Modulation Of Laser Beam Research Articles

Nonlinear phenomenon in nanocrystallites produced by laser-induced etching of silicon

Self-phase modulation of continuous wave argon-ion laser beam by a medium containing silicon nanocrystallites is reported here. Refractive index of nanocrystallites shows nonlinear behavior with the intensity of probing laser beam when size of nanocrystallites is decreased below 8 nm. A simple quantitative explanation for the observed optical fringes is given in terms of distribution of sizes of nanocrystallites and intensity of laser beam. Nonlinear optical response of silicon nanocrystallites is found to have a strong correlation with the size of nanocrystallites, which are determined by the Raman scattering and photoluminescence using two-dimensional quantum confinement effects of phonons and electrons, respectively.

Characterization of static and dynamic optical actuation of Al microbeams by microscopic interferometry techniques

Out-of-plane optical actuation of thin Al cantilever microbeams was investigated as a function of laser beam power, spot size, modulation frequency and location on the beam. Several microscopic interferometry techniques including quantitative time-averaged interferometry and phase shifting stroboscopic interferometry were used to determine the resonant frequencies, vibration mode shapes and optically induced mean deflection of the microbeams. Transient response measurements by stroboscopic interferometry are also demonstrated. Experimental results show that on-wafer, non-contact, full-field dynamic measurements can be performed up to at least several hundred kHz although a large light power is necessary to actuate Al microbeams. It is demonstrated that dynamical optical actuation tends to saturate as a function of laser beam power and is maximum when the laser spot is located below 1/3 of the beam length. It is also evidenced that fundamental resonant frequencies measured with optical actuation are lower than that measured with piezoelectric actuation. Results are discussed by considering that actuation is dominated by photothermal effects.

Electron beam modulation using a laser-driven photocathode

Coherent synchrotron radiation may lead to a microwave instability on an electron bunch at wavelengths much smaller than the bunch length. It is possible that ripples (prebunching) on the electron bunch distribution may seed this instability. We report on research exploring this effect using a longitudinally modulated drive laser to generate a modulated electron beam. Our first step is to develop simulations that will help us study the beam generation process using PARMELA. Preliminary experiments on laser beam and electron beam modulation, conducted at the Source Development Laboratory at the National Synchrotron Light Source, show modulation at frequencies in the terahertz regime is attainable. Longitudinal prebunching may enhance the performance of FEL or other radiative devices in the terahertz regime. Alternatively, longitudinal control over the electron beam might be an effective method of suppressing coherent synchrotron radiation instabilities that cause beam break-up.

Polarization-Modulated Second Harmonic Generation in Collagen

Collagen possesses a strong second-order nonlinear susceptibility, a nonlinear optical property characterized by second harmonic generation in the presence of intense laser beams. We present a new technique involving polarization modulation of an ultra-short pulse laser beam that can simultaneously determine collagen fiber orientation and a parameter related to the second-order nonlinear susceptibility. We demonstrate the ability to discriminate among different patterns of fibrillar orientation, as exemplified by tendon, fascia, cornea, and successive lamellar rings in an intervertebral disc. Fiber orientation can be measured as a function of depth with an axial resolution of ∼10 μm. The parameter related to the second-order nonlinear susceptibility is sensitive to fiber disorganization, oblique incidence of the beam on the sample, and birefringence of the tissue. This parameter represents an aggregate measure of tissue optical properties that could potentially be used for optical imaging in vivo.

Transverse mode of an atom laser

The transverse mode of an atom laser beam that is outcoupled from a Bose-Einstein condensate is investigated and is found to be strongly determined by the mean--field interaction of the laser beam with the condensate. Since for repulsive interactions the geometry of the coupling scheme resembles an interferometer in momentum space, the beam is found show filamentation. Observation of this effect would prove the transverse coherence of an atom laser beam.

