# Plasmons convert light into a voltage

## Plasmons convert light into a voltage

Like with the "Hall Effect", the properties of "Plasmons" can be explained better with Mathis' Charge Field perhaps?

Mathis wrote:And that leads us to a further realization. My last illustration (of the grey rainbow) was done on a piece of paper, then scanned, but it could have just as easily been done in Photoshop. Either way, we have a surface reflecting or emitting variable amounts of light. But that surface is also particulate. A surface is a thing, and all things have charge fields; so we are not without a charge field here. Can we postulate a charge field variation then, with grey bands on a piece of paper? Yes, we can. Let us start with a white sheet of paper. The charge field is pretty equal and stable across the sheet, since the sheet itself is not variable. Likewise for the light hitting it. If we see the sheet as uniformly white, the incoming light is not variable either. What happens when we draw a grey band on the white sheet? The reflectivity of the sheet is altered, and more photons are absorbed. This means that the charge field in the grey band is being tamped down a bit. Not only is the reflected light less dense in that area, the emitted charge field is less dense. So we have a double variation to work with.

This means that we do not need material edges to cause diffraction or refraction. Edges of dark and light also work for the same reason. All we require is density variations, and we have shown those in both cases. This is what Goethe was noticing when he first scanned his room with his prism. This is why he knew Newton could not be right. Goethe could not explain the mechanics underneath the diffraction he saw, but he was quite thorough in cataloguing the effects. He saw that Newton's theory of bending was very incomplete, since it could in no way explain refraction by non-material edges. To explain refraction and diffraction mechanically requires the unified field and density variations, variations Newton did not have.

http://milesmathis.com/rain2.html

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Plasmons convert light into a voltage

http://physicsworld.com/cws/article/news/2014/oct/30/plasmons-convert-light-into-a-voltage

A new way of creating a voltage by shining light on a solid has been developed by researchers in the US and Europe. Unlike most photovoltaic devices, the new system does not rely on semiconductors but rather on surface plasmons in tiny metal nanostructures. The team is now working to create new types of devices that convert light into electrical energy.

Surface plasmons are collective excitations of electrons at the surface of a metal that interact very strongly with light. As a result, plasmons are of great technological interest as an interface between photonics and electronics. This interaction is strongest at the plasmon-resonance frequency, which is defined by the size and shape of an object and its charge density. In 2009 Paul Mulvaney and colleagues at the University of Melbourne in Australia applied an electrical potential to gold nanoparticles, and found that they could tune the plasmon-resonance frequency by injecting or removing electrons.

Sweeping laser

In the new work, applied physicist Harry Atwater and colleagues at California Institute of Technology, together with researchers in the Netherlands, show that the reverse can also occur: a surface potential can be induced by using light to modify the charge density of a nanoparticle. The team made its plasmonic material by attaching gold nanorods with a plasmon-resonance wavelength of 550 nm to an indium-tin-oxide substrate. Then the researchers fired a tuneable laser at the structure, and swept the laser wavelength from 480 nm to 650 nm. During illumination, the electric potential on the surface of the material was monitored using the conductive tip of an atomic force microscope.

When the laser was on resonance with the surface plasmon, no voltage was induced. Irradiation either side of the resonant frequency, however, did produce a voltage. When the wavelength was below 550 nm a negative potential was measured on the gold nanorods, while longer-wavelength light created a positive potential. The team found that the magnitude of the potential related to the rate at which the light absorbance changed with respect to the frequency of the light. The largest potential (which was negative) was produced by illumination at 500 nm. Atwater offers a thermodynamic explanation for this observation: "If you shine light on the structure, free-energy minimization will cause the structure to try to adjust its charge density to bring itself into resonance with the exciting light." The researchers have dubbed this phenomenon the plasmoelectric effect.

Successful model

The team then used this model to predict the frequency at which the maximum potentials should be generated in its set-up, and found broad agreement with its experimental results. The researchers also checked that the model could be applied generally, by testing it in a different type of plasmonic material: a thin gold sheet studded with a periodic pattern of 10 μm holes mounted on a glass substrate. This too showed a plasmoelectric effect, with the peak negative and positive potentials as predicted by the model.

