Frequency degenerate oscillation in semilinear photorefractive oscillator with reflection gratings

The oscillation intensity of a semilinear photorefractive oscillator with reflection gratings is calculated for different cavity losses, different pump intensity ratios and different crystal orientations. Distinct from an oscillator with transmission gratings, this oscillator always has a soft excitation in the vicinity of the threshold. The theoretical results are in reasonable agreement with that of experimental studies with BaTiO₃:Co as a photorefractive crystal.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Instability of single-frequency operation in semilinear photorefractive coherent oscillators

The transition of the single-frequency oscillation of a semilinear photorefractive coherent oscillator for sufficiently large coupling strengths into two-frequency oscillation is predicted and is observed experimentally. The critical value of the coupling strength at which the bifurcation occurs is a function of pump intensity ratio and cavity losses. The supercritical bifurcation in the oscillation spectrum is analogous to the second-order phase transition.     

Frequency detuning dependence of oscillation condition for a semilinear photorefractive optical resonator in a photorefractive material with reflection gratings

Frequency detuning dependence of four-beam coupling in a photorefractive crystal pumped with two counter-propagating waves for a semilinear coherent optical resonator on the oscillation conditions has been analyzed in the case of non-degenerate-wave mixing under the slowly varying amplitude approximation method. Self oscillation can be achieved when the gain arising from the four-beam coupling is large enough to overcome the cavity loss. The effects of frequency detuning (i.e., non-degeneracy), dielectric constant and photoconductivity of the photorefractive materials on the performance of the semilinear photorefractive coherent resonator with the reflection grating configuration have also been studied in detail. The phase-conjugate reflectivity of the pumped crystal and oscillation intensity has been calculated for different input pump beam intensity ratio, intensity reflectivity of the conventional mirrors, degenerate energy coupling strength of the interacting beams. It has been found that for the higher value of the photoconductivity σ_p(>2.0 pS/cm) of photorefractive crystal, the semilinear resonator can oscillate at almost any frequency detuning (Ω) of the oscillation beam with respect to the fixed frequency of the pump waves whereas for the lower value of photoconductivity σ_p(<0.1 pS/cm) oscillation occurs only when the frequency detuning is limited to small region around Ω = 0. But reverse of the case is found for dielectric constant (?), pump intensity ratio (p) and conventional mirror reflectivity (R).     

Semilinear coherent oscillator with reflection-type photorefractive gratings

The saturation intensity, oscillation spectrum, and temporal dynamics of oscillation are studied for a semilinear coherent oscillator with two counterpropagating pump waves and a photorefractive crystal with dominating reflection gratings. The instability of the single-frequency oscillation spectrum is revealed, similar to that known for an oscillator with dominating transmission gratings. The experimental manifestation of this transition in the output characteristics of the oscillator is favourably compared with numerical calculations. PACS 42.65.Hw; 05.45.-a; 42.65.Pc; 42.65.Sf     

Thermal and optical properties of Tm: Li6Gd(BO3)3 crystal: A potential candidate for 1.83 μm lasers

Single crystal of Tm³⁺: Li₆Gd (BO₃)₃ was grown by the Czochralski method. The heat capacity was measured from 308 to 673 K. The absorption spectra of the crystal in three mutually perpendicular arbitrary directions were measured at room temperature. Based on the Judd-Ofelt theory and the spectra measured in three mutually perpendicular directions, the intensity parameters Ω_t (t=2, 4, 6), the line strengths, the oscillator strengths, the radiative rates, radiative lifetimes and fluorescent branching ratios were calculated. We calculated the emission cross-section by the reciprocity method and also obtained the gain cross-section.     

Threshold Analysis of a THz-Wave Parametric Oscillator

The parametric gain of a terahertz wave parametric oscillator （TPO） is analyzed. Meanwhile the expression of TPO threshold pump intensity is derived and theoretically analyzed with different factors. The effective interaction length between the pump wave and Stokes wave is calculated, and particular attention is paid to the coupling efficiency of the pump wave and Stokes wave. Such an analysis is useful for the experiments of TPO.     

Theoretical oscillator strengths for some transitions in Si(II), Ge(II), Sn(II) and Pb(II) spectra

Calculations are reported of non-relativistic and relativistic oscillator strengths for the principal, sharp and diffuse series as well as some lifetimes in the spectra of singly-ionized silicon, germanium, tin and lead. The radial integrals were computed by employing the wave functions obtained from a semiempirical approach which included exchange effects (the semiempirical X$\text{α}$ method). Good agreement was obtained with available experimental and theoretical data. A comparison between non-relativistic and relativistic oscillator strengths confirms the importance of the spin-orbit interaction of the optical electron, as well as other relativistic effects for the oscillator strengths, even for the spectra of moderately heavy ions. Some systematic trends in the calculated oscillator strengths and line-strength ratios were investigated along the spectral series, as well as along the sequence of homologous ions.     

