Ultrafast Research Articles

An overview of the use of non-titanium MXenes for photothermal therapy and their combinatorial approaches for cancer treatment.

Since the initial publication on the first Ti3C2T x MXene in 2011, there has been a significant increase in the number of reports on applications of MXenes in various domains. MXenes have emerged as highly promising materials for various biomedical applications, including photothermal therapy (PTT), drug delivery, diagnostic imaging, and biosensing, owing to their fascinating conductivity, mechanical strength, biocompatibility and hydrophilicity. Through surface modification, MXenes can mitigate cytotoxicity, enhance biological stability, and improve histocompatibility, thereby enabling their potential use in in vivo biomedical applications. MXenes are also known for their ability to absorb light in the near-infrared (NIR) region and generate heat by localised surface plasmon resonance (LSPR) effects and electron-phonon coupling. Optical excitation laser pulses result in hot photocarrier distribution in MXenes, which quickly transfers surplus energy to the crystal lattice and results in the internal conversion of light into heat with nearly 100% efficiency. The relaxation of hot carrier distribution by electron-phonon interactions leads to the cooling of the lattice by dissipating thermal energy to the surrounding environment. This heating effect of MXenes makes them potential photothermal agents (PTAs), particularly for PTT applications. The adjustable surface of MXenes and their high surface area-to-volume ratios are ideal for the combinatorial approach of PTT along with drug delivery, photodynamic therapy (PDT), bone regeneration and other applications. Since non-Ti MXenes are more biocompatible than Ti MXenes, they are promising candidates for different biomedical applications. This comprehensive review provides a concise overview of the current research patterns, properties, and biomedical applications of non-Ti MXenes, particularly in PTT and its combinatorial approaches.

Multi-watt picosecond 1.7 µm Tm-doped fiber laser amplification system

We report on a multi-watt, high-repetition-rate picosecond 1.7 µm Tm-doped fiber (TDF) laser amplification system. The seed oscillator is a figure-9 passively mode-locked TDF laser, which delivers a pulse train with a center wavelength of 1738nm and a fundamental repetition rate of ∼85 MHz. After a pre-amplifier and two stages of TDF amplifiers, the output power can be amplified to 5.2 W at a pump power of 10 W, corresponding to a slope efficiency of 52.1%. The output pulse duration is 33.87 ps and the pulse energy is 61 nJ. These results demonstrated that it is an effective method for achieving high-power ultrafast fiber laser source at 1.7 µm waveband, which would be a promising candidate for diverse applications such as polymer welding, bioimaging, mid-infrared laser generation and medical applications.

Nonlinear Refraction and Absorption in Polymers Used for Femtosecond Direct Laser Writing.

Direct laser writing (DLW) has been recognized as a unique technique for three-dimensional (3D) prototyping with resolution beyond the diffraction limit. One trend in DLW technologies is the use of polymers, given their favorable mechanical properties and optical quality, rendering them promising for the next generation of nonlinear photonic devices. However, absorptive properties that facilitate DLW processes may also hinder the performance of polymers as all-optical devices. Furthermore, the increasing use of ultrashort pulse lasers makes refractive index dispersion a factor of fundamental importance in studying candidate materials for broadband applications. Here, we study the suitability of resins commonly used in DLW for all-optical broadband applications. We provide linear and nonlinear refractive and absorptive spectra of acrylate-based resins with different compositions, and IP-S and IP-Dip from Nanoscribe. We found nonlinear refractive indices four times higher than reported values for fused silica, one of the most widely used photonic materials, and we evaluate the potential of these materials using group velocity dispersion and figures of merits as comparative metrics.

Femtosecond Laser-Induced Photothermal Effects of Ultrasmall Plasmonic Gold Nanoparticles on the Viability of Human Hepatocellular Carcinoma HepG2 Cells.

