fisica e... vol28 / no1-2 / anno2012 > 39

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1 fisica e... the policryps holographic structure A new polymeric - liquid crystalline composite template for optical, electro-optical and all-optical applications Cesare Paolo Umeton Laboratorio Italiano Cristalli Liquidi LICRYL (CNR-IPCF) Centro di Eccellenza Materiali Innovativi e Funzionali CEMIF.CAL Dipartimento di Fisica - Università della Calabria, Arcavacata di Rende (CS), Italy The acronym POLICRYPS indicates a micro-composite structure made of slices of almost pure polymer alternated to films of well aligned Nematic Liquid Crystal (NLC), with a spatial periodicity that can be settled in the range mm. It can be utilized for transmitting, diffracting or reflecting an impinging light beam with negligible scattering losses, while the optical effects of a spatial modulation of the refractive index (from polymer to NLC) can be switched on and off both by applying an electric field of few V/mm or, in some cases, by irradiating the sample with a light beam of suitable wavelength. In different geometries, the POLICRYPS can be exploited as a switchable diffraction grating, a switchable optical phase modulator and a switchable beam splitter. 1 The POLICRYPS structure In the last decades, great attention has been devoted to the realization of switchable holographic structures in liquid crystalline composite materials [1]. In particular, it has been shown that diffraction gratings based on Holographic Polymer Dispersed Liquid Crystals (HPDLC), that means droplets of Nematic Liquid Crystal (NLC) dispersed in a polymer matrix (see insert 1), are low cost and can exhibit good performances [2]. However, applicationoriented utilization of these devices is very often limited by their strong scattering of light, due to the circumstance that the average size of NLC droplets is, in general, comparable to the wavelength of the impinging light. For this reason, it has been recently designed, fabricated, patented and deeply investigated a new kind of holographic grating called POLICRYPS (acronym of POlymer LIquid CRYstal Polymer Slices), which is made of slices of almost pure polymer alternated to films of uniformly aligned NLC. These structures present a sharp morphology of high optical quality and, if used as diffraction gratings, they can exhibit values of the diffraction efficiency as high as 98%. vol28 / no1-2 / anno2012 > 39

2 fisica e... Insert 1 Liquid Crystals and Polymer Dispersed Liquid Crystals Organic molecules with an elongated shape can exhibit, in a given range of temperature, a Liquid Crystal (LC) phase, with physical characteristics that are typical of both liquids and crystals. Depending on the particular geometry the LC molecules organize, this intermediate phase is indicated with different names, the most known and studied in application oriented investigation being the Nematic Liquid Crystal (NLC) phase. In this particular configuration, the elongated (or calamitic ) molecules do not show any long-range order related to the position of their center of mass, while they try to orient their long axis along a preferred direction, identified by a versor n, which is referred to as the nematic director (of course, n coincides with n). The orientational behavior of NLC molecules is reflected in all the (tensorial) macroscopic characteristics of the NLC phase and is, therefore, responsible for all those anisotropic properties that are of interest for applications. In particular, from the optical point of view, the NLC is a uniaxial, birefringent medium whose optical axis is directed along n, with a complete rotational symmetry around it. It is worth noting that: a) The orientation of n is determined, in general, by the surfaces of the (glass) slabs which form the cell containing the NLC. This orientation can be strongly affected, however, by the interaction with a magnetic or an electric (static or even optical) field. Application of a suitable external field can be utilized, therefore, to realize a particular change in the anisotropic properties of the material; this characteristics of NLCs is the basis of their exploitation in device applications. b) Above a given temperature, indicated as the Nematic-Isotropic transition temperature, the NLC phase disappears and the material exhibits only those characteristic that are typical of an isotropic liquid. Of particular interest is the case of micrometric droplets of NLC that are uniformly dispersed in a polymeric matrix. This liquid crystalline composite material, called Polymer Dispersed Liquid Crystal (PDLC), can be obtained by curing with a UV/visible light a homogeneous mixture of NLC, monomer and curing agent and, once the curing process has come to an end, the sample exhibits optical and electro-optical properties that cannot be attributed to any of its single components (NLC and polymer). In PDLCs, the orientation of the NLC director inside the single droplet depends on different geometrical parameters, like the shape and the dimension of the droplet, and possible director configurations are represented in fig. I1-1 a, b, c. The bipolar configuration (fig. I-1 c) can be obtained by suitably adjusting those parameters and is of particular interest for some applications. Indeed, for this configuration, the symmetry axis of each droplet is, in general, randomly oriented but, due to the dielectric anisotropy of the NLC, the director tends to reorient when the droplet is acted on by an external electric field (fig. I1-2 a, b). Depending on the strength and initial orientation of this field with respect to the axes of individual droplets, some of them do perfectly align with the field direction, while other ones can only partially or not at all align; above a given field value, which is in the V/mm range, all the droplet axes are, in fact, aligned with the external field direction. The peculiarity of a PDLC system is that of scattering different amounts of an impinging light beam intensity, depending on the strength of an external electric field that is, eventually, applied to the system. In fact, by simply adjusting this value, a PDLC film (tens of mm thick) can continuously pass from a highly scattering, opaque, state to a