Threshold Fluence Research Articles

Carbon nanotube paper with different polymer composition for laser ablation propulsion.

Laser ablation propulsion is an important micro-propulsion system for microsatellites. Polymers with carbon added and carbon-based nanomaterial have been demonstrated as propellants with high impulse coupling coefficient (Cm). Among them, the carbon nanotube film exhibits a low ablation threshold fluence of 25 mJ/cm2, which shows its potential for propulsion under low laser fluence. In this study, we investigate carbon nanotube papers (CNTPs) as propellants for laser ablation propulsion. Here four types of CNTPs have been included: S-CNTP (composed of single-walled carbon nanotubes, SWCNTs) and M-CNTP1 (composed of multi-walled carbon nanotubes, MWCNTs) and polymer composited CNTP of M-CNTP2 (30% MWCNTs) and M-CNTP3 (8% MWCNTs). SEM shows that S-CNTP and M-CNTP1 feature a network structure of carbon nanotubes while M-CNTP2 and M-CNTP3 have polymer-filled solid surfaces. Notably, M-CNTP3 exhibited a high Cm of 58.1 µN/W under a laser fluence of 1.09 J/cm2. Time-resolved plasma spectroscopy revealed a reduced C2 Swan band emission for M-CNTP3. Thermogravimetric analysis (TGA-DSC) further showed that the polymer's decomposition temperature contributes to the enhanced Cm value for M-CNTP3. These findings suggest that the performance of CNTP-based composite materials as propellants is closely related to the type and quantity of carbon nanotubes, providing an alternative propellant for microsatellite propulsion under low laser fluence conditions.

Lasing in Crystals Based on 7‐Azaindole‐Phenylhydrazone Organoboron Compounds

AbstractThe development of efficient organic solid‐state lasers requires an in‐depth understanding between chemical structure, intermolecular interactions in the crystal phase, and optical and electronic properties. This study highlights these closed dependencies in novel 7‐azaindole phenylhydrazone derivatives and their corresponding boron complexes, by deploying a combined approach of experimental techniques and theoretical calculations (Density Functional Theory and Time‐Dependent Density Functional Theory) in the solvated and solid‐state phase. Notably, it is found that when these compounds, which are weakly emissive in solution, are processed into crystalline microfibers, they experience a sharp emission enhancement and exhibit laser action at low pump fluence thresholds. This is achieved by partially inhibiting structural relaxation, which drives non‐radiative decay, a critical factor for effective lasing, highlighting the potential of these materials for future optoelectronic applications.

Nanobubble nucleation by pulsed laser illumination of colloidal gold nanoparticles.

This study expands upon a technique our team previously developed for generating nanobubbles on demand with a collimated pulsed laser beam. This work highlights how the controlled addition of gold nanoparticles enhances nanobubble generation efficiency in water, even at laser intensities well below the threshold for multiphoton ionization. Specifically, we investigated the influence of nanoparticles of three distinct sizes on the laser fluence threshold for bubble nucleation and the lifetime of the resultant nanobubbles. Our findings confirm that nanoparticles with a diameter of 60nm exhibit the greatest nucleation efficiency, achieving nearly 45% at a fluence threshold of around [Formula: see text]. Interestingly, nanoparticle size did not impact the nanobubble lifetime.

Laser melting, evaporation, and fragmentation of nanoparticles: Experiments, modeling, and applications

This review examines the processes of laser heating, melting, evaporation, fragmentation, and breakdown of metal nanoparticles, as well as the dependences and values of the threshold laser parameters that initiate these processes. Literature results are analyzed from experimental studies of these processes with gold, silver, and other nanoparticles, including laser surface melting and evaporation of nanoparticles and Coulomb fragmentation of nanoparticles by ultrashort laser pulses. A theoretical model and description of the thermal mechanisms of mentioned processes with metal (solid) nanoparticles in a liquid (solid) medium, initiated by the action of laser pulses with the threshold fluences, are presented. Comparison of the obtained results with experimental data confirms the accuracy of the model and makes it possible to use them to evaluate the parameters of laser thermal processing of nanoparticles. Applications of the processes include the laser melting, reshaping, and fragmentation of nanoparticles, the formation of nanostructures and nanonetworks, the laser processing of nanoparticles located on substrates, and their cladding on surfaces in various laser nanotechnologies. The use of laser ignition, combustion, and incandescence of nanoparticles is discussed, as is the use of nanoparticle-triggered laser breakdown for spectroscopy. These laser processes are used in photothermal nanotechnologies, nanoenergy, laser processing of nanoparticles, nonlinear optical devices, high-temperature material science, etc. In general, this review presents a modern picture of the state of laser technology and high-temperature processes with nanoparticles and their applications, being focused on the latest publications with an emphasis on recent results from 2021–2024.

