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 mechanisms1,2, whereas much room remains for further studies in solids3–5. 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 plasmas6, diagnostics of laboratory plasmas7 and collective excitations in condensed matter8. However, induced transparency in plasmas remains relatively unexplored9. 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 leftand 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. The free electrons in the conduction band of doped narrow-gap semiconductors, for example, InSb, InAs and HgCdTe, behave as classic solid-state plasmas and have been examined through a number of infrared spectroscopy studies10,11. Owing to the low electron densities achievable in these materials and to the electrons’ small effective mass and high mobility, most of the important energy scales (the cyclotron energy h̄ωc, the plasma energy h̄ωp, the Fermi energy EF, intra-donor transition energies and so on) can all lie within the same narrow energy range from ∼1 to 10meV, or the terahertz frequency range (1 THz is equivalent to 4.1meV). The interplay between these material properties, which are tunable with magnetic field, doping density and/or temperature, make doped narrow-gap semiconductors a useful material system in which to probe and explore new phenomena that can be exploited for future terahertz technology12–14. Here, we have used a time-domain terahertz magnetospectroscopy system15 (see the Methods section) with a linearly polarized, coherent terahertz beam to investigate magnetoplasmonic effects in a lightly n-doped InSb sample that shows a sharp plasma edge at ∼0.3 THz at zero magnetic field as well as sharp absorption and dispersion features around the cyclotron resonance (ωc/2π∼ 2 THzT−1). These spectral features can be sensitively controlled by changing the magnetic field and temperature, owing to the very small effective masses of electrons and low thermal excitation energy in this narrow-gap semiconductor. Furthermore, long decoherence times (up to 40 ps) of electron cyclotron oscillations give rise to sharp interference fringes and coherent beating between different normal