Borate Crystals for Nonlinear Optical and Laser Applications: A Review

e development of borate-based single crystals for laser and frequency conversion applications is reviewed.e basic idea behind nonlinear optics and the role of anionic groups in the borate crystals are summarized.e properties of borate crystals—BBO, LBO, CBO, KBBF, SBBO, CLBO, YCOB, GdCOB, GdYCOB, KAB and LCB—are discussed. e growth and characterization of several rare earth-based borate crystals are mainly focused. Several borate crystals are grown from the melt techniques and a few crystals are grown adopting the �ux technique. Many rare earth-based borate crystals are extensively used in device applications as they exhibit the frequency conversion ability along with high laser-induced damage tolerance.


Demand for UV and Visible
Radiations. e development of lasers has played a key role in the past �ve decades for the development of mankind in various �elds and to reach several technological advancements. e demand for laser beams in the ultraviolet and visible regions is growing enormously. e laser beams in the UV and visible regions �nd applications in several industries, medical surgeries, data storage, optical communication, and entertainment purposes. Advances in semiconductor photolithography, for example, are creating demand for 158 and 193 nm coherent light sources, while emerging micromachining and material-processing applications also need deep-UV laser radiation. In addition, research scientists would like a widely tunable coherent light source down to 200 nm for laser spectroscopy and photochemical synthesis.
Although excimer lasers can emit some isolated wavelengths of coherent light in the UV and deep-UV spectral region with a high average output power, compact and efficient solid-state lasers with nonlinear optical (NLO) crystals in this spectral region are still needed. Important solid-state bene�ts include narrow bandwidth, improved beam quality, tunability, and relative ease of handling. e performance of solid-state lasers in the UV and deep-UV spectral regions depends heavily on efficient NLO crystals, such as the borate deep-UV crystals that are being developed over the last two decades.

Emphasis of the Present
Review. e recent developments in NLO borate crystals for the generation of high power visible and ultraviolet laser radiations are reviewed. e basic principles behind nonlinear optical materials are also dealt with. e classi�cation of borate crystals based on the "anionic group theory" is discussed. Few important aspects involved in the growth of borate crystals and their properties are also reviewed and presented. e borate based NLO crystals are classi�ed into three categories.  [1]. A new �eld-nonlinear optics-came into being. Franken and his coworkers were the �rst to realize the nonlinear optical e�ect when they observed light at twice the frequency of a ruby laser ( = 693.4 nm) from a quartz crystal which was subjected to the ruby laser radiation. e practical observation of nonlinear optical phenomena would not have been observed if the lasers were not invented. is can be explained as follows. e �eld strengths of the conventional light sources used prior to the advent of lasers was of the order of 10 3 V/m. But the interatomic �eld strengths lie in the range between 10 7 and 10 10 V/m. Hence, the conventional light sources are very less intense to affect the atomic �elds to the extent of altering the optical parameters associated with it. e unique property of lasers, its coherence, helped in achieving radiations with the intensities of the order of 10 10 V/m feasible. Hence, lasers serve as useful and essential tool in the �eld of nonlinear optics and other related interesting phenomena such as Kerr effect, Pockels effect, second-or higher-harmonic generations, and so on.
e origin of NLO processes is the response of a nonlinear dielectric medium to an oscillating electric �eld. For example, when a dielectric material is subjected to an electromagnetic (e.m) radiation, the propagation of the e.m wave through the material produces changes in the spatial and temporal distribution of electric charges due to the interaction between the e.m wave, electrons, and atoms. is perturbation creates electric dipoles whose manifestation is macroscopic polarization (P). When the applied electric �eld (E) is very small, the induced polarization can be expressed as, where 0 is the permittivity of free space ( 0 = 8.854 × 10 −12 C 2 s 2 /kg m 3 ) and (1) is the linear susceptibility term which is responsible for the optical properties such as absorption, index of refraction, dispersion, and birefringence of the medium. When the applied electric �eld is highly intense and comparable to the interatomic �eld, the induced polarization is given by, where (2) , (3) are the second-and the third-order nonlinear susceptibilities and their magnitude decreases as their order increases ( (1) : (2) : (3) = 1 : 10 −8 : 10 −16 ). e second order susceptibility term ( (2) ) gives rise to second-harmonic generation, frequency mixing and parametric generation and the third order susceptibility term ( (3) ) is responsible for the third harmonic generation, Stimulated Raman scattering, optical bistability, and phase conjugation. For the purpose of reliable laser frequency conversion, NLO crystals with the following properties are desired: large NLO coefficient, moderate birefringence, small walk-off effect, large angular, spectral and temperature bandwidths, wide transparency in the wavelength of interest, high laserinduced damage threshold, ease of growth, low material cost, good chemical stability, and good mechanical stability.
Till 1975, extensive research on NLO crystals based on the P-O, I-O, and Nb-O bonds were carried out. e widely studied NLO crystals include KDP (KH 2 PO 4 ), LN (LiNbO 3 ), LiIO 3 , and so forth as they ful�ll the above listed requirements to a good extent. With the advent of potassium pentaborate, KB 5 O 8 4H 2 O, crystal in the year 1975, tremendous attention was directed to grow boratebased crystals for frequency conversion purposes.

