Pump Pulse Research Articles

An analytical model of label-free nanoscale chemical imaging reveals avenues toward improved spatial resolution and sensitivity

Atomic force microscopy-infrared spectroscopy (AFM-IR) is a photothermal scanning probe technique that combines nanoscale spatial resolution with the chemical analysis capability of mid-infrared spectroscopy. Using this hybrid technique, chemical identification down to the single molecule level has been demonstrated. However, the mechanism at the heart of AFM-IR, the transduction of local photothermal heating to cantilever deflection, is still not fully understood. Existing physical models only describe this process in few special cases but not in many of the types of sample geometries encountered in the practical use of AFM-IR. In this work, an analytical expression for modeling the temperature and photothermal expansion process is introduced, verified with finite element simulations, and validated with AFM-IR experiments. This method describes AFM-IR signal amplitudes in vertically and laterally heterogeneous samples and allows studying the effect of position and size of an absorber, pump laser repetition rate and pulse width on AFM-IR signal amplitudes and spatial resolution. The analytical model can be used to identify optimal AFM-IR experimental settings in conventional and advanced AFM-IR modes (e.g., tapping mode, surface-sensitive mode). The model also paves the way for signal inversion based superresolution AFM-IR.

Speckled Laser Pump–Thermoreflectance Microscopy Probe to Measure and Study Micro/Nanoscale Thermal Transport: Numerical Simulation

Abstract Nondiffusive thermal transport in solids and their micro/nanostructures is a key subject in the research of micro/nanoscale heat conduction. A number of laser and optical techniques to measure or capture the nondiffusive behaviors of heat carriers have been developed, such as transient thermoreflectance, time-domain thermoreflectance (TDTR), transient thermal grating (TTG), and so on. Here, we propose a novel method to study micro/nanoscale heat transport, namely, speckled laser pump–thermoreflectance microscopy probe. In this technique, micrometer to few hundred nanometer size random heat spots are generated by a speckled laser pump pulse, and the time–space evolution of heat spots are recorded by thermoreflectance microscopy images of the probe pulses arriving at different delay times. Fourier transform is applied to analyze the thermoreflectance images and extract the thermal decaying time for different spatial frequencies and along different in-plane directions. Thermal conductivity at different spatial frequencies, which includes the nondiffusive transport information, is obtained in this way. By numerically performing simulation of anisotropic Brownian motion and solving phonon Boltzmann transport equations under the initial condition of random heat spots, we retrieve the preset anisotropic thermal conductivity and the nondiffusive behavior of reduced thermal conductivity with increasing spatial frequencies, proving the validity of this technique. The innovative method can also be applied to study electron and spin transports, and holds the potential to facilitate the experimental research and understanding of nanoscale energy transport.

Wavelength Conversion Process of Intra-Pulse Stimulated Raman Scattering in Near-Zero Negative Dispersion Range

In the near-zero negative dispersion region of highly nonlinear fiber, the process of wavelength conversion based on the mechanism of intra-pulse stimulated Raman scattering is sensitive to the parameters of pumping pulse and fiber length under the combined effects of nonlinearity and dispersion. Therefore, we experimentally demonstrate the process in detail by using conventional soliton pulses with three sets of pulse parameters and two highly nonlinear fiber lengths of 400 m and 500 m. The experimental results show that, under the combined action of dispersion and several types of nonlinear mechanisms, the wavelength conversion processes are apparently different when using pulses with different parameters to pump different lengths of highly nonlinear fibers. Specifically, the separation degree of the frequency-shifted pulse spectrum and pumping pulse spectrum, and the corresponding redshift rate and pump power consumption all show significantly different results. The experimental results can guide the selection of more suitable parameters for the pumping pulse and the length of highly nonlinear fiber to achieve a better effect of wavelength redshift or spectrum broadening for various practical applications.

1 joule arbitrary pulse shape hybrid laser source

A hybrid laser source with a profiled laser pulse shape is presented. It is designed with a fiber pulse shaping system and a two-stage solid-state amplifier based on two gain modules with pulsed transverse diode pumping. The ability to control the pulse shape of the hybrid laser source pulse is demonstrated, and rectangular and stepped pulse shapes with a radiation wavelength of 1064.15 nm, duration of 20 ns, repetition rate of 2 Hz, and output energy of 1 J are obtained. The resulting shapes are achieved by controlling the temporal shape of the pulses of the master oscillator to compensate for the effect of gain saturation in solid-state amplifiers. The dynamics of the pulse shape evolution as a function of the electrical pump energy in the gain modules is traced.

