Na<sub>5</sub> [B<sub>2</sub>P<sub>3</sub>O<sub>13</sub>] (NBP) is a promising nonlinear optical crystal. It exhibits a wider transparency range from UV to far-IR.
And its absorption edge lies at 186nm. The forbidden band gap is about 6.67eV. The time domain spectra in the range of
0.25-2.5 THz were measured. The power absorption coefficient and refractive index were studied. The refractive index is
linear with frequency from 0.4THz to 1.8THz. And the value increases slowly from 1.8 to 2.1THz. The properties of the
sample show that it is possible to apply it to optical device.