Laser Stark Spectroscopy and Isotopic Separation

A potentially useful scheme for photoselective isotope enrichment is presented. The selective laser excitation step is accomplished by utilizing the Stark effect to shift molecular transitions into resonance with laser frequencies. The method is demonstrated for isotope separation of ¹³C, ¹⁵N and ²H. Laser Stark spectroscopy can sensitively detect as few as 10⁹ molecules in the optical path, thus the isotopic separation can be achieved from industrial sample mixtures containing natural abundances. A comparison of the isotopic enrichment efficiency using the present method of “exact” resonance and the previously used method of “near” resonance excitation is also given.     

Lattice dynamics in two-photon-excited CdS studied by picosecond time-resolved X-ray diffraction

Lattice dynamics and radiative processes in single-crystal cadmium sulfide induced by two-photon excitation with a femtosecond laser are investigated. The development of lattice expansion is directly observed by picosecond time-resolved X-ray diffraction. The obtained lattice dynamics are explained on the basis of a thermally induced impulsive-strain model. The model calculation indicates that two- and more-photon absorption processes occur and that reflectivity rapidly increases under laser irradiation. In photoluminescence spectroscopy, the spectra for TW cm⁻² excitation are shifted to lower energy and show an additional shoulder at 2.35 eV. Furthermore, emission due to Fabry-Perot laser modes with self-formed cavities was observed under 11 TW cm⁻² excitation. The discrepancy between carrier densities deduced from the lattice expansion and the PL spectra indicates that the predominant process at a higher carrier density is not radiative recombination, but Auger recombination followed by lattice heating.     

Evaluation of Limiting Characteristics of Laser Separation of Silicon Isotopes

Isotopically selective photoionization of silicon atoms by laser radiation using tunable dye lasers is considered. The scheme of atomic ionization with two-photon excitation via the resonance level of one of the high states with subsequent excitation of the autoionization state is proposed. The calculations carried out show that the up-to-date development of laser facilities allows one to evaluate the ultimate production of the highly enriched ²⁸Si isotope at a level of 0.1 g/h at a radiating power of 1 W for each wavelength. The excitation selectivity appears to be high (about 100) within the framework of the radiation problem and is actually defined by the conditions of ion extraction, background atom concentration, charge exchange, and so on rather than by the radiation processes.     

Measured Cd III Stark widths

The Stark FWHM (Full-Width at Half of the Maximal line intensity, W) have been measured for 16 doubly ionized cadmium (Cd III) spectral lines in a pulsed helium discharge in the wavelength interval between 200 nm and 304 nm. The helium discharge was created in the linear low pressure pulsed arc operated at 19 000 K electron temperature and 5.0×¹⁰²² m−3 electron density. The cadmium atoms were sputtered from the cadmium (99.9% purity) cylindrical plates located in the linear part of the discharge tube. The high density of the Cd III ions is boosted by cascade ionization processes via the well populated Cd II energy levels due to the Penning and charge exchange effects. The shapes of the Cd III lines are recorded using a spectrograph (McPherson model 209, 1.33 m focal-length with 2400 grooves/mm holographic grating) and intensified CCD camera (Andor DH740-18F-03) as a high-sensitive detection system. The Stark parameters introduced in this Letter are the first published experimental Cd III Stark widths.     

Raman study of BiFeO3 with different excitation wavelengths

Raman spectra of bismuth ferrite (BiFeO₃) over the frequency range of 100-1500 cm⁻¹ have been systematically investigated with different excitation wavelengths. The intensities of the two-phonon modes are enhanced obviously under the excitation of 532 nm wavelength. This is attributed to the resonant behavior when incident laser energy closes to the intrinsic bandgap of BiFeO₃. The Raman spectra of BiFeO₃ excited at 532 nm were measured over the temperature range from 77 to 678 K. Besides the abnormal changes of the peak position and the linewidth of the A₁ mode at 139 cm⁻¹, the prominent frequency shift, the line broadening and the decrease of the intensity for the two-phonon mode at 1250 cm⁻¹ were observed as the temperature increased to Néel temperature (T_N). All these results indicate the existence of strong spin-phonon coupling in BiFeO₃.     

Greater signal, increased depth, and less photobleaching in two-photon microscopy with 10 fs pulses

The fundamental advantages to using ultrafast (?100 fs) laser pulses in two-photon microscopy for biomedical imaging are seldom realized due to chromatic dispersion introduced by the required high numerical aperture microscope objective. Dispersion is eliminated here by using the multiphoton intrapulse interference phase scan (MIIPS) method on pulses with a bandwidth greater than 100 nm full width at half maximum. Higher fluorescence intensity, deeper sample penetration, and improved signal-to-noise ratio are demonstrated quantitatively and qualitatively. Due to the higher signal intensity obtained after MIIPS compensation, lower laser power is required, which decreases photobleaching. The observed advantages are not realized if group delay dispersion is compensated for while higher-order dispersion is not. By using a pulse shaper and taking advantage of the broad spectrum of the ultrafast laser, selective excitation of different cell organelles is achieved due to the difference in nonlinear optical susceptibility of different chromophores without requiring an emission filter wheel. Experiments on biological specimens, such as HeLa cells and mouse kidney tissue samples, illustrate the advantages to using sub-10 fs pulses with MIIPS compensation in the field of two-photon microscopy for biomedical imaging.     

