Dynamic Beam Switching by Liquid Crystal Tunable Dielectric

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Dynamic beam switching by liquid crystal tunable dielectric metasurfaces Andrei Komar, Ramon Paniagua-Dominguez, Andrey E. Miroshnichenko, Yefeng Yu, Yuri S. Kivshar, Arseniy I. Kuznetsov, and Dragomir N. Neshev ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01343 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018

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Dynamic beam switching by liquid crystal tunable dielectric metasurfaces Andrei Komar,∗,† Ramón Paniagua-Domínguez,∗,‡ Andrey Miroshnichenko,¶ Ye Feng Yu,‡ Yuri S. Kivshar,† Arseniy I. Kuznetsov,‡ and Dragomir Neshev† †Nonlinear Physics Centre, Research School of Physics and Engineering, The Australian National University, Canberra ACT 2601, Australia ‡Data Storage Institute, A*STAR, 138634 Singapore ¶School of Engineering and Information Technology, University of New South Wales, Canberra ACT 2600, Australia E-mail: [email protected]; [email protected]

Abstract Dynamic steering of laser beams by ultra-thin optical metasurfaces is a significant research advance for possible applications in remote ranging and sensing. A unique platform for such important functionalities is offered by dielectric metasurfaces that have the highest transmission efficiency. However, the realization of dynamically tunable metasurfaces still remains a challenge. Here we experimentally demonstrate the dynamic switching of beam deflection by a silicon-nanodisks dielectric metasurface infiltrated with liquid crystals. In particular, we show the switching of a laser beam from zero to a 12◦ angle with an efficiency of 50% by heating the metasurface to modify the liquid crystal state from nematic to isotropic. Our results open important opportunities for tunable ultra-thin beam steering metadevices.

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Keywords optical metasurface, tunable metasurface, beam steering, dielectric resonators, liquid crystals All-dielectric metasurfaces have received great attention in the past years, 1–3 and have been established as a platform for efficient manipulation of optical beams. Advances in the design and fabrication of such dielectric metasurfaces have led to the development of several ultra-thin optical metadevices, including flat lenses, 4–7 beam converters, 8–10 deflectors 9,11–14 and holograms. 15–17 Composed of periodic or aperiodic latices of dielectric nanoparticles, such metasurfaces exhibit low absorption in the infra-red and visible spectral ranges. Low losses allow nanoparticles to exhibit Mie-type resonances with a higher quality-factor in comparison to their plasmonic counterparts. Furthermore, Mie-type resonances in dielectric nanoparticles offer two independent families of resonant modes — electric and magnetic. The far-field interference of these two types of resonant modes lead to fundamentally new effects, such as, unidirectional scattering, 18,19 unconventional reflection behaviour associated with the generalized Brewster effect 20 and near-unity transmission in the so-called Huygens’ regime. 9,10,21–23 Operating in the Huygens’ regime of the dielectric metasurfaces enables the combination of near-unity transmission together with a full-range of phase modulation, thus being the key to enabling functional dielectric metasurfaces with nearly 100% efficiency. Most functional dielectric metasurfaces to date are based on static designs, defined through geometrical parameters, such as nanoparticle shape, size, and array layout. However, in many applications it is crucial to enable dynamic tunability of the device functionality with time. For example, the focal distance of a camera lens needs to be changed when taking pictures of objects at different distances; the position of the ranging beam in driver-less vehicles needs to scan different directions. Therefore, implementing dynamic control over the response of the metasurfaces is of paramount importance for their practical implementation. The development of tunable metasurface optics was first focused on enabling different mechanisms for metasurface tunability. 24 Currently, the focus is shifting towards the demon-

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(b) Figure 1: Concept of beam switching metasurfaces infiltrated with liquid crystals. (a) Nematic state. (b) Isotropic state. stration of tunable functionalities, while high efficiency and the possibility to individually control the response of each nanoantenna in a metasurface remain major challenges. 25 Several schemes for the tunability of resonant dielectric metasurfaces have been proposed enabling mechanical deformation of the structures or a change of the refractive index of either the dielectric resonators or surrounding media. These include the mechanical or electrical deformation of elastomers, 26,27 electrical 28 or thermal 29,30 tuning of semiconductors, as well as the use of phase-change materials, 31,32 graphene, 33 or liquid crystals (LCs) as embedding

