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Nonlinear optical characteristics of DADP crystals with different deuterium
Optical Materials 91 (2019) 17–22
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Nonlinear optical characteristics of DADP crystals with different deuterium a,b,c
d
a
a
Yafei Lian , Lili Zhu , Jin Huang , Xiangxu Chai , Dongting Cai Xun Sunb,c,∗∗, Xiaodong Jianga,∗
a,b,c
T
b,c
, Mingxia Xu ,
a
Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang, 621900, China State Key Laboratory of Crystal Materials, Shandong University Jinan, 250100, China Key Laboratory of Functional Crystal Materials and Device (Shandong University), Ministry of Education Jinan, 250100, China d School of Materials Science and Engineering, Shandong Jiaotong University, Jinan, 250300, China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: D)ADP crystals Nonlinear absorption Nonlinear refraction
DADP crystals with different deuterium as 0%, 30%, 50%, and 70% were grown via traditional temperature reduction method and the samples were gained from the as-grown crystals in the directions of Ι -type (θ = 90°,φ = 45°). The transmittance spectrum, UV absorption spectrum, and Raman spectrum were implemented to describe the properties of DADP crystals, and the deuteration of ADP crystals makes a difference on the absorption peaks in these spectrums. Particularly, the optical band gap Eg decreases with the accretion of deuterium content. The third-order nonlinear optical characteristics were systematically represented and tested by means of Z-scan measurements with Pico-second pulse laser irradiation at λ = 355 nm, with the results listing the reverse saturable absorption and self-focusing effect. The investigations may contribute to replenish and elucidate the application and laser-induced damage of DADP crystals.
1. Introduction
succeeded in the multi-photon absorption and laser-induced damage of ADP, KDP and DKDP crystals [13–16]. Distinct steps were observed in the damage threshold of DKDP at photon energies of 2.55eV or 3.9eV associated with the multi-photon absorption [13]. Absolute measurements of two-photon absorption (2 PA) coefficients were presented by John Reintjes and Robert C. Eckardt and the value of β = 11 ± 3 × 10−11cm/W at 266.1 nm for ADP and 2.7 ± 0.7 × 10−11cm/W for KDP were determined [14]. Wang et al. [15] investigated the third-order nonlinear optical (NLO) properties of ADP crystal at different wavelengths and different orientations. In our early works, nonlinear characters of thick samples in different directions of ADP crystals were investigated, with the conclusions weaker nonlinear characters correspond to greater LDT, and the nonlinearities and LDT possess the same anisotropy [16]. However, little work has been done on the NLO properties of DADP crystals with different deuterium. In this work, DADP crystals with different deuterium as 0%, 30%, 50% and 70% were grown via traditional temperature reduction method and the samples were gained from the as-grown crystals in the directions of Ι -type (θ = 90°,φ = 45°). The transmittance spectrum, UV absorption spectrum and Raman spectrum were implemented to describe the properties of DADP crystals, and the deuteration of ADP
Ammonium dihydrogen phosphate (NH4H2PO4, ADP) belonging to scalenohedral (twelve faced) class of tetragonal crystal system [1] is a typical isomorph of potassium dihydrogen phosphate (KH2PO4, KDP) crystal which is the only nonlinear optical material used in inertial confinement fusion (ICF) engineering [2], and can be applied for frequency conversion such as the third or fourth harmonic generation [3] due to its large nonlinear optical (NLO) coefficient, high transmittance and high laser damage threshold (LDT) [4,5]. Compared with KDP crystals, deuterium KDP crystals can efficiently reduce the Stimulated Raman Scattering (SRS) caused by high power laser [6]. And as for ADP crystals, the deuterium is additionally able to increase the infrared cutoff edge and the transmittance in the near-infrared region [7]. Nowadays, the laser-induced damage is the bottleneck for the nonlinear optics such as DKDP crystals, ADP crystals and fused silica [8–10]. And as Eg/hω < 4 or pulses are in the ps and fs regime, intrinsic damage is generally dominated by the multi-photo generation of free electrons when multi photons are absorbed by one single electron called multiphoton absorption [11]. The high order nonlinear processes are connected with refractive index change and light absorption in a strong electromagnetic field [12]. In recent years, many efforts have
∗
Corresponding author. Corresponding author.State Key Laboratory of Crystal Materials, Shandong University Jinan, 250100, China. E-mail addresses: [email protected] (X. Sun), [email protected] (X. Jiang).
∗∗
https://doi.org/10.1016/j.optmat.2018.12.018 Received 26 September 2018; Received in revised form 13 December 2018; Accepted 14 December 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
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ΔΦ 0 = kn2 I0 Leff
crystals makes a difference on the absorption peaks in these spectrums. Particularly, the optical band gap Eg decreases with the accretion of deuterium content. The third-order nonlinear optical characteristics were systematically represented and tested by means of Z-scan measurements with Pico-second pulse laser irradiation at λ = 355 nm, with the results listing the reverse saturable absorption and self-focusing effect. The investigations may contribute to replenish and elucidate the application and laser-induced damage of DADP crystals.
Where ΔΦ0 , k and n2 are the nonlinear phase shift at the focus, the wave vector equaling to 2π/λ and the nonlinear refractive index, respectively. As known, the third-order nonlinear processes include the nonlinear absorption (imaginary part) and the nonlinear refraction (real part) as [22,27].
χ (3) = χR(3) + iχI(3)
[7]
2. Experimental
χR(3) (esu) = cn02 n2/120π 2 (m2 / W )
[8]
2.1. Samples preparations
χI(3) (esu) = c 2n02 β /(240π 2ω)(m / W )
[9]
χR(3) χI(3) ,
c and n0 represent the third-order nonlinear Where χ (3) , susceptibility, real part of χ (3) related to the nonlinear refractive index, imaginary part concerned with the nonlinear absorption coefficient, the speed of light in a vacuum and the linear refractive index.
The ADP and DADP crystals were prepared from aqueous solution via the traditional growth method by temperature reduction [17]. Z-cut seed plates with a size of 50 mm × 50 mm × 10 mm were gained from the other as-grown crystals and the growth producers are similar with Ref. 17. In addition, the samples tested in our experiments were cut from the ADP and DADP crystals in the directions of Ι -type (θ = 90°,φ = 45°) and were processed and polished to a size of 10 mm × 10 mm × 2 mm.
