Chen Deying

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.

Optical Constants of SiO2 Films Deposited on Si Substrates

SiO2 films were deposited on single-crystalline silicon substrates by ion beam sputtering technology. Optical constants of SiO2 films are calculated from spectroscopic ellipsometry data, transmittance spectra and reflectance spectra by WVASE32 software, and the best fitted method is obtained for calculating optical constants of dielectric materials in the ultraviolet-visible-infrared (UV-VIS-IR) range. In the UV-VIS-NIR spectral range, refractive indices of SiO2 films are calculated separately by both ellipsometry data and reflectance spectra, and the obtained results are almost the same. Complex dielectric functions of SiO2 films in the IR spectral range are accurately calculated with infrared transmission spectra using the GenOsc model. The obtained accuracy complex refractive index of SiO2 films in the wavelength region from 0.19 μm to 25 μm is of great importance for the design of high quality coatings, such as ultra-low loss coating.

The thermal effect in diode-end-pumped continuous-wave 914-nm Nd:YVO4 laser

The thermal effect and the heat generation in diode-end-pumped continuous-wave 914-nm Nd:YVO4 lasers are investigated in detail. A theoretical model of a diode end-pumped solid-state laser is constructed to analyse the influence of fractional thermal loading on the thermal effect in the Nd:YVO4 laser based on finite element analysis. The thermal focal lengths and the end-surface deformations of laser rods in Nd:YVO4 quasi-three-level and four-level lasers are measured and compared with the results obtained by ordinary interferometry for the demonstration of higher thermal loading in 914-nm laser. Finally the fractional thermal loading in the Nd:YVO4 quasi-three-level laser is calculated by matching the experimental and the simulated end deformations.

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.

Conical bubble photoluminescence from rhodamine 6G in 1, 2-propanediol

A modified U-tube conical bubble sonoluminescence device is used to study the conical bubble photoluminescence. The spectra of conical bubble sonoluminescence at different concentrations of rhodamine 6G (Rh6G) solution in 1,2-propanediol have been measured. Results show that the sonoluminescence from the conical bubbles can directly excite Rh6G, which in turn can fluoresce. The light emission of this kind is referred to as conical bubble photoluminescence. The maximum of fluorescence spectral line intensity in the conical bubble photoluminescence has a red shift in relative to that of the standard photo-excited fluorescence, which is due to the higher self-absorption of Rh6G, and the spectral line of conical bubble photoluminescence is broadened in width compared with that of photo-excited fluorescence.

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.