Consumer Resistance to Innovations

Consumer resistance to innovations: the marketing problem and its solutions This article describes the major barriers which create customer resistance to innovations. This understanding is important because of the high rate of new product failure. A major cause for this is consumer resistance, although consumers are pro-innovation. It’s a normal, instinctive response of customers. This article suggests marketing strategies to overcome these barriers. Innovation resistance can appear in customers because it disrupts their established routines and they can be happy with the current status quo.The higher the discontinuity of an innovation, the higher the resistance is likely to be. Also, consumers have their own belief structure. An innovation can conflict with this which can results in resistance. There are various characteristics of innovation resistance. First, there are different groups regarding to the timing of adoption of an innovation (Innovators, Early Adopters, Early Ma jority, Late Majority and Laggards). Second, resistance varies in degree; (1) inertia (they may feel disinclined to adopt the innovation), (2) active resistance, (3) very active resistance.Third, resistance is influenced by the degree of change/discontinuity and/ or the extent to which it conflicts with consumer’s belief structure. There are two kinds of barriers which create consumer resistance. The article gives some explanation how to undo these barriers: 1. Functional barriers Usage barrier: the most common reason for customer resistance to an innovation is that it’s not compatible with existing workflows, practices or habits. The more existing habits have to change, the more the resistance will be. UNDO: develop a systems perspective to market the innovation.The innovating firm has to estimate how its new product will fit into the existing system, by looking at the whole operation. A second strategy is to integrate the innovation into the preceding activity or pro duct. Finally, overcome usage barriers by making the innovation mandatory through government legislation: lawmakers have to say they are convinced that customers will benefit from the innovation. Value barrier: there has to be a good performance-to-price value compared with product substitutes, otherwise there is no incentive for customers to change.UNDO: provide significant performance value over existing alternatives. Second, reduce the manufacturing costs of the innovation and decrease the price of the product. Third, add value to the innovation by successful positioning the product. Risk barrier: every innovation can have potential side effects that cannot be anticipated. Customers will try to postpone the innovation until they can learn more about is. There are 4 main types of risk inherent in an innovation. (1) physical risk: harm to person or property (2) economic risk: the higher the cost, the higher the perceived economic risk. 3) functional risk: customers can worry that t he innovation may not work properly because it’s not been fully tested. (4) social risk: customers can feel scared to face peer ridicule when they adopt the innovation. UNDO: Offer the innovation on a trial basis to potential customers. Second, show testimonials from experts who objectively evaluate the innovation. Third, package the innovation under a well-known name. 2. Psychological barriers Traditional barrier: innovations can create a cultural change for the customer. The more the customer deviate from traditions, the more resistance there can be.UNDO: understand and respect cultural traditions. Second, educate customers/market education. Third, use change agents. Once industry leaders adopt the innovation, the rest will do the same. Image barrier: this is a perceptual problem that arises out of stereotyped thinking. If associations the product class or where its manufactured is unfavorable, there can be created a barrier to adoption. UNDO: Suggest to people that its sil ly to carry such stereotypes and make fun of the image. Second, create a unique image for the product of service (for example using a cowboy theme).Third, associate the innovation with someone/something with a positive public image. Each of these strategies for countering the barrier for innovation may be classified into one of the five types: product strategy, communication strategy, pricing strategy, market strategy and coping strategy. The answer to successful innovation lies in understanding the causes of resistance and cope with these causes. S. Ram & Jagdish N. Sheth (1989) â€Å"Consumer resistance to innovations: the marketing problem and its solutions†, The Journal of Consumer Marketing, Vol. 6 Iss: 2; pp. 5-14

Native American Literature Essay

Native American literature is made up of two different types of literature, the oral traditions and the newer written traditions. From these two types are many different styles that make up the many different tribes of the Native American culture. Storytelling has long been an important aspect of all Native American cultures. It is through storytelling that the Native Americans are able to pass down their traditions and cultural identities. Oral traditions as well as the newer written traditions play an important part in understanding the cultural make-up of a tribe as well as establishing the historical significances of each individual tribe. The oral traditions of a tribe provided the heritage and memories of the tribe. It contained the actions, behaviors, relationships, and practices that encompass the social, economic, and spiritual identities of the people. These stories were related to each generation, keeping intact the beliefs and important aspects of the tribe. Storytellers learn their stories from other storytellers and from experience. Their stories change with the speaker and with time and with circumstance. Each story is told from a subject-position which affects the telling of the story (Leen, 1995). Storytelling is an event in which the people gather and information is shared through orations for both social and educational purposes. The same tale told in different tribes will be significantly different because each tale contains the important beliefs and stylistic differences of the individual tribe. An example of this can be found in the Trickster tales. Throughout just the Plains Indians, the Trickster takes many forms, such as the spider for the Dakota tribe or the coyote for the Kiawa tribe. However, the story or the moral of these stories is often the same, serving to teach or provide information necessary to keep the beliefs of the tribe intact. Oral traditions of