光屈折率の、フォトリフラクティブ

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/11/09 16:35:04」(JST)

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The photorefractive effect is a nonlinear optical effect seen in certain crystals and other materials that respond to light by altering their refractive index.^[1] The effect can be used to store temporary, erasable holograms and is useful for holographic data storage.^[2]^[3] It can also be used to create a phase-conjugate mirror or an optical spatial soliton.

Mechanism

The photorefractive effect occurs in several stages:

A photorefractive material is illuminated by coherent beams of light. (In holography, these would be the signal and reference beams). Interference between the beams results in a pattern of dark and light fringes throughout the crystal.
In regions where a bright fringe is present, electrons can absorb the light and be photoexcited from an impurity level into the conduction band of the material, leaving an electron hole (a net positive charge). Impurity levels have an energy intermediate between the energies of the valence band and conduction band of the material.
Once in the conduction band, the electrons are free to move and diffuse throughout the crystal. Since the electrons are being excited preferentially in the bright fringes, the net electron diffusion current is towards the dark-fringe regions of the material.
While in the conduction band, the electrons may with some probability recombine with the holes and return to the impurity levels. The rate at which this recombination takes place determines how far the electrons diffuse, and thus the overall strength of the photorefractive effect in that material. Once back in the impurity level, the electrons are trapped and can no longer move unless re-excited back into the conduction band (by light).
With the net redistribution of electrons into the dark regions of the material, leaving holes in the bright areas, the resulting charge distribution causes an electric field, known as a space charge field to be set up in the crystal. Since the electrons and holes are trapped and immobile, the space charge field persists even when the illuminating beams are removed.
The internal space charge field, via the electro-optic effect, causes the refractive index of the crystal to change in the regions where the field is strongest. This causes a spatially varying refractive index grating to occur throughout the crystal. The pattern of the grating that is formed follows the light interference pattern originally imposed on the crystal.
The refractive index grating can now diffract light shone into the crystal, with the resulting diffraction pattern recreating the original pattern of light stored in the crystal.

Application

The photorefractive effect can be used for dynamic holography, and, in particular, for cleaning of coherent beams. For example, in the case of a hologram, illuminating the grating with just the reference beam causes the reconstruction of the original signal beam. When two coherent laser beams (usually obtained by splitting a laser beam by the use of a beamsplitter into two, and then suitably redirecting by mirrors) cross inside a photorefractive crystal, the resultant refractive index grating diffracts the laser beams. As a result, one beam gains energy and becomes more intense at the expense of light intensity reduction of the other. This phenomenon is an example of two-wave mixing. It is interesting that in this configuration, Bragg diffraction condition is automatically satisfied.

The pattern stored inside the crystal persists until the pattern is erased; this can be done by flooding the crystal with uniform illumination which will excite the electrons back into the conduction band and allow them to be distributed more uniformly.

Photorefractive materials include barium titanate (BaTiO₃), lithium niobate (LiNbO₃), vanadium doped zinc telluride (ZnTe:V), organic photorefractive materials, certain photopolymers, and some multiple quantum well structures.

There were even claims that an amplifier based on photorefractive crystals can have less than the minimum quantum noise that is typical for optical amplifiers of any kind.^[who?]

References

^ J. Frejlich (2007). Photorefractive materials: fundamental concepts, holographic recording and materials characterization. ISBN 978-0-471-74866-3.
^ Peter Günter, Jean-Pierre Huignard, ed. (2007). Photorefractive materials and their applications. ISBN 978-0-387-34443-0.
^ Pochi Yeh (1993). Introduction to photorefractive nonlinear optics. Wiley series in pure and applied optics. ISBN 0-471-58692-7.

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English Journal

The efficacy of co-treatment with suberoylanilide hydroxamic Acid and mitomycin C on corneal scarring after therapeutic keratectomy: an animal study.

