Highly diffractive, reversibly fast responsive gratings formulated through holography

Abstract

Versatile and reversibly rapid responsive one-dimensional photonic crystals with a diffraction efficiency of 97%, which consisted of uniformly 273 ± 48 nm wide liquid crystal belts within transmission holographic polymer dispersed liquid crystal (HPDLC) gratings, were formulated by a facile single step holography based on a hyperbranched monomer. The effect of the monomer average functionality on the photopolymerization kinetics and the electro-optical performances as well as the grating morphologies was investigated. The results show that the low intrinsic viscosity of the hyperbranched monomer accounts for a well-structured morphology in terms of providing a prolonged gelation time for the liquid crystals to diffuse from the light illumination region during the holographic polymerization induced phase separation. Another intriguing observation is that with an increase in the hyperbranched monomer loading, the diffraction efficiency of the HPDLC gratings gradually increases from zero to an average of 94% and then levels off. This is quite different from previous results that gave less than a 50% diffraction efficiency when the monomer average functionality was larger than 4.

---

1. Introduction

Most natural optical phenomena like iridescence are created by the periodic distribution of materials and refractive indices. By mimicking nature, photonic crystals have been extensively investigated for applications such as sensors, displays and optical analysis.^1–11 Despite many approaches like self-assembly,^12–15 inkjet printing,¹⁶ spin-coating^7,17,18 and lithography^14,19 being proposed to develop artificial periodic features, holography is regarded as the most promising technique for fabricating mass-productive, large area and defect-free photonic crystals.¹⁹ Newly developed photopolymerization systems will further extend the capability of holography to form a 3D complex photonic structure temporally and spatially.^20–22 Through holographic photopolymerization, versatile and striking photonic crystals based on holographic polymer dispersed liquid crystals (HPDLCs) have been developed.^{10,19,23–27} Analogous to other traditional photonic crystals, HPDLC gratings exhibit a “photonic bandgap” that disallows light to travel through at the wavelength of the stopbands.^25,27 Moreover, as one type of reversibly responsive functional material,^28–31 light diffraction in these HPDLC gratings can be switched by an electric or thermal stimulus.^10,19,32

Typically, HPDLC gratings are constructed using multiple coherent laser beams to illuminate homogeneous mixtures that consist of photopolymerizable monomers, liquid crystals and photoinitiators. During the construction, the monomers are consumed by rapid polymerization at the bright interference fringes while the liquid crystals diffuse to the dark areas within the interference patterns due to their chemical potentials becoming different from the growing polymers, resulting in a phase separation between the polymer regions and the liquid crystal domains. Finally, HPDLC gratings with a spatial periodic distribution of chemical materials and their corresponding refractive indices are formed. For applications in tunable lasers,^33–35 graded rainbow-colored films,³⁶ plasmon coupling,³⁷ or in some biological areas,^10,38 it is highly desirable to enhance the phase separation and the refractive index contrast between the LC-rich domains and the polymer-rich regions during holographic recording. Thus well-structured HPDLC gratings can be formed with good performances such as high diffraction efficiency, low driving voltage and response time.^39–42 One effective way is to change the monomer type and the average functionality.^40,43–48 Besides the crosslinking density and the elastic force of the polymers giving a sufficient chemical difference from the LCs, the main hindrance for the holographic polymerization induced phase separation is monomer gelation. Only if the LC diffusion is much faster than the LC nucleation, and if the latter occurs earlier than the polymer network gelation, can a perfect segregation structure be formed.³⁹ In order to enhance the phase separation, Caputo et al. have suggested that it is helpful to heat the holographic syrups to a temperature higher than that of the LC nematic–isotropic transition during holography,⁴⁹ but this is complicated and energy-consuming. The integration of nematic LCs into high resolution submicron or even nanoscale polymer structures which show a sharp and uniform morphology along with a high diffraction efficiency by one-step holography remains a challenge, so researchers have turned to two-step fabrication methods whereby a polymer fringe is initially prepared by molding the lithography before embedding with LCs.⁵⁰ Obviously, the two-step strategy is complicated, and unpractical for two or three-dimensional HPDLC construction. Few strategies have been proven to be efficient and straightforward for constructing highly diffractive HPLC photonic crystals with complete phase separation that show a sharp and uniform morphology. Meanwhile, few reports have obtained a diffraction efficiency higher than 90% at a high loading of multi-functional monomers.

