One of the daunting problems in nanophotonics is how to efficiently transport photons from the macro-world to the nano-world and back. This difficulty is rooted in the well-known diffraction tendency of electromagnetic waves. To tackle this issue, various plasmonic antennas have drawn much attention for applications including surface-enhanced Raman scattering (SERS), photovoltaics, and nonlinear signal generation. However, such metallic antennas suffer from large metallic absorption and/or radiation losses and the resulting coupling efficiencies are rather low due to their poor directional emission.
Recently, we have proposed an optical antenna based on photonic-band-gap confinement (See Figure 1). Specifically, the photonic-crystal dipole mode was engineered to have broad spectral response (Q~70) and well-directed vertical-radiation by introducing a plane mirror below the cavity. A surprisingly large local electric-field intensity enhancement ~4,500 is expected from the proposed design for a normally incident planewave. Furthermore, we developed a very accurate analytic model based on coupled-mode theory, which predicts that the enhancement can easily be over 100,000 by employing a reasonably high-Q (~ 5,000) dipole mode cavity.
The proposed photonic-crystal optical antenna has several promising applications. As has been demonstrated by using various nanoplasmonic structures, the expected electric-field intensity enhancement over 100,000 means that it can be used to obtain strongly enhanced nonlinear signals in applications including SERS, the second- and third harmonic generation, and more importantly, high-harmonic generation at the extreme-UV regime for ultra-high resolution spectroscopy and nanolithography. A dense array of photonic-crystal antennas can be fabricated on the same chip, where their resonances can be tuned ‘lithographically’. A phased array antenna with highly-directional vertical beaming can be made by adopting the concept of a coupled-resonator optical waveguide (CROW). Furthermore, the proposed free-space resonant coupling scheme can be used for photovoltaic cell design – towards wireless-driving nanoelectric devices. The fact that the optical force exerted will be proportional to the total electromagnetic energy stored in a cavity implies that the photonic-crystal antenna can also be applied in optomechanics - optomechanical cooling and phonon lasing as well as all-optical manipulation of RF signals.
References
- Chang, H. -., Kim, S.-H., Lee, Y. -., Kartalov, E. & Scherer, A. (2010). A Photonic-Crystal Optical Antenna for Extremely Large Local-Field Enhancement. Optics Express, 18(23), 24163.