Pump Pulse Research Articles

Gain-Switched Er-Doped Fluoride Fiber Laser at ~3.75 μm

We demonstrate a pulsed Er-doped ZBLAN fiber laser operating at 3.75 μm based on the gain-switching scheme. A diffraction grating is introduced as a wavelength selection component to enable stable lasing in this long-wavelength region that deviates from the emission peak of 4F9/2→4I9/2 transition in Er3+. Different from the conventional gain-switching behavior where the pulse repetition frequency of the output laser is same as the that of the pump, the gain-switched laser demonstrated here shows a variable pulse repetition frequency, which accounts for 1/n (n = 4, 3, 2) of the pump pulse repetition frequency, in response to the 1950 nm pump power. The output pulse characteristics, including average output power, repetition frequency, pulse duration, and peak power, are investigated in detail. Over 200 mW average output power at 3.75 μm was obtained at 12 W of 1950 nm pump power. This work demonstrates that the Er-doped ZBLAN fiber laser, in combination with gain-switched scheme, is a feasible and promising approach to generate powerful pulsed emission > 3.7 μm.

Frequency comb generation via synchronous pumped χ(3) resonator on thin-film lithium niobate

Resonator-based optical frequency comb generation is an enabling technology for a myriad of applications ranging from communications to precision spectroscopy. These frequency combs can be generated in nonlinear resonators driven using either continuous-wave (CW) light, which requires alignment of the pump frequency with the cavity resonance, or pulsed light, which also mandates that the pulse repetition rate and cavity free spectral range (FSR) are carefully matched. Advancements in nanophotonics have ignited interest in chip-scale optical frequency combs. However, realizing pulse-driven on-chip Kerr combs remains challenging, as microresonator cavities have limited tuning range in their FSR and resonance frequency. Here, we take steps to overcome this limitation and demonstrate broadband frequency comb generation using a χ(3) resonator synchronously pumped by a tunable femtosecond pulse generator with on-chip amplitude and phase modulators. Notably, employing pulsed pumping overcomes limitations in Kerr comb generation typically seen in crystalline resonators from stimulated Raman scattering.

Generating quantum correlated photon pairs in a hybrid silicon–BTO platform

Here, we show photon pair generation from ring resonator and waveguide structures in a hybrid silicon–BTO on an insulator platform with a pulsed pump. Our analysis of single photon and coincidence generation rates show that spontaneous four-wave mixing is comparable to that expected from SOI devices with similar characteristics and find γeff of (14.7 ± 1.3) and (2.0 ± 0.3) MHz/mW2 for ring resonator and waveguide structures, respectively.

Frequency Conversion of Femtosecond Ti:Sapphire Laser Pulse to the Long-Wave Mid-IR Range with BaGa2GeSe6 Crystal

Generation of ultrashort mid-IR pulses spanning from 8.5 to 10.5 μm with high energy (up to 4.5 μJ) was experimentally demonstrated through difference frequency generation in BaGa2GeSe6 crystal pumped by 100-fs 0.95-μm Ti:sapphire laser pulses. Optical damage threshold and two-photon absorption coefficient were determined for this pump pulses. Frequency conversion efficiency reached 0.24% at 1.85 mJ pump pulse energy and was decreased at higher one. Estimations indicate that application of 15 mm in diameter wide-aperture BaGa2GeSe6 sample will allow one to increase pump pulse energy up to ~10 mJ, and to increase the mid-IR pulse energy up to 24 μJ at the same efficiency.

