A Simple Interferometric Method for Measuring the Fidelity of Phase Conjugated Beams

Abstract

One of the most interesting properties of the process of nonlinear optical phase conjugation is the capability of compensating for aberrations. The quality of compensation can be determined by investigation of the wavefront. A simple interferometric method for measuring fidelity of phase conjugated beams which allows a quantitative evaluation with respect to the phase distribution is described. This method was used to investigate the quality of phase conjugation by four-wave mixing in saturable absorbers. Illustrative examples are presented for the case of a phase conjugating and an ordinary mirror.     

Picosecond Phase Conjugation and Two-wave Coupling in Strontium Barium Niobate

Abstract

Picosecond phase conjugation and two-wave coupling in cerium-doped strontium barium niobate at 778 nm were studied. The phase conjugation reflectivity at 778 nm was measured to be R _pc = 58%. It is close to that pumped by a continuous-wave pump source. The temporal width t _pc and spectral width δγ_pc of the phase conjugate pulse were measured to be the same as that of the pump pulse. The intensity coupling gain of two-wave coupling as a function of time delay τ_D between two beams was also studied.     

Phase conjugation in four-wave mixing of cw radiation under conditions of strong self-interaction

An experimental investigation is made of the influence of strong self-interaction of the reference waves on phase conjugation in four-wave mixing of cw radiation in a medium with a thermal nonlinearity. It is shown that the thermal lens formed by the Gaussian reference beams reduces the nonlinear reflection coefficient appreciably (several-fold) and causes a deterioration in the phase conjugation quality. Pis’ma Zh. Tekh. Fiz. 23, 60–63 (August 12, 1997)     

High Reflectivity Broad Band Self-pumped Phase Conjugator at Near Infrared Wavelengths Using SBN : Ce

Abstract

A high reflectivity broad-band SBN : Ce phase conjugator with an external ring cavity in the near infrared wavelength region, 706 to 830 nm, is reported. Phase conjugation reflectivity was measured to be greater than 60% in the wavelength region, 706-820 nm, and as high as 67% at 716 nm. To our knowledge, this is the highest phase conjugation reflectivity by a SBN : Ce self-pumped phase conjugator. Both c.w. and square-wave pump sources were used. No significant changes of the phase conjugation reflectivity were observed when the pump source was changed from c.w. to a square wave at each different wavelength. The wave form of the phase conjugate wave follows the pump square wave. The build-up time of the phase conjugate wave under a square-wave pump was found to be twice that under a c.w. pump when the peak power of the square wave was the same as the power of the c.w. beam.     

High-intensity Effects in Self-pumped Photorefractive Phase Conjugation Using Nanosecond Pulses

Abstract

This paper reports on self-pumped photorefractive phase conjugation using repetitively pulsed high intensity (MW cm^?2) pulses of nanosecond duration. An intensity-dependent phase conjugate reflectivity is measured experimentally. In the corner self-pumped geometry the direction of beam fanning and corner self-pumping is observed to reverse above a critical intensity. Formation of a higher order spatial mode is reported for the external loop self-pumped geometry.     

Phase Conjugation by Degenerate Four-wave Mixing in Inverted Dye Solutions

Abstract

Phase conjugation has been observed in an amplifying medium of rhodamine 6G dye molecules in solution, excited either by flashlamps or by a laser. It is found that the mechanism involved in phase conjugation is degenerate four-wave mixing through gain saturation, and reasonable agreement obtained with the theory of saturable amplifiers. A phase-conjugate signal was observed over the tuning range of the dye system (580–605 nm) and a reflectivity up to a maximum of 25% was obtained.     

The study of bridge phase conjugation in photorefractive Tb:Cu:KNSBN crystal

Abstract

Photorefractive bridge phase conjugation is studied theoretically by two-region coupled-wave theory. The dependence of phase conjugate reflectivity and transmissivity on the input intensity ratio is measured. When two incoherent beams symmetrically enter opposite Tb:Cu:KNSBN crystal faces, the maximum reflectivities are higher than 351%, 236% and 147% for incident angles of 17.5°, 27.5 and 39° respectively; we also obtain a transmissivity higher than 58% for an incident angle of 33.5°. A comparison between the experimental data and theoretical results is also given.     

A new possibility for separate measurement of the imaginary and real parts of the third-order nonlinear optical susceptibility of initially isotropic media

A new possibility for separate measurement of the imaginary nd real parts of the cubic optical susceptibility (and for measurement of its phase) of isotropic resonant media is demonstrated theoretically. The theoretical conclusions show good agreement with recent experimental data. Pis’ma Zh. Tekh. Fiz. 23, 14–20 (October 12, 1997)     

A phase-conjugate Twyman-Green interferometer with increased sensitivity for laser beam collimation

Abstract

A Twyman-Green interferometer having a phase-conjugate mirror in one arm and double mirror in another arm is described to measure divergence and to check the collimation of a laser beam. It uses an internally self-pumped phase conjugation in BaTiO₃ crystal as phase-conjugate mirror. The use of double mirror gives a dual interference field which results in improved sensitivity in collimation testing. The dual-field interferogram is also useful in easily distinguishing between the divergent and convergent nature of the test beam. Measurement results are presented.     

