Time- and angle-resolved photoemission spectroscopy with wavelength-tunable pump and extreme ultraviolet probe enabled by twin synchronized amplifiers.

We describe a setup for time- and angle-resolved photoemission spectroscopy with wavelength-tunable excitation and an extreme ultraviolet probe. It is enabled by using the 10 kHz twin Ti:sapphire amplifiers seeded by the common Ti:sapphire oscillator. The typical probe energy is 21.7 eV, and the wavelength of the pump excitation is tuned between 2400 and 1200 nm by using the optical parametric amplifier. The spectral width of the extreme ultraviolet probe is 53 meV, and the time resolution is dependent on the wavelength for the pump, better than 60 fs for the pump energy >0.7 eV. This system enables the pump energy to be matched with a specific interband transition and to probe a wider energy-momentum space. We present the results for the prototypical materials of highly oriented pyrolytic graphite and Bi2Se3 to show the performance of our system.

Highly CEP-stable optical parametric amplifier at 2†µm with a few-cycle duration and 100 kHz repetition rate.

We develop a BiB3O6 (BiBO)-based optical parametric amplifier in the spectral region around 2 µm using a Yb:KGW amplifier operating at 100 kHz. The two-stage degenerate optical parametric amplification results in a typical output energy of 30 µJ after compression, spectrum covering 1.7-2.5 µm range, and a pulse duration fully compressible down to 16.4 fs, corresponding to 2.3 cycles. Due to the inline difference frequency generation of the seed pulses, the carrier envelope phase (CEP) is passively stabilized without feedback over 11 hours at the level below 100 mrad including a long-term drift. Short-term statistical analysis in the spectral domain further shows a behavior qualitatively different from that of parametric fluorescence, indicating high degree of suppression of optical parametric fluorescence. The high phase stability together with the few-cycle pulse duration is promising for the investigation of high-field phenomena such as subcycle spectroscopy in solids or high harmonics generation.

Real-time observation of the Woodward-Hoffmann rule for 1,3-cyclohexadiene by femtosecond soft X-ray transient absorption.

The stereochemistry of pericyclic reactions is explained by orbital symmetry conservation, referred to as the Woodward-Hoffmann (WH) rule. Although this rule has been verified using the structures of reactants and products, the temporal evolution of the orbital symmetry during the reaction has not been clarified. Herein, we used femtosecond soft X-ray transient absorption spectroscopy to elucidate the thermal pericyclic reaction of 1,3-cyclohexadiene (CHD) molecules, i.e., their isomerization to 1,3,5-hexatriene. In the present experimental scheme, the ring-opening reaction is driven by the thermal vibrational energy induced by photoexcitation to the Rydberg states at 6.2 eV and subsequent femtosecond relaxation to the ground state of CHD molecules. The direction of the ring opening, which can be conrotatory or disrotatory, was the primary focus, and the WH rule predicts the disrotatory pathway in the thermal process. We observed the shifts in K-edge absorption of the carbon atom from the 1s orbital to vacant molecular orbitals around 285 eV at a delay between 340 and 600 fs. Furthermore, a theoretical investigation predicts that the shifts depend on the molecular structures along the reaction pathways and the observed shifts in induced absorption are attributed to the structural change in the disrotatory pathway. This confirms that the orbital symmetry is dynamically conserved in the ring-opening reaction of CHD molecules as predicted using the WH rule.

100-mJ class, sub-two-cycle, carrier-envelope phase-stable dual-chirped optical parametric amplification.

Based on dual-chirped optical parametric amplification (DC-OPA) and type-I BiB3O6 (BiBO) crystals, the generation of >100 mJ, 10.4 fs, 10 Hz, carrier-envelope phase (CEP)-stable laser pulses, which are centered at 1.7 µm, was demonstrated producing a peak power of 10 TW. CEP-dependent high harmonic generation (HHG) was implemented to confirm the sub-two-cycle pulse duration and CEP stabilization of infrared (IR) laser pulses. As far as we know, the obtained pulse energy and peak power represented the highest values for sub-two-cycle CEP-stable IR optical parametric amplification. Additionally, the prospects of achieving high-energy water window isolated attosecond pulses (IAPs) via our developed laser source were discussed.

Optical parametric amplification of phase-stable terahertz-to-mid-infrared pulses studied in the time domain.

We report optical parametric amplification (OPA) of low-frequency infrared pulses in the intermediate region between terahertz (THz) frequency and mid-infrared (MIR), i.e., from 16.9 to 44.8 THz (6.7-17.8 µm). The 255-fs laser output of the Yb:KGW regenerative amplifier is compressed to 11-fs pulses using a multi-plate broadening scheme, which generates THz-to-MIR pulses with a spectrum extending to approximately 50 THz by intra-pulse differential frequency generation (DFG) in GaSe. The THz-to-MIR pulses are further amplified using a two-stage OPA in GaSe. The temporal dynamics and photocarrier effects during OPA are characterized in the time domain. Owing to the intra-pulse DFG, the long-term phase drift of the THz-to-MIR pulses after two-stage OPA is as small as 16 mrad during a 6-h operation without any active feedback. Our scheme using the intra-pulse DFG and post-amplification proposes a new route to intense THz-to-MIR light sources with extreme phase stability.

