Pump Pulse Research Articles

Dynamic distributed Brillouin sensing based on pump frequency-agile quadrupling modulation

We propose a dynamic distributed Brillouin sensing based on a frequency-agile quadrupling method. A common low-bandwidth direct digital frequency synthesizer (DDS) is employed to generate a few hundred MHz of frequency-agile pump pulses for Brillouin gain spectrum (BGS) scanning, and polarization multiplexing technology is utilized to further decrease the bandwidth requirement for the frequency-agile microwave signal to one-quarter. This method has low requirements for electronic devices and significantly reduces costs in systems. Static and dynamic measurement capabilities of 2 km fiber were demonstrated experimentally, the vibration frequencies of 183 Hz were measured with a sampling rate of 400 Sa/s.

Theoretical Description of Pump-Probe Experiments in Charge-Density-Wave Materials out to Long Times

We describe coupled nonequilibrium electron-phonon systems semiclassically—Ehrenfest dynamics for the phonons and quantum mechanics for the electrons—using a classical Monte Carlo approach that determines the nonequilibrium response to a large pump field. The semiclassical approach is expected to be accurate, because the phonons are excited to average energies much higher than the phonon frequency, eliminating the need for a quantum description. The numerical efficiency of this method allows us to perform a self-consistent time evolution out to very long times (tens of picoseconds), enabling us to model pump-probe experiments of a charge-density-wave (CDW) material. Our system is a half-filled, one-dimensional (1D) Holstein chain that exhibits CDW ordering due to a Peierls transition. The chain is subjected to a time-dependent electromagnetic pump field that excites it out of equilibrium, and then a second probe pulse is applied after a time delay. By evolving the system to long times, we capture the complete process of lattice excitation and subsequent relaxation to a new equilibrium, due to an exchange of energy between the electrons and the lattice, leading to lattice relaxation at finite temperatures. We employ an indirect (impulsive) driving mechanism of the lattice by the pump pulse due to the direct driving of the electrons. We identify two driving regimes, where the pump can either cause small perturbations or completely invert the initial CDW order. Our work successfully describes the ringing of the amplitude mode in CDW systems that has long been seen in experiment but never successfully explained by microscopic theory. We also describe the fluence-dependent crossover that inverts the CDW order parameter and changes the phonon dynamics. Finally, we illustrate how this method can examine a number of different types of experiments including photoemission, x-ray diffraction, and two-dimensional (2D) spectroscopy. Published by the American Physical Society 2024

Stimulated Raman scattering of a broadband chirped Ti:sapphire laser pulse in calcite seeded by a narrowband nanosecond Nd:YAG laser pulse

Stimulated Raman Scattering (SRS), pumped by a broadband (i.e., compared to the bandwidth of the material excitation) chirped 50-ps pulse, with Stokes seeding by a 20-ns narrowband pulse, is experimentally and theoretically investigated. In the experiments, a femtosecond-class 0.95 μm Ti:sapphire laser system and a Q-switched 1.064 μm Nd:YAG laser were used for pumping and seeding SRS in a calcite (CaCO3) crystal. This material was chosen because its Raman resonance frequency (∼1089 cm−1) is near to the frequency difference between the pump and seed radiation. It is shown that, despite a narrowband seed, the generated Stokes pulse spectrum mimics the pump pulse spectral width. The observed SRS conversion efficiency saturates at 40%, with a weak dependence on the seed pulse energy and on the detuning of the pump-seed frequency from the Raman resonance. Theoretical modeling confirms the observed effects and permits prediction of the characteristics of the investigated system as its parameters are varied.

