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Abstract
Silicon-based all-dielectric metamaterials (SAMs), with advantages like low loss and simple structure, are attracting more and more attention. However, SAMs usually suffer from narrow bandwidth and low tunability, and thereby their applications are seriously impeded. In this work, we propose and experimentally demonstrate a tunable SAMs in terahertz (THz) ranges by covering the SAMs with a layer of active medium, strontium titanate (STO). It shows that the THz responses of SAMs can be thermally tuned due to the temperature-dependent permittivity of STO. This work provides a convenient route to tunable SAMs from THz to optical ranges.
© 2017 Optical Society of America
1. Introduction
Metamaterials, also called artificially structured medium, are becoming more and more attractive because of their exotic properties like negative index, near zero index and giant index, as well as their considerable applications such as perfect absorber, invisibility cloak, superlens and so on [1–5]. Traditional metamaterials usually consist of subwavelength metallic structures, which however suffer from serious ohmic loss, complex structure and highly anisotropic responses [6–8]. A promising method to solve these problems is employing all-dielectric metamaterials. It is well-known that various resonances including magnetic and electric resonances can be induced in high-index and low-loss particles, which provide a new platform for manipulating and steering light. With advantages like low loss and simple structure, all-dielectric metamaterials, especially the silicon-based all-dielectric metamaterials (SAMs), have attracted increasing attention [9–19]. This is because silicon-based metamaterials have low loss in broad spectrum of light and are compatible with current nano- and micro- fabrication.
Recently, increasing studies are focused on the SAMs in THz ranges. Yang et al. [17] demonstrated a series of high-qualify Mie resonances in silicon grating structure. Headland et al. [18] investigated a terahertz magnetic mirror with silicon resonator antennas. Silicon metasurface based absorbers have been employed for uncooled terahertz imaging by Fan et al. [19]. However, up to now, the tunable SAMs in THz ranges have not been investigated. From the viewpoint of practical applications, tunable metamaterials are highly desired in switch, modulator, filter and etc. Thereby, it is necessary to develop a practical and simple method to obtain SAMs in THz ranges.
In this work, a tunable SAMs in THz ranges is proposed and experimentally demonstrated by covering it with a layer of active medium, strontium titanate (STO). It shows that the terahertz response of SAMs can be thermally tuned based on the temperature-dependent permittivity of STO.
2. Design and Preparation
The considered silicon-based all-dielectric metamaterial in THz range is shown in Fig. 1, which actually is a silicon grating structure. The magnetic field direction of incident THz wave is along the silicon rod (along the y direction), as shown in Fig. 1(a). The corresponding geometry parameters are provided in Fig. 1(b), where the silicon grating period p is 200 μm and the size of silicon rod is 100 μm × 100 μm (a × a). According to the previous work [17], various magnetic resonances can be induced in such structure due to the high index and low loss of silicon. However, the resonances usually are difficult to be tuned due to the stable optical properties of silicon in THz ranges. It is well-known that STO is a special active material [20], whose frequency dependent complex relative permittivity can be expressed as:
where is high-frequency bulk permittivity ( = 9.6), is oscillator strength (),is the angular frequency. and are the soft mode frequency and damping factor, which can be expressed as: Clearly, and are temperature (T) dependent parameters. As a result, STO has temperature dependent complex relative permittivity. Thereby it provides a possibility to construct a tunable SAMs with the help of STO film. In this paper, STO/silicon composite structure is employed to tune the THz response of SAMs, as shown in Fig. 1(c), where thin STO film is deposited on the silicon grating. It is worth noting that this structure is simply, which can be obtained readily by traditional silicon etching and film deposition processes such as evaporation, magnetron sputtering and solution-based methods.
Fig. 1 (a) Schematic illustration of SAMs. (b) Top view of SAMs and corresponding geometry parameters. (c) Tunable silicon/STO all-dielectric metamaterial in THz ranges. The STO film is represented by blue colour.
The preparation of tunable SAMs starts with the fabrication of silicon grating structure. The corresponding process flow is provided in Fig. 2. Firstly, ultrathin intrinsic silicon wafer (100 μm thickness) is cleaned with standard silicon cleaning process [Fig. 2(a)]. The photoresist is spin-coated on the silicon wafer and common lithography is carried out [Figs. 2(b) and 2(c)]. Finally, deep silicon etching technology is employed to obtain a silicon grating and then the photoresist is removed by acetone [Fig. 2(d)]. The employed deep silicon etching technology is similar to the one employed by Fan [19]. The micro photograph of fabricated SAMs is shown in Fig. 3. The STO film is sputtered on the fabricated SAMs with a magnetron sputtering system. The STO ceramic target (>99.99% purity) with a diameter of 75 mm is prepared by sintering at 1400 °C for 2 h. The film is deposited with mixing flowing of 30 sccm Ar (>99.99% purity) and 1 sccm O2 (>99.99% purity) under pressure of 2.5 Pa for 1 h (RF power 160W). Finally, the as-deposited STO film is further crystallized in air at 850 °C for 2 h.