Multiscale modeling of short pulse laser induced amorphization of silicon

Silicon surface amorphization by short pulse laser irradiation is a phenomenon of high importance for device manufacturing and surface functionalization. To provide insights into the processes responsible for laser-induced amorphization, a multiscale computational study combining atomistic molecular dynamics simulations of nonequilibrium phase transformations with continuum-level modeling of laser-induced melting and resolidification is performed. Atomistic modeling provides the temperature dependence of the melting/solidification front velocity, predicts the conditions for the transformation of the undercooled liquid to the amorphous state, and enables the parametrization of the continuum model. Continuum modeling, performed for laser pulse durations from 30 ps to 1.5 ns, beam diameters from 5 to 70 μm, and wavelengths of 532, 355, and 1064 nm, reveals the existence of two threshold fluences for the generation and disappearance of an amorphous surface region, with the kinetically stable amorphous phase generated at fluences between the lower and upper thresholds. The existence of the two threshold fluences defines the spatial distribution of the amorphous phase within the laser spot irradiated by a pulse with a Gaussian spatial profile. Depending on the irradiation conditions, the formation of a central amorphous spot, an amorphous ring pattern, and the complete recovery of the crystalline structure are predicted in the simulations. The decrease in the pulse duration or spot diameter leads to an accelerated cooling at the crystal–liquid interface and contributes to the broadening of the range of fluences that produce the amorphous region at the center of the laser spot. The dependence of the amorphization conditions on laser fluence, pulse duration, wavelength, and spot diameter, revealed in the simulations, provides guidance for the development of new applications based on controlled, spatially resolved amorphization of the silicon surface.

Quantifying UV-induced photodamage for longitudinal live-cell imaging applications of deep-UV microscopy.

Deep-UV microscopy enables high-resolution, label-free molecular imaging by leveraging biomolecular absorption properties in the UV spectrum. Recent advances in UV-imaging hardware have renewed interest in this technique for quantitative live cell imaging applications. However, UV-induced photodamage remains a concern for longitudinal dynamic imaging studies. Here, we quantify UV phototoxicity with several cell types at notable UV wavelengths. We find that the fluence required for cell death via UV phototoxicity with continuous UV exposure varies with cell type and wavelength from ∼0.5µJ/µm2 to 2µJ/µm2, but is independent of typical illumination power/radiant flux of UV microscopy (e.g., 0.1-20 nW/µm2). We also show results from fractionation studies that reveal cell repair following UV exposure, which increases the tolerance to UV radiation by a factor of 2 or more, depending on the fractionation paradigm. Results further show that UV tolerance exceeds ANSI guidelines for maximum permissible exposure. Finally, we calculate imaging limits for a typical application of UV microscopy, such as hematology analysis. Together, this work provides UV fluence thresholds that can serve as guidelines for nondestructive, longitudinal, and dynamic deep-UV microscopy experiments.

A multi-method study of femtosecond laser modification and ablation of amorphous hydrogenated carbon coatings

We present a study on femtosecond laser treatment of amorphous hydrogen-containing carbon coatings (a-C:H). The coatings were deposited on silicon wafers by a plasma-assisted chemical vapour deposition (PA-CVD), resulting in two different types of material with distinct properties (referred to as “absorbing” and “semi-transparent” coatings in the following). The samples were laser-treated with single fs-laser pulses (800 nm center wavelength, 35 fs pulse duration) in the ablative regime. Through a multi-method approach using topometry, Raman spectroscopy, and spectroscopic imaging ellipsometry, we can identify zones and thresholds of different fluence dependent effects and have access to the local dielectric function. The two coating materials react significantly different upon laser treatment. We determined the (non-ablative) modification threshold fluence for the absorbing coating as 3.6×10-2 Jcm−2 and its ablation threshold as 0.22 Jcm−2. The semi-transparent coating does not show such a low-fluence modification but exhibits a characteristic interference-based intra-film ablation mechanism with two distinguishable ablation thresholds at 0.25 and 0.28 Jcm−2, respectively. The combination of tailored layer materials and correlative imaging spectroscopic methods delivers new insights into the behaviour of materials when treated with ultrashort-pulse laser radiation.Graphical abstract

Ultrasensitive and Stretchable Strain Sensors Based on Laser-Induced Graphene With ZnO Nanoparticles.

