Spectral engineering of surface plasmon waveguides for THz quantum cascade lasers

This report investigates the optical spectral behaviour of periodically FIB (Focused IonBeam) etched plasmon waveguides for a terahertz quantum cascade laser (THz QCL).The plasmon waveguide considered is due to Kohler [1]. Optical confinement is achievedusing doped GaAs and Au layers surrounding the active region, which also double as thedevice electrical contacts. Selective periodic etching away of the top contact layer disruptsthe plasmon mode supported by this layer and results in a band-stop transmissionchracterstic. Photonic states are depleted in the band stop region, which is designed to bein the gain wavelength range of the THz QCL. This encourages single mode lasing at theedge of the band-stop region with a high degree of side mode suppression. To calculateshape of the transmission characteristic, Maxwell???s equations need to be solved for theguided and radiation modes supported by the periodic waveguide structure. This involvesfinding the refractive indices of the materials used to fabricate the waveguide using aDrude-Lorentz model. The refractive indices are then used to calculate the effectiverefractive indices of etched and unetched cross-sections. The TM (transverse magnetic)radiation modes for the complete structure can then be solved, yielding the transmissionand reflection characteristics. These methods are described using both commercialsoftware and models developed from first principles. The effect of gain is consideredusing a simple approximation. It was found that a combination of using the commercialsoftware to find the effective indices and a transfer-matrix waveguide model yielded theexpected transmission characteristic for both a Fa bry-Perot (FP) cavity and theperiodically etched structure. The combined effect of FP facets and the etched structureon this characteristic needs further work to confirm. A more sophisticated gain modelwould be needed to produce a complete THz QCL model.