Controlling light with metamaterial-based nonlinear photonic crystals

SUPPLEMENTARY INFORMATION DOI: 10.1038/NPHOTON.2015.17 Controlling light with metamaterial-based nonlinear photonic crystals Nadav Segal, Shay Keren-Zur, Netta Hendler, Tal Ellenbogen * Department of Physical Electronics, Fleischman Faculty of Engineering, Tel-Aviv University, Tel-Aviv 69978, Israel Corresponding author e-mail address: tellenbogen@tauex.tau.ac.il; nadavsegal@mail.tau.ac.il Supplementary Section I: Numerical simulation based on a hydrodynamic model The Hydrodynamic Model The hydrodynamic model describes the nonlinear optical response of the charge carriers in the metal. It is based on modeling the electron fluid by Euler s equation where n(r,t) represent the electron fluid density and v(r,t) represent the electron velocity field. vv tt + (vv )vv + γγvv = ee ββ2 (EE + vv BB) nn (1) γγ electron collision rate mm ee nn ββ Fermi velocity vv FF JJ = eenn (JJ = eeeevv) (2) mm ee electron effective mass After expanding all fields in a perturbative manner, it is possible to extract the nonlinear current terms which depend on the bulk polarization (1, 2 and refs. within): JJ NNNN = iiii [tt (PP nn 0 ee 1 PP 1 ) + nn 1 3ωω+iiii (PP 2 2ωω+iiii 1 ) 2 ] (3) NATURE PHOTONICS www.nature.com/naturephotonics 1

Supplementary Figure 1: Numerical simulation. (a) Illustrates one of the simulated SRR geometrical configurations. The height of the SRR is 30nm. (b) Currents (arrows) and near-field radiation patterns (color scale) from a uniform array for FH and (c) SH. The near-field radiation patterns were taken 100nm above the SRRs. In the simulation we insert the obtained linear polarization in b into Equation 3 to produce the currents and radiation at the SH shown in c. (d) Inverted particles produce nonlinear radiation pattern with phase shift. This is the mechanism used to construct the NLMPCs. Supplementary Figure 2: Illustration of point-of-view of simulation results shown in Fig. 1 of the manuscript. (a) Simulation results of near-field at the FH and emitted SH for uniform arrays. (b) Illustration of the simulation point-of-view. 2

Supplementary Section II: Two-dimensional nonlinear metamaterials photonic crystals and nonlinear Raman-Nath diffraction The ability to control the phase of the nonlinear output is used here to generate one- and twodimensional periodic and quasi-periodic nonlinear metamaterial photonic crystals. This is identical to conventional nonlinear photonic crystals 3,4 in which the manipulation of the material symmetry is usually obtained by electric field poling of the nonlinear crystal structure or by special molecular beam epitaxial growth 4. In this work this is achieved by fabricating metaatoms with inverted symmetry (See Supplementary Figure 3a). As shown above and in the main text inverted SRRs impose an opposite phase on the nonlinear generation. For example, taking a periodic 1D structure of the SRRs with inversion period Λ will effectively multiply the generated SH wave-function with the term e iδφ = sign (cos 2πx ). By performing a Fourier transform this modulation is transformed from the real space into the reciprocal space, with discrete RLVs, k Λ = m 2π Λ Λ. The RLVs in conventional nonlinear photonic crystals (NLPCs) are usually designed to phase-match (or momentum-match) the nonlinear interaction. For example, for a second harmonic generation process, full momentum conservation (nonlinear Bragg diffraction) leads to 2k 1 + m K Λ = k 2, where k 1 is the incident wave vector, K Λ = m 2π Λ K Λ is RLV with Λ being the period of grating, and m is an integer number. In the case of sufficiently thin NLPC (which is the case in the current work) only the transverse component of nonlinear Bragg s law 2k,1 + m K Λ = k,2 needs to be satisfied. This regime is the nonlinear Raman-Nath diffraction regime 5. For the more general case of oblique incidence of the pump beam, the SH wave (with a wave vector k 2 ) is emitted at angles of propagation θ 2,m which satisfy sin(θ 2,m ) = 2 k 1 sin(θ k 1 ) + m K Λ = sin (θ 2 k 1 ) + m λ 1. The momentum 2 2Λ matching diagrams of nonlinear Raman Nath diffraction are illustrated in Supplementary Figure 3. Although the NLMPCs that are studied here experimentally have 1D or 2D modulations, the concept of matching the momentum in the interaction by using RLVs of the NLMPCs can be extended also to 3D periodic or quasi periodic NLMPCs in analogy to conventional NLPCs 4 as shown below in Supplementary Section IV. 3

