Laser Pulse Repetition Frequency Research Articles

Experimental study on femtosecond laser ablation of 4H–SiC substrate

Silicon carbide (SiC) is an ideal substrate for manufacturing high-power electronic devices and microwave devices and has broad application prospects. The surface treatment of SiC wafers plays a critical role and faces challenges in the semiconductor industry. Among the multiple treatment methods, the laser-based method has gradually attracted the attention of scholars. Therefore, this research uses a femtosecond laser to ablate 4H–SiC sliced wafers and analyzes the influence of key parameters, such as laser pulse energy, defocus amount, repetition frequency, and scanning intervals, on the laser ablation depth, width, and surface morphology. Scanning electron microscopy and laser coherence-focused microscopy were used to characterize the laser ablation surface. When the defocus amount was 6 mm, the microgroove profile formed by the laser was U-shaped. The scanning interval parameters had to be optimized to obtain the optimized surface roughness. The results show that the optimized surface roughness (Sa ) was 0.267 μm, and brittle fracture areas such as microcracks and pits on the original surface were removed. Effective removal facilitates further material surface processing, which provides valuable insights for similar researchers and benefits for the semiconductor industry.

Heat accumulation and phase transition induction in a VO2 thin film by a femtosecond pulse-periodic radiation.

The kinetics of optical switching due to the insulator-metal phase transition in a VO2 thin film is studied experimentally at different laser pulse repetition frequencies (PRFs) in the NIR range and compared with temperature kinetics obtained through the thermal conductance calculations. Two switching processes have been found with characteristic times <2 ms and <15 ms depending on the PRF; the former is explained by the accumulation of metallic domains remaining after a single-pulse phase transition, and the latter is referred to the heat accumulation in the film. Consequently, the dynamics of the microscopic domains is leading in the initiation of phase transition under pulse-periodic conditions compared to the macroscopic heat transfer. The reverse transition at the radiation turn-off depends on the PRF with a time coefficient of 17.5 µs/kHz and is determined by the metallic domains' decay in the film. The results are important for understanding the nature of the insulator-metal transition in thin films of VO2 as well as using them in all-optical switches of pulse-periodic laser radiation.

Parameter optimization of microjoule high pulse repetition frequency laser induced breakdown spectroscopy for aluminum alloy identification

In the last decade, the analysis and recycling of aluminum alloy continue to raise additional concerns. A large number of aluminum alloy grades and their similar appearance makes it difficult to distinguish these visually manually. A fast and effective real-time in-situ classification method is urgently needed to guide the recycling and reuse of aluminum alloy. In this paper, we propose a laser-induced breakdown spectroscopy (LIBS) based on a microjoule high repetition frequency laser, which significantly reduces the volume and cost of the instrumentation required to realize LIBS compared to conventional LIBS. We used polished flat aluminum alloys for seven wrought alloy classes as samples and designed two spectral acquisition modes based on the existing experience, and further studied the influence of experimental parameters on the spectrum (the fixed mode and the motion mode). When the integration time is 80 ms and the integration windows is 80 to 160 ms, the intensity and stability of the spectral peak can be effectively improved in the fixed mode, when the integration time is 50 ms and the movement speed is 7 mm/s, the spectral peak intensity and stability of the spectrum can be significantly improved in the motion mode. Considering the practical application, we combined with back propagation artificial neural network (BP-ANN) to classify samples from two manufacturers, and the highest classification accuracy after optimization is 97.71%.

Factors Influencing the Repeated Transient Optical Droplet Vaporization Threshold and Lifetimes of Phase Change, Perfluorocarbon Nanodroplets.

