A tunable light source in the 370 nm range based on an optically stabilized,frequency-doubled semiconductor laser

A tunable harmonic output power of 18 W at a wavelength of =370 nm is obtained by resonance-enhanced frequency doubling of an optically-stabilized semiconductor laser. A commercially available AlGaAs laser diode which emits a maximum power of 10 mW at =740 nm is operated in an extended-cavity configuration. Dispersion prisms are used in the extended cavity to obtain longitudinal-mode selection with low loss of optical power. The output is focussed into an optically isolated high-finesse ring resonator which contains a LiIO₃ crystal for second-harmonic generation. One potential application of this laser source is the optical excitation and laser cooling of ytterbium in an ion trap. In a related demonstration experiment, the frequency-doubled diode laser is applied to excite the =369.5 nm ² S _1/2-² P _1/2 transition of ytterbium ions in a hollow-cathode discharge.     

Two-step laser excitation of the highly excited even-parity states of atomic mercury

We report term energies and quantum defects of highly excited even-parity states of mercury in the 83 876–84 140 cm^-1 energy range, employing a two-step laser excitation scheme via the S₀↦6s6p³P₁ inter-combination transition. Two dye lasers, pumped by a common Nd:YAG laser, were frequency doubled by BBO crystals and used to record the spectra in conjunction with a thermionic diode ion detector. Our new observations include the much extended D₂ (22 ≤n ≤52) series and a few members of the S₁ (24 ≤n ≤30) Rydberg series. Members of the D₂ Rydberg series with such a high n value are reported for the first time. The relative intensities of the D₂ and S₁ transitions (m = 4, 5 and 6) of group II-B elements excited from the P₁ inter-combination states are also discussed.     

Frequency-mixing scheme for the production of tunable narrowband XUV radiation (91–95 nm): application to precision spectroscopy and predissociation in diatomic molecules

Tunable narrowband extreme ultraviolet radiation in the range 91–95 nm is produced by sum-frequency mixing of the outputs of a visible pulsed dye amplifier (seeded by a ring dye laser) and of a seeded second-harmonic Nd:YAG laser and subsequent frequency tripling in a gas jet of xenon. The capability of this scheme to provide tunable narrowband extreme ultraviolet radiation is demonstrated in several spectroscopic studies. The bandwidth of this system (0.01 cm^-1) is deduced from a recording of absorption spectra of the 4p⁵(²P_1/2)6d, J=1 line in krypton. The applicability of the system for gas-phase molecular spectroscopic studies is demonstrated in recordings of the Werner bands (4,0) in H₂ and (5,0) in D₂ at unprecedented absolute accuracy. Line-broadening studies are performed on the b¹_u⁺,v=5 valence state in N₂, yielding a lifetime of 210±25 ps. A singlet–triplet perturbation, giving rise to an accidental predissociation in an excited ¹ Rydberg state in carbon monoxide at an excitation energy of 107680 cm^-1, is analyzed in high resolution. PACS 42.65.Ky; 32.80.Rm; 33.20.Ni; 33.80.Gj; 42.60.By     

Optical bistability in fluorescein dyes

Optical bistability has been observed in highly concentrated fluorescein dye solutions and in thin (1 m) doped polymeric films. At concentrations larger than 10^–5 mole/l dye dimers are formed. For fluorescein dye, the dimer-monomer equilibrium constant is 10⁵ l/mole so that most of the dye species are in the dimer form. At 480 nm the dimer absorption cross section is 10^–18 cm²/molecule, while that for the dye monomer molecule is 7.6×10^–17 cm²/molecule. Upon laser excitation dimers dissociate to form monomers thus providing a highly nonlinear laser induced absorption. This high nonlinear absorption coefficient can be utilized for optically bistable response of the dye system.Optical bistability was observed by placing dye solutions or dye thin films inside a Fabry-Perot resonator and exciting it with 480 nm dye laser pulses of 10 ns duration. The effect is more pronounced in 10^–4 mole/l fluorescein than in 10^–6 mole/l fluorescein in which dimer formation is not that efficient.In disodium fluorescein no significant dimer formation is observed even at 10^–3 mole/l dye concentration. The observed bistability both in solution and in thin films can be explained in terms of recent models for optical bistability in nonlinearly absorbing molecular systems.     

