The Role Of Electric-field Induced Double-frequency Absorption In Two-photon Response

Two-photon response includes the two-photon absorption (TPA) and doubled-frequency absorption (DFA). Both TPA and DFA nonlinear process occur for photons whose energy hv is less than the semiconductor energy gap E_g, but greater than E_g/2 .The TPA means that two photons are absorbed simultaneously. And the DFA means that non-phase matching double frequency effect of the incident lightwave occurs in semiconductor material and then the doubled-frequency photons are absorbed. The essential of DFA is the single photon absorption of double-frequency photon. In macroscopical effect, whether TPA or DFA, both of them show the result that absorbing two photon to produce transmission, so we call "two-photon response" by combing these two effect together. Corresponding to the optical detector made by these two effects, it is difficult to differentiate the two effects because of their resemblance such as their responsibilities are both proportion to the square of incident intensity in experiments, etc. But they are different from each other in theory, the distinction of these two effects is that TPA has a certain feature of three-order nonlinear optics phenomenon but the DFA has a certain feature of two-order nonlinear optics phenomenon. In general, they have different request to incident light intensity, TPA need higher incident light intensity than DFA. Corresponding to centrosymmetric crystal whose second-order nonlinear optical susceptibilities are zero, so no DFA exist but TPA exist in them. But at surface or interface, the inversion symmetry is broken and a second-order dipole response can exist so DFA can appear. In addition, an applied electric field can also destroy its centrosymmetric and arose DFA, which we call it electric-field induced DFA.Since the first report of using two-photon response optical detector to construct autocorrelator to measure ultrashort pulse in 1992, the research of two-photon response optical detector get more and more recognition, especially Si two-photon response optical detector, because its response wavelength range include all optical communication wavelength, and it can construct Si base optical-electric integration device combine with Si base integrate circuit. So it is very significance to study Si two-photon response optical detector. Si is a kindof center-symmetric crystal, the physical mechanism of two-photon response is not well know and it cannot concluded which is dominant or both of them exists in optical detector made of such material. Most people think that TPA is the main mechanism of two-photon response before 1996, some person speculate on exist DFA according to reflect double frequency light from Si surface at the same year, but up to now we cannot see their further report about it. Our research shows that the DFA induced by electric field is the main physical mechanism of two-photon response.The theory foundation of DFA is the center of inversion will vanish when a centrosymmetric crystal existing in an invariableness electric field due to the action of electric field, so the second-order nonlinear optical effects include double-frequency effect can occur. Field induced effective second-order nonlinear susceptibilities equal to the product of the three-order susceptibilities tensor and electric-field vector. Si is belong to m3m point group, we discuss the form from theory analysis about field-induced effective second-order nonlinear susceptibilities when the direction of electric field along <100>, <110> and <111> respectively and find that they have the same form with 4mm , mm2 and 3m point group respectively. These results further explain the disappearance of center of inversion. Optical rectification and double-frequency effect which both belong to second-order nonlinear optical effect simultaneity. So we must study double-frequency effect and optical rectification effect if we want to study double-frequency absorption simultaneity. Field-induced effective second-order nonlinear susceptibilities have certain relationship to electric field. However, TPA have no relationship to optical rectification effect, and have no relationship to applied electric field. So we can use such theory to differentiate DFA and TPA from experiment. The following experiments verify field-induced DFA is the main physical mechanism of Si two-photon response detector.We measure the relationship curve between the two-photon response and incident light power of Si commercial optical-electric detector. The result shows when the incident light power extrapolate to its value is zero, but the optical current is not equal to zero and above the origin. We have read some reference before and seen similar situation, some person explain it is the effect of mixed light, however, during our experiment, the mixed light already been removed by us. And we also find in some other reference, when light power extrapolate to its value was zero, the optical current was underside the origin. So if take the physical mechanism of two-photon response as TPA, it could not explain above experiment results better. But it can get reasonable explanation using single photon absorption after doubled-frequency. It was due to optical rectification ofsecond-order nonlinear optics effect-existence. Because the direction of electric field produced by optical rectification were different, so when the curve extrapolate to incident light power was zero but the optical current was not zero, may be it lie above the origin or underside the origin.We fabricate Si Metal-Semiconductor-Metal planar two-photon response optical detector. Such device usually be taken as optical conductive device before, but we found it could be taken as optical voltage too. We also find correspond to different point of optical sensitivity plane had different optical sensitivity feature, and had a certain rule. Such phenomena had not been reported before. After analysis, we think the reason of this is due to the contact barrier between metal and semiconductor, and the result of self-build field. As the existence self-build field it must turn up DFA. During our experiment, we observe that the magnitude of optical current had a certain relationship with electric field. The stronger the electric field is, the bigger the optical current, vice versa , the weaker the electric field is, the smaller the optical current. Such situation shows that DFA take the main role, and the TPA has no relationship with electric field.We fabricate Si MSM face to face type two-photon response optical detector. It must exist second-order nonlinear optical effects due to the existence of self- field, so two effects must be existed at the same time those are double-frequency effect and optical rectification effect. During our experiment, let incident plumb MSM plane, and we really observe optical rectification signal, and through a fitting curve, we find that the experimental results are in good agreement with the theoretical analysis. The experimental result also verify the electric-field induced DFA is really existence in Si two-photon response photon-electric detector.Electric field induced DFA of second order nonlinear optical effects take the main role in two-photon response, and two-photon response must rely on the intensity of electric field mightily. We fabricate Si MSM face to face photoelectric detector in order to research the relationship between optical current and the intensity of applied electric field, measure the relationship between two-photon response and electric field. The results shows that the relationship between optical current and applied electric voltage is nonlinear, and it can get a reasonable explanation from the theory of "space-charge limited current". Such result further verify the fact that DFA is the main physical mechanism of two-photon response.GaAs is a cubic crystal with 43 m symmetry, the form of its third order...