Ultrafast Research Articles

Ultra‐Broadband Visible‐DUV Supercontinuum Generation by Non‐Resonant Coherent Raman Scattering in Air

AbstractTo date, supercontinuum light in the visible and near‐infrared ranges is readily realizable by the optical Kerr effect through self‐phase modulation of ultrashort laser pulses in transparent media. However, it is still a challenge to extend the supercontinuum spectrum down to the deep‐ultraviolet (DUV) range, which is particularly needed for exploring ultrafast dynamics in chemistry, materials, and biology. Here, an approach of non‐resonant coherent Raman scattering is developed to generate ultra‐broadband visible‐DUV supercontinuum in ambient air with a spectral range spanning over 250 nm and a wavelength down to 220 nm. A rovibrational coherence is established in air molecules by filamentation of a near‐infrared femtosecond 800 nm pulse and two femtosecond Raman laser pulses at 267 and 400 nm are introduced into the coherent media to induce non‐resonant coherent Stokes and anti‐Stokes Raman scatterings, which serve as the spectral bridges to link the neighboring Raman pump laser spectra, resulting in ultra‐broadband supercontinuum light. The mechanism is further verified by examining the broadening of picosecond N2+ laser lines with narrow bandwidths (10–30 cm−1), which forms a supercontinuum spectrum spanning over 150 nm. The work provides a viable route for the establishment of coherent DUV supercontinuum in the gas media at designed wavelength ranges.

A Refined Model for Ablation Through Cavitation Bubbles with Ultrashort Pulse Lasers

A Refined Model for Ablation Through Cavitation Bubbles with Ultrashort Pulse Lasers

Ultrafast laser direct writing of in-line polarizers based on nano-gratings

Ultrafast laser direct writing of in-line polarizers based on nano-gratings

Near-complete extraction of maximum stored energy from large-core fibers using coherent pulse stacking amplification of femtosecond pulses

High field science relies on ultrashort pulse lasers with multi-joule pulse energies for studying light–matter interactions under extreme conditions and for driving particle accelerators and secondary radiation sources of x rays, gamma rays, neutrons, positrons, muons, and protons. Next-generation laser drivers will require a 103−104 times increase in pulse repetition rates, producing multi-joule energies at multi-kilowatt average powers to enable practical applications in nuclear engineering, advanced materials, medicine, biology, homeland security, and high-energy physics. Spatially coherently combined femtosecond fiber lasers are recognized as a pathway to these next-generation drivers, with significant practical advantages including high efficiency and the possibility of compact integration. However, chirped pulse amplification in fibers is capable of extracting only a small fraction (usually ∼1%) of the maximum stored energy. Here we demonstrate near-complete maximum stored energy extraction with low accumulated nonlinearity from a large-core fiber amplifier using coherent pulse stacking amplification. We have amplified a 81-pulse stacking burst in a 85 µm core chirally coupled core Yb-doped fiber, extracting up to 9.5 mJ (∼90% of stored energy) with <4.5 radians of accumulated nonlinear phase, temporally combined this burst into a single pulse, and achieved 4.2 mJ pulses of 313 fs bandwidth-limited duration after compression. This represents, to our knowledge, the highest energy extracted and compressed into a femtosecond pulse from a single fiber amplifier, enabling approximately two orders of magnitude size reduction of future high-energy coherently spatially combined fiber laser arrays.

Exploring Ultrafast Carrier Dynamics in Photoelectrochemical Water Splitting Using Transient Absorption Spectroscopy

AbstractHydrogen fuel produced from water splitting using sunlight remains one of the cleanest and most sustainable energy sources. However, photoelectrochemical hydrogen production faces several challenges, including poor sunlight absorption, deficient charge carrier lifetime and transfer rates, and very sluggish kinetics of the water oxidation half‐reaction. To address these challenges, researchers have concentrated on enhancing the efficiency of photoelectrochemical water splitting. A critical factor in this enhancement is a deeper understanding of the fundamental processes involved in photoabsorption and the subsequent oxidation evolution reaction. The advent of ultrafast lasers has enabled the detailed tracking of charge carriers within materials after sunlight absorption, including their separation and migration to the surface for water oxidation. Ultrafast transient absorption spectroscopy is a powerful technique that allows real‐time observation of the behavior of excited states and charge carriers on femtosecond to nanosecond timescales. Recent studies utilizing ultrafast transient absorption spectroscopy are explored to investigate the dynamics of charge carriers in photoelectrochemical water splitting, providing insights into the mechanisms that lead to the design of enhanced photoanodes with improved efficiency.

