Recently it was reported that free-space propagating, ultrashort-pulsed polychromatic beams with orbital angular momentum (OAM) show a spectral Gouy rotation (SGR) of red- and blue-shifted areas around singularities. In femtosecond laser experiments with different types of spiral phase gratings, pulse propagation in spectral domain was studied with high resolution and sensitivity. By analyzing maps of spectral moments it was found that the interference of multiple OAM beams leads to a periodical revival of SGR by diffractive Talbot self-imaging. If the wavefront twist of the sub-beams is synchronized (co-rotating vortices), an optimum performance is found. In contrast, SGR echoes of counter-rotating beams are periodically distorted by destructive interference. Thus, the fine structure of self-imaged spectral maps enables to sort partial beams from interference patterns by even extremely weak imprinted vorticity information. It may further have implications for highly nonlinear processes and opens new prospects for applications in metrology, optical computing, or interferometry.
Spatially resolved spectroscopy of vortex beams is able to test the state of optical systems, to decode specific information or to sensitively indicate light-matter interactions. Spectral maps of ultrashort vortex pulses generated by hybrid diffractive-reflective spiral phase plates were studied experimentally and theoretically. Local spectral maps were detected by high-resolution scanning with a fiber-coupled spectrometer. Distributions of spectral centers of gravity and second moments were analyzed for femtosecond pulses. Gouy rotation of characteristic spectral features in the proximity of a phase singularity as a function of propagation distance was indicated in the spectral domain. Angular rotation was found to be modulated by weak oscillations. Analysis of spectral meta-moments indicates a fast switching and twisting behavior of spatial chirp.