Measurement of backscattered electric field of chipless radio frequency identification tag based on Rydberg atoms

Chipless radio frequency identification tags have been widely used in many areas, such as vehicle recognition and identification of goods. Near-field measurement of a chipless radio frequency identification tag is important for offering the precise spatial information of the backscattered field of tag. In this paper, we demonstrate the angle discrimination of a line-shape chipless radio-frequency identification tag via the near-field measurements of scattered electric fields in two orthogonal directions. Two laser beams with different frequencies counter propagate and pass through a roomtemperature caesium vapor. A Rydberg ladder-type system is formed in the experiment, which includes three levels, namely 6S1/2, 6P3/2, 51D5/2. The electromagnetically induced transparency of transmission of probe light, which is locked to the transition of 6S1/2↔ 6P3/2, is observed when the frequency of coupling light varies nearby the transition of 6P3/2↔ 51D5/2. When the 5.366 GHz microwave electric field that is resonant with the transition between two adjacent Rydberg states 51D5/2↔ 52P3/2 is applied to the caesium vapor cell by using a standard-gain horn antenna, the transmission signal of probe laser splits into two peaks, which is known as Autler-Townes splitting. The splitting between the transmission peaks is proportional to the microwave electric field strength at the position of laser beam. The spatial distribution of backscattered microwave electric field of the chipless radio-frequency identification tag is obtained through varying the position of the laser beam. The spatial resolution of near-field measurement approximately equals λMW/12, where λMW is the wavelength of the measured microwave electric field. The distributions of the electric field strength in two orthogonal directions show the clarity difference while the angle of radio-frequency identification tag is changed. The scattered electric field strength of the identification tag is strongest when the angle of line-shape tag is the same as that of the polarization of the horn antenna. Moreover, the scattered field strength of identification tag in the incident field direction of the horn antenna increases as the measured position and the identification tag get closer to each other. The scattered electric field distributions in the vertical direction are almost constant at the different angles between the incident electric filed and identification tag. The fluctuation of spatial distribution of the scattered electric field strength is attributed to the Fabry-Pérot effect of microwave electric field in the vapor cell. And the geometry of vapor cell results in the minor asymmetric distribution of scattered field. The simulation results from the electromagnetic simulation software are accordant with the experimental results. The novel approach to near-field measurement of identification tag will contribute to studying and designing the chipless radio-frequency identification tag and complex circuits.