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Updated: May 17, 2026

Direct Imaging of Laser-driven Ultrafast Molecular Rotation
Published on: February 4, 2017
Observation of a 2-µm flat-top beam carrying orbital angular momentum by intracavity incoherent superposition
Abstract:
Flat-top beams exhibit symmetrical intensity and steep edge in cross-section with wide application. Especially, orbital angular momentum (OAM) endows it with the potential to achieve uniform angular momentum transfer or phase encoding. In this Letter, we observed a flat-top beam carrying +ℏ OAM by intracavity incoherent superposition in an X-cavity Tm: YAG ceramic laser. The corresponding M2 factors are 2.24 and 2.08 with a typical output power of 470 mW around ~2011.2 nm. Simultaneously, a radially polarized vortex beam (RPVB) with +ℏ OAM was observed. Further simulations and experiments revealed that a flat-top beam is formed by the incoherent superposition of RPVB and a Gaussian beam. Using a homemade M-Z interferometer, we observed vortex stripes' chirality reversal before and after the focal plane of the convex lens. To the best of our knowledge, this is the first observation of a flat-top beam with OAM at 2 µm wavelength.
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