Propagation Of Laser Pulses Research Articles

Repetition‐Rate and Wavelength Flexible Femtosecond Laser Pulse Generation (Laser Photonics Rev. 18(11)/2024)

Repetition‐Rate and Wavelength Flexible Femtosecond Laser Pulse Generation (Laser Photonics Rev. 18(11)/2024)

Femtosecond Mode-Locking and Soliton Molecule Generation Based on GaAs Saturable Absorber

Abstract In the past few years, research on advanced ultrafast photonic devices has maintained great interest throughout the laser community. As a semiconductor material with excellent nonlinear saturation absorption characteristics, GaAs has been used in solid-state and fiber lasers as a mode-locker. However, pulse widths that have been reported in the searchable published literature are all longer and the shortest is also tens of picoseconds. Femtosecond pulse widths, desired for variety of applications, have not yet been reported in GaAs-based pulsed lasers. In this work, we further explore the nonlinear characteristics of GaAs that magnetron sputtered onto the surface of the tapered fiber and its application in the generation of femtosecond laser via effective dispersion optimization and nonlinearity management. With the enhanced interaction between evanescent wave and GaAs nanosheets, mode-locked soliton pulses as short as 830 fs are generated at 4.64 MHz repetition rates. As far as we know, this is the first time that femtosecond-level pulses are generated with a GaAs-based SA. In addition, soliton molecule, including dual-pulse state are also realized under stronger pumping. This work demonstrates that GaAs-based photonic device has good application prospects in effective polymorphous ultrashort pulsed laser generation.

Nonlinear optical research on 2D NbSe2 nanosheets and their ultrafast photonics applications

Two-dimensional (2D) NbSe2 is a new material with a variety of excellent properties. In this article, 2D NbSe2 nanosheets are prepared using liquid phase exfoliation (LPE) and spin coating methods. At the same time, the properties of NbSe2 were calculated by using density functional theory (DFT), exploring the changes in the electronic band structure of NbSe2 with the number of layers, and studying the optical properties of NbSe2. The nonlinear optical properties caused by the Pauli blocking effect and the absorption spectra are studied through typical nonlinear testing techniques and an ultraviolet–visible-near-infrared (UV–VIS-IR) spectrophotometer. In addition, 2 µm solid-state pulse lasers have important applications in a variety of fields. For the first time, 2D NbSe2 nanosheets are prepared as saturable absorbers (SA) and applied them to solid-state lasers as nonlinear optical modulation devices, successfully achieving the generation of ultra-short pulse lasers with a pulse duration of 445.4 ps in 2 µm band. Our research results prove that 2D NbSe2 nanosheets is a promising nanomaterial, can be prepared into nonlinear optical modulation devices with excellent performance, and show great application potential as ultrafast photonic devices. It is beneficial to the miniaturization of solid-state pulse lasers in subsequent applications.

Guiding properties of Bessel–Gaussian and super-Gaussian pulses in inhomogeneous parabolic plasma channels

The guidance and stable propagation of laser pulses over many Rayleigh lengths are crucial for the plasma electron acceleration in the laser wakefield accelerators. Using plasma channels with specific characteristics can lead to the proper guidance of the laser pulse. Here, quasi-three-dimensional particle-in-cell (PIC) simulations are performed to investigate the guidance of Bessel–Gaussian pulse (BGP) of zeroth order and super-Gaussian pulse (SGP) of 3rd and 4th orders in an axially and radially inhomogeneous plasma channel. The effects of the channel radius and depth, the laser wavelength and initial spot size, and the plasma channel inhomogeneity on the guidance of the laser pulse are also examined. The results indicate that the guidance of a laser pulse in the plasma channel depends on the pulse profile, and under certain conditions, the pulses can be guided with the least variation of spot size in the inhomogeneous plasma channel. It is shown that the channel depth and the initial laser spot size are very effective in pulse guiding, as the values of these parameters increase, the pulse guidance is done better. In addition, the results show that the guidance of laser pulse is dependent on the type of plasma inhomogeneity represented by three different kinds of initial conditions, as considering the nonlinear-axial inhomogeneity in the parabolic plasma channel can lead to more convergence than the axially homogeneous and linear-axially plasma density profiles.

