Observation of a Sub-10-Hz Linewidth 88Sr+ 2S1/2–2D5/2 Clock Transition at 674 nm

In this paper, a pair of diode lasers at 674 nm, locked to two independent ultralow-expansion high-finesse cavities, have been characterized by monitoring the beat frequency between them. The individual laser linewidth of 1.4 Hz for a 3-s averaging time broadened to 4 Hz at 30 s. The relative frequency stability of the beat at 1s is 2.5 10^-15. One of the lasers has been used to interrogate the ⁸⁸Sr^{+ 2}Si_1/2-²D _5/2 transition at 674 nm, and a transition linewidth of 9 Hz has been observed     

CO2/OsO4 lasers as frequency standards in the29 THz range

The CO₂ lasers, frequency stabilized on OsO₄ transitions around 29 THz, are successfully used as secondary references for visible frequency synthesis and also for high-accuracy spectroscopy in the infrared domain. The BNM-LPTF CO₂/OsO₄ frequency standards exhibit high metrological performances: a long term reproducibility of ±10 Hz (1σ=2×10^-13) and a stability σ_y=6.6×10^-14/√τ with a Flicker plateau of 4×10^-15 (Δν/ν=0.1) for 300 s to 10³ s averaging times. These results and a preliminary study of the main effects which can induce frequency shifts are reported     

Frequency shifts of iodine stabilized He-Ne lasers at higherharmonic order stabilization

The frequency shifts of an ¹²⁷I₂ stabilized He-Ne laser at 633 nm have been measured at the third, fifth, and seventh harmonic stabilization using a digital signal processing lock-in amplifier. The observed frequency shifts confirmed the previous theoretical prediction that the laser frequency depends on the harmonic order of stabilization. The lower effects of modulation amplitude shift, power shift, and iodine pressure shift at the fifth harmonic stabilization indicate a potential improvement of the frequency reproducibility of the laser standard. A model is suggested to describe the power shifts of the laser. The mechanism explains the lower power shift of some lasers when they are operated at relatively high intracavity power     

Properties of iodine cells contaminated by the heteromolecule127I129I [He-Ne laser stability]

The influence of contamination of the iodine cell by the heteromolecule is discussed. The results of the numerical model are compared with measured values for both the third (THT) and fifth (FHT) harmonic method of detection. Contamination causes a substantially lower frequency shift in the case of the FHT     

Frequency dependent electrical conductivity and dielectric relaxation behavior in amorphous (90V2O5–10Bi2O3) oxide semiconductors doped with SrTiO3

We report on the experimental results of frequency dependent a.c. conductivity and dielectric constant of SrTiO₃ doped 90V₂O₅–10Bi₂O₃ semiconducting oxide glasses for wide ranges of frequency (500–10⁴ Hz) and temperature (80–400 K). These glasses show very large dielectric constants (10²–10⁴) compared with that of the pure base glass (≈10²) without SrTiO₃ and exhibit Debye-type dielectric relaxation behavior. The increase in dielectric constant is considered to be due to the formation of microcrystals of SrTiO₃ and TiO₂ in the glass matrix. These glasses are n-type semiconductors as observed from the measurements of the thermoelectric power. Unlike many vanadate glasses, Long's overlapping large polaron tunnelling (OLPT) model is found to be most appropriate for fitting the experimental conductivity data, while for the undoped V₂O₅–Bi₂O₃ glasses, correlated barrier hopping conduction mechanism is valid. This is due to the change of glass network structure caused by doping base glass with SrTiO₃. The power law behavior (σ_ac=A(ω^s) with s<1) is, however, followed by both the doped and undoped glassy systems. The model parameters calculated are reasonable and consistent with the change of concentrations (x).     

