Higher order mode entanglement in a type II optical parametric oscillator.

Nonclassical beams in high order spatial modes have attracted much interest but they exhibit much less squeezing and entanglement than the fundamental spatial modes, limiting their applications. We experimentally demonstrate the relation between pump modes and entanglement of first-order Hermite Gauss modes (HG₁₀ entangled states) in a type II OPO and show that the maximum entanglement of high order spatial modes can be obtained by optimizing the pump spatial mode. To our knowledge, this is the first time to report this. Utilizing the optimal pump mode, the HG₁₀ mode threshold can be reached easily without HG₀₀ oscillation and HG₁₀ entanglement is enhanced by 53.5% over HG₀₀ pumping. The technique is broadly applicable to entanglement generation in high order modes.