Laser-induced photothermal therapy using gold nanoparticles (AuNPs) has emerged as a promising approach to cancer therapy. However, optimizing various laser parameters is critical for enhancing the photothermal conversion efficacy of plasmonic nanomaterials. In this regard, the present study investigates the photothermal effects of dodecanethiol-stabilized hydrophobic ultrasmall spherical AuNPs (TEM size 2.2 ± 1.1 nm), induced by a 343 nm wavelength ultrafast femtosecond-pulse laser with a low intensity (0.1 W/cm2) for 5 and 10 min, on the cell morphology and viability of human hepatocellular carcinoma (HepG2) cells treated in vitro. The optical microscopy images showed considerable alteration in the overall morphology of the cells treated with AuNPs and irradiated with laser light. Infrared thermometer measurements showed that the temperature of the cell medium treated with AuNPs and exposed to the laser increased steadily from 22 °C to 46 °C and 48.5 °C after 5 and 10 min, respectively. The WST-1 assay results showed a significant reduction in cell viability, demonstrating a synergistic therapeutic effect of the femtosecond laser and AuNPs on HepG2 cells. The obtained results pave the way to design a less expensive, effective, and minimally invasive photothermal approach to treat cancers with reduced side effects.

Gain-managed nonlinear amplification in an erbium-doped fiber

To our knowledge, we report the first experimental demonstration of gain-managed nonlinear (GMN) amplification of femtosecond pulses in an erbium-doped fiber amplifier (EDFA). We investigated the GMN amplification using two different seed sources, operating at wavelengths of 1530 and 1560 nm. We obtained broadband output spectra spanning the entire C- and L-bands (1530–1620 nm). Our study shows that, in contrast to Yb-doped fiber amplifiers, it is not critical to choose a seed wavelength located at the short-wave edge of the gain bandwidth of the amplifier. Efficient and broadband GMN amplification can also be achieved with a seed located at 1560 nm wavelength.

UWB Chaotic Pulse-Based Ranging: Time-of-Flight Approach

Nowadays, indoor positioning using ultra-wideband (UWB) signals is actively being developed with the aim of implementing existing ideas and solutions, improving their performance, and searching for new measurement schemes. This paper proposes an approach to estimating the distance between wireless nodes by measuring radio signal propagation time using UWB chaotic radio pulses and UWB transceivers. This type of signal is a simple and practically interesting alternative to radio carriers of other types of UWB signals, which are based on packets of pulses (usually ultra-short pulses). The practical interest is caused by the noise-like nature of chaotic radio pulses, as well as their immunity to multipath fading and ease of generation. The aim of this work is to analyze such a system and identify the fundamental limitations inherent in the proposed approach. This paper describes a wireless system for measuring the signal propagation time based on the envelope of chaotic radio pulses using the SS-TWR (Single-Sided Two-Way Ranging) method. A difference scheme is used to determine the range. The characteristics of the proposed system are studied experimentally. The factors related to the threshold scheme for determining the time of arrival of a radio signal that introduce a systematic error into the measurement results are revealed, and approaches to correcting their influence are proposed.

High-energy tunable ultraviolet pulses generated by optical leaky wave in filamentation

Ultraviolet pulses could open up new opportunities for the study of strong-field physics and ultrafast science. However, the existing methods for generating ultraviolet pulses face difficulties in fulfilling the twofold requirements of high energy and wavelength tunability simultaneously. Here, we theoretically demonstrate the generation of high-energy and wavelength tunable ultraviolet pulses in preformed gas-plasma channels via the leaky wave emission. The output ultraviolet pulse has a tunable wavelength ranging from 91 nm to 430 nm, and an energy level up to sub-mJ. Such a high-energy tunable ultraviolet light source may provide promising opportunities for characterization of ultrafast phenomena, and also an important driving source for the generation of high-energy attosecond pulses.

Characterizing ultrashort pulses with photon energies above 1.12 eV based on transient absorption in silicon thin films

Abstract Frequency-resolved optical switching (FROSt) is a phase-matching-free characterization technique for ultrashort pulses based on transient absorption in semiconductors. So far, this technique has been limited to characterizing pulses with photon energies smaller than the bandgap of the semiconductors used. In this work, we extend the method to characterize pulses of photon energy greater than the bandgap of the semiconductor used for characterization. We demonstrate this by characterizing ultrashort visible pulses and supercontinuum (SC) using silicon (Si) thin films deposited on a sapphire substrate. We also demonstrate that visible light sources up to a repetition rate of 250 kHz can be characterized using these samples. Therefore, this study highlights the potential of FROSt as a suitable technique for the temporal characterization of weak visible to infrared (IR) pulses, including high harmonics generated in solids.

