# Nanotechnology Questions and Answers – Optical Tweezers

This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Optical Tweezers”.

1. What was the first application of optical tweezers in the field of bioscience?
a) Synthesis of artificial cells
b) Trapping of an individual E.coli bacterium
c) Measurement of viscoelasticity of biopolymers
d) Sorting of cells

Explanation: The first application of optical tweezing was demonstrated by Arthur Ashkin and Joseph M. Dziedzic in the late 1980s. This technology was used to optically trap an individual Escherichia coli bacterium and the tobacco mosaic virus (TMV) .

2. Which of the given expressions correctly represent the relative trap frequencies for the radial and axial directions?
a) ωrz = 4ω0/mλ2
b) ωzr = 8mπ3/P0λ
c) ωrz = (√2)ω0π/λ
d) ωzr = 3P0/mω04

Explanation: Inorder to approximate Gaussian beam potential in both radial and axial directions of the beam, the intensity profile has to be extended to second order in z and r. Hence, for solving harmonic frequencies, they can be written as:
ωr = √{8P0/πmω04} and ωz = √{4P0λ2/mπ3ω06}
Therefore, the relative trap frequencies for the radial and axial directions can be given as :-
ωrz = (√2) ω0π/λ

3. What is the expansion of LOT?
a) Left Occiput Transverse
b) Laser Optic Technology
c) Lower Order Term
d) Lensless Optical Trapping

Explanation: LOT stands for Lensless Optical Trapping. This refers to the use of extended optical landscape patterns by evanescent field optical tweezers. They simultaneously guide a large number of particles into a preferred direction using a wave guide.

4. They possess the unique capability of trapping particles that are optically reflective and absorptive. This statement corresponds to which of the following laser beam modes used in optical tweezers?
a) Conventional TEM00 Gaussian beams
b) Hermite – Gaussian beams (TEMxy)
c) Bessel beams
d) Laguerre – Gaussian (LG) beams (TEMpl)

Explanation: Optical tweezers use different kinds of laser beam modes. One of them is Laguerre – Gaussian (LG) beams(TEMpl) . Tweezers using these beams have the unique capability of trapping particles that are optically reflective and absorptive. These beams possess a well – defined orbital angular momentum that rotates particles, without any external mechanical or electrical influence on the beam.

5. Which of the following is not an application of optical tweezers?
a) Study cell motility
b) Construct tissue like networks of artificial cells
c) Fabrication of synthetic and semi – synthetic polymers
d) Measure viscoelastic properties of biopolymers

Explanation: Optical tweezers have proven useful in different scientific fields. They have been used in the construction of tissue – like networks of artificial cells. They have found their applications in cell sorting, probing cytoskeleton, measuring viscoelasticity of biopolymers and initiating biochemical reactions by fusing synthetic membranes into one another. Inorder to study cell motility, optical tweezers have been used.

6. What is the force applied on a single charge in an electromagnetic field during electric dipole approximation of particles for optical trapping?
a) Spring force
b) Nuclear force
c) Frictional force
d) Lorentz force

Explanation: The force applied on a single charge present in an electromagnetic field is known as Lorentz force. It can be described as the combination of magnetic and electromagnetic force on a point charge due to the presence of electromagnetic fields. It is given by the expression;
F1=q(E1+dx1/dt*B)

7. What is the most suitable range of numerical aperture of the objective used in optical tweezers?
a) 3.5 – 4.5
b) 1.2 – 1.4
c) 10 – 15
d) 7.8 – 9.2

Explanation: Optical tweezers have a microscope objective as one of its components. The choice of this objective is very important. For a stable trap the gradient force should be greater than the scattering force. This gradient force depends on the numerical aperture (NA) of the objective. Suitable objectives typically have an NA between 1.2 and 1.4.

