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Abstract
Nonlinear optical α-Ca(HCOO)2 crystals were synthesized by the solution evaporation growth method. The crystals with good transparency in the visible light region were grown with a maximum size of 7 mm × 6 mm × 4 mm and were studied using single crystal X-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV-Vis), Fourier-transform infrared spectroscopy (FT-IR), and FTIR-Raman spectroscopy. The crystals are orthorhombic with a space group of Pbca having unit-cell dimensions a = 10.238(4) Å, b = 6.305(2) Å, c = 13.456(5) Å, α = β = γ = 90°, V = 868.6(5) Å3, and Z = 8. The as-grown crystals exhibit polyhedral morphology consisting of {210}, {111}, and {100} facets. The nonlinear absorption property of the crystals was explored by open-aperture Z-scan measurements under the excitation of femtosecond laser pulses at the wavelength of 800 nm. The α-Ca(HCOO)2 crystals exhibit saturable absorption. The effective nonlinear absorption coefficient under the low-intensity approximation is discovered to be β = −5.09 cm/GW.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nonlinear optical crystals have been widely employed in modern optical science and technology for frequency conversion of laser light. They have also had a significant impact on laser technology, optical communication, and optical data storage technology [1–4]. Metal formate crystals, such as NaHCOO, Sr(HCOO)2, Ba(HCOO)2, and LiHCOO·H2O [5–8], have demonstrated nonlinear optical properties, and have potential applications in harmonic generation and optical mixing devices. Most of them are focused on the second harmonic generation (SHG) and laser damage threshold studies of metal formate crystals [9–12].
Calcium formate exhibits four different modifications: α, β, γ, δ. The structure of α-Ca(HCOO)2 is more stable at room temperature than other structures [13]. The orthorhombic α-Ca(HCOO)2 single crystal has been reported as a potential frequency converter material based on stimulated Raman scattering (SRS) for Raman laser converters with rather high gain coefficients [14]. Further study of orthorhombic calcium formate is valuable to explore its optical properties.
In this work, we report on the observation of the nonlinear absorption property in calcium formate crystals. The structure of the crystal was studied using XRD; chemical structures of the calcium formate crystals were investigated using FTIR and FTIR-Raman; the transmittance spectra of the crystals were obtained with UV-Vis; and the nonlinear absorption property of the crystal was investigated by femtosecond-pulsed Z-scan measurements.
2. Experimental
2.1. Preparation of α-Ca(HCOO)2 single crystals
α-Ca(HCOO)2 single crystals were grown by solution method with slow evaporation technique. First, 17.2 g Ca(HCOO)2 (99.9%) was dissolved in 100 mL distilled water in a beaker covered with a perforated polythene paper. The growth vessel was then kept in a water bath at 50 °C. After 48 hours, transparent single crystals of α-Ca(HCOO)2 were harvested.
2.2. Characterization
The structure of α-Ca(HCOO)2 was characterized by X-ray powder diffraction with Cu Kα radiation (λ = 1.5406 Å) on a Bruker AXS D8 DISCOVER X-ray diffractometer at 40 kV and 40 mA. Diffractograms were recorded in a 2θ range from 10° to 50° with scanning steps of 0.02° and an exposure of 4 s per step. Single-crystal data sets of the α-Ca(HCOO)2 sample were collected at 296 K in a Bruker Nonius x8 ApexII X-ray diffractometer with a CCD detector and using monochromatic Mo Kα1 radiation (λ = 0.71073 Å). Relevant data are shown in Table 1.
Table 1. Fractional coordinates for α-Ca(HCOO)2 at T = 296 K.
To determine the composition of the synthesized sample, FT-IR spectrum was recorded using a Nicolet 5700 FT-IR spectrometer by KBr pellet technique in the range of 500-4000 cm−1 at room temperature. The FTIR-Raman spectrum was recorded using Nicolet Raman 960 with a 1064 nm laser. The transmittance spectra of α-Ca(HCOO)2 was recorded at room temperature with a Shimadzu UV-3600 spectrophotometer.
