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Abstract
We demonstrate volume holographic recording at a wavelength of 640 nm in a photopolymerizable nanoparticle-polymer composite (NPC) film dispersed with ultrahigh refractive index hyperbranched-polymer (HBP) organic nanoparticles. We employ a new photosensitizer-initiator system consisting of cyanine dye, triazine compound and borate salt for efficient radical generation in the red. We investigate the electron transfer and radical generation processes of the system by measuring fluorescence quenching and photopolymerization dynamics to find the optimum composition of the system for volume holographic recording. We show that recorded volume gratings of 0.5-µm spacing possess the saturated peak-to-mean refractive index modulation amplitudes as large as 3×10−2 at a readout wavelength of 640 nm. Our results show the usefulness of photopolymerizable HBP-dispersed NPCs for volume holographic recording materials for various photonic applications including security and color holograms, and volume Bragg grating devices in head-mounted displays.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
A novel photonic nanocomposite material, the so-called photopolymerizable nanoparticle-polymer composite (NPC) [1], is a holographic photopolymer material dispersed with nanoparticles at high concentrations. Holographic grating formation in such an NPC material is different from the photopolymerization-driven diffusion mechanism of conventional all-organic and binder-based photopolymer [2–4]. As shown in Fig. 1, holographic light exposure initiates the photopolymerization reaction of monomer molecules by photo-generated free radicals in the bright regions of the light intensity-interference fringe pattern.Since the chemical potential of monomer decreases in the bright regions due to their transformation to the formed polymer, the diffusion of monomer from the dark to the bright regions takes place during holographic light exposure. At the same time photo-insensitive nanoparticles experience the counterdiffusion from the bright to the dark regions since they are not consumed by light and their chemical potential increases in the bright regions. Then, the photopolymerization-driven mutual diffusion facilitates holographic assembly of nanoparticles [5] that continues until the process completes. If refractive indices of the formed polymer and nanoparticles are different from each other, the spatial refractive index modulation (i.e., a refractive index phase grating) is formed due to their density differences between the bright and the dark regions.
So far, NPC materials dispersed with inorganic nanoparticles such as TiO$_2$ [6,7], SiO$_2$ [8], ZrO$_2$ [7,9], ZnS [10], zeolites [11] and semiconductor quantum dots [12,13] have been reported. Refractive indices of these inorganic nanoparticles are generally much different from those ($\sim$1.5) of the formed polymer hosts so that a volume holographic grating with large saturated peak-to-mean refractive index modulation amplitude ($\Delta n_\textrm {sat}$) of the order of 1$\times 10^{-3}$ or a bit larger and high dimensional stability [14] can be obtained. They can be used for numerous photonic applications that require high dynamic range gratings ($i. e.$, large $\Delta n_\textrm {sat}$) and efficient beam manipulations of non-electromagnetic matter waves [1]. These include holographic data storage [15], head-mounted display [16], holographic optical elements [17], holographic spatial-spectral filters [18], and slow-neutron beam control [19–21]. We also reported NPC materials dispersed with hyperbranched polymer (HBP) [22] acting as an organic nanoparticle because of its size and refractive-index controllability and good compatibility with host monomer. So far we developed holographic NPC gratings using various types of HBPs (refractive indices of 1.51 and 1.61 at 532 nm) and photopolymers with free radical mediated chain-growth, cationic ring-opening and thiol-ene polymerizations [23–25]. In all these works holographic NPC gratings of 1-$\mu$m spacing were recorded in the green (532 nm) and they had $\Delta n_\textrm {sat}$ as large as 8$\times 10^{-3}$ at a readout wavelength of 532 nm. Details of these NPC materials are described in [1]. Other organic nanoparticles using carbon allotropes such as carbon nanotubes and nanodiamonds were also reported very recently [26,27].
Later, we developed a new NPC material dispersed with ultrahigh refractive index (1.82 at 532 nm) HBP for volume holographic recording in the green [28–30]. We demonstrated recording of volume holographic gratings of 1-$\mu$m (0.5-$\mu$m) spacing with $\Delta n_\textrm {sat}$ as large as $4.5\times 10^{-2}$ ($3\times 10^{-2}$) at 532 nm [30]. Such NPC volume gratings can be a candidate for volume Bragg grating (VBG) devices to be implemented, for example, in head-mounted displays for augmented and mixed reality [31–33], where VBGs must provide high diffraction efficiencies with wide Bragg apertures (e.g., $\Delta n_\textrm {sat}>2\times 10^{-2}$ and the grating thickness of 10-$\mu$m order or thinner). Moreover, if they have panchromatic recording capabilities all in R, G and B bands, they also find other holographic applications such as color and security holograms as well as VBGs for head-mounted displays [34–37]. In quest for the realization of panchromatic recording sensitivities of our NPC material dispersed with ultrahigh refractive index HBP we describe here volume holographic recording in the HBP-dispersed NPC material codoped with a new three-component photosensitizer- initiator system consisting of cyanine dye, triazine compound and borate salt for efficient radical generation in the red. We investigate the electron transfer and radical generation processes of the system by measuring fluorescence quenching and photopolymerization dynamics to find its optimum composition for volume holographic recording in the red. It is shown that the NPC material of 25 vol.% HBP dispersion doped with the optimum composition of the system can produce a plane-wave holographic transmission volume grating of 0.5-$\mu$m spacing with $\Delta n_\textrm {sat}$ as large as 3$\times 10^{-2}$ at 640 nm and at a recording intensity of 10 mW/cm$^2$.