Supplementary Figure 3: Nonlinear Metamaterials Photonic Crystal and Raman-Nath diffraction. (a) Exemplified real space and the appropriate reciprocal space of NLMPC with one-dimensional modulation. (b) Phase-matching conditions for nonlinear Raman Nath diffraction for normal and (c) oblique incidence. Supplementary Section III: Experimental setup Supplementary Figure 4: Experimental setup. SP- shortpass filter; LP- Longpass filter; P- polarizer; /2- Half waveplate; OPO- optical parametric oscillator. 4

Supplementary Figure 5: Spectral measurement of the SH. Measurement of the SH emitted from a uniform array which was illuminated with FH with 1200 nm wavelength. The spectrometer was Andor Shamrock 303i with Newton 970 camera. Supplementary Section IV: Increasing conversion efficiency by three-dimensional NLMPCs The ability to generate materials with artificial nonlinearity is exciting; however so far the nonlinear metamaterials that were studied were made from extremely thin surfaces. One way to increase the total nonlinear conversion efficiency is to use advanced fabrication methods 6 to construct three-dimensional metamaterials that provide longer interaction lengths. However, in the case of multilayered metamaterials there are some considerations that have to be taken into account, including phase matching of the nonlinear interaction, reflection, transmission and absorption of the waves in the multilayered structures. These considerations make the wave dynamics in nonlinear multilayered metamaterials much more complex in comparison to conventional nonlinear materials. In the following section we describe the problem in more details, analyze analytically and numerically the major implications of stacking multiple layers for forming three-dimensional nonlinear metamaterials and NLMPCs and show that by understanding the fundamental problems of this configuration the total conversion efficiency can be increased substantially by 5

using multilayered structures. In our studies we show about 500-fold increase of the total power conversion efficiency in comparison to the performance of a single layer resonant SRR structure which was studied before 1. We believe that additional future studies can lead to even better improvements in the conversion efficiencies of multilayered nonlinear metamaterials. Quasi-phase-matching: Momentum mismatch between the waves (for SHG Δk = k SH 2k FH ), that is caused from material dispersion, usually leads to accumulated phase mismatch as the waves propagate resulting in a destructive interference of the generated nonlinear signal. Therefore, without phase matching, the nonlinear interaction is inefficient even for thick, three-dimensional, nonlinear materials. In conventional materials the common solution to the phase matching problem is to use birefringence 7 in uniaxial or biaxial nonlinear crystals, or quasi-phase-matching (QPM) in poled NLPCs 8. NLPCs are widely used to control nonlinear optical interactions since in addition to generating efficient nonlinear signal, they prevent walk-off of the interacting beams and enable adjusting other aspects of the interaction such as the direction of the generated beams 3 and their spatial shape 9,10. Moreover, NLPCs can be used for active devices 11 and efficient generation of entangled photon pairs 12. Our work presented in the manuscript demonstrates experimentally two-dimensional NLMPCs and gives motivation to create three-dimensional NLMPCs and to use them for QPM and additional manipulations of nonlinear interactions. Supplementary Figure 6 illustrates three different phase matching schemes in threedimensional metamaterials based on multiple SRRs layers. Perfect phase matching can be achieved by placing different SRR layers of the same orientation spaced at coherence lengths (L c = 2π/Δk ) or by placing the SRR layers at half coherence lengths and flipping the orientation of the SRRs (and phase of nonlinear output) in successive layers. QPM can be achieved by closely packing the SRR layers and flipping different regions of the structures (at half coherence length distance). This is the same method which is used for construction of conventional NLPCs where the inversion of (2) is used to impose π phase shifts on the locally generated radiation. Using QPM for the multilayered metamaterial structure will result in much higher conversion efficiency per propagation length, since it allows packing many more SRR layers in the same region in space. 6

