Propagation of Electromagnetic Waves in Slab Waveguide Structure Consisting of Chiral Nihility Claddings and Negative-Index Material Core Layer

The dispersion equation of an asymmetric three-layer slab waveguide, in which all layers are chiral materials is presented. Then, the dispersion equation of a symmetric slab waveguide, in which the claddings are chiral materials and the core layer is negative index material, is derived. Normalized cut-off frequencies, field profile, and energies flow of right-handed and left-handed circularly polarized modes are derived and plotted. We consider both odd and even guided modes. Numerical results of guided low-order modes are provided. Some novel features, such as abnormal dispersion curves, are found.


Introduction
Left-handed materials (LHMs), also known as negative-index materials (NIMs), were demonstrated theoretically in 1968 by Veselago [1]. These double negative materials with simultaneously negative electric permittivity () and magnetic permeability () have abnormal properties, such as a negative index of refraction, sub wavelength imaging, backward wave propagation, and reverse Doppler and Cherenkov effects . These materials cannot be found in nature but are man-made. Because of their novel properties, Pendry and his co-authors worked hard to generate NIM and finally developed the concept of a perfect lens [9]. The main idea to fabricate NIMs is to treat the permittivity and permeability separately. Pendry used wire array structure to generate negative permittivity and split-ring resonator (SRR) structure to generate negative permeability [11,13]. At the time when it was quite difficult to obtain a negative refractive index of refraction in the visible region, Shelby experimentally fabricated the first NIM in the microwave region [23]. More and more attention to this kind of metamaterial has been drawn due to its abnormal properties and possible applications in many fields such as cloaking [24], micro strip patch antenna [25], wave absorbers [26], and biochemical sensors [27][28][29][30][31][32][33][34][35][36].
A material with high optical activity can also have a negative index of refraction. This property makes a material capable of revolving the plane of incident polarized light due to the asymmetrical molecular structure. In the beginning of 19th century, chiral media were discovered and studied due to their optical activity. At the interface between the chiral material and achiral material, bi-refraction occurs because of the existence of two different modes caused by the chiral medium. A chiral medium can have a negative index of refraction. If the chirality coefficient (κ) is strong and greater than the index of refraction at least near the resonant frequency, one eigen-wave in the chiral medium turns out to be a backward wave, and the phenomenon of negative refraction in the chiral material occurs. Therefore, chiral materials can achieve negative refraction, from which we call chiral negative refractive index medium [37].
The Greek word "chiral" means "hand": our hands are mirror images and they cannot be superimposed on each other no matter how hard we try. A chiral object cannot be superimposed on its mirror image neither by translation nor by rotation. The modes in a chiral bulk material can be divided into right-handed circular polarization (RCP) and left-handed circular polarization (LCP). When a linearly polarized light propagates through a chiral material, it is decomposed into two orthogonal modes known as RCP and LCP. Upon leaving the chiral medium, the two modes recombine again, and the output is linearly polarized. The effect of the chiral medium is to rotate the plane of polarization with an angle depending on the thickness of the chiral material. Due to the novel features of chiral media, intensive interest has been focused on waveguide structures comprising chiral materials [38][39][40][41][42][43]. Waveguide structures comprising chiral media were first studied by P. Pelet [38]. They have unique propagating properties such as hybrid modes.
In this work, the properties of guided modes in slab waveguides in which the claddings are chiral nihility and the core is left-handed metamaterial are investigated theoretically. The characteristic equations, field profile, and energy flow in the waveguide layers of several low-order guided modes in the chiral nihility waveguide structure are presented. Some unusual properties such as abnormal dispersion behavior in the chiral nihility waveguide are observed.

Dispersion relations of slab chiral waveguides
The structure of an asymmetric three-layer slab chiral waveguide is shown in Fig. 1. The film has a permittivity ε 2 , permeability µ 2 , chirality 2  , and thickness d. Regions 1 and 3 are semi-infinite chiral materials with different refractive indexes and chirality. The electric and magnetic fields in the chiral material are coupled because of the chirality parameter.
for odd guided modes and for even guided modes.

Dispersion of three-layer symmetric slab achiral core and chiral cladding waveguides
When the geometry of the waveguide consists of an achiral core and chiral claddings, where 2 0   , the above parameters become   for even guided modes. When the claddings are chiral nihility media, in which the permittivity and permeability tend to be zero, the above parameters become 1 The dispersion relation given in (7) is divided into two equations as for RCP odd modes and for LCP odd modes, where m is mode number. As can be seen, m starts from 0 in RCP odd modes and from 1 in LCP odd modes.
Also, the dispersion relation given in (8) is divided into two equations which take the form as for RCP even modes and 1/ 2 for LCP even modes. As can be seen, in contrast to odd modes, m starts from 1 in RCP odd modes and from 0 in LCP odd modes, because of the handedness of chiral meta-material.

Guided modes in chiral nihility claddings and NIM core waveguide
Consider that Region 1 and Region 3 shown in Fig. 1 are chiral nihility materials and the core is NIM. The dispersions (7) and (8) can be splited into two dispersion equations corresponding to two modes: RCP and LCP. The characteristic relation, cut-off frequencies, field profile, and energy in the different layers of the waveguide structure of RCP and LCP modes are found. We consider both odd and even modes.

