Optical spin-to-orbital angular momentum conversion
and its applications in quantum optics and photonics

Can a ray of light rotate around its own axis while it propagates in a straight line? The answer, surprising to many, is yes! Actually, there are two different ways for the light to rotate. More technically, one can say that there are two different kinds of "angular momentum" that can be transported by light along its propagation: the "spin" angular momentum (associated with the circular polarization states) and the "orbital" angular momentum (associated with the helical shape of the optical wavefront, see figure above; by the way, did you know that a wave can have a helical-shaped wavefront?).

Before 2006, spin and orbital angular momentum of light were regarded as independent of each other. In 2006 our group made the important discovery that there is a way to transfer angular momentum from the spin to the orbital kind ("conversion of optical angular momentum") and invented and demonstrated a simple device (international patent pending) for doing it in a controlled way, based on the liquid crystals technology. This result has been reported in our first paper “Optical spin-to-orbital angular momentum conversion in inhomogeneous anisotropic media”, Phys. Rev. Lett. 96, 163905 (2006), that was also highlighted in Physical Review Focus (see link), and then studied in further detail in several other papers. To demonstrate a helical wave front generated by angular momentum conversion, we let it interfere with a spherical reference beam, obtaining nice spiral fringes like those shown in the figure on the right. These fringes can be switched from one handedness to the opposite one by inverting the input polarization handedness of the light.

We are currently working on the photonic applications of this novel optical technology that we introduced, particularly in the quantum optics field. We are collaborating on this topic with the quantum optics group of Roma La Sapienza (Dr. Fabio Sciarrino, Prof. Francesco De Martini) and obtained already some very promising results. Exploiting the spin-orbital couplings taking place in the q-plate, we have demonstrated the transfer of a qubit of quantum information from the space of photon polarizations to the OAM and vice versa [Phys. Rev. Lett. 103,  013601 (2009)] and we have performed the first quantum cloning of OAM-encoded photon qubits using the Hong-Ou-Mandel effect [Nature Photonics 3, 720-723 (2009)].

Although this research line originated in the Nonlinear Optical Spectroscopy lab, the experimental activity associated with this project is currently performed mainly in the Nonlinear optics of liquid crystals lab, led by Prof. Enrico Santamato (both labs collaborate strictly and belong to the Optics in Soft Matter group), and in Rome. However, there are thesis subjects available on this project, both for the dottorato di ricerca (PhD) and for the laurea magistrale (MSc), also under the supervision of Prof. Marrucci. Contact Lorenzo Marrucci for getting more details about them.

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Last updated: 23-feb-2011