Laser and absorption saturation measurements of Cr crystals, pumped by broadband pulsed 940 nm radiation

The characteristics of a 940 nm pumped Cr⁴⁺:forsterite laser have been investigated using a pulsed colour centre laser as the excitation source. Laser threshold, slope efficiency and temporal behaviour have been determined for high and low Cr⁴⁺ doped material. 940 nm pumping is found to be more efficient than 532 and 730–770 nm excitation. Discrepancies with 1064 nm pumping are attributed to excited state absorption. Absorption saturation measurements have been used to assess the potential applications of Cr⁴⁺:forsterite, Cr⁴⁺:YAG and Cr⁴⁺: Lu₃A1₅O₁₂ (LuAG) as passive Q-switches for Nd lasers in the 940 nm region.     

A numerical analysis of gain characteristics of Er-doped Al2O3 waveguide amplifiers

The gain characteristics of erbium-doped Al₂O₃ waveguide amplifiers are investigated by solving numerically rate equations with upconversion effects and propagation equations. We obtained the dependence of gain of erbium-doped Al₂O₃ waveguide amplifiers on the waveguide length, erbium concentration and pump power at different pumping wavelengths (980 and 1480 nm). The performance of amplifiers pumping at 1480 and 980 nm are compared. It is shown that 980 nm pumping has higher gain and higher pumping efficiency. The parameters of waveguide amplifiers have been optimized. A optical gain of 43 dB can be achieved for a optimum waveguide length of 8.25 cm and 5.8 × 10²⁰ cm^–3 Er concentration pumped with 100 mW at 980 nm, that is a gain of 5.2 dB/cm.     

Low gas pressures obtained by means of sorbents

The paper describes the use of active charcoal and a 13X molecular sieve at 78°K to pump all-glass apparatuses with a small volume (up to 5000 cm³) from atmospheric pressure to a pressure region of 10^–2 mm Hg. At this pressure an outgassed second pumping stage (also sorbent) permits pressures from 10^–6 to 10^–7 mm Hg to be obtained. Active charcoal was also used as a two-stage fore pump (p=10^–6 mm Hg) for a one-stage mercury diffusion pump and in this system pressures of 5 to 10×10^–10 mm Hg were achieved.     

Single line high efficiency tunable fir generation by resonant four-wave mixing in CH3 pumped by a multiatmosphere tunable CO2 laser

By pumping CH₃F with a high pressure tunable TE-CO₂ laser, the resonant four-wave mixing process (RFWM) generates a very efficient tunable single line FIR emission at the Raman frequency. This result is strictly related to the spectroscopic structure of the CH₃F molecule. By means of this process, a tunable FIR emission on a 0.1 cm^–1 bandwidth 150 kW (8 mJ) single line, is obtained which can be used for many FIR multiphoton applications.     

New far infrared laser lines in optically pumped CD3F

Summary Using a high pressure CO₂-laser we have studied laser action in¹²CD₃F stimulated by R-branch pumping. We used an arrangement with low-feedback mirrors as suitable for Raman laser action. However, no Raman laser action has been observed in¹²CD₃F though similar experimental conditions were met as for¹²CH₃F and¹³CH₃F. The reason for the different behavior of the gases is not yet clear.Instead of Raman laser action we found 15 FIR resonant laser lines with 13 of them unknown up to now in the frequency range from 50 cm^–1 to 68 cm^–1. We reached FIR pulse energies up to 900 J, corresponding to a photon conversion efficiency of 12%. For our arrangement the optimum operation pressure varied from 40 torr at low J values (J=36) to 70 torr at high J values (J=49).     

Tritium isotope separation by CO2-laser irradiation at low temperatures

Tritium isotope separation by CO₂-laser induced multiphoton dissociation of CTF₃ is investigated. For the optimization of the performance of this working substance, trifluoromethane, the conditions to yield high-selectivity at high-operating pressure and low-critical fluence for complete dissociation are studied using our deconvolution procedure. The irradiation conditions are varied over the following ranges; wavenumber: 1052–1087 cm^–1, gas temperature: 25°C to –78°C, CHF₃ pressure: 5–205 Torr. The selectivities exceeding 10⁴ are observed for 85–205 Torr CHF₃ at –78°C by the irradiation at 1057 cm^–1.     

