Plasmonics and Optics Research in Ramki Kalyanaraman's group

Plasmonics is the field that deals with the resonant interaction of light with materials. The resonance occurs when light frequencies corresponding to the natural frequency of collective and coherent electron oscillations are incident on the conductive material. This field is very important towards technologies in biosensing, chemical sensing, surface enhanced Raman sensing, waveguiding, solar energy harvesting, sub-diffraction limited optics, etc.

CHALLENGES: As will be noted by studying the table on the right, currently there are very few materials that show practical plasmonic behavior in air, in fact only two, Au and Ag. However, Au is very expensive and, though Ag is much superior to Au, it degrades rapidly in ambient air and other corrosive environments. So our plasmonics work is aimed at discovering new materials for visible wavelength applications by a combination of materials design and modeling, synthesis, and characterization.

A New Plasmon Metal: AgCo Plasmon modeling in nanocomposites Nonlinear optical mixing model Figure of merit for photonics

A New Plasmon Metal: AgCo

In this collaborative research between UT and SIUE we are discovering new plasmonic metals. Innovative and cost-effective nanomaterials which improve upon or introduce new functionalities within small dimensions are seen as a driving force for technological advancement. One such category of nanomaterials are bimetallic systems made by combining immiscible metals of different functional properties. They have applications as sensing and therapeutic materials, can accelerate detection rates of superparamagnetically-tagged molecules, enable chemical sensing through magneto-optics, and potentially improve high-speed optical communication circuits as well as energy harvesting devices. We are investigating AgCo bimetals as a new plasmonic metal bimetals through synthesis of new types of nanostructures, such as the vertically stacked structure shown in Fig. 1 and measurement of their plasmon stability, wavelength tunability and sensing ability. This systems continues to show significantly better properties then pure Ag.

R. Sachan et al Adv Mat 2013; R. Sachan et al Nanotechnology 2012

Fig. 2 shows a plot of the change in the width of the plasmon peak with time, an indication of the degradation of the particles due to oxidation and corrosion. As evident Ag nanoparticles decay in air rapidly and lose their ability to show a strong plasmonic resonance. Therefore, sensing and other useful functions in air or other oxidizing ambient must be either done within a short time after preparation or by stabilizing the Ag surface. However, if on uses AgCo bimetals, the Ag plasmons can last more then 10 times longer as shown by the red curve in Fig. 1. In fact after 8 months in air (~6000 hrs), the AgCo plasmon still shows a plasmon peak intensity while the pure Ag has nearly completely degraded. One implication is that AgCo nanoparticles could be a cheaper and better new material for biological and chemical sensing based on localized surface plasmon resonance (LSPR) or Surface Enhanced Raman Sensing (SERS) techniques.

R. Sachan et al Adv Mat 2013

One of the most important aspects of selecting plasmonic materials for sensing applications is its sensitivity towards refractive index change of the external environment. Sensitivity is defined as the rate of change in LSPR wavelength with refractive index, usually in units of nm/RIU. In Fig. 4, variation in the λLSPR as a function of refractive index of external dielectric environment is presented for various size and composition of pure Ag and bimetallic Ag-Co nanoparticle arrays. This measurement indicated that the sensitivity of the Ag-Co bimetallic arrays, which ranged between 51 and 105 nm/RIU, was better or comparable to pure Ag particles of similar shape and size despite the bimetallic particles having a broader LSPR peak as compared to pure Ag.

R. Sachan et al Nanotechnology 2012

Plasmon & Optical modeling in nanocomposites

In this research with Prof. Garcia at SIUE we are developing models to describe the plasmonic and optical behavior of nanocomposites. Predicting the optical properties of multi-material composites, such as the array of nanoparticles on a surface, as shown in Fig. 1, is an important task towards designing better materials for solar energy harvesting and other optical and plasmonic applications. Our work is focussed on mean field models to describe the optical behavior of such materials. We have made significant progress at describing the plasmon behavior of biemtals such as AgCo as well as other systems using a simple self-consistent mixing model based on a modified Maxwell-Garnett approach.

H. Garcia et al Phys. Rev. B 2007; H. Garcia et al Plasmonics 2012

Nonlinear mixing in nanocomposites

Figure of Merit for Photonic Materials

In this research with Prof. Garcia at SIUE we are coming up with ways to optimize and select materials for plasmon-photon applications. In recent years considerable research has gone into the study of all-optical switching devices for photonic applications, where the requirement of the material had been a large nonlinear refractive index, low index change due to thermal effects, and small two-photon absorption. There is significant interest in using nanocomposites comprising of metal nanoparticles in dielectric host for such applications because of their nonlinear response and because of their potential use in applications below the diffraction limited regime. Here, we have come up with a compound figure of merit based on simultaneously optimizing the thermal (Fig. 1), nonlinear and two-photon absorption characteristics.

H. Garcia et al Appl. Phys. Lett. 2006

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