2. Materials and experimental methods
2.1 Materials and sample preparation
We employed ultrahigh refractive index HBP by the polycondensation of a diamine monomer with 2, 4, 6-trichloro-1, 3, 5-triazine in N, N-dimethylacetamide as reported previously [28,38]. It was further processed by the end-capping reaction with aniline. Details of the synthesis process and the material properties are described in Supplement 1. Host monomer, 4-hydroxybutyl acrylate (4-HBA, purity > 97.0%), was purchased from Tokyo Chemical Industry Co. Ltd. Cross-linking monomer, ethoxylated dipentaerythritol hexaacrylate (A-DPH, purity $\ge$ 99.0%), was purchased from Shin-Nakamura Chem. Co. Ltd. As similar to three-component photoinitiator systems used for photopolymerization at visible wavelengths [39], we employed a three-component photosensitizer-initiator system consisting of cyanine dye, triazine compound and borate salt. Red photosensitizing cyanine dye, 3, $3'$-Dipropylthiadicarbocyanine Iodide [DiSC$_3$(5), purity $>$ 98.0%], was purchased from Tokyo Chemical Industry Co. Ltd. Note that methylene blue, a well-known red photosensiting dye for holographic photopolymer [36,40,41], was undissolved in our present NPC formulation. Triazine compound acting as an electron acceptor, 2,4,6-Tris(trichloromethyl)-1,3,5-triazine (TCT, purity $>$ 95.0%), was purchased from Wako Pure Chemical Co. TCT was previously used for red-sensitive holographic photopolymer in combination with squarilium dye sensitizer [42,43]. Borate salt compound acting as an electron donor, tetrabutylammonium butyltriphenylborate (N3B, purity N/A), was received from SHOWA DENKO K. K. Karenz. All of them were used as received without further purification step. Chemical structures of all these materials are illustrated in Fig. S1 of Supplement 1.
We added ultrahigh refractive index HBP in a powder form to a monomer blend consisting of single functional 4-HBA as host monomer with low viscosity and multifunctional A-DPH as crosslinking monomer with high photochemical reactivity used in our previous NPC system [29,30]. A-DPH possesses ethylene oxide chains and also lowers the viscosity of a monomer blend, by which it facilitates crosslinking reactions and mutual diffusion effectively. Refractive indices of HBP were 1.82 and 1.80 at 532 and 633 nm, respectively, very high for organic materials due to its triazine and aromatic ring unit structure. Refractive indices of 4-HBA and A-DPH were 1.452 and 1.478, respectively, at 589 nm. The volume ratios of HBP, 4-HBA and A-DPH in NPC syrup were 20:77:3 (referred to as 20 vol.% HBP) and 25:72:3 (referred to as 25vol.% HBP), respectively. The doping concentration of DiSC$_3$(5) was fixed to 2 wt.% with respect to A-DPH in these NPC syrup. TCT and N3B were doped in NPC syrup at various relative molar ratios in such a way that DiSC$_3$(5):TCT:N3B was shown as 1:$i$:$j$ ($i$=0, 8, 10 and $j$=0, 2, 4, 6, 8). Note that 1:10:8 was the maximum doping concentration for N3B to be uniformly mixed in NPC syrup. All components were added to a 20-ml glass vial at the same time without any solvent. The sample preparation procedure was carried out under orange illumination by a high pressure sodium vapor lump at 589 nm to avoid the photoexcitation of DiSC$_3$(5). They were mixed by a tube roller mixer at 50$^\circ$ C and at 40 rpm for longer than 48 hours in the dark. The resultant uniformly NPC syrup was cast on a glass substrate loaded with a $\sim$5-$\mu$m spacer and was covered with another glass substrate, followed by annealing at 60$^\circ$ C for 10 min. to prepare an NPC film sample.
2.2 Spectral absorption
Linear absorption measurements at varied HBP concentrations were done before and after uniform curing by an incoherent green LED light source with its peak wavelength of 525 nm by using UV-visible spectrometer (V-630, JASCO).
