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Purdue Univ, Prof. Shalaev, http://cobweb.ecn.purdue.edu/~shalaev/
Univ Central Florida, CREOL, Prof Kik, http://sharepoint.optics.ucf.edu/kik/OSE6938I/Handouts/Forms/AllItems.aspx
28. Selected Modern Applications28. Selected Modern Applications
(Optical crystal 과는 다른 개념이다.)
Λ
1-D
2-D
3-D
We know the origin of electronic Energy Band Gaps
Gap in energy spectra of electrons arises in periodic structureOrigin of energy gap : Bragg reflection of electron waves
22
2E k
m= Conduction band
Valence band
Band gap
Energy of free electrons Electron energy in crystal
Periodic lattice structure
a ~ nm
Wavelength does not correspond to the period
Reflected waves are not in phase.
Wave propagates through.
Wavelength corresponds to the period.
Reflected waves are in phase.
Wave does not propagate inside.
Bragg reflection = Bragg diffraction = Bragg scattering
Bragg reflection in crystals
Incident wave
Wavewave is such that
Origin of the energyband gap
We know the Bragg condition : 2 ( )B Bna sinλ θ= ⋅
2B naλ =2
BB
ka
π πλ
= =If θ = 90 deg.
a
In same way, we may define a new terminology : Photonic Band Gap (PBG)
Dispersion relation of a EM wave in free space
ω
c kn
ω =
H L H L H L
Bragg reflection from a periodic index structure
Photonic band gap (PBG)
PBG
Photons with energy in the PBG does not propagate inside the structure.
a ~ wavelength
Therefore, the Photonic crystals mean
Air band
Dielectric band
Band Gap
periodic structures with photonic band gaps (PBG)and their lattice constants are comparable to wavelength
0π/a
ω
k
Natural Opals
aka πλ ==2
1. Dispersion curve for free space
2. In a periodic system, when half the wavelength corresponds to the periodicity
the Bragg effect prohibits photon propagation.
3. At the band edges, standing waves form, with the energy being either in the high or the low index regions
4. Standing waves transport no energy with zero group velocity
PBG formationPhotonic band gap
n1 n2 n1 n1 n1n2 n2
Dispersion relation
n1: high index materialn2: low index material
bandgap
0π/a
standing wave in n1
standing wave in n2
4. Standing waves transport no energy with zero group velocityω
k
Dispersion curve = Photonic band structure
Dispersion Relation
This reduced range of wave vectors is called the “Brillouin zone”
Plot the dispersion curves for both the positive and the negative sides, and then shift the curve segments with |k|>π/a upward or downward one reciprocal lattice vectors.
Dispersion curve = Photonic band structure
2D Photonic band structure
2-D Photonic Crystals
1. In 2-D PBG, different layer spacing, a, can be met along different direction. Strong interaction occurs when λ/2 = a.
2. PBG (Photonic band gap) = stop bands overlap in all directions
Band Diagram
Stop band
Air band
Dielectric band
2D Photonic band structure
Four Possible Functionalities of PBG1. Use of Stop Band
1. Stop Band: Use PBG as high reflectivity omni-directional mirror (PBG waveguides)
Stop band
1. Stop band
2. Dielectric Band: Uses the strong dispersion available in a photonic crystal(dispersion engineeringwith form birefringence)
Dielectric band
2. Use of Dielectric Band
2. Dielectric band
2. Dielectric band
2. Dielectric band
3. Use of Air Band
3. Air band
3. Air Band : Couples to radiativemodes for light extraction from high-efficiency LEDsand fiber coupling.
Air band
3. Air band
4. Use of Defect Band
4. Defect band
4. Defect Band : Couples to waveguide/cavity modes for spectral control such as PBG point defect laser or PBG line defect filter, etc.
Defect band
Defects in PBG4. Defect band
Line Defect PBG Waveguide
Waveguide modes exist within the bandgap.
Photons are prohibited in the 2D PBG,which lead to lossless confinement of photons in the line defect area.
Defect modes in stop band
4. Defect band
4. Defect band
3D Photonic materialsS.Noda, Nature (1999) K. Robbie, Nature (1996)
3D Photonic band structure
E. Yablonovitch, PRL(1989)
Artificial Phonic StructureE.Yablonovitch et al., PRL (1987, 1991)
Fabrication of artificial fcc material and band gap structure for such
material.
3D Photonic band structure
Artificial Opal
Artificial opal sample (SEM Image)Several cleaved planes of fcc structure are shown
3D Photonic band structure
Fabrication of artificial opals
Silica spheres settle in close packed hexagonal
layers
There are 3 in-layer positionA – red; B – blue; C –green;Layers could pack infcc lattice: ABCABC or ACBACBhcp lattice: ABABAB
3D Photonic band structure
Inverted Opals
Inversed opals obtain greater dielectric contrast than opals.
3D Photonic band structure
PCF
Photonic Crystal Fibers
PCF
PCF
PCF