Hyperbranched monomers are good choices to formulate HPDLC gratings because hyperbranched architectures are a non-entangled architecture, and of low intrinsic viscosity, low hydrodynamic radius and high free volume.^51,52 These properties are profitable to delay gelation and enhance the migration of both the monomers and the LCs, resulting in an improved segregation, a high index contrast and consequently a high diffraction efficiency. Kim's group has fabricated HPDLC gratings with a diffraction efficiency close to 80% by loading 0.05 wt% hyperbranched monomer with a functionality of 16 and a molecular weight of 1750.⁴⁸ In this paper, we introduce a low viscous hyperbranched monomer 6361-100 (functionality of 8, average molecular weight of 650 and viscosity of 245 mPa s at room temperature). When its loading was higher than 31 wt%, we obtained one-dimensional HPDLC photonic crystals with a uniform and sharp morphology and as high as 97% diffraction efficiency.

2. Experimental

2.1 Chemicals

2-Ethylhexyl acrylate (EHA, purity: >99%) was purchased from Acros Organics. The solubilizer N-vinylpyrrolidone (NVP, purity: 99%) was supplied by Aldrich. The hyperbranched monomer 6361-100 (functionality f = 8 and average molecular weight = 650) was donated by Eternal Chemical Co., Ltd (China). The liquid crystal P0616A (Δn_{(589 nm, 20 °C)} = 0.22, T_NI = 58 °C) was obtained from Shijiazhuang Chengzhi Yonghua Display Material Co., Ltd (China). We used all chemicals as received without further purification.

2.2 Holographic mixtures preparation

All the monomer mixtures utilized in this study are given in Table 1. Homogeneous mixtures containing consecutive monomer mixtures, 1.9 wt% of a type II photoinitiator containing coumarine and 33.3 wt% LCs P0616A, were obtained by ultrasonication at 50 °C for 1 h.

Table 1 Compositions of the monomer mixtures and the corresponding average functionality f_av

Entry	EHA (wt%)	NVP (wt%)	6361-100 (wt%)	fav
1#	49.9	10.3	4.6	1.5
2#	45.0	10.3	9.5	2.0
3#	40.9	10.2	13.7	2.5
4#	36.4	10.2	18.2	3.0
5#	31.8	10.3	22.7	3.5
6#	27.2	10.3	27.3	3.9
7#	22.5	10.7	31.6	4.4
8#	18.2	10.2	36.4	4.9
9#	13.6	10.3	40.9	5.4
10#	9.3	10.4	45.1	5.9

---

2.3 Holographic recording

Homogeneous mixtures were introduced by capillary action into visibly transparent LC cells with a 10 μm cell gap and an indium-tin oxide (ITO) electronic layer coated on both the inner surfaces. A 441.6 nm He–Cd laser was split into two equal beams, each with an intensity of 8.8 mW cm⁻², and then directed at the LC cells for 20 s at an angle of θ_set = 24° before being blocked by a timer shutter. All recorded holograms were post-cured under a mercury lamp for 300 s. The grating period Λ and Klein-Cook parameter Q are 1060 nm and 16.5 respectively according to eqn (1) and (2), indicating that our gratings belong to the Bragg regime type (volume gratings) and their properties can be predicted by Kogelnik's coupled-wave theory,^32,43,53where, λ_writing is the wavelength of the holographic recording beams, L is the grating thickness and n₀ is the average refractive index of the materials, roughly equal to 1.5.

