Plasmons Surface Plasmon Resonance Plasmonic Effects and Applications Introduction During the last two decades many researches devoted to develop optical sensors for the measurement of chemical and biological quantities. Large variety of optical methods have been used in chemical and biosensors, among them, surface Plasmon resonance. In these sensors, a desired quantity is determined by measuring the refractive index, absorbance and fluorescence properties of analyte molecules. What is Plasmon? The quanta of waves produced by collective effects of large numbers of electrons in matter when the electrons are disturbed from equilibrium. In other word, the quantum of Plasma Oscillation is called Plasmon The name plasmon derives from the physical plasma as a state of matter in which the atoms are ionized. At the lowest densities this means an ionized gas, or classical plasma; but densities are much higher in a metal, or quantum plasma, the atoms of a solid metal being in the form of ions. In both types of physical plasma, the frequency of plasma-wave oscillation is determined by the electronic density. In a quantum plasma the energy of the plasmon is its frequency multiplied by Planck's constant, a basic relationship of quantum mechanics. Metals provide the best evidence of plasmons, because they have a high density of electrons free to move. 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. Again, the geometry of the metal film plays an important role in plasmon frequency. For example gold, has plasmon frequency in the deep ultraviolet, but geometric factors bring it close to the visible. In doped semiconductors, the plasma frequency is usually in the infrared. The plasmon energy for most metals corresponds to that of an ultraviolet photon. However, as mentioned above for some metals like silver, gold, the alkali metals, and a few other materials, the plasmon energy can be sufficiently low to correspond to that of a visible or near-ultraviolet photon. This means there is a possibility of exciting plasmons by light. If plasmons are confined upon a surface, optical effects can be easily observed. In this case, the quanta are called surface plasmons, and they have the bulk plasmon energy as an upper energy limit. Surface plasmons are those plasmons that are confined to surfaces and that interact strongly with light resulting in a polariton. 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). Surface plasmons were first proposed to explain energy losses by electrons reflected from metal surfaces. Since then, numerous experiments have involved coupling photons to surface plasmons. Potential applications extend to new light sources, solar cells, holography, Raman spectroscopy, microscopy, and sensors. Surface plasmons on a plane surface are non-radiative electromagnetic modes, that is, SPs cannot be generated directly by light nor can they decay spontaneously into photons. The origin of the non-radiative nature of SPs is that the interaction between light and SPs cannot simultaneously satisfy energy and momentum conservation. This restriction can be circumvented by relaxing the momentum conservation requirement by roughening or corrugating the metal surface. Other method is to increase the effective wave vector (and hence momentum) of the light by some means (discussed later).
Surface Plasmon Resonance The excitation of surface plasmons by light is denoted as a surface plasmon resonance (SPR) for planar surfaces or localized surface plasmon resonance (LSPR) for nanometer-sized metallic structures. Surface plasmons, also known as surface plasmon polaritons, ( coupling between photon and an excitation of a material) and are surface electromagnetic waves that propagate parallel along a metal/dielectric interface. For surface plasmons to exist, the complex dielectric constants of the two media must be of opposite sign. This condition is met in the IR-visible wavelength region for air/metal and water/metal interfaces (where the real dielectric constant of a metal is negative and that of air or water is positive). Typical metals that support surface plasmons are silver and gold, but metals such as copper, titanium, or chromium can also support surface plasmon generation. Surface Plasmon resonance (SPR) is a non-destructive analysis technique, which is used in the investigation of thin layers of molecules upon a material surface. More specifically SPR is capable of detecting changes in the refractive index (n) occurring near the surface of a metal (within ~200nm). It is a physical process, which occurs when plane polarized light hits a metal film under total internal reflection conditions. When a light beam, traveling from a dense to a less dense medium, strikes the surface of a prism this causes the light to bend towards the interface plane. As depicted in the figure changing the angle of incidence changes the resulting light until a critical angle is reached. Upon reaching the critical angle all the incoming light is reflected within the prism, this is referred to as total internal reflection (TIR). Light is not generated during TIR, however the electrical field of the photons extends approximately a quarter of the wave length beyond the reflecting surface.
Transmitted (refracted) light θ t k t n 2 Evanescent wave k i Incident light θ i θ i k r n > n 1 2 θ c θ c θ i >θ c TIR Reflected light (a) (b) (c) Light wave travelling in a more dense medium strikes a less dense medium. Depending on the incidence angle with respect to θ c, which is determined by the ratio of the refractive indices, the wave may be transmitted (refracted) or reflected. (a) θ i < θ c (b) θ i = θ c (c) θ i > θ c and total internal reflection (TIR). 1999 S.O. Kasap, Optoelectronics (Prentice Hall) When the prism is coated with a material with an infinitely high index of refraction, past a certain critical angle, total internal reflection occurs for all light reflected. In addition to this all light sent toward the surface is reflected away from the surface. However, when light is totally internally reflected off the inside of a prism there is a probability that some of the light will exist outside the surface of the prism. This light is called the evanescent wave.
