Fs Pulses Research Articles

A design for terahertz-driven isolated sub-femtosecond electron pulses

Abstract Isolated electron pulses are pivotal in resolving ultrafast dynamics. In this study, we present a design for generating isolated ultrashort electron pulses using a terahertz resonator capable of achieving subfemtosecond pulse durations. Our numerical simulations demonstrate that terahertz field enhancement within the thin resonator could enable generation of isolated electron pulses as short as 817 attoseconds from a 500-fs pulse. The design can also be extended to direct current with two additional capacitors. We analyze key design parameters to provide empirical guidance for experimental implementations, with the potential of delivering ultrafast single electron pulses compactly and cost-effectively.

Real‐Time and Site‐Specific Perturbation of Dynamic Subcellular Compartments Using Femtosecond Pulses

Understanding laser interactions with subcellular compartments is crucial for advancing optical microscopy, phototherapy, and optogenetics. While continuous‐wave lasers rely on linear absorption, femtosecond (fs) lasers enable nonlinear multiphoton absorption confined to the laser focus, offering high axial precision. However, current fs laser delivery methods lack the ability to target dynamic molecular entities and automate target selection, making them incapable of performing real‐time perturbation of mobile or complexly distributed biomolecules. Additionally, existing technologies separate fs pulse delivery and imaging, preventing simultaneous recording of cellular responses. To overcome these challenges, this study introduces fs real‐time precision opto‐control (fs‐RPOC), which integrates a laser scanning microscope with a closed‐loop feedback mechanism for automated, chemically selective subcellular perturbation. Fs‐RPOC achieves superior spatial precision and fast response time, enabling single‐ and sub‐organelle microsurgery of dynamic targets and localized molecular modulation. By applying a pulse‐picking method, fs‐RPOC independently controls laser average and peak power at any desired subcellular compartment. Targeting mitochondria, fs‐RPOC reveals site‐specific molecular responses resulting from fs‐laser‐induced reactive oxygen species formation, H2O2 diffusion, and low‐density plasma generation. These findings offer new insights into fs laser interactions with subcellular compartments and demonstrate fs‐RPOC's potential for precise molecular and organelle regulation.

High energy nonlinear pulse compression to sub-50 fs by a hybrid bulk-air multi-pass cell

We demonstrate a high energy nonlinear pulse compression scheme based on a hybrid bulk-air multi-pass cell (MPC), achieving a high-power ultrafast source with 45 fs pulse duration, 130 W average power at 200 kHz repetition rate, and near-diffraction-limited beam quality. The impact of air nonlinearity accumulation in the MPC on beam quality is investigated to maintain good beam quality at high pulse energy, which suggests the adoption of pre-chirped and circularly-polarized pulses can effectively improve the input pulse energy and suppress the spatial quality degradation. The nonlinear mode-matching is also adopted to minimize the impact of the self-focusing effect of solid-thin-plates on q-preserving propagation. Although both solid plates and air contribute to the spectral broadening, the hybrid MPC is still an all-solid-state configuration without a pressure-controlled cell, providing a compact and economical approach for the nonlinear pulse compression at several hundred microjoules (µJ) energy levels. To the best of our knowledge, this is the highest pulse energy from the solid-state MPC configuration.

Real-Time and Site-Specific Perturbation of Dynamic Subcellular Compartments Using Femtosecond Pulses

Understanding laser interactions with subcellular compartments is crucial for advancing optical microscopy, phototherapy, and optogenetics. While continuous-wave (CW) lasers rely on linear absorption, femtosecond (fs) lasers enable nonlinear multiphoton absorption confined to the laser focus, offering high axial precision. However, current fs laser delivery methods lack the ability to target dynamic molecular entities and automate target selection, limiting real-time perturbation of biomolecules with mobility or complex distribution. Additionally, existing technologies separate fs pulse delivery and imaging, preventing simultaneous recording of cellular responses. To overcome these challenges, we introduce fs real-time precision opto-control (fs-RPOC), which integrates a laser scanning microscope with a closed-loop feedback mechanism for automated, chemically selective subcellular perturbation. fs-RPOC achieves superior spatial precision and fast response time, enabling single- and sub-organelle microsurgery of dynamic targets and localized molecular modulation. By applying a pulse-picking method, fs-RPOC independently controls laser average and peak power at any desired subcellular compartment. Targeting mitochondria, fs-RPOC reveals site-specific molecular responses resulting from fs-laser-induced ROS formation, H2O2 diffusion, and low-density plasma generation. These findings offer new insights into fs laser interactions with subcellular compartments and demonstrate fs-RPOC’s potential for precise molecular and organelle regulation.

