‘Nanoantennas’ could bring sensitive detectors and optical circuits

Researchers have shown how tiny wires and metallic spheres might be arranged to form ‘nanoantennas’, a development that could lead to a new class of ultrasensitive sensors.

Engineers from Purdue University have demonstrated through mathematical simulations that nanometer-scale antennas with certain geometric shapes should be able to make possible new sensors capable of detecting single molecules in a chemical or biological agent.

Such an innovation could result in detectors that are, in some cases, millions of times more sensitive than current technology.

The nanoantennas in the simulations are made of metal wires and spheres only about 10 nanometers thick. They are an example of ‘left-handed’ materials, meaning they are able to reverse the normal behaviour of visible light and other forms of electromagnetic radiation. This property means that such materials might be used to create a so-called ‘super lens’ that drastically improves the quality of medical diagnostic images.

‘All of the work in this area so far has been done in the microwave spectral range,’ said Vladimir Shalaev, a professor in Purdue’s School of Electrical and Computer Engineering. ‘We believe that this is the first project for how these types of materials can be used in the visible range of the electromagnetic spectrum.’

The researchers showed how the nanoantennas could be created by arranging pairs of tiny wires parallel to each other. That arrangement, in theory, enables the nanoantennas to achieve a ‘negative index of refraction,’ said Shalaev.

Light and other forms of radiation bend as they pass through a material. Physicists measure this bending of radiation by its ‘index of refraction.’ The larger a material’s index, the slower light travels through it, and the more it bends, or changes direction when going from one material to a different one. Because left-handed materials bend light in precisely the opposite direction as right-handed materials, they are said to have a ‘negative index of refraction.’

‘With these new types of materials, it may be possible to accomplish better performance than all existing materials, in terms of making images and manipulating light,’ Shalaev said.

The nanoantennas work by using clouds of electrons (plasmons), all moving in unison as if they were a single object instead of millions of individual electrons.

Researchers hope to use nanoantennas to create more compact, faster circuits and computers that use photons, instead of electrons for carrying signals. Photons travel much faster than electrons, but, unlike electrons, they do not possess an electrical charge. This lack of an electrical charge makes it far more difficult to manipulate photons.

‘Because electrons are negatively charged particles, it’s easy to manipulate them,’ Shalaev said. ‘You just apply a field and they start moving.’

‘It turns out that, by employing these plasmonic nanomaterials, you should be able to manipulate light,’ continued Shalaev. ‘You can guide light. You can basically simulate all the basic fundamental properties of electronic circuits, but in this case photons start to work.’

Such photonic circuits could usher in a new class of ultrasensitive sensors that detect tiny traces of chemicals and biological materials, making them useful for a host of applications.

‘This could be a way to dramatically enhance sensitivity in detecting molecules,’ Shalaev said. ‘That’s a great goal. These plasmonic nanomaterials accumulate electromagnetic energy in extremely small areas, nanoscale areas. It’s like focusing light in areas much smaller than the wavelengths of light.

‘Conventional lenses cannot focus light in an area smaller than the wavelength of the light. When you use these plasmonic nanomaterials, which act like nanoantennas, you do focus light in areas much smaller than the wavelengths. This means that metallic nanostructures might be able to detect even a single molecule of a substance, which is not possible with conventional optics.’

Purdue researchers plan to take the work a step further by creating the nanoantennas and conducting experiments to support the theoretical calculations, Shalaev said.