Perfluorocarbon nanodroplets (PFCnDs) are sub-micrometer emulsions composed of a surfactant-encased perfluorocarbon (PFC) liquid and can be formulated to transiently vaporize through optical stimulation. However, the factors governing repeated optical droplet vaporization (ODV) have not been investigated. In this study, we employ high-frame-rate ultrasound (US) to characterize the ODV thresholds of various formulations and imaging parameters and identify those that exhibit low vaporization thresholds and repeatable vaporization. We observe a phenomenon termed "preconditioning", where initial laser pulses generate reduced US contrast that appears linked with an increase in nanodroplet size. Variation in laser pulse repetition frequency is found not to change the vaporization threshold, suggesting that "preconditioning" is not related to residual heat. Surfactants (bovine serum albumin, lipids, and zonyl) impact the vaporization threshold and imaging lifetime, with lipid shells demonstrating the best performance with relatively low thresholds (21.6 ± 3.7 mJ/cm2) and long lifetimes (t1/2 = 104 ± 21.5 pulses at 75 mJ/cm2). Physiological stiffness does not affect the ODV threshold and may enhance nanodroplet stability. Furthermore, PFC critical temperatures are found to correlate with vaporization thresholds. These observations enhance our understanding of ODV behavior and pave the way for improved nanodroplet performance in biomedical applications.

Simultaneous distributed acoustic and temperature sensing system based on ultra-weak chirped fiber Bragg grating array.

The quasi-static temperature and dynamic acoustic signals based on ultra-weak chirped fiber Bragg grating (CFBG) array is proposed and experimentally demonstrated, which can be employed to detect distributed acoustic and temperature signals simultaneously. Distributed temperature sensing (DTS) was realized by cross-correlation operation to measure the spectral drift of each CFBG, and distributed acoustic sensing (DAS) was achieved by gauging the phase difference between adjacent CFBG. Using CFBG as the sensor unit can protect acoustic signals from the fluctuations and drifts induced by temperature change without deterioration of signal-to-noise ratio (SNR). Applying least square mean adaptive filter (AF) can enhance harmonic frequency suppression ratio and increase the SNR of system. In the proof-of-concept experiment the SNR of acoustic signal is higher than 100 dB after the digital filter and frequency response is from 2 Hz to 1.25 kHz with repetition frequency of laser pulses of 10 kHz. Temperature measurement from 30°C to 100°C is achieved with demodulation accuracy of ±0.8°C. The spatial resolution (SR) of two-parameter sensing is 5 m.

Second harmonic generation laser scanning microscopy with intrinsic first-order modulation

The contrast or signal-to-noise ratio (SNR) in images is a crucial parameter that determines the quality of images in second harmonic generation laser scanning microscopy (SHG-LSM). With a better image contrast/SNR, SHG-LSM can be performed at a faster frame rate or deeper tissue locations without compromising image quality. In this work, we present a novel strategy based on the SHG signals of first-order modulation (1 M) to enhance the image contrast/SNR. Notably, a photodetector (i.e., photomultiplier tube) has a better signal SNR during the photon-to-electron conversion process at the frequency of 1 M, which is equivalent to the laser pulse repetition frequency. The improved signal SNR thus enhances the image quality, which agrees well with the results of the measured electrical spectra. A remarkable increment of image contrast by more than a factor of two has been obtained in images by using 1 M. In addition, the image acquisition time is also shortened by a factor of 2.5 compared to that for acquiring images with similar contrast taken without signal modulation. Furthermore, by analyzing the dependencies of image contrast/SNR on sample depth, we demonstrate that the images obtained with 1 M have better quality than those obtained without signal modulation at the same imaging depth.

Researching and integrating colour laser marking system on a metal surface using high power fiber laser with MOPA configuration

This paper presents the results of research and fabrication of a colour laser marking device on metal surfaces using a high-power fiber laser with a MOPA (Master oscillator power amplifier) configuration by intergrating semi-finished modules. Using our manufactured colour laser marking device, the effects of laser parameters such as laser power, laser pulse repetition frequency, beam scanning speed, line width on forming colour process have been researched and evaluated explicitly to provide the optimal set of parameters for the colours on each specific stainless steel and titanium metal.

