A. Knoesen

Hybrid guided modes in uniaxial dielectric planar waveguides

Rigorous electromagnetic methods are applied to calculate the propagation constants and the fields of the allowed guided modes in uniaxial slab waveguides for an arbitrary orientation of the optic axis. In addition to the familiar TE and TM modes, there are hybrid guided modes. These hybrid guided modes can be divided into three distinct types: homogeneous pure guided modes, inhomogeneous pure guided modes, and leaky guided modes. The conditions for the existence of these pure guided and leaky guided modes are derived. A quantitative method of classifying the hybrid modes in terms of the limiting decoupled TE and TM cases within that mode where the ordinary and extraordinary polarizations propagate independently is introduced. For each mode the propagation constant has a continuous band of allowed values as a function of optic axis orientation. Metal-indiffused positive-birefringent (lithium tantalate) and metal-in-diffused and proton-exchange negative-birefringent (lithium niobate) planar waveguides are treated for illustration. This analysis of hybrid guided modes is important, not only for the design and use of anisotropic waveguides, but also suggests a new class of waveguide devices based on material birefringence. >

Trapping forces in a multiple-beam fiber-optic trap

Transverse and axial trapping forces are calculated in the ray optics regime for a multiple-beam fiber-optic light-force trap for dielectric microspheres located both on and off axis relative to the beam axis. Trap efficiencies are evaluated as functions of the effective index of refraction of the microspheres, normalized sphere radius, and normalized beam waist separation distance. Effects of the linear polarization of the electric field and of beam focusing through microlenses are considered. In the case of a counterpropagating two-beam fiber-optic trap, using microlenses at the distal ends of the fiber to focus the beams may somewhat increase the trapping volume and the axial stability if the fiber spacing is sufficiently large but will greatly reduce the stiffness of the transverse force. Trapping forces produced in a counterpropagating two-beam fiber-optic trap are compared with those generated in multiple-beam fiber-optic gradient-force traps. Multiple-beam fiber-optic traps use strong gradient forces to trap a particle; therefore they stabilize the particles much more firmly than do counterpropagating two-beam traps.

Ultrashort-pulse second-harmonic generation. II. Non-transform-limited fundamental pulses

A general theory of second-harmonic generation, including all the effects of group-velocity dispersion, is given for coherent ultrashort pulses with arbitrary shapes and carrier chirps. Ultrashort-pulse second-harmonic generation is analyzed for transform-limited fundamental pulses. The effects of intrapulse group-velocity dispersion (IGVD) on the second-harmonic (SH) pulse shape are investigated for parameters representative of popular phase-matchable crystals and wavelength, including Ti:sapphire lasers. In phase-matched structures IGVD at the SH cannot be neglected for pulses approaching 10 fs. It results in a spectral quadratic phase on the SH and in some cases can shorten the pulse. External dispersive shaping of the SH pulses distorted by group-velocity mismatch (GVM) is examined, and some pulse shortening is found possible. It is shown that the effect of IGVD at the SH wavelength on the pulse is similar to that of the spectral quadratic phase provided by an external pulse shaper. Group-velocity-matched configurations are also investigated. IGVD at both the fundamental and the SH wavelengths is found to limit the optimum thickness of the nonlinear medium. A measure of the interaction length in which the pulse width of the fundamental pulse is preserved in the SH is introduced. It is defined in terms of the GVM and the pulse bandwidth for phase-matched structures and in terms of the IGVD and pulse bandwidths for group-velocity-matched configurations.

Light-induced orientation in azo-polyimide polymers 325 degrees C below the glass transition temperature

We report on light-induced nonpolar orientation of sidechain nonlinear-optical polyimides [glass transition temperatures (Tg) up to 350 °C] containing no flexible connectors or tethers. The nonlinear-optical azo-dye chromophore is incorporated through the donor substituent as a part of the polymer backbone. This impressive photoinduced orientation occurs at room temperature, i.e., at least 325 °C below the Tg of one of the polymers that we investigated. Furthermore, it is shown that, after photoinduced ordering, this polymer must be heated to 350 °C to induce main-chain movement and to erase the orientation, whereas the direction of orientation of the chromophores can easily be controlled at room temperature simply by choice of the appropriate polarization of the irradiating light. This light-induced orientation process is shown to be useful for storing images in waveguides.