2.4 Measurements

For understanding the photopolymerization kinetics, approximately 10 mg of the holographic syrups were micropipetted into a TA aluminum liquid pan, placed on the photo-differential scanning calorimeter (P-DSC) sample holder (Q2000, TA instruments) and allowed to equilibrate at the cure temperature of 30 °C for 1 min, and then isothermally exposed for 10 min under a 50 mL min⁻¹ nitrogen purge to a monochromatic 441.6 nm light with an intensity of 17.6 mW cm⁻². An empty aluminum liquid pan was used as the reference. The rate of polymerization R_p and the monomer conversion α(t) can be calculated by eqn (3) and (4) because the isothermal heat flow recorded by the P-DSC comes from the consumption of C=C double bonds:⁵⁴where, dH/dt, ΔH(t), m, x_i, f_i and M_i are the exothermic heat flow at the reaction time t, the enthalpy change up to time t, the total mass of the holographic mixtures, the individual weight fraction, and the functionality and the molecular weight of the monomer i in the mixture, respectively. The standard reaction enthalpies of the C=C double bonds, ΔH₀ are 86 190 and 53 900 J mol⁻¹ for the acrylates and NVP respectively.

Electro-optical tests were implemented by monitoring the first-order diffraction intensity variation by a LCT-5016C liquid crystal display parameter tester (instrument from North LC Engineering Research and Development Centre, China), under a square wave voltage across the HPDLC cells with a frequency of 1 kHz and amplitudes of 0–180 V. A 2 mW collimated, and as high as 500:1 p-polarized, He–Ne laser (λ_probe = 632.8 nm, made by Thorlabs, USA) probed the cells at their Bragg angles. The ratio of the first-order diffraction intensity to the sum of the zero- and first-order diffraction intensities was defined as the diffraction efficiency.

The morphologies of the HPDLC gratings were observed by removing a piece of glass, extracting the LCs using analytical pure n-hexane for 2 days, spraying with a thin layer of platinum, and then mounting on the sample stage of a Sirion 200 field emission scanning electron microscope (FESEM) (FEI, Netherlands).

3. Results and discussion

3.1 Effect of average functionality f_av on the reaction kinetics, electro-optical performances and morphologies of HPDLC gratings

The degree of phase separation is closely related to the photopolymerization kinetics of holographic mixtures during holography. Generally, an increase in monomer functionality leads to an increase in the crosslinking density and the elastic force of a polymer network, thus causing an enhanced phase separation and the narrow distribution of liquid crystal droplets.⁴⁴ On the other hand, a high monomer functionality slows down the polymerization rate and the monomer conversion since the high viscosity and early gelation hinders monomer migration.⁴⁰ To maintain a balance, researchers usually mix low functional monomers with high functional monomers to generate a high crosslinking density and elastic force as well as delayed gelation. The curves of polymerization rate R_p against monomer conversion α for mixtures with different average functionalities f_av are displayed in Fig. 1. Although the figure is similar to that reported before,⁴⁰ there are diverse features worth mentioning. With an increase of f_av from 1.5 to 5.4, α decreases by 45.9% (i.e., from 45.3% to 24.5%); by contrast, the maximum R_p firstly increases from 0.0048 to 0.0055 s⁻¹ when f_av increases from 1.5 to 2.5 and then decreases to 0.0037 s⁻¹ when f_av reaches 5.4. Since high functional monomers have more reactive sites to devote to the formation of polymer networks, the increase in the amount of the high functional monomers significantly increases the crosslinking density of the three-dimensional polymer network and decreases the mobility of the reacting species, which in turn diminishes the polymerization rate R_p and the monomer conversion α. The mixtures with f_av < 2.0 have a relatively low reaction rate which can be ascribed to the presence of a large amount of monofunctional monomers (>60 wt%) and prevalent bimolecular termination.⁴⁰

Fig. 1 Rate of polymerization R_p vs. monomer conversion α of the holographic mixtures with varied average functionality f_av: 1#, 1.5; 2#, 2.0; 3#, 2.5; 4#, 3.0; 5#, 3.5; 7#, 4.4; 9#, 5.4.

