PLASMONICS

ABSTRACT:

     In this review we discuss about how the light waves can be used with the nano scale components to generate plasmons and transfer data at high speed. The technology explained below is the combination of both Electronics and Photonics which is used to employ optics in integrated electronic chips and how data can be transferred through busiest areas of integrated circuits using nano scale wires. 

The discussion revels the role of NANOTECHNOLOGY for the implementation of PLASMONICS. Finally we discuss about the applications of PLASMONICS in the fields of Medicine, Military, Biochemical, and Biological imaging etc.

INTRODUCTION:

With the increasing quest for transporting large amount of data at a fast speed along with miniaturization, both electronics and photonics are facing limitations. Light is derful medium for carrying information.

Optical fibers now span the globe, guiding light signals that convey voluminous streams of voice communications and vast amounts of data. This gargantuan capacity has led some researchers to prophesy that photonic devices which channel and manipulate visible light and other electromagnetic waves could someday replace electronic circuits in microprocessors and other computer chips. Unfortunately, the size and performance of photonic devices are constrained by the diffraction limit; because of interference between closely spaced light waves, the width of an optical fiber carrying them must be at least half the light's wavelength inside the material. For chip-based optical signals, which will most likely employ near-infrared wavelengths of about 1,500 nanometers (billionths of a meter), the minimum width is much larger than the smallest electronic devices currently in use; some transistors in silicon integrated circuits, for instance, have features smaller than 100 nanometers.

 Today everyone needs huge data to be transferred at a very high speed with miniature devises, but one contradicts to another, if data increases then speed will going to decrease and component size will going to increase and vise versa. Photonic components such as fiber-optic cables can carry a lot of data but these are bulky compared to the electronic circuits. Electronic components such as wires transistors can be incredibly small but carry less data.

A problem holding back the progress of computing is that with miss matched capacities and sizes; the two technologies are hard to combine in a circuit. We can cobble them together but a single technology that has both the speed of photonics and smallness of electronics is “plasmonics”.

Plasmon waves are interesting because they are at optical frequencies. The higher frequency of the wave, the more information you can transport. This technology squeezes electromagnetic waves into minuscule structures will yield a new generation of superfast computer chips and ultrasensitive molecular detectors.

What is plasmonics?

            Plasmonics is also called as” light on a wire”, would allow transmission of data at an optical frequencies along the surface of a tiny metallic wires, despite the fact that the data travels in form of electron density distributions rather than photons. Plasmonics refers to the investigation, development and applications of enhanced electromagnetic properties of metallic nanostructures. The term ‘plasmonics’ is derived from ‘plasmons’, which are the quanta associated with longitudinal waves propagating in matter through the collective motion of large number of electrons.

By directing light waves at the interface between a metal and a dielectric (a nonconductive material such as air or glass) can, under the right circumstances, induce a resonant interaction between the waves and the mobile electrons at the surface of the metal. (In a conductive metal, the electrons are not strongly attached to individual atoms or molecules.) In other words, the oscillations of electrons at the surface match those of the electromagnetic field outside the metal. The result is the generation of surface plasmons--density waves of electrons that propagate along the interface like the ripples that spread across the surface of a pond after you throw a stone into the water

We can creatively designing the metal-dielectric interface they can generate surface plasmons with the same frequency as the outside electromagnetic waves but with a much shorter wavelength. This phenomenon could allow the plasmons to travel along nanoscale wires called interconnects, carrying information from one part of a microprocessor to another.

      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 charge of the conduction electrons are balanced by an equal concentration of positive charge of the ion core. A plasma oscillation in a metal is a collective longitudinal excitation of the conduction electron gas against a background of fixed positive ions with plasma frequency.

     Surface plasmons are dense waves of electrons –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 subwave length size means that they take up less space. Plasmonics, then, is the technology of transmitting this light like waves along nano scale wires. With every wave, we can, in principle, carry information.

Benefits 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 microprocessors. Because of the dual nature of the light, it behaves as particle and as well as wave. So

Plasmonics allows us to use both particle and wave properties of light.  

