Introduction
No one can deny that photonic (opto-electronics) devices have made a great progress and achievements in the whole world over the past ten years.
We can see these devices on our daily life when we deal with or see for example: displays, optical communication devices, solar cells, and data storage.
We can see this also in organic light emitting diode (OLED) technology which is used in cheap and flexible displays.
The flexibility of organic chemistry and with the recent optical technology allows us to design molecules and polymers for new applications.
In these Report we will talk about the importance of the photonic devices and their physics and how they are used in optical communications also we are talking in these report about the difficulties that faced the scientists in developing this technology and how they were able to get rid of these difficulties.
Hi speed electro-optic fibers are now very important to our life for many applications that we all use in our normal life.
We will discuss the material that these devices made of. How could we choose it? And what the best one nowadays is?
Also, we will know the challenge between lithium and polymers in this field and each one's advantages and disadvantages.
Developments in polymer optical fiber (POF) and a lot of researches have been made about it and that enable us to develop our devices in highly increasing rate.
Organic and polymeric optoelectronic devices have come a long way over the lastdecade.
We talk in this report a bout the progress that has been made in the area of optical communications. Photovoltaic solar cells are a clean energy source that could reduce dependence on fossil fuels.
Investments that have been made in these devices have set the stage for development of truly cost-effective thin film photovoltaic. After a decade of intensive research aimed at finding organic materials with large carrier mobilities, high hole mobilities have now been demonstrated in several organic system.
Work on organic field effect transistors, was initially undertaken to develop an organic thin film transistor (OTFT). Researchers have undertaken a number of efforts that use the organic semiconductor pentacene, which has allowed impressive mobilities in transistor structures to be achieved.
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Optical Communication
The invention of the Laser and detector technology fueled the great achievements in optical networks over the past ten years also with availability of high performance devices that execute functions as filtering, modulation, attenuation and switching no one can deny that next generation of optical components or instruments will be highly integrated and of the minimum cost available.
And this can be achieved by using polymers in telecom applications.
Passive and thermo-optic devices
As Polymers optical waveguides have the ability to incorporate various functions such as lithography and molding they are widely used in optical devices but there are some difficulties that have slowed polymer waveguide technology to be used in optical communications.
The most important difficulties have been:
1- High single-mode optical waveguide loss (>1 dB/cm) at wavelengths such as 1,310 nm and 1,550 nm.
2- Inability to pass some of the standard industry Telcordia tests.
3- Excessive sensitivity to the environment temperature changes.
A lot of trials have been made to fabricate low loss single-mode waveguide loss polymers which lead to great progress in this industry as shown in fig.1
Two important points helped in reaching this achievement:
1- The reduction of hydrogen content in the polymer would lead to the lowest absorption limited results.
2- High glass transition temperature (Tg) was not necessarily to create low-loss optical polymer.
Molecular vibrational overtones have high absorption strength in the 1-2 µm region and that from which the relation between absorption and hydrogen content came.
Most of the energy is absorbed in these vibrations (especially when light atoms are involved) as the energy is directly proportional to 1/õ and
µ= (m1 m2) / (m1+m2) where µ is the reduced mass of the two atoms making the vibration and m1 and m2 is the masses of the two atoms.
When atom1 is hydrogen and atom2 is carbon it is well known that m1<<m2 so it can be neglected in the addition and µ ≈ m1 which will lead to high energy absorption.
We only can reduce this energy by substituting hydrogen by halogens such as fluorine, chlorine and bromine in which they have masses that aren’t neglected when compared with that of carbon and so the absorbed energy will decrease by reducing hydrogen content in the polymer.
In the second point scientists realized that using high Tg was of low priority as using it will increase the stress on the waveguide of the polymer and will disprove the coefficient of thermal expansion (CTE)
of the polymer so they decided to use conventional dry etching processes to make waveguides and a polymer substrate was used to greatly improve the CTE match which leads at the end to lower the stress and to easily create low-loss optical polymer.
