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2Physics Quote:
"Many of the molecules found by ROSINA DFMS in the coma of comet 67P are compatible with the idea that comets delivered key molecules for prebiotic chemistry throughout the solar system and in particular to the early Earth increasing drastically the concentration of life-related chemicals by impact on a closed water body. The fact that glycine was most probably formed on dust grains in the presolar stage also makes these molecules somehow universal, which means that what happened in the solar system could probably happen elsewhere in the Universe."
-- Kathrin Altwegg and the ROSINA Team

(Read Full Article: "Glycine, an Amino Acid and Other Prebiotic Molecules in Comet 67P/Churyumov-Gerasimenko"

Sunday, July 02, 2017

An Efficient Energy Transfer Mechanism on the Nanoscale Using Plasmonic Nanoparticles

From Left to Right: (top row) Larousse Khosravi Khorashad, Lucas V. Besteiro, Eva-Maria Roller, (bottom row) Claudia Pupp, Tim Liedl, Alexander O. Govorov
Authors: Larousse Khosravi Khorashad1, Lucas V. Besteiro1, Eva-Maria Roller2, Claudia Pupp2, Tim Liedl2, Alexander O. Govorov1

1Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, United States 

2Department of Physics and Center for NanoScience, Ludwig-Maximilians-Universität München, 80539 Munich, Germany

Research in electronic devices is pushing the size of transistors below the 10 nm mark, but at this scale information processing faces difficulties in terms of heat dissipation, as well as in controlling quantum effects such as electron tunneling. An alternative to electron-based computing lies in the creation of all-optical computational devices. Although we are still far from producing functional processors, today we show how plasmonic structures are viable tools not only for the nanoscale localization of electromagnetic energy but also its transfer, contrary to what their typically high dissipation rates would have us believe.

Plasmonics, in general terms, is the field of Physics that studies the interaction of light with materials, mostly at the nanoscale, that support collective electronic oscillations [1]. These plasmonic materials are typically metallic nanoparticles (NPs) or heavily doped semiconductors that exhibit free-electron behavior. When light, as an electromagnetic wave, is incident on the NPs, the free electron gas of the NPs is excited and oscillates as a result of the polarization of the electromagnetic wave. These excited plasmonic oscillations typically have resonances in the visible spectral range and the resonance frequencies depend on the type, shape and size of the plasmon-supporting material.

Plasmonic materials afford us the capability of manipulating light and funneling its energy at the nanoscale, and has enabled the creation of novel devices, from optical metamaterials [2] to efficient nanoscale heat generators [3]. For all-optical computational devices, it is challenging to transfer and manipulate photonic energy at the nanoscale. Here, photon transfer is not feasible using traditional waveguides such as glass fibers, which require sizes comparable to the wavelength of the confined light. Plasmonic nanostructures are remarkably effective in localizing the electromagnetic energy of light, but these structures suffer from high energy losses due to the high rate of electron scattering with the background charges in the material. In our recent work [4] we show how this essential limitation can be successfully circumvented by carefully considering the interaction of different material systems with distinct plasmonic resonances.
Figure 1. TEM image of a trimer fabricated with the DNA origami technique, with a 30 nm AgNP placed between two 40 nm AuNPs. Below there is an energy diagram of the trimer. The AgNP resonance is not fully excited, acting as an efficient energy transmitter, coupling the outer AuNPs. Figure adapted from Ref. [4]

By carefully arranging two gold NPs (AuNPs) and a silver NP (AgNP) in a linear chain , our model system takes advantage of the different optical spectra of gold and silver [4]. Figure 1 shows a transmission electron microscopy (TEM) image of one of the experimentally fabricated trimers, illustrating the arrangement of our proposed hybrid nanostructure. The two 40 nm AuNPs are spatially separated by a 38 nm gap, partially occupied by a 30 nm AgNP that leaves two 4 nm interparticle gaps. To achieve this accurate NP arrangement we have used the DNA origami technique [5,6] to build a rigid bundle of 14 interconnected, parallel DNA double helices. At three points along this DNA structure, attachment sites for each particle are built, consisting of single-stranded DNA (ssDNA) extensions with orthogonal sequences for each material. Figure 1 also includes a schematic energy diagram of the trimer, which illustrates the main effect explored in the paper. The two AuNP plasmon states are only weakly coupled due to the distance separating them, but the connecting AgNP acts as a bridge for their interaction. More importantly, it mediates the transfer with a plasmonic resonance of different energy than the resonance of the AuNPs. In this way, the AgNP plasmon state is not fully occupied and it functions as a virtual state for the plasmon to travel from one AuNP to the other.

