Experimental investigation of the effect of collisions and temperature on degenerate four-wave mixing

An experimental investigation (including a comparison with a simple theoretical model) of the effect of buffer-gas composition, pressure and temperature on resonant Degenerate Four-Wave Mixing (DFWM) has been performed. The DFWM signal from NO in a quartz cell was measured and the effect of quenching as well as elastic (phase-changing) collisions was studied by varying the total and partial pressures of N₂ and CO₂ as buffer gases. It was found that the DFWM signal first slowly increased with buffer-gas pressure (up to 10 mbar) and then rapidly decreased. It was furthermore found that the DFWM signal was considerably less sensitive to quenching collisions as compared to Laser-Induced Fluorescence (LIF) (for laser intensities approximately equal to half the DFWM saturation intensity of the transition). On the other hand, while LIF is virtually insensitive to elastic collisions, DFWM displays a larger sensitivity to elastic collisions than to quenching collisions. The DFWM saturation intensity was found to increase with buffer-gas pressure (although slower than expected). When varying the temperature of the gas composition, it was found that the DFWM signal decreased markedly with increasing temperature. This decrease is too fast to be explained solely by a change in the population of the molecular state probed by the laser.     

Two-Step degenerate four-wave mixing as a means to decrease pre- and post-filtering effects in optically thick media

The use of crossed beam 2-Step Degenerate Four-Wave Mixing (2S-DFWM) for decreasing pre- and post-filtering effects under optically thick conditions has been investigated. 2S-DFWM is a technique in which the DFWM process is performed between two excited states of which the lower one is populated by an ordinary laser excitation from a low lying, highly populated state. Experiments were performed on Au in an acetylene/air flame. We have shown that under conditions where ordinary (one-step) DFWM experiments are significantly affected by pre- and post-filtering effects (i.e., partly absorption of the pump, probe or signal beams prior to or after the interaction region) the 2S-DFWM technique can give virtually interference free signals. A variety of different laser beam and flame configurations have been investigated. It was found that the use of a crossed beam geometry, where the first step exciting laser is incident upon the interaction region perpendicular to the DFWM beams, completely eliminated severe pre- and post-filtering effects occurring for an ordinary single-step DFWM scheme in an optically thick medium.     

Ultrafast time-resolved coherent degenerate four-wave-mixing spectroscopy for investigating molecular dynamics in different states

In this paper, ultrafast time-resolved coherent degenerate four-wave-mixing (DFWM) spectroscopy is performed to investigate molecular dynamics in the gaseous phase. Laser pulses lasting for 40 fs are used to create and monitor different vibrational eigenstates of iodine at room temperature (corresponding to a low saturation pressure of about 35 Pa). Using an internal time delay in the DFWM process resonant with the transition between the ground X-state and the excited B-state, the vibrational states of both the electronically excited and the ground states are detected as oscillations in the DFWM transient signal. The dynamics of either the electronically excited or ground state of iodine molecules obtained are consistent with the previous high temperature studies on the femtosecond time-resolved DWFM spectroscopy.     

Effects of acid anions,major metallic elements,and chlorides on Li detection by DFWM in graphite furnace

In trace Li analysis with degenerate four-wave mixing （DFWM） method, acid anions and major metallic elements are dominant interferences in Li-eontaining samples. To better use DFWM technique to analyze trace Li in actual samples, we study their effects on Li DFWM signal intensity. It is found that K, Cs, and Ni can enhance the Li DFWM signal, SO4^2, PO^34-, Cl-, and Ca can cause significant suppression, and NO3-, Mg, Ba, Sr, and Na almost have no effects. Finally, we use HaBOa to eliminate the depressive effects of chlorides on Li DFWM signal. The result is also of reference in other trace elements analysis with DFWM.     

