Investigation of NO detection in flames by the use of polarization spectroscopy

Various aspects of detection of NO molecules in a premixed H(2)/N(2)O flame were investigated by the use of polarization spectroscopy. The signals were recorded in the A(2)Σ(+)-X(2)II (0, 0) band of NO at 226 nm. Excitation spectra for circularly and linearly polarized pump beams are presented to illustrate the influence of the polarization state. The signal dependencies on the pump- and the probe-beam intensities were also investigated, as well as the influence of the setup geometry with respect to the pump-beam focusing and the crossing angle between the pump and the probe beams. A preliminary evaluation of temperature based on spectra recorded by the use of the Boltzmann plotting approach was performed. Finally the possibilities of two-dimensional imaging of the NO molecules were demonstrated.     

Stable isotope ratio analysis at trace concentrations using degenerate four-wave mixing with a circularly polarized pulsed probe beam

Stable isotope analysis based on vectorial optical-phase conjugation by resonant degenerate four-wave mixing (D4WM) is reported by using a D4WM method with vertically polarized pump beams and a circularly polarized probe beam. Since the polarization of the signal beam is different from that of the pump beams, the background radiation is suppressed more effectively. Excellent sensitivity, high spectral resolution, and efficient optical detection make this an effective and unusually convenient nonlinear spectrometric method for the analysis of trace amounts of stable isotopes. Using an excimer-pumped pulsed dye laser, the fine structures of lithium are examined. A detection limit of 2.5 ng/mL lithium is observed while a Doppler-free resolution is maintained by using transient "coherent-grating" based D4WM spectroscopy.     

Vectorial four-wave mixing due to optical Kerr effect in polycrystalline CdTe

We study experimentally the polarization properties of the wave generated by means of degenerate four-wave mixing in polycrystalline CdTe using forward-box phase matching configuration and picosecond laser pulses with a wavelength of 1064 nm. The dependencies of the wave polarization generated due to the optical Kerr effect on the polarization combinations of the input beams are presented. We show that diffracted light polarization depends on the polarization of both recording beams, and the effect of each recording beam on the diffracted beam polarization is different depending on the mutual position of the recording beams and the probe beam. It was found that virtually any polarization of the generated beam could be obtained by proper choice of the recording and probe beam polarization. These results could make the polycrystalline media with third-order non-linearity a cheap and effective alternative to single crystals in non-linear devices for ultrafast all-optical control of polarization.     

Thin-cell sub-Doppler spectroscopy by spatially separated beam method and pump-probe method

We experimentally demonstrate two methods that improve the resolution of sub-Doppler spectroscopy using a 1 mm thick vapor cell. The linewidths of the observed spectra are approximately 1 order of magnitude narrower than the Doppler width. The first method involves using a 1 mm thick cell filled with Rb atomic vapor and two spatially separated laser beams. By employing the same principle, we also demonstrate that it is possible to achieve the same resolution by using the pump and probe pulses of a single beam. The latter method enables us to construct a simple and robust optical setup for sub-Doppler spectroscopy.     

KNSBN:V晶体材料制备及光诱导吸收研究

成功地生长、制备出了一系列含不同掺钒浓度的钾钠铌酸锶钡（ＫＮＳＢＮ：Ｖ）晶体样品。测量了在不同光强、光偏振和掺钒浓度下ＫＮＳＢＮ晶体的光诱导吸收变化。实验结果显示,ＫＮＳＢＮ：Ｖ晶体存在较强的光诱导吸收,且其光诱导吸收变化明显依赖于泵浦束光强,探测束光偏振和晶体的掺钒量,对于探测束光偏振平行和垂直于ｃ轴的情况其值分别可达０．５５和０．２２ｃｍ＾－１。对实验结果进行了解释。     

Spatial effects in two-photon excitation transient absorption spectroscopy

We investigate the influence of the transverse mode of the pump beam and the geometry of the pump–probe configuration on the femtosecond two-photon excitation transient absorption (TA) spectroscopy. Strong modulation of the transient TA dynamics may result from the multi-peak distribution of the transverse mode of the pump beam, the large pump beam size, and the large separation angle between the pump and the probe beams, which is sensitive to the pulse length. Quantitative characterization of this kind of spatial effect is of general importance for the correct interpretation of the photophysical mechanisms that can be revealed by the two-photon TA measurements.     

