Free-surface photopolymerizable recording material for volume holography

Tina Sabel; Susanna Orlic; Karl Pfeiffer; Ute Ostrzinski; Gabi Grützner

doi:10.1364/OME.3.000329

1. Introduction

Photopolymers are highly attractive materials for numerous current and future applications. Based on holographically fabricated structures with a periodic modulation of the refractive index, these include three-dimensional photonic crystals [1,2], optical data storage technologies [3], diffractive and holographic optical elements (DOEs and HOEs) [4], devices in optical networks, such as (De-) Multiplexers [5] or narrow-spectral bandwidth filters [6] and holographic interferometry [7].

The scope of applications for photosensitive polymers would significantly expand with the possibility of combination and integration with diverse materials and substrates. The properties of particular interest for many emerging applications concern layer formatting and sandwiching in multilayer configurations. Free-surface samples that can be adhered to a wide range of substrates allow novel integrated optics by multiple-interface options. In addition, relatively thick layers are required to ensure high performance in terms of diffraction efficiency, angular selectivity, fill factor, and storage capacity. In order to fulfill those requirements, we develop a new material based on epoxy guest-host systems. With similar systems we have demonstrated the generation of single-mode waveguides by UV-mask exposure fabrication [8]. The material composition is also suitable for the incorporation of self-organized materials, such as nanoparticles. This provides the potential for improved systems with additional functionality [9–11] or for polymeric templates [12].

SU8 is well known for lithography applications [13]. Cost- and time-efficient, it is commonly used for the fabrication of micro- and nano-electromechanical systems (MEMS/NEMS) [14]. It forms layers up to 2 mm thick and enables high resolution and high aspect ratio. With good adhesion on silicon and glass, high chemical resistance and biological compatibility, it shows great promise as a basis for advanced integrated systems [15].

However, with no diffusing component SU8 is not qualified for use in volume holography. By doping with epoxy monomers, we combine the advantages of SU8 with the advantages of a diffusing guest-host system. This enables all-optical processing and avoids the need of post-exposure chemical treatment. Furthermore, the optical properties can be matched to specific requirements [16]. However, the capability of three-dimensional holographic structuring remains challenging. In addition, new material compositions for highly functional components should offer high photosensitivity, sufficient inducible local refractive index change, good dimensional stability and high resolution.

While commonly photopolymer samples and films are fixed in layer thickness and covered with glass plates [17], we apply a spin-coating fabrication technique. This allows a high flexibility in substrate choice, layer thickness and surface design, and opens diverse possibilities for new applications, especially for integrated optics. Among those are multilayer devices and holographic stacks, as for example proposed by [18]. Stacks may consist of diverse photonic materials. Multiple layers may be fabricated in separate processing steps. With functionalities such as wide bandwidth, multilayer configurations might complement multiplexing techniques.

This paper is structured as follows. Section 2 presents details on the material composition and sample preparation. Methods used for holographic recording and optical analysis are described in section 3. This includes the investigation of the material’s dimensional stability as well as the influence of the refractive index contrast and the layer thickness on the maximum diffraction efficiency and on the angular response. Results on the material photoresponse, energetic sensitivity, angular selectivity and resolution are presented in section 4. Finally, we discuss the appearance of nonsinusoidal refractive index profiles as well as the influence of the exposure duration.

2. Material composition and sample preparation

Synthesizing a volume holographic recording material with the attribute of a free surface imposes several requirements on the material composition. Three-dimensional holographic patterning in stable uncovered layers entails antithetic exigencies. A diffusing guest-host system enables a light-induced refractive index modulation. At the same time, the material system must be able to form volumetrically stable and homogeneous layers. Furthermore, the host and guest components are to differ in refractive index as much as possible, nonetheless being chemically compatible with each other. The molecular weight of the guest components must be small enough to allow diffusion, but high enough to form layers. Finally, the reproducibility in chemical formulation and sample preparation has to be ensured.