Detection of NO in a spark-ignition research engine using degenerate four-wave mixing

We report the first application of degenerate four-wave mixing (DFWM) to combustion diagnostics in a methane-fuelled internal combustion research engine. Combustion-generated NO in the spark-ignited engine was detected using scanning narrowband DFWM in a modified forward folded BOXCARS geometry. The resulting spectra of the X²Π-A²Σ⁺(0,0) band at 226 nm display an excellent signal-to-noise ratio. Extension of the technique to different engine operating conditions and to time-resolved multiplex DFWM is discussed. Received: 3 May 2001 / Revised version: 1 October 2001 / Published online: 29 November 2001     

Degenerate four-wave-mixing spectroscopy in NaH

We performed DFWM spectroscopy on X ^–1+–A ¹⁺ transitions in NaH produced in an indirect photochemical reaction between Na(3p) and H₂ and detected v=1, 2 and 3 ground state vibrational levels of NaH molecules, whereas with resonance enhanced CARS, we observed v=0 levels only. This different sensitivity can be explained by considering the Franck-Condon-factors and the relevant damping coefficients for the corresponding transitions in the NaH molecule. Time resolved DFWM spectroscopy showed that NaH(v=1) molecules effectively live much longer than Na(3p) atoms which merely follow the laser excitation pulse.     

噁英鎓类高聚物薄膜非线性折射率的测量及其时间分辨研究

本文报道有机杂环化合物(口恶)英(钅翁)掺杂高聚物PMMA薄膜的光学特征,在600～1000nm波长范围内,透射率为80～90%,材料折射率n=1.554,利用反向传播简并四波混频法测得该薄膜的非线性光学系数x~(3)为10~(-10)e.s.u.,并观测到后向泵浦束被延迟情况下,前向泵浦光束与探测光束形成光栅瞬态弛豫时间分辨曲线,它的弛豫时间为30ps.     

Optical phase conjugation by resonant degenerate four-wave mixing in liquid phase medium

In this paper, frequency-doubled Nd:YAG laser's phase conjugation has been investigated experimentally using resonant degenerate four-wave mixing (DFWM). The iodine solution was used as resonant medium. In this way, the energy of the DFWM phase conjugation beam arrived at 43μJ/pulse while the total pump beam's energy reached 8mJ/pulse. The relation between the pulse energy of the DFWM phase conjugation beam and that of the pump beam was investigated. We also measured the optical field distributions of pump beam and DFWM phase conjugation beam, from which it can be noticed that the DFWM phase conjugation can improve the laser beam quality. At the same time, we observed the influence of the pump beam's disturbance on the DFWM signal.     

Femtosecond Time-Resolved Third-Order Nonlinear Response of 4-(N, N-Diethylamino)-β-Nitrostyrene Solutions in the Nonresonant Region—Experimental Evidence of Group-Velocity Dispersion Effec—

We measure the femtosecond time-dependent, third-order nonlinear optical response for 4-(N, N-diethylamino)-β-nitrostyrene (DEANST) dissolved in N, N-dimethylformamide (DMF) from 0.1 to 1.4 M concentrations in the nonresonant region using time-resolved degenerate four-wave mixing (DFWM) spectroscopy. It is found that the DFWM signal profile with delayed responses depends on the concentration. This concentration dependence is determined to be mainly due to the group-velocity dispersion effect of the DEANST solution at the high concentration and the nonlinear response of the DMF solvent at the low concentration, but not due to the change in third-order nonlinear response of DEANST itself.     

Degenerate four-wave mixing experiments in Methyl green dye-doped gelatin film

Degenerate four-wave mixing (DFWM) was performed in Methyl green dye-doped gelatin films using continuous-wave laser radiation () generated by a He-Ne laser of total power 35 mW. Various parameters which influence the phase-conjugate (PC) signal during the DFWM process were studied. The PC signal contributions from induced holographic transmission and reflection gratings were measured. We observed a maximum PC beam reflectivity of 0.13% in these dye-doped gelatin films.     

