1. Introduction

Usually, a NLO crystal oriented along phase matching (PM) direction can only efficiently convert a particular wavelength with a narrow linewidth, which is known as the spectral acceptance bandwidth. It is a characteristic parameter of NLO material, and related to the refractive index dispersions. Large spectral acceptance bandwidths are needed for the frequency conversions of ultrashort pulse lasers and inertial confinement fusion laser drivers [1–8].

By time now, different approaches have been employed to compensate group velocity dispersion and improve the frequency conversion efficiency of broadband laser, including spectral angular dispersion compensation [9–12], crystal-cascaded matching [13,14], retracing point matching [15,16],quasi-phase-matched and group-velocity-matched (QPM-GVM) [17–19], as well as chirped pulses matching [20,21]. Among them, the retracing point matching scheme, i.e. S-NCPM, is the simplest, which is very convenient for practical applications. However, only limited NLO crystals have been investigated for this technique, such as DKDP, MgO:LN, BBO, and LBO [15,18,22,23]. As we have known, for the most popular two solid laser wavelengths, 1064 nm of Nd:YAG and 1053 nm of Nd:glass, the only reported S-NCPM frequency conversion materials are DKDP crystals with different deuterium contents [15].

As an analogue of KDP crystal, ADP has larger effective nonlinear optical coefficient and higher laser damage threshold. The deuterated ADP crystal has similar PM properties with DKDP crystal, so it is feasible to use DADP crystal to realize S-NCPM frequency doubling of 1~1.2 μm lasers, at the same time bring higher conversion efficiency and larger output energy than DKDP. In this paper, this topic was studied for the first time, large sizes, high quality DADP crystal was successfully grown, the deuterium content that can realize S-NCPM frequency doubling for Nd:YAG laser is determined to be 25%, and the corresponding deuterium content for Nd:glass laser is predicted to be 17%.

2. Theoretical analysis

Before crystal growth, a theoretical analysis is needed to be performed to estimate the deuterium content of DADP crystal for target wavelength, 1064 nm. According to the refractive index dispersion parameters of Zernike [24], the retracing wavelength of pure ADP crystal is calculated to be 1030 nm, which is very close to our measured value of 1027 nm. According to the refractive index data of Kirby [25], the corresponding wavelength of a 96% deuterated ADP crystal is 1161 nm. For KDP type crystals, the linear variation discipline between retracing wavelength and deuterium content was demonstrated in reference [15]. Therefore, a linear fitting can be given to partially deuterated ADP crystals, as shown in Fig. 1. The wavelength of 1064 nm corresponds to the deuterium content of 25%, so our target crystal is determined to be 25% deuterated ADP.

 

Fig. 1 Calculated retracing wavelength as a function of deuterium content for partially deuterated ADP crystal.

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3. Crystal growth and sample preparation

With point seed aqueous solution method, we grew DADP crystal in a 5000 mL glass crystallizer. The deuterated aqueous solution was obtained by dissolving high-purity NH₄H₂PO₄ raw material in a certain proportion of heavy water and deionized water. For partially deuterated KDP or ADP crystals, it has been well-known that the deuterium contents in crystal and solution have great discrepancies [26–29]. According to the previous experience, the segregation coefficient in DADP crystal and its solution is ~0.83 under our growth conditions. So, to achieve 25% deuterated ADP crystal, the deuterium content in prepared solution was set to be 30%. The solution was filtered with a polysulfone ultrafiltration membrane whose pore diameter was 0.22 µm. After setting the temperature reduction procedure, DADP crystal started growing from an a-cut ADP point seed. Comparing with the growth of z-cut point seed, the present method is helpful for getting high quality single crystal with good crystallization integrity. Crystallization process was performed in a temperature interval of 5 ºC, and the growth rate was 3.5 mm/d. The crystal was rotated in the 'forward-stop-backward' mode with a speed of 77 r/min. The temperature was controlled by a Shimada controller (Model FP21) with an accuracy of ± 0.1 ºC. As shown in Fig. 2, no visible macroscopic defects were observed in the obtained DADP crystal. With this crystal we processed a 10 × 10 × 10 mm³ experimental sample, along the type-I PM direction (θ = 41$°$, ϕ = 45$°$). The detailed cutting style was demonstrated in Fig. 3. The cutting accuracy was less than 10' and the transmittance faces of the sample were polished but uncoated.

 

Fig. 2 As-grown DADP crystal.

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Fig. 3 Processing schematic of DADP crystal.

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4. Property testing

The rocking curve of DADP sample was measured by a high resolution X-ray diffractometer (Bruker, D8 Discover). One high quality ADP sample with the same processing style was used as the reference. Both of them were cut along the type-I PM direction of 1064 nm, i.e. (θ = 41$°$, ϕ = 45$°$). As demonstrated in Fig. 4, these two crystals exhibit similar, sharp and clear peaks, with full widths at half maximum (FWHMs) of 15.1”, 16.7”, respectively. It indicates that the DADP crystal has very good crystalline perfection.

 

Fig. 4 Rocking curves of DADP (a) and ADP (b) crystals.

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With a spectrophotometer (Hitachi UH4150), the transmission spectrum of DADP crystal was measured, and the result was shown in Fig. 5. The sample was z-cut with a thickness of 2 mm, which was optical polished before measurement. Its ultra-violet transmission cutting edge is about 200 nm, and the infrared transmission region extends to 1600 nm around. The wide transmission spectrum is favorable for various optical and nonlinear optical applications.

 

Fig. 5 Transmission spectrum of DADP crystal.

