Tunable Polarization-Selective Absorption by Gating Ultrathin TiN Films in the Near-Infrared Region

Abstract

Ultrathin titanium nitride (TiN) is a novel material platform for constructing active metasurfaces in the near-infrared region (NIR). Here, we realized tunable polarization-selective absorption by gating ultrathin TiN in an Ultrathin TiN Grating Metasurface (UTGM) and a gold resonator/TiN film Hybrid Metasurface (GTHM), respectively. The TM wave absorption (0.96) was much larger than that of the TE wave in the UTGM. When the carrier density decreased by 12%, the near-perfect TM absorption peak blue-shifted by 0.3 μm. Similarly, the linear dichroism (0.96) peak in GTHM blue-shifted by 0.12 μm when gating ultrathin TiN film. Active metasurfaces with tunable polarization-selective absorption have huge potential in dynamic integrated electro-optic devices in NIR.

1. Introduction

In recent years, metasurfaces have achieved unique properties such as beam manipulation [1], phase modulation [2], anomalous reflection [3], and optical encryption [4]. However, most conventional metasurfaces constructed with noble metals (Au [5] and Ag [6]) are static, and their EM characteristics are fixed once fabricated. Meanwhile, dynamic light manipulation is in high demand, especially in applications for dynamic polarization conversion, real-time bio-sensing, dynamic imaging, and numerous other areas. Recently, many studies have achieved dynamic electromagnetic (EM) control by integrating tuning materials or structures into static metasurfaces [7,8]. The dominant tuning materials include phase-change materials (Ge₂Sb₂Te₅ [9] and VO₂ [10]), two-dimensional materials (Graphene [11]), transparent conducting oxides (ITO [12]), semiconductor materials (silicon [13]), etc.

In spite of gaining active dynamic EM control, these active metasurfaces have their deficiencies, originating from corresponding tuning constituents. In general, most of these active metasurfaces work in the mid-infrared and THz regions. Only a few crystal liquid [14], ITO [12] and acousto-optic modulation-based active metasurfaces [15] have been reported in the visible and NIR, and face the challenges of size limitation and energy inefficiency. Because the visible and NIR wavelengths are of utmost importance for sensing and optical signal processing technologies, the development of active metasurfaces is still important at these wavelengths.

Recently, ultrathin TiN films have offered an alternative material platform for realizing dynamic EM manipulation in the NIR [16,17,18,19]. In 2018, Shah et al. grew 2 nm-thick TiN films and investigated their optical properties [20]. By using a conventional electric gating technology with a nanosecond switching time, the carrier density in ultrathin TiN films could be efficiently modulated by 12%, which is a realistic experiment-based number [21]. TiN also features high durability and a high melting point; it is also compatible with existing semiconductor fabrication processes [16]. The high-quality growth and tunable optical response make ultrathin TiN an attractive material for constructing active metasurfaces in the visible and NIR wavelengths. We have already explored dynamic phase modulation with the gated ultrathin TiN films [22].

In this letter, we achieve tunable polarization-selective absorption by gating a TiN film in two representative nanostructure designs. The Ultrathin TiN Grating Metasurface (UTGM) and gold resonator/TiN film Hybrid Metasurface (GTHM) represent these two different designs, which are proposed to realize tunable polarization-selective absorption by gating ultrathin TiN. The first design (UTGM) is based on etching a specific pattern in an ultrathin TiN film and obtaining an optical response from the gated TiN pattern. The other design (GTHM) combines a set of conventional Au resonators with an ultrathin TiN film that controls the Au resonators. The proposed designs offer novel approaches towards the realization of reconfigurable metasurfaces and active meta-devices in NIR.

2. Design and Simulations

Both designs for tunable polarization-selective absorption are analyzed numerically. The polarization spectra are calculated using the frequency domain finite element method (FEM) with the CST Microwave Studio. A tetrahedral mesh is adopted in our simulation. We used the periodic boundary conditions (unit cell) in ±x (±y) directions and perfectly matched layers (add space) in ±z directions, respectively. The complex permittivity of Au was experimentally obtained and fitted by a Drude model with two Critical Points (D2CP) [23,24]. The refractive index of Al₂O₃ is 1.76. The permittivity of the ultrathin TiN film is modeled with a Drude–Lorentz model, which consists of one Drude and one Lorentz oscillator retrieved from the experimental data [25].

$$\epsilon (\omega )={\epsilon }_{\infty }-{\displaystyle \frac{{\omega }_p^2}{{\omega }^2+i{\Gamma }_D\omega }}+{\displaystyle \frac{f_L{\omega }_L^2}{{\omega }_L^2-{\omega }^2-i{\Gamma }_L\omega }}$$

