Conventional elastomers doped with benzophenone derivatives as effective media for all-optical fabrication of tunable diffraction elements

Abstract

Simple and effective approach for the fabrication of volume diffraction gratings (VDGs) using different widely used elastic materials is demonstrated for the first time. The method consists in the introduction of UV-photoactive compounds into the ready-to-use elastomeric films with following gratings inscription by a holographic technique. As photoactive dopants a number of methyl-substituted benzophenone derivatives have been tested. Polydimethylsiloxane- and acrylate-based elastic polymers as well as styrene-ethylene-co-butylene-styrene copolymer are exploited as elastomeric matrices. The diffraction gratings of different geometrical parameters are successfully fabricated in all developed elastic materials. The highest value of the refractive index modulation of about 1 × 10⁻³ is achieved. The technique enables to create also slanted gratings and multiple gratings. The use of such gratings as strain-controllable elements for managing of light is demonstrated. Moreover, elastic reflection gratings have been produced for the first time using conventional polydimethylsiloxane (PDMS) material. The proposed approach provides a very flexible, low-cost and effective tool for all-optical fabrication of complex diffraction structures in the broad class of commercially available elastomeric materials that undoubtedly gives a strong impact to the field of soft tunable optics, photonics, and spectroscopy.

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1. Introduction

Nowadays one of the important tasks of materials science is the development of controllable and easily processable “smart” materials for optics and photonics. One of the most promising materials are elastic polymers, such as polydimethylsiloxanes (PDMS), which thanks to their huge and reversible elastic deformations and good optical quality are widely used for the production of flexible and stretchable optical diffraction elements and devices: gratings, couplers, filters, lenses, beam steering devices etc.^1–8 Particular attention is focused on the diffraction gratings as the most simple and versatile element for light dispersion. In most studies the gratings manufactured in elastic materials (mostly PDMS) using a replica molding technique are surface relief gratings (SRGs), i.e. the surface of the elastomer is periodically corrugated. This method has been extensively used for fabrication of elastomer-based SRGs for polymer distributed feedback lasers,^9,10 modulators,¹¹ opto-microfluidics¹² or pressure sensing.¹³ Another type of diffraction gratings which has distinctive optical performance is volume diffraction gratings (VDGs) operating in the Bragg or Raman–Nath regimes. These gratings are formed by periodically modulated refractive index in the volume of the material while the surface of the sample does not change at all. The VDGs have many advantages such as very high diffraction efficiency, DE, (up to 100% in the first order, in a case of the Bragg regime), dependence of their diffractive performance on the incident angle of light, the wavelength and the grating thickness and also their ability to operate in transmission and reflection modes. There are a great number of the contributions dedicated to the materials for the recording of VDGs.^14–18 At the same time, the variety of optical materials for recording of VDGs with sufficient elasticity for tuning in a wide range is very narrow and is represented in a few studies only. Bunning et al. used so called holographic polymers dispersed liquid crystalline (HPDLC) materials for fabrication of tunable volume reflection grating, but such materials are rather complex and expensive, and provide only minor parameters alterations under the action of the external forces.¹⁹ There is also azobenzene-containing elastic material for VDGs (styrene-butadiene-styrene triblock copolymer) which requires complex multistep synthesis and has a number of other drawbacks, for example, the coloring of material (due to the absorption of the azobenzene groups) and poor gratings stability.^20a Recently we have reported the material based on PDMS for optical recording of VDGs.²¹ The method consists in the introduction of the benzophenone (BPh) molecules in PDMS at the stage of pre-polymer with following curing and gratings structuring by UV-holography or with an amplitude mask. This multi-step method is a time-consuming and has obvious limitation by using elastic materials of different chemical nature. In the current contribution we have modernized and unified the method of introduction of the photosensitive molecules into elastomers that significantly expands the number of suitable elastic polymers of different chemical nature as well as photoactive dopants (Fig. 1). We have demonstrated the role of covalent bonding of the photoactive agent and its diffusion during the gratings formation process. For the first time we have successfully fabricated tunable reflection gratings in elastic material. The abilities of a new method are illustrated through the manufacturing various diffraction optical elements (tunable volume gratings, multiplex gratings, planar light couplers, light switchers). It should be stressed that the approach enables to exploit the commercially (ready-to-use) elastic films or other elastomer products. Low cost and easy processing of the materials together with all-optical structuring technique makes the proposed approach of the fabrication of VDGs very perspective from a practical point of view.

Fig. 1 Chemical structures of the benzophenone (BPh) derivatives used as photosensitive dopants.