3. Results and discussion 3.1. Spectrum measurements The transmittance spectra and UV absorption spectra of (D) ADP crystals are shown in Fig. 1. And the transmittance spectra show high transmittance with few differences between the others in the range of 400 nm-1200nm, indicating none macro defects consist in the samples and the samples were well polished. In the near-infrared region, the transmittance of samples increases with the accretion of deuterium (D) content, owing to the substitution of D to H. The transmittance spectra illustrate that three absorption peaks exist in near-IR, namely about 1070 nm, 1300 nm and 1584 nm, as shown in Fig. 1(a). Referring to Ref. [28], the 3739.74 cm−1 (2674 nm), 3251.55 cm−1 (3075 nm) peak is assigned to be the symmetrical stretching of OeH bond (γs-OH) and NeH group (γs-NH4), respectively. Therefore, the 1070 nm peak exists in the third overtone region of the NeH (D) bond and the 1584 nm peak lies in the second overtone region of the NeH (D) bond. Besides, the 1300 nm peak is in the second overtone of the OeH (D) bond. These absorptions decrease as the deuteration take places, leading to the augmentation of transmittance in near-IR. With the increase of deuterium content, the three absorption peaks have a redshift for that the D atom shows a higher quality than H atom and the vibration of N-D bond abates as the substitution in process. The transmittance of (D) ADP crystals between 190 and 400 nm decrease in some extent, by virtue of the reason that metal impurity ions i.e. Cr3+, Fe3+ et al. and instinct defects (vacancy, hydrogen, Frenkel pair and interstitial oxygen et al.) [29–31] exist in the DADP crystals and the defect energy levels may absorb the UV light [32]. Fig. 1(b) illustrates the UV absorption spectrum, from which the absorption peaks of the (D) ADP crystals are all about 199 nm (20 spectrums were tested and the average absorption peak is 199 nm). Besides, the relationship between hν vs. (αhν) 2 is gained from Fig. 1(b), as shown in Fig. 1(c). There can be found unphysical steps in UV–vis absorption plots, near c.a 300 nm and 4eV accordingly in Fig. 1(b) and (c), which are supposed to be related to the spectrophotometer operation. Besides, the optical band gap of DADP crystals with different deuterium content can be calculated as listed in Table 1. The optical band gap Eg decreases with the accretion of deuterium content. As supposed, the deuterium renders the crystals imperfect leading to the decrease of the optical band gap. Raman spectra of (D) ADP crystals with different deuterium are shown in Fig. 1(d). It can be seen from the figure that the strongest Raman peak of the ADP crystal is located at 925 cm−1, which is caused by the asymmetric stretching vibration of the PO4 tetrahedral group (γas-PO4) [33]. Besides, with deuterium, the Raman peak split at 925 cm−1 into two peaks, and the intensity of the scattering peak is significantly reduced. The position of the main peak of the scattering has a redshift as the content of strontium increases, that is, moving towards the low wave
2.2. Experiment procedures In order to the transmittance spectrum with a test band of 200–2000 nm at room temperature of samples was tested by Hitachi U3500 spectrograph. And the UV absorption spectrum was measured via LS5-Lambda 950 at the range of 190–800 nm. Room temperature Raman spectra were obtained by microscopic Raman spectroscopy (Renishaw in via) using a 532 nm laser over the range of 100–1800 cm−1. The third-order nonlinear optical characteristics were investigated by utilizing the Z-scan technique in Pico-second regime and the experimental set-up and procedures were described in many already published papers in detail [18–21]. Besides, the beam waist radius in our experiments is about 20 μm. 2.3. Theories As focusing an intense light source on the tested samples, intensity dependent absorption is induced by the light field and two or more photons are absorbed leading to the appearance of multi-photon absorption process, which can be expressed as [22].
α(Ι ) = α + βI + γI 2 + ⋅⋅⋅+⋅⋅⋅
(1)
Where I, α, β and γ is the laser energy intensity, the linear absorption coefficient, the nonlinear two-photon absorption coefficient, and the three-photon absorption coefficient, respectively. Now a Gauss beam spread along the normal of crystal faces, the 2 PA coefficient can be calculated as [23–25]. ∞
T2PA =
∑
[−β2 I0 Leff /(1 + z 2/ z 02)]n
n=0
(n + 1)3/2
(2)
Leff = [1 − exp(−α 0 L)]/ L
(3)
z 0 = πω0 / λ
(4)
Where T2PA, β2, I0, z, z0, Leff, α0, L, ω0 and λ are the normalized transmittance, 2 PA coefficient, the peak intensity, the sample position, the diffraction length of focused beam, the effective sample thickness, linear absorption coefficient, sample thickness, the waist radius and the wavelength of the light beam respectively. What is more, the normalized transmittance of the closed-aperture Z-scan can be expressed as [26].
T=1+
4ΔΦ 0(z / z 0) [(z / z 0)2 + 9][(z / z 0)2 + 1]
[6]
[5] 18
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Fig. 1. (a). Transmittance spectra of the ADP and DADP samples; (b). UV absorption spectra of (D) ADP crystals; (c) hν (eV) vs. (αhν) 2 relationship of (D) ADP crystals; (d) Raman spectra of (D) ADP crystals. (There can be found unphysical steps in UV–vis absorption plots, near c.a 300 nm and 4eV accordingly in (b) and (c), which are supposed to be related to the spectrophotometer operation).
Table 1 The Spectrum measurements of (D) ADP crystals. Spectrum Transmittance spectrum
UV absorption Raman spectrum
3rd NeH 2nd OeH 2nd NeH Eg (eV) γas-PO4 δ-OD
0%
30%
50%
70%
1066 1295 1581 5.72 922 –
1069 1308 1583 5.68 919 –
1072 1315 1585 5.66 912 944
1075 1323 1588 5.64 900 963
number. And the newly small peak moves towards the high wave number, as shown in Table 1. The appearance of the new peak is due to the substitution of hydrogen by deuterium, which causes the appearance of the in-plane bending vibration of the O-D hydrogen bond (δOD). However, the substitution reduces the vibration frequency of the hydrogen bond, and the originally strong symmetric stretching vibration mode weakens and moves to the low frequency, and the weak bending vibration gradually appears and moves toward the high frequency.
