Showing papers in "Ultrafast science in 2022"

Air-Laser-Based Standoff Coherent Raman Spectrometer

Abstract: Among currently available optical spectroscopic methods, Raman spectroscopy has versatile application to investigation of dynamical processes of molecules leading to chemical changes in the gas and liquid phases. However, it is still a challenge to realize an ideal standoff coherent Raman spectrometer with which both high temporal resolution and high-frequency resolution can be achieved, so that one can remotely probe chemical species in real time with high temporal resolution while monitoring the populations in their respective rovibronic levels in the frequency domain with sufficiently high spectral resolution. In the present study, we construct an air-laser-based Raman spectrometer, in which near-infrared femtosecond (fs) laser pulses at 800 nm and cavity-free picosecond N2+ air-laser pulses at 391 nm generated by the filamentation induced by the fs laser pulses are simultaneously used, enabling us to generate a hybrid ps/fs laser source at a desired standoff position for standoff surveillance of chemical and biochemical species. With this prototype Raman spectrometer, we demonstrate that the temporal evolution of the electronic, vibrational, and rotational states of N2+ and the coupling processes of the rovibrational wave packet of N2 molecules can be probed.

High-Sensitivity Gas Detection with Air-Lasing-Assisted Coherent Raman Spectroscopy

Abstract: Remote or standoff detection of greenhouse gases, air pollutants, and biological agents with innovative ultrafast laser technology attracts growing interests in recent years. Hybrid femtosecond/picosecond coherent Raman spectroscopy is considered as one of the most versatile techniques due to its great advantages in terms of detection sensitivity and chemical specificity. However, the simultaneous requirement for the femtosecond pump and the picosecond probe increases the complexity of optical system. Herein, we demonstrate that air lasing naturally created inside a filament can serve as an ideal light source to probe Raman coherence excited by the femtosecond pump, producing coherent Raman signal with molecular vibrational signatures. The combination of pulse self-compression effect and air lasing action during filamentation improves Raman excitation efficiency and greatly simplifies the experimental setup. The air-lasing-assisted Raman spectroscopy was applied to quantitatively detect greenhouse gases mixed in air, and it was found that the minimum detectable concentrations of CO2 and SF6 can reach 0.1% and 0.03%, respectively. The ingenious designs, especially the optimization of pump-seed delay and the choice of perpendicular polarization, ensure a high detection sensitivity and signal stability. Moreover, it is demonstrated that this method can be used for simultaneously measuring CO2 and SF6 gases and distinguishing 12CO2 and 13CO2. The developed scheme provides a new route for high-sensitivity standoff detection and combustion diagnosis.

Nature-Inspired Superwettability Achieved by Femtosecond Lasers

Abstract: Wettability is one of a solid surface’s fundamental physical and chemical properties, which involves a wide range of applications. Femtosecond laser microfabrication has many advantages compared to traditional laser processing. This technology has been successfully applied to control the wettability of material surfaces. This review systematically summarizes the recent progress of femtosecond laser microfabrication in the preparation of various superwetting surfaces. Inspired by nature, the superwettabilities such as superhydrophilicity, superhydrophobicity, superamphiphobicity, underwater superoleophobicity, underwater superaerophobicity, underwater superaerophilicity, slippery liquid-infused porous surface, underwater superpolymphobicity, and supermetalphobicity are obtained on different substrates by the combination of the femtosecond laser-induced micro/nanostructures and appropriate chemical composition. From the perspective of biomimetic preparation, we mainly focus the methods for constructing various kinds of superwetting surfaces by femtosecond laser and the relationship between different laser-induced superwettabilities. The special wettability of solid materials makes the femtosecond laser-functionalized surfaces have many practical applications. Finally, the significant challenges and prospects of this field (femtosecond laser-induced superwettability) are discussed.