2. Experimental part

2.1. Preparation of the elastomeric polymer films

Several commercially available elastomer materials were used as polymer matrices for the fabrication of VDGs. The films of PDMS of Sylgard 184 (Dow Corning) were obtained by a standard procedure. Firstly, the “base” (liquid silicon rubber) and “curing agent” (catalyst mixture) were mixed with a weight ratio of 10 : 1. Then the mixture was degassed in vacuum for 1 hour, introduced into the glass cell of a fixed thickness controlled by Mylar spacers by capillary action and after that was cured in oven at 70 °C for 5 hours. After curing, the glass cell was split off to get a free standing PDMS film. Silicon elastomer Elastosil® 2030 (Wacker), hereinafter – Elastosil, was delivered in the form of 400 μm thick film and was used as received. Acrylic-based elastomer films VHB 4905, hereinafter – VHB, with the thickness of 550 μm were purchased by 3M. Poly(styrene-b-ethylene-co-butylene-b-styrene), hereinafter – SEBS, was delivered by Hexapol TPE. The SEBS films were made by doctor-blading of the polymer solution in toluene on a glass substrate. After the deposition the films were dried at room temperature for 2 days. All elastic films used were transparent and exhibited good optical quality. The refractive index of pure PDMS, Elastosil, VHB and SEBS films was measured with Abbe refractometer as 1.414, 1.413, 1.473 and 1.489, respectively.

2.2. Photosensitive dopants

Benzophenone (BPh), 4-methylbenzophenone (4MBPh), 3-methylbenzophenone (3MBPh), 4,4′-dimethylbenzophenone (DiMBPh), 4-hydroxybenzophenone and allyl bromide were purchased by Sigma-Aldrich and used without additional purification. 4-Allyloxybenzophenone (alBPh) was synthetized by etherification of 4-hydroxybenzophenone with allyl bromide according to previously described procedure.^20b

2.3. Preparation of the photosensitive elastomeric films

In order to make the elastic films sensitive to UV light they were modified with photoactive molecules. PDMS, Elastosil and VHB films were placed in chloroform solution of benzophenone derivative with an appropriate concentration for several minutes (usually 30 min) to let them swallow. Then the films were pulled out of the solution and dried at room temperature for several hours in order to completely remove the solvent. Depending on the concentration of chloroform solution the total content of dopant in elastic polymer matrix can be varied up to 4 wt%. The concentration of BPh in the elastic films was calculated from absorbance of the samples. For this purpose the series of PDMS films with known BPh concentrations were prepared as previously described²¹ and used for the calibration (see Fig. S1 in the ESI†). For the preparation of the SEBS photoactive films, 1 wt% of BPh was added to the toluene solution of SEBS before its blade-casting. The PDMS films with covalently bonded BPh molecules was produced by dissolving of 1 wt% of alBPh in a “base” component of Sylgard 184 at 80 °C; the following steps are the same as for the pure PDMS films fabrication (see above).

2.4. Optical structuring

UV holographic exposure technique was applied for the VDGs fabrication. The transmission unslanted gratings were recorded by a symmetric exposure to the interference pattern of two mutually coherent s-polarized beams of equal intensities. Recording intensity was 25 mW cm⁻² unless otherwise mentioned. The scheme of a holographic set-up is provided in Fig. S2a (ESI).† A laser (Genesis CX355-100 SLM OPS Laser-Diode System, Coherent) operating at a wavelength of 355 nm was used as a coherent light source. The grating period, Λ, was varied by changing the angle (2θ_rec) between the interfering beams. The gratings with fringes slanted with respect to the film surface were recorded by rotating the sample concerning to the bisector of the angle between the recording beams (Fig. S2a, in the ESI†). A non-actinic probe beam from a He–Ne laser at λ_t = 632.8 nm with an output power of 15 mW (Thorlabs) positioned at the Bragg angle was used for a real-time monitoring of the evolution of the grating formation. The diffraction efficiency (η) of the transmission gratings was determined as a ratio of intensity of the diffracted beam and a sum of intensity of the transmitted and diffracted beams. The refractive index modulation amplitude of the grating, Δn, was estimated using the experimental values of DE and Kogelnik equation for phase transmission Bragg gratings with a sinusoidal refractive index modulation and orthogonal grating planes (eqn (1)):^22awhere θ_B is the Bragg angle in the material; d – thickness of the film. The values of Δn were measured at the readout wavelength, λ_t = 632.8 nm.

The reflection gratings were inscribed using the exposure configuration shown in Fig. S2b (ESI).† In this case the recording beams were directed from the opposite sites on the sample. The change in the grating period and the fringes configuration was carried out by varying of the recording angle (2θ_rec) and through the rotation of the sample. It should be additionally mentioned that both holographic setups were highly protected against vibrations.

2.5. Measurements

Absorption and transmission spectra of the elastic films were measured using a TIDAS spectrometer (J&M). The intensity of laser light was measured by a power meter LaserMate-Q (Coherent). For the measurements of the optical properties of the elastic VDGs at different mechanical strain, a handmade setup consisting of two film-holder units and two micro-moving platforms installed on a precisely rotatable stage was used (see Fig. S3 in the ESI†). Two actuators with the micrometer screws provide the mechanical movement with an accuracy of 10 μm. The strain values was determined as 100% × (l − l_o)/l_o, where l_o initial film length, l – stretched film length.

3. Results and discussions

3.1. General principles of the VDGs formation in elastomers

The preparation of photosensitive elastic films consists in a soaking of the ready-to-use commercial elastomeric films in a solution containing a low molecular mass photoactive component (benzophenone derivatives) with following removal of the solvent (details are given in Experimental part). This method is extremely simple and suitable for modification of a wide range of flexible elastic materials for which the main requirement is an excellent optical quality.

