Far Field Investigations on Quantum Cascade Lasers

Quantum cascade lasers (QCLs) are unipolar semiconductor lasers operating in the mid infrared region. Their emitted wavelength is defined by the separation of intersubband levels which can be tailored by band structure engineering. Due to the fact that the maximum conduction band offset in the GaAs/AlGaAs material system is 380 meV, the lowest directly accessible wavelength is roughly 7 μm. One way to reach lower wavelengths is to use intracavity nonlinearities. With sum frequency (SF) generation or second harmonic (SH) generation respectively it is possible to cover the spectroscopically important band between 3.5 μm and 7 μm. Unlike conventional interband semiconductor lasers QCLs do not suffer from the problem of strong absorption of the frequency doubled light because the emission energies (100 – 300 meV) are well below the band gap of the hosting GaAs (EGap = 1.4 eV).