Adaptive self-aligning, bi-directional interconnection using double phase conjugation in Rh:BaTiO 3

The results of an experimental demonstration of a very efficient adaptive interconnection between two low power lasers, using a double phase-conjugate mirror (DPCM) in Rh:BaTiO 3 are presented. Using low power incident beams, high phase conjugate reflectivity (∼120%) and, simultaneously, high coupling transmission efficiency (∼50%) of the double phase conjugation process have been measured. After subtracting the losses in the crystal, this diffraction efficiency corresponds to an almost complete deflection of one input laser beam into the direction of the other one. We compare our experimental results on phase conjugate reflectivity and coupling transmission efficiency with analytical predictions of DPCM and find they are in agreement. The build-up time of the interconnection channel was measured taking into account the competition of DPCM with self-pumped phase conjugation. The robustness of the interconnection to out-of-plane angular changes of the direction of one of the input beams is demonstrated. The influence of the laser beam modulation on the interconnection efficiency is studied. We show that the interconnection efficiency is very good even at very strong modulation of the input beams.

Self-polarization and migratory polarization in thin lead zirconate-titanate films

This paper reports on a study of the electrical properties of 0.7–1-µm-thick textured PZT ferroelectric films prepared by rf magnetron sputtering of a PbZr0.54Ti0.46O3 target which additionally contained 10 mol % lead oxide. Such films are shown to feature a combination of a self-polarized state and migratory polarization. The totality of the data obtained suggest that the films had n-type conduction. As shown by the laser beam modulation technique, the polarization was distributed nonuniformly in depth, with most of the poled state localized near the lower interface of the thin-film ferroelectric capacitor. The mechanism underlying the onset of this self-polarization is related to the charging of the lower interface of the structure by electrons, which occurs during the cooling following the high-temperature treatment of the PZT film, and to poling of the bulk of the film by the charged interface. This mechanism of the self-polarization of ferroelectric films is believed to have a universal character.

Self- and cross-phase modulation of high-intensity laser beams emerging from a diamond-turned KDP wedge

The Laboratory for Laser Energetics plans to install KDP wedges on each beam line of the OMEGA laser system in order to improve the on-target laser uniformity by decreasing the instantaneous speckle through spatial averaging of the two resultant orthogonally polarized beams. The proposed wedge-production method, diamond turning, produces small residual scratch marks, causing each beam to acquire a pseudorandom phase perturbation. In addition, the orthogonally polarized beams interfere such that their combined polarization state continuously cycles through all elliptical states along any transverse plane. Since the nonlinear refractive index depends on the polarization state, intense beams accumulate a periodic phase perturbation that is greater for linear polarization. Propagation of both types of phase perturbation yields an intensity modulation that tends to be larger in the neighborhood of linear polarization, through a combination of diffraction and self- and cross-phase modulation. However, one- and two-dimensional calculations using a symmetrized split-step Fourier method demonstrate that diamond-turned KDP wedges are not a significant source of intensity modulation under OMEGA laser conditions. Installation of diamond-turned rather than polished wedges will reduce costs without adversely affecting the system performance.

Photothermal Radiometric Frequency-Swept Studies of Kinetics Process In P-Si Wafers

Frequency-swept (chirped) infrared photothermal radiometry was combined with conventional single-frequency modulation of an Ar ion laser beam to yield a quantitative study of the surface-state annealing process induced by the low fluence laser beam on n and p-type Si wafers. The appearance of signal transient was found to be strongly dependent on the electronic quality of the wafer surface and was absent in the thermally oxidized p-si wafer. The low injection carrier minority lifetimes and diffusion coefficient were not affected by the laser-surface interaction, but the front surface recombination velocity strongly decreased with the time exposure, in the case of n-Si (positive transient). It was found that in the case of p-Si wafers with negative transient both carrier-lifetime and front surface recombination velocity exhibit changes. A two trap rate model was advanced to explain the positive transient behavior in terms of surface-state annealing and carrier injection.

Fifth-order corrected field descriptions of the Hermite-Gaussian (0,0) and (0,1) mode laser beam.

In this paper, we extend the work of Barton and Alexander [J. App. Phys. 66, 2800 (1989)] on the fifth-order corrected field expressions for a Hermite-Gaussian (0,0) mode laser beam to more general cases with adjustable parameters. The parametric dependence of the electron dynamics is investigated by numerical methods. Finally, the fifth-order corrected field equations for the Hermite-Gaussian (0,1) mode are also presented.

