Power Nanosecond Laser Research Articles

Multiphoton-initiated laser writing of semiconductors using nanosecond mid-infrared pulses

The advent of more powerful mid-infrared nanosecond laser sources allows using them as attractive tools in material laser processing. For semiconductor bulk processing, laser intensities must remain modest to avoid detrimental nonlinear propagation and pre-focal plasma screening effects. Accordingly, nanosecond pulses are usually appropriate to locally initiate energy deposition by multiphoton absorption and subsequent modification, hardly achievable with ultrashort pulses. Employing a 2.8-μm 2.5-ns laser source, we explore the possibility to modify silicon and other semiconductors. With interactions initiated by three-photon absorption, we modify and determine the surface and bulk modification thresholds of Si, GaAs, and InP. For Si, compared to previous studies at telecom wavelengths, we report on a reduction of the energy threshold for volume modification with processing depth, and repeatable rear-surface modifications. Compared to Si, we find for GaAs and InP important fluctuations in the modification responses, which we attribute to an absorption mechanism initially triggered by a greater presence of defects. We also study germanium, as this narrower bandgap semiconductor very interesting for mid-infrared electro-optical applications is transparent at the newly introduced wavelength. Despite the ability to modify the surface, the bulk of Ge remains inaccessible in this two-photon regime, mainly attributed to beam aberrations and nonlinearities. Nonetheless, we demonstrate the ability to process Si chips integrating Ge layers. All these results showcase the potential of mid-infrared wavelengths to offer new degrees of flexibility for 3D laser processing technologies turned to silicon photonics and microelectronics packaging.

PROSPECTIVE LITHIUM ALUMINOSILICATE GLASS-CERAMIC MATERIALS FOR PULSE RANGEFINDERS

The relevance of creating multi-functional nanosecond pulsed laser devices with a generation wavelength that lies in the conditionally eye-safe region of the spectrum of 1.5÷1.6 μm has been established. The prospects of using the mode of passive Q-switching when receiving powerful nanosecond laser pulses with a radiation divergence close to the diffraction one have been determined. Materials for shutters that function in the mode of passive Q-switching were analyzed: solid-state nonlinear optical materials based on single crystals of yttrium-aluminum garnet doped with cations of neodymium, chromium, vanadium and other Nd- or Yb-doped crystals in the wavelength range of 0.8–1.2 μm. The promising use of oxide single crystals activated by Co2+ ions in four-coordinated oxygen positions for MgAl2O4, LiGa5O8, LaMgAl11O19 has been confirmed. The feasibility of developing transparent glass-ceramics based on aluminomagnesium and lithium gallium spinel for pulsed ranging was determined, taking into account its commercial availability, the simplicity of the production technology, as well as the homogeneity of the distribution of the activator in the volume. The feasibility of creating a new type of nonlinear optical materials based on nanostructured heat-resistant lithium aluminosilicate glass ceramics with spinel nanocrystals activated by Co2+ ions under low-temperature heat treatment conditions, has been determined. The purpose and tasks of the work are formulated, which consist in justifying the choice of the system and compositions of glasses for obtaining glass-ceramic materials as a passive laser shutter for powerful lasers that operate at a wavelength of 1.54 μm and are characterized by high heat resistance. The choice of the lithium aluminosilicate system R2O – RO – RO2 – P2O5 – R2O3 – SiO2 based on phase-forming SiO2, Al2O3, Li2O, modifying K2O is substantiated; RO – MgO, ZnO, CaO, SrO, BaO components and a complex crystallization catalyst of P2O5, CeO2, TiO2, SnO2, ZrO2 and ZnO. The structural criteria for the glass matrix were formulated. Compositions of model glasses were designed taking into account the calculated parameters fSi > 0.22, Kсr ≥ 3.5, Ktr ≥ 2.1, and the probability of obtaining transparent glass-ceramic materials with a sitallized structure for pulse rangefinders was established on their basis.

