Femtosecond Laser Research Articles

Generation of terahertz radiation with a specified spectral profile from a mosaic combined organic crystal

We present a method for the generation of terahertz (THz) pulses with a high optical-to-THz conversion efficiency and a smooth spectrum. The method is based on the optical rectification of near-infrared femtosecond laser pulses with a wavelength of 1240 nm in a mosaic combined crystal, consisting of two pieces of nonlinear organic crystals, OH1 and DSTMS. The mosaic combined crystal produces a relatively smooth spectrum in the 0.3–4.5 THz range with no pronounced absorption dips and with a maximum in a spectral amplitude at about 2 THz. The peak field strength is 9.6 MV/cm for an effective crystal diameter of 4.3 mm.

Intentional or Designed? The Impact of Stance Attribution on Cognitive Processing of Generative AI Service Failures.

With the rapid expansion of the generative AI market, conducting in-depth research on cognitive conflicts in human-computer interaction is crucial for optimizing user experience and improving the quality of interactions with AI systems. However, existing studies insufficiently explore the role of user cognitive conflicts and the explanation of stance attribution in the design of human-computer interactions. This research, grounded in mental models theory and employing an improved version of the oddball paradigm, utilizes Event-Related Spectral Perturbations (ERSP) and functional connectivity analysis to reveal how task types and stance attribution explanations in generative AI influence users' unconscious cognitive processing mechanisms during service failures. The results indicate that under design stance explanations, the ERSP and Phase Locking Value (PLV) in the theta frequency band were significantly lower for emotional task failures than mechanical task failures. In the case of emotional task failures, the ERSP and PLV in the theta frequency band induced by intentional stance explanations were significantly higher than those induced by design stance explanations. This study found that stance attribution explanations profoundly affect users' mental models of AI, which determine their responses to service failure.

Dual-Function Femtosecond Laser: β-TCP Structuring and AgNP Synthesis via Photoreduction with Azorean Green Tea for Enhanced Osteointegration and Antibacterial Properties.

β-Tricalcium phosphate (β-TCP) is a well-established biomaterial for bone regeneration, highly regarded for its biocompatibility and osteoconductivity. However, its clinical efficacy is often compromised by susceptibility to bacterial infections. In this study, we address this limitation by integrating femtosecond (fs)-laser processing with the concurrent synthesis of silver nanoparticles (AgNPs) mediated by Azorean green tea leaf extract (GTLE), which is known for its rich antioxidant and anti-inflammatory properties. The fs laser was employed to modify the surface of β-TCP scaffolds by varying scanning velocities, fluences, and patterns. The resulting patterns, formed at lower scanning velocities, display organized nanostructures, along with enhanced roughness and wettability, as characterized by Scanning Electron Microscopy (SEM), optical profilometry, and contact angle measurements. Concurrently, the femtosecond laser facilitated the photoreduction of silver ions in the presence of GTLE, enabling the efficient synthesis of small, spherical AgNPs, as confirmed by UV-vis spectroscopy, Transmission Electron Microscopy (TEM), and Fourier Transform Infrared Spectroscopy (FTIR). The resulting AgNP-embedded β-TCP scaffolds exhibited a significantly improved cell viability and elongation of human bone marrow mesenchymal stem cells (hBM-MSCs), alongside significant antibacterial activity against Staphylococcus aureus (S. aureus). This study underscores the transformative potential of combining femtosecond laser surface modification with GTLE-mediated AgNP synthesis, presenting a novel and effective strategy for enhancing the performance of β-TCP scaffolds in bone-tissue engineering.