While the devices reported by the team simply produce a potential difference when illuminated, the team is now working on a device that will deliver usable electrical energy and thereby function as a solar cell. Atwater believes that such a device could complement traditional semiconductor photovoltaic cells: "Any given single-material solar cell can only convert power from photons that have energy greater than the band-gap energy," he says, "[Our device] could potentially be used behind a conventional photovoltaic cell to harvest the infrared part of the spectrum, because I can design a plasmonic structure to have a resonance at pretty much any frequency."

Fascinating physics

Nano-optics specialist Thomas Ebbesen of the University of Strasbourg, says: "I find it to be very impressive work. If something like this could become efficient as an energy conversion process that would of course be technologically important. But independent of that, I find the underlying physics very interesting just from a thermodynamic point of view."

Last edited by Cr6 on Sun Sep 13, 2015 1:28 am; edited 5 times in total

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## Re: Plasmons convert light into a voltage

HOW DOES SURFACE PLASMON RESONANCE WORK?
http://www.bionavis.com/technology/spr/

http://www.bionavis.com/technology/spr/animation3/
http://www.bionavis.com/products/spr-navi-specifications/

General

Surface Plasmon Resonance has been established as a powerful method to monitor label-free biomolecular interactions in liquids. However, today with MP-SPR, it can deliver well beyond kinetics and equilibrium constants.

In this page, we summarize the basic theory behind SPR. You can find here information on surface plasmons, different SPR configurations and modelling.

Surface plasmons

Excitation of surface plasmons is based on total internal reflection when an incident beam of p-polarized light strikes an electrically conducting gold layer at the interface of a glass sensor with high RI (Refractive Index) and an external medium (gas or liquid) with low RI. At a given angle, the excitation of surface plasmons takes place resulting in a reduced intensity of the reflected light. A slight change at the interface (e.g. a change in refractive index or formation of a nanoscale film thickness) will lead to a change in SPR signal, allowing precise measurements of thin film properties as well as surface molecular interactions in real-time.

General

In all three cases, momentum matching between the plasmon and the incoming photon, i.e. excitation of plasmons is evidenced by a drop in intensity of reflected light when the angle of resonance is approached. Figure below shows an example of the SPR curve measured for silver with the aid of the Kretschmann configuration. Such resonant behavior gives an advantage in biosensor applications, because the value of the resonant angle ΘR is a sensitive function of the dielectric constants of the two contacting media. Due to this property, the surface plasmon resonance can be utilized in monitoring surface reactions, as every new ad-layer formed on the metal surface causes changes in dielectric function of medium ε1, establishing new resonance angle ΘR'. The shape of the whole resonance curve, i.e. its depth and width depends on optical absorption within the metal and on radiation losses resulting from surface roughness.

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## Re: Plasmons convert light into a voltage

A good read before the following article:

Rainbows, Prisms, and non-edge Diffraction: A Rehabilitation of Goethe

by Miles Mathis
http://milesmathis.com/rain2.html
http://milesmathis.com/rainbow.html
http://milesmathis.com/rain3.pdf
---------------
Published online 6 June 2014
Light scattering and surface plasmons on small spherical particles

Xiaofeng Fan1, Weitao Zheng1 and David J Singh1,2

1College of Materials Science and Engineering, Jilin University, Changchun 130012, China
2Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6056, USA

Correspondence: Professor XF Fan or Professor DJ Singh, College of Materials Science and Engineering, Jilin University, Changchun 130012, China. E-mail: xffan@jlu.edu.cn; singhdj@ornl.gov

Received 18 November 2013; Revised 17 March 2014; Accepted 21 March 2014

Abstract

Light scattering by small particles has a long and interesting history in physics. Nonetheless, it continues to surprise with new insights and applications. This includes new discoveries, such as novel plasmonic effects, as well as exciting theoretical and experimental developments such as optical trapping, anomalous light scattering, optical tweezers, nanospasers, and novel aspects and realizations of Fano resonances. These have led to important new applications, including several ones in the biomedical area and in sensing techniques at the single-molecule level. There are additionally many potential future applications in optical devices and solar energy technologies. Here we review the fundamental aspects of light scattering by small spherical particles, emphasizing the phenomenological treatments and new developments in this field.

