I Why is BBO the most used crystal for second-order nonlinear optics?

[James2018]

TL;DR

There are many nonlinear crystals that can be used for entangled photon generation and other second-order nonlinear frequency-mixing processes

In this software, SNLO https://as-photonics.com/products/snlo/, http://www.spdcalc.org/, you can see that Beta Barium Borate is not the only crystal capable of spontaneous parametric down-conversion, second-harmonic generation and sum frequency mixing and difference frequency mixing. In fact crystals like ammonium dihydrogen phosphate, potassium dihydrogen phosphate, lithium iodate https://opg.optica.org/oe/fulltext.cfm?uri=oe-19-11-10351&id=213837 and even urea crystals https://patents.google.com/patent/US4639923A/en are some crystals that can be used for SHG, OPO, SFG and DFG nonlinear processes.

Yet beta barium borate is the only crystal that is the most widely used and receives most attention. Can anyone explain to me why?

 

[berkeman]

You will likely get better answers from others, but since there were no replies yet, I did a Google search on your thread title, and this was the first hit on the list:

https://optics.org/products/P000022372

β-BBO Nonlinear Crystal—a widely used nonlinear crystal for frequency conversion in the ultraviolet, visible and near-infrared

As one of the most important nonlinear optical crystals, beta-barium borate (β-BaB2O4，β-BBO) combines many outstanding features such as its high nonlinear optical coefficients, low group-velocity dispersion, broad transparency range (189–3500 nm) and high damage threshold. This unique combination ensures β-BBO crystal a promising candidate for a wide range of nonlinear optical applications such as frequency converters and optical parametric oscillators. In the realm of quantum optics, β-BBO crystal can be used to generate entangled photon pairs and ten-photon entanglement.

BBO is a negative uniaxial crystal, which provides phase matching for various second-order interactions almost over its entire transparency range (from 185 nm to 3.3 µm, as deduced from the transmittance measurements using crystal samples of several mm thickness), making it a widely used crystal for nonlinear frequency conversion in the ultraviolet, visible and near-infrared. In that regard, BBO is the most important nonlinear crystal for near infrared optical parametric chirped pulse amplifiers, which currently deliver few optical cycle pulses with high average and ultrahigh peak powers.