storytelling change not only from tribe to tribe but also from generation to generation. Each storyteller will alter or change pieces of the traditional story to allow the stories to continue to captivate and entertain the audience. It is important that each story be relatable to the generation in which it is being told so that the information and histories found within the story will be considered relevant and be remembered. In John Roger’s Return to White Earth he speaks of his mother relating a story to him and his siblings. He writes, â€Å"As Mother talked, we children forgot all about what we were so eager to hear†¦ We listened eagerly to know what would happen next in the story. † (Return to White Earth, p. 56). That movement of stories through generations and the evolving of stories over time thread all the individuals’ experiences together to weave a shared identity. Trying to capture the essence of the oral tradition in written form is a near impossible task. Vizenor tells us, â€Å"Some of these diverse oral narratives have been translated and of course, is that written translation, even when the languages are similar, is not a representation of oral performances, and even the best translations are scriptural reductions of the rich oral nuances† (Native American Literature, 1995, p. 6). It is impossible to recreate the emotional and visual aspects associated with the oral storytelling of the American Indian. When the oral traditions were first written, the white man was usually writing these tales through an interpreter. These written works lost much of their meaning through the translation. Even though the white man had begun to recognize the historical importance of the oral tales of the Native American, they still often viewed them as primitive. With the inability of the white writer to fully understand the traditions, heritage, or social morality found within the tale, many oral traditions were presented as being silly or incredible tales told by an uneducated people. Luther Standing Bear wrote: White men who have tried to write stories about the Indian have either foisted on the public some bloodcurdling , impossible â€Å"thriller†; or if they have been in sympathy with the Indian, have written from knowledge which was not accurate and reliable. No one is able to understand the Indian race like an Indian (My People, The Sioux, p. 33). The definition of Native American literature is closely tied to what people think constitutes the essence of Native American identity. Three views stand out in this highly contested debate: those of legal bloodlines, cultural traditions, and bicultural production. According to the Annenberg Foundation, Native American literature, then: Would be those works written by someone who legally is Native American, regardless of their content or style. A second perspective links Native American identity and literature with the preservation of cultural traditions. Literary critics who rely on this view focus on aspects of â€Å"traditional† Indian culture in contemporary American Indian literature, such as the continuance of oral traditions. A third trend in Native American studies defines American Indian identity and literature not in terms of what it preserves (whether it be blood or culture), but rather as a bicultural mixture of Native and European American people and traditions (Native Voices, 2013). Luther Standing Bear believes that the only true knowledge about Native American’s lives, beliefs, and cultures must come from Native Americans immersed in cultural traditions. He says: The American Indian has been written by hundreds of authors of white blood or possibly by an Indian of mixed blood who has spent the greater part of his life away from a reservation. These are not in a position to write accurately about the struggles and disappointments of the Indian (My People, The Sioux, p. 33). Some Native Americans have argued that since their indigenous cultures have always assimilated aspects of other cultures, even aspects of other Native American cultures, to be Indian is to be bicultural, or multi-cultural. Many American Indians define themselves not primarily as â€Å"Native Americans† but as members of a specific tribe, each with their own separate history and culture, yet still very much Native American. There is a strong belief that the Native American culture is disappearing, being replaced by aspects of other cultures, particularly those of the white man. N. Scott Momaday reflects: Now that I can have her only in memory, I see my grandmother in the several postures that were and hope, having seen many things†¦ I do not speak Kiowa, and I never understood her prayers, but there was something inherently sad in the sound (The Way to Rainy Mountain, p. 63). Even though he relates strongly to his Native American ancestry, Momaday admits that aspects of his own tribe are already lost to him. The Native American literary tradition has multiple layers, encompassing the historical traditions of old while addressing the struggles and inaccuracies found today. Le Anne Howe best addresses the struggles of the Native American to find their place in the literary world as well as the struggle to maintain their own cultural identity within a society that sees them as the minority. She quotes Edward Galeano saying, â€Å"Throughout America, from north to south, the dominant culture acknowledges Indians as objects of study, but denies them as subjects of history. Indians have folklore, not culture, they practice superstitions, not religion, they speak dialects, not languages, they make crafts not arts†¦ † (Mocassins Don’t Have High Heels, p. 202). It is through these thoughts that today’s Native American writers try establish understanding of their people through their works while trying to maintain the cultural traditions of their history, passing them on for the next generations. References Annenberg Foundation. (2013). Native Voices. http://www. learner. org/amerpass/unit01/pdf/unit01ig. pdf retrieved August 19, 2013 Howe, Le Anne. (1995). Moccasins Don’t Have High Heels. Native American Literature. A Brief Introduction and anthology. New York, NY: Addison-Wesley p. 199 Leen, M. (1995). An art of saying: Joy Harjo’s poetry and the survival of storytelling. American Indian Quarterly,19(1),http://search. ebscohost. com. ezproxy. apollolibrary. com/login. aspx? direct=true&db=lkh&AN=9508220366&site=ehost-live retrieved August 19, 2013 Luther Standing Bear, (1928) My People, The Sioux. Native American Literature. A Brief Introduction and anthology. New York, NY: Addison-Wesley p. 33 Momaday, N. Scott. (1969). The Way to Rainy Mountain.

