# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
## 4 Nonlinear Photonics Devices
### All-Optical Digital Devices: Switches, Gates, Flip-Flop Units, and Optical Transistors
However, use of a high-Q cavity makes the operation slow because the light charging speed is slow for a cavity having a high Q. Although the carrier effect has enabled fast operation at a few ns [46], a tradeoff between low power and high speed due to the high Q of the cavity remains. An approach based on nonlinear plasmonic slot waveguides was used to address the well-known tradeoffs between all-optical switching speeds and associated energy requirements [143]. Graphene's ultrafast saturable absorption (SA), leading to transmission of a signal pulse when a control pulse overlapped with it, was achieved, with an ultrafast response time of 260 fs. All-optical logic gates and flip-flops are other fundamental elements for optical computing [22], even if still, at the beginning of the last decade, there were doubts about their feasibility and their possibility of competing with microelectronics [144]. Such skepticism was not fully justified, even if, still five years later, realization of an all-optical flip-flop required coupling two optical triode switches made by a nonlinear etalon and  
Figure 9: Schematic illustration of an all-optical switch made of an add–drop microring resonator. The dotted line represents the transmittance spectrum of a cold cavity. The solid line is the transmittance when inputs are applied. A resonant shift occurs due to the optical Kerr effect. Two different wavelengths, \(\lambda_{1}\) and \(\lambda_{2}\), are used for the operation. On the left column \(\lambda_{1}\) will drop (high) when only \(\lambda_{1}\) is inputted (high).) On the middle column \(\lambda_{2}\) will not drop (low) when only \(\lambda_{2}\) is inputted (high). On the right column \(\lambda_{2}\) will drop (high) when both \(\lambda_{1}\) and \(\lambda_{2}\) are inputted (high). As a result, \(\lambda_{2}\) can be switched off and on by turning \(\lambda_{1}\) signal on and off. Reprinted with permission from [22] © The Optical Society. gradient-index lenses [19]; to date, however, several all-optical logic devices have been demonstrated [145; 146; 147; 27; 148; 149; 25]. Extremely-low-power all-optical bistability was demonstrated by utilizing silicon PhC nanocavities [46]. Bistability was obtained by the plasma effect of carriers generated by two-photon absorption, with a bistable threshold power of 0.4 mW, a set pulse energy of 74 fJ, and a switching speed of <100 ps. Design and fabrication of a large-scale memory in InGaAsP/InP and Si devices were discussed in Ref. [140]. A large number (128) of serially integrated PhC Si-nanocavities were fabricated, and the experimental characterization showed that bistable memory operation was achieved for 105 bits in this chip, with bias power for this operation in the range 79-400 \(\upmu\)W. The proposed device involves a parallel configuration, where side-coupled bit memory cavities are placed along a bus waveguide. Electronic transistors, as the basic unit of logic circuits, have succeeded in supporting large-scale integrated circuits for computers. Any complex logic circuit can express a combination of three basic logic gates, AND, OR, and NOT, and these basic gates can be constructed with a universal transistor. Although electronic logic gates have enabled creation of integrated circuits with high density and functionality, optical logic gates cannot reach the far requirements of large-scale optical computing circuits even today [149]. On the other hand, optical transistors, as the core hardware of optical gates, so far have not been effectively exploited. Moreover, the universal optical transistor does not seem to exist or be practical for optical gates. Many different approaches to optical transistors have been proposed. Much early work was based on optical bistability [150] using nonlinear optical phenomena, mostly in resonators, whereas others worked on the optically controlled switching of light-exploiting single molecules [151] or quantum dots [152].