Fast femtosecond laser ablation for efficient cutting of sintered alumina substrates

Fast, accurate cutting of technical ceramics is a significant technological challenge because of these materials' typical high mechanical strength and thermal resistance. Femtosecond pulsed lasers offer significant promise for meeting this challenge. Femtosecond pulses can machine nearly any material with small kerf and little to no collateral damage to the surrounding material. The main drawback to femtosecond laser machining of ceramics is slow processing speed. In this work we report on the improvement of femtosecond laser cutting of sintered alumina substrates through optimisation of laser processing parameters. The femtosecond laser ablation thresholds for sintered alumina were measured using the diagonal scan method. Incubation effects were found to fit a defect accumulation model, with F_th,1=6.0 J/cm² (±0.3) and F_th,∞=2.5 J/cm² (±0.2). The focal length and depth, laser power, number of passes, and material translation speed were optimised for ablation speed and high quality. Optimal conditions of 500 mW power, 100 mm focal length, 2000 µm/s material translation speed, with 14 passes, produced complete cutting of the alumina substrate at an overall processing speed of 143 µm/s – more than 4 times faster than the maximum reported overall processing speed previously achieved by Wang et al. [1]. This process significantly increases processing speeds of alumina substrates, thereby reducing costs, making femtosecond laser machining a more viable option for industrial users.     

Enhanced ionization of perpendicularly aligned H2 in intense laser fields

Using three-dimensional classical ensembles, we have investigated the enhancement of double ionization of perpendicularly aligned H₂ molecules by a 800 nm laser pulse with intensity ranging from 1 × 10¹⁴ W/cm² to 6 × 10¹⁴ W/cm². The simulated results show that double ionization probability of H₂ strongly depends on R and reaches a maximum at an intensity independent critical distance R_C ≈ 5 a.u. Furthermore, the enhancement of double ionization is more pronounced in the cases of weaker or stronger fields. These results, a well indication of the influence of molecular structures and laser–molecule interactions on double ionization of diatomic molecules, are analyzed in detail and qualitatively explained based on the field-induced barrier suppression model and back analysis.     

Investigation of optimum condition in oxygen gas-assisted laser cutting

Laser cutting characteristics including power level and cutting gas pressure are investigated in order to obtain an optimum kerf width. The kerf width is investigated for a laser power range of 50–170 W and a gas pressure of 1–6 bar for steel and mild steel materials. Variation of sample thickness, material type, gas pressure and laser power on the average cut width and slot quality are investigated. Optimum conditions for the steel and mild steel materials with a thickness range of 1–2 mm are obtained. The optimum condition for the steel cutting results in a minimum average kerf width of 0.2 mm at a laser power of 67 W, cutting rate of 7.1 mm/s and an oxygen pressure of 4 bar. A similar investigation for the mild steel cutting results in a minimum average kerf width of 0.3 mm at the same laser power of 67 W, cutting rate of 9.5 mm/s, and an oxygen pressure of 1 bar. The experimental average kerf is about 0.3 mm, which is approximately equal to the estimated focused beam diameter of 0.27 mm for our focusing lens (f=4 cm and 100 W power). This beam size leads to a laser intensity of about 1.74×10⁹ W/m² at the workpiece surface. The estimated cutting rate from theoretical calculation is about 8.07 mm/s (1.0 mm thickness and 100 W power), which agrees with the experimental results that is 7.1 mm/s for 1.0 mm thickness of mild steel at the laser power of 88 W.     

Low-threshold continuous-wave Tm3+:YAlO3 laser

We describe a continuous-wave, low-threshold Tm:YAlO₃ (Tm:YAP) laser operating at 1945 nm with incident threshold pump powers in the 10–20 mW range. The z-cavity containing a 2-mm-long Tm:YAP crystal with 4 at.% Tm³⁺ concentration was end pumped by a continuous-wave Ti:sapphire laser at 795 nm. Tight focusing of the pump and the laser beams enabled low-threshold operation. The power performance of two different cavity configurations with 5-cm radius (R = 5 cm cavity) and 10-cm radius (R = 10 cm cavity) curved mirrors was tested. The best performance was obtained with the R = 10 cm configuration, where, the incident threshold pump power could be lowered to 10 mW after optimizing the polarization direction of the pump beam and by employing double pumping. Theoretical estimation of the threshold power was in good agreement with the experimental observations. The laser could be further tuned from 1842 to 1994 nm.     

