Pressure dependence of small signal gain and saturation intensity of a gold-vapor laser using various buffer gases in gain medium

A pair of gold-vapor laser (627.8 nm) in an oscillator-amplifier configuration was used to investigate the small signal gain, g₀, and saturation intensity, I_s, as amplifying parameters, versus pressure at various types of buffer gas. It was shown that the small signal gain decreases and saturation intensity increases linearly with increasing the pressure. Moreover, the values of these parameters are different using various gas mixtures in gain medium. Both parameters were estimated to be more at helium buffer gas atmosphere than that of neon or their mixed ones.     

Behavioral studies of gain and saturation energy density in a N2 laser with corona preionization

By the use of an oscillator-amplifier (OSC-AMP) TE N₂ laser system, both operating with corona preionizers, the laser parameters (small-signal gain g ₀ and saturation energy density E _s) have been measured at different N₂ gas pressure and for different states of the AMP preionizer. The details of our measurements are presented. In addition, the effect of He buffer gas on the laser parameters has also been investigated; it was found that both laser parameters remain almost constant up to 150 Torr of He gas pressure, indicating that He mainly affects the discharge uniformity. Finally, with the variation of the N₂ laser gain values in the literature, we found that the g ₀-N₂-laser parameter depends strongly on the length of the laser channel. Based on the most recent measurements, a graph showing this dependency is introduced.     

Small-signal gain dependence on active medium length and axial thermal profile in metal-vapor lasers

We measure the small-signal gain coefficient g ₀ of a gold vapor laser for a 627.8 nm transition through the implementation of a discharge-driven oscillator-amplifier configuration. We study the gain dependence on the length of the laser active medium. In addition, we measure the output power of a copper-vapor laser for two different thermal insulator distributions and simulate the thermal-length profile to determine the effective length of the active medium. We show that the small-signal gain coefficient depends on the effective length of the active medium and do not depend on the physical (actual) length of the active medium.     

Nitrogen lasers: Optical devices of variable gain coefficient

A N₂-laser system, consists of an oscillator and an amplifier of different electrode lengths, ranged from 5 to 31 cm, operating under the optimized gas pressures conditions and 14 kV input voltage, was used to measure small signal gain, g₀, and saturation energy density, E_s. It was found that g₀-parameter follows a profile of the type m + n/l_AMP, where l_AMP is the effective electrode length of the laser amplifier, and (m, n) are some constants. So, by reducing the active length a gain value as high as ∼1.2 cm⁻¹ for l_AMP = 5 cm was obtained. The proposed function, with a rather good approximation, can be used to explain almost all the reported g₀-values of laser systems with moderate current densities. For explaining more details of the reported gain coefficients, the g₀l_AMP-parameter was introduced, where it was realized that with a good approximation a Gaussian profile is a suitable function for explaining the reported g₀l_AMP-products. Furthermore, we found that at a constant input voltage the saturation energy density and output energy density, E_out, regardless of the type of N₂-lasers and their operational conditions, are linearly correlated. Based on this observation, and by operating the system at different input voltages, the functional dependence of the measured E_s/E_out-values with respect to the operational voltages is introduced. Details of our present observations, along with the previously reported g₀, and E_s-measurements are presented graphically, or tabulated. The results are giving some interesting features of TE/TEA N₂-lasers for lasers g₀, E_s predictions which are valuable for laser designs and also for further theoretical investigations.     

Effects of inert gases on the degenerate four-wave-mixing spectrum of NO2

We describe the effects of He, Ar and N₂ on the resonant degenerate four-wave-mixing spectrum of NO₂. We report results obtained using the phase-conjugate and forward-geometry experimental configurations for various laser intensities and bandwidths. We find that the effect of buffer-gas pressure on the reflectivity of the laser-induced grating depends critically on the relative value of the laser intensityI to the saturation parameterI _sat. WhenI I _sat the four-wave-mixing signal initially decreases with increasing buffer-gas pressure. However, at pressures above ca. 100 Torr the signal increases. WhenI I _sat the signal is found to increase with buffer-gas pressure even at the lowest pressures studied. These observations do not agree with the standard model of degenerate four-wave mixing in the gas phase. We have investigated the source of these effects by employing different polarisation geometries of the pump and probe laser fields, and conclude that thermal gratings are responsible for the increase in signal observed at high buffer-gas pressure. This conclusion is supported by a simple gas kinetic model.     

