Morikawa Junko, Orie Akihiro, Hashimoto Toshimasa, Juodkazis Saulius
Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Japan.
Opt Express. 2010 Apr 12;18(8):8300-10. doi: 10.1364/OE.18.008300.
Temperature diffusivity of laser micro-structured regions in sapphire is determined by a temperature wave method with a lateral resolution reduced to 10 microm using a directly sputtered micro-sensor and heater. A record high reduction of the temperature diffusivity of sapphire by 12% from its (1.26+/-0.02) x 10(-5)m(2)/s in-bulk value inside the femtosecond laser-structured volumes is determined; in a BK7 glass (4.8x10(-7) m(2)/s), a 2% decrease of the thermal diffusivity has been observed. Origin of the reduction is consistent with disorder and scattering of phonons around the laser photo-modified micro-volumes. The stress-induced birefringence is directly measured by polariscopy together with its radial distribution, and azimuthal orientation of the polarization ellipsis near the laser structured regions in sapphire. The maximum birefringence of Deltan approximately 1x10(-3) is achieved without crack formation and corresponds to a local stress of ~1.3 GPa. The stress (and birefringence) decay radially with a single-exponential constant of tau(R) = 24 microm while the azimuthal orientation of the polarization ellipsis is spiraling around the laser structured volume. Such structures are promising in waveguiding and lasing applications of optical vortices where spatial control of birefringence and optical activity are required.
利用直接溅射的微传感器和加热器，采用横向分辨率降低至约10微米的温度波方法，测定了蓝宝石中激光微结构区域的热扩散率。在飞秒激光结构化区域内，蓝宝石的热扩散率从其体值（1.26±0.02）×10⁻⁵米²/秒创纪录地大幅降低了12%；在BK7玻璃（约4.8×10⁻⁷米²/秒）中，观察到热扩散率降低了2%。这种降低的起源与激光光改性微体积周围声子的无序和散射一致。通过偏光显微镜直接测量应力诱导的双折射及其径向分布，以及蓝宝石中激光结构化区域附近偏振椭圆的方位取向。在不形成裂纹的情况下实现了最大双折射Δn约为1×10⁻³，对应于约1.3吉帕的局部应力。应力（和双折射）以τ(R)=24微米的单指数常数径向衰减，而偏振椭圆的方位取向围绕激光结构化体积呈螺旋状。这种结构在需要对双折射和光学活性进行空间控制的光学涡旋的波导和激光应用中很有前景。
