# Nanotechnology Questions and Answers – Optical Tweezers – Set 2

This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Optical Tweezers – Set 2”.

1. How is the harmonic potential approximation related to AC stark shift in the case of two-level atom?
a) ΔE = (3πc2Γμ){I(r, z)}/2ω03ծ
b) ΔE = H(t+T)
c) ΔE = (-ħΔ/2)±[ħ√(Ω22)]/2
d) ΔE = Φ{Ꜫ, t+(2π/w) }

Explanation: The harmonic potential experienced in the case of two-level atom is related to its AC stark shift or Autler – Towers effect as ΔEAC stark = (3πc2Γμ){I(r, z)}/2ω03ծ, where (Γ) is the natural line width of the excited state, (μ) is the electric dipole coupling, (ω0) is the frequency of transition and (ծ) is the difference between the laser frequency and the transition frequency.

2. Why are optical cell rotators preferred to standard optical tweezers?
a) Ends of fiber are not molded
b) Possess a total-internal-reflection geometry
c) Decoupling of trapping from imaging optics
d) Gradient force traps particles the transverse direction

Explanation: Optical cell rotators are advantageous over standard optical tweezers in decoupling of trapping from imaging optics. The modular design, the high compatibility of divergent laser traps with biological samples and the decoupling of trapping of the optical cell rotators lay path for the greater potential in the field of medical science and research.

3. How can the optical trapping of dielectric objects of dimensions far less than the wavelength of light be carried out?
a) Treatment of simple ray optics
b) Treatment of time dependent Maxwell equation
c) Treatment of particles as electric dipoles in the electric field
d) Treatment of time harmonic Maxwell equation

Explanation: For optical trapping of dielectric particles having dimensions far less than that of wavelength of light, particles are treated as electric dipoles in an electric field. While for particles having dimensions much greater than wavelength of light, a simple ray optics treatment is done. However, for particle dimensions within an order of magnitude of the beam wavelength, the treatment of either time dependent or time harmonic Maxwell equations using appropriate boundary conditions is applied.

4. Which of the following components is not likely to be found in an optical tweezer setup?
a) Boiler
b) Position detector
c) Condenser
d) Microscope objective

Explanation: An optical tweezer setup contains the following components:- a laser, a microscope objective, a beam expander, a position detector, a microscope illumination coupled to a CCD camera, a condenser and some optics to steer beam location in the sample plane.

5. Fluorescence optical tweezers are used to investigate the disaggregation machines in action.
a) True
b) False

Explanation: Fluorescence optical tweezers can manipulate and image fluorescent samples. They have been applied for simultaneous sensing and imaging of dynamic protein complexes using long and strong tethers. They have also been used in the investigations of disaggregation machines that are in action.

6. Optical traps are utilized for the detection of sub-nanometer displacements for sub-micron dielectric particles.
a) True
b) False

Explanation: Optical traps from the tweezers are quite sensitive and thus have the ability to manipulate and detect sub-nanometer displacements for sub-micron dielectric particles. Owing to this ability they have found their usage in single molecule studies. This can be done by interacting with a bead that has been attached to the molecule such as DNA, proteins, enzymes etc.

7. The Gaussian beam profile is not defined by which of these formulas?
a) P0 = (1/2)πI0ω02
b) ꞷ(z) = ω0√{1+(z/zR)2}
c) ZR = πω02
d) F = (1/2)α▽E2

Explanation: The intensity of a Gaussian beam profile can be characterized by the wavelength (λ), minimum waist (ω0) and power of the beam (P0). It can be defined using these expressions:
I. P0 = (1/2)πI0ω02
II. ꞷ(z) = ω0√{1+(z/zR)2}
III. ZR = πω02
IV. I(r, z) = I00/ꞷ(z))2 e-2(r2)/(ꞷ2)(z)

8. Which of the following does not lead to multiple traps in multiplex optical tweezers?
a) Using acoustic-optic deflectors to split a single laser beam into hundreds of tweezers
b) Time sharing a single beam of laser among multiple tweezers
c) Intrinsic rotating mechanism and orbital angular momentum of light
d) Splitting the beam into orthogonally polarized beams

Explanation: Multiplex optical tweezer has a setup that uses one laser to create multiple traps. Two traps can be made by splitting the laser beam into two orthogonally polarized beams. Time sharing a single laser beam among several optical tweezers results in many optical traps. However, to generate hundreds of optical traps acousto-optic deflectors or galvanometer – driven mirrors can be used.

9. While manipulating dielectric particles, laser light tends to apply a force on the particles in the beam. Why does this happen?
a) Conservation of mass
b) Conservation of momentum
c) Conservation of energy
d) Conservation of resources

Explanation: While manipulating dielectric particles, laser light tends to apply a force on the particles in the beam along the direction of propagation of the beam by virtue of conservation of momentum. The photons impart momentum to the dielectric particles after being scattered by them and this is known as the scattering force. This leads to the shift of the particles slightly downstream from the beam waist.

10. Who invented the optical tweezers?
a) Philip H. Jones
b) Dr. Donna Strickland
c) Onofrio M. Maragò
d) Dr. Arthur Ashkin

Explanation: Dr. Arthur Ashkin was born in September 1922. He pioneered optical tweezing, a method used to hold and move microscopic objects such as atoms and nanoparticles. He was even lauded with the 2018 Nobel Prize in physics for the development of optical tweezing.

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