Capillary discharge soft X-ray laslng in Ne-like Ar pumped by long current pulses

A capillary discharge soft X-ray laser operating at 46.9 nm on the transition 3p-3s (J = 0-1) of the Ne-like Ar has been realized by exciting the active medium with a long half-cycle duration current pulse of 140 ns. The current is produced by discharging a 10 nF water dielectric capacitor, initially charged to voltages lower than 200 kV by a six stage Marx generator, through a 15-cm long capillary channels. The laser amplification has been obtained by properly adjusting all the other experimental parameters. Utilizing a 3-mm in diameter Al₂O₃ capillary channel initially filled with 0.3 torr of Ar pressure, a laser beam, which has a 4-mrad divergence and a time duration of 1.3 ns, is characterized by a gain of 0.6±0.1 cm^-1. The stability of the plasma compression followed by the laser emission is verified. Received 13 December 2001     

Polarization-dependent gain and saturation energy density in a TE N2-laser amplifier

The polarization-dependent gain, g₀, and saturation energy density, E_s, in a TE N₂-laser amplifier were measured, using an oscillator-amplifier laser system for different amplifier electrode gap separations, d_AMP, of 7, 9 and 4 mm and gas pressure of p = 77, 60, and 165 Torr, respectively. It was realized that for the amplifier with the gap separation of 7 and 9 mm, where the pd_AMP-value has its optimum-value of 54 Torr cm, the gain-coefficient for the input beam with the polarization parallel to the discharge electrodes (P-polarized beam) is slightly higher than the case when the beam polarization is perpendicular to the discharge electrodes (S-polarized beam). In this case, the depolarization ratio for d_AMP = 7 mm is the range of ∼0.998 to ∼0.962 as the input voltage increases from 12 to 15.5 kV, having a minimum of 0.937 around 14 kV. For the E_s-parameter, the reversed order is true. Also, it was found that the saturation energy densities for three states of polarization are linearly related to the output energy densities, having different slopes of 0.11, 0.14, and 0.17 for R (randomly), P- and S-polarization, respectively. The present measurement supports qualitatively the prediction of polarization-inhomogeneity model for the stimulated emission cross-section, showing that randomly oriented dipoles exhibit slightly larger gain on the direction of the electric field.     

Simultaneous detection of atomic and molecular hydrogen using a tunable ArF excimer laser

We report preliminary experiments which demonstrate the simultaneous detection of both atomic and molecular hydrogen using a single tunable ArF laser. The tunable ArF laser was modified to lase simultaneously at two wavelengths by the addition of a second grating to the oscillator stage as in earlier work by Ketterle et al. for KrF operation. To our knowledge, this is the first demonstration of dual-wavelength capability with a tunable ArF excimer. The H atom diagnostic utilized tunable ArF excimer laser output at 193.29 nm which was Raman shifted in D₂ (first Stokes) to generate 205.14 nm radiation which then excited the H atom to its (3s, 3d) states via a two-photon transition. Fluorescence was detected at 656 nm on the (3s, 3d) 2p transition. The second wavelength from the tunable excimer was used to excite several rovibronic LIF transitions of hydrogen molecule via two-photon absorption on the E ¹ _g ⁺ X ¹ _g ⁺ (2,0) band. Fluorescence occurs at 750 nm and 830 nm on the (E ¹ _g ⁺ B ¹ _g ⁺ ) transitions.     

Multiline laser probing of CO:He, CO:N2, and CO:O2 active media in a wide-aperture pulsed amplifier

Multiline laser probing was applied for the diagnostics of the active medium of a wide-aperture carbon monoxide laser amplifier. The temporal behavior of the small-signal gain in the active medium of a pulsed electron-beam sustained discharge CO laser amplifier was studied with a specially designed probe CO laser in a wide range of ro-vibrational spectral lines (from V = 5 up to V = 31). The results were analyzed in order to reconstruct the temporal behavior of gas temperature and vibrational populations in CO-containing gas mixtures, CO:He and CO:N₂, and oxygen-rich CO: O₂ mixtures. __________ Translated from Preprint No. 13 of the P. N. Lebedev Physical Institute, Moscow (2005).     