Laser-induced graphene (LIG) has attracted considerable attention for its use in flexible and stretchable sensors, owing to its electrical/mechanical properties and scalable fabrication processes. Although laser scanning facilitates the formation of LIG and its strain sensor, the strain-sensing sensitivity enhancement of LIG remains limited by the material's properties and structural design. In this study, we demonstrate a substantial improvement in sensitivity that was achieved by fabricating a LIG using ZnO nanoparticle (NP)-assisted photothermal enhancement. The results show that ZnO NPs selectively reduce the threshold fluence needed to convert polyimide (PI) into LIG. By transferring the LIG formed on PI to poly(dimethylsiloxane), we fabricate a stretchable strain sensor with ultrahigh sensitivity and a gauge factor of 1214 at 10% strain, which is approximately 60 times higher than the gauge factor without ZnO NPs. Using the selective graphenization properties of LIG, a flexible, dual-sided integrated sensor sheet that is equipped with flexible strain and ultraviolet (UV) sensors is demonstrated. This sheet enables simultaneous monitoring of UV intensity and joint bending angles of sports wearable devices. We validated the developed sensors by attaching them to a runner's body to monitor and simulate forefoot and heel strikes, demonstrating the sensor's ultrahigh sensitivity and long-term stability without the need for a camera. These findings highlight the potential of the proposed method for developing multifunctional sensor applications with ultrahigh sensitivity and stability.

Investigation of ablation threshold and microchannel fabrication on stainless steel using ultrafast laser

ABSTRACT Ultrafast laser processing is a highly versatile tool for creating microscale structures in many materials. The ablation and micromachining characteristics of AISI 304 stainless steel were examined using a femtosecond-pulsed laser. Parameters such as power, scanning speed, transverse overlap, scanning strategies, and shielding gases were systematically varied. The ablation threshold fluence for SS 304 was calculated as 0.16 J/cm2. Two ablation regimes were observed: the gentle regime, where smooth crater walls were formed under low pulse energy (below 20 µJ), and the strong regime, characterized by rough walls created under high pulse energy (above 20 µJ). The optimal parameters for fabricating microchannels on SS 304 surfaces comprised a laser power of 100 mW, a scanning speed of 30 mm/s, a line-to-line distance of 5 µm, and a parallel scan strategy along the major axis. This study provides a practical approach for enhancing the microchannel fabrication process on stainless steel.

A Nançay Radio Telescope study of the hyperactive repeating FRB 20220912A

Abstract The repeating fast radio burst source FRB 20220912A was remarkably active in the weeks after its discovery. Here we report 696 bursts detected with the Nançay Radio Telescope (NRT) as part of the Extragalactic Coherent Light from Astrophysical Transients (ÉCLAT) monitoring campaign. We present 68 observations, conducted from October 2022 to April 2023, with a total duration of 61 hours and an event rate peaking at $75^{+10}_{-9}$ bursts per hour above a fluence threshold of 0.59 Jy ms in the 1.2 − 1.7-GHz band. Most bursts in the sample occur towards the bottom of the observing band. They follow a bimodal wait-time distribution, with peaks at 33.4 ms and 67.0 s. We find a roughly constant dispersion measure (DM) over time (δDM ≲ 2 pc cm−3) when taking into account ‘sad-trombone’ drift, with a mean drift rate of −8.8 MHz ms−1. Nonetheless, we confirm small ∼0.3 pc cm−3 DM variations using microshot structure, while finding that microstructure is rare in our sample – despite the 16 μs time resolution of the data. The cumulative spectral energy distribution shows more high-energy bursts (Eν ≳ 1031 erg Hz−1) than would be expected from a simple power-law distribution. The burst rate per observation appears Poissonian, but the full set of observations is better modelled by a Weibull distribution, showing clustering. We discuss the various observational similarities that FRB 20220912A shares with other (hyper)active repeaters, which as a group are beginning to show a common set of phenomenological traits that provide multiple useful dimensions for their quantitative comparison and modelling.

In Silico Evaluation of Nanosecond Laser Treatment of Pigmented Lesions Based on Skin Optical Properties Using a Model of Melanosome Disruption Threshold Fluence.

