Subharmonics in BSO

In 1988, Mallick et al.[1] found self-generated subharmonics in Bi12SiO20 (BSO). They used two coherent almost linear pump beams (wave vectors k→ p and k→ p ′ , see Fig. 1) with grating vector K→ and frequency detuning Ω, impinging on a 1 cm thick BSO crystal. For a certain frequency detuning, they observed a third beam in the middle between the two pump beams with intensity as high as 40% of the pump beam intensity. The length of the grating vector for the grating written by one of the pump beams and the third beam was approximately K/2 and therefore they called the new beam a K/2-subharmonic.