Interference-induced terahertz transparency in a semiconductor magneto-plasma

Maximum modulation of light transmission occurs when an opaque medium is suddenly made transparent. This phenomenon occurs in atomic and molecular gases through different mechanisms(1,2), whereas much room remains for further studies in solids(3-5). A plasma is an illustrative system showing opacity for low-frequency light, and light-plasma interaction theory provides a universal framework to describe diverse phenomena including radiation in space plasmas(6), diagnostics of laboratory plasmas(7) and collective excitations in condensed matter(8). However, induced transparency in plasmas remains relatively unexplored(9). Here, we use coherent terahertz magneto-spectroscopy to reveal a thermally and magnetically induced transparency in a semiconductor plasma. A sudden appearance and disappearance of transmission through electron-doped InSb is observed over narrow temperature and magnetic field ranges, owing to coherent interference between left- and right-circularly polarized terahertz eigenmodes. Excellent agreement with theory reveals long-lived coherence of magneto-plasmons and demonstrates the importance of coherent interference in the terahertz regime.