Asymmetric transmission of terahertz waves using polar dielectrics

: Asymmetric wave transmission is a Lorentz reciprocal phenomenon, which can appear in the structures with broken symmetry. It may enable high forward-to-backward transmittance contrast, while transmission for one of the two opposite incidence directions is blocked. In this paper, it is demonstrated that ultrawideband, high-contrast asymmetric wave transmission can be obtained at terahertz frequencies in the topologically simple, i.e., one-or two-layer nonsymmetric gratings, which are entirely or partially made of a polar dielectric working in the ultralow-ε regime inspired by phonon-photon coupling. A variety of polar dielectrics with different characteristics can be used that gives one a big freedom concerning design. Simple criteria for estimating possible usefulness of a certain polar dielectric are suggested. Contrasts exceeding 80dB can be easily achieved without a special parameter adjustment. Stacking a high-ε corrugated layer with a noncorrugated layer made of a polar dielectric, one can enhance transmission in the unidirectional regime. At large and intermediate angles of incidence, a better performance can be obtained owing to the common effect of nonsymmetric diffractions and directional selectivity, which is connected with the dispersion of the ultralow-ε material. At normal incidence, strong asymmetry in transmission may occur in the studied structures as a purely diffraction effect.


Introduction
The interest to the asymmetric wave transmission has been growing over last years. This is a Lorentz reciprocal phenomenon, which manifests itself in strong difference between the forward and the backward transmission in the structures with broken spatial inversion symmetry for illumination directions that differ by 180 degrees, e.g., see [1][2][3][4][5][6][7]. In spite of that the functionalities associated with nonreciprocity, e.g., optical isolation cannot be obtained in the frame of this mechanism, there are many others, which are realizable in various nonsymmetric structures made of linear, isotropic, passive materials [8,9]. In particular, the achievable operation regimes include single-beam deflection, diodelike transmission, and dual-and multibeam splitting [2,3,6,7]. In fact, asymmetric wave transmission is possible due to additional transmission channels, to which the incident wave is either coupled or uncoupled, depending on the illumination side. Such a channel may be created by involving a higher diffraction order or polarization state, which differs from that of the incident wave. Generally speaking, all these regimes can be explained in terms of one-way mode conversion. Various performances of nonsymmetric structures for asymmetric wave transmission in acoustic, microwave, and optical ranges have been suggested to this time. They include photonic crystal gratings [3,4,[10][11][12], stacked hole arrays with one-side corrugations [13], metallic gratings with subwavelength slits [1,5,7,14], gratings that represent a one-fraction structure or a two-fraction structure obtained by either stacking two layers, one of which is corrugated, or embedding one single-fraction grating into another [2,15,16], and ultrathin structures based on subwavelength resonators that enable polarization manipulation [14,17]. Performance in terms of the asymmetry and band width can be especially strong when dispersion contributes to the resulting mechanism. Its potential has been demonstrated, for example, in photonic crystal gratings, where various regimes of wideband unidirectional transmission, i.e., those with transmission vanishing in a wide frequency range for one of the two opposite illumination directions have been obtained [3,10]. Moreover, inclining interfaces, as in photonic crystal prisms, allows one to further extend variety of the regimes that are achievable in the frame of the asymmetric wave transmission mechanism [6,18,19]. Unusual features relevant to the directional selectivity, which is inspired by the common effect of diffraction and dispersion, as in photonic crystal gratings, may include but are not restricted to one-way Rayleigh-Wood anomalies [11], unidirectional splitting [20], and reflection-free diodelike transmission [3]. On the other hand, it has been demonstrated in case of photonic crystal gratings that unidirectional transmission can be a purely diffraction effect, i.e., it can be achieved without the use of a peculiar dispersion [12,21]. Most of the known designs require structuring and/or assembling to obtain a proper configuration. Performances that can be easily fabricated and integrated with other components invoke materials whose properties could match the requirements to the asymmetric wave transmission without structuring. To this end, one should mention ultralow-ε ( ε ranging from 0 to 1) and ultralow-index materials (refractive index ranging from 0 to 1) [22][23][24]. For such a material, equifrequency dispersion contours have a circular shape and are narrower than in air. Thus, they enable unidirectional transmission. Nonsymmetric gratings based on Drude materials have been proposed [2,16]. However, the range of 0 1 ε < < corresponds for most of metals to petahertz frequencies. At lower frequencies, Drude dispersion can be achieved by utilizing a wire medium, i.e., the structuring is still required [22].
If the terahertz frequency range is targeted, polar dielectrics can be suggested as a reasonable alternative, because they may show 0 1 ε < < in a wide frequency range [25]. The existence of such a range is connected with the coupling of photons with transverse optical phonons that leads to the appearance of polaritons [25]. The ranges of (ultra) high ε can appear in the frame of this mechanism for the same materials but at lower frequencies. In the last two decades, various periodic structures containing polar dielectrics, e.g., photonic crystals, metamaterials, hole arrays, and distributed microcavities have extensively been studied [26][27][28][29][30][31][32][33]. In particular, the periodicity relevant multiple excitonic polaritons and polaritonic gaps have been demonstrated in one-dimensional photonic crystals [31,34,35]. The polaritonic gap and strong dependence of permittivity on frequency occur also in nonstructured polar dielectrics, where they have nothing to do with the periodicity. Both structured and nonstructured polar dielectrics can be useful for various applications related to light manipulation.
In this paper, we will study diffraction on nonsymmetric gratings, which are either partially or entirely made of polar dielectrics. Comparison of zero-and higher-order transmission and reflection at the corrugated-side illumination (forward case) and the noncorrugated-side illumination (backward case) will be carried out in order to confirm the existence and detect the basic features of unidirectional transmission, which is expected to appear in these gratings. Two types of the grating configurations will be studied, which differ from each other in that whether the resulting configuration is single-fraction, i.e., it contains a polar dielectric only, or it is two-fraction, i.e., one of the layers is made of a polar dielectric while the other one is made of a high-ε material to enhance diffraction. A simple approach to the comparison of various polar dielectrics is proposed that allows one selecting a material with a desired width of the ultralow-ε range and, hence, with that of the unidirectional transmission range. The main attention will be paid to the unidirectionality relevant regimes arising due to the common effect of diffraction at the interfaces and dispersion of the polar dielectric. Besides, operation regimes will be studied, which are connected with diffraction only. Calculations are carried out by using an integral equation technique [36].