The mechanism of the formation of VDGs is proposed based on investigation of the PDMS films doped by BPh. Due to a highly elastic state of the polymer matrix the incorporated low molar mass substances can quite freely move within the matrix that was evidenced by high diffusion coefficients.^21,22b,c On the other hand, this feature also provides a relatively high compatibility of the material components without phase separation. Regardless to the route of the introduction of the photo-active dopant into elastic polymers the behavior of such materials under the influence of light is invariable.

The action of a spatially modulated UV light (e.g. holographic exposure) on the PDMS films doped by BPh causes its photo-attachment to the polymer network through the formation of alfa-alkylbenzhydrazol in the areas of the constructive interference (bright areas). Simultaneously, because of the concentration gradient the dopant diffuses from the areas of the destructive interference (dark areas) to the bright areas where the photo-attachment of BPh takes place (Fig. 2a). As a result the alternating regions of high and low content of alfa-alkylbenzhydrazol are formed that creates a sufficient modulation of the average refractive index that in turn provides efficient light diffraction. Thus, the driving force of the formation of the VDGs in elastomers is the diffusion of a photoactive dopant and its photo-attachment to the polymer matrix. Moreover, if the diffusion of a dopant will be suppressed, for example by its covalent bonding to the elastic network, it results in lower refractive index modulation and, as a consequence, smaller diffraction efficiency (Fig. 2b). The latter case will be discussed in details below. It should be underlined that the VDGs formed are stable, i.e. long-time UV post-exposure and thorough washing of the films with the gratings in chloroform does not lead to the change in DE that is very important for the application. It can be associated only with the fact that the photoproduct is covalently bonded to the matrix.

Fig. 2 Schematic representation of the gratings formation in elastomer matrix with (a) and without (b) diffusion of the benzophenone molecules.

3.2. Influence of the benzophenone concentration on gratings properties

In this section the influence of the concentration of BPh in PDMS on the optical properties of the gratings will be discussed. By immersion of the 220 μm thick PDMS films in the chloroform solutions of BPh with different concentrations a series of the samples with different BPh content ranging from 0.5 to 4 wt% has been obtained. It should be noted that the gratings in the films containing 4 wt% of BPh were recorded within half an hour after the samples preparation because of crystallization of dopant. Fig. 3a shows the kinetics of the grating recording in the prepared samples. It is clearly seen that the increasing concentration leads to an increase in DE and also accelerates the gratings formation. Fig. 3b displays the values of the refractive index modulation (Δn) calculated according to the eqn (1) and the rate constants of the recording processes. The increase in Δn values is connected with higher content of the photo-attached BPh products in the area of the constructive interference that provides, consequently, higher refractive index modulation. Obviously, the rate of the DE growth increases due to a more efficient absorption of the material at the writing laser wavelength (355 nm).

Fig. 3 (a) Kinetics of the VDGs recording in the PDMS films with different concentrations of BPh; (b) maximal values of the refractive index modulation amplitude (Δn) and the recording rate constants versus BPh concentration in PDMS. Grating period – 853 nm; film thickness – 220 μm.

Furthermore the effect of the thickness of the PDMS films and the intensity of UV light on the dynamics and Δn values of the gratings were studied. The results are shown in Fig. S4 in the ESI.† It was revealed that the increase in the film thickness results in an almost constant Δn whereas the increasing intensity of UV light from 10 to 40 mW cm⁻² causes the increase in Δn by ∼56%. Moreover, the materials under study make it possible to record transmission VDGs in a very broad range of the spatial periods (from 300 nm to 4 μm) with approximately the same values of Δn. Thus, the approach proposed is quite flexible and enables to purposefully tailor the parameters of the gratings according to the requirements of proper practical applications. For example, by an uncomplicated variation of the recording parameters one can achieve almost 100% of DE, as it is shown in Fig. S5.†

3.3. Different benzophenone derivatives and polymer elastic matrices for the fabrication of VDGs

A series of methyl-substituted benzophenones, 4-methylbenzophenone (4MBPh), 3-methylbenzophenone (3MBPh), 4,4′-dimethylbenzophenone (DiMBPh), as well as 4-allyloxybenzophenone (alBPh), were used as photoactive dopants. Their chemical structures are shown in Fig. 1; absorbance spectra are gathered in Fig. S6.† Compared with the more hydrophilic benzophenone derivatives such as hydroxyl- and methoxy-substituted benzophenones methyl-substituted benzophenones exhibit a sufficient compatibility with elastic (usually hydrophobic) matrices and therefore they were selected as the photoactive dopants.

In order to demonstrate the impact of the dopant diffusion on the grating properties, alBPh was synthesized. Having a terminal allyl group alBPh is capable to react with hydrogensiloxane groups yielding of Si–C bonds and thereby becoming covalently attached to the polymer network of PDMS.