Few-femtosecond soft X-ray transient absorption spectroscopy with tuneable DUV-Vis pump pulses

Achieving few-femtosecond resolution for a pump-probe experiment is crucial to measuring the fastest electron dynamics and for creating superpositions of valence states in quantum systems. However, traditional UV-Vis pump pulses cannot achieve few-fs durations and usually operate at fixed wavelengths. Here, we present, to our knowledge, an unprecedented temporal resolution and pump tuneability for UV-Vis-pumped soft X-ray transient absorption spectroscopy. We have combined few-fs deep-UV to visible tuneable pump pulses from resonant dispersive wave emission in hollow capillary fiber with attosecond soft X-ray probe pulses from high harmonic generation. We achieve sub-5-fs time resolution, sub-fs interferometric stability, and continuous tuneability of the pump pulses from 230 to 700 nm. We demonstrate that the pump can initiate an ultrafast photochemical reaction and that the dynamics at different atomic sites can be resolved simultaneously. These capabilities will allow studies of the fastest electronic dynamics in a large range of photochemical, photobiological and photovoltaic reactions.

Multiwavelength highly transient stimulated Raman scattering on dual Raman modes in Sr(MoO4)0.8(WO4)0.2 and Sr(MoO4)0.4(WO4)0.6.

For the first time to our knowledge, multiwavelength, highly transient, single-pass stimulated Raman scattering with a low wavelength spacing on dual (stretching and bending) Raman modes in Sr(MoO4)0.8(WO4)0.2 and Sr(MoO4)0.4(WO4)0.6 solid solutions in a range of 1000-1300 nm (transparence window of biological tissue) under ultrafast chirped pulse laser pumping is comparatively investigated in the interests of multicolor two-photon imaging of a living tissue. For both the solid solutions, the optimum range (1-5 ps) of chirped pump pulse durations for multiwavelength Raman conversion on dual Raman modes was wider than for SrMoO4 (2-3 ps) due to the higher integral cross section of the bending Raman mode. Higher efficient SRS conversion took place at negative chirping of the pump pulse with its stretching from 0.25 ps up to 5 ps due to the compensation of a positive chirp caused by nonlinear phase modulation with total Raman conversion efficiency of up to 36% for Sr(MoO4)0.8(WO4)0.2 and 49% for Sr(MoO4)0.4(WO4)0.6. The highest number (five) of Stokes components in the desired range (1000-1300 nm) was observed in the optimum Sr(MoO4)0.4(WO4)0.6 solid solution, which has the Raman modes with comparable intensities.

Research on picosecond synchronization control between synchrotron radiation X-ray pulse and femtosecond laser pulse on SSRF

The laser-pumped X-ray detection method is a crucial tool for investigating molecular dynamics, allowing researchers to study ultrafast structural changes within materials. This method demands extremely high precision in the synchronization and delay timing between the pump pulse and the probe pulse. In this study, we explored picosecond (ps)-level clock synchronization control between synchrotron radiation X-ray pulses and femtosecond (fs) laser pulses at the BL05U (d-Line) of the Shanghai Synchrotron Radiation Facility (SSRF). Utilizing the X-ray pulses generated by the 40 ps/0.3 mA electron beam bunch from the SSRF and femtosecond laser pulses with high repetition rates and high emission power, we conducted synchronization experiments. A custom-developed picosecond time synchronization control system and a high dynamic range X-ray streak camera with picosecond time resolution were used as detectors, enabling ultra-precise time synchronization monitoring of X-ray and laser pulses. This setup achieves higher time resolution without the need for photodiodes and oscilloscopes. Using the X-ray pulses as a stationary reference frame, we adjusted the clock synchronization control system to delay the laser pulses and measured their relative timing changes with the X-ray streak camera. The experiments demonstrated a time resolution better than 10 ps, providing a foundation for achieving 60 ps time resolution in laser-pumped X-ray detection experiments at the SSRF.

