Deep UV laser etching of GaN epilayers grown on sapphire substrate

The laser etching of GaN epilayers on sapphire substrate was carried out using a deep ultraviolet pulsed laser (157 nm wavelength, 20 ns pulse width). The quality and morphology of the etched GaN surface were evaluated by scanning electron microscopy, atomic force microscopy and scanning profilometer. Quadrate micro-hole and micro-trenches etched by the 157 nm laser exhibited clean and smooth edges, sharp side walls and very small heat affected zone (HAZ). In order to achieve controllable high-quality etching, the laser and processing parameters, such as laser repetition rate, scan speed, were systematically investigated and optimized. The mechanism analysis shows that, direct photoionization or photo-chemical reaction play predominant role within 157 nm laser etching of GaN epilayers.     

Microstructure and optical absorption properties of Au-dispersed CoO thin films

Au nanoparticles dispersed cobalt monoxide (CoO) composite films were fabricated via a chemical solution approach combined with a spin-coating processing. Au particles were spherical approximately and uniformly dispersed in the amorphous CoO matrix. The mean diameters of Au particles in the Au/CoO thin films with 40 mol% Au are about 30 nm. The optical absorption peaks due to the surface plasmon resonance (SPR) of Au particles were observed in the UV-vis absorption spectra in the wavelength range of 550-650 nm. The SPR peaks exhibit a red shift and intensify with increasing Au content from 10 to 40 mol%, but show a blue shift and weaken from 40 to 60 mol%. The band gap E_g decreases with increasing Au contents from 10 to 40 mol% but increases by further increasing Au content.     

Embedded phase change material microinclusions for thermal control of surfaces

The performance and lifetime of sensors, microelectronic devices, mini air vehicle components and other systems can benefit from maintenance of a constant temperature profile or a local temperature that does not exceed a pre-selected, critical value during short-term transient loading events. In this work, the utility of surfaces featuring phase change materials (PCMs) encapsulated within micro-reservoirs was evaluated as a passive thermal management system for mitigation of transient temperature spikes. Patterned silicon substrates with 40 μm diameter, 10 μm deep trenches were prepared by reactive ion etching in an Ar/CF₄ plasma. The features were filled with an organic phase change material possessing a high latent heat and known to undergo melting at a target operating temperature of approximately 60 °C. An infrared microscope was used to produce temporally and spatially resolved temperature maps of the surface during heating. When a constant heat flux of approximately 200 W m^− 2 was used for uniform heating of the sample area from 50 to 75 °C the PCM encapsulated materials demonstrated an isothermal plateau period lasting 5–8 s near the PCM melting point. A similar plateau was also observed during cooling below the PCM melting point. From the isothermal time during heating the areal thermal energy storage density was estimated to be approximately 800 J m^− 2, close to that predicted by the calculated volume of PCM contained within the sample. The effects of PCM inclusions on surface temperature gradients during pulsed laser heating were also investigated. An infrared (980 nm) laser at a fixed power and repetition rate was focused into a 1 mm diameter beam on surfaces with and without PCM encapsulation for rapid heating (analogous to heating from resistive contacts in microelectronic circuits) of different duration from 0.5 to 5.0 s. The surface temperature stayed below the desired temperature limit for 100 mW laser pulses lasting up to 2 s in duration. Transient temperature profiles of Si/PCM surfaces showed that micro-scale volumes of embedded phase change material stored sufficient quantities of heat to stabilize and otherwise control surface temperatures for potentially useful periods of time in selected applications.     

Kinetics of picosecond laser treatment of silver nanoparticles on ITO substrate

In this work, the effect of ITO substrate on Ag NP size, morphology and photoinduced absorption depending on the time of 30 ps laser treatment were explicitly studied. Silver nanoparticles with an average diameter of ∼40 nm supported on indium tin oxide (ITO) were irradiated with a tightly focused pulsed laser (doubled frequency beam) at 532 nm. The size transformation of silver nanoparticles induced by a single pulse of Nd:YAG laser (λ = 532 nm, pulse width = 30 ps) was directly observed by an electron scanning microscopy (FE-SEM) on indium tin oxide surface. Simultaneously the change in the absorption and the corresponding derivatives are also presented. These morphological changes are accompanied by a significant change in the optical absorption properties of the array. This study demonstrates that picosecond laser irradiation is an excellent technique to operate and control the properties of nanostructured materials on solid supports.     