Comparative study of spectral broadening and few-cycle compression of Yb:KGW laser pulses in gas-filled hollow-core fibers

While industrial-grade Yb-based amplifiers have become very prevalent, their limited gain bandwidth has created a large demand for robust spectral broadening techniques that allow for few-cycle pulse compression. In this work, we perform a comparative study between several atomic and molecular gases as media for spectral broadening in a hollow-core fiber geometry. Exploiting nonlinearities such as self-phase modulation, self-steepening, and stimulated Raman scattering, we explore the extent of spectral broadening and its dependence on gas pressure, the critical power for self-focusing, and the optimal regime for few-cycle pulse compression. Using a 3-mJ, 200-fs input laser pulses, we achieve 17 fs, few-cycle pulses with 80% fiber energy transmission efficiency. The optimal parameters can be scaled for higher or lower input pulse energies with appropriate gas parameters and fiber geometry.

Efficient, broadly tunable, hollow-fiber source of megawatt pulses for multiphoton microscopy.

Three-photon fluorescence microscopy (3PM) has driven rapid progress in deep-tissue imaging beyond the depth limit of two-photon microscopy, with impacts in neuroscience, immunology, and cancer biology. Three-photon excitation places a premium on ultrashort pulses with high peak power in the 1300- and 1700-nm wavelength bands, which allow deepest imaging. The inefficiency and cost of current sources of these pulses present major barriers to the use of 3PM in biomedical research labs. Fiber sources of such pulses could potentially alleviate these problems, but the peak-power limitations of optical fibers have limited their use in 3PM. Here, we describe a fiber-based source of femtosecond pulses with multi-megawatt peak power. Femtosecond pulses at 1030 nm are launched into an antiresonant hollow-core fiber filled with argon. By varying only the gas pressure, pulses with hundreds of nanojoules of energy and sub-100 fs duration are obtained at wavelengths between 850 and 1700 nm. This approach is a new route to an efficient and potentially low-cost source for deep-tissue imaging. In particular, 960-nJ and 50-fs pulses are generated at 1300 nm with a conversion efficiency of 10%. The nearly 20-MW peak power is an order of magnitude higher than the previous best from a femtosecond solid-core fiber source at 1300 nm. As an example of the capabilities of the source, these pulses are used to image structure and neuronal activity in a mouse brain as deep as 1.1 mm below the dura.

75 fs-pulse duration, ∼200 nm-span, distributed dispersion managed 2 μm ultrafast fiber laser

75 fs-pulse duration, ∼200 nm-span, distributed dispersion managed 2 μm ultrafast fiber laser

Time-resolved Coulomb explosion imaging of vibrational wave packets in alkali dimers on helium nanodroplets.

Vibrational wave packets are created in the lowest triplet state 13Σu+ of K2 and Rb2 residing on the surface of helium nanodroplets, through non-resonant stimulated impulsive Raman scattering induced by a moderately intense near-infrared laser pulse. A delayed, intense 50-fs laser pulse doubly ionizes the alkali dimers via multiphoton absorption and thereby causes them to Coulomb explode into a pair of alkali ions Ak+. From the kinetic energy distribution P(Ekin) of the Ak+ fragment ions, measured at a large number of delays, we determine the time-dependent internuclear distribution P(R, t), which represents the modulus square of the wave packet within the accuracy of the experiment. For both K2 and Rb2, P(R, t) exhibits a periodic oscillatory structure throughout the respective 300 and 100ps observation times. The oscillatory structure is reflected in the time-dependent mean value of R, ⟨R⟩(t). The Fourier transformation of ⟨R⟩(t) shows that the wave packets are composed mainly of the vibrational ground state and the first excited vibrational state, in agreement with numerical simulations. In the case of K2, the oscillations are observed for 300ps, corresponding to more than 180 vibrational periods with an amplitude that decreases gradually from 0.035 to 0.020Å. Using time-resolved spectral analysis, we find that the decay time of the amplitude is ∼260ps. The decrease is ascribed to the weak coupling between the vibrating dimers and the droplet.

On the theory of spectral compression-assisted optical temporal differentiation

Bandwidth limitation represents a significant factor that degrades the performance of optical devices. The dimensions, composition and configuration of optical devices impose intrinsic constraints on processing broadband optical pulse signals. The enhancement of the response bandwidth of optical devices represents a significant challenge. In this study, we put forward the theory of self-similar spectral compression (SSSC), which involves solving the nonlinear Schrödinger equation with variable coefficients by using the Taylor expansion and residual theorem. The spectral waveform can be precisely preserved in the process of SSSC, leading to a predictable compression factor without pedestals. To demonstrate the effectiveness of the proposed SSSC, we present a case study by designing an on-chip optical time-domain differentiator (OTD) system including a silicon-based tapered spiral waveguide. A 200-fs chirped pulse is well differentiated at multiple orders in the OTD system. Although the linear loss of spiral waveguide has a detrimental impact on SSSC, the broadband spectrum can still be self-similarly compressed, leading to a reduction of differentiation deviation of 22.5 times. The proposed SSSC theory offers valuable guidance for designing all-optical signal processing systems with high spectral resolution and low signal error.

