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The origin of the pseudogap in many strongly correlated materials has been a longstanding puzzle. Here, we present experimental evidence that many-body interactions among small Holstein polarons, i.e., the formation of bipolarons, are primarily responsible for the pseudogap in (TaSe4)2I. After weak photoexcitation of the material, we observe the appearance of both dispersive (single-particle bare band) and flat bands (single-polaron sub-bands) in the gap by using time- and angle-resolved photoemission spectroscopy. Based on Monte Carlo simulations of the Holstein model, we propose that the melting of pseudogap and emergence of new bands originate from a bipolaron to single-polaron crossover. We also observe dramatically different relaxation times for the excited in-gap states in (TaSe4)2I (∼600 fs) compared with another 1D material Rb0.3MoO3(∼60 fs), which provides a new method for distinguishing between pseudogaps induced by polaronic or Luttinger-liquid many-body interactions.

Nanostructuring on length scales corresponding to phonon mean free paths provides control over heat flow in semiconductors and makes it possible to engineer their thermal properties. However, the influence of boundaries limits the validity of bulk models, while first-principles calculations are too computationally expensive to model real devices. Here we use extreme ultraviolet beams to study phonon transport dynamics in a 3D nanostructured silicon metalatticewith deep nanoscale feature size and observe dramatically reduced thermal conductivity relative to bulk. To explain this behavior, we develop a predictive theory wherein thermal conduction separates into a geometric permeabilitycomponent and an intrinsic viscouscontribution, arising from a new and universal effect of nanoscale confinement on phonon flow. Using experiments and atomistic simulations, we show that our theory applies to a general set of highly confined silicon nanosystems, from metalattices, nanomeshes, porous nanowires, to nanowire networks, of great interest for next-generation energy-efficient devices.

Topological magnetic monopoles (TMMs), also known as hedgehogs or Bloch points, are three-dimensional (3D) non-local spin textures that are robust to thermal and quantum fluctuations due to the topology protection1–4. Although TMMs have been observed in skyrmion lattices1,5, spinor Bose–Einstein condensates6,7, chiral magnets8, vortex rings2,9and vortex cores10, it has been difficult to directly measure the 3D magnetization vector field of TMMs and probe their interactions at the nanoscale. Here we report the creation of 138 stable TMMs at the specific sites of a ferromagnetic meta-lattice at room temperature. We further develop soft X-ray vector ptycho-tomography to determine the magnetization vector and emergent magnetic field of the TMMs with a 3D spatial resolution of 10 nm. This spatial resolution is comparable to the magnetic exchange length of transition metals11, enabling us to probe monopole–monopole interactions. We find that the TMM and anti-TMM pairs are separated by 18.3 ± 1.6 nm, while the TMM and TMM, and anti-TMM and anti-TMM pairs are stabilized at comparatively longer distances of 36.1 ± 2.4 nm and 43.1 ± 2.0 nm, respectively. We also observe virtual TMMs created by magnetic voids in the meta-lattice. This work demonstrates that ferromagnetic meta-lattices could be used as a platform to create and investigate the interactions and dynamics of TMMs. Furthermore, we expect that soft X-ray vector ptycho-tomography can be broadly applied to quantitatively image 3D vector fields in magnetic and anisotropic materials at the nanoscale.

The broad laser bandwidth and excellent material properties of titanium-doped sapphire (Ti:sapphire) have proved ideal for small- and largescale ultrafast lasers and systems. (1) Incorporating cryogenic cooling into an ultrafast [...]

Heat management is crucial in the design of nanoscale devices as the operating temperature determines their efficiency and lifetime. Past experimental and theoretical works exploring nanoscale heat transport in semiconductors addressed known deviations from Fourier’s law modeling by including effectiveparameters, such as a size-dependent thermal conductivity. However, recent experiments have qualitatively shown behavior that cannot be modeled in this way. Here, we combine advanced experiment and theory to show that the cooling of 1D- and 2D-confined nanoscale hot spots on silicon can be described using a general hydrodynamic heat transport model, contrary to previous understanding of heat flow in bulk silicon. We use a comprehensive set of extreme ultraviolet scatterometry measurements of nondiffusive transport from transiently heated nanolines and nanodots to validate and generalize our ab initiomodel, that does not need any geometry-dependent fitting parameters. This allows us to uncover the existence of two distinct time scales and heat transport mechanisms: an interface resistance regime that dominates on short time scales and a hydrodynamic-like phonon transport regime that dominates on longer time scales. Moreover, our model can predict the full thermomechanical response on nanometer length scales and picosecond time scales for arbitrary geometries, providing an advanced practical tool for thermal management of nanoscale technologies. Furthermore, we derive analytical expressions for the transport time scales, valid for a subset of geometries, supplying a route for optimizing heat dissipation.

