Silicon based plasmonic coupler.

Plasmonics is a field in which the light matter interaction can be controlled at the nanoscale by patterning the material surface to achieve enhanced optical effects. Realisation of micron sized silicon based plasmonic devices will require efficient coupling of light from an optical fibre grating coupler into silicon compatible plasmonic waveguides. In this paper we have investigated a silicon based plasmonic coupler with a very short taper length, which confines and focuses light from a broad input fibre opening into a plasmonic waveguide at the apex of the structure. A simple transfer matrix model was also developed to analyse the transmission performance of the coupler with respect to its key physical parameters. The proposed plasmonic coupler was optimised with respect to its different structural parameters using finite element simulations. A maximum coupling efficiency of 72% for light coupling from a 6.2 µm wide input opening into a 20 nm slit width was predicted. The simulated result also predicted an insertion loss of ≈ 2.0 dB for light coupling into a 300 nm single mode SOI waveguide from a plasmonic structure with a 10.4 µm input opening width and a taper length of only 3.15 µm. Furthermore, the application of the optimised plasmonic coupler as a splitter was investigated, in which the structure simultaneously splits and couples light with a predicted coupling efficiency of ≈ 37 % (or a total coupling efficiency of 73%) from a 6.22 µm input opening into two 50 nm wide plasmonic waveguides.

Adiabatic mode coupling between SiGe photonic devices and SOI waveguides.

We describe the coupling between optical modes of silicon-on-insulator SOI waveguides and Ge/SiGe quantum well modulators using an eigenmode expansion method. Laterally tapered features in the epitaxial layers are investigated for adiabatic optical coupling, and we find that there is a critical width range of the Ge/SiGe structure of 200-300 nm, where the taper angle should be minimised. We identify optimised taper profiles, which, for 1-µm-wide waveguides, allow the length of an adiabatic taper to be reduced from 250 µm for a simple linear profile to 40 µm for the optimised structure.

Modulation of the absorption coefficient at 1.3 µm in Ge/SiGe multiple quantum well heterostructures on silicon.

We report modulation of the absorption coefficient at 1.3 µm in Ge/SiGe multiple quantum well heterostructures on silicon via the quantum-confined Stark effect. Strain engineering was exploited to increase the direct optical bandgap in the Ge quantum wells. We grew 9 nm-thick Ge quantum wells on a relaxed Si0.22Ge0.78 buffer and a contrast in the absorption coefficient of a factor of greater than 3.2 was achieved in the spectral range 1290-1315 nm.

Terahertz ambipolar dual-wavelength quantum cascade laser.

Terahertz frequency quantum cascade lasers (THz QCLs) are compact solid-state sources of terahertz radiation that were first demonstrated in 2002. They have a broad range of potential applications ranging from gas sensing and non-destructive testing, through to security and medical imaging, with many polycrystalline compounds having distinct fingerprint spectra in the terahertz frequency range. In this article, we demonstrate an electrically-switchable dual-wavelength THz QCL which will enable spectroscopic information to be obtained within a THz QCL-based imaging system. The device uses the same active region for both emission wavelengths: in forward bias, the laser emits at 2.3 THz; in reverse bias, it emits at 4 THz. The corresponding threshold current densities are 490 A/cm(2) and 330 A/cm(2), respectively, with maximum operating temperatures of 98K and 120 K.