Lastly, I am also interested in exploring fundamental physics from Gravitational Waves (GWs). This constitutes a highly promising avenue for discovering new physics given the planned improvements of LIGO/Virgo \cite{KAGRA:2013rdx} as well as other upcoming experiments such as LISA or BBO. For example, the first detection of GWs has already led to a bound on the propagation speed of tensor modes cT, thereby ruling out plenty of models in modified gravity. A particular focus of my efforts concerns GWs with a spectrum at high frequencies f>kHz. The reason is simple: known astrophysical objects are incapable of emitting frequencies f>10kHz which is why such signatures are either associated with events in the early Universe (such as preheating or phase transitions) or with novel astrophysical objects (such as primordial black holes), see in particular this article for challenges and opportunities of GW searches at high frequencies. It is hence imperative to scrutinise all available sources of high-frequency gravitational waves. String Theory naturally provides a variety of sources through the plethora of moduli and axion fields. Among others, I will investigate first-order phase transitions in the post-inflationary era where bubble walls collide and thereby produce GWs. Further, the formation and interaction of (non-)topological solitons, especially in the context of baryogenesis, gives rise to exciting dynamics and signatures that deserve further scrutiny.