Theoretical background
First, let us consider the general dispersion features of polar dielectrics in the connection with the conditions required for obtaining of unidirectional transmission. schematics of (b) two-layer and (c) single-layer nonsymmetric gratings containing polar dielectrics; for two-and single-layer gratings, D is the maximal thickness and L is grating period; for two-layer grating, h is thickness of the non-dispersive dielectric layer.
Owing to phonon-photon coupling and polariton excitation, polar dielectrics behave as a strongly dispersive medium whose permittivity depends on frequency as follows [26]: where 0 ε is the static dielectric constant, ε ∞ is the high-frequency limit of dielectric constant, and γ is the absorption relevant factor. One of the basic features of polar dielectrics is the existence of the polaritonic gap, which corresponds to the range of 0 P ε < , see Fig. 1 (2) To obtain dispersion relevant unidirectional transmission, the working frequency range should correspond to 0 1 P ε < < , provided that the surrounding medium is air. One can see in Fig. 1(a) that the corresponding frequency range is located on the right of the polaritonic gap, whereas Accordingly, the expected width of the range of unidirectional transmission is estimated for To compare different polar dielectrics from the point of view of their possible usage for achieving unidirectionality, we introduce the following dimensionless criterium: It gives the relative width of the frequency band, in which unidirectional transmission is expected to appear. From Eq. (5), it is seen that the stronger the difference between T ω and L ω , and the smaller value of ε ∞ , the larger ζ is. The values of T ω , L ω , 0 ε and ε ∞ can be found in the textbooks [25,37]. However, it is worth noting that these values may differ from one book to another. The values of / L T ω ω , ε ∞ and ζ have been compared for various polar dielectrics. Results of the comparison are presented in Table 1, based on the data from Table 3 in Ref [25]. For the sake of definiteness, we take here 2 A = , i.e., 0 0 . 5 P ε < < . One can see that there are materials, for which / 2 L T ω ω > or/and 2 ε ∞ < , and thus the values of ζ can be rather large.
for this material. Note that these values differ from those given for LiF and promise even a better performance as compared to that based on the data from [25].
In the next section, we consider two configurations, which are referred to as configuration A and configuration B. They are schematically shown in Figs. 1(b) and 1(c), respectively. Configuration A consists of two layers: the noncorrugated layer made of LiF and the corrugated layer made of a dielectric with 5 . These values and sinusoidal corrugations of the depth of D h − are kept throughout the paper. Configuration B differs from configuration A only in that it is entirely made of LiF. Both configurations are illuminated from either the corrugated or the noncorrugated side at angle θ , which is measured in the counter-clockwise direction from the normal to the illuminated interface.
To characterize asymmetry in transmission and reflection, we use