Fig. 4a shows the kinetic curves of the DE of the VDGs recorded in the 220 μm PDMS films doped with 2 wt% of BPh, 3MBPh, 4MBPh, DiMBPh and in the film containing 1 wt% of covalently linked alBPh (higher amount of a alBPh was not possible to introduce because of its low solubility in the “base” component of Sylgard 184). Our observations on the solubility of dopants in “base” component of Sylgard 184 reveal that their compatibility with PDMS matrix reduces in a series of BPh > 3MBPh > 4MBPh > DiMBPh > alBPh. It can be seen from Fig. 4a that the smallest value of Δn, ∼8 × 10⁻⁵, was obtained for the alBPh modified PDMS film that is assigned with a covalent attachment of the dopant to the polymer network preventing its diffusion. In fact the refractive index modulation is provided only due to the difference in the refractive indices of benzophenone and alfa-alkylbenzhydrazol (Fig. 2b). Such refractive indices' difference is considerably lower than one between the polymer matrix and alfa-alkylbenzhydrazol acting in the case of a non-suppressed diffusion. As it was also expected the homogeneous illumination of the alBPh based gratings with UV light leads to an almost complete erasure of the grating. This phenomenon is associated with photoreaction in “dark” areas of the grating yielding to linked alfa-alkylbenzhydrazol that, in turns, leads to decrease in refractive index modulation contrast.

Fig. 4 (a) Kinetics of the VDGs recording in the PDMS films modified with different benzophenone derivatives. Concentration of BPh, 3MBPh, 4MBPh and DiMBPh in PDMS films is equal to 2 wt%, whereas concentration of alBPh is 1 wt%. (b) Kinetics of the VDGs recording in the PDMS, Elastosil, VHB and SEBS elastic films doped with 1 wt% of BPh. Grating period – 853 nm.

All other samples where nothing prevents the dopant diffusion have showed significantly higher values of Δn, from 6 × 10⁻⁴ to 1 × 10⁻³. The kinetics curves presented in Fig. 4a show that the samples containing BPh and 3MBPh possess almost the same Δn whereas the substituted in para position dopants 4MBPh and DiMBPh exhibit higher Δn. The given phenomenon can be explained by the stabilization of the π–π* charge transfer form of BPh exited triplet state²³ by methyl-group in the para position of the aromatic ring that makes BPh radical less reactive. The detailed description and the schemes of the resonant radical structures are presented in the ESI (Fig. S7†). Slightly less radicals' activity enables their more complete diffusion from dark to bright regions of the interference pattern during the grating recording which leads to higher Δn. Thus, it was shown a crucial role of dopant diffusion in the gratings formation mechanism and it was also found that the 4-methyl substituted derivatives of BPh provide the highest values of Δn and, consequently, the highest DE.

Let us to demonstrate the possibility to apply our approach to other commercially available elastic polymers. The choice of Elastosil, VHB and SEBS elastomers was dictated by the fact that these materials are widely used for the production of dielectric elastomeric actuators (DEAs) or artificial muscles^10,24–26 (i.e. devices converting energy of electric field to the mechanical work) and furthermore they have a good optical quality. The possibility of the production of VDGs in these materials would enable to create single-layer (or monolith) electro-controllable optical elements.

The modification of VHB and Elastosil films was carried out in the same manner as the modification of PDMS (see Experimental part). SEBS was doped with BPh at the stage prior the casting of the films. The concentration of BPh was kept in all cases the same for the correct comparison of the results and also due to the fact that the increase of the dopant content in Elastosil and SEBS films causes its crystallization.

Chemical nature of the selected elastic matrices should be also discussed extra. VHB and Elastosil ready-to-use films represent chemically cross-linked networks on the basis of polyacrylates and polydimethylsiloxanes, respectively. SEBS is a tri-block copolymer (styrene-ethylene-co-butylene-styrene), wherein the network is physical, i.e. network nodes are formed by polystyrene microphase. So, Fig. 4b shows the kinetics of the gratings recording in PDMS, Elastosil, VHB and SEBS matrices doped with 1 wt% of BPh. As it can be seen the use of the elastic matrices of a completely different chemical nature allows us fabricating the VDGs with rather high values of Δn, from 3 × 10⁻⁴ to 5 × 10⁻⁴. The doped VHB samples demonstrate practically the same value of Δn as PDMS, while Elastosil and SEBS possess smaller Δn. Lower Δn of the Elastosil grating is related to the fact that the elastomer is already doped by silica particles (doping was made at a production stage for the improvement of the dielectric properties of the material), thus, this matrix has less free volume in comparison with PDMS that partially hinders the diffusion of the dopant during holographic exposure. The relative low Δn obtained for the SEBS samples can be explained by its high refractive index (∼1.489) providing lower contrast of the refractive indices of the matrix and the linked-photoproduct (Fig. 2a). Despite to the high refractive index of polyacrylic elastic VHB matrix the elevated Δn values were observed. It can be governed by the polarity of the matrix which stabilizes the π–π* charge transfer form of the BPh exited triplet state²⁷ reducing its activity analogous to the influence of electron-donor substituent in para position of BPh that, consequently, increases an diffusion of a dopant during the grating recording.

