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Holographic sensing of organic vapor is characterized at transmission and reflection geometries in ZSM-5 nanozeolites dispersed acrylamide photopolymer. Nano-zeolites as absorption medium are dispersed into the polymer to enhance the absorptivity to organic vapor. Obvious enhancements of spectrum strength are observed during the sensing process. Two primary factors causing the enhancement, absorption of nanozeolites and photopolymerization induced by broadband white light, are analyzed experimentally. Significant increment provides a quick and intuitive identification strategy for holographic sensing. Accompanying with the wavelength blue-shift, the shrinkage of sample is measured quantitatively under homogeneous white light. It is further demonstrated that the significance of nanozeolites absorption. Finally a theoretical model with mutual diffusion is used to simulate the swelling process. This study provides significant foundation for the application of holographic sensor.
© 2015 Optical Society of America
1. Introduction
Photopolymer as attractive material for holographic storage and optical element has much attention [1–5]. The interesting in holographic analytic sensitive medium for environmental monitory is increasing [6–9]. Operational principle of holographic sensor is that a grating firstly is recorded in sensitive medium, then the medium absorbs analytes and brings about a series of change in holographic optical characteristics, including fringe space, refractive index modulation, or average refractive index of holographic gratings, etc [10–12]. Subsequently, the diffractive spectrum is shifted by the binder swelling and diffraction efficiency of gratings also is changed. These processes provide a novel sensing method and a probability for determining the dependences of holographic characteristics on environmental factors. Further, when holograms are recorded in the sensitive medium, the change of environmental factors can result in the color or strength change of holographic image [13,14]. The holographic image with high color quality brings about a simple, quick and intuitive identification strategy for environmental factors. The holographic sensor as cheap visual monitoring method accelerates the applicability development of holographic optical element.
Recently, nanozeolites as absorption medium are dispersed into photopolymer to enhance the sensibility of holographic sensor [15–17]. The complementary spatial redistribution between monomers and nanoparticles can be formed due to the mutual diffusion mechanism under coherent exposure [5, 18–20]. Further the absorption of nanozeolites can bring about the change of diffraction efficiency. This absorption can directly reflect the vapor characteristics. In experiments, during absorption process the reduction of diffraction efficiency is observed by I. Naydenova research group [15–17]. They think that a part of change is attributed to the absorption of nanozeolites [21]. However, the detailed and regular experimental results and corresponding further quantitative analysis are absent. Specially, the analysis aimed at organic vapor response and theoretical simulation has seldom been studied.
In this paper the spectrum responses of holographic sensor in sensing acetone vapor are characterized quantitatively. Nanozeolites as absorption medium provided a novel sensing way to enhance the change of diffraction efficiency. The enhancements of spectrum strength are observed. The relations between vapor characteristics and holographic properties of gratings are determined.
2. Materials and experimental setup
The holographic sensitive photopolymer is acrylamide system, which has much attention in the field of holographic storage and optical elements in recent years [22–26]. The material consists of acrylamide monomer (AA), triethanolamine (TEA) initiator, NN’methylene bisacrylamide (BAA) crosslinking agent, Methylene Blue (MB) dye for red laser, and polyvinyl alcohol (PVA) binder (1788 type). To enhance the sensitivity, nanozeolite is used. The ZSM-5 nanozeolites are purchased from Shanghai Shen Tan New Chemical Materials Co. Ltd. Its average size is 250nm, the ratio of Si to Al is 40. The primary components weight proportions are AA 10wt%, TEA 30wt%, BMA 5wt%, MB 0.1wt%, ZSM-5 0.0wt%~0.4wt%, respectively. The rest component is PVA binder. The preparation procedure of holographic sensor is described as follows. PVA powder is dissolved into deionized water to form 10w/v% mixed solution at temperature 70 °C. Other components, namely, AA, TEA, BAA, MB, ZSM-5 nanozeolites are weighted into another bottle according to the proportion. Then the PVA solution is mixed into the bottle, and stirred until the mixed liquid become transparent. The blue solution is spread on glass sheet and dried 36-48h at relative humidity 30% for holographic sensing. For transmission grating, the thickness of sample is 240μm after drying process. In order to obtain high sensitivity of reflection hologram, the recording mediums are formed using spin coating machine. The corresponding low and high speeds are 500r/min and 1500r/min. After drying process, the thickness of sample is 7μm (reading microscope) for recording reflection grating and sensing organic vapor.
