Plasmonics - Online Article

Introduction

The development of chip-scale electronics and photonics has led to remarkable data processing and transport capabilities that permeate almost every facet of our lives. Plasmonics is an exciting new device technology that has recently emerged. It exploits the unique optical properties of metallic nanostructures to enable routing and manipulation of light at the nanoscale. A tremendous synergy can be attained by integrating plasmonic, electronic, and conventional dielectric photonic devices on the same chip and taking advantage of the strengths of each technology.

Why Plasmonics?

The ever-increasing demand for faster information transport and processing capabilities is undeniable. Our data-hungry society has driven enormous progress in the Si electronics industry and we have witnessed a continuous progression towards smaller, faster, and more efficient electronic devices over the last five decades. The scaling of these devices has also brought about a myriad of challenges. Currently, two of the most daunting problems preventing significant increases in processor speed are thermal and signal delay issues associated with electronic interconnection1-3. Optical interconnects, on the other hand, possess an almost unimaginably large data carrying capacity, and may offer interesting new solutions for circumventing these problems4,5. Optical alternatives may be particularly attractive for future chips with more distributed architectures in which a multitude of fast electronic computing units (cores) need to be connected by high-speed links. Unfortunately, their implementation is hampered by the large size mismatch between electronic and dielectric photonic components. Dielectric photonic devices are limited in size by the fundamental laws of diffraction to about half a wavelength of light and tend to be at least one or two orders of magnitude larger than their nanoscale electronic counterparts. This obvious size mismatch between electronic and photonic components presents a major challenge for interfacing these technologies. Further progress will require the development of a radically new chip-scale device technology that can facilitate information transport between nanoscale devices at optical frequencies and bridge the gap between the world of nanoscale electronics and micro scale photonics. Researchers are pioneering such a technology called 'plasmonics' or 'light on a wire'.

Plasmonics as a new device technology

Metal nanostructures may possess exactly the right combination of electronic and optical properties to tackle the issues outlined above and realize the dream of significantly faster processing speeds. The metals commonly used in electrical interconnection such as Cu and Al allow the excitation of surface plasmon-polaritons (SPPs). SPPs are electromagnetic waves that propagate along a metal-dielectric interface and are coupled to the free electrons in the meta.

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An SPP propagating along a metal-dielectric interface. These waves are transverse magnetic in nature. Their electromagnetic field intensity is highest at the surface and decays exponentially away from the interface. From an engineering standpoint, an SPP can be viewed as a special type of light wave propagating along the metal surface.

From an engineering standpoint, an SPP can be viewed as a special type of light wave. The metallic interconnects that support such waves thus serve as tiny optical waveguides termed plasmonic waveguides. The notion that the optical mode ('light beam') diameter normal to the metal interface can be significantly smaller than the wavelength of light has generated significant excitement and sparked the dream that one day we will be able to interface nanoscale electronics with similarly sized optical (plasmonic) devices.

Plasmonics is derive from 'plasmons', which are the quanta associated with longitudinal waves propagating in matter through the collective motion of large numbers of electrons.

Plasma is a medium with equal concentration of positive and negative charges, of which at least one charge type is mobile. In a solid, the negative charges of the conduction electrons (i.e., electron gas) are balanced by an equal concentration of positive charge of the ion cores. A plasma oscillation in a metal is a collective longitudinal excitation of the conduction electron gas against a background of fixed positive ions with a plasma frequency.

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Surface plasmons are density waves of electrons-picture bunches of electrons passing a point regularly-along the surface of a metal. Plasmons have the same frequencies and electromagnetic fields as light, but their sub-wavelength size means they take up less space. Plasmonics, then, is the technology of transmitting these light-like waves along nanoscale wires.

Working Of Plasmonics

Plasmon waves are of particular interest because these are at optical frequencies .the higher the frequency of the wave, the more the information we can transport. Optical frequencies are about 100000 times greater than the frequency of today's electronic micro processors.

The key is using a material with low refractive index, ideally negative such that the incoming electromagnetic energy is reflected parallel to the surface of the material and transmitted along its length as far as possible. For this reason, plasmonics is frequently associated with nanotechnology.

Plasmonics describes how ultra small metallic structures of various shapes capture and manipulate light and provides a practical design tool for nanoscale optical components. the fact that light interacts with nanostructures overcome the belief held for more than a century that light waves couldn't interact with any thing smaller than their own wavelengths.

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Research has shown that nanoscale objects can amplify and focus light in ways scientist never imagined. The 'how' of this involves plasmons - ripples of waves in the ocean of electrons flowing across the surface of metallic nanostructures. The type of plasmons that exists on a surface is directly related to its geometric structure. When light of a specific frequency strikes a plasmon that oscillates at a compatible frequency, the energy from light is harvested by the plasmon, converted into electrical energy that propagates through the nanostructure and eventually converted back into light.