This study aimed to evaluate the efficacy and safety of nanosecond laser treatment of pigmented lesions in silico using a model of melanosome disruption threshold fluence (MDTF) based on skin optical properties. Particle size analysis and scanning electron microscopy were performed to determine the threshold fluence for melanosome disruption using a nanosecond laser. By inputting the obtained threshold fluence into the MDTF model and considering the variability in skin optical properties, irradiation parameters were calculated and compared with the results from clinical studies. The threshold fluences for 532 and 755 nm nanosecond laser irradiation were determined to be 3.0 and 15.0 J/cm2, respectively. In silico analysis showed that the incident fluence for moderately pigmented skin should be 1.2 times that for lightly pigmented skin, whereas it should be 50% lower than that for lightly pigmented skin to achieve the same level of energy deposition. Clinically applied fluences for moderately pigmented skin are at the low end of the calculated range of values, suggesting that the clinical fluence is chosen to minimize energy deposition in normal tissues. Our results showed that the MDTF model can be used to evaluate nanosecond laser treatments and provide clinical guidance on fluence settings based on laser-tissue interactions in moderately pigmented skin. The in silico method can, therefore, provide a robust and quantitative retrospective evaluation of the treatment effects that accounts for variation in irradiation parameters among patients by combining the MDTF model with the in vivo optical properties of individual skin types.

Femtosecond Laser Ablation and Delamination of Functional Magnetic Multilayers at the Nanoscale.

Laser nanostructuring of thin films with ultrashort laser pulses is widely used for nanofabrication across various fields. A crucial parameter for optimizing and understanding the processes underlying laser processing is the absorbed laser fluence, which is essential for all damage phenomena such as melting, ablation, spallation, and delamination. While threshold fluences have been extensively studied for single compound thin films, advancements in ultrafast acoustics, magneto-acoustics, and acousto-magneto-plasmonics necessitate understanding the laser nanofabrication processes for functional multilayer films. In this work, we investigated the thickness dependence of ablation and delamination thresholds in Ni/Au bilayers by varying the thickness of the Ni layer. The results were compared with experimental data on Ni thin films. Additionally, we performed femtosecond time-resolved pump-probe measurements of transient reflectivity in Ni to determine the heat penetration depth and evaluate the melting threshold. Delamination thresholds for Ni films were found to exceed the surface melting threshold suggesting the thermal mechanism in a liquid phase. Damage thresholds for Ni/Au bilayers were found to be significantly lower than those for Ni and fingerprint the non-thermal mechanism without Ni melting, which we attribute to the much weaker mechanical adhesion at the Au/glass interface. This finding suggests the potential of femtosecond laser delamination for nondestructive, energy-efficient nanostructuring, enabling the creation of high-quality acoustic resonators and other functional nanostructures for applications in nanosciences.

From Ultrafast Photoinduced Small Polarons to Cooperative and Macroscopic Charge-Transfer Phase Transition.

We study by femtosecond infrared spectroscopy the ultrafast and persistent photoinduced phase transition of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 ⋅ 0.2H2O material, induced at room temperature by a single laser shot. This system exhibits a charge-transfer based phase transition with a 75 K wide thermal hysteresis, centred at room temperature, from the low temperature Mn3+-N-C-Fe2+ tetragonal phase to the high temperature Mn2+-N-C-Fe3+ cubic phase. At room temperature, the photoinduced phase transition is persistent. However, the out-of-equilibrium dynamics leading to this phase is multi-scale. Femtosecond infrared spectroscopy, particularly sensitive to local reorganizations through the evolution of the frequency of the N-C vibration modes with the different characteristic electronic states, reveals that at low laser fluence and on short time scale, the photoexcitation of the Mn3+-N-C-Fe2+ phase creates small charge-transfer polarons [Mn2+-N-C-Fe3+]* within ≃250 fs. The local trapping of photoinduced intermetallic charge-transfer is characterized by the appearance of a polaronic infrared band, due to the surrounding Mn2+-N-C-Fe2+ species. Above a threshold fluence, when a critical fraction of small CT-polarons is reached, the macroscopic phase transition to the persistent Mn2+-N-C-Fe3+ cubic phase occurs within ≃ 100 ps. This non-linear photo-response results from elastic cooperativity, intrinsic to a switchable lattice and reminiscent of a feedback mechanism.

