Time diffraction of optical helicity
If in a classic double-slit interference experiment the light that passes though one slit is in an orthogonal polarisation state to that of the second slit, no visible interference fringes emerge. Interference fringes are instead present in a different observable that relates to the difference in energy carried by two orthogonal polarisation components, causing spatially varying polarisation patterns: the optical helicity density in the case of linearly polarised fields. Here, we develop a generalised treatment of interference in space and time, and experimentally demonstrate the helicity patterns produced by interference of orthogonally polarised pulses, which would be produced by two temporal slits in time diffraction observations. The developed approach may be important for understanding and applications of complex polarisation patterns and polarisation effects in time varying media.