This is the third and final volume of a three volumes book series devoted to photorefractive effects, photorefractive materials and their applications. Since the publication of our first two Springer books on "Photorefractive Materials and Their Applications" (Topics in Applied Physics, Vols 61 and 62) almost 20 years ago a lot of research has been done in this area. New and often unexpected effects have been discovered, theoretical models developed, known effects could be finally explained and novel applications had been proposed. We believe that the field has now reached a high level of maturity, even if research continues in all areas mentioned above and with new discoveries arriving quite regularly.
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The first volume is devoted to the description of the basic effects leading to photoinduced refractive index changes in electro-optical materials. In the second volume the status of the most recent developments in the field of photorefractive materials is reviewed and the parameters, which govern the photorefractive nonlinearity are highlighted.
This third volume deals with the applications of the photorefractive effects and of materials. Starting about 35 years ago the attractivity of the photorefractive effect for data storage, for optical metrology, optical signal processing and nonlinear optical applications has been recognized. One of the main reasons for this is the large nonlinearity or refractive index change, which can be induced by low light intensities by using the photoinduced space-charge fields in electro-optical materials. Many new concepts have been demonstrated in the laboratories over all these years. Several of these concepts have been proved useful also in other areas of nonlinear optics. Particularly interesting was the observation of a large energy from pump beams to the signal beam in two- and fourwave mixing experiments. This effects lead to coherent amplification of a waveform covering spatial information and to self-pumped optical phase conjugation with applications in the area of wavefront correction of self-induced optical resonators.
This and the other two volumes on photorefractive effects, materials and applications have been prepared mainly for researchers in the field, but also for physics, engineering and materials science students. Several chapters contain sufficient introductory material for those not so familiar with the topic to obtain a thorough understanding of the photorefractive effect. We hope that for researchers active in the field these books should provide a useful reference source for their work.
While literally the word photorefraction may describe all kinds of photo-induced changes of the refractive index of a material and therefore any photo-induced phase grating would belong to this category, it has become customary in the literature to consider only a smaller class of materials as being photorefractive. These materials possess two important properties: they are photoconductive and exhibit an electro-optic effect. Photoconductivity ensures charge transport, resulting in the creation of a space-charge distribution under inhomogeneous illumination. The electro-optic effect translates the internal electric fields induced by the inhomogeneous space-charges into a modulation of the material refractive index. This is the main mechanism for photorefraction in inorganic and organic crystals [1]. In polymers and liquid crystals, the concept of photorefraction has been expanded to include refractive index changes governed by a field-assisted molecular reorientation of the chromophores. The photorefractive effect is also distinguished from many other mechanisms leading to optically induced refractive index gratings by the fact that it is an intrinsically non-local effect, in the sense that the maximum refractive index change does not need to occur at the spatial locations where the light intensity is largest.
To obtain a dynamic holographic 3D display, several approaches are possible: either using an electronically controlled phase modulator (such as a liquid crystal or acousto-optic modulator) [17,18,19,20], or using a refreshable holographic recording material, such as photorefractive polymer. The present review article focuses on this latter case, specifically the development of photorefractive materials and their use for holographic 3D display.
This seemingly simple effect: a light-induced reversible refractive index change gives rise to a plethora of macroscopic observations and applications, such as self-focusing [25], beam fanning [26], two beam coupling [27], four wave mixing [28], holographic data storage [29], image processing [30], image through scattering media [31], and refreshable holographic recording [32]. For an extended review of the different applications of photorefractive materials, see, for example, References [33,34].
Because the photorefractive effect is based on electronic properties of the material, it is fully reversible, allowing for the recording, the erasing, and the refreshing of holograms at will. To do so, two coherent laser beams are intersecting each other inside the material. One beam is homogeneous and is referred as the reference beam, while the other beam carries the information as an intensity modulation and is referred as the object beam. When these two beams intersect, they form an interference pattern whose frequency is defined by their angle, and its modulation amplitude given by their respective intensities. This interference pattern is copied in the material by the photorefractive effect as a refractive index change.
