Abstract
The creation of photonic Dirac cones by accidental degeneracy in the Brillouin-zone center was recently reported for both metamaterials with localized electromagnetic resonant states and dielectric photonic crystals without well-defined resonance. Based on the anticipation that there should be a common physical origin in this phenomenon, we systematically examined the relation between mode symmetries and shapes of dispersion curves for both systems. The result strongly suggests the presence of universality of mode symmetries that enable the creation of photonic Dirac cones irrespective of the details of the sample structure.
© 2012 Optical Society of America
1. INTRODUCTION
Photonic Dirac cones, or the isotropic linear dispersion relation around certain points in the Brillouin zone, have been attracting considerable interest during the last five years, since they offer various novel possibilities for physics and engineering. Haldane et al. [1,2] pointed out the presence of photonic Dirac cones on the Brillouin-zone boundary of two-dimensional triangular-lattice photonic crystals due to their structural symmetry and discussed unidirectional propagation of surface modes caused by time-reversal symmetry breaking due to the Faraday effect. Ochiai and Onoda extended the discussion to honeycomb-lattice photonic crystals [3]. Zhang proposed optical simulation of Zitterbewegung, or trembling motion, in particle physics by propagating an optical pulse of the Dirac point frequency [4]. A pseudodiffusive transmission whose intensity was inversely proportional to the thickness of the photonic crystal was found by Sepkhanov et al. [5] and numerically demonstrated by Diem et al. [6].
Recently, Nam et al. analyzed the wave propagation in a periodic metal-dielectric waveguide array, which may be regarded as a one-dimensional photonic crystal, to show the presence of the linear dispersion relation, or a one-dimensional Dirac cone, in the Brillouin-zone center by adjusting the coupling constants between the waveguides [7]. Huang et al. reported that Dirac cones can also be created in the Brillouin-zone center of two-dimensional dielectric photonic crystals by accidental degeneracy of two bands [8]. They showed for square- and triangular-lattice photonic crystals that combinations of a nondegenerate band and a doubly degenerate band yield a Dirac cone together with an auxiliary quadratic dispersion surface. Because the Dirac point in the Brillouin-zone center is equivalent to a zero effective refractive index [8], it has much potential for various applications such as scatter-free waveguides [9] and lenses of arbitrary shapes [10].
On the other hand, we showed by tight-binding approximation and group theory that we can also create Dirac cones in the Brillouin-zone center of metamaterials, which are characterized by well-defined electromagnetic resonant states localized in their unit structures, by accidental degeneracy of two modes [11,12]. We proved that the combination of and modes of square-lattice metamaterials and the combination of and modes of simple-cubic-lattice metamaterials create a Dirac cone with auxiliary quadratic dispersion surfaces [11]. We further proved that the combination of and modes of triangular-lattice metamaterials yields double Dirac cones, or a pair of identical Dirac cones, without an auxiliary quadratic dispersion surface [12].
Because the presence of Dirac cones was found in two extreme systems, i.e., dielectric photonic crystals for which the extended-wave picture is appropriate and metamaterials with localized electromagnetic resonant states for which the tight-binding picture is appropriate, it is highly likely that there is a common physical origin in this phenomenon irrespective of the details of the sample structure.
In this paper, based on this anticipation, we systematically examine the relation between the symmetry of modes and the shapes of dispersion curves generated by accidental degeneracy for both dielectric photonic crystals and metamaterials with localized electromagnetic resonant states by numerical calculation and tight-binding approximation, respectively. We show that the two calculations give the same results, which strongly suggests the presence of universality of mode symmetries that enable the creation of photonic Dirac cones irrespective of the details of the sample structure, irrespective of the presence or absence of localized resonant states, and irrespective of the approximation used in the calculation.
