Femtosecond Laser Pulse Energy Research Articles

Generation of high-energy near-infrared femtosecond laser pulses by optical parametric amplification in YCOB crystals

"The energy of near-infrared femtosecond laser pulses generated by optical parametric amplification (OPA) in β-barium borate (BBO) crystals pumped by femtosecond Ti:sapphire lasers is restricted to few-mJ due to the 20 mm limited diam- eter of available crystals. In a type I collinear OPA with an yttrium calcium oxoborate (YCOB) crystal, pumped with femtosecond Ti:sapphire lasers at 800 nm wavelength, the 1300 nm signal wavelength and 2080 nm idler wavelength are located in the normal and anomalous group velocity dispersion range, respectively. Due to the small group velocity mismatch (GVM) between signal and idler pulses, as broad as 27 THz gain bandwidth can be obtained in a 3-mm YCOB crystal at 100 GW/cm2 pump intensity. A high parametric gain is the result of the increased parametric interaction length due to the different signs of GVM between pump-signal and pump-idler pulses. More than 10-mJ energy femtosecond laser pulses at 1300 nm wavelength can be generated by OPA in YCOB crystals with larger than 50 mm clear aperture."

Effect of laser pulse energy on atomic lines and molecular bands in femtosecond LIBS of aluminum

This study explored the characteristics of atomic and molecular emissions in Al plasmas generated by femtosecond lasers. The influence of laser pulse energy on Al atoms and AlO molecules time-integrated spectra was examined. As laser pulse energy increased, the intensity of Al atoms increased, while the emission of AlO molecules first increased and then remained unchanged. Secondly, the vibration temperature of AlO was determined and found to result in strong emission at a low temperature. Finally, the influence of laser pulse energy on the time-resolved spectra of Al atom and AlO molecule was explored, finding consistency with the time-integrated emission. These results indicate that the Al atom and AlO molecule have different sensitivities to femtosecond laser pulse energy. This research enriches our knowledge of femtosecond laser-plasma interactions and has potential applications in material processing and spectroscopic analysis.

Laser Direct Writing of Visible Spin Defects in Hexagonal Boron Nitride for Applications in Spin-Based Technologies

Optically addressable spins in two-dimensional hexagonal boron nitride (hBN) attract widespread attention for their potential advantages in on-chip quantum devices, such as quantum sensors and quantum networks. A variety of spin defects have been found in hBN, but no convenient and deterministic generation methods have been reported for other defects except the negatively charged boron vacancy (VB−). Here we report that by using femtosecond laser direct writing technology, we can deterministically create spin defect ensembles with spectral range from 550 to 800 nm on nanoscale hBN flakes. Positive single-peak optically detected magnetic resonance (ODMR) signals are detected in the presence of a magnetic field perpendicular to the substrate, and the contrast can reach 0.8%. With the appropriate thickness of hBN flakes, substrate, and femtosecond laser pulse energy, we can deterministically and efficiently generate a bright spin defect array. Our results provide a convenient deterministic method to create spin defects in hBN, which will motivate more endeavors for future research and applications of spin-based technologies such as a quantum magnetometer array.

High-Density Weak in-Fiber Micro-Cavity Array for Distributed High-Temperature Sensing With Millimeter Spatial Resolution

