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ENLIGHTEN (Jurnal Bimbingan dan Konseling Islam)

A mesoscopic blue-detuned light ring for Bose-Einstein Condensates based on conical refraction

Alex Turpin, Juan Polo, Yu. V. Loiko, J. Küber, F. Schmaltz, Todor K. Kalkandjiev, V. Ahufinger, G. Birkl, J. Mompart

Published Jan 1, 2015• 0 citations
OpenAlex

Abstract

Conical refraction (CR) is an optical phenomenon observed in biaxial crystals (BCs) that has been a subject of renewed interest since the last decade [1], despite being predicted by W.R. Hamilton in 1832. CR occurs when a light beam propagates along one of the optic axes of a biaxial crystal with birefringence R0 = l·α, where l is the length of the crystal and α is its conicity [2]. Under such conditions, an input focused Gaussian beam with beam waist radius w0, is transformed into two concentric bright rings at the focal image plane of the system, as long as the condition R0 >> w0 is satisfied. These bright rings are separated by a thin ring of null intensity known as the Poggendorff dark ring (PDR), see Fig. 1.

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Conical refraction (CR) is an optical phenomenon observed in biaxial crystals (BCs) that has been a subject of renewed interest since the last decade [1], despite being predicted by W.R. Hamilton in 1832. CR occurs when a light beam propagates along one of the optic axes of a biaxial crystal with birefringence R0 = l·α, where l is the length of the crystal and α is its conicity [2]. Under such conditions, an input focused Gaussian beam with beam waist radius w0, is transformed into two concentric bright rings at the focal image plane of the system, as long as the condition R0 >> w0 is satisfied. These bright rings are separated by a thin ring of null intensity known as the Poggendorff dark ring (PDR), see Fig. 1.
We present an efficient three-dimensional dark-focus optical trapping potential for neutral atoms and Bose-Einstein condensates. This ``optical bottle'' is created by a single blue-detuned light field exploiting the phenomenon of conical refraction occurring in biaxial crystals. The conversion of a Gaussian input beam to the bottle beam has an efficiency of close to 100% and the optical setup requires the addition of the biaxial crystal and a circular polarizer only. Based on the conical-refraction theory, we derive the general form of the potential, the trapping frequencies, and the potential barrier heights. We present experiments on confining a 87Rb^{87}\mathrm{Rb}87Rb Bose-Einstein condensate in three dimensions. We determine the trap shape, the vibrational frequencies along the weak axis, as well as the lifetime of ultracold atoms in this type of potential.
We propose a novel gravito-optical atom trap composed of a blue-detuned, conical dark-hollow-beam propagating upward and the gravity field. When cold atoms are loaded into the conical dark-hollow-beam from a magnetic-optical trap and bounce inside the hollow beam, they experience an efficient hollow-beam induced Sisyphus cooling and repumping-beam induced geometric cooling. Our investigation shows that an ultracold and dense 87 Rb atomic sample with an equilibrium 3D temperature of about 1 μK can be obtained and the appearance of an optically-trapped Bose-Einstein condensation may be possible in this pure optical trap.
We propose an optical lattice for cold atoms made of a one-dimensional stack of dark ring traps. It is obtained through the interference pattern of a standard Gaussian beam with a counterpropagating hollow beam obtained using a setup with two conical lenses. The traps of the resulting lattice are characterized by a high confinement and a filling rate much larger than unity, even if loaded with cold atoms from a magneto-optical trap. We have implemented this system experimentally, and demonstrated its feasibility. Applications in statistical physics, quantum computing, and Bose-Einstein condensate dynamics are conceivable.
Matter wave interferometry offers a novel approach for high precision measurements, such as the determination of physical constants like the local gravity constant g or the fine-structure constant. Since its early demonstration, it has become an important tool in the fields of fundamental and applied physics. The present work covers the implementation of matter wave interferometers as well as the creation of novel guiding potentials for ultra-cold ensembles of atoms and Bose-Einstein condensates for this purpose. In addition, novel techniques for the manipulation of atoms with Bragg lattices are presented, serving as elements for interferometry. The measurements in this work are performed with a Bose-Einstein condensate of 25000 87 rubidium atoms created in a crossed optical dipole trap. The crossed optical dipole trap is loaded from a magneto-optical trap and allows a measurement every 25 s. This work introduces the novel technique of double Bragg diffraction as a tool for atom optics for the first time experimentally. The creation of beamsplitters and mirrors for advanced interferometric measurements is characterized. An in depth discussion on the momentum distribution of atomic clouds and its influence on double Bragg diffraction is given. Additionally experimental results for higher-order Bragg diffraction are explained and double Bragg diffraction is used to implement a full Ramsey-type interferometer. A second central result of this work is the implementation of novel guiding structures for ultra-cold atoms. These structures are created with conical refraction, an effect that occurs when light is guided along one of the optical axis of a bi-axial crystal. The conical refraction crystal used to operate the novel trapping geometries is a KGd(WO4)2 crystal that has been specifically cut orthogonal to one of the optical axis. Two regimes are discussed in detail: the creation of a toroidal matter wave guide and the implementation of a three-dimensional dark focus. Additional geometries accessible with conical refraction are introduced and possible applications are shown. The first regime characterized in detail is the creation of a toroidal wave guide for ultra-cold atoms and Bose-Einstein condensates. With the aid of a lightsheet potential atoms are trapped in a quasi two-dimensional ring geometry. Inside of the geometry atoms are accelerated, decelerated and held for extended storage times of up to two seconds. First attempts for the implementation of a Mach-Zehnder-type interferometer in a toroidal trap are presented. The second regime shown is the creation of a three-dimensional dark focus that is used to trap atoms in a repulsive confinement of light. The parameters of the dark focus are investigated in detail. Future application of a two-dimensional array of dark foci is shown by demonstrating the respective light field.
A new geometry of optical lattice is proposed, namely a lattice made of a 1D stack of ring traps. It is obtained though the interference pattern of two counterpropagating beams: one of the beam is a standard gaussian beam, while the other one is a hollow beam obtained through a setup with two conical lenses. The resulting lattice is shown to have a high filling rate and a good confinement, so that it could be loaded directly from a MOT with applications in the domain of quantum computing, or with a Bose-Einstein condensate, which would have in this case a 1D ring geometry.