transparent one. The underlying mechanism is quite simple: due to the optical anisotropy of the NLC droplet, the refractive index experienced by an impinging light beam depends on the angle between the droplet axis and the light polarization direction. In general, for a system with randomly oriented droplet axes, going from droplet to droplet this angle assumes all possible values, and light transverses a medium which exhibits random variations of the refractive index; as a consequence, if the droplet dimension is comparable to the wavelength of the impinging light, this one is highly scattered by the medium, which appears opaque. Application of an external electric field of suitable strength alignes all the droplet axes; in this case, all of them exhibit the same refractive index and the scattering of the impinging light depends only on the difference between the droplet and the polimeric matrix refractive indices. By choosing a suitable polymer, it is possible to make this difference negligible for a given light polarization: in this case, when applying the external elecric field, the PDLC film becomes optically uniform and therefore transparent for the impinging light. PDLC-based, switchable, diffraction gratings or Holographic Polymer Dispersed Liquid Crystals (HPDLC) gratings are realized if the intensity of the curing UV/visible light acting on the initial mixture of NLC, monomer and curing agent is not uniform, but follows the spatial modulation of an interference pattern. Indeed, under suitable conditions of total curing intensity and exposure time, after that the curing process has come to an end, the realized sample is made of almost pure polymer stripes alternated to films of polymer that are rich of NLC droplets of very small dimension. In the absence of an applied external field, the optical axes of NLC droplets in the droplet-rich films are randomly aligned, thus scattering a probe light beam impinging on the sample; on the other hand, the almost pure polymer stripes are transparent: the system acts, therefore, as a diffraction grating. By aplying an electric field, high enough to reorient almost all the the NLC droplets, a suitable choice of the polymer utilized for the fabrication of the sample can yield that an impinging probe beam of opportune polarization sees no difference between the polymer and the NLC droplet refraction indices: the system becomes optically uniform and the diffraction grating is, in fact, switched off. Fig. I1-1 Different configurations of the director orientation in PDLC droplets. a) radial configuration; b) axial configuration; c) bipolar configuration. Fig. I1-2 Effect of an external electric field on the director orientation of a PDLC droplet. a) electric field off; b) electric field on. 40 < il nuovo saggiatore

3 C. P. Umeton: the policryps holographic structure The standard procedure that enables realization of a high-quality POLICRYPS sample exploits the high diffusivity exhibited by NLC molecules when they are in the isotropic state; this enables to avoid the formation and separation of the nematic phase during the curing process [3, 4] (see insert 2 for the related theoretical model). The main fabrication steps are the following. A homogeneous syrup of NLC, monomer and photo-initiator is heated above the Nematic-Isotropic (N-I) transition temperature of the NLC component; the sample is then cured with the interference pattern of a UV radiation. After the curing process has come to an end, the sample is slowly cooled down to room temperature, below the Isotropic-Nematic transition point. The experimental set-up exploits an active stabilization system for suppression of vibrations [5] and is presented in fig. 1. An Ar-Ion laser is the source of a singlemode radiation at the wavelength l B = mm. The output light beam is broadened up to a diameter of about 25 mm by the beam expander, BE, and divided into two parts of almost equal intensity by the beam splitter, BS. Then, the two beams overlap at the entrance plane of the sample cell S, thus giving rise to a curing interference pattern, whose spatial period can be easily settled in the range L = mm, simply by adjusting the interference angle 2q cur. A stabilization system, which exploits the devices shown in the sketched setup, enables an active control of the vibrations the sample undergoes while being cured. The photodiode PD 3 reveals the intensity of the diffracted/reflected beams that are wave-coupled by the test grating placed above the sample; the detected signal is sent to a computerized feedback system, leaded by a software that is based on a Proportional- Integral-Derivative (PID) protocol. This drives a mirror-holder whose position can be controlled by a piezoelectric mechanism, used in feedback configuration. This stabilization system has proved to be able to continuously compensate for those optical path length changes that can take place due to slow variations in environmental conditions such as room pressure, temperature or humidity. Fig. 1 Optical holographic setup for UV curing gratings with stability check: P, polarizer; λ/2, half-wave plate; BE, beam expander; BS, beam splitter; 2θ cur, total curing angle; M, mirrors; S, sample. Inset: reference grating (positioned immediately below the sample area) that enables the stability check. vol28 / no1-2 / anno2012 > 41