Linear and nonlinear optical properties of femtosecond laser inscribed waveguides into GLS glass

Gallium lanthanum sulfide (GLS) glass is a promising material for mid-infrared photonics due to its wide transmission window and its high nonlinear refractive index that is almost three orders of magnitude higher than that of fused silica. In this paper, we present the results of a detailed study into the linear and nonlinear optical properties of waveguides fabricated in GLS glass via ultrafast laser direct-inscription using three different techniques: cumulative heating in the thermal regime as well as multi-scan and half-scan in the athermal regime. Using quadriwave lateral shearing interferometry, we fully characterized the refractive index profiles of such inscribed waveguides and found no difference between half-scan and multi-scan writing which indicates the absence of laser-induced stress in this soft glass in stark contrast to fused silica. In terms of nonlinearity, we utilized self-phase modulation (SPM)-induced spectral broadening experiments at mid-IR wavelengths to demonstrate that waveguides fabricated in the athermal regime preserve the high intrinsic nonlinearity of the GLS bulk material, outperforming those written in the thermal regime based. These findings pave the way for the fabrication of fiber-coupled optical waveguide chips for nonlinear mid-infrared photonics.

Optical pulse generation with programmable positions in an actively mode-locked optical cavity

A novel, to our knowledge, approach for generating optical pulses with programmable positions in an actively mode-locked (AML) optical cavity is proposed and experimentally demonstrated. In the AML, active mode locking occurs when the cavity gain exceeds one, and the cavity round trip time equals integer multiples of the repetition period of the electrical driving signal. The generated optical pulses are present at the positions where the optical cavity operates at its maximum transmission points. By periodically controlling the cavity gain with the driving signal, the maximum transmission points can be manipulated using a periodic arbitrary waveform, allowing the positions of the generated optical pulses within one round trip time to be programmed. Thanks to the superposition of the circulating optical field within the optical cavity, the pulse width of the generated pulses can be greatly compressed. In a proof-of-concept experiment, optical pulses with programmable pulse numbers, pulse intervals, and pulse position distributions in one period are generated by programming the driving signals. Optical pulse compression is also successfully verified. Optical pulses with an approximate 34-ps pulse width, ultra-low pulse interval (about 200 ps), and linear positional distribution are achieved.

Low-noise 2-GHz figure-9 fiber laser based on passive harmonic mode-locking

We have proposed and demonstrated the generation of a high-repetition-rate ultrashort pulse with long-term stability and low noise based on a harmonic mode-locked (HML) figure-9 fiber laser. Different HML orders from the 2nd to the 13th are generated by adjusting the pump power, net dispersion, and wave plate angles. A 2-GHz HML pulse is obtained with a 155-MHz fundamental repetition rate in a Yb-doped fiber laser, and the corresponding supermode suppression level is as high as 50 dB. The average power of the output pulse is nearly 200 mW, and the compressed pulse duration is 535 fs. To the best of our knowledge, 2 GHz and 196 mW represent, respectively, the highest repetition rate and output power in a figure-9 fiber laser. This work offers a potential pathway to achieve high-repetition-rate ultrashort pulses in the GHz range with high power and low noise levels.

Noise‐Like Pulse Seeded Supercontinuum Generation: An In‐Depth Review For High‐Energy Flat Broadband Sources

AbstractAs the need for compact, cost‐effective, and reliable laser sources continues to rise, fiber lasers have gained widespread interest in science and technology. In recent years, passively mode‐locked fiber lasers (PMLFLs) have emerged as pivotal tools for generating ultrashort pulses, propelling advancements across various domains including communication, manufacturing, medicine, defense, and security. Amongst the various types of lasing states supported by a PMFL, the emphasis in this review is on the noise‐like pulses (NLP) and their potential applications in supercontinuum generation (SCG). Interestingly, the quasi‐stationary operation of the NLP envelope containing numerous chaotic sub‐pulses has facilitated relatively high energy and broad bandwidth compared to standard mode‐locked laser pulses. Moreover, the NLP generation goes beyond a specific cavity arrangement, the nature of mode‐locking or cavity dispersion. Therefore, through this review, the foremost aim is to report the differences in NLPs across various experimental settings reported so far and highlight the strategies beneficial for high‐energy and broadband NLP development directly from a fiber oscillator. Secondly, the application of NLP as a seed laser is examined to stimulate SCG in different types of fibers, underlining the improved supercontinuum characteristics over the conventional ultrashort pulse pumping schemes. Finally, the benefit of NLP‐seeded SCG for various bio‐medical and industrial applications are highlighted, thanks to the broader and flatter continuum achievable through compact experimental settings.