The monomer average functionality dependent diffraction efficiency is shown in Fig. 2, where, η is almost zero when f_av is less than 2.0, and increases gradually to its largest value of 97% with an increase of f_av to 4.9, then decreases to 91% with the further growth of f_av to 5.9. Using eqn (5) and Origin software 8 (OriginLab, USA) we can calculate that the average maximum η_max is equal to 94%, the diffraction growth factor k and the critical average functionality f_c are 3.54 and 3.48 respectively. Compared to the reports from Ramsey⁴³ and De Sarkar et al.,⁴⁰ our results show a higher η at high f_av because the hyperbranched monomer is used here which offers a low viscosity, so that the LCs have sufficient time to diffuse out before system vitrification.

Fig. 2 Diffraction efficiency η of the HPDLC gratings against average functionality f_av from 1.5 to 5.9.

The significance of the crosslinking density and the elastic force of the HPDLCs is demonstrated by the field-dependent η of the HPDLC gratings. As visualized in Fig. 3, when f_av is less than 3.0, there is no optical response even when an electric field as high as 180 V is applied across the cells. By contrast, when f_av increases from 3.5 to 4.9, the threshold voltage E₉₀ and the saturated voltage E₁₀ for switching HPDLC gratings⁴⁰ decrease from 59.7 and 141.2 V to 35.9 and 70.5 V, respectively, however, they rise to 89.6 and 167.9 V respectively when f_av further increases to 5.9. When we add more of the high-functional monomer to increase f_av, the enlarged crosslinking density and elastic force lead to a greater phase separation,^40,43 resulting in the remarkable improvement of the refractive index contrast n_LC − n_p and the volume fraction of the LCs ϕ in the LC-rich region, and consequently a larger η according to eqn (6).^53,55 The enlarged segregation also increases the LC droplet size and increases the low frequency conductivity ratio σ_LC/σ_P between the LC-rich and the polymer-rich regions, the former causes a lower critical driving voltage E_c while the latter gives a higher one in accordance with eqn (7).³² On the other hand, the more we introduce the high functional monomer, the higher the viscosity of the mixtures, which causes a depressed segregation since it hinders the LC diffusion. Eventually, the LC droplet size and the low frequency conductivity ratio σ_LC/σ_P are reduced; the former leads to a larger E_c while the latter gives a smaller one. Therefore, both the threshold voltage E₉₀ and the saturated voltage E₁₀ firstly decrease and then increase with an increase of f_av, depending on which factor is more prominent.

where, θ_B is the Bragg angle, λ_probe is the wavelength of the probe beam, Δε and k₃₃ represent the dielectric anisotropy and pure bend elastic force constant of the LCs, respectively; b and [small script l] represent the length of the semi-major axis and the aspect ratio (defined as the specific value of the semi-major axis length to the semi-minor axis length of the LC droplets), respectively.

Fig. 3 The field-dependent η of the HPDLC gratings vs. an applied field with different f_av: 3#, 2.5; 4#, 3.0; 5#, 3.5; 6#, 4.0; 7#, 4.4; 8#, 4.9; 9#, 5.4; 10#, 5.9.

These field-dependent η results are in accordance with the time-response curves explored in Fig. 4. The contrast ratio ranges from 1.7 to 8.1 (Table 2). The rise time τ_on is around 2 ms for all the gratings except that of 4.5 ms given for the grating with a f_av of 5.9. It can be explained that τ_on not only depends on the LC droplet size and the aspect ratio but also mainly relies on the electric field strength E according to eqn (8).³² By contrast, the decay time τ_off slightly increases from 4.5 to 7.5 ms when f_av increases from 3.5 to 4.9 and then decreases to 5.5 ms along with a further increase in f_av. The decay time of the HPDLC gratings is described by eqn (9):³²

where, γ₁ is the rotational viscosity coefficient of the LCs. Combining this with the switching voltage results and the decay time outcomes given in Fig. 3 and 4 and Table 2, we can conclude that the LC droplet size firstly increases and then decreases.

