Coherent vacuum ultraviolet generation by frequency mixing in glass hollow waveguides

Efficiency for vuv generation in a glass hollow waveguide has been compared with that obtained by tight focussing of the primary radiation. Coherent vuv radiation is generated by resonant four-wave difference frequency mixing and four-wave parametric processes in xenon. At vuv wavelengths close to the 5p-6s resonance, the efficiency of generation was found to be enhanced by a factor of up to 2.5 under phase-matched conditions. The measured enhancement is limited by losses due to absorption of the generated radiation and by propagation losses of the primary beams along the waveguide.     

Continuous-wave,single frequency,second harmonic generation in sodium vapour

Single frequency, second harmonic radiation has been generated continuously in sodium vapour using resonant enhancement provided by the two photon transition 3S-4D at 578.7 nm, and a transverse magnetic field to remove the isotropy of the vapour. Phase matching and linewidth effects have been studied. The generation profile of the vapour is shown to be Doppler limited.     

830 nm emission in sodium vapor

We demonstrate that the intriguing 830 nm coherent emission, which is observed when sodium vapor is pumped with a high-power pulsed laser tuned near the 3S4D two-photon transition, is due to an axially phase-matched six-wave mixing process. This conclusion is based upon the observation of emission near 584 nm, which is coupled to the 830 nm emission in the six-wave mixing process: ₁+₂=2_L–_4D4P –_4P3D . In addition, we have observed coherent emission near 1.16 m, which is due to an analogous process involving cascade through the 4S (as opposed to the 3D) state. We calculate the wavelengths of all photons involved in these processes using the standard formulas of parametric wave-mixing theory, and show that they can be predicted to within experimental uncertainties. Finally we report observations of significant blue shifts of the 830 nm and 1.16 m emissions in a mixed sodium-potassium vapor. These shifts can be readily understood by considering the effect of the potassium on the frequency-dependent refractive index of the vapor. Due to these results, other recent interpretations of the 830 nm emission as stimulated excimer emission on the Na₂ 1³ _g ⁺ 1³ _u ⁺ band must now be rejected.     

Axially phase-matched parametric four- and six-wave mixing in potassium vapor

Systematic experimental observations of axially phase-matched parametric four- and sixwave mixing processes have been made in potassium vapor pumped on or near the 4S–6S two-photon resonance. An IR up-conversion with a beta barium borate crystal was used for the detection of IR emissions. All the predicted processes have been observed. It is found that the excitation spectra of the emissions for those processes observed in this work are quite different from the excitation spectra of similar processes in sodium vapor. A dye-dependence of some emissions generated by the axially phase-matched parametric six-wave mixing processes has been found by using different dyes in the dye laser. The reason for the dye-dependence is discussed.     

Broadly tunable near-infrared six-wave mixing processes in potassium vapor

We report the observation of several six-wave mixing processes which result in broadly tunable coherent emission in the wavelength range 1.20–1.45 m. These emissions are produced in potassium vapor which is simultaneously pumped by two pulsed dye lasers. One set of processes is produced when the frequency of the first laser is fixed to the potassium 4S»6S two-photon transition frequency, while the frequency of the second laser is tuned. The second set of processes is observed when both lasers are tuned, but with their sum frequency fixed to the 4S»6S two-photon transition. Peak output energies of 10 nJ/pulse have been observed.     

Ninth-order nonlinear polarization and vuv generation in Hg vapor

Interference of resonant ninth-order and nonresonant seventh-order nonlinear polarizations in Hg atoms has been studied, and direct conversion of fundamental ir radiation into the vuv has been obtained. The interference has been observed in the form of a Fano-Boitler contour. The efficiency of generation by ninth-order processes appears to be higher than that by seventh-order.     

Two-photon resonant four-wave mixing in atomic sodium vapour

Using the 4d- and 5s-states of sodium, two different types of two-photon resonance enhanced four-wave mixing have been observed. In the first type 2v _L =v _IR +v _pIR radiation near 2.34 μm and UV radiation near 320 nm has been detected. In the second typev _P −v _S =v _L +v _IR the UV radiation produced in the first parametric process acted as a pump for stimulated electronic Raman scattering via the 4p-state of sodium. Resonantly enhanced by the two-photon transition 3s–4s, radiation near 1.18 μm was produced in this scheme. The realisation of a tunable coherent IR source based on these four-wave mixing processes is discussed.     

Intracavity frequency mixing in pulsed optical parametric oscillators for the efficient generation of continuously tunable ultraviolet radiation

Received: 4 May 1998/Revised version: 8 July 1998     

Frequency mixing in a gas-filled waveguide for VUV light generation

Advantages of nonlinear frequency-mixing in gas-filled waveguides to generate vuv radiation are analyzed. Under phase-matching conditions the waveguides are shown to provide a much higher conversion efficiency (up to 10²÷10⁴ times) as compared to the tight focusing into the bulk gas medium. Estimations of the improvements for cw and pulse vuv generation are presented.     