What features of Cloudstreet contribute to its textual integrity Essay Example

What features of Cloudstreet contribute to its textual integrity Essay Example What features of Cloudstreet contribute to its textual integrity Essay What features of Cloudstreet contribute to its textual integrity Essay Essay Topic: Literature True Grit Cloudstreet, written by Tim Winton, has all the requirements and qualities of a text with textual integrity. Textual integrity consists of structure, form and language which create a meaning. Cloudstreet is a family saga which employs a generally linear and sometimes circular structure. It meets the criteria of a saga in two ways. Firstly, it takes place over 4 generations, and even though the oldest generation is only embodied through memories and dreams it does help the narrative achieve resolution by bringing the reader closer to the Cloudstreet characters. Secondly the novel takes place over 25 years and speaks of events that actually took place in the time the novel was set; WWII, Nedlands Murders. Through Wintons use of chapters and sub headings the readers do not notice time passing but the important details instead. The structure is mostly linear but there are times which incorporate flashbacks or are in dream-mode. These modes are important for the reader to fully understand each character and their way of thinking. It is important that past events be recognised as they are an essential part of the story. Such is when Quick is out shooting roos and Fish comes to him in a Harvey oranges box with tomato stakes for oars. Across the still waters of the sunburnt crop wherein lies Quick Lamb breathing without help He took my bloody dog (page 201). This shows that the water essence is always with each character. The fact that the house was originally inhabited by Indigenous girls who subsequently were abused, helps the reader to understand why Spiritual Fish hears the house breathing and crying and how he talks to dark ghosts. All of Wintons characters are restless and seek meaning in a world where an explanation isnt easy to find. Their dialogue defines their class of people; the characters use idioms like carn in place of come on and the recognisable events such as the Nedlands Murders give the text, textual integrity. Winton creates viable, believable and realistic characters by using dialogue, specific language techniques and point of view. Winton uses multiple narrators; Omniscient, stream of consciousness (Fish, Oriel) and interior monologue (Rose, Lester). Its Fishs spiritual narration that lets the reader in on otherwise inaccessible information which is unattainable from the other often inarticulate characters. Inarticulateness, Australian vernacular and alcoholism, were Australian traits evident in many novels and plays written about Post-War Australia such as The Touch of Silk by Betty Roland and Summer of the Seventeenth Doll by Ray Lawler. This shows Cloudstreet was a true depiction of Post-War, poorly educated, working class, Aussie battlers and shows its viability and textual integrity. Winton enhances the textual integrity of his novel by giving the characters distinctly Australian qualities and they fit the Western Australian war time period they are placed in. The characters are all laconic and use colloquialisms such as that on page 399; Lon Lamb has taken a sickie off work Dolly is sarcastic and cynical. When Sam lost his fingers all Dolly could say was; His bloody working hand. A man could hardly pick his nose with a thumb and half a pointer. They were done for; stuffed, cactus. Thank you, Lady Luck, you rotten slut. Wintons Characters carefully chosen names evoke an image into the readers mind of what the character should be. These ideas are immediately vanquished. Take Rose, a rose is a delicate flower with fine features and a beautiful aura but Rose Pickles is a hateful woman, though beautiful she is spiteful and hard. Dolly is just like her daughter, in that she is hardened. Dolly is also an alcoholic and sex-crazed woman who spends most of her time at the pub drinking and vying for male attention. These two strong women contradict their pleasant names and also hate each other. Fish, like the creatures, observe the immeasurable depths of experience and all he can think about is the river. The surnames of the two Cloudstreet families are Pickles and Lamb. A club sandwich with so many characteristics all piled under the same roof of Cloudstreet. Cloudstreet is written using a paradox. Although Fish is intellectually disabled he is the main narrator in the book. Once favourite, who after the accident became stuck somewhere like hes half in and half out. Not all of Fish had come back and in his half state of life hes become a spiritually liberated and insightful being. His refusal to acknowledge his mothers presence is ironic because it if wasnt for her stubborn and sheer will power Fish would never have come back at all. Winton uses symbolism and themes (the black man, house and river) as a central part of Cloudstreet, especially the river. All the major life changing events take place at the river; Fish drowns then gets brought back to life, Rose and Quick acquaint themselves, Quick and his mother open up to each other and then finally Fish dies there. The setting has been given life by Winton as he explores Australia. He uses personification to show a connectedness between humanity and the land. He also uses personification when he writes about the house that moans and groans with the lost souls of the indigenous Australians. The house along with the river is the place where everything starts and finishes (circular structure). The house brings the two families together and its the place where the two families really are brought together when Quick and Rose make love to each other and Dolly and Lester have sex. Fish also understands the room that nobody else can bear to enter. He hears the house crying at night and he feels its pain. Winton also in a way personifies the pig. There is no real reason for the pig except comical relief but Fish understands it, for its his spiritual side that allows this. The pig speaks in its own tongue much like the Glossolalia of Pentecost in the New Testament. Another biblical reference. Cloudstreet has many biblical over-tones such as: The Lambs are believers in the bible, Beryl, and even a sub heading, Disciples on page 247 relates to the bible. Another theme Winton explores is women and men in this time. Australia was a male centred society and the values of women were restricted to their sexuality and ability to be nurturers. Dolly and Oriel have opposing views about the men in their lives. Dolly desires the mens attention where Oriel battles through life despite men. The women are the rock in the unstable lives of their families, they shape the children, and men and make everything function while the men create predicaments and problems. Oriel and Dolly; however different their views, are the symbols of the plight of women in a manly world. Beyond Cloudstreet lies a world that is male orientated, where within the walls of their home, everything is shaped by women. The men live in their past and memories and are largely unaware of the real world around them. E. G: Lester relieves his glories at the ANZAC club. Although when an emergency arises, the men find the answers because they are spur-of-moment characters. Such is when Lester lends Sam money for his debt. The men may be good with physical encounters but they make themselves scarce for emotion catastrophes. Cloudstreet has textual integrity. It does not chop and change its ideals throughout the novel and its main themes stay the same. Everything Cloudstreet entails is true and viable. There is no doubt that Cloudstreet has textual integrity.