Basic features of transmission and reflection
In Fig. 2, forward transmittance and forward-to-backward transmittance contrast, 10 20 log / , are presented for the two configurations, which are expected to enable unidirectional transmission. A rather large value of θ has firstly been taken to ensure wideband unidirectionality. In the presented example, 0 T → > , 0 T ← ≈ and 70dB T C > at least at 14. 4 19 kL < < . This kL -range corresponds to 28.2THz 37.2THz f < < , whereas 24 m L = μ . In turn, the polaritonic gap extends from 1.5 kL π ≈ to 14 kL ≈ . Owing to the non-dispersive dielectric layer, the two-layer (two-fraction) grating is characterized by higher transmission than the one-layer (one-fraction) grating, but T C is higher for the latter. This is not a surprising feature, since the ultralow-ε material occupies a larger volume in this case.
In Fig. 2(a), the order 2 m = − appears first, so that respectively. Thus, the conventional order in appearance of higher orders that corresponds to Eq. (6) is broken here. In Fig. 2 where the orders 1 m = − and 3 m = − start contributing to T → . Since transmission is entirely suppressed within the considered kL -range at the noncorrugated-side illumination [see Fig. 2(c)], asymmetry in transmission occurs also in the actual th m -order frequency thresholds (wavelength cutoffs), i.e., they differ for the two utilized illumination directions. On the contrary to Fig. 2, there is no anomaly in threshold frequencies (cutoff wavelengths), starting from (up to) which m r → contributes to R → . At the same time, 0 0.9 R r ← ≈ > in the considered kL -range, for the both configurations A and B. Hence, reflections are not affected by the corrugations of the exit interface, i.e., the grating behaves in the reflection regime similarly to a noncorrugated ultralow-ε layer. As a result, strong asymmetry takes place in reflection and, in particular, in the frequency thresholds.
In Fig. 3, the dependencies of m t → , T → , and T C on kL are presented for the same parameter settings as in Fig. 2, but now for smaller values of θ . The effect of decrease of θ is well seen from the comparison of Figs. 2 and 3. In particular, some narrowing of the unidirectional transmission range, weaker difference between configurations A and B in terms of max T → , and increase of T → for configuration B should be noticed. For the values of θ used in Fig. 3, unidirectionality is still a wideband effect. For a larger part of the unidirectional transmission range, higher contrasts are observed for configuration B. For instance, this takes place at 16.2 kL = in Fig. 3(c), where Hence, directional selectivity can occur in a wide range of θ variation that enables tunability. Further decrease of θ results in that the order 0 m = is coupled at smaller values of kL than in Fig. 3, in accordance with the general theory of the diffraction inspired unidirectional transmission [38]. This leads to that the unidirectionality disappears, provided that the coupling for zero order is not vanishingly weak.

Decreasing grating period and thickness
Now let us consider the basic effects arising due to decrease of / T L c ω at fixed other parameters. In fact, this means that we take a smaller value of L , and − since / D L is assumed to be fixed − also a smaller value of D . Shift of the unidirectional transmission range towards smaller values of kL , significant single-beam transmission due to lower nonzero diffraction orders, and change of the sequence in appearance of higher orders are among the expected effects. First, we take   . Hence, wideband unidirectionality with a rather high T → can also be obtained for configuration B, i.e., when using a single-layer grating, at least at intermediate θ .  Fig. 2(c), T C is now higher for configuration A, over a larger part of the unidirectional transmission range. Asymmetry in reflection is also observed in this case. It manifests itself in the similar way as described above, e.g., within a wide frequency range. For the sake of completeness, we consider diffraction at smaller values of θ and the same / T L c ω as in Fig. 6 Fig. 7(a), at 6.5 7.5 kL < < in Fig. 7(b), and at 6.5 8.5 kL < < in Fig. 7(c). To further increase max T → , additional optimization is required.  In Fig. 8, m r → , m r ← , R → and R ← vs kL are shown for the same case as in Fig. 7(b) and for the corresponding case of noncorrugated-side illumination. Besides, 0 t and T ← are presented in Fig. 8(b). One can see that the contribution of m r → to R → strongly differs from that of m r ← to R ← . In particular, 0 R r ← ≈ over a wide kL -range, while all of the orders 0, 1, 2, 3 m = − − − contribute to R → . At 12 kL > , there are no significant reflections. At the same time, most part of the incident-wave energy can be converted into a reflected higher-order beam, as occurs in Fig. 8(a)  Unidirectional transmission is observed in this case with It is worth noting that the polaritonic gap is located here at 0. 4 3.78 kL π < < where 6.5 m L ≈ μ . For the comparison purposes, Fig. 10  9.4THz f < < . As expected, this range is narrower than for LiF, because of smaller / L T ω ω and larger ε ∞ . In spite of this, NaCl can be a reasonable choice when a lower frequency range is targeted.