Phase Synchronism in the Generation of Harmonics in Gas Taking into Account Changes in the Wave Vector Detuning during the Propagation of an Intense Femtosecond Laser Pulse

Phase Synchronism in the Generation of Harmonics in Gas Taking into Account Changes in the Wave Vector Detuning during the Propagation of an Intense Femtosecond Laser Pulse

Efficient general method for numerically modeling laser pulse propagation, overlap, and lifetime effects in amplifiers

An efficient numerical time-dependent general method is developed to address incoherent pulse overlap and lifetime effects in laser amplifiers. The alternating propagation-population laser energetics method (APPLE) has been validated against a semi-discrete coupled rate equation numerical method (SDRE) and analytic formalisms in bounding cases. APPLE is based on decoupled rates applied to a time-dependent framework where both space-time-dependent populations and pulse energetics are consistently updated in each time step. A significant advantage of APPLE lies in its conceptual simplicity, ease of implementation, and relatively small computational cost. SDRE tracks the populations through coupled rates and uses the method of lines to discretize the hyperbolic partial differential transport equations allowing for use of ordinary differential equation solvers. With reasonably sized mesh, we report both energetic and power pulse shape relative differences on the order of one percent between the models over a large range of initial conditions.

Sub-attosecond-precision optical-waveform stability measurements using electro-optic sampling

The generation of laser pulses with controlled optical waveforms, and their measurement, lie at the heart of both time-domain and frequency-domain precision metrology. Here, we obtain mid-infrared waves via intra-pulse difference-frequency generation (IPDFG) driven by 16-femtosecond near-infrared pulses, and characterise the jitter of sub-cycle fractions of these waves relative to the gate pulses using electro-optic sampling (EOS). We demonstrate sub-attosecond temporal jitter at individual zero-crossings and sub-0.1%-level relative amplitude fluctuations in the 10-kHz–0.625-MHz band. Chirping the nearly-octave-spanning mid-infrared pulses uncovers wavelength-dependent attosecond-scale waveform jitter. Our study validates EOS as a broadband (both in the radio-frequency and the optical domains), highly sensitive measurement technique for the jitter dynamics of optical waveforms. This sensitivity reveals outstanding stability of the waveforms obtained via IPDFG and EOS, directly benefiting precision measurements including linear and nonlinear (infrared) field-resolved spectroscopy. Furthermore, these results form the basis toward EOS-based active waveform stabilisation and sub-attosecond multi-oscillator synchronisation/delay tracking.

One-Step Fabrication of 0D Cs4PbBr6 Perovskite with Nonlinear Optical Properties for Ultrafast Pulse Generation.

Low-dimensional lead halide perovskites demonstrate remarkable nonlinear optical characteristics attributed to their distinctive physical structures and electronic properties. Nevertheless, the investigation into their nonlinear optical properties remains in its incipient stages. This study addresses this gap by precisely controlling solvent volumes to synthesize both 0D Cs4PbBr6 and Cs4PbBr6/CsPbBr3 perovskites. Remarkably, as saturable absorbers, both pure Cs4PbBr6 and Cs4PbBr6/CsPbBr3 composites exhibit favorable nonlinear optical properties within the C-band, showcasing modulation depths of 9.22% and 16.83%, respectively. Moreover, for the first time, Cs4PbBr6 and Cs4PbBr6/CsPbBr3 composites have been successfully integrated into erbium-doped fiber lasers to realize the mode-locking operations. The utilization of the Cs4PbBr6/CsPbBr3 composites as a saturable absorber that enables the generation of conventional soliton mode-locked laser pulses with a pulse duration of 688 fs, and a repetition frequency of 10.947MHz at a central wavelength of 1557nm. Cs4PbBr6 is instrumental in generating laser pulses at a frequency of 10.899MHz, producing pulse widths of 642 fs at the central wavelength of 1531.2nm and 1.02ps at the central wavelength of 1565.3nm, respectively. The findings of this investigation underscore the potential utility of 0D Cs4PbBr6 and Cs4PbBr6/CsPbBr3 composites as promising materials for optical modulation within fiber laser applications.

