K2Al2B2O7 Research Articles

Structure and Optical Properties of K0.67Rb1.33Al2B2O7 Crystal

A UV nonlinear optical (NLO) crystal K0.67Rb1.33Al2B2O7 (KRABO) has been obtained by the top-seeded solution growth method for the first time. Its structure is formed of coplanar [BO3]3− triangle groups and [Al2O7]8− groups, which are built from two [AlO4]5− tetrahedra sharing one vertex. Rb and K atoms have the same coordinates and locate between each layer in a disorderly fashion. The crystal has a larger NLO effect compared with its analog K2Al2B2O7 (KABO), as evidenced by powder second harmonic generation (SHG) test. The shortest SHG phase-matching wavelength is down to 231 nm according to the first-principle calculation, which indicates that KRABO is possible for the fourth harmonic generation of Nd:YAG at 266 nm.

Spectroscopic study of red-light-emitting centers in K2Al2B2O7: Fe single crystals

We report on spectroscopic study of red-light-emitting centers in K2Al2B2O7 (KABO) single crystals containing ca. 2ppm of Fe3+. Owing to the low Fe3+-concentration, KABO does not show noticeable absorption due to Fe3+d–d-transitions in the visible spectral region, but it exhibits the charge-transfer (CT) UV-absorption bands O–Fe at 4.7, 5.7 and 6.5eV. The red photoluminescence at 1.675eV (FWHM=0.173eV) is due to intracenter 4T1 (4G)→6A1 (6S) transitions in Fe3+ ions. Because of partial overlapping of the fundamental absorption edge of the crystal, where mobile excitons are created, and a broad CT absorption band at 6.5eV, the most intensive red emission occurs at 7K upon excitation in the excitonic energy region. The presence of two nonequivalent Al2O7 clusters in KABO lattice provides two different types of red-light-emitting centers in the form of Fe3+ ion occupied the Al3+ tetrahedral site. Superposition of their luminescence bands determines both the spectrum and temperature dependence of red emission in KABO at T=7–80K: two bands with the ratio of intensities of ca. 2:1 are 20meV-shifted relative to each other; two-stage thermal quenching obeys the Mott law with ET=9 and 20meV.

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K2Al2B2O7 (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21eV) measured at 7K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5eV were detected and assigned to charge-transfer absorption from O2− to Fe3+ ions; the intrinsic PL band at 3.28eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54eV were separated; the strong PL band at 1.72eV was revealed and attributed to a radiative transition in Fe3+ ion.

Luminescence and electronic excitations in crystals K2Al2B2O7 with defects

This paper reports on the results of the comprehensive study of the dynamics of electronic excitations in K2Al2B2O7 (KABO) crystals, obtained by low-temperature luminescence vacuum ultraviolet spectroscopy with nanosecond time resolution upon photoexcitation by synchrotron radiation. For the first time, the data have been obtained on the photoluminescence (PL) decay kinetics, PL spectra with time resolution, PL excitation spectra with time resolution, and reflection spectra at 7 K; the intrinsic nature of PL at 3.28 eV has been established; luminescence bands of defects have been separated in the visible and ultraviolet spectral regions; an intense long-wavelength PL band has been detected at 1.72 eV; channels of the formation and decay of electronic excitations in K2Al2B2O7 crystals have been discussed.

Crystal growth and optical properties of non-UV absorption K2Al2B2O7 crystals

In this paper, large and transparent K2Al2B2O7 (KABO) crystals were grown using NaF as a flux in a non-oxygen atmosphere for the first time. Through optimization of the growth condition, bulk KABO crystal has been obtained with good quality. The transmittance spectrum shows that the abnormal UV absorption from 200 to 300nm of KABO crystals could be effectively eliminated by this new method. The non-UV absorption KABO crystal is promising for the application of fourth harmonic generation of Nd:YAG lasers. A 280mW average output power was obtained through fourth harmonic generation of Nd:YAG laser with 10kHz, 10ns pulse and a 8.85-mm-thick KABO crystal. The conversion efficiency up to 33.4% of second harmonic generation from 532 to 266nm has also been achieved with a 25ps, 10Hz laser system and the same crystal. To our knowledge, this is the highest result at present.