Highly accurate laser wavelength meter based on Doppler effect

We have built an accurate wavelength meter based on a Michelson interferometer characterized by a high stability velocity moving system. The unknown wavelength is determined from the Doppler frequency shifts of the output beams of the Michelson interferometer. The reference laser is a frequency stabilized helium-neon laser. A counting resolution of 2.6 × 10⁻⁹ for an integration time of 30 s has been obtained. The apparatus has been used to determine the wavelength of a second frequency stabilized helium-neon laser and the result has been compared to those given by two different methods: frequency beating in regards to the national reference and using a commercially available scanning-Michelson wavemeter. Taking into account the statistical errors, we achieved a relative accuracy on the unknown wavelength of 6.4 × 10⁻⁸ at 1σ.     

Resonance Ionization Mass Spectrometry (RIMS) with Pulsed and CW-Lasers on Plutonium

The detection of long-lived plutonium isotopes in ultra-trace amounts by resonance ionization mass spectrometry (RIMS) is a well-established routine method. Detection limits of 10⁶ to 10⁷ atoms and precise measurements of the isotopic composition have been achieved. In this work multi-step resonance ionization of plutonium atoms has been performed with tunable lasers having very different output intensities and spectral properties. In order to compare different ways for the resonance ionization of plutonium broadband pulsed dye and titanium:sapphire lasers as well as narrow-band cw-diode and titanium:sapphire lasers have been applied for a number of efficient excitation schemes. It has been shown, that for identical excitation schemes the optical isotope selectivity can be improved by using cw-lasers (bandwidths < 10 MHz) instead of pulsed lasers (bandwidths > 2 GHz). Pulsed and cw-laser systems have been used simultaneously for resonance ionization enabling direct comparisons of pulsed and continuous ionization processes. So far, a three-step, three-color laser excitation scheme has been proven to be most practical in terms of efficiency, selectivity and laser wavelengths. Alternatively a newly discovered three-step, two-color excitation scheme which includes a strong two-photon transition from an excited state into a high-lying autoionizing state yields similar ionization efficiencies. This two-photon transition was characterized with respect to saturation behavior and line width.     

Investigations of Yb-doped optical fiber using selective laser excitation

The method of tunable site-selective laser excitation was used for a study of the spectral and fluorescence characteristics of Yb³⁺ optical centers in laser optical fiber. Decay curves and positions of fluorescence spectra maxima at different wavelengths of selective laser excitation and positions of excitation spectra maxima at different fluorescence selective registration provide new information on lifetimes and Stark energy distribution among inhomogeneously broadened Yb³⁺ lines. The obtained Stark splitting energy distribution diagram demonstrates the tuneability of Yb³⁺ laser oscillation wavelength under tunable excitation of a pump laser.     

Thermal stability and spectroscopic properties of Er-doped lead fluorogermanate glasses

The thermal characterization and spectroscopic properties of Er³⁺-doped 0.6GeO₂-(0.4-x)PbO-xPbF₂ glasses were investigated experimentally. With the replacement of PbO by PbF₂ the thermal stability of glasses is improved and the infrared fluorescence intensity at 1530 nm is increased. The Judd-Ofelt intensity parameters, radiative transition rates, and fluorescence lifetimes of the excited ⁴I_13/2 level of Er³⁺ ions were calculated from Judd-Ofelt theory. The asymmetric ligand field around Er³⁺ ions resulted from the incorporation of PbF₂ into germanate glasses, broadens the infrared emission spectra at 1530 nm. Upconversion luminescence in the investigated glasses was observed at room temperature under the excitation of 976 nm laser diode. The glass 0.6GeO₂-0.3PbO-0.1PbF₂ exhibits the maximum upconversion emission intensity, while no frequency upconversion luminescence was observed in the 0.6GeO₂-0.4PbO glass. The quadratic dependence of the green and red emissions on excitation power indicates that two-photon absorption contributes to the visible emission under the 976-nm excitation.     