Implementation of intensity-modulated laser diodes in time-resolved, pump-probe fluorescence microscopy

We present the implementation of intensity-modulated laser diodes for applications in frequency-domain pump-probe fluorescence microscopy. Our technique, which is based on the stimulated-emission approach, uses two sinusoidally modulated laser diodes. One laser (635 nm) excites the chromophores under study, and the other laser (680 nm) is responsible for inducing stimulated emission from excited-state molecules. Both light sources are modulated in the 80-MHz range but with an offset of 5 kHz between them. The result of the interaction of the pump and the probe beams is that a cross-correlation fluorescence signal at 5 kHz is generated primarily at the focal volume. Microscope imaging at the cross-correlation signal results in images with high contrast, and time-resolved high-frequency information can be acquired without high-speed detection. A detailed experimental arrangement of our methodology is presented along with images acquired from a 4.0-mum-diameter fluorescent sphere and TOTO-3-labeled mouse STO cells. (TOTO-3 is a nucleic acid stain.) Our results demonstrate the feasibility of using sinusoidally modulated laser diodes for pump-probe imaging, creating the exciting possibility of high-contrast time-resolved imaging with low-cost laser-diode systems.     

Highly efficient continuous-wave operation of a Nd:YAG rod laser that is side pumped by p-polarized diode laser bars

We report on the performance of highly efficient, high-power continuous-wave (CW) Nd:YAG lasers that are side pumped by p-polarized diode laser beams. In this configuration pump light is directly coupled into the Nd:YAG rod through a threefold symmetric gold-coated flow tube. The polarization direction of our pump diode bars is perpendicular to the rod's axis (p-polarized). In a closed coupled resonator, a maximum output power of 195 W in multimode operation is obtained for a pump power of 423 W, which corresponds to an optical-to-optical efficiency of 46% and an electrical-to-optical efficiency of 23%. This is, to the best of our knowledge, the highest electrical-to-optical efficiency reported for a CW diode side-pumped Nd:YAG laser. By the pump-power leakage analysis method, we measured the pumping efficiency to be approximately 94%. The high efficiency of the system can be attributed to wing pumping, which results in uniform pump-light distribution and better pumping efficiency because of the use of p-polarized pump beams.     

Crossed two-beam coherent anti-stokes Raman spectroscopy in dispersive media

Coherent anti-Stokes Raman spectroscopy (CARS) is a nonlinear optical wave mixing process that is used in gas-phase systems to determine the energy distribution of the probed species (usually N2) and, through a fitting procedure, the temperature giving rise to it. CARS signal strengths are maximized when the phase matching condition is met. Because gases are generally non-dispersive, this phase matching condition can be found geometrically as a function of the crossing angles between the CARS beams and their wavelengths. In addition, perfect phase matching in non-dispersive media occurs automatically for collinear beams. To improve spatial resolution, however, intersecting the laser beams is desirable. Being a third-order process, phase matching for CARS in gases typically requires three input laser beams. This paper discusses and demonstrates the issues of phase matching for CARS when the medium is dispersive, and the ability for CARS phase matching to occur with only two crossed laser beams (one pump and one probe). This two-beam X-CARS in dispersive media can be used as an alignment tool for gas-phase CARS and may be relevant as a simpler diagnostic in high-pressure environments. The paper also discusses the effects of non-ideal phase matching in dispersive and non-dispersive media.     

Anisotropic Photoexcited Carrier Dynamics in (100)-, (001)-, and (110)-Oriented YBCO Films by Polarized Ultrafast Optical Spectroscopy

We report, the time-resolved ultrafast optical spectroscopy results obtained from a series of well characterized (001)-, (100)- and (110)-oriented YBa₂Cu₃O_7– (YBCO) superconducting films prepared by pulsed laser deposition. The well-controlled film orientation has allowed us to probe the relaxation dynamics along various crystal orientations using respectively polarized pump and probe laser beams. The significant anisotropies in both the magnitude of the characteristic relaxation time and the temperature dependence of the photo-induced transient reflectance change indicate the nature of relaxation channel might be intrinsically different along different orientations. The implications of the results on the fundamental characteristics concerning superconducting gaps are discussed.     

