Strongly correlated electron-photon systems.

An important goal of modern condensed-matter physics involves the search for states of matter with emergent properties and desirable functionalities. Although the tools for material design remain relatively limited, notable advances have been recently achieved by controlling interactions at heterointerfaces, precise alignment of low-dimensional materials and the use of extreme pressures. Here we highlight a paradigm based on controlling light-matter interactions, which provides a way to manipulate and synthesize strongly correlated quantum matter. We consider the case in which both electron-electron and electron-photon interactions are strong and give rise to a variety of phenomena. Photon-mediated superconductivity, cavity fractional quantum Hall physics and optically driven topological phenomena in low dimensions are among the frontiers discussed in this Perspective, which highlights a field that we term here 'strongly correlated electron-photon science'.

Designing and controlling the properties of transition metal oxide quantum materials.

This Perspective addresses the design, creation, characterization and control of synthetic quantum materials with strong electronic correlations. We show how emerging synergies between theoretical/computational approaches and materials design/experimental probes are driving recent advances in the discovery, understanding and control of new electronic behaviour in materials systems with interesting and potentially technologically important properties. The focus here is on transition metal oxides, where electronic correlations lead to a myriad of functional properties including superconductivity, magnetism, Mott transitions, multiferroicity and emergent behaviour at picoscale-designed interfaces. Current opportunities and challenges are also addressed, including possible new discoveries of non-equilibrium phenomena and optical control of correlated quantum phases of transition metal oxides.

Photoinduced Electron Pairing in a Driven Cavity.

We demonstrate how virtual scattering of laser photons inside a cavity via two-photon processes can induce controllable long-range electron interactions in two-dimensional materials. We show that laser light that is red (blue) detuned from the cavity yields attractive (repulsive) interactions whose strength is proportional to the laser intensity. Furthermore, we find that the interactions are not screened effectively except at very low frequencies. For realistic cavity parameters, laser-induced heating of the electrons by inelastic photon scattering is suppressed and coherent electron interactions dominate. When the interactions are attractive, they cause an instability in the Cooper channel at a temperature proportional to the square root of the driving intensity. Our results provide a novel route for engineering electron interactions in a wide range of two-dimensional materials including AB-stacked bilayer graphene and the conducting interface between LaAlO_{3} and SrTiO_{3}.

Cavity-Mediated Electron-Photon Superconductivity.

We investigate electron paring in a two-dimensional electron system mediated by vacuum fluctuations inside a nanoplasmonic terahertz cavity. We show that the structured cavity vacuum can induce long-range attractive interactions between current fluctuations which lead to pairing in generic materials with critical temperatures in the low-kelvin regime for realistic parameters. The induced state is a pair-density wave superconductor which can show a transition from a fully gapped to a partially gapped phase-akin to the pseudogap phase in high-T_{c} superconductors. Our findings provide a promising tool for engineering intrinsic electron interactions in two-dimensional materials.

Measuring non-equilibrium dynamics in complex solids with ultrashort X-ray pulses.

Strong interactions between electrons give rise to the complexity of quantum materials, which exhibit exotic functional properties and extreme susceptibility to external perturbations. A growing research trend involves the study of these materials away from equilibrium, especially in cases in which the stimulation with optical pulses can coherently enhance cooperative orders. Time-resolved X-ray probes are integral to this type of research, as they can be used to track atomic and electronic structures as they evolve on ultrafast timescales. Here, we review a series of recent experiments where femtosecond X-ray diffraction was used to measure dynamics of complex solids. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Theory of Enhanced Interlayer Tunneling in Optically Driven High-T_{c} Superconductors.

Motivated by recent pump-probe experiments indicating enhanced coherent c-axis transport in underdoped YBCO, we study Josephson junctions periodically driven by optical pulses. We propose a mechanism for this observation by demonstrating that a parametrically driven Josephson junction shows an enhanced imaginary part of the low-frequency conductivity when the driving frequency is above the plasma frequency, implying an effectively enhanced Josephson coupling. We generalize this analysis to a bilayer system of Josephson junctions modeling YBCO. Again, the Josephson coupling is enhanced when the pump frequency is blue detuned to either of the two plasma frequencies of the material. We show that the emergent driven state is a genuine, nonequilibrium superconducting state, in which equilibrium relations between the Josephson coupling, current fluctuations, and the critical current no longer hold.

Electronic-structural dynamics in graphene.

We review our recent time- and angle-resolved photoemission spectroscopy experiments, which measure the transient electronic structure of optically driven graphene. For pump photon energies in the near infrared ([Formula: see text]), we have discovered the formation of a population-inverted state near the Dirac point, which may be of interest for the design of THz lasing devices and optical amplifiers. At lower pump photon energies ([Formula: see text]), for which interband absorption is not possible in doped samples, we find evidence for free carrier absorption. In addition, when mid-infrared pulses are made resonant with an infrared-active in-plane phonon of bilayer graphene ([Formula: see text]), a transient enhancement of the electron-phonon coupling constant is observed, providing interesting perspective for experiments that report light-enhanced superconductivity in doped fullerites in which a similar lattice mode was excited. All the studies reviewed here have important implications for applications of graphene in optoelectronic devices and for the dynamical engineering of electronic properties with light.