This paper is organized as follows. In Section 2, the methods for calculating the dispersion curves of metamaterials and photonic crystals are briefly described. In Section 3, the shapes of the dispersion curves thus calculated are presented in the case of accidental degeneracy for various combinations of mode symmetries. A summary of the present study is given in Section 4. In Appendix A, we show the formulation of the tight-binding approximation with the magnetic fields of the resonant states of a single unit structure of metamaterials as basis functions. The method for deriving mutual relations among different electromagnetic transfer integrals is also described. The shapes of the dispersion curves thus calculated for metamaterials of the and symmetries are presented in Appendices B and C, respectively.
2. THEORY
For the analytical description of dispersion curves of periodic metamaterials by tight-binding approximation, electromagnetic transfer integrals defined by the following equation play the main role [11,12]:
where () denote the magnetic field of resonant states of a unit cell, are lattice vectors, is the volume on which the periodic boundary condition is imposed, and is the operator that defines the eigenvalue problem of the electromagnetic field: where is the position-dependent dielectric constant, is the eigen angular frequency, and is the light velocity in free space. The magnetic permeability is assumed to be unity, since we do not deal with magnetic materials.We analytically solve the secular equation of the electromagnetic field for wave vector in the vicinity of the Brillouin-zone center by taking into account ’s for the nearest-neighbor lattice points [11,12]. The spatial symmetry of the resonant states results in many relations among ’s and simplifies the secular equation (see Appendix A). In the case of accidental degeneracy of and modes of triangular-lattice metamaterials with (regular triangle) symmetry, for example, we can thus analytically solve the secular equation that is quartic in [12]. For numerical calculations of the dispersion relation of photonic crystals, a conventional plane-wave method is used [13].
3. RESULTS AND DISCUSSION
First, we examine the triangular lattice of symmetry. In Table 1, the shapes of dispersion curves generated by accidental degeneracy on the point of the Brillouin zone are summarized for metamaterials. As we mentioned previously, we obtain double Dirac cones by combining and modes [12]. On the other hand, we found in the present study that (, ), (, ), and (, ) combinations yield a Dirac cone with an auxiliary quadratic dispersion surface. For the rest of the mode combinations listed in Table 1, we obtain two or three different quadratic dispersion curves (see Appendix B for details).
For the photonic crystal, we assume a periodic triangular array of circular air cylinders in a uniform material with a dielectric constant of 12.6 (GaAs). First, we examine the structural parameters for materializing accidental degeneracy. Figure 1 shows the second to fifth lowest eigenfrequencies on the point for transverse electric (TE) polarization with its electric field perpendicular to the cylinder axis. In this figure, the vertical axis denotes the eigen angular frequency normalized by the lattice constant of the photonic crystal and the light velocity in free space, while the horizontal axis denotes the radius of the air cylinder normalized by the lattice constant. The spatial symmetry of the magnetic field is also given to each mode.
Figure 1 shows that there are six values of the normalized radius that materialize accidental degeneracy of eigenfrequencies. Dispersion curves for five of the six cases are given in Figs. 2(a)–2(e). They are calculated for in the -to- and -to- directions in the Brillouin zone [see Fig. 2(f)]. The sixth case (normalized radius of 0.494) is the degeneracy of the and modes and gives a feature similar to Fig. 2(a).
Accidental degeneracy of the and modes results in double Dirac cones. As shown in Fig. 2(a), their dispersion in the vicinity of the point is linear and isotropic and the slopes of the two dispersion lines are the same on the point. For the combination of and modes [Fig. 2(b)] and that of and modes [Fig. 2(e)], accidental degeneracy results in the formation of a Dirac cone with an auxiliary quadratic dispersion surface. Two of the three dispersion curves are linear and isotropic in the vicinity of the point, while the third curve is quadratic. Finally, for the combination of and modes [Fig. 2(c)] and that of and modes [Fig. 2(d)], accidental degeneracy results in three quadratic dispersion curves. All these features are the same as the case of triangular-lattice metamaterials listed in Table 1, which was analytically proved assuming the presence of localized resonant states in each unit structure.