Dispersion irrelevant asymmetry in transmission
Up to now, we considered asymmetric wave transmission connected with the common effect of dispersion and diffraction at inclined incidence. As known from the earlier works, asymmetry due to the common effect can be quite strong also at normal incidence [3,20]. In particular, it might occur in the gratings based on photonic crystals or materials with hyperbolic type dispersion. However, this regime cannot be obtained in the non-structured configurations based on polar dielectrics, for which asymmetric wave transmission is associated with ultralow ε and, thus, with non-hyperbolic type of dispersion. On the other hand, recent studies show that asymmetry in transmission can be strongly pronounced, even if dispersion is not hyperbolic, i.e., it does not force zero order to be uncoupled [12,21]. Thus, it can be a fully diffraction effect. Furthermore, in this case, stronger transmission often appears at the noncorrugated-side illumination, on the contrary to the above-considered examples. Similar regimes have already been studied in the nonsymmetric dielectric photonic crystal gratings at 0 θ = [12] and 0 θ > [21]. Dependencies of m t → , m t ← , T → , T ← , and T C on kL are presented in Fig. 11, for the same configurations as in Figs. 2 and 3 but for 0 θ = . One can see that a well pronounced contrast can be achieved within several narrow frequency ranges for configuration A, whereas asymmetry in transmission is weakly pronounced for the corresponding configuration B. For example, this situation occurs at 16.5 kL = , i.e., for one of the dips of 20dB Fig. 11(c), where 0.4 T ← = and 1 0.
It is expected that this regime of asymmetric wave transmission can be obtained for various polar dielectrics, including those with a narrow range of ultralow ε , see Table 1. Indeed, if it is not necessary that zero order is formally uncoupled, the requirement of a wide range of 0 1/ P A ε < < , 1 A > can be cancelled. Thus, more polar dielectrics can be considered as viable candidates to design gratings that are similar to the configurations A and B. In Fig. 12, an example of the dependencies of m t → , m t ← , T → , T ← , and T C on kL is presented for the grating that differs from configuration B only in the used polar dielectric. Here, it is GaAs, which has 8.12THz  At the chosen value of / T L c ω , the polaritonic gap is located at 2 6 . 7 7 kL π < < . Since / 1.08 L T ω ω ≈ and ε ∞ is rather large, wideband unidirectional transmission due to the common effect of dispersion and diffraction is not expected to appear. However, as seen in Fig. 12, asymmetry in transmission can be quite strong within a narrow range near 9.2 kL = , where 1  ω ω nor small ε ∞ is necessary for obtaining of a strongly pronounced asymmetry.

Conclusions
Merging the recently developed asymmetric wave transmission theory with the peculiar dispersion of polar dielectrics creates a proper platform to realize wideband directional selectivity at terahertz frequencies. In spite of that nonreciprocal operation regimes cannot be achieved in this framework because additional transmission channels (higher orders in our case) are needed to accept the re-directed energy, it looks very promising for manipulation of terahertz waves. Based on the obtained results, the main features of asymmetry arising in transmission and reflection have been demonstrated for the nonsymmetric gratings, which contain polar dielectrics working in the ultralow-ε regime inspired by phonon-photon coupling. The best pronounced asymmetry results from the common effect of asymmetric coupling and diffractions at the two interfaces, on the one hand, and the ultralow-ε relevant dispersion features, on the other hand. It can be obtained at large and intermediate angles of incidence. Adding a high-ε corrugated layer is required for obtaining of significant transmission at large angles, while both single-fraction and two-fraction nonsymmetric grating can be used at intermediate angles. However, transmission can be quite strong for one of the two incidence directions and can vanish for the opposite direction, even if dispersion does not force zero order, which is responsible for the symmetric transmission component, to be uncoupled. This regime is especially important for small angles, at which the common effect based mechanism is not possible. It can be achieved both with and without the use of the high-ε corrugated layer, depending on the chosen material and geometrical parameters.
Hence, co-existence of dispersion relevant and irrelevant unidirectional regimes is not a unique feature of more complex nonsymmetric structures, e.g., photonic crystal gratings. The main regimes of such structured configurations can be replicated in simpler configurations but optimization can be required in order to achieve high transmittance for one of the two directions. Although only two types of simple nonsymmetric gratings have been considered here, more advanced structures could be suggested that exploit the same mechanism. Adjustment of thickness-to-period ratio allow us controlling contribution of desired and unwanted diffraction orders within the unidirectional range.