Direct laser writing of diffractive optics in glass.

Low-power lasers and a novel thermal expansion process in semiconductor-doped glasses are used to fabricate diffractive optical elements.

External-feedback effects in high-gain scattering media.

We report on experimental studies of external-feedback effects on high-gain scattering media. We explain experimental results for the pump energy required for laser action as a function of the separation between a mirror and the media by use of Monte Carlo simulations and integration of the laser equations.

Speckle-mirror laser.

We report on a new laser feedback mechanism that uses speckle-pattern phenomena to create highly dispersive feedback. This speckle mirror is capable of creating laser emission as narrow as that from laser cavities using a number of dispersive elements such as gratings and prisms. Contributions from the peaks of intensity of the speckle patterns and a limited pumping-beam diameter are the basic conditions for the observation of this effect.

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.

Maskless photoencoded selective etching for glass-based microtechnology applications.

Two-photon excitation of carriers in boron E'-center-containing borosilicate glasses results in a photoencoding of selectively etchable regions. Using a turbulent etching process followed by polishing, we have demonstrated a number of patterning capabilities for microtechnology applications such as ultrafast capillary electrophoresis chips and rapid prototyping of diffractive optical elements.

Direct measurement of photoinduced charge distribution responsible for second-harmonic generation in glasses.

We have found that borosilicate glasses that produce second-harmonic generation after simultaneous irradiation by fields at omega and 2omega exhibit postexposure selective etching. We use the charge specificity of the selective etching process to spatially map the photoexcited electron distributions responsible for second-harmonic generation. The results show that nearly symmetric nondipolar charge distributions exist for some regimes of encoding and that, at higher intensities, a nearly dipolar distribution is produced.

Radiative coupling between photonic paint layers.

Intense isotropic laser emission from an optically pumped scattering gain medium has been used to induce isotropic lasing in an adjacent scattering gain medium at a longer wavelength. The energy-transfer mechanism between the media involved has been studied with a simple model based on radiation-transfer theory. The applications of this effect for wavelength-domain coding are also discussed.

Direct observation of the effective chi((2)) grating in bulk glasses encoded for second-harmonic generation.

The effective chi((2)) grating responsible for second-harmonic generation in bulk glasses was spatially resolved with a charge-selective etching technique. We observed a spatially varying structure with a period equal to the coherence length of the material. The measured period of the grating showed excellent agreement with the expected value. In addition, cross-sectional etching patterns show that in this regime of encoding intensities the f lux responsible for the intense electric field is not unidirectional as predicted by most models.

Injection locking of photonic paint.

The narrow-linewidth laser emission from a scattering gain medium can be forced to oscillate in a narrower emission bandwidth with the introduction of a seed in much the same way as the injection-seeding technique used to produce single-mode oscillation from conventional lasers. The system exhibits complete spectral collapse at all wavelengths other than that of the seed and a large enhancement of the isotropic emission peak intensity. A theory explaining the emission characteristics and the minimum threshold seed energy required for locking is presented.

Photoimpulsive dispersive relaxation of the effective chi((2)) in binary lead-silicate glasses.

We have performed a detailed investigation of the readout decay of optically encoded second-harmonic generation in bulk lead-silicate glasses. The data show that over several decades of readout pulses, the relaxation is a stretched exponential in the pulse number. A new photoimpulsive dispersive transport model, in which the experimental data on the order of the optical process and the depth of the trap distribution below the mobility edge have been included, is used to explain the results.

Optically controlled imaging phase mask element.

We demonstrate a new optical element consisting of a phase mask made of a nonlinear material. The element uses the nonlinearity to produce an intensity-dependent lens that controls its imaging properties. We demonstrate this element by optically controlling the imaging of an array produced by a diffractive spot generator in direct contact with a thin semiconductor-doped glass slab. The image at 632.8 nm is controlled by 514.5-nm light from an argon-ion laser.

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.

Nonlinear Christiansen filter.

We demonstrate a nonlinear Christiansen filter in both the steady state and the transient regimes and in both the self- and the cross-modulation modes. Experiments in which a thermal nonlinearity was used agree well with predictions based on a modified scattering theory model.

Laser action in polymeric gain media containing scattering particles.

Optical pumping of polymer sheets containing laser dyes and TiO(2) nanoparticle scatterers results in emissions that exhibit laser behavior with linewidths as low as 4 nm. The input-output characteristics, effects of index matching on the emission, damage thresholds, wavelength stability, and possible applications of these new materials are discussed.

Photonic textile fibers.

The characterization of the emission from nylon fibers containing laser dyes and TIO(2) nanoparticle scatterers indicates laser behavior with linewidths as low as 4 nm in 200- to 800-µm fibers. These materials can be used to produce lasing textiles, which can be used to produce photonic codes for a variety of civilian and military applications.

Multiphoton micrometer-scale photoetching in silicate-based glasses.

Silicate-based glasses containing approximately 10% by weight boron or more have been observed to exhibit selective etching in hydrofluoric acid solution after direct exposure to intense visible laser light at 532 nm. The observation of a ring-shaped etch pattern in samples exposed to solid Gaussian beams suggests that selective etching is related to a charge-diffusion process rather than to local light-induced defect generation. The technique has so far resulted in the maskless production of micrometer-scale features with submicrometer depths.

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.