A caged existence

Solitons maintain their confinement as they are manipulated to change direction

© (2006) Nature Phys.

Optical solitons are prisoners of their own making. Thanks to nonlinear interactions with the medium in which they travel, these solitary waves are able to ‘self-trap’ — in other words, they keep their beam shape and resist the urge to spread out during propagation. This curious imprisonment could be useful for carrying information reliably over optical networks. Now a group of English and Italian researchers has shown that these solitons can be controlled at will when they travel in a nonlinear medium¹.

To date, optical solitons have been studied in a range of one- and two-dimensional nonlinear systems. What Marco Peccianti and colleagues have done is extend the domain to liquid crystals, in particular, they have studied what happens at the interface between two such crystals. Liquid crystals offer an added bonus: the fact that their molecules reorient themselves gives rise to a particularly large optical nonlinearity, which is useful for sustaining solitons. Moreover, the orientation of the molecules can be controlled by using an applied voltage, making it an easily tunable system.

Peccianti and colleagues confine a thick layer of liquid crystals between two glass slides. They then apply a different voltage to different parts of the crystal, which induces varying levels of molecular reorientation in the two regions and thus different refractive indices. By tuning the voltage difference over a range of less than one volt, the researchers demonstrate unprecedented control over the soliton waves at the interface. The waves can be made to refract and reflect by angles up to 18° and 22° respectively, or they can simply pass straight through. What is striking is that the light beams doggedly maintain their self-confinement throughout. This ability of solitons could prove to be invaluable for use in signal processing and optical circuits.