Reducing the hydrogen content and using conventional dry etching leads to lowering the waveguide loss to a level suitable for high levels of optical integration and that was achieved on the silica-on-silicon region in fig 1 and that also leads to lowering the electrical power consumption for devices that uses this technology
Electro-optic devices
The standard material for high speed electro-optic (EO) modulation has long been lithium niobate, because of its relatively large electro-optic coefficient (r=30 pm/V), good wavelength technology and relatively good stability. But because lithium niobate's electro-optic effect derives primarily from the motion of nuclei in the crystal's lattice, at high modulation rates, the efficiency of the response begins to diminish. Also the velocity mismatch between the optical and the electrical wave's lithium niobate traveling wave modulators lead to complex, narrow bandwidth designs. EO polymers, in contrast, drives their EO effect entirely from electronic motion and have virtually no velocity mismatch, making them ideal for high speed devices; instead, EO polymer modulators operating at greater 100 GHz have been demonstrated. Progress in the design of the organic dye molecules that give EO polymer their EO coefficient has lead to the demonstration of material with at telecommunications wavelengths, which make it possible to build modulators with low operating voltage.
But EO polymers are still challenged by several important issues, including high fiber-to-waveguide coupling loss, relatively poor thermal stability and low optical damage thresholds. The low optical damage thresholds are directly rated to the photo oxidative stability of the dye molecule, while the thermal stability is related to the freedom of the motion of the dye molecule in the host polymer matrix.
The dye molecule must be oriented in the host polymer by raising the polymer to a temperature near its Tg and then applying a relatively strong electric field (50-100 V/µm) to it. The film is the cooled while the field is still being applied, which results in many of the dye molecules being locked in place; over time, however, they will deorient because of the natural statistical fluctuations in the polymer. To impede deorientation, the dye molecule must be covalently bound to the polymer matrix; this can be done either prior to fabrication or through a cross-linking process during fabrication.
High propagation loss and high fiber-to-waveguide coupling losses have been difficult to address. One approach to reduce these losses is to dope the EO dye molecule into a lower loss matrix material, preferably a low molecular weight prepolymer that mixes well with the dye molecule, thereby limiting aggregation-induced scattering effects. Promising host materials in this respect include organically modified sol-gels, which have been used to demonstrate several hybrid device structures. A typical system consists of an organic component and an inorganic component which by adjusting the ratios of these components, the refractive index of the sol-gel can be adjusted so that appropriate core and cladding material can be used.
In fig.2, we show a schematic of a sol-gel based EO phase modulator developed at University of Arizona. It consists of sol-gels UV patterned to form a trench that is then filled in with the EO dye molecule doped sol-gel that comprises the core layer. The EO dye molecule is chemically modified to provide for direct incorporation into the sol-gel matrix, leading to increased thermal stability which has been verified by experiment. The inset evidences good single-mode propagation through the waveguide structure.
A variation on the theme is shown in fig.3.In this case, fiber input and output areas of the waveguide modulator consist of passive MAPTMS-based sol-gel waveguides; this configuration provides for the ability to match the waveguide mode very well to standard optical fiber. The UV-patterning capabilities of the sol-gels are used to create an adiabatic vertical taper that gradually moves the waveguide mode up into an electro-optic polymer layer and then back down into the passive sol-gel wave guide at the exit of the device.
Components and fibers for optical access
The penetration of optical fiber into homes – both from the outside (fiber-to-the-home) and on the inside (home and vehicular networks) – is finally occurring, especially in Asia but now also in the United States. While most of the current systems use conventional micro-optic and fused fiber components, intensive development of polymer-based filters, splitters and transceivers is underway.
At the same time, polymer optical fiber (POF) is increasingly being developed in high-end automobiles as well as in the final 100 meters of fiber communication networks. While wireless access solutions are receiving increasing attention, there limitations will be come more apparent as they are more widely adopted, and POF will claim its niche in home networks and enterprise applications at Gbit/s data rates and a few hundred meter length scales. The best multimode POF, one based on perfluoropolymers, has achieves losses on the order of 30 dB/km at 1,300 nm, the wavelength of choice for many access applications.
