YAG Nanosecond Laser Pulses Research Articles

Amplified spontaneous emission of 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole molecule embedded in various polymer matrices

Results of studies on the amplified spontaneous emission (ASE) phenomenon in 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP) molecules in four different polymeric matrices are reported. We have analyzed ASE spectra coming from thin films of DCNP-matrix samples when excited by the Nd:YAG nanosecond pulsed laser doubled in frequency (λ=532nm). We report on ASE characteristics in function of different excitation pulse energy densities evaluating ASE thresholds, exponential gain coefficients and reporting the influence of the specific matrix–dye interactions on the photo-degradation process of the dye.

Silicon surface microstructures created by 1064 nm Nd∶YAG nanosecond laser

We investigated the evolution of surface microstructures created on single crystal silicon wafers by the cumulative Nd ∶YAG nanosecond laser pulses (wavelength 1064 nm ) in different atmospheres (N2, air and vacuum). Micropore structure and the fracture lines are formed after irradiation of a few laser pulses,compared with ripple structures created by laser pulses of wavelengths of 532 and 355 nm. The fracture line structure is different for (111) and (100) silicon. The fracture lines have 60°and 120° intersections for (111) silicon. For (100)-oriented silicon wafers, two sets of fracture lines intersect at 90° to form a grid that divides the surface into rectangular blocks with side length of from 15 to 20 μm. We think that phase explosions are responsible for the growth of micropore structure. The fracture lines are mainly due to thermal stress. Finally, We studied the formation of microstructures under different atmospheres, and the results show that it is closely related to the etching and growth rate.

Simultaneous Raman Spectroscopy−Laser-Induced Breakdown Spectroscopy for Instant Standoff Analysis of Explosives Using a Mobile Integrated Sensor Platform

A novel experimental design combining Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) in a unique integrated sensor is described. The sensor presented herein aims to demonstrate the applicability of a hybrid dual Raman-LIBS system as an analytical tool for the standoff analysis of energetic materials. Frequency-doubled 532 nm Nd:YAG nanosecond laser pulses, first expanded and then focused using a 10x beam expander on targets located at 20 m, allowed simultaneous acquisition of Raman-LIBS spectra for 4-mononitrotoluene (MNT), 2,6-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), C4 and H15 (plastic explosives containing 90% and 75% of RDX by weight, respectively), and Goma2-ECO (Spanish denominated dynamite class high explosive mainly composed of ammonium nitrate, nitroglycol, and dinitrotoluene among other compounds), sodium chlorate, and ammonium nitrate. With the use of a Cassegrain telescope, both Raman and LIBS signals from the same laser pulses were collected and conducted through a bifurcated optical fiber into two identical grating spectrographs coupled to intensified charge-coupled device (iCCD) detectors. With the use of the appropriate timing for each detection mode, adjustment of the laser power on the beam focal conditions is not required. The ability of the present single hybrid sensor to simultaneously acquire, in real time, both molecular and multielemental information from the same laser pulses on the same cross section of the sample at standoff distances greatly enhances the information power of this approach.

Evolution of silicon surface microstructure induced by Nd:YAG nanosecond laser

We investigate the evolution of surface microstructure created on single crystal silicon wafer by irradiation of cumulative Nd:YAG nanosecond laser pulses (wavelength 532 or 355 nm) in vacuum. The wavy structures are formed on silicon surface by 532 or 355 nm laser on early stage of laser pulse irradiation, and nearly concentric but slightly disordered semi-circular rings are formed under 355 nm laser pulse irradiation. With the number of laser pulses increasing, the structures gradually evolved into the concave-convex structures which are similar to the beads. And finally quasi-ordered arrays of conical spikes are formed. The growth of these microstructures depends on the capillary waves and self-organization of structures. We find that the cross-ring structures are caused by the superposition of some wavy structures created by capillary waves in the process of 355 nm laser pulses irradiation.

Analysis of polycyclic aromatic hydrocarbon sequences in a premixed laminar flame by on‐line time‐of‐flight mass spectrometry

A time-of-flight mass spectrometer in reflectron configuration has been used for the real-time detection of combustion products. The products of a premixed laminar C2H4/O2 flame at atmospheric pressure were sampled along its axis, diluted with inert gas and carried to the ion source as a molecular beam under minimal perturbation. Electron ionization and different optical ionization sources are compared. Photoionization was achieved with laser radiation from a Nd:YAG nanosecond pulsed laser at two different wavelengths in the UV range (266 and 355 nm). The mass spectra obtained using laser wavelength of 355 nm and electron ionization present a series of ions regularly spaced by 18 m/z units up to m/z 2000. This series allowed precise calibration of the instrument for compounds of high molecular weight. Information on the chemical nature of the analyzed species has been obtained by comparing mass spectra produced with different ionization methods. In order to better understand the growth mechanisms, polycyclic aromatic hydrocarbon sequences have been analyzed by fast Fourier transform of the mass spectra.

