Ultrafast Research Articles

Theoretical analysis of the phase characteristics in few-cycle laser coherent beam combining

To meet the increasing demand for high-energy (Joule-level) few-cycle pulses in strong-field physics research, coherent beam combining (CBC) presents an effective method to further enhance the power of few-cycle pulse sources. However, due to the broad spectrum and ultrashort pulse duration of few-cycle pulses, achieving efficient beam combining is particularly challenging. In this paper, we employ the angular spectrum (ASM) method to numerically simulate the propagation of few-cycle pulses in a focusing system. By constructing a three-dimensional (2D + 1D) model, we accurately quantify the effects of laser parameters such as carrier-envelope phase (CEP) difference, time delay (TD), dispersion between beams, and combining configurations on the combining efficiency and CEP shift. The study results provide design guidelines for high energy few-cycle laser systems based on coherent beam combining and reveal the possibility to significantly enhance the combining efficiency while maintaining high CEP stability, which will finally provide a reliable light source for advancing the frontier of strong-field physics.

Non-collinear broadband phase-matching for dual-chirped optical parametric amplification

Dual-chirped optical parametric amplification (DC-OPA) is a variant of OPCPA used to produce intense ultrafast pulse. In this study, we comprehensively explore the quantitative relationship between non-collinear broadband PM geometry and linear chirp coefficients. Compared to OPCPA, non-collinear broadband DC-OPA demonstrates different design strategies. In the proof-of-principle experiment based on a specific oppositely configured DC-OPA design, ∼3 times enhancement in the bandwidth of the generated mid-IR idler pulse is observed compared to the collinear PM situation, which is well consistent with the theoretical predictions. The presented results may provide valuable insights for the design of all types of broadband DC-OPA systems.

Detection of low-power RF signals using active mode-locking based coupled optoelectronic oscillator

A novel method to detect and amplify broadband low power radio frequency (RF) signal with high gain based on coupled optoelectronic oscillator (COEO) is proposed and experimentally demonstrated. When the frequency of the weak signal matches the COEO oscillation mode, it can obtain the high gain of active mode-locking in the COEO optical loop. The system can selectively detect and amplify RF signals from 1 to 10 GHz by observing the optical pulse repetition rate state. The experimental results show that sensitivity of detection as low as −89 dBm with a maximum gain of 34.32 dB at 1.002 GHz, and an average gain of 30.52 dB for low power RF signals. The real application of the method for detecting low power Quadrature Phase Shift Keying (QPSK) modulation is investigated and the spectral waveforms is discussed.

Ultrafast Laser-Induced Thickness-Limited Sintering of Metal Nanoparticle Film for Ultrathin Flexible Microelectronic Devices.

Laser sintering of metal nanoparticles (NPs) has been widely used in flexible microelectronic device fabrication, wherein the sintered layer thickness is a key factor affecting the mechanical stability and conductivity. In this work, ultrathin flexible electronic circuits on flexible substrates with robust bonds and excellent conductivity have been fabricated through ultrafast laser-induced thickness-limited sintering of the metal NP film. When the laser fluence is below the damage threshold of the metal NP film, sintered layer thickness can be controlled by the laser parameters. The maximum thickness of the dense sintered NP layer is limited to 355, 421, 491, 527, and 647 nm at laser pulse durations of 0.3, 5, 10, 15, and 20 ps, respectively. This thickness-limited sintered layer is mainly determined by the plasmonic photothermal absorption of metal NPs and heat transfer within the NP layer. Due to the nonthermal process under intense ultrafast laser irradiation, the metal-polymer interaction can be further enhanced with minimal damage on substrates. The resistivity of the as-received Ag NP film decreases to 6.32 μΩ·cm after laser sintering at a pulse duration of 20 ps. Meanwhile, the relative resistance of the NP film increases to 2.7, 1.7, and 1.03 after 105 bending cycles, 500 tape peeling cycles, and water flow impinging for 60 min, respectively. This thickness-controlled ultrathin Ag NP film fabricated by ultrafast laser sintering exhibits excellent mechanical robustness and electrical conductivity, which shows great promise in ultrathin flexible microelectronic devices.