On-chip quasi-light storage for long optical delays using Brillouin scattering

Efficient and extended light storage mechanisms are pivotal in photonics, particularly in optical communications, microwave photonics, and quantum networks, as they offer a direct route to circumvent electrical conversion losses and surmount bandwidth constraints. Stimulated Brillouin Scattering (SBS) is an established method to store optical information by transferring it to the acoustic domain, but current on-chip SBS efforts have limited bandwidth or storage time due to the phonon lifetime of several nanoseconds. An alternate approach known as quasi-light storage (QLS), which involves the creation of delayed replicas of optical data pulses via SBS in conjunction with a frequency comb, has been proposed to lift the storage time constraint; however, its realization has been confined to lengthy optical fibers, constraining integration with on-chip optical elements and form factors. Here, we present an experimental demonstration of QLS on a photonic chip leveraging the large SBS gain of chalcogenide glass, achieving delays of up to 500 ns for 1 ns long signal pulses, surpassing typical Brillouin storage processes' acoustic lifetime by more than an order of magnitude and waveguide transit time by two orders of magnitude. We experimentally and numerically investigate the dynamics of on-chip QLS and reveal that the interplay between the acoustic wave that stores the optical signal and subsequent optical pump pulses leads to a reshaping of the acoustic field. Our demonstrations illustrate the potential for achieving ultra-long storage times of individual pulses by several hundred pulse widths, marking a significant stride toward advancing the field of all-optical storage and delay mechanisms.

Theoretical model of femtosecond coherence spectroscopy of vibronic excitons in molecular aggregates.

When used as pump pulses in transient absorption spectroscopy measurements, femtosecond laser pulses can produce oscillatory signals known as quantum beats. The quantum beats arise from coherent superpositions of the states of the sample and are best studied in the Fourier domain using Femtosecond Coherence Spectroscopy (FCS), which consists of one-dimensional amplitude and phase plots of a specified oscillation frequency as a function of the detection frequency. Prior works have shown ubiquitous amplitude nodes and π phase shifts in FCS from excited-state vibrational wavepackets in monomer samples. However, the FCS arising from vibronic-exciton states in molecular aggregates have not been studied theoretically. Here, we use a model of vibronic-exciton states in molecular dimers based on displaced harmonic oscillators to simulate FCS for dimers in two important cases. Simulations reveal distinct spectral signatures of excited-state vibronic-exciton coherences in molecular dimers that may be used to distinguish them from monomer vibrational coherences. A salient result is that, for certain relative orientations of the transition dipoles, the key resonance condition between the electronic coupling and the frequency of the vibrational mode may yield strong enhancement of the quantum-beat amplitude and, perhaps, also cause a significant decrease of the oscillation frequency to a value far lower than the vibrational frequency. Future studies using these results will lead to new insights into the excited-state coherences generated in photosynthetic pigment-protein complexes.

Dynamically enhanced Autler–Townes splitting by orthogonal XUV fields

Abstract Based on numerical solutions of the time-dependent Schrödinger equation, we theoretically investigate the photoelectron spectrum of hydrogen atoms ionized by a pair of ultrashort, intense, and orthogonally polarized laser pulses with a relative time delay in a pump–probe configuration. The pump pulse resonantly excites electrons from the 1s and 2p levels, inducing Rabi oscillations. The resulting dynamically enhanced Autler–Townes (AT) splitting is observed in the photoelectron energy spectrum upon interaction with the second probe pulse. In contrast to the previous parallel-polarization scheme, the proposed orthogonal-polarization configuration enables the resolution of dynamically enhanced AT splitting over a considerably wider range of probe photon energies.

Manipulation of high-frequency spin waves in ferromagnetic thin films via magnetic torques induced by ultrafast demagnetization

Spin-based technologies demand the effective excitation and control of high-frequency spin waves in order to increase the operational speed of magnonic logic circuits. An available option for the high-frequency spin wave is the perpendicular standing spin waves (PSSWs) in ferromagnetic thin films. However, the practical utilization of a PSSW is still challenging due to a few unsolved critical issues such as its relatively low amplitude and simultaneous excitation of other PSSW modes. Here, efficient excitation and control of a first-order PSSW mode are demonstrated in Ni80Fe20 thin films using a double-laser-pulse scheme. Based upon our experimental and theoretical investigations, it is found that the precession amplitudes of the Kittel and PSSW modes show strong dependence on equivalent torques (i.e., the averaged torques during fast relaxation process after the second pulse excitation) applied to these two modes. Thus, selective excitation and amplification of the first-order PSSW mode can be realized by manipulating the equivalent torque, i.e., introducing a second laser pulse with an appropriate arrival time and laser fluence. Moreover, the spectral tuning of this single PSSW mode can be achieved by proportionately adjusting the laser fluence of two pump pulses. These findings may pave the way for the realization and construction of future high-performance spintronic devices.