Pulsed 532 nm laser wirestripping: Removal of dye-doped polyurethane insulation

Removal of rhodamine 6G doped polyurethane insulation coated onto 50 m diameter wire is shown to proceed efficiently and cleanly by irradiation with 532 nm Q-switched pulses from a Nd:YAG laser. The stripping action produced by this method is similar in quality to excimer laser wirestripping. Several experimental parameters were explored including fluence, pulse duration, dye concentration, and the number of incident pulses. Acceptable stripping conditions were obtained for a 3–5 s exposure at 10 Hz, using a dye concentration of 10% by weight, and 12 n pulses at 650 mJ/cm². Nearly 0.5 m/pulse is removed at this fluence, which exceeds the threshold fluence of 600 mJ/cm² by only 50 mJ/cm². The measured 532 nm absorption coefficient of the 10% dye-doped polyurethane was 4×10⁴ cm^–1. Lower fluences and/or dye concentrations produced inadequate stripping, while shorter duration pulses caused unacceptable melting of the thin gold layer which covered the copper core of the wire. Pulse-by-pulse photographs of the stripping action clearly show melting of the dye/polymer insulation, and thermal rollback of the insulation near the stripped end. Regardless, excellent edge definition is obtained by this method.     

Spectroscopic trace-gas sensor with rapidly scanned wavelengths of a pulsed quantum cascade laser for in situ NO monitoring of industrial exhaust systems

Development of a pulsed quantum cascade laser (QCL)-based spectroscopic trace-gas sensor for sub-part-per-million detection of nitric oxide (NO) and capable of monitoring other molecular species such as CO₂, H₂O, and NH₃ in industrial combustion exhaust systems is reported. Rapid frequency modulation is applied to the QCL to minimize the influence of fluctuating non-selective absorption. A novel method utilizes only a few laser pulses within a single wavelength scan to probe an absorption spectrum at precisely selected optical frequencies. A high-temperature gas cell was used for laboratory evaluation of the NO sensor performance. A noise-equivalent sensitivity (1) of 100 ppb × m/ at room temperature and 200 ppb × m/ at 630 K was achieved by measuring the NO R(6.5) absorption doublet at 1900.075 cm^–1.     

High-resolution laser spectroscopy on H2 at 97–98 nm

A narrowband tunable eXtreme UltraViolet (XUV) laser source is used for a high resolution study of the Lyman (B ¹ _u ⁺ X ¹ _g ⁺ ) band system of molecular hydrogen. Seven rotational transitions of two vibrational bands, (10,0) and (11,0), in the wavelength range from 97.2–98.3 nm have been investigated for the first time under sub-Doppler molecular beam conditions. A calibration procedure using the I₂ standard in the visible yielded an absolute frequency accuracy of 0.02 cm^–1. The obtained H₂ transition frequencies provide a calibration standard in the extreme ultraviolet wavelength region.     

Intense spontaneous amplified emission of Li2 diffuse violet bands in the 4400 Å region

We report observation of intense spontaneous amplified radiation of Li₂ diffuse violet band in the 4100–4900 Å region. The radiation is strongly enhanced when the lithium vapor in a heat pipe is optically pumped with a pulsed dye laser with the output wavelength tuned to near the Li 2s–4s two-photon resonance transition. The diffuse violet band can probably be assigned to a recently reported triplet bound-free (2³ _g a ³ _u ⁺ ) transition. It is found that the productions of the molecular diffuse band are contributed from Li atoms as well as Li₂ molecules. The excitation functions and their dependence on laser power density are presented and the mechanisms for producing the diffuse bands are discussed.Department of Physics, National Central University, Chung-Li, Taiwan, Republic of Chinabl]References     

Laser-induced fluorescence detection of singlet CH2 in low-pressure methane/oxygen flames

Methylene, CH₂, is a chemically important intermediate in hydrocarbon combustion but has previously eluded optical detection in a combustion environment. The CH₂ signal as a function of height above the burner surface in a premixed, laminar, methane/oxygen flame (5.6 Torr and fuel equivalence ratio 1.05) is measured by laser-induced fluorescence (LIF) in the B ₁ – ã¹ A ₁ electronic system. The ã state which lies 3165 cm^–1 above the ground state is populated at the high temperatures of the flame (800–1800 K). Although less than one photon for each laser pulse is detected, we can unambiguously attribute the LIF features in the region 450 to 650 nm to CH₂ by both scanning the excitation laser and dispersing fluorescence. LIF temperatures and CH and OH LIF concentration profiles are also obtained for the flame. The CH₂ radical concentration maximum occurs closer to the burner than that of either OH or CH, as expected from models of methane combustion chemistry.     

NaK 22Σ+ → 11Σ+ band optically pumped laser near 1.02 μm

Optically pumped laser emission has been observed on the NaK 2(A)¹⁺ 1(X)¹⁺ electronic state transition. The emission occurs between 1.015 and 1.035 m when a sodium-potassium heat-pipe oven is pumped with 695–745 nm pulsed dye laser radiation. The laser emission occurs on many ro-vibrational transitions without the use of cavity mirrors. However, the addition of a simple cavity increases both the number of observed lasing transitions and the amplitude of the emission on each line. We report our results for the dependence of the emission intensity on pump laser power, oven temperature, and buffer gas pressure.     

