Optical sources having a strongly scattering gain medium providing laser-like action

DETAILED DESCRIPTION OF THE INVENTION In order to describe in detail a number of novel embodiments of this invention, a description is first made of experimental results which clearly illustrate the novel properties of an embodiment of the medium of this invention.
Experimental Description Experiments were performed on solutions containing a 1.
times.
10.
sup.
-3 M to 2.
5.
times.
10.
sup.
-3 M concentration of rhodamine 640 dye in methanol with varying amounts of either TiO.
sub.
2 (rutile) or Al.
sub.
2 O.
sub.
3 (.
alpha.
-phase) nanoparticles.
The TiO.
sub.
2 particles had a mean diameter of 250 nm and the Al.
sub.
2 O.
sub.
3 particles were either 280 nm or 33 nm in diameter.
The TiO.
sub.
2 particles were coated with a layer of Al.
sub.
2 O.
sub.
3 to prevent flocculation.
Based on the particle sizes and densities, it was determined that these colloids should exhibit sedimentation times of 14.
2 hours, 6.
6 hours and 882 hours over a 1 cm length, respectively.
These times were considerably longer than the approximately 30 minute to 1 hour experiment times.
In addition, the total surface area available for adsorption of the dye molecules to the nanoparticles was determined.
Specifically, it was found that the TiO.
sub.
2 particles had a 13.
4 m.
sup.
2 /gram available surface area for accommodating dye molecules.
This value indicates that at a particle density .
rho.
.
about.
10.
sup.
10 /cm.
sup.
3, approximately 1% of the dye molecules of a 2.
5.
times.
10.
sup.
-3 M dye solution can be accommodated on the nanoparticle surfaces.
This upper limit effectively eliminates the possibility that surface effects play a significant role in the observed laser-like properties of the colloidal solution.
The optical scattering properties of the nanoparticles were in the Mie regime.
The scattering cross-sections at the peak dye emission wavelength of .
about.
617 nm were computed using the full Mie solutions and found to be far too small to exhibit any morphological resonances.
This is believed to be primarily due to the small size parameter, x=ka, where k is the emission wavevector in methanol and a is the particle radius