R branch tuning characteristics of the13CH3F Raman FIR laser

Summary We described a¹³CH₃F Raman laser pumped by a grating tuned 20 atmospheres CO₂ laser. The emission characteristics of the¹³CH₃F laser extends from 14 cm^–1–35 cm^–1 and from 49 cm^–1–72 cm^–1; about 65% of these frequency ranges can be covered with tunable radiation. The characteristics shows a strong dependence on the rotaional quantum numbers of the states involved in the Raman laser transitions and, within each tuning interval, on the frequency offset with respect to the frequencies of resonant transitions. We obtained, at 51 cm^–1, a maximum FIR laser pulse energy of about 800 J (at a pump energy of 200 mJ), corresponding to a photon conversion of about 8%. In some cases we have observed simultaneous emission at a Raman and a cascade frequency. In addition, FIR emission power dependence on¹³CH₃F gas pressure and pump pulse power were investigated for different J quantum numbers.     

Practical separation of silicon isotopes by IRMPD of Si2F6

Large-scale silicon isotope separation based on the IRMPD of natural Si₂F₆ has been carried out using a commercially available high power CO₂ TEA laser and a flow reaction system. The decomposition product SiF₄ containing 19–33% of ³⁰Si was obtained at a production rate of 1.5×10^–2–2.6×10^–2 mol·h^–1, depending on experimental parameters such as laser wavelength, laser fluence, pressure, and flow rate. SiF₄ containing 12% of ²⁹Si was obtained under slightly different conditions, i.e., at a shorter wavelength than that for ³⁰Si. When 39% of Si₂F₆ was decomposed at a slow flow rate, residual Si₂F₆ was found to have 99.7% of ²⁸Si. The production rate was 4.2×10^–2 mol·h^–1.     

PbO–Bi2O3–Ga2O3–BaO glasses doped by Er as novel materials for IR emission

Infra-red luminescence (at wavelengths about 1600 and 2500 nm) from Er³⁺ ions embedded in PbO–Bi₂O₃–Ga₂O₃–BaO glass hosts is reported for room and helium liquid temperatures. The substantial influence of energy transfer processes between the host and Er³⁺ ions is shown experimentally through the dependences of photoluminescence on light polarization and excitation wavelength. Only the application of the polarized pumping YAG–Nd laser beam (λ=1060 nm) stimulates substantial luminescence with quantum efficiency up to 24%. The role of phonon-relaxation subsystem in the observed luminescence is discussed.     

Independent UHF plasma source and possibility of filling open magnetic trap

A stationary UHF plasma source, its characteristics and possibility of filling open magnetic trap with plasma injected from it have been described. Plasma is created in the source at frequency of 2400 MHz (supplied power is up to 150 W) in the electron cyclotron resonance (ECR) regime under working gas pressure 10^–5–10^–2 Torr. By changing discharge conditions one can change the injected plasma density from 10⁹ to 10¹² cm^–3, at the temperatureT _e=2–10 eV. The possibility of efficient plasma injection from the source into the open magnetic trap of various configurations is shown experimentally. Plasma characteristics in the trap are presented under various experimental conditions. It is established that plasma parameters can be easily changed in the trap.     

Pressure dependence of the spectral lines of a high power, high pressure atomic fluorine laser pumped by a charge transfer from He2

Various transitions of a pressure tunable atomic fluorine laser have been observed over a rather wide spectral range, 634.8–755.2 nm, by exciting the atmospheric pressure of helium containing 0.2% F₂ in a preionized Blumlein discharge device with a charging voltage of 10–15 kV. The intensity of two quartet transitions, 739.9 and 755.2 nm, increases with pressure, whereas that of the other quartet and doublet lines, 634.8, 641.4, 712.8 and 731.1 nm, decreases with increasing atmospheric fill-pressure. The pumping mechanisms at high pressures appear to be different from those at relatively low pressures.     

Diode-pumped colour-centre lasers tunable in the 1.5 µm range

Under pumping with a 990 nm, 1 W laser diode, continuous-wave (cw) tunable laser emission in the 1.5 µm wavelength range was obtained from two different colour centres: Tl⁰(1) in NaCl:Tl⁺ and (F ₂ ⁺ )_H in NaCl:OH^–. The results are compared to those recently obtained with a similar apparatus and Tl⁰(1) centres in KCl:Tl⁺. The highest output power (30 mW) and the broadest tuning range (1.48–1.68 µm) were achieved with (F ₂ ⁺ )_H centres.     

