Actual applications at the moment are mostly imaging, the 'nudie scanners' at airports and THz imaging at various frequencies is in use for food inspection as it can easily show blemishes/defects in produce that would otherwise be invisible. I read a paper once on detecting counterfeit money. Analysis of airborne contaminants / weather prediction are other possible applications. For telecom it has been considered (at very early stages) for 6G due to the huge bandwidth available. Really there are a lot of 'possible' applications but implementing them has proven difficult either because it requires advancements in materials or other risky aspects or because it's beaten by existing technologies at the moment.

With terrahertz rectifiers you can use antennas and diodes to convert light to DC.

https://en.wikipedia.org/wiki/Optical_rectenna