Optical nanocubes have utility in biosensors

Cubic-shaped optical nanoantennas developed by scientists at Monash University (Victoria, Australia) offer the potential to measure food safety, identify pollutants in the air, and quickly diagnose and treat cancer. The "nanocubes" direct an ultra-narrow beam of light where it is needed, with little or no loss due to heating and scattering.

Related: Laser-like optical amplifier portends external transmission of bio signals

The nanocubes, which are composed of insulating rather than conducting or semiconducting materials, are easier to fabricate as well as more effective than spherical versions, says Debabrata Sikdar, a postgraduate student at Monash University who led the work.

Sikdar's paper, published in the Journal of Applied Physics, presents analysis and simulation of 200 nm dielectric (nonconductive) nanocubes placed in the path of visible and near-infrared (NIR) light sources. The nanocubes are arranged in a chain and the space between them can be adjusted to fine-tune the light beam as needed for various applications. As the separation between cubes increases, the angular width of the beam narrows and directionality improves, the researchers say.

Schematic representation of unidirectional cubic nanoantennas inducing directionality to omnidirectional nanoemitters (light sources; e.g., spasers, quantum dots), to precisely focus light with adjustable beam width and intensity, which can be tuned by adjusting the length of nanocube chain or intercube spacing. These ultra-narrow directional beams can play multiple roles in lab-on-a-chip devices such as illumination sources in microfluidic analysis or minute deflection registers in nanocantilever based sensors. All these signals are further detected in the photodetectors and get processed by on-chip signal processing circuitry for bio-molecular identification. (Image courtesy of D. Sikdar and M. Premaratne/Monash University)

"Unidirectional nanoantennas induce directionality to any omnidirectional light emitters like microlasers, nanolasers, or spasers, and even quantum dots," Sikdar says. Spasers are similar to lasers, but employ minute oscillations of electrons rather than light. Quantum dots are tiny crystals that produce specific colors, based on their size, and are widely used in color televisions. "Analogous to nanoscale spotlights, the cubic antennas focus light with precise control over direction and beam width," he said.

The nanocubes have the potential to revolutionize nano-electromechanical systems (NEMS). "These unidirectional nanoantennas are most suitable for integrated optics-based biosensors to detect proteins, DNA, antibodies, enzymes, etc., in truly portable lab-on-a-chip platforms of the future," Sikdar says. "They can also potentially replace the lossy on-chip IC (integrated circuit) interconnects, via transmitting optical signals within and among ICs, to ensure ultrafast data processing while minimizing device heating," he adds.

Sikdar and his colleagues plan to begin constructing unidirectional cubic NEMS antennas in the near future at the Melbourne Center for Nanofabrication. "We would like to collaborate with other research groups across the world, making all these wonders possible," he said.

Full details of the work appear in the Journal of Applied Physics; for more information, please visit http://dx.doi.org/10.1063/1.4907536.

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