Improved temporal characteristics for post-compressed pulses via application-tailored nonlinear polarization ellipse rotation.

Intense ultrashort laser pulses with high temporal quality are essential for fundamental science. Nonlinear polarization ellipse rotation (NER) is one way to ensure this high temporal quality, by suppressing weaker signals beyond the duration of the main pulse up to a few orders of magnitude. Post-compression schemes have revolutionized ultrafast lasers, enabling the generation of pulses with durations beyond the limit supported by laser gain media. However, high compression ratios lead to the formation of new pre- and post-pulses. Both NER and post-compression depend on the optical Kerr effect. This makes the combination of the two in a single setup both advantageous and straightforward. While NER cannot suppress the new pre- and post-pulses generated in post-compression, we show via simulations and experimental data that by combining the two, it is possible to shape the output spectrum and counteract temporal contrast degradation.

Optically Modulated Nanofluidic Ionic Transistor for Neuromorphic Functions

Neuromorphic systems that can emulate the behavior of neurons have garnered increasing interest across interdisciplinary fields due to their potential applications in neuromorphic computing, artificial intelligence and brain‐machine interfaces. However, the optical modulation of nanofluidic ion transport for neuromorphic functions has been scarcely reported. Herein, inspired by biological systems that rely on ions as signal carriers for information perception and processing, we present a nanofluidic transistor based on a metal‐organic framework membrane (MOFM) with optically modulated ion transport properties, which can mimic the functions of biological synapses. Through the dynamic modulation of synaptic weight, we successfully replicate intricate learning‐experience behaviors and Pavlovian associate learning processes by employing sequential optical stimuli. Additionally, we demonstrate the application of the International Morse Code with the nanofluidic device using patterned optical pulse signals, showing its encoding and decoding capabilities in information processing process. This study would largely advance the development of nanofluidic neuromorphic devices for biomimetic iontronics integrated with sensing, memory and computing functions.

Photonic compressive sensing of microwave signals with enhanced compression ratio and frequency range via optical pulse random mixing

Photonic compressive sensing of microwave signals with enhanced compression ratio and frequency range via optical pulse random mixing

Excitonic-Vibrational Interaction at 2D Material/Organic Molecule Interfaces Studied by Time-Resolved Sum Frequency Generation

The hybrid heterostructures formed between two-dimensional (2D) materials and organic molecules have gained great interest for their potential applications in advanced photonic and optoelectronic devices, such as solar cells and biosensors. Characterizing the interfacial structure and dynamic properties at the molecular level is essential for realizing such applications. Here, we report a time-resolved sum-frequency generation (TR-SFG) approach to investigate the hybrid structure of polymethyl methacrylate (PMMA) molecules and 2D transition metal dichalcogenides (TMDCs). By utilizing both infrared and visible light, TR-SFG can provide surface-specific information about both molecular vibrations and electronic transitions simultaneously. Our setup employed a Bragg grating for generating both a narrowband probe and an ultrafast pump pulse, along with a synchronized beam chopper and Galvo mirror combination for real-time spectral normalization, which can be readily incorporated into standard SFG setups. Applying this technique to the TMDC/PMMA interfaces yielded structural information regarding PMMA side chains and dynamic responses of both PMMA vibrational modes and TMDC excitonic transitions. We further observed a prominent enhancement effect of the PMMA vibrational SF amplitude for about 10 times upon the resonance with TMDC excitonic transition. These findings lay a foundation for further investigation into interactions at the 2D material/organic molecule interfaces.

