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We study the Kerr nonlinearity of quantum cascade lasers (QCLs) by coupling resonant and off-resonant mid-infrared (mid-IR) femtosecond (fs) pulses into an active QCL waveguide. We observe an increase in the spectral width of the transmitted fs pulses as the coupled mid-infrared (mid-IR) pulse power increases. This is explained by the self-phase modulation effect due to the large Kerr nonlinearity of QCL waveguides. We further confirm this effect by observing the intensity dependent far-field profile of the transmitted mid-IR pulses, showing the pulses undergo self-focusing as they propagate through the active QCL due to the intensity dependent refractive index. We experimentally estimate the nonlinear refractive index n2 of a QCL to be ∼8 × 10−9 cm2/W using the far-field beam profile of the transmitted pulses. The finite-difference time-domain simulations of QCL waveguides with Kerr nonlinearity incorporated show similar behavior to the experimental results.

We temporally resolved the ultrafast mid-infrared transmission modulation of quantum cascade lasers (QCLs) using a near-infrared pump/mid-infrared probe technique at room temperature. Two different femtosecond wavelength pumps were used with photon energy above and below the quantum well (QW) bandgap. The shorter wavelength pump modulates the mid-infrared probe transmission through interband transition assisted mechanisms, resulting in a high transmission modulation depth and several nanoseconds recovery lifetime. In contrast, pumping with a photon energy below the QW bandgap induces a smaller transmission modulation depth but much faster (several picoseconds) recovery lifetime, attributed to intersubband transition assisted mechanisms. The latter ultrafast modulation (>60 GHz) could provide a potential way to realize fast QCL based free space optical communication.

Second harmonic generation (SHG) pulses at λ ∼ 2.25 µm have been obtained from a 4.5 µm quantum cascade laser (QCL) when it is resonantly pumped by transverse magnetic polarized 120 fs, λ = 4.5 µm pulses through the QCL’s front facet at room temperature. The measured SHG spectrum narrows when the bias across the QCL increases due to the electron population re-distribution and subband realignment. The expected quadratic dependence of the SHG with pump power is observed but saturates at higher pump powers. The linear to nonlinear power conversion efficiency is calculated (∼2 µW/W2) and compared with theoretical calculation. This experiment provides an alternative way of investigating the carrier dynamics in QCLs.