Liquid Crystal Photonic Structure

Article pubs.acs.org/JPCC

Low-Voltage Tuning in a Nanoparticle/Liquid Crystal Photonic Structure Luigino Criante† and Francesco Scotognella*,‡ †

Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, Via Giovanni Pascoli, 70/3, 20133 Milano, Italy Dipartimento di Fisica, Istituto di Fotonica e Nanotecnologie CNR, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

‡

ABSTRACT: In this study, we propose the fabrication and characterization of nanoparticle based multilayer photonic crystals infiltrated with a nematic liquid crystal. We show a very simple infiltration of the liquid crystal in the porous structure, controlled by monitoring the photonic band gap during the infiltration process. Tunability with electric field (by aligning the liquid crystal director) has been observed at very low applied voltage, with a blue shift of the photonic band gap of 8 nm at only 8 V. The presented results could be very interesting for realization of low cost and portable switching devices for high density integrated optics.

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can be easily recorded spectroscopically or even by eye.20,21 A very interesting approach in order to obtain a photonic structure with high refractive index contrast, is the fabrication of multilayer photonic crystal by using metal oxide nanoparticles, starting from their colloidal dispersion. Nanoparticle based multilayer photonic crystals are reported by using silicon dioxide (also silicalites), titanium dioxide, zinc oxide, and tin oxide.7,22 In these photonic crystals, beside the peculiar properties of each oxide nanoparticle that can be exploited for optical and sensing applications,23 it is possible to obtain very high refractive index modulations only alternating two or more suitable nanoparticle materials. An interesting consequence is the capability to achieve significant photonic band gap efficiency with a thickness of just a few micrometers. Furthermore, the inclusion of molecular and polymeric materials in the pores between the nanoparticles can be studied, in order to allow the realization of organic/inorganic composite materials.13 In this way, a combination of inorganic structures, characterized by a wide refractive index variation among the different compounds, with organic material versatility, might undertake the aforementioned technological challenges in high density integrated optics. The infiltration of liquid crystals in photonic structures is an interesting example of organic/inorganic composite useful for integrated optics. In these structures, the photonic band gap can be tuned with temperature24−26 and applied electromagnetic field,27−32 owing to the optical properties of liquid crystals. In particular, even if the tunability with applied electric field can be easily obtained with common suppliers, a proper choice of materials should be done in order to obtain an intense photonic band gap and a low applied voltage. The photonic band gap efficiency is proportional to Δnd, where Δn is the

INTRODUCTION The replacement of electronic switches and processes by all optical devices is one of the toughest technological challenges in the last decades. Future key technologies should make possible the achievement of very high density integrated optical circuits allowing a significant electro-optic tuning capability. The main problems associated with high density integration are the high operation speed, power dissipation, and low voltage circuit design.1 Moreover, despite of the huge progress in nanoand micro-optical circuit manufacture (such as in lab on a chip (LOC) systems), the cost effectiveness, ease of manufacturability, and portability of the devices remain largely unaddressed.2 Photonics crystals may open the way to overcome these problems due to ease of fabrication, the possibility of obtaining compact tunable and switchable optoelectronic devices and the flexibility in wavelength design. They are complex dielectric structures that show variations of the refractive index on a length scale comparable to the wavelength of light. In such structures with an ordered dielectric periodicity, for a certain range of energies and certain wave vectors, light is not allowed to propagate through the medium.3−6 Photonic crystals exist in nature or can be fabricated using a wide range of techniques, with the dielectric periodicity in one, two, and three dimensions.7−9 In the one dimension case, simple and lowcost fabrication techniques can be used, as for instance, spin coating or coextrusion.7,10 Nowadays, these materials are extensively studied since they find application in several fields, including photonics for low threshold laser action, high bending angle waveguide, superprism effect, sensors, and optical switches.11−19 Recently, by combining these types of fabrication with simple bottom-up self-assembly fabrication strategies, several research groups have realized photonic crystals that are based on intrinsically functional layer materials whose response to external stimuli translates directly into a color-change, that © 2012 American Chemical Society