Fast plasma production by intense femtosecond extreme ultraviolet and x-ray pulses

We develop a semiempirical model to describe the creation of an electron plasma in matter at extreme conditions, as excited by high-intensity, extreme ultraviolet (EUV) or x-ray radiation pulses currently available at free-electron laser (FEL) facilities. In typical FEL experiments, EUV and x-ray pulses are used to investigate highly excited states of matter produced by optical pump pulses. The pump pulses, promoting electrons from delocalized valence band states, excite the system often through multiphoton absorption processes and/or shock wave propagation. Instead, the use of FEL radiation as a pump allows triggering electron transitions from bound, core states to nearly free levels above the Fermi energy, promptly generating a highly excited state, toward the warm dense matter regime. We model such an excitation process starting from the optical properties of the analyzed materials and first-principle considerations, with the aim of motivating further studies in this scheme. We apply our model to predict the differences in the plasma creation process in insulators and metals, at varying exciting pulse wavelength and fluence, and we interpret the results in terms of the different nature of the investigated systems. Furthermore, we explore the effect of the finite core hole lifetime on the plasma formation and relaxation dynamics. We show the model is suited for the interpretation of real experimental data obtained from a prototypical insulating crystal (LiF) in a EUV FEL pump–optical transmission probe experiment. Published by the American Physical Society 2025

Over 100-kW peak power picosecond pulse generation at 2924 nm via efficient single-pass optical parametric generation with broadband tunability.

We report a high peak power mid-infrared (MIR) source via efficient optical parametric generation (OPG) in a piece of 50-mm-long MgO: PPLN crystal pumped by using a near-infrared (NIR) narrow-band picosecond laser source. The highest peak power of the idler pulse can reach ∼109.86 kW with a duration of ∼7.3 ps and wavelength of ∼2924 nm. Both the signal and idler pulses can be agilely tunable in repetition rate, from 500 kHz to 4 MHz, depending on the pump pulses. Through adjusting the operation temperature and switching the applied grating of the MgO: PPLN crystal, the emission wavelengths of the signal and idler can be broadband-tunable across the NIR range of 1430-1741nm and the MIR range of 2773-4156 nm, respectively. The root-mean-square errors of the delivered signal and idler average power are less than 1% for all operational parameters. Our result provides a simplified yet agile solution of a coherent MIR source featuring a high pulse peak power, customizable emission wavelength, exceptional operation stability, etc.

Decoupling Carrier Dynamics and Energy Transport in Ultrafast Near-Field Nanoscopy.

Ultrafast near-field optical nanoscopy has emerged as a powerful platform to characterize low-dimensional materials. While analytical and numerical models have been established to account for photoexcited carrier dynamics, quantitative evaluation of the associated pulsed laser heating remains elusive. Here, we decouple the photocarrier density and temperature increase in near-field nanoscopy by integrating the two-temperature model (TTM) with finite-difference time-domain (FDTD) simulations. These results reveal that the electron-phonon coupling in a silicon film after femtosecond laser excitation is most pronounced within approximately 3 ps─substantially shorter than the photocarrier decay time scale at tens of picoseconds. Moreover, the coupled TTM-FDTD method indicates that ultrafast laser heating can cause up to a 14% variation in the near-field signal at a 220 μJ/cm2 pump pulse fluence. Our numerical results are further validated by transient experiments, highlighting the potential of this method for investigations of carrier and thermal phenomena in emerging nanomaterials and nanodevices.

Magnetic Bistable Dome Actuators for Soft Robotics with High Volume Capacity and Motion Stability.

Magneto-responsive soft actuators hold significant promise in soft robotics due to their rapid responsiveness and untethered operation. However, controlling their deformations presents challenges because of their inherent flexibility and high degrees of freedom. Here, we present a magnetically driven bistable dome-shaped soft actuator that simplifies deformation by limiting it to two distinct states. The actuator achieves controlled state transitions by switching the orientation of external magnetic fields. We investigate the design strategy and magnetization styles of the dome-shaped soft actuator. Additionally, we analyze their effects on state transitions. The bistable dome undergoes significant volume changes reliably and smoothly during deformation, and its natural curvature makes it suitable for tasks involving rolling motion. We demonstrate the actuator's effectiveness in various applications, including an array of bistable domes for controlled actuation, a magnetically driven pulse pump with integrated check valves, and a ball-shaped bistable robot capable of efficient rolling locomotion and fluid manipulation. Our design significantly enhances the versatility and efficiency of bistable soft robotic systems, highlighting their potential for tasks such as liquid collection and release.