Excimer laser projection-patterned deposition of Al via photolytically driven decomposition of trialkylamine alane as adsorbate precursor

Aluminum films have been deposited from trimethylamine alane (CH₃)₃N · AlH₃ and triethylamine alane (C₂H₅)₃N · AlH₃ precursors on Al₂O₃, GaAs and quartz substrates using XeCl (=308 mn) and KrF (=248 nm) excimer lasers. Substrate surface irradiation induces the decomposition of the precursor in the adsorbed phase. The technique allows projection patterned deposition of Al at room temperature. Al deposition proceeds in two steps: surface nucleation, which is a pure photolytic process, and the succesive Al growth, which is photolytically driven but is thermally activated at low laser energy densities (<60 mJ/cm²). The nucleation process strongly depends on the substrate and laser wavelength. Mirror like Al films are deposited at rates up to about one Al monolayer per pulse which corresponds to rates up to 2 m/min when photolyzing at 100 Hz. Al films with good adhesion and resistivities down to 7.5 cm (2.5 times bulk), were deposited. The process has good spatial selectivity. Patterns with 1 m resolution have been generated.     

High-resolution isotope shift measurement of the MgI 1 S 0 - 3 P 1 intercombination transition

We measured the isotope shift of the MgI intercombination line 3s3p ³ P ₁-3s ^{2 1} S ₀ (=457 nm) for the three stable isotopes ²⁴Mg, ²⁵Mg and ²⁶Mg. The measurement was performed by optical Ramsey spectroscopy (respectively saturation spectroscopy for ²⁵Mg) on a magnesium atomic beam. The rf precision of the measurement was achieved by using optical sideband techniques for the stabilization and tuning of a dye laser relative to an ultrastable cavity.     

Frequency of lamb-dip-stabilized 1.52 μm He-Ne lasers

A ³He-²⁰Ne and a ³He-²²Ne 1.52 m laser were frequency stabilized to the Lamb-dip of their respective gain curve and provided a resettability of 2 MHz. Using these lasers, absolute frequencies for the ²⁰Ne and ²²Ne laser transitions were measured by interferometric frequency comparison with an I₂-stabilized He-Ne laser at 0.633 m. A least-square analysis which accounts for the linear frequency dependence of the laser gas pressure yielded two parameters which can reproduce the measured frequencies within an accuracy of ±1.0·10^–8 at 1.52 m.     

Stark shifts and fine structure of the Li 3 D states

⁶Li 3²D _{3/2, 5/2} states were studied using a diode laser to first excite the 2P _3/2 state and a dye laser to populate the 3²D _{3/2, 5/2} states. The dye laser was modulated by an electro-optic modulator and intersected an atomic beam that passed through a field free region and subsequently through a uniform electric field. A value of 1084.24±0.20 MHz was found for the 3²D fine structure splitting. The scalar and tensor polarizabilities were determined to be α (3D _3/2 ) = - 3.753±0.015, α ₂ (3D _3/2 ) = 2.893±0.017, α (3D _5/2 ) = - 3.772±0.008 and α ₂ (3D _5/2 ) = 4.058±0.013 MHz/(kV/cm)². Received 26 September 2002 / Received in final form 22 January 2003 Published online 11 March 2003 RID="a" ID="a"e-mail: wlaser@yorku.ca     

Laser projection-patterned etching of (100) GaAs by gaseous HCl and CH3Cl

Laser projection-patterned etching of GaAs in a HCl and CH₃Cl atmosphere performed using a pulsed KrF-excimer laser (=248 nm, =15 ns) and deep-UV projection optics (resolution 2 m) is reported. The etching process carried out in a vacuum system having a base pressure of 10^–6 mbar is shown to result from a purely thermochemical reaction. Etching takes place in two steps: (i) between the laser pulses, the etchant gas reacts with the GaAs surface-atomic layer to form chlorination products (mainly As and Ga monochlorides), (ii) local laser surface heating results in the desorption of these products allowing further reaction of the gas with the surface. The influence of the etching parameters (laser energy density, gas pressure and pulse repetition rate) on the etch rate and the morphology of the etched features was studied. Etch rates up to 0.15 nm per pulse, corresponding to the removal of 0.5 GaAs molecular layer, are achieved. The spatial resolution of the etching process is shown to be controlled by the heat spread in the semiconductor and by the nonlinear dependence of the etch rate on the surface temperature. As a result, etched features smaller or larger than the projected features of the photomask are achieved depending on the laser energy density. Etched lines having a width of 1.3 m were obtained at low fluences by the projection of 2 m wide lines onto the GaAs surface.     