3.2. Nonlinear absorption and nonlinear refraction The results of open-aperture Z-scan measurements namely the normalized transmittance versus the position Z are listed and fitted well via equation (2) and are shown in Fig. 2(a). The points are the experimental data and the solid lines are the fitted curves. In Fig. 2(a), the fitted lines represent the two-photon absorption of ADP, 30% DADP, 50% DADP and 70% DADP crystals, respectively and show the valley characteristic, indicating the existence of reverse saturable absorption of (D) ADP crystals. The depth of the valley stands for the magnitude of nonlinear absorption, showing an increasing tendency with the deuterium adding up. The diffraction length z0 of focused beam at 355 nm is about 3.54 mm as shown in Table 2. From the results, it can be seen the nonlinear absorption increases greatly as deuterium content augments. For the nonlinear refraction, the normalized transmittance of
Fig. 2. Z-scan measurements results of ADP samples at 355 nm. (a) openaperture (b) closed-aperture. The points are the experimental data and the solid lines are the fitted curves.
closed-aperture Z-scan measurements with the peak laser intensity of about 39 GW/cm2 is performed in Fig. 2(b). The dotted lines and solid lines are on behalf of the experimental data and the fitting results 19
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Table 2 Nonlinear absorption parameters, nonlinear refractive parameters and the third-order nonlinear susceptibility at 355 nm. Wavelength
sample
β2 cm·GW−1 ( × 10−2)
n2 m2 W−1 ( × 10−19)
Imχ3 esu ( × 10−14)
Reχ3 esu ( × 10−14)
χ(3) esu ( × 10−14)
355 nm
ADP 30%DADP 50%DADP 70%DADP
3.26 8.01 9.99 13.8
4.69 4.88 5.90 8.18
3.50 8.61 10.7 14.8
2.84 2.95 3.57 4.95
4.51 9.10 11.3 15.6
± ± ± ±
0.06 0.12 0.17 0.10
± ± ± ±
0.08 0.11 0.24 0.26
± ± ± ±
0.06 0.13 0.18 0.11
± ± ± ±
0.05 0.06 0.14 0.16
± ± ± ±
0.08 0.14 0.22 0.19
two electrons go back immediately and the others fall down afterword. Supposing the laser fluence is high enough beyond the laser-induced damage threshold, more and more photons are absorbed and the activated electrons cannot relax in a short time. As a result, the accumulation of the excited electrons takes place and the initial process of laser-induced damage subsequently appears. Fig. 2(a) performs that the valley depths of the absorption curves vary with the deuterium changing. That is the nonlinear absorption coefficient β increases greatly as deuterium content augments and β of 70% DADP is one order of magnitude larger than that of ADP. What we think is that the β is associated with the band gap Eg. The nonlinear absorption can be regarded as a probability event, in which the photons of the same number are more potentially absorbed when the band gap is narrower, contributing to a larger nonlinear absorption coefficient. Open-aperture Z-scan measurements results of ADP crystal at 355 nm with different peak intensities are shown in Fig. 4(a). And from the results of β vs. I0 listed in Fig. 4(b), it can be seen that there is a
adopting formulas (4), respectively. The valley-peak configurations in the results indicate nonlinear refractive index of ADP crystals at 355 nm is positive [34], which means as the (D) ADP crystals are irradiated by the high power laser, the self-focusing effect will be induced. Besides, the valley-to-peak value shows the change of the nonlinear refraction properties [35]. And the real part of the third-order susceptibility representing the nonlinear refraction can be calculated by Eq. (8). Then the third-order susceptibility is achieved as listed in Table 2.
3.3. Discussions As illustrated in Fig. 2, the normalized transmittance of Z-scan measurements is well fitted by formulas (2) and (5), which are the twophoton absorption theories. As typical isomorphs of DKDP crystals, ADP and DADP crystals are all wide band gap nonlinear optical materials, with a theoretical band gap of 6.96 eV at room temperature [36]. However, many factors i.e. supersaturation, the stability of the solution, hydrodynamic conditions, the cooling rate and the impurities ions et al. [37,38] may affect the growth of the (D) ADP crystals. For example, with the supersaturation levels increasing, the stability of the solution may decrease, contributing to the addition of defects in crystals; impurity composition in the solution will lead to a distortion of the lattice as absorbed during the growth process. Therefore, all the micro defects result in the occurrence of the abundant defect energy levels which can deduce the intrinsic band gap of (D) ADP crystals, meeting well with the experimental data from the UV absorption spectrum. Besides, the photon energy at 355 nm is 3.49 eV and multi-photons cross sections for two, third, and fourth order absorption in wide band gap materials are on the order of 10−50 cm4 s, 10−81 cm6 s2, and 10−114 cm8 s3, respectively [25,39]. Therefore, two-photon absorption (2 PA) is predominant during the nonlinear process of (D) ADP crystals. As shown in Fig. 3, the process of 2 PA is well described. Before the laser-induced damage occurs, the nonlinear absorption shows a process of dynamic equilibrium until the laser does not irradiate the solid samples after relaxation. In the real crystals, there are many defect levels formed by the defects as said in the above and the defect levels lie between the valence band (VB) and conduction band (CB). As the laser irradiates, the electrons in the VB or in the defect levels may absorb the photons and begin to transition to the upper energy levels; then they fall back to the defect level or the VB. This process keeps equilibrium with a short relaxation time. For example, ten electrons jump into the CB at a certain time, with
Fig. 4. Open-aperture Z-scan measurements results of ADP crystal at 355 nm with different peak intensity I0 (GW/cm2) (a) and the nonlinear absorption coefficient β (b). As the peak intensity is lower than 32.0 GW/cm2, the nonlinear absorption process does not take place and the β is zero. As the peak intensity adds up, the valley depth and β increase overtly. The normalized transmittance curve drops abruptly and almost breakdown near the focal point, connoting that the laser-induced damage occurs at this peak intensity.
