ANFFL logo - go to ANFFL Home Australian National Fabrication Facility

ACT Node and WA Node

Latest News

Nanoantennas: Manipulating light at the nanoscale - ACT Node

March 2013

Antennas are all around in our modern wireless society: they are the front-ends in satellites, cell-phones, laptops and other devices establishing communication by sending and receiving electromagnetic waves. While all these devices typically operate at frequencies from 300 GHz to as low as
3 kHz, according to Maxwell's equations the same principles of directing and receiving electromagnetic waves should work at various scales independently of the wavelength.

Thus, one may naturally ask "Can an antenna send a beam of light?" And the answer is "Yes, a nanoantenna can!".

However, nanoantennas have even more to offer than this: They can concentrate light in ultra-small nanoscopic volumes, thereby strongly enhancing interaction with nanoscale matter. Plus, they can efficiently link these spatially localised near fields with propagating optical fields and, by reciprocity, vice versa. Based on these principles nanoantennas are expected to play an important role in key applications like efficient quantum-light sources, photovoltaics, nonlinear optics, single-molecule detection, and as transmitting and receiving devices for on-chip optical networks. Yet, given the small dimensions of nanoantennas, their precision fabrication still remains a challenge and relies on state-of-the-art nanotechnology.

Figure 1
Figure. 1: (a) Design sketch and (b) scanning electron micrograph of a tapered Yagi-Uda nanoantenna for broadband unidirectional emission enhancement [1].

Nonlinear Physics Centre (NLPC) researchers at ANU, using ANFF ACT Node facilities, are developing next-generation optical nanoantennas for the unprecedented control of light at the nanoscale, including control of the bandwidth, directionality, and complex polarization state of the light emitted or absorbed by the nanoantennas. For example, Figure 1 (left) shows a tapered Yagi-Uda nanoantenna fabricated at the ANFF ACT facilities using electron-beam lithography, gold evaporation and a lift-off procedure [1]. This nanoantenna has been predicted to provide broadband unidirectional emission enhancement from nanoemitters like quantum dots placed in direct vicinity of the gold nanorod ends. In addition, this type of nanoantenna is expected to offer a number of intriguing new functionalities like multichannel sensing and cascaded four-wave mixing. Crucially for the performance of this design, the centre-to-centre distance between neighbouring antenna elements is only 80 nm. NLPC researchers were able to reproducibly obtain such small distances while preserving a high structure quality by using a cold development procedure of the electron-beam resist at -10°C. Also, the realised taper angle α of the fabricated nanoantenna is 6.6 degrees, which has been found to optimise the nanoantenna's directivity in numerical simulations. Using transmittance spectroscopy, and comparing the collected spectra to those of untapered reference structures, NLPC researchers were able to confirm the broad spectral bandwidth of this arrayed nanoantenna, laying the foundation for directional broadband quantum light sources and the nanontenna's other unique applications.

Despite their ability to provide many useful properties, planar nanoantennas like the tapered design shown in Figure 1 have serious limitations, e.g. when it comes to out-of-plane directionality or for interacting with circularly polarized light. Circularly polarized light, in particular, is inherently connected to chiral structures, which are, by definition, three-dimensional (3D). However, it is technologically important as it has favourable properties for propagation in rough environments and is, for example, successfully used in current technology for watching 3D films in theatres (the light seen by the two eyes differs in its 'handedness', a property which, unlike linear polarization, is not destroyed when the viewer tilts their head). In fact, while nanoantenna designers have drawn lots of inspiration from RF antenna geometries in the past, some of the most successful macroscopic antennas, like highly directional dish antennas or (chiral) helical antennas, are actually 3D and cannot be transferred to the nanoscale by standard planar fabrication schemes. This lack of a flexible and robust 3D metal nanofabrication scheme also affects research into a variety of other dimensionality effects, like 3D tapers for light, magnetic coupling between nanoantenna elements, or dimensionality effects in fractal structures, which are currently almost completely unexplored for nanoantennas.

Figure 2 Figure 3
Figure 2: Broadband nanoantennas have recently received a lot of attention from the media because of their unique application opportunities and interesting physical behavior. NLPC results were highlighted on the front cover of the December 2012 issue of Physica Status Solidi Rapid Research Letters [1]. Figure 3: (a) Schematic of the process steps of the hybrid three-dimensional nanofabrication approach:
A glass substrate is pre-patterned using two-photon direct laser writing. Next, the photoresist structure is
sputter-coated with Indium-Tin-Oxide (ITO) to make it conductive for electron-beam lithography and it is
spin-coated with the electron-beam resist PMMA. The PMMA is then exposed by electron-beam lithography.
A precision alignment procedure can be performed before electron-beam lithography in order to precisely place
the written pattern on top of the photoresist template. After PMMA development, a gold evaporation step
and a lift-off procedure are performed, altogether resulting in out-of plane gold structures
with truly nanoscopic feature sizes [2].

In order to overcome these limitations NLPC researchers have developed a novel hybrid fabrication approach combining the three-dimensionality from two-photon direct-laser writing with the small feature sizes, excellent metal quality, and capability of selective metallisation from electron-beam lithography based fabrication schemes [2]. A sketch illustrating the individual steps of this approach is shown in Figure 3 (a) above. Figure 3 (b-e) displays a variety of out-of-plane plasmonic nanostructures the NLPC have realised using this approach. In contrast to planar gold nanorods, the nanorods they have fabricated are curved out into the third dimension, supporting not only an electric dipole mode, but also a ring current which gives rise to a magnetic optical response. NLPC researchers studied the optical properties of their fabricated structures using trans-mittance spectroscopy and found a very good agreement with theoretical predictions for this magnetic mode. Their results pave the way towards experimental realisation of a range of other previously out of reach 3D nanoantennas with unique functionalities that rely on entering the third dimension.

Article supplied and written by Dr Isabelle Staude, Research Fellow, Nonlinear Physics Centre, Research School of Physics & Engineering, ANU.

References:
[1] I. Staude, I. S. Maksymov, M. Decker, A. E. Miroshnichenko, D. N. Neshev, C. Jagadish, and Yu. S. Kivshar, "Broadband scattering by tapered
nanoantennas", Phys. Status Solidi RRL 6, 466-468 (2012).
[2] I. Staude, M. Decker, M. J. Ventura, C. Jagadish, D. N. Neshev, M. Gu, and Yu. Kivshar, "Hybrid High-Resolution Three-Dimensional Nanofabrication for Metamaterials and Nanoplasmonics", Adv. Mater. Early View DOI: 10.1002/adma.201203564 (2012).