Plasmonic Enhancement of Single Methylene Blue Molecule Fluorescence Near Individual

Gold Nanorods

Tianchu LiangOptical Science GroupNanoBiophysics GroupUniversity of Twente

2014-08-19

Introduction

Image (left) from Resonant Plasmonic Enhancement of Single-Molecule Fluorescence by Individual Gold Nanorods.Image (right) from Jesse Mak’s Thesis.

• Single Molecule imaging and spectroscopy requires high photon counts by emitters.

Introduction-Emitters• Methylene Blue

http://en.wikipedia.org/wiki/Methylene_blue

Images from Jesse Mak’s Thesis, Fig 2.1.

Introduction-Plasmonic Enhancement

• Excitation Enhancement– Confine the light field to nano structure’s near

surface.– The shape of nano structure is important.

• Emission Enhancement– Altering emitters radiative and nonradiative decay

rates of the emitter molecules.

Excitation Enhancement

Images from Jesse Mak’s Thesis, Fig 3.3, Fig 3.4.

• Optical properties described by a frequency dependent dielectric function.

• Plasmonic resonances in quasi-static structures.• Plasmonic resonances in elongated structures.

Fabry-Perot Resonantor

Emission Enhancement

Intrinsic quantum yield:

New quantum yield:

Emission Enhancement

Images from Jesse Mak’s Thesis, Fig 4.1.

Quantum yield of an emitter near a gold sphere, as a function of their separation dis- tance, for emitters with different intrinsic quantum yields (Φ0q ).

Methylene Blue has 1%-2% quantum yield.

My job over the summer

• Separate emission enhancement from the total enhancement.

• Possible quenching caused by emitters sticking to the nanorods.

Experiment-SetupNanorod sample covered with solution

Objective

640nm Laser

Dichroic Filter

664nm Long pass

APD

Experiment-Nanorod sample

Images from Jesse Mak’s Thesis。

Experiment-MB solution

• 102.8nM of Methylene blue in glycerol.

Pick the right nano rod

Images from Jesse Mak’s Thesis.

Experiment

• Hit the same nano rod with two polarizations, along short axis, and along long axis. – Short axis-> Emission Enhancement.– Long axis-> Emission Enhancement + Excitation

Enhancement.• For a certain polarization, vary the light focus

height: aiming at nano rods, slightly above nano rods, a lot higher than the nano rods.

Results-Emission Enhancement

Results-Total Enhancement

Results-With varying light focus heightEnhancement factors

Distance from the nanorods (nm)

Emission Enhancement

Total Enhancement

Conclusion

• At the maximal enhancement, the total enhancement is about 1.41 times stronger than the emission enhancement.

• As the light focus ascend from nanorods to 600nm above, the total enhancement tends to decrease. But the emission enhancement is maximum at 150nm above the nanorods.

• There might be quenching for emission enhancement when emitters are too close to the nano rods.

Future work

• More data to be taken.• Autocorrelation (FCS) curves may give useful

information.

Acknowledgements

• University of Twente• Optical Science Group, NanoBiophysics Group• Jord Prangsma• Amin Abolghassemi Fakhree• Jennifer Herek