Materials for nonlinear optics

A team of scientists and engineers headed by V. I. Katzman and V. I. Bespalov was established in IAP RAS more than 30 years back, one of major results of which was a unique technique of growing large-aperture nonlinear optical elements from water-soluble crystals (KDP, α-LiIO3 and their deuterated isomorphs), including rapid growth of profiled large-aperture crystals and their precision finishing by the method of single-point diamond turning. Currently V. V. Lozhkarev heads this department.
Iinterest in this technology has markedly increased in the recent years with the advent of new generations of lasers with high pulse energy, ultrahigh peak power, and ultrashort pulses, including:
• megajoule laser systems for thermonuclear and hybrid power engineering consisting of a great number of laser channels with pulse energy of 1—10 kJ (the USA, France, UK, Japan, China and Russia);
• multiterawatt and petawatt lasers for advanced medical and research applications;
• femtosecond laser systems for technological, medical and research applications.
Construction of giant megajoule laser complexes for laser-driven fusion is currently under way in the USA (National Ignition Facility), France (Megajoule Facility), Japan (Gekko) and in other countries. In Russia, the "UFL-2M" project was started in RFNC – VNIIEF in 2012. Each facility of this class comprises several hundred elements made of nonlinear optical KDP crystals with an aperture of 40 × 40 cm.
Physical foundations and the technique of growing large KDP crystals with growth rate ten times that of the traditional technology (currently the growth rate is up to 2 cm per day) were developed at IAP RAS. The technique enables growing monosector crystal billets of needed orientation, making production almost wasteless and ensuring crystals of high optical quality. The combination of the properties of this crystalline material makes it hard to machine.
A KDP crystal is a soft and extremely brittle material (inferior to glasses in brittleness and cracking) with high thermal expansion coefficient and low thermal conductivity. It is hygroscopic and has a pronounced anisotropy of the mechanical elasticity tensor. Optical finishing of KDP elements is performed by single-point diamond turning of the crystal surface by a diamond tool fixed in a rotating spindle of an ultraprecision machine.

Room for growing crystals
in crystallizers of different size

Area of optical processing of KDP crystals —
single-point diamond turning machines

KDP crystals are also used in laser complexes with ultrahigh peak power for parametric amplification of pulses. They are key optical components that ensure a record high multipetawatt power level. Such projects are currently implemented in Europe (10 PW VULCAN, ELI) and in Russia (PEARL-10, IAP RAS). Nonlinear optical elements made of DKDP crystals of high optical quality (produced at IAP RAS) are used as parametric amplifiers of broadband radiation of femtosecond lasers, as pump second harmonic generators, phase-inverting plates, and birefringent wedges.
Studies aimed at improving the temporal contrast of output radiation of petawatt femtosecond laser complexes have become highly topical. Ultrathin large-aperture KDP elements for frequency conversion and contrast enhancement of laser pulses are in high demand in all femtosecond laser systems of multiterawatt power level the number of which is several dozen worldwide.

450 l crystallizer for growing 46 × 46 × 6 cm crystals
Linear absorption index of a DKDP sample (97.2%)
1.9 cm thick

Precision finishing of crystal elements is of primary importance for obtaining elements with enormous difference of geometric dimensions (aspect ratio 200:1 and more), which requires the development of the special technology.
Recently, highly deuterated KDP crystals are in high demand in the world market of laser elements needed for electro-optic shutters of high average power lasers. A technique for rapid growth of such crystals with the degree of deuteration over 97% and low absorption at a wavelength of 1054 nm was developed at IAP RAS.

Photo and shadowgraph of a DKDP crystal 160×130×130 mm in size