The experimental setup for holographic sensing is asymmetric two beam coupling geometry that described in Fig. 1. The recording light wavelength operated at He-Ne 633nm. There is a slanted angle of the gratings in the recording process. A broadband light source by means of fiber is collimated and incident into sensitive medium in the opposite direction relative to the recording red beam. The diffraction spectrum and gratings strength are measured using AvaSepc-3648 fiber spectrometer, which is purchased from Avantes co. After recording the gratings, the sensitive medium is transferred into a glass box. The acetone vapor concentration can be controlled and changed from 0~5000ppm in the box. In order to obtain comparable quantitative results, the atmosphere temperature and relative humidity are kept at 25°C and 30% for all the experiments. In Fig. 1, the schematic diagrams for holographic sensing in transmission and reflection gratings are shown. During sensing process, the nanozeolites absorb the organic vapor molecules and bring about the change of diffraction efficiency. Simultaneously, the swelling of grating space can results in the spectrum shift.
Fig. 1 Experimental setup for holographic recording and schematic diagram for acetone vapor sensing process. (a) Transmission recording geometry, (b) Reflection recording geometry.
3. Results and discussions
3.1 Spectrum response of transmission gratings
Acetone vapor responses of slanted transmission gratings are investigated experimentally. The slanted transmission gratings are recorded in our AA/ZSM-5/PVA system. The angle between two recording beams is 60 degree, and the slant angle of sample is 10 degree. After recording a transmission grating, the medium is placed into a glass box to sense the acetone vapor for a certain period 3 minutes. Then the sample is restored in original position for measuring diffraction spectrum. Then the corresponding acetone vapor concentrations can be changed for next operation. The vapor concentration change interval is 1000ppm, and its range is 0-5000ppm. The spectrum response of grating as a function of acetone vapor concentrations can be observed.
The diffraction spectrum of slanted transmission gratings with various acetone vapor concentrations are shown in Figs. 2(a)-2(c) are corresponding to the photopolymer sample with ZSM-5 nanozeolites concentrations 0.0wt%, 0.1wt% and 0.3wt%, respectively. The diffraction efficiency is relative to the incident white light sources as reference signal from spectrometer. It is indicated that the changes of diffraction efficiencies in undoped polymer are weak and even neglected. In our multiple repeating experiments, the reduction of diffraction efficiency is also observed in undoped polymer. The corresponding reason could be that the acetone vapor osmoses into the binder, bounds with the polar molecules and brings about the reduction of average refractive index. On exposure spot, spatial modulated distribution of photoproduct and residual monomer may be destroyed by the acetone vapor. The decreased viscosity of binder also accelerates the monomer diffusion and photoproduct counter-diffusion to reduce the refractive index modulation. The broadband white light source with total intensity 4mW at output end of fiber can slightly induce the photochemical reaction and enhance corresponding modulation depth. However, relating to the decay effect, the enhancement of uniform white light for the grating can be neglected. Due to the high initial grating strength, this means a lot of monomers already are consumed during exposure process. A few of residual monomers limits the grating further enhancement resulted by monomer photopolymerization.
Fig. 2 Holographic spectrum response of transmission gratings in sensing acetone vapor. (a), (b) and (c) are corresponding to ZSM-5 concentrations 0.0wt%, 0.1wt% and 0.3wt%. The label represent the corresponding peak wavelength and diffraction efficiency.