Research on the ablation mechanism and ablation threshold of CMC-SiCf/SiC by using dual-beam coupling nanosecond laser

Due to the excellent properties of high hardness, oxidation resistance and high temperature resistance, silicon carbide fiber silicon carbide ceramic matrix composite (CMC-SiC f /SiC) is a typical difficult-to-process material, and is a high-performance advanced material in the aerospace field. In this paper, two groups of ablation experiments (experiment 1 and experiment 2) were performed on CMC-SiC f /SiC using a dual-beam coupling nanosecond laser, and the ablation morphology was observed by confocal laser microscope. The dual-beam coupling angle of experiment 2 is obtained by experimental method. And through the method of calculation, we get the dual-beam coupling angle of experiment 1 and experiment 2. According to the dual-beam coupling ablation mechanism, based on the theoretical calculation model of non-destructive method D 2 -ln P 0 , combined with the Equivalent Diameter Calculation Method (EDCM) and Equivalent Area Calculation Method (EACM), the laser ablation threshold corresponding to different beam waist size was calculated and compared. The results show that the ablation region of CMC-SiC f /SiC surface can be divided into three parts: ablation boundary, recast layer area and SiO 2 coverage area. When the pulse energy increases gradually from 300 μJ to 1500 μJ, the variation trend of hole depth is first increase, second decrease, increase again, and finally decrease. The angle between two laser beams affects the waist radius, which in turn affect the laser ablation threshold. The waist of the dual-beam coupling is elliptical, and the orifice of the ablation hole is elliptical. When the waist radius of nanosecond laser is 57 μm, the laser ablation threshold is calculated to be 3.12 J/cm 2 . The main factors affecting the laser ablation threshold are laser pulse repetition frequency ( f ), beam waist radius ( ω 0 ), laser pulse width ( τ ), minimum laser power ( P th ), and laser wavelength ( λ ).

Stimulated Resonant Spin Amplification Reveals Millisecond Electron Spin Coherence Time of Rare-Earth Ions in Solids.

The inhomogeneity of an electron spin ensemble as well as fluctuating environment acting upon individual spins drastically shorten the spin coherence time T_{2} and hinder coherent spin manipulation. We show that this problem can be solved by the simultaneous application of a radio frequency (rf) field, which stimulates coherent spin precession decoupled from an inhomogeneous environment, and periodic optical pulses, which amplify this precession. The resulting resonance, taking place when the rf field frequency approaches the laser pulse repetition frequency, has a width determined by the spin coherence time T_{2} that is free from the effects of inhomogeneity and slow nuclear spin fluctuations. We measure a 50-Hz-narrow electron spin resonance and milliseconds-long T_{2} for electrons in the ground state of Ce^{3+} ions in the yttrium aluminum garnet (YAG) lattice at low temperatures, while the inhomogeneous spin dephasing time T_{2}^{*} is only 25ns. This study paves the way to coherent optical manipulation in spin systems decoupled from their inhomogeneous environment.

Impact of laser pulse repetition frequency on nucleation and growth of LiNbO3 thin films

This paper reports the impact of the laser pulse repetition frequency on growth processes, morphological and electro-physical parameters of nanocrystalline LiNbO3 thin films obtained by the pulsed laser deposition technique. It was found that the nucleation process in LiNbO3 films could controllably change by increasing the laser pulse repetition frequency. The film obtained at the repetition frequency of 4 Hz consists of local islands and clusters with a diameter of 118.1 ± 5.9 nm. Nanocrystalline films, grown at the repetition frequency of 10 Hz, possess a continuous granular structure with a grain diameter of 235 ± 11.75 nm. Achieved results can be used for the development of promising “green” energy devices based on lead-free piezoelectric energy harvesters.