Pulsed optical pumping in electron spin vapor

Optically pumped atoms have essential applications in magnetic sensing, physics research, and inertial navigation. The polarization gradient caused by light absorption dramatically influences the performance of quantum sensors based on optical pumping. This paper presents a novel approach to address the trade-off between polarization gradient and sensitivity by introducing the pulsed optical pumping method. The interplay of pulsed optical pumping, relaxation, and diffusion processes leads to a more homogeneous distribution of electron spin. Experimental results indicate that the pulsed optical pumping technique increases the optically pumped magnetometer (OPM) signal strength by 12.8% and increases the nuclear spin coherence time by 12.29% in optically pumped co-magnetometer (OPCM). This technology has broad applications in sensors based on the optical pumping effect, such as OPM, OPCM, and nuclear magnetic resonance gyroscope, and holds immense promise for future advancements in the optical pumping field.

THz generation and spatiotemporal variation of two cross‐focused Gaussian laser pulses in plasma

AbstractThe impact of spatial and temporal evolution of nonlinear mixing of two Gaussian laser pulses propagating in plasma on generation of terahertz (THz) radiation have been investigated taking into account the ponderomotive nonlinearity. By calculating the modified electron density of plasma caused by the finite ponderomotive force of the pump lasers and using wave equation and paraxial ray approximation, two coupled governing equations for temporal and spatial pulse‐width parameters have been derived. The electric field of the THz wave as a result of nonlinear current density induced by the beat ponderomotive force of the pulses was extracted. Combined effects of initial laser and plasma parameters on the behavior of self‐compression and self‐focusing as well as THz radiation generation were investigated. The numerical results indicated a considerable spatiotemporal compression takes place within a specific range of laser intensity, exhibiting a saturation intensity point where the compression process reaches its maximum extent. It is observed that the generated THz radiation also strongly depends on the spatiotemporal dynamics of the pump pulses. The maximum THz amplitude corresponds to the strongest pump pulse compression extent.

Laser-pump-resistive-probe technique to study nanosecond-scale relaxation processes

Standard optical pump-probe methods analyze a system’s temporal response to a laser pulse within sub-femtoseconds to several nanoseconds, constrained by the optical delay line’s length. While resistance is a sensitive detector in various fields, its measurements are typically slow (>µs) due to stabilization requirements. We suggest here a time-resolved pump-probe technique that combines an optical pump pulse and a rectangular electrical probe pulse through the sample, measuring transmission in a 50 ohm matched circuit with a digital oscilloscope. This allows electrically driven delays from nanoseconds to seconds. Demonstrations include studying heat-induced changes in a thin amorphous VO x film and carrier relaxation in a CdS photoresistor, showcasing potential applications in heat transfer, biochemical reactions, and gradual electronic transformations.

Keldysh crossover in one-dimensional Mott insulators

Recent advancements in pulse laser technology have facilitated the exploration of nonequilibrium spectroscopy of electronic states in the presence of strong electric fields across a broad range of photon energies. The Keldysh crossover serves as an indicator that distinguishes between excitations resulting from photon absorption triggered by near-infrared multicycle pulses and those arising from quantum tunneling induced by terahertz pulses. Using a time-dependent density-matrix renormalization group, we investigate the emergence of the Keldysh crossover in a one-dimensional (1D) Mott insulator. We find that the Drude weight is proportional to photo-doped doublon density when a pump pulse induces photon absorption. In contrast, the Drude weight is suppressed when a terahertz pulse introduces doublons and holons via quantum tunneling. The suppressed Drude weight accompanies glassy dynamics with suppressed diffusion, which is a consequence of strong correlations and exhibits finite polarization decaying slowly after pulse irradiation. In the quantum tunneling region, entanglement entropy slowly grows logarithmically. These contrasting behaviors between the photon-absorption and quantum tunneling regions are a manifestation of the Keldysh crossover in 1D Mott insulators and provide a novel methodology for designing the localization and symmetry of electronic states called subcycle-pulse engineering.