Light with spatiotemporal orbital angular momentum (ST-OAM) is a recently discovered type of structured and localized electromagnetic field. This field carries characteristic space–time spiral phase structure and transverse intrinsic OAM. Here, we present the generation and characterization of the second harmonic of ST-OAM pulses. We uncover the conservation of transverse OAM in a second-harmonic generation process, where the space–time topological charge of the fundamental field is doubled along with the optical frequency. Our experiment thus suggests a general ST-OAM nonlinear scaling rule, analogous to that in conventional OAM of light. Furthermore, we observe that the topology of a second-harmonic ST-OAM pulse can be modified by complex spatiotemporal astigmatism, giving rise to multiple phase singularities separated in space and time. Our study opens a new route for nonlinear conversion and scaling of light carrying ST-OAM, with the potential for driving other secondary ST-OAM sources of electromagnetic fields and beyond.

Metalattices are artificial 3D solids, periodic on sub-100 nm length scales, that enable the functional properties of materials to be tuned. However, because of their complex structure, predicting and characterizing their properties is challenging. Here we demonstrate the first nondestructive measurements of the mechanical and structural properties of metalattices with feature sizes down to 14 nm. By monitoring the time-dependent diffraction of short wavelength light from laser-excited acoustic waves in the metalattices, we extract their acoustic dispersion, Young’s modulus, filling fraction, and thicknesses. Our measurements are in excellent agreement with macroscopic predictions and potentially destructive techniques such as nanoindentation and scanning electron microscopy, with increased accuracy over larger areas. This is interesting because the transport properties of these metalattices do not obey bulk predictions. Finally, this approach is the only way to validate the filling fraction of metalattices over macroscopic areas. These combined capabilities can enable accurate synthesis of nanoenhanced materials.

Optical interactions are governed by both spin and angular momentum conservation laws, which serve as a tool for controlling light–matter interactions or elucidating electron dynamics and structure of complex systems. Here, we uncover a form of simultaneous spin and orbital angular momentum conservation and show, theoretically and experimentally, that this phenomenon allows for unprecedented control over the divergence and polarization of extreme-ultraviolet vortex beams. High harmonics with spin and orbital angular momenta are produced, opening a novel regime of angular momentum conservation that allows for manipulation of the polarization of attosecond pulses—from linear to circular—and for the generation of circularly polarized vortices with tailored orbital angular momentum, including harmonic vortices with the same topological charge as the driving laser beam. Our work paves the way to ultrafast studies of chiral systems using high-harmonic beams with designer spin and orbital angular momentum.

We demonstrate numerically and experimentally that intense pulses propagating in gas-filled capillaries can undergo localization in space and time due to strong plasma defocusing. This phenomenon can occur below or above the self-focusing threshold P_cr as a result of ionization-induced refraction that excites higher-order modes. The constructive interference of higher-order modes leads to spatiotemporal localization and resurgence of the intensity. Simulations show that this confinement is more prominent at shorter wavelength pulses and for smaller capillary diameters. Experiments with ultraviolet pulses show evidence that this ionization-induced refocusing appears below P_cr and thus represents a mechanism for spatiotemporal confinement without self-focusing.

High-harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, thus far, the shortest isolated attosecond pulses have only been produced with linear polarization, which limits the range of physics that can be explored. Here, we demonstrate robust polarization control of isolated extreme-ultraviolet pulses by exploiting non-collinear high-harmonic generation driven by two counter-rotating few-cycle laser beams. The circularly polarized supercontinuum is produced at a central photon energy of 33 eV with a transform limit of 190 as and a predicted linear chirp of 330 as. By adjusting the ellipticity of the two counter-rotating driving pulses simultaneously, we control the polarization state of isolated extreme-ultraviolet pulses—from circular through elliptical to linear polarization—without sacrificing conversion efficiency. Access to the purely circularly polarized supercontinuum, combined with full helicity and ellipticity control, paves the way towards attosecond metrology of circular dichroism. Circularly polarized isolated extreme-ultraviolet pulses are generated by exploiting non-collinear high- harmonic generation driven by two counter-rotating few-cycle laser beams. The numerical simulation predicts a linear chirp of 330 attoseconds.

