Au|SiO2|Yb:Er:Y2O3 core|shell optical nanoantenna: experiment & simulation
V. Jankovic, J. Chang
Department of Chemical Engineering, University of California, Los Angeles, Engineering V-1011,
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  Au face and directs the growth of Au nanorods in the  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:Y2O3 nanoparticles. 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.