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media. 34–36 The use of LCs is an attractive approach as it relies on well-developed techniques from the LC display (LCD) industry. LCs possess high optical birefringence that can be controlled by temperature or an electric field. This optical anisotropy has been previously used for light beam routing in bulk LCs with excellent results. 37–41 All of these properties make LC-tunable metasurfaces a practical realization of tunable metadevices, which is likely to be favored by the display industry today. To date, however, LC tunability has only been demonstrated with homogeneous metasurfaces, while the LC tunability of functional metadevices, such as beam deflectors, lenses or holograms, has not been investigated. Here, we experimentally demonstrate a switching of the beam deflection with the use of a gradient all-dielectric metasurface infiltrated with LCs. We show that we can effectively switch the beam diffraction from the metasurface from the zero-th to the first diffraction order with an efficiency of 50%. The tunability is enabled by heating of the LC such that it changes its internal structure. At low temperature, the LC is in its nematic phase (Fig. 1a), when the elongated LC molecules are arranged parallel to each other. At higher temperatures (Fig. 1b) the LC arrangement changes to isotropic. Due to a difference in the refractive indexes between the LC nematic and isotropic states, the metasurface can either transmit light straight through or deflect it at a fixed angle, as shown in Fig. 1. In this way, dynamic switching between different beam propagation directions can be achieved.

Results and discussion To achieve tuning of an LC-infiltrated metasurface, we first test the properties of a homogeneous nanodisk array. We therefore design (see the Supporting Information) and fabricate sub-diffractive silicon-disk arrays operating in the Huygens’ regime, e.g. for a spectral overlap of the electric and magnetic dipole resonances of the nanodisks to achieve near-unity transmission. 21 The array consists of silicon nanodisks with a diameter d = 300 nm, the

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Figure 2: (a) Schematic of a Si nanodisk array metasurface. (b) SEM image of a homogeneous metasurface. (c,d) Experimentally measured (solid curves) and simulated (dashed curves) transmission spectra (zero-th order) of the homogeneous metasurface for the nematic (room temperature) and isotropic state (60◦ ), respectively. The green and yellow shadings depict the magnetic and electric dipolar contributions to the scattering of each nanoparticle in both liquid crystal states. distance between them is 130 nm and the height is h = 130 nm, as depicted in Fig. 2a. The metasurface was fabricated using e-beam lithography and subsequent reactive-ion etching. The substrate was quartz with a thickness of 0.4 mm. The scanning electron micrograph (SEM) image of this sample is shown in Fig. 2b. The exact dimensions were chosen such that the Huygens’ regime is satisfied for the isotropic state. Next, we infiltrate the metasurface with a LC. For our experiments, we chose E7 LC from Merck, that is well-investigated and widely used in the LC display industry. It has a positive anisotropy and an index change of ∼ 0.15 for the transition from the nematic (ne ) to isotropic state (ni ) in the visible and the near infrared wavelength regions. 42 The critical temperature at which the E7 LC becomes isotropic is 58 ◦ C. Using this metasurface as a bottom substrate, we build an LC cell with a thickness of 5 µm determined by spacers (round plastic balls with