Nonlinear
Optical Borate Crystals. e large family of borate compounds is a suitable chemical playground now adopted by many materials scientists, because the extremely wide variability of borate crystal chemistry allows the creation of various different structure types [2]. Furthermore, among all the borate structures reported till date, 36% are noncentrosymmetric, while among the reported inorganic crystal structures there are in total only 15% of noncentrosymmetic structure [3].
Borate crystals are superior in UV applications to other commonly used NLO materials such as potassium di hydrogen phosphate (KDP) or lithium niobate (LN) because of their high transmittance at wavelengths down to 155 nm combined with higher damage threshold. A comparison of selected NLO materials is given in the Table 1. e �rst borate crystal described for UV light generation was potassium pentaborate ( [4]. However, intense research work on borate crystals was initiated only aer the development of -BaB 2 O 4 (BBO) and LiB 3 O 5 (LBO). ere are several borate crystals available today which cater the need of optical industry [5][6][7][8][9][10].
e optical properties of borate crystals can be related to their molecular structure. A few borate crystals are mentioned in the Table 1 as examples. ese crystals are constructed from the three basic structure units: Figure  1. A model called the "anionic group theory" was designed by Chen and his coworkers and is used to understand the relation between composition, structure of borate materials, and the related NLO properties [11].
Due to the planar hexagonal structure of the (B 3 O 6 ) 3− anionic group, borate crystals constructed from this basic unit have greater (2) compared to crystals composed of (B 3 O 7 ) 5− and (BO 3 ) 3− anionic groups. us in terms of NLO coefficients, the (B 3 O 6 ) 3− group is the most suitable as the basic structure unit of NLO borate crystals followed by (B 3 O 7 ) 5− group and then (BO 3 ) 3− group. However, the UV absorption edge of the borate crystals constructed from the (B 3 O 6 ) 3− group occurred at a longer wavelength (e.g., BBO) when compared to those constructed from (B 3 O 7 ) 5− group (e.g., LBO, CBO, CLBO  3 with good optical quality were grown and reported [12]. Intense research on the incongruently melting borate crystals is also in progress. Due to the fact that a few incongruently melting borate materials are transparent down to vacuum-UV region, single crystal growth of these materials is undertaken. e family MBe 2 BO 3 F 2 , with M = Na, K, is an example of a structure type with isolated [BO 3 ] triangles, crystallizing in the noncentrosymmetric space group R32. e potassium compound KBe 2 BO 3 F 2 (KBBF) was �rst . ese crystals exhibit the hexagonal crystal system and were grown by the TSSG technique. ese crystals are grown by slow cooling with the cooling rate of 1-2 ∘ C/day. Crystals with the dimensions of 7 × 7 × 3 mm with good optical quality are obtained and reported. SBBO (Δn = 0.062 at = 589 nm) and KBBF crystals (Δn = 0.072 at = 589 nm) are attractive candidates in this aspect. However, because of the weak binding between the layered structural units, KBBF is difficult to grow and is mechanically fragile [13]. e SBBO has strong covalent bonds between beryllium atoms and oxygen atoms in adjacent layers. is makes SBBO mechanically stronger and relatively easy to grow compared to KBBF. However, beryllium is toxic, which makes crystal growth inconvenient. us, it is important that the Be atoms in SBBO be replaced by a nontoxic element. Sasaki and his coworkers had attempted to replace the (BeO ) − with (AlO ) − and had replaced Sr + with M + (M + = Li + , Na + , K + , Rb + and Cs + ) according to the concept of ionic compensation (Al + + K + → Be + + Sr + ). In this way, potassium aluminum borate crystal with the chemical formula K Al B O (KAB) was discovered. KAB has the spatial arrangement similar to that of SBBO.

KBBF, SBBO, and KAB
e structure of the KAB crystal is trigonal with the P321 space group. e lattice parameters of the crystal are a = b = 8.5657 Å, c = 8.463 Å, V = 537.7 Å , and Z = 3 [14]. e KAB crystals are grown from �ux technique. Different �uxes such as B O , K CO , and K CO -B O , alkali halides such as KF and NaF were used for the growth of KAB crystals and are reported. e KAB crystal is transparent from 180-3600 nm. e thermal property of the KAB crystal is also studied. e linear thermal expansion coefficient of the KAB crystal along the x, y, and z directions are very lesser. e speci�c heat values of KAB crystal at 47.6 ∘ C and 294.6 ∘ C are 1.0084 J/g ∘ C and 1.39 J/g ∘ C [15].