Received: September 12, 2012 Revised: September 18, 2012 Published: September 18, 2012 21572

dx.doi.org/10.1021/jp309061r | J. Phys. Chem. C 2012, 116, 21572−21576

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RESULTS AND DISCUSSION In Figure 1a, the scanning electron microscope image of the fabricated SiO2/ZrO2 multilayer photonic crystal is shown. The

amplitude modulation of the refractive index and d is the thickness of the sample. To increase the thickness would be detrimental since the electrical tuning will require, inevitably, a high applied electric voltage.28,29 Instead, to obtain a lowvoltage tuning photonic crystal with good band gap efficiency, the best way is the use of materials (or a combination of them) that allow a high refractive index contrast (Δn) and to arrange an efficient device with only a few micrometers of thickness. In this study, we report the fabrication and the optical characterization of multilayer photonic crystals composed by silicon dioxide and zirconium dioxide made with spin coating technique and subsequently infiltrated with a liquid crystal. The optical characterization of the photonic crystal has been corroborated with theoretical analysis, employing the transfer matrix method (TMM). The infiltration of nematic liquid crystal mixture in the nanoparticle based porous structure is a very simple process and has been monitored in real time by the photonic band gap shift toward longer wavelengths. The wavelength tuning capability of the device, by applying an external alternate voltage, has been also performed. Owing to the small thickness of the sample, we have observed a blue shift of the photonic band gap of 8 nm by applying a very low voltage, i.e., 8 V.

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Article

EXPERIMENTAL SECTION

The multilayer photonic crystal has been fabricated by starting from colloidal dispersion of silicon dioxide and zirconium dioxide. Both silicon dioxide and zirconium dioxide colloidal dispersions are purchased from Alfa Aesar. Silicon (zirconium) dioxide nanoparticles have an average size of 100 nm and are dispersed in ethylene glycol (water) with a concentration of 30% (10%) in weight. E7 liquid crystal mixture, composed of 4cyano-4-n-pentyl-biphenyl (5CB), 4-cyano-4-n-heptylbiphenyl (7CB), 4-cyano-4-n-octyloxy-biphenyl, and 4-cyano-4-n-pentylp-terphenyl, was purchased at Merck Liquid Crystal. In the visible range (from 650 to 400 nm) and at room temperature, the extraordinary refractive index (ne) increases from 1.73 to 1.80, while its ordinary refractive index (no) also increases slowly from 1.52 to 1.54.33 Its isotropic refractive index (at 589 nm) niso is ∼1.575, while in the near-infrared region, Δn is about 0.186 at 1.55 μm.34 The photonic multilayer has been realized by spin coating technique. The sample has been deposited on indium tin oxide (ITO) glass substrate, previously sonicated for 10 min in acetone and isopropanol. The spin coater is a Laurell WS-4006NPP-Lite, and the speeds of rotation were 6000 and 3000 rpm for silicon dioxide and zirconium dioxide, respectively (with a rotation time of 30 s). After each layer deposition, the sample was heated at a temperature of 350 °C for 20 min in air. Morphological characterization of the samples (before infiltration) has been done by Alpha Step profilometer and JEOL scanning electron microscope (SEM). The optical characterization has been performed by using a tungsten lamp and concave grating spectrometer, from Stellarnet (spectral resolution 1.5 nm). Infiltration has been done with electronically controlled Teflon oven (CaLCTech). We have used a linearly polarized light for the optical characterization of the photonic structure (even if we have observed that, in our case, light polarization has not significantly affected the optical properties of the structure).

Figure 1. (a) SEM cross-section image and (b) experimental (red line) and simulation (by transfer matrix method, black line) transmission spectrum of 6 bilayer zirconium dioxide/silicon dioxide nanoparticle photonic crystal. The fundamental, second, and third order of the photonic band gap are indicated.