     The key is using a material with low refractive index, ideally

Negative, such that the incoming electromagnetic energy reflected parallel to the surface of the material and transmitted along its length as far as possible. There exists no natural material with a negative refractive index, so nano structured materials must be used to fabricate effective plasmonic devices. For this reason, plasmonics is frequently associated with nanotechnology.

              Plasmonics describes how ultrasmall metallic structures of various shapes capture & manipulate light & provides a practical design tool for nanoscale optical components. The fact that light interacts with nanostructures overcomes the belief held for more than a century that light waves couldn’t interact with anything smaller than their own wavelengths.

        Research has shown that nanoscale objects can amplify & focus light in ways scientists 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 plasmon that exits on a surface is directly related to its geometric structure.

            Since 2001, there has been an explosive growth of scientific interest in the role of plasmons in optical phenomena including guided-wave propagation and imaging at the sub wavelength scale, nonlinear spectroscopy and ‘negative index’ metamaterials. The unusual dispersion properties of metals near the plasmon resonance enables excitation of surface modes and resonant modes in nanostructures that access a very large range of wavevectors over a narrow frequency range, and accordingly, resonant plasmon excitation allows for light localization in ultra-small volumes. Noble metal, especially gold (Au) and silver (Ag) nanoparticles exhibit unique and tunable optical properties on account of their surface plasmon resonance (SPR). This feature constitutes a critical design principle for light localization below the free space wavelength and opens the path to truly nanoscale plasmonic optical devices.

     Plasmons are generated when, under the right conditions light strikes a metal. The electric field of light jiggles the electrons to the light’s frequency, setting of density waves of electrons. The process is analogous to how the vibrations of the larynx jiggle molecules in the air into density waves experienced as sound.  

     When light of a specific frequency strikes a plasmon that oscillates at a compatible frequency, the energy from the light is harvested by the plasmon, converted in to electrical energy that propagates through the nanostructure & eventually converted back in to light.

Research & Development: -

     The field of plasmonics, which has existed for only a few years, has already attracted researchers from the industry & government. Studying the way light interacts with metallic nanostructures will make it easier to design new optical materials & device from the bottom up, using metal particles of specially tailored shapes. One primary goal of this field is to develop new optical components & systems that are of the same size as today’s smallest integrated circuits & that could ultimately be integrated with electronics on the same chip.

     The research shows that the equations that determine the frequencies of plasmons in complex nanoparticles are almost identical to the quantum mechanical equations that determine the energies of electrons in atoms & molecules called ‘Plasmon Hybridisation’. Just as quantum mechanics allows scientist to predict the properties of complex molecules, research shows that the properties of plasmons in complex metallic nanostructures can be predicted in a simple manner. The findings are applicable not only to nanoshells but also to nanoscale wave guides & other nano photonic structures.

The ultimate goal of research and development is to demonstrate plasmonics in action on a standard silicon chip and make working plasmonic components. The next step will be to integrate the components with an electronic chip to demonstrate plasmonic data generation, transport and detection.

      Plasmon waves behave on metals much like light waves behave on glass, meaning that plasmonic engineers can employ all the ingenious that photonic engineers use to cram more data down a cable- such as multiplexing, or sending multiple waves. Meanwhile, because plasmonic components can be crafted from the same materials that the chip makers use today, we can make all the devices needed to route the light around a processor or other kind of chip. These would include plasmon sources, detectors and wires as well as splitters and even transistors.

Limitations of plasmonics:

The potential of plasmonics right now is mainly limited by the fact that plasmons can typically travel only several millimeters before they peter out. Chips meanwhile are typically about a centimeter across, so plsmons can’t yet go the whole distance.

     The distance that a plasmon can travel before dying out before is a function of several aspects of the metal. But for optical transfer through a wire of any metal, the surface of contact with surrounding materials must be as smooth a possible and the metal should not have impurities.

      For most wavelengths of visible light, aluminium allows plasmons to travel farther than other metals such as gold, silver, copper. It is some what ironic that aluminium is the best metal to use because the semiconductor for industry recently dumped aluminium in favour the copper the better electrical conductor as its wiring of choice. But since we know that the metals will change their properties when they are fabricated on a nano scale, for example gold will change its color when converted in to its nanoparticals. So in future by research we may come to know about the metal which has greatest Surface characteristics suitable for transporting plasmons. So with the help of nanotechnology we can implement plasmonics satisfactorily.                                      

      Another thing that we must consider here is the heat. Since electronics chips will generate heat, the plasmonics also will generate some heat but the exact amount is not yet known. Even if plasmonics runs as hot as electronics, it will still have the advantage of a higher data capacity in the same space.