Speeds of sound and isentropic compressibilities of (2-ethoxyethanol + ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol) binary mixtures at 298.15 K

The speed of sound (u) has been obtained at a frequency of 8.3 MHz in {CH₃CH₂OCH₂CH₂OH + HOCH₂CH₂(OCH₂CH₂)_nOH}for n = 0, 1, 2, and 3 over the whole composition range of studied binary liquid mixtures, at T = 298.15 K. The speed of sound values were combined with those of our previous results for densities and viscosities to obtain isentropic compressibility (κ_s), free volume (V_f), and intermolecular free length (L_f). From all these data excess isentropic compressibility (κ_s^E), excess free volume (V_f^E) and excess intermolecular free length (L_f^E) as well as the deviations of the speed of sound (Δu) were obtained. The results are interpreted in terms of molecular interactions occurring in the solutions.     

Structural, optical and morphological studies on laser ablated nanostructured WO3 thin films

Thin films of tungsten trioxide (WO₃) are prepared by reactive pulsed laser deposition (PLD) technique on glass substrates at three different substrate temperatures (T_s). The structural, morphological and optical properties of the deposited films are systematically studied using X-ray diffraction (XRD), grazing incidence X-ray diffraction (GIXRD), micro-Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV-VIS spectrophotometry techniques. X-ray diffraction analysis reveals that crystalline WO₃ films can grow effectively even at 300 K at an oxygen pressure of 0.12 mbar. All the films deposited at various T_s exhibit mixed oxide phase consisting of orthorhombic and triclinic phase of tungsten oxide with a preferred orientation along (0 0 1) lattice plane reflection. Micro-Raman results are consistent with X-ray diffraction findings. The SEM analysis shows that deposited films are porous and crystalline grains are of nano-metric dimension. The effect of T_s on mean surface roughness studied by AFM analysis reveals that mean surface roughness decreases with increase in T_s. The optical response of WO₃ layers measured using UV-VIS spectrophotometry is used to extract the optical constants such as refractive index (n), extinction coefficient (k) and optical band gap (E_g), following the method of Swanepoel.     

Role of the wall ablation in the operation of a 46.9 nmAr capillary discharge soft x‐ray laser

A capillary discharge pumped soft x‐ray laser operating at 46.9 nm on the 3p–3s transition of the Ne‐like Ar has been realized by pumping the active medium with a relatively slow current pulse (dI/dt ≈ 6 · 10¹¹ A/s). In order to study the role of the ablation in the production of the laser effect, the intensity of the amplified 46.9 nm line has been investigated using the same pumping current pulses in the plastic (polyacetal) and ceramic (Al₂O₃). We showed that the ablation of the capillary walls is unfavorable both for the compression and stability of the plasma and consequently for the soft x‐ray laser production. The amplification and lasing effects are observed only in the ceramic channel. The measurements of the line intensity at 46.9 nm showed the lasing with a gain‐length product of ≈ 9, a laser pulse energy of ≈ 5 μJ, a pulse duration of 1.3 ns and a beam divergence of ≈ 3.5 mrad. In addition, effect of the scaling of the time of lasing with the initial plasma diameter was demonstrated experimentally and compared with a one‐dimensional MHD model.     

Role of gas- and condensed-phase kinetics in burning rate control of energetic solids

A simplified two-step kinetics model for the combustion of energetic solids has been used to investigate the effect of gas-phase activation energy on flame structure and burning rate and the role of gas- versus condensed-phase kinetics in determining burning rate. The following assumptions are made: a single-step, unimolecular, high activation energy decomposition process which is overall relatively energetically neutral, is followed by a highly exothermic single-step, bimolecular, gas-phase reaction with arbitrary activation energy, E? _g. The results show that at extremely low (<10⁴ Pa) or high (>10¹² Pa) pressures the burning rate is controlled by the condensed-phase reaction kinetics for any E?_g. At intermediate pressures (10⁵-10¹⁰ Pa) gas reaction kinetics contribute strongly to the burning rate. In this pressure range the value of E?_g plays an important function in determining the role of gas- and condensed-phase reactions: for high E?_g a gas-phase kinetically controlled regime exists; for low E?_g both condensed and gas-phase kinetics are important. The limiting behaviour of asymptotically large E?_g (gas kinetically controlled burning rate) occurs at about E?_g=20 kcal mol^?1 for parameters representative of HMX, while the vanishingly small E?_g behaviour occurs near E?_g. Previous comparison with burning rate and temperature profile data has suggested that the small-E?_g limit is the more accurate of the two extremes. This may imply that the important (burning rate influencing) primary gas reaction zone near the surface has more the character of a chain reaction mechanism than the classical high activation energy thermal decomposition mechanism. To the degree that the low-E?_g chain reaction model is a better approximation than the high-E?_g thermal decomposition model, the possibility exists that the chemistry of either reaction zone, including the molecular structure of the material, might be exploited for favourable tailoring of burning rate. The low-E?_g model also provides a rational mechanistic explanation of observed trends in burning rate temperature sensitivity with pressure and temperature for materials like HMX in terms of a gradual transition from mixed gas- and condensed-phase kinetic control to condensed-phase only kinetic control as the pressure decreases.     