Modeling and optimization on Nd:YAG laser turned micro-grooving of cylindrical ceramic material

Nd:YAG laser turning is a new technique for manufacturing micro-grooves on cylindrical surface of ceramic materials needed for the present day precision industries. The importance of laser turning has directed the researchers to search how accurately micro-grooves can be obtained in cylindrical parts. In this paper, laser turning process parameters have been determined for producing square micro-grooves on cylindrical surface. The experiments have been performed based on the statistical five level central composite design techniques. The effects of laser turning process parameters i.e. lamp current, pulse frequency, pulse width, cutting speed (revolution per minute, rpm) and assist gas pressure on the quality of the laser turned micro-grooves have been studied. A predictive model for laser turning process parameters is created using a feed-forward artificial neural network (ANN) technique utilized the experimental observation data based on response surface methodology (RSM). The optimization problem has been constructed based on RSM and solved using multi-objective genetic algorithm (GA). The neural network coupled with genetic algorithm can be effectively utilized to find the optimum parameter value for a specific laser micro-turning condition in ceramic materials. The optimal process parameter settings are found as lamp current of 19 A, pulse frequency of 3.2 kHz, pulse width of 6% duty cycle, cutting speed as 22 rpm and assist air pressure of 0.13 N/mm² for achieving the predicted minimum deviation of upper width of ?0.0101 mm, lower width 0.0098 mm and depth ?0.0069 mm of laser turned micro-grooves.     

Patterning of Aluminium thin film on polyethylene terephthalate by multi-beam picosecond laser

High speed patterning of a 30 nm thick Aluminium thin film on a flexible Polyethylene Terephthalate substrate was demonstrated with the aid of Computer Generated Holograms (CGH׳s) applied to a phase only Spatial Light Modulator. Low fluence picosecond laser pulses minimise thermal damage to the sensitive substrate and thus clean, single and multi-beam, front side thin film removal is achieved with good edge quality. Interestingly, rear side ablation shows significant Al film delamination. Measured front and rear side ablation thresholds were F_th=0.20±0.01 J cm⁻² and F_th=0.15±0.01 J cm⁻² respectively. With laser repetition rate of 200 kHz and 8 diffractive spots, a film removal rate of R>0.5 cm² s⁻¹ was demonstrated during patterning with a fixed CGH and 5 W average laser power. The effective laser repetition rate was f_eff~1.3 MHz. The application of 30 stored CGH׳s switching up to 10 Hz was also synchronised with motion control, allowing dynamic large area multi-beam patterning which however, slows micro-fabrication.     

Structural and vibrational properties of PVT grown Bi2Te3 microcrystals

High-quality Bi₂Te₃ microcrystals have been grown by physical vapor transport (PVT) method without using a foreign transport agent. The microcrystals grown under optimal temperature gradient are well facetted and they have dimensions up to ～100 μm. The phase composition of grown crystals has been identified by X-ray single crystal structure analysis in space group R3?m, a=4.3896(2) Å, b=30.5019(10) Å, Z=3 (R=0.0271). Raman microspectrometry has been used to describe the vibration parameters of Bi₂Te₃ microcrystals. The FWHM parameters obtained for representative Raman lines at 61 cm^?1 and 101 cm^?1 are as low as 3.5 cm^?1 and 4.5 cm^?1, respectively.     

Sonochemical assisted hydrothermal synthesis of ZnO: Cr nanoparticles loaded activated carbon for simultaneous ultrasound-assisted adsorption of ternary toxic organic dye: Derivative spectrophotometric,optimization, kinetic and isotherm study