Infrared degenerate four-wave mixing and resonance-enhanced stimulated Raman scattering in molecular gases and free jets

2 H₂), methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) in a cell under equilibrium conditions and cooled in free jet expansions. For methane at room temperature the detection limit was 2×10¹² molecules per cm³ and quantum state, enabling the detection of trace species with a spatial resolution of 1 mm²×30 mm. In an attempt to study transitions in the ν₁+ν₃ and 2ν₂+ν₃ combination bands of CO₂ or N₂O, it was not possible to observe any DFWM signal. Instead a surprisingly strong, backward- and forward-directed emission was found which could not be attributed to the DFWM process. The signal arising from this emission was more than 2 orders of magnitude stronger than the DFWM signals obtained for other molecules. The frequencies of the emitted radiation were found to correlate with the transitions ν₁+ν₃→ν₁ and 2ν₂+ν₃→2ν₂, respectively. Our investigations lead to the conclusion that the emission can be explained by stimulated Raman scattering, resonantly enhanced by transitions to the combination levels ν₁+ν₃ and 2ν₂+ν₃. This process seems to suppress the generation of DFWM signals. Received: 1 October 1996/Revised version: 6 January 1997     

Phase conjugation in fluorescein film by degenerate four-wave mixing and holographic process

Phase conjugation is performed in a fluorescein film by a cw Ar ion laser of the wavelength 0.488 μm in a degenerate four-wave mixing (DFWM) configuration. It becomes apparent that the phase conjugate wave is generated simultaneously by not only DFWM but also a holographic process and that the DFWM and holographic components of the phase conjugate wave have different time constants.     

Photoinduced High-Density Phase of Exciton-Polariton Masses and their Coherent Propagation at Stacking Fault Interface in BiI 3

Photoinduced high-density phase of exciton-polariton masses excited at a two-dimensional stacking fault interface in BiI 3 has been studied by observing the degenerate four-wave-mixing (DFWM) signals. It was found that the increase in the dephasing rate due to exciton-exciton collisions is suppressed above a threshold excitation density. Above such density, a coherently propagating component of the exciton masses was detected on the space-resolved DFWM signals obtained by applying the spatially isolated two-spot excitation with a picosecond laser. In this scheme the off-diagonal element of the density matrix of the excitons at the one exciting laser spot propagates to the other, bringing about nonlinear polarization to yield the DFWM light. Time evolution of the space-resolved DFWM signals reveals evidently the macroscopic expansion of the exciton-polariton masses in a photoinduced condensed phase. Energy splitting of the DFWM spectra obtained with a narrow-band laser shows a repulsive interaction between the excitons.     

Degenerate four-wave mixing in nitrogen dioxide: Application to combustion diagnostics

Single shot degenerate four wave mixing (DFWM) images of the distribution of nitrogen dioxide (NO₂) doped into a propane/air flame at concentrations of the order of 7000 ppm have been obtained. These images indicate the relative concentration of NO₂ in different parts of the flame with an estimated spatial resolution of 150 m.Initial experiments were performed using NO₂ in a glass cell with nitrogen buffer gas. DFWM signals were generated using both the frequency doubled output of a pulsed ND:YAG laser and the tunable blue output of an excimer pumped dye laser. The signal was investigated as a function of laser power, NO₂ concentration and buffer gas pressure. In addition, spectra of NO₂ in the region 450 to 480 nm were obtained.Signals were then sought in both a cold air/NO₂ gas flow and an ignited mixture of propane and air seeded with NO₂, using a DFWM imaging geometry. The resulting images from the flame demonstrate the disappearance of the NO₂ molecules in the flame interaction zone.This work was done when previously employed by AEA Technology at Harwell     

Rotational and vibrational temperature determination by DFWM spectroscopy

Using the DFWM technique we determined rotational and vibrational temperatures of NaH molecules in a dynamical system consisting of NaH, Na, H and H₂ for several heat-pipe oven temperatures. We applied a new way to determine the dependence of the DFWM signalI _c ^int on the power of the transition dipole momentµ without previous knowledge of the temperature. The revealed dependence isI _c ^int µ ⁴. In this case the laser intensities were found to be of the same order as the saturation intensity in accordance with simple DFWM theory. A usual Boltzmann plot was employed for the determination of the relevant rotational temperatures, which displayed higher values than the oven temperature.     