Strong self- and cross-phase modulation effects in chromium-doped KTiOPO4 crystals

The presence of chromium impurities in the crystal matrix of chromium-doped KTiOPO4 is shown to be at the origin of a strong cubic optical nonlinearity of this material. As a result, a pronounced self-phase modulation of a Gaussian laser beam at 532 nm is observed in the far field as a diffraction pattern consisting of alternating dark and bright rings. The induced nonparabolic and polarization sensitive refractive index change is also used to modulate a weak probe laser beam at 633 nm. A simple theoretical model incorporating a laser heating process and a thermo-optical effect accounts for the main features observed in our experiments and elucidates the mechanisms leading to the diffraction pattern formation.

Optical pumping and modulation techniques with a molecular Ramsey–Bordé interferometer

Using the example of a matter-wave interferometer with K2 molecules, we present different methods to simplify the observed interference structures within a hyperfine pattern for improving phase measurements of the interference structure. We consider optical pumping for depletion of specific components to simplify the observed spectra. In connection with a lock-in amplifier, amplitude modulation as well as frequency modulation of laser beams in the interferometer are investigated for better detection of the interference pattern. As a follow-up treatment, digital filtering is demonstrated.

CO2 and Nd–YAG laser beam welding of 5754–O aluminium alloy for automotive applications

Federal regulations have recently been enacted to reduce significantly the atmospheric pollution caused by motor vehicles. This has compelled automotive manufacturers to improve the fuel efficiency of cars and light trucks by using lightweight materials such as aluminium. The focus of the present work is to develop welding procedures for autogenous CO2 (continuous mode) and Nd–YAG (continuous mode) laser beam welding of 5754–O aluminium alloy. The mechanical and microstructural characteristics of the welded joints were evaluated using tensile tests, microhardness tests, optical microscopy, and chemical analysis. Results indicate that this alloy can be autogenously laser welded with full penetration, minimum surface discontinuities, and little if any loss of magnesium through vaporisation from the fusion zone. The total elongation (all weld metal) in the longitudinal direction for 5754–O laser welds produced using 5 kW CO2 and 3 kW continuous wave (CW) Nd–YAG shows a slight decrease with increasing travel speed. Studies indicate that the decreasing tendency is probably due to the orientation of the grains with respect to the loading direction. The welds produced using the 5 kW CO2 laser at travel speeds between 127 and 212 mm s-1 displayed a total longitudinal elongation of 19.13–15.12% and those produced using the 3 kW CW Nd–YAG laser at travel speeds between 85 and 148 mm s-1 displayed a total longitudinal elongation of 22.6–18.15%, compared with the base metal value of 28.1%. An observation of great interest was that the weld surface condition did not have any effect on the ductility of the 5754–O aluminium alloy studied in the present investigation.

Self-modulation of an intense laser beam in near-critical homogeneous plasma

An investigation of longitudinal intensity modulation of a relativistic laser beam in near-critical homogeneous plasma (ne>14ncr) is performed in this paper. On the basis of a nonlinear Schrödinger equation of electromagnetic vector potential, a model equation of the intensity profile is derived via the variational approach from Action and Lagrangian. Longitudinally uniform and longitudinally modulated pulse structures are viewed as different solutions of this model equation. For these two different types of pulse structure, their relative stability under different structure parameters is numerically studied. Numerical results indicate that this relative stability depends closely on the transverse structures of the laser pulse.

Dynamics of Optical Nonlinearity in Water-in-Oil Microemulsion

Strong optical nonlinearity is reported in water-in-oil microemulsion. Its time evolution is studied by self phase modulation of a Gaussian laser beam by an optically thin film. The material is WAD (water/AOT/decane, where AOT denotes sodium bis-diethylsulfosuccinate) far from critical points and near the percolative transition from electrically insulating to electrically conducting. A simple mathematical model is introduced for a quantitative description of the results. Time constants are shown to be too short to be due to the diffusion of spherical clusters. Electrostriction of rod-like clusters and their consequent diffusion could be involved in the development of the phenomenon.

