Evanescent and Gradient force on an optical waveguide

[Sciencestd]

Let's say that on the surface of the cladding we have evanescent field due to the total internal reflection between the core and the cladding. The refractive indices of the the core is 1.45 and the refractive index of the cladding is 1.4, and I want to use the gradient force of the evanescent field (Evanescent field exponentially decaying above the surface), so I casted particles that immersed in toluene which has refractive index of 1.5, do you think it will still evanescent field and not frustrated evanescent field because the refractive index of toluene is higher?!
Please answer me it is crucial.

 

[Paul Colby]

How thick is the cladding? Much will depend on how far down the evanescent wave has decayed at the outer cladding surface.

[edit] Actually, one could measure this by immersing the fiber in the medium and measuring the change in transmission loss.

 

[Paul Colby]

Another point, once into the 1.5 index the field will likely be non evanescent. I would suggest simplifying the problem by considering a plan geometry rather than a cylindrical one. Much easier to solve and it would give a feel for the answer.

The measurement of transmission loss I'm assuming a non evanescent field in the toluene.

 

[Sciencestd]

How thick is the cladding? Much will depend on how far down the evanescent wave has decayed at the outer cladding surface.

[edit] Actually, one could measure this by immersing the fiber in the medium and measuring the change in transmission loss.

Let's say the cladding is so thin and the evanescent field is bit large comparing to the cladding, that a portion of evanescent field will reach the droplet...,
can you please explain the point about transmission loss...

 

[Paul Colby]

Let's say the cladding is so thin and the evanescent field is bit large comparing to the cladding, that a portion of evanescent field will reach the droplet...,

There is a lab experiment people do in school where the hypotenuses of two 45 degree prisms are brought within very close proximity. A laser beam reflecting off one will tunnel to the second based on how far the evanescent wave has decayed outside the reflection surface. Basically the beam is no longer evanescent after it jumps the boundary.

can you please explain the point about transmission loss...

Sure. One can check if a given fiber leaks light into toluene by measuring the light lost when a section of the fiber is immersed. For any given fiber my bet is near no light at all will leak cause it wouldn't be a very good fiber design if it did.

I get the feeling your interest hinges on the field being evanescent in the toluene. I don't see why it would be. To make certain I would do a planar waveguide calculation as a sanity check first. If you're contemplating single mode fiber a full cylindrical geometry wouldn't be too difficult.

 

[Paul Colby]

You might have a look at

https://www.rp-photonics.com/lp_modes.html

 

[Andy Resnick]

Let's say that on the surface of the cladding we have evanescent field due to the total internal reflection between the core and the cladding. The refractive indices of the the core is 1.45 and the refractive index of the cladding is 1.4, and I want to use the gradient force of the evanescent field (Evanescent field exponentially decaying above the surface), so I casted particles that immersed in toluene which has refractive index of 1.5, do you think it will still evanescent field and not frustrated evanescent field because the refractive index of toluene is higher?!
Please answer me it is crucial.

In the n=1.5 region, any EM field that leaks through the cladding will be propagating, not evanescent. Are you trying some sort of optical trapping?

 

[Sciencestd]

In the n=1.5 region, any EM field that leaks through the cladding will be propagating, not evanescent. Are you trying some sort of optical trapping?

Yes exactly to trap particles... So they will not feel gradient force...?!
by the way how would the mode field get changed in this case?!

 

[Sciencestd]

Thank you so much Paul Colby for your answers, it really gave me direction in thinking..
I'm just now looking how would the mode field changed in region where I cast Toluene..

 

[Paul Colby]

There are high index fiber, 1.6 or greater that might be of interest?

http://przyrbwn.icm.edu.pl/APP/PDF/114/a114zA11.pdf

 

[Sciencestd]

There are high index fiber, 1.6 or greater that might be of interest?

http://przyrbwn.icm.edu.pl/APP/PDF/114/a114zA11.pdf

The point that I have to use a waveguide made in special way with specefic refractive indices as mentioned above. I casted a droplet of toluene contains molecules..., then I got aggregation above the waveguide... I thought it is because gradient force caused by evanescent field... but unfortunately it seems not because the toluene has higher refractive index... now I have to explain why I got aggregation up the waveguide...!
(and next step to use solution with lower refractive index)

 

[Paul Colby]

I can only guess what you're talking about. If there is data, we need to know about it to comment. Other than that, who can help?

 

[Andy Resnick]

Yes exactly to trap particles... So they will not feel gradient force...?!
by the way how would the mode field get changed in this case?!

That's unclear to me- I am aware of some similar approaches for 'nano-tweezers' but don't know too many details:

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-12-12-2429
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-22-13-16322
https://www.osapublishing.org/ol/abstract.cfm?uri=ol-21-21-1768
https://www.sciencedirect.com/science/article/pii/S0022407312002877

 

[Sciencestd]

I can only guess what you're talking about. If there is data, we need to know about it to comment. Other than that, who can help?

The particles has close absorption resonance at the light coupled in the waveguide... and if we say that a gaussian beam propagates in the fiber so anyway there is a gradient force in and out the plane (because the gaussian beam is decaying... So the question is does these two thing causes to molecules to aggregate on the waveguide!

 

[Sciencestd]

That's unclear to me- I am aware of some similar approaches for 'nano-tweezers' but don't know too many details:

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-12-12-2429
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-22-13-16322
https://www.osapublishing.org/ol/abstract.cfm?uri=ol-21-21-1768
https://www.sciencedirect.com/science/article/pii/S0022407312002877

Thank you so much man.. the articles are so useful..one of the articles I think it's so close to my case! we''ll see..