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Walk-off effects, originating from the difference between the group and phasevelocities, limit the efficiency of nonlinear optical interactions. Whiletransverse walk-off can be eliminated by proper medium engineering,longitudinal walk-off is harder to avoid. In particular, ultrafast twin- beamgeneration via pulsed parametric down-conversion (PDC) and four-wave mixing(FWM) is only possible in short crystals or fibres or in double-path schemes.Here we show that in high-gain PDC, one can overcome the destructive role ofboth effects and even turn them into useful tools for shaping the emission. Inour experiment, one of the twin beams is emitted along the pump Poynting vectoror its group velocity matches that of the pump. The result is dramaticallyenhanced generation of both twin beams, with the simultaneous narrowing ofangular and frequency spectrum. The effect will enable efficient generation ofultrafast twin photons and beams in cavities, waveguides, andwhispering-gallery mode resonators.

Giant Narrowband Twin-Beam Generation along the Pump Energy Propagation