Self-frequency Doubling Crystal Research Articles

Helix-constructed polar rare-earth iodate fluoride as a laser nonlinear optical multifunctional material.

The first trivalent rare-earth iodate fluoride nonlinear optical (NLO) crystal, Y(IO3)2F (YIF), was successfully designed and synthesized, featuring polarization-favorable helical chains constructed from trans-YO6F2 polyhedra and IO3 groups. It exhibited a suitable balance of a wide transparency range of 0.26-10.0 μm, high laser damage threshold (LDT) of 39.6 × AgGaS2, and moderate second harmonic generation (SHG) effect of 2 × KDP. A series of doped RE:YIF (RE = Pr, Nd, Dy, Ho, Er, Tm, and Yb) crystals were easily synthesized benefiting from the spring-shaped helix structure, which possess wide absorption and emission peaks as well as long lifetime, especially in the visible and near-infrared regions. Particularly, the remarkable fluorescence properties of Nd and Yb doped YIF crystals are comparable to and even better than those of traditional self-frequency doubling (SFD) crystals such as YAB, YCOB, and GdCOB. Thus, these RE-doped YIF crystals are promising laser SFD crystals. This work also indicated that constructing helical chains should be an effective strategy for the design of inorganic polar materials.

Three-wavelength green generation in Nd:GdCOB slab crystal based on self-frequency conversion

We report on a multi-self-frequency conversion in one Nd3+-doped GdCa4O(BO3)3 (Nd:GdCOB) slab crystal, including self-sum-frequency (SSF) at 538 nm and self-frequency doubling (SFD) at 530 and 545 nm from two fundamental infrared wavelengths at 1060 and 1091 nm for the first time. The total output power of the three green lasers was up to 1.7 W with a 0.9 W SSF output at 538 nm. To our knowledge, this is the highest total green output power and the highest SSF power at 538 nm via the multi-self-frequency conversion process in SFD crystal. Moreover, a theoretical simulation of the temperature distribution in the slab by considering the pump power induced thermal effect was implemented, leading a deep insight into the physical mechanism of multi-wavelength generation.

Polarized spectroscopic properties of a potential self-frequency doubling crystal, Nd 3+:BaCaBO 3F

In this paper, the polarized absorption and fluorescence spectra of Nd 3+ ion in the BaCaBO 3F non-linear optical crystal have been investigated at room temperature. The Judd–Ofelt theory, extended to anisotropic system, has been applied to calculate the phenomenological intensity parameters, spontaneous transition probabilities, branching ratios and radiative lifetimes. The maximum emission cross-sections have been calculated to be 7.22 × 10 −20 cm 2 and 15.21 × 10 −20 cm 2 at 1068 nm for π- and σ-polarization using the Füchtbauer–Ladenburg (F–L) equation. The fluorescence lifetime of 4F 3/2 manifold is equal to 57.7 μs, and the quantum efficiency is 26.8%, which is higher than those of most of other borates. The potential of Nd 3+:BaCaBO 3F as a new self-frequency doubling laser crystal has also been evaluated preliminarily. Appreciable self-absorption at the second-harmonic wavelength of Nd 3+:BaCaBO 3F crystal should be considered for further laser operations.

Optical properties of Nd3+-doped La2CaB10O19 crystals with different Nd3+ concentrations

La2CaB10O19 (LCB) is a new NLO crystal, in which Nd3+ can be doped with different concentrations to be self-frequency doubling crystals. We obtained seven LCB crystals with different Nd3+ concentrations grown by flux method. Refractive indices, absorption spectra and fluorescence spectra of the seven LCB crystals with various Nd3+ doping concentrations have been measured and evaluated. It has been found that the 5mol% Nd doped LCB crystal is most appropriate to be a laser material, by fitting the absorption spectrum with the Judd–Ofelt theory, radiative lifetime τr of Nd3+ in the 5mol% Nd doped LCB crystal is determined to be 237.8μs. For the 5mol% Nd doped LCB crystal, the absorption cross section at 799nm and the emission cross section at 1050nm are 4.52×10−20 and 1.29×10−19cm2, respectively. The measured fluorescence lifetime τf of the 5mol% Nd doped LCB crystal is 85μs and its quantum efficiency is 35.7%.

