Focal-shape effects on the efficiency of the tunnelionization probe for extreme laser intensities

We examine the effect of laser focusing on the effectiveness of a recently discussed scheme[M.F.Ciappina et al.,Phys.Rev.A 99,043405(2019)and M.F.Ciappina and S.V.Popruzhenko,Laser Phys.Lett.17,025301(2020)]for in situ determination of ultrahigh intensities of electromagnetic radiation delivered by multi-petawatt laser facilities.Using two model intensity distributions in the focus of a laser beam,we show how the resulting yields of highly charged ions generated in the process of multiple sequential tunneling of electrons from atoms depend on the shapes of these distributions.Our findings lead to the conclusion that an accurate extraction of the peak laser intensity can be made either in the near-threshold regime,when the production of the highest charge state happens only in a small part of the laser focus close to the point where the intensity is maximal or through the determination of the points where the ion yields of close charges become equal.We show that for realistic parameters of the gas target,the number of ions generated in the central part of the focus in the threshold regime should be sufficient for a reliable measurement with highly sensitive time-of-flight detectors.Although the positions of the intersection points generally depend on the focal shape,they can be used to localize the peak intensity value in certain intervals.Finally,the slope of the intensity-dependent ion yields is shown to be robust with respect to both the focal spot size and the spatial distribution of the laser intensity in the focus.When these slopes can be measured,they will provide the most accurate determination of the peak intensity value within the considered tunnel ionization scheme.In addition to this analysis,we discuss the method in comparison with other recently proposed approaches for direct measurement of extreme laser intensities.

Radiative loss of coherence in free electrons:a long-range quantum phenomenon

Quantum physics rules the dynamics of small objects as they interact over microscopic length scales.Nevertheless,quantum correlations involving macroscopic distances can be observed between entangled photons as well as in atomic gases and matter waves at low temperatures.The long-range nature of the electromagnetic coupling between charged particles and extended objects could also trigger quantum phenomena over large distances.Here,we reveal a manifestation of quantum mechanics that involves macroscopic distances and results in a nearly complete depletion of coherence associated with which-way free-electron interference produced by electron-radiation coupling in the presence of distant extended objects.This is a ubiquitous effect that we illustrate through a rigorous theoretical analysis of a two-path electron beam interacting with a semi-infinite metallic plate and find the inter-path coherence to vanish proportionally to the path separation at zero temperature and exponentially at finite temperature.The investigated regime of large distances originates in the coupling of the electron to radiative modes assisted by diffraction at material structures but without any involvement of material excitations.Besides the fundamental interest of this macroscopic quantum phenomenon,our results suggest an approach to measuring the vacuum temperature and nondestructively sensing the presence of distant objects.

Realizing Attosecond Core-Level X-ray Spectroscopy for the Investigation of Condensed Matter Systems

Unraveling the exact nature of nonequilibrium and correlated interactions is paramount for continued progress in many areas of condensed matter science. Such insight is a prerequisite to develop an engineered approach for smart materials with targeted properties designed to address standing needs such as efficient light harvesting, energy storage, or information processing. For this goal, it is critical to unravel the dynamics of the energy conversion processes between carriers in the earliest time scales of the excitation dynamics. We discuss the implementation and benefits of attosecond soft x-ray core-level spectroscopy up to photon energies of 600 eV for measurements in solid-state systems. In particular, we examine how the pairing between coherent spectral coverage and temporal resolution provides a powerful new insight into the quantum dynamic interactions that determine the macroscopic electronic and optical response. We highlight the different building blocks of the methodology and point out the important aspects for its application from condensed matter studies to materials as thin as 25 nm. Furthermore, we discuss the technological developments in the field of tabletop attosecond soft x-ray sources with time-resolved measurements at the near and extended edge simultaneously and investigate the exciting prospective of extending such technique to the study of 2-dimensional materials.

Free-electron interactions with van der Waals heterostructures: a source of focused X-ray radiation

The science and technology of X-ray optics have come far,enabling the focusing of X-rays for applications in highresolution X-ray spectroscopy,imaging,and irradiation.In spite of this,many forms of tailoring waves that had substantial impact on applications in the optical regime have remained out of reach in the X-ray regime.This disparity fundamentally arises from the tendency of refractive indices of all materials to approach unity at high frequencies,making X-ray-optical components such as lenses and mirrors much harder to create and often less effcient.Here,we propose a new concept for X-ray focusing based on inducing a curved wavefront into the X-ray generation process,resulting in the intrinsic focusing of X-ray waves.This concept can be seen as effectively integrating the optics to be part of the emission mechanism,thus bypassing the effciency limits imposed by X-ray optical components,enabling the creation of nanobeams with nanoscale focal spot sizes and micrometer-scale focal lengths.Specifically,we implement this concept by designing aperiodic vdw heterostructures that shape X-rays when driven by free electrons.The parameters of the focused hotspot,such as lateral size and focal depth,are tunable as a function of an interlayer spacing chirp and electron energy.Looking forward,ongoing advances in the creation of many-layer vdw heterostructures open unprecedented horizons of focusing and arbitrary shaping of X-ray nanobeams.

