Intracavity optical trapping with fiber laser
Author
Kalantarifard, Fatemeh
Advisor
Volpe, Giovanni
Date
2019-06Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
After Ashkin's seminal works, optical trapping has been a powerful technique for
capturing and manipulating sub micro particles not only in physics research fields
but also in biology and photonics. Standard optical tweezers consists of a single
beam with Gaussian or profile which focused by a high numerical aperture (NA)
water or oil immersion microscope objective. Typically, objective with NA>1.2 is
used to provide strong enough gradient forces being able to overcome Brownian
uctuations and gravity and trap the particle stably. On the other hand, compare
with high NA, trapping with low NA, has its own advantage and among all the
advantages, low local heating of the sample has a particular interest in molecular
biology and manipulating living cells. The main concern is that the interaction
of trapping laser beam and biological object induces a damage on the specimen
which is mainly due to light absorption of the sample. It is, therefore, recommended
to use NIR (near infrared ) wavelength due to its minimal absorption by
water and biological objects. Other important factors that must be considered, to
secure the viability of the cell, are spot size of the focused beam and laser power
at the sample plane. Thus, it deserves an effort to look for new configurations
with low NA with the capability of creating 3D confinement. Standard optical
tweezers rely on optical forces that arise when a focused laser beam interacts with
a microscopic particle: scattering forces, which push the particle along the beam
direction, and gradient forces, which attract it towards the high-intensity focal
spot. Importantly, the incoming laser beam is not affected by the particle position
because the particle is outside the laser cavity. Here, we demonstrate that
intracavity nonlinear feedback forces emerge when the particle is placed inside the
optical cavity, resulting in orders-of-magnitude higher confinement per unit laser
intensity on the sample. We first present a toy model that intuitively explains
how the microparticle position and the laser power become nonlinearly coupled:
The loss of the laser cavity depends on the particle position due to scattering,
so the laser intensity grows whenever the particle tries to escape. We describe a
simple toy model to clarify how the nonlinear feedback forces emerge as a result
of the interplay between the particle's motion and the laser's dynamics. It also
quantifies how and to what extent this scheme reduces the average laser power
to which a trapped particle is exposed. In this model, the power and hence trapping
force are considered to be zero for small particle displacements. However,
in reality they have small values that do operate the trap even when the particle
is near the equilibrium position. Thus, we need an accurate description of the
coupling between the laser and the trapped particle thermal dynamics at equilibrium
to compare with experiments. In particular, accurate simulations can help
to associate an effective harmonic potential to the optical trap for small displacements
from the equilibrium position, and hence to define a meaningful stiffness
using the standard calibration methods based on the thermal
uctuations of a
trapped particle. We therefore present a series of numerical simulations based on
an extended theoretical model, including highly realistic descriptions of the laser
dynamics, optical losses incurred by the particle, and the particle's Brownian motion
in order to gain a quantitative understanding of the dynamics of intracavity
optical trapping and to guide the experiments. Finally, guided by the simulation
results, we have built an experimental setup to prove the operational principle
of intracavity optical trapping and experimentally realize this concept by optically
trapping microscopic polystyrene and silica particles inside the ring cavity
of a fiber laser. One of the major advantages of the intracavity optical trapping
scheme is that it can operate with very low-NA lenses, with a consequent large
field-of-view, and at very low average power, resulting in about two orders of
magnitude reduction in exposure to laser intensity compared to standard optical
tweezers. When compared to other low-NA optical trapping schemes, positive
and negative aspects can be considered, such as in terms of trap stiffness and average
irradiance of the sample. These features can yield advantages when dealing
with biological samples. Ultra-low intensity at our wavelength can grant a safe,
temperature controlled environment, away from surfaces for micro
uidics manipulation
of biosamples. Accurate studies on Saccharomices cerevisiae yeast cells in
near-infrared counterpropagating traps and standard optical tweezers have found
no evidence for a lower power threshold for phototoxicity. We observed that we
can 3D trap single yeast cells with about 0:47 mW, corresponding to an intensity
of 0:036 mW m2, that is more than a tenfold less intensity than standard
techniques.