Institute of Optical and Electronic Materials

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Electro-optic modulation

Introduction

The increasing bandwidth demand in information technology and limitations of cable bound information transfer have inspired many researchers to explore new solutions to the interconnect problem. Since many years fiber communication is the standard technology for long distance data transfer due to it’s large available band width and low losses. In order to take advantage of the large bandwidth also for rack to rack or chip to chip communication basic building blocks such as detectors and modulators need to be integrated into a CMOS compatible technology. Many contributions have been made to realize electro-optic modulators based on absorption of free charge carriers in silicon waveguides. Although modulators up to 40 GHz bandwidth have been demonstrated these devices typically have a length of several hundred µm [1,2]. It has been shown that silicon photonic circuits can be significantly enhanced if combined with electro-optic polymers allowing for more compact modulators which can operate at lower voltages [3].

Goals

Our previous works have demonstrated electro-optic modulation in a silicon photonic crystal covered by an electro-optic polymer. The device had a length of less than 10 µm and we have shown a bandwidth of more than 40 GHz [4].

This project focusses on the improvement of the previously shown photonic crystal modulators. This can be done in three different ways:

  • Incorporating resonators into the photonic crystal modulators with Q-factors of 10k or more, the principle has been demonstrated earlier but the Q-factors reported were only 2.6k
  • Improving the poling process for the electro-optic polymer. The strength of the EO effect in the polymer depends critically on the poling step. It has been observed that poling in silicon nanostructures is much less efficient than in a thin film. The reasons for this behavior as well as possible solutions are investigated in this project.
  • Using improved materials with stronger electro-optic effect. Due to collaboration with the world leading group on electro-optic polymers we have access to new experimental materials which offer stronger electro-optic effects.

Results

 

We have shown in previous publications that the concept of a photonic crystal being modulated by means of an electro-optic polymer has several advantages. It features small footprint and an electrical bandwidth of 100 GHz. Theoretically modulations voltages as low as 1V should be feasible with the material parameters published in literature [5]. A scanning electron microscope image of the fabricated photonic crystal modulator is given in figure 1.

Figure 1. Scanning electron image of a photonic crystal electro-optic modulator.


Based on the given structures a proof of concept for low frequencies has been published but both the quality factor of the structure and the electro-optic effect of the polymer were lower than what was expected from the literature [6]. Later we showed that modulations frequencies up to 40 GHz can be measured in these structures [4].  A photo of the structured wafer with high frequency electrodes and a microscope image of the modulator are given in figure 2.

Figure 2. Photo of a structured silicon wafer with RF electrodes (l) and light microscope image of a photonic crystal electro-optic modulator (r).


Recently we have shown that using electron beam bleaching as a method for defining resonances in the photonic crystals yielded Q-factors of 20k which helps to reduce the modulation voltage [7].

List of publications

Wülbern, J.-H. 40 GHz electro-optic modulation in hybrid silicon–organic slotted photonic crystal waveguides, Optics Letters 35, 16, 2753-2755 (2010)

Wülbern, J.-H. Electro-optical modulator in a polymerinfiltrated silicon slotted photonic crystal waveguide heterostructure resonator, Optics Express 17, 1, 304-313 (2009)

Wülbern, J.-.H Electro-optic modulation in slotted resonant photonic crystal heterostructures, Applied Physics Letters 94, 24, 241107-3 (2009 )

Prorok, S. Photonic crystal cavity definition by electron beam bleaching of chromophore doped polymer cladding, Proc. SPIE 8425, Photonic Crystal Materials and Devices X, 842518 (2012)

Responsible

Stefan Prorok

Collaborations:

Prof. Dr. Seth Marder, Georgia Tech, School of Chemistry and Biochemistry, Atlanta, Georgia

Prof. Dr. Alex K.-Y. Jen,University of Washington , Department of Materials Science & Engineering, Seattle, Washington

References

  1. Ziebell, M. 40 Gbit/s low-loss silicon optical  modulator based on a pipin diode, Optics Express 20, 10, 10591-10596, (2012)
  2. Liu, A. High-speed optical modulation based on carrier depletion in a silicon waveguide, Optics Express 15,  2, 660-668 (2007)
  3. Ding, R. Demonstration of a low VπL modulator with GHz bandwidth based on electro-optic  -polymer-clad silicon slot waveguides, Optics Express 18, 15, 15618-15623 (2010)
  4. Wülbern, J.-H. 40 GHz electro-optic modulation in hybrid silicon–organic slotted photonic crystal waveguides, Optics Letters 35, 16, 2753-2755 (2010)
  5. Wülbern, J.-H. Electro-optical modulator in a polymerinfiltrated silicon slotted photonic crystal waveguide heterostructure resonator, Optics Express 17, 1, 304-313 (2009)
  6. Wülbern, J.-.H Electro-optic modulation in slotted resonant photonic crystal heterostructures, Applied Physics Letters 94, 24, 241107-3 (2009 )
  7. Prorok, S. Photonic crystal cavity definition by electron beam bleaching of chromophore doped polymer cladding, Proc. SPIE 8425, Photonic Crystal Materials and Devices X, 842518 (2012)