Spatiotemporal shaping of high power laser pulses based on stimulated brillouin scattering

We proposed and numerically demonstrated the combination of temporal and spatial shaping of high power nanosecond laser pulses based on stimulated Brillouin scattering (SBS) in this paper. With the intensity-dependence characteristic of SBS, the higher intensity parts of the laser beam obtain higher reflectivity, and the incident non-uniform high power laser beam would be well smoothed. A parameter adjustable feedback control loop was used to tailor the output temporal profile by pre-compensating the temporal profile of the input pump. In our numerical simulation, a 3 ns super-Gaussian shaped single-frequency laser pulse with a 527 nm wavelength was used as the pump. And the heavy fluorocarbon FC-70 was chosen as the Brillouin medium. Simulation results show that the laser spatial modulation can be significantly pulled down when the energy efficiency is maintained above 90% in our beam smoothing system with suitable laser intensity. The flat-toped laser pulses both in temporal and spatial domain were demonstrated to be achievable simultaneously. The method proposed here paves a simple and effective way to optimizing the near field pattern and temporal shape of high power laser systems.

Development and analysis of scalable laser machined surface utilizing low power nanosecond laser on Ti-6Al-4V alloy

ABSTRACT A low-power fiber laser system was utilized for machining a scalable surface area on the Ti-6Al-4V alloy surface through the track overlapping process. The effect of laser power and track overlapping on laser machined surfaces was investigated. The experimental results revealed that the laser machined surface characteristics, i.e. machining depth, resolidified layer thickness, heat affected zone (HAZ) size, and surface roughness, depend on the average laser power and track overlapping. The increased laser power from 30 W to 50 W at 50% track overlapping increases the machining depth from 88 µm to 981 µm due to a higher energy supply. Further, laser processing at higher power, i.e. 50 W, shows a significant increase in resolidified layer (up to 727 µm). The laser machined surface generated at higher power and track overlapping increases the surface roughness. The laser machined surface developed at 30 W and 50% track overlapping indicates surface roughness (Ra) of 2.9 µm and increases to 6.5 µm for surface processing at 80% track overlapping. Further, the surface roughness of the laser machined increases up to 14 µm for laser processing at higher power (50 W) and high track overlapping (80%).

Wave-like destruction on the input surface of optical media by powerful nanosecond laser pulses as a manifestation of stimulated radial scattering in the surface layer

The wave-like damage structures on the entrance surface of optical media, which observed in various experimental studies with nanosecond laser pulses, are analysed. Sometimes such structures appear as a system of straight lines intersecting at the an-gles defined by the crystallography of a sample. It is proposed that stimulated radial scattering in a surface layer of a sample is responsible for their formation. Keywords: stimulated radical scattering, wave beating in the surface layer, plastic deformation.

Investigation of Multiparameter Laser Stripping of AlTiN and DLC C Coatings.

The lifetime and properties of cutting tools and forming moulds can be prolonged and enhanced by the deposition of hard, thin coatings. After a certain period of usage, the coating will deteriorate. Any remaining coating must be removed prior to successful recoating. Laser stripping is a fast and environmentally friendly coating removal method. In this paper, we present laser removal of two types of coatings deposited on a 1.2379 tool steel substrate, namely, an AlTiN coating with high hardness and a DLC C coating with a small coefficient of friction (COF). A powerful nanosecond laser was employed to remove the coating from the substrate with high efficiency, along with suitable residual surface roughness. Measurements were taken of surface roughness, removed depth, and working time on a stripped area of 1 cm2. The samples were evaluated under a microscope, with a 3D profilometer, and by EDS chemical analysis. Successful removal of the coating was confirmed by optical analysis, but detailed chemical characterisation showed that about 30% of the coating element may remain on the surface. Moreover, a working time of less than 7.5 s per cm2 was obtained in this study. In addition, it was shown that the application of a second low energy, high frequency laser beam pass leads to remelting of the peaks of the material and reduced surface roughness.

Application progress of the stimulated Brillouin scattering phase conjugate mirror in high power nanosecond lasers