A Horizontal Double‐Sided Copper Metallization Technology Designed for Solar Cell Mass‐Production

ABSTRACTIn the international photovoltaic market, the “carbon footprint” label has received great attention, and low‐carbon footprint products are widely popular. Compared with metallic silver, copper has the characteristics of low carbon emissions and low cost, which can effectively reduce the carbon footprint. Based on this, this article reports a horizontal double‐sided copper metallization technology. This technology can not only metalize the front and back sides of various types of silicon solar cells at the same time but also has fast speed, good uniformity, and simple process, making it suitable for the industrial mass production of solar cells. This article describes the structure and manufacturing process of TOPCon solar cells patterned with an ultrashort pulse laser and metalized using this novel horizontal double‐sided copper metallization technology. In this batch, average efficiency reached above 26%.

Conventional soliton and noise-like pulse emissions at 1.98μm utilizing tungsten trioxide as mode-locker

In this paper, we experimentally demonstrate the dual pulse emissions of conventional soliton (CS) and noise-like pulse (NLP) in thulium-holmium co-doped fiber laser using tungsten trioxide saturable absorber (WO3-SA). The CS pulse delivered 2[Formula: see text]ps width, centered at 1982.8[Formula: see text]nm. Increasing the pump power and alteration of polarization states switched the CS to the NLP regime with 1990.0[Formula: see text]nm central wavelength, 17.2[Formula: see text]nm spectral bandwidth, and 396[Formula: see text]fs spike width riding on top of a wide picosecond pulse pedestal in the time domain. This work suggests the capability of WO3-SA as a pulse initiator for future dual-source ultrashort pulses based on an all-fiber laser cavity design.

Fast Gain Dynamics in Interband Cascade Lasers

AbstractInterband Cascade Lasers (ICL) have matured into a versatile technological platform in the mid‐infrared spectral domain. To broaden their applicability even further, ongoing research pursues the emission of ultrashort pulses. The most promising approach is passive mode‐locking with a fast saturable absorber. However, despite vast attempts no passive mode‐locked ICL has been demonstrated up to date. In this study, pump‐probe measurements are conducted on the ICL gain, demonstrating that 60–70% of its dynamic behavior is characterized by a rapid recovery time of 2 ps. The findings explain why passive mode‐locking of ICLs has not succeeded so far, shedding a new light on ICL dynamics as strategies are proposed to overcome the current limitations.

Pulsed polarized vortex beam enabled by metafiber lasers

Pulsed polarized vortex beams, a special form of structured light, are generated by tailoring the light beam spatiotemporally and witness the growing application demands in nonlinear optics such as ultrafast laser processing and surface plasma excitation. However, existing techniques for generating polarized vortex beams suffer from either low compactness due to the use of bulky components or limited controlment of pulse performance. Here, an all-fiber technique combining plasmonic metafibers with mode conversion method is harnessed to generate high-performance pulsed polarized vortex beams. Plasmonic metafibers are utilized as saturable absorbers to produce Q-switched pulses with micro-second duration, while the offset splicing method is employed to partially convert the fundamental transverse mode (LP01\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{01}$$\\end{document}) to higher-order mode (LP11\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{11}$$\\end{document}). Eventually, a polarized vortex beams laser is achieved at the telecom band with a repetition frequency of 116.0 kHz. The impact of geometrical parameters including period of metafibers and offset of splicing on the spatiotemporal properties of pulsed polarized vortex beams is systematically investigated. Our findings could pave the way for design, control and generation of all-fiber pulsed polarized vortex beams, and also offer insights into the development of other types of structured laser sources.

Coherent dynamics in an optical quantum dot with phonons and photons

Genuine quantum-mechanical effects are readily observable in modern optomechanical systems comprising “classical” (bosonic) optical resonators. Unique features and advantages of optical two-level systems (qubits) for optomechanics, however, have not been so thoroughly explored. We experimentally demonstrate these advantages using charge-controlled InAs quantum dots (QDs) in surface-acoustic-wave resonators. We coherently control QD population dynamics using engineered optical pulses and mechanical motion, i.e., using both phonons and photons. As a first example, at moderate acoustic drive strengths, we demonstrate the potential of this technique to maximize fidelity in quantum microwave-to-optical transduction. Specifically, the scheme is tailored to enhance mechanically assisted photon scattering over the direct detuned photon scattering from the QD. Spectral analysis reveals distinct scattering channels related to Rayleigh scattering and luminescence in our pulsed excitation measurements, which lead to time-dependent scattering spectra. Quantum-mechanical calculations show good agreement with our experimental results, together providing a comprehensive description of excitation, scattering, and emission in a coupled QD-phonon system. These results highlight unique opportunities to expand the functionality of quantum optomechanical systems.