Learn About NASAs Pathfinder Mission to Mars

Learn About NASAs Pathfinder Mission to Mars Meet Mars Pathfinder The Mars Pathfinder was the second of NASAs low-cost planetary Discovery missions to be launched. It was an ambitious way to send a lander and a separate, remote-controlled rover to the surface of Mars and demonstrated a number of innovative, economical, and highly effective approaches to spacecraft and mission design of a planetary landing mission. One reason it was sent was to show the feasibility of low-cost landings at Mars and eventual robotic exploration.   Mars Pathfinder was launched on a Delta 7925 on December 4, 1996. The spacecraft entered the Martian atmosphere on July 4, 1997 and took atmospheric measurements as it descended. The entry vehicles heat shield slowed the craft to 400 meters per second in about 160 seconds. A 12.5-meter parachute was deployed at this time, slowing the craft to about 70 meters per second. The heat shield was released 20 seconds after parachute deployment, and the bridle, a 20-meter-long braided Kevlar tether, deployed below the spacecraft. The lander separated from the back shell and slid down to the bottom of the bridle over about 25 seconds. At an altitude of about 1.6 kilometers, the radar altimeter acquired the ground, and about 10 seconds before landing four air bags inflated in about 0.3 seconds forming a 5.2-meter-wide diameter protective ball around the lander. Four seconds later at an altitude of 98 meters the three solid rockets, mounted in the backshell, fired to slow the descent, and the bridle was cut 21.5 meters above the ground. That released the airbag-encased lander, which dropped to the ground. It bounced about 12 meters into the air, bouncing at least another 15 times and rolling before coming to rest approximately 2.5 minutes after impact and about a kilometer from the initial impact site. After landing, the airbags deflated and were retracted. Pathfinder opened its three metallic triangular solar panels (petals) 87 minutes after landing. The lander first transmitted the engineering and atmospheric science data collected during entry and landing. The imaging system obtained views of the rover and immediate surroundings and a panoramic view of the landing area. Eventually, the landers ramps were deployed and the rover rolled onto the surface.   The Sojourner Rover The Pathfinders rover Sojourner was named  in honor of Sojourner Truth, a 19th-century abolitionist and champion of womens rights. It operated for 84 days, 12 times longer than its designed lifetime of seven days. It investigated rocks and soil in the area around the lander.   The bulk of the landers task was to support the rover by imaging rover operations and relaying data from the rover to Earth. The lander was also equipped with a meteorology station. Over 2.5 meters of solar cells on the lander petals, in combination with rechargeable batteries, powered the lander and its onboard computer. Three low-gain antennas extended from three corners of the box and a camera extended up from the center on a 0.8-meter high pop-up mast. Images were taken and experiments performed by the lander and rover until 27 September 1997 when communications were lost for unknown reasons. The landing site in the Ares Vallis region of Mars is at 19.33 N, 33.55 W. The lander has been named the Sagan Memorial Station, and it operated nearly three times its design lifetime of 30 days. Pathfinders Landing Spot The Ares Vallis region of Mars is a large flood plain near Chryse Planitia. This region is one of the largest outflow channels on Mars, the result of a huge flood (possibly an amount of water equivalent to the volume of all five Great Lakes) over a short period of time flowing into the martian northern lowlands. The Mars Pathfinder mission cost approximately $265 million including launch and operations. Development and construction of the lander cost $150 million and the rover about $25 million. Edited and updated by Carolyn Collins Petersen.