Stimulated Brillouin Scattering (SBS) is a third-order nonlinear optical effect, the reflected beam of which has the property of phase conjugation. The reflected beam is time inversed on amplitude, phase and polarization with the incident beam and maintains the same wavefront as the incident light. In the Master Oscillator Power Amplifier (MOPA) nanosecond laser system, the thermal effect in the laser amplifiers and a large number of optical elements in the optical path cause serious wavefront distortion of the transmitted beam, which not only deteriorates the beam quality, but also limits the possibility of further power improvement. Stimulated Brillouin Scattering Phase Conjugate Mirror (SBS-PCM) is utilized widely in this kind of laser system for the fact that using SBS-PCM by making the beam transmit along a round trip in amplifiers which bring in serious wavefront distortion compensates the distortion in real time, therefore is capable of optimizing the beam quality and improving the output power, so as to promote the development of nanosecond laser system in the direction of both high power and high beam quality. In this paper, the basic principle and phase conjugation characteristics of SBS-PCM were briefly introduced theoretically. Then the characteristics and application scope of different SBS-PCM media were compared. The research progress by domestic and foreign institutions on SBS-PCM and the typical application and development process of SBS-PCM in high power nanosecond lasers were summarized. Finally, the development trend of SBS-PCM was prospected.

Laser induced micro-cracking of Zr-based metallic glass using 1011 W/m2 nano-pulses

Low power nanosecond laser irradiation has potential applications in surface processing of metallic glasses (MGs). However, the response of MGs to a relatively low power intensity (1011 W/m2) has not been investigated. In this study, surface microstructures and characteristics of Zr-based MG after low power nanosecond laser irradiation were studied. Micro-cracks with depth in nanoscale and width in microscale were observed on the irradiated MG surface. The effects of laser parameters on the formation and evolution of micro-cracks were further investigated, and surface characteristics were characterized by using X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The results showed that under the peak laser power intensity of 2.3 × 1011 W/m2, when increasing the scanning speed from 1 to 5 mm/s, the maximum width of micro-cracks reduced from 3 μm to be less than 1 μm; further increasing to 10 mm/s, no micro-cracks appeared. When increasing the laser power intensity, the micro-groove structure would be formed with pileup around it and line cracks, open cracks and laser pulse tracks on it. During multi-line laser scanning, the pulse overlap rate also affected the formation and distribution of micro-cracks. At last, the formation mechanism of micro-cracks was discussed. These results would be meaningful for understanding the laser-MG interaction as well as guiding the selection of laser parameters for various applications.

Symmetrical phthalocyanine bearing four triptycene moieties: Synthesis, photophysical and singlet oxygen generation

Novel metal-free (7) and zinc(II) (8) phthalocyanine derivatives bearing four substituted triptycene moieties were synthesized and fully characterized by various spectroscopic methods including 1H-NMR, 13C-NMR, mass, FT-IR and UV-vis. Photophysical and photochemical properties such as electronic absorption, fluorescence emission, fluorescence lifetimes and singlet oxygen generation were investigated in toluene. The UV-vis spectra of 7 and 8 exhibited absorption bands in the visible region. The fluorescence lifetimes of 7 and 8 phthalocyanines were found to be 6.2 and 3.6 ns, respectively. The ability of the examined zinc(II) (8) and metal-free (7) to produce the singlet oxygen has been determined by both chemical and direct methods. While the chemical method 1,3-diphenylisobenzofuran (DPBF) as a singlet oxygen quencher was used in the first method, the direct determination has been done by recording the weak emission band of the singlet oxygen in the near-infrared region (1270 nm). From the direct process, the quantum yields of the singlet oxygen were found to be 0.28 (for 7) and 0.60 (for 8) and similar values were also obtained by the chemical method. The fluorescence quenching behavior of the studied phthalocyanine compounds (8 and 7) by electron-accepting 1,4 benzoquinone (BQ) was also studied in toluene. The powerful nanosecond laser flash photolysis technique was also employed to record the transient absorption spectra of the long-lived triplet states of the investigated compounds.

The modified SRRS threshold criteria for high peak power laser pulses in long air-path transmission considering the near-field beam quality

This paper demonstrates that the stimulated rotational Raman scattering (SRRS) threshold for high peak power laser pulses propagating through a long air path can be influenced strongly by the near-field quality of the laser beams, and the relationship between the SRRS threshold and the near-field beam quality (i.e., spatial intensity modulation index and contrast ratio) can be evaluated quantitatively. By using our three-dimensional numerical model, which can describe the spatial–temporal evolution behaviors of SRRS and is verified by previously published SRRS experimental data, the criteria of the safe transmission distance for high peak power nanosecond laser pulses are obtained, and the modified SRRS threshold criterion formulas considering the near-field beam conditions are presented.