Low temperature pulsed laser deposition of textured γ′-Fe4N films on Si (1 0 0)

Low-temperature reactive pulsed laser deposition (PLD) was used to prepare iron nitride films. The textured γ′-Fe₄N films with (0 0 1)-orientation were deposited on Si (1 0 0) substrate with Fe buffer layer at a substrate temperature as low as 150 °C. The (0 0 1)-oriented γ′-Fe₄N film grew on the Fe buffer layer with a 3.5-nm thick amorphous interlayer, which eliminated the lattice mismatch stress between them. The films showed a columnar granular morphology with an average lateral grain size of approximately 110 nm. The films exhibited good soft magnetic properties with a high in-plane M_r/M_s value of 0.84. The magneto-optic Kerr effect results indicated an in-plane magnetic isotropy and confirmed the high remnant ratio of the γ′-Fe₄N films.     

Internal structure of clusters of partially coherent nanocrystallites in Cr-Al-N and Cr-Al-Si-N coatings

Nano-sized clusters consisting of strongly preferentially oriented, partially coherent nanocrystallites were observed in Cr-Al-N and Cr-Al-Si-N coatings deposited using cathodic arc evaporation. Microstructure analysis of the coatings, which was done using the combination of X-ray diffraction (XRD) and transmission electron microscopy with high resolution (HRTEM), revealed furthermore stress-free lattice parameters, size and local disorientation of crystallites within the nano-sized clusters in dependence on the aluminium and silicon contents, mean size of these clusters and the kind of structure defects. Within the face-centred cubic (fcc) Cr_{1 − x − y}Al_xSi_yN phase, the stress-free lattice parameter was described by the equation a = (0.41486 − 0.00827 · x + 0.034 · y) nm. The size of individual crystallites decreased from ∼ 11 nm in Cr_0.92Al_0.08N to ∼ 4 nm in Cr_0.24Al_0.65Si_0.10N. These nanocrystallites formed clusters with the mean size between 36 and 56 nm. The mutual disorientation of the partially coherent nanocrystallites forming the clusters increased with increasing aluminium and silicon contents from 0.5° to several degrees. The disorientation of neighbouring nanocrystallites was explained by the presence of screw dislocations and by presence of phase interfaces in coatings containing a single fcc phase and several phases, respectively.     

Effective segregation during Czochralski growth and spectral properties of Nd in GSAG crystal

GSAG and Nd:GSAG crystals were grown by standard Czochralski technique. The refractive index of GSAG in the wavelength range 500-3000 nm, effective segregation coefficient and absorption cross sections of Nd³⁺ in GSAG were determined by optical absorption method. The effective segregation coefficient was calculated to be 0.525. The spectroscopic and laser properties of Nd:GSAG crystal were studied by Judd-Ofelt analysis. The Judd-Ofelt parameters were calculated to be Ω₂ = 1.32 × 10⁻²¹ cm², Ω₄ = 2.93 × 10⁻²⁰ cm² and Ω₆ = 3.91 × 10⁻²⁰ cm². With these intensity parameters, the values of absorption and emission oscillator strengths, transition probabilities, fluorescence ratios and radiative lifetimes were obtained. The large fluorescence ratio of |[4F]3/2〉 is very favorable for laser operation at 942 nm. These results confirm that Nd:GSAG is a suitable laser material for water vapor detection by DIAL.     

Pulsed laser deposition of transparent ZnO/MgO multilayers

We report on the structural, optical and electrical properties of ZnO/MgO multilayers grown by pulsed laser deposition technique. The film thickness of ZnO sublayer (t_ZnO) was found to have great impact on the properties of ZnO/MgO multilayers. Investigations reveal the structural phase transition from wurtzite phase to cubic phase with corresponding decrease in ZnO thickness. The optical transmittance of the multilayers is over 80% in the visible region and there is a gradual shift of absorption edge towards a longer wavelength with corresponding increase in ZnO sublayer thickness. Two absorption bands at around 400 nm and 270 nm were observed in the transmission spectra of ZnO/MgO multilayers for similar ZnO and MgO layer thickness, which has been ascribed to phase separation to hexagonal and cubic phases. The calculated optical band gap E_g shows a widening from 3.51 eV to 6.23 eV with corresponding decrease in ZnO sublayer thickness from 100 nm to 23 nm, which in turn leads to an increase in resistivity in ZnO/MgO multilayers. These results provide important information for the design and modeling of ZnO/MgO optoelectronic devices due to their adjustable bandgap energies.     

