Single emitters coupled to bow-tie nano-antennas

Approaching a metallic tip to a single quantum emitter quenches the photolumi-
nescence by opening non-radiative decay channels for the excited-state. This is
one of the main problems in nano-optics research which prohibits optical studies
on single emitters in contact with the tip with high resolution and high sensitivity.
In this thesis I have shown that a bowtie nanoantenna can be used to overcome
the quenching problem at the single chromophore level. Bowtie antenna tip in-
teracts with the dipole of the single emitter. This is most probably the ¯rst study
on this type of measurements which opens new pathways for many disciplines.
Semiconductor nanocrystals were selected as a single emitter system due to their
relatively high photo-stability. Based on °uorescence confocal studies, satura-
tion behavior of single CdSefZnSg nanocrystal (NC) is studied under one- and
two-photon excitation. In one-photon excitation (1PE) laser wavelength of 532
nm and in two-photon excitation (2PE) laser wavelength of 830 nm were used
to excite the °uorophore. Due to the broad distribution in photoluminescence
(PL) intensity of nanocrystals, power dependence studies were done based on
an average over » 90 nanocrystals. Using focused ion beam, bowtie antennas
are sculptured at the apices of silicon nitride AFM tips, which were fully-coated
with a homogeneous layer of 40 nm of aluminum ¯lm. Details of structuring
procedures used to fabricate well-de¯ned bowtie antennas with smallest possible
feedgaps are described. Interaction of bowtie nanoantennas with single semicon-
ductor nanocrystals, is investigated using PL intensity and excited-state lifetime
of the nanocrystal as two intrinsic signatures of the single emitter. Proximity of
the feedgap of a bowtie nanoantenna to a single nanocrystal under one-photon
excitation leads to enhanced emission in addition to enhanced excitation. This
e®ect is shown, by increasing the PL intensity of the nanocrystal and shortened
lifetime, in contact with the bowtie antenna feedgap. These results were com-
pared with a fully-coated tip which lead to complete quenching of the nanocrystal
PL. Thus, the observed e®ects in PL intensity and lifetime of the nanocrystal in
contact with the antenna are originated from the metallic nanostructure. Under
two-photon excitation, PL intensity is enhanced but there is no change in the
lifetime of the nanocrystal in contact with the bowtie antenna. This is caused by
enhanced excitation through the antenna, induced by enlarging the absorption
cross section of the nanocrystal in contact with the antenna. Since, °uorescence
of single nanocrystal in contact with the bowtie antenna is "not quenched", more
detailed studies on their interaction were performed. Under two-photon excita-
tion absorption cross section of one nanocrystal was measured with and without
the presence of antenna. Free nanocrystal showed a two-photon absorption cross
section in the order of 6:3£10¡37cm4s which in contact with the bowtie antenna
increased to 20:2£10¡37cm4s. This proves that enhanced excitation observed in
2PE is caused by a larger absorption cross section of the system induced by the
antenna structure. Emission polarization of nanocrystals was studied under 1PE
using polarization microscopy. From these studies, in- and out-of-plane angles as
well as the absolute value of the projection of the transition dipole moment on
the sample plane were determined. Results showed in contact with the bowtie
antenna, the in-plane angle turns towards the orientation of the antenna. This
is induced by the strong dipole of the antenna in contact with the nanocrystal.
Moreover, modulation depth and the absolute value of the transition dipole were
increased dramatically in contact with the bowtie nanostructure. The results
show that antenna/NC system has a highly polarized emission, whose polar-
ization direction is determined by the antenna dipole. Photon antibunching of
nanocrystals under 1PE was done with and without the presence of the antenna
tip. Shorter lifetime of the excited-state in contact with the bowtie antenna
immediately appears in antibunching results. This shows that the "dead time"
for single photon generation, caused by excitation-recombination cycles, is much
shorter in contact with the antenna. Therefore, a nanocrystal in contact with
the bowtie antenna is a more e±cient single photon source. Moreover, taking
into account the emission polarization of the antenna/NC system, polarization of
single photons generated from the nanocrystal in contact with the antenna can
be tuned by antenna orientation. Thus, single photons provided by antenna/NC
system can have strong potentials in quantum cryptography. As a result, coupling
single quantum emitters (here nanocrystal) to bowtie nanoantennas will produce
a new type of emitter with widely adjustable photophysical properties, which can
be called a "tunable superemitter". Emission characteristics of the antenna/NC
system is highly determined by the coupling intra-superemitter.