2.3 Fluorescence quenching measurement
Fluorescence quenching characteristics were measured under a cw excitation beam at a selected wavelength of 640 nm (i.e., our holographic recording wavelength) by a spectrofluorometer (FP-8300, JASCO). Glass sample cells of 10-$\mu$m gap were employed in the measurement. Spectral and temporal fluorescence signals were detected for NPC samples of 20 vol.% HBP dispersion at different DiSC$_3$(5):TCT:N3B. The impact of TCT and N3B on fluorescence quenching was evaluated by using DiSC$_3$(5), TCT and N3B dissolved in 4-HBA, where the concentration of DiSC$_3$(5) was 1.83$\times 10^{-3}$ mol/L. The least-squares fitting of the data was done by the following Stern-Volmer equation [44]:
where $I_{F,0}$ ($I_{F}$) is the intensities of fluorescence in the absence (presence) of a quencher, either TCT or N3B, $\tau _0$ is the lifetime of the excited singlet state of a photosesitizing dye DiSC$_3$(5), $k_Q$ is the quenching rate constant of the quencher and $[Q]$ is the concentration of the quencher.2.4 Photopolymerization kinetics
Photopolymerization kinetics were measured to quantify their photo-induced polymerization rate ($R_\textrm {p}$) and conversion ($\alpha _\textrm {p}$). We used a commercially available photocalorimeter (Q200, TA Instrument) equipped with a refrigerated cooling system (RCS 90, TA instrument) that can maintain the isotherm condition at 25$^\circ$ C. Each NPC syrup had the same formulation as that for holographic recording. It was dripped on an uncovered Tzero aluminum pan (TA Instrument). The weight of each NPC syrup in the pan was approximately 1-1.5 mg. The sample chamber of the photocalorimeter was purged with nitrogen gas for approximately 1 hour prior to light exposure to avoid oxygen inhibition. Photopolymerization was initiated by a loosely focused light beam from a 200W Hg-Xe lump through a 640 nm bandpass filter and a light-guiding fiber. The light intensity on each NPC syrup was varied from 1 to 10 mW/cm$^2$ in the measurement. Since $R_\textrm {p}$ is proportional to the number of reacted monomer units that can be measured from the reaction heat flow as a function of curing time ($t$) in s, $R_\textrm {p}$($t$) in s$^{-1}$ for a blend of acrylate monomers is given by [45]
where $dH/dt$ is the heat flow in J s$^{-1}$, $f_i$ is the number of C=C double bonds of the $i$th acrylate monomer and $m_i$ is the mole of the $i$th acrylate monomer in an NPC syrup. $\Delta H_{0i}$ in J mol$^{-1}$ is the standard exothermic heat from the $i$th acrylate monomer, which is ideally released under complete polymerization of 1 mol of an acrylate group in the range of 78-86 kJ mol$^{-1}$ [46]. The time-dependent $\alpha _\textrm {p}$ of a blend of acrylate monomers is also calculated from the exothermic heat flow over the irradiation time by the following equation: where the cumulative heat flow $\Delta H(t)$ is given by the time integration of $dH/dt$ from 0 to $t$. We assumed $\Delta H_{0i}\equiv \Delta H_{0}=$80 kJ mol$^{-1}$ for all acrylate monomers in our calculation as used previously [47,48].2.5 Spectral absorption
Linear absorption measurements were carried out by a UV-visible spectrometer (V-700, JASCO) before and after uniform curing under incoherent red LED light illumination at its peak wavelength of 660 nm . Each NPC film sample before and after uniform curing had the same formulation as that for holographic recording.
2.6 Refractive index measurement
Refractive indices of cured NPC film and polymer mixture film samples were measured by a temperature-controllable Abbe refractometer (DR-M2, ATAGO). We employed the red LED light source to avoid the formation of holographic noise gratings during spatially uniform curing. After curing, one of the glass plates was removed for the refractive index measurement. The refractive index of HBP was evaluated by measuring cured NPC film samples at different concentrations of HBP with a help of the Lorentz-Lorenz formula and at 25$^\circ$ C.
2.7 Viscosity measurement
Viscosities of NPC syrup before curing were measured as a function of HBP concentration by a viscometer (DV-II+Pro, Brookfield) at the rotation speed of 100 r.p.m. and at 25$^\circ$ C.
2.8 Holographic recording and characterization
We used a holographic recording setup to record an unslanted and plane-wave transmission phase grating in an NPC film sample at 0.5-$\mu$m spacing by two mutually coherent beams of equal intensities from a diode-pumped solid-state laser (Cobolt Bolero, HÜBNER Photonics) operating at a wavelength of 640 nm. The experimental setup is illustrated in Fig. 2. The linearly polarized laser beam passed through a tandem combination of a half-wave plate (W) and a prism polarizer (P) to obtain an s-polarized beam. It was was expanded and collimated with a diameter of approximately 10 mm by a beam expander (BE), and then it was split into two beams by a half-mirror (HM) to record a phase grating in an NPC sample. A low-intensity diode-pumped frequency-doubled Nd:YVO$_4$ laser (Compass 315M-100, COHERENT) operating at a wavelength of 532 nm was employed as an s-polarized probe beam to monitorthe buildup dynamics of an NPC grating being recorded since the three-component photosensitizer-initiator system was insensitive in the green. Transmitted 0th- and forward diffracted 1st-order signals were measured by detectors (D).