When the sensor is placed into a glass box for sensing, the corresponding sensing time periods are changed, namely, 1min, 3min, and 5min. The various sensing time periods are used to optimize the sensitivity of sensor. The initial and saturation curves in a set of experiments are described. Then a new sample is used to achieve the same sensing process at various sensing time periods. A long period can improve the absorbility and enhances the variation range of holographic characterization. The vapor concentration change interval still is 1000ppm. Acetone vapor spectrum responses of holographic sensor in two dimensional forms are shown in Fig. 3. The two solid lines are represented the spectrum responses at initial and saturation absorption situation. It is indicated that the change of diffraction efficiency is weak and even its reduction (in Fig. 3(c) at 5min interval) is caused in undoped polymer. In ZSM-5 nanozeolites dispersed polymer, the strong enhancements of spectrum strength are presented. The optimized time interval is 3min and corresponding 1.6-fold maximum increament of diffraction are observed. The enhacenemt can be attributed to the enhancement of modulation depth which casued by absorption of ZSM-5 nanozelolites. Moreover, the peak’s wavelength shift can be attributed to the the swelling of binder and grating space.
Fig. 3 Acetone vapor spectrum response of holographic sensor in two dimensional forms. (a)-(c) presente the ZSM-5 concentration 0.0wt%. (d)-(f) presente ZSM-5 concentration 0.1wt%. The sensing time periods are 1min, 3min, 5min for absorbing vapor.
In our repeating experiments, the enhancements of spectrum strength throughout are observed. It is implied that the enhancement isn’t accidental event. The possible reasons can be summarized as follows. Firstly, acetone vapor absorption by ZSM-5 nanozeolites can improve the refractive index modulation. The refractive index of acetone (1.36) is less than the monomer’s (acrylamide 1.55) and photoproduct’s (poly(acrylamide) 1.65) indexes. Under inhomogeneous recording illumination, the spatial distribution of ZSM-5 nanoparticles complemented to the monomer distribution can be formed due to mutual diffusion between nanoparticle and monomer. A lot of nanoparticles are assembled at dark region and absorbed the acetone vapor. Acetone molecules with low refractive index can further reduce the index of dark region, and accelerate the enhancement of modulation depth. Therefore it means that the absorption of nanozeolites is a significant factor for the enhancement of grating strength.
Secondly, low viscosity of binder can accelerate the mutual diffusions of components. In the polymer, the primary mobile components are monomer molecules and nanoparticles. During recording process, the spatial modulated distributions of monomer and nanoparticles are formed. In type acrylamide system, the high diffusion coefficient (10−18-10−16m2/s) bring about the diffusion process is completed at a short period after exposure [24]. The spatial distribution of residual monomer almost can be considered as uniform [1]. Moreover, the counter diffusion of nanozeolites from bright into dark region could be reduced the refractive index modulation. Therefore the diffusion processes for enhancing the modulation depth is limited. Thirdly, the effect of broadband white light on the grating strength may be another reason for the enhancement. The white light source can accelerate the photochemical reaction. This process is similar as light-cure process. Comparing with the diffusion process, the residual monomers at dark and bright region can be consumed by white light. The corresponding average refractive index and modulation depth are slightly enhanced. Moreover, the light-thermal effect can evaporate partial acetone molecules osmosed into the binder. The extravasation of acetone molecules from nanozeolites hole is relatively difficult than that from binder. The molecular diameter of acetone is less than the aperture of ZSM-5 nanozeolites. It means there are strong absorption between acetone molecules and nanoparticles. Therefore the evaporation of acetone at bright region can accelerate the enhancement of modulation depth. In summary, the enhancement of diffraction efficiency can be attributed to the absorption of ZSM-5 nanozeolites and monomer consumption by uniform white light. Furthermore, the absorption of acetone can be considered as a primary factor for the enhancement.
