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
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. ).
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.