Future applications:-

         For millennia, alchemists and glassmakers have unwittingly taken advantage of plasmonic effects when they created stained-glass windows and colorful goblets that incorporated small metallic particles in the glass. The most notable example is the Lycurgus cup, a Roman goblet dating from the fourth century A.D. and now held in the British Museum. Because of plasmonic excitation of electrons in the metallic particles suspended within the glass matrix, the cup absorbs and scatters blue and green light--the relatively short wavelengths of the visible spectrum. When viewed in reflected light, the plasmonic scattering gives the cup a greenish hue, but if a white light source is placed within the goblet, the glass appears red because it transmits only the longer

       Before all plasmonic chips are developed, plasmonics will probably be integrated with conventional silicon devices. ‘Plasmonic wires will act as high band width free ways across the busiest areas of the chip’.

        Plasmon printing is a new approach to lithographic printing that takes advantage of the resonantly enhanced optical intensity in optical near field of metallic nanoparticals, and that could enable printing of deep sub wavelength features using conventional photoresist and simple visible/ultraviolet light sources.

       Plasmonics has also been used in biosensors. When a particular protein or DNA molecule rests on the surface of a plasmon-carrying metallic materials, it leaves its characteristics signature in the angle at which it reflect the energy.

       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.

     The strongly enhanced SPR scattering from Au nanoparticles makes them useful as bright optical tags for molecular-specific biological imaging and detection using simple dark-field optical microscopy. On the other hand, the SPR absorption of the nanoparticles has allowed their use in the selective laser photothermal therapy of cancer. The sensitivity of the nanoparticle SPR frequency to the local medium dielectric constant has been successfully exploited for the optical sensing of chemical and biological analytes. Plasmon coupling between metal nanoparticle pairs forms the basis for nanoparticle assembly-based biodiagnostics and the plasmon ruler for dynamic measurement of nanoscale distances in biological systems.

     Ultimately it may be possible to employ plasmonic components in a wide variety of instruments, using them to improve the resolution of microscopes, the efficiency of light-emitting diodes (LEDs) and the sensitivity of chemical and biological detectors. We can also consider medical applications, designing tiny particles that could use plasmon resonance absorption to kill cancerous tissues, for example. And some researchers have even theorized that certain plasmonic materials could alter the electromagnetic field around an object to such an extent that it would become invisible.

Although not all these potential applications may prove feasible, investigators are eagerly studying plasmonics because the new field promises to literally shine a light on the mysteries of the “nanoworld”.

        Totally, if we master this technology it will give rise to a new era of miniaturisation and speed with huge data transfer. The existing scenario depicts the fact that PLASMONICS is going to emerge as the next device technology incorporated with nano scale devices.

 Conclusion:-

Ø Plasmonics combines the two technologies     Photonics’ and Electronics

Ø Both miniaturisation and speed with huge data transfer can be achieved using this technology.

Ø  This technology is also known as ‘light on a wire’.

Ø Surface plasmons are dense waves of electrons –bunches of electrons passing a point regularly along the surface of a metal.

Ø Plasmons have the same frequencies and electromagnetic fields as light, but their subwave length size means that they take up less space.

Ø Optical frequencies are about 100000 times greater than the frequency of today’s electronic microprocessors.

Ø To implement this technology we must use materials with negative refractive index. Since none of the material has negative refractive index so nano structured materials must be used.

Ø For most wavelengths of visible light, aluminium allows plasmons to travel farther than any other metal.

Ø Plasmons can travel only several millimeters before they loose their energy.

Ø Plasmonics allows us to transfer the data through busiest areas of integrated circuits.

Ø Plasmonics can be implemented only with the help of “nanotechnology”.

Ø Since we are about to perfect the art of nanotechnology it seems that the golden words of Richard Fainman “There is lot of space at the bottom” has been proven.