High-spatial-resolution distributed sensing at high temperature is crucial in many industrial areas, such as aero-engines, nuclear power, furnaces, and fuel cells. Here, we propose and demonstrate a large-scale multiplexed high-density weak in-fiber micro-cavity (MC) array for distributed high-temperature sensing with millimeter spatial resolution. The proposed in-fiber MC, featured by an intrinsic Fabry-Perot interferometer (IFPI) with a short cavity length of 100 μm and a low peak reflectivity of ∼−55 dB, was formed by two weak reflectors created in a conventional single-mode fiber (SMF) by using femtosecond laser point-by-point inscription. Several high-density MC arrays, consisting of identical weak IFPIs over 1000, were fabricated by using different femtosecond laser pulse energy to investigate the transmission loss (TL) of MC arrays. The experimental result shows that the TL induced by a single MC could be low as 0.0009 dB. Moreover, the high-temperature performance of the MCs was studied via cyclic heating and cooling between room temperature and 1000 °C, showing a temperature sensitivity of −2.29 GHz/°C (i.e., 18.4 pm/°C). Furthermore, distributed high-temperature sensing was demonstrated by employing the fabricated in-fiber MC array with the demodulation of optical frequency domain reflectometry, and a high spatial resolution of 1 mm was achieved at a high temperature of 1000 °C. As such, the proposed high-density weak in-fiber MC arrays are suitable for distributed high-temperature sensing in harsh environment, and hence have wide prospection of application in the fields of aerospace, nuclear power, metallurgy, and electrochemical industry.

Fabrication of Cylindrical Microlens by Femtosecond Laser‐Assisted Hydrofluoric Acid Wet Etching of Fused Silica

Micro‐optical elements play a key role in various advanced devices, including biomedical, photovoltaic, 3D display, and optical communication devices. As one of the typical micro‐optical elements, cylindrical microlens (CML) can modulate the wavefront phase of the incident light in only one direction. The process methods of CML have been developed and various methods are proposed for CML fabrication. For the simple and versatile fabrication of CML on hard brittle materials, femtosecond laser direct writing combined with hydrofluoric acid (HF) wet etching for removal of laser‐modified regions in fused silica is proposed here for high‐quality CML fabrication. A high‐accuracy CML can be processed by adjusting the HF etching time, femtosecond laser pulse energy, and the position of the laser exposure point. Crossed‐shaped CML can be realized by accurate control of the single laser pulse exposure positions at the intersection in two directions. A series of alphabet‐shaped CML, showing this fabrication method's flexibility is made. The characterization of the CML under a microscope system reveals a good optical focusing ability. The proposed method offers a practical route toward the manufacture of CML on hard brittle materials.

Gas-phase pressure measurement using femtosecond laser-induced grating scattering technique.

Gas-phase pressure measurements remain challenging in situations where local pressure rapidly changes or in hostile environments such as turbulent combustion. In this work, we demonstrate the implementation of the recently developed femtosecond laser-induced grating scattering (fs-LIGS) technique for pressure measurement in ambient air. With an overall femtosecond laser pulse energy of 185 μJ, fs-LIGS signals were generated for various gas pressure ranging from 0.2 to 3.0 bar. By theoretically fitting the signal and extracting the time constant of the stationary density modulation damping, the pressure is successfully derived. The derived values were compared to the gauge pressure, which shows a quasi-linear dependence with a slope of 0.96, suggesting the feasibility of the fs-LIGS technique for gas-phase pressure measurements.

Processing and Profile Control of Microhole Array for PDMS Mask with Femtosecond Laser.

Polydimethylsiloxane (PDMS) is hailed as one of the foundational materials that have been applied to different products in various fields because of its chemical resistance, low cost, excellent flexibility, and high molding capability. With the aim to achieve surface texture with high efficiency by means of electrochemical micromachining with PDMS mask, a femtosecond laser is utilized to process a precision array of micro-through-holes on PDMS films as the molds. The ablation process of PDMS with a femtosecond laser was investigated via numerical simulation verified with experiments indicating a laser energy density of 4.865 mJ/mm2 as the ablation threshold of PDMS with the melting temperature of 930 K. The spiral scanning path with optimized radial offset was developed to ablate materials from the PDMS film to form the laminated profiles, and a tapered through hole was then formed with multilayer scanning. The profile dimension and accuracy were examined as control targets in terms of laser pulse energy and scanning speed, showing that a 12 μJ femtosecond laser pulse energy and 1000 mm/s scanning speed could bring about a nearly circular laminating profile with expected smaller exit diameter than the entry diameter. All the cross-section diameters of the microcone decreased with the increase of laser scanning speed, while the taper increased gradually and then saturated around a laser scanning speed of 800 mm/s due to the energy absorption resulting in smaller ablation in diameter and depth.

