Photon Storage in a Dynamic Two-Ring-Two-Bus System

We propose a novel dynamic two-ring-two-bus system to achieve photon storage. We have demonstrated numerically that the photon can be stopped and released by tuning the ring coupled to two buses in a short time. The two-ring-two-bus system is fabricated on the silicon-on-insulator platform, with the Q factor changing significantly when shifting one resonance. Due to the flexibility and simplicity, it is a promising candidate for the future optical storage and buffering device.


Introduction
The ever-increasing demand to store light for long time using compact devices, has led to a variety of approaches to manipulating light. Ideally, an optical buffer should have not only a large delay, but the delay should be constant over a broad bandwidth. Optical memory schemes have been demonstrated using slow light including both optical [1] [2] [3] and electromagnetically induced transparency (EIT) [4] [5]. Some investigations have demonstrated the storage time within the resonator can become larger than the inverse of the bandwidth of the input pulse by introducing a time-changing Q-factor [6] [7]. Notomi and Mitsugi [8] showed that the physical effect behind this method is the adiabatic tuning of an oscillator, such as a guitar string. In addition, it is also possible to stop light by dynamically changing a coupled resonator system as was predicted by Yanik and Fan [5].
In this paper, we investigate an optical storage device based on two mutually coupled ring resonators, with one ring coupled to two waveguides (i.e. Optics and Photonics Journal ring-bus-ring-bus, 2R2B). The storage time can be longer than the photon lifetime in the system by dynamically tuning the refractive index of resonator. The way of changing the refractive index is flexible. The dynamic optical storage is demonstrated using numerical simulations. We choose the silicon microrings as the platform because it has a high Q factor and a small mode volume, and can be fabricated on a chip. We get the quality value changed significantly.

Theoretical Analysis
The schematic structure is shown in Figure 1. The transmission coefficients in the ring 1-bus and ring 1-ring 2 coupling regions are denoted as 1 t and 2 t (the coupling coefficient is respectively. Using the transfer functions of the one-ring system [9], the through (T) and drop (D) transfer functions of the two-ring-two-bus system can be given by Figure 1. Example of a figure caption    narrow resonance, as seen in Figure 2(c). The light is mostly located in ring 2 and the Q factor of the system increases significantly (Figure 2(d)).

Simulation and Experiment
The   Then, we fabricated the two-ring-two-bus system in silicon. The SEM image of device is shown in Figure 5(a). The gap between two rings is 0.26 μm, and R1 = 28 μm, R2 = 21 μm. The desired results are obtained. Figure 5(b) shows the measured through and drop transmission before tuning ring 1. It is a class-like EIT resonant peak and there is a symmetric resonance splitting around 1530.61 nm. When we decrease the radius of ring 2, the resonant wavelength of ring 2 is blue-shifted, resulting in two greatly separated resonances for the system. It is found that the Q factor of the system is significantly increased from 10204 to 19132. The results imply that if the refractive index of the microring is changed by injecting pump pulse, the system will be able to store and release optical pulse dynamically.

Conclusion
In summary, we have demonstrated an optical buffer based on a two-ring-two-bus structure by tuning a ring. The photon storage and release are demonstrated numerically in the structure. The Q value changing from 10,204 to 19,132 is realized by shifting the resonance peak of ring 1. Our buffer is independent of the mechanism used for index tuning, and the tuning time is smaller (a) (b) (c) than the photon time, such as carrier injection and Kerr effect. The captured pulse can be held long enough to exceed the constraints of the delay-bandwidth product imposed on the passive all-optical approaches. Although the buffering time was shorter than those of previous studies, we believe that it can be improved as the Q factor of the ring increases.