11. Optical Tweezers Flashcards
What are optical tweezers?
A tool that manipulates nano- or micron-scale objects and measures very small forces by trapping particles within focused laser light.
How are objects moved using optical tweezers?
By moving the laser beam.
Give one use of optical tweezers
To measure the forces exerted by individual DNA molecules attached to small polymer beads.
What type of wave is light?
Electromagnetic (interacting transverse waves)
As light propagates through a vacuum/medium it carries _______.
Energy
Give the equation for an electric field oscillation
E = electric field
E0 = amplitude
ω = angular frequency
t = time
k = wavenumber
x = position
Give the equation for the time-averaged intensity of light
<i> = time-averaged intensity
c = speed of light
ε = permittivity
n = refractive index
<E²> = electric field magnitude</i>
Small dielectric particles experience a _____ when illuminated with visible light.
Force
When dielectric particles are placed in an intensity gradient, they will move to the region of ______ intensity.
Higher
Give the equation for the instantaneous change in energy associated with placing a particle in a laser beam
U = energy
α = polarisability
E = electric field
Give the equation for the time-averaged change in energy associated with placing a particle in a particle in a laser beam
U = energy
α = polarisability
c = speed of light
ε = permittivity
I = intensity
Give the equation for the force on a dielectric particle in an intensity gradient in one dimension
F = force
U = energy
x = position
α = polarisability
c = speed of light
ε = permittivity
n = refractive index
I = intensity
Give the equation for the force on a dielectric particle in an intensity gradient in three dimensions
F = force
U = energy
x/y/z = position
α = polarisability
c = speed of light
ε = permittivity
n = refractive index
I = intensity
The force acting on a dielectric particle in a laser beam always acts along the direction of _________ intensity gradient, so particles move to regions of _______ intensity.
Increasing
Higher
What shape is the beam profile of a laser?
Gaussian in the radial direction
Give the equation for the intensity of a laser
I = intensity
I0 = amplitude
x/y = position
w0 = half-width of beam
r = radius
What shape can a laser profile be approximated as (for small displacements)
A quadratic intensity profile
Give the equation for the approximated intensity of a laser
I = intensity
I0 = amplitude
r = radial position
w0 = half-width of the beam
Give the equation for the approximate force experienced by a particle in a laser beam
F = force
k = spring constant
r = radius
α = polarisability
I0 = amplitude
c = speed of light
ε = permittivity
n = refractive index
w0 = half-width of beam
How can optical trapping occur in three dimensions?
By changing the area of the beam near the focus so that the intensity of light decreases on either side, trapping the particle in the z-direction as well as x and y.
Give the equation for the numerical aperture
NA = numerical aperture
n = index of refraction
θ = beam angle
How does numerical aperture (NA) impact the intensity gradient?
Lenses with a higher numerical aperture maximise the intensity gradient.
What is a quadrant photodiode?
A photodiode arrangement that detects deflections in optical traps in both the x and y directions.
What are the two types of quadrant photodiodes?
- Shadowing (for large particles)
- Fringe pattern (for small particles)
Give two ways in which an optical trap can be calibrated
- Measure the mean square displacement of the particle under thermal motion.
- Measure the displacement of the particle under known forces.
What is the precision of optical trapping forces?
pN precision