FM-to-AM conversion measurement for high power nanosecond lasers

Through numerical simulations we show that the spectral content of amplitude modulations induced by a transfer function converting frequency modulations required for high-power lasers may be very broad. Hence, measurement of FM-to-AM conversion should be first done in the spectral domain to remove unwanted transfer functions at low frequency scale and then in the time domain to obtain an accurate value.

Laser Cutting of CFRP with a Fibre Guided High Power Nanosecond Laser Source – Influence of the Optical Fibre Diameter on Quality and Efficiency

For the development of a robot based laser cutting process of automotive 3D parts consisting of carbon fibre reinforced plastics (CFRP), investigations with a newly developed fibre guided nanosecond pulsed laser with an average power of PL = 1.5 kW were conducted. In order to investigate the best combination of quality and process time 2 different optical fibres were used, with diameters of df = 400 μm and df = 600 μm. The main differences between the two setups are the resulting focal diameter and the maximum available pulse energy up to EP = 80 mJ. In a first instance, a comparable investigation was performed with both fibres for a constant pulse overlap. For each fibre the minimum required line energy was investigated and cuts were performed, distributed over the complete parameter range of the laser source. The influences of the fibre diameter on the quality and efficiency of the cutting process are summarized and discussed.

Experimental study of the critical point region of aluminum under the action of the powerful nanosecond laser pulse

This paper is a report on the novel experimental method of the study of the thermodynamic parameters of thin aluminum films in the critical point region. The controlled supercritical state of aluminum is achieved for the first time as a result of the heating of these films by the absorption of the powerful nanosecond pulse of Q-switched Nd:YAG laser at the fundamental wavelength. The possibility is demonstrated to find simultaneously the temporal dependencies of the temperature, of the pressure and of the density of aluminum during the experiment with the thin aluminum films confined at both sides by the quartz glass substrates. These dependencies are obtained taking into account the nonlinear dependence on the incident laser intensity of the light reflection coefficient from the irradiated surface of aluminum. For the first time the thermodynamic cooling cycle of aluminum after its heating by the powerful nanosecond laser pulse is plotted in the space of variables’ temperature—pressure and temperature—density that get into the supercritical region.

Picosecond laser with 11 W output power at 1342 nm based on composite multiple doping level Nd:YVO4 crystal

We report results of design and optimization of high average output power picosecond and nanosecond laser operating at 1342nm wavelength. Developed for selective micromachining, this DPSS laser is comprised of master oscillator, regenerative amplifier and output pulse control module. Passively mode-locked by means of semiconductor saturable absorber mirror and pumped with 808nm wavelength Nd:YVO4 master oscillator emits 12.5ps pulses at repetition rate of 55MHz with average output power of ∼100mW. The four-pass confocal delay line forms a longest part of the oscillator cavity in order to suppress thermo-mechanical misalignment. Picked from the train seed pulses were injected to the cavity of regenerative amplifier based on composite Nd:YVO4 crystal with diffusion-bonded segments of multiple Nd doping concentration end-pumped at 880nm wavelength. Laser produces pulses of ∼13ps duration at 300kHz repetition rate with average output power of 11W and nearly diffraction limited beam quality of M2∼1.03. Attained high peak power ∼2.8MW facilitates conversion to the 2nd, 3rd and 6th harmonics at 671nm, 447nm and 224nm wavelengths with 80%, 50% and 15% efficiency respectively. Without seeding the regenerative amplifier transforms to electro-optically cavity-dumped Q-switched laser providing 10ns output pulses at high repetition rates with beam propagation factor of M2∼1.06.

Synthesis, photophysical and photochemical properties of novel phthalocyanines substituted with triptycene moieties

New metal-free and zinc(II) phthalocyanines containing four triptycene moieties were synthesized and characterized by 1H NMR, mass, IR, electronic spectroscopies and elemental analysis as well. The steady-state absorption spectra of the investigated compounds exhibited the main absorption bands in the visible region, at around 700nm. The fluorescence spectra and quantum yields were recorded by steady-state emission measurements. The powerful nanosecond laser flash photolysis technique was employed to record the transient absorption spectra of the long-lived triplet states of the investigated compounds. The singlet oxygen generation and photodegradation properties of the target phthalocyanines under light irradiation were also investigated in tetrahydrofuran solution.