Fig. 4 The time-response of the HPDLC gratings to a 180 V pulsed voltage with different f_av: 5#, 3.5; 6#, 4.0; 7#, 4.4; 8#, 4.9; 9#, 5.4; 10#, 5.9.

Table 2 Electro-optic parameters of the HPDLC gratings with consecutive f_av

Entry	fav	E90 (V)	E10 (V)	τon (ms)	τoff (ms)	Contrast ratio
5#	3.5	59.7	141.2	2.0	4.5	2.4
6#	3.9	48.8	92.0	2.0	5.5	1.7
7#	4.4	48.7	85.4	2.0	4.5	5.9
8#	4.9	35.9	70.5	1.5	7.5	4.4
9#	5.4	71.5	129.5	2.0	6.0	8.1
10#	5.9	89.6	167.9	4.5	5.5	7.2

---

Another factor responsible for the electro-optical behaviors of the HPDLC gratings is the anchoring force of the polymer networks. An irregular surface might give a higher anchoring force owing to a larger specific surface area of the LC droplets and accordingly increase the switching field; τ_off of the HPDLC gratings with a nondroplet morphology is also dominantly determined by the surface interaction parameter.⁵⁶ Fig. 5 illustrates the FESEM images of several HPDLC gratings, since the LCs are removed before microscopy, the darker regions represent the original LC domains. From these images, we can see that if f_av is less than 2.0, no gratings can be observed; with an increase in f_av, the degree of phase separation increases, and uniform gratings with a sharp edge are generated, especially for that with a f_av of 4.4, where the ratio of the LC belts to the grating period is 28.0%, roughly close to the concentration of the LCs in their liquid mixture (33.3%). Especially, when f_av = 4.9, similar structures to that of Fig. 5c are obtained. The periodical and uniform structure in Fig. 5b and c makes the HPDLC gratings form one-dimensional photonic crystals, which show great potential for applications in low-threshold and high output tunable lasers. Further increasing the high-functional monomer seems to broaden the LC-rich region,⁴⁰ however, it hinders diffusion and produces smaller LC droplets which are dominantly distributed in the LC-rich region. As displayed in Fig. 5d, LC droplets with an average diameter of 74 ± 22 nm can be found when f_av = 5.4, mainly because the higher viscosity causes earlier gelation which consequently hinders LC migration. It's worth mentioning that the grating pitch obtained from the FESEM images is smaller than that expected, a possible explanation is that polymerization shrinkage causes that difference.

Fig. 5 FESEM images of the HPDLC gratings with varied f_av: (a) 2.0; (b) 3.0, Λ = 976 ± 27 nm, LC belt width is 254 ± 41 nm; (c) 4.4, Λ = 976 ± 31 nm, LC belt width is 273 ± 48 nm; (d) 5.4, Λ = 976 ± 50 nm, LC-rich region width is 279 ± 54 nm, and LC droplet size is 74 ± 22 nm.

Perfect phase separated fringes not only depend on the crosslinking density and the elastic force, but also primarily rely on polymer network gelation. Both the crosslinking density and the elastic force dramatically increase while the gelation time becomes shorter when the amount of the high-functional (f = 8) monomer 6361-100 increases. When f_av is around 4.4, a uniform grating with submicron LC belts and a sharp edge is obtained. The relatively low viscosity (245 mPa s) of the hyperbranched monomer affords the LC molecules sufficient time to migrate across distances of the order of Λ/2 prior to polymer network vitrification, consequently, belts in the dark region squeezed by growing polymer networks are formed.^39,43

3.2 Angle-dependent diffraction of one-dimensional HPDLC photonic crystals

The LC director rotates during holographic polymerization, resulting in preferential LC alignment. As shown in Fig. 6, we discover that p-polarized diffraction in the one-dimensional photonic crystals is much larger than s-polarized. Based on Kogelnik's coupled-wave theory⁵³ (eqn (10)–(12)), calculated results given in Table 3 show that the index modulation n₁ for a p-polarized beam is nearly twice that for a s-polarized beam, in addition, the fitted grating thickness L is in quantitative agreement with the designed parameters.where, θ is the incident angle.