A deep understanding of the basic multimode optical propagation characteristics of both step index and graded index POF has been achieved throw a series of detailed studies. This work has shown that light propagation in multimode POF is qualitatively different from that in silica multimode optical fiber; on of the primary distinctions is that multimode POF is much more proficient at mode mixing than silica fiber, by virtue of the increased light scattering present in multimode POF. In parallel with these developments, POF has been developed for nonlinear optical applications, as it is possible to dope nonlinear dye molecule into the core of POF and thereby enable unique in-fiber devices.
Materials and devices for photonic applications
Organic and polymeric optoelectronic devices have come a long way over the last decade, In the February issue of OPN; we summarized the progress that has been made in the area of organics in optical solar cells, organic light emitting diode (OLED) technology for flexible displays and organic optical data storage. OLEDs have already achieved significant penetration in the commercial market for small, inexpensive, flexible displays.
Organic solar cells
Photovoltaic solar cells have garnered much interest in both the public and private sectors because they are a clean energy source that could potentially reduce dependence on fossil fuels.
There has been increased deployment of "first generation" photocells, which are homojunction devices based on crystalline silicon and are not cost-effective enough to provide sustainable growth in an open market environment. However, the extensive investments that have been made in these devices have set the stage for development of truly cost-effective thin film photovoltaics, and a number of companies are pursuing this goal.
For typical AM1.5 solar radiation, a power conversion efficiency of 10 percent to 20 percent would be required; AM1.5 represent the solar radiation flux when the sun is about 45o from directly over-head. To date, the best devices have achieved conversion efficiencies in the range of 5 percent, compared with the 20 percent to 25 percent available from crystalline silicon solar cell.
The problems faced by the organic solar cell effort, as well as the potential remedies, can best be understood by examining the formal expression for the composite external efficiency ηE of a solar cell:
ηE=ηaηdηc
In this equation, ηa is the photon absorption efficiency, ηd the exciton dissociation efficiency and ηc the charge collection efficiency.
The exciton dissociation efficiency ηd can be enhanced by using a charge (electron) collector like c60. Similarly, ηc in organics is generally quite low, and represents an area that has been the focus of much effort, aimed primarily at preventing the recombination of electrons and holes before they reach their respective electrodes.
After a decade of intensive research aimed at finding organic materials with large carrier mobilities, high hole mobilities have now been demonstrated in several organic system. The evaporated organic crystal pentacene has demonstrated mobilities as high as 35 cm2/V-s (volts-seconds) at room temperature as a result of an improved purification process that greatly reduces the number of traps. For low-cost, solvent-based processing, disco tic liquid crystals that self-assemble into ordered one-dimensional columnar stacks have demonstrated hole mobilities greater than 3 cm2/V-s.
The dyes consist of a perylene diimide core and 3, 4, 5-tridodecylphenyl substituents (see fig. 4, inset) that confer liquid crystalline properties to these materials over a wide temperature range (-10oC to220oC ).polarized microscopy (fig.4) and x-ray studies indicate that these molecules self-assemble into hexagonal columnar stacks.
Organic heterojunction solar cell are very similar in construction to OLED, in that they typically consist of a glass substrate, an indium tin oxide (ITO) transparent conducting electrode, a hole transport layer, an electron transport layer and a top metal electrode.
The basic structure of the device is illustrated in fig, 4.The researchers employed a glass substrate with a 1,500-AO-thick ITO anode with a sheet resistance of 15 ~/sq. In just published results, this team has now achieved 5.7 percent efficiency using two hybrid planar-mixed molecular heterojunction cells in series.
Organic electronics and field effect transistors
Work on organic field effect transistors, which began in the late 1990s, was initially undertaken to develop an organic thin film transistor (OTEF) that could be used with OLED technology to produce inexpensive, flexible displays.