Effect of the Förster interaction and the pulsed pumping on the quantum correlations of a two quantum dot-microcavity system in the strong coupling regime

The quantum correlations of a system of two quantum dots with Föster interaction (Γ) in a microcavity with strongly coupled dissipation and a single mode of the electromagnetic field and driven by a laser pulse were studied theoretically, using the formalism of the master equation in Lindbland form. The energy eigenvalues of the system were studied as a function of detuning for the first and second excitation varieties. Concurrence (C), formation entanglement (EoF), mutual information (I) and quantum discord (Q) are studied as a function of time considering different values of Föster coupling, varying the pump times of the simulated laser pulse and pulse intensity. We found a discrepancy between EoF and C as entanglement quantifiers, noting that concurrence reaches much higher values than EoF; so concurrence can indicate results that are well above the EoF. The presence of the Föster interaction favors that the quantum discord is the dominant correlation in the system, which indicates that the system maintains quantum correlations even when the entanglement of the system has disappeared, but that it is affected by the increase in the laser pump time.

All-Optical Control of Vortex Lasing in Silicon-Organic Lattices.

This paper reports a silicon-organic hybrid lattice that can lase with vortex emission and allow all-optical control. We combine an array of amorphous silicon nanodisks with gain from dye molecules in organic solvents to generate vortex lasing from bound states in the continuum under pulsed optical pumping. Irradiating the device with an additional continuous wave green laser beam can cause optical heating in silicon and lead to negative change in the refractive index of the organic solvents; meanwhile, the green laser beam can provide additional gain. Dynamic tuning of the lasing wavelength is achieved by varying the intensity of the controlling beam. Furthermore, the vortex beam lasing can be switched to single-lobed beam lasing by moving the controlling spot to break the in-plane symmetry within the pumping spot. Our findings could shed new light on active silicon topological devices.

Optimized terahertz generation in BNA organic crystals with chirped Ti:sapphire laser pulses.

We report on the efficient generation of intense terahertz radiation from the organic crystal N-benzyl-2-methyl-4-nitroaniline pumped by chirped Ti:sapphire femtosecond laser pulses. The THz energy and spectrum as a function of the pump fluence and duration of the chirped laser pulses are studied systematically. For the appropriate positively chirped pump pulses, a significant boost in the THz generation efficiency by a factor of around 2.5 is achieved, and the enhancement of high-frequency components (>1 THz) shortens the THz pulse duration. Via complete characterization of THz properties and transmitted laser spectra, this nonlinear behavior is attributed to the extended effective interaction length for phase matching as a result of the self-phase modulation of the intense pump laser pulses. Numerical calculations well reproduce the experimental observation. Our results demonstrate a robust, efficient, strong-field (up to several MV/cm) THz source using the common sub-10 mJ and sub-100 fs Ti:sapphire laser systems without optical parametric amplifiers.

Ultrafast Four‐Wave Mixing Phase‐Matched by Transient Nonlinear Phase Modulation in a MAPbBr3 Single Crystal

AbstractA degenerated four‐wave‐mixing (FWM) process in single‐crystal (CH3NH3)PbBr3 (MAPbBr3) is reported, where 150‐fs laser pulses at 1.33 µm are employed as the pump. Two pump photons interact with a single crystal, producing another two photons with higher and lower energies, respectively. One of the FWM‐generation sidebands is tuned from ∼1.23 to 1.21 µm and the other from ∼1.43 to 1.48 µm for the center wavelength, as the pump fluence is increased from 2.6 to 11.69 mJ cm−2. The self‐phase modulation induced by the strong pump pulses through the optical Kerr effect is responsible for the tuning dynamics. Transient spectroscopy not only verifies the FWM scheme for the interacting waves but also reveals the interference dynamics between the FWM‐generated sidebands and the probe pulse. In particular, the angular dependence of the FWM generation supplies direct evidence for the phase‐matching geometry. Using experimental data, a nonlinear refractive index coefficient of 1.19 × 10−14 cm2 W−1 at 1.33 µm for single‐crystal MAPbBr3 is determined.

Structure and spin of the low- and high-spin states of Fe2+(phen)3 studied by x-ray scattering and emission spectroscopy.