Cerium oxide for L-band Q-switched pulse laser generation

Cerium oxide (CeO2), a rare earth metal oxide belonging to the lanthanide group is a mature engineered nanoparticle that has been developed for various industry applications. The nanoparticle features defects in its lattice structure, which is highly advantageous for bandgap tuning, and is acclaimed for its least cytotoxicity among other metal oxides. In this work, CeO2 is employed as a saturable absorber (CeO2-SA) in the generation of an L-band Q-switched erbium-doped fiber laser. Q-switching was attained at 33.33 mW pump power threshold with a central wavelength of 1600.512 nm. Within 33.33 mW to 153.20 mW pump power range, the Q-switched laser obtained pulse repetition rates between 11.00 to 34.48 kHz, and pulse widths between 39.0 to 12.0 µs, with a high maximum pulse energy of 410 nJ. The Q-switched fiber laser remained stable over a 45-minute observation time at 153.20 mW pump power. The room temperature and direct mechanical deposition fabrication technique of the CeO2-SA in this work demonstrates the extreme simplicity and low cost of the device, which would reduce the overall cost and facilitate a wider technology adoption of pulse laser systems.

Manganese dioxide as wide adaptive ultrafast photonic device for pulsed laser generation

Manganese dioxide as wide adaptive ultrafast photonic device for pulsed laser generation

Repetition‐Rate and Wavelength Flexible Femtosecond Laser Pulse Generation

AbstractCompared to mode‐locked oscillators, gain‐switched diodes (GSD) have the key advantage of repetition‐rate agility. Yet, large pulse duration and poor coherence of the GSD pulses greatly limit their applications. Here, a GSD‐pumped Raman fiber amplifier is demonstrated, which can effectively generate femtosecond laser pulses with both repetition‐rate and wavelength agility. It is proved that both nonlinear optical gain and single‐frequency seed in the fiber amplifier play critical roles for improving the coherence of the generated laser pulses. In the experimental demonstration, pumped by 1065 nm GSD pulses, the nonlinear fiber amplifier can generate highly coherent 1121 nm femtosecond Raman pulses with up to 80.2% conversion efficiency. The Raman pulses can maintain high performance within the repetition‐rate tuning range from 1 to 150 MHz. The potential for generating 1178 nm femtosecond Raman pulses is also demonstrated with an optical conversion efficiency of 63.8%. This all‐fiber based femtosecond laser with both repetition‐rate and wavelength agility is a promising light source for applications such as nonlinear microscopy and micromachining.

Interference mechanism of plasma self-organization in transparent dielectrics under the intense femtosecond laser pulse exposure

A new mechanism of plasma self-organization in transparent dielectrics with wide bandgap exposed to the intense tightly focused laser radiation was revealed, which causes the generation of 3D periodic ring structures with subwavelength period both along the laser pulse propagation and in the radial direction. The mechanism involves formation of dense plasma burst in the pre-focal region that provides efficient scattering of the incident wave. The interference of a plane incident laser wave in the focal region and a divergent reflected one will form the standing wave pattern with local minima and maxima of laser field both in the direction of the incident wave propagation and perpendicular to it producing the ring patterns of effective ionization regions in the dielectric volume. Analytical and numerical simulations of the process of laser wave scattering on a near-spherical plasma object with dimensions both smaller and larger than the laser radiation wavelength are performed to verify the proposed model.

Efficient laser-driven proton acceleration from a petawatt contrast-enhanced second harmonic mixed-glass laser system