In Fig. 3, structural parameters for realizing accidental degeneracy are examined for transverse magnetic (TM) polarization with its magnetic field perpendicular to the cylinder axis. The spatial symmetry of the magnetic field is also given to each mode.
Let us make one remark here. The TM polarization is often referred to as polarization, and its electric field, which is parallel to the cylinder axis, is discussed traditionally. Because the electric field is a genuine vector and the magnetic field is an axial vector, their transformation by symmetry operations is generally different. When we denote the symmetry operation and its matrix representation by and and the character of transformation of the electric (magnetic) field by (), we have the following relation [13]:
Because of this difference between and , the irreducible representation of the electric field is generally different from that of the magnetic field. In the present study, we always refer to the symmetry of the magnetic field to avoid confusion.There are two values of the normalized radius that materialize accidental degeneracy for the TM polarization as shown in Fig. 3. The combination of and modes [Fig. 4(a)] results in the creation of a Dirac cone with an auxiliary quadratic dispersion surface, whereas the combination of and modes [Fig. 4(b)] results in the formation of three quadratic dispersion curves. Both features agree with the case of metamaterials listed in Table 1.
Now we proceed to the case of square lattices of (regular square) symmetry. Table 2 summarizes the shapes of dispersion curves of periodic metamaterials generated by accidental degeneracy on the point. As we mentioned previously, we obtain a Dirac cone with an auxiliary quadratic dispersion surface by combining and modes [11], whereas (, ) and (, ) combinations yield two quadratic curves [12]. We further found in the present study that (, ) and (, ) combinations give a Dirac cone and an auxiliary quadratic dispersion surface and that (, ) and (, ) combinations yield two quadratic dispersion curves (see Appendix C for details).
On the other hand, for square-lattice photonic crystals of the symmetry composed of circular air cylinders in a uniform material with a dielectric constant of 12.6, we found three values of the normalized radius that materialize accidental degeneracy on the point. Dispersion curves for these three cases are shown in Fig. 5. Their shapes agree with the metamaterials listed in Table 2.
Thus, in all cases that we could examine numerically, dielectric photonic crystals and metamaterials with localized electromagnetic resonant states have the same relation between the mode symmetries and the shapes of the dispersion curves generated by accidental degeneracy in the vicinity of the point. This fact strongly suggests the presence of universality in combinations of mode symmetries that enable the creation of photonic Dirac cones and double Dirac cones irrespective of the details of the sample structure. This knowledge is quite useful for designing specimens with Dirac cones to realize many applications described in Section 1. However, we should note that the universality has not been proved rigorously yet, since the present study is partly based on numerical calculation for a limited number of examples. Three-dimensional systems in particular were not analyzed, although we expect a similar universality of mode symmetries. So, the rigorous proof of the universality remains a future problem.
4. CONCLUSION
We analytically solved secular equations of periodic metamaterials of the and symmetries with localized electromagnetic resonant states by tight-binding approximation and obtained the shapes of dispersion curves in the vicinity of the Brillouin-zone center. Thus we clarified the combinations of mode symmetries that materialize Dirac cones and double Dirac cones by accidental degeneracy. We also examined dispersion curves of dielectric photonic crystals numerically by the plane-wave expansion method. By changing a structural parameter of assumed samples, we found 11 cases of accidental degeneracy on the point and compared the shapes of dispersion curves with the metamaterials.
In all 11 cases, the relation between the mode symmetries and the shape of the dispersion curves was common to metamaterials with localized electromagnetic resonant states and dielectric photonic crystals without well-defined resonant states, which strongly suggests the presence of universality of mode symmetries that enable the creation of photonic Dirac cones irrespective of the details of the sample structure, irrespective of the presence or absence of localized resonant states, and irrespective of the approximation used in the calculation.