Ultra fast melting process in femtosecond laser crystallization of thin a-Si layer

In this paper, we investigated the mechanism of crystallization induced by femtosecond laser irradiation for an amorphous Si (a-Si) thin layer on a crystalline Si (c-Si) substrate. The fundamental, SHG, THG wavelength of a Ti:Sapphire laser was used for the crystallization process. To investigate the processed areas we performed Laser Scanning Microscopy (LSM), Transmission Electron Microscopy (TEM) and Imaging Pump-Probe measurements. Except for 267 nm femtosecond laser irradiation, the crystallization occurred well. The threshold fluences for the crystallization using 800 nm and 400 nm femtosecond laser irradiations were 100 mJ/cm² and 30 mJ/cm², respectively. TEM observation revealed that the crystallization occurred by epitaxial growth from the boundary surface between the a-Si layer and c-Si substrate. The melting depths estimated by Imaging Pump-Probe measurements became shallower when the shorter wavelength was used.     

Upconversion fluorescence property of Er/Yb-codoped novel bismuth-germanium glass

Infrared-to-visible upconversion fluorescence property of Er³⁺/Yb³⁺-codoped novel bismuth-germanium glass under 975 nm LD excitation has been studied. Intense green and red emissions centered at 525, 546 and 657 nm, corresponding to the transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively, were observed at room temperature. The quadratic dependence of the 525, 546 and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs. The structure of the bismuth-germanium glass has been investigated by peak-deconvolution of FT-Raman spectrum, and the structural information was obtained from the peak wavenumbers. This novel bismuth-germanium glass with low maximum phonon energy (∼750 cm⁻¹) can be used as potential host material for upconversion lasers.     

Doppler characterization of laser inter-satellite links for optical LEO satellite constellations

Because of ensuring very low propagation delay between satellites, and providing global space-based broadband network services, low earth orbit (LEO) satellite constellations with laser inter-satellite links (ISLs) are considered to be the enabling technology to satisfy the increasing data traffic demand. However, significant Doppler can be observed by the onboard terminals on the ISLs, due to the high relative speed of the two communicating LEO satellites. This paper describes an analytic derivation of the Doppler wavelength shift measured by the terminal onboard a satellite on the signal transmitted through the ISLs. The Point-Ahead Mechanism of the optical ISLs is considered in the analytical expression of the Doppler wavelength shift. Then, in terms of the ISLs characteristics of the satellite constellations, the expression of Doppler wavelength shift is deduced into two aspects. First, for the full time accessing ISLs, it evolves as a function of the constellation parameters. Thus, the Doppler characterization for two kinds of interorbit full time accessing ISLs of LEO satellite constellations is analyzed. Second, for the intermittent accessing ISLs, the expression of Doppler wavelength shift is given as a function of the minimum ISL distance between two communicating satellites. And the visibility duration of the destination satellite at the source satellite is estimated for the intermittent ISLs. This work is helpful to evaluate the design of constellation networking.     

Isotope selective loading of an ion trap using resonance-enhanced two-photon ionization

We have demonstrated that resonance-enhanced two-photon ionization of atomic beams provides an effective tool for isotope selective loading of ions into a linear Paul trap. Using a tunable, narrow-bandwidth, continuous wave (cw) laser system for the ionization process, we have succeeded in producing Mg⁺ and Ca⁺ ions at rates controlled by the atomic beam flux, the laser intensity, and the laser frequency detuning from resonance. We have observed that with a proper choice of control parameters, it is rather easy to load a specific number of ions into a string. This observation has direct applications in quantum optics and quantum computation experiments. Furthermore, resonant photo-ionization loading facilitates the formation of large isotope-pure Coulomb crystals. Received: 21 December 1999 / Published online: 11 May 2000     

Study of the neutron deficient 182–190Pb isotopes by simultaneous atomic- and nuclear-spectroscopy

Mean-square charge radii and magnetic moments have been measured for the neutron deficient lead isotopes, ^182–190Pb. The measurement was performed at the ISOLDE online mass separator, using the in-source resonance ionization spectroscopy technique. The wavelength of the first excitation step for the resonance ionization laser ion source (RILIS) was scanned over the resonance(s) whilst the α- and γ-ray spectra from the decay of the Pb isotopes were recorded as a function of the wavelength. The isotope shift and, in the case of odd-A isotopes, the hyperfine splitting were deduced. The rms-charge radii of the very neutron deficient Pb isotopes follow the smooth trend of the heavier isotopes. This finding indicates a spherical shape for the lead ground states at the neutron mid-shell (N = 104), where the excitation energy of the oblate 0⁺ state in the even isotopes reaches its minimum.     

