Institut für Optische und Elektronische Materialien

Third Order Nonlinear Photonics

Introduction

An optical communication technology with large bandwidth necessitates the ability to switch light with light. To this end third order nonlinear effects such as Cross-Phase-Modulation (XPM) and Four-Wave-Mixing (FWM) have been employed. In on-chip communication networks silicon is widely accepted as the material to fabricate guiding structures, because there is a mature fabrication technology for silicon, and due to its large refractive index silicon allows to confine light in small volumes. We are using slotted silicon waveguides [1] that have been infiltrated with third order nonlinear polymers provided by our collaborators at Georgia Tech [2, 3] and silicon strip waveguides that have been covered with chalcogenides (Ge-Sb-Se, Ge-As-Se) [4]. High mode confinement in slotted photonic structures and wires yields large intensities, which is beneficial for the efficiency of nonlinear processes. Also there are no free carriers in polymers or chalcogenides, which reduces the nonlinear losses and makes polymer infiltrated slotted waveguide structures and chalcogenide covered strip waveguides a promising scheme to have highly efficient nonlinear processes.

Goals

-          Design, fabrication and characterization of polymer infiltrated slotted waveguide structures.

-          Parametric amplification and highly efficient wavelength conversion in micro-photonic waveguides.

Methods

Simulation methods:

-          Finite Integration Technique (CST Micro Wave Studio, www.cst.com)

-          Finite Difference Mode Solver (wgms3D, [5])

-          Finite Difference Time Domain (MEEP, [6])

-          Split-Step Fourier Method (to be published)

Manufacturing methods

-          Electron beam lithography of SOI wafers

-          DUV lithography of SOI wafers

-          ICP

-          Spin coating of polymers

-          Pulsed laser deposition of chalcogenides

Experimental methods:

-          Transmission characterization with end-fire and grating coupling scheme.

-          Pulse shape characterization using interferometric cross-correlation.

Results

As a polymer we are using a 7C-TCF chromophore that has been doped in PMMA. This chromophore has been optimized in terms of nonlinear refractive index.

By using nonlinear polymer functionalized silicon slotted waveguides we can confine light in very small areas and at the same time guide the light predominantly in the nonlinear polymer. Effective areas as low as A_eff = 0.13 µm² can be achieved which are even smaller than those observed in silicon nanowires (A_eff = 0.41 µm²) (cf. [7]). Here we have used the definition of effective area reported in[7].

We investigate the nonlinear process of (degenerate) four wave mixing which involves the interaction of four waves of three different wavelengths, denoted pump, signal and idler, where the two identical pump fields are far stronger than the other two. We characterize the performance of the waveguides by conversion efficiency which is defined as
G_i = P_idler(z = L) / P_signal (0).

Figure 1: Electrical field of the TE polarized mode in a silicon slotted waveguide infiltrated with polymer. The arrow indicates the direction of the electrical field.	Figure 2: Electrical field of the TM polarized mode in a silicon waveguide covered with chalcogenide. The arrow indicates the direction of the electrical field.

Slotted waveguides with slot width of 90nm have been fabricated and coated with 7C-TCF-TOA in PMMA. In the experiment only the TE polarized mode as shown in Fig. 1 is used. So far a conversion efficiency of -14dB could be achieved in a 750µm long waveguide with a pump peak power of 1W and a signal peak power of 0.2W, even though a conversion efficiency of -1.32dB would have been expected at the excitation conditions used. Measures to close the gap between simulation and experiment are undertaken now.

In a first attempt strip waveguides with Ge-Sb-Se cladding have been fabricated and tested. In the experiment only the TM polarized mode as shown in Fig. 2 is used. A conversion efficiency of -8dB could be observed with the presently used Ge-Sb-Se composition, a pump peak power of 1W and a signal peak power of 0.11W in a waveguides of 7.1mm length. In simulation a conversion efficiency of -8.15dB would have been expected if n₂=25∙10^-18m²/W and β_TPA=0.1cm/GW are assumed. Presently optimization of chemical composition and deposition as well as optimization of design are investigated in order to enhance conversion efficiency.