13.4 fs, 0.1 Hz OPCPA Front End for the 100 PW-Class Laser Facility

Abstract: Here, we report the recent progress on the front end developed for the 100 PW-class laser facility. Using 3 stages of optical parametric chirped-pulse amplification (OPCPA) based on lithium triborate (LBO) crystals, we realized a 5.26 J/0.1 Hz amplified output with a bandwidth over 200 nm near the center wavelength of 925 nm. After the compressor, we obtained a pulse duration of 13.4 fs. As the compression efficiency reached 67%, this OPCPA front end could potentially support a peak power of 263 TW at a repetition rate of 0.1 Hz. To the best of our knowledge, among all the 100 TW-level OPCPA systems, it shows the widest spectral width, the shortest pulse duration, and it is also the first OPCPA system working at a repetition-rate mode.

Theoretical Insights into Ultrafast Dynamics in Quantum Materials

Abstract: The last few decades have witnessed the extraordinary advances in theoretical and experimental tools, which have enabled the manipulation and monitoring of ultrafast dynamics with high precisions. For modeling dynamical responses beyond the perturbative regime, computational methods based on time-dependent density functional theory (TDDFT) are the optimal choices. Here, we introduce TDAP (time-dependent ab initio propagation), a first-principle approach that is aimed at providing robust dynamic simulations of light-induced, highly nonlinear phenomena by real-time calculation of combined photonic, electronic, and ionic quantum mechanical effects within a TDDFT framework. We review the implementation of real-time TDDFT with numerical atomic orbital formalisms, which has enabled high-accuracy, large-scale simulations with moderate computational cost. The newly added features, i.e., the time-dependent electric field gauges and controllable ionic motion make the method especially suitable for investigating ultrafast electron-nuclear dynamics in complex periodic and semiperiodic systems. An overview of the capabilities of this first-principle method is provided by showcasing several representative applications including high-harmonic generation, tunable phase transitions, and new emergent states of matter. The method demonstrates a great potential in obtaining a predictive and comprehensive understanding of quantum dynamics and interactions in a wide range of materials at the atomic and attosecond space-time scale.

Bessel Terahertz Pulses from Superluminal Laser Plasma Filaments

Abstract: Terahertz radiation with a Bessel beam profile is demonstrated experimentally from a two-color laser filament in air, which is induced by tailored femtosecond laser pulses with an axicon. The temporal and spatial distributions of Bessel rings of the terahertz radiation are retrieved after being collected in the far field. A theoretical model is proposed, which suggests that such Bessel terahertz pulses are produced due to the combined effects of the inhomogeneous superluminal filament structure and the phase change of the two-color laser components inside the plasma channel. These two effects lead to wavefront crossover and constructive/destructive interference of terahertz radiation from different plasma sources along the laser filament, respectively. Compared with other methods, our technique can support the generation of Bessel pulses with broad spectral bandwidth. Such Bessel pulses can propagate to the far field without significant spatial spreading, which shall provide new opportunities for terahertz applications.

Distributed Kerr Lens Mode-Locked Yb:YAG Thin-Disk Oscillator

Abstract: Ultrafast laser oscillators are indispensable tools for diverse applications in scientific research and industry. When the phases of the longitudinal laser cavity modes are locked, pulses as short as a few femtoseconds can be generated. As most high-power oscillators are based on narrow-bandwidth materials, the achievable duration for high-power output is usually limited. Here, we present a distributed Kerr lens mode-locked Yb:YAG thin-disk oscillator which generates sub-50 fs pulses with spectral widths far broader than the emission bandwidth of the gain medium at full width at half maximum. Simulations were also carried out, indicating good qualitative agreement with the experimental results. Our proof-of-concept study shows that this new mode-locking technique is pulse energy and average power scalable and applicable to other types of gain media, which may lead to new records in the generation of ultrashort pulses.

