In this paper, we report how to use femtosecond laser illumination to inscribe micro/nano structures, particularly the permanent periodic refractive index change/modulation in functional crystalline materials, such as lithium niobate and Ti: sapphire crystals. First, we introduce the motivations to create micro/nano structures in functional crystalline materials, which include the enhancement of light and material interaction, leading to new functionalities such as a spectrally tunable electro-optic device. Second, we discuss the physical mechanism of femtosecond laser illumination induced permanent refractive index change. The extreme high light intensity at the focusing spot of femtosecond laser beam can induce a rapid melting/re-solidification process in the functional crystalline materials, consequentially generating a permanent refractive index change in these materials via Kovacs effect. Third, we provide a detailed description of the experimental setup and procedure used to inscribe permanent periodic refractive index modulated patterns in functional crystals. We use the direct femtosecond laser writing method due to its simplicity and versability. Fourth, we discuss the experimental results confirming that we can indeed inscribe permanent periodic refractive index change in functional electro-optic lithium niobate crystals and the Ti: sapphire lasing medium. Finally, we will conduct and report experimentations to confirm the applications of micro/nanostructured functional crystalline materials in highly compact and fast-speed tuning lasers and spectral filters in future.
In this paper, we report the growth of ytterbium-doped lithium niobate (Yb:LiNbO3) crystalline fibers by laser heated pedestal growth (LHPG) method. In the growth, first, a Yb:LiNbO3 bulk crystal is grown by melting the Yb:LiNbO3 powder in a platinum (Pt) crucible at 1300 °C. Second, a Yb:LiNbO3 bulk crystal is cut/lapped/polished in a rectangularly shaped bar, which can serve as the feed rod of LHPG. Finally, a Yb:LiNbO3 fiber is drawn using the LHPG method, in which the Yb:LiNbO3 bar is served as the feed rod, and a pure LiNbO3 crystal bar is served as the seed rod. The major properties of the Yb:LiNbO3 fiber are evaluated, including (1) identifying the composition of the fiber by energy dispersive spectroscopy (EDS), (2) determining the crystalline structure by X-ray diffraction (XRD), and (3) measuring the absorption spectrum by a spectrophotometer. The results of characterization experiments confirm that the grown fiber is indeed Yb:LiNbO3 crystalline fiber. This unique crystalline fiber has multifunctional capability, including (1) a very effective low quantum defect lasing medium; (2) an electro-optically tunable medium, in which the refractive index of medium can be quickly tuned by the applied external electric field; and (3) a light guiding medium that offers the advantage of low driving voltage and power. The multifunctional capability of this crystalline fiber makes it very useful for a variety of applications such as highly compact, high-efficiency tunable lasers and filters.
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