Third-harmonic Generation Experiments Research Articles

Spatially resolved atomic hydrogen concentrations and molecular hydrogen temperature profiles in the chemical-vapor deposition of diamond

We report here a direct measurement of the spatially resolved atomic hydrogen concentration profiles during hot-filament-assisted chemical-vapor deposition (HFCVD) of diamond films. The ground-state hydrogen (1s 2S1/2) atoms generated in this process are monitored by an optical four-wave-mixing technique, third-harmonic generation (THG). For THG, a 364.6 nm dye laser beam is focused into the HFCVD reactor and the third-harmonic radiation near resonant with the Lyman-α (2p 2P0j↔1s 2S1/2) transition in atomic hydrogen at 121.6 nm is observed. The resultant THG intensity and THG peak shift with respect to the Lyman-α transition are both dependent on hydrogen atom concentration. Titration experiments based on the reaction NOCl+H→HCl+NO were conducted to obtain absolute hydrogen atom concentrations from the relative concentrations determined in the THG experiment. Spatially resolved molecular hydrogen temperature and concentration profiles obtained by coherent anti-Stokes Raman scattering in a similar HFCVD reactor are reported. The observed H atom concentrations exceed computed equilibrium concentrations based on the measured gas temperatures. Transport of the atomic hydrogen from the hot filament surfaces is discussed and diffusion is shown to be the principal mechanism controlling the H atom distribution.

Special Issue on Nonlinear-Optic Waveguide Materials and Devices. Polymer Materials for Third-Order Nonlinear-Optical Devices.

Material design concept of polymeric materials applicable for 3rd-order nonlinear optical devices is reviewed. Firstly, the importance of distinguishing molecular origins for high 3rd-order optical nonlinearity is stressed. Molecular structures and preparation methods of majorπ-conjugated polymer films with potentially high optical nonlinearity are introduced. Then, relations between molecular structures and 3rd-order nonlinear susceptibilities are discussed. Stress is put on coherent 3rd-order nonlinearity, which can be examined with nonlinear sus-ceptibility spectra obtainable from third-harmonic generation experiments.

Third-harmonic generation of polyacetylene.

We present an exact calculation of third-harmonic generation (THG) for a one-dimensional electron-lattice system. The result shows that once the conjugated length of a polyene is finite, the transitions between inside-band electronic states will be important. A random distribution of conjugated lengths is used to simulate the polymer chains of polyacetylene. The average spectrum of THG distinctly demonstrates two peaks: one is the three-photon resonance exactly at one-third of the energy gap; the other sits near half of the energy gap, which is induced by the transitions between the inside-band states. This spectrum agrees quite well with that of THG experiments for trans-polyacetylene.

Atmospheric influences in optical third-harmonic generation experiments

Third-harmonic generation in a thin solid plate surrounded by a gas is investigated. We conclude that atmospheric contributions can be suppressed in third-harmonic generation from thin solid plates. A phase-sensitive measurement of the third-order susceptibility of gases is proposed. We calibrated the third-order susceptibility of air to be X(3)a=(3.4±1.5)×10−18 esu with a phase uncertainty of ±10°.

AB INITIO study of third-order nonlinear optical properties of the H2 and D2 molecules.

Third-order dynamic hyperpolarizabilities (\ensuremath{\gamma}) for ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ are calculated using James-Coolidge-type wave functions for several internuclear distances and frequencies. From them, third-order susceptibilities (\ensuremath{\chi}) relevant to dc Kerr, degenerate four-wave mixing, electric-field-induced second-harmonic generation and third-harmonic generation experiments are determined. Agreement with the electric-field-induced second-harmonic generation experimental data is very good: ${\ensuremath{\chi}}_{?}^{(3)}$=792, 821, and 959 a.u. for \ensuremath{\lambda}=6943, 6328, and 4880 A\r{}, compared with the experimental values: 805, 828, and 973 a.u. Other aspects such as dispersion formulas, Kleinman symmetry, and vibrational contributions are also investigated and discussed.

Frequency dependent nonlinear optical properties of molecules

Various nonlinear optical polarizabilities are derived and evaluated by time dependent Hartree–Fock theory (TDHF). The recursive nature of the TDHF theory is exploited to develop formulas that are applicable in any order. The theory is applied to evaluate dispersion effects for the series of molecules CH4, CH3F, CH2F2, CHF3, and CF4. Comparisons are made with results obtained from dc-induced, second-Harmonic generation, and third-Harmonic generation experiments. Additional applications are reported for H2 and HF.

Optical third-harmonic generation in the fluorinated methanes and sulfur hexafluoride

Optical third-harmonic generation experiments are used to determine nonlinear polarizabilities for the fluorinated methanes and sulfur hexafluoride. An ambiguity present in previous measurements of third-harmonic generation in gases is resolved.

Orientational and electronic contributions to the third-order susceptibilities of cryogenic liquids

Alternating current Kerr effect and third-harmonic generation (THG) are reported for the cryogenic liquids CO, O2, N2, and Ar pumped by CO2 TEA laser radiation. The THG experiments measure the hyperpolarizability, while the Kerr effect measurements probe the total nonresonant third-order susceptibility χ(3), arising from both molecular orientation and electronic hyperpolarizability. The THG process in liquid CO is dominated by a vibrational two-photon resonant contribution for the CO2 R(6) pump line at 9.35 μm. By studying the interference between the resonant and nonresonant contributions in liquid CO–O2 mixtures the hyperpolarizability of liquid O2 has been determined relative to the two-photon resonant χ(3) of liquid CO. These measurements give the first experimental confirmation for liquid media that the two-photon resonance contribution to polarized scattering consists of both narrow and broad linewidth components associated with, respectively, the isotropic and anisotropic Raman polarizabilities.