Factor 30 Pulse Compression by Hybrid Multipass Multiplate Spectral Broadening

Abstract: As ultrafast laser technology advances towards ever higher peak and average powers, generating sub-50 fs pulses from laser architectures that exhibit best power-scaling capabilities remains a major challenge. Here, we present a very compact and highly robust method to compress 1.24 ps pulses to 39 fs by means of only a single spectral broadening stage which neither requires vacuum parts nor custom-made optics. Our approach is based on the hybridization of the multiplate continuum and the multipass cell spectral broadening techniques. Their combination leads to significantly higher spectral broadening factors in bulk material than what has been reported from either method alone. Moreover, our approach efficiently suppresses adverse features of single-pass bulk spectral broadening. We use a burst-mode Yb:YAG laser emitting pulses with 80 MW peak power that are enhanced to more than 1 GW after postcompression. With only 0.19% rms pulse-to-pulse energy fluctuations, the technique exhibits excellent stability. Furthermore, we have measured state-of-the-art spectral-spatial homogeneity and good beam quality of M 2 = 1.2 up to a spectral broadening factor of 30. Due to the method’s simplicity, compactness, and scalability, it is highly attractive for turning a picosecond laser into an ultrafast light source that generates pulses of only a few tens of femtoseconds duration.

High-Flux 100 kHz Attosecond Pulse Source Driven by a High-Average Power Annular Laser Beam

Abstract: High-repetition-rate attosecond pulse sources are indispensable tools of time-resolved studies of electron dynamics, such as coincidence spectroscopy and experiments with high demands on statistics or signal-to-noise ratio, especially in case of solid and big molecule samples in chemistry and biology. Although with the high-repetition-rate lasers such attosecond pulses in a pump-probe configuration are possible to achieve, until now only a few such light sources have been demonstrated. Here, by shaping the driving laser to an annular beam, a 100-kHz attosecond pulse train (APT) is reported with the highest energy so far (51 pJ/shot) on target (269 pJ at generation) among the high-repetition-rate systems (> 10 kHz) in which the attosecond pulses were temporally characterized. The on-target pulse energy is maximized by reducing the losses from the reflections and filtering of the high harmonics, and an unprecedented 19% transmission rate from the generation point to the target position is achieved. At the same time, the probe beam is also annular, and low loss of this beam is reached by using another holey mirror to combine with the APT. The advantages of using an annular beam to generate attosecond pulses with a high average power laser is demonstrated experimentally and theoretically. The effect of nonlinear propagation in the generation medium on the annular-beam generation concept is also analyzed in detail.

Probing Molecular Frame Wigner Time Delay and Electron Wavepacket Phase Structure of CO Molecule

Abstract: The time delay of photoelectron emission serves as a fundamental building block to understand the ultrafast electron emission dynamics in strong-field physics. Here, we study the photoelectron angular streaking of CO molecules by using two-color (400+800 nm) corotating circularly polarized fields. By coincidently measuring photoelectrons with the dissociative ions, we present molecular frame photoelectron angular distributions with respect to the instantaneous driving electric field signatures. We develop a semiclassical nonadiabatic molecular quantum-trajectory Monte Carlo (MO-QTMC) model that fully captures the experimental observations and further ab initio simulations. We disentangle the orientation-resolved contribution of the anisotropic ionic potential and the molecular orbital structure on the measured photoelectron angular distributions. Furthermore, by analyzing the photoelectron interference patterns, we extract the sub-Coulomb-barrier phase distribution of the photoelectron wavepacket and reconstruct the orientation- and energy-resolved Wigner time delay in the molecular frame. Holographic angular streaking with bicircular fields can be used for probing polyatomic molecules in the future.

Birefringence-Managed Normal-Dispersion Fiber Laser Delivering Energy-Tunable Chirp-Free Solitons

Abstract: Chirp-free solitons have been mainly achieved with anomalous-dispersion fiber lasers by the balance of dispersive and nonlinear effects, and the single-pulse energy is constrained within a relatively small range. Here, we report a class of chirp-free pulse in normal-dispersion erbium-doped fiber lasers, termed birefringence-managed soliton, in which the birefringence-related phase-matching effect dominates the soliton evolution. Controllable harmonic mode locking from 5 order to 85 order is obtained at the same pump level of ~10 mW with soliton energy fully tunable beyond ten times, which indicates a new birefringence-related soliton energy law, which fundamentally differs from the conventional soliton energy theorem. The unique transformation behavior between birefringence-managed solitons and dissipative solitons is directly visualized via the single-shot spectroscopy. The results demonstrate a novel approach of engineering fiber birefringence to create energy-tunable chirp-free solitons in normal-dispersion regime and open new research directions in fields of optical solitons, ultrafast lasers, and their applications.