Optical transmittance and pulse shape discrimination of polystyrene/poly(methyl methacrylate)-based plastic scintillators

Optical transmittance and pulse shape discrimination of polystyrene/poly(methyl methacrylate)-based plastic scintillators

Fabrication of hafnium-based nanoparticles and nanostructures using picosecond laser ablation

This work presents a unique and straightforward method to synthesise hafnium oxide (HfO2) and hafnium carbide (HfC) nanoparticles (NPs) and to fabricate hafnium nanostructures (NSs) on a Hf surface. Ultrafast picosecond laser ablation of the Hf metal target was performed in three different liquid media, namely, deionised water (DW), toluene, and anisole, to fabricate HfO2 and HfC NPs along with Hf NSs. Spherical HfO2 NPs and nanofibres were formed when Hf was ablated in DW. Hf ablated in toluene and anisole demonstrated the formation of core–shell NPs of HfC with a graphitic shell. All NPs exhibited novel optical reflectance properties. Reflectance measurements revealed that the fabricated NPs had a very high and broad optical absorption throughout the UV–vis–NIR range. The NPs synthesised in toluene exhibited the best absorption. The successful fabrication of Hf NSs with the formation of laser-induced periodic surface structures (LIPSS) with low spatial frequency (LSFL) and high spatial frequency (HSFL) orthogonal to each other was also demonstrated. The LSFL and HSFL both exhibited quasi-periodicity. This work presents a simple way to fabricate HfO2 and HfC NPs and provides insight into their morphological and optical characteristics paving way for their applications in future.

Exploring the dynamics of H atom guested endofullerene embedded in plasma with ultrashort chirped laser pulses

In this work, the effects of an ultrashort laser pulse on the excitation and ionization dynamics of a hydrogen guested endofullerene system embedded in a quantum plasma environment under spherical encompassment are investigated. The interaction of the plasma environment is considered within the more general exponential cosine screened Coulomb (MGECSC) potential model, and the excitation and ionization dynamics are analyzed through plasma screening parameters. For endohedral confinement, the relevant model that aligns with experimental data and is most suitable for static endohedral encapsulation is the Woods–Saxon potential model. By considering different numerical ranges of the parameters in this model, the effects of various forms of fullerenes are thoroughly explained through the analysis of confinement depth, spherical shell thickness, inner radius and the smoothing parameters. The effects of the characteristic properties of the laser pulse, such as its intensity and frequency, on the probability dynamics are also discussed. All parameters and their respective ranges are important for optimizing system performance. Additionally, the alternatives of all parameters related to the plasma-embedded endofullerene system for probability dynamics are considered. In this context, the findings cause new ideas in the controlled excitation and ionization processes of endofullerene systems embedded in a quantum plasma environment and provide a significant foundation for future experimental studies.

Advanced optimized detection scheme based on a neural network in a bandwidth-limited IM/DD system.

In this Letter, we propose a high-performance optimized detection scheme based on a neural network (NN) in a receiver digital signal processing (DSP) for bandwidth-limited intensity modulation and direct detection (IM/DD) transmission systems. The NN-based optimized detection scheme consists of two components, an NN-based lookup table (NN-LUT) and an NN-based log-maximum a posteriori estimation with a fixed number of surviving state (NN-MAP) decoder. The NN-LUT provides more accurate and sufficient a priori information (PI) to the decoder than the conventional filter-form PI without increasing computational complexity. The NN-MAP decoder can be optimized for detection using the NN-form PI. The proposed scheme is implemented in a 122-Gbps IM/DD optical 4-level pulse amplitude modulation (PAM-4) system with a 3.7-GHz 3-dB bandwidth. The results demonstrate that the proposed NN-MAP scheme can enhance receiver sensitivity by 1.4 dB under the same complexity compared to conventional optimized detection, achieving a 20% forward error correction threshold. To the best of our knowledge, this represents the inaugural instance of NN being employed for optimized detection.