Efficient Terahertz Harmonic Generation with Coherent Acceleration of Electrons in the Dirac Semimetal Cd_{3}As_{2}.

We report strong terahertz (∼10^{12} Hz) high harmonic generation at room temperature in thin films of Cd_{3}As_{2}, a three-dimensional Dirac semimetal. Third harmonics are detectable with a tabletop light source and can be as strong as 100 V/cm by applying a fundamental field of 6.5 kV/cm inside the film, demonstrating an unprecedented efficiency for terahertz frequency conversion. Our time-resolved terahertz spectroscopy and calculations also clarify the microscopic mechanism of the nonlinearity originating in the coherent acceleration of Dirac electrons in momentum space. Our results provide clear insights for nonlinear currents of Dirac electrons driven by the terahertz field under the influence of scattering, paving the way toward novel devices for high-speed electronics and photonics based on topological semimetals.

Few-cycle laser driven reaction nanoscopy on aerosolized silica nanoparticles.

Nanoparticles offer unique properties as photocatalysts with large surface areas. Under irradiation with light, the associated near-fields can induce, enhance, and control molecular adsorbate reactions on the nanoscale. So far, however, there is no simple method available to spatially resolve the near-field induced reaction yield on the surface of nanoparticles. Here we close this gap by introducing reaction nanoscopy based on three-dimensional momentum-resolved photoionization. The technique is demonstrated for the spatially selective proton generation in few-cycle laser-induced dissociative ionization of ethanol and water on SiO2 nanoparticles, resolving a pronounced variation across the particle surface. The results are modeled and reproduced qualitatively by electrostatic and quasi-classical mean-field Mie Monte-Carlo (M3C) calculations. Reaction nanoscopy is suited for a wide range of isolated nanosystems and can provide spatially resolved ultrafast reaction dynamics on nanoparticles, clusters, and droplets.

Ultrafast Unbalanced Electron Distributions in Quasicrystalline 30° Twisted Bilayer Graphene.

Ultrafast carrier dynamics in a graphene system are very important in terms of optoelectronic devices. Recently, a twisted bilayer graphene has been discovered that possesses interesting electronic properties owing to strong modifications in interlayer couplings. Thus, a better understanding of ultrafast carrier dynamics in a twisted bilayer graphene is highly desired. Here, we reveal the unbalanced electron distributions in a quasicrystalline 30° twisted bilayer graphene (QCTBG), using time- and angle-resolved photoemission spectroscopy on the femtosecond time scale. We distinguish time-dependent electronic behavior between the upper- and lower-layer Dirac cones and gain insight into the dynamical properties of replica bands, which show characteristic signatures due to Umklapp scatterings. The experimental results are reproduced by solving a set of rate equations among the graphene layers and substrate. We find that the substrate buffer layer plays a key role in initial carrier injections to the upper and lower layers. Our results demonstrate that QCTBG can be a promising element for future devices.

Optical parametric amplification of carrier-envelope phase-stabilized mid-infrared pulses generated by intra-pulse difference frequency generation.

We report on a wavelength-tunable optical parametric amplifier (OPA) from 2.7 to 3.8 µm seeded with carrier-envelope phase (CEP) stabilized pulses generated by intra-pulse difference frequency generation (DFG) using a commercial Yb:KGW chirped-pulse amplifier. The Yb:KGW laser's output pulses are spectrally broadened in two-stage multi-plate pulse compression from 0.8 to 1.25 µm, which are compressed down to a sub-two-cycle duration of 6.5 fs using chirp mirrors. CEP-stabilized mid-infrared pulses are produced in intra-pulse DFG of the spectrally broaden pulses around 1.03 µm and parametrically amplified in KTiOAsO 4 crystals. The output energy and temporal duration of the OPA output pulses range from 31 to 56 µJ and from 102 to 203 fs, respectively. The root mean square value of their CEP errors is measured to be 101 mrad.

All-optical characterization of the two-dimensional waveform and the Gouy phase of an infrared pulse based on plasma fluorescence of gas.