Fig. 2. Experimental setup for two-beam interference holographic recording. M, mirror; P, polarizer; W, half-wave plate; HM, half-mirror; S, electronic shutter; D, detector.
We measured the diffraction efficiency ($\eta$) that was defined as the ratio of the 1st-order diffracted signal to the sum of the 0th- and 1st-order signals. Three to five repeated measurements were done under each experimental condition. The grating thickness ($\ell$) and $\Delta n_\textrm {sat}$ at a probe wavelength of 532 nm were extracted by least-squares curve fits to the saturated $\eta$ ($\eta _\textrm {sat}$) as a function of Bragg-angle detuning ($\Delta \theta _B$) from a phase-matched Bragg angle ($\theta _B$) all in an NPC film with the following analytical formula for an unslanted and uniform transmission phase grating derived from the Kogelnik’s coupled-wave theory [49,50], which is valid in the Bragg diffraction regime [51]:
3. Results and discussion
Prior to the presentation of our main results on the holographic recording properties of HBP-dispersed NPC film samples, we first describe results of fluorescent quenching and photopolymerization kinetics measurements. To this end, according to Kabatc et al.’s explanation, [39] we briefly describe the radical generation mechanism of the three-component photosensitizer-initiator system as illustrated in Scheme S1 of Supplement 1. Borate salt, N3B, acts as an electron donor and meets a photoexited cyanine dye, [DiSC$_3$(5)]$^*$, by thermal diffusion, so that [DiSC$_3$(5)]$^*$ and N3B primarily form the exciplex, [DiSC$_3$(5) N3B]$^*$. This happens because the cationic nature of [DiSC$_3$(5)]$^*$ tends to attract the anionic nature of N3B rather than a triazine compound, TCT. Then, they dissociate into a radical pair, an N3B radical and a DiSC$_3$(5) radical, by electron transfer from N3B to DiSC$_3$(5) as the primary photochemical reaction. The former is continuously cleaved into a trinaphtyl boran compound and a butane radical that reacts with acrylate monomer. On the other hand, TCT acts as an electron acceptor so that the electron transfer from the DiSC$_3$(5) radical to TCT is more feasible, leading to reduction of TCT and oxidation of the DiSC$_3$(5) radical. As a result, the former forms an anionic TCT radical, followed by its cleavage action leading to the formation of a triazinyl radical and a chloride ion for initiation. The oxidation of the DiSC$_3$(5) radical results in the generation of the singlet ground state DiSC$_3$(5), allowing it for participating again in the primary photochemical process (i.e., photo-recycling). These overall process is depicted in Scheme S1 of the Supplement 1. The synergic reaction of such multiple radical generation and photo-recycling would lead to efficient chain-growth polymerization of acrylate monomer at its rate higher than that by the two-component photosensitizer-initiator system, either DiSC$_3$(5) and TCT or DiSC$_3$(5) and N3B. In the former case the radical generation and therefore photopolymerization rates would be low due to the cationic and electrically neutral nature of DiSC$_3$(5) and TCT, respectively. In the latter case an N3B radical derived from the dissociation of [DiSC$_3$(5)]$^*$ provides an initiating butane radical for acrylate monomer after the cleavage of an N3B radical. However, resultant growing radical monomer would be rapidly terminated by a DiSC$_3$(5) radical since such a cyanine dye radical is known as an inhibitor [39], a similar role of Rose Bengal radicals in the radical generation process by green light [10,30,40]. This inhibiting action may be observed by rapid quenching of fluorescence from [DiSC$_3$(5)]$^*$ and by low photopolymerization efficiency. We examined a scenario of such photochemical reactions as shown below.
3.1 Fluorescence quenching measurement
Fluorescence spectra for NPC samples of 20 vol.% HBP dispersion were measured at different DiSC$_3$(5):TCT:N3B. It was found that peak wavelengths were between 693 and 699 nm weakly depending on DiSC$_3$(5):TCT:N3B (see Fig. S2 of Supplement 1). We also examined the fluorescence quenching dynamics for NPC samples doped only with DiSC$_3$(5) and with either DiSC$_3$(5) and TCT or DiSC$_3$(5) and N3B. It was observed that fluorescence quenching for an NPC sample doped only with DiSC$_3$(5) slightly took place due to dye fading. It was found that fluorescence quenching for NPC samples doped with DiSC$_3$(5) and TCT increased with increasing TCT due to the generation of initiating triazinyl radicals from TCT radicals via the redox reaction between DiSC$_3$(5) radicals and TCT [42,43,52] [see Fig. S3(a) of Supplement 1]. More significant fluorescence quenching could be observed for NPC samples doped with DiSC$_3$(5) and N3B since both DiSC$_3$(5) and butane radicals could be generated via the formation of [DiSC$_3$(5) N3B]$^*$ as a result of the cationic and anionic nature of DiSC$_3$(5) and N3B, respectively [see Fig. S3(b) of Supplement 1]. Such a difference in fluorescence quenching between TCT and N3B was shown in Fig. 3, where Stern-Volmer plots are shown as a function of relative molar concentration of either TCT or N3B to DiSC$_3$(5). Coefficients of determination ($R^2$) on the least-squares linear fits were 0.96002 and 0.49756 with N3B and TCT, respectively. Although the latter fit is not good due, mainly to a large departure of one data point at the relative molar concentration=4, it clearly shows that quenching by N3B is stronger than that by TCT; $k_Q$ for N3B is four times larger than that for TCT. This result supports the quenching mechanism of the three-component photosensitizer-initiator system.