Rare-Earth-Based Borate Crystals.
Borate crystals are not only employed for frequency conversion applications but also as self-frequency doubling (SFD) active laser sources in the recent years. Rare-earth-based borate crystals are employed both for NLO and SFD applications.

Huntite Family
Crystals. e families of borate crystals with the general chemical formula, RAl (BO ) with R = Y, Nd, Sm, Eu, Tb, Dy, Er, and RX (BO ) with R = Gd, Sm, X = Cr, Al are termed as "huntite" family crystals. e emissions of sharp bright lines from these crystals were observed. ese crystals are chemically stable, nonhygroscopic, and have high hardness. ese crystals melt incongruently and are grown from �ux methods only. All these crystals are trigonal with the space group R32. e widely studied huntite-type borate crystal is YAl (BO ) (YAB) [16]. ese crystals are doped with Er, Nd, and Yb ions for making them as SFD crystals. It is commonly grown by top seeded solution growth (TSSG) method.
YAB is a noncentrosymmetric crystal and as early as in 1974, it was reported as a very effective second-harmonic generating material. Furthermore, owing to its good chemical stability and the possibility of substituting Y + ions with other lanthanide ions, namely Nd + , Yb + , and Er + , it is a good material for laser applications. e nonlinear optical properties of this material along with lasing properties led to the fabrication of numerous systems generating red, green, and blue lights due to self-frequency doubling effect [17]. ey also possess relatively large two-photon absorption. ese compounds are promising candidate as second-and third-order optical materials [18]. At the same time they are good matrices for different rare-earth ions [19,20]. Reports are also available on the Nd + -, Tb + -, Yb + -, and Er + -doped YAB crystals [21][22][23].
Further, in the family of borate crystals with huntite structure, research on crystals such as NdAl (BO ) (NAB), ErAl (BO ) (ErAB), or YbAl (BO ) (YbAB) is carried out in the present years [24]. e melting points of these materials are below 1300 ∘ C. ese crystals are grown by �ux techniques and are mainly employed in SFD purposes.  [25]. Later intense work on the growth and characterization of this family of crystals with various rare-earth elements such as La, Nd, Gd, Er, and Y were carried out and reported. e RECOB family of crystals appears to be attractive candidates for NLO applications as they possess the noncentrosymmetric structure which is an essential parameter for any NLO material. Widely studied RECOB crystals include YCOB, GdCOB, and LCOB crystals. e RECOB crystals with the rare-earth ions with electronic con�gurations 4f , 4 give rise to electronic transitions in the visible region that would interfere with the expected NLO properties.

Rare-Earth Calcium
e RECOB crystals melt congruently and were conventionally grown by the Czochralski and Bridgman techniques of crystal growth. e melting temperatures of the RECOB crystals increase with a decrease in the ionic radii of the rareearth ion present in it [26]. Accordingly, the melting points of LCOB, GdCOB, and YCOB single crystals are 1410, 1480, and 1510 ∘ C, respectively [27].
e RECOB single crystals exhibit the monoclinic crystal structure with the C space group. ey are biaxial crystals. ese crystals offer the advantage in providing suitable sites for doping them with "laser-active" ions, since the widely used "laser-active" ions such as neodymium, erbium, and ytterbium have similar ionic radii and occur in the trivalent state as that of the rare-earth ion present in the RECOB crystals [28,29]. e �ux growth of yttrium calcium oxy borate (YCOB) single crystals by �ux technique was also carried out by our group earlier and reported [30]. e growth and characterization of pure and Nd 3+ present lanthanum calcium oxy borate (LCOB) were performed and reported [31]. A comparison between YCOB, LCOB, and GdCOB crystals are summarized in Table 2. 3.6. Rare Earth Calcium Borate (RCB) Crystals. Another emerging borate-based crystal family is rare-earth calcium borate (RCB) crystals, with the general chemical formula R 2 CaB O 9 (R represents rare-earth element). In this series of crystals, only crystal growth of pure and doped lanthanum calcium borate (LCB) are performed and reported in literature. ere are no reports available on any other materials in the RCB family of crystals. e LCB crystal is reported to be insensitive to moisture, has high hardness (6.5 mhos), and is transparent from 180 nm to 3300 nm. Moreover, the laser damage threshold (LDT) value of the LCB crystal is also very high (11.5 GW/cm 2 for 1064 nm, 8 ns radiation) [32]. Various reports on "laser-active-" ion-doped LCB crystals are also available in literature [33].

Conclusion
Growth and material characteristics of several borate-based single crystals were discussed. Borate crystals are grown from melt and �ux techniques. Anionic group theory plays an essential theory for selecting borate materials for nonlinear optical applications. Several borate-based crystals act as desirable host materials for fabricating lasers. Borate crystals offer themselves as suitable candidates for both nonlinear optical and laser applications.