How does a dielectric interact with a laser beam?

  • #1

Homework Statement


A laser tweezer is a laboratory instrument, which uses highly focused laser beams to ‘trap’, hold or move small sized objects. The principle of the operation is that in the focal spot, the light intensity is inhomogeneous, and acts on the particle with a force that points from the low intensity region towards the high intensity region.
In this problem below, the object to be trapped is a nano-ball made of latex, which is insulating, has no net electrical charge, and is much smaller than the wavelength of the light. The ball is compact, homogeneous with mass m, radius R and of relative dielectric constant of ##ε_r##.The nano-ball is placed into a well focused, polarized laser beam (see figure). Let us approximate the laser light
in all points of the focal region as a plane wave moving in the x direction, with angular frequency of ω and with an amplitude which varies point by point. The time averaged intensity of the laser light can be approximated as $$ I=
I_0 \left( 1- \frac {x^2} {a^2 } - \frac {y^2} {b^2} - \frac {z^2} {b^2} \right) $$ in the region of |x| ≪ a; |y| ≪ b; |z| ≪ b (here a, b > 0).
a) Let us determine the coordinates of the equilibrium position of the ball, that is, the point where the trapping
force and the force from radiation pressure are equal. We can assume that the distance of the equilibrium position
from the origin of the coordinate system is much smaller than the parameters a and b, but much larger than the ball radius R. Let us use the laws of Maxwellian electrodynamics.

Homework Equations


Intensity
##I= \frac {P} {A}##
Where P and A denotes the power and cross sectional area respectively .
And the force of radiation is defined as
##F=\frac {Ic} {A}##

The Attempt at a Solution


Due to symmetry of the problem,we can choose z=0 for equilibrium position.
Now the force of radiation is $$F_{rad} =\left( \frac {I_0c} {\pi R^2} \right)\left( 1- \frac {x^2} {a^2} - \frac {y^2} {b^2} \right) $$
But I can't figure out what will be the expression for trapping force. And how this ball will interact with the laser beam.
 

Attachments

  • IMG_20181205_022059_150.jpg
    IMG_20181205_022059_150.jpg
    5.6 KB · Views: 205
Last edited:

Answers and Replies

  • #2
Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.
No,I don't
 

Related Threads on How does a dielectric interact with a laser beam?

  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
1
Views
6K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
0
Views
4K
  • Last Post
Replies
1
Views
3K
Replies
1
Views
4K
Replies
1
Views
809
  • Last Post
Replies
0
Views
2K
  • Last Post
Replies
0
Views
1K
Top