Veselago and negative index

All transparent or translucent materials that we know of possess positive refractive index--a refractive index that is greater than zero. However, is there any fundamental reason that there should not be materials with negative refractive index? This question was asked by Victor Veselago, a Russian physicist. In 1968, Veselago published a theoretical analysis of the electromagnetic properties of materials with negative permittivity and negative permeability. The electric permittivity and the magnetic permeability are commonly used material parameters that describe how materials polarize in the presence of electric and magnetic fields. Maxwell's equations relate the permittivity and the permeability to the refractive index as follows:

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The sign of the index is usually taken as positive. However, Veselago showed that if a medium has both negative permittivity and negative permeability, this convention must be reversed: we must choose the negative sign of the square root. This reversal of the refractive index can seem confusing. As an example, it is often said that the velocity of a wave in a material is given by c/n, where c is the speed of light in vacuum. The implication of a negative index, then, is that the wave travels backwards. An electromagnetic wave can be depicted as a sinusoidally varying function that travels to the right or to the left as a function of time. A wave is incident on a positive index material (the reflected wave has been ignored). The greater index of the second medium implies that the wavelength decreases (by a factor of 1/n); however, to maintain the same phase at the interface as a function of time, the speed of the wave must also be reduced, again by a factor of 1/n.

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When the refractive index is negative, the speed of the wave--given by c/n--is negative and the wave travels backwards toward the source. Yet, we would reasonably expect that since energy is incident on the material from the left, the energy in the material should likewise travel to the right, away from the interface. This seeming paradox is resolved, as Veselago showed, by realizing there are more ways to define the velocity of a wave. The definition c/n is well known as the phase velocity and determines the rate at which the peaks (or zeros) of a wave pass a given point in time. But this is not most relevant definition of a wave's velocity: we can also define the group, energy, signal and front velocities, and these generally differ from the phase velocity.

When the refractive index of a material does not vary with the wavelength of light that travels through it, then all of the velocity definitions above are the same and we can intuitively use the index as a measure of the wave's velocity. However, when a material is dispersive--has an index that varies with wavelength--then the various definitions of velocity no longer agree and we can no longer determine the actual velocity of the wave, or at least the rate at which energy is transported, from the value of the refractive index alone. So, even though the positive and negative index materials in the figure above seem to display drastically different behaviors, a calculation of the group or the energy velocity reveals that energy is actually flowing to the right in both cases. Thus, as Veselago showed, the phase and energy velocities are opposite in a negative index material.

History Of Negative Refractive Index

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Early publications prior to 2000 laid the groundwork for the most recent developments in negative refraction.

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In 2000, negative index metamaterials were introduced, as well as the concept of the "perfect lens" by John Pendry.

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In 2001, negative refraction by a metamaterial wedge was demonstrated.

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2002 was a controversial year for negative refraction in metamaterials; however, 2002 also saw ideas to realize negative refraction in photonic crystals.

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In 2003, two more Snell's law experiments helped to decide the controversy in favor of negative index.

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2004 saw improvements in fabrication, leading to a negative index lens and higher frequency structures.

Plasmon And Their Structure

In physics, the plasmon is the quasiparticle resulting from the quantization of plasma oscillations. They are a hybrid of the electron plasma (in a metal or semiconductor) and the photon. Thus, plasmons are collective oscillations of the free electron gas at optical frequencies. A plasmon is basically just an oscillation of the conduction electrons in a metal. This definition suggests that plasmons are strictly quantum mechanical entities, but many of their important properties can be derived directly from Maxwell's Equations.

It is the classical electro dynamical picture that is implied in most of the modern literature on plasmons. And for simulations of plasmons in complex geometry no free electrons are simulated, but simply the dielectric constant for a given frequency is used (or the local impulse response if you use time instead of frequency). No nonlocal interaction (like the one which is needed for high precision UV lithography lens simulation) is needed.

Surface Plasmons

Surface plasmons are those plasmons that are confined to surfaces and that interact strongly with light resulting in polariton. The interface between a conductor and an insulator is where surface plasmons propagate; bound to the surface between the two, they exponentially decay into both media. They occur at the interface of a material with a positive dielectric constant with that of a negative dielectric constant (usually a metal or doped dielectric). They play a role in Surface Enhanced Raman Spectroscopy and in explaining anomalies in diffraction from metal gratings (Wood's anomaly), among other things. Surface plasmon resonance is used by biochemists to detect the presence of a molecule on a surface.

More recently surface plasmons have been used to control colours of materials. This is possible since controlling the materials surface shape controls the types of surface plasmons that can couple to it and propagate across it. This in turn controls the interaction of light with the surface. This has been done both for visible light and for microwave radiation. Much research goes on first in the microwave range because at this wavelength material surfaces can be produced mechanically as the patterns tend to be of the order a few cm. To produce optical range surface Plasmon effects involves producing surfaces which have features >400nm. This is much more difficult and has only recently become possible to do in any reliable or available way.

