(3bs) Core|Shell Optical Nanoantenna

Au|SiO₂|Yb:Er:Y₂O₃core|shell
optical nanoantenna: experiment & simulation  

V. Jankovic, J. Chang

Department of Chemical Engineering, University of California, Los
Angeles, Engineering V-1011,

ABSTRACT

The conversion of electromagnetic (EM) energy from free
propagating radiation to localized energy and vice versa in the radio frequency
(RF) and microwave domains is accomplished with the use of antennas. Optical
antennas are analogous to their RF and microwave counterparts, but there are
crucial differences in their physical properties and scaling behavior because
metal is a highly dispersive material with finite conductivity at optical
frequencies.             Optical antennas
are not driven by galvanic transmission lines like RF antennas, instead, localized
oscillators such as atomic emitters are brought close to the feed point of the
antennas, and electronic oscillations are driven capacitatively.

 In this work, Au nanoparticles of different shapes (spheres, rods and stars)
were used as antenna elements, Er³⁺ ions in an Y₂O₃
host matrix were used as atomic emitter antenna driving elements while the capacitative gap between the antenna element and the atomic
emitter was controlled by deposition of an ultra-thin SiO₂ inner
shell between the Au nanoparticle and the Yb:Er:Y₂O₃ outer shell. Au nanospheres were grown by nucleating Au seeds using a
sodium borohydride in the presence cetyltrimethylammoniumbromide (CTAB) micelles and aging the
resulting Au seeds (<2nm) over a few days thus producing monodisperse
spheres of radius 10nm and an extinction peak around 520nm. Au nanorods were grown by adding a small amount of freshly
prepared seed solution to a growth solution of Au(I) ions, and CTAB where the
CTAB micelle preferentially adsorbs on the [100] Au face and directs the growth
of Au nanorods in the [111] direction. Au nanorod aspect ratios from 2 to 5 were achieved by varying
the concentration of the reducing agent, ascorbic acid, resulting in plasmon resonances that ranged from 650nm to 850nm, ideal
for coupling with rare earth ion based emitter materials. Au nanostars with extinction peaks ranging from 550nm to 650nm
were synthesized using the same protocol used for the nanorods
except that the order of the precursor addition was changed. A 4-5nm silica
spacer layer was deposited through a controlled TEOS hydrolyzation
reaction and was shown to be effective in preventing quenching yet enabling
energy coupling between the Au nanorod and the RE-ion
doped oxides. Spatially and compositionally controlled Yb:Er:Y₂O₃
outer shells were deposited using both wet chemistry methods and  radical enhanced atomic layer deposition
(RE-ALD).

Upconversion (UC) spectral, power
dependence and radiative lifetime measurements with 532nm,
750nm 980 nm and 1064nm laser excitation were used to assess the coupling of the
Au optical antenna to the emitter ions as a function of antenna shape, spacer
layer thickness and spectral and spatial mode overlap efficiency. Preliminary
optical characterization showed a 2X earlier onset of upconversion
with 980nm excitation for Yb:Er:Y₂O₃
coupled to an Au nanorod antenna compared to pure (uncoupled)
Yb:Er:Y₂O₃nanoparticles. Power
dependence measurements with 980nm excitation showed a >5 slope indicating a
multi-photon absorption induced luminescence process for the Au-coupled erbium
and a <2 slope for the uncoupled erbium, indicating a two photon absorption
(expected for erbium with 980nm excitation). These optical antenna core|shell particles have potential application in bio-imaging
and light trapping for solar and sensor applications.