Efficient laser-driven plasma acceleration of ion beams requires precision control of the target–plasma profile, which is crucial to optimize the laser energy transfer. Along the laser propagation direction, this can be achieved by tailoring the temporal structure of the laser pulse. We show for the first time that frequency-doubling of a short pulse (hundreds-femtosecond range) petawatt-class mixed-glass laser system, which results in temporal intensity contrast enhancement, enables surface and volumetric laser–energy coupling, and the acceleration of proton beams from few-nanometer-thick foil targets. Experimentally, maximum ion energies and laser-to-proton energy conversion efficiencies were found to be both maximized at optimum laser and target conditions manifested when the normalized target density nearly equalizes the normalized laser vector potential, which is in agreement with theory and simulations. These signatures are recognized as a unique indication of the interaction between ultra-intense laser pulses with high temporal intensity contrast and ultra-thin nanometer-scale targets. Transverse modulations of accelerated proton beams in the form of bubble- and ring-like structures measured in the thinnest targets provide additional evidence of volumetric laser-driven particle acceleration regimes and transitional features in ultra-thin foil targets specific to laser–plasma interactions characterized by a high temporal intensity contrast. These results open avenues in the generation of high contrast laser pulses from short-pulse-femtosecond petawatt mixed-glass laser systems and demonstrate the feasibility of this technique for applications requiring high laser intensity contrast with high efficiency.

Time-resolved ARPES with probe energy of 6.0 eV and tunable MIR pump at 250 kHz

In this paper, we present a laser source designed specifically for time- and angle-resolved photoemission spectroscopy (TR-ARPES) investigations of light-induced electron dynamics in quantum materials. Our laser source is based on a ytterbium-doped laser that seeds an optical parametric amplifier (OPA) followed by a difference frequency generation (DFG) stage. This configuration enables the generation of tunable near-infrared and mid-infrared laser pulses (1.5 to 8 μm - 0.82 to 0.15 eV) at 250 kHz of repetition rate, serving as the pump for TR-ARPES measurements. The remaining energy of the laser is used to generate the ultraviolet 6 eV probe pulses, which prompt the material to emit photoelectrons. We demonstrate the long-term stability of the source, as well as the characterization of the beam profiles and pulse durations. Additionally, we present preliminary TR-ARPES results obtained on Bi2Te3, a prototypical 3D topological insulator. This paper illustrates the capability of our laser source to probe electronic dynamics in quantum materials.

Cherenkov second harmonic generation of femtosecond laser pulses in a homogeneous nonlinear crystal

In experiments on second harmonic (SH) generation (SHG), a conical structure of radiation has been observed. In the present study, a non-stationary theory of SH excitation of ultrashort laser pulses with phase modulation has been developed, which explains the properties of such a structure as Cherenkov radiation. Under phase-mismatched interactions, a maximum of the SH spectrum is observed at the Cherenkov angle, which is determined by the ratio of the SH and laser radiation phase velocities. It is shown that tightly focused laser beams are preferred to observe Cherenkov SHG. The SH spectral width depends on the group velocity mismatch and is more complicated on the excited radiation spectrum. The SH energy can be proportional to the crystal length or group delay length depending on their ratio. We also demonstrate that a complex angular distribution of spectral components (an angular chirp) appears within the SH cross-section.

Nonlinear optical absorption of TiTe2 nanosheets and its application for pulse laser generation

Two-dimensional TiTe2 materials have excellent optical and electrical properties due to their unique semi-metallic properties and are widely used in detectors, photocatalysis, and field effect tubes. However, their potential in field of nonlinear optics (NLO) and ultrafast photonics has not been explored. In this paper, TiTe2 nanosheets are prepared by liquid-phase exfoliation, and their excellent nonlinear optical properties are being demonstrated. Z-scan measurements show that their nonlinear absorption coefficients are -6.83 ± 0.52 and -15.10 ± 0.82 cm GW-1 at 1560 and 2000nm, respectively, exhibiting saturable absorption properties. Two saturable absorbers (SA) based on TiTe2 nanosheets are prepared, one by encapsulating TiTe2 nanosheets in a polyvinyl alcohol (PVA) film and the other by depositing them on D-shape fiber (DSF). Finally, a Q-switched operation is achieved using TiTe2-PVA SA, with the maximum output power of 14.2 mW, corresponding to a pulse energy of up to 212.9 nJ. Stable mode-locked operation is achieved using TiTe2-DSF SA with a signal-to-noise ratio and pulse duration of 70 dB and 788 fs, respectively. Notably, this is the first time that TiTe2 nanosheets have been used as SA. The results suggest that TiTe2 nanosheets are a high-performance nonlinear material that provides a new impetus for the development of NLO and ultrafast photonics.