The characterization of the temporal waveform of few-cycle laser pulses is an indispensable part in strong-field physics and attosecond science. Recently, a simple waveform-characterization technique called TIPTOE (tunneling ionization with a perturbation for the time-domain observation of an electric field) has been demonstrated for measuring linearly polarized few-cycle pulses. We theoretically and experimentally show that TIPTOE can be extended to resolve more characteristics of an optical waveform: the two-dimensional polarization and the Gouy phase. Based on the plasma fluorescence of a gaseous medium, we achieve all-optical and spatially resolved measurements of the waveform of an infrared pulse. This detection method enables the remote characterization of a waveform without the need to place an apparatus near the focal point of the laser beam. The proposed approach represents a simple and powerful method for conducting waveform diagnostics on few-cycle laser sources.

Nonlinear propagation effects in high harmonic generation in reflection and transmission from gallium arsenide.

High harmonic spectroscopy in solids is emerging as a new tool to investigate ultrafast electron dynamics in the presence of strong optical fields. However, the observed high harmonic spectra do not usually reflect the microscopic origin of high harmonic generation (HHG) because of nonlinear and/or linear propagation effects. Here, we systematically investigate the HHG in reflection and transmission from gallium arsenide exposed to intense mid-infrared optical pulses. In transmission geometry, we find that the properties of high harmonics are drastically changed by nonlinear effects during the propagation of even tens of micrometers. Especially, the nonlinear absorption and/or nonlinearly induced ellipticity of the drive pulses as well as a cascade nonlinear mixing significantly alter the high harmonic signals in the case of the transmission geometry, making an extraction of the microscopic electron dynamics of gallium arsenide difficult. On the contrary, in reflection geometry, we obtain HHG spectra that are free from propagation effects, opening a general approach for high harmonic spectroscopy.

Photo-induced semimetallic states realised in electron-hole coupled insulators.

Using light to manipulate materials into desired states is one of the goals in condensed matter physics, since light control can provide ultrafast and environmentally friendly photonics devices. However, it is generally difficult to realise a photo-induced phase which is not merely a higher entropy phase corresponding to a high-temperature phase at equilibrium. Here, we report realisation of photo-induced insulator-to-metal transitions in Ta2Ni(Se1-xSx)5 including the excitonic insulator phase using time- and angle-resolved photoemission spectroscopy. From the dynamic properties of the system, we determine that screening of excitonic correlations plays a key role in the timescale of the transition to the metallic phase, which supports the existence of an excitonic insulator phase at equilibrium. The non-equilibrium metallic state observed unexpectedly in the direct-gap excitonic insulator opens up a new avenue to optical band engineering in electron-hole coupled systems.

Polarization-Resolved Study of High Harmonics from Bulk Semiconductors.

The polarization property of high harmonics from gallium selenide is investigated using linearly polarized midinfrared laser pulses. With a high electric field, the perpendicular polarization component of the odd harmonics emerges, which is not present with a low electric field and cannot be explained by the perturbative nonlinear optics. A two-dimensional single-band model is developed to show that the anisotropic curvature of an energy band of solids, which is pronounced in an outer part of the Brillouin zone, induces the generation of the perpendicular odd harmonics. This model is validated by three-dimensional quantum mechanical simulations, which reproduce the orientation dependence of the odd-order harmonics. The quantum mechanical simulations also reveal that the odd- and even-order harmonics are produced predominantly by the intraband current and interband polarization, respectively. These experimental and theoretical demonstrations clearly show a strong link between the band structure of a solid and the polarization property of the odd-order harmonics.

Generation of sub-two-cycle CEP-stable optical pulses at 3.5 µm from a KTA-based optical parametric amplifier with multiple-plate compression.

We demonstrate the generation of 21 fs (1.8 optical cycle), 45-µJ, carrier-envelope phase (CEP)-stable optical pulses with an octave-spanning spectrum from 2.2 to 4.9 µm. Multi-cycle output pulses (120 fs, 149 µJ, 3.5 µm, and 300 Hz) from a KTA-based optical parametric amplifier are compressed down to the sub-two-cycle regime using YAG and Si plates. The single-shot CEP stability of the compressed pulses is measured to be 283 mrad for 500 s. This robust and compact scheme realizes a field strength of 282 MV/cm with f/11 focusing, opening a new regime of attosecond strong-field physics in solids.

Attosecond streaking measurement of extreme ultraviolet pulses using a long-wavelength electric field.

Long-wavelength lasers have great potential to become a new-generation drive laser for tabletop coherent light sources in the soft X-ray region. Because of the significantly low conversion efficiency from a long-wavelength light field to high-order harmonics, their pulse characterization has been carried out by measuring the carrier-envelope phase and/or spatial dependences of high harmonic spectra. However, these photon detection schemes, in general, have difficulty in obtaining information on the spectral phases, which is crucial to determine the temporal structures of high-order harmonics. Here, we report the first attosecond streaking measurement of high harmonics generated by few-cycle optical pulses at 1.7 µm from a BiB3O6-based optical parametric chirped-pulse amplifier. This is also the first demonstration of time-resolved photoelectron spectroscopy using high harmonics from a long-wavelength drive laser other than Ti:sapphire lasers, which paves the way towards ultrafast soft X-ray photoelectron spectroscopy.

