Ultraviolet enhancement cavity for ultrafast nonlinear optics and high-rate multiphoton entanglement experiments

Ultrafast, ultraviolet light pulses are a key tool for spectroscopic studies (for example, molecular formation(1,2) and carrier dynamics in semiconductors(3)) as well as a source for non-classical states of light(4-13). The power required for many nonlinear processes makes amplifier systems mandatory, which significantly reduces the available repetition rate and thus often lengthens the experimental acquisition time. Here we adopt techniques recently developed for the infrared regime(14-16) to design the first enhancement cavity for femtosecond ultraviolet pulses. An average ultraviolet power of more than 7 W at a repetition rate of 81 MHz is now available to pump a nonlinear crystal inside the cavity, applied here to implement a powerful source for high-rate experiments with entangled multiphoton states. The field enhancement enables a new scale of experiments in photonic quantum logic and in nonlinear optics research, for example, to operate optical parametric amplifiers at high repetition rates or to create high-harmonic-frequency combs(14-16).