# Off-Resonance Control and All-Optical Switching: Expanded Dimensions in Nonlinear Optics
## 7 Discussion  
The primary discoveries associated with the off-resonant modification of optical processes were entertained in connection with their potential deployment in spectroscopic and photophysical measurements. At the simplest level, additional control is afforded by the capacity to vary the intensity,  
Figure 6: The number of emitted photons, plotted as log \(n\), against the pumping rate in the absence (red line) and presence of an off-resonant beam. The irradiance of the probe beam is \(2\times 10^{11}\) W cm\({}^{-2}\) (green) and \(4\times 10^{11}\) W cm\({}^{-2}\) (blue). The lasing threshold, as indicated by the sudden increase in \(n\), reduces with increasing intensity of the probe beam. With suitable pumping, a shift to above-threshold operation is observed (upper dotted lines). Modified from the original image in [96]. polarization, wavelength, and even pulse duration of the off-resonant beam, representing an expanded parameter space for experimentation. Although it has yet to receive detailed analysis, it is not only the intensity of individual bands in a spectrum that may exhibit change under suitable conditions: A modification to line-shapes may also be anticipated. The possibility of achieving all-optical switching now represents one of the more recently attractive propositions for application. Although the speed of ultrafast communications and computer processing continues unabated, they have until now still been essentially subject to Moore's Law. For many years, it has been known that the long-sought goal of all-optical switching has the capacity to revolutionize telecommunications and computing, since the relatively slow and energetically inefficient opto-electronic conversions that form bottlenecks in present technology can be circumvented. Therefore, unsurprisingly, numerous implementation strategies have been proposed, some of which are now briefly discussed. One technique involves the optical control of the refractive index of a material, so that a secondary beam travelling though the medium can be modified in speedily reversible manner [97]. Closely related are systems that exploit electromagnetic induced transparency [98; 99; 100], in which a material becomes transparent within a narrow range of the absorption spectrum. Similar all-optical switching arrangements are based on Mach-Zehnder interferometry [101; 102]. Other schemes make use of semiconductor nanoparticles [103; 104; 105], nanoplasmonic waveguides [106; 107; 108; 109], fiber Bragg gratings [110; 111; 112] or nanocavities in photonic crystals to strongly enhance optical nonlinearities [113; 114]. Moreover, based on irradiation of rubidium vapor, there is a development that uses counter-propagating laser beams to induce an optical pattern that rotates on application of a switching laser [115]. Compared to the approach we describe, many of these methods suffer the disadvantage of requiring optical structures and configurations substantially larger than typical molecular dimensions. Moreover, although there are indeed other specifically molecule-based methods, such as several based on the photocycle of bacteriorhodopsin [116; 117], their speed is significantly constrained by the kinetics of state interconversion, each with a finite (typically picosecond) lifetime. The progress of these and many other all-optical systems have been discussed in two excellent reviews [118; 119]. It is still clear, at present, that no single scheme has been adopted as the one that will be useful in a practicable device in the near future. The prospect we have identified of controlling resonance energy transfer without requiring a microcavity [120] is most likely to involve all-optical switching implemented in a synthetic heterostructure. Building such a device requires exploitation of the recent advances in the construction of quantum dot arrays--see, for example, [121; 122]--here, with juxtaposed arrays sandwiching an optically transparent spacer layer.