(1)

where ${\epsilon }_{\infty }$ is the permittivity at high frequency, ${\omega }_{\mathrm{L}}$ is the resonant frequency of the Lorentz oscillator, $f_L$ is the strength of the Lorentz oscillator, ${\Gamma }_D$ and ${\Gamma }_L$ are the damping rates, and ${\omega }_p$ is the plasma frequency, which depends on the change in carrier density, as follows:

$${\omega }_p^2={\displaystyle \frac{N_0\left(1+\Delta N/N_0\right)e^2}{m^{*}{\epsilon }_0}}$$

(2)

where $N_0$ is the density of the unperturbed free electron, $\Delta N/N_0$ is the relative change in carrier density, and m* and ${\epsilon }_0$ represent the electron effective mass and the permittivity of free space, respectively. The Drude term captures the optical response of free carriers, while the Lorentz term accounts for inter-band transitions.

2.1. The UTGM Design

First, we explored the UTGM design, which could be obtained by etching an ultrathin TiN film; see Figure 1a. The proposed 2 nm-thick TiN grating structure was placed on the Au substrate separated by a 210 nm-thick Al₂O₃ spacer. The carrier density of ultrathin TiN was electrically tuned in the Ionic-Gated Transistor (IGT) structure, as shown in Figure 1b, where the ion gel emulates an oxide layer [26]. The geometric parameters of TiN grating, such as period and width, will influence the polarization-selective absorption to different degrees. After careful optimization by sweeping geometric parameters, the period of the TiN grating array isset as p = 81 nm. The width and thickness of the TiN grating are 51 nm and 2 nm, respectively.

Assuming that incident waves polarized along the x- and y-axis are transverse magnetic (TM) and electric (TE) waves, the incident and reflected fields can be related via Jones calculus [27]:

$$\left(\begin{array}{l}{E}_r^x\\ E_r^y\end{array}\right)=\left(\begin{array}{l}{r}_{xx} r_{xy}\\ r_{yx} r_{yy}\end{array}\right)\left(\begin{array}{l}{E}_i^x\\ E_i^y\end{array}\right)=R_{lin}\left(\begin{array}{l}{E}_i^x\\ E_i^y\end{array}\right)$$

(3)

where $E_i^x,\mathrm{and}{E}_r^y$ are the incident and reflected electric fields polarized in the x direction, respectively. Similar notations with a superscript of y represent the field polarized in the y direction. The reflection matrix R_lin of linear polarization is expressed as

$$R_{lin}=\left(\begin{array}{l}{r}_{xx} r_{xy}\\ r_{yx} r_{yy}\end{array}\right)$$

(4)

where the matrix elements r_xx(yy) and r_xy(yx) represent co- and cross-polarization reflection coefficients, respectively. As shown in Figure 2a, the cross-polarization reflection coefficients of UTGM completely overlap, but the co-polarization reflection coefficients are different. Thus, while there is a dip of r_xx at 1.8 μm, r_yy remains at the high value above 0.8. The polarization reflection spectra are calculated according to the relationship with reflection coefficients $R_y={\left|r_{xy}\right|}^2+{\left|r_{yy}\right|}^2$ and $R_x={\left|r_{yx}\right|}^2+{\left|r_{xx}\right|}^2$. R_y and R_x match the behavior of r_yy and r_xx (Figure 2b), while R_y remains above 0.62; there is a minimum of R_x at 1.8 μm.

Furthermore, the polarization-selective absorption can be examined ($A=1-R-T$). Since the gold substrate blocks the transmission of incident waves, the absorption is simplified as $A=1-R$. As shown in Figure 2c,d, before gating the ultrathin TiN film, the shapes of TE and TM absorption spectra are similar, but the absorption maxima are different. The maximum absorption of the TM wave is 0.96 at 1.8 μm (Figure 2c). However, the maximum absorption of the TE wave (0.36) at 1.6 μm (Figure 2d) is much lower than that of the TM wave. The absorption difference between the TE and TM waves, represented by linear dichroism (LD), indicates the polarization-selective absorption in the UTGM design. LD originates from the selective excitation of electric dipole resonance by TE and TM waves [28].

By gating ultrathin TiN, the response wavelength of the polarization-selective absorption could be dynamically tuned. As TiN carrier density decreases by 12% from the unperturbed value, the peak wavelength of TM absorption blue-shifts 300 nm from 1.8 to 1.5 μm in Figure 2c. As the TiN carrier density decreases, the plasma frequency ω_p of ultrathin TiN decreases. Therefore, the reduced metallicity of ultrathin TiN leads to the blue-shift of TM absorption. Similarly, the resonant wavelength of TE wave could be dynamically tuned in Figure 2d. With the carrier density decreasing by 12%, the peak wavelength blue-shifts by 100 nm. The different tuning capabilities between TM and TE absorption enable dynamic control of the response wavelength.