Introduction

The production, control, manipulation and use of light are at the core of many technologies. Light scattering plays key roles in all of these. Of course, the scattering of light by small particles has a long history, where it was studied in contexts such as cumulus clouds, the color of the sky and rainbows, and used in various glass artifacts and windows from the middle ages.1 The remarkable fact is that such a classical topic is the basis of many fundamentally new and unexpected scientific and technological advances. The key is the current focus on the nanoscale and especially near-field effects at the nanoscale, while much of the older classical study was oriented towards the accessible far-field behavior.

More specifically, there have been fascinating developments in regard to the light scattering by nanosized particles, including metal particles and surfaces, where localized surface plasmons can be excited leading to optical resonance phenomena.2,3,4,5 Small particles with surface plasmons can be used to detect the fluorescence of single molecules,6,7 enhance Raman scattering,8 resonantly transfer energy of excitons9 and create nanosized quantum amplifiers of optical energy. Potential practical uses include10 small-scale sensing techniques,11,12 numerous biomedical applications,13 manipulation of light for solar energy technologies14 and others.

Here we provide a short review emphasizing the nano-optics of small particles, near-field effects and the fundamental theoretical basis for their description. We begin with a review of the classical light scattering theory for spherical particles based on the quasistatic (Rayleigh) approximation and the general Mie theory. Scattering by dielectric particles is discussed along with the new topic of optical trapping. We discuss plasmon resonances and light scattering on small metallic particles, which is a subject that has been renewed by a series of new findings, including anomalous scattering with an inverted hierarchy of resonances and Fano resonances. The breakdown of the general Drude model for dielectric function at very small particle sizes and the resulting effects are discussed. Finally, we review the stimulated radiation from the surface plasmons of small particles along with concepts for new kinds of lasers based on nanolasing related to surface plasmons coupled to an active medium (so-called spasers).

We start with a summary of the basic concepts that remain useful in understanding light scattering, focusing on the case of spherical particles. Light scattering by small particles is one of fundamental problems of electrodynamics. As mentioned, it is a classical subject for which theory was developed long ago. This theory includes both the near and far field description. However, until recently the near field was inaccessible to experiments, and the interest was focused on far field effects. Now with advances in nanotechnology and nano-optics, the richness of the near field theory is being exploited. This includes the production by scattering of very high light intensities with spatial variations shorter than the wavelength—a phenomenon that enables rich new physics, both linear and nonlinear, at the nanoscale.

The physical understanding of light scattering by small particles began with the electric dipole concept, introduced by Lord Rayleigh in 1871.15 One starts with the assumption that the electromagnetic phase is constant over the region of interest, which is natural since the size of small particle considered is less than the wavelength of light. Then the homogeneous field of the incident light induces a polarization, which in turn results in light scattering. Higher order scattering modes, such as quadrupole and octupole, are not considered at this level. The polarization (i.e. the induced dipole) of materials in response to electromagnetic fields is determined by the dielectric function.

The dielectric function of a material (at energies above the phonon energies) is determined by its electronic structure. It is practical to calculate dielectric functions from first-principles band theory and often good agreement with experiment is found.16,17 However, for analyzing optical properties, it is often useful to approximate the optical properties of solids using the classical harmonic oscillator formalism introduced by Lorentz. In the Lorentz model, the dielectric function of non-conducting materials can be expressed as1

Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

where f and ω0 are a phenomenological oscillator strength and frequency representing the bound electrons and γ is a damping constant. This naturally leads to the Sellmeier formula for the refractive index, 1/(n2−1)=−A/λ2+B, where n is the refractive index, λ is the wavelength and A and B are material-dependent quantities. This formula and generalizations (e.g., to two or more oscillators) are very effective in fitting the optical constants of real materials.18

Many physical phenomena can be very simply understood even in the simplest one oscillator theory. For example, the dispersion of light by prisms or water drops is explained by the frequency dependence of the refractive index. This follows the normal dispersive behavior (refractive index increases with energy) for materials like glass and water. This originates in the fact that the energy (ω0) of the effective oscillator for transparent materials such as these is generally much larger than the frequency of visible light.19 For metals, the contributions of free electrons need to be added. This yields a model known as the Lorentz–Drude model. This model has1

Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

where the sum on j is over different oscillators. The free electron part is due to the electron plasma of the metal, which is described by the parameters ωp and γe which represent the resonant frequency and damping constant of bulk plasma.

