AVS2011 Session NS-MoA: Frontiers in Nanophotonics and Plasmonics

We fabricated arrays of Au nanoparticles (NPs), 180 nm in diameter and 20 nm high on indium-tin-oxide coated glass by electron-beam lithography. Subsequently, the nanoparticle arrays were coated with a 60 nm VO₂ film by pulsed laser ablation of vanadium metal targets in 10 mTorr oxygen (O₂) background gas, then annealed for 45 minutes at 450^oC in 250 mTorr of O₂. Using a Peltier heater and thermocouple mounted on a copper sample holder, temperature-dependent extinction of the array was measured using plain VO₂ film as a reference to determine the LSPR wavelength and linewidth during the SMT.

The LSPR wavelength of the NPs was 1000 nm in the semiconducting state and approximately 840 nm in the metallic state, thus overlapping the VO₂ electronic transitions from the occupied vanadium 3d_|| band to the empty 3d­_π band centered at approximately 885 nm. As the film undergoes the SMT, the split 3d­­_|| bands merge and, with the 3d_π band, form the metallic VO₂ conduction band. Since the Au NPs are sensitive to changes in both the real and imaginary parts of the VO₂ local dielectric function, they serve as a direct probe of the SMT. The results show a 30% decrease in plasmon dephasing time during the transition due to an increase in carrier-carrier scattering in the VO₂. Both Maxwell-Garnett and Bruggeman effective-medium theories predict the decrease in dephasing time during the SMT; however, a linear theory is a more accurate model for the hysteresis in the LSPR wavelength.

Core/shell and core/multi-shell nanoparticles with luminescent Gd₂O₃:Eu³⁺ were successfully synthesized by a high boiling-point alcohol (polyol) and solution precipitation methods, respectively. The hetero-structured nanoparticles with Eu doped Gd₂O₃ exhibited intense ⁵D₀-⁷F₂ photoluminescence (PL) from Eu³⁺ after calcination at 600 ˚C for 2h in air. Photoluminescence excitation (PLE) data showed that while a small fraction of the emission resulted from direct excitation of Eu³⁺, most of the excitation resulted from adsorption in the Oxygen to Europium charge-transfer band (CTB) between 225 and 275 nm. Gd₂O₃:Eu³⁺/Y₂O₃ core/shell nanoparticles exhibited PL intensities up to 40% larger than from bare Gd₂O₃:Eu³⁺ nanoparticles and SiO₂/Gd₂O₃:Eu³⁺/Y₂O₃ core/multi-shell samples showed quantum yield (QY) up to 72% larger than that of SiO₂/Gd₂O₃:Eu³⁺ core/single-shell nanoparticles. The increased PL and QY were attributed to reduced non-radiative recombination based on longer luminescence decay time. Potential applications of the nanoparticles as scintillation radiation detectors will be discussed.

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. 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.

Optical cavities can tightly confine light in the vicinity of optical emitters, enhancing the interaction of light and matter. The modes or optical states of the cavity can be precisely designed and engineered, and in recent years there has been remarkable progress in demonstrations of ‘cavity quantum electrodynamics (cQED)’ in solid state platforms. Such progress has been primarily for cavities fabricated in dielectric materials, with a steady improvement in cavity quality, with quality factors, Q, in excess of 10⁴ – 10⁶ realized for cavities with coupled emitters [1],[2]. These high Q-coupled emitter systems have demonstrated heralded single photon emission [3], ultra-low threshold lasing [4] and strong light-matter coupling [5],[6].

Metal-based optical cavities would have inherently lower Q’s (and greater loss) than dielectrics; however, metal cavities utilizing surface plasmon polaritons (SPPs) can have sufficiently small mode volume to produce a substantial Q/V, the quantity relevant for high Purcell factors, a measure of the light-matter interaction. This talk will focus on such plasmonic cavities, with optical modes formed within the gap of the two metal layers which defined the cavity [7]. Initial structures comprised silver (Ag) nanowires (NW), 70 nm in diameter and 1 - 3 µm in length, placed into close proximity to a Ag thin film substrate, with the NW axis parallel to the substrate surface. Optically active material was interposed between the nanowire and the Ag substrate: this comprised one to two monolayers of PbS colloidal quantum dots, clad on top and bottom by thin dielectric layers of varying composition and thickness. The fluorescence spectrum of PbS quantum dots within the gap was strongly modified by the cavity mode, with peak position in quantitative agreement with numerical calculations, and demonstrating Q values of ~ 60.

