| Back Issues |
|
|
|
|
ACS Members can sign up to receive C&EN e-mail newsletter.  |
|
|
 Join ACS |
|
|||||||||
|
OPTICAL MATERIALS
Tunable infrared lasers based on nanocrystals may become available soon. Researchers at Los Alamos National Laboratory in New Mexico have shown that amplified spontaneous emission--an important milestone along the road to making practical lasers--can be achieved in the near-IR region using nanocrystalline lead salts. The advance may be especially useful in optical telecommunications, remote sensing, and related applications that depend upon light in that wavelength region.
But now, Los Alamos postdoctoral fellows Richard D. Schaller and Melissa A. Petruska and team leader Victor I. Klimov have demonstrated that a novel synthesis procedure for preparing lead selenide nanocrystals in a titania matrix leads to high-quality optical films capable of generating amplified spontaneous emission in the near-IR region [J. Phys. Chem. B, published online Nov. 21, http://dx.doi.org/10.1021/jp0311660]. "It's an important advance, especially for technological applications," says Alexander Efros, a staff scientist at the Naval Research Laboratory in Washington, D.C. By adjusting the size of nanocrystals synthetically, scientists can tune the light emitted from quantum dot sources, Efros explains. That type of flexibility should make it possible to adjust the output of future optical devices so that scattering and absorption interferences can be avoided. Three years ago, the Los Alamos group and their coworkers reported that CdSe nanocrystals could be made to lase in the visible region (C&EN, Oct. 23, 2000, page 78). Motivated by this success, the Los Alamos team and other research groups sought to use nanocrystals of lead salts to produce IR laser light. But despite some indications that lead-based nanocrystals could work in that application, the lasing effect remained elusive. Two fundamental shortcomings make it difficult to achieve lasing in PbSe. One of the problems is that several of PbSe's lowest electronic states have equivalent (degenerate) energy levels. In addition, excitations that potentially could lead to IR emission can be quenched rapidly by a fast competing process known as Auger recombination, which does not lead to light emission. "The nonradiative mechanism steals energy from the lasing transition," explains Mark I. Stockman, a physics professor at Georgia State University, Atlanta. Stockman adds that despite these problems, the Los Alamos group has shown via "a direct and convincing demonstration" that lasing is still possible and can be achieved in a practical manner. Klimov and coworkers credit their synthesis method, which includes colloidal chemistry techniques and amphiphilic polymers, with producing optical films of high enough quality to observe the lasing effect. According to the Los Alamos group, the results indicate that the lack of success previously in developing the required lasing conditions in nanocrystalline lead salts was due to material quality issues, not intrinsic physical limitations.
|
|||||||||
|
Chemical & Engineering News |
|||||||||
 |
||||||||
|
||||||||
|
||||||||
|
||||||||
|
||||||||
|
||||||||
|
||||||||
|
||||||||
