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Compact ultraviolet (UV) laser devices would be useful in applications such as optical storage, photocatalysis, sterilization, and ophthalmic surgery, and many researchers are devoting considerable effort to finding materials with larger direct band gaps than that of GaN for use in UV devices. We have found that hexagonal boron nitride (hBN) is a promising material for UV laser devices because of its direct band gap nature in far-UV region. A pure hBN single crystal, which proves it to be a direct band gap material by showing a dominant cathodoluminescence peak at 215 nm (Figure) and a series of s-like exciton absorption bands around 215 nm, was obtained under high pressure and high temperature by employing a temperature gradient method. Based on the absorption spectrum, the band gap energy of 5.971 eV and the quasi-two dimensional exciton binding energy of 149 meV were finally obtained. In addition, UV lasing at 215 nm by accelerated electron excitation was demonstrated at room temperature as proved by the enhancement of the longitudinal mode and threshold behaviour of the excitation current dependence of the emission intensity. The details on this facinating properties of the material can be found in a recent article of Nature Materials vol 3, p.404 (2004).
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