The majority of OTFT work has been based on the solution processible semi conducting conjugated polymer regioregular poly (3-hexylthiophene) (P3HT), in which field effect hole mobilities of 0.1 cm2/V-s have been achieved, along with ON-OFF ratios of higher 106. Fig, 5 illustrates a typical P3HT construction using the "bottom-contact" approach. The P3HT can be deposited by either dip coating or spin coating.
Some of the drawbacks to the P3HT approach, which uses a thick gate dielectric, are that the threshold operating voltage has been in the range of 20 V and that mobilities are still relatively low. To address these issues, researchers have undertaken a number of efforts that use the organic semiconductor pentacene, which has allowed impressive mobilities in transistor structures to be achieved.
A recent report described an OTEF based on pentacene and a very thin self-assembled monolayer gate dielectric based on 1E-phenoxyoctadecyl/tricholosilane. This system has achieved field effect mobilities of 1 cm2/V-s, threshold voltages of -1.3V, ON-OFF ratios higher 106, and sub threshold swings matching those of state-of-the-art silicon metal oxide semiconductor field effect transistors.
Organic light emitting diodes
OLEDs have progressed remarkably quickly from materials research to widespread commercialization over the past 10 years. Phillips and Sony have aggressive plans lo further the technology's reach from its current use in small displays, such as those found in cell phones, to 30 inch or larger televisions by the latter part of this decade. Among the many advantages of OLED displays are their excellent color and contrast, wide viewing angle and, most important, potential for very low cost production.
Research in this area is now focused on the development of more efficient OLEDs, the demonstration of novel low-cost fabrication approaches and white light OLEDs for applications such as solid-state lighting. Recent efforts to increase efficiency have focused on the materials comprising the OLED, the mechanism for luminescence (fluorescence vs. phosphorescence), and interfaces between the charge transport layers and electrodes and out coupling of light from the high index layers comprising the OLEDs. OLEDs are generally most efficient in the green to red parts of the spectrum and work less well in the blue.
For small organic molecule OLEDs, the use of phosphorescent dopants has led to significantly increased efficiency. Among the best results for green emission arc those reported in a multilayer structure by He et al., where 45 lm/W has been obtained at 1,000 cd/m2 bright- ness and an operating voltage of 3,1V; the corresponding external quantum efficiency was 13.5 percent.
In recent work, the same team reported achieving quantum efficiencies of 19.3 percent, with 50 lm/W even at 4.000 cd/m2 brightness, 14. Highly efficient green polymer LEDs using singlet fluorescence have been described recently, 15 where 2H cd/A was achieved at 2,650 cd/m2 brightness (1.8V).
These results are expected to be improved upon by incorporating scattering panicles that outcouple light without significant spectral dispersion, as a good deal of the light generated within the OLED does not make it out of the mm due to waveguiding effects.
Many high-volume processes have been considered for the manufacture of OLEDs, including screen printing, 16 ink jet printing, thermal transfer, cold welding and direct writing.
Ink jet printing provides much simpler approach to color displays compared to conventional processes.
It is expected that these approaches and others that are still under development will be used for the mass manufacture of lightweight flexible, low-power consumption displays, electronic paper and
other novel media.
All-Optical Processing Archival Optical Data Storage
Polymers have been essential to holography since its inception, both as photopolymer recording media and for the embossed holograms on bank and credit cards that we all carry around in our wallets. Today's digital world is constantly driving demands for increased capacity storage technologies; In Phase Technologies has developed a volume holographic archival storage technology that is ready for commercial introduction.
Three-dimensional volume holographic data storage is used in photopolymer media to potentially achieve storage densities of 1 Tb/ in2 with transfer rates greater than 200 MB/s. Such densities are enabled by a novel two photopolymer chemistry approach, in addition to special techniques for making exceptionally flat (λ/10) surfaces that provide high storage densities in cubic photopolymer media with volumes of tens of mm3 .
