ANFFL logo - go to ANFFL Home Australian National Fabrication Facility

ACT Node and WA Node

Latest News

Taking Glass Nanowires to the Max

March 2011

All-optical processing in highly non-linear waveguides is currently a subject of considerable interest as it enables the single channel data rate in a telecommunications network to far exceed those imposed by the limited bandwidth of electronics. All-optical processors utilise nonlinear optical phenomena in waveguides to manipulate light using processes such as four-wave mixing (FMW). FWM can be used for wavelength conversion; demultiplexing; performance monitoring; dispersion compensation; etc in an optical network.

To achieve efficient processors, optical nanowires made from highly nonlinear materials, like silicon or chalcogenide glass, are required since these structures provide a strong nonlinear response because the optical field is confined with a sub-micron waveguide "core" which enhances the light intensity. Furthermore, the small transverse profile can reduce the footprint of the device and also allows the optical dispersion to be engineered to achieve devices with THz bandwidth. However, whilst there has been a lot of interest in using silicon waveguides as the nonlinear material, its strong two-photon absorption (TPA) and free carrier absorption (FCA) degrade performance.

As a result, Xin Gai, a PhD student supervised by Prof. Luther-Davies, working within the Centre for Ultra-high bandwidth Devices for Optical Systems (CUDOS) at the Laser Physics Centre (LPC), ANU has developed devices based on chalcogenide glasses because they offer extremely high optical non-linearities without TPA and FCA.

In the work reported here, the LPC designed and fabricated highly non-linear nanowires from a new Ge11.5As24Se64.5 chalcogenide glass. In order to achieve nonlinear processing over the widest bandwidth, the nanowires need to be designed to achieve near zero optical dispersion. Using a finite-difference time-domain (FDTD) mode solver, the dispersion parameter was calculated as a function of wavelength and film thickness as shown in Figure 1. The zero-dispersion point is indicated by the blue curve. Finally, a structure 630nm wide and 500nm thick was selected for fabrication.

Figure 1 Figure 2
Figure 1. Dispersion parameter as a function of wavelength
and film thickness. The blue curve represents zero-dispersion.  
Figure 2. SEM pictures of nanowires.
Inset shows hybrid glass top cladding.

To fabricate the nanowires, 500nm thick Ge11.5As24Se64.5 was deposited onto silicon oxidized substrate by thermal evaporation, and PMMA was spin-coated onto the film as a resist. E-beam lithography, using the Raith 150 machine at the ANFF ACT Node facility, was used to transfer the waveguide pattern into the resist. Using the special fixed beam moving stage (FBMS) capability of this machine allowed for stitching errors between different write fields to be eliminated, reducing optical loss in the nanowires. The pattern was then transferred into the glass film using ICP dry etching with CHF3. A 5nm layer of Al2O3 was deposited by atomic layer deposition to enhance the adhesion and passivate the etched surfaces. At the end of the processes, a hybrid glass was spun onto the structure as a top cladding. The SEM picture of nanowires is shown in Figure 2.

To test the design, researchers measured the non-linear parameter of the nanowire using continuous wave FWM (Figure 3a). The conversion efficiency from signal to idler was measured with an optical spectrum analyser and confirmed to vary with the square of the pump power in the nanowires (Figure 3b). A value for the non-linear parameter of 136,000(Wkm)-1 was obtained - this is the highest value ever achieved in a glass waveguide!

To demonstrate the effectiveness of the nanowire as a nonlinear device LPC researchers used it to generate supercontinuum requiring only an 18mm long nanowire driven with 20W peak power pulses 1ps long. This confirmed the strong nonlinear response and that correct dispersion had been obtained in the design (Figure 3c).

Devices such as these will find wide application in high speed and all-optical nonlinear processing in the future.

Figure 3a Figure 3b Figure 3c
Figure 3. (left to right) - (a) The CW FWM spectrum, (b) Square law between pump power and conversion efficiency,
(c) Spectrum of supercontinuum generation in 18mm nanowires.

Story courtesy Xin Gai and Prof. Barry Luther-Davies - Laser Physics Centre, The Australian National University.