Privatization of Prisons Essay Example | Topics and Well Written Essays - 750 words

Privatization of Prisons - Essay Example Private prisons purpose was to house inmate thus reduce congestion in public prisons then paid by the government using taxpayer prison revenue. Despite privatization being profit generation source, private prisons have benefits and disadvantages to the public and prisoners. As a result, the essay will deduce and come up with the judgment on whether the government should embrace private prisons or abolish the idea. Positively, private prisons reduce congestion of prisoners in public prisons in United States. Mason indicates that many public prisons in America house inmate more than the required number because of lack of an alternative place to keep the criminals (4-7). For instance, some of California prisons facility have about 300, 000 prisoner compared to the required number of 170,000 prisoners. Additionally, such high numbers of prisoners hinders effective rehabilitation of prisoners because of lack of sufficient rehabilitation space, tools, and personnel found in the prisons. Fo r this reason, the federal and state government adopting the idea of private prisons was important. This will reduce the congestion in public prisons and ensure effective rehabilitation of all the prisoners hence curbing the level of prison violence. Private prisons served as a cost effective method in the budget of federal and state government in the United States. ... Mason posits that states with higher number of private prisons enjoy lower cost of housing prisoners in the state (8-10). Therefore, with abundant prison facilities in United States, criminal rate may reduce in most states. This is because there will be no waste of time in delay of convicting criminals because of lack of space in public prisons. However, private prisons also have several disadvantages. There are high reports of violence and mistreatment of prisoners in private facilities. Most private prisons have limited incentives that could curb recidivism and violence among prisoners (American Civil Liberties Union 23). Moreover, most private prisons focuses on profit creation and not the safety and decent living of the prisoners hence hindering the process of rehabilitation and accelerating violence level among prisoners. In addition, the management team in private prisons may not have sufficient training personnel to handle cases of violence and recidivism hence, becoming a thr eat in the rehabilitation of prisoners. Thus, such reasons may inhibit the number of prisoners admitted in private prisons while may affect the profit generation goal of the private prison. Despite the private prison goal being profit generation to the public and shareholders, most private prisons experience limited profits level. The amount of profit earned causes the private prisons to overlook the intensity of training the prison security personnel (American Civil Liberties Union 20-22). Therefore, this affects the way in which the private prison personnel may handle rude prisoners and curb the level of prisoners escaping experienced in from private prisoners. Moreover, the surrounding community may not benefit from the revenue gained by the private prison in town. Hence,

Topic will be based on which book you choose Term Paper

Topic will be based on which book you choose - Term Paper Example The constant commercial pommeling that teenagers are exposed to regularly makes them give in to the pressures of being considered cool by their peers, which in turn works in the corporations’ favor (Quart 32). In addition to all the above, the growing need to incorporate sex in some of these advertisements is making its way into the lives of teenagers and younger children. Recent sources in different organizations can attest to this. This paper will examine aspects of the book and relate it to an advertisement, and the impact it might have on its target market. At a particularly young age, teenagers and pre-teenagers are forced into different loyalty schemes by corporations that use their peers to market and sell their products. In Branded, the author brings into account the fact that corporations are having a higher stake in schools than they possibly should. In the past, countless groups, including parents and politicians, were at the forefront trying to resist the notion of advertising in schools. However, the current situation of sponsorship that most corporations boast of is making this a difficult task. Corporations, at present, have more shares in schools than any other party, and they are using this to their advantage. Brand-afflicted teenagers are now everywhere, but there might be time to save some of the young ones from the menace that is branding (Quart 47). Target audience Victoria’s Secret is one brand that is affiliated with the imagery of sex and sex appeal. In most of their advertisements, models and people wearing their lingerie appeal to the adults and market that tends to grasp the concept of being sexy. However, lately the brand has been moving toward a different market altogether. In a recent advertisement, â€Å"Bright Young Things,† the brand has brought to light their aim for a younger market. Teenagers and pre-teens who want to be considered cool are now looking toward Victoria’s Secret line of lingerie in a bid to look older, or more sophisticated. Parents are being forced to give in to the pressure of their young children who demand such clothing, and it is obvious that once children catch on to this brand, there is no turning back. It is next to impossible for the children not to remain loyal to the brand, if they start when young (Lutz 1). Aspects of the demographic The income and lifestyle of teenagers and pre-teens cannot be questioned as it is not impossible to pinpoint exactly where they get their income, or lifestyle choices. These aspects are influenced directly by their parents, immediate family members, and even friends. The little income that teenagers may receive is probably from allowances that parents offer their children once they partake of certain activities. However, as the issue of branding and marketing takes precedence in most schools and areas (Quart 53), teenagers are looking for different avenues which can work toward increasing their capital. It is this aspec t that sheds light on why teenagers look for different jobs during the summer and spring-breaks. More money means more savings, which ultimately leads to lifestyle autonomy. Parents have little or no say in the matter once their children start buying their own clothes, using their hard-earned money. Alissa Quart takes the reader into the processes used by corporations to attract teens and pre-teens into their branding world, and why it is particularly easy for teens to succumb to these schemes. The first aspect that comes into play is the use of people their own age to