Detection of atomic hydrogen in flames using picosecond two-color two-photon-resonant six-wave-mixing spectroscopy

We report an investigation of two-color six-wave-mixing spectroscopy techniques using picosecond lasers for the detection of atomic hydrogen in an atmospheric-pressure hydrogen-air flame. An ultraviolet laser at 243 nm was two-photon-resonant with the 2S(1/2) <-- <-- 1S(1/2) transition, and a visible probe laser at 656 nm was resonant with H(alpha) transitions (n=3 <-- n=2). The signal dependence on the polarization of the pump laser was investigated for a two- beam polarization-spectroscopy experimental configuration and for a four- beam grating configuration. A direct comparison of the absolute signal and background levels in the two experimental geometries demonstrated a significant advantage to using the four-beam grating geometry over the simpler two-beam configuration. Picosecond laser pulses provided sufficient time resolution to investigate hydrogen collisions in the atmospheric-pressure flame. Time-resolved two-color laser-induced fluorescence was used to measure an n=2 population lifetime of 110 ps, and time-resolved two-color six-wave-mixing spectroscopy was used to measure a coherence lifetime of 76 ps. Based on the collisional time scale, we expect that the six-wave-mixing signal dependence on collisions is significantly reduced with picosecond laser pulses when compared to laser pulse durations on the nanosecond time scale.     

Performance improvement in a directly 879 nm dual-end-π-polarized-pumped CW and pulsed GdVO₄/Nd:GdVO₄ laser

A continuous-wave and Q-switched 10 kHz GdVO?/Nd:GdVO? laser by 879 nm laser-diode pumping was demonstrated. We combined dual-end-π-polarized-pumping, half-wave plate (HWP) insertion, composite laser crystals, and dual Q-switching techniques to obtain a 1.06 μm nanosecond laser with high temporal and spatial quality. A polarization beam splitter was used to divide the unpolarized pump beam into two linearly polarized beams to increase the absorption efficiency. The output performance was improved obviously by inserting a HWP into the cavity. The pulse duration was compressed and the peak power was increased for a doubly Q-switched laser, in contrast to a single acousto-optical Q-switched laser under the same conditions.     

Optical Kerr coefficient measurement in coiled high-birefringent fibers

The optical Kerr effect in coiled high-birefringent fibers was measured based on a double-beam polarimetric method. A Q-switched Nd:YAG laser, operating at 1.064 mum (FWHM of 80 ns at 1 kHz), was used as the pump beam and a cw 0.633-mum He-Ne laser was used as the probe beam with its polarization fixed at 45 degrees with respect to the birefringent axis whereas orientation of the linearly polarized pump light varied. The phase shifts induced by an intense pump beam in a short bow-tie high-birefringent fiber were determined for different fiber lengths coiled into 15- and 30-cm-diameter drums. It was found that the induced phase shift changes drastically with the state of polarization of the pump light. A strong dependence of the phase shift on orientations of linear pump polarization is attributed to differential losses of eigenmodes peculiar to birefringent axes. Therefore, optical Kerr coefficients remain unchanged regardless of the dependence of the nonlinear response of the coiled high-birefringent fibers on pump polarization.     

Investigation of nonlinear dynamics in CdS by time-resolved nondegenerate pump–probe system with phase object

The time-resolved nondegenerate pump–probe system with phase object is employed for investigation of nonlinear absorption and refraction dynamics in CdS. The 532?nm laser beam with 21?ps duration is used as the excitation and the laser beams of 600 and 680?nm with 10?ps duration from optical parametric generation are used for probing. The experimental results at both probe wavelengths show free-carrier absorption and large free-carrier refraction along with two-photon absorption and bound electronic optical Kerr effect. By numerically fitting the experimental data based on the nondegenerate pump–probe theory, the nondegenerate two-photon absorption coefficient, the nondegenerate Kerr coefficient, the free-carrier decay time, the free-carrier absorptive cross-section and free-carrier refractive coefficient at different wavelengths are all determined.     

Fiber delivered two-color picosecond source through nonlinear spectral transformation for coherent Raman scattering imaging

We demonstrate a two-color, fiber-delivered picosecond source for coherent Raman scattering (CRS) imaging through nonlinear spectral transformation. The wavelength tunable picosecond pump is generated by nonlinear spectral compression of a prechirped femtosecond pulse in a fiber wavelength division multiplexer (WDM). The 1064-nm synchronized picosecond Stokes pulse is generated through pulse carving of a continuous wave laser, nonlinear spectral broadening in 100-m standard single-mode fiber, and subsequent dispersive compression with a fiber compressor. The pump and Stokes beams are combined and delivered by the fiber WDM. CRS imaging of mouse skin is performed to demonstrate the practicality of this source.     

