Third-harmonic Conversion Efficiency Research Articles

Harmonic conversion of large-aperture 105-μm laser beams for inertial-confinement fusion research

To provide high-energy, high-power beams at short wavelengths for inertial-confinement fusion experiments, we routinely convert the 1.05-microm output of the Nova, Nd:phosphate-glass, laser system to its second- or third-harmonic wavelength. We describe the design and performance of the 3 x 3 arrays of potassium dihydrogen phosphate crystal plates used for type-II-type-II phase-matched harmonic conversion of the Nova 0.74-m diameter beams. We also describe an alternate type-I-type-II phasematching configuration that improves third-harmonic conversion efficiency. These arrays provide conversion of a Nova beam of up to 75% to the second harmonic and of up to 70% to the third harmonic.

Tunable, Coherent Vacuum Ultraviolet Radiation for Photoionization Mass Spectrometry

Coherent vacuum ultraviolet radiation between 118 and 129 nm (10.5 and 9.6 eV) is generated by third-harmonic conversion of radiation between 355 and 390 nm. The conversion efficiency of a single negatively dispersive rare gas (xenon or krypton) is compared to the efficiency of a mixture of a negatively dispersive gas with a positively dispersive gas (argon). The rare-gas mixtures are found to give significantly higher third-harmonic conversion efficiencies. They also have much narrower wavelength tuning ranges than the single gases. Optimum gas pressures, mixing ratios, and conversion efficiencies are tabulated at selected wavelengths. The photoionization characteristics of compounds that exhibit little or no parent ion abundance with conventional 70-eV electron impact ionization are evaluated with tunable vacuum ultraviolet radiation. n-Alkanes, alkenes, aldehydes, amines, carboxylic acids, ethers, and ketones are ionized without significant fragmentation by using wavelengths close to the ionization thresholds. Alcohols and esters, however, fragment extensively, even at the ionization threshold.

Experimental and theoretical investigation of third-harmonic generation in phase-matched dye solutions

The phase-matched third-harmonic light generation in dye solutions is studied experimentally and theoretically. In the experiments picosecond light pulses of a passive mode-locked Nd-glass laser are converted to the third-harmonic frequency. A third-harmonic conversion efficiency of up to 4×10−4 was achieved for one of the dyes investigated (1,3,3,1′,3′,3′-hexamethylindocarbocyanine iodide in hexafluoroisopropanol). The theoretical calculations determine the influence of various dye and solvent parameters on the conversion efficiency. The conversion efficiency is found to be limited by excited-state absorption of pump laser light and third-harmonic light from the S1-state to higher singlet states. The S1-state is mainly populated by two-photon absorption. Amplified spontaneous emission may reduce the limiting effects of excited-state absorption. Phase changes caused by the non-linear refractive index and the refractive index dispersion within the spectral bandwidth of the laser pulses reduce the conversion efficiency. Under ideal conditions conversion efficiencies up to 10% may be achieved.

Picosecond third-harmonic light generation in?-BaB2O4

The type-II phase-matched third-harmonic light generation in a β-BaB2O4 crystal is studied experimentally. A passively mode-locked Nd: phosphate glass laser is used as a pump source. At a pump pulse peak intensity ofI10=5×1010W/cm2 a third-harmonic conversion efficiency of a percent is obtained. A theoretical discussion of phase-matched third-harmonic generation in crystals of the symmetry group of β-BaB2O4 (trigonal class 3) is given. The effective nonlinear susceptibility ϰeff for type-II phase-matching is determined.

Two-photon coherent propagation and third-harmonic generation

Calculations of third-harmonic generation in conditions of coherent two-photon propagation in lithium vapor are presented that predict shapes, frequencies, and intensities for the fundamental pulse yielding an optimal third-harmonic energy conversion efficiency of 8%.

Third-order nonlinearities in molecular hydrogen: Two-photon resonance enhanced third-harmonic generation and Raman scattering

In this paper, we investigate the effects of quantum mechanical interferences on third-order susceptibilities in molecules. First principle calculations for molecular hydrogen are presented and shown to agree with results derived from experimental stimulated Raman gain and spontaneous Raman cross-section data. We predict that 10 percent third-harmonic conversion efficiency in H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> at 1 atm without phase matching requires a laser intensity of 150 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.81 μm. As little as 5.9-MW power is sufficient when the beam is properly focused. We suggest that resonance Raman scattering (RRS) is a possible scheme for experimentally investigating the interference effects, which tend to reduce the strength of third-order nonlinear susceptibilities.

Infrared third-harmonic generation in phase matched CO gas

Resonance-enhanced third-harmonic generation (THG) of CO 2 laser radiation has been studied in CO and NO. It is shown that the coherence length for THG in CO can be increased by adding SF 6 and that phase matching occurs for a CO:SF 6 ratio of 208:1. The pressure dependence of the third-harmonic conversion efficiency in CO-SF 6 mixtures has been measured up to 11 atm, and the results are in good agreement with calculated values for the coherence length and the third-order susceptibility tensor.