Quantum cascade lasers are versatile semiconductor laser sources which are able to cover the mid-infrared and THz ranges of the electromagnetic spectrum. At these very long wavelengths, surface-plasmons can be used for waveguiding. In fact, the longer the wavelength, the lower the propagation losses are; thus the use of surface-plasmon metallic waveguides for semiconductor lasers becomes possible in the mid-infrared and it is definitely advantageous in the THz.
In this talk, we will show how surface-plasmon waveguides offer the opportunity to implement new functionalities (single mode, well-behaved surface-emission…) for semiconductor lasers with a simple technological approach.
In the mid-infrared, we report on new surface-plasmon waveguides which exhibit modes with low propagation losses. This is obtained by periodically structuring the metallic top cladding in asymmetric surface-plasmon waveguides. Counter intuitively, the electric-field maxima of the low-loss modes are located below the metallic fingers, and not below the air-gaps. We have proven this concept on surface-plasmon quantum-cascade lasers operating at λ≈7.5 micrometers, but the concept can be applied at shorter wavelengths. In addition, measurements performed with apertureless near-field scanning optical microscopy reveal the presence of the optical mode as an evanescent electric field above the metal, signature of surface-plasmon generation by electrical injection.
In the THz, we demonstrate photonic-crystal THz quantum cascade lasers, where the photonic-crystal is implemented via the sole patterning of the device top metallization. Single-mode laser action is obtained in the (2.55 – 2.88) THz range, and the emission far-field exhibits a small angular divergence, thus providing a solution for the quasi-total lack of directionality typical of THz semiconductor lasers based on metal-metal ridge waveguides. We also demonstrate that the emission far-field can be predictably engineered.