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OPTICAL SENSING USING NANO-STRUCTURES Waleed S. Mohammed
BU-CROCCS Bangkok Univerisy
SENSING
Sensor
Environment
Something measureable
SENSOR
Selective interaction
Transducer
Environment
D
Something measureable
TRANSDUCER
Correlates change of a physical parameter to a
measurable value:
Refractive index to light intensity
Capacitance to Voltage drop
Chemical reaction to color
Chemical reaction to electrical current
TRANSDUCER
Transducer
Optical Electrical Chemical
D :: refractive index
:: attenuation
D :: Resistance
:: Capacitance
OPTICAL TRANSDUCER: REFRACTIVE INDEX
Refractive index
Phase
Scattering
Guidance
Resonance
Intensity levels
Detection
Electrical current
SCATTERING
Light scatters in different direction due to surface roughness
Surface roughness
n1
ns
Incident region
Substrate
SCATTERING CHANGE WITH INDEX
Two extremes
n1
ns
n1
ns
ns >> n1 ns == n1
n1
ns
In between
SCATTERING MEASUREMENT
Direct measurement (1)
Detector
SCATTERING POLAR PLOT
Polar plot of scattering pattern
weak scattering
Strong scattering
SCATTERING MEASUREMENT
Direct measurement (2)
Dete
cto
r
Dete
cto
r
POLAR PLOT OF SCATTERING PATTERN
REFRACTIVE INDEX AND SCATTERING
Change of index changes the scattering pattern
In direct measurement, polar plots change with the
environment.
INDIRECT MEASUREMENT OF SCATTERING
Leakage from waveguide
In waveguides, light is trapped in the guiding region through total internal reflection
nf
ns
n1
INDIRECT MEASUREMENT OF SCATTERING
Leakage from waveguide
Surface roughness causes light to scatter outside the waveguide (leakage)
nf
ns
n1
INDIRECT MEASUREMENT OF SCATTERING
Power reduces by distance nf
ns
n1
z
Outp
ut pow
er
z
n1
Dete
cto
r
EXPERIMENTAL EXAMPLE
Roughness on nano-wires
C. Viphavakit, N. Atthi, S. Boonruang, C. Themistos, M. Komodromos, W. S. Mohammed, and B. M. Azizur Rahman,
"Realization of a polymer nanowire optical transducer by using the nanoimprint technique," App. Opt. Vol. 53, No. 30 (2014)
EFFECT OF REFRACTIVE INDEX ON ATTENUATION
Attenuation is reduced with increase of refractive index
C. Viphavakit, N. Atthi, S. Boonruang, C. Themistos, M. Komodromos, W. S. Mohammed, and B. M. Azizur Rahman,
"Realization of a polymer nanowire optical transducer by using the nanoimprint technique," App. Opt. Vol. 53, No. 30 (2014)
INDIRECT MEASUREMENT OF SCATTERING (2)
Side coupling: part of the scattered light is coupled
to the waveguide
nf
ns
n1
Dete
cto
r
Incident light
qc
COUPLING THROUGH SCATTERING
Scattered angles higher than critical angle are coupled to the waveguid
EFFECT OF INDEX CHANGE ON OUTPUT POWER
Coupled light leaks through scattering too
Higher n1
less leakage
higher output power
Low n1
higher leakage
lower output power
nf
ns
n1
De
tecto
r
Incident light
zo
COMPARING TWO CASES
nf
ns
n1
De
tecto
r
Incident light
zo
nf
ns
n1
De
tecto
r
Incident light
z
Almost no alignment required
alignment required
INDUCED SURFACE ROUGHNESS
Polishing
Growth of nano-rods
ASE OSA
Red light scattered from fiber
124.762μm
100μm
(a)
Medium: Air Medium: Glucose solution, DI water
(b)
h h h h
ncore
nclad
n
SIDE COUPLING TO FIBER WITH NANORODS
EXPERIMENTAL MEASUREMENT OF LEAKAGE
H. Fallah, M. Chaudhari, T. Bora, S. W. Harun, W. S. Mohammed, and J. Dutta, "Demonstration of Side Coupling to
Cladding modes through Zinc oxide Nanorods grown on Multimode Optical Fiber," Optics letters Vol 38 Issue 18 (2013)
DEMONSTRATION OF METHANOL SENSING
REMEMBER
Selective interaction
Transducer
Environment
D
Something measureable
Refractive index
Optical transd.
Optical power
SELECTIVE INTERACTION
Optical transducer is not a sensor unless the
selective interaction process is performed
It typically means surface processing for selective chemical or biological binding
CURRENT WORKING EXAMPLE
Binding or protein on ZnO nano-rods
Measure the scattering process using the direct
method (2) before and after binding
Correlate the change of the polar plot with the concentration
BEFORE AND AFTER BINDING
Dete
cto
r
Dete
cto
r
SUMMARY (1)
Sensors:
Transducer
Selective interaction
Transducer:
Electrical
Optical
Chemical
Transducer transfers the environment change into something measurable
SUMMARY (2)
Change of environment:
Refractive index
Absorption
Refractive index change can affect:
Phase through
Scattering properties
Phase changes are measured with interferometry
SUMMARY (3)
Scattering can be measured by:
Direct method:
Rotating the detector (Nephlometer)
Rotating the sample
Indirect method:
Leakage from waveguid
Side coupling to fiber
THANK YOU