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Abstract
Ultrathin titanium nitride (TiN) is a novel material flatform for constructing active metasurfaces in the near-infrared region (NIR). In this paper, we numerically achieved the dual-channel dynamic modulation for polarization-dependent absorption by adjusting incident angles and gating ultrathin TiN films. To enhance the active modulation effect of ultrathin TiN film, the “卍” shaped gold resonator is integrated with ultrathin TiN film for constructing a metasurface Salisbury screen (MSS). By electrical reducing TiN carrier concentration by 12%, the absorption peak of TE wave blueshifts 100 nm. The magnitude and the frequency of the polarization-dependent absorption are independently tuned by changing incident angles or the carrier concentration in TiN film. The ultrathin TiN film based polarization-dependent metasurface enabling versatile dual-channel modulation has great potentials in the applications of optical communication, information encryption, real-time imaging etc.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Metasurfaces, as two-dimensional metamaterials, could arbitrarily manipulate electromagnetic (EM) waves over a wide spectral range and shows many exotic properties such as super-resolution imaging [1], anomalous refraction [2], vortex beam generation [3], hologram imaging [4] and so on. However, the fixed operating wavebands and single EM manipulation function of these passive metasurfaces greatly limit the applications in sensing detection [5], dynamic polarization control [6], optical encryption [7], etc. Thus, it is highly desirable to realize active metasurfaces, whose functionalities and operating wavelengths could be dynamically tuned by applying external stimuli. At present, the methods for dynamic EM manipulation mainly includes mechanical tuning of stretchable substrate [8] and microfluid [9,10], incorporating electronic elements (PIN, MEMS diodes) [11–13], thermal tuning of phase change materials (GST, VO2) [14–16], electrical tuning of 2D materials (graphene, MoS2) [17,18] optical tuning of semiconductor [19] and so on [20–22]. However, most of these tunable materials work in the longer wavelength regions including mid-infrared and THz wave bands.
To address the limitations of response wavebands, much effort has been made to find suitable tuning methods in the visible and near-infrared (NIR) regions. With the development of nanofabrication methods, the sizes of metal components in plasmonic devices have been shrinking, now approaching only a few monolayers in thickness. In 2017, Deesha Shah et al. fabricated epitaxial, ultrathin TiN films with thicknesses down to only 1-2 nm by using DC reactive magnetron sputtering [23]. The optical properties of atomically thick TiN films can be tailored by electrical [24] and thermal [25] stimuli. Except for the excellent electrical control capability, TiN films are also compatible with oxide semiconductor (CMOS) [26] and possess remarkably durable metallic properties [27,28] and a progressive red shift of plasmonic energy, which can enhance the control over plasmonic properties [29]. These advantages make ultrathin TiN film an attractive material platform for realizing dynamic EM manipulation in the visible and NIR. Currently, there are some reports about tunable metasurfaces using ultrathin TiN films. For example, Huan Jiang et al. proposed an Au strip-TiN film hybrid metasurface which can achieve a deep 337° phase modulation at 1550 nm by reducing the TiN carrier density [30].
In addition, dynamic polarization control plays an important role in real-time biosensing, dynamic display, and numerous other areas. As we all know, the symmetry broken nanostructures will exhibit intrinsic chirality [31], which could be used for polarization control. Also, the method of oblique incidence could break the symmetry of structure system and gain extrinsic chirality [32]. The essence of polarization dependence is that the symmetry of the original structure is broken due to the change of the incident angles, thereby affecting specific optical responses. At present, many metasurfaces has achieved excellent tuning ability on extrinsic chirality [33,34]. In 2020, Yijia Huang et al. achieve giant LD and CD effects simultaneously on a metasurface with simple metal-insulator-metal geometry by changing the direction of incident EM waves [33].
In this paper, we proposed a dual-channel dynamically modulated polarization-dependent metasureface in NIR by integrating “卍” shaped gold resonator with ultrathin TiN films. Under oblique incidence, the metasureface could absorb almost all the incident TM wave but reflect incident TE wave. By changing the carrier concentration in ultrathin TiN film or incident angles, the magnitude and the resonance wavelength of polarization-dependent absorption could be dynamically switched. The dual-channel active modulation for polarization-dependent absorption enhances the modulation flexibility of the active metadevices in NIR, which will promote the development and application of photonic devices for dynamic polarization control.