Non-equilibrium control of complex solids by nonlinear phononics.

We review some recent advances in the use of optical fields at terahertz frequencies to drive the lattice of complex materials. We will focus on the control of low energy collective properties of solids, which emerge on average when a high frequency vibration is driven and a new crystal structure induced. We first discuss the fundamentals of these lattice rearrangements, based on how anharmonic mode coupling transforms an oscillatory motion into a quasi-static deformation of the crystal structure. We then discuss experiments, in which selectively changing a bond angle turns an insulator into a metal, accompanied by changes in charge, orbital and magnetic order. We then address the case of light induced non-equilibrium superconductivity, a mysterious phenomenon observed in some cuprates and molecular materials when certain lattice vibrations are driven. Finally, we show that the dynamics of electronic and magnetic phase transitions in complex-oxide heterostructures follow distinctly new physical pathways in case of the resonant excitation of a substrate vibrational mode.

Phonon-pump extreme-ultraviolet-photoemission probe in graphene: anomalous heating of Dirac carriers by lattice deformation.

We modulate the atomic structure of bilayer graphene by driving its lattice at resonance with the in-plane E_{1u} lattice vibration at 6.3 µm. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES) with extreme-ultraviolet (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 E_{1u} vibration is driven, which we use to explain the anomalous dynamics observed in the experiment.

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.

Non-equilibrium Dirac carrier dynamics in graphene investigated with time- and angle-resolved photoemission spectroscopy.

We have used time- and angle-resolved photoemission spectroscopy (tr-ARPES) to assess the influence of many-body interactions on the Dirac carrier dynamics in graphene. From the energy-dependence of the measured scattering rates we directly determine the imaginary part of the self-energy, visualizing the existence of a relaxation bottleneck associated with electron-phonon coupling. A comparison with static line widths obtained by high-resolution ARPES indicates that the dynamics of photo-excited carriers in graphene are solely determined by the equilibrium self-energy. Furthermore, the subtle interplay of different many-body interactions in graphene may allow for carrier multiplication, where the absorption of a single photon generates more than one electron-hole pair via impact ionization. We find that, after photo-excitation, the number of carriers in the conduction band along the ΓK-direction keeps increasing for about 40 fs after the pump pulse is gone. A definite proof of carrier multiplication in graphene, however, requires a more systematic study, carefully taking into account the contribution of momentum relaxation on the measured rise time.

Pulse shaping in the mid-infrared by a deformable mirror.

We present a pulse-shaping scheme operating in the mid-infrared (MIR) wavelength range up to 20 µm. The spectral phase is controlled by a specially designed large stroke 32-actuator deformable mirror in a grating-based 4f configuration. We demonstrate the shaper capability of compressing the MIR pulses, imparting parabolic and third-order spectral phases and splitting the spectral content to create two independent pulses.

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.

Coherent single-cycle pulses with MV/cm field strengths from a relativistic transition radiation light source.

Terahertz (THz) pulses with energies up to 100 µJ and corresponding electric fields up to 1 MV/cm were generated by coherent transition radiation from 500 MeV electron bunches at the free-electron laser Freie-Elektronen-Laser in Hamburg (FLASH). The pulses were characterized in the time domain by electro-optical sampling by a synchronized femtosecond laser with jitter of less than 100 fs. High THz field strengths and quality of synchronization with an optical laser will enable observation of nonlinear THz phenomena.

Transient photoinduced 'hidden' phase in a manganite.

Photoinduced phase transitions are of special interest in condensed matter physics because they can be used to change complex macroscopic material properties on the ultrafast timescale. Cooperative interactions between microscopic degrees of freedom greatly enhance the number and nature of accessible states, making it possible to switch electronic, magnetic or structural properties in new ways. Photons with high energies, of the order of electron volts, in particular are able to access electronic states that may differ greatly from states produced with stimuli close to equilibrium. In this study we report the photoinduced change in the lattice structure of a charge and orbitally ordered Nd(0.5)Sr(0.5)MnO(3) thin film using picosecond time-resolved X-ray diffraction. The photoinduced state is structurally ordered, homogeneous, metastable and has crystallographic parameters different from any thermodynamically accessible state. A femtosecond time-resolved spectroscopic study shows the formation of an electronic gap in this state. In addition, the threshold-like behaviour and high efficiency in photo-generation yield of this gapped state highlight the important role of cooperative interactions in the formation process. These combined observations point towards a 'hidden insulating phase' distinct from that found in the hitherto known phase diagram.

Single-shot detection and direct control of carrier phase drift of midinfrared pulses.