Label-free identification and differentiation of different microplastics using phasor analysis of fluorescence lifetime imaging microscopy (FLIM)-generated data

As plastic pollution is becoming an increasing worldwide problem, a variety of different techniques for the detection and in-depth characterization of plastics, including spectroscopy and chromatography methods, were introduced to the public. Recently we presented fluorescence lifetime imaging microscopy (FLIM) a new approach for the identification and characterization of microplastics using their fluorescence lifetime (τ) for differentiation. A very powerful extension of the recently established FLIM could be phasor analysis, which allows data representation in an interactive 2D graphical phasor plot thereby enabling a global view of the fluorescence decay in each pixel of the measured image. Microplastic particles generated from six different types of plastics were subjected to excitation wavelengths of 440 nm, upon which specific fluorescence lifetimes as well as the photon yield were determined using FLIM and phasor analysis. We could show that phasor analysis for FLIM with a laser pulse repetition frequency of 40 MHz was able to generate specific locations in the phasor plot for the plastics for fast differentiation, e.g. resulting in well-defined phasor plot positions for ABS at 3.019 ns, PPE at 6.239 ns, PET bottle from Germany at 2.703 ns and PET bottle from USA at 2.711 ns. Phasor analysis for FLIM proves to be a fast, label-free, and sensitive method for the identification and differentiation of plastics also with the aid of visualization variation enabling techniques such as heat treatment of plastics.

Optimization of wide-field second-harmonic generation microscopy for fast imaging of large sample areas in biological tissues

Second-harmonic generation (SHG) microscopy is a label-free imaging method that can be used to visualize the detailed arrangement of collagen structures in biological tissues. Here, we sought to optimize the speed of microscopic SHG image acquisition of macroscopic fixed tissue sample areas by employing the wide-field imaging with a high power and medium, 1 MHz pulse repetition frequency laser in combination with a mechanical sample scanning. Unlike in the conventional laser-scanning microscopy, the optimum of the wide-field acquisition entails an interplay between the size of the illuminated area and the intensity of the generated signal. We delineate quantitative procedures to set the image parameters for the maximum speed of the tiled image acquisition, and also describe the possible optimization of the laser parameters for further enhancement of the speed of acquisition.

GPU implementation of photoacoustic short-lag spatial coherence imaging for improved image-guided interventions.

.Significance: Photoacoustic-based visual servoing is a promising technique for surgical tool tip tracking and automated visualization of photoacoustic targets during interventional procedures. However, one outstanding challenge has been the reliability of obtaining segmentations using low-energy light sources that operate within existing laser safety limits.Aim: We developed the first known graphical processing unit (GPU)-based real-time implementation of short-lag spatial coherence (SLSC) beamforming for photoacoustic imaging and applied this real-time algorithm to improve signal segmentation during photoacoustic-based visual servoing with low-energy lasers.Approach: A 1-mm-core-diameter optical fiber was inserted into ex vivo bovine tissue. Photoacoustic-based visual servoing was implemented as the fiber was manually displaced by a translation stage, which provided ground truth measurements of the fiber displacement. GPU-SLSC results were compared with a central processing unit (CPU)-SLSC approach and an amplitude-based delay-and-sum (DAS) beamforming approach. Performance was additionally evaluated with in vivo cardiac data.Results: The GPU-SLSC implementation achieved frame rates up to 41.2 Hz, representing a factor of 348 speedup when compared with offline CPU-SLSC. In addition, GPU-SLSC successfully recovered low-energy signals (i.e., ) with mean ± standard deviation of signal-to-noise ratios of (compared with with conventional DAS beamforming). When energies were lower than the safety limit for skin (i.e., for 900-nm wavelength laser light), the median and interquartile range (IQR) of visual servoing tracking errors obtained with GPU-SLSC were 0.64 and 0.52 mm, respectively (which were lower than the median and IQR obtained with DAS by 1.39 and 8.45 mm, respectively). GPU-SLSC additionally reduced the percentage of failed segmentations when applied to in vivo cardiac data.Conclusions: Results are promising for the use of low-energy, miniaturized lasers to perform GPU-SLSC photoacoustic-based visual servoing in the operating room with laser pulse repetition frequencies as high as 41.2 Hz.

