Roadmap on optical sensors.

Optical sensors and sensing technologies are playing a more and more important role in our modern world. From micro-probes to large devices used in such diverse areas like medical diagnosis, defence, monitoring of industrial and environmental conditions, optics can be used in a variety of ways to achieve compact, low cost, stand-off sensing with extreme sensitivity and selectivity. Actually, the challenges to the design and functioning of an optical sensor for a particular application requires intimate knowledge of the optical, material, and environmental properties that can affect its performance. This roadmap on optical sensors addresses different technologies and application areas. It is constituted by twelve contributions authored by world-leading experts, providing insight into the current state-of-the-art and the challenges their respective fields face. Two articles address the area of optical fibre sensors, encompassing both conventional and specialty optical fibres. Several other articles are dedicated to laser-based sensors, micro- and nano-engineered sensors, whispering-gallery mode and plasmonic sensors. The use of optical sensors in chemical, biological and biomedical areas is discussed in some other papers. Different approaches required to satisfy applications at visible, infrared and THz spectral regions are also discussed.

Temperature dependence of the dielectric function of dehydrated biological samples in the THz band.

Terahertz technology has demonstrated enormous potential for the analysis of biological systems and the diagnosis of some medical conditions, given its high sensitivity to detect water content. In previously published papers, effective medium theories are used to extract the water content from the terahertz measurements. When the dielectric functions of water and dehydrated bio-material are well known, the volumetric fraction of water can be left as the only free parameter in those effective medium theory models. While water complex permittivity is very well known, the dielectric functions of dehydrated tissues are normally measured for each individual application. In previous studies, it has been traditionally assumed that, unlike water, the dielectric function of the dehydrated tissues is temperature independent, measuring it only at room temperature. Yet, this is an aspect that has not been discussed and that is relevant in order to get THz technology closer to clinical and in-the-field applications. In this work, we present the characterization of the complex permittivity of dehydrated tissues; each studied at temperatures ranging from 20°C to 36.5°C. We studied samples of different organism classifications to have a wider confirmation of the results. We find that, in each case, the dielectric function changes of dehydrated tissues caused by temperature are smaller than for water across the same temperature interval. Yet, the changes in the dielectric function of the dehydrated tissue are not negligible and should, in many cases, be taken into account for the processing of terahertz signals that interact with biological tissues. While this study gives a first introduction into the probable relevancy of temperature-dependent optical behavior of biological samples, this work only focuses on the experimental proof for this relationship and will, therefore, not give a deeper analysis of how the underlying models have to be modified.

Layer separation mapping and consolidation evaluation of a fifteenth century panel painting using terahertz time-domain imaging.

Over time, artworks often sustain paint layer separation and air gaps within their internal structure due to storage conditions and past restoration efforts. Because of this, paint layer consolidation interventions are an essential activity for art conservators. However, it is difficult to determine the exact location and the extent of layer separation on a piece of art in a non-invasive way, and even more difficult to evaluate the success of a consolidation intervention. In this work, a fifteenth-century wood panel painting was analyzed using terahertz time-domain imaging before and after it was consolidated. Using the terahertz data, it was possible to determine the areas on the artwork in need of consolidation and aid the intervention. The analysis of the after data allowed for the control and determination of the success of the consolidation effort in a non-destructive manner.

The effect of angular dispersion on THz data transmission.

One of the key distinctions between legacy low-frequency wireless systems and future THz wireless transmissions is that THz links will require high directionality, to overcome the large free-space path loss. Because of this directionality, optical phenomena become increasingly important as design considerations. A key example lies in the strong dependence of angular radiation patterns on the transmission frequency, which is manifested in many different situations including common diffraction patterns and the emission from leaky-wave apertures. As a result of this effect, the spectral bandwidth at a receiver is nonlinearly dependent on the receiver's angular position and distance from the transmitter. In this work, we explore the implications of this type of effect by incorporating either a diffraction grating or a leaky wave antenna into a communication link. These general considerations will have significant implications for the robustness of data transmissions at high frequencies.

Terahertz imaging demonstrates its diagnostic potential and reveals a relationship between cutaneous dehydration and neuropathy for diabetic foot syndrome patients.

Diabetic foot syndrome, a long term consequence of Diabetes Mellitus, is the most common cause of non-traumatic amputations. Around 8% of the world population suffers from diabetes, 15% of diabetic patients present a diabetic foot ulcer which leads to amputation in 2.5% of the cases. There is no objective method for the early diagnosis and prevention of the syndrome and its consequences. We test terahertz imaging, which is capable of mapping the cutaneous hydration, for the evaluation of the diabetic foot deterioration as an early diagnostic test as well as ulcers prevention and tracking tool. Furthermore, the analysis of our terahertz measurements combined with neurological and vascular assessment of the patients indicates that the dehydration is mainly related to the peripheral neuropathy without a significant vascular cause.

Terahertz beam steering using active diffraction grating fabricated by 3D printing.

In this article, we propose variable-period diffraction gratings for terahertz frequencies. The design, fabrication and characterization of such devices are presented. Our measurements show the possibility to actively shift of the deflection angle for each frequency using this device. We also demonstrated that, when driven by a speaker, these variable gratings can be used for active beam steering with potential application in terahertz communications.

Empirical comparison between effective medium theory models for the dielectric response of biological tissue at terahertz frequencies.

We study the use of three effective medium theory models, namely Maxwell-Garnett, Bruggeman, and Landau-Lifshitz-Looyenga, for the dielectric response of biological tissue in the terahertz band of the electromagnetic spectrum. In order to accomplish our objectives, we performed measurements on water-dehydrated basil binary mixtures encompassing the entire concentration range, and we further analyze the dielectric function with the models. Our results indicate that the Landau-Lifshitz-Looyenga and Bruggeman models provide marginally better fit to the experimentally measured dielectric function in the terahertz band. We further discuss the biological relevance of the models in the context of our experimental data based on their fundamental assumptions.