Compared to other holographic recording materials, such as dichromated gelatin or silver halide, that need post-processing for the hologram to appear, or to photopolymers that record permanent holograms, photorefractive materials represent an interesting platform for the development of an updateable 3D display. However, to do so requires several specific characteristics:
In polymer material, the detailed analysis of the molecular energy levels has led to a better understanding of the charge transport mechanism and space charge field formation [58]. Most notably, the comparison between the time constant of polyvinylcarbazole (PVK)-based material versus poly(acrylic tetraphenyldiaminobiphenyl) (PATPD)-based polymer showed that deep charge trapping can slow down the response time of the material [44]. In Figure 2, the energy levels of PVK and PATPD-based photorefractive materials are presented. On the left panel, it can be seen that, because the HOMO of the PVK polymer is relatively low (5.92 eV), deep traps are present in the form of chromophores 7-DCST (4-homopiperidinobenzylidenemalononitrile) and DBDC (3-(N,N-di-n-butylaniline-4-yl)-1-dicyanomethylidene-2cyclohexene) molecules with levels at 5.90 eV and 5.62 eV, respectively. These deep traps attract the charges, slowing down their transportation along the polymer manifold. The PVK-based photorefractive material has a response time measured in the hundreds of milliseconds. On the right panel, the PATPD HOMO level is too high (5.43 eV) for the holes to be trapped into the 7-DCST or DBDC molecular levels, allowing for a faster charge transport and a photorefractive effect with a response time measured in the tens of milliseconds.
The polymer matrix used in photorefractive compounds is not only responsible for the structural integrity of the material but is also used to support charge transport. While a large array of photoconductive polymers has been used over time, the two most predominant matrices found in the literature are PVK and PATPD (see Figure 3 for their chemical structure). Both of these materials are hole conductors, meaning that the excited electrons stay in place in the excited LUMO level of the sensitizer, and it is the holes that are transported over the polymer chain by a hopping mechanism.
More recent works have shown that very high sensitivity can be obtained with poly(triarylamine)s (PTAA) compounds, thanks to their high dipole moment that allows for a large concentration of chromophores to be loaded in the polymer matrix [72,73]. Another approach calls for reducing the amount of space the chromophore needs to align in the field. By controlling the chromophore free volume, researchers have demonstrated a faster photorefractive response [74,75].
To improve the efficiency and shorten the response time of photorefractive polymers, plasticizer molecules, such as BBP (benzyl butyl phtalate) or ECZ (ethyl carbazole), are often added into the compound (see Figure 3 for their chemical structure). The plasticizer molecules lowers the glass transition temperature (Tg), which improves the mobility of the chromophores [41,63,76]. However, a side effect of lowering the Tg is to make the material susceptible to crystallization over time. The transformation from amorphous to crystallized manifests itself in polymer by making the material opaque, which is obviously a problem for optical applications. Fortunately, the crystallization mechanism is fully reversible, and the transparency of the sample can be restored by heating it above the Tg for a few seconds. The sample then needs to be quenched to room temperature to avoid immediate re-crystallization.
The other advantage of the photorefractive polymers that makes them highly suited for 3D display is their relatively high sensitivity. Sensitivity is one of the most important characteristics of any holographic recording material, dynamic or permanent, and is defined as:
The unique feature of photorefractive materials over any other dynamic holographic recording mechanism, such as molecular hole burning, molecular reorientation, or photobleaching, is that the sensitivity of photorefractive media can be improved by increasing the external applied voltage. The voltage and, thus, the efficiency is only limited by the dielectric breakdown value (Emax) at which the material experiences a catastrophic failure.
The dynamic capability of photorefractive materials is often demonstrated by recording the changing images displayed on a spatial light modulator (SLM) [60,87]. However, the image coming from these SLMs is only 2D. Therefore, the hologram recorded in the holographic material using SLMs is also 2D and lacks depth information. To display 3D images, another approach needs to be taken, one of which is holographic stereograms. 2ff7e9595c
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