Phase control in coherent population distribution in molecules

A chirped laser pulse transfers population from one level to another level accessible by one photon dipole transition. We have used a pair of phase-locked chirped pulses of same frequency instead of a single chirped pulse to achieve phase control over the population transfer and thus creating coherent population distribution in hydrogen molecule. Simultaneous actions of the phase controlled interference and rapid adiabatic passages due to chirped pulses lead to the control in population transfer from the ground X(v = 0, j = 0) level to the C(v = 2, j = 1) level. We have extended this two-level system to a three-level 1 + 1 ladder system for population transfer from the X level to the J(v = 2, j = 2) level via the C intermediate level using two pairs of phase-locked laser chirped pulses and have achieved laudable control over the coherent population distribution.     

Investigation of gas-dynamic processes in optical discharges with an ablating polymer wall in air and vacuum conditions

Experimental research results are presented on the formation dynamics and macrostructure of optical discharges of condensed matter of a polymer series ((C₂F₄) _n , (CH₂O) _n ) under the action of a femtosecond laser (τ _0.5 ∼ 45–70 fs) pulses (I ₀ ∼ 10¹³−10¹⁵ W/cm²) in the UV — NIR spectral region (λ ∼ 266, 400, 800 nm) under air and vacuum conditions. Electron density distributions in the near-surface area of the optical discharge, vapor expansion, and velocities of shock-wave front propagation are determined for the first time by precise laser pulse micro-interferometry with high spatial and time resolution. The correspondence is shown of the values of the laser ablation spectral-energy threshold, as determined by interference microscopy and the interferometry of a gas-plasma flow. An estimation technique for the total momentum of light-erosion gas-plasma flow in the sub-nanonewton range is proposed and implemented for the first time. The results of comparative analysis are presented on the laser radiation conversion efficiency at different stages of femtosecond optical discharges.     

Laser Heating of Metals

Abstract

The Sprite KrF/Raman laser system has been developed, over the last 12 years, into one of the world's brightest laser sources. It is now a fully scheduled user facility delivering more than 1900 shots per year to a dedicated target chamber. A laser development programme is also supported, addressing the future requirements of the high-power laser community. Sprite has traditionally been operated as a KrF-pumped Raman laser, delivering 10 ps pulses of very high brightness (～ 10²⁰W cm^?2 sterad^?1) and exceptional prepulse contrast ratio (< 10¹⁰). Direct amplification of pulses as short as 3 ps is practical in the Sprite KrF chain, and a chirped pulse amplification scheme has now been implemented delivering 300 fs pulses to target with a power of 1 TW. The next major upgrade to the system will be the installation of a new 40 cm aperture amplifier, Titania, designed to deliver up to 400 J in four Raman pulses.     

Longitudinal magnetic relaxation in superfluid 3He

Longitudinal magnetic relaxation in both ³He-A and ³He-B is studied in a variety of magnetic fields and three different geometries. Relaxation as described by Leggett and Takagi is observed for particular geometries in both ³He-A and ³He-B within a very limited range of magnetic field and temperature near T _c. Under certain other conditions the longitudinal magnetization in ³He-A does not relax monotonically in time. Relaxation in ³He-B very near the critical temperature is shown to be remarkably different following nominal 90 and 180 pulses. Other observations using RF pulses with a time-dependent frequency provide evidence for a negative frequency shift in ³He-B at large tipping angles in certain geometries.Supported by the U.S. Department of Energy under contract number EY-76-S-03-0034, P.A. 143.     

The ablation threshold in amorphous diamondlike carbon films exposed to an ArF excimer laser radiation

An experimental study of the laser-induced ablation threshold in amorphous diamondlike carbon films is reported. The aim is to assess the possibility of using the material as a photoresist in vacuum-ultraviolet laser lithography. Grown on silicon substrates, 10-nm films were irradiated by 20-ns pulses of a 193-nm ArF excimer laser with variable pulse energy per unit area E _p. It is found that the etch rate is very low if E _p < 20 mJ/cm², whereas a single pulse suffices to remove the film completely if E _p=60 mJ/cm². This is attributed to an increase in the thermal ablation component.     

Angularly-resolved High-order Harmonic Generation in Helium

Abstract

Harmonics up to 119th have been generated in helium using a 1·053 μm, 1 ps chirped pulse amplification laser at intensities up to 3 × 10¹⁴W cm^?2, and their far-field spatial distributions have been measured. In the presence of considerable ionization, complex angular distributions are found for harmonics in the plateau region, while near the cut-off, their distribution narrows to approximately that predicted by lowest-order perturbation theory. The experimental results are found to be in good agreement with the predictions of a numerical propagation model which incorporates photoelectron dispersion and strong-field atomic data for the ionization rates and dipole moments.     