Glass catfish inspired subaquatic abrasion-resistant anti-fouling window fabricated by femtosecond laser electrodeposition

Abstract Transparent materials utilized as underwater optical windows are highly vulnerable to various forms of pollution or abrasion due to their intrinsic hydrophilic properties. This susceptibility is particularly pronounced in underwater environments where pollutants can impede the operation of these optical devices, significantly degrading or even compromising their optical properties. The glass catfish, known for its remarkable transparency in water, maintains surface cleanliness and clarity despite exposure to contaminants, impurities abrasion, and hydraulic pressure. Inspired by the glass catfish's natural attributes, this study introduces a new solution named subaquatic abrasion-resistant and anti-fouling window (SAAW). Utilizing femtosecond laser ablation and electrodeposition, the SAAW is engineered by embedding fine metal bones structures into a transparent substrate and anti-fouling sliding layer, akin to the sturdy bones among catfish’s body. This approach significantly bolsters the window's abrasion resistance and anti-fouling performance while maintaining high light transmittance. The sliding layer on the SAAW’s surface remarkably reduce the friction of various liquid, which is the reason that SAAW owns great anti-fouling property. The SAAW demonstrates outstanding optical clarity even after enduring hundreds of sandpaper abrasion, attributed to the fine metal bone structures bearing all external forces and protect the sliding layer of SAAW. Furthermore, it exhibits exceptional resistance to biological adhesion and underwater pressure. In a green algae environment, the window remains clean with minimal change in transmittance over one month. Moreover, it retains its wettability and anti-fouling properties when subjected to a depth of 30 meters of underwater pressure for 30 days. Hence, the SAAW prepared by femtosecond laser ablation and electrodeposition presents a promising strategy for developing stable optical windows in liquid environments.

Frequency-resolved measurement of two-color air plasma terahertz emission

We investigated the far-field terahertz beam profile generated from an air plasma induced by two-color femtosecond laser pulses. Under our experimental conditions (filament length shorter than the dephasing length between the two-color pulses), using electro-optic sampling in both ZnTe (0.2-2.2 THz) and GaP (0.4-6.8 THz) crystals, and ultra-broadband ABCD technique (1-17.5 THz), we determined that the THz beam exhibits a unimodal beam pattern below 4 THz and a conical one above 6 THz. This experimental finding is consistent with theoretical studies based on the unidirectional pulse propagation equation, which predict the transition of THz emission from a flat-top profile to a conical one due to the destructive interference of THz waves emitted from the plasma filament. Our results also underscore the importance of accounting for experimental artifacts, such as photo-excited losses in silicon resulting in on-axis THz absorption along with the influence of drilled mirrors, in characterizing the complex spatial and frequency-dependent behavior of two-color plasma-induced terahertz emission.

Cystoid macular edema after low-energy femtosecond-assisted cataract surgery.

This study aimed to evaluate the occurrence of pseudophakic cystoid macular edema (PCME) post low-energy femtosecond laser-assisted cataract surgery (FLACS) in a high-volume surgical setting. The medical records of 242 FLACS patients were retrospectively reviewed. The central subfield macular thickness (CSMT) was measured via optical coherence tomography (OCT) before and 4-6weeks after surgery, and the results were compared for PCME detection. Macular edema was defined as a 10% increase in CSMT, a new onset of intraretinal fluid, or a decrease in visual acuity (VA). VA development in PCME patients was reviewed at 2-3months and 6months. The median patient age was 72years (49-92years). Among 242 eyes, seven eyes (2.89%) developed PCME. The median preoperative CSMT in these eyes was 255μm (minimum 231μm, maximum 326μm), whereas the median CSMT at 4-6weeks after surgery was 317μm (minimum 255μm, maximum 463μm). 4- to 6-week postoperative visual acuity decreased in comparison with 1-week postoperative values in three eyes of two patients, remained stable in two patients, and improved in one patient, whereas one patient did not return for his 1-week appointment but improved from 0.4 to 0.2 logMAR 2.5months postoperatively. By 3-6months, all eyes with PCME had gained visual acuity in comparison with their preoperative values. None of the PCME patients had diabetes or used prostaglandin analogues. Three patients were receiving anticoagulation medication. A 2.89% incidence of PCME after low-energy FLACS matched published standard phacoemulsification rates. In our series of uncomplicated cases, PCME caused only a transient postoperative decrease in visual acuity. What is known Increased prostaglandin levels have been detected in the aqueous humour of cataract patients after femtosecond laser application. Prostaglandins are mediators of inflammation. Femtosecond lasers come in low energy and high energy variants. There is contrasting evidence of increased incidence of PCME after femtosecond laser assisted cataract surgery (FLACS) What is new The incidence of PCME after low-energy FLACS in our high volume surgical setting is 2.89% Low-energy FLACS does not seem to have a causative effect on PCME.