Model of a multiple-line distributed-feedback dye laser

The theoretical basis for simultaneous oscillation of 2N - 3 laser lines is due to interference of N (for all even N > or = 2) pump beams in a distributed-feedback dye laser is described. Multiple gratings are produced in a dye solution by interference patterns of N/2 pairs of a frequency-doubled Nd:YAG laser. N/2 pairs of mutually time-delayed pulses induce multiple gratings of different periodicities, of which 2N - 3 gratings support oscillation of 2N - 3 lines and the remaining gratings, because of their larger periods, cannot support Bragg scattering. The maximum number of laser lines depends on the mutual delay between adjacent pairs of beams, coherence, states of polarization, pulse lengths, and of course the number of pulses. For three pairs of excitation beams derived from the same source through wave-front or amplitude phase division techniques, the output lasing lines varied from a minimum of three to a maximum of nine. This research was carried out by pumping of a dye solution with two, four, and six pulses, but the principle may be extended to multiple output lines, depending on the number of pump pulses and on the gain of the dye solution.     

Spectral intensity and phase changes of short laser pulses by two-photon absorption

We calculated third order non-linear polarization to estimate the two-photon absorption of non-interacting two-level molecules in the transmission-type degenerate pump–probe geometry. The spectral intensity and the phase changes of the laser pulses when passing through a thin dielectric slab composed of the molecules were considered. We also investigated the effect of the decay rate of the molecules and the chirp of the pulses on their spectral intensity and phase changes.     

Development of multipoint vibrational coherent anti-Stokes Raman spectroscopy for flame applications

A novel technique for coherent anti-Stokes Raman spectroscopy (CARS) measurements in multiple points is presented. In a multipass cavity the pump and Stokes laser beams are multiply reflected and refocused into a measurement volume with an adjustable number of separated points along a line. This optical arrangement was used in a vibrational CARS setup with planar BOXCARS phase-matching configuration. The CARS spectra from spatially separated points were recorded at different heights on a CCD camera. Measurements of temperature profiles were carried out in the burned gas zone of a premixed one-dimensional flame to demonstrate the applicability of this method for temperature measurements in high-temperature regions. The ability to measure in flames with strong density gradients was demonstrated by simultaneous measurements of Q-branch spectra of N2 and CO in a Wolfhard-Parker burner flame. Interference phenomena found in multipoint spectra are discussed, and possible solutions are proposed. Merits and limitations of the technique are discussed.     

Light-induced diffraction in thermally reduced congruent and near-stoichiometric Er:LiNbO3 crystals

A polarization-isotropic two-colour light-induced diffraction (LID) phenomenon from thermally reduced congruent and near-stoichiometric Er:LiNbO₃ crystals was observed when these crystals were exposed to superposed 632.8?nm probe and 488?nm pump beams. The LID is characterized by expansion of the probe beam spot and the appearance of alternate bright and dark diffraction rings at higher pump light intensities. This phenomenon is absent not only in all other Er-doped crystals which either were as grown or had experienced other post-growth treatments such as vapour transport equilibration (VTE), oxidation and strong annealing but also in a pure VTE-treated crystal even though the crystal was subjected to the same reduction procedure. The LID characteristics of five reduced crystals, which have different Li-to-Nb ratios and different doping levels of Er³⁺, were studied in detail by changing the light intensity, polarization state and wave vector direction of both the pump and the probe beams. The origin of the LID was investigated from the viewpoints of the red fluorescence of Er³⁺ excited by a 488?nm pump beam, light-induced scattering and light-induced thermal nonlinearity. The results have shown that the thermal nonlinearity mechanism is preferred as it can reasonably explain almost all the observed experimental phenomena.     

Optical nonlinearities of gallium nitride

Luminescence, induced absorption and degenerate four wave mixing experiments are performed on GaN epilayers grown on a sapphire substrate by MOCVD. We measure the nonlinear behaviour of the luminescence spectra near the excitonic resonance, by using an excitation at 4.026 eV from an excimer laser. At low intensities of excitation, spectra show a saturation of the I₂ line due to the finite donor density in the sample. Higher intensities of excitation induce collision process between photo-created particles. Using a dye laser as a pump beam, we measure the induced variation of absorption of a probe beam as a function of the intensity and of the wavelength of the excitation. With increasing intensities of the pump beam, curves show a red-shift of the absorption edge and of the excitonic resonance. Pulsed degenerate four-wave mixing experiments were performed using the third harmonics of a picosecond Nd-YAG laser at 3.492 eV. A characteristic time of 16 ps has been measured, which is independent of the temperature, of the fringe spacing and of the intensity of the pump beams.