A class of universal relations for constrained,isotropic elastic materials

Summary A class of universal relations for all kinematically constrained, isotropic, elastic materials is described by the equationSB=BS relating the symmetric extra stress and the Cauchy-Green deformation tensors. This rule generates easily at most three universal relations for all kinematically admissible deformations of any constrained, isotropic body for which these tensors are nondiagonal. New universal formulae for homogeneous, compressible and incompressible materials reinforced by inextensible fibers in a variety of arrangements are presented for several kinds of homogeneous and nonhomogeneous, controllable universal deformations.     

Rheology and flow-induced liquid crystal phase transitions in thermotropic polyethers

This work investigates the interdependence of the phase behaviour, viscosity, temperature, molecular weight and shear rates of thermotropic liquid crystalline polyethers. The viscosity of the isotropic and nematic phases are quantitatively compared; a positive variation in viscosity with respect to temperature is found, with the isotropic viscosity being about an order of magnitude higher than the liquid crystalline viscosity. The dependence of viscosity upon molecular weight of well defined fractions is investigated in both the liquid-crystal and isotropic phases. In the liquid crystalline state the viscosity scales with M ^3.5–5. Variations in the viscosity due to temperature changes affect the isotropic phase more than the liquid crystal phase. No evidence for a negative first normal stress difference is seen. Finally, it is shown how the phase diagram of the material can be altered by shearing the material in the isotropic phase. This is evident by the onset of a shear thinning region at temperatures slightly above T _i, which can be attributed to the formation of shear induced liquid crystallinity.     

Sorbitan Ester Organogels for Transdermal Delivery of Sumatriptan

ABSTRACT

The partial phase behavior, rheological, and drug release characteristics of an organogel (OG) composed of water, isooctane and sorbitan esters, sorbitan monopalmitate (Span-40) and poly(oxyethylene)sorbitan monostearate (Polysorbate-60) were studied. Phase diagrams showed decreasing areas of optically isotropic organogel region depending on the surfactant ratio, K_w and drug incorporation. The nonbirefringent, clear isotropic solution suggested the reverse micellar/microemulsion nature of the organogel without any molecular ordering. The increase in drug concentration in OGs leads to increase in the viscosity and sol-gel transition temperature (T_g). Fractal dimension (d_f) values calculated for different compositions suggested that the density of the tubular network increases with increasing drug concentration in OGs. The release rate of the drug from OGs was found to be non-Fickian through the dialysis membrane. The permeation rate of sumatriptan from pig skin was 0.231 mg/h/cm² (781.9 nmol/h/cm²). The study indicates potential of OG as a reservoir system for transdermal drug delivery.     

Prism-coupled excitation of multiple Tamm waves

Abstract

A Tamm wave is guided by the interface of a homogeneous dielectric material and a periodically nonhomogeneous dielectric material, both isotropic. We found theoretically that multiple Tamm waves of different linear polarization states can be guided by such an interface incorporated in a prism-coupled configuration. The Tamm waves have different linear polarization states, phase speeds, and spatial profiles, but the same frequency. The narrow absorptance peaks associated with the excitation of Tamm waves in the prism-coupled configuration offer the application as chemical sensors with very high sensitivity.     

Elastic Property Measurement Using Rayleigh-Lamb Waves

Abstract

A nondestructive technique is described for the measurement of elastic constants of isotropic plates using ultrasonic Rayleigh-Lamb waves. The experimental method employs continuous harmonic waves and a pair of variable-angle contact transducers in pitch-catch mode. The phase velocity of the R-L waves at a particular frequency is determined from the phase shift over a measured path length. This simple experimental technique can measure phase velocity over the range 1–10 mm/μs with an error of less than 0.5% over a frequency range of 50 kHz-2 MHz. Individual symmetric and antisymmetric modes can be generated through the selection of transducer angle and frequency. Young's modulus and Poisson's ratio for the material are calculated from measurements of frequency and phase velocity by a nonlinear least squares solution to the dispersion equations. The sensitivity of the nonlinear least squares function to the measurement region of the dispersion curve is investigated. It was found that estimations of material properties are more accurate and less sensitive to small experimental errors when only selected frequencies and R-L modes are used in the least squares calculation. This technique is demonstrated with several isotropic materials and with both thick (6 mm) and thin (0.8 mm) plates. Values for elastic constants determined by the contact transducer Lamb wave technique compare favorably with values measured using the pulse-echo-overlap method. The uncertainty in measurements of Young's modulus and Poisson's ratio was less than 1% and 2%, respectively. The technique has advantages over more traditional methods for measuring elastic properties when it is desirable to use wavelengths greater than the plate thickness, when properties may vary with frequency, or when it is necessary to measure in-plane elastic properties of thin plate structures.     

