  Two-dimensional electron gasses (2DEG) in InAs quantum wells proximitized by
aluminum are promising platforms for topological qubits based on Majorana zero
modes. However, there are still substantial uncertainties associated with the
nature of the electronic states at the interfaces of these systems. It is
challenging to probe the properties of these hybridized states as they are
buried under a relatively thick aluminum layer. In this work, we have
investigated a range of InAs/In$ _{1-\text{x}} $Ga$ _\text{x} $As
heterostructures with Al overlayers using high precision time-domain THz
spectroscopy. Despite the thick metallic overlayer, we observe a prominent
cyclotron resonance in the magnetic field that can be associated with the
response of the interfacial states. Measurements of the THz range complex
Faraday rotation allow the extraction of the sign and magnitude of the
effective mass, the density of charge carriers, and scattering times of the
2DEG despite the close proximity of the aluminum layer. We discuss the
extracted band parameters and connect their values to the known physics of
these materials.
