My Research and Inventions Surface Plasmons

### AlGaAs waveguide (lambda=1550 nm)

#### The AlGaAs waveguides grown on GaAs are used to delver light from/to an optical fiber to/from a plasmonic waveguide. Optical field distribution in these waveguides should be adjusted to be as similar as possible to the field distribution in a plasmonic waveguide. The GaAs substrates is transparent at this wavelength. However, due to high refractive index of GaAs(n=3.375), which is higher than the refractive index of the waveguide core, light is leaking into GaAs causing optical loss of waveguide mode. At a cutoff the mode effective refractive index is equal to refractive index of the clad layer. The leakage into the GaAs substrate becomes large.

The difference between AlGaAs waveguide between wavelengths of 800 nm and 1550 nm is that at wavelength=1550 nm the GaAs substrate is transparent.

## 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.

3. If in AlGaAs waveguide at wavelength of 800 nm the loss is due to absorption of light by GaAs substrate, at wavelength of 800 nm the loss is due to the leakage of light into the GaAs substrate. The optical loss causing by the leakage of light into the substrate is very difficult to calculate by FEM.

## Waveguide structure Fig.1 Structure of AlGaAs waveguide

refractive index of AlGaAs(x=0.5) clad layer =3.051

refractive index of AlGaAs(x=0.3) core layer =3.23

refractive index of GaAs substrate =3.375

## Dependence on core layer thickness

In the case of Fig.6, for TM0 mode the cutoff thickness is about 410 nm   Fig.3 Animated figure.Distribution of optical field inside AlGaAs waveguide for TM0 mode (left) and TE0 mode (center). Animated parameter is thickness of AlGaAs core layer. Waveguide width is 2 um. Effective refractive index of TM0 and TE0 modes (right). Optical loss due to leakage into GaAs was ignored.

## Dependence on waveguide width   Fig.6 Animated figure. Distribution of optical field inside AlGaAs waveguide for TM0 mode (left) and TE0 mode (center), Animated parameter is waveguide width. Effective refractive index of TM0 and TE0 modes (right). Optical loss due to leakage into GaAs was ignored.

## Calculations

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

*.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