thicknesses of the layers can be extracted from profilometry measurements and confirmed by the SEM cross-section image. The sum of the thicknesses of the SiO2 (light gray in the SEM image) and the ZrO2 layer (dark gray in the SEM image) shaping a unit cell thickness of Λ = 380 nm (Λ is called the pitch of the photonic crystal), such that the overall multilayer thickness is around 2300 nm (six bilayers). Figure 1b shows the experimental (red line) and simulated (black line) transmission spectrum of the multilayer photonic crystal. The first order photonic band gap arises at 1100 nm (at the border of the detectable spectral window by the experimental apparatus), a second order at 565 nm, and a third order at 410 nm, according to the Bragg−Snell law for this structure. We attributed the decrease of experimental transmission, at shorter wavelengths, mainly to a significant scattering of light due to the relatively large size of the nanoparticles in the zirconia dioxide layer (100 nm). As a matter of fact, it is possible to characterize a multilayer photonic crystal in accordance with the Bragg regime (or twowave regime) diffraction only if a thick grating (or volume grating) has been obtained. An exact definition of a thick grating has been given in ref 35, and the two useful conditions to be fulfilled are Q = K 2λd /(2πnm) > 1 and ρ = λ 2 /(ΛnmΔn) ≥ 10 21573

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where K = 2π/Λ is a grating vector, Λ is the grating spacing, λ is the vacuum wavelength spacing, nm is the average refractive index, d is the sample thickness, and Δn is the index modulation for the dielectric grating. In our case (six bilayers), the two above conditions are fulfilled beginning already in the visible range. Besides, we point out that the peak positions of the Bragg diffraction high orders are strictly affected by the dispersion of the materials: the refractive index dispersion, especially at high frequency, induces a red shift of the peak position of second and third Bragg diffraction order comparing to their theoretical positions according to the first Bragg law, as shown in Figure 1b. In order to perform a deep study of the optical properties of the multilayer, we have used the transfer matrix method36−38 taking into account the dispersion of all used materials. For the silicon dioxide refractive index dispersion, we have used the Sellmeier equation reported in ref 39, while for the zirconium dioxide, we have used tabulated data from ref 40. For the liquid crystal E7, the Cauchy equation that usually described the dispersion relationship between refractive index and wavelength has been taken from Li et al.33 The simulation is in good agreement with the experimental data, testified by matching of the three orders of Bragg peak wavelength position with the simulated ones. No scattering losses in the simulation model have been considered. We take into account only the constant loss due to the ITO coated glass support. Further theoretical studies will be devoted to the losses in such photonic systems. In the simulations, we use as parameters the layer thicknesses and their porosity. The calculated thicknesses fit well with the SEM image, whereas we can tentatively conclude that the porosity of SiO2 nanoparticle layer is about 25% and the porosity of ZrO2 nanoparticle layer is about 50%. In addition, we present, in figure 2, a high magnification optical microscope

Figure 3. Transmission spectra at the different liquid crystal (LC) infiltration times: before infiltration (black line); infiltration at room temperature (red line); temperature increase up the LC isotropic phase (blue line); after infiltration at room temperature (magenta line). λim indicates the photonic band gap, with i being the spectral position and m the Bragg diffraction order.

agreement with the analytical model based on transfer matrix method, the poor observed diffraction efficiency indicates that the refractive index modulation magnitude in the infiltrated device is significantly reduced. A first reason could be ascribed to increased losses due to scattering leading to a shallow photonic band gap. A second reason might be a partial mild infiltration of liquid crystals in the first nanoparticle layers, leading to a thinner photonic structure with a superimposed superficial defect [note that the photonic band gap efficiency is defined by η = tanh2((πΔnd)/(2λnm)), ref 41]. Here, we report a transmission spectrum after a time of 60 s where we can observe the difficulty for the liquid crystal to enter quickly inside the pores at room temperature. By increasing the temperature (third spectrum, blue line), up to the isotropic phase transition, the rate of infiltration immediately increases, and the deep notch on the transmission spectra corresponding to the third order photonic band gap continues the enhancement up to its maximum efficiency value (more than 17%). At this temperature, the liquid crystal E7 is characterized by one refractive index niso = 1.575, and in the isotropic phase, scattering losses are lower. Moreover, the infiltration process becomes easier and faster, as reported in the literature.27 This is confirmed from a continuous increase of the transmissivity at all wavelengths and very fast photonic band gap formation (in correspondence of the isotropic phase transition). After the infiltration procedure, reaching the room temperature (last spectrum in Figure 3, magenta line), we have observed that the liquid crystal turns back into the nematic phase (note the scattering increase at shorter wavelength). Furthermore, it is remarkable that the third order photonic band gap shows an additional red shift of about 5 nm, related to an increase of the effective liquid crystal refractive index [defined as (2no + ne)/3,

Figure 2. High magnification optical microscope images (100× objective) of silicon dioxide and zirconium dioxide nanoparticle layers. White voids depict the nanoparticle interpores.