Thus, the presented results demonstrate the capability to apply our approach (the material modification with a corresponding optical structuring) to a fairly wide range of elastic materials of diverse nature. It allows the fabrication of VDGs of different parameters those can be easy integrated into a variety of modern tunable micromechanical devices and systems, for example, into DEAs. Our next work will be focused on this kind of single-layer electro-driven optical devices.

3.4. Performances of elastic VDGs under external strain

Since the materials under study are elastic and can be stretched reversibly to a very large extent the inscribed VDGs are also able to vary their parameters in a wide range by a strain that, in turns, can be successfully used to produce tunable optical diffraction elements. In this section we illustrate the functionality of the obtained VDGs of proper geometrical parameters under mechanical strain and discuss their possible applications.

According to the Bragg condition (2Λ sin θ_B = λ_t) the highest diffraction efficiency is observed only at a strictly defined incident angle of light and at a proper wavelength therefore volume Bragg gratings are mostly (but not always) used for manipulation of coherent monochromatic light. Mechanical or electrical control of the gratings parameters (like Λ, d, θ) gives the possibility to realize tunable diffraction elements. Due to its high flexibility the holographic patterning method allows fabricating the diffraction structures of different geometries, for example, slanted gratings, multiplex gratings, reflection gratings, etc.

Fig. 5a shows a lateral shift of the diffracted beam of the PDMS grating with fringes orthogonal to the grating surface under a mechanical strain. A strain causes the elongation of the spatial period of the grating that in turn changes the diffraction angle of the laser beam used (λ_t = 532 nm). It is clear seen that at a 100% mechanical strain the diffraction angle is halved that is in consistent with the theory (red dashed line in Fig. 5a). It means that an 100% elongation of the grating leads to the increase in the grating period also for of about 100% as it is shown in the ESI (Fig. S8a†). Interestingly, a small decrease in DE occurs under a strain because of the reasonable reduction of the film thickness (see Fig. S8b in the ESI†). The inset in Fig. 5a shows the change of the diffraction pattern under the grating stretching from 0% up to 100%. Thus, the elastic VDGs can be used, for example, as beam steering devices. The advantage of a volume Bragg grating for such applications is the presence of only one movable signal (−1^st diffraction order) instead of multiple diffraction orders as in the case of conventional SRGs. Another notable feature of the studied elastic VDGs is that the switching between the Bragg and Raman–Nath diffraction regimes can be achieved under a strain by simple selection of the sample parameters (see Fig. S9 in the ESI†).

Fig. 5 (a) Evolution of the diffraction Bragg angle of the PDMS based VDG under strain. Inset shows the diffraction patterns at different strain (grating period – 1.95 μm; film thickness – 220 μm); (b) disappearing of the diffracted signal under a small strain of the PDMS based VDG (grating period – 530 nm; film thickness – 400 μm).

Intrinsic spectral selectivity of volume Bragg gratings, which depends on the period and thickness, allows separating of a narrow spectral band from a wide polychromatic spectrum. We have already demonstrated that the tunable volume PDMS Bragg gratings in combination with a white light source could be used for wavelength-adjustable spectral applications.²¹

The angular selectivity of the stretchable Bragg VDGs can be used as well. Small deviations of the grating parameters along with the parameters of the input beam cause the mismatch with the Bragg condition that leads to change of the diffraction performance. Fig. 5b shows the disappearing of the diffracted beam at a very small stretching (incident angle is fixed) of the grating of a 400 μm thickness and period of Λ = 531 nm. It can be seen that for the given VDG 0.5% strain only is enough for a complete “switching off” of the diffracted beam (see also the diffraction patterns presented in the inset of Fig. 5b). At this stretching the grating period increases from 531 nm (the corresponding Bragg angle is θ_B = 30.06° for λ_t = 532 nm) to of 533.7 nm (the corresponding θ_B will be ∼29.87°). The difference of the Bragg angles for the grating with Λ = 531 and 533.7 nm is ∼0.19° that is larger than an angular selectivity of gratings of similar thickness which is equal to 0.16°.²¹ Therefore it causes breach of the Bragg condition and, consequently, the disappearance of the diffracted beam. Hence, if the elastic film with a volume Bragg grating has a length of, for example, 1 cm, only a several tens microns of grating elongation is required in order to “switch off” the diffracted signal, that can be simply provided by conventional piezoelectric actuators. This approach can be used to create fast optical modulators for optical sensors, spectroscopy, switchers or spectral filters in elastic optical fibers. The parameters of the gratings can be easy adjusted to satisfy proper application requirements.

Some applications require normal incidence of the input light on the device (for example, display back-light systems) that is not possible to realize using transmission Bragg gratings with the fringes perpendicular to the surface. For example, in order to provide normal incidence of coherent beam on the grating so-called slanted VDG can be used. The developed materials allow fabricating the volume structures with variable spatial arrangement of the refractive index modulation pattern. The slanted VDGs of different periods (1 μm–300 nm) and the slant angles up to 45° were fabricated. The DE almost does not differ from that of the gratings with non-slanted fringes. Moreover, not only single but also triplex²⁸ elastic VDG having three fringes set of three different slant angles was successfully produced (see description in the ESI, Fig. S10†).