Mechanisms for Optical Limiting

Mechanisms for Optical Limiting Chapter 2 2.1. Reverse Saturable Absorption In the mid 1960s shortly after the invention of the laser, many researchers were investigating dyes for potential application to Q-switching of the laser cavity. For this application, dyes were sought that would bleach to transparency under intense illumination (saturable absorbers). Guiliano and Hess [2a] in 1967 were investigating vat dyes and their modified cousins and noted some examples that not only did not bleach to transparency but instead darkened at high intensities. This was the first recognition of the property of reverse saturable absorption (RSA). Reverse saturable absorption generally arises in a molecular system when the excited state absorption cross section is larger than the ground state cross section. The process can be understood by considering a system that is modeled using three vibronically broadened electronic energy levels, as shown in figure 2.1. The cross section for absorption from the ground state 1 is à ¯Ã‚ Ã‚ ³1. à ¯Ã‚ Ã‚ ³2 is the cross section for absorption from the first excited state 2 to the second excited state 3. The lifetime of the first excited state is à ¯Ã‚ Ã‚ ´2 (seconds). Figure 2.1: Three level and Four level models for RSA As light is absorbed by the material, the first excited state begins to become populated and contributes to the total absorption cross section. If à ¯Ã‚ Ã‚ ³2 is smaller than à ¯Ã‚ Ã‚ ³1, then the material becomes more transparent or ‘bleaches’ i.e. it is a saturable absorber. If à ¯Ã‚ Ã‚ ³2 is larger than à ¯Ã‚ Ã‚ ³1, then the total absorption increases, and the material is known as a reverse saturable absorber. This behavior is shown in figure 2.2 Figure 2.2: Plot of the incident intensity versus the transmitted intensity of a typical three level RSA material. The change in intensity of a beam as it propagates through the material is: , (2.1) Where z is the direction traversed, NT is the total number of active molecules per area in the slice dz, N2 is the population of level 2 and the population of level 3 has been neglected. Initially, the material obeys Beer’s law when 2 is unpopulated, and the transmission is constant as the incident fluence is increased. The slope is given by. At a sufficiently high fluence, however, the first excited state 2 becomes substantially populated and in the limit of complete ground state depletion the slope again becomes constant at the new value of. The optical limiting action is not truly limiting, as the fluence, which is transmitted, is still increasing with increasing incident fluence, but it does so more slowly. If the ratio à ¯Ã‚ Ã‚ ³2/à ¯Ã‚ Ã‚ ³1, is sufficiently large, however, the new transmission will be small and in a properly designed system the dynamic range of the sensor will be greatly extended. The three level diagrams describe the simplest case for RSA materials but can generally only be applied for subnanosecond pulses and under circumstances such that transitions from the second excited state are negligible. The energy states involved in three level materials usually consists of singlet states and the transitions are all allowed. The transition cross sections are therefore large, but a disadvantage is that de-excitation is rapid (à ¯Ã‚ Ã‚ ´2 is small). This necessitates larger intensities for long pulses to activate the nonlinearity through populating the excited electronic state. Fortunately, on longer timescales in some systems, significant intersystem crossing to other states can occur from the first excited state. In this case the five level diagrams shown in figure 2.1 is applicable. The excited state 4 is usually a triplet or other long-lived state, and for long pulses it can act as a metastable state that accumulates population during the pulse. The lifetime of 4 gives an indication of the maximum pulse width for which the material is efficient to act as an optical limiter. Pulses with duration longer than the metastable state allow some of the metastable molecules generated by the leading edge of the pulse to decay to the ground state before the trailing edge have passed, thereby reducing the RSA. In most systems, à ¯Ã‚ Ã‚ ´3 and à ¯Ã‚ Ã‚ ´5 are very small and significant populations of 3 and 5 do not accumulate. Therefore, N3 and N5 can be set to zero, considerably simplifying the dynamical equations describing. The equations representing the full five level models are given below by: (2.2) (2.3) (2.4) (2.5) (2.6) (2.7) and (2.8) Where hà ¯Ã‚ Ã‚ ® is the energy per photon, I is the intensity of the pulse and stimulated emission has been neglected. The latter assumes that optical coupling to the excited states is well above the bottom of the vibronic manifolds and that relaxation from the optically-coupled states to the bottom of the manifolds occurs on a time scale that is much shorter than the pulse duration. To completely understand the response of an RSA device, these equations must be solved as the pulse propagates through the material. The material parameters necessary to solve the equations are à ¯Ã‚ Ã‚ ³1, à ¯Ã‚ Ã‚ ³2, à ¯Ã‚ Ã‚ ³4, à ¯Ã‚ Ã‚ ´2, à ¯Ã‚ Ã‚ ´4 and à ¯Ã‚ Ã‚ ´24. For optimum optical limiting performance, certain parameters need to be maximized. The ratio of the excited state absorption to the ground state, à ¯Ã‚ Ã‚ ³2/à ¯Ã‚ Ã‚ ³1, à ¯Ã‚ Ã‚ ³4/à ¯Ã‚ Ã‚ ³1 should be large to minimize the transmission of the limiter at high incident intensity. For maximum efficiency, the lifetime of the triplet state (à ¯Ã‚ Ã‚ ´2) and the intersystem crossing rate l/à ¯Ã‚ Ã‚ ´24 should be large to populate the triplet state and maintain the population throughout the pulse. By the mechanism of RSA we get better performance on optical limiting. 