用Judd-Ofelt理论计算TmP5O14晶体中Tm3+的光谱参数

根据Judd-Ofelt理论,测得了Tm3+ 在TmP5O14晶体中的强度参数分别为Q2=:1.50×10-20cm2,Q4=1.51×10-20cm2,Q6=0.916×10-20cm2,计算的振子强度Pcal.与实验值Pexp.符合较好,平均根方误差为2.60×10-7。求得Qλ后,计算了J多重激发态之间的振子强度,自发辐射跃迁速率,辐射寿命和荧光分支出,与YAlO3:Tm3+,YAG:Tm3+、Y2O3:Tm3+等晶体和掺Tm3+玻璃的光谱参数进行了比较,并就3H4→3H6,3F4→3H5,1D2→3H4和1G4→3H4的跃迁进行了对比和讨论。观察到Tm3+在不同介质中的Pexp.与强度参数的总和∑τλ=τ2+τ4+τ6存在Pexp=α∑τλ+b的关系。     

Theoretical simulations of emission spectra of Fe^7＋ and Fe^＋8

The energy levels, oscillator strengths, spontaneous radiative decay rates, and electron impact collision strengths are calculated for Fe VIII and Fe IX using the recently developed flexible atomic code （FAC）. These atomic data are used to analyse the emission spectra of both laboratory and astrophysical plasmas. The nf-3d emission lines have been simulated for Fe VIII and Fe IX in a wavelength range of 6-14 nm. For Fe VIII, the predicted relative intensities of lines are insensitive to temperature. For Fe IX, however, the intensity ratios are very sensitive to temperature, implying that the information of temperature in the experiment can be inferred. Detailed line analyses have also been carried out in a wavelength range of 60-80 nm for Fe VIII, where the solar ultraviolet measurements of emitted radiation spectrometer records a large number of spectra. More lines can be identified with the aid of present atomic data. A complete dataset is available electronically from http：//www.astrnomy.csdb.cn/EIE/.     

Relativistic oscillator strengths for some transitions in Cu(I), Ag(I) and Au(I)

Relativistic oscillator strengths have been calculated for transitions in the principal, sharp and diffuse series of Cu(I), Ag(I) and Au(I) spectra. The computations have been performed by employing a semiempirical method which includes exchange and core-polarization effects. A comparison is presented for the calculated ?_ik values with experimental and other theoretical data. The influence of core-polarization effects on oscillator strengths is discussed.     

Spectroscopic and photoluminescence characteristics of Dy ions in lead containing sodium fluoroborate glasses for laser materials

Lead containing calcium zinc sodium fluoroborate (LCZSFB) glasses doped with different concentrations of trivalent dysprosium ions were prepared and investigated by the XRD, FTIR, optical absorption, photoluminescence and decay curve analysis. The experimentally determined oscillator strengths have been determined by measuring the areas under the absorption peaks and the Judd–Ofelt (J–O) intensity parameters were calculated using the least squares fit method. From the evaluated J–O parameters the radiative transition probability rates, radiative lifetimes and branching ratios were calculated for ⁴F_9/2 excited level. Room temperature photoluminescence spectra for different concentrations of Dy³⁺-doped LCZSFB glasses were obtained by exciting the glass samples at 386 nm. The intensity of Dy³⁺ emission spectra increases with increasing concentration of 0.1, 0.25, 0.5 and 1.0 mol% and beyond 1.0 mol% the concentration quenching is observed. The measuring branching ratios are reasonably high for transitions ⁴F_9/2→⁶H_15/2 and ⁶H_13/2, suggesting that the emission at 484 and 576 nm, respectively, can give rise to lasing action in the visible region. From the visible emission spectra, yellow–blue (Y/B) intensity ratios and chromaticity color coordinates were also estimated. The lifetimes of ⁴F_9/2 metastable state for the samples with different concentrations were also measured and discussed.     

Relativistic model-potential oscillator strengths and transition probabilities for 4ƒn6s−4ƒn6p transitions in Eu(II), Tb(II), and Ho(II) in J1 j coupling

The lowest $\text{4?}{}^{\mathrm{n}}\text{6s?4?}{}^{\mathrm{n}}\text{6p}$ transitions are studied for the Eu(II) (n=7), Tb(II) (n=9), and Ho(II) (n=11) spectra, where the J₁j coupling is an acceptable approximation. The relativistic radial integrals, required to evaluate the oscillator strengths and transition probabilities, are calculated with the model-potential method, which includes also core-polarization effects. The similarities observed in oscillator strengths for transitions with given ΔJ but different J values are discussed and explained. The computer oscillator strengths are compared with those obtained with the Coulomb approximation and it is found that the latter are only 11–12% lower. The core polarization influence on oscillator strengths is also investigated and the 19–21% decrease in oscillator strengths due to this effect is predicted. This result may, however, be overestimated because of some deficiencies in our procedure.     

Chiroptical spectra of molecules with nearly degenerate electronic states

The influence of vibronic interactions on the chiroptical spectra associated with pairs of nearly degenerate electronic transitions in chiral systems is examined on a formal theoretical model. We consider the special case in which two nearly degenerate electronic states are coupled by a single non-totally symmetric vibrational mode. Formal expressions are developed for the rotatory strengths of individual vibronic transitions in this coupled system. Calculations based on these expressions are carried out for a large number of parameter sets appropriate for various energy spacings between the unperturbed electronic states, vibronic coupling strengths, oscillator (vibrational mode) frequencies, and electronic rotatory strengths. The calculated results demonstrate the profound influence of vibronic interactions on the sign patterns and intensity distributions within the rotatory strength spectrum associated with the two coupled electronic states. The implications of these results for interpretations of circular dichroism spectra are discussed.     