Optical frequency mixing in planar waveguides: Influence of crystal orientation

Optical frequency mixing through nonlinear mode coupling in a planar waveguide, fabricated with a cubic or uniaxial dielectric, is discussed. A threewave interaction is examined and the optimum crystal orientations that lead to the highest efficiency are determined in those particular cases in which they do not depend on the field distribution of the modes. The analysis is carried out in detail for cubic crystals; sensitivity to misalignments is also studied and numerical results are reported.     

Generation of tunable cw infrared radiation by difference frequency mixing

Tunable cw infrared radiation has been generated by noncollinear difference frequency mixing in LiIO₃, using the output of an argon ion laser at 514 nm or 488 nm and the intracavity radiation of a Rhodamine 6 G dye laser. Infrared powers up to 4 μW in multimode and 0.5 μW in single-mode operation tunable in the range of 2.3 μm to 4.6 μm have been obtained. Properties and possibilities of the noncollinear mixing geometry and the intracavity mixing system are discussed.     

Observation of 280 nm and 298 nm ultraviolet emissions in sodium vapor

Dye laser output around 578 nm is utilized to pump sodium vapor confined in a heatpipe. Fixed wavelength ultraviolet emissions at 280 nm and 298 nm are observed for the first time and can be explained by anti-Stokes Raman scattering and two photon emission respectively. In addition, there are tunable emission lines around 280 nm and 298 nm, the frequencies of which vary with pump laser wavelength. These tunable lines can be explained by four-wave mixing schemes.     

Molecular enhanced four-wave parametric generation in sodium atom

A number of resonances have been observed in the excitation spectrum of the uv resonance lines of sodium. The mechanism proposed includes absorption by molecular states, energy transfer to excited atomic levels and four-wave parametric generation enhanced by radiative decay of the excited states.     

Picosecond phase-conjugation by degenerate four-wave mixing in sodium vapour

Phase-conjugated waves (PCW), via degenerate four-wave mixing (DFWM) with picosecond laser pulses having a FWHM of 80 ps have been obtained in sodium vapour on both sides of theD ₂(3² S _1/23² P _3/2) transition at 589 nm. The maximum reflectivity of the PCW signals was 60%. The reflectivity of the PCW has been measured as a function of the temperature, the detuning and the pump and probe intensities. Because of the very strong nonlinear behaviour of theD ₂ transition the appearance of the PCW is accompanied by strong self-focusing and defocusing of the beams, which give a rather complicated line-shape of the detuning curve of the PCW. By varying the delay between the beams, the lifetime of the transient volume grating has been, measured to be 7.4 ns. The results are in qualitative agreement with the theoretical model of Abrams and Lind.     

Atom-density distribution in a metal vapor cell studied by third-harmonic generation

The density distribution of Cs atoms in a metal-vapor cell designed for nonlinear optical and spectroscopic measurements has been investigated by the THG technique. For high heater powers the desnity-distribution function was found to display a dip in the central part of the cell. Explanation of the phenomenon and criteria to eliminate it are proposed.     

Tunable near-infrared radiation by difference frequency mixing in a lithium tetraborate crystal

Non-collinear difference frequency mixing is reported for the first time in a lithium tetraborate (Li₂B₄O₇) crystal, and is used to generate near-infrared radiation in the range 1700–2300 nm. Received: 13 July 2000 / Published online: 10 January 2001     

Fifth harmonic generation in sodium vapor

We report experimental investigation of direct fifth harmonic of Nd³⁺: glass laser radiation (1060 nm) in metal vapor. Using various mixtures of sodium vapor and xenon in order to achieve a phase-matching, we observed direct as well as cascade nonlinear processes. The first measurement of effective fifth-order nonlinear susceptibility χ⁽⁵⁾_eff in atoms, is performed.     

Seventh harmonic generation in sodium vapor

We present experimental evidence of both direct and cascade seventh harmonic generation by collimated laser beam in atomic systems. The phase-matching condition is fulfilled. The variation of seventh harmonic signals with sodium vapor pressure is described.     

Difference frequency generation in proustite from 11 to 23 μm

Tunable infrared radiation was produced in the spectral range 11 μm to 23 μm by nonlinear mixing of two ruby-pumped dye lasers in proustite. A peak power of about 3 watts was observed at 19 μm. A 7^o crystal angle was sufficient to achieve phase matching over the entire wavelength range. Ordinary wave absorption measurements in proustite correlate well with the observed difference frequency output, once allowance has been made for the convergence of the dye laser beams. This research was supported by the Energy Research Development Agency under contract E29-2-3597 and by the Office of Naval Research contract No. N00014-76-C-0431.     

Continuous-wave mid-infrared laser sources based on difference frequency generation

We report on recent developments of widely tunable, continuous-wave (CW) laser sources based on difference-frequency generation (DFG) in nonlinear optical materials. The state-of-the-art of CW DFG technology will be reviewed. Applications of the DFG-based laser source to high-resolution molecular spectroscopy and trace gas detection will be presented. New development trends of DFG will be discussed. To cite this article: W. Chen et al., C. R. Physique 8 (2007).