By Els Parton and Niels Verellen, imec, Belgium
You could compare it to a radio antenna, except for the fact that an optical antenna works with light instead of radio waves. Researchers at imec and KU Leuven made an optical antenna that is shaped like a boomerang and made out of silicon. The antenna scatters different colors in different directions. This unique effect can come in handy for all kinds of applications that use light waves on a chip, from sensors to optical communication, solar cells, holographic lenses, and even quantum computers.
Red To The Left, Green To The Right
The new optical antenna scatters light in a direction that depends on the wavelength of the light. For example, when you illuminate the antenna with red light, it bends to the left, while green light bends to the right. This color-specific scattering of light is unique in the photonics world.
The optical antenna redirects incident light in different ways for different colors. By adjusting the shape and size of the antenna, it can be tuned to specific wavelengths.
The fabrication of optical antennas is not straightforward, since it concerns nanoscale dimensions. The typical size measures a few hundred nanometers. Optical antennas usually are made of noble metals, such as gold or silver. But recently, silicon has entered the spotlight as an interesting material for low-loss antennas.
In 2013, imec’s biophotonics research group made the smallest golden nanoantenna that scatters light into one direction, a so-called directional antenna. This one also had the shape of a boomerang. Other research groups focus on symmetrical shapes, such as spheres or cylinders. However, asymmetrical-shaped optical antennas show unique properties. This is why a silicon antenna with an asymmetrical boomerang shape was made.
While the metallic antenna can only scatter one color in a specific direction, its silicon counterpart can scatter different colors in different directions. You could compare it to a very small prism, though there is an important difference between antenna and prism scattering: antenna scattering is not based on the conventional refraction of light, but on the interference between resonance modes in the antenna. Other advantages of the silicon boomerang antenna (as compared to the metallic one) are its more efficient light scattering and lower absorption losses.
The color-dependent scattering of light is a unique property that is shows promise for several applications. Nowadays, color prisms or diffraction gratings often are used in optical instruments. These instruments are made up of many separate optical components, each of which has to be aligned, making the instruments relatively large. Imagine if you could integrate all these functionalities on a chip. You would end up with an extremely compact optical system that is easy to use.
An important advantage of the silicon antenna is that it works for visible and near-infrared light. These wavelengths are commonly used in life science applications for the detection of molecules — just think of fluorescent labels, Raman spectroscopy, etc. The antenna could be used in a multiplexing experiment in which two fluorescent labels need to be detected (e.g., a green and red label). By placing two photodetectors underneath the antenna, one could easily read out the signal strength of both the red and the green light, separately.
The optical antenna is made of silicon, the same material as electronic chips. Therefore, it is relatively easy and cheap to integrate this building block on a chip (e.g., as part of an on-chip photonic circuit). These kinds of circuits are used for on-chip communication or the detection of biomolecules in optical biosensors. Think of them as extremely high-speed highway networks, where information is carried by light waves that move through waveguides. To control the traffic, you need some kind of traffic light. The newly developed antenna could do that job by directing red light to the left, and green light to the right. Current photonic components performing similar tasks have footprints that are easily ten times the size of the boomerang antenna.
Additionally, there will be more applications for the optical antenna — all applications, in fact, in which light is used and the focus is on a compact, silicon-based system. Examples include 2D lenses, holograms, solar cells, etc.
What Is This Antenna’s Future?
This new optical antenna concept will be further developed for specific applications. It will be fabricated using other materials to enable it to address other wavelength ranges. Also, the antenna shape can be optimized for specific applications. Whereas the current antenna is a passive structure, it will be further developed into an ‘active’ component that changes its properties according to an external pulse. For example, by applying a voltage to an active antenna, one could make red light go to the right, instead of to the left. One thing is for sure: there are a lot of opportunities ahead with this new antenna concept.
Three of the imec and KU Leuven researchers that developed the new antenna concept. From left to right: Niels Verellen, Dries Vercruysse, and Jiaqi Li.
The Big Picture: Flat Optics For Life Science Applications
The optical antenna is a perfect example of a new trend in the photonics domain: flat optics. Flat optics for visible light are especially important for biophotonics, where photonic components are used to make integrated life science systems. Nanostructures, with a certain shape and material placed in a specific configuration, can mimic the functionality of lenses, color splitters, etc. However, nanostructures offer the advantage that the resulting structure is much more compact, cost-effective, and often more efficient.
With flat optical components, one can make a stack of functionalities: a lens layer, on top of a color splitter layer (i.e., the optical antenna), on top of a detector layer. Combine this stack on a silicon chip, and the result is a complete detection platform for visible light. Moreover, this detection system is easy to integrate with other electronic chip functionalities, and can be mass fabricated in existing fabs.
A good example of such an integrated system is the cell sorter chip that imec’s Liesbet Lagae and her team are developing in the framework of her ERC grant (Liesbet Lagae is program director Life Science Technologies). It’s a compact and easy-to-use system that can sort and analyze individual cells (e.g., cancer cells) in blood. The system consists of several silicon-based building blocks: microfluidic channels, heating elements, mixers, bubble generators, etc. Flat optics also are an essential part of this compact system. For example, a stack of lens, color splitter, and detector layer could be used to detect cells with different fluorescent labels in a fast and compact way.
Flat optics are a relatively new domain in biophotonics. Another better-known domain is the one of photonic circuits (PICs). Here, very compact photonic components are connected one after another to guide light through the circuit; the technology is used for applications like detecting biological molecules. For PICs, imec has set up a SiN photonics platform for visible light applications, which companies can use and tailor to their specific needs. Concerning flat optics, the research centre develops silicon-based flat lenses — for use in DNA sequencers and cytometry, for example — and optical antennas / color splitters. The latter is a unique building block, not yet widely available, nor well-described in literature.
The paper “All-dielectric antenna wavelength router with bidirectional scattering of visible light” was published in Nano Letters. See http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.6b01519
Els Parton, PhD, is a science editor at imec, where she is responsible for authoring and editing the research organization's numerous technical documents and publications. She received a PhD in Bioscience Engineering from the University of Leuven. She can be reached at +32 16 281467 or email@example.com.
Niels Verellen, PhD, is postdoctoral research fellow for the FWO Flanders affiliated to imec and KU Leuven. His research is focused on nanoscale photonics and optical quantum sensing. He received a PhD in physics from the KU Leuven. He can be reached at +32 16 281010 or firstname.lastname@example.org.