The Study on the Propagation of a Driving Laser Through Gas Target Using a Neural Network: Interaction of Intense Laser with Atoms

High-order harmonic generation is one of the ways to generate attosecond ultra-short pulses. In order to accurately simulate the high-order harmonic emission, it is necessary to perform fast and accurate calculations on the interaction between the atoms and strong laser fields. The accurate profile of the laser field is obtained from the propagation through the gas target. Under the conditions of longer wavelength driving lasers and higher gas densities, the calculation of the laser field becomes more challenging. In this paper, we utilize the driving laser electric field information obtained from numerically solving the three-dimensional Maxwell’s equations as data for machine learning, enabling the prediction of the propagation process of intense laser fields using an artificial neural network. It is found that the simulation based on frequency domain can improve the accuracy of electric field by two orders of magnitude compared with the simulation directly from time domain. On this basis, the feasibility of the transfer learning scheme for laser field prediction is further studied. This study lays a foundation for the rapid and accurate simulation of the interaction between intense laser and matter by using an artificial neural network scheme.

Transfer matrix model of beam propagation and optimization method for bulk multi-pass cell

Abstract Bulk multi-pass cell (MPC) is an effective technique used for spectral broadening and temporal compression in the fields of ultrafast optics. In an actual experiment, due to mode-mismatching, the beam profile changes at each pass transmitting through the medium, which will damage the optical elements and has a negative impact on the nonlinear effects. In this paper, based on the symmetry configuration of MPC and ABCD transfer matrix, we propose the ABCD transfer matrix model for beam propagation and adjusted optimization method for input beam. To verify the model, the result is compared with the theoretical value of the resonator. The beam propagation and B-integral before and after mode-matching are calculated. The results demonstrate that the mode-matching adjustment method significantly improves beam quality and nonlinear effects during transmission. This technique provides a potential tool for the design, experiment and evaluation in the generation of ultrashort pulse.

Graphene Oxide and Reduced Graphene Oxide Saturable Absorbers: Advancements in Erbium-Doped Fiber Lasers for Mode-Locking and Q-Switching

Graphene oxide (GO) and reduced graphene oxide (rGO) have emerged as robust materials in the development of SAs for erbium-doped fiber lasers (EDFLs). Their exceptional optical properties, such as broadband absorption and fast recovery times, make them ideal candidates for achieving ultrashort pulse operation in EDFLs. With its higher oxygen content, GO offers greater nonlinearity and a tunable absorption spectrum, while rGO, yielded through chemical reduction, exhibits enhanced electrical conductivity and higher saturable absorption. These properties facilitate the generation of ultrashort pulses in EDFLs, which are highly desired for various medical imaging, telecommunications, and material processing applications. This review paper comprehensively analyzes the advancements in GO and rGO SAs in the context of EDFLs for mode-locking and Q-switching applications. The performance of EDFLs utilizing GO and rGO SAs is critically evaluated, focusing on key parameters, such as modulation depth, pulse duration, repetition rate, average power, pulse energy, peak power, and signal-to-noise ratio. Additionally, this review delves into the various synthesis methods of GO and rGO thin film, highlighting their impact on the optical properties and performance of SAs. The discussion on techniques to integrate the SAs into laser cavities includes direct deposition of nanoparticles/thin-film-based SAs, tapered-fiber-based SAs, and D-shaped SAs. Furthermore, the paper explores the challenges encountered during the fabrication of ideal GO and rGO SAs, with issues related to uniformity, stability, and tunability, along with proposed solutions to address these challenges. The insights provided offer valuable guidance for future research aimed at enhancing the performance of EDFLs using GO/rGO SAs.

Neural-network enabled octave-spanning coherent diffraction imaging

Ultrafast lasers, providing the shortest pulses worldwide, have been playing a vital role in the ultrafast imaging technology. The temporal resolution has been increasing rapidly in recent years but finally reaches its limit—the pulse width approaches photoperiods, causing significant broadening of spectral bandwidth. The state-of-the-art high harmonics generation based attosecond lasers, with pulse widths reaching ∼50 attoseconds, present octave-spanning spectra. This brings a major challenge to traditional imaging methods, as they result in unbearable chromatic aberrations. To address this challenge, we propose the neural-network approach for broadband imaging and demonstrate its effectiveness empirically by facilitating rapid coherent diffractive imaging under octave-spanning supercontinuum illumination. The proposed method remains effective when deployed with three-octave-spanning spectra, supporting both continuous and comb-like profiles as indicated by simulations. Such lensless imaging method, applicable to both extreme ultraviolet and soft x-ray sources, potentially provides an approach to attosecond imaging.

