Studies Of Electric-field-Induced Pockels Effect And Optical Rectification In Silicon

Silicon is a kind of conventional semiconductor that has the widest applications and the most advanced technology. It is not only the main microelectronics material but also the prior material for integrated optics and integrated optoelectronics. At present, silicon optoelectronics and silicon photonics have become one of attractive researches. An ideal silicon crystal does not possess Pockels effect because of its inversion symmetry. Thus, it cannot be used for manufacturing high-speed electro-optic modulators or optical switches that are key devices for the optical communication. In recent years, silicon electro-optic modulators based on the plasma dispersion effect are manufactured successfully, but the modulation speed of these modulators is not high enough because of such limits as the carrier lifetime, the carrier diffusion and drift. Even though the modulation frequency can be increased up to GHz by improving the electric and optical configuration of the silicon modulators, this modulation speed is not comparable with that of the electro-optic modulator based on Pockels effect yet. Moreover, complicated optical and electric configurations require complex technology. So it is very significant to investigate the electric-field-induced Pockels effect in silicon.In this thesis, the dc electric field is used for breaking the inversion symmetry of silicon, as a result, the electric-field-induced Pockels effect and optical rectification occur in silicon, and these effects are researched theoretically and experimentally.I. Studies of electric-field-induced Pockels effectWe mainly investigate Pockels effect induced by the applied dc electric field in silicon.(1) Applying the theory of nonlinear optics and tensor calculation, we deduce the change of the symmetry of silicon biased by the dc electric field. The results show that the symmetry of silicon will reduce from O h group to C4 v, C2 v, and C3 v group when the uniform dc electric field applied along the direction of [100],[ 011], and [111] respectively, thus, silicon will possess electric-field-induced second-order nonlinear optical effect, and the effective second-order nonlinear susceptibility equal to the inner product of the third-order nonlinear susceptibility and the dc electric field, that is ? e(f2f ) ? ?(3)? E.(2) The polarization is analyzed when the dc electric field, the low-frequency modulation field and the optical field simultaneously exist in silicon, and the change of refractive index of silicon is deduced further. The electric-field-induced Pockels effect is theoretically demonstrated. Furthermore, we verify the change of the symmetry of silicon broken by the built-in electric field and the applied dc electric field, and experimentally prove the electric-field-induced Pockels effect.(3) (111)-cut near-intrinsic silicon is used for samples which have the MISIM structure in experiments. With the Bias Tee circuit configuration, the dc electric field and the low-frequency modulation field are applied on the sample at the same time, and it is observed that the electric-field-induced Pockels effect is obviously enhanced by the applied dc electric field. The measured electro-optic signal is linearly increased with the dc bias rising, which is in accordance with the theory.Ⅱ. Studies of electric-field-induced optical rectificationOptical rectification can be taken as the reverse effect of Pockels effect. The electric-field-induced optical rectification is also found when we observe the electric-field-induced Pockels effect in silicon. Moreover, the anisotropy of the electric-field-induced optical rectification and the effect of the dc electric field on the optical rectification are studied.(1) According to the theory of nonlinear optics, the dependences of the electric-field-induced optical rectification on the polarization azimuth of the input linearly polarized beam are analyzed when the symmetry of silicon is reduced and belongs to C4 v, C2 v and C3 v group respectively.(2) In experiments, a (111)-cut near-intrinsic silicon crystal is used for the sample which has the MISIM configuration, and the input linearly polarized laser beam propagates along the direction of [110]. The optical rectification induced by the built-in electric field underneath (111) surface is detected. The anisotropy of the observed electric-field-induced optical rectification shows that the space charge region below (111) surface possesses the characteristics of C 3v point group symmetry, which agrees with the theoretical analysis.(3) Experimental results indicate that the applied dc electric field can considerably enhance the observed optical rectification in silicon, and the signals of optical rectification are also enlarged linearly with increasing the applied dc bias, which is owed to the fact that the effective second-order nonlinear susceptibility ? e(f2f) is proportional to the dc electric field. According to the anisotropy of the measured optical rectification, it is also demonstrated that the symmetry of silicon biased by the dc electric field with the direction of [111] still possesses the characteristics of C 3v point group.These researches will greatly promote the development of silicon optoelectronics, and will provide the theoretical and experimental evidences for manufacturing the novel silicon-based optoelectronic devices. These methods and results are also worthy of being referred to investigate the nonlinearity of other materials with the inversion symmetry.