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precise diameters). We deposit an alignment layer only on the top substrate. The alignment layer is a ‘Nylon 6’ polymer and a rubbing process is used as an alignment technique. The induced alignment is planar. The silicon metasurface, which is the bottom substrate of the LC cell, is not covered with an alignment layer, as the required rubbing might damage the nanostructures. While the LC alignment on top of the metasurface is not perfect, our experiments demonstrate that more than 90% of the LC molecules are aligned parallel to the alignment direction defined by the top surface. In our optical measurements, we chose the polarization of the incident light along the LC orientation to excite the extraordinary wave inside the cell. For heating the LC cell and controlling its temperature, we use a temperature controller with a control loop (see description in the Supporting Information), so the measurements for different temperatures are stable. Figures 2c and 2d show the experimentally measured (solid curves) and simulated (dashed curves) transmittance spectra of the zero-th order for both nematic (room temperature) and isotropic (heated sample) states of the LC. The simulation results, obtained using Lumerical software are performed for disks of 290 nm diameter, which is slightly smaller than the experimentally fabricated sample, having diameters of 300 nm. This small discrepancy can be explained by the formation of a thin native oxide layer on the surface of the Si disks that decreases the effective size and is consistent with previous works. 10 The experimental setup for measurements of the transmittance spectra is shown in Fig. S2 of the Supporting Information. To reach the LC isotropic state, the LC cell was heated to 60 ◦ C. At room temperature (Fig. 2c) the transmission spectrum exhibits a pronounced dip at about 810 nm. This dip is due to the partial overlap between the electric and magnetic dipolar resonances of the nanoparticles, as shown with green and yellow shading, respectively. After heating, the resonant magnetic and electric response almost fully overlap (Fig. 2d), which results in high transmittance, as predicted for the case of the Huygens regime. 21 In the region of 810 nm, for example, the transmittance rises from about 45% to nearly 75%. The experimental results are in good agreement with the numerical simulations, thus confirming the physical

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Figure 3: Six-element gradient metasurface. (a) Radii of disks that constitute a unite cell of the beam-deflecting metasurface. (b) SEM image of the fabricated metasurface. phenomena of spectral shifting of the resonances during the LC phase change. After proving the tunability of our Huygens’ dielectric metasurface, we proceed to testing the tunability of a functional gradient metasurface, such as a beam deflector. 9 In such a beam deflector, the intensity of the transmitted light is close to unity for the operational wavelength of 800 nm, while its phase is varied such that it forms a binary blazed grating with six phase steps. 3,9 Each step incorporates the phase delay of 0, π/3, 2π/3, π, 4π/3 and 5π/3, successively. To determine the exact diameters of the silicon nanodisks, which define these phase values, first we calculated the transmittance of regular arrays with a fixed gap and a height, while varying the disk diameters. The metasurface is designed to operate as a beam deflector when the LC is in the isotropic state. The refractive index used in the simulations corresponds to that of the E7 LC above the isotropic phase transition temperature. The results are presented in Supporting Information (see Fig. S1). We find that the nanodisk radius takes values from 115 nm to 150 nm to imprint the desired phase delays in the incoming wave, at the operating wavelength of 780 nm. The corresponding values and the schematic of the unit cell are given in Fig. 3a. Overall, the device deflects the incident beam at the angle of 12◦ . It is worth noting that the thickness of the metasurface is only 130 nm, which is much thinner than the operational wavelength in free space. The ultra-small thickness is the main advantage of metasurfaces compared to other diffractive optical elements, which makes them attractive for large-scale 7


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Figure 4: Numerically simulated transmittance spectra for different diffraction orders: (a) Isotropic state of LC, (b) Nematic state of LC. Comparison of zero (c), first (d) and second (e) diffraction orders transmittance for both LC states. nanofabrication. The SEM image of the fabricated gradient metasurface is shown in Fig. 3b. The height and pitch of the nanodisks are the same as in the homogeneous array described above, however, the diameters of the nanodisks vary such that different phases are imprinted at different spatial locations, while preserving high transmission of the incident beam. This results in a phase gradient that mimics a blazed grating, which deflects an incident beam of certain wavelength into the first diffraction order of the grating (see Fig. 1). After defining the required nanodisk diameters, numerical simulations have been done to characterize the performance of the resulting beam-bending metasurface. The results are presented in Fig. 4a together with the performance results when the LC is switched to the nematic state, see Fig. 4b. In this latter case, the LC is modeled as an anisotropic medium with the extraordinary axis directed along the electric-field polarization direction, which in both cases is perpendicular to the super-cell direction. The values of the extraordinary