8. Find out the incorrect statement from the following.
a) There is no difference in minimum power needed for stable optical trapping of microscale and nanoscale particles
b) Optical tweezers are non – contact manipulation techniques for 3D nano positioning of arbitrary objects
c) Most of the optical traps operate with Gaussian beam (TEM00 mode) profile intensity
d) Translating bean steering lens leads to an extra degree of translational freedom

Explanation: Optical tweezers are the non – contact manipulation techniques for the 3D nano positioning of arbitrary objects. These tweezers make use of various types of beams for optical trapping. The most common one is TEM00 Gaussian beams. The setup of tweezers comprise of optics that can provide extra degree of translational freedom by translating the Beam Steering lenses. The minimum power needed for stable optical trapping differs for micro scale and nanoscale particles.

9. Identify an object from the given options that has not been levitated successfully.
a) Tungsten
b) Sodium carbonate
c) Black liquor
d) Nickel

Explanation: Levitation for metallic micro – spheres has not been achieved properly. For macroscopic objects levitation is only theoretically possible, and can be enhanced with nano – structuring. However, till date certain materials such as tungsten, nickel, aluminium oxide and black liquor have been successfully levitated.

10. Define optical levitation.
a) Holding up of an object in air or vacuum against gravitational force
b) Force of attraction occurring between the particles that are to be tweezed
c) Weakest force to exist between any two particles on Earth
d) Consequence of the curvature of space time caused by uneven distribution of mass

Explanation: Optical levitation refers to the floating of object in air or vacuum without additional support. Inorder to levitate a particle in air, the downward force of gravity must be balanced by the forces originating from photon momentum transfer. This prevents the lateral and vertical instabilities so as to form a stable optical trap that can hold microscopic particles in suspension.

11. How can you express the scattering force under the Rayleigh approximation?
a) Fscat(r) = q[E1+{(dx1/dt)×B}]
b) Fscat(r) = α[E2+{(dB/dt)×a3}]
c) Fscat(r) = (8πn0k4a6(m2-1/m2+2)2I(r))/3c
d) Fscat(r) = q[E1(x, y, z)–E2(x,y,z)+(dx/dt)]

Explanation: While optical trapping, the scattering force of light works against the gradient force in the axial direction of the trap. This scattering can be expressed under the Rayleigh approximation as;
Fscat(r) = (8πn0k4a6(m2-1/m2+2)2I(r))/3c
Where (a) is particle radius, (n0) is its refractive index, (m = n0/n1), which is the relative refractive index
between the particle and the medium, and c is the speed of light.

12. Optical binding of chiral nanoparticles are independent of the polarization of laser beams.
a) True
b) False

Explanation: Research on optical binding for a system of chiral nanoparticles has shown the dependency of its magnitude on certain factors. These include the polarization of the laser beam and the handedness of the interacting particles themselves.

13. Choose a statement from the given options that best describe the evanescent field.
a) Excitation of optical modes in a short piece of fiber allowing non – trivial trapping geometries
b) Optical field where leakage takes place during total internal reflection
c) Organization of the microparticles trapped within a monochromatic laser beam
d) A true 3D optical trapping based on a single fiber

Explanation: An evanescent field is a kind of an optical that leaks during total internal reflection, resulting in light fading off at an exponential rate. A continuous evanescent field can be created in optical tweezers when light is passing through an optical wavelength.

14. What factor is responsible for the attractive or repulsive force provided by optical tweezers?
a) The size of the particles to be tweezed
b) RI between particle and surrounding medium
c) The nature of the particles to be tweezed
d) The type of laser beam used in the tweezers

Explanation: Optical tweezers are instruments making use of laser beams to hold and move minute objects like nanoparticles, atoms, droplets etc. The laser light used by the tweezers provides either an attractive or repulsive force depending on the relative refractive index of particles and surrounding medium.

15. Identify the most common choice of laser in optical tweezers for working with biological specimens.
a) Ruby laser
b) Argon laser
c) Helium – silver (HeAg) laser
d) Nd: YAG laser

Explanation: An optical tweezer makes use of a laser beam usually an Nd: YAG laser of 1064nm wavelength for working with biological specimens. The choice is owing to the low absorption coefficient of the specimens at this wavelength. The low absorption can minimize the damage made to the specimens, during the process.

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