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

Terawatt-attosecond hard X-ray free-electron laser at high repetition rate

Ångstrom and attosecond are the fundamental spatiotemporal scales for electron dynamics in various materials. Although attosecond pulses with wavelengths comparable to the atomic scales are expected to be a key tool in advancing attosecond science, producing high-power hard X-ray attosecond pulses at ångstrom wavelengths remains a formidable challenge. Here, we report the generation of terawatt-scale attosecond hard X-ray pulses using a free-electron laser in a special operation mode. We achieved 9 keV single-spike X-ray pulses with a mean pulse energy of around 180 μJ, exceeding previous reports by more than an order of magnitude, and an estimated average pulse duration of 200 as at full-width at half-maximum. Exploiting the unique capability of the European XFEL, which can deliver ten pulse trains per second with each containing hundreds of pulses at megahertz repetition rates, this study demonstrates the generation of attosecond X-ray pulses at a 2.25 MHz repetition rate. These intense high-repetition-rate attosecond X-ray pulses present transformative prospects for structural and electronic damage-free X-ray measurements and attosecond time-resolved X-ray methodologies, heralding a new era in ultrafast X-ray science.

Influence and mechanism of ultrafast laser-textured Cu substrate on wetting behavior of SAC305 solder

Periodic arrays of grids, micro-cones, and grooves were successfully fabricated on Cu substrate using ultrafast laser technology. The wetting behavior of SAC305 solder on micro-nano structured Cu substrates was investigated, and the influence mechanism of superficial micro-nano structure was discussed using classical wetting model and molecular dynamics (MD) simulation. The results indicate that compared to polished surface, all the superficial micro-nano structures on Cu substrate improve the wettability and wetting kinetics of SAC305 solder. Especially, grid structure shows the most significant improvement, and the effect is further enhanced with the increase of processing depth. When the processing depth of grid structure is 25 μm, the smallest wetting angle of 15.1° and wetting equilibrium time of 29 s were achieved. The wetting process of SAC305 solder on Cu substrate is composed of rapid reaction stage, stable diffusion state, and equilibrium stage. Rn ∼ t relationship curve reveals that the influence of micro-nano structure on wetting kinetics mainly occurs in the rapid reaction stage. The enhancement of wettability and wetting kinetics from superficial micro-nano structure can be attributed to the introduced extra capillary force and the improved atomic diffusion rate on the wetting interface.

Nanoscale phase transformation in SiC wafer by spatiotemporal tailored ultrafast laser pulses for multiple optical data encryption

Color center has shown great potential in optical data encryption due to its tunable optical performance. In this work, precise and controllable implantation of color centers within 4H-SiC wafers has been demonstrated by spatiotemporal tailored ultrafast laser irradiation. Distinct microstructures via micro-ablation, micro-cracking and internal phase transformation can be achieved by precisely regulating the spatial energy input in SiC wafer. The orientation of generated microstructures can be accurately controlled by tuning the laser polarization directions. Those color centers can be further customized by varying the defocusing distances and pulse durations. Wherein, nanoscale amorphous carbon layer with thickness ~ 100 nm was achieved by ultrafast laser induced internal phase transformation, which was ascribed to an enhanced near-field optical effect. Due to the thin-film interference, various colors can display from pink to light-green in those laser modified samples with different carbon nanolayer depths. With the highly flexible and precise modification in SiC crystal, multiple colors can therefore be implanted to encode optical information at small scale, allowing high density and complex information storage. This represents a significant advance in controlled phase transformation for nanoscale color centers fabrication, which is promising in multiple optical data encryption.

Multi-state broadband spectrum and ultra-short pulse spatiotemporal mode-locked Yb-doped multimode fiber laser

In the multimode fiber laser, a phenomenon of multi-state broadband spectrum and ultra-short pulse spatiotemporal mode-locking (STML) is obtained. The dissipative soliton, multiple pulses, harmonic pulse and bound soliton can be observed by rotating the polarization controllers(PCs). The spectral bandwidth of dissipative soliton is 19.04 nm at the central wavelength of 1039.40 nm, and the pulse width is 3.58 ps, acquiring the widest spectrum and the shortest pulse in an all-fiber dissipative soliton STML laser. As the STML state changes from dissipative soliton to bound soliton, spectral bandwidth decreases slightly. The spectral width and modulation period of the loose bound soliton with central wavelength of 1038.24 nm are 18.03 nm and 0.76 nm, respectively. We realize the conversion between the multiple states, which provides a scheme for the study of nonlinear soliton dynamics.