High-Sensitivity Visualization of Ulitrafast Carrier Diffusion by Wide-Field Holographic Microscopy

Ultrafast transient microscopy is a key tool to study the photophysical properties of materials in space and time,but current implementations are limited to≈1-μm fields of view,offering no statistical information for heterogeneous samples.Recently,we demonstrated wide-field transient imaging based on multiplexed off-axis holography.Here,we perform ultrafast microscopy in parallel around a hundred diffraction-limited excitation spots over a≈60-μm field of view,which not only automatically samples the photophysical heterogeneity of the sample over a large area but can also be used to obtain a 10-fold increase in signal-tonoise ratio by computing an average spot.We apply our microscope to study the carrier diffusion processes in methylammonium lead bromide perovskites.We observe strong diffusion due to the presence of hot carriers during the first picosecond and slower diffusion afterward.We also describe how many-body kinetics can be misleadingly interpreted as strong diffusion at high excitation densities,while at weak excitation,real diffusion is observed.Therefore,the vast increase in sensitivity offered by this technique benefits the study of carrier transport not only by reducing data acquisition times but also by enabling the measurement of the much smaller signals generated at low carrier densities.

Integrated colloidal quantum dot photodetectors with color-tunable plasmonic nanofocusing lenses

High-sensitivity photodetection is at the heart of many optoelectronic applications,including spectroscopy,imaging,surveillance,remote sensing and medical diagnostics.Achieving the highest possible sensitivity for a given photodetector technology requires the development of ultra-small-footprint detectors,as the noise sources scale with the area of the detector.This must be accomplished while sacrificing neither the optically active area of the detector nor its responsivity.Currently,such designs are based on diffraction-limited approaches using optical lenses.Here,we employ a plasmonic flat-lens bull’s eye structure(BES)to concentrate and focus light into a nanoscale colloidal quantum dot(CQD)photodetector.The plasmonic lenses function as nanofocusing resonant structures that simultaneously offer color selectivity and enhanced sensitivity.Herein,we demonstrate the first CQD photodetector with a nanoscale footprint,the optically active area of which is determined by the BES;this detector represents an exciting opportunity for high-sensitivity sensing.

Simple experimental procedures to distinguish photothermal from hot-carrier processes in plasmonics

Light absorption and scattering of plasmonic metal nanoparticles can lead to non-equilibrium charge carriers,intense electromagnetic near-fields,and heat generation,with promising applications in a vast range of fields,from chemical and physical sensing to nanomedicine and photocatalysis for the sustainable production of fuels and chemicals.Disentangling the relative contribution of thermal and non-thermal contributions in plasmon-driven processes is,however,difficult.Nanoscale temperature measurements are technically challenging,and macroscale experiments are often characterized by collective heating effects,which tend to make the actual temperature increase unpredictable.This work is intended to help the reader experimentally detect and quantify photothermal effects in plasmon-driven chemical reactions,to discriminate their contribution from that due to photochemical processes and to cast a critical eye on the current literature.To this aim,we review,and in some cases propose,seven simple experimental procedures that do not require the use of complex or expensive thermal microscopy techniques.These proposed procedures are adaptable to a wide range of experiments and fields of research where photothermal effects need to be assessed,such as plasmonic-assisted chemistry,heterogeneous catalysis,photovoltaics,biosensing,and enhanced molecular spectroscopy.

Quantum nonlocal effects in individual and interacting graphene nanoribbons

We show that highly doped graphene ribbons can support surface plasmons at near-infrared frequencies when their width is in the nanometer range,leading to important nonlocal and finite quantum-size corrections,such as sizable blueshifts.The magnitude of these effects is assessed by comparing classical and quantum-mechanical models to describe graphene plasmons.More precisely,we examine individual and interacting 6–8 nm wide zigzag and armchair ribbons doped to 0.4–1.5 eV Fermi energies.We find a strong influence of nonlocal effects on the orientation of graphene edges,with plasmons in zigzag ribbons undergoing strong quenching when their energy is below the Fermi level.Nonlocality is also affecting the hybridization between ribbon plasmons in dimers and arrays for separations below a few nanometers.Remarkably,the removal of a single row of atomic bonds in a ribbon produces a strong plasmon frequency shift,whereas the removal of bonds along an array of rows separated by several nanometers in an extended sheet causes a dramatic increase in the absorption.Besides the fundamental interest of these results,our work supports the use of narrow ribbons to achieve electro-optical modulation in the near infrared.

