Photophysical properties and laser performance of photostable UV laser dyes

The photophysical properties such as singlet absorption and fluorescence spectra, the fluorescence quantum yield and fluorescence decay time as well as the laser performance data such as the tuning range, the conversion efficiency, and the photochemical stability of 12 sterically hinderedp-quaterphenyls have been measured in ethanol and/or dioxane at room temperature. The sterically hinderedp-quaterphenyls exhibit shorter laser dye emissions in the 330–380 nm range than the parent compoundp-quaterphenyl. The conversion efficiencies of the sterically hinderedp-quaterphenyls range between 1 and 21%. The photochemical stability of the sterically hinderedp-quaterphenyls is by a factor of 10 to 20 better in dioxane than in ethanol. 2-Methyl-5-tert. butyl-p-quaterphenyl and 2,5,2,5-tetramethyl-p-quaterphenyl are among the most stable UV laser dyes known today.     

Laser performance of perylenebis (dicarboximide) dyes with long secondary alkyl chains

The laser performance and related photophysical properties of two very soluble perylene dyes with long chain secondary alkyl groups were investigated in cyclohexane solution. With a dye laser as pump source a tuning range of 555–580 nm was obtained at an optimum concentration of 3×10^–4 M. The quantum efficiencies (=0.29 and 0.21) were better than 1/2 that of rhodamine 6G. No photodegradation was observed over an excitation period of several hours.     

Photophysical properties and laser characteristics of a new rigid aminocoumarin dye lasing in the blue-green region

The photophysical properties of a new dye, 7-Diethyl AminoCoumarin with a Rigid substitution in the 3-position (referred to as DARC) have been studied in three solvents: dioxane, DMF and DMSO. The dye has been found to have a fluorescence quantum efficiency (_fl) between 0.40 and 0.80 in these solvents. The dye-laser performance of this dye has also been investigated in dioxane, DMF and DMSO, under nitrogen-laser pumping and compared with that of the commercially available standard laser dye, Coumarin 515 (C-515). A tuning range of nearly 70 nm was obtained in the blue-green region with an efficiency up to 80% of that of the standard dye. The observed characteristics of the dye are explained in terms of the sructural rigidization of the dye in the 3-position which inhibits the formation of the Twisted Intramolecular Charge Transfer (TICT) conformation in the excited state leading to an enhancement of the _fl and a considerable improvement in the laser performance.Research carried out at U.D.C.T., Bombay, India     

Photophysical parameters for potential vapour-phase dye-laser media

The fundamental photophysical parameters of the organic scintillators, anthracene, 9, 10 dimethyl antracene (DMA), 9, 10 diphenyl anthracene (DPA), 2-(1-napthyl)-5-phenyloxazole (α-NPO) and 2, 2′-p-phenyl bis (5-phenyloxazole) (POPOP), were investigated in the liquid and vapour phases. Their ground state, triplet-triplet and fluorescence spectra were determined and accurate extinction coefficients obtained. Photodecomposition of POPOP vapour under 337 nm excitation was investigated in detail and a photodecomposition quantum yield of 1.1×10⁻² obtained at 568 K. The POPOP triplet state in the vapour phase is quenched by the addition of 1, 3, 5, 7 cyclo-octatetraene and a quenching rate constant of 2.9×10¹⁰ M⁻¹ s⁻¹ was determined.     

Photophysical and photochemical switches based on twisted intramolecular charge transfer (TICT) states

TICT states, accessible in twisted multichromophoric systems, are nonradiative funnels to the ground state in many dyes. By controlling these funnels, faster saturable absorbers for subpicosecond laser pulses can be developed. Oriented assemblies of TICT molecules, as in liquid crystalline polymers, are expected to exhibit light-induced macroscopic charge separation. Chemical approaches to supramolecular bistable species are also shown.     

Effect of laser dye deterioration on performance

Loss of gain in a dye medium as a result of decomposition by pump photons is considered in terms of disappearance of dye molecules and the appearance of a new, single chemical product having absorption in the fluorescence band. The cases of small signal gain and saturated gain are applied to a single pass through a transversely excited amplifier. Loss of output, defined as “quantum yield of laser deterioration”,Q _L , is related to the true quantum yield of molecular destruction of the dye,Q _M , and other known parameters. For the experimentally common, saturated gain condition, the smallestQ _L can be isQ _M , which requires a photochemically bleachable dye with high gain, preferably at high concentration, in a long cavity, operating at high injection and pumping fluxes. Reversing these conditions, the highest valueQ _L can be, compared withQ _M , is unrestricted. This work was performed under the auspices of the U.S. Energy Research and Development Administration. Contract No. W 7405 Eng 48.     