Thermophysical Properties of a New Nonlinear Optical Na3La9O3(BO3)8 Crystal

New nonlinear crystal Na3La9O3(BO3)8 (NLBO) with good optical quality and weighing 45 g with size of 38 × 30 × 20 mm3 has been grown by the optimized top-seeded solution growth (TSSG) method. Thermophysical properties including melting point, specific heat, thermal expansion, thermal diffusion, and hardness were performed on the crystal to determine thermal behaviors along different crystallographic axes. The specific heat was measured to be 0.45−0.70 J·g−1·K−1 in the temperature range from 20 to 300 °C, which is lower than that of β-BaB2O4(BBO), LiB3O5 (LBO), and CsLiB6O10 (CLBO). The Vickers hardness were measured to be 1052.67 kg/mm2 for (100) and 1239.67 kg/mm2 for (001) faces, which are greater than most of the commonly used borate crystals such as BBO, LBO, CsB3O5 (CBO), BiB3O6 (BIBO), KBe2(BO3)F2 (KBBF), K2Al2B2O7 (KABO), etc. The thermal expansion coefficients were measured to be αx = αy = 7.8 × 10−6 K−1, and αz = 15.3 × 10−6 K−1, resulting in weaker expansion anisotropy than those of BBO, LBO, BIBO, CBO, CLBO, KBBF, La2CaB10O19 (LCB), etc.

Nonlinear optical properties of some newly developed crystals for measurement of ultrafast laser pulses

Second harmonic generation (SHG) in nonlinear optical crystal is well-established technique for the measurement of the pulse-width of ultrafast femtosecond laser pulses. However, only selected nonlinear crystalline materials are suited for this purpose due to the limitations in transparency cut-off, limited phase-matching wavelength range, and large difference between the group velocities of the fundamental and harmonic waves. It is found that some newly discovered crystals, such as CsLiB6O10 (CLBO), K2Al2B2O7 (KABO), Li2B4O7 (LB4), and KBe2BO3F (KBBF) have some advantageous characteristics for use in ultrafast nonlinear optical applications. Here we have presented several linear and nonlinear optical parameters including phase-matching angle, angular and spectral acceptance bandwidths, walk-off angle, nonlinear coupling coefficient, and group-velocity mismatch for generation of second harmonic of ultrafast laser radiations by employing type-I and type-II SHG techniques in these crystals.

(K1-xNax)2Al2B2O7 with 0 ≤ x < 0.6: A Promising Nonlinear Optical Crystal

Up to about 60% K+ in K2Al2B2O7 (KABO), a well-known nonlinear optical material, can be substituted by Na+ with the structure type being retained. AlO4 tetrahedra and BO3 triangles in the Na-substituted compound tilt to adjust to the local distribution of alkali-metal atoms, which explains the large range of homogeneity. When the Na+ substitution is higher than 50%, the decomposition and volatility of KABO are strongly inhibited and the sample melts congruently at about 940 °C, which makes it possible to grow bulk crystals from the melt directly. The second harmonic generation property of KNaAl2B2O7 is close to that of KABO. Taking the crystal growth into account, (K1-xNax)2Al2B2O7 (0 ≤ x < 0.6) seems to be a more promising nonlinear optical material than KABO.

The development of new borate-based UV nonlinear optical crystals

The developments of nonlinear optical (NLO) crystals in the last decade are reviewed from two aspects. In the theoretical part, on the ab initio plane-wave pseudopotential energy method, calculation of the optical responses, including the second harmonic generation coefficients, a computer-program package, and a real-space atom-cutting method are described and employed to evaluate the degree of approximation of the anionic group theory for nonlinear optical crystals. The ab initio method combined with the anionic group theory gives much more insight into the understanding of the relationship between the optical responses and microscopic structures of NLO crystals, borate-based NLO crystals in particular. In the experimental part, we describe how to use the anionic group theory method and an experimental evaluation system to develop a series of borate-based UV-NLO crystals such as KBe2BO3F2 (KBBF) and K2Al2B2O7 (KABO). The capability of these new UV-NLO crystals in producing deep-UV harmonic output is evaluated and determined. Finally, recent advances in the generation of deep- and vacuum-UV harmonic output with these new NLO crystals are reported.

