Brian L. Justus

Nanochannel Array Glass

The fabrication and characterization of a glass containing a regular parallel array of submicrometer channels or capillaries are described. The capillaries are arranged in a two-dimensional hexagonal close packing configuration with channel diameters as small as 33 nanometers and packing densities as high as 3 x 1010 channels per square centimeter. The high-temperature stability of the nanochannel glass array is well suited as a host or template for the formation of quantum confined semiconductor structures or as a mask for massively parallel patterned lithographic applications.

Some characteristics of a droplet whispering-gallery-mode laser

We report lasing characteristics of 40-60-microm-diameter Rhodamine 590/water solution droplets pumped by a 20-nsec-duration Q-switched laser. The Rhodamine/water solution provides a useful model system for studying the properties of oscillators based on whispering-gallery-wave spherical cavities. The low threshold for lasing, 10(4) W/cm(2) for 10(-4) M solutions, is consistent with particle size and a cavity Q factor of 10(4). Portions of the droplet lase purely in transverse electric (TE) modes, while other portions contain both TE and lower-Q transverse magnetic modes. In the far field, the lasing droplet approximates a coherent point source emitting in all directions.

Remote optical fiber dosimetry

Abstract Optical fibers offer a unique capability for remote monitoring of radiation in difficult-to-access and/or hazardous locations. Optical fiber sensors can be located in radiation hazardous areas and optically interrogated from a safe distance. A variety of remote optical fiber radiation dosimetry methods have been developed. All of the methods take advantage of some form of radiation-induced change in the optical properties of materials such as: radiation-induced darkening due to defect formation in glasses, luminescence from native defects or radiation-induced defects, or population of metastable charge trapping centers. Optical attenuation techniques are used to measure radiation-induced darkening in fibers. Luminescence techniques include the direct measurement of scintillation or optical excitation of radiation-induced luminescent defects. Optical fiber radiation dosimeters have also been constructed using charge trapping materials that exhibit thermoluminescence or optically stimulated luminescence (OSL).

Nonlinear optical properties of quantum-confined GaAs nanocrystals in Vycor glass

Quantum‐confined nanocrystallites of GaAs are fabricated in porous Vycor glass and the bound electronic nonlinear refractive index, the two‐photon absorption coefficient, and the refraction from carriers generated by two‐photon absorption are simultaneously determined using the Z‐scan method and compared to those of bulk GaAs. The measured nonlinear refractive index is an order of magnitude larger than that of bulk GaAs at 1060 nm.

Broadband thermal optical limiter

The limiting behavior of nigrosin dye dissolved in carbon disulfide was investigated in an f/5 defocusing geometry using 6 ns duration 532 nm laser excitation. Nigrosin dye is a broadband visible light absorber that is used here in conjunction with the large thermal nonlinearity of carbon disulfide solvent to defocus intense incident visible light. A limiting threshold energy of 40 nJ, corresponding to a fluence of only 100 mJ/cm2 in the solution, was observed with device absorption adjusted to 53%.

Excited-state absorption-enhanced thermal optical limiting in C 60

We report on optical limiting of Q-switched Nd:YAG radiation at 532 nm in an f/5 defocusing geometry using liquid solutions of C60 in 1-chloronaphthalene. The nonlinear optical-limiting mechanism is C60 excited-state absorption and enhanced thermal lensing in the solvent. A limiting threshold energy of 65 nJ, corresponding to an incident fluence of ∼0.3 J/cm2 at the focus in the solution, was observed with a device absorption of 25%.

Laser‐heated radiation dosimetry using transparent thermoluminescent glass

Laser‐stimulated thermoluminescence emission is reported from a novel transparent glass phosphor exposed to γ‐ray or ultraviolet radiation. Laser‐heated radiation dosimetry measurements, using this effect, are reported. A unique laser heating method permits the stimulation of the thermoluminescence without significantly raising the bulk temperature of the glass.

Excitonic optical nonlinearity in quantum‐confined CuCl‐doped borosilicate glass

The nonlinear refractive index n2 is reported for quantum‐confined CuCl microcrystallites in borosilicate glass near the Z3 excitonic resonance. Induced index changes were estimated using Kramers–Kronig analyses of absorption bleaching data from 370 to 388 nm. The nonlinearity was found to increase with increasing particle radius over the range 22 to 34 A, in agreement with theoretical predictions.

Characterization of a fiber‐optic‐coupled radioluminescent detector for application in the mammography energy range

Fiber-optic-coupled radioluminescent (FOC) dosimeters are members of a new family of dosimeters that are finding increased clinical applications. This study provides the first characterization of a Cu doped quartz FOC dosimeter at diagnostic energies, specifically across the range of x-ray energies and intensities used in mammographies. We characterize the calibration factors, linearity, angular dependence, and reproducibility of the FOC dosimeters. The sensitive element of each dosimeter was coupled to a photon counting photomultiplier module via 1 m long optical fibers. A computer controlled interface permitted real-time monitoring of the dosimeter output and rapid data acquisition. The axial-angular responses for all dosimeter models show nearly uniform response without any marked decrease in sensitivity. However, the normal-to-axial angular response showed a marked decrease in sensitivity of about 0 degrees C and 180 degrees C. In most clinical applications, appropriate dosimeter positioning can minimize the contributions of the varying normal-to-axial response. The FOC dosimeters having the greatest sensitive length provided the greatest sensitivity, with greatest to lowest sensitivity observed for 4.0, 1.9, 1.6, and 1.1 mm length sensitive elements. The average sensitivity of the dosimeters varies linearly with sensitive volume (R2=95%) and as a function of tube potential and target/filter combinations, generally exhibiting an increased sensitivity for higher energies. The dosimeter sensitivity as a function of tube potential had an average increase of 4.72 +/- 2.04% for dosimeter models and three target-filter combinations tested (Mo/Mo, Mo/Rh, and Rh/Rh) over a range of 25-31 kVp. All dosimeter models exhibited a linear response (R2 > or = 0.997) to exposure for all target-filter combinations, tube potentials, and tube current-time product stations evaluated and demonstrated reproducibility within 2%. All of the dosimeters examined in this study provided a response adequate for the accurate measurement of doses in clinical mammography applications.

Nonlinear absorption and refraction of quantum confined InP nanocrystals grown in porous glass

Single‐beam Z‐scan experiments have been used to measure the two‐photon absorption coefficient β and bound electronic nonlinear refractive index γ of composites of InP nanocrystals grown in 40 and 150 A Vycor porous glass. These materials were also studied with two‐beam time‐resolved Z scans to confirm that the nonlinearities are instantaneous with respect to the ∼100 ps pulses used. The magnitudes of the nonlinearities in the 150 A sample are found to be similar to those of bulk InP when scaled by the volume fraction of deposited material. For the 40 A sample the ratio γ/β is enhanced by a factor of fourteen compared to bulk InP.