To show the mechanism of the polarization-selective absorption in the UTGM design. Figure 3a,b present the E-field distributions excited by the TM wave at 1.8 μm. The arrows indicate the direction of electric field vectors. The electric field is mainly enhanced in the gap between adjacent TiN grating strips, which indicates that the localized surface plasmon resonance is excited. By contrast, with the incidence of the TE wave, there is no obvious resonance excited, as in the electric field shown in Figure 3c,d. The electric field of the TE wave in Figure 3c,d is much weaker than that of the TM wave in Figure 3a,b. The weaker electric field explains the lower absorption of the TE wave in Figure 3d.

2.2. The GTHM Design

The ultrathin TiN film can also be integrated with gold resonators. Here, an array of the C-shape gold resonator/TiN film hybrid metasurface is proposed in Figure 4a for tunable polarization-selective absorption. To achieve polarization selectivity, we used the C₄ symmetry broken C-shaped resonators. In this design, the tunability effect of the ultrathin TiN film is enhanced by integrating the array of C-shaped gold resonators with ultrathin TiN film and constructing a metasurface Salisbury screen (MSS); see Figure 4b. As shown in Figure 4c, in a unit cell, a 40 nm-thick C-shaped gold resonator integrated with a 2 nm-thick ultrathin TiN film was placed on a Au substrate separated by a 200 nm thick Al₂O₃.

To estimate the tuning capability of ultrathin TiN, the polarization spectra of the GTHM design with various carrier densities are calculated. The co-polarization reflection coefficients have a huge difference, and the cross-polarization ones (r_xy and r_yx) overlap completely in Figure 5a. Specifically, the co-polarization reflection coefficient of r_xx maintains a very high value above 0.98 in the whole waveband, but there is a dip of r_yy with a value of 0.1 at 1.8 μm. Furthermore, the polarization absorption spectra are calculated in Figure 5b,c. Before gating ultrathin TiN film, the absorption curves of TE and TM waves show totally different shapes by comparing Figure 5b with Figure 5c. The absorption of TM maintains a low, near-zero value, while the maximum absorption approaches almost 1 at 1.8 μm.

We estimated the absorption difference (LD) between the TE and TM waves, expressed as $LD=A_y-A_x$ [28]; see Figure 5d. Before gating ultrathin TiN, the maximum LD was 0.95 at 1.8 μm, and LD remained above 0.7 between 1.6 and 2.2 μm. The large LD indicates the giant polarization-selective absorption in the GTHM design. To study the tuning capability of polarization-selective absorption, the absorption of TE and TM waves and LD with different TiN carrier densities was calculated (Figure 5b–d). As TiN carrier density decreases by 12%, there is no change in TM absorption in Figure 5b. The absorption peak of the TE wave blue-shifts by 120 nm after decreasing the carrier density by 12% vs. the unperturbed value. The difference in tuning of TE vs. TM absorption contributes to the dynamic control of LD. This difference results in a 120 nm blue-shift of LD by decreasing TiN carrier density by 12% in Figure 5d.

The mechanism of tunable polarization-selective absorption in the GTHM design is illustrated with the electric and magnetic field maps. The E-field excited by a TE wave at 1.8 μm in Figure 6a is much stronger than that of the TM wave in Figure 6c. The E-field of the TE wave is mainly located in the gap between the C-shaped resonators and the slit of the C-shaped resonator. The field enhancement in the gap between C-shaped resonators is from the typical gap plasmon mode, and is similar to the one in the metal grating structure [22]. As for the field enhancement in the slit within the unit C-shaped resonator, the circular magnetic field vector verified the excitation of electric dipole resonance, as shown in Figure 6b. The common effect of gap plasmonic mode and the electric dipole resonance inter- and intra-C-shaped resonators result in the high TE absorption. In contrast, there is no obvious field enhancement under TM wave incidence; the weak electric and magnetic fields are shown in Figure 6c,d, leading to near-zero TM absorption.

As expected, the resonance difference between TE and TM waves also induces distinct tuning for TE and TM absorption. Following the absence of resonance under the excitation of the TM wave, the ultrathin TiN films totally lose their tuning capability, which is consistent with the absorption spectra in Figure 5b. By contrast, under TE wave incidence, the resonances’ inter and intra-C-shaped resonators could both be disturbed by carrier density variation.

3. Conclusions

In summary, we achieved tunable polarization-selective absorption in NIR by gating ultrathin TiN in the UTGM and GTHM designs, respectively. The resonant wavelengths of both types of metasurfaces could be effectively tuned by gating the ultrathin TiN films without changing their geometry. The proposed two approaches towards the realization of innovatory active metasurface devices in NIR will enable the development of future integrated photonic and optoelectronic systems.