Fig. 6 Angle sensitive diffraction plots of the HPDLC photonic crystals with f_av of 4.4.

Table 3 Polarized diffraction parameters of the HPDLC photonic crystals with f_av of 4.4

Probe beam	λ (nm)	L (μm)	θB (degree)	n1
p-Polarized	632.8	9.37	17.9	0.027
s-Polarized	632.8	9.60	17.6	0.014

---

The fascinating angle-dependent diffraction and response time make our photonic crystals here capable of display and laser modulation, although the switching field should be further reduced for practical applications. Fig. 7 explores the diffraction and transmission states of the one-dimensional HPDLC photonic crystals based on the hyperbranched monomer 6361-100, which clearly shows an ultrahigh contrast, i.e., we cannot see the school logo of HUST at the diffraction angle, while we see that pattern unambiguously through the grating at the transmission angle.

Fig. 7 Diffraction (left) and transmission (right) states of HPDLC transmission gratings, f_av = 4.4. The incident light comes from a LC computer screen.

4. Conclusions

In summary, based on the hyperbranched monomer 6361-100, we investigated the effect of the average monomer functionality f_av on the photopolymerization kinetics of holographic mixtures, the diffraction efficiency η, the electro-optical behaviors and the morphologies of the corresponding HPDLC transmission gratings. With an increase of f_av from 1.5 to 5.9, both the reaction rate and the monomer conversion decrease, however, η reversibly increases from zero to its average maximum of 94%, and the critical average functionality f_c is equal to 3.48. Meanwhile, the driving voltage to switch off diffraction firstly decreases and then increases as a result of competition between the crosslinking density, the elastic force and the gelation of the polymer networks. When f_av equals 4.4, a HPDLC photonic crystal possessing uniform liquid crystal belts with a width of 273 ± 48 nm and η of 97% is given; in addition, its threshold and saturated voltages are 48.7 and 85.4 V, the rise time and decay time are 2.0 and 4.5 ms respectively. A uniform nondroplet belt morphology dominates and benefits from the relative low viscosity of the hyperbranched monomer 6361-100 which offers a sufficient diffusion time for the LCs to migrate on the order of half the grating period prior to polymer vitrification.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The authors appreciate financial support by the Outstanding Youth Fund of the National Natural Science Foundation of China (50825301) and thank the HUST Analytical and Testing Center for characterization assistance. H.Y. Peng expresses his grateful appreciation for a China Scholarship Council award (201206160040) and technical support from TA instruments and Future S&T (Shenzhen) Co., Ltd.