Laser-induced surface decomposition of Al2O3 excited in the V-center absorption band

The primary products of desorption from Al₂O₃ surface excited by laser pulses (pulse duration τ∼15 ns; wavelength λ=354 nm, radiant power density P/S<10⁸ W/cm²) in the V-center absorption band were studied by the time-of-flight (TOF) spectroscopy. The TOF spectra show evidence of the desorption of one “ cold” (T ₁=300 K) and two “hot” (T ₂=1000 K, T ₃=4300 K) groups of oxygen molecules with the Maxwell velocity distributions, as well as of the hot Al and O atoms with nonequilibrium energy distributions (E ₁=0.37 eV, E ₂=0.38 eV). A model describing the oxygen desorption as initiated by the electron transitions is suggested, in which escape of the cold O₂ molecules from the surface is related to discharge of the O ₂ ⁻ anions adsorbed on the V-centers, desorption of the hot atoms is attributed to discharge of the surface O⁻ anions, and the appearance of the hot O² molecules is related to the associative desorption of two O⁻ anions localized at the same V-center discharged by a pair of excitons.     

Synchronized mode-locked erbium fibre lasers with intracavity dispersion control using chirped in-fibre Bragg gratings

Abstract

We demonstrate a dual-wavelength fibre laser system using chirped fibre Bragg gratings as reflectors and dispersive elements. The system produces two synchronized trains of soliton pulses with rms jitter of 620 fs.     

Laserstrukturierte nitridische und oxinitridische Beschichtungen abgeschieden mittels Mittelfrequenz‐Magnetron‐Sputtering

The presented work adresses the investigation of the influence of the variation of laser parameters such as pulse energy and the number of pulses on uncoated and coated surfaces with nitride and oxynitride coatings. Three different oxygen gas flows j(O₂) = 10 sccm, j(O₂) = 15 sccm and j(O₂) = 20 sccm in the coating process and correspondingly three different resulting coating compositions with x_{j(O2) = 10 sccm} = (39.8±4.2) atomic‐%, x_{j(O2) = 15 sccm} = (31.6±1.4) atomic‐% and x_{j(O2) = 20 sccm} = (58.4±6.0) atomic‐% were investigated. The analysis of the structure depth using confocal laser scanning microscopy showed that the structure depth is increased with increasing pulse energy and number of pulses. The line‐like interference structures with constant periodicity were detected by scanning electron microscopy. In addition, laser‐induced periodic surface structures with higher spatial frequency and smaller periodicity can be observed more clearly on all coated surfaces in the vertical direction. For coating with x_{j(O2) = 20 sccm} = (58.4±6.0) atomic‐% a formation of microcracks on the flanks of the interference structure is observed after laser structuring.     

Fabrication of Ag and Cu nanowires by a solid-state ionic method and investigation of their third-order nonlinear optical properties

Ag and Cu nanowires were separately fabricated in a direct current electric field using a solid-state ionic method, and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Their optical nonlinearities induced by 8 ns laser pulses from a frequency-doubled, Nd:YAG laser at 532 nm, were investigated using the Z-scan technique. Experimental results indicate the metal nanowires have obvious positive refractive nonlinearities and reverse saturated absorption behaviors. The self-focusing behaviors of Ag and Cu nanowires can be attributed to Kerr-induced self-focusing of laser radiation, the nonlinear refractive indexes of Ag and Cu nanowires are n₂ = 1.7 × 10^− 11 esu and n₂ = 2.4 × 10^− 11 esu respectively, and the two-photon process of Ag and the one-photon process of Cu are responsible for the difference between Ag and Cu nanowires suspended in de-ionized water in nanosecond nonlinear absorptions.     

Quantitative evaluation of B1 insensitivity in nonadiabatic frequency‐selective fat‐suppression RF pulse techniques

Nonadiabatic frequency‐selective fat‐suppression radiofrequency (RF) pulses are simpler than adiabatic RF pulses because nonadiabatic RF pulses are only amplitude modulated. The specific absorption rate (SAR) is lower. However, nonadiabatic RF pulses tend to be sensitive to B₁ inhomogeneity. The purpose of this research was to evaluate whether conventional adiabatic RF pulses can replace nonadiabatic RF pulse techniques. The B₁ insensitivities of nonadiabatic frequency‐selective fat‐suppression RF pulse techniques were calculated by using the Bloch equation, and their effectiveness was evaluated by simulation using the measured B₁ field map. The B₁ insensitivities were compared quantitatively. The B₁ insensitivities of the nonadiabatic RF pulse techniques were ±5% (CHESS), a maximum of ±20% (inversion recovery RF pulse with the best inversion time), ±25% (RF pulse train with two subpulses), and ±44% (RF pulse train with three subpulses). The RF pulse train was the most effective. The B₁ insensitivity of different nonadiabatic RF pulse techniques was specified quantitatively. These results can be used to judge which nonadiabatic RF pulses can replace adiabatic RF pulses. Nonadiabatic RF pulses can reduce the SAR without compromising the image quality and would be useful in higher field‐strength MRI. © 2015 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 25, 86–91, 2015     