Corona poling of nonlinear polymer thin films for electro‐optic modulators

We have developed an experimental technique for corona poling a nonlinear polymer thin film for use in electro‐optic modulators. This method eliminates the pinhole and surface roughness damage that has been observed when corona poling a nonlinear polymer deposited on a conducting substrate. Electro‐optic modulators have been constructed to compare this corona poling technique to the contact poling technique commonly used with electro‐optic devices. Experimental results are presented that demonstrate that the corona poled electro‐optic devices were poled with electric fields near the dielectric breakdown of the thin polymer films—fields that are often difficult to achieve with contact poling.

Correlation between polymer architecture and sub-glass-transition-temperature light-induced molecular movement in azo-polyimide polymers: influence on linear and second- and third-order nonlinear optical processes

We report on light-induced linear and second- and third-order nonlinear optical effects in high-glass-transition-temperature (Tg) photosensitive nonlinear optical azo polyimides. We present evidence of light-induced orientation of azo chromophores at room temperature in very high-Tg polyimides (Tg up to 350 °C) even though the chromophore is firmly embedded into the polymer backbone. We show that the isomerization reaction and the light-induced polar and nonpolar orientation depend on the molecular structure of the unit building blocks of the polymer. The mechanism of the photoassisted poling process is clarified, and it is shown how the linear and second- and third-order nonlinear optical effects can be controlled by light.

Dispersive models for the finite-difference time-domain method: design, analysis, and implementation

Digital signal processing techniques are used to design, analyze, and implement discrete models of polarization dispersion in finite-difference time-domain (FDTD) simulations. These methods are warranted by the extreme importance of dispersion to the propagation behavior of femtosecond-duration optical pulses. Input-invariant and frequency-approximation methods of designing discrete analogs of continuous-time dispersive electromagnetic systems are presented. Our methods are shown to unify existing design techniques. The inherent accuracy of each dispersive design is quantified by truncation error and frequency response methods, and stability analysis of the total FDTD system is found through root locus techniques. Implementation of the design is accomplished by algebraic manipulation of the system function in the frequency domain, resulting in canonical, partial fraction, or cascade structures that minimize the number of stored variables and provide a trade-off between efficiency and sensitivity to finite precision.

Electromagnetic propagation at interfaces and in waveguides in uniaxial crystals

The use of surface-impedance and surface-admittance concepts for analyzing reflection and refraction at an isotropic dielectric interface (first developed about 1938) is extended to include an interface between uniaxial birefringent dielectrics. Total internal reflection and the polarizing (Brewster) angle at an anisotropic interface are shown to be naturally explainable in terms of surface impedance (for TM polarization) and surface admittance (for TE polarization). The allowable modes in an integrated optical uniaxial asymmetric dielectric slab waveguide are also shown to be directly obtainable using the surface impedance/admittance approach. Numerical examples are presented.

Use of Fabry-Pérot devices for the characterization of polymeric electro-optic films

Electro‐optic properties of thin polymeric films can be studied by incorporating the films into the spacer layers of transmission‐mode Fabry–Perot devices formed using metal mirrors. The metal mirrors, which also serve as electrodes, are used for the application of an electric field for poling the polymeric material and for subsequently modulating the index of refraction through the electro‐optic effect. A model for transmission modulation is developed, and experimental characterization of the devices by angles tuning at a fixed wavelength is presented. Suitable approximations can be made to allow determination of the net trapped charge and the electro‐optic coefficients of the spacer layer. This paper demonstrates the use of polymeric thin‐film etalon devices for the study of new materials and indicates their potential use as electro‐optic devices.

Reversible optical storage utilizing pulsed, photoinduced, electric‐field‐assisted reorientation of azobenzenes

We demonstrate a method of permanent optical recording of digital data which exploits a fast photoisomerization of nonlinear molecules, followed by a slow permanent alignment within a polymer. Write and erase cycles are initiated by rapidly photoisomerizing azobenzene molecules into an intermediate state with a larger mobility. The molecules align or randomly orient within the polymer depending on the presence or absence of an electric field. During orientation, relaxation to a stable isomer occurs and the alignment becomes permanent. The recorded information can then be nondestructively read by second‐harmonic generation. Nanosecond optical exposures were used, demonstrating that extremely fast recording rates are possible.