Gap plasmon mode distributed feedback lasers

Unlike conventional dielectric photonic structures, metal coated or plasmonic structures can confine light on a sub-wavelength scale. It has been shown that despite high internal losses, lasing is possible in plasmonic cavities, with dimensions well below the diffraction limit of light. Due to the small size and high current density, it is possible that future metallic nano-lasers will have switching speeds in the terahertz range. Until now, integration of these lasers with other optical components was difficult since they are fully enclosed by a metal layer of several hundreds of nanometers thick. This thesis discusses the possibility of incorporating distributed feedback in plasmonic waveguides to control the wavelength of operation, to control the emissive properties of the devices and to enable coupling to waveguides. The research which was carried out can be divided in three major parts. The first part involves the analysis and design of plasmonic waveguide lasers. The optical properties of plasmonic waveguides were determined using FDTD and FEM techniques. The FDTD simulations were carried out in 2D and 3D and included the presence of gain, absorption, material dispersion and imperfections in shape. The dependence on structural and material properties was studied. After studying the behavior of basic plasmonic waveguides, the incorporation of distributed feedback through vertical groove gratings was investigated. The wavelength dependence and feedback strength were determined. Also the behavior of resonant cavities, in which this distributed feedback was incorporated, was investigated. Finally, the threshold gain requirements and spontaneous emission enhancement were determined. The second part of the project consisted of developing a fabrication process for the plasmonic waveguide lasers. Existing processing techniques, used for wafer-scale fabrication of photonic integrated circuits, have been modified to make them suitable for the fabrication of plasmonic devices. A novel, high-resolution electron beam lithography process was developed capable of defining structures with 50 nm feature sizes in the III-V material system. The effect of various processing steps on surface and material quality was studied. A full fabrication run of the devices was carried out and the devices have been mounted afterwards. Some of the devices went through additional processing steps in order to open their end-facets (using focused ion beam milling). The final part of the project involved the characterization of the fabricated devices. A new measurement setup was built, in which the devices could be imaged and measured at the same time. The measurement setup is suitable for characterization through the substrate and through an open end-facet of the device. Measurements can be carried out at temperatures ranging from 4K to room temperature, in a continuous flow cryostat. Electrically pumped, surface plasmonic lasers have been realized, with core waveguide widths well below the diffraction limit of light (min. 100 nm). The lasers have been characterized though the substrate as well as via an open end-facet. Threshold currents as low as 400 µA for 100 µm long devices have been observed. The distributed feedback lasers show line-widths below 0.5 nm (limited by the resolution of the spectrometer) and have a side-mode suppression ratio of over 20 dB. Their emission wavelength could be tuned over a 100 nm range by changing the period of the distributed feedback by 60 nm. The devices have initially been characterized at cryogenic temperatures (80K), using continuous current injection, and in a later stage also at room temperature (> 295K), using pulsed current injection.