Chromium doped zinc oxide nanoparticles (ZnO: Cr-NPs) was synthesized by ultrasonically assisted hydrothermal method and characterized by FE-SEM, XRD and TEM analysis. Subsequently, this composite ultrasonically assisted was deposited on activated carbon (ZnO: Cr-NPs-AC) and used for simultaneous ultrasound-assisted removal of three toxic organic dye namely of malachite green (MG), eosin yellow (EY) and Auramine O (AO). Dyes spectra overlap in mixture (major problem for simultaneous investigation) of this systems was extensively resolved by derivative spectrophotometric method. The magnitude of variables like initial dyes concentration, adsorbent mass and sonication time influence on dyes removal was optimized using small central composite design (CCD) combined with desirability function (DF) approach, while pH was studied by one-a-time approach. The maximized removal percentages at desirability of 0.9740 was set as follow: pH 6.0, 0.019 g ZnO: Cr-NPs-AC, 3.9 min sonication at 4.5, 4.8 and 4.7 mg L⁻¹ of MG, EY and AO, respectively. Above optimized points lead to achievement of removal percentage of 98.36%, 97.24%, and 99.26% correspond to MG, EY and AO, respectively. ANOVA for each dyes based p-value less than (<0.0001) suggest highly efficiency of CCD model for prediction of data concern to simultaneous removal of these dyes within 95% confidence interval, while their F-value for MG, EY and AO is 935, 800.2, and 551.3, respectively, that confirm low participation of this them in signal. The value of multiple correlation coefficient R², adjusted and predicted R² for simultaneous removal of MG is 0.9982, 0.9972 and 0.9940, EY is 0.9979, 0.9967 and 0.9930 and for AO is 0.9970, 0.9952 and 0.9939. The adsorption rate well fitted by pseudo second-order and Langmuir model via high, economic and profitable adsorption capacity of 214.0, 189.7 and 211.6 mg g⁻¹ for MG, EY and AO, respectively.     

The study on the nonlinear optical response of Sudan I

The nonlinear optical properties of Sudan I were investigated by a single beam Z-scan technique. The Sudan I ethanol solution exhibited large nonlinear refractive indices under both CW and pulse laser excitations. The nonlinear refractive indices of Sudan I were in the order of ?10^?8 cm²/W under CW 633 nm excitation and ?10^?6 cm²/W under CW 488 nm excitation, respectively. Under the excitation of a pulse 532 nm laser, the nonlinear refractive index n₂ was calculated to be 1.19 × 10^?14 cm²/W. It was discussed that the mechanism accounting for the process of nonlinear refraction was attributed to the laser heating for the CW laser excitation and the electronic effect for the pulse excitation. Moreover, the second hyperpolarizability of Sudan I was also estimated in this paper.     

Remote sensing estimation of Chlorophyll a and suspended sediment concentration in turbid water based on spectral separation

Chlorophyll a and suspended sediment are important indicators of water quality, and remote sensing estimation of them is difficult due to the optical complexity of turbid water. The spectrum above water surface is influenced by phytoplankton, suspended sediment and colored dissolved organic material in water, thus spectral separation is important before estimating one specific component. Based on the field experiment of pond water and Taihu lake, China, this study calculated the Gaussian parameters of Chlorophyll a (Chla) and suspended sediment (SS) through spectral decomposition, and then these parameters were used to separate the mixed spectrum of water samples from pond water and Taihu lake. After spectral separation, the Chla estimation model based on the peak height at 650 nm has high accuracy (R² = 0.78, RMSE = 4.80 mg/m³), better than the band-ratio model; the SS estimation model based on the peak height at 811 nm (R² = 0.82, RMSE = 6.80 mg/L) performs better than the single-band model. Results in this study indicate that spectral separation based on Gaussian parameters is a good method for Chla and SS estimation in turbid lake water.     

Fine structure of far infrared spectrum of ethanol in the lowest OH-torsional vibrational trans-state (e0) of ethanol

In this report the spectroscopic results for far infrared Fourier transform spectrum corresponding to the b-type transitions within the lowest lying trans-substrate (e₀) have been presented. The calculated matrix elements connecting various K-levels suggest that ΔK = 1 transitions within the trans- subs-state should be quite strong but the transitions between the trans state to the gauche states would quite week (practically non-existent). This was also concluded by previous studies using microwave and millimeter wave regions (Pearson et al., 1982; Millar, 1995). The assignments were confirmed by direct observations at the spectrum and the agreement between the observed and calculated spectrum using precise energy levels reported by Pearson et al. (1982). All the strong ^RR and some ^RQ branch lines starting from K = 10 ← 9 through K = 24 ← 23 have been identified. State dependent expansion parameters for all the 15 sub-bands have been presented. These parameters can reproduce the experimental wave numbers within experimental uncertainty. An atlas for about 450 transition lines corresponding to transitions within the e₀ torsional–vibrational species has been prepared. To our knowledge this is the first time the high resolution far infra-red spectral region study for ethanol have been performed.     