The important application of diffraction gratings is coupling of free-space light into optical waveguides or fibers and vice versa. This is mostly realized with the prism or the grating coupling approaches.^7,29 Due to their compact size and planarity the grating couplers prevail over prism couplers. For most applications the SRGs which are usually produced by interference or electron-beam lithography followed by wet or dry etching of semiconductors or polymeric waveguides are exploited.⁷ Volume holographic gratings based on cheap and soft polymeric materials look as interesting candidates for this application.^30,31 In the current work we propose using our materials and method of slanted VDGs fabrication for elastic light couplers.

When the light wave propagates in the waveguide the corresponding diffraction angle (β) should satisfy β ≥ θ_c = arcsin(1/n), where θ_c is the angle of the total internal reflection. The period and the slant angle of the grating were selected taking into account θ_c (44.9°) in the doped PDMS (n ≈ 1.415). The grating coupler with a period of 316 nm and a slant angle 27° was recorded in 220 μm PDMS film containing 2 wt% of BPh. In-coupling of green and red coherent light into the glass substrate at 20° and 30.5° incident angles, correspondingly, is shown in Fig. 6a. If white collimated light is used one can selectively in-couple light of different wavelengths in a substrate by the rotation of the grating. The rotation of the grating by about 22° results in the selective in-coupling of light within broad visible spectral range, 408–630 nm, due to the variation of the incident angle of white light (Fig. 6b). The effect is clearly demonstrated in the photos of the sample at the top of Fig. 6b.

Fig. 6 (a) In-coupling of green and red laser light into glass substrate by means of the elastic VDG coupler; (b) dependence of the wavelength of in-coupled light via the incident angle of white light; illustration of the tuning of in-coupled light (at the top); (c) transmission spectra of the VDG coupler at different strains; (d) dependence of the wavelength of in-coupled light via a strain; the incident angle of light is 8°; (e) tuning of the in-coupled laser light at a mechanical strain of the VDG coupler (i.e., elongation of the grating period), the incident angle of light is 14.5°; corresponding schematic representations (at the right). In all cases the same elastic slanted VDG was used.

Furthermore, a free-standing film with the grating coupler under study was investigated upon a mechanical strain. The evolution of spectrum of the transmitted beam at variable mechanical stretching at a constant incident angle of white light is depicted in Fig. 6c. Fig. 6d shows the corresponding dependence of the wavelength of in-coupled light via a strain. It was found that the 50% strain enables to control the wavelength of coupling light in the range of 400–750 nm, i.e. in the whole visible range. To illustrate the effect, the studied grating was tested using three (RGB) laser beams all aligned along one axis and incident at the angle of 14.5° on the sample in order to in-couple firstly a blue light (475 nm) into the films at 0% strain (Fig. 6e, at the top). When the sample is stretched by 13% the in-coupled blue light disappears and the Bragg condition will be satisfied for a green light, hence, its in-coupling is observed. The red laser beam was in-coupled at a ∼23% strain as shown in Fig. 6e (at the bottom). The video of this experiment is available in the ESI (Video S1.mpg†). The mechanical strain increases the period of the structure and provides the Bragg condition consecutively for blue, green and red wavelengths. So, it shows that the proposed elastic VDGs can operate as devices for controllable in/out-coupling of light into planar waveguides in a very wide spectral range that can be extremely useful for optics, photonics and display technology. It is noteworthy that the VDGs presented here are sufficiently simpler in terms of materials comparing with elastic HPDLCs¹⁹ and azobenzene-containing liquid crystalline elastomers.^20a,32,33 In the terms of the gratings performances we can sign out the following advantages of our elastic VDGs: higher efficiency (up to ∼100% in one order); higher spatial resolution (of about 140 nm for the reflection gratings, as will be demonstrated below); higher fully reversible tunability (more than 100%, depending on the used elastomer); no degradation of the grating performances at elevated temperatures and non-sensitivity to light polarization. Comparing with alternative methods for the production of elastic gratings, e.g. using femtosecond pulses or ion beams,^34–36 the proposed approach is non-destructive and, consequently, diffraction elements possess higher optical quality, and moreover, the gratings of large area can be produced by a single-step exposure and also with much higher spatial resolution.

3.5. Elastic reflection VDGs

Above we have demonstrated different tunable diffractive elements based on unslanted and slanted holographic transmission VDGs. The other kind of VDGs that is also very attractive for proper applications is the reflection gratings (inset in Fig. 7a) with fringes parallel or slightly titled with respect to the surface of the grating. In this case the diffracted beam crosses the front surface of the grating.³¹ Such gratings possess much smaller periods compared to the transmission ones and require high dimensional stability of the material during the structuring. These gratings are mostly used as a narrow-band spectral or angular filter, beam deflectors/magnifiers, etc. To our knowledge only several works are devoted to elastic reflection gratings.¹⁹ Here we present our first results on the fabrication of reflection VDGs in doped PDMS. The gratings were recorded using a modified holographic set-up as shown in Fig. S2b† (see also Experimental section).

