N. M. Lawandy

Theory of laser action in scattering gain media.

A laser model based on feedback produced by scattering has been developed to explain the narrow linewidth emission and input-output behavior observed in scattering gain media. The model is based on the transient two-level laser equations and includes the detailed spectral properties of the dye gain system. Monte Carlo methods were employed to calculate the threshold gain required for modeling the input-output and linewidth emission characteristics.

Dynamics of the formation of Bragg gratings in germanosilicate optical fibers

Abstract The dynamics of the formation of Bragg gratings in germanosilicate optical fibers is examined using a material rate equation coupled to spatio-temporally evolving counter-propagating light fields. Theories for the microscopic material changes which give rise to the index change are discussed and the differential equations describing the process are numerically integrated to yield solutions for the evolution of a dynamic grating and the counter-propagating electric fields. Results for the time dependent reflectivity are compared with previously reported experimental findings.

Interface reflection effects in photonic paint

Experiments on laser action in scattering systems with gain reveal that even in the nondiffusive regime interface reflection effects do not enhance linewidth collapse. Instead we show that in dye systems replacing the cell wall with a mirror causes broadening and that removing Fresnel reflection at the surface results in narrower emission lines.

Optically encoded phase-matched second-harmonic generation in semiconductor-microcrystallite-doped glasses

We observed that the simultaneous injection of the fundamental and second-harmonic fields (1.06 μm and 532 nm) into semiconductor-doped glasses results in the evolution of a permanent, quasi-phase-matched, second-harmonic signal that is 105 times the initial background value. Using samples as long as 5 cm, we obtained second-harmonic signals that are visible under room lights, corresponding to a conversion efficiency of 10−6 for mode-locked, Q-switched input pulses. Results are presented for a variety of experiments that shed light on the basic physics of the effect. These include polarization dependence, IR and second-harmonic preparation intensity effects, thermal erasure, and the application of external static-electric fields. The results indicate that the most likely mechanism is the encoding of a periodic internal electric field, which results in a phase-matched electric-field-induced second-harmonic generation process.

Understanding bichromatic emission from scattering gain media.

Intense optical pumping of solutions of high-gain laser dyes and TiO(2) nanoscatterers in methanol is found to result in narrow-linewidth bichromatic emission. Experimental studies of the long-wavelength emission peak show that weak pumping of the scattering gain medium by the primary lasing emission results in a random system that lases at longer wavelengths. Measurements with other dyes show that the bichromatic emission effect is very general.

COHERENT BACKSCATTERING FROM HIGH-GAIN SCATTERING MEDIA

We report on experimental observations of coherent backscattering signals from high-gain scattering media in the regime where significant amplification takes place over one transport length. Our samples consist of polymer sheets containing optically pumped dyes as the amplifying medium, with TiO(2) nanoparticles providing the scattering. The width of the backscattering cones narrows with increasing amplification, while the enhancement factor remains unchanged.

Single-step laser fabrication of refractive microlenses in semiconductor-doped glasses

Exposure of semiconductor-doped glasses to above-band-gap focused cw laser radiation results in the controllable formation of microlenses. A laser-driven thermal runaway is proposed to explain the low powers required for the process. A linear array of eight identically prepared microlenses is characterized and shown to exhibit diffraction-limited performance.

Quantum electrodynamics in photonic crystals

A macroscopic quantum electrodynamic formalism has been developed for a three-dimensional periodic dielectric lattice made of a linear, non-magnetic, isotropic, locally homogeneous, and lossless medium. The usual choice of Coulomb gauge can be shown to be inappropriate for the study of QED processes in photonic crystals. In the absence of charged particles and in a modified Coulomb gauge, the scalar potential can be forced to be zero and a vector potential is obtained which is consistent with Maxwells equation and satisfies Blochs theorem and the dispersion relation first derived by Ho et al. In the presence of sources, using a standard formalism from the appropriate lagrangian, the hamiltonian can be developed in a parallel way with the minimal coupling hamiltonian. However, several differences are present and the atomic hamiltonian is not the same as that of an isolated atomic system due to the fact that the scalar potential is not a solution of the Poisson equation, and the ordering of particle and radiation operators in the interaction hamiltonian is important. In the long wavelength approximation, the effective interaction hamiltonian for an infinitely heavy atom resembles that of the Coulomb gauge. In addition to the complete suppression of spontaneous emission within the forbidden band, we predict significant modifications outside the forbidden bands. The emission rate is found to be strongly position dependent and the atoms can emit light even in the direction of the oscillation, an effect which is totally absent in the free space case.

Light induced transport and delocalization in transparent amorphous systems

Abstract We show that intense nonresonant light fields such as those present in optical fiber may cause localized electronic states to become extended. A threshold criteria is derived for the effect and its implications for change transfer in glasses is discussed. In addition we show that such effects may be important for understanding the yet unexplained efficient second harmonic generation which occurs in germanosilicate optical fibers.

High density optically encoded information storage using second harmonic generation in silicate glasses

Abstract Multiple bits of information have been stored in the frequency domain in bulk glass using optically encoded χ (2) gratings. The information is read out by measuring second harmonic generation from the encoded glass as a function of wavelength. Information densities in excess of 10 8 bits/cm 2 should be possible using this effect. The storage is stable under readout conditions, and the information can be erased and rewritten.