Relations between photophysical and lasing properties of rhodamines in solid polymeric matrices

2 laser) pulses. In these solid media the rhodamine chromophore shows a photophysical behaviour similar to that in a liquid solution, and its photostability clearly increases when the chromophore is bound to the polymeric chain. These new solid materials show laser efficiencies that depend on the HEMA/MMA ratio, in some cases with values close to 20%. Received: 15 May 1996/Revised version: 26 November 1996     

UV excimer laser photochemistry of hybrid organometallic compounds of gallium

The gas phase ultraviolet (UV) excimer laser induced photolysis of the gallium-alkyls Ga(t-C₄H₉) _n ^– (CH₃)_3–n (n=0, 1, 2, 3) was studied, using photolysis wavelengths of 308, 248, and 193 nm. The photofragments Ga, GaH, and GaCH₃ were detected by laser ionization time-of-flight mass spectroscopy, while the hydrocarbon products CH₄, C₂H₆, HC(CH₃)₃ and H₂C=C(CH₃)₂ were identified using Fourier transform infrared (FTIR) spectroscopy. The formation of the GaH photofragment, and a high olefin-to-alkane product ratio, for Ga(t-C₄H₉)₂(CH₃) and Ga(t-C₄H₉)₃ are interpreted to indicate a -hydrogen elimination process. However, -hydrogen elimination only occurs after fission of the weakest Ga-C bond, thus no -hydride elimination is observed for Ga(t-C₄H₉)(CH₃)₂. Detection of C₂H₆ for Ga(CH₃)₃ and Ga(t-C₄H₉)(CH₃)₂, but not for Ga(t-C₄H₉)₂(CH₃), shows that under our experimental conditions the formation of ethane is as a result of the reductive elimination of the methyl groups, and is not due to the recombination of two free methyl radicals.     

Laser characteristics of a new laser dye: 7-Diethylamino-3-Styryl Benzimidazo (1,2-a) Quinoline

The laser characteristics of a new dye, 7-Diethyl-amino-3-Styryl Benz31 yimidazo (1,2-a) Quinoline (DSBQ) have been investigated in two solvents: DiMethylSulfOxide (DMSO) and DiMethylFormamide (DMF) by excitation with a pulsed nitrogen laser and compared with that of the standard dye rhodamine B. The lasing range of the dye is nearly 40 nm with maxima at 635 nm and 625 nm in DMSO and DMF, respectively. The efficiency of this dye (DSBQ) has been found to be better than half that of rhodamine B.     

Fast degradation of fullerenes by ultraviolet laser radiation

The first observation of fullerene C₆₀ ultraviolet photolysis in hexane solution was published two years ago [1]. Similar further experiments realized with an ultraviolet lamp and solar light gave inconsistent results with ambiguous interpretations. We report the unexpectedly fast and efficient degradation of the fullerenes in n-hexane solutions, induced by an XeCl-excimer laser. Well-defined experimental conditions and good reproducibility in these experiments allow us to estimate the minimal value of the quantum yield of fullerene photolysis.     

Lasers excited by photodissociation: The thalliumiodide laser

This review is concerned with lasers excited by photodissociation of molecules. The principle of photolytic excitation is described and a number of existing lasers is presented. As an example the properties of the thallium laser such as spectral and temporal emission, achievable gain and efficiency are discussed in detail.     

A broadly tunable IR waveguide Raman laser pumped by a dye laser

Design and performance characteristics of a tunable ir waveguide Raman laser pumped by a high power dye laser are described. Using SRS up to third Stokes order in compressed H₂, the wavelength range from 0.7 to 7 μm has been covered without gaps. Taking advantage of a waveguide structure in the scattering chamber, energy conversion efficiencies better than 1% with power levels in excess of 80 kW for the third Stokes component have been achieved. The experimental results support the theoretical predictions that a 4-wave parametric process is responsible for the production of the third Stokes component.     

Dopant-induced excimer laser ablation of poly(tetrafluoroethylene)

Clean ablation of poly(tetrafluoroethylene) (PTFE) at etch rates in excess of 7µm/pulse has been achieved with an excimer laser using 308nm radiation and a 25 ns pulse width. This was accomplished by doping the ultraviolet-transparent PTFE polymer with polyimide. Ablation rates were investigated as a function of fluence in the range from 1 to 12J/cm² and dopant levels up to 15% (wt/wt). Results show that at a given fluence there exists an optimum absorption coefficient _max, for which maximum ablation rates are achieved. The value of _max was found to decrease with increasing fluence. The relationship between _max and fluence was determined from existing ablation rate models and found to compare favorably with empirical results.     