Fig. 3. Stern-Volmer plots of fluorescence quenching of DiSC$_3$(5) by TCT and N3B, respectively. Solid lines are the least-squares linear fits of Eq. (1) to the data.
Figure 4 shows temporal traces of fluorescence signals normalized by those at $t=0$ (i.e., onset of the photoexcitation) for uncured NPC samples of 20 vol.% HBP dispersion at different DiSC$_3$(5):TCT:N3B. These fluorescence signals were taken at peak wavelengths corresponding to different DiSC$_3$(5):TCT:N3B. It can be seen that decays of fluorescence signals strongly depend on whether or not TCT and N3B coexist. The fluorescence decay is most significant in the presence of N3B without TCT as also observed in Fig. S3(b) of Supplement 1. This result indicates that N3B acts as a quencher most effectively. However, the coexistence of TCT and N3B relaxes quenching due to the photo-recycling of DiSC$_3$(5) by TCT as mentioned earlier.Therefore, it is expected that $R_\textrm {p}$ and $\alpha _\textrm {p}$ (and therefore $\Delta n_\textrm {sat}$ as seen later) increase when doping of N3B increases in the presence of TCT.
Fig. 4. Temporal traces of normalized fluorescence (FL) signals at an excitation wavelength of 640 nm for NPC samples of 20 vol.% HBP dispersion at different DiSC$_3$(5):TCT:N3B.
3.2 Photopolymerization kinetics
Figure 5 shows parametric plots of $R_\textrm {p}$ vs. $\alpha _\textrm {p}$ at curing intensities of (a) 5 mW/cm$^2$ and (b) 10 mW/cm$^2$ for NPC syrup of 20 vol.% HBP dispersion at different DiSC$_3$(5):TCT:N3B. These curing intensities correspond to those in holographic recording as shown later. It can be seen that peak values for $R_\textrm {p}$ generally increase with increasing the curing intensity and that final values for $\alpha _\textrm {p}$ are close to 0.9 for NPC syrup at DiSC$_3$(5):TCT:N3B=1:10:6 and 1:10:8. It can also be seen that the value for $\alpha _\textrm {p}$ maximizing $R_\textrm {p}$ (near the gelation point) is the largest for the NPC syrup at DiSC$_3$(5):TCT:N3B=1:10:8. These trends suggest that DiSC$_3$(5):TCT:N3B=1:10:8 gives the longest delay in the gelation throughout the formation of cross-linking networks during the photopolymerization process. This means that this molar combination efficiently facilitates the mutual diffusion of HBP nanoparticles and monomer molecules under holographic exposureto form a large spatial density modulation of the HBP nanoparticle density, resulting in large $\Delta n_\textrm {sat}$ as seen later. On the other hand, $R_\textrm {p}$ significantly decreases in the absence of N3B [DiSC$_3$(5):TCT:N3B=1:10:0] and of TCT [DiSC$_3$(5):TCT:N3B=1:0:8] due to strong quenching as seen in Fig. 4 and Fig. S3 of Supplement 1, resulting in small $\Delta n_\textrm {sat}$ as also seen later.
Fig. 5. $R_\textrm {p}$ vs. $\alpha _\textrm {p}$ at curing intensities (a) 5 mW/cm$^2$ and (b) 10 mW/cm$^2$ for NPC syrup of 20 vol.% HBP dispersion at different DiSC$_3$(5):TCT:N3B.