Evaluation of correlation between chemical dosimetry and subharmonic spectrum analysis to examine the acoustic cavitation

Currently several therapeutic applications of ultrasound in cancer treatment are under progress which uses cavitation phenomena to deliver their effects. There are several methods to evaluate cavitation activity such as chemical dosimetry and measurement of subharmonic signals. In this study, the cavitation activity induced by the ultrasound irradiation on exposure parameters has been measured by terephthalic acid chemical dosimetry and subharmonic analysis. Experiments were performed in the near 1 MHz fields in the progressive wave mode and effect of duty cycles changes with 2 W/cm² intensity (I_SATA) and acoustic intensity changes in continuous mode on both fluorescence intensity and subharmonic intensity were measured. The dependence between fluorescence intensity of terephthalic acid chemical dosimetry and subharmonic intensity analysis were analyzed by Pearson correlation (p-value < 0.05). It has been shown that the subharmonic intensity and the fluorescence intensity for continuous mode is higher than for pulsing mode (p-value < 0.05). Also results show that there is a significant difference between the subharmonic intensity and the fluorescence intensity with sonication intensity (p-value < 0.05). A significant correlation between the fluorescence intensity and subharmonic intensity at different duty cycles (R = 0.997, p-value < 0.05) and different intensities (R = 0.985, p-value < 0.05) were shown. The subharmonic intensity (μW/cm²) significantly correlated with the fluorescence intensity (count) (R = 0.901; p < 0.05) and the fluorescence intensity due to chemical dosimetry could be estimated with subharmonic intensity due to subharmonic spectrum analysis. It is concluded that there is dependence between terephthalic acid chemical dosimetry and subharmonic spectrum analysis to examine the acoustic cavitation activity.     

Ultrastructural analysis of the low level laser therapy effects on the lesioned anterior tibial muscle in the Gerbil

Low level laser therapy (LLLT) is known for its positive results but studies on the biological and biomodulator characteristics of the effects produced in the skeletal muscle are still lacking. In this study the effects of two laser dosages, 5 or 10 J/cm², on the lesioned tibial muscle were compared. Gerbils previously lesioned by 100 g load impact were divided into three groups: GI (n = 5) controls, lesion non-irradiated; GII (n = 5), lesion irradiated with 5 J/cm² and GIII (n = 5), lesion irradiated with 10 J/cm², and treated for 7 consecutive days with a laser He–Ne (λ = 633 nm). After intracardiac perfusion, the muscles were dissected and reduced to small fragments, post-fixed in 1% osmium tetroxide, dehydrated in increasing alcohol concentrations, treated with propylene oxide and embedded in Spurr resin at 60 °C. Ultrafine cuts examined on a transmission electron microscope (Jeol 1010) revealed in the control GI group a large number of altered muscle fibers with degenerating mitochondria, intercellular substance containing degenerating cell fragments and budding blood capillaries with underdeveloped endothelial cells. However, groups GII and GIII showed muscle fibers with few altered myofibrils, regularly contoured mitochondria, ample intermembrane spaces and dilated mitochondrial crests. The clean intercellular substance showed numerous collagen fibers and capillaries with multiple abluminal processes, intraluminal protrusions and several pinocytic vesicles in endothelial cells. It was concluded that laser dosages of 5 or 10 J/cm² delivered by laser He–Ne (λ = 633 nm) during 7 consecutive days increase mitochondrial activity in muscular fibers, activate fibroblasts and macrophages and stimulate angiogenesis, thus suggesting effectivity of laser therapy under these experimental conditions.     