Integration of Multifocal Microlens Array on Silicon Microcantilever via Femtosecond-Laser-Assisted Etching Technology.

Micro-opto-electromechanical systems (MOEMSs) are a new class of integrated and miniaturized optical systems that have significant applications in modern optics. However, the integration of micro-optical elements with complex morphologies on existing micro-electromechanical systems is difficult. Herein, we propose a femtosecond-laser-assisted dry etching technology to realize the fabrication of silicon microlenses. The size of the microlens can be controlled by the femtosecond laser pulse energy and the number of pulses. To verify the applicability of this method, multifocal microlens arrays (focal lengths of 7–9 μm) were integrated into a silicon microcantilever using this method. The proposed technology would broaden the application scope of MOEMSs in three-dimensional imaging systems.

An experimental investigation of scan trajectory into the underwater femtosecond laser polishing SiC ceramic

Silicon carbide (SiC) ceramics, due to their excellent mechanical properties and oxidation resistance at elevated temperature, have a widespread application in microelectronics and aerospace. However, the machining of SiC ceramics into the desired structural ceramic components presents a challenge because of their high hardness and high brittleness. In this paper, a novel method of underwater femtosecond laser polishing SiC ceramics was proposed. We investigated the influence of scan trajectory, such as scanning pitch and polishing mode, and femtosecond laser pulse energy on surface roughness and polishing depth during underwater polishing processing. The experimental results indicated that the polishing depth decreased with the increasing scanning pitch, and then tending to saturate. A deep cavity was obtained under the condition of high laser pulse energy and small scanning pitch during polishing. However, the obtained surface quality was rough at this condition, which could be used for rough polishing operation because the high material removal rate is the primary concern. In addition, the cross-scanning polishing mode achieved the better surface quality without cracks and pits than the transverse polishing mode, but the polishing efficiency was low. In addition, under appropriate machining conditions, the smooth polished surface with surface roughness 0.76 µm was obtained by using 15 µm scanning pitch together with high laser pulse energy and cross-scanning polishing mode.

Aberrated moving soliton in nonlinear photonic crystal

ABSTRACTBased on computer simulation, we demonstrate the nonlinear localization of a femtosecond laser pulse energy in 1D- and 2D photonic structures at the inclined incidence of a beam (or several beams) on a face of the photonic structure consisting of layers or rectangular elements. We show that, in 1D photonic crystal (PC), the energy localization is due to the formation of the moving soliton with the aberrated wave front – the aberrated soliton, which moves inside the photonic structure, being reflected many times from the PC faces, and its appearance depends on the photonic structure elements, ambient medium and the incident laser pulse shape. The dependence of the light energy localization on the sizes of the rectangular elements is investigated. We demonstrate controlling the light energy localization due to the incidence of several beams on the photonic structure and discuss the possibility of observing the described phenomena in a physical experiment.

Simulation of femtosecond laser ablation sapphire based on free electron density

Femtosecond laser has become an effective method for processing super hard materials, due to the multiphoton ionization and avalanche ionization induced by its high peak power. A FEM model with Fokker-plank equation and Drude equation was established to study the space-time evolution of free electron density caused by the ionization. The study focuses on investigating the temporal and spatial evolution law of free electron density induced by femtosecond laser pulse and its influence on absorption coefficient, reflectivity and ablation depth. It was shown that there is a dramatic increase of sapphire surface absorption coefficient and reflectivity within picosecond, and femtosecond laser pulse energy is rapidly absorbed in a very small depth about 140 nm. When the single pulse energy exceeds a certain threshold (8 J/cm2), the predicated ablation depth is basically not affected by the incident laser pulse energy and remains unchanged, which are in agreement with the experimental data. The FEM model is applicable with advantages of relative simplicity and acceptable precision.

Single-Step Laser Plasmonic Coloration of Metal Films.

