Evanescent and Gradient force on an optical waveguide

In summary: The problem with using toluene as the medium is that it has a higher refractive index than air, and the evanescent field would be dissipated as the droplet passed through the cladding. This is why aggregation may occur up the waveguide.
  • #1
Sciencestd
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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.
 
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  • #2
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.
 
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  • #3
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.
 
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  • #4
Paul Colby said:
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...
 
  • #5
Sciencestd said:
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.

Sciencestd said:
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.
 
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  • #7
Sciencestd said:
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?
 
  • #8
Andy Resnick said:
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?!
 
  • #9
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..
 
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  • #11
Paul Colby said:
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)
 
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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?
 
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  • #13
Sciencestd said:
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
 
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  • #14
Paul Colby said:
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!
 
  • #15
Andy Resnick said:
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..
 

1. What is an evanescent force on an optical waveguide?

An evanescent force on an optical waveguide is a phenomenon that occurs when an optical waveguide, such as a fiber optic cable, is placed near a surface. The force is created by the interaction between the light waves traveling through the waveguide and the electric field of the surface. This force can cause the waveguide to be attracted or repelled from the surface, depending on the properties of the surface and the waveguide.

2. How does the gradient force affect the behavior of an optical waveguide?

The gradient force is a component of the evanescent force that is caused by a variation in the intensity of the electric field along the surface of the waveguide. This force can cause the waveguide to be pulled towards areas of higher intensity or pushed away from areas of lower intensity. This can result in changes in the trajectory or stability of the waveguide, which can be useful for manipulating the position and movement of the waveguide in optical systems.

3. What factors affect the strength of the evanescent and gradient forces on an optical waveguide?

The strength of the evanescent and gradient forces on an optical waveguide can be affected by several factors, including the distance between the waveguide and the surface, the angle at which the waveguide is incident on the surface, the properties of the surface material, and the properties of the waveguide material. Additionally, the wavelength and polarization of the light traveling through the waveguide can also impact the strength of these forces.

4. Can the evanescent and gradient forces be controlled or manipulated?

Yes, the evanescent and gradient forces on an optical waveguide can be controlled and manipulated through various methods. For example, the strength of these forces can be adjusted by changing the distance between the waveguide and the surface, or by altering the properties of the waveguide or surface materials. Additionally, the use of external fields, such as electric or magnetic fields, can also be used to manipulate these forces.

5. What are some practical applications of evanescent and gradient forces on optical waveguides?

The evanescent and gradient forces on optical waveguides have a wide range of practical applications. They are commonly used in optical tweezers, which can manipulate small particles using the forces exerted by a focused laser beam on an optical waveguide. These forces are also utilized in biosensors, where changes in the evanescent and gradient forces can be used to detect and measure the presence of specific molecules on the surface of the waveguide. Additionally, these forces can be used in microfluidic systems to control the movement of fluids through tiny channels, and in optical communications to steer and manipulate light signals within waveguides.

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