Photodiode properties of molecular beam epitaxial InSb on a heavily doped substrate

Photodiodes of InSb were fabricated on an epitaxial layer grown using molecular beam epitaxy (MBE). Thermal cleaning of the InSb (0 0 1) substrate surface, 2° towards the (1 1 1) B plane, was performed to remove the oxide. Photodiode properties of МВЕ-formed epitaxial InSb were demonstrated. Zero-bias resistance area product (R₀A) measurements were taken at 80 K under room temperature background for a pixel size of 100 μm × 100 μm. Values were as high as 4.36 × 10⁴ Ω/cm², and the average value of R₀A was 1.66 × 10⁴ Ω/cm². The peak response was 2.44 (A/W). The epitaxial InSb photodiodes were fabricated using the same process as bulk crystal InSb diodes with the exception of the junction formation method. These values are comparable to the properties of bulk crystal InSb photodiodes.     

Internal evaluation of impregnation treatment of waterlogged wood; relation between concentration of internal materials and relaxation time using magnetic resonance imaging

The purpose of this study is to clarify the degree of impregnation resulting from treatment of internal waterlogged wood samples using MRI. On a 1.5 T MR scanner, T₁ and T₂ measurements were performed using inversion recovery and spin-echo sequences, respectively. The samples were cut waterlogged pieces of wood treated with various impregnation techniques which were divided into different concentrations of trehalose (C₁₂H₂₂O₁₁) and polyethylene glycol (PEG; HO-(C₂H₄O)_n-H) solutions. Then these samples underwent impregnation treatment every two weeks. From the results, we found that the slope of the T₁-concentration curve using linear fitting showed the value of the internal area for PEG to be higher than the external area; internal, − 2.73 ms/wt% (R² = 0.880); external, − 1.50 ms/wt% (R² = 0.887). Furthermore, the slope of the T₁-concentration curve using linear fitting showed the values for trehalose to have almost no difference when comparing the internal and the external areas; internal, − 2.79 ms/wt% (R² = 0.759); external, − 3.02 ms/wt% (R² = 0.795). However, the slope of the T₂-concentration curve using linear fitting for PEG showed that there was only a slight change between the internal and the external areas; internal, 0.26 ms/wt% (R² = 0.642); external, 0.18 ms/wt% (R² = 0.920). The slope of the T₂-concentration curve did not show a change in linear relationship between the internal and the external areas; internal, 0.06 ms/wt% (R² = 0.175); external, − 0.14 ms/wt% (R² = 0.043). In conclusion, using visualization of relaxation time T₁, it is possible to obtain more detail information noninvasively concerning the state of impregnation treatment of internal waterlogged wood.     

Effect of high intensity ultrasound on the fermentation profile of Lactobacillus sakei in a meat model system

The objective of this study was to investigate the efficacy of high intensity ultrasound on the fermentation profile of Lactobacillus sakei in a meat model system. Ultrasound power level (0–68.5 W) and sonication time (0–9 min) at 20 °C were assessed against the growth of L. sakei using a Microplate reader over a period of 24 h. The L. sakei growth data showed a good fit with the Gompertz model (R² > 0.90; SE < 0.042). Second order polynomial models demonstrated the effect of ultrasonic power and sonication time on the specific growth rate (SGR, μ, h⁻¹) and lag phase (λ, h). A higher SGR and a shorter lag phase were observed at low power (2.99 W for 5 min) compared to control. Conversely, a decrease (p < 0.05) in SGR with an increase in lag phase was observed with an increase in ultrasonic power level. Cell-free extracts obtained after 24 h fermentation of ultrasound treated samples showed antimicrobial activity against Staphylococcus aureus, Listeria monocytogenes, Escherichia coli and Salmonella typhimurium at lower concentrations compared to control. No significant difference (p < 0.05) among treatments was observed for lactic acid content after a 24 h fermentation period. This study showed that both stimulation and retardation of L. sakei is possible, depending on the ultrasonic power and sonication time employed. Hence, fermentation process involving probiotics to develop functional food products can be tailored by selection of ultrasound processing parameters.     

