Abstract
Wave transport is one of the most interesting topics related to quasicrystals. This is due to the fact that the translational symmetry strongly governs the transport properties of every form of wave. Although quasiperiodic structures with^{1,2,3,4} or without^{1,5,6,7} disorder have been studied, a clear mechanism for wave transport in threedimensional quasicrystals including localization is missing^{8,9}. To study the intrinsic quasiperiodic effects on wave transport, the time invariance of the lattice structure and the lossfree condition must be controlled^{10,11}. Here, using finitedifference methods, we study the diffusivelike transport and localization of photonic waves in a threedimensional icosahedral quasicrystal without additional disorder. This result appears at odds with the wellknown theory^{12} of wave localization (Anderson localization), but we found that in quasicrystals the short mean free path of the photonic waves makes localization possible.
Main
The first discovery of a quasicrystal^{13} disproved the longstanding conjecture in condensed matter physics that only crystalline materials with translational symmetry could be densely packed and highly ordered. In crystalline materials the waves with wavelengths commensurate with the crystal’s periodicity can transmit without scattering loss, leading to ballistic transmission. Disordered materials can be contrasted with ordinary crystals. Because of frequent scattering, wave transport in disordered materials is usually described by random walks leading to diffusive transmission, for example, Ohm’s law^{14}. Considering the wave nature of electrons, Anderson predicted that if the degree of structural randomness is sufficiently large, the wave interference will result in complete halting of electrons, the socalled Anderson localization^{15}, and the transmission coefficient will decrease exponentially with increasing sample thickness^{16}. Because of the mixed structural characteristics—for example, the lack of translational symmetry of the disordered media and the highly ordered structure of the ordinary crystals—a critical question has been raised regarding wave transport in quasicrystals, including localization, which has not been thoroughly answered^{17}.
To the best of our knowledge, this is the first demonstration of the intrinsic localization of photonic waves in a threedimensional (3D) quasicrystal without additional disorder. Photonic wave localization in a 3D icosahedral quasicrystal is carefully investigated by photonic wave transmission utilizing finitedifference methods. The diffusive transport and localization of photonic waves in the quasicrystal are revealed by widely accepted approaches^{18,19}. We characterize the localization phenomena by analysing the spatial and temporal evolution of photonic waves. The localization mechanism is elucidated using the photonic band structures of quasicrystal approximants.
An icosahedral quasicrystal structure can be built according to the substitution rules^{20} as shown in Fig. 1a–i and further detailed in Supplementary Figs 1–5. The rhombic triacontahedron, indicated in purple in Fig. 1a, constitutes a large proportion of an icosahedral quasicrystal. Rhombic triacontahedrons are derived from the intersection of five cubes (Fig. 1j)^{21}. The parallelogram shape planes of the rhombic triacontahedrons can be placed on the three faces of the five cubes. The planes derived from the parallelograms are expected to form effective Brillouin zone faces and give rise to Bragg scattering.
Transmission spectra are obtained using a finitedifference timedomain code^{22}. A diamond structure is also compared as a reference. The photonic bandgap of the diamond on the first Brillouin zone boundary overlaps nicely with the lowintensity spectral regime (Fig. 2e). Thus, lowtransmission footprints originate from the photonic bandgap^{23}. Similar lowtransmission footprints are found in the quasicrystal (Fig. 2c), exhibiting good agreement with the Bragg scattering from the effective Brillouin zone faces with a plane distance of 0.85d, where d is the rod length of the rhombic triacontahedron (Fig. 1k). On the basis of interpretation of the diamond structure, we suggest that photonic bandgaps are formed on the effective Brillouin zone faces. However, the bandgap is very narrow and loosely defined thus exhibiting characteristics of pseudogaps^{16}. Indeed, a previous study^{3} demonstrated that a quasicrystal has pseudogaps. Since localized states can exist in the pseudogap regime^{16}, the wave localization is expected to occur in the lowintensity spectral regime of the quasicrystal.
Based on the transmission spectra timedependent transmission profiles in the lowtransmission regime are calculated. In the case of the quasicrystal for 30° rotation (fivefold symmetry axis), the average transmitted intensity decays exponentially during longer periods of time than 45° and 90° rotation cases, followed by a delayed decay (Fig. 2f). The timedependent intensity profiles for 45° and 90° rotations exhibit more noticeable delays. This delay originates from wave localization because localized waves have longer lifetimes in the structure^{24}. Wave propagation along a lowersymmetry axis experiences significant delay in timedependent transmission (comparing the 45° and 90° rotation cases). The localization is dependent on the wavepropagation direction in the quasicrystal, and lowersymmetry directions favour localization. Figure 2g shows that the transmission intensities in the diamond decrease much more quickly than those in the quasicrystal, clearly indicating ballistic propagation^{25}.