Odd modes
Assume the core layer is negative-index material, thus the parameter n 2 has a negative value, so the dispersion relation given by (7) can be divided into two equations as for RCP odd modes and for LCP odd modes. It is noted that, in contrast to (9) and (10), m starts from 0 in LCP odd modes and from 1 in RCP odd modes.
We can obtain the normalized cutoff frequencies (V ) by setting in the dispersion relations (13) and (14) for RCP odd modes and for LCP odd modes. The derivation of the power flow (Poynting vector) is given in details in Appendix B for odd modes.
From Appendix B, the energy flow along the z-axis can be expressed as for RCP (upper sign) and LCP (lower sign) odd modes.
Inspection of (17) reveals that S z is positive for RCP odd modes and negative for LCP odd modes in the surroundings, and positive for both RCP and LCP odd modes in the guiding layer. On the other hand, we use NIM guiding layer which makes the energy flux negative for both RCP and LCP odd modes.

Even modes
We again assume a NIM in the core of negative index n 2 , so the dispersion relation given in (8) for LCP even modes.
We can obtain the normalized cutoff frequencies (V) by setting (18) and (19)   for RCP even modes and for LCP even modes. The derivation of the power flow (Poynting vector) is given in details in Appendix C for even modes. From Appendix C, the enery flow along the z-axis can be expressed as for RCP (upper sign) and LCP (lower sign) even modes, respectively. It is obvious from (22) that S z is positive for RCP even modes and negative for LCP even modes in the cladding, and positive for both RCP and LCP even modes in the core. On the other hand, we use the NIM core which makes the energy flux negative in the core for both RCP and LCP even modes.

Results and discussion
We can calculate the propagation constants numerically from the dispersion relations (13), (14), (18), and (19), then the electromagnetic fields and the energy flow distribution can be calculated. In this section, we use numerical values for the parameters in the core and cladding as 1 1       , and 2 0.

Dispersion curves
The dispersion properties of odd and even guided low-order modes are illustrated in Fig. 2 odd and even modes, the dispersion curves increase monotonically. The effective refractive index incerases monotonically with the normalized frequency. The normalized cutoff frequencies (points C 1 and C 2 where eff 1 n  ) satisfy (16) or (21). However, for RCP odd and even modes, the dispersion curves are no longer increasing monotonically, but are bent. The cutoff frequencies where eff 1 n  are not the minimum frequencies that waves can propagate. Considering the fundamnetal mode where m = 0, there is one solution below cutoff frequency (Point 1 C ) for RCP even mode in some frequency region. If we consider the first guided mode, m = 1, there exist two solutions below cutoff frequencies (Points C 2 and C 3 ) for both RCP even and odd modes in some frequency region. Therefore, the cutoff frequencies are no longer really "cutoff". The real "cutoff" frequencies correspond to the minimum frequencies (critical Points B and D) that guided wave can propagate. As the normalized frequency increases from the critical Points B and D, dispersion curves split into two branches, which the modal index increases with an increase in the normalized frequency for upper branch and decreases to eff 1 n  for lower branch.  Fig. 2 Dispersion curves of guided modes in the chiral nihility cladding and negative-index material core waveguide.

Odd guided modes
The field profile which shows the amplitudes of electric and magnetic fields is illustrated in Fig. 3 at normalized thichness 0 2.2 k d  for RCP odd first mode. Figure 4 illustrates the energy flux distribution at normalized thichness 0 2.2 k d  for RCP odd first mode. As can be seen from Fig. 3, E z and H z are odd functions of x (sin form) and E x , E y , H x , and H y (cos form) are even functions of x. z S is positive in the cladding and is negative in the core due to the NIM core material. However, there are two propagation constants at 0 2. The field profile is plotted in Fig. 5 whereas and energy flux distribution is shown in Fig. 6 at normalized thickness 0 2 k d  for LCP odd fundamental mode. As Fig. 5 shows, E z and H z are odd functions of x (sin form), and E x , E y , H x , and H y (cos form) are even functions of x . As can be seen from Fig. 6, z S is negative in both the guiding layer and the surroundings due to the negative refractive index of the guiding layer.  Fig. 5 Field profiles at 0 2 k d  for LCP odd fundamental mode. Fig. 6 Energy flux at 0 2 k d  for LCP odd mode when m = 0.

Even guided modes
The field profile is plotted in Fig. 7 whereas the energy flux distribution is shown in Fig. 8 at 0 0.5 k d  for RCP even fundamental mode. As clearly shown in Fig. 7, E z and H z are even functions of x (cos form), and E x , E y , H x , and H y are odd functions of x (sin form). It is obvious from Fig. 8 that S z is positive in the surroundings and is negative in the guiding layer as expected.  The field profils are shown in Fig. 9, and the energy flux distribution is illustrated in Fig. 10 at 0 3 k d  for LCP even first mode. As clearly seen from Fig. 9, E z and H z are even functions of x (cos form), and E x , E y , H x , and H y (sin form) are odd functions of x. As can be seen from Fig. 10, S z is negative in both the guiding layer and the surroundings.

Conclusions
The dispersion equations of three-layer asymmetric and symmetric chiral slab waveguides are derived. We first assume the three layers are chiral media. Guided modes in a special case of chiral nihility claddings, and NIM core waveguides are presented in details. Both the dispersion equations for odd and even RCP and LCP guided modes in the chiral nihility waveguides are examined. For odd and even guided modes, the dispersion relations, normalized cutoff frequencies, field profiles, and energy flow of RCP and LCP modes are derived and plotted. A numerical results for typical chirality parameters of several guided modes are given. Some novel features such as abnormal dispersion curves in the chiral nihility waveguides are mentioned.

Appendix A
The solutions of the longitudinal-field component in (4) can be written as for odd guided modes and   When the structure shown in Fig. 1