Ultrasensitive strain sensor of dual parallel FPIs utilizing femtosecond laser processing and Vernier effect

This study proposes and develops a highly sensitive fiber optic strain sensor utilizing femtosecond laser processing and the Vernier effect. The sensor features two parallel Fabry-Pérot interferometers (FPI), with each FPI consisting of an air bubble and a tapered fiber optic cascade. FPI 2 serves as the sensing cavity, while FPI 1 acts as the reference cavity. The strain sensing capability of a single FPI 2 is measured at 6.67 p.m./με, while the parallel configuration of FPI 2 with FPI 1 achieves a sensitivity of 68.3 p.m./με, resulting in an amplification of 10.24. Furthermore, the proposed sensor demonstrates excellent repeatability, low-temperature cross-sensitivity, simple fabrication, and cost-effectiveness, making it a promising tool for strain measurement applications.

Superhydrophobic anti-icing/de-icing SR/CNTs surfaces hot-embossed with femtosecond laser machined mold

Anti-icing/de-icing surfaces are dispensable in reducing accidents and economic losses caused by accumulated ice and frost on outdoor facilities in cold and humid environments. In this work, a low-cost, reproducible process was presented to fabricate superhydrophobic anti-icing/de-icing surfaces. For this purpose, concave micro-structures were firstly created on the mold steel surface using femtosecond laser machining, and then the corresponding convex micro-structures were prepared on the surface of a silicone rubber (SR) composite mixed with multi-wall carbon nanotubes (CNTs) using hot embossing. Thanks to the presence of unique micro-scale peak-ridge structures, the as-prepared SR/CNTs surfaces possess superhydrophobic characteristic. Long icing delay time was endowed by the large volume of air pockets inside micro-structures, and the icing time was significantly extended from 67 to 416 s, which is ascribed to the greatly inhibited heat transfer between the droplet and the surface. Moreover, the SR/CNTs surface had excellent photothermal conversion capability with the presence of photothermal particles CNTs, and the temperature of the surface increased rapidly from 21.6 to 86.1 °C in 5 min under a standard sunlight irradiation. In addition, the SR/CNTs surface also possesses superior photothermal defrosting and de-icing performance as the ice and frost on the surface can be quickly melted under a standard sunlight intensity irradiation and the de-icing rate is up to 0.92 mg/s. Finally, the good reusability of the mold steel and the resistance to mechanical wear and chemical corrosion of superhydrophobic SR/CNTs surface found in this work make the proposed approach promising for industrial mass production and anti-icing/de-icing applications in harsh environments.

Grating compressor optimization aiming at maximum focal intensity of femtosecond laser pulses

It is shown that the optimal geometry of a Treacy compressor is the full-aperture compressor, in which the beam size at the first diffraction grating is equal to its length. Despite the energy losses and greater size of the focal spot, such a compressor provides considerably higher (by 1.5–2 times) focal intensity than an energy lossless compressor. Decreasing the density of grooves from 1200–1400/mm to about 1000/mm also increases the focal intensity by tens of percent. The constructed theory is generalized to the full-aperture two-grating compressor, which is the best design due to the angle of incidence on the first grating being smaller than the Littrow angle. Two gratings with a length of 138 cm allow obtaining an intensity of 4.09 × 1024W/cm2 and 5.01 × 1024W/cm2 in the focus of F/2 parabola for the projects XCELS and SEL-100PW, reaching the 139 PW and 174 PW power.