Rapid growth of ultra thin SiO2 films by a large-area electron beam

Rapid growth of ultra thin oxide films (40–180Å) of silicon using a low-energy large-area electron beam has been performed with a pressure ratio of 31 (O₂/He) and a total pressure of 0.5–0.7 Torr. A higher oxidation rate of about 625Ų/s is found for shorter irradiation time of the e-beam in the e-beam dose range 0.75–3 Coulomb/cm² and at lower substrate temperature 540–740°C. AES and XPS demonstrated a rapid electron-stimulated oxidation process of the Si surface. For the grown ultra thin oxide films, C-V characteristics, dielectric strength, uniformity of the film over the entire Si wafer and its thickness as a function of the processing time of the e-beam are also presented.     

A laser infrared source of nanosecond pulses tunable from 1.4 to 22 μm

A reliable source of coherent ns pulses of infrared radiation continuously tunable between 1.4 and 22 m has been designed and built with the aim of developing a time-resolved infrared vibrational spectroscopy for species adsorbed on surfaces. The system is based on a Nd: YAG-laser and dye-laser combination which drive difference mixing processes in a sequence of nonlinear optical crystals (two LiNbO₃, and a CdSe or AgGaS₂). The system operates at MW peak power levels above 2500 cm^–1, at kW power levels from 1000–2500 cm^–1 and at 10–100 W levels down to 450 cm^–1. These power levels are certainly sufficient for spectroscopic purposes, and at shorter wavelengths molecular pumping and applications requiring high-power should be possible. Vibrational spectra of a monolayer of CO adsorbed on Pt in an electrochemical cell have been obtained in an initial application of this source.     

Radiation of the Plasma of a Transverse Discharge in a Helium–Krypton–Elegas Mixture

We carried out a spectroscopic investigation of the degradation of the active medium of a pulsed-periodic KrF emitter based on a He/Kr/SF₆ mixture (P = 10–150 kPa) with pumping by a transverse volumetric discharge. The plasma radiation spectra in the range 200–620 nm at different stages of degradation of the working mixture and the dynamics of the radiation of inert gases as well as of the products of decomposition of SF₆ molecules in the plasma are studied. It is shown that since the number of discharge pulses is 10⁴, rather effective formation of excited sulfur molecules is observed which decompose with emission in the spectral range 260–550 nm. This can be employed for developing a wideband lamp based on the system of KrF(B–X; D–X), S₂(B–X), and S₂(f–a) bands.     

Far infrared emission spectra from stratospheric hydrogen peroxide

Synthetic emission spectra from two stratospheric altitude observations have been analyzed for the presence of H₂O₂ in the far infrared region. The calculations are made with a high spectral resolution (10^–3 cm^–1 or 10^–4 cm^–1) greater than those in experimental measurements which are in the region of 3.10^–3 cm^–1. Spectra cover a spectral interval between 40 and 120 cm^–1 showing the best features of H₂O₂ susceptible to observation in a stratospheric spectrum. The optimum conditions for identification have been considered. Using the variations in H₂O₂ abundance in the measurement data and photochemical models, the H₂O₂ features detection limits have been studied.     

Efficient generation of FIR radiation by optical pumping of D2 18O

In this paper we show that D₂ ¹⁸O vapour, optically pumped with a continuously tunable high pressure CO₂ laser, is an excellent source for far infrared radiation. Both high photon conversion coefficients and broad Raman gain regions were found for a large number of new laser transitions spread over the frequency range from 25 cm^–1 to 240 cm^–1. We demonstrate that these Raman gain regions can be used to generate far infrared laser pulses with high intensity and durations of about 100 ps.     