Preparation and photoluminescence properties of RE:Na 3La 9O 3(BO 3) 8 ( RE=Er, Yb) crystals

Using Na 2CO 3–H 3BO 3–NaF as fluxes, transparent RE:Na 3La 9O 3(BO 3) 8 (abbr. RE:NLBO, RE=Er, Yb) crystals have been grown by the top seed solution growth (TSSG) method. The X-ray powder diffraction analysis shows that the RE:NLBO crystals have the same structure with NLBO. The element contents were determined by molar to be 0.64% Er 3+ in Er:NLBO, 2.70% Yb 3+ in Yb:NLBO, respectively. The polarized absorption spectra of RE:NLBO have been measured at room temperature and show that both Er:NLBO and Yb:NLBO have a strong absorption bands near 980 nm with wide FWHM (Full Wave at Half Maximum) (21 nm for Er:NLBO and 25 nm for Yb:NLBO). Fluorescence spectra have been recorded. Yb:NLBO has the emission peaks at 985 nm, 1028 nm and 1079 nm and the emission peak of Er:NLBO is at 1536 nm. Spectral parameters have been calculated by the Judd–Ofelt theory for Er:NLBO and the reciprocity method for Yb:NLBO, respectively. The calculated values show that Er:NLBO is a candidate of 1.55 μm laser crystals and Yb:NLBO is a candidate for self-frequency doubling crystal.

Growth and spectral properties of self-frequency doubling crystal, Nd:Ca9.03Na1.08La0.62(VO4)7

The non-doped and doped Nd3+ of Ca9.03Na1.08La0.62(VO4)7 crystals were grown by the Czochralski technique. The effective segregation coefficients of Na+ and Nd3+ ions in the crystal were measured to be about 0.5 and 1.1, respectively. The XPS analysis of Ca9.03Na1.08La0.62(VO4)7 crystal indicates that the vanadium in the crystal is a mixture of V4+ (1.46 at. %) and V5+ (98.54 at. %). The hardness of Nd:Ca9.03Na1.08La0.62(VO4)7 crystal is about 383.1 VDH. Nd:Ca9.03Na1.08La0.62(VO4)7 crystal exhibits similar thermal expansion coefficients along the a (11.2×10-6 K-1) and c (13.7×10-6 K-1) axes, indicating a low thermal expansion anisotropy (αc/αa≈1.2). The qualitative frequency-doubling experiment shows that the doping of Na+ ion can help reduce the scattering of frequency-doubling light, and the intensity of SHG for Ca9.03Na1.08La0.62(VO4)7 crystal is found to be about 3.5 times as large as that of KDP. The polarized absorption and fluorescence spectra are analyzed based on Judd–Ofelt theory, which exhibits that the π-polarized absorption and stimulated emission cross sections are 6.07×10-20 cm2 with an FWHM 12.0 nm at 810 nm and 1.42×10-19 cm2 at 1069 nm, respectively. The fluorescence lifetime is 115 μs at room temperature. All the results indicate that Nd:Ca9.03Na1.08La0.62(VO4)7 crystal is a candidate of self-frequency doubling laser material.

Optical assessment on a new self-frequency doubling crystal: neodymium-doped lanthanum calcium borate

A new, to our knowledge, neodymium-doped lanthanum calcium borate crystal (Nd3+:La2CaB10O19, Nd:LCB) has been grown by the flux method. The spectroscopic parameters for Nd:LCB crystal have also been calculated based on Judd-Ofelt theory. The intensity parameters Omega_t are Omega_2 = 4.96×10^−20 cm², Omega_4 = 3.94×10^−20 cm², and Omega_6 = 3.71×10^−20 cm². The radiative lifetime taur is 349 µs. The absorption band at 800 nm has a FWHM of 12 nm. The absorption cross section is sigmaa = 2.40×10^−20 cm², and the emission cross sections at 1070 nm are sigma_X = 3.02×10^−19 cm², sigma_Y = 2.33×10^−19 cm², and sigma_Z = 2.68×10^−19 cm². The measured fluorescence lifetime tauf is 64 µs, and the quantum efficiency epsilon is 18.3%. In comparison with other Nd-doped laser crystals, the calculated parameters show that Nd:LCB crystal satisfies the fundamental spectral condition for laser emission.

Characterisation of nonlinear conversion and crystal quality in Nd- and Yb-doped YAB

Variable crystal quality affects the laser performance of many self-frequency doubling crystals, particularly those of the yttrium aluminum borate family. In this report we characterize nonlinear frequency conversion in Yb:YAB and demonstrate a simple non-destructive technique for measuring crystal quality. By imaging the nonlinear conversion using a CCD camera we observe phase matching characteristics similar to that obtained in quasi-phase-matched crystals. These effects are attributed to stacking faults in the structure of the YAB crystal during crystal growth. We believe that such defects cause the large variability in self-doubled performance reported for Nd- or Yb-doped YAB and that our technique may be used as a nondestructive measurement of crystal quality.

