Noncritical phase-matching (NCPM) fourth harmonic generation (FHG) of a 1053 nm laser was realized at room temperature on an ammonium dihydrogen phosphate (ADP), 35% deuterated DADP and 62% deuterated potassium dihydrogen phosphate (DKDP) crystals grown by point-seed rapid growth method. The NCPM temperature is 33.7°C for ADP, 32.1°C for DADP, and 31°C for DKDP crystal. The temperature, angle tuning properties, and performance in NCPM FHG of ADP DADP and DKDP crystals are calculated and compared in detail. The 2ω-to-4ω conversion efficiency has been demonstrated up to 67% with ADP crystal, 63.45% with DADP crystal, and 63.78% with DKDP crystal. Large-aperture high-efficiency fourth harmonic generation would be realized by improving the temperature control and crystal quality with all these crystals and all these results could be a good reference in crystal chosen of NCPM FHG.
© 2017 Optical Society of America
Nowadays, efficient laser frequency conversion gets a lot of world-wide attention in new energy community due to its applications in internal confinement fusion (ICF) system [1,2], which has many advantages compared with the traditional nuclear fusion methods. Phase matching between interacting waves is regarded a necessary condition for achieving high conversion efficiency . Potassium dihydrogen phosphate (KDP) crystals are well known excellent nonlinear optical material due to the high transmittance from infrared to ultraviolet regions, adequate nonlinear-optical coefficient, high laser damage threshold and availability in large crystal size. Therefore, the KDP crystals and their isomorphs are preferred for fourth harmonic generation (FHG) [4,5]. Among all KDP type crystals, partially deuterated dihydrogen phosphate (KD*P or DKDP), ammonium dihydrogen phosphate (ADP) and partially deuterated ammonium dihydrogen phosphate (AD*P or DADP) crystals are three kinds of crystals suitable for noncritical phase matching (NCPM) FHG [6–13]. Compared with critical phase-matching, NCPM has many advantages including a broad angular tuning range, no beam walk-off and high utilization of the as-grown crystal . In previous research work, NCPM FHG has been mainly reported in DKDP crystals, whose refractive index can be adjusted continuously by altering the deuteration level or crystal temperature [6–10]. Compared with DKDP, ADP has a larger nonlinear optical (NLO) coefficient, a higher laser damage threshold and a higher conversion efficiency . However, it suffered a narrow temperature acceptance and larger transverse stimulated Raman scattering (TSRS) gain. Meanwhile, studies on deuterated ADP crystals showed that DADP crystal can effectively decrease the spontaneous Raman scattering intensity and still holds reasonable conversion efficiency . However, to the best of our knowledge, there is no comparison about the NCPM FHG properties in these three kinds of KDP type crystals yet. In this work, ADP, DADP and DKDP crystals were grown by point-seed rapid growth method under the similar growth parameters and room temperature NCPM FHG was realized in all these crystals. The corresponding three kinds of crystals with relatively high transmission and conversion efficiency were chosen to compare the NCPM FHG behavior.
2. Experiment setup
2.1 Crystal growth
ADP, DADP and DKDP crystals were grown by the point-seed rapid growth method from supersaturated aqueous solutions and the deuterium contents of solution was 35% for DADP, and 70% for DKDP. The deuterated level in DKDP crystal (Dc) is 62% calculated by Eq. (1) . Extra pure NH4H2PO4 and KH2PO4 salt were dissolved in the heavy water and deionized water to obtain solutions with expected deuterated degrees. The deuteration level in solution (Ds) is calculated using the Eq. (2) , where n(D) is the mole amount of deuterium and n(H) is the mole amount of hydrogen.
The growth system consists of 5000 ml glass crystallizer placed in a controlled temperature water bath. The solution was filtered by 0.22 μm micro-porous membrane before overheating. Z-cut seed was overheated before being put into the solution. The crystallization procedure was operated within a temperature range of 53 °C to 45 °C and the velocity of the temperature reduction was controlled by considering the supersaturation of the solution. The forward-stop-backward rotation mode was adopted with a speed approximately 70 rpm. The period of crystal growth was about 10 days. Photos of crystals are shown in Fig. 1 and all the crystals are transparent without visible macroscopic defects, and the corresponding growth parameters are listed in Table 1.
2.2 Preparation of samples
In the NCPM FHG, the ADP, DADP and DKDP crystals were processed along the type-I NCPM direction which was at 90° to the Z-axis (θ = 90°) and at 45° to the X-axis (ϕ = 45°). The cross sections of these crystals were 15mm × 15mm and the thickness of the samples was 6.02 mm for ADP crystal, 6.20 mm for DADP crystal and 7.50 mm for DKDP crystal. Their transmittance faces were well polished but uncoated. In the measurement of Raman spectrum, both the incidence of excitation light and detecting of Raman scattering are along Z direction, therefore, the (001) parallel surface of the samples are polished to increase transmittance.