FULLY ANALYTICAL SOLUTION FOR THE FIRST SPECTRAL MOMENT OF A FLUOROPHORE IN SOLUTION

One of the common methods for assessing the solvation dynamics of a fluorescent system in a polar medium is the analysis of the first spectral moment behavior. So there is a need to build a model that takes into account important physical processes and at the same time is not demanding in terms of calculations. In this paper, an approach is proposed that considers the parameters of the exciting laser pulse and intramolecular vibrational relaxation for constructing the solvation dynamics. A method for building an analytical solution for the dynamics of the first spectral moment is presented. This model is free of numerical methods using, and it is a simple construction built by basics functions and an additional error function. The influence of all key parameters on the system relaxation is illustrated. It is shown that delocalization in time of the pump pulse suppresses the oscillatory behavior of the first moment and hides information about the damped intramolecular mode. The similar effect is also observed with the diffusion and inertial processes contribution insreasion to the reorganization energy at early stages of relaxation. The presented model can be generalized to take into account quantum transitions between vibrational sublevels of high-frequency intramolecular vibrations.

Temporal-spatial manipulation of bi-focal bi-chromatic fields for terahertz radiations

Mixing the fundamental (ω) and the second harmonic (2ω) waves in the gas phase is a widely employed technique for emitting terahertz (THz) pulses. The THz generation driven by bi-chromatic fields can be described by the photocurrent model, where the THz generation is attributed to free electrons ionized by the ω field, and the 2ω field provides a perturbation to break the symmetry of the asymptotic momentum of free electrons. However, we find that the THz radiation is amplified by one order of magnitude when driven by bi-focal bi-chromatic fields, which cannot be explained only using the photocurrent model. Meanwhile, present measurements demonstrate that the THz radiation mainly originates from the plasma created by the 2ω pulses instead of the ω pulses. Energy transfer from the 2ω beam to the THz beam during the THz generation has been observed, validating the major contribution of the 2ω beam. Furthermore, the THz bandwidth has been observed to extensively exceed the bandwidth of the pump pulse, not be explained by the photocurrent model as well. These counterintuitive results present a significant challenge for understanding strong-field nonlinear optics and simultaneously expanding various applications.

Photo-physical mechanism of near-IR femtosecond laser-induced refractive-index change in PMMA

Micromodification in bulk undoped polymethylmethacrylate (PMMA) by single focused (numerical aperture (NA) = 0.25), 1030-nm 250-fs laser pump pulses was explored by pump self-transmittance; optical, 3D-scanning confocal photoluminescence (PL); Raman micro-spectroscopy; and optical polarimetric and interferometric microscopy. Starting from the threshold pulse energy Eth = 0.4 ± 0.1 μJ (peak laser intensity Ith ≈ 8 TW/cm2), visible bright micro-voxels emerged inside PMMA at the 100 ÷ 300-μm depth, with their PL-acquired dimensions increasing versus pulse energy. Optical phase change was interferometrically measured in the voxels at the 532-nm wavelength, exhibiting versus the pulse energy the isotropic refractive index increase Δn = +(4 ÷ 10) × 10−4, and a new 1640-cm−1 peak of C=C vibrations emerged in the Raman spectra. Pump self-transmittance measurements demonstrated the predominating eight-photon absorption (excited energy level ≈ 9.7 eV, coefficient β8 ≈ 3 × 10−5 cm13/TW7) at the sub-threshold I < Ith, implying photoionization of the PMMA chains (the ionization potential of MMA molecule ≈ 9.7 eV). At higher peak intensities I > Ith, inverse brems-strahlung absorption (coefficient ∼103cm−1) of near-critical micro-plasma (density >5 × 1020 cm−3) predominates over the multi-photon PMMA absorption, providing the bulk energy density >6 × 102 J/cm3 and the temperature rise ΔT > 2.2 × 102 K, which are sufficient for PMMA (de)polymerization near the equilibrium bulk temperature TP ≈ 220°C. These results uncover the quantitative mechanism of fs-laser modification of PMMA, justifying the previous qualitative findings and enabling controllable energy deposition during fs-laser PMMA micromachining of diverse functional applications.

Multi-microjoule broadband terahertz generation in the organic crystal BNA pumped by ytterbium laser pulses compressed via a gas-filled hollow-core fiber.