The average intensity distribution of the quasicrystal shows its lattice structure and the diffraction peaks that are irrelevant to the structure (Fig. 3a). The logarithm of the intensity crosssection reveals two aspects of transport characteristics in the quasicrystal. An average intensity profile close to the Gaussian shape corresponds to diffusive transport^{26}, while an exponential curve indicates the localization. The 30° rotation case is very similar to the Gaussian shape indicating the diffusivelike transport. The 90° and 45° cases, corresponding to the lowersymmetry axes, deviate from the Gaussian shape and exhibit sharper linear curves implying stronger localization, where the fit to the exponential function yields shorter localization lengths (Fig. 3a and Supplementary Table 2)^{26}. The intensity profile for the diamond structure is quite distinct from those of the quasicrystal (Fig. 3b). A wide range of the profile for the diamond exhibits periodic spike patterns that reflect the lattice structure due to its ballistic transport.
We further calculate the effective beam width. The beamwidth changes as a function of propagation distance quantitatively show the confinement of the propagating waves (Fig. 3c). For the diamond structure, the beam width grows rapidly up to approximately L = 4 cm due to the radiation from the beam centre and then increases slowly. As the distance increases, the propagating eigenstates^{27} exclusively allow electromagnetic waves to travel through the diamond, where the speckle pattern of Fig. 3f shows the eigenstates. The exponents of the width variation are approximately 1.05 for 0° rotation and 0.89 for 90° rotation. A value close to 1.0 indicates ballistic transport^{3,26}. The beam width change of the quasicrystal is substantially different from that of the diamond. The beam width increases slowly indicating stronger confinement, and the slope of the curve smoothly changes. The calculated exponents for 30°, 45° and 90° are close to 0.5, indicating that wave propagation in the quasicrystal is diffusivelike^{3,26}.
Similar behaviour is observed in the azimuthally averaged intensities as a function of the propagation distance (Fig. 3d–f). The initial intensity profile for the quasicrystal is close to the Gaussian distribution, whereas in the middle of the propagation, a pronounced peak near the beam centre position appears exhibiting localized waves in the quasicrystal (Fig. 3d, e). The final intensity profile for the 30° rotation is more similar to the Gaussian profile than that of the 45° rotation, indicating more diffusive transport. Propagation along the lowsymmetry axis exhibits a more localized distribution and larger deviations from the Gaussian profile. These results agree with those from the timedependent profiles. For the diamond structure, the shape with periodic spike patterns is invariant and broad (Fig. 3f).
A wellknown criterion for Anderson localization in disordered materials is the Ioffe–Regel condition^{12}, kl^{∗} ≤ 1, where k is a wavevector, and l^{∗} is the transport mean free path. The calculated mean free paths in the quasicrystal are very small, and kl^{∗} is close to 5 at frequencies of 0.558 and 0.906, where the localization occurs (Table 1 and Fig. 4a). The low kl^{∗} for the 45° rotation implies a better probability of localization. A recent experimental study^{28} demonstrated 3D Anderson localization under a similar condition (kl^{∗} < 5). At the low frequency, 0.453, the kl^{∗} values exceed 5 and photonic wave localization is weakened. The mean free path shortens as the frequency increases, and localization is favoured at higher frequencies. Therefore, the main driving force of localization in the quasicrystal is the short mean free path.
Photonic band structures of the 1/1 approximant unit cell are studied to understand the mean free path variation in the quasicrystal. For comparison, the band structures of an empty cubic unit cell, that is, the structurefree bands, are overlapped to separate artefacts from the supercell. The structurefree bands indicate photonic wave propagation in a homogeneous medium. Two important features can be observed in the band structures: convergence of the quasicrystal photonic bands to the structurefree bands at low frequencies (lower part of Fig. 4e) and band flattening at high frequencies (upper part of Fig. 4e). Larger band flattening corresponds to slower group velocity and more scattering events. Thus, increased scattering events at high frequencies cause decreasing scattering mean free path^{29} and increase the possibility of wave localization in light of the Ioffe–Regal condition.
The degree of band flattening in the approximant changes with varying the wavevector direction and is highly related to the quasicrystal symmetry. To investigate symmetry relationships between the approximant and the quasicrystal, we superimpose the stereoprojection of the approximant unit cell on the icosahedral structure (Fig. 4c). We examine the band flattening from the Γ point to the three points (Fig. 4f). The band flattening phenomena become more significant at lowersymmetry points, and the wave transmission in the lowsymmetry direction is suppressed, leading to stronger localization. Similar behaviours are further observed in the 2/1 approximant unit cell (Supplementary Fig. 15).
Our results indicate the universal features of wave transport in 3D quasicrystals, including electron and phonon, and should stimulate future research on the transport properties of quasicrystalline materials. In contrast to disordered materials, quasicrystalline structures can be precisely engineered to enable the control of wave localization for future photonic applications, such as lasers.
Methods
Preparation of a 3D icosahedral quasicrystal and a diamond.
