Potassium dihydrogen phosphate (KDP) crystal has been widely used as a nonlinear optical (NLO) material in inertial confinement fusion (ICF) equipment. For ultra-violet (UV) frequency conversion, the transverse stimulated Raman scattering (TSRS) is a critical limiting factor for enhancing output energy and conversion efficiency. In the past several decades, many researchers have focused on the Raman properties of KDP crystals.^{[1– 6]} In fact, some ICF equipment, like the National Ignition Facility (NIF) of America, have used a partially deuterated KDP (DKDP) crystal to relieve the TSRS problem.^[7]

In recent years, many advantages have been found in ammonium dihydrogen phosphate (ADP) crystal as compared with KDP crystal, such as larger nonlinear optical (NLO) coefficient, higher laser damage threshold, and bigger angular acceptance. Nevertheless, the stimulated Raman scattering (SRS) characteristics of ADP-type crystals must be evaluated first for practical applications. The spontaneous Raman scattering and SRS effect of ADP crystal have been reported before, ^{[8, 9]} and the pump wavelength is visible (532 nm) or near infrared (1064 nm). In the present paper, by comparing the SRS property at third-harmonic generation (THG) wavelength of Nd:YAG laser (355 nm), we find that the UV Raman gains are very close for ADP and KDP crystals, further confirming the application potential of ADP-type crystals in UV NLO fields.

Single crystals of ADP and KDP are grown from NH₄H₂PO₄ and KH₂PO₄ solution using the rapid crystal growth method.^[10] In the growth process, the velocity of temperature-reduction is maintained to be 0.1 ° C/day. The crystal rotated in the mode of “ forward-stop-backward” with a speed of about 77 rpm.^[11] The pristine crystals are carefully cut along the a, b, and c axes, each with a size of 17 mm× 17 mm× 14 mm with a precision of 1′ , the specimens are then optically polished.

The experiments of spontaneous Raman scattering are conducted with a HR800 Raman spectrometer (France), the pump wavelength λ _p is 633 nm, and the results are shown in Fig. 1. The information about the spectral line near 920 cm^{− 1} is also listed in Table 1. It attributes to the symmetrical vibration of [PO₄] tetrahedron group. It can be seen that the line width Δ Ω _R(ADP) = 28 cm^{− 1} and Δ Ω _R(KDP) = 23 cm^{− 1}. Compared with KDP, ADP exhibits a more intense and broader Raman line.

Fig. 1.

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	Fig. 1. Room temperature spontaneous Raman scattering spectra of ADP and KDP crystals. The inset shows the details in a range from 700 cm− 1 to 1200 cm− 1, in which the solid line denotes ADP crystal and the dotted line denotes KDP crystal, respectively.

For steady state stimulated Raman scattering (SRS), the gain coefficient g_SS is determined by the Raman properties of a nonlinear medium^[12]

where N is the number of scattering centers (here, N_ADP = 0.944 × 10²² molecules/cm³, N_KDP = 1.035 × 10²² molecules/cm³), λ _p and λ _s are the pump and Stokes wavelengths (λ _p = 633 nm, λ _s(ADP) = 730.14 nm, λ _s(KDP) = 671.96 nm), n_S is the refractive index at λ _s (n_S(ADP)= 1.5202 at 672.32 nm, n_S(KDP) = 1.5060 at 671.96 nm) and d σ /dΩ is the Raman scattering cross section. Thus, referencing to Table 1 the steady state Raman gain coefficients of ADP and KDP crystals at 633-nm pump condition can be calculated, and the result of g_SS(ADP) is 1.58 times larger than that of g_SS(KDP). On the other hand, the value of the product (dσ /dΩ )(Δ Ω _R)^{− 1} can be characterized as a peak intensity Σ _peak in the measured spontaneous Raman scattering spectrum^[12]

From Table 2 and Eq. (2), it follows thatby using Eqs. (1) and (2), the result of g_SS(ADP)/g_SS(KDP) can be obtained to be 1.58.

Table 1.

Table 1.

Table 1. Raman spectroscopic parameters of ADP and KDP crystals. Material Raman freq./cm− 1 Raman linewidth/cm− 1 Scattering cross section/a.u. ADP 924 28 100a KDP 916 23 46.5 a The scattering cross section of ADP crystal is set as the integral intensity of the spectral line near 920 cm− 1, and its reference value is 100.

Table 1. Raman spectroscopic parameters of ADP and KDP crystals.

At room temperature, both ADP and KDP crystals have the same point group D_2d, which belongs to space group (see Fig. 4). But the bonds in the structures of these two crystals are very distinct, which can be seen in Fig. 5.^[15] The hydrogen bonds (dashed lines in Fig. 5), which link the neighboring groups, and the chemical bonds inside groups (solid lines between P, O, and H atoms in Fig. 5), are very similar in these two isomorphs, i.e., the numbers, lengths, strengths, directions of the bonds in the two groups are approximately the same. However, the bond strengths between the

Fig. 4.

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	Fig. 4. Spatial structures of ADP crystal (a) and KDP crystal (b).

Fig. 5.

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	Fig. 5. Bond structures of ADP (a) and KDP (b) crystals, [15] All of the chemical bonds are represented by different kinds of lines labeled with bond length (Å ).

adjacent anions and cations are very different, while the bond number and bond direction are still analogous. The strong K– O ionic bond is substituted with a weaker N– H· · · O hydrogen bond in ADP structure than in KDP. Such a difference in structure makes a larger Raman mode frequency of ADP than that of KDP (see Table 1), which could be responsible for slowing down the dephasing processes and reducing the homogenous linewidth.^{[12, 16]}

In this paper, the Raman gain coefficients of ADP and KDP crystals are compared with each other. The results show that although in the visible waveband g_ss(ADP) is obviously larger than g_ss(KDP), for the ultra-violet THG wavelength of Nd:YAG laser (355 nm) g_ss(ADP) is almost the same as g_ss(KDP). From the variation trend we can speculate that at the FHG wavelength 266-nm g_ss(ADP) is very likely to be smaller than g_ss(KDP), which needs more experiments to be confirmed. The present research results show that in the UV light region the SRS effect of ADP is comparable to or even weaker than that of KDP. The present research results also show that, as a powerful competitor to KDP crystal, ADP crystals could be used in large aperture THG or FHG processes.