Isotope shifts and hyperfine structure in the transitions in calcium II

The isotope shift and hyperfine structure in the three - transitions in Ca II have been studied by fast ion beam collinear laser spectroscopy for all stable Ca isotopes. The metastable 3d states were populated within the surface ionization source of a mass separator with a probability of about 0.1%. After resonant excitation to the 4p levels with diode laser light around 850 nm the uv photons from the transitions to the ground state were used for detection. Hyperfine structure parameters A and B for the odd isotope ⁴³Ca, as evaluated from the splittings observed, agree well with theoretical predictions from relativistic many-body perturbation theory. Field shift constants and specific mass shift constants were extracted from the measured isotope shifts and are discussed in comparison with expectation values from theory. Received: 19 September 1997 / Revised: 5 December 1997 / Accepted: 27 January 1998     

Isotope shifts and hyperfine structure in the transitions of gadolinium

High-resolution resonance ionization mass spectrometry has been used to measure isotope shifts and hyperfine structure in all (J = 2-6) and the transitions of gadolinium (Gd I). Gadolinium atoms in an atomic beam were excited with a tunable single-frequency laser in the wavelength range of 422-429 nm. Resonant excitation was followed by photoionization with the 363.8 nm line of an argon ion laser and resulting ions were mass separated and detected with a quadrupole mass spectrometer. Isotope shifts for all stable gadolinium isotopes in these transitions have been measured for the first time. Additionally, the hyperfine structure constants of the upper states have been derived for the isotopes ^{155, 157} Gd and are compared with previous work. Using prior experimental values for the mean nuclear charge radii, derived from the combination of muonic atoms and electron scattering data, field shift and specific mass shift coefficients for the investigated transitions have been determined and nuclear charge parameters for the minor isotopes ^{152, 154} Gd have been calculated. Received 18 November 1999     

Energy transfer and upconversion luminescence in Tm/Yb co-doped lanthanum-zinc-lead-tellurite glasses

A series of Tm³⁺/Yb³⁺ co-doped lanthanum-zinc-lead-tellurite (TPZL) glasses pumped by a 980 nm laser diode (LD) were demonstrated to obtain a high efficiency of infrared-to-visible upconversion. Effects of PbO content on the thermal stability, structure and upconversion properties of Tm³⁺/Yb³⁺ co-doped TPZL glasses had been investigated. The efficient visible upconversion fluorescences corresponding to the ¹G₄→³H₆, ¹G₄→³F₄ and ³H₄→³H₆ transitions of Tm³⁺ were observed under 980 nm excitation. The upconversion intensities of blue, red and near infrared emissions in Tm³⁺/Yb³⁺ co-doped TPZL glasses were obviously enhanced with increasing PbO content. The dependence of upconversion intensities on excitation power and the possible upconversion mechanisms had been evaluated by a proper rate equation model. Population density in different levels and coefficients of the energy transfer rate C_Di (i=2, 4, 6) between Tm³⁺ and Yb³⁺ were estimated by fitting the simulated curves to the measured ones. The obtained three energy transfer coefficients C_D2, C_D4, and C_D6 were determined to be 5.7×10⁻¹⁷, 1.3×10⁻¹⁶ and 8.6×10⁻¹⁷ cm³/s, respectively.     

Light-induced ultrafast phase transitions in VO2 thin film

Vanadium dioxide shows a passive and reversible change from a monoclinic insulator phase to a metallic tetragonal rutile structure when the sample temperature is close to and over 68 °C. As a kind of functional material, VO₂ thin films deposited on fused quartz substrates were successfully prepared by the pulsed laser deposition (PLD) technique. With laser illumination at 400 nm on the obtained films, the phase transition (PT) occurred. The observed light-induced PT was as fast as the laser pulse duration of 100 fs. Using a femtosecond laser system, the relaxation processes in VO₂ were studied by optical pump-probe spectroscopy. Upon a laser excitation an instantaneous response in the transient reflectivity and transmission was observed followed by a relatively longer relaxation process. The alteration is dependent on pump power. The change in reflectance reached a maximum value at a pump pulse energy between 7 and 14 mJ/cm². The observed PT is associated with the optical interband transition in VO₂ thin film. It suggests that with a pump laser illuminating on the film, excitation from the d_θ,? - state of valence band to the unoccupied excited mixed d_θ,?-π* - state of the conduction band in the insulator phase occurs, followed by a resonant transition to an unoccupied excited mixed d_θ,?-π* - state of the metallic phase band.     

Development of laser ablation assisted resonant ionization mass spectrometry for isotope analysis

Stable isotope analysis using Laser Ablation Assisted Resonant Ionization Mass Spectrometry (LA-RIMS) is discussed. For the case of the ⁸⁷Sr/⁸⁶Sr isotope ratio measurements with a single color two-photon excitation scheme, the mass discrimination effects, which occur in LA-RIMS, are analyzed and a precision of 0.6% was extracted. For preparation of LA-RIMS on Hf isotopes, a novel two-color resonant excitation scheme with subsequent non-resonant ionization has been established using a high repetition-rate Ti:Sapphire laser system.