4 fisica e... Insert 2 Chemical - diffusive model Realization of diffraction gratings in LC composite materials by means of a UV interference pattern can be modeled by taking into account both chemical and mass diffusion processes that occur during the formation of the structure. The model performs an analysis of the obtained system of equations with the aid of numerical techniques. In this way, two control parameters are singled out, which govern the whole process and it is shown that both HPDLC and POLICRYPS morphologies can be obtained. It is considered a sample of length L in z and infinite in x and y directions, filled with a mixture of liquid crystal, monomer, and initiator in concentration C, M and I, respectively. The sample is cured by the interference pattern W (x) of two coherent UV beams and the pattern (grating) wave vector q is assumed to be directed along the x-direction. By utilizing a classical reaction scheme for the radical polymerization, assuming that all mass transfer processes are driven by the conventional Fick diffusion and following a list of quite reasonable assumptions [4], the whole process is governed by three coupled equations: where. the first Fourier component of n (x). A solution of this kind is plotted in fig. I2-1 as a function of B and G parameters for N 0 = 4. It is evident that two regions of high Dn values (high grating efficiency) exist: a) B << 1. In this condition (fast curing regime), the polymerization process is quite fast, so that polymer chains grow before monomer diffusion takes place. The high number of radicals created in the bright fringes often recombine with each other and form a new photo-initiator molecule before forming polymer chains; on the other hand, the few radicals created in the dark fringes can capture a great number of monomers before a chain is closed. Several SEM analyses of HPDLC gratings realized in these experimental conditions indicate that the width of polymer slices well coincides with theoretical predictions. b) G << 1 with B > 1. In this zone (called slow curing regime ), the polymerization reaction is quite slow and monomers are allowed to diffuse across the fringes before reacting; it is more probable for a chain to be closed by a radical than to get a new monomer; thus, because of the availability of a great number of radicals, polymerization takes place mainly in the bright fringes. SEM analyses of POLICRYPS gratings realized in these experimental conditions confirm theoretical predictions. It is possible to conclude that diffusion is the main mechanism that determines which kind of grating (HPDLC or POLICRYPS) is going to be obtained in a given experimental condition. In order to fabricate a HPDLC grating, both low curing intensity and low temperature values are requested, while realization of a POLICRYPS grating needs, mainly, a high temperature during the curing process. The system is written in a reduced form, which enables an immediate insight into the features of the investigated phenomena: s = C/T, m = M/T and n = P/T are the relative concentrations of components (P stands for the polymer concentration and T for the total molecular concentration), while t = {(k p /k t )[k t gw (x) I] 1/2 }t is a dimensionless time; k p and k t are the chemical prolongation and termination constants for the polymer formation reactions, and t is the time; g represents the activation probability of the initiator molecules when acted on by the radiation. Furthermore, in the above equations, D is the monomer diffusion constant and N 0 stands for the least number of monomer molecules that are needed for the formation of an immobile polymer chain. The local intensity of the curing pattern is written as W (x) = W 0 [1 + msin(x)], where W 0 = (I 1 + I 2 ) is the total intensity (I 1 and I 2 being the intensities of individual beams) and m = 2 [I 1 I 2 ] 1/2 /(I 1 + I 2 ) is the fringe contrast. Furthermore, q = 2p/L, where L is the fringe spacing, and x = qx. An analytical solution of the above system can be obtained only when the diffusion processes are negligible (a condition called fast curing regime ) and, in general, a numerical approach is needed. In this framework, a central derivative scheme can be used to perform spatial derivatives (with a spatial step that has to ensures the stability of solutions for each B value), while a second-order Runge-Kutta scheme can enable calculation of temporal derivatives. Thus, the modulation Dn of the normalized polymer concentration across the fringes, which determines the diffraction efficiency of the grating, can be calculated as Fig. I2-1 The modulation Dn of the polymer concentration is presented as a function of parameters B and G on a three-dimensional surface. 42 < il nuovo saggiatore

5 C. P. Umeton: the policryps holographic structure Indeed, residual fluctuations are of the order of 6 7 nm, which corresponds to the sensitivity of the used piezo-system. It is, finally, worth noting that the sample temperature is controlled by a hot-stage. Very recently, the initial single-step POLICRYPS fabrication procedure has been implemented in a new multi-steps procedure which enables both realization of an excellent morphology and possibility of putting any kind of liquid crystalline material between the polymeric slices [6]. When the usual first step is completed with a low-quality, low-cost NLC, an etching process of the sample is carried out by immerging it (without opening the cell) in a water solution of tetrahydrofuran (THF). In this way, the solvent washes out the NLC from the polymeric structure by capillary flow. For short time intervals (3 4 h), the THF acts as a selective agent, removing the NLC and any eventual unpolymerized component without affecting the regularity of the polymer slices. The process takes place above the Nematic- Isotropic transition temperature of the NLC (65 C), thus ensuring a low viscosity of this component. In fig 2, the sample appears to be made of sharp polymer slices separated by empty channels; this is a clear confirmation that the NLC has been completely removed. In a third step, the channels can be filled, by capillary flow, with the new, desired liquid crystalline material, which may include photoresponsive, cholesteric or ferroelectric liquid crystals, for the desired application. Indeed, low scattering losses and good switchability of POLICRYPS open a wide range of possible utilizations. As a matter of fact, it has been demonstrated that not only the POLICRYPS behaves as a good switchable diffraction grating, but it can also be suitable for several applications, depending on the way a light beam impinges on, and propagates through, the structure. Fig. 2 Polarizing Optical Microscope view of the POLICRYPS structure during the micro-fluidic etching process at different time scale (a-d). Polymer template after removing the NLC (f); the flat diffraction efficiency behavior versus the impinging probe polarization (e) and temperature (g) demonstrates the absence of NLC in the sample. vol28 / no1-2 / anno2012 > 43