Review of high-precision femtosecond laser materials processing for fabricating microstructures: Effects of laser parameters on processing quality, ablation efficiency, and microhole shape

Femtosecond lasers are promising tools for achieving high-precision processing of thin materials without causing any thermal surface damage and bulk distortion. However, thermal damage can occur even with ultrashort laser pulses. This is because of high electron penetration depth and heat accumulation at high fluence and high repetition rate. Nanoparticle redeposition can be dramatically altered with variation in repetition rate. The symmetry of microholes and ablation efficiency vary with laser polarization. The laser wavelength affects the ablation efficiency and surface roughness. Therefore, understanding these laser–matter interactions that depend on the laser parameters is essential for high-precision laser processing. This article reviews laser–matter interactions in the 64FeNi alloy, as well as analytical models for designing the desired hole size and taper angles. This can help establish strategies for creating various high-precision microstructures using femtosecond lasers.

Mode conversion in graded-index few-mode fiber via hollow cylindrical long-period fiber gratings.

We demonstrate the realization of mode conversion using hollow cylindrical long-period fiber gratings (LPFGs) inscribed in graded-index few-mode fibers (FMFs) by a femtosecond laser. By precisely shaping the refractive index modulation into a hollow cylindrical structure, we enable efficient coupling from the fundamental mode to high-order modes (LP11, LP02, LP21, LP12, LP31, LP03, and LP22 modes). The method achieves high-efficiency and low-loss mode conversion across various mode groups, marking a first for graded-index FMFs with different grating periods. The influence of the hollow cylindrical LPFG radius on mode conversion efficiency and spectral characteristics is thoroughly investigated. Additionally, incorporating a linear polarizer allowed for distinguishing between LP modes within a mode group, enhancing the tunability of the mode converter. These mode conversion devices have significant potential for applications in mode gain equalization and mode scrambling for mode division multiplexing (MDM) systems.

Transient dielectric function dynamics driven by coherent phonons in Bismuth crystal

In this study, we investigate the transient dynamics of the dielectric function in photoexcited bismuth using a double-probe experiment. We demonstrate how the A1g coherent phonon mode influences both the real and imaginary components of the dielectric function, extending to a quasi-steady state. Additionally, we compare this transient behavior to ellipsometry measurements taken at varying crystal temperatures under equilibrium conditions. Our findings reveal that bismuth crystals, when excited by femtosecond laser pulses, enter a quasi-stationary state that follows the coherent phonon oscillations. This transient state is distinct from a liquid phase or a simple thermalized state. We interpret this quasi-steady state as a heated, excited state where electron-hole recombination has not yet occurred.

KeraRing Implantation with Femtosecond Laser in Keratoconus Treatment at Different Stages

Objective: To investigate the reliability and effectiveness of femtosecond laser-assisted KeraRing (Mediphacos, Belo Horizonte, Brazil) implantation in treating keratoconus. Methods: Intrastromal corneal ring segments (KeraRing, Mediphacos, Brazil) were implanted in 15 eyes of 14 patients unable to tolerate contact lenses. Femtosecond laser (Intralase, 60 Hz) was used for corneal tunnel creation. Based on the distribution of ectatic area on the cornea, dual segments were implanted in 9 eyes, while single segments were implanted in 6 eyes. Preoperative and postoperative uncorrected visual acuity (UCVA), best-corrected visual acuity (BCVA), objective and subjective refraction, and topographic corneal curvature (K) values of the cases were compared using the Wilcoxon test. Results: The mean age of patients was 25±7.46 (range: 15-39) years, with a mean postoperative follow-up period of 10.8±7.37 (range: 1-24 months). BCVA improved in all eyes during the follow-up period, increasing from a mean of 0.12±0.1 to 0.38±0.24 (p

Femtosecond Laser Assisted Chemical Ionization Mass Spectrometry: Toward Sub-ppq Detection Limits for Organic Molecules.