Study of anomalous lasing behavior on the twoJ=0−1 transitions in Ne-like V

We present an experimental report on the recent observation of lasing at 26.1 and 30.4 nm on the 3p ¹ S ₀–3s ³ P ₁ [termedG((0–1)] and 3p ¹ S ₀–3s ¹ P ₁ [termedE(0–1)] transitions in Ne-like V, in which the normally weakG(0–1) transition was observed to lase more strongly than theE(0–1) transition. The experiment was performed on the Asterix IV iodine laser with a prepulse 5.23 ns before the main pulse. At a target length of about 2.5 cm, it was found that, while theG(0–1) andE(0–1) lines have comparable intensities in V, theE(0–1) line dominates spectra from Mn, Cr, Ti and Sc, which have adjacent nuclear charges. It was also found that the two lasers in V also have different temporal histories and spatial distributions. This is in contrast to the LASNEX + XRASER simulation, which predicts virtually similar temporal and spatial behavior for the two transitions. On leave from: Shanghai Institute of Optics and Fine Mechanics, P.O. Box 800211, Shanghai, People's Republic of China     

Optical,photoelectric and electric properties of single-crystalline Sb2Se3

Measurements of absorption coefficient in the region of the absorption edge, of spectral distribution of photoconductivity and dependence of electrical conductivity upon temperature on Sb₂Se₃ single crystals are given. The absorption of light was proved to correspond to indirect forbidden transitions. The value of optical gapE _g ^opt =(1·11±0·02) eV forE a andE c was determined. From photoconductivity and conductivity measurements the values of the gaps areE _g ^opt =1·11 eV andE _g ^el =1·04 eV. The anisotropy of the electrical conductivity parallel and perpendicular to the cleavage plane is 2·2.     

Relation between the reduced and unreduced hardness in nanomicroindentation tests

Hooke’s law is generalized to the case of arbitrary elastic or plastic indentation , where ɛ=q/E _r is the elastic strain, q is the average pressure over the contact area, E _r is the reduced elastic (Young’s) modulus, A is the projected area of the contact, w ₁ is the deformation in elastic indentation by a flat punch. On this basis a relation is obtained between the reduced hardness H and unreduced hardness H _h, which depends on the ratio w _s/w₁=m _s; w _s is the elastic deformation along the perimeter of the indent, and m _s≅0.78. It is shown that the correction ΔE _r to the elastic modulus E _r determined from the condition of linearity of the initial part of the unloading diagram, is ΔE _r=0.27(ΔP/P _m), where ΔP is the value used in the calculation of E _r for the length of the linear part of the diagram, reckoned from the maximum load P _m. It is shown that for metallic construction materials of medium hardness one has q=HM, where HM is the Meyer hardness. With increasing HM and increasing angle ϕ at the tip of the indenter, the ratio HM/q grows by an exponential law. Zh. Tekh. Fiz. 69, 42–48 (July 1999)     

Magnetic field effects on the hydrogen abstraction reaction of triplet 4-methoxybenzophenone with thiophenol as studied by a picosecond laser flash photolysis technique

The hydrogen abstraction reaction of triplet 4-methoxybenzophenone with thiophenol at 265 K has been studied with a newly developed picosecond laser flash photolysis apparatus under magnetic fields of 0–1.7 T. The decay rate constant of the radical pair generated was found to increase from 3.42 × 10⁹ s^?1 to 4.15 × 10⁹ s^?1 with increasing the magnetic field from 0 to 1.7 T. The observed magnetic field effects can be explained by the Δg mechanism. Using the simple kinetics model with the Δg mechanism, the rate constant of the escape process from the pair (k _esc) and two rate constants for the T-S spin conversion process (k _T-S) at 0 and 1.7 T were found to be 1.97 × 10⁹ s^?1, 1.45 × 10⁹ s^?1, and 2.12 × 10⁹ s^?1, respectively. From the magnetic field dependence on k _T-S, the difference in the g values of the 4-methoxybenzophenone ketyl and phenylthiyl radicals was estimated to be 0.0087.     