The prism described above is generally coated with a thin metal film placed in contact with the base of the prism (usually the reflection site), e.g. gold. The use of a metal sensing surface in SPR is critical as this technique capitalizes upon the fact that metals contain electrons, which behave as a continuous sea of charge. This "sea" of charge can undergo charge-density oscillations, plasmons, at the surface of the conductor, particularly at a surface in contact with an insulator. Furthermore a molecular layer of interest can be coated onto the thin metal film on the side opposite the prism. When a particular type of light (from the light source) strikes the metal sensor, surface plasmon waves (SPW) are generated at the interface between the conductive metal and the insulating molecular layer. In addition to the generation of the SPWs, light is also reflected off of the metal surface. As indicated earlier at TIR, all the energy from the incident light wave will be transferred to the reflected light wave. However, at a particular angle past the point of TIR, which results in the SPR angle, a majority of the incidence light energy will interact with the generated SPW s. This results in a phenomenon called resonance. At resonance, the reflected light intensity will be
minimal; this intensity corresponds with the SPR angle (the intensity of the reflected light may be measured using the photo-detector. The SPR angle is dependent upon several factors, including: properties of the metal film, the wavelength of the incident light and the refractive index of the media on either side of the metal film i.e. molecular layer in contact with the metal sensing surface; (the refractive index is sensitive to temperature, therefore it is important to perform the measurements at defined temperatures as well). The metal film used must have conduction band electrons capable of resonating with the incoming light at a suitable wavelength. Metals that satisfy this condition are silver, gold, copper, aluminum, sodium and indium. In addition, the metal on the sensor surface must be free of oxides, sulfides and should not react to other molecules on exposure to the atmosphere or liquid. The thickness of the metal layer is also of great importance. Above an optimum thickness the dip in reflective light becomes shallow, and below an optimum thickness the dip becomes broader; thus affecting the SPR angle.
Wavelength vs Reflectance Techniques to Induce Surface Plasmon Resonance Several configurations of SPR devices exist, and serve as sensors. These optical devices are capable of exciting the SPWs and are also used to interrogate the SPR. The configurations that are known and used today are the following: Surface plasmon resonance sensors using optical prism couplers Surface plasmon resonance sensors using grating couplers Surface plasmon resonance sensors using optical waveguides Surface plason resonance sensors based on optical fibers Surface plasmon resonance sensors using optical prism couplers A very suitable geometry for sensors using attenuated total reflection (ATR) is the Kretschmann Prism. The Kretschmann prism is used to measure reactions on a sensor chip attached to a prism. The apparatus consists of a sensor chip, a light source, a light detector, and a prism also referred to as the Kretschmann Prism.
- In order to promote evanescent waves, rather than coating the prism with a material with a high index of refraction, a sensor chip is attached to the prism with a thin layer of metal. In this scenario, waves are present in the sea of free electrons in the metal. when the plasmons have similar properties to that of the evanescent wave, the two couple resulting in SPR. SPR uses energy, therefore the intensity of the light which reflects back from the surface is less that that of the
incident on the surface. This intensity may be measured in order to determine the occurrence of SPR. Furthermore when a sensor chip is fabricated such that it is capable of changing the nature of its surface plasmon in the presence of an analyte, the presence or concentration of this analyte may be determined. Most sensors are operated in the following manner: Monochromatic light is directed through the prism through a range of angles which all cause total internal reflection. The sensor chip is coated with receptors to a specific analyte. The concentration of the analyte present on the opposite surface of the sensor chip modifies the resonant frequency of the Surface Plasmon. The intensity of the reflected light vs. incident angle will have a minimum that corresponds to the resonant frequency. From the location and magnitude of this minimum the concentration of the analyte can be determined. S urface plasmon resonance sensors using optical grating In this technique, the incident electromagnetic radiation is directed towards a medium whose surface has a spatial periodicity (D) similar to the wavelength of the radiation, for example a reflection diffraction grating. The incident beam (red) is diffracted producing propagating modes which travel away from the interface (blue) and evenescent modes which exist only at the interface. The evenscent modes have wavevectors parallel to the interface similar to the incident radiation but with integer 'quanta' of the grating wavevector added or subtracted from it. These modes couple to Surface Plasmons (green), which run along the interface between the grating and the ambient medium.