2.3 FHG experiment
The experimental setup for NCPM FHG is shown in Fig. 2, and the detailed experimental setup is explained in Ji’s report . We used a Nd: YLF laser with wavelength of 1053nm, repetition rate of 1Hz, and the pulse width of 50 ps. The treated samples were placed into a copper chamber which was tightly sealed to control the temperature with an accuracy of 0.1 °C. The copper chamber is placed on an adjustable frame that can rotate around the direction of θ = 90° angle with ϕ = 45° and the angular sensitivity of the crystal can be measured in this way. The second harmonic generation (SHG) was obtained by a Type-I PM direction (41°, 45°)-cut KDP crystal and then frequency converted to UV 4ω light in a FHG crystal. The fundamental energy and 2ω energy were measured by several beam splitters and energy calorimeters. An energy calorimeter with a quartz prism is used to detect the generated fourth harmonic radiation.
3. Spontaneous Raman scattering
All the KDP type crystals used in large aperture laser system are vulnerable to TSRS effects due to their relatively large Raman cross section values associated with the totally symmetric mode of the PO4 group. It has been found that partial deuteration is an effective way to reduce Raman scattering [15–17]. This solution stems from the splitting of the totally symmetric breathing mode upon deuteration into two modes that have lower Raman cross sections. The spontaneous Raman scattering spectra were obtained using a Labram HR800 micro-Raman system (Jobin-Yvon Inc., France). A pristine KDP crystal was also tested for comparison. As shown in Fig. 3, the strongest Raman frequency shifting peak of the ADP crystal is located at 926 cm−1, which corresponds to the totally symmetric vibration of the PO4 group. With the crystal deuterated, the peak shifts and splits into two peaks. For 35% deuterated ADP, the strongest Raman frequency shifting peaks are located at 915 cm−1 and 950 cm−1, where its intensity (3559) at 915 cm−1decreased by 24% compared with the intensity (4687) of 926 cm−1 peak of the ADP crystal. For the 62% deuterated KDP the peaks are at 895 cm−1 and 967 cm−1, where its intensity (2452) at 895 cm−1 decreasing by 47.2% compared with the intensity (4642) of the 918 cm−1 peak of the pristine KDP crystal. All these results agree well with Ji’s research , which confirm the accuracy of our results. In addition, it is found that, considering the phase matching, there exists an optimum deuterium content to minimize the intensity of strongest Raman peak [17-18].
4. Temperature tuning
Noncritical phase matching can be realized by adjusting the crystal temperature to make the fourth harmonic generation phase-matching angle to aim the direction θ = 90°. Figure 4 shows the temperature tuning curves of these three crystals and all curves are fitted by the function sin2x/x2 to evaluate the FWHM of the curves. The noncritical phase-matching temperature (referred as TNCPM) was found to be 33.7 °C with a full-width at half-maximum (FHWM) temperature bandwidth of 0.7 °C in ADP crystal. The same FWHM temperature bandwidth was found in DADP crystal with its TNCPM of 32.1 °C. As a contrast, the NCPM temperature is found to be 31 °C and the FWHM temperature bandwidth is 2.6 °C in DKDP. Because the crystal length was 6.06 mm for ADP crystal, 6.20 mm for DADP crystal and 7.50 mm for DKDP crystal, the FWHM temperature bandwidth and temperature phase-mismatch sensitivity (βT) of each crystal were calculated to be 0.42 °C·cm and 13.12 °C−1·cm−1 for ADP, 0.43 °C·cm and 12.82 °C−1·cm−1 for DADP, and 1.95 °C·cm and 2.85 °C−1·cm−1 for DKDP, respectively.
One can see that NCPM was realized at room temperature in all these three crystals which was easy to control the temperature to achieve high-efficiency and stable frequency conversion. The FWHM of DKDP crystal is much larger than that in ADP and DADP crystal. Temperature phase-mismatch sensitivity results show that ADP crystal and DADP crystal are much sensitive to temperature than DKDP crystal (about more than 4 times) and the temperature phase-mismatch sensitivity of these two crystals is considerable. All these results are in good agreement with those ADP crystals reported in , DADP crystals  and DKDP crystals in .
5. Angle tuning
All the crystals temperature was fixed at their NCPM temperature which was 33.7 °C for ADP crystal, 32.1 °C for DADP crystal and 31 °C for DKDP crystal, the angular sensitivity of phase-matching (PM) was measured by monitoring the fourth harmonic output as the crystal rotated near θ = 90°. Figure 5(a) shows that the angular tuning curve of DKDP crystal had a symmetric double peak when the temperature of crystal is lower than its NCPM temperature. When the temperature rises above its TNCPM, the conversion efficiency will decrease. And once the temperature fixed at its TNCPM, the angle tuning curve has only one peak and maintains high conversion efficiency over a wide range. This is another way to determine the NCPM temperature of crystals. Figure 5(b) shows the angle tuning curve of all these three kinds of crystals. All curves are fitted by the function sin2x/x2 to evaluate the FWHM of the curves. The FWHM angular acceptance of DKDP is as wide as 55 mrad and the corresponding external acceptance bandwidth of 47.6 mrad·cm1/2 with an angular phase mismatch sensitivity (βθ) is 9.8 × 10−3 mrad−2 cm−1. Considering the thickness of our experiments is greater than that of S. T. Yang’s , it’s reasonable to find that our results are slightly higher. All these crystals had similar sensitivity to angle deviation and ADP crystals show a higher efficiency in the stable region of small angle deviation. The corresponding external angular acceptance and angular phase-mismatch sensitivity were calculated to be 42.04 mrad·cm½ and 12.60 × 10−3 mrad−2·cm−1 for ADP while 44.49 mrad·cm½ and 11.25 × 10−3 mrad−2·cm−1 for DADP, respectively.