In-situ and ex-situ investigations of pulsed laser ablation of Y target

High purity yttrium was ablated by using frequency quadrupled ultra-violet pulses of a Nd:YAG laser (λ = 266 nm, τ_FWHM = 7 ns) with power density of about 1 GW/cm². Laser ablation process was studied using in-situ mass spectrometry of the ablated species in combination with ex-situ analyses of both target surface and deposited films. An increase on the Y ablation rate was found at the beginning, followed by a significant drop with increasing of the number of laser pulses per site until it reaches a constant value after 40 pulses per site. Initial topographic changes on the target surface, observed by scanning electron microscope investigations, and plasma shielding effect could be the origin of these changes on the ablation rate. Careful time-integrated and -resolved mass spectrometric studies of the laser ablated material indicate evident hydridation and oxidation processes in gas phase of ablated yttrium. These results clearly suggest that high purity metallic thin films can be deposited only after a deep and prolonged laser cleaning treatment of the target surface. The present parametric studies are aimed and tailored to prepare photocathodes based on Y thin films to be used in RF photoinjectors.     

Nonlinear optical properties of gold nanoparticles synthesized by ion implantation in sapphire matrix

Single crystal Al₂O₃ substrates have been implanted with 160-keV Au⁺ to a dose of 0.6 × 10¹⁷ or 1.0 × 10¹⁷ cm⁻², with a postimplantation annealing for 1 h at 800°C in air. The obtained composite layers were studied by the method of linear optical reflection; the nonlinear optical characteristics were determined by the RZ-scan technique using picosecond radiation pulses of an Nd:YAG laser operating at 1064 nm. The appearance of a characteristic surface optical plasmon resonance band in the linear reflection spectra was indicative of the formation of gold nanoparticles in a subsurface layer of ion-irradiated Al₂O₃. It is shown that the synthesized particles are responsible for the observed manifestations of nonlinear refraction. The composite layers were characterized by the nonlinear refractive index (n ₂) and the real part of the third-order nonlinear susceptibility (Reϰ ⁽³⁾).     

Reverse Saturable Absorption Induced by Phonon‐Assisted Anti‐Stokes Processes

In materials showing reverse saturable absorption (RSA), optical transmittance decreases at intense laser irradiation. One approach to application of these materials is to protect the sensors or human eyes from laser damage. To date, research has mainly concentrated on thin films and suspensions of graphite and its nanostructure (including nanotubes, graphene, and graphene oxides), which are mainly used as an optical limiter for nanosecond laser pulses. Moreover, thin individual pieces of semiconductor usually exhibit increased transmittance due to saturable absorption when the laser energy (E_laser) is higher than the band gap (E_B). Here, it is shown that indirect gap semiconductor WSe₂ exhibits high RSA on exposure to a femtosecond laser under E_laser > E_B near band gap excitation, which is attributed to the longitudinal optical phonon‐assisted anti‐Stokes transition by the annihilation of phonons and the absorption of photons. An optical limiting threshold (≈21.6 mJ cm^?2) lower than those reported for other optical‐limiting materials currently for femtosecond laser at 800 nm is observed.     

Variances of locally Minimum Variance Quadratic Unbiased Estimators (“MIVQUE's”) of Variance Components

Minimum variance quadratic unbiased estimators (MIVQUE's) of variance components from unbalanced data are functions of the components they are to estimate. To use the MIVQUE expressions for estimation under the one-way classification random model, for example, the unknown between- and within-treatments components, σ² _a and σ² _e , must be replaced by a priori estimates σ² _ao and σ² _eo the resulting estimators are called “MIVQUE's.” For the one-way classification, expressions for the variances and covariance of the “MIVQUE's” are obtained under normality. Numerical comparisons indicate that when σ² _a /σ² _e > 1 (approximately) and unless σ² _ao /σ² _eo Q σ² _a /σ² _e , (a) the “MIVQUE's” have variances near their lower bounds, and (b) the “MIVQUE” of σ² _a is more efftcient than the ANOVA estimator. When σ² _a /σ² _e < 1, the “MIVQUE's” are more dependent on accurate specification of σ² _ao /σ² _eo . The “MIVQUE” and ANOVA estimator of σ² _e have nearly equal variances unless σ² _eo /σ² _eo σ² _a /σ² _e , when the ANOVA estimator has smaller variance.     

