Gerald R. Meredith

Third‐order optical susceptibility determination by third harmonic generation. I

A method for determining the third‐order nonlinear susceptibility of liquids using third harmonic generation (THG) is presented. A review of molecular polarizability and discussion of the relationship to susceptibilities are presented. All materials, including air and windows of the liquid cell, can make substantial contributions to the total THG intensity; this technique accordingly accounts for their effects. The method relies on use of a cell with wedged windows and a wedged liquid chamber which when placed in a vacuum chamber and translated across the optical path generates complex THG intensity fringes. By analysis of the latter, the liquid susceptibility is extracted. Several simple neat liquids are used to demonstrate the method.

Third‐order susceptibility determination by third harmonic generation. II

A simple method for determining the third‐order nonlinear optical susceptibility of solutions by optical third harmonic generation (THG) is presented. The method utilizes the weak absorption of light by most liquids at the 1.91 μm wavelength of the fundamental employed in this work. This property simplifies the complex interferences between THG arising in all materials (e.g., the cell windows and the solution) in the focal region. χ(3)1111 (−3ω,ω,ω,ω) values of several neat organic liquids are reported. The results are discussed in relation to bond and group additivity in the orientationally averaged second hyperpolarizability.

Second‐order cascading in third‐order nonlinear optical processes

Because cascaded second‐order processes make substantial qualitative and quanitative differences to the results of third‐order nonlinear optical experiments, a formalism for their treatment is presented. The symmetry dictates concerning the occurrence and relationships of magnitudes of cascading are tabulated for the higher symmetry crystal classes. Angular momentum considerations are applied to the situations allowing circularly polarized light waves.

Small computer‐based system for determination of second‐ and third‐order optical nonlinearities

An apparatus is described which has been designed for the purpose of measuring second‐ and third‐order nonlinear optical susceptibilities of electronic character. The optical power derives from a Q‐switched Nd3+/YAG laser which through a variety of frequency converters allows thorough characterization in the spectral window between the high vibrational states in the near infrared and the low electronic transitions in the visible or near UV. Data collection, treatment, and analysis are performed by a small computer/controller. As example of the potentiality of the apparatus the observation of multiple cascading in summation third‐order wave mixing by crystalline quartz is reported.

Characterization of quasi-crystal structure by optical frequency doubling

Abstract Solutions of spiropyrans in aliphatic solvents undergo a unique sequence of reactions leading to aggregated colloidal species, submicron size globules. following irradiation with ultraviolet light. The highly polar merocyanine molecular species obtained as photoproduct is incorporated into both crystalline nuclei and amorphous coatings of the globules. These globules possess large permanent dipole moments and consequently develop a highly ordered thread-like structure when formed in the presence of a static electric field. causing these media to exhibit second-order non-linear optical effects. Second- harmonic-generation studies are reported.

Solvent dependence of apparent third‐order hyperpolarizability in paranitroaniline

Optical third harmonic generation has been used to characterize solutions of paranitroaniline (pNA) in five different solvents. Results were analyzed to extract an apparent third‐order hyperpolarizability of pNA. Substantial variation of this quantity was observed, far exceeding the uncertainty of each solution determination. This unusual behavior was attributed to cascaded second‐order interactions through a second harmonic local field. The analysis is in substantial agreement with electric field induced second harmonic (EFISH) generation characterization of the second‐order hyperpolarizability of pNA when corrected via molecular orbital calculations for differences in dispersion and anisotropies. Solutions of 2‐methyl‐4‐nitroaniline were also studied and the cascading model suggests that the increase of second‐order nonlinearity over pNA is not generally as large as previous EFISH measurements suggest.

Local field cascading in third-order non-linear optical phenomena of liquids

Abstract Contributions to the third-order non-linear polarization response of molecular liquids due to cascaded second-order processes are discussed. The cascading occurs entirely because of the presence of a reaction field to the second-order polarization of non-centrosymmetric molecules. Application of a simple dielectric continuum model is used to investigate relative magnitudes, polarization anisotropy and resonance behavior.

Lower order effects in nonlinear two‐ and three‐photon resonance spectroscopies

A semiclassical description of the simultaneous processes of two‐photon absorption, two‐photon excitation, and second harmonic generation in noncentrosymmetric crystals is presented. The results are generalized to include three‐photon processes and third harmonic generation in centrosymmetric media. Consequences for molecular crystal excitation spectroscopy are emphasized. Similar results and observations are made for third‐order wave mixing in noncentrosymmetric crystals.

Disproof of extreme enhancement of electronic third order hyperpolarizability in TCNQ

The orientational average of electronic third‐order hyperpolarizability in 7,7,8,8‐tetracyanoquinodimethane (TCNQ), γTCNQ, was investigated by optical third harmonic generation. Characterization of solutions containing TCNQ established limits on γTCNQ which, when compared to nonlinear polarizability of other compounds, are much smaller than recent calculations [E. F. McIntyre and H. F. Hameka, J. Chem. Phys. 70, 2215 (1979)] suggest.

Electric field induced second harmonic generation: Atmospheric effects

Abstract Interference in the measurement of electric field induced second harmonic by glass is observed due to the nonlinearity of surrounding air. Vibration diagrams predict an amplitude reduction and phase shift in the Maker fringes from those which would appear in the absence of the air. These effect are confirmed by experiments in which helium is used to approximate a nonpolarisable medium.