| Summary: | The second-harmonic generation (SHG) from Si<sub>1-x</sub>Ge<sub>x</sub> alloy films has been investigated by near-infrared femtosecond laser. Recognized by s-out polarized SHG intensity versus rotational angle of sample, the crystal symmetry of the fully strained Si<sub>0.83</sub>Ge<sub>0.17</sub> alloy is found changed from the O<sub>h</sub> to the C<sub>2</sub> point group due to the inhomogeneity of the strain. Calibrated by double crystal X-ray diffraction, the strain-induced χ<sup>(2)</sup> is estimated at 5.7 × 10<sup>-7</sup> esu. According to the analysis on p-in/s-out SHG, the strain-relaxed Si<sub>0.10</sub>Ge<sub>0.90</sub> alloy film is confirmed to be not fully relaxed, and the remaining strain is quantitatively determined to be around 0.1%.
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