We meet the requirements with a material composition based on an epoxy oligomer. Both the host and guest molecules have epoxy functional groups. The corresponding mechanism of polymerization is a cationic ring-opening polymerization (CROP). The oligomer host-system features an aromatic bond of the epoxy group. In case of the monomer guest, the epoxy group is bound aliphaticly. The refractive indices of the host and guest components at the sodium-d-line are $n_{host} >1 .58$ and $n_{guest} \approx 1 .46$ , respectively. Guest and host components are compatible with each other in all proportions. A sensitized photo-acid-generator (PAG) is used, causing crosslinking by cationic polymerization at 405 nm [15].

Figure 1 shows the wavelength-dependent attenuation of the material in solution. The photopolymer samples feature excellent transmission in the visible spectrum.

Fig. 1 The spectral optical attenuation of the material solution features excellent transmission in the visible spectrum.

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To fabricate free-surface samples without a protective layer but with sufficient dimensional stability, we limit the dope concentration. As a result, we receive tack-free coatings up to a guest content of 17% by weight. The viscosity for spin coating (24.000 m Pa s) is adjusted by dilution with solvent. In contrast to other systems [19], the solvent does not function as diffusing component. This allows us to prepare samples in form of dry film layers. Rotation speed of ${500 - 4000 min}^{-1}$ enables the creation of defect-free layers with thickness ranging from 50 µm up to more than 250 µm. We identify the glass transition temperature to be $T_{G} =25°C$ . This ensures sufficient mobility for monomer diffusion while crosslinking reduces the mobility of the matrix. Fixation by UV flood exposure (dose 350 mJ/cm²) and hard bake up to 140°C results in $T_{G} =130°C$ .

3. Optical structuring and characterization techniques

Optical techniques we use to investigate the fundamental properties and behavior of the developed photopolymer system are primarily based on the recording and analysis of one-dimensional, plane-wave volume holograms. Both transmission- and reflection-type holographic gratings are created and detected for the purpose of optical characterization. Volumetric hologram recording is done by two freely propagating beams within a holographic exposure setup. The light source is a linearly s-polarized 405 nm diode laser. Exposure time is controlled by an electromechanical shutter. Laser power is adjusted between 0.4 and 1 mW per beam, while the beam diameter is 2 mm. The exposure time varies from 1 to 20 seconds. After exposure and a setting time of about 20 minutes the samples are fixed by UV flood cure.

Recorded holographic gratings are analyzed by observing the diffracted light in a rotation-scan setup, as for example in [20]. The transmitted signal of a 543 nm HeNe laser is detected while the hologram under test is rotated. Angular resolution is below 0.01 degrees. The probe beam used for detection is significantly smaller in diameter than the recording beam. Using smaller probe beams, for example, with a diameter of 0.2 mm, entail less signal variation and deliver more precise results [21].

From the angular resolved transmission we derive the efficiencies of particular diffraction orders. The angular response includes information about the grating period via

Λ (λ, θ) = \frac{λ}{2 n \sin θ} .

We determine volume shrinkage in the direction of the grating vector, comparing the grating period

Λ (λ_{p}, θ_{p})

, derived from the rotation scan, with the period of the exposure interference pattern

Λ (λ_{e x p}, θ_{e x p})

. Herein

λ_{e x p}

and

λ_{p}

are the exposure and probe beam wavelength, respectively.

θ_{e x p}

and

θ_{p}

are the differences of the Bragg angles, given by the exposure geometry and derived from the rotation scan, respectively. The volumetric shrinkage derived from the angular response is 2% in average. This ensures good dimensional stability [21].

By comparison of the angular resolved diffraction efficiency with a rigorous solution of the coupled wave theory (RCWT) [22], we derive the layer thickness d and the refractive index contrast $Δ n$ . The diffraction efficiency $η$ for a probe wavelength $λ$ and the probe angle $ϑ_{p}$ is given by:

η = s i n^{2} (\frac{π d Δ n}{2 λ \cos (ϑ_{p})}) .