A Space‐Time Knife‐Edge in Epsilon‐Near‐Zero Films for Ultrafast Pulse Characterization

AbstractEpsilon‐near‐zero (ENZ) materials have shown strong refractive nonlinearities that can be fast in an absolute sense. While continuing to advance fundamental science, such as time varying interactions, the community is still searching for an application that can effectively make use of the strong index modulation offered. Here, the effect of strong space‐time index modulation in ENZ materials is combined with the beam deflection technique to introduce a new approach to optical pulse characterization that is termed a space‐time knife edge. It is shown that in this approach, temporal and spatial information of a Gaussian beam can be extracted with only two time resolved measurements. The approach achieves this without phase‐matching requirements (<1 µm thick) and can achieve a high signal to noise ratio by combining the system with lock‐in detection, facilitating the measurement of weak refractive index changes (Δn 10−5) for low intensity beams. Thus, the space‐time knife edge can offer a new avenue for ultrafast light measurement and demonstrates a use case of ENZ materials. In support of this, temporal dynamics for refractive index changes in non‐colinear experiments opening avenues are outlined for better theoretical understanding of both the spatial and temporal dynamics of emerging ENZ films.

Ultrafast Carrier and Lattice Cooling in Ti2CTx MXene Thin Films.

Metallic MXenes are promising two-dimensional materials for energy storage, (opto)electronics, and photonics due to their high electrical conductivity and strong light-matter interaction. Energy dissipation in MXenes is fundamental for photovoltaic and photothermal applications. Here we apply ultrafast laser spectroscopy across a broad time range (femto- to microseconds) to study the cooling dynamics of electrons and lattice in emerging Ti2CTx thin films compared to widely studied Ti3C2Tx thin films. The carrier cooling time in Ti2CTx is persistently ∼2.6 ps without a hot-phonon bottleneck. After hot carrier cooling is completed, the transient absorption spectra of Ti2CTx MXene can be described well by the thermochromic effect. Heat dissipation in MXene thin films occurs over hundreds of nanoseconds with thermal diffusivities ∼0.06 mm2 s-1 for Ti2CTx and ∼0.02 mm2 s-1 for Ti3C2Tx, likely due to inefficient interflake heat transfer. Our results unravel the energy dissipation dynamics in Ti2CTx films, showcasing potential applications in energy conversion.

High-power, frequency-doubled all-polarization-maintaining fiber laser system at 925 nm.

We demonstrate a high-power 925-nm pulsed laser system based on a frequency-doubled, all-polarization-maintaining (PM) fiber laser source operating at 1.8 µm. The seed is a figure-9 mode-locked oscillator, which incorporates a nonlinear amplifying loop mirror. After power scaling and pulse compression, the 1.8-µm laser source can provide femtosecond pulses with a repetition rate of 31.3 MHz and an output power of 2.24 W. Through frequency doubling in a nonlinear crystal, the 925-nm laser delivers a pulse duration of 503 fs and an output power of 818 mW, which is the highest power provided by all-PM fiber laser systems at this wavelength, as far as we know. Furthermore, this 925-nm all-PM fiber laser is employed as the excitation light source for two-photon microscopy (TPM) and second-harmonic generation (SHG) microscopy.

