We placed active magnetic metamaterials on metallic surface to implement a tunable reflector with excellent agile performance. By incorporating active elements into the unit cells of the magnetic metamaterial, this active magnetic metamaterial can be tuned to switch function of the reflector among a perfect absorber, a perfect reflector and a gain reflector. This brings about DC control lines to electrically tune the active magnetic metamaterial with positive loss, zero loss and even negative loss. The design, analytical and numerical simulation methods, and experimental results of the tunable reflector are presented.
© 2014 Optical Society of America
Metamaterials offer an entirely new route to design of effective material properties. These artificial materials implemented by geometric unit cells are enriching studies in the field from conventional microscopic materials. A number of metamaterials from microwave to optical frequencies [1–4] for various applications, including cloaks [5–7], absorbers [8–10], lenses  microwave circuits  and antennas , have been proposed in recent years. Although exotic electromagnetic properties have attracted more research on metamaterials, the loss factor inherently limits their engineering practice. Thus, some active media or devices were brought in for loss reduction [14–16]. However, the negative refraction will disappear when the losses are compensated or significantly reduced by active media [17, 18]. Alternatively, more applications of these active metamaterials as gain medium have been proposed. For example, an active metamaterial device demonstrated real-time control and manipulation of THz radiation . The control capability is achieved by means of incorporating a Schottky diode and the transmission of THz wave is enhanced by the amount of gate bias applied to the diode. Another application can be in modifying the reflectivity of an object from a high value to a low value instantly . Currently, switchable reflectors/absorbers are employed to tune the reflectivity value by shifting frequencies of absorption peak [21–23] and these reflectors can be switched from being perfect absorber to perfect reflector. Yet, a truly smart absorber demands wider tuning capabilities . In this paper, we describe a method to achieve reconfigurable absorptivity/reflectivity by the novel use of a controllable gain medium. We can tailor the reflectance of a metallic surface not only from negative values to zero, but also to positive ones at an instance.
2. Design, fabrication and measurement of tunable reflector
In this work, we demonstrate a tunable reflector with function of the reflector among a perfect absorber, a perfect reflector and a gain reflector. As shown in Fig. 1(a), metallic reflectors with conventional material slabs usually exhibit reduced reflectivity, while; with gain medium, amplified reflectivity in Fig. 1(b) can be realized from gain reflector, (i.e. the power of reflected wave from the reflector is stronger than the incident one). The proposed tunable reflector incorporates a tunable active magnetic metamaterial as gain medium on a metallic surface (Fig. 2). The active magnetic metamaterial is based on conventional magnetic metamaterials of periodic split rings , and active elements are loaded between two rings to constitute a unit cell . The active elements provide magnitude amplification and phase shifting of flowing current from sensing ring to driven ring, and DC control lines are used to electrically tune active elements. With suitable DC control voltages, the active magnetic metamaterial exhibits properties of a gain medium with tunable effective permeability of positive loss, zero loss, and even negative loss.
The tunable reflector is implemented using PCB (printed circuit board) technology. Due to the relatively large impedance of inductance on the ring compared to the input impedance of the active elements, the current in the driven ring is limited. To increase the input voltage of the active elements, and decrease the loading impedance of the active elements, a capacitor-loaded split ring is used in active magnetic metamaterial. The dimensions of the capacitor-loaded split ring are shown in the inset of Fig. 3. The rings are periodically and vertically placed on the metallic surface. The total thickness of one unit cell is only 5.1cm (~0.1λ), which is electrically small. The active elements include a power amplifier (minicircuits, model: VNA-28), a voltage tunable phase shifter (minicircuits, model: JSPHS-661), and a voltage tunable attenuator (minicircuits, model: SVA-2000) to modify flowing current from sensing ring to driven ring. The fabricated reflector with six periodic units of the active magnetic metamaterials on metallic surface is shown in Fig. 4. The measurement configuration is shown in Fig. 5. A TEM cell was used as the test fixture. Quasi-TEM wave can be excited and most of the E field and H field are horizontally and vertically distributed in the TEM cell at measured frequencies, respectively. As shown in Fig. 6, the measurement of the TEM cell loaded with six periodic units approximately simulates the free space measurement of doubly infinite periodic units with normal incident TEM waves.