Observation of Fibre Laser Instabilities Through Modulation of the Cavity Optical Length

Abstract

A Nd-doped, single-mode, double-clad fibre laser (FL) is coiled around a resonant piezoelectric ceramic ring; this induces modulation of the cavity optical length and two distinct classes of laser intensity instabilities are observed as the ring drive voltage V _P-P is increased. The first class involves a slowly varying intensity, with characteristic time scale in the microsecond range. For fixed drive-voltage frequency f _m ≈ 20 kHz, we consistently observe subharmonics f _m/2 and f _m/4 in the FL output intensity spectrum; noise in the system precludes the observation of higher subharmonic bifurcations. Small-amplitude random relaxation oscillations of the unmodulated FL are strongly amplified as V _P-P is increased and transition to a state of large-amplitude chaotic relaxation oscillations occurs for V _P-P ≈ 4 V; a value of 2·6 has been measured for the correlation dimension of the chaotic attractor. The second class of laser instability occurs simultaneously for V _P-P ? 10 V, and the FL output consists of a train of pulses about 2 ns wide at a repetition rate close to the cavity frequency f _cav ≈ 15 MHz and modulated by the slowly varying chaotic envelope described above.     

Consideration of the residual stress distributions in fatigue crack growth calculations for assessing welded steel joints

To better understand the crack closure and propagation, an analytical model is established. The residual stress effect on fatigue crack growth equations has been considered using the residual stress intensity factor (SIF) (K_res). The joint geometries, residual stress distributions (σ_res) and residual stress ratio (R_res) were considered also. K_res are calculated using the analytical weight function (WF) method and different residual stress distributions. It is to be emphasized that the current approach is little investigated. This is because the WF has already been developed to calculate SIF for an existing crack. The current approach calculates K_res for the crack that initiates and propagates until failure. Different stress distributions have been used, and R_res is defined. The validity of using the WF has been shown. SIF due to applied load (K_app) and applied stress ratio (R_app) have been considered. Fatigue crack growth rate was investigated in accordance with the current approach. The results have been verified and benchmarked.     

Performance of diode-end-pumped Cr, Nd:YAG self-Q-switched and Nd:YAG/Cr:YAG diffusion bonded lasers

Self-passively Q-switching of a diode-pumped Cr,Nd:YAG, where the Cr⁴⁺ is used as a saturable absorber for the 1064 nm laser emission is reported. The maximum average output power was obtained using an output coupler of R=86%. The self-Q-switched diode pumped laser yielded 1.86-W average output power with low threshold pumping power (≈1.7-W), average slope efficiency of ≈34%, pulse duration of about 14–16 ns, and modulation frequency ranging from 2.4 to 73 kHz, depending on the input pumping power. These results are the highest reported for self-Q-switched lasers. Higher slope efficiency (42%) and shorter Q-switched pulses were obtained for a Q-switched Nd:YAG/Cr⁴⁺:YAG diffusion bonded laser. A comparison of the codoped Cr,Nd:YAG laser performance, with that of a diffusion bonded laser is reported and analyzed.     

2.12 Dielectric dispersion and polarization effects in thin transition metal oxide films

Dielectric properties of thin polycrystalline and amorphous films of NiO, NiO(Li), CoO, CoO(Li), Nb₂O₅ and TiO₂ in the thickness range 0.14–15 ωm were studied as a function of frequency, temperature, applied voltage and dopant concentration in the frequency range 3–10⁸ Hz. These films were prepared either by anodization or thermal oxidation and in some cases by reactive sputtering. Experimental data for the real ?₁ and imaginary ?₂ parts of the dielectric constant, ac conductivity σ(ω) and loss tangent tan δ indicate the existence of several dispersion mechanisms. Cole-Cole plots and the modified Debye relations were used to derive the values for relaxation times at low frequencies. The frequency dependence of σ(ω) in the frequency region 10³–10⁶ Hz where σ(ω)αωⁿ indicates that the dispersion mechanism is due to charge carriers hopping between sites with either correlated or non-correlated potentials. The observed quadratic dependence of σ(ω) on frequency in the frequency region 10⁶–10⁸ Hz is attributed to a Debye type loss resulting from thermally activated hopping. Polarization effects at lower frequencies <10³Hz indicate the existence of Schottky barriers at the metal-insulator interfaces in agreement with Simmons theory of MIM structures.