Femtosecond coherent anti-Stokes Raman scattering （CARS） suffers from poor selectivity between neighbouring Raman levels due to the large bandwidth of the femtosecond pulses. This paper provides a new method to realize the selective excitation and suppression of femtosecond CARS by manipulating both the probe and pump （or Stokes） spectra. These theoretical results indicate that the CARS signals between neighbouring Raman levels are differentiated from their indistinguishable femtosecond CARS spectra by tailoring the probe spectrum, and then their selective excitation and suppression can be realized by supplementally manipulating the pump （or Stokes） spectrum with the π spectral phase step.
Niobic tellurite glass doped by silver chloride nanocrystal was prepared with the melting-quenching and heat treatment method, and the self-trapped exciton absorption band of the silver chloride nanocrystal was observed at 532 nm in the UV-visible absorption spectrum. The glass structure characteristics were investigated by Raman spectroscopy, and the mechanism of self-trapped exciton was analyzed by Jahn-Teller model. Its optical limiting was measured with 532 nm picosecond laser pulses, and the corresponding nonlinear absorption coefficient was measured with open-aperture Z-scan. The experimental results showed that optical limiting at 532 nm was attributed to free carrier absorption between the self-trapped state and the continuum band.
Dependence of surface-enhanced Raman scattering （SERS） from Calf thymus DNA on anions is investigated. With the silver colloid, the bands at 732, 960 and 1333 cm^-1 for adenine （A）, 1466 cm^-1 for deoxyribose, and 1652 cm^-1 for the C=O group of thymine （T） are observably enhanced. With the presence of the Cl^- or SO4^2- anions, the bands at 732 and 1326/1329 cm^-1 for the symmetric stretching and skeletal vibrational modes of adenine （A） are dramatically enhanced, and the enhancement effect with the SO4^2- ion is more than that with the Cl^- ion. The experimental results show that the DNA molecule can be adsorbed on the silver colloid particles through the C6N and N7 of adenine （A） the C=O of thymine （T） and deoxyribose. Moreover, the formed hydrogen bonding of the Cl^- or SO4^2- ions to the C6NH2 group of adenine （A） can induce larger C6N electronegativity, which is favor for the C6N/N7 cooperative adsorption on the （Ag）^＋ colloid particles.
Degenerate four-wave mixing measurements, using the 35 ps pulses at 532 nm, have been employed to investigate the third-order nonlinear optical parameters of two chromium tricarbonyl complexes η6-bonded to 3-amino-9-ethylcarbazole at either the NH2-substituted aryl ring (1) or the unsubstituted ring (2) and their precursor 3-amino-9-ethylcarbazole (AECz). The second-order hyperpolarizability γ of the compounds 1 and 2 were found to be 42.9×10-31 and 35.9×10-31 esu, respectively, approximately one order of magnitude greater than AECz. The relation between the molecular structure and second-order hyperpolarizability of the compounds 1 and 2 was explored in detail based on the three-level model and the density functional theory (DFT) calculation. The theoretical results indicate that the spatial distri-bution of electron density has the profound role in the third-order nonlinear optical properties.
A self-organized thin film of a cyanine dye is fabricated by the spin-coating technique and is characterized by ultraviolet-visible spectroscopy, infrared （IR） spectroscopy, small-angle X-ray diffraction, eUipsometer, and atomic force microscopy （AFM）. The nonlinear optical properties of the thin films are investigated by degenerate four wave mixing （DFWM） technique. The cyanine dye thin film sample exhibits high optical nonlinearities （χ^（3） = 2.55 × 10^-12 esu）, and the mechanism is analyzed by the exciton coupling theory.
To study the efficacy and side effects of antitumor drug by the method of Raman spectroscopy, the cancerous （SGC-7901） and normal （GES-1） gastric cells were treated with 0, 25-, 100-, and 200-mg/L 5-fluorouracil （5-Fu） for 24 h, respectively, then Raman spectra of cells were recorded. The excitation wavelength was 514.5 nm and the Raman spectra in the region of 500 - 1800 cm^-1 were recorded. For the gastric cancer cells, as the concentration of 5-Pu increases, the band at 1094 cm^-1 attributed to the symmetric stretching vibration mode of PO2^- in the DNA backbone gradually decreases, and the intensity ratio of the band at 1315 cm^-1 to that at 1340 cm^-1 （I1315/I1340） shows the ascending trend, and the ratio of the band area at 1655 cm^-1 to that at 1450 cm^-1 （A1655/A1450） shows the slight ascending trend. For the normal gastric cells, these peaks also appear changes, however, the changes are weaker than those for the cancer cells. In SGC-7901 cells, 5-Pu can interfere with the DNA synthesis and result in the reduction of the DNA content. Besides, it can affect the unsaturation degree of the hydrocarbon chains and alter the external environment of guanine and adenine residues in cancer cells. The changes of Raman spectra for normal gastric cells reveal the side effect of 5-Fu.
A femtosecond laser pulse can be tailored to control the two-photon transitions using the ultra-fast pulse-shaping technique. This paper theoretically and experimentally demonstrates that two-photon transitions in molecular system with broad absorption line can be effectively controlled by square phase-modulation in frequency domain, and the influence of all parameters characterizing the square phase-modulation on two-photon transitions is systemically investigated and discussed. The obtained results have potential application in nonlinear spectroscopy and molecular physics.
