Emergence of Chalcopyrites as Nonlinear Optical Materials
- PDF / 2,369,778 Bytes
- 7 Pages / 576 x 777.6 pts Page_size
- 54 Downloads / 237 Views
16
is generally attributed to A.F. Ioffe and N. A. Gory unova of the A.F. Ioffe PhysicoTechnical Institute (IPT) in St Petersburg, Russia. Table I lists some of their relevant properties, and it includes a short list of some other familiar crystals for comparison. Chalcopyrites have many properties similar to those of other semiconductors such as Si, CdTe, and GaAs. This is because all of these semiconductors have essentially the crystal structure of diamond. The chalcopyrite structure is uniaxial as a result of a slight tetragonal distortion from the isotropic cubic zincblende structure. This distortion gives rise to the very important property of birefringence. Birefringence is arguably the most important materials parameter
for the high-average-power laser application. This fact is demonstrated in fable I, which indicates that GaAs is superior in every other way to ZnGeP2. Only its lack of birefringence rules out its use as a NLO laser crystal. A typical energy-band diagram, in this 5 instance for CdGeAs2, appears in Figure 1. It is very similar to that for GaAs under uniaxial compression. In both cases, the fourfold degeneracy of the valence band of these diamondlike semiconductors is lifted by the resulting lower symmetry produced by a tetragonal distortion. As a result, the optical transitions become polarization-specific. These materials possess the remarkable property of pleochroism (many-colored). Practically speaking, these anisotropies allow the chalcopyrites to have a much more complete knowledge of their environment in device configurations than their isotropic counterparts. The Vikings were the first to exploit pleochroism in a practical device. Twilight stones—naturally occurring birefringent cordierite crystals—were used as navigation aids on cloudy, overcast days. These crystals were used to locate the sun via analysis of cloud-scattered sunlight that is selectively polarized. History Since 1975 there has been very little R&D in the United States on the chalcopyrite semiconductors. The most significant early U.S. work was accomplished by a group at Bell Laboratories. It is summarized in a book by Shay and 6 Wernick. However at the IPT there has been an extensive chalcopyrite R&D program since the discoyery of the transistor in 1947. Concurrently since the 1980s,
Table I: A Partial List of Infrared Nonlinear Semiconductors and Their Properties.
Compound GaAs AgGaS2 AgGaSe 2 AgGaTe2
2nd Order Nonlinear Optical Coefficient (picometers/V) 90 11 33 51*
Transparency Range (microns) 1.1-17 0.48-11.4 0.76-17 0.91-23
75 217 -18
0.74-12 2.7-18 0.07-24
ZnGeP2 CdGeAs 2 CdSe
Absorption Coefficient @9 μm
(cm 1 ) 0.005
Data Loading...