Dynamics of self-focusing and self-phase modulation of elliptic Gaussian laser beam in a Kerr-medium

Using a direct variational technique involving elliptic Gaussian laser beam trial func- tion, the combined effect of non-linearity and diffraction on wave propagation of optical beam in a homogeneous bulk Kerr-medium is presented. Particular emphasis is put on the variation of beam width and longitudinal phase delay with the distance of propagation. It is observed that no stationary self-trapping is possible. The regularized phase is also seen to be always negative.

CO 2 and Nd:YAG laser beam welding of 6111-T4 aluminum alloy for automotive applications

There are two important trends that are currently taking place in the forming of sheet metal into automotive body components. One of these trends is the substitution of aluminum sheet alloys for steel. The other trend is the use of tailor welded blanks. Currently, there exists strong motivation for research to combine these technologies in order to produce tailor welded aluminum blanks. The focus of the current study is to develop welding procedures for autogenous CO2 (continuous mode) and Nd:YAG (continuous and pulsed mode) laser beam welding of 6111-T4-aluminum alloy. The mechanical and microstructural characteristics of the welded joints were evaluated using tensile tests, microhardess tests, optical microscopy, and chemical analysis. Results indicate that this alloy can be autogenously laser welded with full penetration, minimum surface discontinuities and little if any loss of magnesium through vaporization from the fusion zone. The total elongation (all weld metal) in the longitudinal direction for the laser welds made using 3 kW CO2 and 2 kW pulsed Nd:YAG show a decreasing trend with increasing travel speed. Studies indicate that the decreasing trend is probably due to the combination of two factors: orientation of the grains with respect to the loading direction and solidification cracking. The total elongation (all weld metal) of the laser welds made using 5 kW CO2 and 3 kW cw Nd:YAG did not show a consistent trend with travel speed. The main reason for this is solidification cracking. The welds made with 2 kW pulsed Nd:YAG with travel speed between 42 and 63 mm/s displayed the highest total longitudinal elongation (20.78% to 16.45%), compared to the base metal values of 27.8%. A very interesting observation was that the weld surface condition did not have any effect on the ductility of the 6111-T4-aluminum alloy studied in this investigation.

Miniature high-repetition-rate TEA CO 2 laser with surface-wire-corona preionization

In this article, an experimental study of a miniature, sealed-off, high-repetition-rate transversely excited atmospheric-pressure (TEA) CO 2 laser with a kind of surface-wire-corona preionization (SWCP) is described. We have utilized an SWCP consisting of SiO 2 dielectric tube and a fine wire strained and attached to the dielectric surface. A BN ceramic material, which has an extremely low coefficient of thermal expansion of about 5 × 10 −7/°C was employed as a supporter of the resonator. A measurement on emission spectra of SWCP has been reported. By applying SWCP to the TEA CO 2 laser, efficient laser operation at an overall efficiency of 9.8% with an output energy of 150 mJ has been achieved from a small discharge volume of 25 cm 3 with an active length of 230 mm. At the pulse repetition frequency of 60 Hz, the TEM 00 mode of laser beam with pulse width of 60 ns was obtained.

Variation of the linewidth of the A0+←X 1Σ+ transition of NaI

A continuous wave (cw) tunable single mode laser beam was propagated perpendicular to a collimated effusive molecular beam of NaI, and the excitation spectra of the A0+(v′,J′)−X 1Σ+(v″,J″) transitions of the NaI molecule were measured by monitoring the intensity of the molecular fluorescence. A large variation of the linewidth and line intensity with the rotational quantum number J was observed. The linewidths of rotationally resolved lines, attributable to lifetime broadening originating from predissociation, were obtained by a fitting of the observed spectral line shape to a Voigt profile.

Direct Fabrication Methods of Surface Relief Electro-Optic Gratings in Azo-Polymer Films

Two fabrication methods of surface relief electro-optic (EO) gratings in azo-polymer films are described: surface deformation by a continuous wave laser and laser ablation by a pulse laser. In the fabrication process, the periodic intensity modulation of laser beams is formed by an interference fringe or a phase mask. The surface relief EO grating is fabricated by poling the surface relief grating (SRG) after the SRG fabrication process. The linear dependence of an applied voltage in the modulation efficiency of the first-order diffraction beam is observed. The proposed methods are effective for highly-efficient surface relief EO grating fabrication.