Two -photon absorption in semiconductors for coherent control of photocurrent ratios and femtosecond pulse characterization

Two-photon absorption (TPA) in semiconductors has been studied for several decades. Although many impressive experiments have been reported, it is still one of most actively researched topics. One major reason is that due to mature semiconductor manufacturing techniques, almost any desired material compositions and device structures can be grown and fabricated. Moreover, semiconductor-based laboratory experiments, if feasible, can be easily transitioned to the mass production stage, unlike those with atoms or molecules in a form of gas or liquid. TPA in semiconductors, therefore, has been researched not only to investigate the material properties that cannot be revealed with single-photon absorption or understand the physics of matter-light interactions but also to embody new practical applications such as optical switching or routing devices. In this thesis, two pioneering semiconductor-based experiments regarding TPA will be discussed. First, I will talk about coherent control of TPA in semiconductors. Here, the coherent objective is the ratio of the two-photon induced photocurrent from one diode to that from the other. This is the first semiconductor-based demonstration, to the best of my knowledge. This technique can be used for the demultiplexer-free detection of chirp-coded signals. Second, the technique of TPA-based femtosecond pulse characterization will be discussed. This technique features a very convenient experimental setup but an innovative approach that has never been used hitherto. Through those experiments ranging from the fundamental physics level to real-world applications, a new potential of TPA in semiconductors may be discovered. Additionally, I will cover several supplemental topics, including the TPA spectrum measurement via interferometric autocorrelation, implementation of the evolutionary algorithms, design of a multiple quantum well diode that may have tunable TPA spectrum, and so on