# Off-Resonance Control and All-Optical Switching: Expanded Dimensions in Nonlinear Optics
## 6 All-Optical Switching
The next goal would be the optical control of a 2D-distributed binary coding--where 1 and 0 denote excited and unexcited molecules, respectively--by manipulating the excitation transfer  
Figure 5: Depiction of optically controlled resonance energy transfer (RET). (**a**) Energy transfer from an excited donor in the upper array of quantum dots to its partner below (spacer material not shown). The transition dipoles lie in the respective planes and are mutually perpendicular. (**b**) Before off-resonant beam enters the system, no energy transfer occurs from the excited quantum dot (red circle) to its partner (green circle). (**c**) On input of the off-resonant beam, energy is transferred from the donor (blue circle) to the acceptor. from multiple molecules in the donor (input) array to their partners in the acceptor (output) array. In other words, the binary pattern on the donor array is controllably transferred to the acceptor array, signifying parallel processing capability [95]. The output array is then'read' by detection of the emission that follows the relaxation of the acceptors. Another all-optical binary system is conceivable via a combination of laser-modified absorption and fluorescence. When conventional absorption and fluorescence cannot undergo linear engagement, i.e., transitions to the electronic excited state are one-photon forbidden by symmetry, nonlinear activation of the process may be possible on input of the off-resonant beam. This all-optical switching principle could lead to the intriguing possibility that binary data is controllably written, through laser-modified absorption, to a molecule with a metastable state and read using the analogous laser-modified emission technique. Such a device will again be confined to the realm of ultrafast processing, in which a temporary data register of a few nanoseconds is sufficiently long-lived. It is noteworthy that the contour maps of Figure 4 also show all-optical switching in that, if the probe is positioned on the \(z\)-axis, no emission is detected when the off-resonant beam is off, but emission is activated at any interval when the beam is switched on. Another feasible scheme is an optical transistor, based on the principle that the off-resonant beam can optically control the amount of stimulated emission from a three-level lasing material pumped just below its threshold. A model analysis demonstrates that an input beam of sufficient intensity modifies the amplification kinetics of the active medium and thereby enhances laser output [96]. For a constant pumping rate, at a level indicated by the dotted vertical line (see Figure 6), the system operates below threshold when the input beam is absent. On introduction of the beam with an irradiance approaching \(2\times 10^{11}\) W cm\({}^{-2}\), output may climb by approximately 14 orders of magnitude, rising to 16 orders if the input intensity is doubled--a phase transition feature that is typical of a laser operating at and above threshold. The beauty of this scheme is that its operation is not limited to any specific material. To summarize, the feasibility of conferring optical nonlinearity upon photoactive materials leads to mechanistic features with a range of potential applications, such as all-optical switching and transistor action.