Indirect photonic transitions for light control in integrated photonics

本研究项目的目标是让光脉冲在具有相对移动折射率前沿的硅慢光波导中强烈相互作用。正确选择慢光波导和前速度可以进行一系列鼓舞人心的物理实验。也就是说，光脉冲可以进行频移、存储一定时间、压缩和展宽。除了上述效应外，光子带跃迁的间接性质使其在本质上是非互易的。因此，光学隔离等物理现象也可以被研究。除了科学兴趣外，该方案的成功实验实现可以为光通信系统和其他领域的光脉冲操纵提供几种新方法。直接跃迁中的频移仅由动态变化的折射率阶跃（即产生自由载流子的功率）决定。这种折射率变化通常在10-3的量级，因此导致1.5µm波长的信号频率变化约为100ghz。间接过渡创造了一个额外的方式，频率控制通过选择的前速度。首先，设想使用两个反向传播信号来演示非互反频移，其中前向传播信号与后向传播信号相比经历不同的频移。这一概念可用于光学隔离，这是当前备受关注的科学技术话题。此外，通过正确组合色散曲线和前速度，可以在很小的折射率阶跃下获得很大的频移。这里我们设想1太赫兹或更多的频移。通过直接转换很难实现可调的时间延迟。通常，信号的频率会发生位移，但群速度保持不变。在光子晶体波导的间接跃迁中，前群速度的选择可以允许跃迁到任何群速度的状态，甚至到零群速度的状态。因此，我们计划通过改变前速度来演示可调的时间延迟。这种间接跃迁也可以用来压缩或拓宽从移动光子晶体前端反射的光脉冲。这种方法有几个优点。首先，可以实现1-10级因子的压缩和展宽。即使采用很小的折射率调制，也可以实现几乎100%的转换效率。同时，中心频率在这个过渡中保持不变，这是一个非常重要的优点。

The goal of this research project is to let optical pulses strongly interact in silicon slow light waveguides with relativistically moving refractive index fronts. The right choice of a slow light waveguide and front velocity can allow a palette of inspiring physical experiments. Namely, the optical pulses can be shifted in frequency, stored for certain time, compressed and broadened. Apart from the listed effects the indirect quality of the photonic band transition causes it to be nonreciprocal in nature. Thus, physical phenomena such as optical isolation can be investigated as well. Apart from scientific interest the successful experimental realization of this proposal can lead to several new approaches for optical pulse manipulation in optical communication systems and other fields. The frequency shift in the direct transition is determined by just the refractive index step (namely the power that generates the free carriers) of the dynamic change. Such refractive index changes are usually in the order of 10-3 and thus lead to frequency changes of approximately 100 GHz for a signal at 1.5 µm wavelength. The indirect transition creates an additional way for frequency control via the choice of the front velocity. First of all, it is envisaged to demonstrate a non-reciprocal frequency shift using two counter propagating signals where the forward propagating signal experiences a different frequency shift as compared to the backward propagating signal. This concept can be used for optical isolation, which is a current scientific and technical topic of high interest. Also, by the right combination of the dispersion curve and the front velocity, very large frequency shifts can be obtained for small refractive index steps. Here we envisage frequency shifts of 1 THz and more.Tunable time delays are difficult to realise with direct transitions. Usually, the frequency of the signal is shifted but the group velocity stays the same. In case of an indirect transition in photonic crystal waveguides the choice of the front group velocity can allow transitions to states of any group velocity or even to a state of zero group velocity. Thus, we plan to demonstrate a tunable time delay by varying the front velocity.The indirect transition can be also used to compress or broaden optical pulses upon reflection from a moving photonic crystal front. There are several advantages of this approach. First of all, compression and broadening factors of the order of 1-10 can be realised. Almost 100% conversion efficiency can be achieved even with a small refractive index modulation. At the same time the center frequency stays the same in this transition, which is a very important advantage.