The discovery that photons in optical vortices—light beams with helical phase fronts and an azimuthal component of the wave vector—can carry orbital angular momentum may lead to wide-ranging applications in optical microscopy, micromanipulation, free-space communication, and quantum information. Techniques for generating optical vortices involve passing free-space light beams through optical elements, including computergenerated holograms, spiral phase plates, inhomogeneous birefringent elements, subwavelength gratings, and nanoantennas.
A key issue for many of the above applications is not just the ability to create optical sources that can control the optical angular momentum of light but also the ability to integrated such optical vortex emitters into photonic integrated circuits. At Glasgow we have been pioneering many new devices using standard silicon processing to produce Si photonic integrated circuits. Recent work with the University of Bristol has demonstrated the use of Si microrings with angular gratings were used to extract light confined to the whispering gallery modes into free space, thereby creating an emitter of optical vortex beams with well defined optical angular momentum. The work has been published in the October edition of Science.
The devices were fabricated in the JWNC cleanroom at Glasgow using electron beam lithography and reactive ion etching to produce the silicon microrings coupled to a silicon waveguide. Previously we have demonstrated that such fabrication techniques have a process variability less than 0.6 nm over distances of centimetres on the silicon chips. This process control is essential to allow high performance Si photonics devices to be produced.