Circular lattice benzene-core PCFs with flat near-zero dispersion for low-power broad-spectrum supercontinuum generation

A circular photonic crystal fiber infiltrated with benzene with different air-hole diameters is proposed as a new supercontinuum light source. Optical properties related to dispersion, effective mode area, nonlinear coefficient, and attenuation of the fundamental mode are investigated numerically. Two optimized structures are selected and verified against supercontinuum generation (SCG) in detail. The first structure (#F1) possesses all-normal dispersion, while the second (#F2) has a zero-dispersion wavelength. The possibility of coherent, octave-spanning SCG is proved by a 40 fs pulse, 1.064 μm wavelength, and 0.45 kW of power in-coupled into the core of #F1. Otherwise, injecting a 90 fs duration, 1.5 μm wavelength, and 0.555 kW peak power pump pulse into #F2 generates a broad SC spanning 0.76–4.23 μm. With the advantages of flat near-zero dispersion, high nonlinearity, low attenuation, and low input power used for SCG, the proposed fibers may lead to new low-cost all-fiber optical systems.

Raman time-delay in attosecond transient absorption of strong-field created krypton vacancy

Strong field ionization injects a transient vacancy in the atom which is entangled to the outgoing photoelectron. When the electron is finally detached, the ion is populated at different excited states with part of coherence information lost. The preserved coherence of matter after interacting with intense short pulses has important consequences on the subsequent nonequilibrium evolution and energy relaxation. Here we employ attosecond transient absorption spectroscopy to measure the time-delay of resonant transitions of krypton vacancy during their creation. We have observed that the absorptions by the two spin-orbit split states are modulated at different paces when varying the time-delay between the near-infrared pumping pulse and the attosecond probing pulse. It is shown that the coupling of the ions with the remaining field leads to a suppression of ionic coherence. Comparison between theory and experiments uncovers that coherent Raman coupling induces time-delay between the resonant absorptions, which provides insight into laser-ion interactions enriching attosecond chronoscopy.

Kinetics of pair-induced quenching in holmium-doped optical fibers

We present a study of the dynamics of the rapid energy transfer upconversion that occurs within pairs or clusters of holmium ions, commonly referred to as pair-induced quenching. The lifetime of excited holmium ion pairs is determined to be around 0.4 µs from time-resolved fluorescence measurements. We develop a rate equation model that incorporates this lifetime and evaluate numerically and experimentally the consequences this finite intra-cluster upconversion rate has on the performance of a nanosecond-pulsed Ho-doped fiber amplifier. We demonstrate a significant improvement in efficiency by utilizing pulsed pumping on a timescale shorter than the measured excited ion pair lifetime.

Efficient tunable cascaded Raman source with all-silica fibers based on 2-µm DSR pulse pumping.

We present an efficient tunable all-silica-fiber 2nd-order cascaded Raman pulse laser utilizing 2-µm dissipative-soliton-resonance (DSR) rectangular pulses for pumping and highly GeO2-doped silica fiber as Raman gain medium. When pumped at 1966.5 nm, the maximum 1st-order Raman optical conversion efficiency is up to 64.4% at 2153 nm, with 92.4% spectral purity and 0.39-W average power. The maximum 2nd-order Raman optical conversion efficiency is 19.3% at 2370 nm, with 39.2% spectral purity and 0.25-W average power. To our knowledge, these conversion efficiencies and spectral purities represent the highest levels achieved in a mid-infrared all-silica-fiber cascaded pulsed Raman laser. Additionally, by adjusting the central wavelength of the DSR seed pulse, the 2nd-order Raman light can be tuned within a range of 41 nm (2354∼2395 nm). Our system provides a simple and easy-to-implement solution for realizing efficient tunable cascaded pulsed Raman lasers in the 2.4-µm band.

