Dr. Vadym Zayets

The AlGaAs waveguides grown on GaAs are used to delver light from/to an optical fiber to/from a plasmonic waveguide. Using visible light makes the fabrication and measurements relatively easy and simple. Optical field distribution in these waveguides should be very similar to the field distribution in a plasmonic waveguide. Experiment is here.

Results in short.

1. Decrease of thickness or width of AlGaAs core weakens confinement of waveguide mode. At the cutoff, the confinement is not sufficient to guide a waveguide mode.

2. Near cutoff, distribution of optical field of a waveguide mode is similar to the optical field of a surface plasmon. Therefore, the coupling between the waveguide mode near cutoff and the surface plasmons is efficient.

 

Dependence on waveguide width

 

The GaAs absorbs light at this wavelength. TM mode have higher absorption, because the optical field of TM mode is mostly inside the AlGaAs clad layer near the GaAs, which is an absorber. The optical field distribution of TM mode is very similar to the optical field distribution of an optical plasmon (example is here).

 

When waveguide width becomes narrow, the confinement of optical mode by the AlGaAs core layer becomes weaker and eventually disappears.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Dependence on thickness of core layer

In the case of Fig.6, for TM0 mode the cutoff thickness is about 210 nm

Fig.6 Animated figure. Waveguide similar to that of Fig.1-3, but thickness of is varied. Waveguide width is 2 um. Distribution of optical field inside AlGaAs waveguide for TM0 mode (left) and TE0 mode (center), Animated parameter is thickness of the AlGaAs core layer. Optical loss of TM0 and TE0 modes (right)

 

 

 

 

 

 

Dependence on thickness of clad layer.

It is desirable to reduce the thickness of clad layer to reduce growth time. However, as can be seen from Fig.4 optical loss significantly increases with the decrease of clad thickness. Thickness of clad layer should be thicker than 2.7 um.

Fig.4 Animated figure. Waveguide similar to that of Fig.1-3, but thickness of clad layer is 2 um. Distribution of optical field inside AlGaAs waveguide for TM0 mode (left) and TE0 mode (center), Animated parameter is waveguide width.

 

 

 

Dependence on etching depth

For the AlGaAs waveguide dry etching I am using ICP-RIE with BCl gas. It is not always easy to control precisely the etching depth. In case of Fig.5, the optical loss of TE mode practically does not depend on the etching depth. The optical loss of TM mode increases with deeper etching.

Fig.5 Animated figure. Waveguide similar to that of Fig.1-3, but etching depth is varied. Waveguide width is 1.7 um. Distribution of optical field inside AlGaAs waveguide for TM0 mode (left) and TE0 mode (center), Animated parameter is etching depth. Optical loss of TM0 mode (right)

 

 

 

 

Waveguide splitter

Waveguide Width=1; nEffect=3.3541; kx_between= 1.8; radius=20 um; length=18 um

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

---

 

Calculations

 

For calculations I have used Matlab or Matlab combined with Comsol Multiphysics

 

Files to download

*.m Matlab+ Comsol Files files

SiWaveguideModeWidth.m modes Si rib waveguide as function of rib width

WaveguideMode.m calculation of waveguide modes of rib waveguide (Comsol file: WaveguideMode.mph)

AlGaAsWaveguide_Scan.m calculation of waveguide modes of rib AlGaAs waveguide. Scan of different parameters. File used RibWaveGeometry.m