Efficient Continuous Wave-laser emission of Pr3+-doped fluorides at room temperature

We report, to our knowledge for the first time, Continuous Wave (CW) laser emission at room temperature of Pr:LiYF₄ (Pr.YLF) at six wavelengths: 522, 545, 607, 639.5, 720 and 907.4 nm. The pump source was an argon-ion laser tuned to a wavelength of = 457.9 nm. The maximum output powers at 522 nm (³P_i ³H₅) and 639.5 nm (³P₀ ³F₂) were 144 and 266 mW, respectively. We also observed CW laser action of Pr:GdLiF₄ (Pr: GLF) at = 639 nm and of Pr:KYF₄ (Pr: KYF) at = 642.5 nm.     

Deuterium separation by multiphoton dissociation of dichlorofluoromethane

Absorption and dissociation probabilities of CHCl₂F and CDCl₂F were investigated by a pulsed CO₂ laser in the wavenumber region of largest selectivity for deuterium. The absorption of CHCl₂F, which is a difference band, can largely be suppressed by cooling. At 200 K and at 920 cm^–1 absorption selectivities up to 4000 were found by extrapolation. In the presence of buffer gas, CDCl₂F can be multiphoton excited nearly like a linear absorber (harmonic oscillator). This is interpreted by a nearly resonant collisional relaxation v₇ to v₂ and by the smallness of the cross anharmonicity x₂₇. The dissociation selectivityS was 24,000 at natural abundance. Such large values were measured by a chromatographic method.S depends only onp _D, the partial pressure of the deuterated species. This dependence is approximately ln Sp _D ^–1 . It can be rationalized by considering only the average energy transferred to the nonresonant molecules by collisions with CDCl₂F. The above functional shape is related to an Arrhenius type law. Comparison with trifluoromethane for D separation shows that CHCl₂F has primarily two advantages: its rapid H-D exchange with water and the less stringent requirements of laser energy and pulse length.A preliminary account of this work has appeared in Quantum Electr.2, 13 (1985) (in Chinese)     

Generation of picosecond pulses in solid-state lasers using new active media

Results are reported of investigations aimed at generating nanosecond radiation pulses in solid-state lasers using new active media having broad gain lines. Passive mode locking is accomplished for the first time in a BeLa:Nd³⁺ laser at a wavelength 1.354 m, and in a YAG:Nd³⁺ laser on a 1.32–m transition. The free lasing and mode-locking regimes were investigated in an alexandrite (BeAl²O₄:Cr³⁺) laser in the 0.72–0.78–m range and in a synchronously pumped laser on F 2- centers in LiF in the 1.12–1.24–m region. The features of nonlinear perception of IR radiation by the eye, using a developed picosecond laser on F₂ ^– centers, are investigated for the first time.Translated from Lazernye Sistemy, pp. 67–86, 1982.     

Luminescence spectra of Eu ions and interstitial oxygen in PbWO4 crystal

The site-selective excitation and emission spectroscopy, and luminescence decay have been investigated under a pulsed, tunable, narrowband dye laser of the ⁵D₀→⁷F₀ region in the europium ions-doped lead tungstate PbWO₄ (PWO) in single crystal. In as-grown sample, the experimental results show that there is only one ⁷F₀→⁵D₀ excitation transition indicating the only one Eu³⁺ site in PbWO₄ lattices. The sequential annealing treatments were conducted to investigate the effects of oxygen components on the microstructure environments of Eu³⁺ in the lattices. The site distribution of Eu³⁺ was changed by the annealing in air atmosphere, which could create new sites in PWO lattices. Confirmation of interstitial oxygen and interpretations of charge compensation mechanism for the observed new sites were discussed in the context of site-selective excitation and emission spectra. The main Eu³⁺ site is related to the charge compensation by the [(Eu_Pb³⁺)-V″_Pb-(Eu_Pb³⁺)] complex; the other minor new sites after annealing are originated from [(Eu_Pb³⁺)-O″_i-(Eu_Pb³⁺)] defects. Emission spectra excited by 355-laser and RT-Raman spectra were also measured.     

Preionization of TEA CO2 laser by sliding discharge

TEA CO₂ laser preionization by plasma sheet formed by discharge sliding over a dielectric surface is described. The preionization electron number density in order of 10⁹cm^–3 was measured in the CO₂ N₂ He=113 gas mixture. The plasma sheet was also tested as a main discharge electrode in TEA CO₂ laser.The authors would like to acknowledge with thanks the current interest and the help of P. Gavrilov and V. Krajíek in experiments.