2.2. Two-Photon Absorption (TPA): Two-photon absorption (TPA) can also be used in a manner similar to RSA to construct optical limiters. In contrast with reverse saturable absorption, TPA is an instantaneous nonlinearity that involves the absorption of photon from the field to promote an electron from its initial state to a virtual intermediate state, followed by the absorption of a second photon that takes the electron to its final state. Since the intermediate state for such transitions is virtual, energy need not be conserved in the intermediate state but only in the final state. The mechanism of TPA can be thought of in terms of the three level RSA model for the case where the lifetime of the intermediate state approaches zero and the ground state absorption is extremely low (highly transparent). The intensity of the beam as it traverses the material is: (2.9) Where z is the linear absorption coefficient and à ¯Ã‚ Ã‚ ¢ is the TPA coefficient which is related to the imaginary part of à ¯Ã‚ Ã‚ £(3) by the equation (SI units): (2.10) Here, à ¯Ã‚ Ã‚ · is the circular frequency of the optical field, n0 is the linear index of refraction, and c is the speed of light in vacuum. The solution to the propagation equation for à ¯Ã‚ Ã‚ ¡= 0 (transparent material at low intensities) is given by (2.11) Where L is the sample length. This clearly demonstrates that the output intensity decreases as the input intensity increases, exactly the behavior that is desired for an optical limiter. The strength of this reduction is explicitly dependent on the TPA coefficient, the incident intensity and the sample thickness. For TPA, the material response is of the order of an optical cycle and is, therefore, independent of the optical pulse length for a fixed intensity. The device will respond virtually instantaneously to the pulse. On the other hand, because of the limited magnitude of à ¯Ã‚ Ã‚ ¢ in existing materials, high intensities are required to realize significant TPA. Since the intensity is essentially the energy density divided by the pulse duration, short pulses are required to achieve limiting with TPA for energy densities that may be high enough to damage an optical sensor. 2.3. Free-Carrier Absorption: This type of limiting occurs in semiconductor materials. Once carriers are optically generated in a semiconductor, whether by single photon or two-photon absorption, these electrons (holes) can be promoted to states higher (lower) in the conduction (valence) band by absorbing additional photons. This process is often phonon assisted, although depending on the details of the band structure and the frequency of the optical excitation, it may also be direct. The phonon assisted phenomenon is referred to as free-carrier absorption, and it is analogous to excited-state absorption in a molecular system. It is clearly an accumulative nonlinearity, since it depends on the buildup of carrier population in the bands as the incident optical pulse energy is absorbed. Free-carrier absorption always plays some role in the operation of a semiconductor limiter, if the excitation process results in the generation of significant free carrier populations in the bands. While it certainly contributes to the limiter performance and its inclusion is important in the precise modeling of the response of such devices, just as in the case of TPA, its importance typically pales in comparison with nonlinear refractive effects, whether the carriers are generated by single photon or two photon transitions. 2.4. Nonlinear Refraction Optical limiters based on self focusing and defocusing form another class of promising devices. The mechanism for these devices may arise from nonlinear refraction associated with carrier generation by either linear or two photon absorption in a semiconductor. Both self focusing and defocusing devices operate by refracting light away from the sensor as opposed to simply absorbing the incident radiation. Compared to strictly absorbing devices, these limiters can, therefore, potentially yield a larger dynamic range before damage to the limiter itself. Figure 2.3 (a) shows the typical device configuration for a self defocusing limiter, while figure 2.3 (b) shows a similar device based on self focusing. A converging lens is used to focus the incident radiation so it passes through the nonlinear medium. This lens provides optical gain to the system, allowing the device to activate at low incident intensities. The output passes through an aperture before impinging on the detector. At low input levels, the nonlinear medium has little effect on the incident beam, and the aperture blocks an insignificant portion of the beam, thus allowing for a low insertion loss for the device. When nonlinear refraction occurs, however, the nonuniform beam profile within the medium results in the generation of a spatially nonuniform refractive index. This acts as either a negative or positive lens, depending on the sign of the refractive nonlinearity, causing the incident beam to either defocus or focus. Figure 2.3: (a) Typical self defocusing optical limiter configuration (b) Typical self focusing optical limiter configuration. In a properly designed system, this self lensing results in significant energy blocked by the system aperture, thereby protecting the sensor. The location of the nonlinear medium is critical to the operation of the refractive limiting device. A self-focusing limiter works best if the nonlinear medium is placed approximately a Rayleigh range before the intermediate focus of the device. When the focusing lens is induced the effective focal length of the device is reduced, and hence a larger beam appears at the exit aperture. For a self-defocusing material, the optimum geometry is approximately one Rayleigh range after the focus. This geometry dependence can be exploited to determine not only the sign of the nonlinear refraction in a given medium, but the magnitude as well. This is the principle behind the so-called Z-scan technique, which has been pioneered by Van Stryland and coworkers [2b,2c]. The technique consists of moving the nonlinear medium through the focal region of a tightly focused beam while measuring the transmittance through an aperture placed in the far field of the focal plane. When the medium is far before the focal plane, no self-lensing occurs. As the medium approaches the focal plane, the high intensity begins to induce a lens in the medium. For a negative nonlinearity, this lens tends to collimate the beam, thereby increasing the transmittance through the aperture. Near the focal plane, even though the intensity is highest, the influence of the induced lens is minimized, resulting in a transmittance comparable to the linear transmittance. This is similar to placing a thin lens at the focus of a beam; this results in minimal effect on the far field beam pattern. As the sample is moved beyond the focal plane, the negative lens tends to increase the beam divergence, resulting in a decrease in the aperture transmittance. As the medium is moved still farther from focus, the intensity again becomes weak enough that the induced lensing is negligible. This sequence results in a change in transmittance with a characteristic peak, followed by a null, followed by a valley as the sample is moved from the input lens, through focus, toward the output lens. For a positive nonlinearity, the pattern consists of a valley, a null, and then a peak. Thus, the sign of the nonlinearity is readily