Fan Rongwei

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.

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.

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.

Degenerate Four-Wave Mixing in SO2 in Free Jets

A rotational excitation spectrum of SO2 [A 1A2(511)← 1A1(000)] at about 33331 cm−1 in free jets was observed by using the forward degenerate four-wave mixing (DFWM). Twelve lines of the G band and 11 ones of the E band were marked based on the rotational constants according to Hamadas result [Can. J. Phys. 53 (1975) 2555]. The relation between the DFWM signal intensity and the pressure of the buffer gas in free-jets was experimentally investigated.

Investigation of energy transfer from pyrromethene dye to cresyl violet 670 in ethanol

In this paper, radiative and nonradiative energy transfer from laser dye pyrromethene 567 (PM567) and pyrromethene 580 (PM580) as donors to cresyl violet 670 (CV670) as acceptor in ethanol are investigated by using the steady-state emission measurement and the second harmonic generation (532 nm, ~ 13 ns) of a Q-switched Nd:YAG laser as the pumping source. The fluorescence intensity of the acceptor is improved due to the introduction of the donors, and the largest enhancement is obtained to be 128% in the PM567:CV670 dye mixture system. Energy transfer parameters, including the radiative and nonradiative energy transfer rate constants (KR and KNR), critical distance (R0), and half quenching concentration ([A]1/2) are investigated using the Stern—Volmer plots, and the acceptor concentration dependencies of radiative and nonradiative transfer efficiencies are also obtained. The values of KR for PM567:CV670 and PM580:CV670 systems are 2032.0×109 Lmol−1s−1 and 2790.4×109 Lmol−1s−1, respectively, and the values of corresponding KNR are 3.3 × 109 Lmol−1s−1 and 4.2×109 Lmol−1s−1, respectively. The results indicate that the dominant mechanism responsible for the energy transfer in the dye mixture systems is of the radiative type.

Medium performance effect on the high output energy from a xenon lamp-pumped pyrromethene-567 solid-state dye laser

In order to obtain a high output energy from a xenon lamp-pumped solid-state dye laser, homogeneities of laser mediums and flatnesses of medium faces with different processing treatments are discussed in the paper. The mediums without aging treatment, which are prepared by using a prepolymer process and have diamond-machined end faces to produce the required optical finish, give a highest laser output of 281.9 mJ with 0.215% slope efficiency at 2.0 × 10−4 mol/L. The best medium lifetime is 21 shots to 50% of original output equating 74.6 kJ/liter.