chapter  4
Photonic Crystal All-Optical Switching
Pages 68

Owing to their unique properties of controlling the propagation states of photons, photonic crystals have been considered to be promising candidates for the study of future integrated photonic devices and integrated photonic chips. Along with the extensive research on the fabrication of high-quality one-, two-, and three-dimensional photonic crystals, various kinds of integrated photonic devices based on photonic crystals have attracted great attention, and have been studied widely and intensively. Nonlinear photonic crystals constructed by third-order nonlinear optical materials are the very important basis for the realization of photonic devices. 4.1 Mechanism of Photonic Crystal All-Optical

SwitchingAnalogous to its electric counterpart, all-optical switching can permit (or prohibit) the propagation of a signal light under the trigger of a control light. Photonic crystal all-optical switching can modulate the propagation states of the signal light based on the interactions of light and matter. As an essential integrated photonic device, photonic crystal all-optical switching plays a very important role in the fields of integrated photonic circuits, optical

interconnection networks, and optical computing systems. In 1994, Scalora et al. presented the concept of photonic crystal all-optical switching [1]. It can be understood simply as following: at the beginning, a signal light is reflected completely by a photonic crystal and cannot propagate through it. The optical switching is in the “OFF” state. Under the excitation of a control light, the signal light can propagate through the photonic crystal. Then the optical switching is in the “ON” state. The propagation states of the signal light are dominated fully by the appearance of the control light. Up to now, various mechanisms have been presented to demonstrate photonic crystal all-optical switching, such as the photonic bandgap shift mechanism, defect mode shift mechanism, and so on. Third-order nonlinear photonic crystals are the essential basis for the realization of all-optical switching devices. 4.1.1 Photonic Bandgap Shift MethodThe photonic bandgap shift method was proposed by Scalora et al. in 1994 [1]. When the nonlinear medium constructing the photonic crystal has a positive third-order nonlinear susceptibility, the frequency of the probe light can be set at the high-frequency edge of the photonic bandgap. At first, the probe light is reflected completely by the photonic crystal and the optical switching is in the “OFF” state. When the pump light is switched on, the refractive index of the nonlinear medium in the photonic crystal increases due to the nonlinear optical Kerr effect, which leads to the increase of the effective refractive index of the photonic crystal. As a result, the photonic bandgap shifts in the low-frequency direction. Then the frequency of the probe light drops into the pass band and can propagate through the photonic crystal. Then the optical switching is in the “ON” state.When the nonlinear medium constructing the photonic crystal has a negative third-order nonlinear susceptibility, the frequency of the probe light should be set at the low-frequency edge of the photonic bandgap. At first, the probe light is reflected completely by the photonic crystal and the optical switching is in the “OFF” state. When the pump light is switched on, the refractive index of the nonlinear medium in the photonic crystal decreases due to the nonlinear optical Kerr effect, which leads to the decrease of

the effective refractive index of the photonic crystal. As a result, the photonic bandgap shifts in the high-frequency direction. Then the frequency of the probe light drops into the pass band and can propagate through the photonic crystal. Now the optical switching is in the “ON” state. The schematic structure of the photonic bandgap shift mechanism is shown in Fig. 4.1.