Photophysical and lasing properties of pyrromethene 567 dye in solid poly(trifluoromethyl methacrylate) matrices with different degrees of crosslinking

The effect of increasing the rigidity of the host matrix by controlled crosslinking of the polymeric chains on the lasing and photophysical properties of pyrromethene 567 (PM567) was evaluated. To this end, the dye was dissolved in 2,2,2-trifluoromethyl methacrylate (TFMA) together with different amounts of the crosslinking monomer ethylene glycol dimethacrylate (EGDMA). Increasing the concentration of this crosslinker monomer resulted in significant increases in both lasing efficiency and photostability. Good correlation between fluorescence and lasing properties has been observed, indicating the existence of an optimum degree of crosslinking for which lasing efficiency and photostabilty reach their highest values. These new solid materials show lasing efficiencies up to 18% when transversely pumped at 534 nm and good photostabilities, with a decrease in the initial laser output of 45% after 10000 pump pulses. Received: 16 February 2001 / Revised version: 9 April 2001 / Published online: 8 June 2001     

Characteristics of a hybridly mode-locked cw dye laser

We present experimental and theoretical results on the characteristics and random variations of subpicosecond pulses generated by a synchronously pumped cw dye laser with saturable absorber. The analysis of the power spectra indicates rapid fluctuations of the pulse duration of 40–50%, energy fluctuations of 3%, and a jitter of the repetition time of 0.1% corresponding to an absolute jitter of 12 ps. The latter is caused mainly by the temporal jitter of the pump laser. A mismatch of the lengths of the dye and the pump-laser cavity can result in a nonstationary mode-locking regime with a periodic change of the pulse parameters. The interpretation of the experimental results are supported by computer simulations of the pulse evolution process.     

Stepwise photoionization of complex organic molecules in the gas phase induced by UV laser radiation

We measured the photoionization cross-sections of anthracene, 1,2-benzanthracene,p-terphenyl, and 2,5-diphenylfuran molecules excited to the singlet and triplet electronic states intermediate in the stepwise two-photon ionization process at 266 nm (all compounds) and at 355, 391 and 417 nm (anthracene).     

Confirmation of far infrared laser assignments from CH3-deformation and CH3-rocking states of optically pumped methanol

High-resolution Fourier transform spectroscopy has been applied to confirm previously proposed assignments for nine far infrared (FIR) laser lines from the CH₃-deformation state of CH₃OH and one line from the CH₃-rocking state. Accurate frequencies are deduced for the observed and other predicted FIR laser transitions. FIR torsional branch frequencies in the ground state which were used in the confirmation are presented. Comments are also made on the OH-bending mode of CH₃OH.     

InAsSb/InAsSbP current-tunable laser with narrow spectral line width

The article describes the manufacture and testing of a new type of semiconductor laser working at low temperatures (12–100 K) in the wavelength range 3200–3300 cm^-1. This kind of laser can be tuned in the modal range up to 6 cm^-1 and is characterized by a narrow spectral line width (about 7 MHz). Received: 12 September 2002 / Final version: 29 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +420-286/591-766, E-mail: civis@jh-inst.cas.cz     

Laser and fluorescence properties of dye-doped sol-gel silica from 400 nm to 800 nm

Lasing and fluorescence properties of ten dyes covering the spectral range from 400 nm to 800 nm were studied in a silica matrix prepared by the sol-gel technique. The dye-doped sol-gel silica samples fluoresced strongly under laser excitation. Many of the samples were also induced to superradiate. In comparison to dyes in alcohol solvents, significant red-shifting was observed in the sol-gel silica fluorescence spectra. Plausible causes of the red-shift are discussed.     

Spectroscopic characterization and laser performance of diode-laser-pumped Nd: GdVO4

We have carried out a detailed spectroscopic characterization of Nd: GdVO₄, a new laser crystal with high effective absorption and emission cross sections. The accidental degeneracy of the upper⁴ F _3/2 laser level decreases the number of emission lines and creates -together with the anisotropic crystal field - high emission cross sections (7.6 × 10^–19 cm² at 1.06 µm and 300 K). In addition, the lines are strongly homogeneously broadened (1.6 nm for the 808.4 nm diode-laser pump transition). The temperature dependences of lifetime, linewidths, and cross sections have been determined. Slope efficiencies up to 57% with respect to the absorbed diode-laser pump power and output powers up to 0.8 W have been achieved at 1.06 µm from a 2 mm long crystal. Intracavity second-harmonic generation, using a KTP crystal, is demonstrated.