Research On Coherent Nonlinearity In Optoelectronic Materials

Optoelectronic conversion has been widely used in photovoltaic application and optoelectronic detection area. For a long time, it is a research hot spot to improve the conversion efficiency. Basically, the optoelectronic conversion process can be regarded as the process of the generation of electron-hole pair and that of the separation of charges. As a result, to improve the efficiency of charge separation and the generation of electron-hole pair is the key factor to improve the total optoelectronic conversion efficiency. In type-Ⅱ energy structure, charge transfer transition occurs after excitation. Compared to traditional hot carrier cooling and carrier migration, the efficiency of charge separation would be improved because of the separation of electron and hole wavefunction in charge transfer transition. On the other hand, to reduce the thermal lost and to improve the carrier generation efficiency, carrier multiplication has been considered the potential method to break Shockley-Queisser limit. As a correspondence to multi-excition generation in semiconductors, in organic materials, singlet fission has been paid much attention in recent years. In this specific thesis, for the study of charge transfer transition and singlet fission, we build a variety of ultrafast spectroscopic methods as ultrafast transient absorption, optical Kerr effect, four wave mixing and two dimensional electronic spectroscopy. With these techniques, we have deeply studied the charge transfer transition in semiconductor nanocrystals and the singlet fission process in certain organic materials.The major findings are summarized as below:1、We identified a giant optical nonlinearity induced by charge-transfer excitonic transition in type-Ⅱ nanocrystals. We performed a comparative study with four-wave mixing technique on type-Ⅰ CdSe/CdS nanocrystals and type-Ⅱ CdSe/ZnTe nanocrystals synthesized with colloidal approaches. We observed optical nonlinearity in type-II nanocrystals with an extended broadband spectral coverage arising from the charge-transfer excitonic transitions. This optical response was further ascribed to the state filling effect with the experiment of transient absorption and time-resolved optical Kerr effect. Our discovery indicates that bandgap engineering, with no harm on the quantum confinement, can tailor the nonlinear response of semiconductor nanocrystals, which is instructive for the design of optoelectronic devices with a broadband response.2、We prove that carrier multiplication in singlet fission can significantly promote nonlinear optical response in organic semiconductors. For decades, optical nonlinearity in conjugate organic semiconductors has attracted tremendous research interest owing to the abundance of molecular structures and the effective cost. The research community has been searching the organic nonlinear materials with a great number of efforts in theoretical calculation and material synthesis for the purpose of all optical switching. In spite of remarkable progresses achieved, the research has been limited in single electron regime (i.e., one photon modulates one electron). Singlet fission is a process that one photon can manipulate two electrons by converting one singlet exciton into a pair of triplet excitons. We observed extremely large optical nonlinearity with susceptibility up to 10-9 esu in pentacene films with dual color time-resolved optical Kerr effect. Remarkably, the slow response time of conventional resonant nonlinearity has been circumvented by employing the extremely short-lived intermediate states relevant to singlet fission. The result is a solid evidence of the presence of intermediate dark state during singlet fission process in crystalline pentacene. We further demonstrated the all-optical switching with the singlet-fission-induced optical nonlinearity.3、Singlet fission holds the potential to break the Shockley-Queisser limit by splitting one singlet exciton into a pair of triplets. Lately, there are numerous researches on implanting this process in the device. However, the mechanism underlying singlet fission remains elusive. An intermediate multiexciton (ME) state was theoretically proposed to connect singlet and triplet pairs, but the exact nature of ME state remains poorly studied. Here, we employed the newly-established two-dimensional electronic spectroscopy (2DES) to study the possible coherent formation of ME state. The setup of 2DES is enabled with ultrashort pulse (< 10 fs) in the visible region from the non-collinear optical parametric amplifiers and the passive phase stabilization with the combination of pair-wise configuration and the pair of wedge glasses. We observed off- diagonal of 2DES signal from tetracene films showing the possible coherent electronic coupling between between ME state and singlet state, implying the quantum coherence may play a critical role in dictating singlet fission in crystalline tetracene.