Figure 2 shows the impact of layer thickness and refractive index contrast on the angular response. Both gratings displayed feature the same index contrast

Δ n = 3.6 \cdot 10^{- 3}

. A variation in layer thickness of only 20% has a strong effect on the angular selectivity. Within the possibilities of material preparation to set arbitrary layer thickness from 50 to 250 μm, this enables the adjustment of the angular response over a wide range.

Fig. 2 Interdependency of index modulation and layer thickness: measurement (dots) and RCW theory (line). Within constant index contrast a variation in layer thickness has a strong impact on angular response.

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4. Material response and performance

The photoresponse of the developed material to exposure fluence is widely linear. As shown in Fig. 3 , there is a rapid increase in the material response with increasing energy density. The response curve corresponds to an exposure time of 15 seconds.

Fig. 3 Linear material response to deposited energy shows a rapid increase of the refractive index contrast with increasing energy density.

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In the case of cationic initiators, it is expected from theory to find the induced index change to be proportional to the absorbed energy [23]. Exposure duration should not have an impact on the degree of polymerization. Figure 4 shows the refractive index contrast versus exposure time for various exposure fluences. We observe a weak dependence on exposure time at low energetic fluence, whereas there is a stronger dependence at higher energetic fluence.

Fig. 4 Refractive index contrast versus exposure time for various exposure fluences shows a constant material response at low energetic fluence.

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In contrast, the energetic sensitivity S shows an influence of the exposure duration. S is defined as the ratio of the refractive index contrast to deposited energy $I \cdot t$ :

S = \frac{Δ n}{I \cdot t} .

Progressions for various exposure fluences are shown in Fig. 5 . We derived optimum exposure duration that is independent on the energetic fluence. The optimum exposure time

t_{o p t}

is in the order of 10 seconds. The material shows high photosensitivity as compared with other epoxy-resin photopolymers [24]. The maximum energetic sensitivity, read from Fig. 5, is S_max = S(

t_{o p t}

) = 11 cm²/J.

Fig. 5 Energetic sensitivity versus exposure time for various exposure fluence shows: Optimal exposure time is independent on the deposited energy.

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Figure 5 also reveals a weak dependence of the sensitivity on exposure intensity I. For short exposure times, the maximal sensitivity is achieved at low intensity. This is similar to characteristics of nanocomposite materials [9]. In the case of longer exposure duration, the behavior is reversed; the sensitivity increases with exposure intensity.

The angularly resolved diffraction efficiency of the first diffraction order is displayed in Fig. 6 . The measured diffracted efficiency is obviously in good agreement with the rigorous coupled wave theory. We reach diffraction efficiency close to 100% with a layer thickness of 200 μm. Angular selectivity better than 1 degree is obtained at a layer thickness of about 250 μm. Among other applications, sharp angular selectivity is very important for multiplexing [25] and all related applications.

Fig. 6 Angular response shows high selectivity: measurement (dots) and RCW theory (line). Exposure time was 6s, fluence 260 mJ/cm². Layer thickness is 280 μm, grating period Λ = 1.7 μm and index contrast $Δ n = 1.8 \cdot 10^{- 3}$ .

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Higher diffraction orders appear in case of overmodulation, but may also be caused by saturation effects [26]. This can be described by the parameter:

R = {| K |}^{2} \frac{D}{F},

where D is the diffusion coefficient, F half the maximum polymerization rate and K the grating vector. A small R parameter results in deviations from the sinusoidal modulation of the refractive index [16]. This may be caused by high polymerization rate, slow diffusion and/or big grating periods. Considering higher orders of the refractive index contrast, those effects can be included in coupled wave theory [27].

