The potential of time-domain Raman spectroscopy in space exploration is discussed. This work is motivated by the emergence of robust, space-qualified femtosecond lasers and by the fact that time-domain detection allows the design of very compact instruments. As is shown, time-domain Raman spectroscopy gives access to the same fingerprint spectrum of minerals as conventional Raman spectroscopy, while avoiding problems such as fluorescence or ambient light backgrounds.

Understanding how light modifies long-range order in quantum materials is key to improving our ability to control functionality. However, this is challenging if the response is heterogeneous. Here we address the most common form of light-induced heterogeneity-surface melting-and measure the dynamics of orbital order in the layered manganite La0.5Sr1.5MnO4. We isolate the surface dynamics from the bulk by measuring the orbital truncation rod and orbital Bragg peak. After photoexcitation, the orbital Bragg peak shows an unusual narrowing, which suggests an increase in correlation length of the probed volume. By contrast, the correlation length at the surface decreases. These differences can be reconciled if the material is heterogeneous, and light melts a less ordered surface. By isolating the surface response, we determine that the loss of long-range order is an incoherent process, which is probably accompanied by the formation of local polarons.

Ultrafast light pulses, if they are sufficiently intense, can induce phase transitions on ultrafast timescales. It is now shown that when a system is first excited by a weak preparatory pulse, this generates local changes in structure that transiently lower the energy barrier to the phase transition, enabling high-speed and energy-efficient transitions.

Ultrafast manipulation of vibrational coherence provides a route to control the structure of solids. However, this strategy can only induce long-range correlations and cannot modify atomic structure locally, which is a requirement for many technological applications such as non-volatile electronics. Here we demonstrate that ultrafast lasers can generate incoherent structural fluctuations that are more efficient for material control than coherent vibrations, extending optical control to a wide range of materials. We observe that local non-equilibrium lattice distortions generated by a weak laser pulse reduce the energy barrier to switch between insulating and metallic states in vanadium dioxide. Seeding inhomogeneous structural fluctuations presents an alternative, more energy-efficient, route for controlling materials that may be applicable to all solids, including those used in data- and energy-storage devices. Controlling phase transitions in solids is crucial for many applications. Ultrafast laser pulses have now been shown to enable the energy-efficient generation of structural fluctuations in VO2 by harnessing the correlated disorder in the material.

Measuring the spectral phase of a pulse is key for performing wavelength resolved ultrafast measurements in the few femtosecond regime. However, accurate measurements in real experimental conditions can be challenging. We show that the reflectivity change induced by coherent phonons in a quantum material can be used to infer the spectral phase of an optical probe pulse with few-femtosecond accuracy.

In the prevalent picture of ultrafast structural phase transitions, atomic motion occurs in a slowly varying potential energy surface adiabatically determined by fast electrons. However, this ignores non-conservative forces caused by electron–lattice collisions, which can substantially influence atomic motion. Most ultrafast techniques only probe the average structure and are less sensitive to random displacements and therefore do not detect the role played by non-conservative forces in phase transitions. Here we show that the lattice dynamics of the prototypical insulator–metal transition of vanadium dioxide cannot be described by potential energy alone. We use the sample temperature to control the preexisting lattice disorder before ultrafast photoexcitation across the phase transition and our ultrafast diffuse scattering experiments show that the fluctuations characteristic of rutile metal develop equally fast (120 fs) at initial temperatures of 100 and 300 K. This indicates that additional non-conservative forces are responsible for the increased lattice disorder. These results highlight the need for more sophisticated descriptions of ultrafast phenomena beyond the Born–Oppenheimer approximation as well as ultrafast probes of spatial fluctuations beyond the average unit cell measured by diffraction. The Born–Oppenheimer approximation is the prevailing assumption for interpreting ultrafast electron dynamics in solids. Evidence now suggests that collisions between electrons and lattice not captured by this approximation play an important role.

Spontaneous C 4 -symmetry breaking phases are ubiquitous in layered quantum materials, and often compete with other phases such as superconductivity. Preferential suppression of the symmetry broken phases by light has been used to explain non-equilibrium light induced superconductivity, metallicity, and the creation of metastable states. Key to understanding how these phases emerge is understanding how C 4 symmetry is restored. A leading approach is based on time-dependent Ginzburg-Landau theory, which explains the coherence response seen in many systems. However, we show that, for the case of the single layered manganite La 0.5 Sr 1.5 MnO 4, the theory fails. Instead, we find an ultrafast inhomogeneous disordering transition in which the mean-field order parameter no longer reflects the atomic-scale state of the system. Our results suggest that disorder may be common to light-induced phase transitions, and methods beyond the mean-field are necessary for understanding and manipulating photoinduced phases.