Dielectric material processing with ultrashort pulses

AbstractFemtosecond lasers have become essential tools in material processing. Thanks to their ultrashort pulse duration, these lasers can process a wide range of materials without causing significant thermal effects, leading to superior quality. Dielectric materials, especially glass and ceramics, are among those that benefit the most from femtosecond laser technology. Traditional processing methods often struggle with these materials, but femtosecond lasers provide a solution with high precision and quality.

Single-shot ultrafast imaging of plasma dynamics induced by dual-angle ultrashort laser pulses with subpicosecond delay

Spatiotemporal manipulation of ultrashort laser pulses is crucial for enhancing laser processing and phonon generation. Optimization of these applications requires ultrafast visualization of the underlying processes. In this study, we induced laser ablation using spatiotemporally manipulated double pulses focused from two angles onto a glass surface with a 0.7 ps interval, and captured the images of its dynamics with 5 sequential frames at a frame interval of 0.8 ps. The observed dynamics suggest that the laser profile reflected on the glass surface is influenced by its topography, which in turn affects the behavior of air breakdown plasmas.

Efficient Ablation, further GHz Burst Polishing, and Surface Texturing by Ultrafast Laser

Efficient Ablation, further GHz Burst Polishing, and Surface Texturing by Ultrafast Laser

Asymmetric phase-split axicon masks for a improved Bessel beam-based glass stealth dicing

The precision glass micromachining technology employing ultrashort laser pulses has reached a state of maturity. This technology enables the precise drilling, cutting, or ablation of various types of glass, achieving accuracy up to a few microns. As laser sources continue to advance in average power and repetition rate, the efficient utilization of laser beam irradiation patterns becomes increasingly crucial. The Bessel beam, known for its ability to create elongated channels in transparent materials because of its invariant focal zone, gains significance in this context. The elongated focal zone of the Bessel beam proves advantageous for expeditious glass cutting, surpassing the efficiency of using a standard Gaussian beam. This study delves into the glass-cutting process by studying asymmetric phase-split diffractive axicon masks to generate such Bessel beams. For this purpose, geometric phase optical elements are created using transparent nanogratings inscribed in the volume of glass. This approach allows high-power asymmetric Bessel-like beams to be generated and experimentally used to investigate beam-shaping performance, ultimately determining the optimal parameter set for applications such as glass stealth dicing.

Supercontinuum saturation of a femtosecond laser filament in pressurized gases.

Filamentation of high-power femtosecond optical pulses in high-pressure gases has gained increasing academic and practical interest from the viewpoint of studying large-scale spectral and temporal transformations occurring with pulsed laser radiation and obtaining super-broadened spectra and extremely short (attosecond) wave packets. Experimentally and theoretically, for the first time to the best of our knowledge, we show that as a result of a 45 fs Ti:sapphire laser pulse filamentation in an optical cell filled with pressurized up to 50 bar nitrogen or argon, the pulse spectrum can reach maximally about eightfold broadening. This limiting pulse spectral width is reached at a gas pressure of about 20 bar and with further pressure increase exhibits saturation and even a slight decrease relative to the limiting value. As a possible reason for this finding, we suppose the increase of pulse energy depletion in the self-created plasma at high gas pressure.

Various pulse nonlinear evolution and dynamics employing CuCrO2 nanocrystals as a promising saturable absorber

Considering that fiber lasers are a far richer nonlinear optical system, a systematical study on various pulse nonlinear evolution and dynamics by one SA has great significance. However, investigating these important pulse nonlinear dynamic evolution processes of CuCrO2 nanocrystals as new SAs applied in ultrafast fiber lasers has never been explored. Herein, we demonstrate the excellent nonlinear optical properties of CuCrO2 nanocrystals and investigate the potential of CuCrO2 nanocrystals for generating various pulse nonlinear dynamic evolution processes. Firstly, the output characteristics of the passively Q-switching pulsed fiber lasers have been regulated by using CuCrO2 nanocrystals-polyimide film and a tapered fiber. The pulse duration and repetition rate were simultaneously compressed by 10 and 222 times, the pulse energy and peak power were increased by 79 and 827 times, respectively. Secondly, we are able to generate chaotic multi-pulse wave packets and optical rogue waves by the polarization mode dispersion of single-mode fiber together with the non-instantaneous relaxation of CuCrO2 nanocrystals. Finally, the transition from chaotic multi-pulse wave packet to giant chirped mode-locked pulse is realized by introducing high nonlinear fibers. Our results show that CuCrO2 nanocrystals are an efficient nonlinear material for studying various nonlinear phenomena, and give a new impetus to the development of nonlinear optics and ultrafast photonics.