Low-Energy Protons in Strong-Field Dissociation of H2+ via Dipole-Transitions at Large Bond Lengths

Abstract: More than ten years ago, the observation of the low-energy structure in the photoelectron energy spectrum, regarded as an “ionization surprise,” has overthrown our understanding of strong-field physics. However, the similar low-energy nuclear fragment generation from dissociating molecules upon the photon energy absorption, one of the well-observed phenomena in light-molecule interaction, still lacks an unambiguous mechanism and remains mysterious. Here, we introduce a time-energy-resolved manner using a multicycle near-infrared femtosecond laser pulse to identify the physical origin of the light-induced ultrafast dynamics of molecules. By simultaneously measuring the bond-stretching times and photon numbers involved in the dissociation of H2+ driven by a polarization-skewed laser pulse, we reveal that the low-energy protons (below 0.7 eV) are produced via dipole-transitions at large bond lengths. The observed low-energy protons originate from strong-field dissociation of high vibrational states rather than the low ones of H2+ cation, which is distinct from the well-accepted bond-softening picture. Further numerical simulation of the time-dependent Schrödinger equation unveils that the electronic states are periodically distorted by the strong laser field, and the energy gap between the field-dressed transient electronic states may favor the one- or three-photon transitions at the internuclear distance larger than 5 a.u. The time-dependent scenario and our time-energy-resolved approach presented here can be extended to other molecules to understand the complex ultrafast dynamics.

High-Harmonic Generation and Correlated Electron Emission from Relativistic Plasma Mirrors at 1 kHz Repetition Rate

Abstract: We report evidence for the first generation of XUV spectra from relativistic surface high-harmonic generation (SHHG) on plasma mirrors at a kilohertz repetition rate, emitted simultaneously with energetic electrons. SHHG spectra and electron angular distributions are measured as a function of the experimentally controlled plasma density gradient scale length L g for three increasingly short and intense driving pulses: 24 fs and a 0 = 1.1 , 8 fs and a 0 = 1.6 , and finally 4 fs and a 0 ≈ 2.1 , where a 0 is the peak vector potential normalized by m e c / e with the elementary charge e , the electron rest mass m e , and the vacuum light velocity c . For all driver pulses, we observe correlated relativistic SHHG and electron emission in the range L g ∈ λ / 20 , λ / 4 , with an optimum gradient scale length of L g ≈ λ / 10 . This universal optimal L g -range is rationalized by deriving a direct intensity-independent link between the scale length L g and an effective similarity parameter for relativistic laser-plasma interactions.

Direct Visualization of Deforming Atomic Wavefunction in Ultraintense High-Frequency Laser Pulses

Abstract: Interaction of intense laser fields with atoms distorts the bound-state electron cloud. Tracing the temporal response of the electron cloud to the laser field is of fundamental importance for understanding the ultrafast dynamics of various nonlinear phenomena of matter, but it is particularly challenging. Here, we show that the ultrafast response of the atomic electron cloud to the intense high-frequency laser pulses can be probed with the attosecond time-resolved photoelectron holography. In this method, an infrared laser pulse is employed to trigger tunneling ionization of the deforming atom. The shape of the deforming electron cloud is encoded in the hologram of the photoelectron momentum distribution. As a demonstration, by solving the time-dependent Schrödinger equation, we show that the adiabatic deforming of the bound-state electron cloud, as well as the nonadiabatic transition among the distorted states, is successfully tracked with attosecond resolution. Our work films the formation process of the metastable Kramers-Henneberger states in the intense high-frequency laser pulses. This establishes a novel approach for time-resolved imaging of the ultrafast bound-state electron processes in intense laser fields.