Supplementary Figure 6: Illustration of different phase matching configurations with multilayered nonlinear metamaterials. (a) Perfect phase matching with uniform multilayered structures. (b) Perfect phase matching with NLMPC. (c) QPM with NLMPC. As mentioned above, there is a major difference between conventional nonlinear materials and the nonlinear metamaterials which were studied up to date. Conventional nonlinear materials that are used to create NLPCs are transparent in the operation wavelength regime, whereas metamaterials working in the plasmonic resonance regime are not transparent. The linear interaction of FH and SH waves with each SRR layer involves partial transmittance (T FH, T SH ), reflectance (R FH, R SH ) and absorbance. The partial transmittance leads to attenuation of the propagating FH and generated SH. In addition, in the case of multilayered structures which are not impedance-matched there are multiple reflections. As a result, depending on the spacing between the layers, optical micro-cavities can be formed. These factors make the interaction more complex compared to conventional nonlinear materials. Supplementary Figure 7 illustrates the problem. 7

Supplementary Figure 7: Illustration of the dynamics of propagating waves through the multilayered structure: As the waves (FH- red; generated SH- green) propagate they are discretely attenuated. In addition, a portion of the waves is reflected back and forth between the layers, resulting in a cavity behavior (dashed lines) for the FH and SH independently. Numerical study: To study numerically the total generation of SH out of multiple layers of SRRs we use a combination of finite element (FEL) simulation and a transfer matrices method 13. The parameters that were taken from the FEL simulation are the linear transmission and reflection from each SRR layer for the FH and the SH, and the nonlinear generation coefficient (using the hydrodynamic model). We took into account both forward and backward generated SH waves. The simulation process is illustrated and described in detail in Supplementary Figure 8. 8

Supplementary Figure 8: Illustration of simulation process for N layers: FEL simulation produces linear transmission and reflection coefficients and the generated SH fields from each single layer. The linear coefficients are used in the transfer matrix formulation to calculate the FH everywhere in the multilayered structure. The local FH in the n th layer is used to calculate the locally generated SH in the n th layer. The locally generated SH is fed to the SH transfer matrix formulation which takes into account all the layers before and after the locally generated SH and calculates the contribution of the locally generated SH in each layer to the output SH. The total output SH is calculated as the sum of contributions of the locally generated SH from each layer after interacting with the entire multilayered structure. Results of numerical study: We started by examining the SHG from a single layer of SRRs with dimensions and unit-cell size which were studied before by Ciraci et al. 1, in a more realistic configuration for a multilayered structure where the structure is embedded in silica glass 14. This added dispersion to the problem (and need for phase matching) and led to a red shift of the resonances. The single layer transmittance and reflectance of the FH (polarized along the base of the SRRs) and SH (polarized along the arms of the SRRs) in this case are shown as the blue lines (F=1) in Supplementary Figure 9a-d. 9

Supplementary Figures 9 a-d: Spectral transmittance and reflectance from a single layer of SRRs. Spectral transmittance of FH (a) and SH (b). Spectral reflectance of FH (c) and SH (d). The FH is polarized along the base of the SRRs and the SH is polarized along the arms of the SRRs. F is a scaling factor of the SRR dimensions (while maintaining the unit-cell size at 180 nm 180 nm). In this case (F=1), in agreement with previous works 1,15, the highest SHG correlates to the resonances and to the dip in the transmission. This is shown as the blue line in supplementary figure 9a. However due to the strong reflection and absorption at these specific geometrical parameters (as shown in Supplementary Figure 9a-d) the power of light passing to the next layer decreases drastically, and the overall efficiency of a multilayered structure is lower than that of a single layer. Therefore in order to be able to accumulate SH through the multilayered structure the transmittance of light through layers needs to be increased. One way to do it is to use smaller SRRs. In addition to providing better transmittance, the larger field enhancements of smaller SRRs enhance their effective conversion efficiency. It was suggested before to use tiny nonlinear meta-atoms that exhibit strong effective conversion and 10

