Title: Artificial Atoms and Antennas
Hans-J. Eisler
Light Technology Institute, University of Karlsruhe (TH)
Kaiserstr. 12, D-76131 Karlsruhe, Germany
hans.eisler@lti.uni-karlsruhe.de
Tailoring the light-matter interaction at the nano-scale is one of the most
fascinating and active research fields of current solid state physics, chemistry and materials science. This activity is driven to a large extent by the
vision on generating, manipulating and directing the flow of photons. Using the fundamental nature of photons as the carrier of information is at
the heart of this research field, much like the early studies on learning how
to move charge carriers in semiconductor materials led to the now existing
information technology. Before we can explore a so-called nano-photonics
based technology as a potentially new and glory area in mankind, we have
to face the challenge of learning how to make the most efficient and most
controllable optical light sources.
Therefore, designing nano-scale materials with size tunable properties yields
quantum-confined semiconductor materials, also referred to as artificial atoms
or quantum boxes.[1] In this sense, these quantum dots may be seen as electronically well-defined light receivers and emitters. Seen from an antenna
engineering point-of-view on the other hand, these quantum nano-structures
are completely mismatched absorbers as well as radiators.
Optimizing the interconversion of free propagating optical waves and highly
localized, enhanced electromagnetic fields is instrumental for pushing the
limit in optical characterization, manipulation, and (quantum) optical information processing. These requirements recently triggered a search for
favorable structures, materials and designs that would be able to fulfill this
task. The recently introduced resonant optical antennas [2] are structures
that exhibit both, dimensions comparable to and much smaller than the
wavelength of light. They excel among other structures by synergistically
combining
i) field-line concentration at a local shape singularity,
ii) optimum impedance matching to freely propagating optical waves
and
iii) resonant collective oscillations of the free electron gas in the antenna
arms.
In this presentation, I will explore the fundamental half-wave resonance of
nanometer-scale gold dipole antennas at optical frequencies. On resonance,
strong field enhancement in the antenna feed gap leads to white-light super continuum generation. The resonance is shifted significantly towards
antenna lengths shorter than one half of the wavelength. This is in contradiction to classical antenna theory, but in accordance with computer simulations that take into account the finite metallic conductivity at optical
frequencies. Moreover, we probe the interaction of a single semiconductor
nanocrystal quantum dot with a bowtie antenna for visible light.[3] The antenna is generated at the apex of an atomic force microscopy tip by focused
ion beam milling. When scanned over a single quantum dot, the nanocrystal
photoluminescence count rate is enhanced while its excited-state lifetime is
decreased. The observations demonstrate that the relaxation channels of a
single quantum emitter can be controlled by coupling to an efficiently radiating metallic nano-antenna.
1. S. Kim, B. R. Fisher, H.-J. Eisler, M.G. Bawendi, Type-II Quantum Dots:
CdTe/CdSe (core/shell) and CdSe/ZnTe (core/shell) Heterostructures,
J. Am. Chem. Soc. 123, 11466 (2003)
2. P. M¨uhlschlegel, H.-J. Eisler , O.J.F. Martin, D.W. Pohl, B. Hecht, Resonant
Optical Antennas, Science 308, 1607 (2005)
3. J. Farahani, H.-J. Eisler, D.W. Pohl, B. Hecht, Single emitter coupled to a
scanning optical antenna: A tunable super-emitter ,
Phys. Rev. Lett., 95, 017402 (2005)