Fig. 3. Schematic diagram of elastic 2 PA process. 20
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describe the properties of DADP crystals, and the deuteration of ADP crystals makes a difference on the absorption peaks in these spectrums. With the increase of deuterium content, the three absorption peaks in the transmittance spectrum have a redshift as D atom shows a higher quality than H atom and the vibration of N-D bond abates as the substitution in process; the optical band gap Eg decreases with the accretion of deuterium content as the UV absorption spectrum perform. With deuterium, the Raman peak split at 925 cm−1 into two peaks and the intensity of the scattering peak is significantly reduced. The position of the main peak of the scattering has a redshift as the content of strontium increases, that is, moving towards the low wave number. And the newly small peak moves towards the high wave number. Besides, The third-order nonlinear optical characteristics were systematically represented and tested by means of Z-scan measurements with Picosecond pulse laser irradiation at λ = 355 nm, with the results listing the reverse saturable absorption and self-focusing effect. The nonlinear absorption coefficient β increases greatly as deuterium content augments and β of 70% DADP is one order of magnitude larger than that of ADP, indicating the β is associated with the band gap Eg. The results indicate that the NLA exists in an interval of the laser peak intensity and in this interval, the β adds up after I0 increasing. It is believed our research is propitious to the application of (D) ADP crystals as frequency conversion devices.
threshold of the appearance of the nonlinear absorption. That is, as the peak intensity is lower than 32.0 GW/cm2, the nonlinear absorption process does not take place and the β is zero. As the peak intensity adds up, the valley depth and β increase overtly. However, the normalized transmittance curve drops abruptly and almost breakdown near the focal point, connoting that the laser induced damage occurs at this peak intensity. The 2 PA theory is forced fitted on this curve to gain the nonlinear absorption coefficient to be about twenty times of that at I0 = 42.5 GW/cm2. As supposed, the process of NLA appears below the fluence of laser induced damage threshold (LDT), otherwise will not arise at some lower fluence. That is, the NLA exists in an interval of the laser peak intensity and in this interval, the β adds up after I0 increasing. Provided that the I0 exceeds above the critical point, the excited energy level induced by multi photons absorption cannot recover and the electron will be ionized, resulting in the occurrence of the laserinduced damage [40,41]. As shown in Fig. 2(b), the distance of valley-to-peak varies with the deuterium changes. The nonlinear refractive index is positive, which means that as the high-power laser irradiate, the NLO crystals become an additional positive lens as a result of the self-focusing effect, which will lead to a larger damage possibility at the rear surface. The parameters of nonlinear refraction are listed in Table 2 and the nonlinear refractive index increases with the addition of deuterium. As known, the nonlinear absorption makes a significant effect on the nonlinear propagation effects such as the self-focusing effect, which can contribute to the reduction of the cross section of high power laser beams which propagate through the crystals [42]. The reduction of laser spot size induced by the self-focusing effect and beam breakup makes local increase of laser irradiance to the levels easily exceeding LIDT and the laser-induced damage driven by self-focusing occurs as soon as irradiance reaches the threshold of self-focusing which is lower than LIDT in most cases [43]. The self-focusing effect takes place as beam has traveled the certain distance within the crystals calling self-focusing distance ZSF [44].
ZSF =
2n 0 ω02 λ
PCR =
π (0.61)2λ 02 8n 0 n2
1 P / PCR − 1
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Financial support from the National Natural Science Foundation of China (Grants No 51402173, 61705204 and 61705206), the Fundamental Research Funds for the Central Universities, Young Scholars Program of Shandong University, Laser Fusion Research Center Funds for Young Talents (NO. LFRC-PD011).
(10)
References
(11)
Where P, PCR is the power of the laser beam and the critical power of self-focusing, respectively. The position of self-focus is not steady and fixed in space. The first self-focus forms at the distance ZSF from the front surface of the samples and the second self-focus follows the first one. The process is repeated to make a track of the self-focus points. According to Eqs. (10) and (11), self-focusing distances of 0%, 30%, 50% and 70% DADP crystals are 1.03 mm, 1.00 mm, 0.91 mm and 0.76 mm, respectively. That means the self-focus point may appear in the samples and can induce the damage if the laser power is beyond the LIDT. However, a larger third-order nonlinear refractive index is necessary to all optical switch materials [45], especially for the organic compounds with a large π-electron conjugation system. Therefore, DADP crystals are promising for applying as optical switch optics, and ADP with some deuterium, to a certain extent, may promote the Kerr nonlinearity in correlation with nonlinear refraction. Besides, appropriate nonlinear absorption and nonlinear refraction properties should all be considered for the application of (D) ADP crystals as frequency conversion devices.
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4. Conclusions In our study, DADP crystals with different deuterium as 0%, 30%, 50%, and 70% were grown via traditional temperature reduction method and the samples were gained from the as-grown crystals in the directions of Ι -type (θ = 90°,φ = 45°). The transmittance spectrum, UV absorption spectrum, and Raman spectrum were implemented to 21