Enhancement of abrasion and corrosion resistance of duplex stainless steel by laser shock peening

The results for laser shock peening of duplex stainless steel (22% Chromium-5% Nickel) using a pulsed Nd:YAG laser (wavelength = 532 nm, pulse width = 8 ns) for the application to high-capacity pumps for reverse-osmosis type seawater desalination plants are reported. By properly selecting the process parameters such as laser intensity of 10 GW/cm², laser pulse density of 75 pulse/mm², and 100 μm thick aluminum foil as a protective coating layer, wear volume and corrosion rate of duplex stainless steel could be reduced by 39% and 74.2%, respectively. The number and size of corrosion pits produced on wear track during copper accelerated acetic acid salt spray test decreased approximately by half as a result of laser shock peening. It is shown that laser shock peening is a practical option to improve abrasion and corrosion properties of a seawater desalination pump parts.     

Substrate temperature influence on boron carbide coatings grown by the PLD technique

Boron carbide films were synthesized by laser ablation technique, using a target of B₄C with 99.9% of purity, varying the substrate temperature between room temperature and 650 °C, in order to produce the hexagonal phase (h-BC). Films were grown on (111)-silicon wafers in an ultra high vacuum system with a base pressure in the order of 10^− 7 Pa. For the films' growth, an atmosphere of (CH₄) at a pressure of 2.5 Pa was used. During the process, the substrate temperature was varied in order to identify the influence of this parameter on the coatings' structure, composition and morphology. XRD analysis did not present peaks of BC, possibly because of the amorphous character of the film that has different phases. Films were characterized by several techniques as in situ Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and ex situ electron diffraction. Results present a concentration of 50 at.% for the sample grown to 650 °C. Electron diffraction showed an interplanar spacing (d₍₀₀₂₎ = 0.334 nm) and also other hkl reflections have been identified. Lattice parameters calculated from the interplanar spacing a = 0.585 nm and c = 1.2 nm obtained for the sample grown at 650 °C are similar to those reports for hexagonal boron carbide.     

Structure, magnetic, and magnetocaloric properties of amorphous and crystalline La0.4Ca0.6MnO3+δ nanoparticles

We report a systematic study of the effects of size reduction on the magnetic and magnetocaloric properties of amorphous and crystalline La_0.4Ca_0.6MnO_3+δ nanoparticles. The materials were synthesized using a modified wet chemical Pechini route, starting with nitrate precursors to produce the perovskite structure. Phase purity, structure, size, and crystallinity were investigated using XRD and TEM. Thermal treatments resulted in nanocrystals with average diameters of 25 nm, 50 nm, and 130 nm, as well as amorphous particles ∼10 nm in diameter. Magnetic measurements revealed broad, second order ferromagnetic transitions in the nanocrystals. As particle size increased from 10 nm to 130 nm, the Curie temperature shifted from 40 K to 255 K. Magnetization, magnetic entropy change (ΔS_M), and refrigerant capacity (RC) also increased with size in the nanocrystalline samples. For a field change of 5 T, the 130 nm particles exhibit a magnetic entropy change of 2.8 J/kg K and a large refrigerant capacity of ∼240 J/kg at 250 K. Interestingly, the 10 nm amorphous particles undergo the sharpest magnetic transition, leading to a larger value of ΔS_M than in the 25 nm or 50 nm crystalline particles. These results reveal that size reduction has a significant impact on the magnetic and magnetocaloric properties of La_0.4Ca_0.6MnO_3+δ.     

Cemented carbide surface modifications using laser treatment and its effects on hard coating adhesion

A pulsed HyBrID copper laser (510 nm, 30 ns, 13.8 kHz) was used for the treatment of cemented carbide substrate before deposition of TiCN/Al₂O₃/TiN coating by the MT-CVD process. The influence of the laser treatment on the surface morphology, surface structure and coating adhesion was investigated based on the laser irradiation dynamics used here. The experimental results showed that a large variety of cemented carbide surface textures could be obtained, depending on the laser intensity and number of applied laser pulses. Moreover, this laser process was found to produce some less carbon non-stoichiometric WC phases such as β-WC_1 − x and α-W₂C. Finally, using the Rockwell C adhesion test as output criteria, two sets of laser parameters were identified that produced a surface with adhesion strength comparable to that of commercial tools pretreated by micro-sandblasting.     