Generating Isolated Attosecond X-Ray Pulses by Wavefront Control in a Seeded Free-Electron Laser

Abstract: We proposed a simple method based on the seeded free-electron laser (FEL) to generate fully coherent X-ray pulses with durations at dozens of attosecond level. The echo-enabled harmonic generation technique is utilized to generate the fully coherent laser pulse covering the water-window range. A wavefront rotation laser is adopted as the seed to tailor the longitudinal contour of the radiation pulse. Due to the sensitivity of seeded FEL to external lasers, this method can effectively inhibit the bunching of the adjacent regions while preserving an isolated bunching in the middle. Sending such an electron beam into a short undulator, simulation results show that ultrashort X-ray pulses with peak power of GW level and pulse duration as short as 86 attoseconds can be generated. The proposed scheme can make it possible to study the electronic dynamic of the valence electrons of which the time scale is about 100 attoseconds and may open up a new frontier of ultrafast science.

Anti-Correlated Plasma and THz Pulse Generation during Two-Color Laser Filamentation in Air

Abstract: The THz generation efficiency and the plasma density generated by a filament in air have been found anti-correlated when pumped by 800 nm+1600 nm two-color laser field. The plasma density near zero delay of two laser pulses has a minimum value, which is opposite to the trend of THz generation efficiency and contradicts common sense. The lower plasma density cannot be explained by the static tunneling model according to the conventional photocurrent model, but it might be attributed to the electron trapping by the excited states of nitrogen molecule. The present work also clarifies the dominant role of the drifting velocity accelerated by the two-color laser field during the THz pulse generation process. The results promote our understanding on the optimization of the THz generation efficiency by the two-color laser filamentation.

Attosecond Delays of High-Harmonic Emissions from Hydrogen Isotopes Measured by XUV Interferometer

Abstract: High-harmonic spectroscopy can access structural and dynamical information on molecular systems encoded in the amplitude and phase of high-harmonic generation (HHG) signals. However, measurement of the harmonic phase is a daunting task. Here, we present a precise measurement of HHG phase difference between two isotopes of molecular hydrogen using the advanced extreme-ultraviolet (XUV) Gouy phase interferometer. The measured phase difference is about 200 mrad, corresponding to ~3 attoseconds (1 as=10−18 s) time delay which is nearly independent of harmonic order. The measurements agree very well with numerical calculations of a four-dimensional time-dependent Schödinger equation. Numerical simulations also reveal the effects of molecular orientation and intramolecular two-center interference on the measured phase difference. This technique opens a new avenue for measuring the phase of harmonic emission for different atoms and molecules. Together with isomeric or isotopic comparisons, it also enables the observation of subtle effects of molecular structures and nuclear motion on electron dynamics in strong laser fields.

Exploring Femtosecond Laser Ablation by Snapshot Ultrafast Imaging and Molecular Dynamics Simulation

Abstract: Femtosecond laser ablation (FLA) has been playing a prominent role in precision fabrication of material because of its circumvention of thermal effect and extremely high spatial resolution. Molecular dynamics modeling, as a powerful tool to study the mechanism of femtosecond laser ablation, still lacks the connection between its simulation results and experimental observations at present. Here we combine a single-shot chirped spectral mapping ultrafast photography (CSMUP) technique in experiment and a three-dimensional two-temperature model-based molecular dynamics (3D TTM-MD) method in theory to jointly investigate the FLA process of bulky gold. Our experimental and simulated results show quite high consistency in time-resolved morphologic dynamics. According to the highly accurate simulations, the FLA process of gold at the high laser fluence is dominated by the phase explosion, which shows drastic vaporized cluster eruption and pressure dynamics, while the FLA process at the low laser fluence mainly results from the photomechanical spallation, which shows moderate temperature and pressure dynamics. This study reveals the ultrafast dynamics of gold with different ablation schemes, which has a guiding significance for the applications of FLA on various kinds of materials.