The breaking of time-reversal symmetry is a crucial ingredient to topological bands. It can occur intrinsically in materials with magnetic order, or be induced by external fields, such as magnetic fields in quantum Hall systems or circularly polarized light fields in Floquet Chern insulators. Apart from polarization, photons can carry another degree of freedom, orbital angular momentum, through which time-reversal symmetry can be broken. In this Letter we pose the question of whether this property allows for inducing topological bands via a linearly polarized but twisted light beam. To this end we study a graphenelike model of electrons on a honeycomb lattice interacting with a twisted light field. To identify the topological behavior of the electrons, we calculate their local markers of Chern number and monitor the presence of in-gap edge states. Our results are shown to be fully analogous to the behavior found in paradigmatic models for static and driven Chern insulators, and realizing the state is experimentally straightforward. With this, our work establishes a mechanism for generating fermionic topological phases of matter that can harness the central phase singularity of an optical vortex beam.

In solids, the response of the lattice to photoexcitation is often described by the inertial evolution on an impulsively modified potential energy surface which leads to coherent motion. However, it remains unknown if vibrational coherence is sustained through a phase transition, during which coupling between modes can be strong and may lead to rapid loss of coherence. Here we use coherent phonon spectroscopy to track lattice coherence in the structural phase transition of V_{2}O_{3}. In both the low and high symmetry phases unique coherent phonon modes are generated at low fluence. However, coherence is lost when driving between the low and high symmetry phases. Our results suggest strongly damped noninertial dynamics dominate during the phase transition due to disorder and multimode coupling.

Using light to control transient phases in quantum materials is an emerging route to engineer new properties and functionality, with both thermal and non-thermal phases observed out of equilibrium. Transient phases are expected to be heterogeneous, either through photo-generated domain growth or by generating topological defects, and this impacts the dynamics of the system. However, this nanoscale heterogeneity has not been directly observed. Here we use time- and spectrally resolved coherent X-ray imaging to track the prototypical light-induced insulator-to-metal phase transition in vanadium dioxide on the nanoscale with femtosecond time resolution. We show that the early-time dynamics are independent of the initial spatial heterogeneity and observe a 200 fs switch to the metallic phase. A heterogeneous response emerges only after hundreds of picoseconds. Through spectroscopic imaging, we reveal that the transient metallic phase is a highly orthorhombically strained rutile metallic phase, an interpretation that is in contrast to those based on spatially averaged probes. Our results demonstrate the critical importance of spatially and spectrally resolved measurements for understanding and interpreting the transient phases of quantum materials.

Abstract We demonstrate a passive all-chalcogenide all-optical perceptron scheme. The network’s nonlinear activation function (NLAF) relies on the nonlinear response of Ge2Sb2Te5 to femtosecond laser pulses. We measured the sub-picosecond time-resolved optical constants of Ge2Sb2Te5 at a wavelength of 1500 nm and used them to design a high-speed Ge2Sb2Te5-tuned microring resonator all-optical NLAF. The NLAF had a sigmoidal response when subjected to different laser fluence excitation and had a dynamic range of −9.7 dB. The perceptron’s waveguide material was AlN because it allowed efficient heat dissipation during laser switching. A two-temperature analysis revealed that the operating speed of the NLAF is ≤ 1 $\le 1$ ns. The percepton’s nonvolatile weights were set using low-loss Sb2S3-tuned Mach Zehnder interferometers (MZIs). A three-layer deep neural network model was used to test the feasibility of the network scheme and a maximum training accuracy of 94.5% was obtained. We conclude that combining Sb2S3-programmed MZI weights with the nonlinear response of Ge2Sb2Te5 to femtosecond pulses is sufficient to perform energy-efficient all-optical neural classifications at rates greater than 1 GHz.

In Mott insulators the evolution of antiferromagnetic order to superconducting or charge-density-wave-like states upon chemical doping underpins the control of quantum phases. Photo-doping can induce similar transitions on the ultrafast timescale, however the response of the spin system has remained elusive. Here, we use 4D-ultrafast optical spectroscopy to extract quantitative magnetic dynamics in the spin-orbit coupled Mott insulator Sr3Ir2O7. We demonstrate that light can non-thermally melt long-range spin order. At low fluences magnetic order recovers within 1 ps despite demagnetization of roughly 50%. However, high fluences induce a crossover to a long-lived demagnetized state without increasing the lattice temperature. We show that the generation of photo-induced spin defects enables a mechanism that stabilizes the demagnetized state which could help expose new transient phases.

Solid-state systems can host a variety of thermodynamic phases that can be controlled with magnetic fields, strain, or laser excitation. Many phases that are believed to exhibit exotic properties only exist on the nanoscale, coexisting with other phases that make them challenging to study, as measurements require both nanometer spatial resolution and spectroscopic information, which are not easily accessible with traditional x-ray spectromicroscopy techniques. Here, we use coherent diffractive imaging spectroscopy (CDIS) to acquire quantitative hyperspectral images of the prototypical quantum material vanadium oxide across the vanadium L 2,3 and oxygen K x-ray absorption edges with nanometer-scale resolution. We extract the full complex refractive indices of the monoclinic insulating and rutile conducting phases of VO2 from a single sample and find no evidence for correlation-driven phase transitions. CDIS will enable quantitative full-field x-ray spectromicroscopy for studying phase separation in time-resolved experiments and other extreme sample environments where other methods cannot operate.