The study of butterflies and beetles has revealed that many of the optical effects (from simple colour to iridescence) are actually produced by natural nanometre length structures which occur in nature.

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Explanation

Plasmons play a large role in the optical properties of metals. Light of frequency below the plasma frequency is reflected, because the electrons in the metal screen the electric field of the light. Light of frequency above the plasma frequency is transmitted, because the electrons cannot respond fast enough to screen it. In most metals, the plasma frequency is in the ultraviolet, making them shiny (reflective) in the visible range. On the other hand, some metals, such as copper, have a plasmon frequency in the visible range, yielding their distinct color. For other metals, such as gold, the plasma frequency lies deeply in the ultraviolet, but geometric factors come into play which reduces the plasmon frequency to the visible. In doped semiconductors, the plasma frequency is usually in the infrared.

Applications of Plasmonics

  1. Before all plasmonic chips are developed, plasmonics will probably be integrated with conventional Silicon devices. Plasmonic wires will act as high bandwidth freeways across the busiest areas of the chip.
  2. Plasmon printing is a new approach to lithographic printing that takes advantage of the resonantly enhanced optical intensity in optical near field of metallic nanoparticles, and that could enable printing of deep sub wavelength features using conventional photoresist and simple visible / ultra violet light sources.
  3. Plasmonics has also been used in biosensors. When a particular protein or DNA molecule rests on the surface of a plasmon - carrying metallic material, it leaves its characteristic signature in the angle at which it reflects the energy.
  4. In the field of chemical sensing, plasmonics offers the possibility of new technologies that will allow doctors, anti- terror squads and environmental experts to detect chemicals in quantities as small as a single molecule.

Structure of plasmonic devices

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A compact source with sub-wavelength spatial resolution provides distinct advantages in a number of applications (microscopy, spectroscopy, optical data storage, lithography and laser processing). Limitations on throughput of near-field scanning optical microscopy (NSOM) fibers have led to work on very-small aperture lasers (VSAL), where a sub-wavelength aperture is placed on the facet of a diode laser. In recent years, much attention has been given to optical antennas, in particular due to their ability to couple light very efficiently to sub-wavelength dimensions. In this work, we implement optical antennas on the facet of a laser, thereby creating a new plasmonic device, termed an active optical antenna.

Barriers and frontiers

The potential of plasmonics right now is mainly limited by the fact that plasmons typically can travel only several millimeters before they peter out. Chips, meanwhile, are typically about a centimeter across, so plasmons can't yet go the whole distance. The distance a plasmon can travel before dying out is a function of several aspects of the metal. But for optimal transfer through a wire of any metal, the surface of contact with surrounding materials must be as smooth as possible and the metal should not have impurities.

For most wavelengths of visible light, aluminum allows plasmons to travel farther than other metals such as gold, silver and copper. It is somewhat ironic that aluminum is the best metal to use because the semiconductor industry recently dumped aluminum in favor of copper-the better electrical conductor-as its wiring of choice. Of course, it may turn out that some kind of alloy will have even better plasmonic properties than either aluminum or copper.

Another classic semiconductor issue that the researchers will have to address is 'heat' Chipmakers are constantly battling to ensure that their electronic chips don't run too hot. Plasmonics also will likely generate some heat, but exactly how much is not yet known. Even if plasmonics run as hot as electronics, they will still have the advantage of having a higher data capacity in the same space.

Electronics face fundamental physical barriers to their data-carrying capacity, but the demands placed on them never seem to stop.

Conclusions

Plasmonics has the potential to play a unique and important role in enhancing the processing speed of future integrated circuits. The field has witnessed an explosive growth over the last few years and our knowledge base in plasmonics is rapidly expanding. As a result, the role of plasmonic devices on a chip is also becoming more well-defined and is captured. This graph shows the operating speeds and critical dimensions of different chip-scale device technologies. In the past, devices were relatively slow and bulky. The semiconductor industry has performed an incredible job in scaling electronic devices to nanoscale dimensions. Unfortunately, interconnect delay time issues provide significant challenges toward the realization of purely electronic circuits operating above ~10 GHz. In stark contrast, photonic devices possess an enormous data-carrying capacity (bandwidth). Unfortunately, dielectric photonic components are limited in their size by the laws of diffraction, preventing the same scaling as in electronics. Finally, plasmonics offers precisely what electronics and photonics do not have: the size of electronics and the speed of photonics. Plasmonic devices, therefore, might interface naturally with similar speed photonic devices and similar size electronic components. For these reasons, plasmonics may well serve as the missing link between the two device technologies that currently have a difficult time communicating. By increasing the synergy between these technologies, plasmonics may be able to unleash the full potential of nanoscale functionality and become the next wave of chip-scale technology.

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Operating speeds and critical dimensions of various chip-scale device technologies, highlighting the strengths of the different technologies.

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