The nonlinear optical absorption (NLA) of 70% deuterated DKDP crystals that were cut along different directions and annealed under different temperatures was measured at the third-harmonic-generation (THG) wavelength (355 nm) of a nanosecond Nd:YAG laser by using the Z-scan method. The nonlinear absorption (NLA) coefficient β was obtained by fitting the experimental data. According to the fitting result, the nonlinear absorption at 355 nm is identified to two-photon absorption. Results indicate that the β value of the type I THG direction (5.6 × 10−2 cm/GW) was close to that of the type II THG direction (5.2 × 10−2 cm/GW). Moreover, the β values of both types were obviously below the data of the Z-axis direction (7.2 × 10−2 cm/GW). Our experiments showed that thermal annealing can effectively decrease NLA coefficients. At the optimum annealing temperature of 140 °C, the β of the type II THG sample was 2.4 × 10−2 cm/GW, which was only 46% that of the unannealed crystal. This work will aid the THG application of DKDP crystals in inertial confinement fusion systems.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Since the construction of the National Ignition Facility Project laser systems, the only viable alternatives for frequency conversion (second, third, and fourth harmonic generators) of large-aperture lasers at wavelengths from the infrared region to the UV region were the ferroelectric crystals discovered by Busch and Scherrer in 1935, specifically the KDP and DKDP. These crystals are effective because of their high damage thresholds, moderate nonlinear optical coefficients, and the ability to grow into large sizes [1–5]. In contrast to a KDP crystal, a DKDP crystal with 70% deuterium content can effectively reduce transverse stimulated Raman scattering, which induces damage to optic components [6–8]. Therefore, 70% deuterated DKDP crystals are the only viable alternatives for third harmonic generator (THG) optics in current inertial confinement fusion systems.
Although DKDP crystals yield optics that display high optical and crystalline quality, laser damage is still observed in the bulk and surface of optics because of the intrinsic and impurity defects at the fluences that fall within the range of the operational fluences [9–12]. This property limits the laser output energy density and service life of the crystal as the large-aperture components are exposed to high irradiance and high-fluence laser pulses [13, 14]. One main reason for this limitation is the absorption of UV light increasing nonlinearly with laser radiation intensity. The absorption of a 1 cm-long KDP crystal at 355 nm increases from 4.4% to 6.3% when the energy density rises from 0.1 J/cm2 to 3 J/cm2 . Nonlinear absorption (NLA) is usually ascribed to multiphoton absorption. During harmonic generation, although the energy loss of the laser beam derived from NLA is a problem, the potential damage to the edges of the optic and the adjacent components (e.g., holders) caused by the amplified NLA is of greater concern . C. W. Carr et al. observed sharp steps in the damage threshold of DKDP crystals at 2.55 and 3.9 eV. They associated these sharp steps with multiphoton absorption. By contrast, in the region between steps, the damage threshold decreased smoothly with decreasing wavelength . Their results suggested that the NLA process dominated the mechanism for damage initiation in the component . Although the NLA of the KDP crystal has been investigated at the wavelengths of 211, 216, 248, 264, 266, and 355 nm [17–20], the NLA of the DKDP crystal has been seldom explored. Moreover, picosecond or femtosecond lasers have been selected to measure the NLA in nearly all of these studies. The reason is the low thermal effects of the picosecond or femtosecond lasers on the reduction of the risk of laser-induced damage (LID) and decreased difficulty of tests. Thus, it is quite necessary to investigate the characteristic of the NLA of DKDP crystal by using a nanosecond Nd:YAG laser stem from the fact that DKDP crystal is mainly applied to nanosecond laser in engineering applications field.
In this paper, the Z-scan technique was employed to measure the NLA of 70% deuterated DKDP that were cut at different directions (Z-direction, type I, and type II) and annealed under different temperatures at the wavelength of 355 nm by a nanosecond laser. This method is highly sensitive and simple in contrast to nonlinear interferometry, degenerate four-wave mixing, and beam distortion method [21, 22]. The NLA coefficient β was obtained by experimental data fitting.