APPENDIX A: FORMULATION
Maxwell’s wave equation for the magnetic field is given by
where is the periodic dielectric constant of the system. The magnetic permeability is assumed to be unity, since we do not deal with magnetic materials. We impose the periodic boundary condition on and assume that is real to make our problem well-defined [13]. Thus, Eq. (A1) leads to an eigenvalue problem.For a single metallic unit structure described by dielectric constant , we assume the presence of localized resonant states, which characterize the optical response of the metamaterial. Thus, their magnetic fields satisfy the following eigenvalue equation:
where operator is defined by the first equality of this equation, is the suffix to distinguish the resonant states, and is the resonance angular frequency. Without loss of generality, we can assume that the eigenfunctions are normalized as follows: where is the volume on which we impose the periodic boundary condition.We assume that the unit cell of the periodic metamaterial has a certain spatial symmetry and denote the group of its symmetry operations by . In Appendices B and C, we deal with two cases of and . By definition, the transformation of the magnetic field by symmetry operation is given by
where is the matrix representation of . For all , we can prove the following equation [12]. where is defined by Eq. (1). Because is an eigenfunction of , it is a basis of an irreducible representation of , so can be expressed by a linear combination of eigenfunctions of the same irreducible representation [14]. Then, the right side of Eq. (A5) must be a linear combination of , so we obtain their mutual relations. We can thus derive many relations among for and , as will be shown in Appendices B and C.Now, we solve the eigenvalue problem given by
by the tight-binding approximation. In Eq. (A6), is a wave vector in the Brillouin zone. The secular equation has the following form [12]: where is a unit tensor and elements of matrix are given by When we analyze the dispersion curves for small around the Brillouin-zone center, the summation for can be limited to the nearest-neighbor lattice points, since other terms give contributions of higher orders of .APPENDIX B: TRIANGULAR LATTICE
1.
In this appendix, we examine the dispersion relation in the vicinity of the Brillouin-zone center for the triangular lattice of unit cells of the symmetry. We denote the origin and the nearest-neighbor lattice points by 0 to 6, as shown in Fig. 6, where symmetry operations of the point group are also shown.
We start with the case of accidental degeneracy of an mode and a mode on the point of the Brillouin zone. We denote the magnetic fields of the and modes by and , respectively. By using all in Eq. (A5), we can derive the following relations:
In addition to these relations, we can generally prove the following equation. where denotes the lattice point given by .From Eq. (A8), we obtain
where which are the square of the eigen angular frequencies at the point divided by for the and modes, respectively. Because the secular equation in the present case is a quadratic equation of , it can easily be solved. When we assume the accidental degeneracy of the two modes at the point—that is, —the dispersion relation for small is where and . gives higher-order corrections. So, the linear term is absent and the dispersion relations are quadratic in in the vicinity of the point.2.
For the combination of an mode (mode 1) and a mode (mode2), we can derive
by examining Eq. (A5) for all . So, there is no interaction between the two modes and the dispersion relations are quadratic for small .3.
modes are doubly degenerate. From group theory [14], we can assume without loss of generality that two eigenfunctions of the mode transform like and . We denote the magnetic field of the mode by for this time and those of the mode by , which transforms like , and , which transforms like . Then, we can derive the following relations from Eq. (A5):
We can further prove that Then, we can obtain the elements of matrix according to Eq. (A8). The secular equation is a cubic equation of . To obtain its analytical solutions, we need a lengthy calculation similar to that presented in [11] The solutions are, however, rather simple when we assume the accidental degeneracy of the and modes at the point. The result is where . Two of the three dispersion curves are linear and isotropic with respect to in the vicinity of the point, and the third one is quadratic. So, we have a Dirac cone with an auxiliary quadratic dispersion surface.4.
modes are doubly degenerate. From group theory [14], we can assume without loss of generality that two eigenfunctions of the mode transform like and . We denote the magnetic field of the mode by and those of the mode by , which transforms like , and , which transforms like . Then, we can derive the relations among ’s by using Eq. (A5). The results are completely the same as the case of . So we have a Dirac cone with an auxiliary quadratic dispersion surface in the vicinity of the point.