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and ordinary refractive indexes correspond to those of the E7 LC at temperature T = 25◦ C. Panels c-e in Fig. 4 show a detailed comparison of the power channeled into the (positive) zero-th, first, and second diffraction orders upon LC switching from the isotropic to nematic state. The results show a power redistribution between the orders, namely, significant reduction of the power channeled into the first order (one enhanced by the gradient metasurface in the isotropic case) and significant increase of the power channeled into the zero-th order (one suppressed by the gradient metasurface in the isotropic case). To experimentally test the proposed design, the beam-bending gradient metasurface was fabricated, and a typical SEM image of several of its unit cells is shown in Fig. 3b. To perform measurements of the beam deflection, we used the experimental setup shown in Fig. S3 of the Supporting Information. To place the cell with a heating element and temperature sensor in front of the microscope objective, we used a Mitutoyo objective with a high working distance of 2 cm and a numerical aperture N A = 0.7. To capture the image, we used a CMOS visible camera. We measure the intensity of each diffraction order for two limiting states of the LC orientation: nematic and isotropic. We take images simultaneously of the -2, -1, 0, +1, +2 diffraction orders, however, the intensity of the -2 and -1 orders is found to be insignificant in both the nematic and isotropic states as compared to the other orders. Therefore, in all our figures we show only the 0, +1, and +2 orders. Based on the numerical simulations (Fig. 4), we can estimate the range of wavelength operation. However, due to minor variations in the dimensions of the fabricated samples this wavelength range can be spectrally shifted. Furthermore, since we use a femtosecond laser with a finite spectral bandwidth of 4.4 nm, it is important to determine the exact operational wavelength range directly from the experiment. We therefore measure the intensity of the zero-th, first and second diffraction orders at different wavelengths. The data for the nematic state are obtained at room temperature (25◦ C), and for the isotropic state – at 60◦ C. The results are presented in Fig. 5 for the nematic (red curves) and isotropic (blue curves) states, respectively. The observed decrease

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Figure 5: Experimental measurements of the intensity transmitted into the zero-th, first and second diffraction orders in the nematic (red) and isotropic (blue) states of the LC cell. The error bars reflect the level of noise in the measurements. of the light transmitted into the zero-th order with temperature, at around 745 nm, is clearly visible. At the same time, the energy going into the first and the higher diffraction orders is increased. This change is less pronounced than in our simulations (Fig. 4d), which is likely due to the incomplete alignment of the LC on the metasurface facet. From these measurements, we directly obtain the working wavelengths of our beam deflector, which is slightly blue-shifted compared to the initial design. To demonstrate the switching properties of our beam-deflection metadevice, we fix the laser wavelength to λ = 745 nm and measure the intensity change of different diffraction orders by increasing the temperature of the LC cell. The intensities of different diffraction orders are shown in Figs. 6a and 6b. The zero-th order has the highest intensity under 25◦ C; all other values are normalized according to this intensity. We observe that the intensity of the zero-th order decreases with the rise in temperature, while the intensity of the first order increases, the second order remains almost unchanged. All other diffraction orders are not shown in the figures because they carry negligible energy as compared to the second order at all measured temperatures. Ideally, a beam deflection metadevice should not only switch the power between ON and OFF states, but also it should not reduce the total transmitted power. The total transmitted power (a sum of the intensities in all diffraction orders) is given in Fig. 6c. During the heating process, the total power decreases by only 20%, that shows the high transmission efficiency of the device.

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does not switch intensity between the orders at all, the F OM number vanishes, even if there is a change in the total intensity. Thus, the absolute value of the F OM number shows the quality of the beam deflection switching. Ideally, we aim to achieve F OM = 1 or close to unity. For our fabricated beam deflector, we achieve F OM = 0.48. The numerical simulations shown in Fig. 4 predicted an F OM of about 0.45, demonstrating a very good match between the theoretical results and the experimental data. In summary, we have demonstrated how to employ LCs for achieving functional tunability of optical metasurface devices. In particular, employing the concept of tunable resonant Huygens’ metasurfaces, we have studied theoretically and demonstrated experimentally a switchable beam deflection metadevice operating in the visible spectrum. By controlling the temperature of the metasurface, we have been able to control the amount of the optical power propagating straight (zero-th order) or deflected at 12◦ C to the normal (first order). We notice that the thickness of such a metadevice is only 5 µm, with the minimal thickness of a liquid crystal layer being limited only by the near field of silicon nanoantennas that is significantly smaller than 1 µm. Thus, switchable beam steering metadevices with a thickness smaller than 1 µm can be realized in practice without compromising their efficiency. We believe our results open new opportunities for dynamically reconfigurable functional metadevices including tunable lenses and dynamic metasurface displays.

