Dynamic silicon photonics
The dynamic manipulation of light relies on the control of the transmission or dispersion of the light confining structure while the signal is still in the system –. The light follows the eigenmodes of the structure as this is manipulated, and thus its characteristics such as frequency and group velocity change accordingly, making frequency conversion possible or even stopping of light. The required fast modulation of the photonic structure can be achieved for instance through a refractive index change of silicon based upon the free-carrier plasma dispersion effect .
Dynamic frequency conversion has been realized in waveguides – as well as in resonators ,,. The frequency of the waveguide mode in the first or of the resonance in the second case is shifted by changing the refractive index of the core material while light is present in the structure, leading to a corresponding conversion of the light frequency. Further, dynamic light storage has been demonstrated in several kinds of structures. In resonant structures, it can be realized by increasing the initially low Q-factor once the signal has entered the device, leading hereby to a corresponding increase of the lifetime of the signal –. In waveguides, light can be stopped by virtue of controlling the shape of the waveguide dispersion –.
An important advantage of a waveguide based system is that the signal pulse can be entirely submitted to the dynamic process because it can be hosted completely in the structure by appropriately choosing the waveguide length. In contrast, this cannot be achieved in systems consisting of resonators coupled to two ports.
Further, different methods for generating the free carriers, which in turn change the refractive index, have been proposed and implemented. Most commonly, free carriers are generated by the linear absorption of a light pulse which is incident on the device from the top ,–,–.
- Development and implementation of concepts for dynamic control of light in waveguide based systems.
- Implementation of dynamic switching by integrating both, the signal and the switching pulse, on-chip.
- Finite Integration Technique (CST Micro Wave Studio, www.cst.com)
- Finite Difference Time Domain (MEEP )
- Electron beam lithography of SOI wafers
- Transmission characterization with end-fire coupling scheme.
- Pulse shape characterization using interferometric autocorrelation.
We have developed a concept for dynamic light storage in slow light waveguides . For a line-defect photonic crystal waveguide, the mode field distributions for different wave vectors might be significantly different from each other if the geometry of the waveguide is properly designed. Figure 1 schematically shows the dispersion of the slow light waveguide, along with the schematic field distributions over the waveguide cross section for two different values of wave vector k. By applying an index change to the grey region of the waveguide, only the frequency of the mode corresponding to wave vector k1 will be shifted, as the mode at wave vector k2 vanishes in this region. The magnitude of this shift depends on the strength of the index change and can be set such that the resulting band has a vanishing derivative and thus vanishing group velocity between points k1 and k2. This concept can be employed to store light. A light signal propagating through the unperturbed structure will be slowed down if the index change is applied.
Further, we have proposed and experimentally implemented a concept for dynamic switching by integrating both, the signal and the switching pulse, on-chip . To this end, we take advantage of structures in which the propagation velocity of light is frequency dependent, and assign the signal pulse a lower propagation velocity compared to the switching pulse. Thus, we allow the switching pulse to overtake a co-propagating signal pulse running ahead while on its way it changes the refractive index of the core material, here silicon, through generation of free-carriers by two-photon absorption, and thus dynamically manipulates the properties of the signal pulse. We have been able to show a dynamic frequency shift of up to 70GHz with a conversion efficiency of up to 25%.
Figure 1: Schematic representation of a mechanism proposed to control the group velocity of a waveguide mode. The black line shows the dispersion diagram for guided light modes. The insets represent a cross section of the waveguide and show the field distribution at different k-points. The grey shaded regions on the waveguide cross section indicate where the change of refractive index is to be applied. The grey flat line is the dispersion relation resulting from applying the change of refractive index. .
Figure 2: Schematic representation of the proposed switching concept. A photonic chip is shown along with access waveguides (black) and a photonic device where light is to be manipulated (green). The switching pulse (black) overtakes the signal pulse in the photonic device and induces a dynamic change.
List of publications
Castellanos Munoz, M., Kanchana, A., Petrov, A. Y. & Eich, M. Dynamic Light Storage in Slow Light Waveguides, IEEE J. Quantum Electron. 48, 862–866, (2012).
Castellanos Munoz, M., Petrov, A. Y. & Eich, M. All-optical on-chip dynamic frequency conversion, Appl. Phys. Lett 101, 141119–4, (2012).
Castellanos Munoz, M., Petrov, A. Y. & Eich, M.; All-optical dynamic frequency conversion in silicon photonic crystal cavities; in Proceedings of SPIE, Photonic Crystal Materials and Devices X, edited by H. R. Miguez, S. G. Romanov, L. C. Andreani & C. Seassal, 84250I (SPIE, 2012).
Prof. Dr. Ernst Brinkmeyer, TUHH, Optische Kommunikationstechnik
Prof. Dr.-Ing. habil Jörg Müller, TUHH, Mikrosystemtechnik
Prof. Dr. Klaus Petermann and Dr. Jürgen Bruns, TU Berlin, Hochfrequenztechnik/Photonik
Prof. Dr. Thomas Krauss, University of St Andrews
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