Nonlinear optical effects such as second- and third-harmonic generation are very important for many practical applications. In fact, green laser pointers use one of these effects: second- harmonic generation allows converting invisible infrared radiation into green light. Typically, nonlinear effects require substantial light intensities, because the light should be intense enough to change material properties—this is why the observation of second-harmonic generation became possible only after the invention of the laser. Moreover, the light should propagate through a significantly thick sample to produce a harmonic signal of significant intensity. However, with the modern trend toward miniaturizing various optical components and making energy-efficient devices, it would seem that the useful implementation of nonlinear effects in these devices will be increasingly difficult.
The current work makes a new step toward creating energy-efficient functional micrometer-scale devices operating in the continuous-wave regime. The authors use a combination of a large number of physical effects to achieve simultaneous generation of two different wavelengths. First, a high-Q cavity is designed in a silicon photonic crystal to enhance the light intensity—the crucial step for efficient nonlinear processes. Second, the photonic crystal is modified around the cavity to allow the excitation of the resonant mode with an incident electromagnetic wave. Bulk silicon possesses only nonvanishing third-order nonlinear susceptibility, and thus the generation of the third harmonic is possible. The second harmonic, however, is generated through nonlinear surface effects.
It is not only hard to carefully design a device that uses such a large number of physical effects, but it is also difficult to interpret the observed behavior. The authors clearly identified the harmonic generation by looking at the dependence of the intensity of the respective harmonic on the intensity of exciting wave. They have also developed a model that describes the complex radiation pattern of the generated light—something that you really need to know if you want to collect it for further use.
This work also demonstrates the maturity of photonic crystal research and technology. For more than a decade, photonic crystal studies have produced fundamental work predicting various fascinating phenomena. At the same time, manufacturing techniques were also developed to meet the requirements for the observation of the predicted effects. Those efforts created a solid background for the success of the current work.
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