The presence of the AgNP is crucial for the coherent transfer of energy from one AuNP (transmitter) to another (receiver) with negligible energy dissipation. The AgNP bridges the large gap between the AuNPs with a virtual energy state that allows the transfer to take place. As we confirmed experimentally, the distance between the AuNPs is enough to impede their coupling in the absence of the mediating particle. In fact, the AuNPs would respond mostly as independent dipoles in the absence of the AgNP. With the three particles in place, the longitudinal plasmonic resonance of the trimer is far from the resonant frequency of the AgNP. Therefore, this mediating particle facilitates the coupling of the two AuNPs and the excitation of the global trimer plasmon mode, while avoiding the dissipation of heat that would appear in the AgNP resonant excitation. Importantly, our simulations show the hot spots that typically occur between the surfaces of metal particles in close proximity, allowing here the effective coupling of their near fields. The coupling is experimentally verified through the observation of the scattering spectra of the trimer, evidencing the hybridization of the plasmonic modes of the NPs. Figure 2 showcases some of the scattering experimental data, alongside results from the simulated system.
Figure 2: (click on the image to view with higher resolution) Experimental (a) and simulated (b) spectra of interacting trimers, compared with a weakly interacting gold NP dimer. The scattering signal is red shifted and strongly enhanced when introducing the mediating silver NP, creating a new resonance for the whole interacting system and evidencing the hybridization of their originally non-interacting states. Figure adapted from Ref. [4]

In a second set of classical electrodynamic simulations, we observed the time resolved transfer inside the trimer after being excited by a single point dipole source, located just outside the chain. We find an extremely fast plasmon passage between the two outer particles, orders of magnitudes faster than the energy transfer observed in typical Förster-type dipole-dipole interactions [7]. By our calculations, we estimate a transfer time of about 5 fs and the oscillation of energy between the two gold particles for more than 40 fs (Figure 3). We have also described the system with a theoretical quantum model in which the three NPs are considered as three quantum oscillators that are coupled to each other via Coulomb interactions. This quantum model predicts the energy shift of the system’s bright plasmon modes that are observed experimentally, as well as the dark modes found in a full electrodynamical simulation.
Figure 3: (click on the image to view with higher resolution) Evolution in time of the excitation in the two AuNPs as obtained from simulations, showing the total electric dipole in each NP after a pulse excitation by a dipole source close to the left AuNP. The diagram on top is a representation of the trimer and the source. The middle AgNP acts as a transmitter. The data in (b), shown in a logarithmic scale, is the total amplitude of the dipolar moment in each gold NP, in which one can appreciate the back-and-forth transfer of the excitation between the AuNPs. Figure adapted from Ref. [4]

In summary, in this research we presented a novel approach toward fast and almost loss-less plasmon transfer between metal nanoparticles, which can have a great impact in applications for energy and information transfer on the nanoscale. We used a simple combination of gold and silver nanoparticles to coherently transfer energy and overcome the large heat dissipation characteristic of plasmonic systems. Our proposed quantum model provides us with a clear understanding of the physics, and our full electrodynamic simulation is in excellent agreement with the experimental measurements performed on this exciting system. This work opens new doors for designing future optical-based computing devices and for the teleportation of data over longer distances at the nanoscale.

[1] Stefan A. Maier, "Plasmonics: fundamentals and applications" (Springer, 2007).
[2] Kan Yao and Yongmin Liu, “Plasmonic metamaterials”. Nanotechnology Rev.iews  3, 177 (2014).  Abstract.
[3] Larousse Khosravi Khorashad, Lucas V. Besteiro, Zhiming Wang, Jason Valentine, and Alexander O. Govorov, “Localization of Excess Temperature Using Plasmonic Hot Spots in Metal Nanostructures: Combining Nano-Optical Antennas with the Fano Effect”. Journal of Physical Chemistry C, 120, 13215 (2016). Abstract.
[4] Eva-Maria Roller, Lucas V. Besteiro, Claudia Pupp, Larousse Khosravi Khorashad, Alexander O. Govorov and Tim Liedl, “Hotspot-mediated non-dissipative and ultrafast plasmon passage”. Nature Physics (published online May 15, 2017). Abstract.
[5] Anton Kuzyk, Robert Schreiber, Zhiyuan Fan, Günther Pardatscher, Eva-Maria Roller, Alexander Högele, Friedrich C. Simmel, Alexander O. Govorov and Tim Liedl, “DNA-based self-assembly of chiral plasmonic nanostructures with tailored optical response". Nature, 483,  311 (2012). Abstract.
[6] Paul W. K. Rothemund, “Folding DNA to create nanoscale shapes and patterns”. Nature, 440,  297 (2006). Abstract.
[7] Volkhard May and Oliver Kühn, “Charge and Energy Transfer Dynamics in Molecular Systems” (3rd Ed. Wiley, 2011). 

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