The influence of Ne isotopes on the single mode operation of He-Neand He-Ne/I2 lasers

When using He-Ne lasers in our experiments, we observed that two adjacent longitudinal modes with the same polarization operate simultaneously with steady output power for each mode when the frequency spacing between them is several times lower than the homogeneous linewidth of the laser transition. In the case of a mixture of ²⁰ Ne and ²²Ne isotopes, an analytical expression for the output power is obtained and calculated for parameters close to the experimental conditions. The theoretical predictions are in agreement with the experimental results. The theory also shows that in the case of pure neon isotope the competition between the longitudinal modes is stronger than in the case of neon isotopes mixture. It is proposed to improve the single mode selection efficiency for He-Ne and He-Ne/I₂ lasers and to enhance their single-mode output power using pure neon isotopes     

Laser microwave double-resonance experiment on trapped 113Cd+ ions

The possibility of using Cd⁺ ions as a microwave frequency standard whose light source is an all-solid state UV laser system is presented. The ground-state hyperfine splitting of trapped ¹¹³Cd⁺ ions is measured using an RF trap technique. The transition is detected using a laser microwave double-resonance method in the presence of He buffer gas. The splitting is determined to be 15 199 862 858(2) Hz. The preciseness of the value was improved over that in previous data by seven orders of magnitude. A light source of 214.5 nm is being developed by the authors for optical pumping and laser-cooling of Cd⁺ ions whose fundamental oscillator is based on diode lasers     

Precision frequency measurement of the 2S1/2 2D5/2 transition of Sr+ with a 674-nmdiode laser locked to an ultrastable cavity

We have measured the frequency of the 5s ²S_1/2 -4d ²D_5/2 clock transition of a single Sr ion confined in a Paul trap. A diode laser locked to an ultrastable Fabry-Perot (F-P) cavity was used to probe the transition with a resolution of 3.5 kHz. The absolute frequency was determined from heterodyne measurements referenced to an iodine stabilized HeNe laser and a CO₂ laser yielding a value for the S-D transition of (444 779 043 963±30) kHz. This work could lead to the development of a new optical frequency standard at 674 nm     

Light shift measurements in a diode-laser-pumped 87Rbmaser

Light shifts in a diode-laser-pumped ⁸⁷Rb maser under the various conditions of laser intensities and frequency detunings were measured, and a linear dependence of the maser frequency on the laser intensities and frequencies was derived. The pulling coefficient of the maser frequency from the laser was found to be about 2.9×10^-3 , and the light shift could be minimized when the laser frequency was adjusted to near the center frequency of the resonance cell     

Photoluminescence of Ga0.94In0.06As0.13Sb0.87 solid solution lattice matched to InAs

The lattice matched Ga_0.94In_0.06As_0.13Sb_0.87 quaternary solid solutions were grown by liquid phase epitaxy on (1 0 0) oriented InAs substrates from In rich melt. The p-type GaIn_0.06As_0.13Sb layers were intentionally undoped and their hole concentration was about p5×10¹⁶ cm⁻³, while n-type GaIn_0.06As_0.13Sb layers were slightly doped with Te and their electron concentration was about n10¹⁷ cm⁻³. Photoluminescence spectra exhibit single unresolved emission band in the spectral region from 0.65 to 0.8 eV for both types. Spectra were decomposed to elementary Gaussian components. The main mechanisms of radiative recombination were determined for both types of material.     

Infrared optical properties of Bi2Ti2O7 thin films by spectroscopic ellipsometry

Bi₂Ti₂O₇ thin films have been grown directly on n-type GaAs (1 0 0) by the chemical solution decomposition technique. X-ray diffraction analysis shows that the Bi₂Ti₂O₇ thin films are polycrystalline. The optical properties of the thin films are investigated using infrared spectroscopic ellipsometry (3.0–12.5 μm). By fitting the measured ellipsometric parameter (Ψ and Δ) data with a three-phase model (air/Bi₂Ti₂O₇/GaAs), and Lorentz–Drude dispersion relation, the optical constants and thickness of the thin films have been obtained simultaneously. The refractive index and extinction coefficient increase with increasing wavelength. The fitted plasma frequency ω_p is 1.64×10¹⁴ Hz, and the electron collision frequency γ is 1.05×10¹⁴ Hz, and it states that the electron average scattering time is 0.95×10⁻¹⁴ s. The absorption coefficient variation with respect to increasing wavelength has been obtained.     

