Electronic & Electrical Engineering
Current research is focused on the development of optical and THz technologies and application of advanced characterization methods, such as THz near-field imaging and spectroscopy. Studies are directed toward understanding the physics of materials and systems with strong light-matter interactions and development of new devices.
Figure 1. High-resolution THz imaging using an integrated near-field probe with a 10μm aperture. Images show E-field ‘snapshots’ near a micro-strip dipole antenna illuminated by a THz pulse (λavg = 600μm). The near-field approach allows imaging and spectroscopic studies on a scale much smaller than the wavelength of THz radiation.
Figure 2. Optical probe for detection of E-field. Using femtosecond optical pulses E-fields with frequencies of several THz can be detected. An electro-optic crystal (GaAs) mounted on a tip of a fibre changes the polarization state of light in the presence of THz field. Electro-optic effect is enhanced when an optical resonator (inset) is placed instead of the bulk crystal. This miniature probe allows local THz spectroscopy and mapping of E-field with spatial resolution of several micrometers.
Graphene and its nanostructures have recently emerged as a promising platform for next-generation nano-plasmonic devices. In the terahertz (THz) frequency range in particular, response of graphene nano-structures is continuously tuneable by varying the carrier density in graphene and the structure size. In the core of these effects are plasmon modes. Due to their local nature, direct experimental detection and investigation of these modes require THz near-field microscopy. In this article, we report on novel THz near-field microscopy investigations of epitaxial multilayer graphene mesas and ribbon arrays in which we observed surface waves. Near-field images reveal that the THz field in vicinity of graphene ribbon arrays can be either reduced or enhanced, depending on the orientation of the ribbon with respect to the polarization of the THz wave and the array periodicity. The observed intriguing properties hold promise for new applications of graphene in a range of THz device applications from tuneable THz filters for THz communications to graphene-based THz sensors. This THz near-field microscopy technique also opens the possibility of non-invasive probing of local THz properties of graphene with sub-wavelength spatial resolution for investigations of surface plasmon phenomena in graphene.
Figure 3: THz near-field image of a graphene square indicating non-uniform carrier density distribution
Figure 4: Space-time THz near-field map showing excitation and propagation of THz surface waves on graphene
This work describes low-loss waveguides for THz radiation (1-3 THz). Cylindrical hollow metallic waveguides with a thin dielectric coating can be designed to support the HE11 mode. These waveguides showed good mode quality with the total loss below 1 dB/m at 2.5 THz. Low-loss THz waveguides open possibilities for new applications in communications and sensing.
To understand the impact of molecular structure on electronic properties of organic solids we investigate crystalline rubrene, which exhibits the highest hole mobility among organics. Using photoluminescence spectroscopy with single- and two-photon excitation and charge transport analysis we found that large variations of the carrier density in rubrene is caused by an oxygen-related impurity, which acts as an acceptor state.
Fabrication of high-reflectivity distributed Bragg reflectors for GaN optical devices, such as GaN VCSELs and microcavities, has long been a challenging task due to the in-plane strain caused by a large lattice constant mismatch between GaN and AlN. This work describes AlGaN/GaN DBR structures, where the tensile and compressive strains are compensated in each multilayer period. This approach relaxes constraints on the DBR design. It allowed fabrication of high-reflectivity (>99%) wide stop-band DBRs.
High-resolution imaging with THz waves requires near-field methods to overcome the diffraction limit. This work describes a miniature electro-optic probe for THz near-field microscopy. A microresonator structure enhances the sensitivity of the probe allowing THz imaging and local spectroscopy with spatial resolution of several micrometers.