Transient pump-probe analysis of pump-induced lensing in laser amplifiers.

Understanding of pump-induced lensing in laser amplifiers is essential for the optimized design of high-power lasers with high spatial quality, but there is usually incomplete knowledge of the interplay between thermal and population induced lensing mechanisms, lensing under lasing and non-lasing conditions, and transient lensing effects under pulsed operation. This paper provides quantitative insight of pump-induced lensing effects by using experimental transient pump-probe measurements in an alexandrite laser amplifier end-pumped by a short pulse pump beam with Gaussian spatial intensity distribution. Lensing results are presented showing a large difference in lensing under both non-lasing and lasing conditions and distinction of the population lens and thermal lens contributions from their different response time. Different pump beam sizes are used to show the variation of the relative strength of the lensing mechanisms. Comparison of experimental results with the analytical transient theory developed in this paper for the Gaussian pump beam gives excellent agreement and quantitative information on the thermal and population contributions to the amplifier lens. This paper provides a methodology for quantitative investigation of pump-induced lensing in general laser amplifier systems, and potentially other classes of optical materials with residual optical absorption.

2 W, 1.5 µm single-mode fiber methane Raman laser pumped by a Yb-doped fiber amplifier.

We report here, to the best of our knowledge, the first 1.5 µm methane-filled fiber Raman laser pumped by a fiber laser. Based on the narrow-linewidth pulsed Yb-doped fiber laser pump source and a 15 m hollow-core fiber filled with 2.5 bar methane, the maximum power of 2.06 W Stokes wave at 1543 nm is obtained. The output laser has a narrow linewidth of 2.3 GHz, and the pulse repetition frequency can be adjusted flexibly. The output shows excellent near-diffraction-limited beam quality with a M2 factor of ∼1.09. This work proves the advantage of the fiber laser pump source with modest peak power and flexible temporal characteristics in 1.5 µm fiber gas Raman laser emission, providing good guidance for generating pulsed fiber source with narrow linewidth and high beam quality.

Toward high-energy few-cycle optical vortices with minimized topological charge dispersion.

A simple approach to generate high-energy few-cycle optical vortices with minimized topological charge dispersion is introduced. By means of numerical simulations, it is shown that, by leveraging the intrinsic properties of optical parametric chirped pulse amplification (OPCPA), clean transfer of topological charge from a high-energy narrowband pump pulse to a broadband idler is feasible under certain particular conditions, enabling the generation of high-energy few-cycle vortex pulses with extremely low topological charge dispersion.

Experimental investigation on cross-beam energy transfer between P-polarized lasers at 45-deg incidence

In this paper, we report the first experimental results on cross-beam energy transfer (CBET) at the Shengguang-II laser facility, in which two p-polarized laser beams irradiated planar targets at 45-deg incidence orthogonally. Frequency shifts of 0 and 0.06% were added between the two beams to investigate the relation of CBET to the degree of frequency shift. Enhancement of scattering was observed in the two-beam configuration with respect to the sum of the two single-beam configurations, indicating CBET was developed by the interaction of the two beams in the cases both with and without the frequency shift. When time delays were added between the seed and the pump pulses, scatter from the seed was enhanced, which further confirmed the existence of CBET in the experiments. The amplifications of the seed light by CBET with and without the frequency shift were sequentially ∼3.3 and ∼2.0 when the two beams were synchronized, and the energy loss due to CBET within the full aperture of the final optics of the pump to the total laser energy was ∼4% with the frequency shift. Calculations suggest that a more than 0.2% frequency shift may be needed to mitigate CBET under experimental laser-plasma conditions.