To simulate the transmission measurement, an icosahedral quasicrystal and a diamond are prepared with vertices connected by rods of length d = 1 cm. The rod diameters are 0.3 cm and 0.4 cm for the quasicrystal and diamond, respectively. The volume fraction of the quasicrystal is about 0.17, and that of the diamond is 0.283.
Finitedifference grid generation.
The 3D structures are converted into finitedifference grids utilizing a freely available code (contact A. H. Aitkenhead (adam.aitkenhead@christie.nhs.uk)) that reads and voxelizes a 3D triangularpolygon mesh in stereolithography (STL) file format. The voxelized result is given in finitedifference grids that can be imported into finitedifference timedomain (FDTD) simulation packages.
Finitedifference timedomain simulation.
The numerical simulations of the transmission and intensity field distribution are performed using MIT Electromagnetic Equation Propagation (MEEP)^{22}, which is freely available code based on the FDTD method. The constituting material for the structures is assumed to be a lossfree polymeric material with the dielectric constant of 2.72. The value of vacuum is 1. As Maxwell’s equations are scale invariant, all of the simulation units can be conveniently chosen depending on the rod length d, where the length and time units are proportional to d, but the frequency unit is inversely proportional to d. For example, the frequency ωd/2πc = 1 corresponds to 30 GHz in the case of d = 1 cm. The number of pixels per distance unit in all simulations is set to 20 pixels cm^{−1} and the overall system size including the perfectly matched layer (PML) reaches 25 × 25 × 25 cm. The PML layer thickness is given by larger than twice the largest wavelength to avoid electromagnetic wave reflections on the interface of vacuum and the PML layer.
The transmission intensities are obtained by rotating the structures along the twofold rotational symmetry axis by 5° as shown in Fig. 2a. A Gaussianpulse source with the temporal width 60 ps is applied to investigate timedependent transmission intensities. The average intensity distributions are obtained after electromagnetic fields in a system are saturated with a planewave source. Further detailed simulation setup and implementation using MEEP are given in Supplementary Information.
Finitedifference frequencydomain simulation.
We developed a finitedifference frequencydomain simulator utilizing parallel Arnoldi Package (ARPACK)^{30} to efficiently calculate the photonic band structures. The governing equation for the frequencydomain Maxwell equation is
where ɛ is the relative permittivity, H is the magnetic fields, ω is the angular frequency, and c is the speed of light. ɛ is set to 2.72 for structures and 1 for vacuum; the same as those of the FDTD simulation. The computational implementation is further detailed in the Supplementary Information.
The mean free path calculation.
To calculate the transport mean free path, l^{∗}, for diffusive wave transport and the scattering mean free path, l_{s}, for ballistic wave transport, a diffusion theory formalism^{31,32} is used. The transmission intensities as a function of propagation distance are obtained after electromagnetic fields in a system are saturated with a planewave source. The absolute transmission values are obtained at multiple positions (Supplementary Fig. 13a). The calculated transmission intensities, T, are fitted using the following equation.
and
where l^{∗} is the transport mean free path, l_{s} is the scattering mean free path, L is the sample thickness (the position of the detector), and τ_{a} is the absorption time indicating the time deviating from the diffusion theory in the timeresolved intensity profile, R is the wall boundary reflectivity, z_{0} is the extrapolation length defined as z_{0} = (2/3)l^{∗}(1 + R)/(1 − R), and D is the diffusion constant. The first term in equation (1) is the diffusive transport contribution, and the second term is the ballistic transport contribution.
In the present work, we assume that internal reflection is negligible following previous work^{33}, since the dielectric constant of the constituting material is small. Thus, z_{0} is set to 0.667. The diffusion constant is 8.43 × 10^{−3} cm^{2} ps^{−1} as obtained from the timeresolved transmission simulation. l^{∗}, l_{s} and τ_{a} are used as fitting parameters. In particular, τ_{a} values are consistent with those of the time deviating from the diffusion theory in Fig. 2f, g and Supplementary Fig. 9.
Data availability.
The data that support the plots within this paper and other findings of this study are available from the corresponding author on request.
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Acknowledgements
This work has been supported by the Korea Institute of Science and Technology (Grant No. 2E26130) and the National Research Foundation of Korea (Grant No. NRF2016M3D1A1021142, NRF2014M3C1A3054143). The authors also acknowledge support from the Disaster and Safety Management Institute funded by the Ministry of Public Safety and Security of the Korean government (Grant No. MPSSCG201602). The calculations were performed using the computational resources of the Korea Institute of Science and Technology Information (KISTI) (Proposal No. KSC2015C2037).
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H.K. prepared 3D structures. S.Y.J. and K.H. generated finitedifference simulation codes, performed simulations, and wrote the paper. All authors contributed to the data analysis and commented on the manuscript.
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Jeon, SY., Kwon, H. & Hur, K. Intrinsic photonic wave localization in a threedimensional icosahedral quasicrystal. Nature Phys 13, 363–368 (2017). https://doi.org/10.1038/nphys4002
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