Methods Fabrication of samples. To fabricate the samples, we deposited a layer of amorphous silicon film with a controlled thickness of 130 nm on a quartz substrate, using the inductively coupled plasma chemical vapor deposition technique (Plasmalab System 380, Oxford Instruments). Thereafter, we patterned hydrogen silsesquioxane (HSQ, Dow Corning, XR-1541006) arrays on the silicon film using electron beam lithography (Elionix, 100 kV) method. The silicon metasurface is finally obtained through silicon film etching using an inductively

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coupled plasma etcher (Plasmalab System 100, Oxford) with Cl2 gas. Experimental setup. To perform measurements of the beam deflection we used the experimental setup shown in Fig. S3 of the Supporting Information. As a light source we used a tunable femtosecond laser with pulse duration of 200 fs and repetition rate of 80 MHz. The spectral full-width at half-maximum of the laser line is 4.4 nm. The intensity of the laser was kept at a very low value to avoid nonlinear effects associated with the high laser peak-power. The lens after the laser focused the beam onto the cell surface to match the diameter of the light beam and the dimensions of the silicon-disk array (75 × 75 µm2 ). A microscope objective with magnification ×100 collects the transmitted light and magnifies the image which we are able to capture with a camera and a tube lens installed at the focal distance in front of the camera. By introducing and removing another lens behind the objective we are able to image the back focal plane of the objective and switch between real and k -space. Numerical simulations. The numerical simulations were carried out using a commercial, Finite-Difference Time Domain electromagnetic solver (Lumerical FDTD). Both in the case of a homogeneous metasurface (having all particles the same) and in that of the beam steering one (having particles of increasing size) a single unit cell (or super-cell) was simulated. We applied Bloch boundary conditions to mimic an infinite system in the boundaries perpendicular to the lateral directions and perfectly matched layers in the boundaries perpendicular to the beam propagation direction to absorb the transmitted and reflected power. Two plane monitors are used to record the fields transmitted through and reflected from the metasurface. The fields are projected into the diffraction orders to compute the intensity channelled into each one of them and normalized to the incident intensity to obtain the diffraction efficiency into each channel. The LC is either modelled as an isotropic or as an anisotropic medium, depending on whether it is in the isotropic or the nematic state. The values of the permittivity for the E7 LC used were taken from the technical specifications of the vendor. In the nematic case, the polarization of the incident field coincides with the

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extraordinary axis of the LC, therefore experiencing a larger dielectric constant. In the case of the beam steering metasurface, the polarization direction is also perpendicular to the super-cell axis. The material parameters used for the silicon were measured by ellipsometry on the deposited sample.

Acknowledgement We acknowledge a financial support from the Australian Research Council through Discovery Projects and Centres of Excellence - CUDOS research programs. We acknowledge the use of the Australian National Fabrication Facility (ANFF) at the ACT Node. DSI co-authors were supported in Singapore by A*STAR SERC Pharos program (Grant 152 73 00025) and National Research Foundation Investigatorship Award (Grant NRF–NRFI2017–01).

Supporting Information Available The supporting information includes details of the numerical simulations of homogeneous Si-disks array as well as experimental setups for transmittance and beam deflection measurements. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Graphical TOC Entry

1 μm Radius (nm) 115 130 135 140 145

π/3 2π/3 π

150

4π/3 5π/3 2π

Phase

For Table of Contents Use Only: Dynamic beam switching by liquid crystal tunable dielectric metasurfaces, Komar et al. Dynamic steering of laser beams by gradient optical metasurfaces infiltrated with liquid crystals.

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Dynamic Beam Switching by Liquid Crystal Tunable Dielectric

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