6 fisica e... 2 The POLICRYPS switchable diffraction grating The basic device that can be realized by using electrically/ optically switchable holographic gratings in liquid crystalline composite materials is an electro/all optical switch, a device which should, in principle, completely diffract or transmit an impinging light beam, depending on the application of an external voltage or irradiation with a suitable light. HPDLCs, which have been actively exploited in the past with the aim of realizing working (electro-optical) prototypes of this kind, still show issues that affect their overall performances; POLICRYPS structures enable overcoming most of these issues. In the following, results are reported related to an experimental comparison between an HPDLC and a POLICRYPS grating, which put into evidence how microscopic features of the realized structure strongly influence the overall performance of the macroscopic device. A standard HPDLC and a single step POLICRYPS grating have been fabricated, both with a fringe spacing L = 1.5 mm. Sample cells, L = 16 mm thick, realized with Indium Tin Oxide (ITO)- coated glass slabs, have been filled with the same initial chemical syrup. This was prepared by diluting the NLC 5CB (CyanoBiphenil) by Merck ( 30% in wt) in the pre-polymer system Norland Optical Adhesive NOA-61. The POLICRYPS grating has been cured by a total UV intensity of 11 mw/ cm 2, acting on the sample for approximately 1000 s at high temperature (e.g. above the N-I transition point of the 5CB liquid crystal), these being the optimal conditions for achieving a high diffraction efficiency and a morphology of good quality [3]. Almost the same UV intensity and curing time proved to be adequate also for the fabrication of the HPDLC grating, but in this case the sample has been cured at room temperature. A weak He-Ne laser beam ( 1 mw at l = 633 nm, with its angle of incidence adjusted for satisfying the Bragg condition for the first order diffracted beam) has been utilized to explore the performances of both gratings. In order to perform a comparison in the same experimental conditions, the impinging intensity I in of the probe beam (before the sample) and the transmitted intensity I tr have been measured before starting the curing process of each sample. Once this process has come to an end and the UV light has been turned off, both the intensity I 0 of the directly transmitted and the intensity I 1 of the first-order diffracted beams have been detected. In this way, it was possible to evaluate the zeroth-order transmittivity T 0 = I 0 / I in, the firstorder transmittivity T 1 = I 1 / I in, the total transmittivity T tot = T 0 + T 1 and the first-order diffraction efficiency, which is usually calculated as h 1 = I 1 /(I 0 + I 1 ). During all the experiments, the intensity of the probe beam was maintained at a low, fixed, value. The first-order diffraction efficiency, measured at room temperature both for POLICRYPS and HPDLC gratings, was: h 1(POLICRYPS) 93% and h 1(HPDLC) 44%. The electro-optical response of both gratings has been investigated by exploiting a low-frequency (500 Hz, square wave) voltage, and results are reported in fig. 3. The behavior of the first order transmittivity T 1 (circles), zeroth-order transmittivity T 0 (squares) and total transmittivity T tot (triangles) is reported versus the appliedelectric-field r.m.s. values, in fig. 3a for the POLICRYPS grating. Fig. 3 Dependence on an applied voltage of the zeroth-order transmittivity T 0 (squares), first-order transmittivity T 1 (circles) and total transmittivity T tot (triangles) for: (a) POLICRYPS grating and (b) HPDLC grating at room temperature. Pictures in the inset show respectively a typical POLICRYPS and HPDLC grating morphology observed with a Polarizing Optical Microscope. 44 < il nuovo saggiatore

7 C. P. Umeton: the policryps holographic structure It is evident that T tot is only slightly lower than 1 and remains approximately the same for all the values of the applied field; this is a clear indication that the grating exhibits negligible scattering losses. The situation is quite different for the HPDLC grating (fig. 3b): in this case, T tot remains well below 1 and increases as the applied field increases. As for the switching field values, the first diffracted beam is almost completely switched off by a field of about 1.5 V/mm applied to the HPDLC grating, while it is needed a value of about 4.3 V/mm to obtain the same effect in the POLICRYPS one. This particular difference can be due to the average size of NLC droplets in the HPDLC; eventually, this size is large enough to ensure low switching fields. This circumstance is confirmed by the observation that both rise and fall times of the HPDLC grating are longer than the ones of the POLICRYPS: about 1.4 ms of the HPDLC against about 0.9 ms of the POLICRYPS for the rise time, and about 10.5 ms of the HPDLC against about 1.1 ms of the POLICRYPS for the fall time. Above values suggest, indeed, a very large average size of HPDLC droplets. It is, however, worth noting that the electro-optical behavior shown in fig. 3a, and its noticeable difference with the one of fig. 3b, represents the best evidence of the good performances of the new structure. Experimental investigation [7] has shown that the dependence of the diffraction efficiency h of a POLICRYPS grating on different physical and geometrical parameters can be interpreted in the framework of the Kogelnik model [8], being given by the following expression: where λ is the vacuum wavelength of the probe radiation, L the cell thickness, e i (i = 1, 1) stands for the i-th Fourier component of the dielectric constant distribution across the fringe, b is the refraction angle of the probe beam inside the sample, T the temperature. h is an oscillating function of y, with a periodical sequence of maxima and minima, which hold 1 and 0, respectively; the argument y depends on the sample thickness, the probe wavelength and is a monotonous decreasing function of temperature both for s and p polarizations of the probe wave (due to the e i dependence on temperature). Of course, the polymer dielectric constant is, in general, temperature dependent too, but this dependence is much weaker than the one of the nematic phase, and can be neglected. It has been recently shown that high quality, optically switchable, POLICRYPS