The peak’s wavelength shift can reflect the swelling and shrinkage of grating. High acetone vapor concentrations can lead to the obvious sample swelling. In Fig. 3, the clear redshift of peak’s wavelength confirmed the swelling. The corresponding quantitative wavelength shift and diffraction change are shown in Fig. 4 at 3min sensing time periods. In Fig. 4(a), the acetone concentrations (0-4000ppm) are corresponding to the sensing time is 1000s. It is similar to other figures. The periodic change of acetone concentration is relative to the periodic time intervals. The linear relations are used to fitting the experimental data. It is indicated that the maximum of wavelength shift closed to 17nm at ZSM-5 0.1wt%. The maximum relative increments of diffraction are closed to 5%, 50% and 40% in the sample with ZSM-5 0.0wt%, 0.1wt%, and 0.3wt%. The definition of relative change is change of diffraction efficiency divided by the initial diffraction efficiency. The results indicate the slight enhancement in undoped polymer comes from broadband white light photochemical reaction. While, in ZSM-5 nanozeolites doped samples, the significant enhancements are attributed to two effects, i.e. absorption of nanozeolites and light consumption of monomer. The absorption of nanozeolites can bring about the reduction of refractive index at dark region and enhancement of modulation depth. However, higher ZSM-5 concentrations (0.3wt% or more) would cause high holographic scattering. In experiments, the significant scattering phenomenon is observed around the transmission light spot. Therefore the nanoparticles with high concentration can scattered incident light and resulted in weak growth (40%) of holographic gratings. Moreover the corresponding slopes of curves are gradually increased as increasing the nanoparticles concentrations. It means more nanozeolites can absorb the acetone vapor more quickly. The change of diffraction efficiency and wavelength shift can be considered as linear proportional to the acetone vapor concentrations, approximately. This assumption can bring about a quantitative relation for the theoretical calculation.
Fig. 4 peak’s wavelength shift and relative diffraction change with various acetone concentrations. (a), (b), (c) are corresponding to ZSM-5 concentration 0.0wt%, 0.1wt%, and 0.3wt%, respectively. The errors from experiemts are 1nm and 5% in wavelength and diffraction, respectively.
Figure 5 shows the wavelength shift and relative change of diffraction efficiency as a function of ZSM-5 nanozeolites concentrations at various acetone concentrations. It is clearly indicated that the changes of diffraction and wavelength shift are reached to maximum around ZSM-5 concentrations 0.2wt%. The primary reason of wavelength shift can be attributed to the swelling of fringe space. The absorption of vapors can reduce the binder viscosity and accelerate the change of grating fringe. But the corresponding change is weak. The significant change of relative diffraction at various nanoparticles demonstrates that nanoparticles results in high absorption of acetone and enhancement of modulation depth. It is implied that the absorption of ZSM-5 nanoparticles is primary reason for the change of diffraction efficiency. Moreover, at high nanoparticles concentration, the slight reduction of diffraction still can be attributed to the high holographic scattering.
Fig. 5 Wavelength shift (a) and relative diffraction change (b) as a function of ZSM-5 concentrations.
3.2 Spectrum response of reflection gratings
Acetone spectrum responses of reflection gratings are characterized by measuring the diffraction spectrum of reflection gratings. In order to obtain obvious spectrum shifts exposed to acetone organic vapor, the thickness of sample is controlled and decreased by spin coating machine. The detailed preparation procedures are described in Materials and Experiments Setup. After drying process, the sample with micron dimension thickness was prepared to record reflection grating. After recording a reflection grating, the medium is placed into a glass box to sensing the acetone vapor for a period 1 minute. The corresponding diffraction spectrum can be measured. Then the acetone vapor concentrations are changed for next sensing process.
Figure 6 shows the spectrum response of reflection gratings. Here Figs. 6(a)-6(c) are corresponding to the polymer with various nanozeolites concentrations, namely 0.0wt%, 0.25wt%, 0.36wt%. The fast response of organic vapor in reflection gratings is obtained. Obvious wavelength shift and change of diffraction efficiency are observed at this condition. The wavelength shift can also be attributed to the swelling of thin film. Maximum of wavelength shift closes to 46nm appeared at ZSM-5 concentrations 0.25wt%. It means the expansion degree at reflection geometry is higher than at transmission geometry. The change trends of diffraction efficiency are reduction at this case. It is attributed to that the ultrathin thickness of polymer only provides slight nanozeolites for the absorption. In this case, the enhancement from absorption of nanozeolites not enough supply the reduction of modulation depth destroyed by acetone vapor. Accompanying the absorption, the swelling of sample also can be caused. The corresponding Bragg detuning maybe reduced the diffraction slightly.
