Voltage applied across nonlinear crystal helps in measuring its properties

Through frequency upconversion, nonlinear optical crystals are one of the key approaches for achieving short wavelengths for high-power and high-energy lasers; as a result, properly characterizing these crystals is important. Now, researchers from the Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences and the University of Chinese Academy of Sciences have described a novel measurement method of the electro-optic (EO) coefficients of two of the most important nonlinear crystals, KH₂PO₄ (KDP) and K(H_1-xD_x)₂PO₄ (DKDP), based on so-called χ⁽²⁾ nonlinear optical technology.¹

Current studies and applications of the EO effect of KDP-family crystals focus on KDP, which has no deuterium, and DKDP, which has varying deuterium content. The EO coefficients of the partially deuterated crystals have been only sporadically reported, although they have crucial applications in many fields. In addition, existing EO coefficient measurement methods are based mainly on the linear optical effect, which requires the polarization stability of the laser light to be strictly maintained throughout the whole measurement system.

Differing deuterium contents measured

In the new study, the Chinese researchers presented a novel method for measuring linear EO coefficients that only depends on the nonlinear process in nonlinear materials. In the technique, an electric field is applied to a KDP-family crystal, which compensates for the phase mismatch caused by variation in angle. For the experiment, Nd:YLF and Nd:YAG lasers with wavelengths of 1053 and 1064 nm, respectively, supplied the fundamental wavelengths. Pulse widths for the two lasers were 8.5 and 7.5 ns, respectively; a half-wave plate and a polarization beamsplitter were used to match the pulse energies for the two lasers at 10 mJ (see figure).

KDP and DKDP crystals with differing deuterium content (0%, 23%, 45%, 68%, 82%, and 90%) were used as fourth-harmonic-generation (FHG) experimental samples; temperature-tuning curves were created and the noncritical phase-matching temperatures found. The researchers determined that, in the presence of an external electric field, the maximum angular acceptance bandwidth for FHG could be increased by a factor of 2.2 and maintain perfect phase matching for a wide range of angles. In an example, noncritical phase matching achieved stable efficiency for a range of only about 0.54°, while voltage-tuned phase matching (VCPM; the new technique) achieved an angular range of 2.35°, which is 4.4X larger. This helped in determining the linear EO coefficients of these crystals, as phase matching is sensitive to the change of refractive index and the change of refractive index is given by an exact formula.

The researchers say that their results can help improve the stability of deep-ultraviolet lasers using FHG.

REFERENCE

1. Z. Sun et al., Opt. Express (2021); https://doi.org/10.1364/oe.415262.