Dynamic holography with photorefractive polymers
The photorefractive effect refers to the generation of index change associated with the combination of a spatial distribution of charges and the electro-optic effect. It was first observed in doped per-ovskite electro-optic crystal such as lithium niobate.
The key components of photorefractive polymer matrix, a sensitizing dye for absorption of incident radiation, an electro-optic chromphore and, often, a plasticizing martial that lowers the glass transition temperature Tg of the hole transport matrix.
Low Tg is desired because the electro-optic effect is actually achieved by application of an electric field that serves to orient the electro-optic
chromophores.
Photorefractive polymers now compete with and in many ways outshine their inorganic counterparts. One state of art photorefractive polymer developed by the university of Arizona and Nitto Denko Technologies is designed for operation at 975 nm, where high power, efficient solid — state laser are readily available.
The devices are 100-mm-thick films of this composite, sandwiched between ITO coated glass slides; fabrication is performed using melt process. Achieving efficient photo refractivity at 975 nm is an important accomplishment .still; it is a considerable distance from the goal of attaining efficient polymer-based photorefractive at eye-safe telecommunications wavelengths (1.55nm) that could enable both free space optical communications and aerospace applications.
In the absence of an ordinary one-photon photorefractive system suitable for this wavelength, several approaches have been developed that take advantage of two-photon absorption for the sensitization process. Clearly, significant research progress has been made and commercialization of photorefractive polymers is already underway.
Researchers have devoted significant effort to synthesizing and characterizing exceptional third order materials, and to a lesser extent, to fabricating actual devices. These materials have also significant linear and nonlinear absorption, as well as poor stability, which limit their device application. The high optical quality of the resulting films and the advantageous anomalous dispersion of the short wave length at 1.55 nm result in robust third harmonic generation (THG).
No one knows which technologies will be the winners. New organic and polymer materials will be developed with new applications giving progress for these remarkable materials.
---------------------------------------------------------------------------------------------------------Conclusion
Optoelectronics is a very important issue in the modern physics.
Optics has a great effect on our daily life and is one of the most important technologies now.
Scientists was having a great problem in the adoption of polymer waveguide technology and that was because of the high waveguide loss of the wave, the inability to pass some of the Telcordia tests and the excessive sensitivity to the temperature changes.
They were able to fix this problem by realization that high hydrogen content was the main reason of the problem and also by the realization that high glass transition was not necessary in the manufacture of low-loss optical polymer.
The fixation of the problem was by substituting the hydrogen by halogens.
Scientists used to use lithium niobate as the best material for making the high speed electro optic modulation as it has many advantages to choose it but researchers found that the electro optic polymers have best EO effect and EO polymers enable them to built modulators with low operating voltage.
Also the EO polymers better than lithium but it still has some drops on it.
Polymer optical fiber is increasingly being developed last years as there limitation will reach few hundreds meters in the wireless access and it will transfer the data at rate of Gbit/s
A series of detailed studies have been made in optical propagation characteristics that enable scientists to know that the light propagation in POF is qualitatively different from the silica optical fiber
Now, it is possible to dope nonlinear dye molecule into the core of POF and thereby enable unique in-fiber devices.
In this issue we focus on the state-of-art in organic solar cells, organic light emitting diode (OLED) technology for flexible displays and organic optical data storage.
There has been increased deployment of "first generation" photocells, which are homojunction devices based on crystalline silicon. The problems faced by the organic solar cell effort, can best be understood by examining the formal expression for the composite external efficiency ηE of a solar cell:
ηE=ηaηdηc
We can find those organic heterojunction solar cells are very similar in construction to OLED. The majority of OTFT work has been based on the solution processible semi conducting conjugated polymer regioregular poly (3-hexylthiophene) (P3HT).Organic thin film transistor (OTFT) that could be used with OLED technology to produce inexpensive, flexible displays.
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