The Krausz work discussed in your article, 'Optical pulses reach attosecond length' (February, p. 17) was presented very deceptively. The title of the paper in Nature is 'Attosecond metrology,' implying [...]

Optical waveguides made from periodically poled materials provide high confinement of light and enable the generation of new wavelengths via quasi-phase-matching, making them a key platform for nonlinear optics and photonics. However, such devices are not typically employed for high-harmonic generation. Here, using 200-fs, 10-nJ-level pulses of 4100-nm light at 1 MHz, we generate high harmonics up to the 13th harmonic (315 nm) in a chirped, periodically poled lithium niobate waveguide. Total conversion efficiencies into the visible–ultraviolet region are as high as 10%. We find that the output spectrum depends on the waveguide poling period, indicating that quasi-phase-matching plays a significant role. In the future, such periodically poled waveguides may enable compact sources of ultrashort pulses at high repetition rates and provide new methods of probing the electronic structure of solid-state materials.

Lensless imaging with short-wavelength light is a promising method for achieving high-resolution, chemically sensitive images of a wide variety of samples. The use of 13 nm illumination is of particular interest for materials science and the imaging of next-generation nanofabricated devices. Prior to this work, there was an unmet need for a microscope that can image general samples with extreme ultraviolet light, which requires a reflection geometry. Here, we fulfill this need by performing lensless imaging using a 13 nm high-harmonic beam at grazing incidence, where most materials are reflective. Furthermore, we demonstrate to our knowledge the first 13 nm reflection-mode lensless microscope on a tabletop by using a compact high-harmonic generation source. Additionally, we present an analytic formalism that predicts when general lensless imaging geometries will yield Nyquist sampled data. Our grazing-incidence ptychographic approach, which we call GLIDER, provides the first route for achieving wide field-of-view, high-resolution, lensless images of general samples with extreme ultraviolet and soft x-ray light.

Coherent diffractive imaging is unique, being the only route for achieving high spatial resolution in the extreme ultraviolet and X-ray regions, limited only by the wavelength of the light. Recently, advances in coherent short-wavelength light sources, coupled with progress in algorithm development, have significantly enhanced the power of X-ray imaging. However, so far, high-fidelity diffraction imaging of periodic objects has been a challenge because the scattered light is concentrated in isolated peaks. Here, we use tabletop 13.5 nm high-harmonic beams to make two significant advances. First, we demonstrate high-quality imaging of an extended, nearly periodic sample for the first time. Second, we achieve subwavelength spatial resolution (12.6 nm) imaging at short wavelengths, also for the first time. The key to both advances is a novel technique called ‘modulus enforced probe’, which enables robust and quantitative reconstructions of periodic objects. This work is important for imaging next-generation nano-engineered devices.

The article focuses on the Ti:sapphire laser pumped directly with a pair of 1.2W 445nm laser diodes. With over 30mW average power at 800 nm and a measured pulsewidth of 15fs, Kerr-lens-modelocked pulses are available with dramatically decreased pump cost. The model explains the observed highly stable Kerr-lens modelocking in spite of the fact that both the mode-locked and continuous-wave modes are smaller than the pump mode in the crystal.

We make direct observations of localized light absorption in a single nanostructure irradiated by a strong femtosecond laser field, by developing and applying a technique that we refer to as plasma explosion imaging. By imaging the photoion momentum distribution resulting from plasma formation in a laser-irradiated nanostructure, we map the spatial location of the highly localized plasma and thereby image the nanoscale light absorption. Our method probes individual, isolated nanoparticles in vacuum, which allows us to observe how small variations in the composition, shape, and orientation of the nanostructures lead to vastly different light absorption. Here, we study four different nanoparticle samples with overall dimensions of ∼100 nm and find that each sample exhibits distinct light absorption mechanisms despite their similar size. Specifically, we observe subwavelength focusing in single NaCl crystals, symmetric absorption in TiO2aggregates, surface enhancement in dielectric particles containing a single gold nanoparticle, and interparticle hot spots in dielectric particles containing multiple smaller gold nanoparticles. These observations demonstrate how plasma explosion imaging directly reveals the diverse ways in which nanoparticles respond to strong laser fields, a process that is notoriously challenging to model because of the rapid evolution of materials properties that takes place on the femtosecond time scale as a solid nanostructure is transformed into a dense plasma.