Attosecond Optical and Ramsey-Type Interferometry by Postgeneration Splitting of Harmonic Pulse

Abstract: Time domain Ramsey-type interferometry is useful for investigating spectroscopic information of quantum states in atoms and molecules. The energy range of the quantum states to be observed with this scheme has now reached more than 20 eV by resolving the interference fringes with a period of a few hundred attoseconds. This attosecond Ramsey-type interferometry requires the irradiation of a coherent pair of extreme ultraviolet (XUV) light pulses, while all the methods used to deliver the coherent XUV pulse pair until now have relied on the division of the source of an XUV pulse in two before the generation. In this paper, we report on a novel technique to perform attosecond Ramsey-type interferometry by splitting an XUV high-order harmonic (HH) pulse of a sub-20 fs laser pulse after its generation. By virtue of the postgeneration splitting of the HH pulse, we demonstrated that the optical interference emerging at the complete temporal overlap of the HH pulse pair seamlessly continued to the Ramsey-type electronic interference in a helium atom. This technique is applicable for studying the femtosecond dephasing dynamics of electronic wavepackets and exploring the ultrafast evolution of a cationic system entangled with an ionized electron with sub-20 fs resolution.

Rapid-scan nonlinear time-resolved spectroscopy over arbitrary delay intervals

Abstract: Femtosecond dual-comb lasers have revolutionized linear Fourier-domain spectroscopy by offering a rapid motion-free, precise and accurate measurement mode with easy registration of the combs beat note in the RF domain. Extensions of this technique found already application for nonlinear time-resolved spectroscopy within the energy limit available from sources operating at the full oscillator repetition rate. Here, we present a technique based on time filtering of femtosecond frequency combs by pulse gating in a laser amplifier. This gives the required boost to the pulse energy and provides the flexibility to engineer pairs of arbitrarily delayed wavelength-tunable pulses for pump-probe techniques. Using a dual-channel millijoule amplifier, we demonstrate programmable generation of both extremely short, fs, and extremely long (>ns) interpulse delays. A predetermined arbitrarily chosen interpulse delay can be directly realized in each successive amplifier shot, eliminating the massive waiting time required to alter the delay setting by means of an optomechanical line or an asynchronous scan of two free-running oscillators. We confirm the versatility of this delay generation method by measuring chi^(2) cross-correlation and chi^(3) multicomponent population recovery kinetics.

Two-Beam Ultrafast Laser Scribing of Graphene Patterns with 90-nm Subdiffraction Feature Size

Abstract: The fabrication of high-resolution laser-scribed graphene devices is crucial to achieving large surface areas and thus performance breakthroughs. However, since the investigation mainly focuses on the laser-induced reduction of graphene oxide, the single-beam scribing provides a tremendous challenge to realizing subdiffraction features of graphene patterns. Here, we present an innovative 2-beam laser scribing pathway for the fabrication of subdiffraction graphene patterns. First, an oxidation reaction of highly reduced graphene oxide can be controllably driven by irradiation of a 532-nm femtosecond laser beam. Based on the oxidation mechanism, a 2-beam laser scribing was performed on graphene oxide thin films, in which a doughnut-shaped 375-nm beam reduces graphene oxide and a spherical 532-nm ultrafast beam induces the oxidation of laser-reduced graphene oxide. The spherical beam turns the highly reduced graphene oxide (reduced by the doughnut-shaped beam) to an oxidized state, splitting the laser-scribed graphene oxide line into 2 subdiffraction featured segments and thus forming a laser-scribed graphene/oxidized laser-scribed graphene/laser-scribed graphene line. Through the adjustment of the oxidation beam power, the minimum linewidth of laser-scribed graphene was measured to be 90 nm. Next, we fabricated patterned supercapacitor electrodes containing parallel laser-scribed graphene lines with subdiffraction widths and spacings. An outstanding gravimetric capacitance of 308 F/g, which is substantially higher than those of reported graphene-based supercapacitors, has been delivered. The results offer a broadly accessible strategy for the fabrication of high-performance graphene-based devices including high-capacity energy storage, high-resolution holograms, high-sensitivity sensors, triboelectric nanogenerators with high power densities, and artificial intelligence devices with high neuron densities.

