Researchers have revealed the first recording of optically encoded audio onto a non-magnetic plasmonic nanostructure, opening the door to multiple uses.
The photographic film property exhibited by an array of novel gold, pillar-supported bowtie nanoantennas (pBNAs) was exploited to store sound and audio files. Compared with the conventional magnetic film for analog data storage, the storage capacity of pBNAs is around 5,600 times larger, indicating a vast array of potential storage uses.
To demonstrate its abilities to store sound and audio files, the researchers led by Kimani Toussaint, an associate professor of mechanical science and engineering, created a musical keyboard or "nano piano," using the available notes to play the short song, "Twinkle, Twinkle, Little Star."
The researchers showed that the pBNAs could be used to store sound information either as a temporally varying intensity waveform or a frequency varying intensity waveform. Eight basic musical notes, including middle C, D, and E, were stored on a pBNA chip and then retrieved and played back in a desired order to make a tune.
"A characteristic property of plasmonics is the spectrum," said Hao Chen, a former postdoctoral researcher in Toussaint's PROBE laboratory and the first author of the paper, "Plasmon-Assisted Audio Recording".
Originating from a plasmon-induced thermal effect, well-controlled nanoscale morphological changes allow as much as a 100-nm spectral shift from the nanoantennas. By employing this spectral degree-of-freedom as an amplitude coordinate, the storage capacity can be improved.
"Moreover, although our audio recording focused on analog data storage, in principle it is still possible to transform to digital data storage by having each bowtie serve as a unit bit 1 or 0," said Chen. "By modifying the size of the bowtie, it's feasible to further improve the storage capacity."
The team previously showed that pBNAs experience reduced thermal conduction in comparison to standard bowtie nano-antennas and easily get hot when irradiated by low-powered laser light. Each bowtie antenna is about 250 nm across, each supported on 500-nm tall silicon dioxide posts. As a consequence, optical illumination results in subtle melting of the gold, and thus a change in the overall optical response. This shows up as a difference in contrast under white-light illumination.
The teams approach, as stated in its paper, was analogous to the method of 'optical sound,' developed circa 1920s: "Although there were variations of this process, they all shared the same basic principle. An audio pickup, e.g., a microphone, electrically modulates a lamp source. Variations in the intensity of the light source is encoded on semi-transparent photographic film as the film is spatially translated. Decoding is achieved by illuminating the film with the same light source and picking up the changes in the light transmission on an optical detector, which in turn may be connected to speakers." In their work, the pBNAs serve the role of the photographic film which can be encoded with audio information via direct laser writing in an optical microscope.
In their approach, the researchers record audio signals by using a microscope to scan a sound-modulated laser beam directly on their nanostructures. Retrieval and subsequent playback is achieved by using the same microscope to image the recorded waveform onto a digital camera, whereby simple signal processing can be performed.
In addition to Toussaint and Chen, co-authors on the PROBE team include Abdul Bhuiya and Qing Ding, graduate students in electrical and computer engineering.