Xia Yuan-Qin

Femtosecond Time-Resolved Resonance-Enhanced CARS of Gaseous Iodine at Room Temperature *

Time-resolved resonance-enhanced coherent anti-Stokes Raman scattering (CARS) is applied to investigate molecular dynamics in gaseous iodine. 40 fs laser pulses are applied to create and monitor the high vibrational states of iodine at room temperature (corresponding to a vapor pressure as low as about 35 Pa) by femtosecond time-resolved CARS. Depending on the time delay between the probe pulse and the pump/Stokes pulse pairs, the high vibrational states both on the electronically ground states and the excited states can be detected as oscillations in the CARS transient signal. It is proved that the femtosecond time-resolved CARS technique is a promising candidate for investigating the molecular dynamics of a low concentration system and can be applied to environmental and atmospheric monitoring measurements.

PM567-Doped solid dye lasers based on PMMA

Polymers are a kind of attractive hosts for laser dyes due to their high transparency in both pumping and lasing ranges and superior optical homogeneity. In this paper solid dye samples based on polymethyl methacrylate (PMMA) doped with different concentrations of 1, 3, 5, 7, 8-pentamethyl-2, 6-diethylpyrromethene-BF2 (PM567) are prepared. The absorption, fluorescence and lasing spectra of the samples are obtained. Wide absorption and fluorescence bands are obtained and a red shift of the maxima of the lasing emission spectra is observed. With the second-harmonic generation of Q-switched Nd:YAG laser (532 nm, ~20ns) pumping the samples longitudinally, the slope efficiencies of the samples are obtained. There is an optimal dye concentration for the highest slope efficiency when the pumping energy is lower than some typical value (~250 mJ), and the highest slope efficiency 35.6% is obtained in the sample with a dye concentration of 2 × 10−4 mol/L. Pumping the samples at a rate of 10Hz with a pulse energy as high as 200 mJ (the fluence is 0.2 J/cm2), the output energy drops to one-half of its initial value after approximate 15500 pulses and the normalized photostability is 5.17 GJ/mol. A kind of solid dye laser which could have some applications is built.

Selective excitation of molecular mode in a mixture by femtosecond resonance-enhanced coherent anti-Stokes Raman scattering spectroscopy

Femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy is used to investigate gaseous molecular dynamics. Due to the spectrally broad laser pulses, usually poorly resolved spectra result from this broad spectroscopy. However, it can be demonstrated that by the electronic resonance enhancement optimization control a selective excitation of specific vibrational mode is possible. Using an electronically resonance-enhanced effect, iodine molecule specific CARS spectroscopy can be obtained from a mixture of iodine—air at room temperature and a pressure of 1 atm (corresponding to a saturation iodine vapour as low as about 35 Pa). The dynamics on either the electronically excited state or the ground state of iodine molecules obtained is consistent with previous studies (vacuum, heated and pure iodine) in the femtosecond time resolved CARS spectroscopy, showing that an effective method of suppressing the non-resonant CARS background and other interferences is demonstrated.

Solid Dye Lasers Based on 2-Hydroxypropyl Methacrylate and Methyl Methacrylate Copolymers

Polymers are a kind of attractive hosts for laser dyes because of their superior optical homogeneity, and high transparency in pumping and lasing range. Copolymers usually have higher damage threshold and better photostability than mono-polymers. Solid dye samples based on copolymer of methyl methacrylate (MMA) with 2-hydroxypropyl methacrylate (HPMA) doped with 1-, 3-, 5-, 7-, 8-pentamethyl-2, 6-diethylpyrromethene-BF2 (PM567) are prepared. Spectra and lasing properties of the samples are studied. Compared to the samples based on monopolymer polymethyl methacrylate (PMMA), enhanced slope efficiency and photostability are obtained in the copolymers. The highest slope efficiency is 45.1%, and nearly one-fold increase of photostability is obtained. The longest useful lifetime of 4390 pumping pulses is presented with the pump energy as high as 183 mJ per pulse at repetition rate of 10 Hz. The results indicate that the laser performances of solid dye mediums can be greatly increased using copolymer of MMA with HPMA as host.

Solid State Dye Lasers Based on Coumarin 440 and Pyrromethene 567 Codoped Polymethyl Methacrylate

Laser dye coumarin 440(C440) is codoped with pyrromethene 567 (PM567) into polymethyl methacrylate (PMMA). The effects of C440 concentration on the performance of the solid state dye medium, including spectra property, slope efficiency and photostability, are studied. When C440 is codoped with PM567 at the same concentration 1 × 10−4 mol/L, the highest efficiency and photostability can be obtained. Compared with the medium based on pure PM567 doped PMMA, about 50% increase in slope efficiency and at least five-fold enhancement in the photostability are observed.

X-Ray and Extreme Ultraviolet Emission from Small-Sized Kr Clusters Irradiated by 150-fs Laser Pulses

x-ray and extreme ultraviolet (EUV) emission from Kr clusters irradiated by 150-fs laser pulses at the peak laser intensity of 5?1015W/cm2 was experimentally investigated. Strong transitions (10?nm-13?nm) from Kr X and Kr IX were observed and some spectral lines from Kr XIII and Kr XIV, which have been predicted to be not produced by optical-field-ionization at the laser intensity used, also appeared. The laser energy absorption and the intensity of x-ray emission started to grow remarkably above the backing pressure of 0.5?MPa and to decrease at the backing pressure of 3?MPa. It is suggested that an optimum backing pressure may exist for Kr clusters heated by 150?fs laser pulses at a certain laser intensity to produce x-ray emission.

Tunable solid-state laser based on modified polymethyl methacrylate with methanol doped with Pyrromethene 580

Solid-state dye material was fabricated by doping a laser dye Pyrromethene 580 (PM580) into the polymer host polymethyl methacrylate. Methanol was also injected into the host to improve the optical properties. The broadband and narrowband laser performances of the sample were studied in this paper. When the selected solid-state dye PM580 was placed in a Shoshan-type oscillator, narrow linewidth operation with a tuning range of 52 nm and good photostability was demonstrated. The narrowband laser output slope efficiency of 42.7% was obtained, and the corresponding broadband laser slope efficiency was 66.0%. To the best of our knowledge, the narrowband slope efficiency and tunable range are the best under similar conditions so far. For broadband and narrowband lasers, the beam quality factors were estimated to be M(y)(2)=10.7 and 3.9 in the vertical direction.

Nonlinear fluorescence properties of DCM dyes irradiated under femtosecond pulses

Fluorescence properties of DCM solution and solid dyes were researched applying femtosecond pulses under linear and nonlinear excitation. The reabsorption was distinct in the solid dyes compared with that in solutions under similar conditions. Nonlinear fluorescence spectra of DCM and rhodamine 590 co-doped tunable laser dyes in ethanol were also researched. The energy transfer of rhodamine 590 to DCM was found to be neglectable in the nonlinear excitation spectra of the co-doped liquid dyes.

Structural Deformation of CO22+ in Intense Femtosecond Laser Fields

The angular distributions of CO+ from the dissociation of CO2+2 and CO+2 in intense femtosecond laser fields (45 fs, about 5 × 1015 W/cm2) are studied at a laser wavelength of 800 nm based on the time-of-flight mass spectra of CO fragment ions. The experimental results show that structural deformation occurs in the charge state of CO2+2 and the CO+2 maintains linear geometrical structure.

Enhanced high harmonic generation driven by two-color laser pulses with two foci

We demonstrate a enhancement of high harmonic generation in CO2 by using two color laser pulses with two foci. The intensity of harmonic H23 increased by a factor of 65 compared to a single focus.