Utilization of structural colors produced by nanosized optical antennas is expected to revolutionize the current display technologies based on an inkjet or a pigmentation-based color printing. Meanwhile, the versatile color-mapping strategy combining the fast single-step single-substrate fabrication cycle with low-cost scalable operation is still missing. We propose lithography-free pure optical approach based on a direct local ablative reshaping of the gold film with nanojoule (nJ)-energy femtosecond laser pulses. Plasmon-color printing at a resolution up to 2.5 × 104 dots per inch satisfying the current visualization demands and data storage capacity is achieved. By controlling only the applied pulse energy, wide gamut of colors in scattering regime was reproduced via tuning the size of the printed nanovoids, which have a polarization- and shape-dependent localized plasmon-mediated scattering. Additionally, brightness of a single pixel was gradually adjusted via varying of the spacing between the printed nanovoids. The presented experimental demonstration opens a new direction toward plasmon-color printing for various applications where durability is required: low-cost cryptography, security tagging, and ultracompact optical data storage.

Ultra-bright keV X-ray source generated by relativistic femtosecond laser pulse interaction with thin foils and its possible application for HEDS investigations

AbstractIt was shown (Faenov et al., 2015b) that the energy of femtosecond laser pulses with relativistic intensity approaching to ~1021 W/cm2 is efficiently converted to X-ray radiation and produces exotic states in solid density plasma periphery. We propose and show by one-dimensional two-temperature hydrodynamic modeling, that applying two such unique ultra-bright X-ray sources with intensities above 1017 W/cm2 – allow to generate shock waves with strength of up to some hundreds Mbar, which could give new opportunities for studies of matter in extreme conditions.

Ultrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm−1 from a metallic spintronic emitter

We explore the capabilities of metallic spintronic thin-film stacks as a source of intense and broadband terahertz electromagnetic fields. For this purpose, we excite a W/CoFeB/Pt trilayer (thickness of 5.6 nm) on a large-area glass substrate (diameter of 7.5 cm) by a femtosecond laser pulse (energy 5.5 mJ, duration 40 fs, and wavelength 800 nm). After focusing, the emitted terahertz pulse is measured to have a duration of 230 fs, a peak field of 300 kV cm−1, and an energy of 5 nJ. In particular, the waveform exhibits a gapless spectrum extending from 1 to 10 THz at 10% of its amplitude maximum, thereby facilitating nonlinear control over matter in this difficult-to-reach frequency range on the sub-picosecond time scale.

Single-pulse perforation of thin transparent dielectrics by femtosecond lasers

The methods of elongation of the effective interaction area (>100 microns) of single femtosecond pulses with transparent dielectrics when focusing in the bulk of material are given. Principal diagrams of transparent materials perforation with single femtosecond laser pulses are proposed. Capability to form cylindrical holes in the transparent dielectrics as a result of material photodegradation subjected to single femtosecond laser pulses was successfully demonstrated. The diameter of through holes made in the polypropylene 50 microns thick film at the energy of femtosecond laser pulses of 5 µJ was 5 µm.

Electro-optic sampling of terahertz waves by laser pulses with an edge-cut spectrum in birefringent crystal.

We demonstrate a terahertz detection technique based on variations of the energy of femtosecond laser pulses with an edge-cut spectrum in birefringent electro-optic crystal-periodically poled lithium niobate (PPLN). The method is compared with the standard electro-optic detection scheme utilizing GaP crystal. The experimental results show that the studied technique is suited for use with birefringent crystals and allows one to achieve a much better response to the terahertz wave radiation at quasi-phase-matching frequencies of PPLN.