Ultrafast opto-mechanical terahertz modulators based on stretchable carbon nanotube thin films

Abstract: For terahertz (THz) wave applications, tunable and rapid modulation is highly required. When studied by means of optical pump-terahertz probe spectroscopy, single-walled carbon nanotubes (SWCNTs) thin films demonstrated ultrafast carrier recombination lifetimes with a high relative change in the signal under optical excitation, making them promising candidates for high-speed modulators. Here, combination of SWCNTthin films and stretchable substrates facilitated studies of the SWCNT mechanical properties under strain,and enabled the development of a new type of an opto-mechanical modulator. By applying a certain strain to the SWCNT films, the effective sheet conductance and therefore modulation depth can be fine-tuned to optimize the designed modulator. Modulators exhibited a photoconductivity change of 3-4 orders of magnitude under the strain due to the structural modification in the SWCNT network. Stretching was used to control the THz signal with a modulation depth of around 100 % without strain and 65 % at a high strainoperation of 40 %. The sensitivity of modulators to beam polarisation is also shown, which might also come in handy for the design of a stretchable polariser. Our results give a fundamental grounding for the design of high-sensitivity stretchable devices based on SWCNT films.

Ultrafast Two-electron Orbital Swap in Li Initiated by Attosecond pulses

Abstract: A universal mechanism of ultrafast two-electron orbital swap is discovered through two-photon sequential double ionization of Li. After a $1s$ electron in Li is ionized by absorbing an EUV photon, the other two bound electrons located on two different shells have either parallel or antiparallel spin orientations. In the latter case, these two electrons are in the superposition of the singlet and triplet states with different energies, forming a quantum beat and giving rise to the two-electron orbital swap with a period of several hundred attoseconds. The orbital swap mechanism can be used to manipulate the spin polarization of photoelectron pairs by conceiving the attosecond-pump attosecond-probe strategy, and thus serves as a knob to control spin-resolved multielectron ultrafast dynamics.

Transient Superdiffusion of Energetic Carriers in Transition Metal Dichalcogenides Visualized by Ultrafast Pump-Probe Microscopy

Abstract: Because of the strong Coulomb interaction and quantum confinement effect, 2-dimensional transition metal dichalcogenides possess a stable excitonic population. To realize excitonic device applications, such as excitonic circuits, switches, and transistors, it is of paramount importance for understanding the optical properties of transition metal dichalcogenides. Furthermore, the strong quantum confinement in 2-dimensional space introduces exotic properties, such as enhanced phonon bottlenecking effect, many-body interaction of excitons, and ultrafast nonequilibrium exciton–exciton annihilation. Exciton diffusion is the primary energy dissipation process and a working horse in excitonic devices. In this work, we investigated time-resolved exciton propagation in monolayer semiconductors of WSe 2 , MoWSe 2 , and MoSe 2 , with a home-built femtosecond pump-probe microscope. We observed ultrafast exciton expansion behavior with an equivalent diffusivity of up to 502 cm 2 s −1 at the initial delay time, followed by a slow linear diffusive regime (20.9 cm 2 s −1 ) in the monolayer WSe 2 . The fast expansion behavior is attributed to energetic carrier-dominated superdiffusive behavior. We found that in the monolayers MoWSe 2 and MoSe 2 , the energetic carrier-induced exciton expansion is much more effective, with diffusivity up to 668 and 2295 cm 2 s −1 , respectively. However, the “cold” exciton transport is trap limited in MoWSe 2 and MoSe 2 , leading to negative diffusion behavior at later time. Our findings are helpful to better understand the ultrafast nonlinear diffusive behavior in strongly quantum-confined systems. It may be harnessed to break the limit of conventional slow diffusion of excitons for advancing more efficient and ultrafast optoelectronic devices.