From Ultrafast Photoinduced Small Polarons to Cooperative and Macroscopic Charge‐Transfer Phase Transition

AbstractWe study by femtosecond infrared spectroscopy the ultrafast and persistent photoinduced phase transition of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 ⋅ 0.2H2O material, induced at room temperature by a single laser shot. This system exhibits a charge‐transfer based phase transition with a 75 K wide thermal hysteresis, centred at room temperature, from the low temperature Mn3+−N−C−Fe2+ tetragonal phase to the high temperature Mn2+−N−C−Fe3+ cubic phase. At room temperature, the photoinduced phase transition is persistent. However, the out‐of‐equilibrium dynamics leading to this phase is multi‐scale. Femtosecond infrared spectroscopy, particularly sensitive to local reorganizations through the evolution of the frequency of the N−C vibration modes with the different characteristic electronic states, reveals that at low laser fluence and on short time scale, the photoexcitation of the Mn3+−N−C−Fe2+ phase creates small charge‐transfer polarons [Mn2+−N−C−Fe3+]* within ≃250 fs. The local trapping of photoinduced intermetallic charge‐transfer is characterized by the appearance of a polaronic infrared band, due to the surrounding Mn2+−N−C−Fe2+ species. Above a threshold fluence, when a critical fraction of small CT‐polarons is reached, the macroscopic phase transition to the persistent Mn2+−N−C−Fe3+ cubic phase occurs within ≃ 100 ps. This non‐linear photo‐response results from elastic cooperativity, intrinsic to a switchable lattice and reminiscent of a feedback mechanism.

Investigation of the laser fluence and wavelength dependence in surface-assisted laser desorption/ionization mass spectrometry using gold nanoparticles.

Surface-assisted laser desorption/ionization (SALDI) mass spectrometry (MS) builds on the use of nanostructured surfaces (e.g., coatings of colloidal nanoparticles) to promote analyte desorption and ionization. The SALDI process is believed to occur mainly through thermal processes, resulting from heating of the nanosubstrate upon absorption of the photon energy, and by assisting ionization steps. Mostly due to the accessibility of the respective hardware, the majority of SALDI-MS studies use standard laser wavelengths for MALDI (i.e., 337 or 355 nm), even though peak absorption of the SALDI nanosubstrate might completely differ from these values. Here, we investigated the wavelength dependence in SALDI-MS to determine if wavelength adjustment would be beneficial, and to provide new experimental data for a better understanding of the SALDI mechanism. To this end, gold nanoparticles (AuNPs) sprayed onto microscope glass slides were employed as SALDI nanosubstrates and L-arginine as a model analyte. In addition, we used 2,5-dihydroxyacetophenone (2,5-DHAP) for classical MALDI-MS using the same experimental setup. Arginine ion signals were recorded as a function of laser wavelength and laser fluence. Mass spectra were acquired in the wavelength range between 310 and 630 nm, including the absorption maximum of the sprayed AuNPs around 550 nm and that of 2,5-DHAP around 380 nm. Laser fluence thresholds for the generation of arginine ions were found to be dependent on the laser wavelength and to inversely correlate with the absorbance profiles of the deposited AuNPs and 2,5-DHAP, respectively. Very differently to MALDI, in SALDI ionization efficiency was found to strictly linearly decrease with increasing laser wavelength. Our results, therefore, corroborate the general assumption that material ejection in SALDI-MS is mainly driven by thermal processes in the low laser fluence range and add new evidence that the ionization process is directly influenced by photon energy when AuNPs are employed as nanosubstrates.

Ultralow radiant exposure of a short-pulsed laser to disrupt melanosomes with localized thermal damage through a turbid medium

Short-pulsed lasers can treat dermal pigmented lesions through selective photothermolysis. The irradiated light experiences multiple scattering by the skin and is absorbed by abnormal melanosomes as well as by normal blood vessels above the target. Because the fluence is extremely high, the absorbed light can cause thermal damage to the adjacent tissue components, leading to complications. To minimize radiant exposure and reduce the risk of burns, a model of the melanosome-disruption threshold fluence (MDTF) has been developed that accounts for the light-propagation efficiency in the skin. However, the light-propagation efficiency is attenuated because of multiple scattering, which limits the extent to which the radiant exposure required for treatment can be reduced. Here, this study demonstrates the principle of melanosome disruption with localized thermal damage through a turbid medium by ultralow radiant exposure of a short-pulsed laser. The MDTF model was combined with a wavefront-shaping technique to design an irradiation condition that can increase the light-propagation efficiency to the target. Under this irradiation condition, melanosomes were disrupted at a radiant exposure 25 times lower than the minimal value used in conventional laser treatments. Furthermore, almost no thermal damage to the skin was confirmed through a numerical simulation. These experimental and numerical results show the potential for noninvasive melanosome disruption and may lead to the improvement of the safety of short-pulsed laser treatment.

