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Second-Harmonic Emission from Quantum Cascade Lasers

The mid-infrared spectral region can be covered by quantum cascade (QC) semiconductor lasers, where the emission wavelength is tailored by bandstructure engineering. For a detailed review on QC lasers see Ref. [1]. Their emission energy is well below the band gaps of the hosting material system. Typical GaAs QC lasers operate in the mid-infrared regime with corresponding energies of ~100 meV, whereas the GaAs band gap is around 1.4 eV. That makes frequency-doubling inside the laser cavity feasible. Such intracavity second-harmonic (SH) generation is not possible in interband semiconductor lasers, where the SH radiation is strongly absorbed. The principle of nonlinear light generation in QC lasers was firstly demonstrated in 2003 [2], since then a lot of progress in this field has been made, such as the improvement of SH generation [3] and the demonstration of third-harmonic generation [4]. A crucial step was the achievement of phase-matching [5], [6], which was demonstrated by means of modal phase-matching in InP-based QC lasers. Another approach for higher conversion efficiencies is quasi phase-matching by periodically modulating the pump current along the QC laser ridge waveguide [7]. Besides up-conversion, other intracavity nonlinear effects are currently being investigated, such as Raman lasing [8] and anti-stokes [9] emission from QC lasers. The above nonlinear effects are due to higher-order susceptibilities of intersubband transitions. Although InP and GaAs, both of which are commonly used as host materials for QC lasers, have nonzero second-order susceptibilities, there is no resulting second-order polarization in the material for QC lasers. That is due to selection rules for intersubband transitions which allow gain only for TM polarized light, which in turn due to crystal symmetry cannot excite nonlinear polarization in the host material. However it was shown that QC lasers grown on +111, substrates, show sum-frequency generation due to bulk nonlinearity [10].