2. Design and simulations
To gain the polarization selectivity, the symmetry of the structure system is broken by oblique incidence [32]. For dynamic polarization-dependent absorption, the “卍” shaped gold resonator and ultrathin TiN film hybrid metasurface (GTHM) is presented in Fig. 1(a). In detail, to enhance the active modulation effect of ultrathin TiN film, the “卍” shaped gold resonator is integrated with ultrathin TiN film for constructing a metasurface Salisbury screen (MSS). The 50 nm-thick “卍” shaped gold resonators and ultrathin TiN film are placed on Au substrate, which is separated by 75 nm-thick dielectric spacer. As shown in Fig. 1(b), the widths and the lengths for the arms of “卍” shaped gold resonators are ${w_1} = 96$nm, ${w_2} = 115$ nm ${l_1} = 360$ nm ${l_2} = 233$nm. The period is $p = 440$nm. To electrically tune the carrier concentration of ultrathin TiN films, ion gel, as the gate dielectric layer, is placed on the top of “卍” shaped resonators to form a field effect transistor (FET) as shown in Fig. 1(c).
Fig. 1. The scheme of the “卍” shaped gold resonator and ultrathin TiN film hybrid metasurface (GTHM). (a) The “卍” shaped periodic array. The (b) top and (b) side view of the unit structure.
The polarization reflection and absorption spectra are numerically calculated by using the frequency domain finite element method (FEM) preformed in CST Microwave Studio. Tetrahedral meshes are adopted in our simulation. The boundary conditions are unit cell in ± x (±y) directions and open (add space) and ± z direction, respectively. The dispersive parameters of gold are derived from Drude model and the conductivity of gold is set as ${\sigma _{Au}} = \textrm{ }4.56\textrm{ } \times \textrm{ }{10^7}\textrm{S/m}$ [35]. The permittivity of dielectric spacer (ZnO) is set as 4. The optical properties of ultrathin TiN film are retrieved from experimental data in Ref. [30]. The optical properties were measured using a variable angle spectroscopic ellipsometry at angles of 50° and 70° at the wavebands from 400 to 2000nm. A Drude–Lorentz model consisting of one Drude oscillator and one Lorentz oscillator was used to fit the measurements:
3. Results and discussions
3.2. Polarization-dependent absorption
By breaking the symmetry of the structure system by oblique incidence, the “卍” shaped gold resonators and TiN film hybrid metasurface (GTHM) shows obvious linear polarization selectivity. Under incident angle of $\theta = {70^\mathrm{^\circ }}$, polarization reflection coefficients of the meatsruface with unperturbed TiN carrier concentration are numerically calculated in Fig. 2(a). The curves of cross-polarized reflection coefficients ${r_{xy}}$ and ${r_{yx}}$ are overlapped and the values are near zero in the whole waveband, but the two co-polarized reflection coefficients ${r_{yy}}$ and ${r_{xx}}$ are very different, leading to obvious polarization-dependent reflection. The polarization reflection are obtained by polarization reflection coefficients as following:
Fig. 2. Under incident angle $\theta = {70^\mathrm{^\circ }}$, the polarization spectra of GTHM with unperturbed TiN. (a) polarization reflection coefficients, (b) polarization reflection, (c) polarization absorption and (d) linear dichroism spectra.
So the reflection spectra of TE (Ry) and TM (Rx) waves are also different as presented in Fig. 2(b). Furtherly, since the gold substrate is thick enough to prevent the penetration of incident light, the polarization absorptions ${A_x}$ and ${A_y}$ could be simplified into
The obvious polarization selectivity for absorption is realized as the spectra shown in Fig. 2(c). The most TE waves will be absorbed by GTHM at 1862nm (${A_y} \approx 1$), while the TM waves will be dominantly reflected. To evaluate the performance of linear polarization selectivity, linear dichroism (LD) defined as the absorption difference of TE and TM waves ($\Delta = {A_y} - {A_x}$) is also plotted in Fig. 2(d). The LD value fluctuates around 0.6, and the maximum LD value is about 0.75 at 1024 nm.
3.2 Dynamic modulation by changing incident angles
Polarization selectivity is closely related to the geometric symmetry of structure system. Here, the polarization-dependent absorption could be dynamically modulated by changing incident angles. As the polarization absorption spectra shown in Fig. 3(a), by changing the incident angles from normal to grazing incidence, the absorption of TE wave gradually decreases especially near 1802nm. In detail, with the normal incidence of TE wave ($\theta = {0^\mathrm{^\circ }}$), absorption is about 0.9 at 1802nm. When the incident angle is increasing to $\theta = {85^\mathrm{^\circ }}$, the absorption is reduced to only 0.14 at the same wavelength. By comparing Fig. 3(a) with Fig. 3(b), the absorption spectra of TE and TM waves are the same under normal incidence, but the absorption spectra of the two linear polarized light are totally different under oblique incidence.
Fig. 3. Under incident angles varied from ${0^\mathrm{^\circ }}$ to ${85^\mathrm{^\circ }}$, polarization-dependent absorption spectra of (a) TE and (b) TM waves.