6. Conversion efficiency
By fixing the crystal temperature and incident angle in their individual NCPM conditions, the 2ω-to-4ω conversion efficiency experiment was conducted with adjusting the energy of the 2ω beam (526nm). Afterwards the generated harmonic pulses and simultaneously incident green pulses were monitored. The theoretical calculation was made to compare with experiment results. The theoretical conversion efficiency can be calculated from:19], λ is the fundamental wavelength, FOM is the quality factor of crystal, deff is the effective nonlinear optical coefficient, Ι is 2ω peak intensity, tanh x is the hyperbolic tangent, LLN is the nonlinear interaction length, L is the crystal length, and η(L) is the 2ω to 4ω conversion efficiency of a certain crystal length.
The calculated nonlinear interaction length (LLN) for ADP DKDP and DADP is 4.19mm 5.28mm and 4.63mm, respectively. The theoretical calculated efficiency for ADP DKDP and DADP crystals is 79.68% 79.11% and 75.91%, respectively. Although the crystal lengths are different in each sample, the experimental results shows that thickness does no effect on the highest conversion efficiency in FHG basically. The highest conversion efficiency is obtained at a certain 2ω peak intensity, and then the nonlinear loss occurs, which resulted in decreasing conversion efficiency. All the crystals exhibited high conversion efficiency, as shown in Fig. 6. The ADP crystal exhibited better conversion efficiency and a larger output energy than the DADP crystal and DKDP crystal almost in the whole range. The largest 4ω energy of 21.84mJ appeared at input 2ω energy of 32.8mJ, which corresponded to the external conversion efficiency of 67% for ADP. While the highest conversion efficiency of DADP and DKDP crystals were 63.45% and 63.78% at their largest input 2ω energy, respectively. Many factors affect the conversion efficiency in the actual FHG process, such as homogeneity of rapid grown crystal, thermo-optical effect, linear absorption, two-photon absorption, etc. All these factors lead to deviations between theoretical and experimental values. However, the trends of FHG conversion efficiency of these three kinds of crystals are basically in consistent with the experimental results. Considering the Fresnel losses caused by the uncoated crystal surfaces and the losses of the quartz prism, the internal conversion efficiency should be slight higher than the test results. Excellent UV transmittance is necessary and important for FHG performance. To ensure the UV transmittance consideration, all the samples were cut from the same position of a crystal and processed with identical test techniques. At the FHG wavelength of 263 nm, the transmittance of ADP is 85.6%, which is slightly higher than that 78.0% of DADP and 79.7% of DKDP. It is observed that higher conversion efficiency appeared in the crystal which retains higher transmittance. However, the ADP DADP and DKDP crystals have close linear absorption coefficient which also decide the conversion efficiency. ADP crystal manifest higher conversion efficiency owing to the larger effective nonlinear coefficient.
In summary, high quality ADP DADP and DKDP crystals were grown by point-seed rapid growth method. NCPM FHG was realized in all these crystals at room temperature. The properties relevant to NCPM FHG of a 1053-nm Nd: YLF laser are summarized in Table 2. Comparing the performance of all these crystals in FHG, the higher conversion efficiency appeared in the crystal which had higher transmittance and superior quality, while the angular acceptance of all these crystals are comparable. The major advantage of DKDP crystal is the reflection in the lower temperature sensitivity (about 1/4 of that in ADP crystal). While the ADP crystal has larger NLO efficiency, higher laser damage threshold and higher crystal homogeneity. Therefore, it is more favorable for ADP crystal to achieve higher output energy with larger conversion efficiency. Compared with ADP crystal, the 32% deuterated ADP crystal can effectively decrease TSRS and maintain a satisfactory conversion efficiency. The highest efficiencies obtained for frequency conversion from 526 to 263 nm is 67% for ADP, 63.45% for DADP and 63.78% for DKDP, respectively. Although each kind of crystal has its own advantages in NCPM FHG, by improving the crystal quality and temperature control precision, all of these crystals can realize large aperture and high efficiency FHG.
National Natural Science Foundation of China (51602165, 11374170); Natural Science Foundation of Shandong Province (ZR2016EMB17); Postdoctoral Science Foundation of China (2015M582055); Applied Research Project of Qingdao (2015130). Shandong Provincial University Key Laboratory of Optoelectrical Material Physics and Devices, Qingdao 266071, China
The author also would like to thank the Taishan Scholar Program of Shandong Province, China.
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