diffraction gratings can be realized, by including a small concentration of PLCs (see insert 3), sensitive in the optical range, in the initial mixture utilized for the fabrication of the structure. Indeed, NLC compositions doped with the azo-lcs (PLCs) of the series BPND (2-bromo- 4-N-ethylpiperazinylphenyl) and CPND (2-chloro-4-Nethylpiperazinylphenyl) (by BEAM engineering) [9, 10] are promising materials for fast and efficient all-optical switching applications due to their high photosensitivity and optical nonlinearity for visible wavelengths, nanosecond response times, and fast relaxation [11]. By utilizing this new material and the multi-step process for the fabrication of the POLICRYPS structure [6], it is possible to realize an all-optical diffraction grating, controlled in the visible range, with a very good phase separation between polymeric slices and NLC materials, which represents a step forward in comparison with previously obtained results [12, 13]. The initial mixture is composed by the UV-sensitive photopolymer NOA 61 (by Norland) and the azo-lc CPND-57, 73% and 27% in weight, respectively. The mixture is injected by capillarity into the cell, consisting of two glass slabs separated by 7.2 mm ball spacers; the pitch of the grating, realized by using the procedure described in [6], is L = 1.6 mm. The all-optical behavior of the sample is investigated by means of the setup shown in fig. 4a, which exploits a green diode laser (pump) emitting at l = 532 nm (in the high absorption range of the mixture spectrum) and a He-Ne probe beam at l 633 nm; this beam impinges at the Bragg angle (11.5 ) and is p-polarized, in order to experience the highest index contrast [13]. Figure 4b shows the significant change in the diffraction efficiency of the azo-lc POLICRYPS induced by the pump green light. When the pump is switched on, the probe light experiences the average refractive index of the NLC, with a value close to the polymer one, and a drop of the diffraction efficiency is observed. When the green pump beam is switched off, a cis-trans photoisomerization of the azo-lc occurs, which induces reorientation of the NLC director, thus restoring the diffractive-index modulation of the sample. The switching behavior is detected by using a periodic (square wave) sequence of on-off pump beam irradiance and proves to be well reversible and repeatable; the intensity of the probe red beam is kept constant. Figure 4b indicates that the multi-step POLICRYPS structure displays the high performance of the azo-lc, which exhibits a fast spontaneous relaxation and an ultrafast photoisomerization response. In addition, due to its high morphological quality, the diffraction grating exhibits high diffraction efficiency and quite short response times. vol28 / no1-2 / anno2012 > 45

8 fisica e... Insert 3 Photosensitive Liquid Crystals and composite structures In the last few years, Photosensitive Liquid Crystals (PLCs), have proved to be promising materials that combine a high reconfigurability (typical of LCs) with optical photosensitivity, a characteristic which is due to a reversible photo-induced isomerization of azobenzene molecules. These photochromic units (and their derivatives) can undergo reversible trans-cis isomerization. The transition from the thermodynamically more stable trans to the cis conformation can be induced by irradiation with UV or visible light and reversed upon heating or irradiation with red/yellow light; transition times may vary from the nanosecond scale (for the single molecular transition), up to few seconds, for micro-domains. Geometrical changes due to photo-isomerization produce concomitant changes in the physical and chemical properties, not only in the PLC itself, but also in the surrounding matrices. When the PLC is embedded in a NLC, the two PLC isomers produce different environments, which are characterized by the two different molecular shapes. The rodlike rigid molecule of the trans form is favorable for the stabilization of the NLC, while the bent cis isomer tends to destabilize the phase structure. Therefore, the ordered NLC phase is isothermally transformable into a disordered isotropic phase by the trans-cis photo-isomerization of the guest PLC; this transformation can dramatically change the refractive index of the material as seen by an impinging light beam. The possibility to fabricate optically controllable photonic devices arises when combing the photosensitivity and reconfigurability of PLCs- NLC mixtures and polymeric structures. Figure I3-1 shows a schematic representation of the optical control of the refractive index contrast in a POLICRYPS grating: By impinging on the structure with a green light laser, it is possible to induce the trans-cis isomerization of the PLC which tends to destabilize the NLC order. Fig. I3-1 Photo-isomerization process inside a POLICRYPS structure. In this condition, the whole structure shows an almost uniform refractive index, since there is almost no difference between the polymer and the disordered-nlc refractive index values; however, when the green beam is turned off, a reverse cis-trans isomerization process takes place and an orientational NLC order (induced by the polymer slices) is well restored, which sets a modulation between the polymer and the ordered-nlc refractive index values. Fig. 4 a) Experimental setup for the observation of all-optical processes in POLICRYPS diffraction gratings containing azo-lc. PD 1,2, photodetectors; HWP, half-wave plate; P, polarizer; ES, electronic shutter. b) Reversible and repeatable changes of the diffraction efficiency of the grating induced by a pump green light. Power density values are indicated in the figure. 46 < il nuovo saggiatore