We investigate the enhanced terahertz generation in the organic crystal BNA when pumped by compressed high-energy ytterbium laser pulses. By compressing the pump pulses from 170 fs down to 43 fs using an argon-filled hollow-core fiber and chirped mirrors, the terahertz conversion efficiency is increased by 2.4 times, leading to the generation of multi-microjoule terahertz pulses with a frequency spectrum almost twice as wide, extending up to 19 THz. These findings showcase a simple and efficient way to generate intense and broadband terahertz pulses by means of an amplified ytterbium laser system.

Tunable FSRS measurements with reduced background signals: Using an etalon filter to generate picosecond pump pulses in the 460-650nm range.

Generating wavelength-tunable picosecond laser pulses from an ultrafast laser source is essential for femtosecond stimulated Raman scattering (FSRS) measurements. Etalon filters produce narrowband (picosecond) pulses with an asymmetric temporal profile that is ideal for stimulated resonance Raman excitation. However, direct spectral filtering of femtosecond laser pulses is typically limited to the laser's fundamental and harmonic frequencies due to very low transmission of broad bandwidth pulses through an etalon. Here, we show that a single etalon filter (15cm-1 bandwidth; 172cm-1 free spectral range) provides an efficient and tunable option for generating Raman pump pulses over a wide range of wavelengths when used in combination with an optical parametric amplifier and a second harmonic generation (SHG) crystal that has an appropriate phase-matching bandwidth for partial spectral compression before the etalon. Tuning the SHG wavelength to match individual transmission lines of the etalon filter gives asymmetric picosecond pump pulses over a range of 460-650nm. Importantly, the SHG crystal length determines the temporal rise time of the filtered pulse, which is an important property for reducing background and increasing Raman signals compared with symmetric pulses having the same total energy. We examine the wavelength-dependent trade-off between spectral narrowing via SHG and the asymmetric pulse shape after transmission through the etalon. This approach provides a relatively simple and efficient method to generate tunable pump pulses with the optimum temporal profile for resonance-enhanced FSRS measurements across the visible region of the spectrum.

Frequency conversion in a hydrogen-filled hollow-core fiber using continuous-wave fields.

In large-area quantum networks based on optical fibers, photons are the fundamental carriers of information as so-called flying qubits. They may also serve as the interconnect between different components of a hybrid architecture, which might comprise atomic and solid-state platforms operating at visible or near-infrared wavelengths, as well as optical links in the telecom band. Quantum frequency conversion is the pathway to change the color of a single photon while preserving its quantum state. Currently, nonlinear crystals are utilized for this process. However, their performance is limited by their acceptance bandwidth, tunability, polarization sensitivity, and undesired background emission. A promising alternative is based on stimulated Raman scattering (SRS) in gases. Here, we demonstrate polarization-preserving frequency conversion in a hydrogen-filled antiresonant hollow-core fiber. This approach holds promises for seamless integration into optical fiber networks and interfaces to single emitters. Disparate from related experiments that employ a pulsed pump field, we here take advantage of two coherent continuous-wave pump fields.

Observation of complete energy transfer by orthogonally polarized Raman scattering in a high-birefringence fiber.

This study reports the observation of complete orthogonally polarized Raman scattering (OPRS) in a 1.0-km high-birefringence fiber (HBF). An incident pump pulse at 1560 nm with an energy of 2.48 nJ and a duration of less than 100 fs is aligned along the slow axis of the HBF. Initially, the incident pump pulse generates a Raman soliton and experiences soliton self-frequency shift (SSFS) along the slow axis. When the energy of the incident pump pulse reaches 0.74 nJ, the Raman soliton in the slow axis induces and amplifies a new, to the best of our knowledge, spectral component referred to as generated pulse in the fast axis through OPRS. Subsequently, the two orthogonally polarized pulses trap each other through cross-phase modulation, temporally overlap, and copropagate along the fiber until the energy of the Raman soliton in the slow axis is depleted. Finally, the generated pulse in the fast axis autonomously undergoes the SSFS process, extending beyond 1770 nm. Notably, the second OPRS process is observed as the energy of the incident pump pulse reaches 1.55 nJ.

Effect on the spectral purity of photon-pairs due to on-chip temporal manipulation of the pump

Photonic Integrated Circuits (PICs) are a promising contender for quantum information technologies. The spectral purity of photons is one of the key attributes of photon-pair sources based on nonlinear optics. We can obtain >99% purity in PIC ring-resonator photon-pair sources by manipulating the pump pulse in the time domain. Here, we have developed a PIC to shape a pulse into dual, triple, and quadruple pulses and investigated the effect of these pump pulse configurations on the purity. Our results show that more complex configurations, compared to a two-pulse configuration, do not result in comparatively higher purity but allow more accurate control over choosing arbitrary values of the purity.