Photoionization studies of the atmospherically important species N and OH at the Elettra synchrotron radiation source

In this paper angularly resolved photoelectron spectra (PES) and constant-ionic-state (CIS) spectra are presented for the atmospherically important species N and OH.The natural width Γ, line shape parameters q and ρ² and discrete oscillator strengths f have been measured for the members of the N*[2s2p³(⁵S), np] (⁴P) ← N(⁴S) autoionizing resonances for n = 5–10. The n = 5 parameters calculated in this work are in good agreement with the values obtained previously whereas for the resonances with n = 6–10 the values of these parameters are reported for the first time. The asymmetry parameter (β) for the first band of N atoms, the N⁺(³P) ← N(⁴S) ionization, has also been measured in the photon energy range of the above autoionizing resonances.For OH, CIS spectra have been recorded for the first photoelectron band corresponding to the ionization OH⁺(X³Σ⁻, v⁺ = 0) ← OH(X²Π, v″ = 0). In these spectra, rotationally partially resolved bands associated with OH*(a¹Δ3d, v′ = 0) ← OH(X²Π, v″ = 0) resonances have been observed. Suggestions for their assignment are made on the basis of their positions and band simulations which use rotational line strength calculations.     

Contact resistance of multi-walled carbon nanotube/natural rubber nanocomposites with metallic ball

This paper reports on the contact resistance (R_c) between carbon filler/natural rubber (NR) nanocomposite and gold ball: three varieties of nanocomposites were prepared from carbon black (CB) and two kinds of multi-walled carbon nanotubes (MWCNTs) with different diameter. R_c of MWCNT/NR nanocomposite was remarkably less than that of CB/NR nanocomposites. The relationship between R_c of MWCNT/NR nanocomposites and applied load was expressed in the formula, R_c=C·P⁻ⁿ (P: load, C and n: constant): for the MWCNTs (diameters of 13 nm)/NR and MWCNTs (diameters of 67 nm)/ NR nanocomposites, they were expressed as R_c=1724·P^−0.6 and R_c=344·P^−0.37, respectively. The former (MWCNT, ϕ13 nm) showed higher R_c than the latter (MWCNT, ϕ67 nm) over whole region of applied load. The mechanical hardness of the former was higher (90 HsA) than that of the latter (82 HsA). Therefore, the smaller contact area between the nanocomposite and gold ball of the former resulted in higher R_c. The apparent specific contact resistivity was calculated from the observed values of R_c and contact area: 130 Ω mm² and 127 Ω mm² for the former (MWCNT, ϕ13 nm) and the latter (MWCNT, ϕ67 nm), respectively.     

Fabrication of type-II InAs/GaSb superlattice long-wavelength infrared focal plane arrays

In this paper, we present an InAs/GaSb type-II superlattice (SL) with the M-structure for the fabrication of a long-wavelength (10 μm range) infrared (LWIR) focal plane arrays (FPA), which are grown by molecular beam epitaxy (MBE). The M-structure is named for the shape of the band alignment while the AlSb layer is inserted into the GaSb layer of InAs/GaSb SL. A 320 × 256 LWIR FPA has been fabricated with low surface leakage and high R₀A product of FPA pixels by using anodic sulfide and SiO₂ physical passivation. Experiment results show that the devices passivated with anodic sulfide obviously have higher R₀A than the un-sulphurized one. The 50% cutoff wavelength of the LWIR FPA is 9.1 μm, and the R₀A is 224 Ω cm² with the average detectivity of 2.3 × 10¹⁰ cm Hz^1/2 W⁻¹.     

Sound insulation property of Al–Si closed-cell aluminum foam sandwich panels

Al–Si closed-cell aluminum foam sandwich panels (1240 mm × 1100 mm) of different thicknesses and different densities were prepared by molten body transitional foaming process in Northeastern University. The experiments were carried out to investigate the sound insulation property of Al–Si closed-cell aluminum foam sandwich panels of different thicknesses and different densities under different frequencies (100–4000 Hz). Results show that sound reduction index (R) is small under low frequencies, large under high frequencies; thickness affects the sound insulation property of material obviously: when the thicknesses of Al–Si closed-cell aluminum foam sandwich panels are 12, 22, and 32 mm, the corresponding weighted sound reduction indices (R_W) are 26.3, 32.2, and 34.6 dB, respectively, the rising trend tempered; the increase of density of Al–Si closed-cell aluminum foam can also increase the sound insulation property: when the densities of aluminum foam are 0.31, 0.51, and 0.67 g/cm³, the corresponding weighted sound reduction indices (R_W) are 28.9, 34.3, and 34.6 dB, the increasing value mitigating.