It has long been recognized that all linear optical phenomena can in principle be modeled by solving the Maxwell's equations with known dielectric functions of the media. This understanding, while correct, by itself yielded relatively little direct physical insight into light scattering, at least prior to the development of modern computers and electromagnetic codes. This is because the vector electromagnetic equations resisted analytic solution, especially for complicated, but interesting, boundary conditions.

The earlier work on light scattering by small particles is mainly from Lorenz, Thomson and Clebsh.20 Actually, the exact solution has been obtained by Clebsh in 1861 in his paper ‘Concerning reflection on a spherical surface’ published in 1863, a year before Maxwell's work about electromagnetic theory of light. The breakthrough in understanding light scattering by spherical structures came from the work of Mie in 1908.21 He obtained a general rigorous solution, on basis of the electromagnetic theory, for the optical scattering by a homogeneous sphere with arbitrary size in a homogeneous medium, whatever the composition of the sphere and medium. The Mie theory solution is also applied directly to the scattering by any number of spheres if the distance between particles is large enough so that there are no coherent phase relations among the scattered light from different particles.19

The general Mie theory of optical scattering is very useful in practice. Initially, interesting problems, such as the origin of rainbows and the solar corona, could be directly answered on basis of the Mie solution.19 So-called corrected Mie theory gives the light scattering by structures with other regular shapes, such as ellipsoids with any size and cylinders with arbitrary radius.19 The basic optical properties of small particles made of different materials have been analyzed in detail via modifications of Mie theory, such as the Gans modification (for spheroidal particles, e.g., plasmonic gold and silver nanoparticles with elongated shapes) and the Maxwell–Garnett equations (providing an effective medium approach).22 Of course, there are also interesting questions related to light scattering by particles that cannot be directly described with Mie theory. An example is the electromagnetic hot spot between two nearby particles, which depends on coherency.

As mentioned, it is remarkable that even though more than 100 years have passed since the introduction of Mie's general theory of light scattering by a sphere, new and exciting physics associated with light scattering by small particles continues to be found.23,24,25,26 Examples include the finding of giant optical resonances with an inverted hierarchy (e.g., the quadrupole resonance is more intense than the dipole) in scattering by small particles with negative dielectric susceptibility and weak dissipation,23 and anomalous scattering with the complicated near-field structures, such as the vortices, unusual frequency dependence, etc.27,28 In addition, the Fano resonances, which are well known in quantum physics, were discovered in optics of small metallic particles.29,30,31 Turning to nanotechnology and nano-optics, there is an increasing focus on control of optical energy in subwavelength structures—a field that is leading to many new ideas and remarkable experiment results.3,32,33,34,35 These include nanolenses36 and nano-antennas.37 The localized surface plasmons in spherical core–shell structures can result in so-called spasers, with resonantly coupled transitions with excitons from dye molecules in the shell layer.38 The coupling between localized surface plasmons from closely separated small particles results in electromagnetic hot spots.39,40,41 Huge light scattering is found on anisotropic spherical particles with an active mechanism.42 These remarkable results based on light scattering by small particles suggest many potential applications, including solar energy technologies14,43 and nanoscale lasers.44

Fano resonance on Mie scattering by a small metal sphere (a) and energy flow in the quadrupole resonance with the singular vortices represented by the Poynting vector field (b). In a, Radar back scattering and forward scattering directions are indicated by the red line and blue line, respectively, the dielectric function ε(ω) is described by Drude model with the dissipation parameter γ=0.001ωp and the particle size is a=0.8c/ωp. In b, q=0.3 and εd=−1.553, the blue line denotes the particle surface, the red lines indicate the separatrix. Figure reproduced with permission, b from Ref. 27 2007 IOP.

Surface plasmons can be amplified by the stimulated emission of radiation in the presence of a gain medium, such as dielectric materials containing excited dye molecules. Lasing depends on the presence of two principal conditions—a cavity for the resonant generation of coherent optical modes and medium with gain due to population inversion. A realization, the so-called spaser (surface plasmon amplification by stimulated emission of radiation) consists of small plasmonic metal particles surrounded by a gain medium such that the linewidth of the light emission from the gain medium overlaps with that of plasmon mode.147 Thus, the losses of energy of the plasmonic particles due to dispersion and radiation can be compensated by light emission of gain medium. The key point is that by using a composite of plasmonic metal particles in a gain producing medium one may obtain a coherent radiation field in a lasing system that is smaller than the wavelength of the light.

