E. W. Seelig

A nanotube-based field-emission flat panel display

A matrix addressable diode flat panel display has been fabricated using a carbon nanotube–epoxy composite as the electron emission source. Field-emission uniformity has been confirmed by measuring the I–V curves of pixels across the panel. This prototype display demonstrates well-lit pixels under ±150 V biasing signals. The “on” and “off” of the pixels are well controlled by the half voltage “off-pixel” method. Further improvement of this technology may lead to easy-to-make and inexpensive flat panel displays.

Microlaser made of disordered media

We have fabricated microlasers with disordered semiconductor nanoparticles. The physical mechanism of optical confinement is based on Anderson localization of light in micrometer-sized random media. Pulsed lasing occurs around 380 nm at room temperature under optical pumping.

Random lasing in closely packed resonant scatterers

We report experimental and theoretical studies of the random lasing threshold and its fluctuation in an ensemble of highly packed spherical dielectric scatterers. The ratio of the sphere diameter to the lasing wavelength was varied in a wide range, which covered the transition from the weak Rayleigh scattering regime to the strong Mie scattering regime. Experimentally, when the diameters of monodispersed ZnO spherical particles changed from less than 100 to more than 600 nm we observed a drastic decrease of the lasing threshold at small-particle size followed by a plateau at large particle size. We attribute this effect to the particle-size dependence of transport mean free path lt, which was deduced from coherent backscattering measurements. Theoretical calculation of lt reproduced experimental behavior. Using the finite-difference time domain method, we obtained the lasing threshold and its standard deviation as functions of particle size in two-dimensional systems. The results of our numerical simulations are in qualitative agreement with the experimental data.

Effect of external feedback on lasing in random media

We have studied the effect of external feedback on random laser action in ZnO polycrystalline thin films. Reinjection of light into scattering-formed cavities strongly influences modes, intensity, and threshold of random lasers. We have compared the effect of external feedback from the side of the film and that from the film surface. Our study opens the possibility of controlling random laser frequencies by external feedback.

Lasing in disordered media

We review our recent work on lasing in active random media. Light scattering, which had been regarded detrimental to lasing action for a long time, actually provided coherent feedback for lasing. We also trapped laser light in micrometer-sized random media. The trapping was caused by disorder-induced scattering and interference. This nontraditional way of light confinement has important application to microlasers. The threshold of random laser can be reduced by incorporating some degree of order into an active random medium. Our calculation result shows that by optimizing the degree of order one can dramatically reduce the threshold of random laser to the values comparable to htose of photonic bandgap defect lasers.