44 research outputs found
From far-field to near-field micro- and nanoparticle optical trapping
Optical tweezers is a very well-established technique that has developed into
a standard tool for trapping and manipulating micron and submicron particles
with great success in the last decades. Although the nature of light enforces
restrictions on the minimum particle size that can be efficiently trapped due
to Abbe's diffraction limit, scientists have managed to overcome this problem
by engineering new devices that exploit near-field effects. Nowadays, metallic
nanostructures can be fabricated which, under laser illumination, produce a
secondary plasmonic field that does not suffer from the diffraction limit. This
advance offers a great improvement in nanoparticle trapping, as it relaxes the
trapping requirements compared to conventional optical tweezers. In this work,
we review the fundamentals of conventional optical tweezers, the so-called
plasmonic tweezers, and related phenomena. Starting from the conception of the
idea by Arthur Ashkin until recent improvements and applications, we present
some of the challenges faced by these techniques as well as their future
perspectives. Emphasis in this review is on the successive improvements of the
techniques and the innovative aspects that have been devised to overcome some
of the main challenges.Comment: 15 pages, 10 figure
Chiral excitation of a single atom by a single photon in a guided mode of a nanofiber
We study the interaction between a single two-level atom and a single-photon
probe pulse in a guided mode of a nanofiber. We examine the situation of chiral
interaction, where the atom has a dipole rotating in the meridional plane of
the nanofiber, and the probe pulse is quasilinearly polarized along the radial
direction of the atom position in the fiber transverse plane. We show that the
atomic excitation probability, the photon transmission flux, and the photon
transmission probability depend on the propagation direction of the probe pulse
along the fiber axis. In contrast, the reflection flux and the reflection
probability do not depend on the propagation direction of the probe pulse. We
find that the asymmetry parameter for the atomic excitation probability does
not vary in time and does not depend on the probe pulse shape
Optical force between two coupled identical parallel optical nanofibers
We study the optical force between two coupled parallel identical nanofibers
using the rigorous array mode theory. We show that the forces of the even array
modes are attractive, while the forces of the odd array modes are repulsive. We
examine the dependencies of the optical forces on the array mode type, the
fiber radius, the light wavelength, and the fiber separation distance. We show
that, for a given power and a given separation distance, the absolute value of
the force achieves a peak when the fiber radius and the light wavelength are
appropriate.Comment: arXiv admin note: text overlap with arXiv:2012.0607
Optical trap for an atom around the midpoint between two coupled identical parallel optical nanofibers
We study the trapping of a ground-state cesium atom in a small region around
the midpoint between two coupled identical parallel optical nanofibers. We
suggest to use a blue-detuned guided light field in the odd
-sine array mode to produce an optical potential with a local
minimum of exact zero at the midpoint between the nanofibers. We find that the
effects of the van der Waals potential on the total trapping potential around
the minimum point are not significant when the fiber separation distance and
the power of the guided light field are large. For appropriate realistic
parameters, a net trapping potential with a significant depth of about 1 mK, a
large coherence time of several seconds, and a large recoil-heating-limited
trap lifetime of several hours can be obtained. We investigate the dependencies
of the trapping potential on the power of the guided light field, the fiber
radius, the wavelength of light, and the fiber separation distance.Comment: arXiv admin note: text overlap with arXiv:2012.0607
Fano-Resonant, Asymmetric, Metamaterial-Assisted Tweezers for Single Nanoparticle Trapping
Plasmonic nanostructures can overcome Abbe's diffraction limit to generate
strong gradient fields, enabling efficient optical trapping of nano-sized
particles. However, it remains challenging to achieve stable trapping with low
incident laser intensity. Here, we demonstrate a Fano resonance-assisted
plasmonic optical tweezers (FAPOT), for single nanoparticle trapping in an
array of asymmetrical split nano-apertures, milled on a 50 nm gold thin film.
Stable trapping is achieved by tuning the trapping wavelength and varying the
incident trapping laser intensity. A very large normalized trap stiffness of
8.65 fN/nm/mW for 20 nm polystyrene particles at a near-resonance trapping
wavelength of 930 nm was achieved. We show that trap stiffness on resonance is
enhanced by a factor of 63 compared to off-resonance conditions. This can be
attributed to the ultra-small mode volume, which enables large near-field
strengths and a cavity Purcell effect contribution. These results should
facilitate strong trapping with low incident trapping laser intensity, thereby
providing new options for studying transition paths of single molecules, such
as proteins, DNA, or viruses.Comment: 28 pages, 7 figure
Enhancement of the quadrupole interaction of an atom with guided light of an ultrathin optical fiber
We investigate the electric quadrupole interaction of an alkali-metal atom
with guided light in the fundamental and higher-order modes of a vacuum-clad
ultrathin optical fiber. We calculate the quadrupole Rabi frequency, the
quadrupole oscillator strength, and their enhancement factors. In the example
of a rubidium-87 atom, we study the dependencies of the quadrupole Rabi
frequency on the quantum numbers of the transition, the mode type, the phase
circulation direction, the propagation direction, the orientation of the
quantization axis, the position of the atom, and the fiber radius. We find that
the root-mean-square (rms) quadrupole Rabi frequency reduces quickly but the
quadrupole oscillator strength varies slowly with increasing radial distance.
We show that the enhancement factors of the rms Rabi frequency and the
oscillator strength do not depend on any characteristics of the internal atomic
states except for the atomic transition frequency. The enhancement factor of
the oscillator strength can be significant even when the atom is far away from
the fiber. We show that, in the case where the atom is positioned on the fiber
surface, the oscillator strength for the quasicircularly polarized fundamental
mode HE has a local minimum at the fiber radius nm, and is
larger than that for quasicircularly polarized higher-order hybrid modes, TE
modes, and TM modes in the region nm
Enabling self-induced back-action trapping of gold nanoparticles in metamaterial plasmonic tweezers
The pursuit for efficient nanoparticle trapping with low powers has led to
optical tweezers technology moving from the conventional free-space
configuration to advanced plasmonic tweezers systems. However, trapping
nanoparticles smaller than 10 nm still remains a challenge even for plasmonic
tweezers. Proper nanocavity design and excitation has given rise to the
self-induced back-action (SIBA) effect offering enhanced trapping stiffness
with decreased laser power. In this work, we investigate the SIBA effect in
metamaterial tweezers and its synergy with the exhibited Fano resonance. We
demonstrate stable trapping of 20 nm gold particles for on-resonant and
off-resonant conditions with experimental trap stiffnesses as high as 4.18
fN/(nm*mW/m and very low excitation intensity of about 1
mW/m. Simulations reveal the existence of two different groups of
hotspots per unit cell of the metamaterial array. The two hotspots exhibit
tunable trap stiffnesses and this is a unique feature of these systems. It can
allow for sorting of particles and biological molecules based on their size,
shape, and refractive index.Comment: 27 pages, 10 figure