Molybdenum gallium carbide coated D-shape fiber for nanosecond dark pulse generation in erbium-doped fiber laser

Molybdenum gallium carbide (Mo2Ga2C), a material belongs to MAX phase group, has garnered significant interest in a wide range of scientific field, including material science, engineering, and laser physics. It is chemically and structurally promising since it exhibits a low shear modulus, superconducting behaviour, and high mechanical stability. Here, we prepared a saturable absorber (SA) based on Mo2Ga2C coated D-shape fiber using solution casting technique. It exhibits outstanding saturable absorption of 7.2%, allowing it to generate dark pulse laser in an erbium-doped fiber laser cavity. We analyzed the laser’s performance and obtained a dark pulse with a pulse energy, peak power, and average output power of 3.9 nJ, 27.5 mW, and 7.3 mW, respectively. This foundational work may introduce a new material as an alternative SA device in fiber laser cavity.

Effect of wavelength on propagation of high-power femtosecond laser pulses in diamond

Numerical modeling of the propagation of high-power femtosecond laser beams in a diamond crystal in the self-focusing mode is carried out. The wavelength of the modeled beams is varied in a wide range (400 nm, 532 nm, 800 nm, and 1064 nm), which covers the most common ultrashort laser systems. For all the wavelengths analyzed, a limitation in the growth of laser energy density inside the diamond crystal with increasing pulse energy is found. This effect has a certain similarity to the effect of optical limitation in laser filaments, but manifests itself outside the filament formation zone and is capable of equalizing the laser energy density, as well as the electron density, inside the extensive pre-focal region.

Nonlinear Optical Saturable Absorption Properties of 2D VP Nanosheets and Application as SA in a Passively Q-Switched Nd:YVO4 Laser.

Two-dimensional (2D) violet phosphorus (VP) plays a significant role in the applications of photonic and optoelectronic devices due to its unique optical and electrical properties. The ultrafast carrier dynamics and nonlinear optical absorption properties were systematically investigated here. The intra- and inter-band ultrafast relaxation times of 2D VP nanosheets were measured to be ~6.83 ps and ~62.91 ps using the pump-probe method with a probe laser operating at 1.03 μm. The nonlinear absorption coefficient βeff, the saturation intensity Is, the modulation depth ΔR, and the nonsaturable loss were determined to be -2.18 × 104 cm/MW, 329 kW/cm2, 6.3%, and 9.8%, respectively, by using the Z-scan and I-scan methods, indicating the tremendous saturable absorption property of 2D VP nanosheets. Furthermore, the passively Q-switched Nd:YVO4 laser was realized with the 2D VP nanosheet-based SA, in which the average output power of 700 mW and the pulse duration of 478 ns were obtained. These results effectively reveal the nonlinear optical absorption characteristics of VP nanosheets, demonstrating their outstanding light-manipulating capabilities and providing a basis for the applications of ultrafast optical devices. Our results verify the excellent saturable absorption properties of 2D VP, paving the way for its applications in pulsed laser generation.

High-output ∼3 μ m MIR pulsed laser enabled by surface state regulation in PtTe2 optical switch

Mid-infrared (MIR) pulsed lasers operating in the ∼3 μm region play a crucial role in various applications, including molecular spectroscopy, ultrafast molecular imaging, and laser-assisted surgery. Despite recent advancements in MIR gain platforms, a notable technological challenge remains in the absence of an effective optical Q-switch. Here, a remarkable optical Q-switch in the 3 μm region based on a Dirac semimetal PtTe2 saturable absorber is realized. By modulating the surface state of PtTe2, the pulsed laser exhibited an increase in average power, escalating from 521 to 588 mW, accompanied by a significant decrease in pulse width from 368 to 187 ns. Nondegenerate pump–probe measurements showed that the recombination rate of the photocarrier in thinner PtTe2 nanoplates was effectively accelerated, primarily attributed to the substantial increase in surface state density, leading to better saturable absorption performance. As the thickness of the PtTe2 nanoplates decreases, the nonsaturable loss decreases from 12% to 3%, while the modulation depth increases from 6% to 12%. The enhanced ultrafast nonlinear absorption enables flexible modulation of saturation absorption parameters, which endows high-performance MIR pulsed laser generation.