Ultrasensitive analysis of organic molecules is crucial for various fundamental research and applications. State-of-the-art techniques for this purpose can achieve detection limits of several hundred ppq (parts-per-quadrillion), while a higher sensitivity is pursued constantly. To achieve this goal, we develop femtosecond laser assisted chemical ionization for mass spectrometry. This technique combines the advantages of femtosecond laser ionization and chemical ionization, either of which enables subppt (parts-per-trillion) mass spectrometry analysis of organic molecules. The results demonstrate that the developed ionization technique, when employed in mass spectrometry, can surpass femtosecond laser ionization by more than 3 orders of magnitude in sensitivity, while still maintaining good versatility and the ability to work under ambient conditions. This work paves the way for subppq analysis of organic molecules in gas phase and even in ambient environments, which can open up new research fields for trace substance analysis in atmospheric environment, clinical diagnosis, biomedical studies, etc.

Figure-8 mode-locked Yb-doped fibre oscillator with a nonlinear amplifying loop mirror and power amplification

Abstract We report an all-polarization-maintaining Yb-doped fibre (YDF) femtosecond (fs) laser source at ∼1 µm comprising a figure-8 mode-locked oscillator and two YDF amplifier stages. By employing a nonlinear amplifying loop mirror, we could achieve dissipative soliton mode-locking with an output of 24.6 mW at a repetition rate of 6.2 MHz. In our experiments, a Raman signal of 16% in the pulse energy helps to obtain stable mode-locked operation in the figure-8 oscillator. The dissipative soliton pulse from the oscillator is stretched by a chirped fibre Bragg grating and is amplified by a two-stage YDF amplifier. After compression, the YDF master-oscillator power-amplifier system yields fs pulses with a pulse energy of 2.3 µJ and a pulse duration of 274.6 fs at a repetition rate of 0.52 MHz. The output pulse energy is limited by the third-order nonlinearity of the fibre amplifier chain. Prospects for further pulse energy scaling are discussed.

Highly Nonlinear Light-Nucleus Interaction.

The interaction between light and atomic nuclei is typically weak and confined within the linear and perturbative regime, which limits the achievable nuclear excitation probability and impedes potential applications such as nuclear optical clocks, nuclear lasers, and nuclear energy storage. In this Letter we show that the interaction between hydrogenlike thorium-229 ions (^{229}Th^{89+}) and contemporary intense lasers propels light-nucleus interaction into a highly nonlinear and nonperturbative regime. This interaction unlocks remarkably efficient nuclear excitation: over 10% of the ^{229}Th nuclei can be excited to the isomeric state by a single femtosecond laser pulse. Moreover, the laser-driven ^{229}Th^{89+} ions radiate multiple wavelengths of light that are high-order harmonics of the laser frequency, akin to the high harmonic generation process from laser-driven atoms but with distinct characteristics. These results pioneer a new frontier for exploring light-matter interaction, furnish a powerful method for efficient control over atomic nuclei, and pave a new way of nuclear coherent light emission.

Laser Control of Specular and Diffuse Reflectance of Thin Aluminum Film-Isolator-Metal Structures for Anti-Counterfeiting and Plasmonic Color Applications

Plasmonic structural color originates from the scattering and absorption of visible light by metallic nanostructures. Stacks consisting of thin, disordered semicontinuous metal films are attractive plasmonic color media, as they can be mass-produced using industry-proven physical vapor deposition techniques. These films are comprised of random nano-island structures of various sizes and shapes resonating at different wavelengths. When irradiated with short-pulse lasers, the nanostructures are locally restructured, and their optical response is altered in a spectrally selective manner. Therefore, various colors are obtained. We demonstrate the generation of structural plasmonic colors through femtosecond laser modification of a thin aluminum film–isolator–metal mirror (TAFIM) structure. Laser-induced structuring of TAFIM’s top aluminum film significantly alters the sample’s specular and diffuse reflectance depending on the fluence value and the number of times a region is scanned. A “negative image” effect is possible, where a dark field observation mode image is a negative of a bright field mode image. This effect is visible using an optical microscope, the naked eye, and a digital camera. The use of self-passivating aluminum results in a long-lasting, non-fading coloration effect. The reported technique could be used in anti-counterfeiting and security applications, as well as in plasmonic color printing and macroscopic and microscopic marking for personalized fine arts and aesthetic products such as jewelry.