Precision interferometric frequency measurement of the 674 nm2S1/2-2D5/2 transition in asingle cold Sr+ ion

The narrow linewidth 674 nm ²S_1/2-² D_5/2 transition in a cold Sr⁺ ion confined within an RF Paul trap has been probed using an optically narrowed 674 nm diode laser offset-locked to a second diode laser stabilized to a high-finesse ultra-low-expansion reference cavity. The transition frequency has been measured by means of interferometric comparison with an iodine-stabilized 633 nm He-Ne reference standard. A preliminary value for the ²S_1/2-²D_5/2 transition line center is 444 779 045 (9) MHz, limited by residual micromotion and low magnetic field Zeeman splitting     

Determining crystallization kinetic parameters of Li2O–Al2O3–SiO2 glass from derivative differential thermal analysis curves

The kinetic parameters such as crystallization activation energy, E, and the frequency factor, ν, of Li₂O–Al₂O₃–SiO₂ glass were determined by a new non-isothermal method. The method is described by the equation

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, where β is the heating rate and T_f is the inflection-point temperature of differential thermal analysis (DTA). The value of T_f is determined as the maximum peak temperature on derivative differential thermal analysis (DDTA) curves. Values of E and ν of Li₂O–Al₂O₃–SiO₂ glass were also determined by two existing non-isothermal methods, namely the Kissinger plot and the Ozawa plot, and compared with those determined by isothermal method. Values of E and ν determined by the proposed equation were 332 kJ/mol and 1.4×10¹³ s⁻¹, respectively. They are excellent agreement with the isothermal analysis results, 336 kJ/mol and 1.8×10¹³ s⁻¹, respectively. In contrast, both the Kissinger equation and the Ozawa equation give much higher values of E and ν.     

Deposition and photoluminescence of sol–gel derived Tb:Zn2SiO4 films on SiO2/Si

Tb³⁺ doped Zn₂SiO₄ films have been deposited on SiO₂ buffered Si wafers by sol–gel method. The structures of these films have been investigated with X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The results revealed that these films were composed of nanometer-size grains with a Willemite structure and had smooth surfaces. Photoluminescence measurements of the films showed a strong emission from ⁵D₄ to ⁷F₅ at 544 nm. The blue emission from ⁵D₃–⁷F_j was depressed because of cross-relaxation effect. The decay kinetics of the ⁵D₄–⁷F₅ green emission was studied and a best fitting was obtained by a double exponential function. The lifetime of the excited ⁵D₄ state is estimated to be 5.2 ms.     

Effect of radiation trapping on the emission properties of Er: I13/2 → I15/2 transition in oxide glasses

The effect of radiation trapping on the emission properties of Er³⁺-doped tellurite and phosphate glasses has been investigated as the function of sample thickness and doping concentration. It was found that radiation trapping exists generally in two glass matrices, even at low doping concentration (0.1 mol% Er₂O₃). The larger effect of radiation trapping in tellurite glasses compared with phosphate glasses is due to its larger emission cross-section at 1.5 μm band and the spectral overlap between the emission and absorption spectra of Er³⁺: ⁴I_13/2 ↔ ⁴I_15/2transition. Due to radiation trapping, the measured lifetime of the Er³⁺: ⁴I_13/2 level in tellurite glasses increases by about 11–37% with increasing the sample thickness at the different erbium doping concentration, while 6–17% for phosphate glasses. And the full-width at half maximum of fluorescence (FWHM) of Er³⁺: ⁴I_13/2 → ⁴I_15/2 transition in tellurite glasses increased by about 15–64% with increasing the sample thickness, while 11–55% for phosphate glasses. It caused a high overestimation on the figure of merits (FOM) for amplifier bandwidth (σ_e × FWHM).     

