OPTIC NANOTECHNOLOGY * CHAPTER 9
9.3 THEORY OF ELECTRO-OPTIC NANOTECHNOLOGY
long time to get these dots to emit light. In the 1990s, researchers were able to get a dark red light.
Since then, other researchers have been able to tune the dots to a higher frequency, thus gaining blue and green light. The applications for this would be beneficial so that we could make full color screens and monitors (Wendy et al., 1999).
9.3 THEORY OF ELECTRO-OPTIC NANOTECHNOLOGY
On the topic of the optical properties of nanomaterials, Shah and Ahmad (2010) have the following to say:
The linear and non-linear optical properties of such materials can be finely tailored by controlling the crystal dimensions and the chemistry of their surfaces. Fabrication technology becomes a key factor for the applications.
This effectively encompasses some of the challenges in electro-optic nanotechnology, including those which are presented in the literature review.
9.3.1 ABSORPTION AND EMISSION TRENDS
The absorption and emission of light in nanomaterials is an important consideration in the production of displays, particularly those of organic light-emitting diodes (OLEDs).
One interesting property of semiconducting materials on the nanoscale is that they have optical properties which differ from the bulk material. It has been noted that as the size of the particles is reduced, there is a shift in the absorption spectra to the blue (Poole & Owens, 2003).
Once again, we see that size and structure play a greater role than chemical identity in this property.
Nanocrystals with larger bandgaps emit lower wavelength colors such as blue and green, whereas low bandgap materials emit higher wavelength colors like red (Shah & Ahmad, 2010).
9.3.2 DISPLAYS
In liquid-crystal displays, the resolution, brightness, and contrast are all dependent on the grain size of the particles being used. It is for this reason that nanomaterials are being investigated for use in such applications (Shah & Ahmad, 2010).
9.3.3 POLYMER-DISPERSED LIQUID CRYSTALS
The polymer-dispersed liquid crystals (PDLCs) are materials on a nano-level that can be thought of as Swiss cheese; there is a sold structure with scattered fragments in between. The cheese itself is a poly- mer substance which has tiny holes that are filled with liquid crystals. Liquid crystals have properties which are associated with both crystals and liquids (Chandrashekar, 1992).
The liquid-crystalline materials are the most crucial part of creating the PDLCs. There needs to be a perfect balance of rigid parts to help align the molecules in one direction and flexible parts to ensure there is fluidity in the liquid crystal (Bryant, 2001).
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There are three major production methods of PDLCs. The first method is encapsulation, also known as emulsification. This method requires that the liquid crystal, polymer, and water be mixed in a solu- tion. Once thoroughly mixed, the water is evaporated leaving the liquid crystals trapped in the polymer layer. The second method of producing PDLCs is phase separation. There are two different types of phase separation, polymerization-induced phase separation and thermally induced phase separation (Malik, Bubnov, & Raina, 2008). The polymerization-induced phase separation has three major steps.
The first is to mix a liquid prepolymer solution and a liquid crystals solution. This solution is mixed into a homogenous state and then the activation is started to form polymers from the prepolymer solution capturing the liquid crystals in between the polymers as it forms. The thermally induced phase separa- tion also has three major steps. The first is to obtain a polymer solution and heat it until the binds of the polymer loosen. Following this, liquid crystals need to be mixed with the heated polymer solution and then mixed to a homogenous state. The solution is then allowed to cool and the polymer will then obtain the liquid crystals that have seeped into the loosened binds.
This material has been paired with electricity to create smart windows (Bryant, 2001). The electrical conducting properties that have been incorporated into the liquid crystals on a nanoscale have allowed the crystals to be controlled by an electric volt. The amount of electricity applied to the liquid crystal will cause a rearrangement on the nanoscale of its structure causing it to scatter or become aligned within the polymer layer. The natural state of a smart window appears opaque due to the random ori- entations of the liquid crystals in the polymer. This state helps to block out sunlight saving energy on air conditioning, because the sunlight can be deflected causing less heat into rooms (Oltean, 2006).
Once an electrical signal is sent by a controlling device, the scattered liquid crystals align allowing the sunlight to be absorbed and the glass appears transparent. This state allows for more natural light saving on cost of lighting and also allows for natural heating for a room. The overall savings can amount to a maximum of 20% savings on electric cost (Oltean, 2006). Another added benefit from smart windows besides saving on electricity cost is that it allows the user privacy and the control of natural sunlight with a flip of a switch.