Surface plasmon resonance sensors using optical waveguides The use of optical waveguides in SPR sensors provides numerous attractive features such as a simple way to control the optical path in the sensor system to suppress the effect of stray light. The process of exciting an SPW in this configuration is similar to that of the Kretschmann ATR coupler. A light wave is guided by the waveguide and, entering the region with a thin metal layer, it evanescently penetrates through the metal layer. If the SPW and the guided mode are phase matched, the light wave excites an SPW at the outer interface of the metal. Theoretically, the sensitivity of waveguide-based SPR devices is approximately the same as that of the corresponding ATR configurations.
Light Light Light n 2 n 2 Light n 1 > n 2 A planar dielectric waveguide has a central rectangular region of higher refractive index n 1 than the surrounding region which has a refractive index n 2. It is assumed that the waveguide is infinitely wide and the central region is of thickness 2a. It is illuminated at one end by a monochromatic light source. 1999 S.O. Kasap, Optoelectronics (Prentice Hall) High order mode Low order mode Intensity Light pulse Cladding Core Broadened light pulse Intensity Axial Spread, Δτ 0 t t Schematic illustration of light propagation in a slab dielectric waveguide. Light pulse entering the waveguide breaks up into various modes which then propagate at different group velocities down the guide. At the end of the guide, the modes combine to constitute the output light pulse which is broader than the input light pulse. 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Surface plasmon resonance sensors based on optical fibers Optical fiber SPR probes present the highest level of miniaturization of SPR devices, allowing for chemical and biological sensing in inaccessible locations. The ability to transmit optical signals over a long distance makes the use of optical fibers very attractive. Fiber optic waveguides have a number of advantages over prism-based sensors. They are inexpensive and can easily be used to make disposable sensors for medical tasks. Fibers are also very small and have no moving parts, giving them a much broader range than the Kretschmann sensors and making multiple sensor arrays a possibility. A fiber optic SPR sensor is built using a large diameter (~400mm) and multimode fiber. Cladding is removed from a portion of the fiber, and a surface plasmon metal layer e.g. silver is deposited instead. The length from which the cladding is removed is dependant upon the diameter of the fiber, and determines the number of reflections occurring at the surface plasmon metal interface. If the length is too short, not enough coupling will occur. If the length is too long, coupling will be very strong and the minimum coupling intensity will be difficult to determine. When light enters a fiber at a specific angle, corresponding to a specific mode, it will propagate through a multimode optical fiber. Although modes are more of an energy distribution, in the fiber, they can also be thought of as angles of total internal reflection as the light bounces back and forth along the fiber. Light which enters the fiber at larger angles (i.e. low-order modes) bounces back and forth at a slow pace, whereas light which enter the fiber at a tighter angles (i.e. higher-order modes) bounces back and forth a fast pace. At low-order modes the energy is distributed in the fiber core, whereas the energy for high-order modes spreads into the cladding, and beyond the waveguide.
y y Cladding Core φ r z Fiber axis n 2 n 1 n The step index optical fiber. The central region, the core, has greater refractive index than the outer region, the cladding. The fiber has cylindrical symmetry. We use the coordinates r, φ, z to represent any point in the fiber. Cladding is normally much thicker than shown. 1999 S.O. Kasap, Optoelectronics (Prentice Hall) Along the fiber 1 Meridional ray Fiber axis 3 1, 3 (a) A meridional ray always crosses the fiber axis. 2 2 1 2 Fiber axis 3 Skew ray 4 5 5 4 1 2 3 (b) A skew ray does not have to cross the fiber axis. It zigzags around the fiber axis. Ray path along the fiber Ray path projected on to a plane normal to fiber axis Illustration of the difference between a meridional ray and a skew ray. Numbers represent reflections of the ray. 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
In order to achieve SPR sensing, as opposed to sweeping through a range of coupling angles in the Kretschmann Prism, the fiber only sweeps through a number of coupling wavelengths. The wavelengths are interrogated, i.e. measuring the amount of each wavelength leaving the fiber, using a broadband, multi-wavelength source e.g. white light. Using a spectrophotometer it is then possible to determine which wavelength coupled with the surface plasmon and how much analyte (species being analyzed) is present. Applications The SPR signal is directly dependent on the change of the refractive index of the medium on the sensor side of the SPR surface. The spectra can be generated for a metal surface with and without a coated molecular layer. Then, the shift in SPR angle between the two can be quantified and used to calculate the thickness or refractive index of the adhered molecules. SPR has proven useful in determining both growth in the thickness of a molecular layer and loss in thickness, even of a single monolayer. Along with its ability to determine the thickness of coated films, SPR has also emerged as a technology in the area of sensors (e.g., for the detection of physical quantities, chemicals and biological purposes). Physical quantities (such as temperature and humidity) can be deduced from changes in refractive index. Chemical sensing can use changes in refractive index to indicate changing concentrations of molecules adhered to the metal surface (as a result of chemical reactions). Biosensing can also use refractive index changes to deduce the occurrence of binding interactions (such as between antigens and antibodies). SPR also provides the important advantage of being able to monitor reactions in real-time, without the need to go through the often complicated process of labeling molecules with fluorescent or radioactive probes. Like all surface analysis techniques, SPR has its limitations in terms of sensitivity (the smallest amount of molecule detectable), resolution (the smallest difference in SPR angle distinguishable) and sample characteristics (geometry, thickness, etc.). However, this technique still provides a remarkable variety of capabilities for the