Formation of hollow micro- and nanostructures of zirconia by laser ablation of metal in liquid

Nanosized zirconia particles were generated by the laser ablation of metal in water and aqueous solutions of sodium dodecyl sulfate (SDS); the structure and morphology of the material were studied by X-ray diffractometry, Raman scattering spectroscopy, and scanning electron microscopy. The cubic, tetragonal, and monoclinic phases of zirconia and an organic-inorganic nanocomposite of Zr-SDS are found in the ablation product upon exposure of zirconium to powerful nanosecond laser pulses with a high repetition rate. Morphologically, the synthesized dioxide is present mainly in the amorphous state. Depending on the experimental conditions, the crystallized part of the dioxide consists of aggregates of rounded dense particles and hollow formations, whole or partially collapsed, close to spherical in shape. It is assumed that vapor-gas bubbles generated during ablation serve as templates for hollow micro- and nanoscale structures.

Dense strongly coupled plasma in double laser pulse ablation of lithium: Experiment and simulation

In a simple method of low power nano-second double pulsed laser ablation experiment in collinear geometry, formation of high density strongly coupled plasma is demonstrated. Using time-resolved measurements of the Stark broadened line width and line intensity ratio of the emission lines, the density and temperature of the plasma were estimated respectively. In this experiment, it is shown that ions are strongly coupled (ion-ion coupling parameter comes out to be >4). For comparison, both single and double pulsed laser ablations are presented. For the estimated experimental plasma parameters, first principle Langevin dynamics simulation corroborates the existence of a strongly coupled regime.

Homogeneous Nucleation-Growth Dynamics Induced by Single Laser Pulse in Supersaturated Solutions

Our original approach consists of observing the dynamics of homogeneous crystal growth throughout the volume of highly supersaturated solutions. The crystal growth is induced at room temperature by a powerful single nanosecond laser pulse and is observed using a synchronized time-resolved optical detection. The supersaturated solution readily transforms after the pulse into a homogeneous gel-like slurry consisting of nanometric crystals. From the analysis of the signals via a kinetic model, we derive the elementary accretion rate constant, the nuclei concentration, and its dependence upon the initial supersaturation concentration. This approach could permit a quantitative study of the homogeneous phase transition dynamics for various other systems.

Strong nonlinear growth of energy coupling during laser irradiation of transparent dielectrics and its significance for laser induced damage

The interaction of high power nanosecond laser pulses with absorbing defects, located in the bulk of transparent dielectric materials and having a multilevel electronic structure, is addressed. The model assumes a moderate localized initial absorption that is strongly enhanced during the laser pulse via excited state absorption and thermally driven generation of new point defects in surrounding material. This model is applied to laser induced damage initiation in the bulk of potassium dihydrogen phosphate crystals (KH2PO4 or KDP) and addresses how during a fraction of the pulse duration the host material around the defect cluster is transformed into a strong absorber that leads to the sufficiently large energy coupling resulting in a damage event. This scenario is supported by time resolved imaging of material modification during the initial phases of laser induced damage in KDP and fused silica.

A mass spectrometric investigation of isomers of butane

Butane is an important industrial chemical in which photo-processes are very important for the initiation of reactions. Recent advances in nanosecond pulsed laser technology have led to high laser intensities being available to researchers to enable these photo-processes to be studied in compounds such as butane. The photo-decomposition, dissociation and combustion mechanisms in the neutral butane molecule have been studied in detail, by investigating the multiphoton (MP) dissociative ionisation of its n- and i-isomers, using a time-of-flight mass spectrometer connected to a high power nanosecond laser system. The laser used was a Nd:Yag with a 5 ns pulse width operated at the fundamental wavelength (1064 nm) and the doubled and tripled wavelengths (532 nm and 355 nm). The fragmentation patterns for the isomers were determined for the three wavelengths as a function of laser intensity. Similar laser intensities of between 10(10) and 10(13) W/cm(2) were used at the three wavelengths: 1064, 532 and 355 nm. The mass spectra of each isomer of the butane molecule display a very weak molecular ion and are dominated by fragment ion peaks. The degree of fragmentation increases as the laser intensity increases. Depending on the wavelength some significant differences in the mass spectra of the two isomers were detected and it has been concluded that the isomerisation of i-butane to n-butane is a process which is faster than the duration of the laser pulse used.