Amplification in one-dimensional random active medium near the lasing threshold

We studied the transmittance of a random amplifying medium near the lasing threshold by using the convergence criterion proposed by Nam and Zhang [Phys. Rev. B 66 73101, 2002] that allows separating the physical solutions of the time-independent Maxwell equations from the unphysical ones and describing critical size L _c of a random system in statistical terms. We found that the dependence of the critical gain ∈ ^″ _c (at which the lasing threshold occurs) as a function of number of layers is configuration-dependent and thus the lasing condition for random media is described by means of the probability of finding of physical solutions and evaluated by averaging over the ensemble of random configurations. By employing this approach we inspect the validity of the two-parameter scaling model by Zhang [Phys. Rev. B 52 7960, 1995], according to which the behavior of the random system with gain is described by relation 1/ξ = 1/ξ ₀ + 1/l _g, where ξ and ξ ₀ are localization length with and without gain, respectively, and l _g = 2/ω∈ ^″, is gain length, where ∈ ^″ is imaginary part of the dielectric constant that represents gain. We show that the range of validity of this relation depends on the ratio of both lengths and also affects the slope of the ln Λ_c(q) (where Λ_c ≡ L _c/ξ ₀ is normalized critical size and q ^?1 ≡ l _g/ξ ₀ is dimensionless gain length). We extend the study of the relation for the critical size L _c by inspecting the dependence of the slope of the ln Λ_c(q) on the strength of the randomness. We interpret its behavior in terms of the statistical properties of the localized states. Namely, by studying of the variance of the Lyapunov exponent λ (the inverse of the localization length ξ ₀) we have found that the slope of the ln Λ_c(q)) reflects the transition between two different regimes of localization with Anderson and Lifshits-like behavior that is known to be indicated by peak in var(λ). We discuss the generalization of two-parameter scaling model by implementing revisited single parameter scaling (SPS) theory by Deych et al. [Phys. Rev. Lett. 84 2678, 2000] which allows describing non-SPS regime in terms of a new scale l _s.

     

Energy band structure and refractive properties of LiRbSO4 crystals

The energy band structure of mechanically free and compressed LiRbSO₄ single crystals is investigated. It is established that the top of the valence band is located at the D point of the Brillouin zone [k = (0.5, 0.5, 0)], the bottom of the conduction band lies at the Γ point, and the minimum direct band gap E _g is equal to 5.20 eV. The bottom of the conduction band is predominantly formed by the Li s, Li p, Rb s, and Rb p states hybridized with the S p and O p antibonding states. The pressure coefficients corresponding to the energies of the valence and conduction band states and the band gap E _g are determined, and the pressure dependences of the refractive indices n _i are analyzed.     

Effect of pressure and mechanical stress on the electronic properties of AlN and GaN

The fundamental properties of the AlN and GaN compounds with a wurtzite structure under external hydrostatic pressure, uniaxial mechanical stress σ_⊥ along the hexagonal axis, and biaxial mechanical stress σ_⊥ in the basal plane of the unit cell have been considered in terms of first-principles calculations in the frame-work of the density functional theory. The pressures of the phase transitions from the structures of wurtzite and zinc blende to the structure of rock salt have been obtained. The behavior of the structural parameters, interband transitions, and positions of the charge neutrality level has been investigated. The calculated pressure coefficients of the band gap are as follows: ∂E _g /∂p = 40.9 meV/GPa, −∂E _g /∂σ _| = −4.2 meV/GPa, and −∂E _g /∂σ _⊥ = 45.2 meV/GPa for AlN and ∂E _g /∂p = 33.0 meV/GPa, −∂E _g /∂σ _| = 23.6 meV/GPa, and −∂E _g /∂σ _⊥ = 9.6 meV/GPa for GaN. The pressure coefficients of the charge neutrality level in almost all cases are substantially smaller than the corresponding values obtained for the band gap E _g .