Femtosecond laser pulse energy accumulation optimization effect on surface morphology of black silicon

Arrays of sharp conical spike microstructures are created by repeatedly irradiating silicon surfaces with focused femtosecond laser pulses in SF6. The absorbance of light is increased to approximately 90% in a wavelength range from the near ultraviolet (0.25 m) to the near infrared (2.5 m) by the microstructured silicon surface. The microstructured surface presents pitch-black because of enhanced absorption with a broad wavelength range, which is called black silicon. The unique microstructure morphology of black silicon surface formed by femtosecond laser can also bring a lot of other surface functions, for example, self-cleaning and field emission. These functions make black silicon highly desirable in solar energy, detectors and other fields. Therefore, the forming mechanism and conditions of fabrication optimization for black silicon microstructure have always been the focus of research. In our work, the sample is moved by motor-controlled stage while the laser beam is fixed. In the case of laser beam scanning, arrays of sharp conical spikes on the silicon are manufactured in 70 kPa SF6. The aim of the experiment is to find how to optimize the distribution of the laser energy in a number of laser accumulation pulses (the combination of single pulse energy and pulse number) to control the surface morphology of the black silicon. Experimental results show that there appears a bottleneck effect of morphology size growth with the increase of laser irradiation (improving the single pulse energy or increasing pulse accumulation number). Excessive energy accumulation brings no extra effect on optimizing and controlling of microstructure morphology on the surface. Based on theoretical results obtained from a physical model we proposed, we find that the reason for this phenomenon is that the microstructure morphology induced by former sequence pulse modulates the laser energy absorption of current laser pulse, and changes the laser ablation efficiency of the current pulse. According to this physical mechanism, we propose a new way of optimizing surface morphology, with fixing the total laser irradiation energy. And the size and distribution of surface morphology can be achieved by optimizing the distribution of the laser energy in a number of laser accumulation pulses. This approach can not only improve the efficiency of silicon surface preparation of microstructures but also reduce the surface defects and damage. Furthermore, the proposed method can reduce the energy consumption in the process of femtosecond machining. It is of great significance for the engineering application of black silicon.

Femtosecond Laser Ablated Polymer SERS Fiber Probe With Photoreduced Deposition of Silver Nanoparticles

In this paper, a surface-enhanced Raman scattering (SERS) spectral probe based on a polymethylmethacrylate (PMMA) polymer fiber is produced by femtosecond laser ablation and photoreduced deposition of silver nanoparticles. It shows that a surface grating structure enabled by femtosecond laser ablation could further improve the SERS signal by around four times larger than the ordinarily roughened structure. By varying femtosecond laser pulse energy and scan surface period in the fabrication process, the enhancement factor of the SERS signal is optimized with laser scan surface period of around 5 μm at 1.2 μJ femtosecond laser pulse energy for a PMMA fiber probe. In particular biochemical analysis, much softer SERS probes are preferred, and our polymer SERS fiber probe shows an alternative method for in situ measurement of the SERS signal and further fabrication of polymer SERS microchip.

Direct Femtosecond Laser Writing of Optical Nanoresonators

A novel method for single-step, lithography-free, and large-scale laser writing of arrays of metallic (Au) and semiconductor (Si) nanoparticles (nanoresonators) has been developed. Changing energy of femtosecond laser pulses and film thickness it is possible to vary diameter of the nanoparticles, while the distance between them can be varied by laser scanning parameters. This method has an advantage over the most previously demonstrated methods in its simplicity and versatility, while the quality of the structures is good enough for many applications.

Influence of femtosecond laser pulse energy on the surface reflection of black silicon in alkaline solution

A highly efficient approach for reducing the reflection of the black silicon surface is demonstrated, in which the black silicon is fabricated in alkaline solution via a femtosecond laser irradiation. The junglelike microstructures are formed on the surface of the black silicon. Compared to the polished silicon, the black silicon can significantly suppress the surface reflection. Throughout the region of visible light, the average minimum reflectance of the blackened surface is about 5.6%. Meanwhile, in order to get an optimal laser energy, we investigated the evolution on silicon surface as a function of incident pulse energy. Taking into account the height of junglelike microstructures and the reflectance of black silicon samples, the optimal laser energy is 1400 μJ. By choosing the right laser energy, it is possible to fabricate the highly absorptive black silicon. These results are of extreme importance in the control of surface morphology and the modification of material surface.