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