# Observation of an Electromagnetically Induced Grating in Cold \({}^{85}\)Rb Atoms
## 1 Introduction  
The success in the production of cold atoms opens up many interesting research fields, among which the electromagnetically induced transparency (EIT) [1; 2; 3] implemented in cold atomic systems has been extensively investigated in recent years [4; 5]. EIT is a kind of quantum coherence effect that makes the absorbing medium transparent to the weak probe field. While studying EIT generally focuses on the traveling wave of the laser field, more recently there has been considerable interest in electromagnetically induced grating (EIG), which replaces traveling wave fields of EIT with strong coupling standing-wave fields [6; 7; 8]. By adopting this method, the dispersion and absorption of the probe laser beam will be modulated spatially in an atomic medium. The EIG configuration is easier to tune and construct than the traditional grating, so it can directly control the properties of light propagation. EIG has lots of potential applications in the area of all-optical communications, such as optical switching [9], optical diodes [10], optical bistability [11], beam splitter [12], and coherently induced photonic band gaps [13].  
Up to now, atoms are still the most appropriate medium to achieve EIG, and several experimental realizations of EIG have been done in thermal atom medium. In particular, the diffraction pattern of the probe laser propagated through the EIG was attracted more attention [14; 15]. The research on the grating controllability, such as structural parameters and the diffraction characteristics, was studied in a rubidium vapor cell [16; 17]. Moreover, the EIG was applied to illustrate the electromagnetically induced Talbot effect in a thermal vapor [18; 19]. Recently, the EIG has beendeveloped in multi-dimensional in thermal atomic system and shows more possibilities in simulation of non-Hermitian physics [20; 21; 22], and the research of the reconfigurable photonic graphene based on EIG will open the way for future experimental exploration of topological photonics [23; 24].  
Compared with the atoms in thermal vapor, the EIG in cold atoms gives more potential for in where the Doppler effect is reduced to negligible. The EIG spectra were first realized experimentally in the cold sodium atoms with a three-level \(\Lambda\) system, which exhibits background-free Lorentzian signal profiles [7]. In addition, by an optical-pumping-transferred ground-state, the transient Bragg diffraction was also observed in cold cesium atoms at the same year [25]. To our best knowledge, there are no more experimental reports on the EIG in cold ensembles. In addition, the two experiments above are focusing on studying the spectrum of the EIG, the research in the visual form of the EIG in cold atoms receives no report. Compared with the spectrum, the visual research can not only observe the diffraction grating of EIG more vividly, but also can intuitively obtain more information about the characteristic parameters under different experimental conditions. In such visual systems, we can get an optimal configuration for desired applications by controlling different experimental parameters [15].  
In this paper, a tunable EIG is realized in a three-level V-type system of \({}^{85}\)Rb cold atoms. In cold atomic medium, the two coupling lasers interfere with each other and generate a strong standing-wave field. Then, a weak probe laser counter-propagated through the cold atoms, and the diffraction pattern of the weak probe field was recorded in a visual form to real-time monitor the EIG. We find that the diffraction characters can be flexibility controlled through the two-photon detuning and the coupling laser power. These results will further motivate the non-Hermitian physics simulation and provide a nondestructive method to image cold atoms.