9 C. P. Umeton: the policryps holographic structure 3 The POLICRYPS switchable optical phase modulator The sharp and high-quality morphology of POLICRYPS suggests a possible use of this structure as a switchable phase modulator [14]. As a matter of fact, examples of such a device are already present in the literature. The basic embodiment is obtained by enclosing a NLC with positive dielectric anisotropy in a cell made of two ITO-coated glasses, treated to give a planar alignment to the NLC director n. Due to the birefringence of the NLC, light with wavelength l, propagating through the structure, is separated into an ordinary and an extraordinary component. If L is the thickness of the sample and Dn indicates the birefringence, the phase difference f between these two waves, measured at the exit of the sample, depends on the value of Dn: f = 2pLDn/l. By applying an external electric field E directed perpendicularly to the glass slabs of the cell, the director n tends to reorient along the same direction of E, thus producing a change in the birefringence, and therefore in the phase difference. This simple device is, however, very sensitive to temperature, a circumstance that represents a limit for an eventual device when the power of the impinging radiation is high; moreover, switching times are, in general, quite long (of the order of 10 ms), and limit the fields of possible applications. An attractive alternative to the discussed system can be represented by POLICRYPS structures: First of all, they exhibit limited scattering losses when acted on by visible light; second, the polymer slices confine and stabilize the NLC molecules, thus increasing the stability of their alignment and third, they can be driven by low voltages, exhibiting short switching times [3]. In the (test) experiment, the grating has a periodicity L = 1.22 mm and thickness L = 6 mm. The above values ensure a very low-efficiency grating, since, in order to be used as a good phase modulator, the grating should, in principle, transmit all the impinging light. The setup utilized for measurements is shown in fig. 5. The focalized light (spot diameter 0.5 mm, power density 1mW/mm 2 ) from a He-Ne laser (l = 633 nm) passes through a vertical polarizer before reaching the POLICRYPS; this is used as a retardation plate, whose orientation angle around the direction of the impinging beam can be manually set to a desired a value by means of a rotation stage. The transmitted laser beam passes, then, through a second (horizontal or vertical) polarizer before being detected by a photo-detector. Assuming, as a reference, the transmitted intensity (zeroth diffracted order) I 0T immediately after the sample when its optical axis (direction of the POLICRYPS channels) is parallel to the axis of the first polarizer, the output intensity after the second polarizer can be measured versus the rotation angle a of the sample axis, both between crossed (I cross ) and parallel (I parallel ) polarizers. Results shown in fig. 6a reveal a behavior that is typical Fig. 5 Experimental setup for the measurement of the POLICRYPS birefringence. P, polarizer; A, analyzer; I inc, totally incidence intensity; I out, output intensity; I 0T and I 1T, zeroth- and the first-order transmitted intensities, respectively; a, angle between the light polarization direction (y-axis) and the grating axis (direction of the POLICRYPS channels) in the yz-plane; PD, photodetector; OSC, oscilloscope. vol28 / no1-2 / anno2012 > 47

10 fisica e... of a retardation wave plate: both intensities are periodic functions of the rotation a. The two curves for I cross and I parallel are drawn by starting from their minimum values, which do not coincide because the sample does not exactly fulfill the half wave plate condition DnL = (m + 1/2)l [14]. It is worth noting that the two curves confirm the absence of liquidcrystal droplets (with dimensions of the order of the probe beam wavelength) inside the structure: otherwise, the impinging light would undergo a strong scattering, becoming depolarized, with the consequent impossibility of obtaining the measured zero value of the transmitted intensity. Results concerning the possibility of switching on and off the birefringence of the sample by applying an external electric field are shown in fig. 6b. Measurements are performed by placing the sample between parallel polarizers, with its optical axis oriented at 45 with respect to their axes. The applied field is increased from 0 V/mm to 7.1 V/mm: due to the director reorientation, the birefringence value is completely turned to zero. The stability of the POLICRYPS used as a retardation plate is demonstrated in fig. 6c, where the birefringence is reported versus the power of an impinging green laser beam (l = 532 nm): The very small observed variations indicate that the stabilization action of the polymeric slices in the POLICRYPS is able to strongly reduce the thermal noise [15] produced by the increasing impinging intensity. The behavior of a POLICRYPS structure, introduced as a phase retarder between a polarizer and an analyzer, can be explained by exploiting the Jones Matrix formalism [16, 17], taking into account also the dichroic behavior of the structure. In this framework, the intensity I out of the light transmitted by the analyzer can be calculated as where I inc is the impinging light intensity, b is the angle between analyzer and polarizer axes, δ is the phase retardation (introduced by the plate) between the two orthogonal components E and E (with respect to the optical axis of the birefringent material) which the electric field, Fig. 6 a) Experimental behavior of the output intensity versus the rotation angle a obtained by placing the sample between crossed (red) and parallel (blue) polarizers. The difference between the level of I 0T and the maximum value reached by I parallel is due to the absorption of the second polarizer. b) Birefringence versus the applied electric field (square voltage pulses at 1 khz). c) Birefringence versus the power of the impinging laser beam. 48 < il nuovo saggiatore