3.3 Spectral absorption
Figure 6 shows spectral absorption coefficients ($\alpha$) for NPC film samples of 20 vol.% HBP dispersion at different DiSC$_3$(5):TCT:N3B before [Fig. 6(a)] and after [Fig. 6(b)] uniform curing by the incoherent red LED light source. It can be seen in Fig. 6(a) that $\alpha$ has a characteristic absorption peak of DiSC$_3$(5) at 677 nm. It can also be seen that peak values for $\alpha$ increase when either TCT or N3B is absent. This trend correlates with that of an increase in fluorescent quenching when either TCT or N3B is absent as shown in Fig. 4. Further investigation is necessary to understand this phenomenon. After intentional curing action $\alpha$ goes to almost zero for the cured NPC film sample without TCT [DiSC$_3$(5):TCT:N3B =1:0:8]. However, the remaining absorption peak after curing is somewhat noticeable at DiSC$_3$(5):TCT:N3B =1:10:4. This result may be related to the photo-recycling mechanism in which TCT oxidizes DiSC$_3$(5) radicals back to their singlet ground state. Available thicknesses $\alpha ^{-1}$ at a holographic recording wavelength of 640 nm is approximately 72 $\mu$m before curing and is 725 $\mu$m after curing, respectively, at DiSC$_3$(5):TCT:N3B =1:10:8, the optimum relative molar concentration ratio maximizing $\Delta n_\textrm {sat}$ as shown later. They are sufficiently thick enough to uniformly record a phase grating of 10-$\mu$m order thickness or thinner along the film thickness direction without substantive absorption loss.
Fig. 6. Spectral absorption coefficients for NPC film samples of 20 vol.% HBP dispersion at different DiSC$_3$(5):TCT:N3B (a) before and (b) after uniform curing by the incoherent red LED light source.
3.4 Holographic recording characteristics
Figure 7 illustrates photographs of a plane-wave transmission grating of 0.5-$\mu$m spacing (approximately 10 mm in diameter) recorded in an NPC film sample of 20 vol.% HBP dispersion at DiSC$_3$(5):TCT:N3B=1:10:8. It can be seen that the grating is highly transparent [Fig. 7(a)] as we showed long available thickness after curing [see Fig. 6(b)]. It can also be seen that the recorded NPC grating produces bright diffracted light [Fig. 7(b)], showing the high diffraction performance.
Fig. 8(a) shows the buildup dynamics of Bragg-matched $\eta$ at a recording intensity of 10 mW/cm$^2$ and at readout wavelength of 640 nm for an NPC film sample of 20 vol.% HBP dispersion at DiSC$_3$(5):TCT:N3B=1:10:8. The inset in Fig. 8(a) shows measured angular selectivity (Bragg-angle detuning) data of $\eta _\textrm {sat}$ at a readout wavelength of 532 nm. The solid curve in green corresponds to the least-squares fit of Eq. (4) to the data. Note that $\eta _\textrm {sat} (0)$ in the inset is higher than that of the buildup dynamics since $\eta _\textrm {sat}(0)$ at 532 nm is generally higher than that at 640 nm due to larger $\Delta n_\textrm {sat}$ and shorter $\lambda$ in Eq. (4). The data fitting gave the best fit value for $\ell$ to be 8.6 $\mu$m. We consider that observed non-zero values of the first sidelobes in the inset are attributed to unnoticeable low light scattering. Figure 8(b) shows the corresponding buildup dynamics of $\Delta n$ evaluated at 640 nm. It can be seen that $\Delta n_\textrm {sat}$ is as large as 2$\times 10^{-2}$. Figure 8(c) shows the buildup dynamics of Bragg-matched $\eta$ at a recording intensity of 10 mW/cm$^2$ and at 640 nm for an NPC film sample of 25 vol.% HBP dispersion at DiSC$_3$(5):TCT:N3B=1:10:8. The inset in Fig. 8(c) shows measured angular selectivity data of $\eta _\textrm {sat}$ at a readout wavelength of 532 nm. The data fitting gave the best fit value for $\ell$ to be 7.1 $\mu$m. Figure 8(d) shows the corresponding buildup dynamics of $\Delta n$ evaluated at 640 nm. It can be seen that $\Delta n_\textrm {sat}$ is as large as 3$\times 10^{-2}$ that is comparable to that at 532 nm for an NPC film sample doped with the green photosensitizer-initiator system [30]. We therefore find that recording of high contrast NPC volume gratings is possible in both the green and the red. The 25 vol.% dispersion of HBP was found to be more or less optimal to maximize $\Delta n_\textrm {sat}$ since HBP dispersion higher than 25 vol.% led to an exponential increase in viscosity of an uncured NPC film as seen in Fig. S4 of of Supplement 1, resulting in our observation of a rapid decrease in $\Delta n_\textrm {sat}$ due to reductions in the mutual diffusion of HBP nanoparticles and monomer and in the thermal diffusion of photosensitizer-initiator components during recording.
Fig. 7. Photographs of a plane-wave transmission grating ($\ell \approx$ 9 $\mu$m) recorded in an NPC film sample of 20 vol.% HBP dispersion at DiSC$_3$(5):TCT:N3B=1:10:8. (a) Top view and (b) side view through a fluorescent light source.
