17,18Momentum-and time-dependent electronic structural dynamics can be measured by time-and angle-resolved photoemission spectroscopy (tr-ARPES). This method has already been applied to quasi-two-dimensional CDW compounds, such as 1T - TaS 2 , 19,20 TbTe 3 , 21 and 1T -TiSe 2 . 22 CDW gaps of various types were seen to melt, 23 and Raman-active amplitude modes were excited. 24In our experiment, 30-fs pulses at 790-nm wavelengths were used to stimulate the sample with a fluence of 0.5 mJ/cm 2 . Synchronized extreme-ultraviolet (EUV) pulses with a photon energy of...

Vacuum polarization, an effect predicted nearly 70 years ago, is still yet to be directly detected despite significant experimental effort. Previous attempts have made use of large liquid-helium cooled electromagnets which inadvertently generate spurious signals that mask the desired signal. We present a novel approach for the ultra-sensitive detection of optical birefringence that can be usefully applied to a laboratory detection of vacuum polarization. The new technique has a predicted birefringence measurement sensitivity of ∆n ∼ 10 −20 in a 1 second measurement. When combined with the extreme polarizing fields achievable in this design we predict that a vacuum polarization signal will be seen in a measurement of just a few days in duration.