We demonstrate the first (to our knowledge) general purpose full-field reflection-mode extreme ultraviolet (EUV) microscope based on coherent diffractive imaging. This microscope is capable of nanoscale amplitude and phase imaging of extended surfaces at an arbitrary angle of incidence in a noncontact, nondestructive manner. We use coherent light at 29.5 nm from high-harmonic upconversion to illuminate a surface, directly recording the scatter as the surface is scanned. Ptychographic reconstruction is then combined with tilted plane correction to obtain an image with amplitude and phase information. The image quality and detail from this diffraction-limited tabletop EUV microscope compares favorably with both scanning electron microscope and atomic force microscope images. The result is a general and completely extensible imaging technique that can provide a comprehensive and definitive characterization of how light at any wavelength scatters from a surface, with imminent feasibility of elemental imaging with few-nanometer resolution.

Photoacoustic spectroscopy is a powerful tool for characterizing thin films. In this paper we demonstrate a new photoacoustic technique that allows us to precisely characterize the mechanical properties of ultrathin films. We focus an ultrafast laser onto a nano-patterned thin film sample, launching both surface acoustic waves (SAWs) and longitudinal acoustic waves (LAWs). Coherent extreme ultraviolet pulses are then used to probe the propagation dynamics of both the SAWs and LAWs. The resulting photoacoustic signal on both short (picosecond) and long (nanosecond) time scales yields important information. In the first 100ps, a fast oscillation followed by an echo signal corresponds to LAWs traveling inside the nanostructures and the thin film, from which the LAW velocities in the two materials can be extracted. On longer time-scales, SAW oscillations are observed. By combining the measured SAW frequency with the wavelength (determined by the nanostructure period) the SAW velocity can be accurately determined, even for very short wavelength surface acoustic waves with very small penetration depths. Using this technique, the elastic properties, including the Young's modulus and Poisson ratio for the thin film, can be obtained in a single measurement, this technique can be extended to sub-10nm thin films.

We implement coherent diffractive imaging (CDI) using a phase-matched high-harmonic generation (HHG) source at 13 nm, demonstrating reconstructed images with a record 22 nm resolution for any tabletop, light-based microscope. We also demonstrate the first reflection-mode CDI using a compact extreme ultraviolet (EUV) source, achieving 100 nm resolution. A clear path towards even higher spatial resolution reflection-mode tabletop imaging using apertured-illumination schemes will be discussed.

Semiconductor metalattices consisting of a linked network of three-dimensional nanostructures with periodicities on a length scale <100 nm can enable tailored functional properties due to their complex nanostructuring. For example, by controlling both the porosity and pore size, thermal transport in these phononic metalattices can be tuned, making them promising candidates for efficient thermoelectrics or thermal rectifiers. Thus, the ability to characterize the porosity, and other physical properties, of metalattices is critical but challenging, due to their nanoscale structure and thickness. To date, only metalattices with high porosities, close to the close-packing fraction of hard spheres, have been studied experimentally. Here, we characterize the porosity, thickness, and elastic properties of a low-porosity, empty-pore silicon metalattice film (∼500 nm thickness) with periodic spherical pores (∼tens of nanometers), for the first time. We use laser-driven nanoscale surface acoustic waves probed by extreme ultraviolet scatterometry to nondestructively measure the acoustic dispersion in these thin silicon metalattice layers. By comparing the data to finite element models of the metalattice sample, we can extract Young’s modulus and porosity. Moreover, by controlling the acoustic wave penetration depth, we can also determine the metalattice layer thickness and verify the substrate properties. Additionally, we utilize electron tomography images of the metalattice to verify the geometry and validate the porosity extracted from scatterometry. These advanced characterization techniques are critical for informed and iterative fabrication of energy-efficient devices based on nanostructured metamaterials.