11 C. P. Umeton: the policryps holographic structure of the impinging wave is decomposed into; it can be evaluated from the expression [17], where parameters H and V depend on the considered material and can reflect a broad range of situations. They can be experimentally evaluated by exploiting the expressions:. 4 The POLICRYPS switchable beam splitter An Optical Beam Splitter (OBS) is an optical device that splits an incident light beam into two or more beams. Different types of OBS exist, which are used for many different purposes [18, 19], but a particularly interesting way to realize inexpensive and large-area, high-quality OBSs is to exploit transmission periodic structures. In this framework, liquid crystalline composite structures like POLICRYPS can be efficiently used for OBS application. Indeed, they are highly responsive to a wide variety of external stimuli like optical, magnetic and electrical fields, exhibiting large birefringence and low absorption coefficient, along with a noticeable optical nonlinearity. Furthermore, thanks to the possibility of combining the properties of NLCs with those of PLCs, particular structures like azo-policryps can be switched on and off by applying either an external electric field or an optical pump beam [12, 13], exhibiting, in this last case, response times that can be reduced to fall in the nanosecond range. This is of particular interest for applications, since fast light responsive devices represent an innovative way to realize an on-chip technology, where the control beam could be engineered to travel together with the signal, with no need of external electrodes. Fig. 7 All-optical OBS and interferometer setup: P, polarizer; HWP, half-wave plate; SRM, semireflective mirror; q int, interference angle; PM, piezomirror; PD, photodetector; L, lens. vol28 / no1-2 / anno2012 > 49

12 fisica e... An azo-policryps based device of this kind is also of great utility for the realization of an innovative tool to investigate material responses. Namely, it enables realization of an interference pattern, whose visibility can be finely varied by means of an external control beam impinging on the OBS. The effect can be exploited to characterize a variety of photoresponsive materials (photoresist, photopolymer, etc.), where adjustability of the impinging light intensity profile is of crucial importance for the investigation of threshold effects like photopolymerization or nonlinear properties. Geometrical parameters of the fabricated [20] azo-policryps structure are L = 6.95 μm in thickness and Λ = 1.57 μm in fringe spacing; according to Kogelnik s theory [8], this grating operates in the Bragg, or thick grating, regime, which means that only one order of diffraction is observed. The experimental setup utilized to exploit the azo-policryps as a finely adjustable, optically controlled, OBS is reported in fig. 7, along with the apparatus needed to produce an interference pattern with a tunable fringe visibility. The azo-policryps grating splits the impinging probe red light into two beams (the transmitted an the diffracted orders, 0 T and 1 T, respectively); these are recombined in a Mach-Zehnder interferometer geometry, a part of the setup that is used to monitor the functionality of the OBS. The diffraction efficiency η of the azo-policryps is optically driven by an external pump source (green diode laser at l = 532 nm); fig. 8 shows the evident change of the diffraction efficiency induced by switching on the pump green light. The switching response of the azo-policryps OBS is detected by using a sequence of on-off pump beam irradiance (P pump = 48 mw/cm 2 ) while the intensity of the probe red beam is kept on at all times (P probe = 0.55 mw/cm 2 ). Characterization of the azo-policryps as a variable OBS is performed by means of the interferometer setup shown in fig. 7. The detector PD enables to monitor the interference pattern (reported in the dark inset of fig. 7) produced by overlapping 0 T and 1 T, beams. The pattern periodicity can be easily adjusted by varying the orientation of the Semi- Reflecting Mirror, thus the angle ϑ int ; in the experiment, this angle is quite small (~ 0.04, the scale is reported in the same dark inset of fig. 7). The ratio R = I 1T / I 0T of the intensities of 1 T and 0 T beams is related to the diffraction efficiency (η) of the azo-policryps trough the equation: η = I 1T /(I 0T + I 1T ) = R/ (1 + R); in this experiment, the polarization of the probe beam and its incident angle are adjusted [5] to obtain a maximum diffraction efficiency value η max 50% (that is to say R max 1) when the pump beam is off. The fringe visibility, defined as V = (I max I min )/(I max + I min ), (where I max and I min are the measured maximum and minimum intensity values of the interference pattern) strongly depends on R. Indeed, in the utilized Fig. 8 Diffraction efficiency changes induced by using a sequence of on-off pump beam irradiance while the intensity of the probe red beam is kept on. Fig. 9 Intensity profile of the interference pattern vs. the piezomirror position. The reversible change of oscillation amplitude is obtained by switching on and off the external pump light. 50 < il nuovo saggiatore

13 C. P. Umeton: the policryps holographic structure geometry, it is easy to see that V = [2 (I 0T I 1T ) 1/2 / (I 0T + I 1T )] γ = [2(R) 1/2 / (1+ R)] γ. Here, γ is the degree of coherence of the two beams [21] and is related to the difference Δl of the optical path lengths of the two beams and to the coherence length l c of the probe laser beam, which turns out to be of the order of 10 cm for the utilized laser source; since Δl does not exceed few μm even when the piezo-mirror PM is shifted back and forward (by few μm), it is reasonable to assume that γ = (1 Δl / l c ) 1. Relating η to R by the equation R = η/1 η and substituting it into the expression of the visibility, yields V = 2[η (1 η)] 1/2. Therefore, the efficiency of the azo-policryps grating η depending on the impinging pump power P pump, the behavior of the tuneable OBS can be investigated by detecting the fringe visibility V versus P pump. Measurements are performed by applying a linear voltage to the piezo-mirror PM included in the interferometric part of the setup of fig. 7; in this way, the optical path length of one of the two arms is modified. This enables a scrolling of the fringe pattern on the PD and a measurement of I max and I min values, without shifting the PD from the top of the impinging Gaussian beams; indeed, a linear movement of the piezo-mirror in the direction perpendicular to the mirror plane corresponds to a shift of the fringe pattern along a direction parallel to the PD surface. The output signal from the PD exhibits the sinusoidal behavior shown in fig. 9. It is evident that the amplitude of the sinusoidal modulation is strongly attenuated when irradiating with the green pump laser (P pump = 48 mw/cm 2 ) over