2.1 Sample preparation
Large-sized DKDP crystals with 70% deuterium content were grown from deuterated aqueous solution through the traditional temperature reduction method. DKDP crystal grew along the Z-direction with a growth rate of 1 mm/day and at temperatures gradually decreasing from 56 °C to 26 °C. Samples with different cutting directions were obtained from the adjacent positions of the same crystal, and type II samples were annealed at different temperatures for 96 h. A new annealing method was implemented using silicone oil under 28 Pa·s as a protective ambient environment . The size of all the samples was 20 mm × 10 mm × 2 mm. The specific cutting schematic and thermal annealing for the DKDP crystals are shown in Figs. 1 and 2, respectively, and Table 1 lists the specific information for these samples.
2.2 Measurement of the NLA
A schematic of the Z-scan experimental set-up is shown in Fig. 3. The original laser was divided into two beams of equal energy by a beam splitter. D1 and D2 (energy meter) were used to detect the energy of the initial laser energy and transmitted laser energy. When the sample moves from end to end along the axis by the platform, the transmitted laser energy decreased initially, reached the minimum at the focus, and then increased. Thus, the D2/D1 ratio was used as a function of the displacement. The absorption coefficient of the material can be expressed as16, 24]. According to the theory of multi-photon absorption, two-photon absorption (2 PA) dominates the NLA when the photon energy hv is in the spectral region of Eg/2 < hv < Eg, whereas, three-photon absorption (3 PA) manifests itself primarily in the region Eg/3 < hv < Eg/2 . Theoretically, the NLA of DKDP crystal at 355nm (3.54 eV) is ascribed to 3 PA. But actually, the energy level splitting and intermediate band caused by impurities existed in DKDP crystal would reduce the order of multiphoton absorption. Thus, the order of NLA should be identified by the fitting result. The normalized transmittance formula T2PA and T3PA are shown in (2) and (3), which are based on the principle of 2PA and 3PA respectively.
3. Results and discussion
The linear transmittance spectra of the sample were measured with a Lamda-950 spectrophotometer at room temperature from 200 nm to 1800 nm. All the samples showed high transmittances from 340 nm to 1000 nm, indicating that the samples had no macroscopic defect and were well polished (Fig. 4). However, in the 200–300 nm band, the transmittance dropped rapidly. One reason is that the impurities (Fe3+, Cr3+, and Al3+) and intrinsic defects (hydrogen vacancy, interstitial oxygen, and Frenkel pair) in the DKDP crystal strengthen UV absorption . Given the cut direction, the transmittance of the Z-direction sample was approximately 3% higher than those of types I and II, which nearly had the same curve. After annealing, the transmittance of the type II samples were improved to different degrees, and the highest transmittance was 91% at 140 °C. Because of samples with different annealing temperature were obtained from the adjacent positions of the same crystal, the effect of metal impurities can be ignored. This result suggests that thermal conditioning can enhance transmittance, and the optimal annealing temperature is 140 °C. The specific values are shown in Table 2.
The linear absorption coefficient α can be evaluated through the linear transmittance in accordance with the following equation :
The dependence of the nonlinear transmission versus the Z (displacement in the Z-direction) obtained through the fitting test data are shown in Figs. 5. Reverse-saturable absorption was observed in samples of different cutting directions, and the test data agreed well with the 2PA fitted curve. This result indicates that the energy level splitting and intermediate band caused by impurities existed in DKDP crystal actually reduce the order of multiphoton absorption, and the 2PA process dominates the NLA in the 70% deuterated DKDP crystal at 355 nm. The NLA coefficients obtained through the 2PA fitting formula are shown in Table 3 and Figs. 6 and 7.
The NLA of the DKDP crystal displayed obvious anisotropy, that is, the NLA in the Z-direction was stronger than those of types I and II (Fig. 6). The NLA in type II was the lowest. This diversity demonstrated that the crystal orientation significantly affects the process of the NLA. Theoretically, 2PA is related to third-order susceptibility, in which the coefficient can be obtained by [27, 28]29]:17]. The specific LID threshold of the different types of DKDP crystals will be measured to support this anisotropy.
