Ultrafast Research Articles

Prospects of Application of Ultrashort Pulses Laser Sources in Medicine

Prospects of Application of Ultrashort Pulses Laser Sources in Medicine

Ultrafast Laser Inkless Full‐Color Printing on Flexible and Thermolabile Substrates

AbstractLaser‐induced structural color technology holds great promise for the mass‐production of structural colors of wide color gamut, high stability, and low cost. However, its application to flexible and thermolabile substrates (such as plastics and paper) is currently hindered by the reliance on high‐temperature processes or metal substrates. Here, an ultrafast laser inkless printing technology is proposed to address this challenge. By optimizing the magnetron sputtering process with a pre‐sintered TiN target, the reflective TiN layer can be prepared at room temperature, on which coated an absorptive TiN layer to produce the TiN hybrid film. Under laser irradiation, this hybrid film is transformed into an “oxide‐absorptive‐reflective” tri‐layer film structure when the upper absorptive TiN layer is oxidized. In this case, the thicknesses of the oxide and absorptive layers can be adjusted by modifying the total accumulated laser fluence. This enables the tuning of the double‐absorption wavelength, thereby obtaining basic structural colors for printing. The high durability of the obtained structural colors is verified through various aging tests. Notably, this technology is successfully applied to flexible and thermolabile substrates, including plastics and cardboard paper, which may further promote the practical application of laser‐induced structural color technology.

Influence of band occupation on electron-phonon coupling in gold

Electron-phonon coupling is a fundamental process that governs the energy relaxation dynamics of solids excited by ultrafast laser pulses. It has been found to strongly depend on the electron temperature as well as on nonequilibrium effects. Recently, the effect of occupational nonequilibrium in noble metals, which outlasts the fully kinetic stage, has come into increased focus. In this study, we investigate the influence of nonequilibrium density distributions in gold on the electron-phonon coupling. We find a large effect on the coupling parameter which describes the energy exchange between electrons and phonons. Our results challenge the conventional view that electron temperature alone is a sufficient predictor of electron-phonon coupling.

In-Volume Glass Modification Using a Femtosecond Laser: Comparison Between Repetitive Single-Pulse, MHz Burst, and GHz Burst Regimes.

In this study, we report, for the first time, to the best of our knowledge, on in-volume glass modifications produced by GHz bursts of femtosecond pulses. We compare three distinct methods of energy deposition in glass, i.e., the single-pulse, MHz burst, and GHz burst regimes, and evaluate the resulting modifications. Specifically, we investigate in-volume modifications produced by each regime under varying parameters such as the pulse/burst energy, the scanning velocity, and the number of pulses in the burst, with the aim of establishing welding process windows for both sodalime and fused silica.

Nonlinear Raman-Nath diffraction in submicron-thick periodically poled lithium niobate thin film

Nonlinear Raman-Nath diffraction (NRND) is a unique diffraction pattern formed when a high-intensity laser interacts with a nonlinear microstructure bulky medium relying only on the transverse phase matching condition. Here, we report on the first experimental observation of NRND in a submicron-thick periodically poled lithium niobate thin film (PPLNTF) by geometric reflection pumped via a near-infrared femtosecond pulse laser. We further observe the evolution of the diffracted signals after broadening of the pump laser via a fused silica plate. We systematically analyze the spectral properties of multi-order second harmonic generation (SHG) diffracted signals exhibiting asymmetric distributions and explicitly clarify their phase matching conditions, simultaneously considering the impacts of the incident pump wavelength, the sample poling period, and the incident angle on the properties of the angular distribution diffracted beams. The realization of NRND phenomena with appreciable on-chip efficiency at a submicron interaction length is mainly attributed to the significant contribution of domain walls to enhance the nonlinear effects along with the modulation of second-order nonlinear susceptibilities χ(2). This NRND scheme provides a high-resolution, non-destructive on-chip microstructure diagnostic method, and even has the potential to develop novel on-chip integrated optoelectronic devices for applications such as precision metrology, biosensing, and spectral analysis.

Sub-ablation-threshold light-induced modification of thin ruthenium layers detected using optical reflectance

In semiconductor device manufacturing, wafer materials may be exposed to intense light sources by optical metrology tools. The desired light fluence often needs to be maximized to levels just below the optical damage threshold of materials deposited on the wafer, such as ruthenium. We, therefore, investigate light-induced permanent structural changes to thin Ru films after exposure to single 400nm wavelength femtosecond pulses in the fluence regime before catastrophic damage. For fluences below that where full-ablation occurs, small optical increases in the reflectivity of up to 4% are observed in the aftermath with a weak probe beam. In this fluence regime, dark-field, scanning electron, and atomic force microscopy images reveal morphological changes such as top-level ablation, where only the top part of the ruthenium layer is ablated whereas the lower part still remains on the substrate, and nanovolcano formation. However, neither top-level ablation nor nanovolcano formation is responsible for the reflection increase. Instead, Electron Backscatter Diffraction reveals that in this low fluence regime where reflectivity increases, Ru grains melt and resolidify into larger grains, which is likely responsible for the observed reflectivity increases. This result is reminiscent of our earlier work on aluminum layers and it suggests that there may be more metals that display this behavior.

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.

Quantum diffusion of pure-quartic solitons in a laser cavity.

Ultrashort pulses experience random quantum motion as they propagate through a mode-locked laser cavity, a phenomenon that inevitably affects the recently introduced pure-quartic solitons. Investigating this process is essential, as quantum-limited noise establishes fundamental performance limits for their application. To date, studies on quantum diffusion and the resulting timing jitter of these solitons remain sparse. In this work, we examine the quantum motion of pure-quartic solitons by injecting amplified spontaneous emission noise into the gain medium, measuring their relative position, and performing analyses using Allan variance and timing jitter power spectral density. Our simulation findings reveal that long-range interactions due to pulse tailing can lead to high-frequency noise, which can be effectively filtered out. In the near-zero dispersion regime, we achieve quantum-limited noise levels significantly less than 1 fs. Good agreement is observed between the analytical model and the simulation results. This work provides insights into the noise levels and mechanisms of pure-quartic solitons and can be extended to more complex scenarios, such as pure-quartic Kerr solitons, and studies on the dynamics of higher-order dispersion solitons.

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.