Superhydrophobicity induced antibacterial adhesion and durability of laser bidirectionally-textured zirconia surfaces with different inclination angles

Femtosecond laser bidirectional texturing with different laser energy densities and inclination angles was performed on zirconia (ZrO2) surfaces. The laser bidirectional texturing models under different inclination angles were constructed to predict the water contact angles (WCAs), and the superhydrophobicity induced antibacterial adhesion mechanism was analyzed. The results revealed that under the combined effect of the optimal laser energy density (7.0 J/cm2) and inclination angle (90°), the laser-textured ZrO2 surface could obtain a micro-nano dual-scale structure composed of periodic regular micro-cones and nanoparticles. This structure not only promoted the adsorption of carbon-containing hydrophobic groups from the stearic acid and the air to form a superhydrophobic surface with excellent chemical/mechanical durability and anti-fouling/self-cleaning abilities, but also minimized the contact area with the bacterial suspension (solid/liquid contact area ratio of 6 %) to enhance antibacterial ability. The optimal stearic acid-modified laser-textured ZrO2 surface exhibited an extremely low adhesive force (Fadhesive=6.69 μN) to the bacterial suspension, and thus achieved the highest antibacterial ratio of 85.3 %. This study is expected to enrich the application of ZrO2 ceramics in the medical field.

Micro-welding of non-optical contact sapphire/metal using Burst femtosecond laser

Recently, welding of dissimilar material, such as transparent material to metal material, is widely used in aerospace, microelectronic packaging, medicine, etc. Ultrafast laser welding provides a new method for high-performance dissimilar material welding, in which the non-optical contact dissimilar material welding has become a new research hot spot. In this paper, we report the effective welding of Fe-36Ni with roughness as high as 1.1 µm to sapphire, which, to our knowledge, is the roughest dissimilar material welding achieved in ultrafast laser. A 238 fs, 1035 nm, 200 kHz Burst mode femtosecond laser (4 sub-pulses with a total energy of 132.5μJ) yields a smooth and uniform interface with a welding strength up to 11 MPa. In this context, the welding performance and mechanism of Burst mode and single pulse mode ultrafast laser are systematically analyzed. The relevant research results are expected to provide theoretical guidance for the welding of high-strength and high-stability dissimilar materials.

3D Printing of Hierarchical Structures Made of Inorganic Silicon-Rich Glass Featuring Self-Forming Nanogratings.

Hierarchical structures are abundant in nature, such as in the superhydrophobic surfaces of lotus leaves and the structural coloration of butterfly wings. They consist of ordered features across multiple size scales, and their advantageous properties have attracted enormous interest in wide-ranging fields including energy storage, nanofluidics, and nanophotonics. Femtosecond lasers, which are capable of inducing various material modifications, have shown promise for manufacturing tailored hierarchical structures. However, existing methods, such as multiphoton lithography and three-dimensional (3D) printing using nanoparticle-filled inks, typically involve polymers and suffer from high process complexity. Here, we demonstrate the 3D printing of hierarchical structures in inorganic silicon-rich glass featuring self-forming nanogratings. This approach takes advantage of our finding that femtosecond laser pulses can induce simultaneous multiphoton cross-linking and self-formation of nanogratings in hydrogen silsesquioxane. The 3D printing process combines the 3D patterning capability of multiphoton lithography and the efficient generation of periodic structures by the self-formation of nanogratings. We 3D-printed micro-supercapacitors with large surface areas and a high areal capacitance of 1 mF/cm2 at an ultrahigh scan rate of 50 V/s, thereby demonstrating the utility of our 3D printing approach for device applications in emerging fields such as energy storage.