The effect of different annealing temperatures on the changes in the NLA values of the type II DKDP crystals is depicted in Fig. 7. We expected that annealing positively influences the crystal optical properties because the NLA of all the annealed samples was lower than that of the unannealed samples. This observation agrees closely with those of other studies [30–33]. The ability of thermal conditioning is supposedly due to the diffusion of impurities (inorganic, organic, and water) or the reconstruction of the structural defects in the crystal by thermal energy . A major difference between KDP and DKDP is that DKDP undergoes a tetragonal-to-monoclinic transition, although the tetragonal phase remains stable from the ferroelectric state at −150 °C up to thermal decomposition at approximately 180 °C. In this transition, the temperature depends on the deuteration ratio. In particular, phase-transition temperature drops as the deuterium content rises and would thus theoretically restrict the application effect of annealing. This difference can be explained by the structural difference. Furthermore, the lattice coefficients of the DKDP crystals gradually change with the x (deuterium content) variation . However, during the thermal annealing, the annealing temperature was unable to reach the theoretical maximum value, and the lower critical annealing temperature of DKDP may have prohibited the crystal from reaching the second critical temperature necessary to alter the defects and reconstruct the metastable state responsible for the crystal optical properties. One reasonable hypothesis is that the defects in the DKDP crystals reduced the barrier heights of the phase transition and the actual temperature of the phase transformation. Moreover, via several annealing experiments, we found that DKDP became opaque beyond 150 °C. Therefore, although the highest annealing temperature of the KDP crystal was able to reach up to 175 °C, which is merely below the destructive tetragonal/monoclinic phase-transition temperature of 180 °C , the highest temperature of the DKDP crystal was unable to exceed 150 °C. Figure 7 and Table 3 show that the NLA of the crystals decreased as the annealing temperature increased, except at 150 °C. Sample E achieved the lowest NLA coefficient, that is, (2.392 ± 0.158) × 10−2 cm/GW. A critical annealing temperature may be supposed to have existed at a point in which the metastable state can be optimally reconstructed. At an increasing annealing temperature, additional thermal energy can be used to revert the metastable atoms to the balanced position when annealing occurs below the critical point. In this case the higher the temperature, the better the annealing effect. If the annealing temperature exceeds the critical temperature, the vibration amplitude of the atoms in equilibrium is increased by excess energy. This phenomenon may be unfavorable to thermal annealing. Collectively, our results revealed that the optimal annealing temperature is approximately 140 °C, which is in good agreement with an early report .
Additional measurements are underway to confirm the change in the crystal structure of the annealing samples and quantitative relation between the NLA and the damage threshold.
The NLA of 70% deuterated DKDP crystal that were cut along different directions (Z, type I, and type II) and under annealing at different temperatures (120 °C, 130 °C, 140 °C, 150 °C) was measured at 355 nm wavelength of a nanosecond Nd:YAG laser through the Z-scan technique. The NLA coefficient β was calculated by fitting formula. The NLA of 70% deuterated DKDP crystal at the wavelength of 355 nm is identified to 2PA absorption according to the fitting result. Two conclusions were reached. First, the NLA of the DKDP crystals showed obvious anisotropy. In particular, the NLA in the Z-direction((7.234 ± 2.42) × 10−2 cm/GW) was the strongest, whereas that in type II((5.164 ± 0.284) × 10−2 cm/GW) was the lowest. Second, annealing positively affected the NLA. Obviously, the NLA of all the annealed samples was lower than that of the unannealed samples, and the optimal annealing temperature for the NLA was approximately 140 °C((2.392 ± 0.158) × 10−2 cm/GW). These findings add substantially to understanding of the influence of NLA on frequency conversion and the LID mechanism of the DKDP crystal. Thus, our study can serve as a reference for the annealing process of DKDP crystals.
National Natural Science Foundation of China (NSFC) (51323002, 51402173); Ministry of Education (MOE) (625010360).
Prof. Wenyong Cheng (Institute of optical center, Shandong University, China) is acknowledged for providing experiment platform.
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