Photoelectron branching ratios and partial ionization cross-sections for CO and N2 in the energy range 18–50 eV

Branching ratios for photoionization to various ion states of CO and N₂ have been measured in the energy range 18–50 eV using an electron—electron coincidence technique at an ejected electron angle of 90°. The branching ratios, which show a marked variation with energy are shown to be in good agreement with the results of conventional photoelectron spectroscopy at those energies where quantitative PES measurements have been made. The results indicate that the effect of the assymmetry parameter, β, is relatively insignificant in the case of CO and N₂. Partial oscillator strengths have been determined from the branching ratios using recent total photoabsorption data. In addition to the expected one electron ion states, others are observed arising from two electron processes and the variation of these cross-sections with energy loss is also reported.     

Optical spectroscopy of Er:YSGG laser crystal

Er:YSGG single crystals with good optical quality were grown by Czochralski method. X-ray diffraction (XRD) measurements show that the crystal lattice parameters of the 30 at.% Er³⁺-doped YSGG crystal were a=b=c=12.4640±0.0065 Å, α=β=γ=90°. The doping concentration of Er³⁺ in YSGG was calculated. The absorption spectrum and photoluminescence spectra of Er:YSGG crystal were studied at room temperature. Based on Judd-Ofelt (J-O) theory, the J-O intensity parameters Ω_t (t=2, 4, 6), the experimental and theoretical oscillator strengths were calculated using the absorption spectrum. The intensity parameters Ω_t (t=2,4,6) obtained to be Ω₂=0.23×10⁻²⁰ cm², Ω₄=0.86×10⁻²⁰ cm², Ω₆=0.37×10⁻²⁰ cm², respectively. With these intensity parameters, the line strengths, oscillator strengths, transition probabilities, fluorescence branching ratios and radiation lifetimes were calculated. The photoluminescence spectra and excitation spectrum of Er:YSGG were measured and studied.     

Determination of the oscillator strengths of Sn(I) in the ultraviolet (2400–4000 Å)

The emission spectrum of Sn(I) generated in a direct current arc plasma (Ar-SnCl₄ mixture) has been studied. The relative socillator strengths were obtained in the u.v. spectral range (2400–4000 Å) from measurements of branching ratios and from the study of radial temperature distribution and radial emission coefficients obtained by side-on measurement. These relative oscillator strengths were converted to an absolute scale by using as reference the lifetime determination of the ³P₁⁰ state. The results are compared with other absolute measurements.     

Manifestation of Curie–Weiss law for optical phase transition

Considerable slowing down is observed for both the temporal development of the coherent oscillation slightly above the threshold and the refractive index grating decay slightly below the threshold for a semilinear photorefractive oscillator with two counter-propagating pump waves. It is shown that in the vicinity of the threshold the reciprocal characteristic time is a linear function of deviation from the threshold coupling strength. This behaviour is similar to an empirical Curie–Weiss law and points to the analogy of the oscillation threshold to a second-order phase transition. Received: 2 April 2001 / Revised version: 23 August 2001 / Published online: 7 November 2001     

Absolute dipole oscillator strengths for photoabsorption and photoionization of carbon disulphide

The absolute dipole oscillator strengths (cross-sections) for photoabsorption and photoionization (total and partial) of CS₂ have been obtained in the 5–40 eV energy range by magic-angle dipole (e, 2e) spectroscopy. Very strong absorption is detected below 20 eV, much of which is attributable to the excitation of molecules decaying by autoionization processes. Analysis of binding energy spectra taken at energy losses above 20 eV reveals extensive satellite structure in the range up to 35 eV. This structure is attributed to many-electron effects consistent with theoretical calculations found in the literature. Photoelectron branching ratios for CS₂ are also reported.     

Intensity of the <Emphasis Type="Italic">f-f</Emphasis> transitions of Nd<Superscript>3+</Superscript>, Er<Superscript>3+</Superscript>, and Tm<Superscript>3+</Superscript> rare-earth ions in calcium niobium gallium garnet crystals

This paper reports on the results of the investigation into the intensities of the f-f transitions of Nd³⁺, Er³⁺, and Tm³⁺ ions in calcium niobium gallium garnet (CNGG) crystals. The values of the oscillator strengths and line strengths obtained for hypersensitive transitions and the intensity parameters Ω _t of the rare-earth ions in the CNGG crystals are compared with the corresponding quantities for crystals of other garnets and some oxide and fluoride crystals. The assumption is made that an increase in the oscillator strengths and line strengths for the hypersensitive transitions and the intensity parameters Ω₂ of the Nd, Er, and Tm ions in the CNGG crystals as compared to those for crystals of other garnets is associated with the specific features revealed in the crystal structure of the calcium niobium gallium garnet, in particular, with the lowering of the symmetry of the positions occupied by rare-earth ions in the crystal structure.