Unraveling the optomechanical nature of plasmonic trapping

Noninvasive and ultra-accurate optical manipulation of nanometer objects has recently gained interest as a powerful tool in nanotechnology and biophysics.Self-induced back-action(SIBA)trapping in nano-optical cavities has the unique potential for trapping and manipulating nanometer-sized objects under low optical intensities.However,thus far,the existence of the SIBA effect has been shown only indirectly via its enhanced trapping performances.In this article,we present the first time direct experimental evidence of the self-reconfiguration of the optical potential that is experienced by a nanoparticle trapped in a plasmonic nanocavity.Our observations enable us to gain further understanding of the SIBA mechanism and to determine the optimal conditions for boosting the performances of SIBA-based nano-optical tweezers.

Spontaneous and stimulated electron-photon interactions in nanoscale plasmonic near fields

The interplay between free electrons,light,and matter offers unique prospects for space,time,and energy resolved optical material characterization,structured light generation,and quantum information processing.Here,we study the nanoscale features of spontaneous and stimulated electron–photon interactions mediated by localized surface plasmon resonances at the tips of a gold nanostar using electron energy-loss spectroscopy(EELS),cathodoluminescence spectroscopy(CL),and photon-induced near-field electron microscopy(PINEM).Supported by numerical electromagnetic boundary-element method(BEM)calculations,we show that the different coupling mechanisms probed by EELS,CL,and PINEM feature the same spatial dependence on the electric field distribution of the tip modes.However,the electron–photon interaction strength is found to vary with the incident electron velocity,as determined by the spatial Fourier transform of the electric near-field component parallel to the electron trajectory.For the tightly confined plasmonic tip resonances,our calculations suggest an optimum coupling velocity at electron energies as low as a few keV.Our results are discussed in the context of more complex geometries supporting multiple modes with spatial and spectral overlap.We provide fundamental insights into spontaneous and stimulated electron-light-matter interactions with key implications for research on(quantum)coherent optical phenomena at the nanoscale.

An ultra-compact particle size analyser using a CMOS image sensor and machine learning

Light scattering is a fundamental property that can be exploited to create essential devices such as particle analysers.The most common particle size analyser relies on measuring the angle-dependent diffracted light from a sample illuminated by a laser beam.Compared to other non-light-based counterparts,such a laser diffraction scheme offers precision,but it does so at the expense of size,complexity and cost.In this paper,we introduce the concept of a new particle size analyser in a collimated beam configuration using a consumer electronic camera and machine learning.The key novelty is a small form factor angular spatial filter that allows for the collection of light scattered by the particles up to predefined discrete angles.The filter is combined with a light-emitting diode and a complementary metal-oxide-semiconductor image sensor array to acquire angularly resolved scattering images.From these images,a machine learning model predicts the volume median diameter of the particles.To validate the proposed device,glass beads with diameters ranging from 13 to 125μm were measured in suspension at several concentrations.We were able to correct for multiple scattering effects and predict the particle size with mean absolute percentage errors of 5.09% and 2.5% for the cases without and with concentration as an input parameter,respectively.When only spherical particles were analysed,the former error was significantly reduced(0.72%).Given that it is compact(on the order of ten cm)and built with low-cost consumer electronics,the newly designed particle size analyser has significant potential for use outside a standard laboratory,for example,in online and in-line industrial process monitoring.

Thermodynamics of spin-1/2 Kagomé Heisenberg antiferromagnet:algebraic paramagnetic liquid and finite-temperature phase diagram

Quantum fluctuations from frustration can trigger quantum spin liquids(QSLs) at zero temperature.However, it is unclear how thermal fluctuations affect a QSL. We employ state-of-the-art tensor network-based methods to explore the ground state and thermodynamic properties of the spin-1=2 kagomé Heisenberg antiferromagnet(KHA). Its ground state is shown to be consistent with a gapless QSL by observing the absence of zero-magnetization plateau as well as the algebraic behaviors of susceptibility and specific heat at low temperatures, respectively. We show that there exists an algebraic paramagnetic liquid(APL) that possesses both the paramagnetic properties and the algebraic behaviors inherited from the QSL. The APL is induced under the interplay between quantum fluctuations from geometrical frustration and thermal fluctuations. By studying the temperature-dependent behaviors of specific heat and magnetic susceptibility, a finite-temperature phase diagram in a magnetic field is suggested, where various phases are identified. This present study gains useful insight into the thermodynamic properties of the spin-1/2 KHA with or without a magnetic field and is helpful for relevant experimental studies.