Crystallization behavior of amorphous GST films under an ultrafast laser irradiation

The crystallization behaviors of amorphous Ge2Sb2Te5 films induced by an ultrafast laser with a time-shaping Gaussian intensity distribution have been studied. The crystalline regions were characterized using optical microscopy, scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. It is found that the region ablated by a single pulse undergoes recrystallization, with a reflectivity higher than that of the non-ablated crystalline region. While preserving the integrity of the film, the diameter of the region with high and uniform reflectivity induced by burst mode is twice that of a single pulse, and the reflectivity is 3 % higher than the 31 % achieved with a single pulse. Additionally, the energy window for laser-induced crystallization expands with an increasing number of burst pulses; specifically, it increases by approximately 2.4 times when the number of sub-pulses is 4 or 5. The Raman results at low pulse energy show a high peak intensity at the 105 cm−1 in related to the vibrations of Te-rich tetrahedra, indicating that the degree of crystallinity in the burst mode region is superior to that achieved with single pulse irradiation. Furthermore, the blue shift of this Raman peak with increased pulse energy further supports that burst mode provides sufficient time for nucleation growth. This work offers insights into achieving more controllable crystallization, which can enhance its applications in phase change memory and other reconfigurable devices.

Abundant families of Jacobi elliptic function solutions and peculiar dynamical behavior for a generalized derivative nonlinear Schrödinger equation employing extended F-expansion method

Abstract This study investigates a generalized derivative nonlinear Schrödinger (GDNLS) equation , demonstrating how ultrashort pulses propagate in a single-mode optical fiber. The extended F-expansion method, which is a modification of Kudryashov’s auxiliary equation approach, is applied in this investigation to generate Jacobi elliptic solutions for the GDNLS. Three distinct solution instances are examined, and a variety of explicit solutions, including breathers, solitary waves, bright/dark solitons, bright-dark interaction solitons, a soliton-like solution, and a rogue-like solution, are obtained. To demonstrate the complex dynamical behavior of GDNLS equation, several representative solutions are chosen and their moduli are shown in three-dimensional, two-dimensional, and contour plots using Maple software.

Optimizing spectral phase transfer in four-wave mixing with gas-filled capillaries

Four-wave mixing (FWM) in gas-filled hollow-core capillaries, a nonlinear optical process that mixes signal and pump photon frequencies to generate idler frequency photons, offers a method for precise spectral phase transfer from signal to idler at ultrashort timescales and extreme powers. However, this regime is challenged by competing linear and nonlinear dynamics, leading to significant trade-offs between spectral phase transfer and conversion efficiency. Our computational investigation focuses on the upconversion of femtosecond pulses from the infrared (IR) to the ultraviolet (UV), a range notoriously difficult to manipulate. We explore an intermediate energy regime that strikes an optimal balance between FWM-mediated phase-transfer fidelity and nonlinear conversion efficiency. By adjusting the energy ratios and spectral phase profiles of the input signal, we achieve conversion efficiencies of approximately 5-15% while maintaining an effective quasi-linear spectral phase transfer. These findings will contribute to establishing first-principles and scaling laws essential for applications such as high-precision imaging, spectroscopy, quantum transduction, and distributed entangled interconnects, facilitating advanced control of ultrafast photonic and electronic wavepackets in quantum materials with unprecedented spatial and temporal precision.

Morphological effects on the photothermal properties and photo-thermionic emission in carbon nanotubes

Carbon nanotubes (CNTs) are efficient nanostructures for fabricating photo-thermionic emission (PTE) cathodes due to their high optical absorbance and the wide range of properties related to their morphological and structural characteristics. In this study, the photothermal and PTE responses of different CNT cathodes were numerically investigated. The CNTs varied in their outer diameters while maintaining the wall thickness. The analysis considered short-pulse laser sources with nanosecond, picosecond, and femtosecond pulse durations. The thermal properties of the CNTs were computed using the phonon density of states by simulating the vibrational phonon modes. The optical irradiances induced nonlinear optical processes from which the photothermal and PTE responses of the CNTs were obtained. Finally, thermal and time decay variables as a function of irradiation and nanotube diameter were analyzed. This work aims to manipulate the linear and nonlinear optical and mechanical properties of the CNTs to achieve multivariable all-optical control systems.