Synthesis and nonlinear optical characterization of copolymers containing alternating 3,4-dialkoxythiophene and (1,3,4-oxadiazolyl)benzene units

We report the synthesis and linear and third-order nonlinear optical (NLO) characterization of two novel copolymers containing alternating 3,4-dialkoxythiophene and 1,4-bis(1,3,4-oxadiazolyl)benzene units. The copolymers were synthesized with a precursor polyhydrazide route. Both copolymers exhibited fluorescence around 430 nm under the irradiation of UV light. The NLO measurements were made with the single-beam Z-scan technique with Nd:YAG nanosecond laser pulses at 532 nm. The nonlinear refractive index of the investigated copolymers was negative, and the magnitude was as high as 10−10 esu. The samples exhibited strong reverse saturation absorption and very good optical limiting properties at the wavelength used. The concentration dependence of third-order NLO parameters was studied. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Morphology and composition of GaN films grown by cyclic-pulsed laser deposition

We describe a detailed study of scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS) analysis to study composition and structure of 500 nm thick polycrystalline GaN samples. The films have been deposited by cyclic-pulsed laser deposition (cyclic-PLD) with a Nd:YAG nanosecond pulsed laser at 1064 nm. SEM pictures of the GaN layers revealed a structure composed of grains with typical dimensions of 200 nm. Coalescence of the grains was more evident for a 1 μm thick sample. EDS mapping of the GaN layer was performed for Ga, N, O, and Al and could be related with the corresponding SEM scan. Both EDS and XPS composition analyses pointed to a Ga rich (or N deficient) GaN layer.

χ(3) measurements and optical limiting in dibenzylideneacetone and its derivatives

We investigated the third order nonlinear optical properties of dibenzylidene acetone (1,5-diphenyl-1, 4-pentadeine-3-one) and its derivatives. The nonlinear measurements were performed by using single beam Z-scan technique with Q-switched Nd:YAG nanosecond laser pulses at 532 nm. Open aperture data for dibenzylidene and its derivatives demonstrate the presence of two-photon absorption at this wavelength. The dependence of χ (3) on donor/acceptor type substituents to the basic compound clearly shows the electronic origin of nonlinearity and hence demonstrates that the mechanism by which the third order nonlinear response enhanced is not by the thermal effects, but instead due to the strong nonlinear absorption and nonlinear refraction of the compounds. The derivatives of the basic compound show very good optical limiting behavior.

Optical limiting effect based on stimulated Brillouin scattering in CCl 4

The optical limiting effect based on stimulated Brillouin scattering (SBS) in a nonlinear medium was investigated. We numerically treated the nonlinear propagation process with a theoretical model, which includes the spontaneous nature of the initiation of SBS and obtained optical limiting effect in the process. Energy limiting, pulse reshaping and stabilization have been demonstrated on SBS mechanism with the nonlinear medium CCl4. The input optical signals were Nd:YAG nanosecond laser pulses with width varying from 16ns to 7ns then to 2ns, the relationship between the transmitted signal and launched pump signal was shown. In the experimental regime, the most stable pulse and a superior energy stabilization of the transmitted pulse were obtained when the laser pulse-width became as short as 2ns. For the energy variation of laser pulses in a wide range of 14-88mJ, the output energy was limited in a quite narrow range 4.5-5.5mJ.

Experimental simulation of a double return‐stroke lightning flash by lasers

A lightning flash is composed of various energetic discharges called return‐strokes which are believed to be responsible for the natural formation NOX in the atmosphere. We report the first experimental attempt in the simulation of such a process. The temporal evolution of electric breakdown in air at atmospheric pressure of two synchronized Nd:YAG nanosecond laser pulses was studied to understand the physical behavior of a double return‐stroke lightning flash by shadowgraphy and interferometry techniques. The temperature of the second return‐stroke reaches about 20,000 K after 1 μs of laser ignition of the second pulse and cools off more rapidly than the first discharge. The cooling rate of the heated gas is about a factor of two faster for the second simulated return‐stroke. The estimated production of nitric oxide of two synchronized laser induced plasmas seems to be not substantially modified as compared with the sum of the isolated single pulses.

Optical Limiting Properties of [60]Fullerene and Methano[60]fullerene Derivative in Solution versus in Polymer Matrix: The Role of Bimolecular Processes and a Consistent Nonlinear Absorption Mechanism

Optical limiting properties of tert-butyl methano[60]fullerene carboxylate were investigated systematically in room-temperature solution at a series of concentrations while the linear transmittance of the solution at 532 nm was kept constant. The results are compared with those of [60]fullerene (C60) obtained under the same experimental conditions. For both C60 and the methano-C60 derivative, optical limiting responses toward the second harmonic of a Q-switched Nd:YAG nanosecond pulsed laser at 532 nm are strongly dependent on the fullerene solution concentrations. The concentration dependence is not related to any special optical effects because the results of chloroaluminum phthalocyanine as a reference in the optical limiting experiments show no such dependence. Instead, the strong concentration dependence in the optical limiting performance of fullerenes in solution is likely due to concentration effects on optical limiting contributions that are associated with bimolecular excited-state processes in the fullerenes. For an examination of the medium viscosity dependence of the bimolecular excited-state processes, optical limiting responses of the methano-C60 derivative in highly viscous nonreactive solvent−polymer blends were determined and compared with those in solution at the same linear transmittances. The optical limiting responses are significantly weaker in the highly viscous media, consistent with medium viscosity effects on diffusional or pseudodiffusional bimolecular excited-state processes. Also consistent with such effects are the results that optical limiting responses of the methano-C60 derivative in poly(methyl methacrylate) polymer films are much weaker than those in room-temperature solution. A reverse saturable absorption mechanism that includes both unimolecular and bimolecular (self-quenching and triplet−triplet annihilation) excited-state processes of fullerenes is proposed. A consistent understanding of the optical limiting properties of fullerenes in room-temperature solution (including the strong concentration dependence), in a highly viscous solvent−polymer blend, and in polymer film is discussed within a single mechanistic framework.