Methods We take Rogers RT5880 copper-clad substrate with a thickness of 1. 57 mm as the substrate of the microstrip cavity and CRLH-TLs. The thickness of the copper layer is 0. 035 mm. A Fabry-Perot (FP) cavity is formed inside a microstrip line. Two SRRs are placed in the cavity and located at the antinode and node of the electromagnetic field in the FP cavity respectively to construct a unidirectional EIT-like structure. The excitation port of the EIT-like effect is determined by the sequence of the antinode and node in the FP cavity. Tunable composite right/left-handed transmission lines (CRLH-TLs) loaded with varactors are added at the two ends of the FP cavity (marked as left and right CRLH-TLs respectively) to change the electromagnetic field distribution in the cavity. By optimizing all parameters, the electrical lengths of CRLH-TLs are quarter wavelength and half wavelength respectively under different bias voltages. Therefore, since the distribution of the nodes and antinodes in the cavity can be switched by changing the electric length of the CRLH-TLs, the sequence of the antinode and node where the two split ring resonators (SRRs) lie in the cavity is also switched, which leads to a switched EIT-like excitation port. Finally, a sample is fabricated and tested to validate the unidirectional EIT-like effect with the electrically switchable excitation port. Results and Discussions This structure realizes the unidirectional EIT-like effect to bring a unidirectional reflection with high contrast ratio. It is validated both in simulation and experiments that the contrast ratio of the unidirectional reflection can reach more than 95%, and the excitation port of the unidirectional EIT-like effect is determined by the sequence of nodes and antinodes in the FP cavity. The capacitance of the varactors in the CRLH-TLs varies along with the bias voltage. Thus, different bias voltages are simulated by setting different capacitance values. In case I, the capacitance of varactors in the left CRLH-TLs is set as 2. 5 pF (C-sl = 2. 5 pF) and that in the right is set as 1. 5 pF (C-sr = 1. 5 pF). The magnitude of the reflection coefficient of port 1 vertical bar S-11 vertical bar and port 2 vertical bar S-22 vertical bar at 3. 97 GHz are 0. 007 and 0. 538 respectively, showing that the EIT-like effect is only excited through port 1. Case II has swapped the capacitance of the varactors in the right and left CRLH-TLs units. Thus the reflection spectra vertical bar S-11 vertical bar and vertical bar S-22 vertical bar will also be exchanged due to the geometric symmetry of the switchable EIT-like effect. At last, the excitation port of the EIT-like effect has been switched to port 2, indicating that switching the bias voltage can achieve a unidirectional EIT-like effect with an electrically switchable excitation port (Fig. 2). When the capacitance of the varactors is set as 1. 5 pF and 2. 5 pF, the transmission amplitudes of the CRLH-TLs are both larger than 0. 7 and angle S-21 are close to -90 degrees and -180 degrees at 3. 97 GHz respectively (Fig. 3). Since the transmission phase difference between the CRLH-TLs units with the capacitance of 2. 5 pF and 1. 5 pF is -90 degrees, once the capacitance of the varactors in the left and right CRLH-TLs is exchanged, the sequence of the nodes and antinodes in the FP cavity is reversed. As a result, the port to excite the unidirectional EIT-like effect is switched (Fig. 4). For the fabricated sample, when the bias voltage on the left and right sides of the CRLH-TLs are V-1 = 0 V and V-2 = 6 V respectively, only when the wave is incident from port 1, the EIT-like effect can be excited. Through exchanging the bias voltages, the unidirectional EIT-like excitation port is switched. This shows that the structure can achieve a unidirectional EIT-like effect with an electrically switchable excitation port. Objective Unidirectional electromagnetically induced transparency-like effect is a special kind of EIT-like effect, which is caused by its asymmetric structure. The EIT-like effect can be excited by the asymmetric structure only when a wave is incident from a certain port. The unidirectional EIT-like effect plays a significant role in realizing directional reflection and transmission and is crucial in unidirectional invisibility. With the development of tunable metamaterials, various kinds of reconfigurable metamaterials are also proposed to realize a tunable EIT-like effect. However, the dynamically switchable unidirectional EIT-like effect has been barely reported. The excitation port of the unidirectional EIT-like effect is usually fixed and determined by the structure topology. To realize a reflection-type unidirectional EIT-like effect, an electrically switchable excitation port based on tunable CRLH-TLs and a two-port microstrip cavity embedded with two SRRs is proposed. The reflection-type EIT-like effect can only be excited when an electromagnetic wave is incident from a certain port. The contrast ratio of the asymmetric reflection coefficient of the two ports in our paper reaches 98. 7%. On this basis, the coupling between the microstrip cavity and the SRRs is dynamically modulated by the tunable CRLH-TLs, thereby changing the excitation port of the unidirectional EIT-like effect. Finally, a unidirectional EIT-like effect with an electrically switchable excitation port is achieved, and the applications of the EIT-like effect in optical storage, optical modulation, sensing, and other fields are promoted. Conclusions We propose a reflection-type unidirectional EIT-like effect with an electrically switchable excitation port, and validate it in simulation and experiments. To switch the excitation port of the unidirectional EIT-like effect, our paper reverses the sequence of nodes and antinodes in the FP cavity by changing the bias voltages of CRLH-TLs on both sides of the cavity. This unidirectional EIT-like effect with an electrically switchable excitation port provides a feasible scheme for tunable asymmetric EIT-like effects and is expected to be applied in directional reflection and multifunctional unidirectional stealth devices.