Filamentation of 100-fs Pulses from a Ti:Sapphire Laser in Gases to Generate a Signal Wave of a Difference Frequency Generator

Filamentation of 100-fs Pulses from a Ti:Sapphire Laser in Gases to Generate a Signal Wave of a Difference Frequency Generator

Optoplasmonic tuneable response by femtosecond laser irradiation of glass with deep-implanted gold nanoparticles

Optoplasmonic tuneable response by femtosecond laser irradiation of glass with deep-implanted gold nanoparticles

Probing coherent phonons in the advanced undergraduate laboratory

Ultrafast optical spectroscopy is an effective experimental technique for accessing electronic and atomic motions in materials at their fundamental timescales and studying their responses to external perturbations. Despite the important insights that ultrafast techniques can provide on the microscopic physics of solids, undergraduate students' exposure to this area of research is still limited. In this article, we describe an ultrafast optical pump-probe spectroscopy experiment for the advanced undergraduate instructional laboratory, in which students can measure coherently excited vibrations of the crystal lattice and connect their observations to the microscopic properties of the investigated materials. We designed a simple table-top apparatus based on a commercial Er-fiber oscillator emitting 50-fs pulses at 1560 nm and at 100 MHz repetition rate. We split the output into two beams, using one of them as an intense “pump” to coherently excite phonons in selected crystals, and the other as a weaker, delayed “probe” to measure the transient reflectivity changes induced by the pump. We characterize the ultrafast laser pulses via intensity autocorrelation measurements and detect coherent phonon oscillations in the reflectivity of Bi, Sb, and 1T-TaS2. We then discuss the oscillation amplitude, frequency, and damping in terms of microscopic properties of these systems.

Cooperative dynamic polaronic picture of diamond color centers

Polarons can control carrier mobility and can also be used in the design of quantum devices. Although much effort has been directed into investigating the nature of polarons, observation of defect-related polarons is challenging due to electron-defect scattering. Here we explore the polaronic behavior of nitrogen-vacancy (NV) centers in a diamond crystal using an ultrafast pump-probe technique. A 10-fs optical pulse acts as a source of high electric field exceeding the dielectric breakdown threshold, in turn exerting a force on the NV charge distribution and polar optical phonons. The electronic and phononic responses are enhanced by an order of magnitude for a low density of NV centers, which we attribute to a combination of cooperative polaronic effects and scattering by defects. First-principles calculations support the presence of dipolar Fröhlich interaction via non-zero Born effective charges. Our findings provide insights into the physics of color centers in diamonds.

All-fiber integrated gain-managed nonlinear amplification system delivers ultrafast lasers with 3.6 MW peak power and 45 fs pulse duration

All-fiber integrated gain-managed nonlinear amplification system delivers ultrafast lasers with 3.6 MW peak power and 45 fs pulse duration

Evidence of non-isentropic release from high residual temperatures in shocked metals measured with ultrafast x-ray diffraction

Shock experiments are widely used to understand the mechanical and electronic properties of matter under extreme conditions. However, after shock loading to a Hugoniot state, a clear description of the post-shock thermal state and its impacts on materials is still lacking. We used diffraction patterns from 100-fs x-ray pulses to investigate the temperature evolution of laser-shocked Al–Zr metal film composites at time delays ranging from 5 to 75 ns driven by a 120-ps short-pulse laser. We found significant heating of both Al and Zr after shock release, which can be attributed to heat generated by inelastic deformation. A conventional hydrodynamic model that employs (i) typical descriptions of Al and Zr mechanical strength and (ii) elevated strength responses (which might be attributed to an unknown strain rate dependence) did not fully account for the measured temperature increase, which suggests that other strength-related mechanisms (such as fine-scale void growth) could play an important role in thermal responses under shock wave loading/unloading cycles. Our results suggest that a significant portion of the total shock energy delivered by lasers becomes heat due to defect-facilitated plastic work, leaving less converted to kinetic energy. This heating effect may be common in laser-shocked experiments but has not been well acknowledged. High post-shock temperatures may induce phase transformation of materials during shock release. Another implication for the study is the preservability of magnetic records from planetary surfaces that have a shock history from frequent impact events.

