THORLABS EDU-OT3, User Manual

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Portable Optical Tweezers Kit Chapter 3: Principles of Optical Tweezers
Rev B, February 15, 2022
Page 15
Influence of the Laser Power
While the numerical aperture affects the ratio of gradient and scattering force, the laser
power directly influences the individual forces. In Equations (13)
𝐹𝐹
𝑡𝑡
≔ 𝐹𝐹
𝑧𝑧
= 𝑛𝑛
𝑚𝑚
𝑃𝑃
𝐵𝐵𝑆𝑆𝐺𝐺𝑚𝑚
𝑐𝑐
𝑄𝑄
𝑡𝑡
and (15),
𝐹𝐹
𝑆𝑆
≡
𝐹𝐹
𝑦𝑦
= 𝑛𝑛
𝑚𝑚
𝑃𝑃
𝐵𝐵𝑆𝑆𝐺𝐺𝑚𝑚
𝑐𝑐
𝑄𝑄
𝑆𝑆
we see that both forces increase proportionally with the laser power
𝑃𝑃
. When the laser
power is increased, each individual force gets stronger. However, their ratio (which
essentially determines if optical tweezing ocurrs or not) remains the same, see Equation
(28).
Combining what we know about the numerical aperture and laser power, we can note that
the increase in trapping strength can be achieved through two factors:
• Increase of the numerical aperture of the objective so that the gradient force
exceeds the scattering force.
• Increase of the laser power , which increases the tweezer’s strength linearly if the
condition
𝐹𝐹
𝑔𝑔 , 𝑡𝑡𝑚𝑚𝑡𝑡
> 𝐹𝐹
𝑠𝑠 , 𝑡𝑡𝑚𝑚𝑡𝑡
is met.
This also tells us that increasing the numerical aperture is the more effective way to
improve trapping than increasing the laser power. While the laser power increases the
trapping force linearly, Figure 9 shows a stronger than linear behavior above the trapping
limit.