Abstract
Walk-off effects, originating from the difference between the group and phase velocities, limit the efficiency of nonlinear optical interactions. While transverse walk-off can be eliminated by proper medium engineering, longitudinal walk-off is harder to avoid. In particular, ultrafast twin-beam generation via pulsed parametric down-conversion and four-wave mixing is only possible in short crystals or fibres. Here we show that in high-gain parametric down-conversion, one can overcome the destructive role of both effects and even turn them into useful tools for shaping the emission. In our experiment, one of the twin beams is emitted along the pump Poynting vector or its group velocity matches that of the pump. The result is markedly enhanced generation of both twin beams, with the simultaneous narrowing of angular and frequency spectrum. The effect will enable efficient generation of ultrafast twin photons and beams in cavities, waveguides and whispering-gallery mode resonators.
Highlights
Walk-off effects, originating from the difference between the group and phase velocities, limit the efficiency of nonlinear optical interactions
Since in nonlinear optical processes it is phase matching that determines the frequency and angular spectrum, interacting pulses are, in the general case, group mismatched and separate in both space and time in the course of propagation. This limits the length of nonlinear interactions for short pulses and focused beams[1,2]
In high-gain parametric down-conversion (PDC), down-converted radiation is exponentially amplified provided that it overlaps in space and time with the pump pulse
Summary
Walk-off effects, originating from the difference between the group and phase velocities, limit the efficiency of nonlinear optical interactions. Since in nonlinear optical processes it is phase matching that determines the frequency and angular spectrum, interacting pulses are, in the general case, group mismatched and separate in both space and time in the course of propagation. This limits the length of nonlinear interactions for short pulses and focused beams[1,2]. Twin-beam bright squeezed vacuum has perfect photon-number correlations between two beams, each of which taken separately has a very broad (thermal) photon-number distribution This non-classical state of light has interesting applications in quantum imaging[3,8] and sensing[9], phase super-resolution[10], macroscopic entanglement[11], enhanced nonlinear interactions and many other fields. The angular spectrum of each twin beam contains less than two modes
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