pack them densely to obtain efficient total conversion, however it was shown that there is a limit on the packing density due to collective effects which reduce the overall conversion efficiency 16. Therefore, as we show next, the route to harness the maximal total conversion efficiencies of multilayered structures is indeed to use layers of small nonlinear meta-atoms which are not densely packed. This will cancel the collective effects that reduce the effective conversion and in addition increase the layer transmittance and allow accumulation of the nonlinear signal. Supplementary Figure 10: Normalized SHG power. (a) From single layer. (b) Maximal total SHG from multilayered structure. The results are normalized to the maximum of the F=1 (SRR dimensions studied by Ciraci et al 1 ) single layer SHG. N denotes the number of layers that yielded maximal SHG. We studied the case of multilayered structures from layers where the SRRs dimensions were rescaled to half (F=0.5) and quarter (F=0.25) of the original SRR dimensions (F=1). Supplementary Figure 9 shows that the resonances become slightly blue shifted and that the total transmission is substantially increased in the cases of F=0.5 (red) and F=0.25 (cyan). The corresponding generated SH power from a single layer normalized to the case of a single (F=1) SRR layer is shown in Supplementary Figure 10a. It can be seen that F=0.5 produces the strongest SH fields from a single layer and that by making the particles smaller (F=0.25) the SHG field that radiates from a single layer is reduced. However in the case of multilayered structure which is phase matched (see Supplementary Figure 6a) the total SH is the strongest for the case of F=0.25 where the SHG is about 500 times stronger than the starting case of F=1. The maximal SHG values for F=1, F=0.5 and F=0.25 were obtained for a single layer, 3 phase matched layers and 21 phase matched layers respectively. 11

The simulated accumulation through the multilayered structure for F=0.25 is shown in Supplementary Figure 11. The FH wavelength in the simulation was 1750 nm and L c in the silica matrix was 83 m. In the case of perfect phase matching the layers were placed at L c separations. In the case of non-phase matched and QPM cases the layers were placed in separations of L c /8 in uniform orientations and in alternating orientation respectively (alternating orientation configuration is shown in Supplementary Figure 6c). The buildup of the SH waves with respect to the number of layers resembles the known dynamics for perfect phase matching, QPM and non-phase matched nonlinear interactions 17 with added losses due to the partial transmittance. Supplementary Figure 11b shows the SH field accumulation in perfect phase matching and QPM stacked layer configurations (shown as configurations a b and c in Supplementary Figure 6). It can be seen that in the QPM case the maximal total SHG is slightly reduced in comparison to the perfect phase matched cases, however it s maximum is obtained much faster which means that it can be used to significantly reduce the total dimensions of the converting device. Supplementary Figure 11: Phase matching with multilayered metamaterials. Total SHG output field for F=0.25 (a) as function of number of layers and (b) as function of propagation length in the structure. From the obtained results we can understand that multilayered structures can be used to significantly enhance the efficiency from nonlinear metamaterials. The best efficiency will be achieved for small particles which are not too closely packed to avoid collective effects. QPM by alternating the orientation of the structures is highly important since it allows getting efficient interaction in much shorter lengths compared to placing layers at L c or L c /2. We believe that this 12

study gives the motivation to use QPM in three-dimensional metamaterials and opens many new directions to study nonlinear dynamics in three-dimensional metamaterials which need further investigations and are out of the scope of this work. Analytical calculations for the case of low reflection (small particles) In order to analyze analytically the effect of propagation through multilayered structure we consider the case of zero reflections which simplifies the picture. Therefore, we can express the value of the propagating SH wave at the n-th layer, as described in the illustration in Supplementary Figure 12 Supplementary Figure 12: Illustration of the effect of the n-th layer on the SHG wave. E 0 is the input FH field; t FH and t SH are the FH and SH transmission coefficients respectively; η is the non-linear conversion coefficient. The total SHG field output (E SH,N ) is the sum over the contributions from each of the layers (after attenuation through the structure), giving the following expression: E E t 2 2N 2 SH, N 0 FH t 1 t t 1 t In order to express the propagation through the dispersive medium, we add the accumulated propagation phase of the interlayer distance L to each of the transmission coefficients and get the following expression: SH 2 FH SH 2 FH N 13

