A unique design of our ultracompact microcavity wavelength conversion device exploits the simple principle that the wavelength conversion efficiency is proportional to the square of the electric field amplitude of enhanced pump light in the microcavity, and expands the range of suitable device materials to include crystals that do not exhibit birefringence or ferroelectricity. Here, as a first step toward practical applications of all-solid-state ultracompact deep-ultraviolet coherent light sources, we adopted a low-birefringence paraelectric SrB4O7 crystal with great potential for wavelength conversion and high transparency down to 130 nm as our device material, and demonstrated 234 nm deep-ultraviolet coherent light generation, whose wavelength band is expected to be used for on-demand disinfection tools that can irradiate the human body.
The bulk laser-induced damage threshold (LIDT) of a strontium tetraborate (SrB4O7, SBO) single crystal has been measured for the first time. Single-shot and multi-shot tests are carried out using a 266-nm deep ultraviolet (DUV) laser. Under single-shot irradiation, the bulk LIDT of SBO crystal is 1.5 times higher than that of a synthetic silica glass. On the other hand, under multi-shot irradiation, the bulk LIDT of SBO is also higher than synthetic silica glass regardless of the number of shots. When the number of shots increased from 102 to 104, SBO crystal’s bulk LIDT has only decreased by less than 11 % compared to 44 % of synthetic silica glass. Although the damage mechanism of SBO crystal under multi-shot irradiation is considered to be due to material modification fatigue, SBO single crystals are found to be excellent optical materials that exhibit bulk laser-induced damage resistance in the DUV region.
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