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Photonic Crystal Waveguides: Simulation and Fabrication. Antonios Giannopoulos ECE 345 December 5, 2003. Overview. Objective Theoretical Background Band Simulation and Results FDTD Simulation and Results Fabrication Process: Ideality vs. Reality Fabrication Results Conclusion.
December 5, 2003
Electrons in atomic crystals
Photons in photonic crystal
-Periodic boundary conditions result in Bloch states.
-In atomic crystal this is realized by an electronic band structure
-Similar thing happens in photonic crystals
*Stephen G. Johnson, MIT
••••Plane Wave expansion method••••
Fourier expansion and
Get linear eigenvalue problem
FDTD = Finite Difference Time domain
Basically discretize time and space
and solve Maxwell’s Equations with ’s instead of
Measure the output
Scanning the wavelength of injected light gives transmittance and thus location of photonic band
Red = air, White = GaAs (n ≈ 3.5)
In plane electric field band gap at 1.55m for a =.558 m
Inject light and see what happens
Simple check for guidance: Does light come out of the side?
If yes then no guidance
If light seems to be guided, check to make sure it is.
Looks like guidance but is very lossy.
There is a simple solution.
Vary width of defect to maximize transmittance
dX=.2 gives 96% transmittance over 8 m
Also need to check for loss due to non-optimal slab thickness
Let slab thickness = d*a. Then scan d.
….Know good design, time to make devices….
SiO2 deposition using PlasmaLab PECVD
Ti deposition using Cooke evaporator
SiO2 etch using PlasmaLab Freon RIE
Use FEI Focused Ion Beam (FIB)
Uses focused beam of Gallium ions to sputter atoms off the surface
Process very sensitive to changes in beam conditions
Use PlasmaTherm Inductively Coupled Plasma (ICP) RIE to etch holes
Etch time ~ 8.5 min
Use 10:1 NH4F:HF buffered oxide etch to remove SiO2 and undercut AlAs.
Etch time ~ 2 min
ICP etches thin oxide left next to holes by FIB.
This results in the GaAs underneath to be etched for part of the process.
-Holes are no longer cylindrical.
-Wider at the top than at the bottom.
Pattern for FIB
14.5 Periods Horizontally, 20 Periods Vertically
Designed for 420nm diameter holes
FIB stigmation problem results in elliptical holes
Beam doesn’t turn off
Holes made too large cause the structure to fall apart after undercut.
Puts upper limit on hole size and thus band gap size
Tried to directly FIB the GaAs surface.
But… sputtered atoms have to go somewhere, i.e. back into the holes
Smaller holes give more sloped sidewall
Tilted so that interior of holes is visible.
Notice sloped sidewalls and non-uniform edge
FIB stigmation adjusted. Holes ended up close to being circle.
Previous picture with normal view