(100×) image of the two different nanoparticle layers in binary image texture. The white voids depict the nanoparticle interpores. Interestingly, the zirconium dioxide layer is less dense with respect to the silicon dioxide one, in accordance with the different porosity values extracted from the TMM. Figure 3 shows the transmission spectra of the photonic structure during the infiltration process. This powerful analysis provides important informations in order to understand the dynamic of the liquid crystal infiltration. From top to bottom, the first spectrum is characteristic of the porous multilayer before infiltration. The second and third orders of the photonic band gap are indicated. The second spectrum (red line) shows the effect of the liquid crystal (LC) infiltration at room temperature: the decrease of transmission all over the spectrum is due to the scattering from the liquid crystal in the nematic phase, which could not be completely infiltrated in the porous structure yet, but partially remains at the multilayer top surface. Moreover, in spite of the fact that the peak position is in 21574

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by the porous silicon.30 By increasing the applied voltage, we have observed a further blue shift, reaching saturation at 10 nm.

for a random orientation of the liquid crystal directors, due to the irregular shapes of pores in the nanoparticle multilayer]. Considering the pitch Λ of the photonic structure, the final wavelength position of the photonic band gap is strictly connected to the refractive index of the used materials. In our case, after the liquid crystal infiltration, the third Bragg order shifts toward the red of about 50 nm. We underline that such third order band gap wavelength position is in agreement with our theoretical prevision taking into account the value of the liquid crystal refractive index defined for a random orientation. In order to tune the position of the photonic band gap with the electric field, we have prepared a device as depicted in Figure 4. On the sample, deposited on glass substrate with ITO

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CONCLUSIONS To conclude, we have presented the fabrication and the optical characterization of a photonic crystal made by metal oxide nanoparticles, infiltrated (with a very simple and fast procedure) with the nematic liquid crystal. By infiltrating the liquid crystal in the porous structure, we have observed a red shift of the photonic band gap, due to the increase of an effective refractive index of the multilayer. The effect of an applied electric field has been studied and we report a shift toward shorter wavelengths of 8 nm, with a relatively low voltage of 8 V, corresponding approximately to an electric field of 3.4 V/μm, 1 order of magnitude smaller with respect to previously reported tunable liquid crystal infiltrated photonic crystals. The performances obtained with this device could be improved by optimization of the photonic structure and through the choice of the most suitable liquid crystals for lower voltage operation. This result could be very interesting for optoelectronic switches42 and for low cost displays:43,44 such color tunability, with low applied electric field, could be optimized toward electrically driven color changing devices, even with portable batteries.

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Figure 4. Scheme of the device for photonic band gap tuning applying an electric voltage. The additional ITO substrate (top of the sample) is superimposed and electrical contacts are applied.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

coating, an additional ITO coated glass has been superimposed [owing to the aforementioned device fabrication, it is a very cumbersome SEM measurement of the electrode distance; therefore, we estimate the electrode distance as minimum as possible, i.e., the photonic crystal thickness that is 2.3 μm, even if such electrode distance is reasonably larger due to a nonperfect adhesion of the additional ITO electrode on the infiltrated photonic crystal], and electrical contacts are applied. Applying a voltage on the device (square wave at 1 kHz), we observed a blue shift of the photonic band gap position (Figure 5). It is noteworthy that we could get a significant shift of 8 nm with a very low voltage, i.e., 8 V. The corresponding applied electric field is about 3.4 V/μm, 1 order of magnitude smaller with respect to what has been reported for similar devices28,29 and lower of about 30% with respect to the best performance

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Professors Francesco Simoni and Guglielmo Lanzani for support and helpful discussions. We acknowledge the Optical Laboratory at SIMAU Department (Università Politecnica delle Marche) for technical support and the project FP7-ICT-248052 (PHOTOFET) for financial support.

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REFERENCES

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Figure 5. Transmission spectrum of the liquid crystal doped photonic crystal for different applied voltages (from 0 to 32 V). The blue shift of the third order photonic band gap is displayed. Inset: position of the third order photonic band gap as a function of the applied voltage. 21575

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