The structure and spin of photoexcited Fe2+(phen)3 in water are examined by x-ray scattering and x-ray emission spectroscopy with 100 ps time resolution. Excitation of the low-spin (LS) ground state (GS) to the charge transfer state 1MLCT* leads to the formation of a high-spin (HS) state that returns to the GS in 725 ps. Density functional theory (DFT) predicts a Fe-N bond elongation in HS by 0.19 Å in agreement with the scattering data. The angle between the ligands increases by 5.4° in HS, which allows the solvent to get 0.33 Å closer to Fe in spite of the expansion of the molecule. The rise in solvent temperature from the return of photoproducts to the GS is dominated by the formation dynamics of HS, 1MLCT* → HS, which is followed by a smaller rise from the HS → GS transition. The latter agrees with the 0.61 eV energy gap E(HS)-E(LS) calculated by DFT. However, the temperature rise from the 1MLCT → HS transition is greater than expected, by a factor of 2.1, which is explained by the re-excitation of nascent HS* by the 1.2 ps pump pulse. This hypothesis is supported by optical spectroscopy measurements showing that the 1.2 ps long pump pulse activates the HS* → 5MLCT* channel, which is followed by the ultrafast return to HS* via intersystem crossing. Finally, the spins of the photoproducts are monitored by the Kβ emission and the spectra confirm that the spins of LS and HS states are 0 and 2, respectively.

Enhancement of Q-Switched Erbium-Doped Fibre Laser Performance using Graphene Oxide-Calcium Oxide Thin Film Saturable Absorbers

This paper presents an experiment utilizing calcium oxide (CaO) from the cuttlefish bone combined with graphene oxide (GO) to create a graphene oxide-calcium oxide (GO-CaO) based saturable absorber (SA) for Q-switched erbium-doped fibre lasers. The experiment aims to investigate the impact of calcium oxide when added to graphene oxide to form the SA thin film. The laser performance of both types of SA was compared, considering their input pump power, repetition rate, pulse width, and output power. The results indicated that the graphene oxide – calcium oxide SA thin film provided the best output power and the lowest threshold input pump power at 10.76 µW and 53.12 mW, respectively. In the characterization, UV-Vis spectroscopy was utilized to determine the number of discrete wavelengths in UV or visible light that the materials absorbed or transmitted. The UV-Vis results indicated that the graphene oxide thin film exhibited an absorbance value of 0.049 AU and a transmittance value of 89.25 % at a wavelength of 1565 nm. In comparison, the GO-CaO SA thin film displayed an absorbance value of 0.037 AU and a transmittance value of 91.89 % at the same wavelength. Raman spectroscopy was also conducted to provide details about the chemical structure and crystallinity of the thin film samples. The results showed high-intensity peaks for the GO thin film at wavenumbers of 1353.74 cm-1 and 1609.85 cm-1, and for GO – CaO thin film at wavenumbers of 1358.56 cm-1 and 1612.18 cm-1. Additionally, SEM was utilized to study the morphology of the thin film samples. Based on the characterization and laser performance results, it was concluded that the addition of calcium oxide to graphene oxide enhances the properties of the SA thin film, leading to improved laser performance due to its higher thermal conductivity.

Scattering Elimination in 2D IR Immune from Detector Artifacts.