Entropy-Mediated Crystallization Manipulation in Glass.

The entropy mediated temperature-structure evolution has attracted significant interest, which is used for the development of functional alloys and ceramics. But such strategy has not yet been demonstrated for development of non-metallic glasses. Herein, the successful application of the entropy engineering concept to non-metallic glass to manipulate its in situ crystallization process is demonstrated. The comparison of the entropy concept in alloys, ceramics, and non-metallic glass is discussed. As a typical example, the activation and preservation of the entropy stabilized effect of a typical niobosilicate glass system at different temperatures are studied. The relation between the micro-configurations and the entropic property is analyzed. Via the entropy engineering strategy, the crystallization of the niobosilicate glass can be manipulated. As a result, the LiNbO3 nanocrystal-in-glass (NiG) composite with high crystallinity is developed, which exhibits 8 times higher nonlinearity compared with the β-BBO crystal. The developed NiG composite is demonstrated for practical application in precise measurement of the duration and phase of ultra-short femtosecond pulse.

Rational function solutions of higher‐order dispersive cubic‐quintic nonlinear Schrödinger dynamical model and its applications in fiber optics

The study explores a series of cubic‐quintic nonlinear Schrödinger equation with higher‐order dispersive characteristics. This equation is also a fundamental equation in nonlinear physics that is used to depict the dynamics of femtosecond light pulses propagating through a medium with a nonlinearity profile characterized by a parabolic function. Symbolic computation is utilized, and the double ‐expansion technique is applied to investigate the mathematical characteristics of this equation. Novel solitons and rational function solutions in various forms of the high‐order dispersive cubic‐quintic nonlinear Schrödinger equation are derived. These solutions have applications in engineering, nonlinear physics and fiber optics, providing insights into the physical nature of wave propagation in dispersive optics media. The results obtained form a basis for understanding complex physical phenomena in the described dynamical model. The computational approach employed is demonstrated to be straightforward, versatile, potent, and effective. Additionally, the presented solutions showcase various intriguing patterns, including kink‐type periodic waves, combined bright‐dark periodic waves, multipeak solitons, and breather‐type waves. This diverse set of solutions contributes to the interpretation of the dynamical model, illustrating its complexity. Moreover, the simplicity and effectiveness of our computational technique make it applicable to solving similar models in physics and other fields of applied science.

Solitary wave solutions, bifurcation theory, and sensitivity analysis in the modified unstable nonlinear Schrödinger equation

This study examines the modified unstable nonlinear Schrödinger model, a fundamental nonlinear physical model vital for depicting optical solitary wave solutions and their evolution in dynamic fiber optics. The propagation of waves in nonlinear dispersive media is of great significance, presenting new opportunities for data processing in communication systems and generating ultrafast light pulses. Applying a wave transformation reduces the nonlinear Schrödinger equation to an ordinary differential equation, and an extended direct algebraic method is used to derive various soliton solutions. The novelty lies in using this method to uncover various soliton forms, which are then explored through graphical representations in 2D, 3D, and contour plots. The phase portraits and bifurcation theory also qualitatively assess the undisturbed planar system. Sensitivity analysis under different initial conditions indicates how the soliton solutions adapt to changes in system parameters. The outcomes confirm that the presented approach effectively evaluates soliton solutions across diverse nonlinear models, contributing to the understanding of wave propagation in slightly stable and unstable media.

Fabrication of a ferrite microcrystal using optical vortex laser induced forward transfer

Optical vortex induced forward transfer (OV-LIFT) allows the crystallization of magnetic nanoparticles enabling the fabrication of a 2-dimensional array of monocrystalline structures with a high spatial resolution. We herein investigate the optimum experimental conditions, such as the viscosity range of the donor and the energy range of the optical vortex pulse required for the crystallization of magnetic nanoparticles through a cavitation processes, including the flight dynamics of donor.