Temperature evolution of Al/PTFE reactive materials irradiated by femtosecond pulse laser

When the laser irradiates the energetic material, the reaction temperature of the material is an important parameter to evaluate the safe processing of femtosecond pulse laser. In order to study the temperature evolutionary process caused by the interaction between femtosecond pulsed laser and reactive materials, by using the femtosecond pulse laser loading system combining with the infrared thermal imager, the radiation temperature distribution induced by laser irradiating traditional formulation Al/PTFE (polytetrafluoroethylene) reactive materials and the safe threshold of laser processing are studied under the condition of different laser pulse repetition frequencies and different laser action durations. The results show that the change rules of the central temperature and edge temperature of the laser irradiation area are different. The radiation temperature irradiated by 500 Hz laser is lower than that of 1000 Hz, and the reactive material is more sensitive when the laser frequency is lower. When the laser pulse repetition frequency is 1000 Hz, deflagration of the reactive material happens at about the moment of 131s. Meanwhile the temperature threshold of deflagration of the reactive material is about 706 K, and the ablation depth is about 4.92 mm.

Repetitive Energy Deposition at a Supersonic Intake in Subcritical and Buzz Modes

This study experimentally investigated the effects of repetitive laser energy deposition using a supersonic intake model with a central conical compression surface and a cross-sectional duct in a Mach 1.92 indraft wind tunnel: especially in the subcritical and buzz modes. A single-pulse energy deposition was observed to suppress the flow separation at the compression surface by “sweeping” a shock wave system with the thermal bubble generated by the energy deposition. The duration of the sweeping effect was approximately in the subcritical mode and in the buzz mode. Furthermore, repetitive deposition of laser pulse energies was observed to moderate instabilities in both modes, and it increased the pressure recovery by as much as 8%; also, the occurrence of buzz was delayed, thereby widening the stable, subcritical regime. In both modes, there was a threshold value for the laser pulse repetition frequency , which corresponded to the duration of the sweeping effect (e.g., in subcritical mode). Below this frequency, the increase in the pressure was proportional to the repetition frequency; whereas above , the effect per single pulse was reduced.

Wave front analysis for enhanced time-domain beamforming of point-like targets in optoacoustic imaging using a linear array

Using linear array transducers in combination with state-of-the-art multichannel electronics allows to perform optoacoustic imaging with frame rates only limited by the laser pulse repetition frequency and the acoustic time of flight. However, characteristic image artefacts resulting from the limited view and a lower SNR when compared to systems based on single-element focused transducers represent a burden for the clinical acceptance of the technology. In this paper, we present a new method for the improvement of image quality based on the analysis of the signal amplitudes along summation curves during the delay-and-sum beamforming process (DAS). The algorithm compares amplitude distributions along wave fronts with theoretical patterns from optoacoustic point sources. The method was validated on simulated and experimental phantom as well as in-vivo data. An improvement of the lateral resolution by more than a factor of two when comparing conventional DAS and our approach could be shown (numeric and experimental phantom data). For instance, on experimental data from a wire phantom, a PSF in the range of 0.18–0.22 mm was obtained with our approach against 0.48 mm for standard DAS. Furthermore, the SNR of a subcutaneous vessel 2.5 mm below the skin surface was improved by about 30 dB when compared to standard DAS.

Simulation of sub-nm carrier profiling by scanning frequency comb microscopy

A mode-locked laser focused on the tunneling junction of a scanning tunneling microscope (STM) superimposes a microwave frequency comb with hundreds of harmonics on the DC tunneling current. Each harmonic, at an integer multiple of the laser pulse repetition frequency, sets the present state-of-the-art for narrow linewidth at its frequency to enable low-noise measurements at an average laser power of several milliwatts. Measurements of the attenuation of the harmonics, which is caused by the spreading resistance, may be used to determine the resistivity of the sample. In Scanning Frequency Comb Microscopy (SFCM) feedback control of the tip-sample distance is based on the power at the harmonics. No DC bias voltage or DC tunneling current is required and the data rate is much higher than that with an STM. Simulations of the spatial distribution of the power dissipated in the sample show the feasibility of non-destructive true sub-nm resolution in the carrier profiling of semiconductors. With no DC bias voltage and no DC tunneling current band-bending and other changes to semiconductor samples in an STM are mitigated and there is a possibility for in vivo microscopy in biology and medicine.

