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
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.
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.
In Fig. 3, another useful property of the AgCo system is shown. The wavelength position of the LSPR can be tuned over nearly 100 nm by controlling the amount of Co in the AgCo bimetal. Thus, significantly more control of LSPR is possible in AgCo as compared to Ag, whose LSPR changes only weakly with size for simple hemispherical shaped nanoparticles, as seen in the figure.
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.
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.
In Fig. 2 the schematic of the binary tree approach to implement the self consistent binary mixing rule to obtain an effective dielectric constant for a mixture of two metals a and b in the host h is shown. This approach can also be used to create an alloy nanocomposite in host h from alloys of metals a and b.
In Fig. 3 the Optical transmission (%) as a function of wavelength for 50%Ag-50%Co nanocomposite particles on a glass susbtrate is shown. The experimental measurements are shown by solid lines, while the theory predictions are shown by the dashed lines. The minima in the curves corresponds to the LSPR position. The inset shows the measured particle size distribution.Very good agreement is seen between model and experimental behavior.
In this research with Prof. Garcia at SIUE we are extending optical models to describe the nonlinear optical behavior of nanocomposites. Accurate prediction of the nonlinear susceptibility of condensed matter systems has been a challenging problem
since the first calculation by Goeppert-Mayer [1931 Ueber elementarakte mit zwei quantenspruengen Ann. Phys. 9 273–95] of the
two-photon absorption cross section for semiconductors, and since the beginning of nonlinear optics research following
the invention of the laser [Maiman T H 1960 Stimulated optical radiation in ruby Nature 187 493
]. In this work we have extend our binary mixing approach to calculate the nonlinear susceptibility (Fig. 1) of a ternary system based on an approach already developed in a weakly nonlinear binary system in the spirit of the effective medium approximation.
In Fig. 2 Absolute value of the nonlinear susceptibility of the Ag:SiO2 system for a volume fraction of 0.12 (solid line), and comparison with experimental values measured by Faccio et al (Europhys. Lett. 43 213). There is a fair agreement with experiment taking into account the fact that the samples have used Na+ ions to control Ag migration following ion implantation.
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.
We simplified the materials selection step with a compound figure of merit (FOMC) for nanocomposites of noble metals in dielectric based on criteria that limit these structures in photonic applications, i.e. thermal heating and two-photon absorption. The device independent results predict extremely large values of FOMC for a specific combination of the metal and insulator dielectric constant given by ϵh = (ϵ1 − ϵ2)/(2), where ϵh is the dielectric constant of the host and ϵ1 and ϵ2 are the real and imaginary parts for the metal. In Fig. 1, the compound FOM for various metals shows peaks at specific wavelengths.