Woo JE1, Park WC, Yoo YH, Kim SW.Author information 1Department of Ophthalmology, Ulsan University of College of Medicine , Ulsan , South Kore and.AbstractAbstract Purpose: This study was undertaken to evaluate the efficacy of co-treatment with suberoylanilide hydroxamic acid (SAHA) and mitomycin-C (MMC) on corneal fibrosis in an in-vivo model. Materials and methods: We examined the effect of co-treatment with MMC and SAHA, a potent histone deacetylase inhibitor (HDACi), on the corneal fibrosis and haze produced in rats following photorefractive keratectomy (PRK). We further examined the toxicity of this co-treatment on human corneal epithelial (HCE) cells in-vitro. Results: The combination of MMC and SAHA efficiently suppressed corneal fibrosis and haze following PRK. At the doses tested, SAHA had no inhibitory effect on human corneal epithelial cell viability, whereas MMC-treated cells showed increased apoptotic changes in Western blot testing and a fluorescence-activated cell sorting system. Co-treatment with SAHA and a low-dose of MMC minimized corneal haze after PRK and produced a significantly lower level of cytotoxic effects than treatment with MMC alone. Conclusions: Treatment with SAHA in combination with a low dose of MMC could be a novel and effective therapeutic strategy to suppress the proliferation of corneal myofibroblasts, thereby inhibiting corneal fibrosis or haze with minimal ocular surface toxicity.
Current eye research.Curr Eye Res.2014 Apr;39(4):348-58. doi: 10.3109/02713683.2013.859272. Epub 2014 Jan 8.
Abstract Purpose: This study was undertaken to evaluate the efficacy of co-treatment with suberoylanilide hydroxamic acid (SAHA) and mitomycin-C (MMC) on corneal fibrosis in an in-vivo model. Materials and methods: We examined the effect of co-treatment with MMC and SAHA, a potent histone deacetylas
PMID 24401036

Inhibitory Effects of PPARγ Ligands on TGF-β1-Induced Corneal Myofibroblast Transformation.

Jeon KI1, Kulkarni A2, Woeller CF3, Phipps RP4, Sime PJ5, Hindman HB6, Huxlin KR7.Author information 1Flaum Eye Institute, University of Rochester, Rochester, New York.2Department of Medicine, University of Rochester, Rochester, New York.3Department of Environmental Medicine, University of Rochester, Rochester, New York.4Flaum Eye Institute, University of Rochester, Rochester, New York; Department of Medicine, University of Rochester, Rochester, New York; Department of Environmental Medicine, University of Rochester, Rochester, New York.5Department of Medicine, University of Rochester, Rochester, New York; Department of Environmental Medicine, University of Rochester, Rochester, New York.6Flaum Eye Institute, University of Rochester, Rochester, New York; Center for Visual Science, University of Rochester, Rochester, New York.7Flaum Eye Institute, University of Rochester, Rochester, New York; Center for Visual Science, University of Rochester, Rochester, New York. Electronic address: huxlin@cvs.rochester.edu.AbstractCorneal scarring, whether caused by trauma, laser refractive surgery, or infection, remains a significant problem for humans. Certain ligands for peroxisome proliferator-activated receptor gamma (PPARγ) have shown promise as antiscarring agents in a variety of body tissues. In the cornea, their relative effectiveness and mechanisms of action are still poorly understood. Here, we contrasted the antifibrotic effects of three different PPARγ ligands (15-deoxy-Δ12,14-prostaglandin J2, troglitazone, and rosiglitazone) in cat corneal fibroblasts. Western blot analyses revealed that all three compounds reduced transforming growth factor (TGF)-β1-driven myofibroblast differentiation and up-regulation of α-smooth muscle actin, type I collagen, and fibronectin expression. Because these effects were independent of PPARγ, we ascertained whether they occurred by altering phosphorylation of Smads 2/3, p38 mitogen-activated protein kinase, stress-activated protein kinase, protein kinase B, extracellular signal-regulated kinase, and/or myosin light chain 2. Only p38 mitogen-activated protein kinase phosphorylation was significantly inhibited by all three PPARγ ligands. Finally, we tested the antifibrotic potential of troglitazone in a cat model of photorefractive keratectomy-induced corneal injury. Topical application of troglitazone significantly reduced α-smooth muscle actin expression and haze in the stromal ablation zone. Thus, the PPARγ ligands tested here showed great promise as antifibrotics, both in vitro and in vivo. Our results also provided new evidence for the signaling pathways that may underlie these antifibrotic actions in corneal fibroblasts.
The American journal of pathology.Am J Pathol.2014 Mar 17. pii: S0002-9440(14)00101-1. doi: 10.1016/j.ajpath.2014.01.026. [Epub ahead of print]
Corneal scarring, whether caused by trauma, laser refractive surgery, or infection, remains a significant problem for humans. Certain ligands for peroxisome proliferator-activated receptor gamma (PPARγ) have shown promise as antiscarring agents in a variety of body tissues. In the cornea, their rel
PMID 24650561

Past and present of corneal refractive surgery: A retrospective study of long-term results after photorefractive keratectomy and a prospective study of refractive lenticule extraction.