determined. While nonlinear absorption has been neglected in this discussion, if present, it must also be accounted for. This is readily done by removing the aperture in the limiter and collecting all the light transmitted by the nonlinear material. This measurement is then insensitive to nonlinear refraction. The response in this case is a valley symmetrically located about the focal plane. It should be noted that nonlinear absorption and induced scattering cannot be distinguished by this technique. The general shape of the Z-scan for a positive index change, negative index change, and a nonlinear absorber or scatterer is shown in figure 2.4 . Figure 2.4: Schematic representation of z-scan results for a negative refractive nonlinearity (dashed curve) and a positive refractive nonlinearity (dotted curve). Both curves have been corrected for absorption. The solid curve shows the result of removing the aperture from the measurement apparatus and collecting all the transmitted light, thus isolating the nonlinear absorption [1e]. 2.5. Induced Scattering Scattering roots from interaction of light with small centers which may be physical particles or simple interfaces sandwiched between non-excited and excited molecular groups. The size of the scattering centers determines whether the scattering will be quite directional or reasonably uniform. Transmission of a medium, for a given solid angle, decreases when scattering centers are induced in the medium by an optical signal. Therefore, this phenomenon of scattering induced by optical signal may be applied to manufacture of optical limiters for sensor protection. Optical limiters based on induced scattering are usually focused on liquid media, as the phenomenon is usually reversible in these media. That is to say, the liquid in the excited state can return to equilibrium with ease in the absence of chemical or structural decomposition. However, in solids, usually irreversible decomposition processes generate the scattering centers which can lead to degradation in the device’s lin ear operation. When light is incident on a particle, the electric charges within the particle oscillate due to its interaction with the electric field. Radiations are then caused by the oscillation. In 1899, Lord Rayleigh originally presented the analytic expression and theory of the elastic scattering of light from particles with dimensions smaller than the wavelength of light. Rayleigh scattering is the name given to the phenomenon. This applies only to particles whose dimensions are quite smaller than the wavelength of light or which are non-absorbing. However, in 1908, Mie developed a theory for particles with dimensions comparable to the wavelength of light or greater [2d]. The transmitted intensity equations of the Mie scattering are notably more intricate than of Rayleigh scattering. In Mie scattering, a bigger percentage of the scattered radiation is in forward direction as the size of the scattering particles increases, implying that limiting based on Mie scattering will not be as effectiv e as Rayleigh scattering. 2.6. Photorefraction Two devices, namely coherent-beam excisor and the beam fanning limiter based on the photorefractive effect are used to limit coherent optical radiation. Materials showing photorefraction should have a nonzero χ(2). The traditional photorefractive mechanism is based on the photorefractive crystal which possesses deep levels that can be excited optically to generate free charge in the conduction or valence band. In a material showing photorefraction, when two coherent beams interfere, additional mobile charge are generated at the peaks of the intensity pattern than at the valleys through photoexcitation of the deep levels of the crsytal. These charges which are photoexcited at the peaks diffuse into the valleys ensuing a variation of charge spatially, in correspondence to the material’s interference pattern. These charges results in an electrostatic space-charge field which gives rise to a change in refractive index through the electro-optic effect in a properly oriented cry stal. Energy coupling and energy exchange can then be achieved between the two beams through the grating generated, which is 90 degrees phase shifted from the intensity of the photon field. A high intensity coherent beam when incident singly on a photorefractive crystal, the energy can be coupled into a large amount of low intensity scattered beams. Fields with new wave vectors are generated inside the crystal by the scattering of the incident beam at the crystal imperfections. The photorefractive gratings are then produced by the interference of the incident field with these scattered fields. Optical signal can later be coupled from the incident beam to the scattered beams through diffraction from these gratings. The light gets preferentially scattered to one side of the crystal as there is a preferred direction of energy transfer for photorefractive gratings which is determined by the direction of the c-axis of the crystal and the charge carriers’ sign. This photorefractive beam fanning phenomenon can be quite efficient in reducing the intensity of the transmitted beam. Construction of an optical limiter using this beam fanning process has been demonstrated by Cronin-Golomb and Yariv [2e]. The photorefractive excisor is another device which provides a weak seed beam to interfere with the incident beam. It is assembled to protect the sensor in such a way that the photorefractive grating produced by the interference of the primary beam with the seed beam at high intensities couples energy from the strong incident beam to the weak seed beam. The speed and efficiency of the device is thus improved. 2.7. Summary All of the nonlinear phenomena discussed above can be used for optical limiting, and figure 2.5 schematically illustrates the application of some of these processes. Figure 2.5 (a) depicts the use of induced absorption, such as reverse saturable absorption, two-photon absorption, and free-carrier absorption. Figures 2.5(b) and 2.5(d) represent, respectively, a self-defocusing limiter, self-focusing limiter, and an induced scattering limiter. Finally, figures 2.5(e) and 2.5(f) illustrate a photorefractive beam fanning limiter and a photorefractive excisor device. While it is often the case that any given material will exhibit multiple nonlinear properties, for simplicity the effects of each individual process have been separately depicted in figure 2.5. Figure 2.5: Some optical limiters based on different mechanisms (a) an induced absorption limiter (b) Self defocusing limiter (c) Self focusing limiter (d) Induced scattering limiter (e) Beam fanning limiter (f) Photorefractive excisor device [1e].