Abstract
Fractional vortex lasers with a fractional topological charge, l, exhibiting a unique fractional radial opening, have been attractive for the optical manipulating of microscopic particles and super-resolution spectroscopy based on stimulated emission depletion. They have also recieved much attention in fundamental physics, including an analogy concerning the quantum flux in the Aharonov–Bohm effect. Such applications of the fractional vortex lasers frequently require wavelength diversity. In particular, tunable fractional vortex lasers in a mid-infrared region, including the eigen frequencies of many molecules originating by the vibration modes, will potentially be attractive for manipulating molecules and molecular spectroscopy with high spatial resolution. Second-order nonlinear frequnecy conversion is a promising method to extend the wavelength range of laser sources. However, the conservation of topological charge, l, in an optical parametric down-conversion process causes a question, i.e., how does the orbital angular momentum of the pump beam divide between the signal and idler outputs? To date, we have demonstrated 2μm fractional vortex output with a topological charge l=0.5 from a 1μm vortex pumped optical parametric oscillator (2μm fractional vortex OPO) formed by a plane-parallel cavity configuration and a nonlinear crystal KTP [1]. (Such the fractional optical vortex operation is never permitted in a conventional stable laser cavity [2].) However, the pulse energy (<0.5mJ) of the fractional vortex output is limited by severe diffraction loss in the plane-parallel cavity configuration. Also, the lasing wavelength of the fractional vortex output was fixed at 2.128μm.
© 2013 IEEE
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