Abstract: Recently, the team of Prof. Xinliang Zhang, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology published new advances of temporal cloak in Nature Communications. The paper is entitled "Field-programmable silicon temporal cloak". This paper is jointly collaborated with Prof. Linjie Zhou from Shanghai Jiao Tong University, Dr. Yunhong Ding from DTU, and Prof. Cheng-Wei Qiu from National University of Singapore.
The invisibility cloak, as a magic concept, has recently attracted great interests in many fields due to its mysterious, fascinating and exciting characteristics. The invisibility cloak can be classified into two types: spatial cloak and temporal cloak. The spatial cloak not only appears in the science fiction works such as "Harry Potter", but also attracts the research interest of researchers, such as exploring different cloaking materials and cloaking technology. These technologies can play an important role in military activities such as stealth aircraft and radar. According to the space-time duality, the spatial cloak is extended to temporal domain, researches have recently described a temporal cloak with temporal gap which can hide some events without detection from the observer.
Since the fantastic concept of temporal cloak was first theoretically proposed by McCall et al in 2011, several breakthrough advances for experimental science in temporal cloak sprung up promptly. Especially, Nature and Nature communications have published the first experiment verification of temporal cloak (Gaeta et al., Nature 481, 62-65, 2012), the first temporal cloak operating at telecommunication data rate (Weiner et al., Nature 498, 205-208, 2013), and temporal spying and concealing process through polarization bypass (J. Fatome et al. Nature communications 5, 4678, 2014). However, until now the state-of-the-art cloaking experiments exhibited only a fixed and small cloaking window with picosecond-level due to the periodicity and aperture limit of time lens, as shown in Fig. 1a. Thus, the periodical and small cloaking window (<200ps) hinders its applications in data shield and secure communication. And, it is more powerful and practical to make the cloaking window field-programmable (namely, the cloaking window can be switched off, switched on, or stretched freely, as shown in Fig. 1b) since different types of optical packets can be hidden freely with the cloaking system.
Figure 1. Comparison between traditional temporal cloak and our field-programmable temporal cloak.
To address this issue, the team of Prof. Xinliang Zhang has reported a major breakthrough to make the temporal cloak programmable, shown in Fig. 1b. They demonstrated a field-programmable temporal cloak with a record cloaking window of nanosecond-level benefiting from a unique electrically controllable silicon-based time lens (experimental setup is shown in Fig.1c). The superior time lens consists of an optical frequency comb and an electrically tuned microring resonator (ET-MRR) acting as a scanning filter, whose output wavelength is proportional to the applied voltage. The electrically controllable time lens is enabled by applying an electrical split sawtooth signal on the ET-MRR and disabled by applying a direct current (DC) electrical signal. This electrically controllable silicon-based time lens has distinct advantages of field-programmable cloaking window, moderate power consumption and compact photonic integration. And the cloaking window can be easily altered by changing the repetition rate and peak voltage of the ET-MRR drive signal.
They demonstrate, for the first time, a field-programmable temporal cloak with potential applications in data shield, enabling to share some public data to the user but conceal other private data in real time. Importantly, they break the periodicity of cloaking window. In the experiment, the cloaking window can be easily opened and closed by setting the split sawtooth and DC signal applied on the ET-MRR, respectively. Without loss of generality, the split sawtooth signal is recorded as cloaking bit 1, representing the state of cloak on, and the DC signal is recorded as cloaking bit 0, representing the state of cloak off. Figure 2 shows some typical experiment results of field-programmable temporal cloak by varying the cloaking bits on the ET-MRR. The first row shows the sequence of cloaking bits at the bit rate of 200 Mbit/s. The second row shows a user-defined sequence event (blue) with dark RZ signals. The third row shows the output waveforms controlled by the cloaking bits. In Fig. 2a, the output waveform is a constant with a low ripple factor, which means all event bits have been hidden successfully since all the cloaking bits (cyan) on. From Figs. 3b and 3c, the switching bits (cyan) is set as random sequences, and the event waveforms are observed at cloaking bit of 0 (bright region), but the event waveforms cannot be observed at cloaking bit of 1 (gray region). Figure 3d shows that the output waveform is the same as the event waveform when the cloak is cloak off. As shown in Fig. 3e, the periodic sequence dark events with half cloaking repetition rate is selectively cloaked by the arbitrary sequence temporal cloak. In addition, an arbitrary sequence dark events with both two-pulse event and one-pulse event are erased by the periodic temporal cloak, as shown in Fig. 3f. It shows that the long cloaking window could hide more pulses, not only a single pulse. Therefore, the field-programmable cloak has been successfully demonstrated.
Figure 2. Field-programmable temporal cloak.
Meanwhile, they opened a time gap waveform with a repetition rate of 200 MHz, and the cloaking window is measured as 3.365 ns (See Fig. 3), 17 times larger than the previous experimental record (196 ps).
Figure 3. Experimental results of a nanosecond-event temporal cloak.
On June 20th, this work was published in Nature Communications. Professor Jianji Dong, Professor Chengwei Qiu and Professor Xinliang Zhang were the corresponding authors. The work was supported by the National Natural Science Foundation of China (No. 61475052, 61622502, 61571186) and the Shanghai Municipal Science and Technology Major Project (2017SHZDZX03).
Reference：F. Zhou, S. Yan, H. Zhou, X. Wang, H. Qiu, J. Dong, L. Zhou, Y. Ding, C.-W. Qiu, and X. Zhang, "Field-programmable silicon temporal cloak," Nature Communications 10, 2726 (2019).
Paper website: https://doi.org/10.1038/s41467-019-10521-5