Especially, the variation trend of TM absorption spectra caused by various incident angles is different from that of TE. With the increasing of incident angles from ${0^\mathrm{^\circ }}$ to ${85^\mathrm{^\circ }}$, the peak absorption of TM wave first increases and then decreases. Meanwhile, the absorption peak of TM wave blue shifts obviously as incident angle increases in Fig. 3(b). In detail, with the incident angle increasing from 0° to 70° the absorption of TM wave at 1871nm is increased to 1 from 0.88 and the peak wavelength almost consistent. Interestingly, when the incident angle is switched from ${70^\mathrm{^\circ }}$ to ${85^\mathrm{^\circ }}$ the strength of the absorption peak is decreased slightly and the peak wavelength blue shifts to 1447 nm from 1821nm. The different variation trends of TE and TM waves lead to giant polarization selectivity. The bigger incident angles, the greater polarization selectivity. Under the incident angle of ${85^\mathrm{^\circ }}$, the LD value, defined as the absorption difference between TE and TM waves, reaches 0.92 at 1464 nm. In fact, with the change of incident angles, the different variation trends of the absorption between TE and TM waves originates from different resonance modes at peak wavelengths.
To explore the mechanism of different variation trend between the absorption of TE and TM waves, the electric field distributions with varying incident angles are plotted at 1802nm. In Fig. 4(a), under normal incidence of TE wave, the electric field excited is mainly enhanced in the slit between unit structures. Due to the construction of Salisbury screen structure [37], the electric field is mainly enhanced in the layer of TiN film. So, the resonance wavelength could be efferently tuned by electrically controlling the carrier concentration of ultrathin TiN film (discuss later). As the vectorial fields (black arrows) shown in y-z plane (Fig. 4(d)), the induced charges at the corners of the adjacent gold resonators represents the excitation of intercellular electric resonance mode (inter-mode). The field enhancement between unit structures via inter-mode leads to the high absorption of TM wave. With the increasing of incident angles from ${0^\mathrm{^\circ }}$ to ${70^\mathrm{^\circ }}$ and then ${85^\mathrm{^\circ }}$, the strength of inter-mode is much weakened as shown in Figs. 4(b), (c), (e), (f). Thus, the absorption of TE wave is gradually weakened as incident angles increase from ${0^\mathrm{^\circ }}$ to ${85^\mathrm{^\circ }}$, leading to the strong dependence of TE absorption strength on incident angles.
Fig. 4. The electric field distributions excited by TE wave at 1802nm. The fields in (a-c) x-y and (d-f) y-z planes under the incident angles of (a), (d) $\theta = {0^\mathrm{^\circ }}$ (b), (e) $\theta = {70^\mathrm{^\circ }}$ and (c), (f) $\theta = {85^\mathrm{^\circ }}$
Under normal incidence, the electric field distribution of TM wave in Figs. 5(a), (d) is similar with that of TE wave in Figs. 4(a), (d). The electric fields are concentrated on the slit of adjacent unit structures but pointing different directions with that of TE wave in Figs. 4(a), (d), which forms intercellular electric resonance mode (inter-mode) with vertical electric moment in x direction. In summary, the absorption spectra of TE and TM waves are overlapped under normal incidence, which further indicates that the resonance mode excited by TM wave is also inter-mode. Not only the strength but also the resonance wavelength of TM absorption spectra could be tuned by varying incident angles (Fig. 3(b)). This is because of the appearance of a new-type resonance on the top of “卍” shaped gold resonators under oblique incidence. However, the resonance mode excited by TM wave under oblique incidence is totally different with that of TE wave.
Fig. 5. The electric field distributions excited by TM waves. The fields in (a-c) x-y and (d-f) y-z planes under the incident angles of (a), (d) $\theta = {0^\mathrm{^\circ }}$ (b), (e) $\theta = {70^\mathrm{^\circ }}$ (the green circle represents inter-mode, the red circle represents intra-mode) and (c), (f) $\theta = {85^\mathrm{^\circ }}$. (g) Schematic diagram of oblique incidence decomposition. The resonance mode under oblique incidence is the superposition of the ones of normal (inter-mode) and grazing (intra-mode) incidences.
With the change of incident angles, the electric field of TM wave between unit cell gradually be weakened, and the enhanced electric field are transferred to Au resonators from the layer of TiN film as shown in Figs. 5(b), (c), (e), (f). When the incident angle becomes grazing incidence ($\theta = {85^\mathrm{^\circ }}$), there is no longer electric dipole resonance between adjacent unit structures, and the induced electric dipole resonance by the charges gathered on the resonator surface becomes dominant, which means a new resonance mode (defined as intracellular electric resonance mode (intra-mode)) is generated under grazing incidence. It is worth mentioning that during the change of incidence angles, the variation of absorption line is continuous without obvious abrupt change, and the coexistence of intercellular electric resonance mode (inter-mode) and intracellular electric resonance mode (intra-mode) can be clearly observed from the vector electric field distribution diagram at oblique incidence ($\theta = {70^\mathrm{^\circ }}$) in Fig. 5(e). Therefore, the resonance mode at oblique incidence is the superposition of the both modes at normal and grazing incidences, as shown in Fig. 5(g).
