Subterahertz difference-frequency generation in terahertz quantum cascade lasers

We have detected subterahertz spectral lines in the emission of free-running GaAs/AlGaAs quantum cascade lasers (QCLs) operating around 3 THz. We have found that the power W(f(d)) of an individual sub-THz line at the frequency f(d) is directly proportional to a power product W(f(i)) x W(f(k)) of the corresponding pair of THz lines having the frequency difference f(i) - f(k) = f(d). The established one-to-one correspondence of each sub-THz line to pair combinations of THz lines proves the nonlinear mechanism of difference-frequency generation (DFG) in the QCL active region. The conversion efficiencies W(f(i) - f(k))/[W(f(i)) x W(f(k))] estimated from the experimental data are about 4 x 10(-3) and 5 x 10(-5) W-1 for the sub-THz lines found at 132 and 310 GHz, correspondingly. Considering nonlinear polarization of QCL active regions as the reason for DFG, we have evaluated the following values of the second-order nonlinear susceptibility chi((2))(132 GHz) approximately equal to 7 x 10(5) pm/V and chi((2))(310 GHz) approximately equal to 4 x 10(4) pm/V from the experimental data. The obtained results pave the way for the use of THz QCLs as sources of sub-THz radiation. Published under an exclusive license by AIP Publishing.