Simulation study on supercontinuum broadening based on the BIC model.

Bound states in the continuum (BIC) refers to waves that are entirely confined within the continuous spectrum of radiation waves without interacting with them. In our study, we attempted to construct a waveguide satisfying BIC conditions by forming a polymer layer on a 4H-SiC substrate, positioned on an S i O 2 insulator. By fine-tuning the waveguide parameters, we minimized losses to the substrate continuum and determined that the lowest loss meeting BIC conditions occurs when the HSQ width is 1.82µm and the 4H-SiC thickness is 440nm. Subsequently, we investigated the supercontinuum generation (SCG) in this waveguide. First, we analyzed the primary linear and nonlinear effects in the SCG process, introducing well-established theoretical frameworks such as the generalized nonlinear Schrödinger equation (GNLSE) for pulse propagation in nonlinear media. We then studied the influence of waveguide parameters on SCG, observing the variations in SCG with different HSQ widths and 4H-SiC thicknesses. Our results indicate that optimal spectral broadening and conversion efficiency are achieved with an HSQ width of 1.82µm and a 4H-SiC thickness of 440nm. In our simulations, the waveguide length was set to 1cm, and the pump pulse was modeled as a Gaussian pulse with a width of 100fs and a peak power of 8W.

Electronic dynamics created at conical intersections and its dephasing in aqueous solution

A dynamical rearrangement in the electronic structure of a molecule can be driven by different phenomena, including nuclear motion, electronic coherence or electron correlation. Recording such electronic dynamics and identifying its fate in an aqueous solution has remained a challenge. Here, we reveal the electronic dynamics induced by electronic relaxation through conical intersections in both isolated and solvated pyrazine molecules using X-ray spectroscopy. We show that the ensuing created dynamics corresponds to a cyclic rearrangement of the electronic structure around the aromatic ring. Furthermore, we found that such electronic dynamics were entirely suppressed when pyrazine was dissolved in water. Our observations confirm that conical intersections can create electronic dynamics that are not directly excited by the pump pulse and that aqueous solvation can dephase them in less than 40 fs. These results have implications for the investigation of electronic dynamics created during light-induced molecular dynamics and shed light on their susceptibility to aqueous solvation.

Ultrafast Mapping of Electronic and Nuclear Structure in the Photo Dissociation of Nitrogen Dioxide.

We investigate the photoinduced dissociation reaction of NO2 → NO + O upon electronic excitation of the X̃2A1 (D0) to the Ã2B2 (D1) state by femtosecond X-ray absorption spectroscopy at the nitrogen K-edge. We obtain key insight into the chemical bond breaking event and its associated electronic structural dynamics. Calculations of the photoinduced reaction allow to assign the transient absorption features at time scales of 10-50 fs to wave packet motions in the excited D1 and ground D0 states, followed by the formation of the NO photoproduct with a 255 ± 23 fs time constant. Our analysis shows that there is no direct correlation between the 1s core levels and the electronic ground and excited states transition energies and the bond elongation of NO2, while en route to dissociation toward the NO + O photoproducts, in the transient nitrogen K-edge spectra. However, simulations predict that for a sufficiently short UV pump pulse, the early wave packet dynamics in the D1 electronic excited state occurring within the first 35 fs along the bending and symmetric stretching modes can be directly mapped in the transient X-ray absorption spectra.

Efficiently catching entangled microwave photons from a quantum transducer with shaped optical pumps

A quantum transducer, when working as a microwave and optical entanglement generator, provides a practical way to coherently connect optical communication channels and microwave quantum processors. Recent experiments on a quantum transducer verifying entanglement between the microwave and optical photons show the promise of approaching that goal. While flying optical photons can be efficiently controlled or detected, the microwave photon needs to be stored in a cavity or converted to the excitation of a superconducting qubit for further quantum operations. However, it remains challenging to efficiently capture or detect a single microwave photon with an arbitrary time profile. This work focuses on this challenge in the setting of an entanglement-based quantum transducer and proposes a solution by shaping the optical pump pulse. By Schmidt decomposing the output entangled state, we show that the microwave-optical photon pair takes a specific temporal profile that is controlled by the optical pump. The microwave photon from the transducer can be nearly perfectly absorbed by a receiving cavity with tunable coupling and is ready to be converted to the excitation of superconducting qubits, enabling further quantum operations. Published by the American Physical Society 2024