Monofluorophosphates: A New Source of Deep-Ultraviolet Nonlinear Optical Materials

Deep-ultraviolet
(DUV) nonlinear optical (NLO) materials that have
attracted considerable renewed interest are in great demand by the
relevant industries, but there is a severe shortage of these materials.
The existence of such a dilemma may be attributed to the extremely
rigorous prerequisite for such materials, including a very wide energy
gap (>6.2 eV), which can be met by only a few borates, phosphates,
and carbonates; among them, only one, KBe2BO3F2 (KBBF), is practically usable. Herein, we theoretically
identify PO3F2– as a novel DUV NLO building
unit based on first-principles studies. In addition, we thoroughly
survey more than 100 known monofluorophosphates and identify three
of them, (NH4)2PO3F, [C­(NH2)3]2PO3F, and NaNH4PO3F·H2O, as promising DUV NLO candidates. To
confirm our prediction, we experimentally synthesize all of them.
Interestingly, these compounds exhibit remarkably strong second harmonic
generations [0.9–1.1 × KH2PO4 (KDP)
at 1064 nm and 0.2–0.3 × β-Ba2B2O4 (BBO) at 532 nm] and high damage thresholds (2.3–3.1
× KDP at 1064 nm). Is it significant that this study offers a
new functional building unit, PO3F, which opens a new avenue
for designing and identifying DUV NLO materials, and thus, new monofluorophosphates
showing better performance are highly anticipated.