18-μJ energy, 160-kHz repetition rate, 250-MW peak power mid-IR OPCPA

Progresses on the development of a high repetition rate mid-IR laser source suitable for the next gen- eration of high-field physics experiments are reported. The presented optical parametric chirped pulse amplification （OPCPA） source currently delivers carrier-envelope phase （CEP）-stable 67-fs duration optical pulses with up to 18-μJ output energy at 160-kHz repetition rate. The focusability of the output beam （M^2 -2） enables peak intensities exceeding 1014 W/cm^2 and the record output energy stability-below 1% power fluctuation over 4.5 h makes this source a key enabler for the strong field physics community.

Rapid and robust control of single quantum dots

The combination of single particle detection and ultrafast laser pulses is an instrumental method to track dynamics at the femtosecond time scale in single molecules,quantum dots and plasmonic nanoparticles.Optimal control of the extremely short-lived coherences of these individual systems has so far remained elusive,yet its successful implementation would enable arbitrary external manipulation of otherwise inaccessible nanoscale dynamics.In ensemble measurements,such control is often achieved by resorting to a closed-loop optimization strategy,where the spectral phase of a broadband laser field is iteratively optimized.This scheme needs long measurement times and strong signals to converge to the optimal solution.This requirement is in conflict with the nature of single emitters whose signals are weak and unstable.Here we demonstrate an effective closed-loop optimization strategy capable of addressing single quantum dots at room temperature,using as feedback observable the two-photon photoluminescence induced by a phase-controlled broadband femtosecond laser.Crucial to the optimization loop is the use of a deterministic and robust-against-noise search algorithm converging to the theoretically predicted solution in a reduced amount of steps,even when operating at the few-photon level.Full optimization of the single dot luminescence is obtained within~100 trials,with a typical integration time of 100 ms per trial.These times are faster than the typical photobleaching times in single molecules at room temperature.Our results show the suitability of the novel approach to perform closed-loop optimizations on single molecules,thus extending the available experimental toolbox to the active control of nanoscale coherences.

Intermittent chaos for ergodic light trapping in a photonic fiber plate

Extracting the light trapped in a waveguide,or the opposite effect of trapping light in a thin region and guiding it perpendicular to its incident propagation direction,is essential for optimal energetic performance in illumination,display or light harvesting devices.Here we demonstrate that the paradoxical goal of letting as much light in or out while maintaining the wave effectively trapped can be achieved with a periodic array of interpenetrated fibers forming a photonic fiber plate.Photons entering perpendicular to that plate may be trapped in an intermittent chaotic trajectory,leading to an optically ergodic system.We fabricated such a photonic fiber plate and showed that for a solar cell incorporated on one of the plate surfaces,light absorption is greatly enhanced.Confirming this,we found the unexpected result that a more chaotic photon trajectory reduces the production of photon scattering entropy.

Thermal manipulation of plasmons in atomically thin films

Nanoscale photothermal effects enable important applications in cancer therapy,imaging and catalysis.These effects also induce substantial changes in the optical response experienced by the probing light,thus suggesting their application in all-optical modulation.Here,we demonstrate the ability of graphene,thin metal films,and graphene/metal hybrid systems to undergo photothermal optical modulation with depths as large as>70%over a wide spectral range extending from the visible to the terahertz frequency domains.We envision the use of ultrafast pump laser pulses to raise the electron temperature of graphene during a picosecond timescale in which its mid-infrared plasmon resonances undergo dramatic shifts and broadenings,while visible and near-infrared plasmons in the neighboring metal films are severely attenuated by the presence of hot graphene electrons.Our study opens a promising avenue toward the active photothermal manipulation of the optical response in atomically thin materials with potential applications in ultrafast light modulation.

Phase-matching-free parametric oscillators based on two-dimensional semiconductors

Optical parametric oscillators are widely used as pulsed and continuous-wave tunable sources for innumerable applications,such as quantum technologies,imaging,and biophysics.A key drawback is material dispersion,which imposes a phase-matching condition that generally entails a complex design and setup,thus hindering tunability and miniaturization.Here we show that the burden of phase-matching is surprisingly absent in parametric microresonators utilizing mono-layer transition-metal dichalcogenides as quadratic nonlinear materials.By the exact solution of nonlinear Maxwell equations and first-principle calculations of the semiconductor nonlinear response,we devise a novel kind of phase-matching-free miniaturized parametric oscillator operating at conventional pump intensities.We find that different two-dimensional semiconductors yield degenerate and non-degenerate emission at various spectral regions due to doubly resonant mode excitation,which can be tuned by varying the incidence angle of the external pump laser.In addition,we show that high-frequency electrical modulation can be achieved by doping via electrical gating,which can be used to efficiently shift the threshold for parametric oscillation.Our results pave the way for the realization of novel ultra-fast tunable micron-sized sources of entangled photons—a key device underpinning any quantum protocol.Highly miniaturized optical parametric oscillators may also be employed in lab-on-chip technologies for biophysics,detection of environmental pollution and security.