Femtosecond laser processing of amorphous silicon films

Femtosecond laser micromachining is a pivotal approach within the dynamic field of laser surface structuring, offering unparalleled precision for materials processing. By exploiting the unique nonlinear light-matter interactions, this technique allows the creation of functional micro- and nanostructures with exceptional accuracy and reproducibility. Of particular interest is its ability to process diverse materials, including glasses, polymers, metals, and semiconductors, with minimal thermal impact and collateral damage. Moreover, femtosecond laser micromachining eliminates the need for traditional lithographic methods, enabling intricate structure fabrication under ambient conditions, thereby revolutionizing microfabrication techniques. This study demonstrates the micromachining using femtosecond lasers at 800 nm wavelength and explores the critical role of process parameters, such as laser pulse energy and the number of pulses in determining the damage threshold fluence, especially considering incubation effects in multi-pulse scenarios. Furthermore, it explores the potential of femtosecond laser micromachining for controlled crystallization in hydrogenated amorphous silicon (a-Si:H), unleashing enhanced electrical and optical properties. With its versatility, precision, and potential for diverse applications, femtosecond laser micromachining holds promise for driving advancements in electronic and photonic devices, particularly in high-efficiency photovoltaics, integrated photonics, and novel optoelectronic technologies.

Estimating the Optically Pumped Threshold Fluence of Thin‐Film Gain Media Using Arbitrary Excitation Beams

Optically pumped threshold fluences are a widely reported metric to benchmark the performance of thin‐film gain media and lasers. Estimating the threshold fluence for nonhomogeneous beams, such as a circular Gaussian excitation, is not trivial since the average fluence depends on the estimated spot diameter. Using an exemplary lead halide perovskite film, the inversion volume at different pump energies is mapped. It is shown that the peak fluence of an arbitrary spatial beam profile is more relevant at the threshold, as it provides an upper bound to the threshold fluence. Also, simple conversion factors to estimate the peak fluence using Gaussian excitation beams are provided and the methodology to arbitrary profiles is extrapolated. Furthermore, it is advocated for using flat‐top or uniform stripe excitations to unambiguously extract the threshold fluence, since these excitations display minor discrepancies between the average and peak fluence, and keep the inversion volume relatively constant during the measurement.

Amorphization and Ablation of Crystalline Silicon Using Ultrafast Lasers: Dependencies on the Pulse Duration and Irradiation Wavelength

AbstractUsing lasers to achieve controlled crystallographic phase changes in silicon with high spatial precision promises new manufacturing solutions in semiconductor technologies, including silicon photonics. Recent demonstrations of improved amorphization thicknesses position ultrafast lasers as an optimum tool to meet current challenges. Here, the literature on silicon transformations is reviewed and complemented with new experimental data. This includes amorphization and ablation response as a function of pulse duration (τ = 13.9 to 134 fs at λ = 800 nm) and laser wavelength (λ = 258 to 4000 nm with τ = 200 fs pulses). For pulse duration‐dependent studies on Si(111), the amorphization fluence threshold decreases with shorter durations, emphasizing the significance of non‐linear absorption in the range of considered conditions. For wavelength‐dependent studies, the amorphization threshold increases sharply from λ = 258 to 1030 nm, followed by near‐constant behavior up to λ = 3000 nm. Conversely, the ablation threshold fluence increases in these specified ranges. Differences in the obtained amorphization thicknesses on Si(111) and Si(100) are also discussed, identifying an anomalously large fluence range for amorphization at λ = 258 nm. Finally, the question of the lateral resolution, shown as independent of the interaction nonlinearity is addressed.

High-density nanodot structures on silicon solar cell surfaces irradiated by ultraviolet laser pulses below the melting threshold fluence

The surface morphology of silicon solar cells irradiated with KrF excimer laser pulses (λ = 248 nm, τ = 20 ns) was investigated below the experimentally observed melting threshold fluence (F th) of 0.47 J cm−2 (±20%). At laser fluences of 0.23–0.48 J cm−2 (equivalent to 0.49F th to 1.0F th), nanodot structures with a height and width of approximately 60–120 nm were periodically formed with an interdot spacing similar to the laser wavelength. The observed nanodot density (29 dots μm2) was higher than that previously obtained at longer wavelengths. Furthermore, crystallinity analysis by micro-Raman spectroscopy revealed a Raman shift of 519.56 cm−1 after irradiation (N= 1500 pulses), compared with 518.27 cm−1 prior to irradiation. A laser fluence of 0.41 J cm−2 ( = 0.87F th) was found to induce compressive stress on the silicon solar cell surface.