E E t 2 SH, N 0 FH t 1 e SH 2 2N 2 i 2N 2 kfh L tfh e N t 1 e t SH 2 FH in kl i kl Where k k 2k SH FH As shown by the formula for the total SHG field output, and as described above, the interlayer distance plays a major role as it comes to phase matching. Supplementary Figure 13a shows the total SHG field output as a function of the inter-layer distance, as calculated by the simulation (green line) for a 10 layers structure of SRRs with F=0.25, FH wavelength of 1750 nm and by the analytical formula (blue line). The maximal values are achieved at L C (83 µm) or when the layers are at the same position which is physically impossible. This is due to the phase mismatch accumulated through the dispersive medium. In addition, up to the highest value at separation of 83 µm, 10 peaks are visible, matching the 10 layers in the structure. This is another consequence of phase matching, but as a result of constructive interference only from a combination of specific layers which are separated by multiples of the coherence length. Another important feature is the modulation at the FH and SH wavelengths in the medium (shown as the fluctuations in the green line), which is caused by the multiple reflections in the structures. This feature does not appear at the analytical solution which does not take into account the reflections. Supplementary Figure 13b shows the accumulation of SH field in a perfectly phase matched structure with respect to the number of layers. The simulation produces higher results since it is taking into account the multiple reflections, which contribute the total generation process in this case. 14

Supplementary Figure 13: Comparison between simulation and analytical results for small particles (F=0.25) (a) Total SHG dependence of a structure of 10 layers of SRRs with uniform orientations on inter-layer distance. (b) SH field accumulation Vs. layer number in a perfect phase matched structure. Supplementary Section V: Illustrations of the SH radiation emitted from the NLMPC based FZP Each pair of adjacent zones of the NLMPC-based FZP consists of SRRs with mirror inversion symmetry and generate SH with an opposite phase. The generated SH creates constructive interference propagates in two main directions, as illustrated in Supplementary Figure 14a, corresponding to m=1 and m=-1 diffraction orders. One diffraction order focuses at 1 mm in front of the lens whereas the other defocuses and creates an imaginary focus 1 mm behind the lens as illustrated at Supplementary Figure 14b. In addition the nonlinear FZP emits focused and defocused waves also backwards. 15

Supplementary Figure 14: Emission of nonlinear FZP (a) Simulation of far-field SH emission from a one-dimensional NLMPC on glass. The asymmetry of emission is due to emission to air on the right and glass on the left. (b) Illustration of the NLMPC-based FZP SH radiation pattern showing forward and backward focusing and defocusing of nonlinear emission. References: 1. Ciracì, C., Poutrina, E., Scalora, M. & Smith, D. R. Origin of second-harmonic generation enhancement in optical split-ring resonators. Phys. Rev. B 85, 201403 (2012). 2. Scalora, M. et al. Second- and third-harmonic generation in metal-based structures. Phys. Rev. A 82, 043828 (2010). 3. Berger, V. Nonlinear Photonic Crystals. Phys. Rev. Lett. 81, 4136 4139 (1998). 4. Arie, A. & Voloch, N. Periodic, quasi-periodic, and random quadratic nonlinear photonic crystals. Laser Photon. Rev. 4, 355 373 (2010). 5. Sheng, Y., Kong, Q., Wang, W., Kalinowski, K. & Krolikowski, W. Theoretical investigations of nonlinear Raman Nath diffraction in the frequency doubling process. J. Phys. B At. Mol. Opt. Phys. 45, 055401 (2012). 6. Soukoulis, C. M. & Wegener, M. Past achievements and future challenges in the development of three-dimensional photonic metamaterials. Nat. Photonics 5, 523 530 (2011). 7. Armstrong, J., Bloembergen, N., Ducuing, J. & Pershan, P. Interactions between Light Waves in a Nonlinear Dielectric. Phys. Rev. 127, 1918 1939 (1962). 16

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