Preparation of hydrophobic surface on steel by patterning using laser ablation process

Currently there is an increasing demand for the application of hydrophobic surface in industrial and biological processes. It has been found that the contact angle with liquid, closely related to hydrophobicity of a solid surface, is largely determined by micro-geometrical structure and chemistry of the said surface. In this investigation, the hydrophobicity of steel surface was achieved by implementing micro-patterns on substrate using laser ablation process, and depositing amorphous carbon (a-C) thin film using magnetron sputtering technique. For the patterning, a short pulse excimer laser with a wavelength of 248 nm was used to etch the substrate surface to form different controlled patterns. Based on the models built by Wenzel and Cassie-Baxter, the depth of etched grooves, d, the clearance between two grooves, a, and the width of grooves, b, were optimized to obtain the largest contact angle. It was found that when a pattern was set at a = 25 μm, b = 50 μm and d = 8 μm, the contact angle of the surface could be increased to about 130°, compared to the 68.5° found from a plain smooth steel surface (R_a ≤ 0.01 µm). As a preliminary investigation, an amorphous carbon (a-C) coating was deposited on the patterned surface. It shows that the contact angle was increased further by about 10°-20° on a patterned surface.     

Development of micro milling tool made of single crystalline diamond for ceramic cutting

In order to machine micro aspheric ceramic molds precisely and efficiently, micro milling tools made of single crystalline diamond (SCD) are developed. Many cutting edges are fabricated 3-dimensionally on the edge of a cylindrical SCD by a laser beam. Flat binderless tungsten carbide mold was cut with the developed tool to evaluate the tool wear rate and its life. Some micro aspheric molds of tungsten carbide were cut with the tool at a rotational speed of 50,000 min⁻¹. The molds were cut in the ductile mode. The form accuracy obtained was about 100 nm P–V and the surface roughness 12 nm R_z.     

On the formation of laser induced self-organizing nanostructures

Laser induced self-organizing rippled nanostructures on steel are formed by femtosecond laser pulses. They are applied as hydrophobic surfaces. A low fluence results in ‘regular ripples’ with a spatial repetition of 300-500 nm, orientated perpendicular to the laser polarization direction. In twinned areas ‘pre-ripples’ with much smaller wavelength (about 150 nm) are observed, with a different orientation. We found indications that the energy absorption depends on the crystal orientation and that pre-ripples are only formed at very low fluence. Pre-ripples initiate on secondary carbides or on grain boundaries. At higher energy, regular ripples initiate in areas with pre-ripples; at even increasing fluence disordered structures are obtained.     

Fabrication of 3D photonic structure on glass materials by femtosecond laser modification with HF etching process

In this study, a novel method of an array of square columns with high aspect ratio (about 10) on quartz and a pattern of complex multi-layered woodpiles on slide glass are firstly fabricated by femtosecond laser inner modification with hydrofluoric acid (HF) etching. The three-dimension (3D) patterns composted with embedded gratings are scribed by femtosecond laser focused inside the bulk materials with the central wavelength of 517 nm, the repetition rate of 100 kHz, the pulse width of 350 fs and the power of 100 mW. For the quartz glass, a high aspect ratio (about 10) structure of squared column array with the width of 10 μm and the height of 100 μm is formed from a grid pattern etched by 15 wt% HF for 15 min and 5 wt% HF for 90 min. For the slide glass, a 3D piled-octahedron structure with the feature size of 10 μm is developed from a multi-layer woodpile pattern etched by 5 wt% HF for 2 h. Moreover, the optical band structures of both 3D structures are calculated by the plane wave expansion method. And then, the defect effect of light propagation with sample paths on square and piled-octahedron structures are designed to verify the optical characteristics by finite difference time domain (FDTD) simulation in this study.     

