Narrow resonances (Δv=2·10−2 cm−1) of multiple-photon IR absorption in ethylene

Narrow peaks of multiple-photon absorption in thev ₇ band of ethylene with the spectral width 0.02 cm⁻¹ and the contrast factor of up to 10² have been observed at the intensity of laser radiation 0.04 and 0.6 MW/cm². The multiple-photon spectra of ethylene in intense IR field have been studied with the use of a quasi-single-mode continuously tunable CO₂ laser. The results of the experiments are interpreted within the model of spepwise molecular excitation due to weak transitions.     

Selective action on nucleic acids components by picosecond light pulses

We report on first attempts for selective action on nucleic acid components, namely uracil and adenine, using picosecond light pulses. Selective action was achieved both in the case of irradiation of mixtures of bases and in the case of irradiation of mixtures of different nucleic acids components, which contain uracil and adenine.     

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).     

Shortening of travelling-wave-amplified spontaneous emission by a transversal pumping scheme

An experimental investigation on shortening of travelling-wave ASE employing a transversal pumping is presented. A N₂ laser with =337.1 nm, pulsewidth: 700 ps is used as a pump source. An ASE pulse with duration of 45, 55 and 45 ps from dye solution of Rh 6G, Rh B and C311, respectively, is obtained. Pulse shapes of output signals generated from these solutions in three different pumping schemes are compared.     

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     

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.     

Generation of femtosecond pulses by two-photon pumping supercontinuum-seeded collinear traveling wave amplification in a dye solution

Visible femtosecond (fs) laser pulses have been obtained in a dye solution with a very simple traveling wave collinear configuration. A femtosecond Ti:sapphire laser (790 nm) pumps the dye solution by two-photon absorption and simultaneously generates supercontinuum, which seeds a light-amplification mechanism. Cross-correlation frequency-resolved optical gating measurements reveal a chirped structure in the dye pulse. The shortest pulse duration achieved is 170 fs and the overall energy efficiency of the process is typically 25%. Received: 22 March 2001 / Revised version: 4 July 2001 / Published online: 19 September 2001     

Solvent dependent characteristics of XeCl pumped UV dye lasers

The characteristics of near uv dye lasers pumped with short duration, 308 nm pulses from a XeCl laser are described with particular emphasis on the parameters influencing high average power operation. The solvent is shown to effect the operation of all dye lasers studied, particularly the photochemical stability. Measurements of the efficiency, as a function of the integrated pump energy have been made for a series of dye-solvent combinations. With the best combinations, average output powers in excess of 1W and conversion efficiencies greater than 25% were obtained at a repetition rate of ∼100 Hz.     

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.     

Inhibition of two-photon absorption due to dipole-dipole interaction in nanoparticles

We have investigated the inhibition of two-photon absorption in photonic crystals doped with an ensemble of four-level nanoparticles. The particles are interacting with one another by the dipole-dipole interaction. Dipoles in nanoparticles are induced by a selected transition. Numerical simulations have been performed for an isotropic photonic crystal. Interesting phenomena have been predicted such as the inhibition of the two-photon absorption due to the dipole-dipole interaction. It has also been found that the inhibition effect can be switched on and off by tuning a decay resonance energy within the energy band of the crystal. A theory of dressed states has been used to explain the results.     

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.     

Influence of collisional coupling on the energy extraction from theB,C, andD state in KrF

Collisional coupling between theB, C, andD states of KrF is measured in a discharge pumped KrF laser. Detection of the time resolved populations in the three states via measurement of the side fluorescence in theB→X, C→A, andD→X bands is used to record the collisional relaxation from theC andD state to theB state during laser emission. The experimentally determined limits for the collisional coupling times are τ_B-C≦1 ns and τ_B-D,τ_C-D<20 ns at total pressures of 2 bar. Investigation of theB→X fluorescence band shows that the vibrational manifold of theB state is not thermalized during laser emission. TheV-V,T coupling time is estimated to be τ_V-V,T=4±2 ns. The influence of collisional coupling on energy extraction in the KrF laser is discussed.     

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     

Feedback controlled self-excitation of optical pulses in a CW dye ring laser

Feedback controlled self-excitation of optical pulses of nanosecond duration has been observed to occur rather reliably in a CW dye ring laser, oscillating in a traveling mode. These observations are described analytically by means of a direct time domain approach. It is shown that a steady-state Gaussian pulse whose time duration determined from the self consistency condition in terms of system characteristics describes accurately the observed pulse behaviors. Supported in part by a NSF Grant.     

Characterization of a Br(2P1/2)–CO2(1001-1000) transfer laser driven by photolysis of iodine monobromide

2 laser operating on the 10⁰1-10⁰0 transition at λ=4.3 μm and pumped by E –V energy transfer from Br(²P_1/2) has been demonstrated. The dynamics and performance of this device were characterized by observing the time-resolved stimulated emission and the steady-state spontaneous side fluorescence after photolysis of IBr or Br₂ by a frequency-doubled Nd:YAG laser or Ar⁺ laser, respectively. Although the E –V excitation kinetics are favorable, rapid vibrational relaxation limits laser action to CO₂ pressures of less than 1 Torr. Numerical modeling of laser pulse shapes and the dependence on IBr and CO₂ pressure and photolysis energy establish a relatively high gain of 0.33%/cm, a CO₂-pressure-dependent optical loss of 0.04–0.06%/cm, and an efficiency of 2×10^-5 4.3-μm-laser photons per incident photolysis photon. The CO₂ fluorescence after photolysis of a fixed Br₂/CO₂ gas mixture decreases as a function of photolysis time by about 30%/h, indicating the photolytic production of an important quencher. Received: 23 June 1997/Revised version: 23 September 1997     

The vibrational relaxation time constant for theB v=0 level of XeCl

The vibrational relaxation time constant for theB _v=0 vibrational level of XeCl^* has been investigated by observing the coupling between theB _v=1 andB _v=0 vibrational levels under saturated lasing conditions. Based on this observation, estimates of the expected extraction efficiency from theB-state have been made.     

Intense ultrashort pulse generation in a two-photon pumped generator-amplifier system

Intense ultrashort light pulses are generated i) by two-photon induced amplified spontaneous emission in a dye generator cell and ii) by two-photon induced seeding pulse amplification of a picosecond light continuum. The generated signals are amplified in a twophoton pumped dye amplifier. A passively mode-locked Nd: glass laser is used as pump source. Ligh pulses in the spectral region between 565 and 630 nm are generated in the dyes rohodamine B, rhodamine 6G, and PYC.     

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.     

An experimental study of self-focusing and self-defocusing of a TEA CO2 laser pulse in CDF3

We observed self-focusing and self-defocusing of a TEA CO₂ laser pulse in CDF₃ vapor under different conditions. The experimental parameters we varied are the pressure inside the interaction cell, the frequency of the laser, the energy and the temporal length of the pulse. We have shown that it is possible to pass from self-focusing to self-defocusing by only increasing the intensity of the laser pulse. We propose a physical model that can explain these experimental results. This model is different from that used to explain the selffocusing of a CO₂ laser in SF₆.