Phase Transitions in Liquid Crystal Doped with Magnetic Particles of Different Shapes in Combined Electric and Magnetic Fields

We have studied the influence of electric and magnetic fields on the orientational structure of ferronematics based on a thermotropic nematic 4-trans-4 \(^{\prime }\) -n-hexylcyclohexyl-isothiocyanato-benzene (6CHBT). The 6CHBT liquid crystal has been dissolved in phenyl isothiocyanate and doped with rod-like or chain-like magnetic particles. In such a mixture, the phase transition from an isotropic to a nematic phase is via a droplet state, i.e., coexistence of nematic and isotropic phases. The obtained results showed that a combination of the electric and magnetic fields can change the character of a phase transition from the isotropic to the nematic phase via the droplet state in such systems. Moreover, magneto-dielectric measurements of structural transitions showed the magnetic field induced a shift of the phase transition temperature from the isotropic to the droplet state.     

Enhancement of the switched phase conjugate reflectivity in a BaTiO3 crystal

We experimentally investigated the response of the photorefractive phase conjugation to a pulsed reading beam in an undoped BaTiO₃ crystal with a four-wave mixing geometry. A single longitudinal-mode Ar⁺ laser was used as a light source. The reading beam with a pulse width of ∼1 s was switched on after writing a steady-state grating in the crystal by two recording beams under a suitable condition and the generated phase conjugation was measured. The phase conjugate output was found to be increased by some factors of magnitude over the initial value for a given peak power and period of the reading pulses. A maximum of the enhanced reflectivity can be controlled by adjusting the interacting beam powers. An increase in the diffraction efficiency was also found with the same read-out process, which plays a key role to increase the phase conjugation as our knowledge.     

On calculating the reflectance and transmittance of light for a simple thick grating structure

Abstract

This paper presents a formulation for calculating the reflectance and transmittance of classical light for a simple structure that contains a rectangularly shaped line grating layer that lies atop a thick transparent or weakly absorbing substrate layer. It is assumed that the substrate thickness is sufficiently large and non-uniform that when the light traverses it is averaged over a large surface area, the averaged field is considered as losing phase coherence and intensities can be added. It is assumed that the optical properties of the media in the various homogeneous regions of the structure are complex, local, linear, isotropic, and non-magnetic. This kind of structure has important applications in the metrology of linewidths for the semiconductor integrated circuit industry.     

Lateral displacement of a beam incident from a uniaxial medium onto a metal

Abstract

The reflection of spatially limited beams at the flat boundary between a uniaxial crystal and a metal is studied theoretically. Incident limited beams are expanded in terms of the ordinary and extraordinary plane waves that can be propagated in uniaxial media. The spatial modifications in ordinary and extraordinary reflected field distributions are numerically calculated. Several effects, including polarization conversion between ordinary and extraordinary waves and field enhancement on the interface due to the excitation of surface waves, are discussed. The numerical calculations of the Goos–Hänchen lateral shift in the reflected fields are compared with the results predicted by the extension to uniaxial media of the stationary phase approximation used for limited beams in isotropic media.     

Third-order Susceptibility Measurements in a Nematic Liquid Crystal—K15

Abstract

In this paper, we report for the first time the determination of the real part of the third-order nonlinear susceptibility, x ⁽³⁾(?ω, ω, 0, 0), measurements in the isotropic phase of a nematic liquid crystal 4′-n-pentyl-4-cyanobiphenyl (K15) from electro-optic Kerr effect experiments. The value of x ⁽³⁾ obtained at 632·8 nm is found to be 1·08 × 10^?18 m² V^?2 corresponding to a temperature 35·5°C and is the highest so far reported to the best of our knowledge.     

Explicit expressions of eigenvalues and eigenvectors for transversely isotropic piezoelectric materials

Summary.  Explicit expressions of eigenvalues and associated eigenvectors in Stroh's theory [1] for transversely isotropic piezoelectric materials under generalized plane deformation are presented. It is shown that the practical calculation for some piezoelectric ceramics yields all eigenvalues which are distinct from each other for generalized x-z plane deformation, where z is the poling axis of a transversely isotropic piezoelectric material. Some numerical results based on the explicit expressions are given, and the orthogonality and closed relation between matrices A and B in Stroh's theory [2], [3] for piezoelectric material are verified numerically. Received May 8, 2002; revised December 14, 2002 Published online: May 20, 2003 The paper is supported by the doctorate foundation of Xi'an Jiaotong University. The science foundation of the Shaanxi Province of China is appreciated, too.