The DC power supplies of the active magnetic metamaterial have three channels, pumped by three control lines, respectively. Channel 1 is the control voltage for the phase shifters (V1) which can be varied from 0V to 12V. Channel 2 is the control voltage for the attenuators (V2) which can also be varied from 0V to 12V. Channel 3 is the supply voltage which is set to be 5V and with amplification gain of around 18dB at 600MHz. The measured reflectivity, when the DC power supplies are off, is shown in Fig. 7. The reflector exhibits near unity absorbance (A(ω)>99%) as perfect absorber , where A(ω) is absorbance, R(ω) is reflectivity and A(ω) = 1-R(ω). We fixed the V2 to a certain value, and then changed V1 from 0V to 12V. Measured results are given in Fig. 8. It shows that the maximum reflectivity increases when the attenuator control voltage increases. In Fig. 9, at 0.625GHz, it is demonstrated that the reflectivity can be tuned from −20dB to + 15dB by carefully selecting control voltages V1 and V2. Thus, the tunable reflector can work as perfect absorber (R(ω)<-20dB), absorbance tunable absorber (−20dB<R(ω)<0dB), perfect reflector (R(ω) = 0dB) and tunable gain reflector (R(ω)>0dB).
3. Analytical simulation method for active magnetic metamaterials
The equivalent circuit of the active magnetic metamaterial unit cell is shown in Fig. 10. Because the ring of magnetic metamaterials particle is electrically small and where denotes the wavenumber in the medium, magnetic dipole moment induced in the ring by incident field can be calculated and the excited current by the incident fields on the ring can be modelled as the equivalent voltage sources Vs,inc and Vd,inc in the equivalent circuit. Vsd and Vds are the equivalent sources caused by mutual coupling. L and C are the self-inductance and self-capacitance of the rings. Zin and Zout are the input and output impedances of the power amplifier, respectively. Vin and Vctl are the input and the output voltages of the power amplifier, respectively. l1 and l2 are the equivalent cable length when the phase shift caused by the connecting cable and the power amplifier is taken into account.
The corresponding current on the sensing and driven rings can be expressed as follows:25]:
When the system is perfect matched and the mutual coupling between rings is negligible, the imaginary part of effective permeability can be expressed as:
The phase delay on two cables () and the gain of amplification (Ar) dominate the sign and magnitude of the μ”, namely, magnetic loss. Thus, the active metamaterial with tunable magnetic loss (especially from positive to negative value) could be realized by suitable control of the active elements according to Eq. (4).
4. Numerical simulation method for tunable reflector
Analytical simulation is good for understanding the mechanism of these active magnetic metamaterials. Furthermore, full-wave numerical simulation will provide results with higher accurate prediction of the tunable reflector. For numerical simulation of the device, a unit cell of the tunable reflector is divided into two parts—active part with active elements and passive part with metamaterial rings and metallic surface. Finite-element-method-based Ansys high-frequency structure simulator (HFSS) is used to predict the properties of split rings with metallic surface. Agilent momentum and modelling work are utilized to simulate the active elements. Every unit cell has two internal connection ports between passive part and active part. One port is between the sensing ring and the active elements and the other is between the driving ring and the active elements. They are nominated as port 2n and port 2n + 1 of the nth unit cell and port 1 is related with external incident wave, which are shown in Fig. 11. Thus, the reflector with n unit cells have (2n)-port network of active part and (2n + 1)-port network of passive part. The S-parameters of two parts are connected, which can be described by the following equations:26]. The n is the number of unit cells. From Eqs. (5) and (6), the reflection coefficient (i.e. b1/a1) can be solved.
In the experiment, we used six periodic units inside a TEM cell to simulate doubly infinite periodic units, as illustrated in Fig. 6. The reflectivity of the tunable reflector was calculated by numerical simulation in Fig. 12. It provides good prediction of the tunable reflector performance. When the attenuator voltage V2 increases (i.e. lower attenuation), the difference between numerical simulation and measurement occurs, which is due to the saturation of the power amplifiers and non-uniform performance of active elements in six unit cells.
In summary, our investigation on active tunable reflector verifies the feasibility of a reflector that is electrically tuned among a perfect absorber, a perfect reflector and a gain reflector by incorporating active magnetic metamaterials with tunable effective permeability. The effective magnetic loss of the active magnetic metamaterial is tunable from positive to negative and the reflectivity from the tunable reflector varies from −20dB to + 15dB. Furthermore, we point out that, although the tunable reflector was experimentally demonstrated at Gigahertz frequencies, it can be easily scaled to Terahertz and infrared frequencies on semiconductor or MEMS/NEMS technologies based on the analytical and numerical simulation methods presented in this paper. As a future work, benefiting from the capacitor-loaded split ring reconfigurability, frequency-agile function could be introduced by incorporating varactor diodes into the split rings. Furthermore, the designed reflector can only interact with TEM or TM wave, new tunable reflector with active electric metamaterials for incident TE waves is under consideration.
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