Furthermore, we qualitatively analyze the causes for generation of intercellular electric resonance mode (inter-mode) and its transformation to intracellular electric resonance mode (intra-mode) based on the coupled mode theory (CMT), namely the generation and disappearance of inter unit cells coupling. According to CMT, two adjacent unit cells could be equivalent to two waveguides, where each waveguide has an electromagnetic wave mode, which we refer to as the waveguide mode. It should be noted that the electric field distributions of two adjacent unit structures are the same at normal incidence, which means two equivalent waveguides and waveguide modes are also identical. However, as shown in Fig. 5, they are all different in the case of oblique incidence, because oblique incidence can cause phase difference. The phase difference between two adjacent unit structures in this structure is 90°. These two waveguide modes could be expressed by ${E_1}(x,y){e^{i(\omega t - {\beta _1}z)}}$ and ${E_2}(x,y){e^{i(\omega t - {\beta _2}z)}}$. The coupling state between two waveguide modes indicates whether two adjacent unit cell can generate intercellular electric resonance mode, and it can be inferred from the following equations. Ignoring interference with any other mode, the coupling equations of the two waveguide modes can be described as follows:
In addition, it is worth mentioning that TE wave also follows CMT, and its coupling efficiency also decreases with increasing incident angle. However, its polarization direction is different from TM wave, the equivalent waveguide is also different from TM wave, causing the coupling efficiency has not decreased to the minimum, so that there is still weak coupling between unit cells during grazing incidence. Thus, there is no obvious new electric resonance mode generated during TE wave.
3.3 Active modulation by gating ultrathin TiN films
The polarization-dependent absorption could be tuned not only by incident angles but also by gating ultrathin TiN films. The polarization-dependent absorption spectra of the metsaurface with different TiN carrier concentration are given out. Due to the polarization-dependent excitation of electric dipole, ultrathin TiN film shows different tunability between the absorption of TE and TM waves. As shown in Fig. 6(a), we use the incident angle $\theta = {70^\textrm{o}}$ as an example, the absorption of TE wave is slightly weakened and has an obvious blue shift (100 nm) while TiN carrier concentration decreases. Due to the construction of Salisbury screen structure [37], the electric field is mainly enhanced in the layer of TiN film. So the resonance wavelength could be efferently tuned by electrically controlling the carrier concentration of ultrathin TiN film. In contrast, there is no similar tunability on the absorption spectrum of TM wave by gating ultrathin TiN film. As shown in Fig. 6(b), the absorption strength and the spectral position are kept unchanged with the varying carrier concentration of TiN.
Fig. 6. Polarization-dependent blue shift. The polarization-dependent absorption spectra of (a) TE and (b) TM waves by gating ultrathin TiN films under the incident angle of $\theta = {70^\mathrm{^\circ }}$.
The polarization-dependent tunability of ultrathin TiN originates from different resonances excited by TE and TM waves. As mentioned above, when the incident wave is oblique $\theta = {70^\mathrm{^\circ }}$, the electric field of TE wave is mostly concentrated inside the slit of adjacent structure, and its induced electric dipole interacts with the TiN film. Accordingly, the TiN film can overwhelm the metal in the absorption competition and absorb most of the trapped light. But for TM wave, most of charge is transferred to the surface of gold resonator, which increases the metal loss and weakens the plasma characteristics of TiN films. This makes the absorption of TM significantly greater than that of TE and unable to be effectively modulated by the TiN carrier concentration like TE wave.
4. Conclusion
In this work, we have numerically demonstrated a dual-channel active modulation for polarization-dependent absorption in NIR by gating ultrathin TiN films and adjusting incident angles. At normal incidence, due to the construction of Salisbury screen, coupling occurs between adjacent unit structures and the incident light are mainly absorbed by the layer of TiN film. As the incidence angle changes from normal incidence to grazing incidence, the resonance mode of TM wave gradually converts to electric dipole resonance, which are gathered on the surface of metal resonators, and the absorption peak also experiences a blue shift from 1821 to 1447 nm. Furthermore, the 100 nm-blue shift of absorption peak for TE wave also can be achieved through reducing TiN carrier concentration by 12%. This dual-channel active metasurface, which can tune TE and TM wave respectively, further improves the accuracy and flexibility of modulation, thus providing a new choice for the design of multi-function metadevices.
Funding
National Natural Science Foundation of China (12004080); Funding by Science and Technology Projects in Guangzhou (202201010540).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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