Fig. 6 Spectrum response of reflection gratings. (a), (b) and (c) are corresponding to ZSM-5 concentrations 0.0wt%, 0.25wt% and 0.36wt%. (d), (e) and (f) are corresponding spectrum responses at initial and saturation state.
The clear comparisons between initial and saturated diffraction efficiencies are shown in Figs. 6(d)-6(f). There is obvious reduction of diffraction efficiency in undoped sample in Fig. 6(d). However, at absorption saturation state, the relative diffraction still keeps a high value at 0.25wt%. The dispersion of nanozeolites can delay the reduction. It means the absorption of acetone still bring about the increasing of modulation depth. Moreover, higher nanozeolites concentrations can result in strong holographic scattering. The corresponding diffraction is reduced by the scattering. Therefore the diffraction is low at saturation in Fig. 6(f).
Figure 7 shows the quantitative wavelength shift and relative change of diffraction with various acetone concentrations. The linear relations are used to fitting the experimental data. The good agreement between experiments and fitting curves demonstrate the sensing process is in linear response region. The corresponding linear trends are similar as the situation in Fig. 4 at transmission geometry. The slopes of curves indicate that high nanoparticles concentrations bring about slow reduction of diffraction efficiency. The low decrement magnitude in diffraction at 0.25wt% can be attributed to the improvement of modulation depth from absorption of nanozeolites. However, higher nanoparticles still provide strong holographic scattering and obviously diffraction reduction. Therefore at 0.36wt%, the fast fall and nonlinear relation are observed. Simultaneously, the responses of wavelength shift are improved by doping nanoparticles. The corresponding linear relations between wavelength and acetone concentrations still can bring about a quantitative expression for numerical calculation.
Fig. 7 Wavelength shift and relative change of diffraction at reflection geometry as a function of vapor concentrations. The symbols are experimental data and the solid lines are linear fitting curves. The error from experiment is 5%.
3.3 Swelling ratio
In order to determine the primary influence factor for the wavelength shift, the swelling behavior of polymer systems is characterized by swelling ratio [28,29]. Then, the swelling kinetics of the polymer system can be determined from swelling kinetic curve. The definition of swelling ratio can be described as
where Wt and W0 represent the t-time and initial dried weight of the polymer materials, respectively. Firstly, the weight of dry polymer film is determined as W0. Then the polymer is placed into a box for absorbing vapor. The absorbing time is 3min and the corresponding weight of wet polymer film is determined as Wt. The swelling ratio as a function of acetone concentration is measured and shown in Fig. 8. The good agreement between linear function and experiments validates the trend of swelling behavior; the corresponding relation can be described aswith Wc1 is slope of curve and Wc2 is constants. Comparing with the wavelength shift, the linear expression of swelling ratio demonstrates the wavelength shift can be attributed to the swelling of fringe space. The gradually increasing slopes can also demonstrate the nanozeolites absorbed more acetone vapor to enhance the weight of medium. The significance of nanozeolites absorption is reflected in swelling ratio curves.Fig. 8 Swelling ratio as a function of acetone vapor concentrations. The symbols are experimental data and the solid lines are linear fitting curves. The error from experiments is 5%.
3.4 Shrinkage under homogeneous white light
In order to further confirm the significance of nanozeolites absorption, the influence of homogeneous white light resource on the grating is measured. The total intensity 4mW of broadband white light source maybe brings about the polymerization of residual monomer. Therefore after recording gratings, the temporal evolutions of spectrum are observed under consecutive white light illumination. The corresponding spectrum response is shown in Figs. 9(a) and 9(d) represent spectrum response of diffraction grating at transmission and reflection geometries, respectively. Figures 9(b) and 9(e) are temporal evolution of peak wavelength shift and corresponding shrinkage. Figures 9(c) and 9(f) are temporal evolution of relative change of diffraction efficiency. The temporal evolutions of wavelength shift and diffraction efficiency are clearly described.
