Use of time-resolved IR reflection and transmission as a probe of carrier dynamics in semiconductors.

We discuss the use of time-resolved IR reflection and transmission for studying semiconductor carrier dynamics. We solve Maxwell's equations for a nonhomogeneous carrier distribution in the limit of small perturbation. We show that the IR probe is sensitive to the spatial profile of the carrier density far below the surface. We present results of measurements for InP:Fe that demonstrate the advantages of this technique.

Preparation of long-coherence-length second-harmonicgenerating optical fibers by using mode-locked pulses.

We have prepared an optical fiber that produces efficient (0.24%) second-harmonic conversion by using modelocked 1.064-microm laser pulses along with a harmonic seeding beam. The effective coherence length increased to 35 cm as compared with only a few centimeters in fibers prepared with mode-locked Q-switched pulses. We show that the input bandwidth of the pump pulse imposes a fundamental limit on the effective coherence length and that this length can be severely reduced by competing nonlinear effects.

Transmission of 3-D images by means of lens guides.

A simple lens guide can be used to present to an observer a lifelike 3-D image of a distant object. The natural way in which the image is presented suggests the use of lens guides within buildings to look outside from interior rooms and in some applications now served by electronic closed-circuit television.

Latent-imaging photopolymer systems.

A new high-resolution latent-imaging photopolymer system is reported for recording thick refractive-index patterns such as volume phase holograms and grating devices. A photosensitive polymerization initiator is chemisorbed on the surfaces of a porous glass matrix (average pore diameter 4 nm). The latent image is recorded by exposing the sensitized glass such that the initiator is selectively destroyed in the desired pattern. During this step, there are only small changes in the optical properties of the material, thus preventing interactions between the image being written and the optical fields writing it. The image is then developed by filling the matrix with a suitable monomer mixture and using a uniform optical exposure to initiate polymerization. During the development step, the spatially modulated initiator concentration, which constitutes the latent image, leads to a corresponding modulation of the refractive index of the polymer, and increases in the amplitude of the image of as much as a factor of 50 (Deltan up to 3 x 10(-4)) have been observed.

Multicomponent photopolymer systems for volume phase holograms and grating devices.

Novel photopolymer systems for the fabrication of high-resolution volume phase holograms and grating devices are reported. Previously reported techniques use a single monomer (or a mixture of similar monomers) and rely solely on density modulation. In contrast, we have found it advantageous to use a mixture of components chosen to have differing reactivities and polarizabilities, which results in a composition modulation. During the image-forming exposure the more reactive monomer is polymerized while the less reactive species is excluded from the irradiated regions. Two-way diffusion, which must be invoked to explain our results, leads to a modulation of the chemical composition and hence of the polarizability of the final material. In some systems the polarizability effect was strong enough to exceed the density effect, even when these factors were in opposite directions. As in previous systems, an over-all exposure is used to fix the images. With appropriate monomer systems, stable images were obtained even when one component was unreactive. The peak-to-peak refractive index differentials achieved in our systems were as high as 1.5%. This is an order of magnitude larger than that reported for initially all liquid systems and 50% larger than that reported for related solid materials. The use of a liquid system enables us to fill small cavities with our materials and then record a high-resolution image. We have made gratings of >3000 lines mm(-1) inside hollow fibers, with 80 microm i.d., filled with a photopolymer material. For white light guided in the core, the expected band-stop filter response was observed.

Technique for measuring refractive index changes in photochromic materials.

We describe a new technique for measuring the refractive index change associated with the photoinduced absorption change in photochromic materials. The technique involves writing a thick holographic grating in a sample of the material and observing the interaction of the grating with the beams used to write it. Only three relative intensity measurements are required, and many of the difficulties of previous techniques are avoided. We have demonstrated the technique in experiments on a photodimer material. These experiments also provide an example of a modulated transmission effect related to the Borrmann effect.