We introduce a scheme for single-shot detection and correction of the carrier-envelope phase (CEP) drift of femtosecond pulses at mid-IR wavelengths. Difference frequency mixing between the mid-IR field and a near-IR gate pulse generates a near-IR frequency-shifted pulse, which is then spectrally interfered with a replica of the gate pulse. The spectral interference pattern contains shot-to-shot information of the CEP of the mid-IR field, and it can be used for simultaneous correction of its slow drifts. We apply this technique to detect and compensate long-term phase drifts at 17 microm wavelength, reducing fluctuations to only 110 mrad over hours of operation.

Retention loss of class v restorations after artificial aging.

PURPOSE: to evaluate whether retention loss of Class V restorations can be simulated in the laboratory and to compare these results with those from clinical trials. MATERIALS AND METHODS: nonretentive v-shaped Class V cavities were prepared on the lingual and buccal side of extracted premolars, half in dentin and half in enamel. Different adhesive systems (AS) were used with the same composite resin (Tetric EvoCeram) and 12 restorations per group: 1-step self-etching AS (AdheSE One, Adper Prompt L-Pop, Futurabond N, Hybrid Bond, iBond, Xeno III, Xeno IV), 2-step self-etching AS (AdheSE), 2-step etch-and-rinse AS (Excite, Prime & Bond NT), 3-step etch-and-rinse AS (Syntac) as well as a conventional glass ionomer (Ketac Fil) with and without conditioner. The comparison groups were the composite without adhesive and a 2-step etch-and-rinse AS (Prime & Bond NT) without etching of enamel and dentin. The restored teeth were submitted to an aging process involving 18 months of water storage, three intermittent phases of thermocycling (TC 10,000 times), and two phases of thermomechanical loading (1 x 640,000 chewing cycles after 12 months, 1 x 1,200,000 chewing cycles after 18 months; 100 N sine-wave force profile, pressing with steel ball without lifting). Retention loss of the restorations was evaluated after every 1000 thermocycles and every 120,000 cycles of thermomechanical loading. The databases MEDLINE and IADR abstracts were used to search for clinical studies on retention loss involving the adhesive systems that were included in the present study. RESULTS: retention loss was only observed in the following groups: composite without adhesive (100% after first 1000 TC), glass ionomer without conditioner (8% after 6 months; 33% after 12 months, 100% after 18 months), adhesive without etching (17% after 6 months, 42% after 12 months). The laboratory results, however, matched with the clinical results only for three adhesive systems (Futurabond NR, Hybrid Bond, Xeno IV, 0% retention loss, 5 studies, observation period between 1.5 and 2 years). CONCLUSIONS: if the materials were applied according to the manufacturer's instructions, no retention loss was observed in the laboratory model. The laboratory model did not reflect the clinical findings.

Microleakage of Class II restorations with different tracers--comparison with SEM quantitative analysis.

PURPOSE: (1) To compare SEM quantitative marginal analysis data with the depth of penetration (DP) of the three most commonly used tracers for microleakage in Class II fillings in vitro; (2) based on the obtained results to calculate the discriminatory power of a sample size of 12. MATERIALS AND METHODS: Standardized large cavities (mesially 1 mm above the CEJ, distally 1 mm below the CEJ, intercuspal distance 70%) were prepared into 36 extracted caries-free first mandibular molars and filled with Tetric EvoCeram/AdheSE, the resin being applied in horizontal layers 2 mm thick. Each increment was light cured for 20 s (1200 mW/cm2). Finishing was performed with fine diamond burs and disks. All teeth were subjected to occlusal loading (1,200,000 cycles, 49 N/1.7 Hz) and simultaneous thermocycling (3000 cycles at 5 degrees C/55 degrees C). The percentage of continuous margin of the cervical dentin and enamel was evaluated on replicas using SEM. The teeth were subjected to tracer penetration with either 0.5% basic fuchsin (24 h, 37 degrees C), 2% methylene blue (24 h, 37 degrees C) or 50% silver nitrate solution (4 h, 37 degrees C, followed by 8-h exposure to a photodeveloping solution and overnight fluorescent light). The teeth were mesiodistally sectioned twice. The depth of tracer penetration was measured with a stereomicroscope and averaged for each site. Due to data inhomogeneity and abnormal distribution, both SEM and DP data were transformed. Sample size calculations were performed based on standard deviation and statistical error estimates. RESULTS: For the dentin margin, there was an acceptable correlation between SEM data and both fuchsin penetration (Pearson: -0.74, p < 0.01) and silver nitrate penetration (Pearson: -0.79, p < 0.01), but not between methylene blue and SEM data. For enamel margins, no significant correlation between SEM and DP data was found for the three tracers. There was statistically no significant difference in tracer penetration between the three tracers (Kruskal Wallis, p > 0.05). For all three tracers, statistically significantly higher penetration occurred at the dentin than at the enamel margin (Wilcoxon, p < 0.05). A sample size of 12 makes it possible to discriminate between materials only when they differ in tracer penetration in the range of 1 mm for enamel and 2 mm for dentinal margins. CONCLUSION: Tracer penetration with fuchsin or silver nitrate showed a moderate correlation with SEM quantitative marginal analysis data at dentinal margins, but not at enamel margins.