Ultrabroad-band phase matching in two recently grown nonlinear optical crystals for the generation of tunable ultrafast laser radiation by type-I noncollinear optical parametric amplification

It was found that the generation of ultrabroad-band femtosecond pulsed laser radiation is possible in two recently grown nonlinear optical crystals, K2Al2B2O7 (KABO) and CsLiB6O10 (CLBO), by employing a type-I noncollinear optical parametric amplification (NOPA) scheme, pumped by Ti:sapphire second harmonic (395 nm) radiation. The phase-matching bandwidths of the process are as large as 152 and 164 THz in KABO and CLBO crystals, with the values of the noncollinear angles (α) between pump input (considered monochromatic) and seed pulses input are 3.3° and 3.0°, respectively. The phase-matching bandwidth calculated for the same type-I NOPA in BBO crystal is 157 THz with α=3.7°. In addition to the large phase-matching bandwidth in CLBO, the major advantages of this crystal are that it can easily be grown to large sizes and it also has a higher threshold against laser damage. The most attractive properties of KABO crystal are that it is nonhygroscopic as well as mechanically robust and easy to cut and polish. Having moderate birefringence the angular and spectral bandwidths of both CLBO and KABO crystals are larger than that of BBO crystal and the deleterious walk-off angles are smaller than that of BBO. The walk-off angles at the pump wavelength of NOPA with CLBO, KABO, and BBO crystals are about 2.1°, 2.7°, and 4.0°, respectively. Here we present the nonlinear optical parametric properties of CLBO and KABO crystals for the generation of ultrabroad band femtosecond laser radiation by the NOPA technique, since we know of no such report so far in the literature.

Growth and properties of K2Al2B2O7 crystal

The flux systems for the growth of crystalline K2Al2B2O7 (KABO) are discussed. KABO can be grown from both KF and NaF flux by a top-seeded growth method. It is easier to grow the crystal in NaF flux than to do so in KF flux. Seed directions influence the quality and morphology of the KABO crystal. The average thermal expansion coefficients are experimentally determined to be 8.4×10−6/K, 1.65×10−5/K along a-axis and c-axis, respectively. The specific heat was measured over a range of temperature 30–295 °C, which is double that of crystalline BBO. Some other data on the chemical and physical properties of KABO are also presented.

Recent development of nonlinear optical borate crystals: key materials for generation of visible and UV light

Abstract The development of nonlinear optical (NLO) borate crystals for generation of visible and UV light is reviewed. We first discussed on the basic principles of laser frequency conversion. Then, we examine the trends in research on NLO crystals. The background and present status of NLO borate crystals are summarized. The main considerations are focused on the discussion of crystals like CsLiB6O10 (CLBO), GdxY1−xCa4O(BO3)3 (GdYCOB) and K2Al2B2O7 (KAB). Properties of related materials like β-BaB2O2 (BBO), LiB3O5 (LBO), KBe2BO3F2 (KBBF), Sr2Be2BO7 (SBBO), CsB3O5 (CBO), GdCa4O(BO3)3 (GdCOB) and YCa4O(BO3)3 (YCOB) are included for comparison. We aim to provide a complete view of developing a new NLO borate material for actual laser applications. This review covers various aspects including the search for new materials, the growth of bulk crystals, the characterization of crystal properties as well as the development of new techniques to overcome obstacles in actual laser application, namely, thermal dephasing and laser-induced damage. Finally, perspectives on NLO borate crystals and all-solid-state UV lasers are evaluated.