New devices based on metamaterials

The research focused mostly on the design of new metamaterials in the Terahertz (THz) frequencies, a waveband located between microwave and infrared. "It is a waveband with enormous potential for applications in biomedicine, radio-astronomy and security -- for the detection of explosives and weapons," pointed out Mr Torres. "Being a waveband of relatively recent use, there is a great lack of efficient devices and this is why the results of our research are contributing to filling this gap." The PhD thesis is entitled "Plasmonics and Metamaterials at Terahertz Frequencies."

The properties of the metamaterials do not arise from their composition but from the shape in which their structure is designed. "In this way, materials can be achieved, for example, with negative refraction index and which bend light in the opposite direction to what occurs in natural materials."

The results of the research can be summed up in three large groups: extraordinary transmission metamaterials, ENZ metamaterials, and optical nano-antennae.

According to the author of the research, the extraordinary transmission metamaterials are very thin metallic structures, made up of very small holes. An optimum layout of these holes enables the light to pass through them, but in a very different way than larger holes do. "We have found a special way of interconnecting these holes through meandering-shaped lines that give these metamaterials additional characteristics." Once the behaviour of the new structure is known, a polariser is designed and manufactured, enabling controlling and changing the direction of the light through the structure. "This polariser is capable of working at two different waveband frequencies, with very good waveband characteristics and a really thin thickness, making it a device that keenly competes with and, in most cases, surpasses commercial devices in performance."

As regards the ENZ metamaterials, their properties have been exploited to design and manufacture metallic lenses with high performances. With these lenses the electromagnetic wave passes through very narrow channels which are very close to each other. "The correct engineering of these channels and of the shape of the lens itself confers on it radiation and energy-focusing properties that are greater than those of classical lenses manufactured with other materials." Mr. Torres has designed and manufactured these lenses with a plane concave profile for working at 0.15 Thz. "It was the first experimental demonstration ever with this type of ENZ metamaterials and, moreover, we have proposed theoretically a more advanced design where the reduction of the volume of the lens has been achieved while maintaining similar performance."

Finally, he has worked on the design of nano-antennas. "This involves devices made up of groupings of metallic nanoparticles (normally gold or silver) that behave in a similar manner to the classic radiofrequency antennas, but at optical frequencies." For his PhD thesis he has designed and manufactured a nano-antenna the uniqueness of which is to provide a waveband greater than the rest of the nano-antennae but with a similar concentration of energy. "In this way, it can be used in multiple-frequency spectroscopy experiments. The same nano-antenna simultaneously gives relevant gain in three different spectroscopy experiments (fluorescence, Raman absorption and infra-red); i.e. instead of using three different nano-antennae for each of the experiments, with this kind design just one nano-antenna is sufficient."