Towards do-it-yourself planar optical components using plasmon-assisted etching.

In recent years, the push to foster increased technological innovation and basic scientific and engineering interest from the broadest sectors of society has helped to accelerate the development of do-it-yourself (DIY) components, particularly those related to low-cost microcontroller boards. The attraction with DIY kits is the simplification of the intervening steps going from basic design to fabrication, albeit typically at the expense of quality. We present herein plasmon-assisted etching as an approach to extend the DIY theme to optics, specifically the table-top fabrication of planar optical components. By operating in the design space between metasurfaces and traditional flat optical components, we employ arrays of Au pillar-supported bowtie nanoantennas as a template structure. To demonstrate, we fabricate a Fresnel zone plate, diffraction grating and holographic mode converter--all using the same template. Applications to nanotweezers and fabricating heterogeneous nanoantennas are also shown.

Plasmon-assisted audio recording.

We present the first demonstration of the recording of optically encoded audio onto a plasmonic nanostructure. Analogous to the "optical sound" approach used in the early twentieth century to store sound on photographic film, we show that arrays of gold, pillar-supported bowtie nanoantennas could be used in a similar fashion to store sound information that is transferred via an amplitude modulated optical signal to the near field of an optical microscope. Retrieval of the audio information is achieved using standard imaging optics. We demonstrate that the sound information can be stored either as time-varying waveforms or in the frequency domain as the corresponding amplitude and phase spectra. A "plasmonic musical keyboard" comprising of 8 basic musical notes is constructed and used to play a short song. For comparison, we employ the correlation coefficient, which reveals that original and retrieved sound files are similar with maximum and minimum values of 0.995 and 0.342, respectively. We also show that the pBNAs could be used for basic signal processing by ablating unwanted frequency components on the nanostructure thereby enabling physical notch filtering of these components. Our work introduces a new application domain for plasmonic nanoantennas and experimentally verifies their potential for information processing.

Reconfigurable nanoantennas using electron-beam manipulation.

Plasmonic nanoantennas have been of increasing interest due to their ability to confine and enhance electric fields in deep sub-wavelength volumes, leading to large near-field optical forces and high refractive index sensitivity. Recently, to enhance the response for sensor applications, metal nanoantennas have been fabricated on pillars. An overlooked consequence of this elevated geometry is the introduction of the mechanical properties, for example, stiffness, as a tunable degree of freedom. Here we demonstrate pillar-bowtie nanoantenna arrays, fabricated on optically transparent SiO2, as a candidate system that couples intrinsic mechanical and electromagnetic degrees of freedom via gradient forces. We show that using a standard scanning electron microscope, individual nanoantenna gap sizes can be controllably tuned down to 5 nm, a factor of ~4 × smaller than what is currently achievable using conventional electron-beam lithography. This approach opens new avenues for fabricating reconfigurable nanoantennas that can inform exciting photonic applications.

Multifunctional plasmonic film for recording near-field optical intensity.

We demonstrate the plasmonic equivalent of photographic film for recording optical intensity in the near field. The plasmonic structure is based on gold bowtie nanoantenna arrays fabricated on SiO2 pillars. We show that it can be employed for direct laser writing of image data or recording the polarization structure of optical vector beams. Scanning electron micrographs reveal a careful sculpting of the radius of curvature and height of the triangles composing the illuminated nanoantennas, as a result of plasmonic heating, that permits spatial tunability of the resonance response of the nanoantennas without sacrificing their geometric integrity. In contrast to other memory-dedicated approaches using Au nanorods embedded in a matrix medium, plasmonic film can be used in multiple application domains. To demonstrate this functionality, we utilize the structures as plasmonic optical tweezers and show sequestering of SiO2 microparticles into optically written channels formed between exposed sections of the film. The plasmonic film offers interesting possibilities for photonic applications including optofluidic channels "without walls," in situ tailorable biochemical sensing assays, and near-field particle manipulation and sorting.

Understanding and controlling plasmon-induced convection.

The heat generation and fluid convection induced by plasmonic nanostructures is attractive for optofluidic applications. However, previously published theoretical studies predict only nanometre per second fluid velocities that are inadequate for microscale mass transport. Here we show both theoretically and experimentally that an array of plasmonic nanoantennas coupled to an optically absorptive indium-tin-oxide (ITO) substrate can generate >micrometre per second fluid convection. Crucially, the ITO distributes thermal energy created by the nanoantennas generating an order of magnitude increase in convection velocities compared with nanoantennas on a SiO2 base layer. In addition, the plasmonic array alters absorption in the ITO, causing a deviation from Beer-Lambert absorption that results in an optimum ITO thickness for a given system. This work elucidates the role of convection in plasmonic optical trapping and particle assembly, and opens up new avenues for controlling fluid and mass transport on the micro- and nanoscale.