Fig. 8. (a) Buildup dynamics of Bragg-matched $\eta$ evaluated at 640 nm for an NPC film sample of 20 vol.% HBP dispersion at DiSC$_3$(5):TCT:N3B=1:10:8 at a recording intensity of 10 mW/cm$^2$. The inset is measured data ($\circ$) of $\eta _\textrm {sat}$ at 532 nm as a function of Bragg-angle detuning, where solid curve is the least-squares fit of Eq. (4) to the data. (b) The corresponding buildup dynamics of $\Delta n$ evaluated at 640 nm. (c) The same as (a) except for an NPC film sample of 25 vol.% HBP dispersion. (d) The corresponding buildup dynamics of $\Delta n$ evaluated at 640 nm.
These NPC film samples doubly doped with TCT and N3B provided efficient radical generation via the cooperative electron transfer mechanism from N3B to [DiSC$_3$(5)]$^*$ and from DiSC$_3$ radicals to TCT as confirmed by the fluorescence quenching and photopolymerization kinetic measurements described earlier. Indeed, it was found that NPC film samples in the absence of either TCT or N3B gave much smaller values for $\Delta n_\textrm {sat}$ and thus $\eta _\textrm {sat}$ as shown in Fig. 9. The absence of N3B causes slower grating buildup speed due to the inefficient generation of anionic TCT radicals giving smaller $R_\textrm {p}$ as seen in Fig. 6(b). The absence of TCT results in rapid grating buildup with its fast decay due to the efficient co-generation of DiSC$_3$(5) and N3B radicals followed by the inhibition by DiSC$_3$(5) radicals, which gives smaller $R_\textrm {p}$ and its rapid decay as seen in Fig. 6(b).
Fig. 9. Buildup dynamics of Bragg-matched $\eta$ for an NPC film sample of 20 vol.% HBP dispersion at (a) DiSC$_3$(5):TCT:N3B=1:10:0 and (b) DiSC$_3$(5):TCT:N3B=1:0:8. In both cases a recording intensity and a probe wavelength were 10 mW/cm$^2$ and 532 nm, respectively.
In order to check the validity of using Eq. (4), valid for the Bragg diffraction regime, in the evaluation of $\Delta n_\textrm {sat}$, let us consider the Klein-Cook parameter ($Q$) given by $2\pi \lambda \ell /n\Lambda ^2$ and the grating strength parameter ($\nu$) given by $\pi \ell \Delta n_\textrm {sat}/\lambda \cos \theta _\textrm {B}$, respectively, where $n$ is the average refractive index of a phase grating [53,54]. When the conditions of $Q\nu /\cos \theta _\textrm {B}> 1$ and $Q/\nu \cos \theta _\textrm {B}> 20$ are met, diffraction occurs in the Bragg diffraction regime [51]. When the conditions of $Q\nu /\cos \theta _\textrm {B}< 1$ and $Q/\nu \cos \theta _\textrm {B}< 20$ are met, diffraction is in the Raman-Nath diffraction regime [55], which features many overlapping diffraction orders in a thin grating, almost independently of the angle of incidence. Otherwise, diffraction is in the intermediate diffraction regime that must be described by the rigorous coupled-wave analysis (RCWA) [56]. Phase gratings with very large $\Delta n_\textrm {sat}$ are sometimes categorized in the intermediate diffraction regime, where RCWA is necessary to evaluate $\Delta n_\textrm {sat}$ from measured $\eta _\textrm {sat}(0)$. In our case $Q\nu /\cos \theta _\textrm {B}$ and $Q/\nu \cos \theta _\textrm {B}$ are 126 (128) and 105 (69), respectively, for the NPC film sample of 20 (25) vol.% HBP dispersion at $\lambda$= 640 nm and $\Lambda$=0.5 $\mu$m and with $\ell$ = 8.6 (7.1) $\mu$m, $\Delta n_\textrm {sat}$ = 2$\times 10^{-2}$ (3$\times 10^{-2}$), $n$=1.549 (1.562) and $\cos \theta _\textrm {B}$=0.77. Therefore, the use of Eq. (4) is valid.
Figure 10 shows a recording-intensity dependence of $\Delta n_\textrm {sat}$ at 640 nm for NPC film samples of 20 and 25 vol.% HBP dispersion at different DiSC$_3$(5):TCT:N3B. It can be seen that an averaged value for $\Delta n_\textrm {sat}$ generally increases with increasing a recording intensity but saturates more or less at 10 mW/cm$^2$. It can also be seen that an averaged value for $\Delta n_\textrm {sat}$ at the HBP concentration of 25 vol.% and at DiSC$_3$(5):TCT:N3B=1:10:8 reaches the saturation close to 3$\times 10^{-2}$. We see that the optimum DiSC$_3$(5):TCT:N3B maximizing $\Delta n_\textrm {sat}$ is at 1:10:8, consistent with our fluorescent quenching and photopolymerization kinetic measurements. Note that 3$\times 10^{-2}$ of $\Delta n_\textrm {sat}$ at a recording and readout wavelength of 640 nm with our present HBP dispersed NPC is larger than 2$\times 10^{-3}$ at a recording and readout wavelength of 633 nm for transmission holograms formed in acrylamide photopolymer [57,58], and it is either comparable to 2.5$\times 10^{-2}$ and 3.0$\times 10^{-2}$ at recording (readout) wavelengths of 633 (623.5) nm and 647 (636.5) nm, respectively, or smaller than 4.2$\times 10^{-2}$ and 4.3$\times 10^{-2}$ at recording (readout) wavelengths of 633 (620) nm and 647 (631.5) nm, respectively, for reflection holograms formed in Du Pont photopolymer [59].
