Researchers at the University of Adelaide, Australia, and the Islamic University of Technology, Bangladesh, report the optimum design for a porous photonic crystal fibre capable of terahertz frequency transmission. Photonic crystal fibres (PCFs), also known as micro-structured optical fibres (MOFs), are a new class of optical fibre. The significance of PCFs is their remarkable optical properties, such as providing single mode operation, high birefringence, and controllable chromatic dispersion characteristics, which are not obtainable using conventional optical fibres. As a result, PCFs are used extensively in different medical imaging applications, for example, in Optical Coherence Tomography (OCT). OCT is an imaging technology that allows non-invasive cross-sectional imaging of biological tissue. Bringing such techniques into the terahertz domain will be assisted by the development of PCFs that operate in the terahertz regime. There are different types of PCFs, including solid core PCF, hollow core PCF and porous core PCF. A solid core PCF experiences large material loss, which is a property that makes these PCFs unsuitable for terahertz signal transmission. In a hollow core PCF, the loss is greatly reduced, but such fibres operate over a narrow frequency range. To overcome the described disadvantages of solid core and hollow core fibres, porous core fibres were introduced. A porous core fibre contains a number of micro-structured air holes in the core. The advantage of the porous structure is that the fibre global parameters, including air hole size, pitch distance (centre-to-centre distance between air holes), core diameter and shape of air holes can be tuned as required for different applications. Thus, for porous fibre PCFs, the effective material loss, birefringence, dispersion, confinement loss, numerical aperture and other modal properties can be obtained by design. Nowadays, PCFs are used in the field of sensing, biomedical imaging, time domain spectroscopy, security, detecting DNA hybridisation and bio-sensing, skin cancer and breast cancer detection, and in communication applications. However, there are no previously reported articles on terahertz PCFs with sufficient numerical aperture (NA) to be used for OCT applications. In this issue of Electronics Letters, Saiful Islam and colleagues propose a terahertz PCF with high birefringence, high NA, and near zero dispersion variation. The measured high birefringence is reported to be due to the elliptical-shaped asymmetrical structure of the air holes inside the core. The asymmetrical structure of air holes created a refractive index difference between the polarisation modes, thus generating birefringence. The observation of high NA is reported to be due to the high birefringence and high modal effective area. Moreover, flattened dispersion is reported due to the waveguide structure and chosen materials that the waveguide was formed of. The kagome-based PCF has the advantage that it has a significantly lower confinement loss than other PCF structures. Terahertz time-domain spectroscopy set-up in the National T-Ray lab at the University of Adelaide. Saiful Islam at the University of Adelaide. The reported simulations in this Letter have allowed for rapid design optimisation that can now be experimentally tested. The major challenge of a PCF design is to anticipate difficulties in fabrication. Thus, the fabrication possibilities of a PCF must be addressed. For accuracy, obtaining correct parameters to describe the structural and optical properties of the fibre, such as refractive index, material absorption loss, relative permittivity and relative permeability, is of great importance. S. Islam states that ‘‘Our PCF design was produced with ease of fabrication in mind, allowing future realisation. Thus, applications in THz-OCT, chemical sensing, and multichannel communications are envisaged.’’ S. Islam and colleagues are currently using photonic crystal fibres to design chemical sensors for skin cancer detection. Non-invasive breast cancer detection is possible via detecting the presence of certain protein markers in human saliva. The authors are also designing sensors to detect hazardous chemicals for security applications. ‘‘We are planning to extend our PCF designs with the use of graphene or gold coasting to exploit surface plasmons for enhanced sensing’’, explains S. Islam. In practice, PCFs are limited to reasonably simple geometries due to difficulties in fabrication. However, emerging techniques exploiting developments in 3D printing are promising methods to overcome current limitations. S. Islam predicts that ‘‘It is possible that in the next decade will we begin to see more sophisticated PCFs with remarkable properties that are not deemed practical in today's technology.’’
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