Studies Of The Electric-field-induced Linear Electro-optic Effect And Its Applications In Silicon Optoelectronics

Silicon is not only the most important microelectronic material, but also theprimary material for the photonic integration and the optoelectronic hyper integration.In recent years, silicon optical modulators as key components of integrated opticalcircuits have been researched widely. Because idea silicon crystals possess theinversion symmetry, and do not have the second-order nonlinear optical effects suchas the linear electro-optic effect, at present, silicon optical modulators are mainlybased on the plasma dispersion effect, the quantum-confined Stark effect, and thethermo-optic effect, etc. In this dissertation, the dc electric field and the stress wereused for breaking the inversion symmetry of silicon, and producing the field-inducedlinear electro-optic effect. Furthermore, a prototype of silicon electro-optic modulatorbased on the linear electro-optic effect induced the electric field or/and strain wasdesigned and manufactured. The field-induced linear electro-optic effect is the novelphysical mechanism for silicon electro-optic modulators, belongs to the electricpolarization, and the response time is about several femtoseconds, so high-propertysilicon electro-optic modulators based on this effect are likely obtained. Theresearches in this dissertation will be helpful to the development of siliconoptoelectronics, and mainly include the following four aspects.(1) According to the theory of classical nonlinear polarization, it is specified thatthe electric-field-induced linear electro-optic effect is one of the third-order nonlinearoptical effects which results from the dc electric field, the ac modulating electric fieldand the optical field in silicon, and the dot product of the dc electric field and thethird-order nonlinear susceptibility can be taken as the effective second-ordernonlinear susceptibility. Thus, the considerable electric-field-induced second-ordernonlinear optical effects can be realized in silicon, as long as the dc electric field (e.g.the built-in field) is strong enough. Furthermore, the electric-field-induced linearelectro-optic effect is investigated experimentally by using a transverse electro-opticmeasuring system in which a near-intrinsic silicon crystal with the configuration of metal-insulator-semiconductor-insulator-metal (MISIM) planar capacitor is used asthe sample. It is verified that the electric-field-induced linear electro-optic signallinearly increases with the applied ac modulating voltage under a constant dc bias,similarly, the electric-field-induced linear electro-optic signal also enlarges with thedc bias under a constant ac modulating voltage.(2) According to the model of atomic shift polarization induced by thenonhomogeneous strain field, it is specified that the strain-induced linear electro-opticeffect in silicon results from the breakdown of the inversion symmetry because of thelattice distortion. Apllying a self-made uniaxial stress device, the relationship betweenthe strain-induced linear electro-optic signal and the strain is quantitively studied, andthe results indicate that the strain-induced second-order nonlinear susceptibility isproportional to the magnitude of strain which the silicon sample can endure. When thestrain is over 10-3, the second-order nonlinear suscepitibility can be larger than 5pm/V.This experimental setup and method can be used for not only determining thestrain-induced second-order nonlinear susceptibility of silicon, but also quantitivelyinvestigating the internal strain at the surface or interface of silicon devices.(3) The physical mechanisms and laws of diversified direct or indirectelectro-optic effects in silicon are specifically analyzed and summarized. Based on thetruth that these electro-optic effects have different polarization dependences andelectric field responses, an experimental method of analyzing and distinguishingamong the linear electro-optic effect, Kerr effect, plasma dispersion effect andthermo-optic effect is put forward, and these effects are measuremed anddistinguished effectively in experiments. The results show that the field-inducedlinear electro-optic effect is the most remarkable in the near-intrinsic silicon sample,and the field-induced linear electro-optic signal is as over four times as those from theKerr effect and the plasma dispersion. So, the field-induced linear electro-optic effectcan be used not only for analyzing the properties of the surfaces and interfaces ofsilicon devices, but also for designing and manufacturing silicon-based electro-opticdevices. (4) Based on the linear electro-optic effect induced by the dc electric field or/andthe strain field, novel siicon electro-optic modualtors are elementarily designed andfabricated. First, the optical waveguide of the modulator is designed and simulated,and the single-mode condition and the insertion loss are mainly considered. Then,silicon Mach-Zender interfence electro-optical modulators in silicon-on-insulator(SOI) are manufactured. Last, some essential parameters of the modualtor aremeasured. The results show that the insertion loss is less than 19dB, the extinctionratio is larger than 7dB, the half-wave voltage is less than 3V, the modulation depth isabove 80%, and the modualtion bandwidth is over 10MHz. Since it is the primaryresearch stage now, the performance of the modulator is unperfect, and needs to befurther improved. The causes for limiting the propertyof the modulator are discussed,and the improvement methods are suggested at the end of the dissertation.In conclusion, many invesigations are original in this thesis, e.g. the study of thephysical mechanism of the electric-field-induced linear electro-optic effect, thequantitive analysis of the relationship between the strain and the strain-induced linearelectro-optic effect by using the self-made uniaxial stress device, the experimentaldistinction of all kinds of electro-optic effects in silicon material, etc. Theseresearches and methods not only enrich the content of silicon optoelectronics andpromote the development of silicon optoelectronics, but also can be refered in theinvestigations of other materials with the inversion symmetry.