the spot of the red light; the oscillation amplitude is restored to its initial value in some ms by turning off the external pump. The behavior of V versus fine variations of P pump is reported in fig. 10, along with measured values of R. Curves can be explained by considering that the rate of the trans-cis isomerization process depends on the number of excited molecules; therefore, the rate of concentration of photoisomerized azo-lc molecules is proportional to the pump power density. This phenomenon directly affects R and therefore V, which varies from 0.94 to 0.2. Fig 10a shows that R values can be finely adjusted between 1 (transmitted and diffracted beams of the same intensity) and 0 (no diffracted beam, the whole impinging intensity is transmitted). It is possible to conclude this review with the assertion that, in the field of electrically switchable devices exploiting liquid crystalline composite materials, the POLICRYPS represents a very promising micro-structure with several possibilities of application, due to the interesting underlying physics. Availability of a reliable theoretical model enables to suitably choose the values of two main physical parameters (molecular diffusion and curing intensity) in such a way that it is possible to fabricate a rigid, stable and sharp polymeric Fig. 10 Beam splitting (a) and fringe visibility (b) are reported vs. the pump power density. The interference pattern acquired with a CCD camera is reported for V = 0.94 (inset 1) and V = 0.2 (inset 2). vol28 / no1-2 / anno2012 > 51

14 frame of channels, filled with uniform films of NLC, which can be eventually doped with PLCs; all reported applications are based on these characteristics. While the sharpness of the structure and the uniformity of the LC films minimize light scattering losses, the application of a relatively low external voltage, or a suitable light, can determine, in a ms time scale, a reorientation of the LC director. This reorientation can be used to realize, tune, and eventually switch on and off, a spatial modulation of the refractive index of the sample: Reported applications exhibit features that are due to the exploitation of this tunability. In fact, depending on the way a light beam propagates through, the POLICRYPS can be used as a switchable diffraction phase grating, a switchable optical phase modulator or a tunable beam splitter. Performances exhibited in all above applications are very interesting and stimulate further investigation. Acknowledgements I am pleased to acknowledge the cooperation of all coauthors of the papers I published on the argument: Roberto Bartolino, Roberto Caputo, Antonio De Luca, Luciano De Sio, Sameh Ferjani, Luigia Pezzi, Giuseppe Strangi, Alessandro Tedesco, Alessandro Veltri, from my Department, Svetlana Serak, Nelson Tabiryan, from Beam Engineering for Advanced Measurements Company (USA), Andrei Sukhov, from the Institute for Problems in Mechanics, Russian Academy of Science, Moscow (Russia). references [1] R. L. Sutherland, V. P. Tondiglia, L. V. Natarajan, T. J. Bunning, W. W. Adams, J. Nonlinear Opt. Phys. & Mater., 5 (1996) 89. [2] L. V. Natarajan, D. P. Brown, W. J. Wofford, V. P. Tondiglia, R. L. Sutherland, P. F. Lloyd, T. J. Bunning, Polymer, 47 (2006) 4411, and references therein. [3] R. Caputo, L. De Sio, A. V. Sukhov, A. Veltri, C. Umeton, Opt. Lett., 29 (2004) [4] A. Veltri, R. Caputo, A. V. Sukhov, C. Umeton, Appl. Phys. Lett., 84, (2004) [5] L. De Sio, R. Caputo, A. De Luca, A. Veltri, C. Umeton, Appl. Opt., 45, (2006) [6] L. De Sio, S. Ferjani, G. Strangi, C. Umeton and R. Bartolino, Soft Matter, 7 (2011) [7] R. Caputo, A. Veltri and C. P. Umeton, A. V. Sukhov, J. Opt. Soc. Am. B, 21 (2004) [8] H. Kogelnik, Bell Syst. Tech. J., 48 (1969) [9] Beam Engineering for Advanced Measurements Co., [10] U. A. Hrozyk, S. V. Serak, N. V. Tabiryan, L. Hoke, D. M. Steeves, B. Kimball, G. Kedziora, Mol. Cryst. Liq. Cryst., 489, 257 (2008). [11] U. Hrozhyk, S. Serak, N. Tabiryan, D. Steeves, L. Hoke, B. Kimball, Proc. SPIE, 7414 (2009) 1. [12] L. De Sio, A. Veltri, C. Umeton, S. Serak, N. Tabirian, Appl. Phys. Lett., 93 (2008) [13] L. De Sio, S. Serak, N. Tabiryan, S. Ferjani, A. Veltri, and C. Umeton, Adv. Mater., 22 (2010) [14] L. De Sio, N. Tabiryan, R. Caputo, A. Veltri, C. Umeton, Opt. Express, 16 (2008) [15] F. Simoni, Nonlinear optical properties of liquid crystals (World Scientific) [16] R. C. Jones, J. Opt. Soc. Am., 31 (1941) 488. [17] R. Caputo, I. Trebisacce, L. De Sio. C. Umeton, Opt. Express, 18 (2010) [18] J. M. Zanardi Ocampo, P. O. Vaccaro, T. Fleischmann, T.-S. Wang, T. Ohnishi, A. Sugimura, R. Izumoto, M. Hosoda, and S. Nashima, Appl. Phys. Lett., 83 (2003) [19] Wei Shen, Ming-Wen Chang, and Der-Shen Wan, Appl. Opt. 35, (1996) [20] L. De Sio, A. Tedesco, N Tabiryan, C. Umeton, Appl. Phys. Lett., 97 (2010) [21] A. Yariv, Quantum Electronics (J. Wiley) Cesare Paolo Umeton Cesare Paolo Umeton was born in Marsala and obtained the Laurea in Fisica at the University of Pisa in Since 2001 he is a Full Professor of Experimental Physics at the University of Calabria, Italy, where he had positions of Assistant Professor of Chemical Physics ( ), Researcher in Chemical Physics ( ), Associate Professor in Structure of Matter ( ). He is the co-editor of a book on Novel Optical Materials and Applications, co-author of six patents on applications of liquid crystalline composite materials and co-author of more than 100 articles in the following fields: Microwave spectroscopy in low-pressure gases, nonlinear phenomena in magnetic resonance, electro-optical and elasto-optical effects in liquid crystals, nonlinear optics in liquid crystals and organo-metallic liquid crystals, nonlinear dynamics and chaos transition in liquid crystals, linear and nonlinear properties of liquid crystalline composite materials, interaction of liquid crystalline composite materials with short laser pulses, thermo-diffusive optical phenomena and photo-refractivity in liquid crystals, spatial solitons in liquid crystal films, diffraction gratings in liquid crystalline composite materials. He is the Chairman of the Meeting Series Novel Optical Materials and Applications (NOMA) and a Fellow of the Optical Society of America. 52 < il nuovo saggiatore