Two-photon Degenerate Four-wave Mixing Research Articles

Atomic oxygen detection in flames using two-photondegenerate four-wave mixing

We report a theoretical and experimental investigation of atomicoxygen detection in an atmospheric-pressure flame by two-photondegenerate four-wave mixing (TPDFWM). We present analyticalexpressions for the TPDFWM signal that are based on the diagrammaticperturbation theory. The description of several polarizationarrangements is given and used in our experiments. Theimportance of the polarization scheme is emphasized and the mostefficient scheme is found by investigating signal and noisecontributions. In this case, thermal grating contributions aregreatly reduced and this allows quantitative concentrationmeasurements in flame to be performed using TPDFWM.

Degenerate four-wave mixing with a tunable excimer laser

We use a simple, forward-geometry degenerate four-wave mixing setup to monitor weak absorptions of various gas-phase molecules with a tunable excimer laser. With this technique, state-selective detection of H(2), CO, H(2)O, and O(2) is demonstrated. The dependence of the forward-geometry degenerate four-wave mixing signals on various experimental parameters (e.g., pressure, laser power) is determined. Several mechanisms for the generation of the observed signals are discussed, including diffraction from density gratings or thermal gratings and two-photon degenerate four-wave mixing.

Observation of the A 2Σ + ← X 2Π [formula omitted] transition of nitric oxide with two-photon degenerate four-wave mixing

A rotationally resolved excitation spectrum of the A 2Σ +−X 2Π 3 2 transition of nitric oxide was observed by use of the two-photon degenerate four-wave mixing (TP-DFWM) process. It was found that the peak positions in the TP-DFWM spectrum are blue-shifted by 0.5 cm −1 compared to corresponding rotational lines in the two-photon laser-induced fluorescence excitation spectrum. From the dependence of the TP-DFWM signal intensity upon the polarization of the input laser fields, the mechanism for the generation of the phase-conjugate beam is discussed.

Two-photon degenerate four-wave mixing (DFWM) for the detection of ammonia: Applications to flames

Two-photon Degenerate four-wave mixing (DFWM) has been used for the detection of ammonia in a cell and in an atmospheric pressure flame. The NH3 molecules were excited from the ground state X to either the C' or the B state and the DFWM signal was recorded simultaneously with the laser-induced fluorescence signal, originating from the C′-A and B-A transitions, respectively. During the flame experiments sequential measurements of NH3 and OH in an NH3/O2 flame were also performed.

Detection of CO molecules using two-photon degenerate four-wave mixing (DFWM)

Abstract Degenerate four-wave mixing (DFWM) has been used to detect CO, excited through a two-photon process. Intensity and pressure dependencies are presented and the potential of using the technique for flame studies is discussed.

Effects of permanent dipole moments in degenerate four-wave-mixing processes.

The main goal of this paper is to analyze a two-photon degenerate four-wave-mixing process that occurs in an isotropic media modeled by homogeneously broadened two-level systems having unequal permanent dipole moments and subject to rotational diffusion. Our theoretical treatment is based upon a third-order perturbative solution of the density-matrix equations and does not require the rotating-wave approximation. The medium being excited by three stationary fields, we give an analytical expression of the macroscopic polarization up to the third order, including permanent dipole moments, orientational diffusion, and field polarizations. We apply these general results to the description of a degenerate four-wave-mixing process by selecting, in the expressions, the contributions in the direction ${\mathbf{k}}_{\mathrm{\ensuremath{\alpha}}}$-${\mathbf{k}}_{\mathrm{\ensuremath{\beta}}}$+${\mathbf{k}}_{\ensuremath{\gamma}}$, ${\mathbf{k}}_{\mathrm{\ensuremath{\alpha}}}$, ${\mathbf{k}}_{\mathrm{\ensuremath{\beta}}}$, and ${\mathbf{k}}_{\ensuremath{\gamma}}$ being, respectively, the wave vectors of the exciting fields. We show that the two-photon degenerate four-wave mixing is dependent on the permanent dipole moments and is not present if these dipole moments are equal or zero. We analyze the influence of rotational diffusion. Different cases of field polarizations are also considered, and we discuss a comparison of the well-known one-photon degenerate four-wave-mixing process and this two-photon process. This process is also discussed in the particular case of phase conjugation.

Two-Photon Coherent-Transient Measurement of the Nonradiative Collisionless Dephasing Rate in SF6via Doppler-Free Degenerate Four-Wave Mixing

This Letter reports, for the first time, an experimental measurement of the collision-less dephasing time of the ${\ensuremath{\nu}}_{3}=2$ level in S${\mathrm{F}}_{6}$ using Doppler-free two-photon degenerate four-wave mixing. The measurement is performed in the time domain by use of a short-pulse C${\mathrm{O}}_{2}$ laser to establish a molecular two-photon coherent state. The coherent state is then probed as a function of time to yield a dephasing time of 1.8 nsec which is related to intramolecular dephasing rates.