9.3.4 DENDRIMERS
Dendrimers are presented in this section because of their suitability for use in electro-optic materials (Cameron et al., 2002). Dendrimers are molecules with a branched mode of growth (Poole & Owens, 2003). Dendrimers can be functionalized with various groups, but for the purpose of electro-optics, light sensitive chromophores are of the greatest interest (Poole & Owens, 2003). Dendrimers can be easily removed from the reaction mixture because of their large size and therefore are easy to work with (Poole & Owens, 2003).
9.3.5 MODULATORS
An application of nanophotonics is the electro-optic modulators which are devices used to modulate or modify a beam of light. Currently they are mainly used in the information technology and telecom- munications industries (e.g., fiber-optic cables). Nanoscale optical communication devices will have increased speed and efficiency, once they can be engineered and used. Nanosize electro-optic modula- tors will be an integral part of a nanoscale communications network (Vlasov, Green, & Xia, 2008).
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9.3.6 PHOTODETECTORS
Photodetectors are electro-optic devices that respond to radiant energy. They are basically sensors of light or other electromagnetic energy. A sensor is an electronic device that converts one type of energy to another for various reasons. Nanoscale size photodetectors will be an integral part of a theoretical nanoscale optical information network (Novotny & Hect, 2006). Nanotechnology creates many new, interesting fields and applications for photonic sensors. Existing uses, like digital cameras, can be enhanced because more “pixels” can be placed on a sensor than with existing technology. In addition, sensors can be fabricated on the nanoscale so that they will be of higher quality and possibly defect free. The end result would be that photos would be larger and more accurate. As part of a communica- tion network, photonic sensors will be used to convert optical data (photons) into electricity (electrons).
Nanoscale photonic sensors will be more efficient and will basically receive similar advantages to other materials constructed under the nanoscale (Novotny & Hect, 2006).
9.3.7 ELECTROPHORETIC DISPLAY SYSTEMS
The main goal of electrophoretic display systems is to produce a high-quality image for low power consumption (Ahn, Yu, Kim, Lee, & Kim, 2008). This display is generally made with titanium dioxide particles dispersed in hydrocarbon oil. The oil used in the display system has a dark-color dye added to it along with surfactants and charging agents. The surfactants will lower the surface tension and inter- facial tension of the particles. This allows for easier movement of the particles through the oil allowing a faster transition time between displayed images (Das, Gates, Abdu, Rose, & Picconatto, 2007). The charging agents are added to allow particles to take on an electric charge and be more responsive even under low volts. Once this mixture has been formed and set between two layers of glass, an electric field can be used to rearrange the charged pigment particles to create visible images. This means that the dark colors will emerge to the viewers in that of the pattern fed into the electronic system, either revealing picture or letter which then can be changed via the electric field. This technology allows for thinner and lighter weight electronic reading devices with low power consumption.
9.3.8 SWITCHES
Electro-optic switches change signals in optical fibers to electrical signals. Typically semiconductor- based, their function depends on the change of refractive index with electric field. This feature makes them high-speed devices with low power consumption. Neither the electro-optic nor thermo-optic optical switches can match the insertion loss, back reflection, and long-term stability of opto-mechanical opti- cal switches. The latest technology combines all-optical switches that can cross-connect fibers without translating the signal into the electrical domain. This greatly increases switching speed, allowing today’s networks to increase data rates. However, this technology is only now in development and deployed systems cost much more than systems that use traditional opto-mechanical switches (Kimble, 2008).
9.3.9 PHOTONIC CRYSTALS
“Photonic crystals are composed of periodic dielectric or metallo-dielectric nanostructures that are designed to affect the propagation of electromagnetic waves (EM) in the same way as the periodic
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potential in a semiconductor crystal affects the electron motion by defining allowed and forbidden electronic energy bands. Simply put, photonic crystals contain regularly repeating internal regions of high and low dielectric constant” (Englund et al., 2007). Photonic crystals (PCs) are used to modify or control the flow of light. PCs may have a novel use in optical data transmission, but are not extremely prominent. They may be used to filter for interference in a fiber-optic cable or increase the quality of the transmission. In addition, they can be used to divide different wavelengths of light. PCs can already be manufactured at close to the nanoscale.
9.3.10 MULTIPLEXERS
A multiplexer is a device for converting many data streams into one single data stream, which is then divided into the separate data streams on the other side with a demultiplexer (Novotny & Hect, 2006).
The main benefit is cost savings, since only one physical link will be needed, instead of many physical links. In nanooptics, multiplexers will have many applications. They can be used as part of a communi- cation network, as well as utilized on a smaller scale for various modern scientific instruments.