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[12] Ilya A. Kulagin, Rashid A. Ganeev, et al., Nonlinear refractive indices and thirdorder susceptibilities of nonlinear-optical crystals, Proc. SPIE 4972 (2003) 182–189. [13] C.W. Carr, H.B. Radousky, S.G. Demos, Wavelength dependence of laser-induced damage: determining the damage initiation mechanisms, Phys. Rev. Lett. 91 (2003) 1–4 127402. [14] John Reintjes, C. Robert, Eckardt, two-Photon Absorption an ADP and KD*P at 266.1 nm, IEEE J. Quantum Electron. QE-13 (9) (1977). [15] Mengxia Wang, Zhengping Wang, Xiangxu Chai, Yuxiang Sun, Tingting Sui, Xun Sun, Xu, Xinguang, third-order nonlinear optical properties of ADP crystal, Mater. Res. Express 5 (5) (2018). [16] Yafei Lian, Jibin Wen, Fang Wang, Tingting Sui, Jin Huang, Xun Sun, Xiaodong Jiang, Nonlinear optical characteristics of an ADP crystal grown in a defined direction, Opt. Mater. Express 8 (Issue 9) (2018) 2614–2624. [17] Lili Zhu, Xiaoyu Gan, Qinghua Zhang, Baoan Liu, Mingxia Xu, Lisong Zhang, et al., Electrical conduction in deuterated ammonium dihydrogen phosphate crystals with different degrees of deuteration, Chin. Phys. Lett. 32 (5) (2015) 107–110. [18] K. Iliopoulos, I. Guezguez, A.P. Kerasidou, A. Elghayoury, D. Branzea, G. Nita, et al., Effect of metal cation complexation on the nonlinear optical response of an electroactive bisiminopyridine ligand, Dyes Pigments 101 (1) (2014) 229–233. [19] B. Kulyk, D. Guichaoua, A. Ayadi, A. El-Ghayoury, B. Sahraoui, Metal-induced efficient enhancement of nonlinear optical response in conjugated azo-based iminopyridine complexes, Org. Electron. 36 (2016) 1–6. [20] B. Kulyk, S. Taboukhat, H. Akdas-Kilig, J.L. Fillaut, Y. Boughaleb, B. Sahraoui, Nonlinear refraction and absorption activity of dimethylaminostyryl substituted bodipy dyes, RSC Adv. 6 (2016) 84854–84859. [21] I. Guezguez, A. Ayadi, K. Ordon, K. Iliopoulos, D.G. Branzea, A. Migalska-Zalas, et al., Zinc induced a dramatic enhancement of the nonlinear optical properties of an azo-based iminopyridine ligand, J. Phys. Chem. C 118 (14) (2014) 7545–7553. [22] D.N. Nikogosyan, Nonlinear Optical Crystals: A Complete Survey, Springer, Cork, Ireland, 2001. [23] M. Sheik-Bahae, A.A. Said, T. Wei, D.J. Hagan, E. Vanstryland, Sensitive measurement of optical nonlinearities using a single beam, IEEE J. Quantum Electron. 26 (4) (1990) 760. [24] R. Desalvo, M. Sheik-Bahae, A.A. Said, D.J. Hagan, E.W. Van Stryland, Z-scan measurements of the anisotropy of nonlinear refraction and absorption in crystals, Optic Lett. 18 (3) (1993) 194. [25] D.S. Corrêa, L.D. Boni, L. Misoguti, I. Cohanoschi, F.E. Hernandez, C.R. Mendonça, Z-scan theoretical analysis for three-, four- and five-photon absorption, Optic Commun. 277 (2) (2007) 440–445. [26] H. Zhang, S. Virally, Q. Bao, L.K. Ping, S. Massar, N. Godbout, et al., Z-scan measurement of the nonlinear refractive index of graphene, Optic Lett. 37 (11) (2012) 1856.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Nonlinear optical characteristics of DADP crystals with different deuterium a,b,c
d
a
a
Yafei Lian , Lili Zhu , Jin Huang , Xiangxu Chai , Dongting Cai Xun Sunb,c,∗∗, Xiaodong Jianga,∗
a,b,c
T
b,c
, Mingxia Xu ,
a
Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang, 621900, China State Key Laboratory of Crystal Materials, Shandong University Jinan, 250100, China Key Laboratory of Functional Crystal Materials and Device (Shandong University), Ministry of Education Jinan, 250100, China d School of Materials Science and Engineering, Shandong Jiaotong University, Jinan, 250300, China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: D)ADP crystals Nonlinear absorption Nonlinear refraction
DADP crystals with different deuterium as 0%, 30%, 50%, and 70% were grown via traditional temperature reduction method and the samples were gained from the as-grown crystals in the directions of Ι -type (θ = 90°,φ = 45°). The transmittance spectrum, UV absorption spectrum, and Raman spectrum were implemented to describe the properties of DADP crystals, and the deuteration of ADP crystals makes a difference on the absorption peaks in these spectrums. Particularly, the optical band gap Eg decreases with the accretion of deuterium content. The third-order nonlinear optical characteristics were systematically represented and tested by means of Z-scan measurements with Pico-second pulse laser irradiation at λ = 355 nm, with the results listing the reverse saturable absorption and self-focusing effect. The investigations may contribute to replenish and elucidate the application and laser-induced damage of DADP crystals.
1. Introduction
succeeded in the multi-photon absorption and laser-induced damage of ADP, KDP and DKDP crystals [13–16]. Distinct steps were observed in the damage threshold of DKDP at photon energies of 2.55eV or 3.9eV associated with the multi-photon absorption [13]. Absolute measurements of two-photon absorption (2 PA) coefficients were presented by John Reintjes and Robert C. Eckardt and the value of β = 11 ± 3 × 10−11cm/W at 266.1 nm for ADP and 2.7 ± 0.7 × 10−11cm/W for KDP were determined [14]. Wang et al. [15] investigated the third-order nonlinear optical (NLO) properties of ADP crystal at different wavelengths and different orientations. In our early works, nonlinear characters of thick samples in different directions of ADP crystals were investigated, with the conclusions weaker nonlinear characters correspond to greater LDT, and the nonlinearities and LDT possess the same anisotropy [16]. However, little work has been done on the NLO properties of DADP crystals with different deuterium. In this work, DADP crystals with different deuterium as 0%, 30%, 50% and 70% were grown via traditional temperature reduction method and the samples were gained from the as-grown crystals in the directions of Ι -type (θ = 90°,φ = 45°). The transmittance spectrum, UV absorption spectrum and Raman spectrum were implemented to describe the properties of DADP crystals, and the deuteration of ADP
Ammonium dihydrogen phosphate (NH4H2PO4, ADP) belonging to scalenohedral (twelve faced) class of tetragonal crystal system [1] is a typical isomorph of potassium dihydrogen phosphate (KH2PO4, KDP) crystal which is the only nonlinear optical material used in inertial confinement fusion (ICF) engineering [2], and can be applied for frequency conversion such as the third or fourth harmonic generation [3] due to its large nonlinear optical (NLO) coefficient, high transmittance and high laser damage threshold (LDT) [4,5]. Compared with KDP crystals, deuterium KDP crystals can efficiently reduce the Stimulated Raman Scattering (SRS) caused by high power laser [6]. And as for ADP crystals, the deuterium is additionally able to increase the infrared cutoff edge and the transmittance in the near-infrared region [7]. Nowadays, the laser-induced damage is the bottleneck for the nonlinear optics such as DKDP crystals, ADP crystals and fused silica [8–10]. And as Eg/hω < 4 or pulses are in the ps and fs regime, intrinsic damage is generally dominated by the multi-photo generation of free electrons when multi photons are absorbed by one single electron called multiphoton absorption [11]. The high order nonlinear processes are connected with refractive index change and light absorption in a strong electromagnetic field [12]. In recent years, many efforts have
∗
Corresponding author. Corresponding author.State Key Laboratory of Crystal Materials, Shandong University Jinan, 250100, China. E-mail addresses: [email protected] (X. Sun), [email protected] (X. Jiang).
