Page 247 - Introduction to Information Optics

232 4. Switching with Optics

Substrate
Contact
-V
f-O

+V
OH

Light out
Light in

Quantum wells

Fig. 4.24. Transverse multiple quantum-well modulator.

Figure 4.24 shows an example of a transverse modulator structure. In a
simplified scheme, a quantum-well structure can be viewed as a thin hetero-
structure of low-bandgap material (GaAs), the thickness of which is roughly
lOnm (about 30 atomic layers), sandwiched between two layers of higher-
bandgap material (AlGaAs). The band diagram of the structure is shown in
Fig. 4.25a. Electrons and holes tend to be localized in the region of lower-

Conduction
Band

Energy gap
Valence
Band

(a) Without electric field (b) With electric field

Fig. 4.25. Band diagram of a quantum well (a) with and (b) without external electrical field.