“…More recently, mid-infrared OFCs have been observed in quantum cascade lasers, both in the mid-infrared and terahertz ranges [9][10][11], and attempts at modeling their dynamics are in progress [12][13][14][15]. Materials with second-order susceptibility, χ (2) , have been used to indirectly replicate or extend an existing frequency comb, otherwise generated, to different spectral regions, exploiting different χ (2) processes, like difference frequency generation [16][17][18][19][20][21][22][23], parametric generation in synchronously pumped optical parametric oscillators (OPOs) [24][25][26][27][28], and harmonic up-conversion [29][30][31]. Quadratic nonlinear crystals have been exploited in laser cavities for mode-locked emission: acting as electro-optic modulator in active mode-locking schemes [32,33], and as nonlinear mirror or nonlinear lens in passive schemes [34][35][36].…”