Fig. 10. Dependence of $\Delta n_\textrm {sat}$ at 640 nm on recording intensity for NPC film samples of 20 and 25 vol.% HBP dispersion at different DiSC$_3$(5):TCT:N3B.
We have seen that our three-component photoinitiator system provides high photosensitivity at 640 nm and the delay in gelation during photopolymerization, facilitating the mutual diffusion of monomer and HBP nanoparticles. This results in an increase in the spatial density modulation of transporting HBP by holographic assembly of nanoparticles during holographic exposure. We can express $\Delta n_\textrm {sat}$ of a holographic phase grating (the first-order peak-to-mean refractive index modulation formed in an NPC material) by [1,27]
where $n_n$ ($n_p$) is the refractive index of nanoparticles (the formed polymer) and $\Delta f$ corresponds to the amplitude of a peak-to-mean spatial density modulation amplitude of nanoparticles at the spatially varying volume fraction of nanoparticles ($f_n$). The latter is either smaller than or equal to the spatially averaged volume fraction of the nanoparticle density ($\bar {f_n}$). The prefactor $a$ is unity for a pure sinusoidal waveform of $f_n$ and is $4\sin (r\pi )/\pi$ for a rectangular waveform of $f_n$, where $r$ is the duty ratio of the concentration distribution in volume between nanoparticle-rich and -poor regions [27]. Although $a$ depends on grating spacing and a recording intensity [60], it would be more or less close to unity under the optimum recording condition maximizing $\Delta n_\textrm {sat}$. Thus, Eq. (5) suggests that an increase in $\Delta n_\textrm {sat}$ can be essentially done by the following ways: Increases in (i) $a\Delta f$ and (ii) $|n_n-n_p|$. We have shown that our NPC film sample of 20 (25) vol.% HBP dispersion at DiSC$_3$(5):TCT:N3B=1:10:8 gives the maximum value for $\Delta n_{sat}$ as large as 2.5$\times 10^{-2}$ (3.0$\times 10^{-2}$) at 640 nm. Using $n_p$ = 1.49 and $n_n$ = 1.80 at 633 nm as well as $\bar {f_n}$ = 0.20 (0.25), we find $a\Delta f$ to be 0.081 (0.097). This means that $a\Delta f/\bar {f_n}$ is 0.41 (0.39) for the NPC film sample of 20 (25) vol.% HBP dispersion, indicating that 41 % (39 %) of 20 (25) vol.% HBP species uniformly dispersed in the NPC film sample counterdiffuse from the bright to the dark regions of the light intensity-interference pattern during holographic exposure. A slight decrease in $a\Delta f/\bar {f_n}$ at 25 vol.% HBP loading may be caused by an exponential increase in the viscosity of NPC prepolymer syrup with increasing the concentration of HBP (see Fig. S4 of Supplement 1). It is consistent with a trend that $\Delta f$ increases with an increase in the ratio of the gelation time to the viscosity of a photopolymer system [10].4. Conclusion
We have demonstrated the red sensitization of photpolymerizable NPC materials dispersed with ultrahigh refractive index HBP for volume holographic recording. We have shown that the three-component photosensitizer-initiator system consisting of DiSC$_3$(5), TCT and N3B provide high photosensitivity at a holographic recording wavelength of 640 nm. It has been shown that co-doping of N3B with TCT plays a crucial role in efficient radical generation and photo-recycling. We have shown that 25 vol.% HBP-dispersed NPC transmission volume gratings of 0.5-$\mu$m spacing possess $\Delta n_\textrm {sat}$ as large as 3$\times 10^{-2}$ at DiSC$_3$(5):TCT:N3B=1:10:8. Such a large $\Delta n_\textrm {sat}$ can be attributed to a large difference between $n_n$ and $n_p$ and to efficient mutual diffusion leading to large $a\Delta f$ by use of appropriately low viscosity of the monomer blend ($\sim 10$ mPa$\cdot$s, see Fig. S4 of Supplement 1) and ultrahigh refractive index HBP. Our investigation of recording similar high contrast volume holographic gratings in the blue is also underway and will be reported elsewhere. The present results suggest the potential use of our HBP-dispersed NPC materials with multi-color photosensitivity for holographic applications including VBGs, color and security holograms.
Funding
Nissan Chemical Corp.
Acknowledgments
Y.T. would like to acknowledge Keisuke Odoi for fruitful discussions throughout the work.
Disclosures
The authors declare no conflicts of interest.
Supplemental document
See Supplement 1 for supporting content.
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