Fig. 2: Field-resolved 2D THz spectroscopy of a free-running QCL.
From: Field-resolved high-order sub-cycle nonlinearities in a terahertz semiconductor laser
a Experimental arrangement. The THz-waveforms EA and EB (blue) are focused onto the QCL facet and the transmitted waveform (EAB, orange) is detected electro-optically. b Electric field emitted by the nonlinear polarization, ENL(t, τ), of the unbiased QCL as a function of the delay times t and τ. The electric field is multiplied by a factor of 10 and color-coded as in c. c ENL(t, τ), as in b, but for biased QCL with a current of IB = 840 mA. d 2D-amplitude spectrum \(\tilde E_{{{{\mathrm{NL}}}}}\left( {\nu _t,\nu _\tau } \right)\) of the nonlinear response (IB = 0) in b. The spectrum is multiplied by a factor of 7 for ντ < −5 THz and ντ > 2.8 THz and color-coded as in e. e 2D-amplitude spectrum \(\tilde E_{{{{\mathrm{NL}}}}}\left( {\nu _t,\nu _\tau } \right)\) of the nonlinear response (IB = 840 mA) in c. For better visibility of the 6WM and 8WM signals, the corresponding part of the spectrum is multiplied by a factor of 7. f Corresponding simulated 2D-amplitude spectrum \(\tilde E_{{{{\mathrm{NL}}}}}\left( {\nu _t,\nu _\tau } \right)\)