Fig. 9 Spectrum response of transmission and reflection grating under homogeneous white light. (a), (d) diffraction spectrum response at transmission and reflection. (b), (e) temporal evolution of peak wavelength shift and corresponding shrinkage. (c), (f) temporal evolution of relative change of diffraction efficiency. The symbols are experimental data and the solid lines are nonlinear fitting curves using exponential function. The error from experiments is 5%.
Based on Bragg condition, the redshift and blueshift represent the swelling and shrinkage of grating space, respectively. The wavelength blueshift means that photo-polymerization and thermal effect from homogeneous light can provided the shrinkages. In our experiments, the primary reason for peaks wavelength shift can be attributed to fringe space change caused by thickness shrinkage. On the other hand, the change of fringe space is proportional to the change of thickness, i.e. shrinkage, which can be described as
where d0 is initial thickness of sample, Δd is change of thickness. Therefore the wavelength shift can directly reflect the shrinkage. The corresponding shrinkage is calculated and shown in Figs. 9(b) and 9(e). It is indicated that the order of magnitude of shrinkage are accorded with the reported value, about 0.6% [27]. This means the homogeneous light bring about the dehydration and shrinkage of binder. In reflection gratings, the short time constant means the fast response. It is attributed to the sample with ultrathin thickness bring about the high sensitivity.Comparing with the shrinkage of grating, the relative diffraction is reduced evidently. The evident reduction also indicates that the homogeneous polymerization of residual monomer isn’t important factor for the enhancement of diffraction during sensing process. The absorption of ZSM-5 nanoparticles can be considered as a primary factor for the enhancement. The photo-polymerization of residual monomer by white light can’t bring about the enhancement of grating strength.
4. Theoretical description
4.1 Mutual diffusion model
Diffusion model used to simulate the sensing process [30]. Radical chain polymerization results in the chain growth away from the initial spot. Consequently, the spatial effect of monomer polymerization is nonlocal [31–33]. The diffusion model with nonlocal response is an effective approach in analyzing the sensing process. The corresponding theoretical equations can be written as
where [M](x,t) and [Z](x,t) are monomer and nanozeolites concentrations, D0 is the mutual diffusion coefficient. The polymerization rate can be depictedwhere V is the visibility, I0 is recording intensity. Here the linear relation between polymerization rate and irradiance is used, i.e. γ = 1. The nonlocal response function R(x,x´) can be expressed aswhere σ1/2 represent the nonlocal response length. After exposed for time t, the concentration of polymerized monomer molecules at location x can be given aswhere [P](x,t) is photoproduct concentration. The corresponding refractive index modulation can be depicted aswhere Ci, i denote the ith components, is the coefficients refractive index of components, ϕ = π is phase difference between complimentary components.4.2 Organic vapor absorption
Based on Fig. 4, the relative change of diffraction efficiency is proportional to the organic vapor concentrations. The corresponding linear relations can be described as
where η1 is the slope of curve and η2 is constant. According to couple wave theory, the diffraction efficiency is directly related to the refractive index modulation. Therefore the corresponding change of modulation depth can be given bywhere δn denotes relative change of modulation depth. It is assumed that the thickness of sample keep a constant during the absorption process, because the absorption is faster than the swelling process. Relating Eq. (10) and (11), the change of modulation depth is related to the vapor concentrations.On the other hand, the refractive index modulation is proportional to the components concentration modulation. During sensing process, the refractive index change of nanozeolites brings about the increment of modulation depth. The Lorenz-Lorenz relations further provide expression of refractive index coefficient,
Equations (10)-(12) indicates the organic vapor directly resulted in the refractive index change of nanozeolites and binder, and finally increased the modulation depth of gratings.4.3 Fringe swelling
Relating the corresponding doping percentages of components, the density of sensitive medium can be considered a constant. Neglecting the expansion paralleled to thickness direction, the swelling ratio can be deduced as