Such plasmonic cavities allow the easy incorporation of a variety of light-emitting active areas, and we have also explored the incorporation of various organic, dye-containing layers within the gap-mode plasmonic cavities. In addition these structures lend themselves to relatively simple modifications of geometry, allowing effective tuning of cavity modes, and also control of modes through the use of photonic crystal geometries, fabricated into metal.

The high Q/V possible for these cavities, and the range of organic and nanocrystalline emitters they can accommodate make these important building blocks for the exploration of light-matter interaction in the solid state.

References

[1] S. Noda, M. Fujita, and T. Asano, Nature Photonics 1 (2007) 449.

[2] B.-S. Song, S.-W. Jeon, and S. Noda, Optics Letter 36 (2011) 91.

[3] P. Michler., et al. Science 290 (2000) 2282.

[4] S. Strauf et al., Phys. Rev. Lett. 96 (2006) 127404.

[5] J. Reithmaier et al.. Nature 432( 2004) 197.

[6] K. Hennessy, A. Badolato, et al. Nature 445 (2007) 896.

[7] K. Russell and E. Hu, Appl. Phys. Lett.97 (2010) 163115.

Using two-photon photoemission electron microscopy (2P-PEEM) we have directly explored the optical fields on a single Ag nanostructure and quantitatively measured the field enhancement factor (FEF). The 2PPE intensity from the Ag nanostructure is enhanced by 2 orders of magnitude with respect to the 2PPE intensity from a smooth and homogeneous Ag thin film. This enhancement is attributed to a localized surface plasmon excitation and resonance of the local field, and the FEF is determined to be around 4. The capability of directly correlating the field enhancement with nanostructures makes 2P-PEEM a promising tool to investigate the fundamental optical properties of nanomaterials.

Magnetic transition nanoparticles (NPs) have been developed and studied by many researchers for bio-imaging and bio-sensing applications [1,2] due to their special optical and magneto-optical (MO) properties. Nevertheless, it is possible to enhance the MO effects of the magnetic NPs by combining them with other materials such as noble metals which exhibit intense localized surface plasmon resonance (LSPR) under certain conditions[3,4]. Here, we present our investigations on LSPR enhanced MO effect in magnetic metal core-noble metal shell NPs, such as core-shell Fe-Ag and Co-Ag NPs. These systems present strong Faraday rotation due to LSPR, nevertheless differences are found among them due to their different optical properties. A blue-shift is experimentally observed in the optical and MO spectra peaks from Fe-Ag to Co-Ag NPs with similar Ag shell concentrations and constant NPs sizes. Also, the absorption and Faraday rotation spectra of Fe-Ag NPs are broader than those of Co-Ag NPs. We explain such differences by means of theoretical studies based on an adaptation of the Maxwell-Garnet model to core shell nanoparticles yielding an excellent agreement with the experimental results. The possibility to understand and tune the properties of core-shell nanoparticles reported here will have significant impact in photonic and plasmonic applications.

This work was financed by DARPA under a grant for the development of novel sensors for bio-defense.

[1] Gilles K. Kouassi and Joseph Irudayaraj, Anal. Chem., 2006, 78 (10), pp 3234–3241.

[2] Jae-Hyun Lee, Yong-Min Huh, Young-wook Jun, Jung-wook Seo, Jung-tak Jang, Ho-Taek Song, Sungjun Kim, Eun-Jin Cho, Ho-Geun Yoon, Jin-Suck Suh and Jinwoo Cheon, Nature Medicine , 2006, 13, pp 95–99.

[3] Lei Wang, Kaida Yang, Cesar Clavero, Andrew. J. Nelson, Kyler J. Carroll, Everett E. Carpenter, and Rosa. A. Lukaszew, J. Appl. Phys., 2010, 107, 09B303.

[4] Lei Wang, Cesar Clavero, Zachary Huba, Kyler J. Carroll, Everett E. Carpenter, Diefeng Gu and Rosa A. Lukaszew, Nano Lett.,2011, 11(3), pp1237–1240.