Fig. 7 (a) Transmittance spectra of reflection gratings of different periods formed in PDMS doped by 2 wt% BPh. Grating periods: 128 nm (solid line), 132 nm (dashed line) and 139 nm (dotted line). The inset schematically shows the reflection grating. (b) Evolution of transmittance spectra during contraction and extension of the elastic reflection grating. (c) Spectral shift of the reflection peak via a strain; the schemes of the contraction/extension processes are shown at the top. The thickness of all samples is 82 μm.

Varying the recording angle and the position of the sample with respect to the interference pattern the gratings possessing the reflection peaks at 355, 363 and 393 nm have been fabricated for the first time in conventional elastic material (Fig. 7a). It is noteworthy that the observed reflection peaks are very narrow (about 3–5 nm) which is caused by high spectral selectivity of inscribed gratings. These results evident the material under study exhibits very high spatial resolution because of the photoinduced reaction on molecular scale that allows forming periodical refractive index modulation structure with the minimal dimension of about 130 nm.

The reflection grating with the period of ∼139 nm was tested in regard to the tunability of its reflection properties. For this purposes the reflection grating was placed on the pre-stretched (25%) supporting PDMS film. In the first experiment the strain of the supporting film was stepwise reduced by 10%. As a result the shift of the reflection maxima towards long wavelengths has been observed (Fig. 7b). Such deformation of the film with the reflection grating on top leads to the grating contraction. Taking into account that the volume of the film is constant it can be concluded that the film thickness increases by contraction. In turns, the increase of the thickness causes the elongation of the grating period as it is schematically illustrated in Fig. 7c (at the top). It is seen in Fig. 7c (solid squares) that the 10% contraction shifts the reflection peak from 392 to 405 nm. In the second experiment the strain of the supporting film was increased that causes a hypsochromic reflection peak shift (Fig. 7b). Obviously, stretching of the supporting film leads also to extension of the reflection grating accompanying with an appropriate decrease of its thickness (Fig. 7c, at the top). It means that the extension reduces the period of the reflection gratings. Fig. 7c (open squares) shows the shift of the reflection peak from 392 to 369 nm by 19.4% strain. Thus, by means of simple contraction–extension of the elastic reflection grating the precise tuning of reflection wavelength by 36 nm can be achieved. Therefore such gratings can be potentially used as stretchable (controllable) narrow-width spectral filters.

4. Conclusion

Simple and effective method of the creation of photosensitive elastic materials and holographic VDGs on the basis of thereof is demonstrated. The approach enables to exploit wide variety of commercial elastomeric polymer matrices of different nature (silicones, acrylates, etc.) as well as different substituted benzophenones as photoactive dopants. The gratings were recorded in new photosensitive elastic materials using UV-holography. The influence of the material formulation on the parameters of the gratings was investigated. Different types of the elastic VDGs fabricated were studied under the mechanical strain. Extremely wide tunability of the elastic gratings was shown. The possible optical devices based on elaborated VDGs of different geometries have been discussed. Not only transmission and slanted gratings but also reflection gratings were successfully fabricated that proves enormous flexibility of this material-structuring approach for the creation of a variety of soft and tunable optical elements. Obviously, the proposed materials offer the ability to design sophisticated and cheap soft diffraction structures like multiband notch filters, holographic lenses, spectral beam combiners, grating couplers, complex beam-splitter, etc.

Acknowledgements

The work has been funded partly by Alexander von Humboldt Foundation and by the German Federal Ministry of Education and Research within the project “EDEL” (FKZ: 03V0881).