Notes and references

1. J. Wang, L. Wang, Y. Song and L. Jiang, J. Mater. Chem. C, 2013, 1, 6048–6058.
2. J. Feng, S. Bian, Y. Long, H. Yuan, Q. Liao, H. Cai, H. Huang, K. Song and G. Yang, J. Mater. Chem. C, 2013, 1, 6157–6162.
3. H. Xu, P. Wu, C. Zhu, A. Elbaz and Z. Z. Gu, J. Mater. Chem. C, 2013, 1, 6087–6098.
4. P. H. C. Camargo, Z.-Y. Li and Y. Xia, Soft Matter, 2007, 3, 1215–1222.
5. Y. Imai, C. E. Finlayson, P. Goldberg-Oppenheimer, Q. Zhao, P. Spahn, D. R. E. Snoswell, A. I. Haines, G. P. Hellmann and J. J. Baumberg, Soft Matter, 2012, 8, 6280–6290.
6. Y. F. Yue, M. A. Haque, T. Kurokawa, T. Nakajima and J. P. Gong, Adv. Mater., 2013, 25, 3106–3110.
7. M. Kolle, A. Lethbridge, M. Kreysing, J. J. Baumberg, J. Aizenberg and P. Vukusic, Adv. Mater., 2013, 25, 2239–2245.
8. Y. Huang, F. Li, M. Qin, L. Jiang and Y. Song, Angew. Chem.,Int. Ed., 2013, 52, 7296–7299.
9. P. Vukusic and J. R. Sambles, Nature, 2003, 424, 852–855.
10. Y. J. Zhao, Z. Y. Xie, H. C. Gu, C. Zhu and Z. Z. Gu, Chem. Soc. Rev., 2012, 41, 3297–3317.
11. Z. Wang, J. Zhang, J. Xie, Y. Yin, Z. Wang, H. Shen, Y. Li, J. Li, S. L. L. Cui, L. Zhang, H. Zhang and B. Yang, ACS Appl. Mater. Interfaces, 2012, 4, 1397–1403.
12. I. Musevic and M. Skarabot, Soft Matter, 2008, 4, 195–199.
13. N. V. Dziomkina and G. J. Vancso, Soft Matter, 2005, 1, 265–279.
14. J. H. Moon and S. Yang, Chem. Rev., 2010, 110, 547–574.
15. J. N. Tisserant, R. Hany, S. Partel, G. L. Bona, R. Mezzenga and J. Heier, Soft Matter, 2012, 8, 5804–5810.
16. L. Cui, Y. Li, J. Wang, E. Tian, X. Zhang, Y. Zhang, Y. Song and L. Jiang, J. Mater. Chem., 2009, 19, 5499–5502.
17. P. Jiang and M. J. McFarland, J. Am. Chem. Soc., 2004, 126, 13778–13786.
18. M. Pichumani, P. Bagheri, K. M. Poduska, W. Gonzalez-Vinas and A. Yethiraj, Soft Matter, 2013, 9, 3220–3229.
19. J. H. Moon, J. Ford and S. Yang, Polym. Adv. Technol., 2006, 17, 83–93.
20. T. Gong, B. J. Adzima, N. H. Baker and C. N. Bowman, Adv. Mater., 2013, 25, 2024–2028.
21. T. Gong, B. J. Adzima and C. N. Bowman, Chem. Commun., 2013, 49, 7950–7952.
22. W. Xi, M. Krieger, C. J. Kloxin and C. N. Bowman, Chem. Commun., 2013, 49, 4504–4506.
23. F. Vita, D. E. Lucchetta, R. Castagna, L. Criante and F. Simoni, Appl. Phys. Lett., 2007, 91, 103114.
24. J. Qi, M. E. Sousa, A. K. Fontecchio and G. P. Crawford, Appl. Phys. Lett., 2003, 82, 1652–1654.
25. V. P. Tondiglia, L. V. Natarajan, R. L. Sutherland, D. Tomlin and T. J. Bunning, Adv. Mater., 2002, 14, 187–191.
26. X. Sun, X. Tao, T. Ye, P. Xue and Y. S. Szeto, Appl. Phys. B: Lasers Opt., 2007, 87, 267–271.
27. E. A. Dorjgotov, A. K. Bhowmik and P. J. Bos, Appl. Phys. Lett., 2010, 96, 163507.
28. R. J. Wojtecki, M. A. Meador and S. J. Rowan, Nat. Mater., 2011, 10, 14–27.
