Terahertz crystals

As was shown in their letter in the previous issue of Electronics Letters, low attenuation and simple integrability confirm the suitability of such crystals for the successful implementation of terahertz technology.

Fabrication and the rest

With its large number of potential applications, terahertz research has experienced rapid growth in recent years. Perhaps most significantly, the scope for THz technology is extremely broad, covering such topics as astronomic and atmospheric spectroscopy and sensing, chemical and biological detection and imaging, security screening, materials research, industry quality control, and next-generation communication networks and radars.

However, as outlined by leading experts in this year’s Electronics Letters special supplement on terahertz technology, there are a significant number of hurdles to overcome before the technology becomes practically and economically viable. Efficient coupling to free space is achieved through effective antenna design, and this alone presents considerable challenges. For example, the intrinsic loss of materials commonly used in antennas increases significantly with frequency, so many conventional antennas working well at microwave frequencies suffer in the THz range. Further, fabrication of THz antennas can be challenging: conductor surface roughness and other non-ideal fabrication tolerances will all impact the performance of THz antennas dramatically. Finally, integration and packaging with other components is difficult owing to tightened tolerance requirements, and a lack of mature calibration standards and procedures hinders accurate testing.  

Owing to their low loss, high coupling efficiency to free space, inherent integration and packaging with other components, electromagnetic crystal-based THz antennas have attracted interest as potential candidates for overcoming these issues and practically implementing this technology.

Crystal Clear

Electromagnetic crystals – or electromagnetic bandgap structures – are a periodic arrangement of dielectric and/or metallic scatterers, so they acquire a gap in their electromagnetic spectrum due to Bragg scattering. Within these bandgaps wave propagation is prohibited so, if an air channel is present, the structure will support wave propagation along the channel in the bandgap frequency, whereas the material loss and radiation loss along the propagation path will be greatly suppressed because of the bandgap cladding. When flared into a horn shape at the interface with air, very good coupling efficiency to free space can be achieved, making this a promising route to high efficiency THz antennas which can be easily integrated with other components to realise fully functional THz systems.

To exploit these advantages, the team fabricated their antenna using MEMS technology on a silicon substrate which is compatible with semiconductor processes, demonstrating one of the first electromagnetic crystal horn antennas operating at THz frequencies. Uniquely, the unit cell of the crystal consists of a 48 × 48 × 241 μm metalised cuboid on a silicon substrate coated with a 200 nm layer of gold.  To overcome the limitations of the testing process, they employed an electronic method instead of the more commonly used photonic methods such as time-domain spectrometry.

Scaling up

While their method has significantly improved the fabrication process, integration, and characterisation of these antennas, the team hopes to make further steps to meet the demands for this technology. For example, refining the fabrication process to achieve better surface smoothness for lower loss, designing and testing other passive components such as filters and power dividers, and investigating efficient and low-cost integration approaches with active devices such as detectors and solid-state based sources, all with the goal of fully integrated THz microsystems.

As well as antenna fabrication and integration, the group has been investigating tailored thermal emission from THz electromagnetic crystals for spectral control and thermal sources, and the THz properties of nanodevices and materials such as carbon nanotubes, graphene and nanostructured dielectrics. Along with their recent demonstration of an all-dielectric THz horn antenna using a polymer jetting technique, they believe these antenna technologies will scale even further – up to several THz – while still integrating with semiconductor devices and other functional components.