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5. Determining the S-NCPM frequency doubling wavelength

A pulsed OPO laser (Continuum, Horizon) was used as the tunable light source, with repetition rate of 10 Hz, pulse width of 6 ns, and linear polarized output along horizontal direction. The crystal samples were fixed on a high accuracy adjusting mount, to guarantee that the initial state was located at the normal incident position. This adjusting mount was installed on a motorized rotary stage with high angular resolution of 5 μrad, which could adjust the incident angle on sample at vertical direction. The room temperature was 20 ºC.

Traditionally, the PM direction was expressed by absolute phase-matching angle, i.e. the angle between PM direction and crystal z-axis. For the sake of convenience, in this part we use relative phase-matching angle to express PM direction, i.e. the angle between PM direction and crystal processing direction, which is denoted as θ_pm. For DADP and ADP crystals, the variation of θ_pm with the fundamental wavelength λ for type-I frequency doubling was shown in Fig. 6. The overall measurement precision of our experimental set-up was less than 0.22 mrad, which was approximately 1/50 of the θ_pm change in the wavelength range 0.95 < λ < 1.15 μm.

 

Fig. 6 Variation of θ_pm as a function of fundamental wavelength λ for 25% DADP (a) and ADP (b) crystals. Discrete blue points: experimental data; Continuous red lines: fitting curves.

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The experimental data can be fitted by the followed equation [15]:

where the parameters λ_n, θ_pm, and C represent the S-NCPM wavelength for which ∂θ_pm/∂λ = 0, the phase-matching angle (referenced to the crystal processing direction) at λ_n, and one half of the curvature ∂ ²θ_pm/∂λ² at λ_n, respectively. The fitted λ_n values of DADP and ADP crystals are 1063 nm and 1027 nm respectively, which are in good agreement with the theoretical calculated values in Fig. 1, i.e. 1064 nm for 25% DADP crystal and 1030 nm for ADP crystal. The small discrepancies may be caused by the temperature and deuterium content deviations. For KDP crystal, the S-NCPM wavelength for type-I frequency doubling was measured to be 1038 nm, which is also close to the fitted value of 1036 nm [15]. From Fig. 6, it can be seen that for both of DADP and ADP crystals, the change of phase-matching angle is < 0.02$°$ in a wavelength range of 40 nm near λ_n, which can be used for efficient broadband second-harmonic-generation (SHG) at respective λ_n. Since our experiments have proved the reliability of the fitted line in Fig. 1, from it one can further speculate that for the type-I broadband SHG of Nd:glass laser (1053 nm) the appropriate deuterium content of DADP crystal will be 17%.

6. SHG experiment

The frequency doubling experiments were performed for DADP and ADP crystals under the same conditions. A KDP crystal with similar PM direction and the same thickness (10 mm) was used as the reference sample. The light source was an Yb-doped femtosecond laser (Pharos, Light Conversion Inc.), whose central wavelength was 1027 nm, spectral bandwidth was 8.7 nm (FWHM), pulse duration was 180 fs, and repetition rate was 1 KHz. All of the samples were adjusted to the optimum SHG PM directions during the experimental processes, and the results were shown in Fig. 7. The maximum SHG output powers of ADP, DADP, and KDP samples were 39.7 mW, 38.2 mW and 34.7 mW, respectively. Correspondingly, the highest SHG conversion efficiencies of these crystals were 52.1%, 48.0% and 44.9%, as exhibited in Fig. 7(a). For ADP and KDP crystals, the effective nonlinear optical coefficients d_eff at type-I SHG direction (θ = 41$°$, ϕ = 45$°$) are 0.31 pm/V and 0.26 pm/V, so ADP presents obviously higher SHG conversion efficiencies than KDP crystal. Because the SHG of ADP at this waveband (1027 nm) is retracing point PM, its conversion efficiencies are also obviously higher than those of DADP crystal. At the same time, its SHG spectral bandwidth is larger than the value of DADP crystal, which are 3.0 and 2.6 nm (FWHM) respectively, as shown in Fig. 7(b). The spectral bandwidth ratio of SHG pulse to fundamental pulse of ADP crystal is 34.5%, which is close to the ideal value of 35.4% (i.e. $\sqrt{2}/4$) for the retracing point PM of a Gaussian light beam [30]. Since the central wavelength of fundamental laser (1027 nm) is adjacent to the retracing point PM wavelength (1038 nm) of KDP crystal, the SHG spectral bandwidth of KDP is almost identical to the bandwidth of ADP.

 

Fig. 7 SHG conversion efficiencies (a) and SHG spectra (b) of different crystals. Inset: the spectrum of fundamental laser.

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In short, the benefits of retracing point PM have been clearly presented by the SHG experiment. For DADP crystals with different deuterium contents, the retracing point PM will bring higher SHG conversion efficiency and broader SHG output spectrum. Meanwhile, comparing with KDP crystal, ADP and DADP exhibit larger optical nonlinearity and more powerful SHG conversions.

7. Conclusions

For the first time, we successfully grew 25% deuterated DADP crystal with dimensions of 7 × 7 × 6 cm³. With this crystal, we realized S-NCPM type-I frequency doubling of 1063 nm, which is almost identical to the wavelength of Nd:YAG laser. Our investigation manifests that for type-I SHG of different fundamental wavelengths in the range of 1027 ~1161 nm, DADP crystal can make the first order wavelength sensitivity vanish by adjusting the deuterium content. So, DADP will be an efficient frequency doubling material for broadband laser whose central wavelength falls in this band. For some popular, important solid laser wavelengths like 1030 nm (Yb:YAG, Yb:YLF), 1053 nm (Nd:glass, Nd:YLF), and 1064 nm (Nd:YAG), the corresponding deuterium contents are 0, 17%, 25%, respectively. Compared with previously reported DKDP crystal that can realize the S-NCPM frequency doubling at the similar waveband (1034 ~1179 nm), DADP crystal possesses larger nonlinearity and higher laser damage threshold, which are possible to help it find more applications.