References

1. L. Maffli, S. Rosset, M. Ghilardi, F. Carpi and H. Shea, Adv. Funct. Mater., 2015, 25, 1656.
2. C. Ghisleri, M. Siano, L. Ravagnan, M. Potenza and P. Milani, Laser Photonics Rev., 2013, 7, 1020.
3. M. Aschwanden and A. Stemmer, Opt. Lett., 2006, 31, 2610.
4. H. C. Jau, Y. Li, C. C. Li, C. W. Chen, C. T. Wang, H. K. Bisoyi, T. H. Lin, T. J. Bunning and Q. Li, Adv. Opt. Mater., 2015, 3, 166.
5. J. L. Wilbur, R. J. Jackman, G. M. Whitesides, E. L. Cheung, K. L. Lee and M. G. Prentiss, Chem. Mater., 1996, 8, 1380.
6. M. Aschwanden, M. Beck and A. Stemmer, IEEE Photonics Technol. Lett., 2007, 19, 1090.
7. A. Kocabas, F. Ay, A. Dana and A. Aydinli, J. Opt. A: Pure Appl. Opt., 2006, 8, 85.
8. J. Missinne, S. Kalathimekkad, B. Van Hoe, E. Bosman, J. Vanfleteren and G. Van Steenberge, Opt. Express, 2014, 22, 4168.
9. J. A. Rogers, M. Meier, A. Dodabalapur, E. J. Laskowski and M. A. Cappuzzo, Appl. Phys. Lett., 1999, 74, 3257.
10. S. Döring, M. Kollosche, T. Rabe, J. Stumpe and G. Kofod, Adv. Mater., 2011, 23, 4265.
11. B. Grzybowski, D. Qin, R. Haag and G. M. Whitesides, Sens. Actuators, A, 2000, 86, 81.
12. O. Schueller, X. Zhao, G. Whitesides, S. Smith and M. Prentiss, Adv. Mater., 1999, 11, 37.
13. K. Hosokawa, K. Hanada and R. Maeda, J. Micromech. Microeng., 2002, 12, 1.
14. F. Bruder, R. Hagen, T. Rölle, M. Weiser and T. Fäcke, Angew. Chem., Int. Ed., 2011, 50, 4552.
15. T. Trout, J. Schmieg, W. Gambogi and A. Weber, Adv. Mater., 1999, 10, 1219.
16. O. Sakhno, L. Goldenberg, J. Stumpe and T. Smirnova, J. Opt. A: Pure Appl. Opt., 2009, 11, 024013.
17. (a) D. Marmysh, A. Stankevich and V. Mogil’nyi, J. Opt. Technol., 2012, 79, 116; (b) U. Mahilny, D. Marmysh, A. Tolstik, V. Matusevich and R. Kowarschik, J. Opt. A: Pure Appl. Opt., 2008, 10, 085302.
18. A. Popov, I. Novikov, K. Lapushka, I. Zyuzin, Y. Ponosov, Y. Ashcheulov and A. Veniaminov, J. Opt. A: Pure Appl. Opt., 2000, 2, 494.
19. (a) D. M. Smith, C. Y. Li and T. J. Bunning, J. Polym. Sci., Part B: Polym. Phys., 2014, 52, 232; (b) S. A. Holmstrom, L. V. Natarajan, V. P. Tondiglia, R. L. Sutherland and T. J. Bunning, Appl. Phys. Lett., 2004, 85, 1949; (c) T. J. Bunning, L. V. Natarajan, V. P. Tondiglia and R. L. Sutherland, Annu. Rev. Mater. Sci., 2000, 30, 83.
20. (a) Y. Zhao, S. Bai, K. Asatryan and T. Galstian, Adv. Funct. Mater., 2003, 13, 781; (b) O. Prucker, C. Naumann, J. Rühe, W. Knoll and C. Frank, J. Am. Chem. Soc., 1999, 121, 8766.
21. A. Ryabchun, M. Wegener, Y. Gritsai and O. Sakhno, Adv. Opt. Mater., 2016, 4, 169.
22. (a) H. Kogelnik, Bell Syst. Tech. J., 1969, 48, 2909; (b) H. Yoshioka, Y. Yang, H. Watanabe and Y. Oki, Opt. Express, 2012, 20, 4690; (c) M. Saito, T. Nishimura, K. Sakiyama and S. Inagaki, AIP Adv., 2012, 2, 042118.
23. (a) G. Porter and P. Suppan, Trans. Faraday Soc., 1965, 61, 1664; (b) S. Christensen, M. Chiappelli and R. Hayward, Macromolecules, 2012, 45, 5237.
24. J. Plante and S. Dubowsky, Int. J. Solids Struct., 2006, 43, 7727.
25. A. O'Halloran, F. O'Malley and P. McHugh, J. Appl. Phys., 2008, 104, 071101.
26. H. Stoyanov, M. Kollosche, S. Risse, R. Waché and G. Kofod, Adv. Mater., 2013, 25, 578.
27. N. Ghoneim, A. Monbelli, D. Pilloud and P. Suppan, J. Photochem. Photobiol., A, 1996, 94, 145.
28. R. Shi, J. Liu, H. Zhao, Y. Liu, Y. Hu, Y. Che, J. Xie and Y. Wang, Appl. Opt., 2012, 51, 4703.
29. (a) P. K. Tien, Appl. Opt., 1971, 10, 2395; (b) M. Dakss, L. Kuhn, P. Heidrich and B. Scott, Appl. Phys. Lett., 1970, 16, 523.
30. N. Zhang, J. Han, X. Li, F. Yang and B. Hu, Appl. Opt., 2015, 54, 3645.
31. R. Collier, Optical holography, Elsevier, 2013.
32. E. Sungur, M. Li, G. Taupier, A. Boeglin, M. Romeo, S. Mery, P. Keller and K. Dorkenoo, Opt. Express, 2007, 15, 6784.
33. M. Devetak, B. Zupančič, A. Lebar, P. Umek, B. Zalar, V. Domenici, G. Ambrožič, M. Žigon, M. Čopič and I. Drevenšek-Olenik, Phys. Rev. E, 2009, 80, 050701.
34. D. Kallepalli, N. Desai and V. Soma, Appl. Opt., 2010, 49, 2475.
35. S. Cho, W. Chang, K. Kim and J. Hong, Opt. Commun., 2009, 282, 1317.
36. R. Huszank, S. Szilasi, I. Rajta and A. Csik, Opt. Commun., 2010, 283, 176.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10918g

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