Vestergaard AH.Author information Faculty of Health Science, University of Southern Denmark, Odense, Denmark; Department of Ophthalmology, Odense University Hospital, Odense, Denmark.AbstractSurgical correction of refractive errors is becoming increasingly popular. In the 1990s, the excimer laser revolutionized the field of corneal refractive surgery with PRK and LASIK, and lately refractive lenticule extraction (ReLEx) of intracorneal tissue, using only a femtosecond laser, has become possible. Two new procedures were developed, ReLEx flex (FLEX) and ReLEx smile (SMILE). Until this thesis, only a few long-term studies of PRK with a relatively limited number of patients had been published; therefore, this thesis intended to retrospectively evaluate long-term outcomes after PRK for all degrees of myopia for a large number of patients. Furthermore, a prospective contralateral eye study comparing FLEX and SMILE, when treating high to moderate degrees of myopia, had not been performed prior to this study. This was the second aim of this thesis. In the first study, results from 160 PRK patients (289 eyes) were presented. Preoperative spherical equivalent ranged from -1.25 to -20.25 D, with 78% having low myopia (<-6 D). Average follow-up time was 16 years (range 13-19 years), making this the longest published follow-up study on PRK patients. Outcomes from eyes with low myopia were generally superior to outcomes from eyes with high myopia, at final follow-up. Seventy-two percent were within ±1.00 D of target refraction, as compared to 47% of eyes with high myopia. However, results from a subgroup of unilateral treated PRK patients indicated that refraction at final follow-up was affected by myopic progression. Fifty percent of eyes with low myopia had uncorrected 20/20 distance visual acuity or better, as compared to 22% of eyes with high myopia. Haze did not occur if attempted corrections were <-4 D, and only eyes with high myopia lost two lines or more of CDVA (corrected distance visual acuity). Eighty-one per cent were satisfied or very satisfied with their surgery. Conclusion: The results support the continued use of the excimer laser for corneal surface ablation as a treatment option for correction of low degrees of myopia, and as the treatment of choice for subgroups of refractive patients (thin corneas, etc.). The results also highlight that treatment of higher degrees of myopia with standard PRK should only be done today under special circumstances, due to low refractive predictability, and high risk of corneal haze. Technological advances since then should be taken into account when comparing these results with contemporary techniques. In the second study, 35 patients were randomized to receive FLEX in one eye and SMILE in the other. Preoperative spherical equivalent refraction ranged from -6 to -10 D with low degrees of astigmatism. A total of 34 patients completed the 6 month follow-up period. Refractive and visual outcomes were very similar for the two methods, as well as tear film measurements and changes in corneal biomechanics. Ninety-seven percent were within ±1.00 D of target refraction, no eyes lost two lines or more of CDVA, and contrast sensitivity was unaffected after both procedures. The changes in higher-order aberrations were also very similar. There were also no differences in tear film parameters 6 months after surgery, although less postoperative foreign body sensation was reported within the first week after surgery in SMILE eyes. Corneal sublayer pachymetry measurements demonstrated equally increased epithelial thickness 6 months after surgery. Contrary to expectations, it was not possible to measure the theoretical biomechanical advantages of a small corneal incision in SMILE as compared to a corneal flap in FLEX. The main differences between FLEX and SMILE were found when the corneal nerves and intraoperative complications were evaluated. Thus, corneal sensitivity was better preserved and corneal nerve morphology was less affected after SMILE, but intraoperative complications occurred more frequently, although without visual sequela. Finally, 97% were satisfied or very satisfied with both their surgeries. Conclusion: The results support the continued use of both FLEX and SMILE for treatment of up to high degrees of myopia. Overall, refractive and visual results for both procedures were good and similar, but from a biological point of view, the less invasive SMILE technique is more attractive, as demonstrated in this study, despite being slightly more surgically demanding than FLEX.
Acta ophthalmologica.Acta Ophthalmol.2014 Mar;92 Thesis 2:1-21. doi: 10.1111/aos.12385.
Surgical correction of refractive errors is becoming increasingly popular. In the 1990s, the excimer laser revolutionized the field of corneal refractive surgery with PRK and LASIK, and lately refractive lenticule extraction (ReLEx) of intracorneal tissue, using only a femtosecond laser, has become
PMID 24636364

Japanese Journal

Photorefractive response and real-time holographic application of a poly(4-(diphenylamino)benzyl acrylate)-based composite

Giang Ha Ngoc,Kinashi Kenji,Sakai Wataru [他]
Polymer journal 46(1), 59-66, 2014-01
NAID 40019921454

Energy Transfer through Photorefractive Spatial Soliton Interaction

Liang Bao-Lai,Guo Qing-Lin,Jiang Yi-Hui,Wang Ying,Zhang Su-Heng,Fu Guang-Sheng
Applied Physics Express 6(6), 062201-062201-4, 2013-06-25
… In this research, we investigate the interaction between colliding photorefractive spatial solitons. … This observation provides preliminary data for optical computation applications via photorefractive spatial solitons. …
NAID 150000107113