∗∗
https://doi.org/10.1016/j.optmat.2018.12.018 Received 26 September 2018; Received in revised form 13 December 2018; Accepted 14 December 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
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ΔΦ 0 = kn2 I0 Leff
crystals makes a difference on the absorption peaks in these spectrums. Particularly, the optical band gap Eg decreases with the accretion of deuterium content. The third-order nonlinear optical characteristics were systematically represented and tested by means of Z-scan measurements with Pico-second pulse laser irradiation at λ = 355 nm, with the results listing the reverse saturable absorption and self-focusing effect. The investigations may contribute to replenish and elucidate the application and laser-induced damage of DADP crystals.
Where ΔΦ0 , k and n2 are the nonlinear phase shift at the focus, the wave vector equaling to 2π/λ and the nonlinear refractive index, respectively. As known, the third-order nonlinear processes include the nonlinear absorption (imaginary part) and the nonlinear refraction (real part) as [22,27].
χ (3) = χR(3) + iχI(3)
[7]
2. Experimental
χR(3) (esu) = cn02 n2/120π 2 (m2 / W )
[8]
2.1. Samples preparations
χI(3) (esu) = c 2n02 β /(240π 2ω)(m / W )
[9]
χR(3) χI(3) ,
c and n0 represent the third-order nonlinear Where χ (3) , susceptibility, real part of χ (3) related to the nonlinear refractive index, imaginary part concerned with the nonlinear absorption coefficient, the speed of light in a vacuum and the linear refractive index.
The ADP and DADP crystals were prepared from aqueous solution via the traditional growth method by temperature reduction [17]. Z-cut seed plates with a size of 50 mm × 50 mm × 10 mm were gained from the other as-grown crystals and the growth producers are similar with Ref. 17. In addition, the samples tested in our experiments were cut from the ADP and DADP crystals in the directions of Ι -type (θ = 90°,φ = 45°) and were processed and polished to a size of 10 mm × 10 mm × 2 mm.
3. Results and discussion 3.1. Spectrum measurements The transmittance spectra and UV absorption spectra of (D) ADP crystals are shown in Fig. 1. And the transmittance spectra show high transmittance with few differences between the others in the range of 400 nm-1200nm, indicating none macro defects consist in the samples and the samples were well polished. In the near-infrared region, the transmittance of samples increases with the accretion of deuterium (D) content, owing to the substitution of D to H. The transmittance spectra illustrate that three absorption peaks exist in near-IR, namely about 1070 nm, 1300 nm and 1584 nm, as shown in Fig. 1(a). Referring to Ref. [28], the 3739.74 cm−1 (2674 nm), 3251.55 cm−1 (3075 nm) peak is assigned to be the symmetrical stretching of OeH bond (γs-OH) and NeH group (γs-NH4), respectively. Therefore, the 1070 nm peak exists in the third overtone region of the NeH (D) bond and the 1584 nm peak lies in the second overtone region of the NeH (D) bond. Besides, the 1300 nm peak is in the second overtone of the OeH (D) bond. These absorptions decrease as the deuteration take places, leading to the augmentation of transmittance in near-IR. With the increase of deuterium content, the three absorption peaks have a redshift for that the D atom shows a higher quality than H atom and the vibration of N-D bond abates as the substitution in process. The transmittance of (D) ADP crystals between 190 and 400 nm decrease in some extent, by virtue of the reason that metal impurity ions i.e. Cr3+, Fe3+ et al. and instinct defects (vacancy, hydrogen, Frenkel pair and interstitial oxygen et al.) [29–31] exist in the DADP crystals and the defect energy levels may absorb the UV light [32]. Fig. 1(b) illustrates the UV absorption spectrum, from which the absorption peaks of the (D) ADP crystals are all about 199 nm (20 spectrums were tested and the average absorption peak is 199 nm). Besides, the relationship between hν vs. (αhν) 2 is gained from Fig. 1(b), as shown in Fig. 1(c). There can be found unphysical steps in UV–vis absorption plots, near c.a 300 nm and 4eV accordingly in Fig. 1(b) and (c), which are supposed to be related to the spectrophotometer operation. Besides, the optical band gap of DADP crystals with different deuterium content can be calculated as listed in Table 1. The optical band gap Eg decreases with the accretion of deuterium content. As supposed, the deuterium renders the crystals imperfect leading to the decrease of the optical band gap. Raman spectra of (D) ADP crystals with different deuterium are shown in Fig. 1(d). It can be seen from the figure that the strongest Raman peak of the ADP crystal is located at 925 cm−1, which is caused by the asymmetric stretching vibration of the PO4 tetrahedral group (γas-PO4) [33]. Besides, with deuterium, the Raman peak split at 925 cm−1 into two peaks, and the intensity of the scattering peak is significantly reduced. The position of the main peak of the scattering has a redshift as the content of strontium increases, that is, moving towards the low wave
2.2. Experiment procedures In order to the transmittance spectrum with a test band of 200–2000 nm at room temperature of samples was tested by Hitachi U3500 spectrograph. And the UV absorption spectrum was measured via LS5-Lambda 950 at the range of 190–800 nm. Room temperature Raman spectra were obtained by microscopic Raman spectroscopy (Renishaw in via) using a 532 nm laser over the range of 100–1800 cm−1. The third-order nonlinear optical characteristics were investigated by utilizing the Z-scan technique in Pico-second regime and the experimental set-up and procedures were described in many already published papers in detail [18–21]. Besides, the beam waist radius in our experiments is about 20 μm. 2.3. Theories As focusing an intense light source on the tested samples, intensity dependent absorption is induced by the light field and two or more photons are absorbed leading to the appearance of multi-photon absorption process, which can be expressed as [22].
α(Ι ) = α + βI + γI 2 + ⋅⋅⋅+⋅⋅⋅
(1)
Where I, α, β and γ is the laser energy intensity, the linear absorption coefficient, the nonlinear two-photon absorption coefficient, and the three-photon absorption coefficient, respectively. Now a Gauss beam spread along the normal of crystal faces, the 2 PA coefficient can be calculated as [23–25]. ∞
T2PA =
∑
[−β2 I0 Leff /(1 + z 2/ z 02)]n
n=0
(n + 1)3/2
(2)
Leff = [1 − exp(−α 0 L)]/ L
(3)
z 0 = πω0 / λ
(4)
Where T2PA, β2, I0, z, z0, Leff, α0, L, ω0 and λ are the normalized transmittance, 2 PA coefficient, the peak intensity, the sample position, the diffraction length of focused beam, the effective sample thickness, linear absorption coefficient, sample thickness, the waist radius and the wavelength of the light beam respectively. What is more, the normalized transmittance of the closed-aperture Z-scan can be expressed as [26].