Temperature- and excitation-dependent photoluminescence in TlGaSeS layered crystals

Photoluminescence (PL) spectra of TlGaSeS layered single crystals have been studied in the wavelength region of 695-1010 nm and in the temperature range of 20-56 K. Two PL bands centered at 773 (1.605 eV, A-band) and 989 nm (1.254 eV, B-band) were observed at T = 20 K. Variations of both bands have been investigated as a function of excitation laser intensity in the range from 4.2 to 111.4 mW cm⁻². These bands are attributed to recombination of charge carriers through donor-acceptor pairs located in the band gap of the crystal. Radiative transitions from deep donor levels located at 0.721 and 1.069 eV below the bottom of conduction band to shallow acceptor levels located at 0.008 and 0.011 eV above the top of the valence band are suggested to be responsible for the observed A- and B-bands in the PL spectra, respectively.     

Effect of deposition temperature on structural, surface, optical and magnetic properties of pulsed laser deposited Al-doped CdO thin films

The objective of this work is to study the influence of deposition temperature on structural, surface, optical and magnetic properties of the Al doped CdO thin films prepared by pulsed laser deposition (PLD) technique. KrF excimer laser (λ = 248 nm, τ_l = 20 ns, ν = 10 Hz, ?_l = 2.5 J/cm²) was employed for the deposition of thin films. It is observed by XRD results that films grown at room temperature and 100 °C show preferential growth along (1 1 1) and (2 0 0) directions while high temperatures (200-400 °C) lead to preferential growth along the (2 0 0) direction only. The optical constants (n, k, α, and optical band gap energy) of films measured by spectroscopic ellipsometry show strong dependence upon deposition temperature. M-H loop of films, measured by vibrating sample magnetometer, deposited at 25 °C and 100 °C show paramagnetic nature while films deposited at temperatures (200-400 °C) exhibit ferromagnetic character. Scanning electron micrographs show degraded elongated grains at lower deposition temperatures, while smooth and compact surface is observed for films deposited at higher deposition temperatures.     

Effect on response time and diffraction efficiency of co-usage azo dye and carbon nanoparticle in nematic liquid crystal

In this work, the formation of the photo-induced grating and nonlinear optical properties such as diffraction efficiency (η), refractive index modulation (Δn), nonlinear index coefficient (n₂) of nematic liquid crystal (E7) doped by azo dye (Methyl Red) and C60 have been investigated by diffraction grating measurements. Diffraction efficiencies of 441 nm pump and 632 nm probe beams were measured in two-wave mixing experiment. Maximum diffraction efficiency was found 26% doped with both 1% MR and 0.5% C60, while cells without C60 had maximum diffraction efficiency of 19% under 30 mW laser illumination. Rise time was found to increase with Methyl Red concentration. The nonlinear index coefficient, n₂, was calculated to be 11 × 10⁻³ cm²/W and highly depend on MR concentration.     

Crystallization and magnetic behavior of nanosized nickel ferrite prepared by citrate precursor method

NiFe₂O₄ nanoparticles have been synthesized by citrate precursor gel formation with subsequent heat treatment. Differential thermal and thermogravimetric (DTA/TG) analyses show that the metal citrates decomposed around 230 °C followed by crystallization of the ferrite. X-ray diffraction (XRD) patterns reveal the formation of the cubic spinel phase in the samples after sintering the gel at 350 °C, 500 °C and 700 °C. For the samples annealed at 350 °C and 500 °C a small amount of α-Fe₂O₃ was detected whereas single phase was obtained for the sample annealed at 700 °C. The lattice constant a for all the samples is comparable to the value of the bulk material. The mean crystallite size D_XRD of the samples determined from XRD line broadening is 26.2-28.5 nm. Transmission electron microscope (TEM) analysis shows that the single-phase particles form clusters with the particle size in the range of 21-82.5 nm and the most probable value D_TEM of 55.4 nm. Magnetic measurements show that its Curie temperature T_C is close to the bulk value while the spontaneous magnetization M_s at 5 K is lower than that of the bulk. The thermal variation of M_s in the temperature range from 5 to 300 K can be best fitted to a modified Bloch T^α law with the exponent value α ≈ 2. The magnetization data are explained with reference to the disordered surface spins and the finite size effects. In this investigated temperature range, the coercive force H_c decreases linearly with increasing temperature. The coercivity mechanism in the nanoparticle sample with broad particle size distribution is expected to be complex and different factors which affect the H_c value were proposed.