Cryptography and chaos seem initially unlikely partners. On the one hand, cryptography is based on hidden information requiring certain “order” to be encoded, transmitted, received and decoded. Chaos, on the other hand, conveys the impression of anarchy and loss of control. However, mathematicians would tell us that both concepts involve dynamic processes highly sensitive to initial conditions.
Complex dynamics are found in optomechanics, which involves the interaction between mechanical modes and light mediated by optical forces. Our experimental model of an optomechanical crystal is a silicon nanobeam patterned by state-ofthe- art silicon technology, in such a way that mechanical modes (phonons) overlap in space with optical modes (photons) under confinement conditions.
Complex dynamics arising from optical non-linearities are observed already in ambient conditions with photons coming from a tuneable laser via a telecommunication optical fibre placed in close proximity to the optomechanical crystal (Fig.1), and the light exiting the fibre captured by a signal analyser. By driving a single optomechanical crystal well into the non-linear regime we show that, as the number of photons stored in the cavity is affected, a chaotic regime is reached which can be smoothly modulated varying the excitation laser parameters. Exploiting the richness of non-linear dynamics we demonstrate accurate control when activating a variety of stable dynamical solutions. The changes in the optical output between the chaotic and coherent regimes are shown in Fig.2.
Our results have repercussions well beyond our own research in phononic circuits. It could allow information to be coded by introducing chaos in the light that carries it. For example, by linking two integrated chips containing equivalent optomechanical cavities with optical fibers, it would be possible to secure information introducing chaos in the light beam at the emitting point and suppressing it at the reception point.