T=1+
4ΔΦ 0(z / z 0) [(z / z 0)2 + 9][(z / z 0)2 + 1]
[6]
[5] 18
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Fig. 1. (a). Transmittance spectra of the ADP and DADP samples; (b). UV absorption spectra of (D) ADP crystals; (c) hν (eV) vs. (αhν) 2 relationship of (D) ADP crystals; (d) Raman spectra of (D) ADP crystals. (There can be found unphysical steps in UV–vis absorption plots, near c.a 300 nm and 4eV accordingly in (b) and (c), which are supposed to be related to the spectrophotometer operation).
Table 1 The Spectrum measurements of (D) ADP crystals. Spectrum Transmittance spectrum
UV absorption Raman spectrum
3rd NeH 2nd OeH 2nd NeH Eg (eV) γas-PO4 δ-OD
0%
30%
50%
70%
1066 1295 1581 5.72 922 –
1069 1308 1583 5.68 919 –
1072 1315 1585 5.66 912 944
1075 1323 1588 5.64 900 963
number. And the newly small peak moves towards the high wave number, as shown in Table 1. The appearance of the new peak is due to the substitution of hydrogen by deuterium, which causes the appearance of the in-plane bending vibration of the O-D hydrogen bond (δOD). However, the substitution reduces the vibration frequency of the hydrogen bond, and the originally strong symmetric stretching vibration mode weakens and moves to the low frequency, and the weak bending vibration gradually appears and moves toward the high frequency.
3.2. Nonlinear absorption and nonlinear refraction The results of open-aperture Z-scan measurements namely the normalized transmittance versus the position Z are listed and fitted well via equation (2) and are shown in Fig. 2(a). The points are the experimental data and the solid lines are the fitted curves. In Fig. 2(a), the fitted lines represent the two-photon absorption of ADP, 30% DADP, 50% DADP and 70% DADP crystals, respectively and show the valley characteristic, indicating the existence of reverse saturable absorption of (D) ADP crystals. The depth of the valley stands for the magnitude of nonlinear absorption, showing an increasing tendency with the deuterium adding up. The diffraction length z0 of focused beam at 355 nm is about 3.54 mm as shown in Table 2. From the results, it can be seen the nonlinear absorption increases greatly as deuterium content augments. For the nonlinear refraction, the normalized transmittance of
Fig. 2. Z-scan measurements results of ADP samples at 355 nm. (a) openaperture (b) closed-aperture. The points are the experimental data and the solid lines are the fitted curves.
closed-aperture Z-scan measurements with the peak laser intensity of about 39 GW/cm2 is performed in Fig. 2(b). The dotted lines and solid lines are on behalf of the experimental data and the fitting results 19
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Table 2 Nonlinear absorption parameters, nonlinear refractive parameters and the third-order nonlinear susceptibility at 355 nm. Wavelength
sample
β2 cm·GW−1 ( × 10−2)
n2 m2 W−1 ( × 10−19)
Imχ3 esu ( × 10−14)
Reχ3 esu ( × 10−14)
χ(3) esu ( × 10−14)
355 nm
ADP 30%DADP 50%DADP 70%DADP
3.26 8.01 9.99 13.8
4.69 4.88 5.90 8.18
3.50 8.61 10.7 14.8
2.84 2.95 3.57 4.95
4.51 9.10 11.3 15.6
± ± ± ±
0.06 0.12 0.17 0.10
± ± ± ±
0.08 0.11 0.24 0.26
± ± ± ±
0.06 0.13 0.18 0.11
± ± ± ±
0.05 0.06 0.14 0.16
± ± ± ±
0.08 0.14 0.22 0.19
two electrons go back immediately and the others fall down afterword. Supposing the laser fluence is high enough beyond the laser-induced damage threshold, more and more photons are absorbed and the activated electrons cannot relax in a short time. As a result, the accumulation of the excited electrons takes place and the initial process of laser-induced damage subsequently appears. Fig. 2(a) performs that the valley depths of the absorption curves vary with the deuterium changing. That is the nonlinear absorption coefficient β increases greatly as deuterium content augments and β of 70% DADP is one order of magnitude larger than that of ADP. What we think is that the β is associated with the band gap Eg. The nonlinear absorption can be regarded as a probability event, in which the photons of the same number are more potentially absorbed when the band gap is narrower, contributing to a larger nonlinear absorption coefficient. Open-aperture Z-scan measurements results of ADP crystal at 355 nm with different peak intensities are shown in Fig. 4(a). And from the results of β vs. I0 listed in Fig. 4(b), it can be seen that there is a
adopting formulas (4), respectively. The valley-peak configurations in the results indicate nonlinear refractive index of ADP crystals at 355 nm is positive [34], which means as the (D) ADP crystals are irradiated by the high power laser, the self-focusing effect will be induced. Besides, the valley-to-peak value shows the change of the nonlinear refraction properties [35]. And the real part of the third-order susceptibility representing the nonlinear refraction can be calculated by Eq. (8). Then the third-order susceptibility is achieved as listed in Table 2.