Consequently, the swelling ratio can directly reflect the thickness change. Moreover, the fringe space Λ of gratings is proportion to the sample thickness d. Therefore, the fringe space can be depicted asSimultaneously, in Fig. 4, the shift of wavelength peak is proportional to the acetone concentration, which can be described asRelating the differentiating results from Bragg diffraction,The wavelength shift can be proportional to the change of fringe space, which is depicted aswhere the other factor can be consider as secondary.4.4 Simulation
The dimensional change is assumed take place in the direction perpendicular to the medium surface. A fractional change in thickness will result in the changing of the tilt angle and grating spacing, which can be derived as [34]
where , Λ0, and , Λ1 are the tilt angle and the fringe grating before and after dimensional change of sample, respectively. The expansion coefficient ρ can be given byThe variable x in the one-dimension diffusion model can be changed approximately asTo simulate the dynamic process of gratings, Eq. (4)-(9) are solved by introducing dimensionless spatial, temporal variables and nonlocal response length xD = KX, tD = kI0t and σD = K2σ. Further the dimensionless space domain ΔxD = 1-2π/N, where N is number of sampling points, the dimensionless time domain ΔtD = 0.4(ΔxD2/RD) are used. The corresponding relations between acetone and wavelength (i.e. fringe space) can be determined in Fig. 4. To illustrate the swelling dynamics, a series of parameters are selected, namely recording angle 120 degree, dimensionless nonlocal response length 0.2, nonlinear coefficient γ = 1. Considering high diffusion coefficient of monomer in acrylamide [23], the ratio RD = D0K2/kI0 = 1 of diffusion rate divided by polymerization rate is selected. The initial concentrations of monomer are normalized for simplifying the numerical calculation.
Spatial and temporal evolutions of components are shown in Fig. 10. The spatial distributions of nanozeolites and monomer are complementary due to the mutual diffusion. At bright region the consumption of monomer bring about enhancement of photoproduct concentration. Simultaneously, the unreacted monomers are diffused from dark region into bright region. Due to the differences of chemical potential between dark and bright regions, nanozeolites can be experienced a counter-diffusion. Accompanying with the mutual diffusion, the fringe space is swelled by absorbing organic vapor. As increasing the acetone concentration, the obvious swelling and increment of modulation depth are shown in Figs. 10(d)-10(f). Consequently, the peak of wavelength can be shifted to match corresponding Bragg condition.
Fig. 10 Spatial and temporal evolutions of components using diffusional model with nonlocal response. (a)-(c) denote redistribution of monomer, nanozeolites, and photoproduct at acetone 1000ppm. (d)-(f) denote change of refractive index modulation with various acetone concentrations, namely 1000ppm, 2000ppm, and 3000ppm.
5. Conclusion
Holographic sensing of organic vapor is investigated at transmission and reflection geometries in ZSM-5 nanozeolites dispersed acrylamide photopolymer. The dispersion of nanozeolites as absorption medium enhanced the absorptivity to organic vapor. The obvious enhancement of spectrum strength is attributed to the absorption of nanozeolites and photopolymerization induced by broadband white light. The absorption of nanozeolites is primary factor for the sensitivity. The ultrathin polymer sample in reflection gratings presents fast response and high sensitivity for organic vapor. Under homogeneous white light the shrinkage of polymer and reduction of diffraction are observed and it is further demonstrated the significance of nanozeolites absorption for the enhancement of spectrum strength. For exploring the physical mechanism, the mutual diffusion model with nonlocal response provides a clear description about the swelling of components and modulation depth. This study provides significant results for the practical application of holographic sensor.
Acknowledgments
The research has been financially supported by National Natural Science Foundation for young (Grant no.61307007), Funds of Tianjin Natural Science (Grant no.13JCYBJC16000), Funds of Tianjin Natural Science for Young (Grant no.13JCQNJC01600), Experimental Technology Innovation Funds (2015SYCX11). University Student's Innovative Training Program, and Found for Outstanding Young Teachers in Tianjin.
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