High-order nonlinear terahertz probing of the two-band superconductor MgB2: Third- and fifth-order harmonic generation

We report on high-order harmonic generation in the two-band superconductor ${\mathrm{MgB}}_{2}$ driven by intense terahertz electromagnetic pulses. Third- and fifth-order harmonics are resolved in time domain and investigated as a function of temperature and in applied magnetic fields crossing the superconducting phase boundary. The high-order harmonics in the superconducting phase reflects nonequilibrium dynamics of the superconducting order parameter in ${\mathrm{MgB}}_{2}$, which is probed via nonlinear coupling to the terahertz field. The observed temperature and field dependence of the nonlinear response allows to establish the superconducting phase diagram.