3.3. Discussions As illustrated in Fig. 2, the normalized transmittance of Z-scan measurements is well fitted by formulas (2) and (5), which are the twophoton absorption theories. As typical isomorphs of DKDP crystals, ADP and DADP crystals are all wide band gap nonlinear optical materials, with a theoretical band gap of 6.96 eV at room temperature [36]. However, many factors i.e. supersaturation, the stability of the solution, hydrodynamic conditions, the cooling rate and the impurities ions et al. [37,38] may affect the growth of the (D) ADP crystals. For example, with the supersaturation levels increasing, the stability of the solution may decrease, contributing to the addition of defects in crystals; impurity composition in the solution will lead to a distortion of the lattice as absorbed during the growth process. Therefore, all the micro defects result in the occurrence of the abundant defect energy levels which can deduce the intrinsic band gap of (D) ADP crystals, meeting well with the experimental data from the UV absorption spectrum. Besides, the photon energy at 355 nm is 3.49 eV and multi-photons cross sections for two, third, and fourth order absorption in wide band gap materials are on the order of 10−50 cm4 s, 10−81 cm6 s2, and 10−114 cm8 s3, respectively [25,39]. Therefore, two-photon absorption (2 PA) is predominant during the nonlinear process of (D) ADP crystals. As shown in Fig. 3, the process of 2 PA is well described. Before the laser-induced damage occurs, the nonlinear absorption shows a process of dynamic equilibrium until the laser does not irradiate the solid samples after relaxation. In the real crystals, there are many defect levels formed by the defects as said in the above and the defect levels lie between the valence band (VB) and conduction band (CB). As the laser irradiates, the electrons in the VB or in the defect levels may absorb the photons and begin to transition to the upper energy levels; then they fall back to the defect level or the VB. This process keeps equilibrium with a short relaxation time. For example, ten electrons jump into the CB at a certain time, with
Fig. 4. Open-aperture Z-scan measurements results of ADP crystal at 355 nm with different peak intensity I0 (GW/cm2) (a) and the nonlinear absorption coefficient β (b). As the peak intensity is lower than 32.0 GW/cm2, the nonlinear absorption process does not take place and the β is zero. As the peak intensity adds up, the valley depth and β increase overtly. The normalized transmittance curve drops abruptly and almost breakdown near the focal point, connoting that the laser-induced damage occurs at this peak intensity.
Fig. 3. Schematic diagram of elastic 2 PA process. 20
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describe the properties of DADP crystals, and the deuteration of ADP crystals makes a difference on the absorption peaks in these spectrums. With the increase of deuterium content, the three absorption peaks in the transmittance spectrum have a redshift as D atom shows a higher quality than H atom and the vibration of N-D bond abates as the substitution in process; the optical band gap Eg decreases with the accretion of deuterium content as the UV absorption spectrum perform. With deuterium, the Raman peak split at 925 cm−1 into two peaks and the intensity of the scattering peak is significantly reduced. The position of the main peak of the scattering has a redshift as the content of strontium increases, that is, moving towards the low wave number. And the newly small peak moves towards the high wave number. Besides, The third-order nonlinear optical characteristics were systematically represented and tested by means of Z-scan measurements with Picosecond pulse laser irradiation at λ = 355 nm, with the results listing the reverse saturable absorption and self-focusing effect. The nonlinear absorption coefficient β increases greatly as deuterium content augments and β of 70% DADP is one order of magnitude larger than that of ADP, indicating the β is associated with the band gap Eg. The results indicate that the NLA exists in an interval of the laser peak intensity and in this interval, the β adds up after I0 increasing. It is believed our research is propitious to the application of (D) ADP crystals as frequency conversion devices.
threshold of the appearance of the nonlinear absorption. That is, as the peak intensity is lower than 32.0 GW/cm2, the nonlinear absorption process does not take place and the β is zero. As the peak intensity adds up, the valley depth and β increase overtly. However, the normalized transmittance curve drops abruptly and almost breakdown near the focal point, connoting that the laser induced damage occurs at this peak intensity. The 2 PA theory is forced fitted on this curve to gain the nonlinear absorption coefficient to be about twenty times of that at I0 = 42.5 GW/cm2. As supposed, the process of NLA appears below the fluence of laser induced damage threshold (LDT), otherwise will not arise at some lower fluence. That is, the NLA exists in an interval of the laser peak intensity and in this interval, the β adds up after I0 increasing. Provided that the I0 exceeds above the critical point, the excited energy level induced by multi photons absorption cannot recover and the electron will be ionized, resulting in the occurrence of the laserinduced damage [40,41]. As shown in Fig. 2(b), the distance of valley-to-peak varies with the deuterium changes. The nonlinear refractive index is positive, which means that as the high-power laser irradiate, the NLO crystals become an additional positive lens as a result of the self-focusing effect, which will lead to a larger damage possibility at the rear surface. The parameters of nonlinear refraction are listed in Table 2 and the nonlinear refractive index increases with the addition of deuterium. As known, the nonlinear absorption makes a significant effect on the nonlinear propagation effects such as the self-focusing effect, which can contribute to the reduction of the cross section of high power laser beams which propagate through the crystals [42]. The reduction of laser spot size induced by the self-focusing effect and beam breakup makes local increase of laser irradiance to the levels easily exceeding LIDT and the laser-induced damage driven by self-focusing occurs as soon as irradiance reaches the threshold of self-focusing which is lower than LIDT in most cases [43]. The self-focusing effect takes place as beam has traveled the certain distance within the crystals calling self-focusing distance ZSF [44].
ZSF =
2n 0 ω02 λ
PCR =
π (0.61)2λ 02 8n 0 n2
1 P / PCR − 1
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Financial support from the National Natural Science Foundation of China (Grants No 51402173, 61705204 and 61705206), the Fundamental Research Funds for the Central Universities, Young Scholars Program of Shandong University, Laser Fusion Research Center Funds for Young Talents (NO. LFRC-PD011).
(10)
References
(11)
Where P, PCR is the power of the laser beam and the critical power of self-focusing, respectively. The position of self-focus is not steady and fixed in space. The first self-focus forms at the distance ZSF from the front surface of the samples and the second self-focus follows the first one. The process is repeated to make a track of the self-focus points. According to Eqs. (10) and (11), self-focusing distances of 0%, 30%, 50% and 70% DADP crystals are 1.03 mm, 1.00 mm, 0.91 mm and 0.76 mm, respectively. That means the self-focus point may appear in the samples and can induce the damage if the laser power is beyond the LIDT. However, a larger third-order nonlinear refractive index is necessary to all optical switch materials [45], especially for the organic compounds with a large π-electron conjugation system. Therefore, DADP crystals are promising for applying as optical switch optics, and ADP with some deuterium, to a certain extent, may promote the Kerr nonlinearity in correlation with nonlinear refraction. Besides, appropriate nonlinear absorption and nonlinear refraction properties should all be considered for the application of (D) ADP crystals as frequency conversion devices.
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4. Conclusions In our study, DADP crystals with different deuterium as 0%, 30%, 50%, and 70% were grown via traditional temperature reduction method and the samples were gained from the as-grown crystals in the directions of Ι -type (θ = 90°,φ = 45°). The transmittance spectrum, UV absorption spectrum, and Raman spectrum were implemented to 21
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