The sub-micrometer thickness n-InSb/i-GaAs epilayers for magnetoresistor applications at room temperatures of operation

Magnetotransport at fields up to 500 mT and LF-noise characteristics are reported for miniature magnetoresistors with ferrite concentrators based on Sn-doped n-InSb/i-GaAs heterostructures grown by MBE. The thickness of the InSb epilayers lie in the range 0.55–1.5 μm giving room temperature mobilities of 2.5–5.5 m² V⁻¹ s⁻¹ with carrier densities of (0.5–1.5)×10¹⁷ cm⁻³. The room temperature magnetoresistance (MR) for our two terminal devices could be as high as 115% at 50 mT which is comparable to the extraordinary MR (ExMR) recently reported in microscopic composite van der Pauw disks four terminal devices [Science 289 (2000) 1530]. In addition, a high signal-to-noise ratio and a good temperature stability of R(B)/R₀=0.5–0.83% K⁻¹ was observed for B<60 mT (below the saturation field B_sat for ferrite). Device resistance stability R₀(T) was equal to 0.27–0.66% K⁻¹ in zero field with a nominal device resistance R₀=197–224 Ω for DC currents in the range I=0.01–1.0 mA. The minimum detectable magnetic field is estimated from the reduced differential MR (∂R/∂B)/R=2000% T⁻¹ at B=31 mT and normalised 1/f current noise power spectral density measured at the same field. The resolution limit B_min=2.6 nT at 10² Hz and B_min=0.82 nT at 10³ Hz. These resolution limits are seven times better than those recently reported for the same material n-InSb/i-GaAs and ferrite fabricated Hall sensors [Magnetotransport and Raman characterization of n-InSb/i-GaAs epilayers, for Hall sensors applications over extremely wide ranges of temperature and magnetic field, Proceedings NGS 10, IPAP Conference Series 2, IPAP, Tokyo, 2001, pp. 151–154].     

High resolution study of anion formation in low-energy electron attachment to SF<Subscript>6</Subscript> molecules in a seeded supersonic beam

Using two variants of the Laser Photoelectron Attachment (LPA) method involving a differentially-pumped, seeded supersonic beam (0.05% and 12.5% of SF₆ molecules in helium carrier gas, nozzle temperatures T₀= 300–600 K, stagnation pressures p₀= 1–5 bar) and mass spectrometric ion detection, we have investigated the energy dependence of anion formation in low-energy electron collisions with SF₆ molecules at high energy resolution. Using the standard LPA method, the yield for SF₆^- as well as SF₅^- and F^- anions was studied with an energy width around 1 meV over the electron energy range 0–200 meV. In addition, a variant of the LPA method with extended energy range (denoted as EXLPA) was developed and applied to measure the yield for SF₆^- and SF₅^- formation over the energy range 0–1.5 eV with an energy width of about 20 meV. The cross-section for formation of SF₆^- decreases by five orders of magnitude over the range 1–500 meV and is only weakly dependent on nozzle temperature. The yield for SF₅^- formation shows — apart from a weak zero energy peak which grows strongly with rising temperature — a broad maximum (located around 0.6 eV for T₀= 300 K and shifting to lower energies with rising T₀) and a monotonical decrease towards higher energies. SF₅^- attachment spectra taken at elevated temperatures exhibit changes with rising stagnation pressure which directly reflect rovibrational cooling of the SF₆ molecules with rising pressure. The SF₅^-/SF₆^- intensity ratio at near-zero energy and the low-energy shape of the broad peak in the SF₅^- spectra are used as thermometers for the internal temperature of the SF₆ molecules in the seeded supersonic beam which (at p₀= 1 bar) are found to be 50–100 K lower than the nozzle temperature. The energy dependence of the yield for F^- formation is similar to that for SF₆^-, but the F^- signals are three to four orders of magnitude lower than those for SF₆^-; in view of the rather high endothermicity of F^- formation the origin of the F^- signals is discussed in some detail.     

Laser properties of coactivated YAP: Nd free of colour centres

YAP:Nd, Cr grown under Ar-H₂ or Ar-He-H₂ atmosphere possesses good energy transfer from Cr³⁺ to Nd³⁺ but suffers from the colour centre formation. The centre formation was completely prevented using further admixture of Ce³⁺ and 10^–4–10^–3 wt. % Fe. Small luminescence quenching of Cr³⁺ or Nd³⁺ due to iron ions is negligible in the presence of Ce³⁺. The crystals may be also heavily doped with Nd³⁺ because the increased pumping efficiency compensates the shortening of the luminescence lifetime. YAP: Nd, Ce, Cr, Fe is advisible active laser material particularly for all the types of pulsed lasers.