The article discusses the history of nonlinear optics which resulted to the development of coherent x-ray lasers. The first experiment in nonlinear optics was done by Peter Franken of the University of Arizona using a ruby laser shortly after the laser's first introduction. On the other hand, the first successful study in high-order harmonic generation (HHG) happened in 2003 with the demonstration of quasi-phase-matching (QPM) of HHG using periodically modulated waveguides.

Researchers are developing tools and techniques that use coherent light at ultrashort wavelengths - in the extreme ultraviolet and soft X-ray regions of the spectrum.

Mega-electron-volt (MeV) electron emission from the interaction of an ultrafast (τ∼29 fs), intense (>10^18 W/cm^2) laser pulse with underdense plasmas has been studied. A beam of MeV electrons with a divergence angle as small as 1° is observed in the forward direction, which is correlated with relativistic filamentation of the laser pulse in plasmas. A novel net-energy-gain mechanism is proposed for electron acceleration resulting from the relativistic filamentation and beam breakup. These results suggest an approach for generating a beam of femtosecond, MeV electrons at a kilohertz repetition rate with a compact ultrafast intense laser system.

We present an analytical, computational, and experimental study of frequency mixing and propagation that takes place in the guided-wave nonlinear frequency conversion of ultrashort pulses from the near infrared to the far and the vacuum ultraviolet. In addition to the standard phase-matching conditions for frequency mixing, we observe that cascaded frequency conversion, in which one of the input frequency components is generated simultaneously, is subject to two additional phase-matching conditions. In cascaded quasi-phase matching, one of the input fields oscillates in strength with a period that matches the phase mismatch. In gain-assisted phase matching, the growth of one of the input fields results in growth of the signal, in spite of a phase mismatch for the final mixing process.

Tunable radiation can be produced by Doppler upshifting light from a laser-generated moving ionization front. We analyze this technique as a method of generating ultrashort tunable light pulses in the UV to extreme UV (XUV) regions of the spectrum and demonstrate the possibility of generating subfemtosecond short-wavelength light pulses. Using realistic and technologically achievable experimental conditions and including dispersion of a pulse reflected from a finite-rise-time multiphoton-ionization-produced ionization front, we show that a single-cycle far-infrared (FIR) pulse can be compressed in time and frequency into a pulse substantially shorter than any light pulse demonstrated to date. We show that arbitrarily large upshifts are achievable using this technique, making it a potentially competitive way of producing tunable XUV radiation. The multiphoton ionization mechanism by which the ionization front is created is an important factor in determining the reflectivity of the ionization front.

Recent advances in the production of ultrashort x-ray pulses by using femtosecond laser-produced plasmas coupled with the development of terawatt ultrashort-pulse lasers may make possible ultrashort-pulse photoexcited x-ray lasers. I examine the creation of a population inversion on the K-α transition of neon at 1.5 nm by using the photoionization scheme first suggested by Duguay and Rentzepis in 1967. It is shown that this laser can be produced by using a pump laser of ~ 10 J in 50 fs, provided that a sufficiently bright laser-produced plasma x-ray source can be created. Recent experimental and theoretical results are discussed that verify the potential feasibility of this scheme.

We demonstrate a compact 20?Hz repetition-rate mid-IR OPCPA system operating at a central wavelength of 3900?nm with the tail-to-tail spectrum extending over 600?nm and delivering 8?mJ pulses that are compressed to 83?fs (<7 optical cycles). Because of the long optical period (∼13?fs) and a high peak power, the system opens a range of unprecedented opportunities for tabletop ultrafast science and is particularly attractive as a driver for a highly efficient generation of ultrafast coherent x-ray continua for biomolecular and element specific imaging.

We demonstrate that phase-matched frequency upconversion of ultrafast laser light can be extended to shorter wavelengths by using longer driving laser wavelengths. Experimentally, we show that the phase-matching cutoff for harmonic generation in argon increases from 45 to 100 eV when the driving laser wavelength is increased from 0.8 to 1.3 μm. Phase matching is also obtained at higher pressures using a longer-wavelength driving laser, mitigating the unfavorable scaling of the single-atom response. Theoretical calculations suggest that phase-matched high harmonic frequency upconversion driven by mid-infrared pulses could be extended to extremely high photon energies.