...

Progress in spasers, including both theory and experiment, has been rapid.44 The original theoretical concept of the spaser was proposed with V-shaped metallic structures and semiconductor quantum dots.10 Following this, a nanolens spaser was proposed with a linear chain structure of metal nanospheres combined with an active medium.150 A proposal for a spaser based on metal cores with an active shell was considered on basis of linear electrodynamics.151 A narrow-diversion coherent radiation on based on the combination of a metamaterial and a spaser was proposed by Zheludev et al.152 The combination of the plasmon of an anisotropic spherical particle and an active medium was also proposed to result in a spaser.42 In experiments, a spaser was realized on a conjugate structure based on a metallic core and a dye-doped dielectric shell38 (Figure . Spasers have also been realized in other nanostructures, such as in CdS nanowires combined with a silver substrate and separated by a MgF2 layer.153 The development of active subwavelength optical elements such as in spasers, is expected to lead not only to diverse applications, but also to new fundamental insights into nonlinear light matter interactions.
...

Conclusions

We have briefly reviewed the theory of light scattering by small spherical particles and aspects of the important progress on light scattering on small spherical particles. It is remarkable that although many of the fundamental aspects of the theory are more than 100 years old, there continue to be new, surprising and useful developments, such as spasers and optical tweezers based on it. The interest 100 years ago was in the far field. While the formalism showed fascinating near field behavior, specifically giant concentrations of electromagnetic energy in regions much smaller than the wavelength and with complex spatial distributions, this was not explored until much more recently. Now these effects are being exploited to yield remarkable new nanoscale effects and potential applications in different science areas, such as high-resolution optical imaging, small-scale sensing techniques, light-activated cancer treatments, enhanced light absorption in photovoltaics and photocatalysis, and numerous biomedical applications. We expect that many more applications will be developed exploiting optical scattering by small particles, and especially nanophotonics applications based on the near-field and far-field applications using linear and nonlinear plasmonic effects.
http://www.nature.com/lsa/journal/v3/n6/full/lsa201460a.html

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## Re: Plasmons convert light into a voltage

http://turroserver.chem.columbia.edu/surfaceplasmons/handbook/chapter2_spr.pdf

CHAPTER 2

Physics of Surface Plasmon Resonance
ROB P.H. KOOYMAN

Biophysical Engineering Group, Faculty of Science and Technology,
University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

2.1 Introduction

In the last two decades, surface plasmon resonance (SPR) has evolved from a
fairly esoteric physical phenomenon to an optical tool that is widely used in
physical, chemical and biological investigations where the characterization of
an interface is of interest. Recently, the field of SPR nano-optics has been added
where metallic structures on a nanoscale can be designed such that they can
perform certain optical functions. This chapter will be mainly concerned with
the more conventional, well-understood SPR theory used in sensor applications
and it will touch upon some of the newer developments relevant for this area.
Essential for the generation of surface plasmons (SPs) is the presence of free
electrons at the interface of two materials – in practice this almost always
implies that one of these materials is a metal where free conduction electrons
are abundant. This condition follows naturally from the analysis of a metal-
dielectric interface by Maxwell’s equations. From this analysis, the picture
emerges that surface plasmons can be considered
as propagating electron density waves occurring at the interface between metal and dielectric. Alternatively, surface plasmons can be viewed as electromagnetic waves strongly bound to this interface; it is found that the surface plasmon field intensity at the interface
can be made very high, which is the main reason why SPR is such a powerful tool for many types of interface studies.

Experimental research on SPs started with electron beam excitation; in 1968,
optical excitation was demonstrated by Otto [1] and Kretschmann and Raether [2].
This last approach turned out to be much more versatile, so in this chapter the
focus will be on the optics of SPR. The following is by no means intended as an
in-depth treatment of surface plasmons, rather it is an attempt to provide a low-
threshold introduction to the physics of SPR for those who are actually
involved in SPR work and want to understand a bit more than ‘‘measuring the shift of the SPR dip."

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