Tunable milli-joule-level 2μm fractional vortex optical parametric amplifier

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 2μπι fractional vortex output with a topological charge l=0.5 from a 1μm vortex pumped optical parametric oscillator (2μπι 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.