Fig. 2 Process flow of SAMs. (a) Ultrathin silicon wafer (100 um thickness) is cleaned with standard silicon cleaning process. (b) Photoresist is spin-coated on the silicon wafer. (c) Common lithography. (d) Deep silicon etching.
3. Results and discussion
CST Microwave Studio, a commercial finite integration time domain package, is employed to obtain the optical responses of designed metamaterials. As for the silicon grating structure without STO film, two obvious dips can be seen in 0.678 THz and 0.903 THz, respectively [Fig. 4(a)]. To give a good insight into these two dips, simulations are carried out to obtain the corresponding magnetic field distribution (across the y-z plane) and magnetic field intensity distribution (across the x-z plane) [Figs. 4(b)-4(d)]. As for the first dip, linear and enhanced magnetic field is induced inside the silicon rod [Fig. 4(b)], resulting in an enhanced magentic field intensity in it [Fig. 4(c)]. With these properties, it can be determined as the first magnetic resonance modeof silicon rod. As for the second dip, two oppsite linear magnetic fields are enhanced inside the silicon rod [Fig. 4(d)], resulting in two enhanced magentic field intensities in it [Fig. 4(e)]. These phenomena suggest that the second magnetic resonance mode is induced. These modes have also been investigated and discussed by Yang [17]. Then, simulations are performed to obtain the THz transmission spectrum of silicon/STO all-dielectric metamaterial at different temperatures, which are also plotted in Fig. 4. Here, the STO film with thickness of 500nm is placed on the surface of silicon grating and four temperatures (250K, 300K, 350K and 400K) are considered. According to Eq. (1), the relative permittivity of STO film at these temperatures can be calculated as 362, 300, 251, 223, respectively, and the corresponding loss angle tangent is about 0.01. The simulated results are plotted in Fig. 4(a), where both the first and second resonances of silicon/STO all-dielectric metamaterial experience red shift compared to the one without STO film (marked by red arrows). This is because the existing of STO film increases the optical thickness of the silicon rods, making the resonances shift to lower frequencies. The resonance frequencies as a function of temperature are plotted in Figs. 5(a) and 5(b), which correspond to the first and second resonances. From these two figures, it can be found that both the first and second resonances shift to high frequency with the increasing temperatures. Quantitatively, when the temperature increases from 250K to 400K, the first resonance shifts from 0.657THz to 0.665THz and the second resonance shifts from 0.853THz to 0.870THz. This can be attributed to the temperature dependent relative permittivity of STO, which decreases with temperature.
Fig. 4 (a) Transmission spectrum of tunable silicon/STO all-dielectric metamaterial at different temperatures. (b)(c) The magnetic field and magnetic field intensity distributions at the first resonance, respectively. (d)(e) The magnetic field and magnetic field intensity distributions at the second resonance, respectively.
Fig. 5 Resonance frequencies with different temperatures: (a) The first resonance (b) The second resonance.
The THz response of the sample is evaluated by a photoconductive switch-based THz-TDS system [17]. In the measurement, the x-polarized THz pulses are perpendicular to the grating lines. The measured THz transmission spectrums for the silicon grating structure without and with STO film at different temperatures are shown in Fig. 6. As for the silicon grating structure witout STO film, two obvious resonance dips are located at 0.702 THz and 0.821 THz, respectively. Similarly, both the first and second resonances of silicon/STO all-dielectric metamaterial experience red shift compared to the one without STO film. Quantitatively, when the temperature increases from 250K to 400K, the first resonance shifts from 0.662THz to 0.695THz and the second resonance shifts from 0.769THz to 0.801THz (Fig. 7). The measured results are consistent with the simulated ones, confirming the thermal tunability of the proposed metamaterials. It should be noted that the slight deviation can be attributed to the fabrications.
Fig. 6 Measured transmission spectrum of tunable silicon/STO all-dielectric metamaterial at different temperatures.
Fig. 7 Measured resonance frequencies with different temperatures: (a) The first resonance (b) The second resonance.
At this point, we have demonstrated that by introducing temperature dependent STO film, SAMs can be effectively tuned thermally. To further improve the tunable performance, the thickness of STO can be increased. In addition, it is well-known that the optical properties of STO can also be tuned electrically. As a result, it is possible to develop a rapid tunable SAMs.
4. Conclusion
This work has provided a convenient route to the tunable silicon all-dielectric metamaterials in THz ranges by covering the SAMs with a layer of STO. It shows that the frequencies of Mie resonances increase with temperature. Both simulated and experimental results suggest that the THz response of silicon all-dielectric metamaterials can be thermally tuned due to the temperature-dependent permittivity of STO.
Funding
National Natural Science Foundation of China (NSFC) (Grant Nos. 61372109 and 51402163), Shenzhen Science and Technology Projects (Grant Nos. XCL201110009, JCY201110096 and JSE201007200050A), and China Postdoctoral Research Foundation (Grant No. 2013M530042).
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