PerspectiveApplied Physics

Plasmonics—turning loss into gain

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Science  22 Jan 2016:
Vol. 351, Issue 6271, pp. 334-335
DOI: 10.1126/science.aad9864
Losses find applications.

Hybrid electrothermoplasmonic nanotweezer for rapid long-range capture and positioning of nanoscale objects on plasmonic hotspots.

ILLUSTRATION: V. ALTOUNIAN/SCIENCE

The light-induced electronic excitations that occur at the surface of metals—plasmons—provide the extraordinary ability to confine electromagnetic energy to the subwavelength scale. Such extreme optical confinement can enhance the light-matter interaction and enable miniaturized optical and optoelectronic devices. However, this confinement requires that plasmonic materials possess free carriers, which unavoidably results in light being lost or absorbed in the system (1). This optical loss has hampered the realization of device designs with ultracompact, on-chip optical components and nanometer-scale resolution imaging. Because of the detrimental effects of plasmonic losses, several avenues are being explored to mitigate the high absorption, such as using gain to compensate for the losses, and synthesizing alternative low-loss plasmonic materials (2). Rather than continuing to pursue low-loss plasmonics approaches, we draw attention to the benefit of losses by high-lighting recent groundbreaking discoveries that were enabled by intrinsic losses in plasmonic systems.

A key consequence of losses in plasmonics is resonant absorption of incident photons to produce local heating of the plasmonic structure. Recently, loss-induced heating was used to address a long-standing challenge in the field of plasmon-enhanced optical tweezing—to dynamically and rapidly load the plasmonic trap on demand (3). The hybrid electrothermoplasmonic nanotweezer (3) combines plasmonic heating and ac electric fields for fast and precise delivery of nanometersized objects to plasmonic hotspots, where they are confined within a few seconds (see the first figure). This device could be used for trapping and analysis of virus and protein samples to improve the sensitivity of nanoscale sensors, as well as trapping and positioning quantum emitters such as quantum dots and nanodiamonds.

Loss-induced plasmonic heating could also play a role in optical data storage and encryption. Prior work has successfully harnessed polarization-dependent local heating and the reshaping of gold nanorods embedded in polyvinyl alcohol for five-dimensional optical recording of images (4). Optical data storage that is amenable to integration in optoelectronic devices was proposed (5) where heat-induced re-shaping of pillar bowtie nanoantennas was used to record images. Heat-assisted magnetic recording (HAMR) is a promising approach for increasing magnetic data storage that could push the limit of data storage beyond 1 Tb/in2 (see the figure, panel B). The conventional design paradigm is to use plasmonic nanoantennas or near-field transducers (NFTs) to focus light onto a sub-50-nm spot that would be subsequently absorbed by the magnetic medium to temporarily heat it and reduce the coercivity in order to write the bits (6, 7). In HAMR, the separation between the NFT and magnetic layer is a few nanometers, and because near-field radiative transfer can be much higher than predicted by the Stefan-Boltzmann law (8), it might be possible to harness near-field heat transfer from the hot NFT to the magnetic film for enhanced magnetic recording.

Plasmonic photothermal therapy represents another practical application of plasmonics that harnesses loss-induced heating of plasmonic nanoparticles to locally heat and destroy cancer cells (9) (see the figure, panel A). Quadrapeutics (10) combines encapsulated drugs, gold nanoparticles, near-infrared short pulses, and x-rays for cancer cell destruction. Central to this emerging technology is the plasmonic nanobubble, which is generated from resonant collective heating of embedded gold nanoparticles that have preferentially accumulated in the cancerous cells. Explosion of the bubble exerts mechanical pressure on cell walls to not only eject the drug payload into the cell cytoplasm but also to destroy the host cancer cells.

Plasmonic heating could also become a crucial component in the renewable energy concept based on the conversion of solar energy to electricity. A key issue limiting the efficiency of solar cells is the loss of sub-bandgap photons, which are not absorbed by the solar cell material, and hence do not contribute to generation of the photocurrent in the cell. Broadband solar absorbers and emitters (7, 11) made of plasmonic resonators can be used to absorb all energy within the solar spectrum and selectively emit to a narrow spectral window within the bandgap of the solar cell. Furthermore, the plasmonic absorbers and emitters could also be used to harvest waste heat. The hot electrons generated following the absorption of the incident photons in plasmonic nanoparticles could be used to enhance chemical reactions such as water splitting, and conversion of solar energy to chemical fuels, thus representing an emerging and actively investigated field with both fundamental and technological relevance (12).

Losses find applications.

(A) Application of local heating of plasmonic nanostructures for photothermal therapy imaging and drug release. (B) Heat-assisted magnetic recording scheme for data storage.

ILLUSTRATION: V. ALTOUNIAN/SCIENCE

Harnessing intrinsic loss in plasmonics could usher transformative technological innovations that would affect several fields, including information technology, life sciences, and clean energy. It is time for the plasmonic community to turn loss into gain.

References and Notes

  1. ACKNOWLEDGMENTS: We acknowledge financial support from NSF Materials Research Science and Engineering Centers grant DMR- 1120923. J. C. N acknowledges partial support from the Purdue Water Institute.
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