An overview of nonlinear material for use in the U

As we have described in the Introduction, we are interested in nonlinear materials that are useful for optical parametric oscillators pumped at wavelengths equal or shorter than 355 nm. This restricts the available nonlinear optical materials to the following; lithium triborate (LB0)[6], barium borate (BB0)[7], urea[8], lithium iodate (LiIC>3)[9], deuterated L-arginine phosphate (d-LAP)[10], potassium dihydrogen phosphate (KDP)[11], and its isomophs. Unfortunately some excellent nonlinear crystals such as KTP[12], KNb03[13] are not included due to limitation in transparency and potential for phase matching. The transmission cutoff wavelength for KTP and KNbOs are 350

nm and 400 nm respectively. It is appropriate to briefly mention other possible materials, such as potassium penta borate tetrahydrate (KB5)[14] and BeS04‘4H20[15]. About 10 years ago, KB5 was the only material used for generating

vacuum UV (VUV 100 nm - 200 nm) coherent radiation by sum frequency mixing.

However, because of its small nonlinear coefficient (dji = 0.046 pm/V) it has recently been completely replaced by BBO and LBO. The nonlinear optical crystal BeS04*4H20 was developed in the 1970s, and more recently has been investigated and mentioned by Kato again last year, for frequency doubling from 532 nm to 266 nm[16]. BeS04*4H20 has not been well characterised, but due to its small nonlinear coefficient (d36 = 0.24 ± 0.25 pm/V) it has attracted little interest.

KDP:

Potassium dihydrogen phosphate (KDP)[11] is a negative uniaxial crystal, belonging to tetragonal system with point group 42m. It has found wide-spread use in the frequency doubling of ultra-high power 1 p,m lasers (as used in plasma fusion experiments), as it is one of the few nonlinear crystal that can be grown with a large enough aperture size. While KDP does possess a high damage threshold (8.4 GW/cm^ for 1.3 ns pulses at 1.053 jim[17]), it has a nonlinear coefficient of only 0.39 pm/V. Furthermore, it can only transmit wavelengths in the range 0.2-1.5 jj,m. Therefore the

CHAPTER 3 Nonlinear optical rmterials

tuning range of a parametric oscillator based on this material is very limited in the

infrared part of the spectrum. This is the reason that while parametric oscillation has been demonstrated using this material, its use has not been wide-spread. KDP shortest SHG phase matchable wavelength is 0.259 p.m, and the shortest SFM phase matchable wavelength is 0.218 jam.

LilOs:

Lithium iodate (LiI03)[9] is a negative uniaxial crystal belonging to the hexagonal system with symmetry point group 6. The transparency range in this material is 0.3- 5.5jam and the nonlinear coefficient at dsi is eleven times that of d36 of KDP (d3i(LiI03) = 4.1 ± 0.2 pm/V). Unfortunately, the optical damage threshold of LiI03

is relatively low, being about 0.5 GW/cm^. Also the absorption in lithium iodate crystals is generally high, of the order of 10 %/cm in the UV. Metal impurities, especially iron, increase this absorption considerably even when present in concentrations of a few ppm. High birefringence and large walkoff angle are another two features. Its use an OPO gain medium is marginal, with optical damage generally occurring close to oscillation threshold.

Urea:

Urea[8] is a positive uniaxial crystal of point group 42m. It has a nonlinear coefficient of 1.4 pm/V, about 4 times that of KDP and a high damage threshold of up to 5 GW/cm^ (1.3 ns pulses at 1.064 pm, single shot). Urea has high transparency in the 200 nm to 1.4 pm band and is consequently of use in the ultraviolet, it e.g. can be used to frequency quadruple the output of a NdrYAG laser and to pump optical parametric oscillators tunable in the visible spectrum band. Until barium borate became

available, it was the dominant material for this use.

d-LAP:

d-LAP[10] is a potentially promising nonlinear optical material. It is easily obtainable in large sizes (70 x 70 x 40 mm^) and has a relatively high nonlinear coefficient (dppp = 0.92 pm/V) and high optical damage threshold. The damage threshold of this material was found to be twice as high as for the best quality commercially available KDP crystals. Its use in optical parametric oscillators is also limited by its transparency range, which is only from 250 nm to 1.3 pm. High absorption in the UV is another problem, being 4 %/cm and 12 %/cm at 355 nm and 266 nm respectively.

BBO:

Barium borate (BB0)[7] is a recently developed material that is of great utility in the ultraviolet, as it transmits wavelengths down to 190 nm. However, BBO is also of use in the near infrared, as it can transmit wavelengths as long as 3.5 pm. BBO has a large

nonlinear coefficient (dll = 2.2 pm/V[18]) comparing well with the other materials that transmit into the ultraviolet BBO possesses a high damage threshold of 10 GW/cm^. It

is fair to say that the direct doubling of wavelengths shorter than 630 nm is now almost exclusively achieved using BBO crystals. The major disadvantage with BBO is the

large walkoff over the majority of its tuning range, making it unusable for low power

applications. BBO's shortest SHG phase matchable wavelength is 205 nm, and the

shortest SFM phase matchable wavelength is 190 nm.

One property that has not been mentioned so far is resistance to atmospheric water vapour. Urea, lithium iodate, KDP and its isomophs, are highly hygroscopic and must therefore either be permanently heated, or mounted in index matching cells. BBO is slightly hygroscopic and therefore should not be left exposed to the atmosphere for any

appreciable length of time. If possible the crystal should be maintained at a temperature

slightly above room temperature to prevent condensation forming on the crystai[5]. LBO:

LBO is a recently newly developed nonlinear material[6]. It has a high transmission in the range from 160 nm to 2.6 pm, and a relatively high nonlinear coefficient (about 3 times that of dg^ of KDP). It does however, possess a remarkably high damage threshold for Q-switched pulse (18 GW/cm^), and can be phase matched in all three

principal planes. It has a sufficient thermal variation of the birefringence to enable it to be temperature tuned to achieve non-critical phase matching. This along with the high

damage threshold allows a high conversion efficiency to be achieved by tightly focusing the pump wave despite the small nonlinear coefficient. As it has, unlike BBO,

appropriate birefringence with corresponding small walkoff angle and large angular acceptance band width, it is the only material suitable for an OPO tunable in the UV, visible and near infrared when pumped with modest UV powers.