Femtosecond dynamics on the nanoscale of intense laser-induced grating plasma

The complex interaction dynamics of intense femtosecond (fs) pulses and their picosecond (ps)-long leading edge with nanostructured solids occur at nanometer (nm) spatial and the fs temporal scales, making them extremely difficult to measure directly. Here, we present pump-probe-based measurements that capture the ultrafast evolution of relativistically intense laser-driven grating plasma on these scales. We measure the transient reflectivity and spectrum of the scattered or diffracted UV-probe pulses from the grating structures with hundreds of fs resolution. Our measurements capture the initial onset of the solid-to-plasma transition and the subsequent grating plasma expansion, a few ps before the peak of the intense fs pulse. We measure the instantaneous position of the electron critical surface, its velocity, and acceleration, which are very crucial for understanding the physics and applications in ion/electron acceleration and high harmonic generation, while also providing valuable benchmarks for simulations. Particle-in-cell simulations corroborate the observations offering further insight into this process.

Coherent molecular vibrational dynamics studied by a 10-fs laser pulse in comparison for selective and nonselective excitation

Various selective excitation methods for molecular vibrations have been developed based on the characteristics of the applied lasers. In the present work, we coherently and selectively excited molecular vibrations up to 3000 cm−1 by irradiation with a sub-10-fs laser pulse. The spectral shaping of this laser pulse enabled us to selectively excite only the target vibrational frequency range (of the model compounds acetonitrile and benzene). Furthermore, the coherent molecular vibrational dynamics were compared between selective and nonselective excitation. The coherence of the molecular vibration persisted until 2–3 ps after photoexcitation, with the lifetime of this coherence being independent of whether the coherent molecular vibration was selective or nonselective excitation.

Post-compression of powerful femtosecond pulses after second harmonic generation

A proof-of-principle experiment of highly efficient (38%) second harmonic generation was implemented at a ∼1TW/cm2 intensity of a 45 fs pulse in a composite nonlinear sample–a 1 mm KDP crystal glued onto a 1 mm fused silica substrate. The attenuated replica of the second harmonic pulse (455 nm) was compressed down to 28.6 fs by means of dispersive mirrors, with a significant reduction in both the wings and the far temporal contrast. The peak power of the second harmonic was ∼74% of the power of the fundamental harmonic, which ensured a three-fold increase in the focal intensity.

Modification of the temporal laser source term in two-temperature model

Simulations on the interaction between laser pulses with materials during pulsed laser ablation require information on the characteristics of both laser and material. While details on material properties are extensively studied, the temporal profile of the laser source term is rarely studied and is generally assumed to be Gaussian. This article explores the effects of non-Gaussian temporal laser source terms (TLSTs) in simulating the heat diffusion within a metal upon irradiation of femtosecond pulsed lasers. We employed the Two-Temperature Model (TTM) to simulate the temperature evolution on the surface of copper upon irradiation of a 100-fs laser pulse with different TLSTs at different laser repetition modes (single-shot, burst, and biburst). We used a constant total fluence of 3J/cm 2 in all cases and subsequently calculated the ablation depths based on the resulting temperature values. Our results show that non-Gaussian TLSTs generate lower electron and lattice temperatures, resulting in shallower crater depths compared to a Gaussian TLST. However, under burst and biburst modes, particularly with higher subpulse number repetition time, the results of non-Gaussian TLSTs approximate those of Gaussian TLSTs. To initiate laser ablation in such cases, higher laser fluence might be required. Our work on non-Gaussian temporal profiles in TTM simulations can be applied to other computational methods in describing laser-matter interaction.

Nonlinear compression of mid-infrared supercontinuum generation in dispersion-engineered As2S5 chalcogenide ridge waveguide

We numerically demonstrated nonlinear compression of mid-infrared (mid-IR) supercontinuum (SC) generation in As2S5 chalcogenide glass (ChG) ridge waveguides, achieving a small dispersion value at the pump wavelength of 2.5 µm by adjusting the waveguide width (normal dispersions of -10.547 ps.nm−1.km−1 and the anomalous dispersions of +5.314 ps.nm−1.km−1). These waveguides were designed using the negative slope of the dispersions with the negative third-order dispersion, which are applied to generate the nonlinear compression of SC generation. Using a 50-fs pulse with the peak power of 2000 W, the two waveguides could compress the maximum pulse peak power of 6900 W (> 3.4 octaves) and 6360 W (> 3.1 octaves) and generate the widest SC spectra, spanning from 1.20 µm to 12.96 µm and from 1.25 µm to >13 µm with only short waveguides 0.85 mm and 1 mm long, respectively. The key process behind SC formation in such ChG waveguides is related to self-phase modulation, four-wave mixing, and nonlinear compression. This particular design is effective, and ChG waveguides can generate high peak power and the widest spectra of SC generation. Moreover, the waveguides are also relatively flexible in design, which is concerned with optical design and engineering, and micro-optical devices. As the ultra-wideband mid-IR SC source, high pulse peak power, very short waveguides, and low-energy pulses (<1 pJ) are important for on-chip mid-IR SC sources, the proposed work would offer the greatest benefits in practical application.