Generation of carrier-envelope phase-stable mid-infrared pulses via dual-wavelength optical parametric amplification.

Carrier-envelope phase-stable mid-infrared pulses spanning from 5 µm to 11 µm with a pulse energy of 5 µJ were produced by difference frequency generation of two-color near-infrared pulses that were produced in a novel inline optical parametric amplifier. The mid-infrared electric waveform was characterized by electro-optic sampling using 6.5-fs pulses at 620 nm.

Wavefront analysis of high-efficiency, large-scale, thin transmission gratings.

Large-scale (180 × 60 × 1 mm(3)) transmission gratings with groove densities of 1250 and 1740 lines/mm have been developed, resulting in diffraction efficiencies above 95%. The throughput of a folded pulse compressor with two large-scale transmission gratings was approximately 80% in a 20-fs Ti:sapphire chirped-pulse amplification (CPA) laser. The parabolic bending of the transmission grating due to anti-reflection (AR) coating was minimized to 2.9 λ at 633 nm by improving the evaporation process. By a simple analysis, we explain why this level of bending does not induce a wavefront distortion through the transmission grating near the Littrow condition while the wavefront from a reflection grating is distorted to nearly twice the bending of the grating. The calculation based on the measured bending shows that both the group delay difference relative to the ideally flat grating from 750 to 850 nm and the spatial pulse front distortion over a 60-mm-diameter input beam are negligible, even when the dispersive beam covers ~140 mm on the grating. The spatial pulse front distortion measured after the compressor was less than the measurement limit (1.5 fs) for a 20-mm-diameter beam, where the beam size in the dispersive direction on the grating was 85 mm.

Carrier-envelope phase-dependent high harmonic generation in the water window using few-cycle infrared pulses.

High harmonic generation (HHG) using waveform-controlled, few-cycle pulses from Ti:sapphire lasers has opened emerging researches in strong-field and attosecond physics. However, the maximum photon energy of attosecond pulses via HHG remains limited to the extreme ultraviolet region. Long-wavelength light sources with carrier-envelope phase stabilization are promising to extend the photon energy of attosecond pulses into the soft X-ray region. Here we demonstrate carrier-envelope phase-dependent HHG in the water window using sub-two-cycle optical pulses at 1,600 nm. Experimental and simulated results indicate the confinement of soft X-ray emission in a single recombination event with a bandwidth of 75 eV around the carbon K edge. Control of high harmonics by the waveform of few-cycle infrared pulses is a key milestone to generate soft X-ray attosecond pulses. We measure a dependence of half-cycle bursts on the gas pressure, which indicates subcycle deformation of the waveform of the infrared drive pulses in the HHG process.

Generation of soft x-ray and water window harmonics using a few-cycle, phase-locked, optical parametric chirped-pulse amplifier.

Multimillijoule, few-cycle, carrier-envelope-phase (CEP)-locked, near-IR pulses at 750 nm from an optical parametric chirped-pulse amplifier are applied to the generation of CEP-dependent, soft x-ray high harmonics around the boron K-edge at 188 eV. The dependence on the CEP manifests the phase coherence of high harmonics preserved in the highest-photon energy ever reported. Multimillijoule optical pulses also allow the extension of the cutoff energy up to 325 eV, exceeding the carbon K-edge of the water window. However, in this spectral range, the CEP dependence of harmonic spectra is not observed, suggesting the degradation of temporal coherence due to the heavy ionization of helium atoms.

Control of the electronic phase of a manganite by mode-selective vibrational excitation.

Controlling a phase of matter by coherently manipulating specific vibrational modes has long been an attractive (yet elusive) goal for ultrafast science. Solids with strongly correlated electrons, in which even subtle crystallographic distortions can result in colossal changes of the electronic and magnetic properties, could be directed between competing phases by such selective vibrational excitation. In this way, the dynamics of the electronic ground state of the system become accessible, and new insight into the underlying physics might be gained. Here we report the ultrafast switching of the electronic phase of a magnetoresistive manganite via direct excitation of a phonon mode at 71 meV (17 THz). A prompt, five-order-of-magnitude drop in resistivity is observed, associated with a non-equilibrium transition from the stable insulating phase to a metastable metallic phase. In contrast with light-induced and current-driven phase transitions, the vibrationally driven bandgap collapse observed here is not related to hot-carrier injection and is uniquely attributed to a large-amplitude Mn-O distortion. This corresponds to a perturbation of the perovskite-structure tolerance factor, which in turn controls the electronic bandwidth via inter-site orbital overlap. Phase control by coherent manipulation of selected metal-oxygen phonons should find extensive application in other complex solids--notably in copper oxide superconductors, in which the role of Cu-O vibrations on the electronic properties is currently controversial.