OPTIMIZATION OF GdCOB:Nd AND YCOB:Nd CRYSTALS FOR SELF-FREQUENCY DOUBLING OUT OF THE PRINCIPAL PLANES

From the linear, nonlinear optical and laser properties of GdCOB:Nd and YCOB:Nd self-frequency doubling crystals, we have found that the slow laser wave at 1060 nm has a higher emission cross-section than the fast one in the whole locus of SH phase matching, meaning that self-frequency doubling should be possible on this whole locus. We have pointed out that the phase matching direction for maximum nonlinear optical coefficient d eff is far from the one for maximum laser emission cross-section. The question of optimum direction for the self-frequency doubling laser was solved with a complete laser modelling. The maximum value of the SH yield occurs at a polar angle θ=68° for GdCOB:Nd , that is to say, close to the value of the optimum nonlinear optical coefficient d eff . For YCOB:Nd , the polar angle at maximum SH yield is shifted at 72°, that is to say, towards the direction of the largest laser emission cross-section. In both cases, we can notice a larger "angular acceptance" on the locus of phase matching for the self-frequency doubling laser than for extra-cavity single pass frequency doubling.

Absorption Spectrum and Optical Parameters of Nd-Doped Gadolinium Aluminium Tetraborate Crystals

Nd-doped gadolinium aluminium tetraborate NdxGd1-xAl3(BO3)4 (NGAB) crystals have been grown by flux method, with large sizes and good optical quality as well as different Nd3+ ion densities of 0 to 10 at.% in the lattice structure. These crystals belong to the trigonal system with point group 32, conforming space group R32. The unit cell constants are a=0.9305(8) and c=0.7248(1) nm, V=0.5436 nm3 for a Nd3+ content of 10 at.%. When Judd–Ofelt (JO) theory of parity-forbidden electric-dipole transitions of rare earth ions on non-centrosymmetric sites is applied to NGAB, it results in a good fit on absorption of the crystal. The three intensity parameters Ωt are fitted as 1.559×10-20, 1.751×10-20 and 2.395×10-20 cm2 at t=2, 4 and 6 respectively, with a root-mean square deviation of 9.35×10-22 cm2. The spectral parameters of NGAB crystal have indicated that it is a promising candidate of self-frequency doubling crystals.

Anisotropic properties of Nd:ReCOB (Re=Y,Gd):A low symmetry self-frequency doubling crystal

The anisotropy of low symmetry Nd:YCOB and Nd:GdCOB crystals, including their linear and nonlinear optical properties, as well as their laser and SFD properties, are discussed in detail, for the first time, in this paper. We have found the optimum directions of doubling-frequency for these crystals are not in the principle planes.

Investigation of diode-pumped, self-frequency doubled RGB lasers from Nd:YCOB crystals

As a new self-frequency doubling crystal, Nd:YCOB demonstrates a great potential for diode-pumped cw visible solid state lasers in a simple hemispherical cavity. We previously reported 62 mW cw green output with less than 1 W diode pump power absorbed in the crystal. Now we present more than 16 mW red cw laser at 666 nm from 5% Nd:YCOB crystal. As far as we know, it is the first time that diode-pumped cw red lasing is achieved by self-frequency doubling. Our effort on blue laser is also addressed.

Ytterbium-doped Ca_4GdO(BO_3)_3: an efficient infrared laser and self-frequency doubling crystal

The surge of interest in ytterbium-doped materials has led to their being proposed as substitutes for their widely used neodymium counterparts for infrared laser emission. Spectroscopic and laser investigations of ytterbium-doped Ca4GdO(BO3)3 (Yb:GdCOB), a new ytterbium-doped crystal, are reported. Laser performances under titanium:sapphire pumping suggest that this material is suitable for schemes that include laser-diode pumping. A maximum slope efficiency of 58.8% with a corresponding laser threshold of 40 mW has been demonstrated with a 1% output coupler. Laser oscillations were also observed for output-coupler transmissions of as much as 10%. Self-frequency doubling of infrared-laser emission has been observed for the first time to our knowledge in an ytterbium-doped medium. The recent demonstration of the interesting self-frequency-doubling properties of neodymium-doped Ca4GdO(BO3)3 suggests that Yb:GdCOB could also be efficient in visible laser emission because of its lack of reabsorption at the second-harmonic wavelength. All these results show that the calcium oxoborate family can be useful compact and efficient laser-diode-pumped visible-laser sources.

Passively Q-switched self-frequency doubling Nd xY 1− xAl 3(BO 3) 4 laser with Cr 4+:YAG saturable absorber

Using a new saturable absorber Cr 4+:YAG as passive Q-switch, we have realized the Q-switched laser running at 0.531 μm with an NYAB self-frequency doubling crystal and measured the single pulse energy and the pulse width under different small-signal transmissions of Cr 4+:YAG and different cavity lengths. Meanwhile, the coupling wave rate equations for Cr 4+:YAG as a passive Q-switch are given and the experimental results agree with the numerical solution of equations.

Growth Zone Boundary in a Self-Frequency Doubling Crystal of Nd3+-Doped YAl3(BO3)4

One of the main defects in the self-frequency doubling crystal of Nd3+-doped YAl3(BO3)4 (NYAB), which may be the key factor to influence the crystal performance, is the growth zone boundary that has been identified by means of x-ray diffraction topography. The characteristic boundary is formed usually by the basal plane and the pyramidal planes or by the pyramidal planes themselves.