characterization of reaction kinetics and thin film properties, with a high degree of sensitivity. Most of the interesting SP-mediated effects happen when the metal surface at which the SP is generated is covered with a dielectric thin film. The presence of even very thin films measurably alters the behavior of the SP reflectivity resonance -- typically shifting the incident angle at which resonance occurs and broadening the reflectivity dip. These effects can be used to make devices. For example, if the film is electro-optically active, one can make an optical modulator; chemical changes in the dielectric overlayer can be used to make a chemical sensor. There are many areas of applications of SPR sensors. For instance they are used for measurements of physical quantities, chemical sensing, and biosensing. Because of the complexity of biological systems and the number of possible interference to chemical nanosensors, the need for added specificity in cellular analyses can arise: nanobiosensors are then employed. Biological receptor molecules (i.e., antibodies, enzymes, etc.) are used to provide added specificity. The different types of bioreceptor molecules that have been used for the fabrication of nanobiosensors include antibodies, oligonucleotides, and enzymes, thereby allowing for the detection of a wide array of analytes. Applications in Technology Plasmons have been considered as a means of transmitting information on computer chips, since plasmons can support much higher frequencies (into the 100 THz range, while conventional wires become very lossy in the tens of GHz.) They have also been proposed as a means of high resolution lithography and microscopy due to their extremely small wavelengths. Both of these applications have seen successful demonstrations in laboratory environment. It is evident that deeply sub-wavelength focal spots cannot be formed through conventional focusing using a lens system or microscope objective. This is due, primarily, to the lack of high-index media at
visible frequencies. What if, however, one was able to achieve a high effective index with conventional optical materials? That is the potential of surface plasmon optics. By employing geometries of conductors (such as metals or doped semiconductors) with dielectrics (such as air or glass), modes at optical frequencies can be created with effective indices of refraction that are orders of magnitude higher than those of the constituent materials. In fact, these indices can be so high as to create X-ray wavelengths (less than 10nm) with visible frequencies. The reason surface plasmon modes can achieve anomalously high wave-vectors at visible frequencies is because they are mediated by electrons rather than free space optical fields. The ability to focus the optical field to deeply sub-wavelength dimensions opens the door to an entirely new class of photonic devices. If one could combine the imaging powers of X-ray wavelengths with the economy and maturity of visible light sources, one could greatly broaden the practical engineering toolbox. Imagine focusing visible photons to spatial dimensions less than ten nanometers. By doing so, electron beam microscopy is immediately displaced by optical microscopy, replacing expensive electron beam sources with inexpensive visible lasers. Beyond simple economics, though, this achievement would allow for the nanoscale imaging of living biological samples. Combining Plasmonics Effects and Photonic Crystals Photonic band structure refers to the modification of the propagation properties of electromagnetic waves traveling through a periodically modulated dielectric. The effects of scattering and interference of the light by the periodic structure would result in a change in the propagation of the waves. The alteration in the propagation properties is particularly significant when the wavelength of the light is approximately equal to the spacing between the dielectric structures. In this regime photonic band gaps--frequency intervals in which no photon modes are allowed--can be created for appropriately designed dielectric arrays. The ability to create volumes of space in which no photons of a given band of energies can exist has a number of fundamental and applied consequences.
Application in Health Science Surface plasmon resonance is used by biochemists to detect the presence of a molecule on a surface. SPR reflectivity measurements can be used to detect DNA or proteins by the changes in the local index of refraction upon adsorption of the target molecule to the metal surface. If the surface is patterned with different biopolymers, the technique is denoted as Surface Plasmon Resonance Imaging (SPRI). For nanoparticles, localized surface plasmon oscillations can give rise to the intense colors of solutions of plasmon resonance nanoparticles and/or very intense scattering. Nanoparticles of noble metals exhibit strong ultraviolet-visible absorption bands that are not present in the bulk metal. Shifts in this resonance due to changes in the local index of refraction upon adsorption of biopolymers to the nanoparticles can be used to detect biopolymers such as DNA or proteins. Areas of interest in this domain are for instance the examination of protein-protein or protein-dna interactions, in order to detect conformation changes in an immobilized protein. In addition to above mentioned, biosensors may also be used to monitor the glucose levels in diabetic patients. The system under study would be based upon direct measurements of the reflection and transmission spectra in the near infrared spectrum.