Force sensor fabrication by AgNWs film using 532 nm pulses laser

This current study investigated the effects of laser direct writing (LDW) on the capacitance force sensor of a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) film mixer with silver nanowire thin films by using a 532-nm laser system. The technique was combined with various scanning speeds (i.e., 800, 1000, and 1200 mm s−1) to investigate the effectiveness of the force sensor. During interdigitated electrode fabrication, the laser fluence and laser pulse repetition frequency were fixed at 5.42 J/cm2 and 40 kHz, respectively. Nonloading force and various loading force states were used to test the function of the electrode under different LDW conditions. A best correlation coefficient of approximately 0.88 confirmed a linear functional dependence between the square of the pattern width and capacitance under the micromachining speed of 1000 mm s−1. Moreover, in the nonloading force state, the capacitance value increased with the increase in the electrode pattern width under a micromachining speed of 800 mm s−1. Regarding the loading force status, the capacitance increased when the loading force increased from 18 to 90 g. When the interdigitated pattern line width was 500 μm and electrode micromachining speed was 800 mm s−1, the maximum capacitance value was 71.5 pF under a loading force of 90 g.

Pulsed Laser Deposition of Aluminum Nitride Films: Correlation between Mechanical, Optical, and Structural Properties

Aluminum nitride (AlN) films were synthesized onto Si(100) substrates by pulsed laser deposition (PLD) in vacuum or nitrogen, at 0.1, 1, 5, or 10 Pa, and substrate temperatures ranging from RT to 800 °C. The laser parameters were set at: incident laser fluence of 3–10 J/cm2 and laser pulse repetition frequency of 3, 10, or 40 Hz, respectively. The films’ hardness was investigated by depth-sensing nanoindentation. The optical properties were studied by FTIR spectroscopy and UV-near IR ellipsometry. Hardness values within the range of 22–30 GPa and Young’s modulus values of 230–280 GPa have been inferred. These values were determined by the AlN film structure that consisted of nanocrystallite grains, strongly dependent on the deposition parameters. The values of optical constants, superior to amorphous AlN, support the presence of crystallites in the amorphous film matrix. They were visualized by TEM and evidenced by FTIR spectroscopy. The characteristic Reststrahlen band of the h-AlN lattice with component lines arising from IR active phonon vibrational modes in AlN nanocrystallites was well detectable within the spectral range of 950–500 cm−1. Control X-ray diffraction and atomic force microscopy data were introduced and discussed. All measurements delivered congruent results and have clearly shown a correlation between the films’ structure and the mechanical and optical properties dependent on the experimental conditions.

Effect of pulsed Nd:YAG laser processing parameters on surface properties of polyimide films

As a cost-effective, facile and scalable technology, direct laser writing technology has been widely reported for modifying the surface properties of polymers. In this work, the modification induced in polyimide (PI) films by the pulsed Nd:YAG laser has been investigated. The formation mechanism of surface morphologies of the laser-induced PI films was clarified through laser Mie scattering. Various laser powers, scanning speeds and pulse repetition frequencies were applied to modify surface properties, including surface morphologies, carbonization characteristics and wettability. Analysis of the morphological changes in the PI film surface layer was performed via a three-dimensional optical microscope. The degree of carbonization was measured using the grey-scale extraction tool in the Adobe Photoshop software. It is found that the laser fluence incident on a PI film is closely related to the density of floating carbonized particles generated by the high-temperature carbonization and the high-pressure release of the excited atoms in PI film. Meanwhile, the density of carbonized particle group and the number of cavities in PI film mainly depend on the unscattered static laser fluence regardless of the dynamic laser fluence. And the degree of laser Mie scattering can be inferred by the degree of carbonization of the cavity-free surface and the distribution ratio of the small-sized cavities with diameters of ≤15 μm. Within the range of the studied processing conditions, the laser power and pulse repetition frequency exhibited a greater sensitivity to the carbonization area rate of cavity defects and the wettability than the scanning speed.