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25th November 200925th November 2009

Physical & Theoretical Chemistry

Cathy Rushworth

Supervisor: Dr. Claire Vallance

Direct absorption measurementsusing fibre-loop cavity ring-downspectroscopy

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Outline

Absorption spectroscopy

Results so far

Challenges ahead

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Absorption spectroscopy

Beer-Lambert Law: eI

I=

0

cl

Usually the path length is ~ 1 cm, but in a chip it can be 1000 times smaller How can we make absorption measurements on sample volumes this

small?

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CRDS with mirrors

)1( clRcd

+

=

In

tens

ity

Time

OKeefe and Deacon 1988

No absorberAbsorber present

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CRDS with fibres

Loock 2002

Fiberguide Industries, Inc.

1

2

n1sin1 =n2sin2Core RI = 1.457Cladding RI = 1.439

)( clLc

d

+

=

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Fibre signal

Light travels in the core and the cladding

In

tens

i ty

Time

Cladding

Cladding modes

Core modes

Start recording signalafter this point

Core

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Sample measurements

Direct absorption

Evanescent wave absorption

HF etching

Taper

CladdingCladding

CoreCore

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Optical set-up

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Sample introduction

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Fibre-loop ring-downs

Water = 224 ns

R6G 0.1125 mM =181 ns

50 um core diameter fibre, l = 24 um,

0.5 pL sample volume!

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End separation in water

200 um

105 um

100 um50 um

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SFS100

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Finding optimum l and d for SFS100

Waechter 2009

For a given abs C,

optimise l and d tomaximise sensitivity

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Lensing

100 um

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Preliminary results SFS50, l = 24 um, d = 4 m

Probed volume ~ 0.5 pL50 uM R6G: Detect ~ 2 fmoles, ~ 1 billion molecules

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Improvements

Minimize loop losses

Separate core and cladding modes

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Contact HF etching in situ

Non contact

Remembering Fibre alignment, Limit to end separation, RI (core) = 1.457

Page 17

Options for fibre chip coupling

Fibre ends

Chip

Chip

Uncleaved fibre

Chip

Fibre ends

Fibre ends

Fibre end

Chip Observationwindows

Plan view

Channel HFflow

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Important considerations

Fibre chip coupling parameters

End separation tolerances

Systems to study

Data acquisition rate?

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Acknowledgements

Thank you for listening

Thank you to Claire, Joao, Bobby and Joanna

Questions?