Interaction of light with
wavelength-scale structures
Alex Argyros
Optical Fibre Technology Centre
University of Sydney
www.oftc.usyd.edu.au/mpof
www.oftc.usyd.edu.au/mpof
Outline
• A bit about my…
– Department
– Research Group
– Self
• Interaction of light with wavelength-scale structures
– Microstructured (polymer) optical fibres
– Photonic bandgap fibres
– Nano-particles and quantum dots in fibres
www.oftc.usyd.edu.au/mpof
Optical Fibre Technology Centre
Physics
USyd
Redfern
OFTC
ATP
www.oftc.usyd.edu.au/mpof
Optical Fibre Technology Centre
www.oftc.usyd.edu.au/mpof
Optical Fibre Technology Centre
• Established in 1989 to study optical fibres
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Multidisciplinary department
20 researchers + technical staff
15 students at any one time
Students mainly from Physics and Engineering
• Microstructured polymer optical fibres (mPOF)
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Invented at the OFTC in 2000
Now…~15 people @ USyd + 10 collaborations internationally
48 publications in 2006…
1 book, 1 book chapter, 12 journal papers, 7 invited conference papers,
19 conference papers, 6 news articles, 2 prizes…
www.oftc.usyd.edu.au/mpof
How I got into research…
• BSc – physics and maths
– Research projects through TSP + summer vacation scholarships
– 2001- Honours in physics
• 2002 – Research Assistant
• 2002-2006 – PhD in physics
• Research Fellow @ OFTC 2006 – ?
– Received 3 year fellowship from Uni.
www.oftc.usyd.edu.au/mpof
Optical Fibres
• Guide light through total internal
reflection
n
• High refractive index core
• Low index cladding
www.oftc.usyd.edu.au/mpof
Microstructured Fibres
or Photonic Crystal Fibres
• Invented in 1996 at the
University of Bath, UK
• Optical fibre made of silica
glass with small holes…
• Can replace holes with solid
rods of different materials, fill
holes with liquid or gas…
• Convenient way of studying
the behaviour of light
www.oftc.usyd.edu.au/mpof
Microstructured Polymer Optical
Fibres
• Invented at the OFTC in 2000
• Fundamentally same as their silica counterparts except for
different
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Material properties
Dopants available
Processing
Design of microstructure, hole size, shape, position etc…
www.oftc.usyd.edu.au/mpof
Examples of mPOF
www.oftc.usyd.edu.au/mpof
Simplest case – modified TIR
• The holes are too small and the light can’t
“see” them
• Cladding is an average of the two materials
polymer + air
• Only supports one mode
– Fundamental mode is too “big” and
gets trapped by the holes
– Other modes slip through the gaps
www.oftc.usyd.edu.au/mpof
Endlessly single-mode
• Normally:
– Short wavelength = relatively larger core
– Larger core = more modes can fit
• In Endlessly single-mode fibres
– Shorter wavelength = relatively larger core
– BUT shorter wavelength = larger gap between the holes (higher
cladding index)
– The two effect cancel each other completely
www.oftc.usyd.edu.au/mpof
Hollow-core fibres
• Hollow core, total internal
reflection not possible
• Guide light using Bragg
reflection or Photonic
Bandgaps
• Simplest case – Bragg Fibre
R
or
T
wavelength
www.oftc.usyd.edu.au/mpof
Hollow-core fibres…
1D – Bragg fibres
Ring-structured Bragg fibres
2D but can behave like 1D
MIT, 2004
www.oftc.usyd.edu.au/mpof
Hollow-core fibres…
• Everything scales linearly with the structure size
• Bandgaps/Bragg reflection shifts to shorter
wavelength
Relative size
1.00
0.70
0.66
0.62
0.38
3rd & 2nd
5th bandgap
4th bandgap
(red+ green = yellow)
www.oftc.usyd.edu.au/mpof
2D-lattice hollow-core fibres
Blaze Photonics, 2004
www.oftc.usyd.edu.au/mpof
Resonances
Ignore for now
Low index
cladding
High index
core
• Approximate the cladding as a set of
high index rods (glass) in a low index
background (air)
• Each rod is a step index fibre
Blaze Photonics, 2004
www.oftc.usyd.edu.au/mpof
Resonances
• Each rod in the cladding supports discrete modes…
• Bring lots of rods together to form bands of modes
• Same idea as the band model for electrons in a solid
Band of modes
n
Photonic
bandgap
One rod
Two rods
Many rods
www.oftc.usyd.edu.au/mpof
Resonances
n
• The larger hole (the core) creates a
defect
• The defect supports it’s own modes
• Similar to doping a semiconductor
Cladding
Core
Cladding
www.oftc.usyd.edu.au/mpof
Resonances
• If two modes have the same n they are resonant
– The light can travel through the core AND cladding
• If two modes have different n they are antiresonant
– The light cannot propagate through the cladding, so it stays
in the core
n
• Anti-Resonant Reflecting Optical Waveguides (ARROW)
Cladding
Core
Cladding
www.oftc.usyd.edu.au/mpof
Resonances
• High index rods (GeO2 + SiO2) in low index (SiO2) background
• Bandgap in the red – core and cladding are antiresonant for
red wavelegnths
• Green light travels through the cladding, red doesn’t
• Difference in refractive index between rods and background is
only 1%
www.oftc.usyd.edu.au/mpof
Resonances
Stop ignoring this!
Low index
cladding
High index
core
• The entire cladding can be
understood in terms of resonances
with the
University of Bath, 2007
– Rods
– Struts
– Holes
www.oftc.usyd.edu.au/mpof
Changing Resonances
• In the simplest case the V parameter determines the
resonances:
V=
2π
λ
rco nco2 − ncl2
• Shift resonance by changing
– Size of structure
– Index contrast in structure
– Fill holes with water to shift resonance to a shorter wavelength
by 61%.
www.oftc.usyd.edu.au/mpof
Conclusion
• Light interact with wavelength-scaled structures in interesting
ways
• This has been used to control light in ways that were
previously impossible
– Using microstructured fibres
• Dopants can be added to add further functionality
• Making something work is one thing….understanding why it
works is another…
www.oftc.usyd.edu.au/mpof
Doping
• Wavelength scaled structures don’t have to be part of the
fibre itself
• Add them
– In the polymer itself
– Fill the holes with a suspension
• Silica nano-particles (~150 of nm)
• Metallic nano-particles (< 100 nm)
• Quantum dots (~10 nm’s)
www.oftc.usyd.edu.au/mpof
Doping
• The light interacts with these dopants to produce other
wavelengths through processes like
– Fluorescence
– Lasing
– Raman Scattering
• Convenient to put them in fibres because the light is guided
by the fibre…
www.oftc.usyd.edu.au/mpof
Functionalizing microstructured
polymer optical fibre
Come to a lunchtime OSA session:
Short presentation by Felicity Cox, a physics PhD student
And of course FREE Pizza!
1pm, Next Tuesday 8th May 2007
Slade Lecture Theatre, School of Physics
Cost is FREE but please RSVP to Bill:
[email protected] or 9351 5978
www.oftc.usyd.edu.au/mpof