Very high Q microwave spectroscopy on trapped171Yb+ ions: application as a frequency standard

A microwave frequency standard based on buffer-gas cooled ¹⁷¹ Yb⁺ ions confined in a linear Paul trap has been demonstrated in prototype form. The standard exhibits a fractional frequency instability characterized by an Allan deviation σ_y (τ)=3.7×10^-13τ^-1/2 for τ<3000 s. Microwave Ramsey fringes with a Q factor of 1.5×10¹³ have been observed     

Spectroscopic evidence of inhomogeneous distribution of Nd in YVO4, YPO4 and YAsO4 crystals

Nd³⁺ : YVO₄ is one of the most interesting laser hosts for micro and diode-pumped solid state lasers. We have studied magnetic and optical properties of Nd³⁺ in three zircon type crystals YMO₄ (M=V, As, P). In particular, Nd³⁺ ions exhibit in the three hosts a multisite character observed in the absorption and emission spectra. However, the emission and its dynamics are strongly dependant on the reabsorption mechanisms. In Nd : YVO₄, single crystals containing 7 ± 1 × 10¹⁸ Nd³⁺ ions/cm³, the lifetime is 95 ± 2 μs in good agreement with the calculated radiative lifetime. Electron Paramagnetic Resonance (EPR) measurements are performed to identify the nature of the different substitution sites for Nd³⁺ ions. Nd³⁺ ions are found to be inhomogeneously distributed in tetragonal D_2d symmetry sites, in isolated ions, “shallow clusters” and pairs. Proportions of the different local environments depend on the total neodymium concentration. For instance, 15% of the Nd³⁺ ions are gathered in Nd³⁺–Nd³⁺ pairs for 7.2 ± 0.2 × 10¹⁹ Nd³⁺ ions/cm³.     

Synthesis and the luminescent properties of europium-activated Ca2SnO4 phosphors

The synthesis and photoluminescent (PL) properties of calcium stannate crystals doped with europium grown by mechanically activated in a high energy vibro-mill have been investigated. The characteristics of Ca₂SnO₄:Eu³⁺ phosphors were found to depend on the amounts of europium ions. The XRD profiles revealed that the system, (Ca_1−xEu_x)₂SnO₄, could form stable solid solutions in the composition range of x = 0–7% after being calcined at 1200 °C. The calcined powders emit bright red luminescence centered at 618 nm due to ⁵D₀ → ⁷F₂ electric dipole transition. Both XRD data and the emission ratio of (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) reveal that the site symmetry of Eu³⁺ ions decreases with increasing doping concentration. The maximum PL intensity has been obtained for 7 mol% concentration of Eu³⁺ in Ca₂SnO₄.     

Light-induced absorption changes in iron-doped LiNbO3

LiNbO₃: Fe and LiNbO₃: Fe,Me (Me = K, Mg, Zn) crystals are illuminated with frequency-doubled pulses of a Q-switched Nd: YAG laser. We detect light-induced absorption changes at high pulse intensities (I > 10⁹Wm^-2) utilizing cw probe lasers of different wavelengths (λ^p = 488, 633, and 785 nm). Intensity dependences as well as relazation processes after illumination are investigated. Absorption changes increase with increasing light intensity. Strongest effects are observed in reduced crystals. Doping with magnesium or zinc and an increasing lithium content, respectively, diminishes light-induced absorption changes. Our results can be described by a two-center charge transport model.     

Growth and spectroscopy of Dy doped in ZnWO4 crystal

Single-crystal ZnWO₄:Dy³⁺ was grown by Czochralski technique. The XRD, absorption spectra as well as fluorescence spectrum are investigated and the Judd–Ofelt intensity parameters Ω₂, Ω₄, Ω₆ are obtained to be 7.76 × 10⁻²⁰ cm², 0.57 × 10⁻²⁰ cm², 0.31 × 10⁻²⁰ cm², respectively. Calculated radiative transition rate, branching ratios and radiative lifetime for different transition levels of ZnWO₄:Dy³⁺ crystals are presented. Fluorescence lifetime of ⁴F_9/2 level is 158 μs and quantum efficiency is 66%.The most intense fluorescence line at 575 nm correlative with transition ⁴F_9/2 → ⁶H_13/2 is potentially for application of yellow lasers.