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Photonics

Improving the Performances of Optical Tweezers by Using Simple Binary Diffractive Optics

Kamel Aı̈t-Ameur, Abdelkrim Hasnaoui

Published Aug 8, 2024• 0 citations
Publisher

Abstract

Usually, optical tweezers for trapping atoms or nanoparticles are based on the focusing of a Gaussian laser beam (GB). The optical trap is characterised by its longitudinal stability (LS), expressed as the ratio of the backward axial gradient and the forward scattering forces. Replacing the GB with a LGp0 beam (one central peak surrounded by p rings) does not improve the LS because the on-axis intensity distribution is the same whatever the mode order p. However, it has been recently demonstrated that a restructured LGp0 beam can improve greatly the LS. In this paper, we consider the restructuring of a LGp0 beam when passing through a simple binary diffracting optical element called a circular π-plate (CPP). For a particular radius of the dephasing zone of the CPP, it is found that the LS is multiplied by a factor corresponding to a relative increase of about 220% to 320%.

DOI
10.3390/photonics11080744

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Usually, optical tweezers for trapping atoms or nanoparticles are based on the focusing of a Gaussian laser beam (GB). The optical trap is characterised by its longitudinal stability (LS), expressed as the ratio of the backward axial gradient and the forward scattering forces. Replacing the GB with a LGp0 beam (one central peak surrounded by p rings) does not improve the LS because the on-axis intensity distribution is the same whatever the mode order p. However, it has been recently demonstrated that a restructured LGp0 beam can improve greatly the LS. In this paper, we consider the restructuring of a LGp0 beam when passing through a simple binary diffracting optical element called a circular π-plate (CPP). For a particular radius of the dephasing zone of the CPP, it is found that the LS is multiplied by a factor corresponding to a relative increase of about 220% to 320%.
We propose a scheme to trap cold atoms (or molecules) by using an improved red- or blue-detuned far-field optical trap, which is formed by an optical system composed of a binary phase plate and a circular aperture illuminated by a plane light wave. We calculate the relative intensity distribution of the far-field trap and study its dependence on the phase $\ensuremath{\varphi}$ of the binary phase plate. Our study shows that the binary phase plate can be used to improve some geometric and optical parameters of the far-field optical trap, and realize the evolution of the far-field optical trap from a red-detuned trap to a blue-detuned one. In particular, when the phase $\ensuremath{\varphi}$ of the binary phase plate is changed from 0 to $\ensuremath{-}\ensuremath{\pi}$, the maximum intensity of the far-field trap and its well depth will be enhanced by $\ensuremath{\sim}4$ times, and the corresponding trapping volume will be increased by $\ensuremath{\sim}8$ times, with respect to a trapping beam obtained from the diffraction by a circular aperture.
An enhanced or improved far-field-diffracted optical dipole trap with a red or blue detuning for cold atoms or molecules on an optical chip is proposed, and our trap scheme is formed by an optical system composed of a binary phase plate and a circular aperture illuminated by a plane light wave. The relative radial and axial intensity distributions of the far-field optical trap and its optical potentials for 87Rb atoms are calculated, and the dependences of the relative intensity of the far-field optical trap and its well depth on the modulated phase φ of the phase plate are studied. Also, the feasibility of our enhanced far-field optical trap is analyzed, and some potential applications of our optical trap are briefly discussed. Our research shows that some geometric and optical parameters of the far-field optical trap can be greatly improved, and the evolution of the far-field optical trap from a red-detuned optical trap to a blue-detuned one can be realized by using our proposed binary phase plate. Particularly, with the change of the modulated phase φ from 0 to −π, the maximum intensity (i.e., the well depth) of the far-field optical trap and its trapping volume will be increased by about four and eight times, respectively. While the modulated phase φ of the phase plate is changed from 0 to π, a red-detuned optical trap will be evolved as a blue-detuned one, and the corresponding trapping intensity and volume will be enhanced by about 4 times. So such an enhanced far-field optical trap scheme has some important applications in the fields of laser cooling and trapping; atomic, molecular, and optical physics; integrated atom or molecule optics; quantum information science; and so on.
Optical tweezers technique has aggrandized the light-matter interaction, whereby researchers have employed light forces from colloidal sciences to biomedical applications. There have been constant quests to integrate novel beams in tweezers system for myriads of novel applications. In this paper, we report optical tweezers system based on self-imaged optical bottle beams, which has some distinct advantages over other conventional beams like Gaussian, Laguerre-Gaussian and Bessel beams in few applications. A self-imaged bottle beam is obtained by superimposing two Bessel beams with varying radial wavevectors. A self-imaged bottle beam possesses three-dimensional intensity-null points along the propagation axis and by varying the radial wavevectors of the Bessel beams number of bottles; location of bottles and the dimension of the bottles can be pre-determined. Moreover, by dynamically varying the parameters of radial wavevector of interfering Bessel beams, the dynamic potential-well obtained are capable of transporting both high and low indices microparticle in conveyor-belt arrangement along the longitudinal axis. Bottle beams belongs to the family of propagation-invariant beams and hence inherent the propagation-invariance and self-reconstruction properties of Bessel beam together with three-dimensional trapping potential contrary to Bessel beam whereby the particles trapped in the central core are loosing defined in the longitudinal axis. Such bottle beams also holds promise applications in blue-detuned optical taps for the optical confinement of laser-cooled atoms in three-dimensional dark region.
In this paper, we propose a novel scheme to produce a longitudinal controllable double-well optical dipole trap for cold atoms (or cold molecules), which is composed of a binary π-phase plate and a lens. The π-phase plate consists of two homocentric rings with equal area and opposite phase (0 and π), and its central region is opaque. When a plane light wave passes through the above optical system, a double-well optical trap will be formed at two sides of the focal point along the optical axis. The outer radius of the binary π-phase plate can be controlled by adjusting the radius of a diaphragm, so that a double-well optical trap will evolve and finally combined into a single-well, or vice versa. We briefly introduce the basic principle and derive several optimal parameters of the trap, and show the dependence of the optical parameters (including intensity distributions) on the geometrical parameters of the system. Our study shows that the proposed controllable optical trap can be used not only to trap cold atoms (or cold molecules) and realize all-optical, double-well or two species Bose-Einstein condensation (BEC), but also to study the trapped-atom (-molecule) interference, or form a 2D array of double-layer optical traps for cold atoms (or molecules), even to prepare a novel 2D optical lattice, and so on.
This thesis describes the experimental and theoretical work that investigated the transfer of orbital angular momentum from light to matter. This was achieved by combining two established areas of laser physics which were "optical tweezers" and Laguerre-Gaussian laser modes. The optical tweezers are essentially a tightly focussed laser beam from a high numerical aperture microscope objective lens, which traps particles in three dimensions just below the beam focus. By incorporating a Laguerre- Gaussian laser mode into the tweezers system, the trapping efficiency was doubled. These improved optical tweezers have been successfully demonstrated both theoretically and experimentally. In addition to the spin angular momentum which is associated with the polarisation state, the Laguerre-Gaussian laser modes also possess orbital angular momentum. The "optical spanners" utilised this property by transferring orbital angular momentum from the laser beam to the trapped particle, causing it to rotate whilst being held in the optical trap. This effect was theoretically modelled and experimentally observed. Using the optical spanners, the spin angular momentum of the laser was used to directly cancel the orbital angular momentum in the beam, which was observed as a cessation in rotation of the trapped particle. This demonstrated the mechanical equivalence of the spin and orbital components of angular momentum in a light beam, and gave experimental evidence for the well defined nature of the orbital angular momentum present in Laguerre-Gaussian laser modes.