3. Results and discussion
Figure 1(a) shows the XRD patterns of the as-prepared crystals, which are consistent with ICDD-PDF NO. 14-0819, indicating an orthorhombic crystal structure. From the XRD measurements of the crystal facets, the facets can be divided into three groups: {210}, {111}, and {100}, as shown in Fig. 1(a). The observed morphology is one of the degenerate cases of the ideal form most likely due to the anisotropic supply of the initial solution. These facets, {210}, {111}, and {100}, have relatively larger lattice-plane-space ratios, and therefore a lower growth rate. This makes these facets more readily found on grown crystals. Figure 1(b) is the schematic drawing of the crystal structure of α-Ca(HCOO)2. Figure 1(c) shows the typical products of grown crystals. Well-developed α-Ca(HCOO)2 crystals are truncated polyhedrons and transparent (Fig. 1(c)).
Fig. 1 (a) X-ray diffraction patterns of α-Ca(HCOO)2 crystal and powder. (b) Crystal structure of α-Ca(HCOO)2. (c) Photograph of α-Ca(HCOO)2 single crystals.
The single crystal XRD data collection of the sample was made on a Bruker Nonius X8 ApexII X-ray diffractometer with Mo Kα1 radiation (λ = 0.71073 Å) at 296 K. α-Ca(HCOO)2, Mw = 130.12, colorless block (0.18 × 0.17 × 0.16 mm), orthorhombic, space group Pbca, a = 10.238(4) Å, b = 6.305(2) Å, c = 13.456(5) Å, α = β = γ = 90°, V = 868.6(5) Å3, Z = 8, Dc = 1.990 g•cm−3, µ = 1.330 mm−1, F(000) = 528, and GOF = 1.042. The refinement on F2 converged to R1 = 0.0179 [I > 2σ(I)], wR2 = 0.0519 (See Table 2). In the α-Ca(HCOO)2 structure, the four O atoms from four different formate groups are coordinated in a square-planar fashion to the Ca2+ ion with two more formate groups placed above and below the square plane (inset of Fig. 1(a)). The two formate groups are non-equivalent. One of the formate groups is coordinated to the Ca 2+ ion acting as a chelate ligand, while the other formate group is coordinated to the Ca2+ ion with only one of the two O atoms [14]. The Ca2+-O distances of the chelate formate groups are found to be longer than the other Ca2+-O distances, as is shown in Table 3. There exist endless chains of the form [… Ca … O – CH – O …]∞, which are cross-linked by the Ca2+ ions. The chains are complicated and the C-H vectors are pointing in four directions [15].
Table 2. Structural parameters (space group, lattice parameters), and reliability factors obtained from the Rietveld refinement data for α-Ca(HCOO)2 at T = 296 K.
Table 3. Selected Bond Lengths and Bond Angles of α-Ca(HCOO)2 at T = 293 K.
The experimental FT-IR spectrum of α-Ca(HCOO)2 is shown in Fig. 2(a). The absorption bands at 770 and 780 cm−1 can be assigned to the O-C-O bending vibration. The strong absorption band observed at 1378 and 1572 cm−1 are assigned to the symmetric and asymmetric C-O stretching mode, respectively. The in-plane C-H bending mode was assigned to the peak at 1420 cm−1. The bands around 2897 cm−1 are assigned to the C-H stretching mode [7].
The FTIR-Raman spectrogram of α-Ca(HCOO)2 is shown in Fig. 2(b). The results of the FTIR-Raman spectrum are in agreement with the results of the FT-IR spectrum. The absorption bands at about 1077 and at 231 cm−1 can be assigned to the H-C-O-Ca and O-C-O-Ca torsive vibration, respectively. The absorption bands at about 254 cm−1 are due to the Ca-O stretching vibration. The absorption band at 291 cm−1 is ascribed to the C-O-Ca bending vibration [16,17]. The presence of two sets of formate ions in the structure of α-Ca(HCOO)2 gives rise to the doubling of the internal modes and to symmetric stretching vibrations of HCOO- with very strong bands at 1352 and 1361 cm−1 [18].