29. Y. Sagara and T. Kato, Nat. Chem., 2009, 1, 605–610.
30. J. M. Spruell and C. J. Hawker, Chem. Sci., 2011, 2, 18–26.
31. S. Bi, H. Peng, S. Long, M. Ni, Y. Liao, Y. Yang, Z. Xue and X. Xie, Soft Matter, 2013, 9, 7718–7725.
32. T. J. Bunning, L. V. Natarajan, V. P. Tondiglia and R. L. Sutherland, Annu. Rev. Mater. Sci., 2000, 30, 83–115.
33. H. Coles and S. Morris, Nat. Photonics, 2010, 4, 676–685.
34. D. Luo, Q. G. Du, H. T. Dai, H. V. Demir, H. Z. Yang, W. Ji and X. W. Sun, Sci. Rep., 2012, 2, 627.
35. D. Luo, X. W. Sun, H. T. Dai, H. V. Demir, H. Z. Yang and W. Ji, Appl. Phys. Lett., 2010, 97, 081101.
36. K. Liu, H. Xu, H. Hu, Q. Gan and A. N. Cartwright, Adv. Mater., 2012, 24, 1604–1609.
37. Y. J. Liu, Y. B. Zheng, J. Liou, I. K. Chiang, I. C. Khoo and T. J. Huang, J. Phys. Chem. C, 2011, 115, 7717–7722.
38. C. E. Hoyle and C. N. Bowman, Angew. Chem.,Int. Ed., 2010, 49, 1540–1573.
39. T. J. Bunning, L. V. Natarajan, V. P. Tondiglia, R. L. Sutherland, R. Haaga and W. W. Adams, Proc. SPIE, 1996, 2651, 44–55.
40. M. De Sarkar, N. L. Gill, J. B. Whitehead and G. P. Crawford, Macromolecules, 2003, 36, 630–638.
41. A. Ogiwara, M. Watanabe, T. Mabuchi and F. Kobayashi, Appl. Opt., 2011, 50, 6369–6376.
42. T. J. White, L. V. Natarajan, V. P. Tondiglia, P. F. Lloyd, T. J. Bunning and C. A. Guymon, Polymer, 2007, 48, 5979–5987.
43. R. A. Ramsey and S. C. Sharma, ChemPhysChem, 2009, 10, 564–570.
44. R. T. Pogue, L. V. Natarajan, S. A. Siwecki, V. P. Tondiglia, R. L. Sutherland and T. J. Bunning, Polymer, 2000, 41, 733–741.
45. T. J. Bunning, L. V. Natarajan, V. P. Tondiglia, G. Dougherty and R. L. Sutherland, J. Polym. Sci., Part B: Polym. Phys., 1997, 35, 2825–2833.
46. M. S. Park and B. K. Kim, Nanotechnology, 2006, 17, 2012–2017.
47. N. H. Nataj, E. Mohajerani, H. Jashnsaz and A. Jannesari, Appl. Opt., 2012, 51, 697–703.
48. J. H. Park and B. K. Kim, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 1255–1261.
49. R. Caputo, L. De Sio, A. Veltri and C. Umeton, Opt. Lett., 2004, 29, 1261–1263.
50. L. De Sio, J. G. Cuennet, A. E. Vasdekis and D. Psaltis, Appl. Phys. Lett., 2010, 96, 131112.
51. W. Gong, Y. Mai, Y. Zhou, N. Qi, B. Wang and D. Yan, Macromolecules, 2005, 38, 9644–9649.
52. X. Y. Zhu, Y. F. Zhou and D. Y. Yan, J. Polym. Sci., Part B: Polym. Phys., 2011, 49, 1277–1286.
53. H. Kogelnik, Bell Syst. Tech. J., 1969, 48, 2909–2927.
54. T. J. White, W. B. Liechty, L. V. Natarajan, V. P. Tondiglia, T. J. Bunning and C. A. Guymon, Polymer, 2006, 47, 2289–2298.
55. R. A. Vaia, C. L. Dennis, L. V. Natarajan, V. P. Tondiglia, D. W. Tomlin and T. J. Bunning, Adv. Mater., 2001, 13, 1570–1574.
56. K. K. Vardanyan, J. Qi, J. N. Eakin, M. De Sarkar and G. P. Crawford, Appl. Phys. Lett., 2002, 81, 4736–4738.

---