Ultrafast refractive index change of novel semiconducting materials measured by frequency domain interferometry
Ultrafast Transient Absorption Spectroscopy is a powerful tool to reveal excited state dynamics and semiconductor photophysics of semiconducting materials on femtosecond (10-15 s) timescales, including carrier recombination, hot carrier cooling, bandgap renormalisation and charge transfer. This technique uses a pump pulse to excite electrons within a material into a higher energy state and measures the state of the material by way of transmission of a probe pulse. This has provided insight into the photoexcitation and charge carrier dynamics of a broad range of materials such as organic semiconductors and halide perovskite materials.
This technique, however, faces two major obstacles. The first obstacle is the inability to distinguish the effects of the excited state absorption and reflection on the transmission of a material. With Transient Absorption measurements, the assumption is made that the change in transmission is the result of the imaginary part of the refractive index alone and does not consider the effect of the real part of the refractive index, which relates to the reflection. This is not an issue for materials with a low real refractive index, however, reflection effects can create significant artefacts on the Transient Absorption signal for materials with a high refractive index, such as perovskite materials. Attempts have been made to measure the excited state refractive index, however, these techniques are model dependent and therefore may not reflect the true excited state refractive index.
The second obstacle is the different requirements in spectrum and pulse energy for the pump and probe pulses used in this experiment. This results in the use of a combination of light sources, such as bulk supercontinuum generation, (Non)collinear Optical Parametric Amplifiers, and/or nonlinear optical fibres to generate these distinct pump and probe pulses. The use of multiple light sources, each with their own multitude of optical components, add to the complexity of the transient absorption spectroscopy system. In this thesis, we address these problems by implementing a Frequency Domain Interferometry system and a Multiple Plate Compression light source.
Firstly, the Frequency Domain Interferometer based on the Michelson interferometer is implemented in an existing Transient Absorption spectrometer. This is discussed in chapter 3, where this system is used to measure the optically induced change of the real part of the refractive index of CsPbBr3 perovskite. From this, the excited state extinction spectrum is accessed, which provides valuable information regarding the suspected reflection artefacts observed in the Transient Absorption signal of this material.
Secondly, the Multiple Plate Compression lightsource is implemented in a Transient Absorption spectrometer in chapter 4. The Multiple Plate Compression is a novel light source capable of generating intense, stable, temporally compressed and spectrally broadband pulses, but has never been applied in the field of ultrafast spectroscopy before. The shot-to-shot stability and spectra of the pulses generated by two multiple plate compression systems are analysed. The Multiple Plate Compression Transient Absorption spectrometer is used on MAPbI3 perovskite to demonstrate the application of this light source in ultrafast spectroscopy. Finally, in chapter 5, the Multiple Plate Compression is used as a single light source for an improved Frequency Domain Interferometer system, based on the Sagnac interferometer. This technique is applied to obtain the optically induced change of the real part of the refractive index of pentacene. These results are then compared to the refractive index change obtained by a novel differential dielectric functions model.
Understanding of the complex refractive index change of materials has great implications for future optoelectronic device architecture. The analysis of the pulses generated by the multiple plate compression show that it is an excellent light source in the field of ultrafast Transient Absorption Spectroscopy. This paves the road for the implementation of this system as a light source in other spectroscopic techniques such as 2-Dimensional Electronic Spectroscopy and Impulsive Stimulated Raman Spectroscopy.