5.
We denote the magnetic field of the mode by and those of the mode by , which transforms like , and , which transforms like , for this time. Then, we can derive the relations among ’s according to Eq. (A5). We obtain the same results as the case of , except
Since the dispersion relation, Eq. (B29), does not depend on the sign of , we obtain the same dispersion, i.e., a Dirac cone with an auxiliary quadratic dispersion surface.6.
We denote the magnetic field of the mode by and those of the mode by , which transforms like , and , which transforms like . Then, the relations among ’s with , are the same as in Appendix B.3. For the rest of ’s, we can derive the following relations:
We can further proveThen, we obtain the elements of matrix in Eq. (A8) and solve the secular equation, Eq. (A7). After a simple but lengthy calculation, we obtain the dispersion relation as follows.
where and is the angle between vector and the axis. So, there are three quadratic dispersion surfaces.7.
We denote the magnetic field of the mode by and those of the mode by , which transforms like , and , which transforms like , for this time. Then, we can derive the relations among ’s according to Eq. (A5). We obtain the same results as the case of , except
Because the dispersion relation, Eq. (B38), does not depend on the sign of , we obtain the same dispersion, i.e., three quadratic dispersion surfaces.8.
We denote the magnetic field of the mode by and those of the mode by , which transforms like , and , which transforms like . Then, the relations derived for ’s are the same as the case of . So, there are three quadratic dispersion surfaces in the vicinity of the point.
APPENDIX C: SQUARE LATTICE
1.
Next, we examine the square lattice of unit cells of the symmetry. We denote the origin and the nearest-neighbor lattice points by 0 to 4, as shown in Fig. 7, where symmetry operations of the point group are also shown.
We start with the case of accidental degeneracy of an mode and a mode in the Brillouin-zone center. We denote the magnetic fields of the and modes by and , respectively. By using all in Eq. (A5), we can derive the following relations:
Then, we obtain the elements of matrix and solve the secular equation. The dispersion relation thus obtained for small in the vicinity of the point is
where is the angle between vector and the axis. So, there are two quadratic dispersion surfaces.2.
For this combination,
So, there is no interaction between the two modes and the dispersion relations are quadratic.3.
Equation (C6) holds for this combination, too. So, the dispersion relations are quadratic with respect to in the vicinity of the point.
4.
We have completely the same relations for ’s as . So, there are two quadratic dispersion surfaces.
5.
modes are doubly degenerate. From group theory [14], we can assume without loss of generality that two eigenfunctions of the mode transform like and . We denote the magnetic field of the mode by and those of the mode by , which transforms like , and , which transforms like . Then, we can derive the following relations by examining Eq. (A5) for all :
Then, from Eq. (A8), elements of matrix are
When we compare the structure of matrix with that for the combination of and modes, Eqs. (9)–(14) of [11], they are the same with each other except the sign of , which is equivalent to changing the sign of . So, the dispersion relation of the case is obtained by changing the sign of in the dispersion relation of . Because the structure of the metamaterial that we assume in this paper is symmetric about the axis due to the symmetry, the dispersion relation is invariant for the change of the sign of . In conclusion, the shape of the dispersion curves is the same for the case and the case, which is a Dirac cone with an auxiliary quadratice dispersion surface.6.
Finally, we examine the combination of a mode and an mode. We denote the magnetic field of the mode by and those of the mode by , which transforms like , and , which transforms like , for this time. Then, the relations among are the same as the case except
The structure of these relations are completely the same as the case, Eqs. (27)–(32) of [11]. So, the accidental degeneracy of the and modes at the point results in a Dirac cone with an auxiliary quadratice dispersion surface.ACKNOWLEDGMENTS
This study was supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Japanese Ministry of Education, Culture, Sports and Technology (Grant No. 22109007).
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