According to the good diaphaneity of α-Ca(HCOO)2, the optical transmittance spectra of the 1.5 mm thick α-Ca(HCOO)2 crystal is shown in Fig. 3. At 800 nm, we obtain the linear absorption coefficient to be α0 = 1.4 cm−1 by the conversion formula , where T and L are the linear transmittance and thickness of the sample, respectively.
Fig. 3 The UV-Vis optical transmittance spectra of α-Ca(HCOO)2 sample through the {210} crystal face.
The nonlinear absorption properties of the sample were carried out by the open-aperture Z-scan technique [19, 20]. The laser pulses used were generated by a Ti:sapphire regenerative amplifier (Coherent Inc.), operating at a wavelength of 800 nm with a pulse duration of τF = 170 fs and a repetition rate of kHz. The spatial distribution of the pulses is nearly Gaussian with the radius of the Gaussian beam waist of ω = 2.00 mm, which is obtained by the beam analyzer (Beamview, Coherent Inc., see Fig. 4). Moreover, the laser pulses had a near-Gaussian temporal profile. In the Z-scan measurements, the laser beam was focused by an achromatic lens with a focal length of f = 150 mm, producing the beam waist at the focus to be . Accordingly, the Rayleigh length of the beam was estimated to be . To perform Z-scan measurements, the sample was scanned across the focus along the z-axis using a computer-controlled translation stage, while the transmitted pulse energies were probed by a detector producing the open-aperture Z-scan trace. The Z-scanner system was calibrated with a piece of CdS bulk crystal, and the experimental uncertainty is within .
For the laser beam with a temporal Gaussian pulse profile, the peak intensity at the focus is obtained by the formula , where ε is the incident energy. Figure 5 shows the open-aperture Z-scan trace of the α-Ca(HCOO)2 sample at I0 = 1.95 GW/cm2. The experimental results indicate that calcium formate crystals exhibit a saturable absorption process [21, 22]. For the samples exhibiting saturable absorption process, the optical intensity loss may be governed by the following differential equations:
Here, α0 is the linear absorption coefficient and IS is the saturable intensity. Models I and II [23, 24] are usually referred to saturable absorption in homogeneous and inhomogeneous broadening systems, respectively.Fig. 5 The open-aperture Z-scan trace of the sample α-Ca(HCOO)2 at I0 = 1.95 GW/cm2. The solid and dotted lines are the theoretical fittings for models I and II, respectively.
To evaluate the nonlinear absorption coefficient and to give an insight on the underlying mechanism for the observed optical nonlinearity, we use two saturable absorption models described in Eq. (1) and (2) to fit our experimental Z-scan trace. Clearly, the profile of the Z-scan trace fitted by model I is in good agreement with the experimental data, indicating that the crystal exhibits saturable absorption effect in an homogeneous broadening two-level system. The best fitting gives the saturable intensity to be IS = 0.275 GW/cm2. Accordingly, the effective nonlinear absorption coefficient under the low-intensity approximation is inferred to be cm/GW. This result indicates that the α-Ca(HCOO)2 crystals exhibit remarkable saturable absorption effects and could be exploited for ultrafast photonic applications in mode-locked and Q-switched lasers.
4. Conclusions
Single crystals of α-Ca(HCOO)2 have been grown by a slow evaporation technique. The material crystallizes in an orthorhombic crystal system with the space group Pbca. UV-Vis studies reveal the wide transparency nature of the crystal. FT-IR and FTIR-Raman studies confirmed the various functional groups and vibrational structures present in the crystal. The nonlinear absorption property of the crystal was investigated by femtosecond-pulsed Z-scan measurements. The nonlinear absorption coefficient of the crystal at 800 nm has been calculated to be β = −5.09 cm/GW. The α-Ca(HCOO)2 crystal exhibits a saturable absorption process indicating its potential applications in short-pulsed laser generations as crucial passive mode-locking or Q-switching elements.
Funding
National Natural Science Foundation of China (No. 51572241, 61704153, 61774019 and 51572033); the Scientific Research Project for the Education Department of Zhejiang Province (No. Y201738294); Science and Technology Department of Zhejiang Province Foundation (No. 2017C37017); Natural Science Foundation of Zhejiang Province (No. LQ18E020003).
Disclosures
The authors declare no conflicts of interest.
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