The effects of collisional quenching on degenerate four-wave mixing

We report a theoretical and experimental investigation of the effects of collisional quenching on resonant degenerate four-wave mixing (DFWM). Using single-mode laser radiation, peak signal intensity measurements were performed on an isolated line in the A – X transition of NO. By using appropriate mixtures of N₂ and CO₂ as buffer gases, we varied the collisional quenching rate over several orders of magnitude while maintaining a fixed total collisional dephasing rate. The mixtures had approximately 100 Torr total pressure and were at room temperature. For I/I _sat approximately equal to 0.02, DFWM intensities were found to be less affected by variations in quench rate than were laser-induced fluorescence (LIF) intensities (I and I _sat are the pump laser and one-photon saturation intensities, respectively). Moreover, for I/I _sat roughly equal to 0.5, DFWM intensities were observed to be nearly independent of quench rate. The results are compared to two theoretical predictions, with good agreement observed. Both theories indicate that the minimum sensitivity of DFWM to quenching occurs near I/I _sat1.     

Analysis of degenerate four-wave mixing spectra of NO in a CH4/N2/O2 flame

4 /N₂/O₂ flame to spectral simulations based on a two-level theory for stationary, saturable absorbers by Abrams et al. Temperatures determined from least-squares fits of simulations to experimental spectra in the A²Σ⁺?X²Π⁺(0,0) band are compared to temperatures obtained from OH absorption spectroscopy and a radiation-corrected thermocouple. We find that DFWM rotational temperatures derived from Q-branch spectra agree with thermocouple and are independent of pump laser intensity for low to moderate saturation (I≈I_sat). However, the temperatures are systematically low and depend on pump intensity if the analysis neglects saturation effects. We demonstrate a method for obtaining an effective pump saturation intensity for use with the two-level model. This approach for analyzing saturated DFWM line intensities differs from previous work in that the use of the theory of Abrams et al. rather than a transition-dipole-moment power law allows treatment of a much wider range of saturation. Based on the observed signal-to-noise ratio an NO detection sensitivity of 25 ppm is projected, limited by a DFWM background interference specific to hydrocarbon flames. Received: 15 September 1998 / Revised version: 18 November 1998 / Published online: 24 February 1999     

Temperature measurement via oh spectrum in a flat C3H8/O2 flame by degenerate four-wave mixing

The degenerate four-wave mixing (DFWM) spectra method was employed to measure the temperature in a premixed propane-oxygen flame via OH radicals. The focus was to study the dependencies of the signal-to-noise ratio and linewidth of DFWM on the crossing angle. It was found that temperature measurements via DFWM could be made with crossing angles up to 13 °. Also, the DFWM signals were detectable for a crossing angle up to 25 ° with acceptable signal-to-noise ratio. The results suggest that it is possible to use DFWM two-dimensional imaging as a realistic diagnostic technique.     

Degenerate four-wave mixing based on excited-state absorption in azo-dye-doped polymer films

Degenerate Four-Wave Mixing (DFWM) based on excited-state absorption in azo-dye doped polymer films is firstly reported. Under a pre-exciting argon laser at 514.5 nm, DFWM is performed using a 632.8 nm HeNe laser which is absorbed only slightly when the molecules are in the ground state. The dynamic behavior of the DFWM signal due to excited-state absorption is also treated theoretically.     

砷化镓晶体中的四波混频

本文报道了以1.06μm调QNd:YAG激光脉冲,采用简并四波混频的方法,在未掺杂的GaAs单晶中产生红外位相共轭短脉冲。研究了该光脉冲的主要特性。用实验的方法探讨了影响共轭光强弱的主要因素。