Highly scattering samples, such as polymer droplets or solid-state powders, are difficult to study via coherent two-dimensional infrared (2D IR) spectroscopy. Previously, researchers have employed (quasi-) phase cycling, local-oscillator chopping, and polarization control to reduce scattering, but the latter method poses a limit on polarization-dependent measurements. Here, we present a method for Scattering Elimination Immune from Detector Artifacts (SEIFDA) in pump-probe 2D IR experiments. Our method extends the negative probe delay method of removing scattering from pump-probe spectroscopy to 2D experiments. SEIFDA works well for all polarizations when combined with the optimized noise reduction scheme to remove additive and multiplicative noise. We demonstrate that our method can be employed with any polarization scheme and reliably lowers the scattering at parallel polarization to comparable levels to the conventional 8-frame phase cycling with probe chopping (8FPCPC) at perpendicular polarization. Our system can acquire artifact free spectra in parallel polarization when the signal intensity is as little as 5% of the intensity of the interference between the pump pulses scattered into the detector. It reduces the time required to characterize the scattering term by at least 50% over 8FPCPC. Through detailed analysis of detector nonlinearity, we show that the performance of 8FPCPC can be improved by incorporating nonlinear correction factors, but it is still worse than that of SEIFDA. Application of SEIFDA to study the encapsulation of Nile red in polymer droplets demonstrates that this method will be very useful for probing highly scattering systems.

A Chirped Characteristic-Tunable Terahertz Source for Terahertz Sensing.

In broadband terahertz waves generated by femtosecond lasers, spatial chirp will be simultaneously produced with the introduction of angular dispersion. The chirp characteristics of the terahertz wave will directly affect the frequency response, bandwidth response, and intensity response of the terahertz sensor. To enhance the capability of terahertz sensors, it is necessary to control and improve the chirped characteristics of broadband terahertz sources. We generate a chirped terahertz wave via optical rectification in a LiNbO3 prism using the technique of pulse front tilt. The effect of the pump-beam spot size on THz generation is systematically studied. The pump's spot size is manipulated using a telescope system. With a pump spot diameter of 1.8 mm, the scanning spectrum of the THz pulse is narrower and is divided into multiple distinct peaks. In contrast, using a pump spot diameter of 3.7 mm leads to increased efficiency in the generation of THz pulses. Also, we investigate the underlying properties governing the generation of chirped terahertz pulses using varying pump pulse spot diameters.

InGaAs/GaAs nano-ridge laser with an amorphous silicon grating monolithically grown on a 300 mm Si wafer.

The monolithic growth of direct-bandgap III-V materials directly on a Si substrate is a promising approach for the fabrication of complex silicon photonic integrated circuits including light sources and amplifiers. It remains challenging to realize practical, reliable, and efficient light emitters due to misfit defect formation during the epitaxial growth. Exploiting nano-ridge engineering (NRE), III-V nano-ridges with high crystal quality were achieved based on aspect ratio defect trapping inside narrow trenches. In an earlier work, we used an etched grating to create distributed feedback lasers from these nano-ridges. Here we deposited an amorphous silicon grating on the top of the nano-ridge. Under pulsed optical pumping, a ∼7.84 kW/cm2 lasing threshold was observed, ∼5 times smaller compared to devices with an etched grating inside the nano-ridge. Compared to the etched grating, the amorphous silicon grating introduces no extra carrier loss channels through surface state defects, which is believed to be the origin of the lower threshold. This low threshold again demonstrates the high quality of the epitaxial deposited material and may provide a route toward further optimizing the electrically driven devices.

Modeling ultrafast anharmonic vibrational coupling in gas-phase fluorobenzene molecules.

In this work, we study the energy flow through anharmonic coupling of vibrational modes after excitation of gas-phase fluorobenzene with a multi-THz pump. We show that to predict the efficiency of anharmonic energy transfer, simple models that only include the anharmonic coupling coefficients and motion of modes at their resonant frequency are not adequate. The full motion of each mode is needed, including the time while the mode is being driven by the pump pulse, because all the frequencies present in the multi-THz pump contribute to the excitation of the non-resonantly excited vibrational modes. Additionally, the model gives us the insight that modes with either A1 or B2 symmetry are more actively involved in anharmonic coupling because these modes have more symmetry-allowed energy transfer pathways.