In this work, we present an achromatic pump-probe setup covering the visible (VIS) to near-infrared (NIR) wavelength regions (500-3000 nm) for few-cycle pulses. Both the pump and probe arms can work either in the VIS or the NIR wavelength regions, making our setup suitable for multi-color, broadband pump-probe measurements. In particular, our setup minimizes time-smearing due to the phase front curvature, an aspect of ultrafast spectroscopy that has been missing from previous works and allowing us to achieve sub-20-fs temporal resolution. We demonstrate the capabilities of our setup by performing measurements on Pr0.5Ca1.5MnO4. We pump and probe in both wavelength regions with a range of pump fluences and demonstrate how the observed dynamics depend strongly on the probe wavelength. Furthermore, the observation of a 16.5 THz phonon demonstrates the high temporal resolution of the setup.

We report a comparative experimental and theoretical study of second harmonic generation from a 20nm-thick indium tin oxide nanolayer in the proximity of the epsilon-near-zero condition. We record the efficiency of the second harmonic signal both as a function of wavelength as well as of the angle of incidence around the epsilon-near-zero crossing point. We compare our experimental results with numerical simulations based on a hydrodynamical model able to capture all major physical mechanisms driving the electrodynamic behavior of conductive oxide layers, with unique aspects of the different nonlinear sources. We found a very good quantitative and qualitative agreement between experiment and theory.

Recent developments in attosecond technology led to tabletop X-ray spectroscopy in the soft X-ray range, thus uniting the element- and state-specificity of core-level x-ray absorption spectroscopy with the time resolution to follow electronic dynamics in real time. We describe recent work in attosecond technology and investigations into materials such as Si, SiO2, GaN, Al2O3, Ti, TiO2, enabled by the convergence of these two capabilities. We showcase the state-of-the-art on isolated attosecond soft x-ray pulses for x-ray absorption near edge spectroscopy (XANES) to observe the 3d-state dynamics of the semi-metal TiS2 with attosecond resolution at the Ti L-edge (460 eV). We describe how the element- and state-specificity at the transition metal L-edge of the quantum material allows to unambiguously identify how and where the optical field influences charge carriers. This precision elucidates that the Ti:3d conduction band states are efficiently photo-doped to a density of 1.9 x 10^21 cm^-3 and that the light-field induces coherent motion of intra-band carriers across 38% of the first Brillouin zone. Lastly, we describe the prospects with such unambiguous real-time observation of carrier dynamics in specific bonding or anti-bonding states and speculate that such capability will bring unprecedented opportunities towards an engineered approach for designer materials with pre-defined properties and efficiency. Examples are composites of semiconductors and insulators like Si, Ge, SiO2, GaN, BN, quantum materials like graphene, TMDCs, or high-Tc superconductors like NbN or LaBaCuO. Exiting are prospects to scrutinize canonical questions in multi-body physics such as whether the electrons or lattice trigger phase transitions.

We demonstrate the first femtosecond-temporal and nanometer-spatial resolution imaging of a light induced phase transition, the insulator-to-metal transition of vanadium dioxide, using time-resolved resonant X-ray holography [1] at the Pohang Accelerator Laboratory X-ray free electron laser. Thin films of VO 2 deposited onto silicon nitride membranes were masked for holography [2] and heated to 325 K, where domains of the rutile metallic phase began to nucleate. Femtosecond 800 nm pulses then drove the system through the phase transition, with the domain growth and nucleation dynamics monitored by coherent X-ray scattering at the vanadium L 2,3 and oxygen K edges ( Fig 1 ). Dynamics at a range of time-scales are observed in the real space videos of the process: we observe nanometer sized domains forming within 200 fs, followed by both sub-picosecond and picosecond dynamics. Real space imaging-spectroscopy was performed on the long-lived transient state at 20 ps time-delay [3] , allowing us to comment on the existence of a proposed nanoscale monoclinic metallic precursor in the ultrafast phase transition [4] , [5] .

Non-thermal breaking of magnetic order via photo-generated spin defects Ernest Pastor†, David Moreno-Mencía†, Maurizio Monti, Allan S. Johnson, Nina Fleischmann, Cuixiang Wang, Youguo Shi, Xuerong Liu, Daniel G. Mazzone, Mark P.M. Dean, Simon Wall 1 ICFO-Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, 08860, Castelldefels, Barcelona, Spain. Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 4 School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China. 5 Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, CH-5232 Villigen, Switzerland 6 Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA.