Optical Antennas
Sidestepping the diffraction limit

An optical antenna is a device that efficiently couples the energy of free-space radiation to a confined region of subwavelength size. While antennas are widespread in the radiowave and microwave regimes they are basically unexplored at optical frequencies. Because nanoscale devices need to interface with optical radiation it is likely that optical antennas will have a broad impact on future technology.

Why Optical Antennas?

The concept of antennas is not new, by any means. They are the enabling technology in cellular phones, satellite communication, and many other devices which use electromagnetic radiation. However, their optical counterpart is basically non-existent in today's technology. Instead, optical radiation is manipulated by redirecting the wavefronts with lenses and mirrors. Consequently, because of diffraction, it appears that optical fields cannot be localized to dimensions much smaller than the optical wavelength. Optical antennas are a solution to the mismatch between the small dimensions of nanoscale devices and the length scale associated with optical wavelengths. It can be expected that optical antennas will be used for artificially enhancing the absorption cross-section or quantum yield of optoelectronic devices (e.g. solar cells), for efficiently releasing energy from nanoscale devices (e.g. LED lighting), and for boosting the efficiency of biochemical detectors relying on a distinct spectroscopic response (Raman scattering, fluorescence, etc. ).

Our Projects

We study optical antennas using both top-down (FIB, e-beam) and bottom-up (colloidal synthesis) approaches. We are interested in understanding fundamental properties and to develop quantitative design strategies for efficient antenna structures.
An optical antenna localizes radiation to subwavelength dimensions but it also interacts with the system under investigation. For example, the excitation rate of a single molecule close to an optical antenna can be strongly enhanced due to the local field enhancement, but nonradiative energy transfer to the metal imposes a loss channel and quenches the fluorescence of the molecule. We measure the fluorescence rate and excited-state lifetime of a single molecule as a function of its distance to the antenna and as a function of its dipole orientation.

In order to assess quantitatively the magnitude of field enhancement near optical antenna structures we are measuring the gradient force acting on a polarizable particle acting as a local sensor. We do this by measuring the deflection of a cantilever with an attached particle. This approach allows us to measure the field enhancement factor and to optimize the properties of optical antennas in an iterative procedure.
With the use of novel top-down nanofabrication (e.g. focused-ion beam milling, electron-beam lithography) and bottom-up self-assembly techniques, fabrication of optical antennas is becoming increasingly feasible. In the future, optical antenna arrays are likely to be used for increasing the efficiency of optoelectronic devices and biochemical sensors. We also see optical antennas as a way to advance the frontiers of metrology and control light-matter interactions on the nanometric scale.

 

This webpage is maintained by nanogroup@optics.rochester.edu