Long-wavelength infrared upconversion time-stretch spectroscopy

High-speed spectroscopy in the molecular fingerprint spectral region (≈6–12 μm) is essential for the detection of ultrafast molecular dynamic processes, rapid combustion analysis, and biological diagnostics. However, ultrafast spectroscopy in the long-wavelength infrared (LWIR) region remains a challenge due to the limitations of laser sources and the lack of ultrafast and sensitive detectors in this wavelength region. Here, we demonstrate broadband LWIR time-stretch spectroscopy, which can realize a single-shot high-speed spectral measurement in a 8–10 μm region, by combining the LWIR femtosecond (fs) light generation and upconversion time-stretching detection with specific dispersive fiber. Broadband tunable fs light generated in the 8–10 μm region is upconverted to the 1.1–1.2 μm near-infrared wavelength via difference-frequency generation with the 1 μm chirped pump pulse. Time-stretch detection of the upconverted light can then be realized by adopting dispersion shifted fiber, which has a superior dispersion-to-loss ratio in the 1.1–1.2 μm wavelength region, as the dispersive medium. As a result, we experimentally demonstrate LWIR time-stretch spectroscopy in the 8–10 μm region with a spectral resolution of 1.07 cm−1, at a rate of 200 kSpectra s−1, which is only limited by the repetition rate of the 1 μm pump source. The demonstration of high-speed time-stretch spectroscopy in the LWIR region would open the possibility in exploring the transient dynamics of molecular fingerprint spectroscopy.

Phase shift of coherent magnetization dynamics after ultrafast demagnetization in strongly quenched nickel thin films

The remagnetization process after ultrafast demagnetization can be described by relaxation mechanisms between the spin, electron, and lattice reservoirs. Thereby, collective spin excitations in form of spin waves and their angular momentum transfer play an important role on the longer timescales. In this work, we address the question whether the magnitude of demagnetization-the so-called quenching-affects the coherency and the phase of the excited spin waves. We present a study of coherent magnetization dynamics in thin nickel films after ultrafast demagnetization using the all-optical, time-resolved magneto-optical Kerr-effect technique. The largest coherent precession amplitude was observed for strongly quenched systems, indicating a well-defined precession phase for all pump pulses at a demagnetization of up to 90% in this system. Moreover, the phase of the excited spin-waves in Ni increases with the pump fluence, indicating a delayed start of the precession during the remagnetization. We compare these findings to recent studies in Ni80Fe20(permalloy), to evaluate the influence of the magneto-elastic coupling and non-linear spin-wave dynamics on the magnetization dynamics.

Suppressing sidechain modes and improving structural resolution for 2D IR spectroscopy via vibrational lifetimes.

Vibrational spectroscopy of protein structure often utilizes 13C18O-labeling of backbone carbonyls to further increase structural resolution. However, sidechains such as arginine, aspartate, and glutamate absorb within the same spectral region, complicating the analysis of isotope-labeled peaks. In this study, we report that the waiting time between pump and probe pulses in two-dimensional infrared spectroscopy can be used to suppress sidechain modes in favor of backbone amide I' modes based on differences in vibrational lifetimes. Furthermore, differences in the lifetimes of 13C18O-amide I' modes can aid in the assignment of secondary structure for labeled residues. Using model disordered and β-sheet peptides, it was determined that while β-sheets exhibit a longer lifetime than disordered structures, amide I' modes in both secondary structures exhibit longer lifetimes than sidechain modes. Overall, this work demonstrates that collecting 2D IR data at delayed waiting times, based on differences in vibrational lifetime between modes, can be used to effectively suppress interfering sidechain modes and further identify secondary structures.

Optimization of Pump Pulse Duration and Cr:ZnSe Crystal Length for Efficient Generation of Tunable Mid-IR Laser Radiation Pumped by Laser Diode at Room Temperature

Optimization of Pump Pulse Duration and Cr:ZnSe Crystal Length for Efficient Generation of Tunable Mid-IR Laser Radiation Pumped by Laser Diode at Room Temperature