Figure 7 shows the transmitted light in a corresponding measurement and the derived index profile. In this case, the saturation effects are caused by very long dark storage times, resulting in high viscosity and consequently slow diffusion rates. By comparison with RCWT simulations, we derive the first, second, and third order refractive index contrast to be: $Δ n^{(0)} = - 4.5 \cdot 10^{- 3}$ , $Δ n^{(1)} = 1.5 \cdot 10^{- 3}$ and $Δ n^{(2)} = 0.5 \cdot 10^{- 3}$ . The corresponding index profile over the lateral position x, displayed in Fig. 7, is calculated via

Fig. 7 Saturation effects. Left: angular resolved measurement (dots) and RCW theory (line). Exposure time was 4.5 s, fluence 700 mJ/cm². Layer thickness is 140μm, grating period Λ = 2.7μm and index contrast $Δ n^{(0)} = - 4.5 \cdot 10^{- 3}$ , $Δ n^{(1)} = 1.5 \cdot 10^{- 3}$ and $Δ n^{(2)} = 0.5 \cdot 10^{- 3}$ . Right: corresponding nonsinusoidal refractive index profile (line) in comparison with sinusoidal interference pattern (dots).

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n (x) = n_{0} + Δ n^{(0)} \cos (| K | x) + Δ n^{(1)} \cos (2 | K | x) + Δ n^{(2)} \cos (3 | K | x) .

The material resolution enables three-dimensional periodic structuring with up to 8000 lines per mm. However, it is expected to find an optimum spatial frequency response between low spatial frequency roll-off and high spatial frequency cut-off [28]. We achieved the optimum material response with grating periods in the micrometer range, resulting in about 500 lines per mm. Nonetheless, the material is designed to record a wide range of pattern spacings, enabling both transmission and reflection holograms [29]. Figure 8 shows light-microscopic images of a transmission grating (left) and reflection grating (right).

Fig. 8 Microscopic imaging: transmission grating (TG, left) and reflection grating (RG, right) in cross-sectional view. The grating periods are Λ = 1.3 μm (TG) and Λ = 130 nm (RG), respectively.

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Imaging of holographic gratings entails high standards on image contrast and resolution. Phase gratings do not feature surface modulations to be mapped by scanning probe microscopy and without conductive species [11] they give no contrast in electron microscopy. With the resolution limit of conventional optical microscopy imaging of reflection gratings seems impossible. To overcome this we implemented imaging in sectional view. The reflection grating planes are nearly parallel to the image plane. The focus plane determines the sectional plane. Altering the adjustment of the focus plane moves the stripes along the sectional-grating vector. The slant angle of the grating planes against the focus plane φ dictates the sectional-grating period via

Λ_{s} = \frac{Λ}{\sin φ} .

Λ_s = 30 μm was read from the image (Fig. 8, right). This corresponds to Λ = 130 nm and a slant angle of φ = 0.25 deg.

5. Conclusions

With epoxy based material composition and limited dope proportions we have developed and optimized a novel photopolymer system for holographic recording, which does not require a protective layer. It exhibits high refractive index contrast (in the order of $5 \cdot 10^{- 3}$ ) at exposure energy densities of about 0.5 J/cm² with good dimensional stability. Diffraction efficiency close to 100% is achieved with 200 μm thick layers. Sharp angular selectivity better than 1 degree is obtained. We demonstrate holographic recording of both transmission and reflection gratings with up to 8000 lines per mm. All-optical processing in combination with additional freedom degrees in functionality makes this material system interesting for a wide range of applications, particularly for integrated optical and optoelectronic components. We now have at our disposal a highly sensitive material for volume holographic recording and three-dimensional optical structuring with the advantage of adhesion on various substrates and widely variable layer thickness. The realization of volumetrically stable, free-surface samples allows the integration with numerous photonic materials in diverse multilayer configurations.

Acknowledgments

We acknowledge support by the Helmholtz Research School on Security Technologies and by the Cobios Foundation. Microscopic recordings are provided by the Zentraleinrichtung Elektronenmikroskopie (ZELMI). T. S. thanks Michael Zschocher for fundamental discussions and assistance.

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Free-surface photopolymerizable recording material for volume holography

Abstract

1. Introduction

2. Material composition and sample preparation

3. Optical structuring and characterization techniques

4. Material response and performance

5. Conclusions

Acknowledgments

References and links

Cited By

Figures (8)

Equations (6)

Optical Materials Express