Heung-Ryoul Noh

Generation of a dark hollow beam by a small hollow fiber

Abstract A new and simple method to generate a dark hollow beam is proposed and demonstrated. This method is based on a micro-collimation technique for the output beam of a hollow optical fiber. In this experiment, the dark spot size of about 50 μm to 100 μm at the propagation distance Z of 100 mm to 500 mm is obtained. The relative divergent angle at the near field is about 6.5 × 10 −5 . The potential applications of the hollow beam in atom optics are also discussed.

Atom optics with hollow optical systems

Abstract Recent theoretical as well as experimental progress in atom optics using optical means—in particular, hollow optical systems such as pyramidal hollow mirror, conical hollow mirror, and hollow-core optical fiber—are presented. These hollow optical systems provide the dark hollow region where atoms can be manipulated in a way so as to minimize the external perturbations due to the optical fields once the atoms are cooled and trapped. Therefore, such systems provide very simple and reliable ways of preparing precooled atoms for further elaborate atom optics experiments and also for introducing them efficiently into low-dimensional space, such as the 2D atomic channel or the 1D atomic wire.

Diffraction-limited dark laser spot produced by a hollow optical fiber.

By using the diffracted field of the LP(11) mode of a hollow-core optical fiber, we have produced a micrometer-sized, focused dark laser spot in the near field of the fiber. The minimum half-width of the dark spot is less than 1 mum . In particular, by masking the hollow core and metal coating the cladding with a microsphere, we blocked the light propagating in the cladding and obtained a clean dark spot, which may be useful in atom-optical experiments such as with atomic lenses, atom traps, and atom switches.

Spontaneous symmetry breaking of population in a nonadiabatically driven atomic trap: an Ising-class phase transition.

We have observed spontaneous symmetry breaking of the population of Brownian particles between two moving potentials in the spatiotemporally symmetric system. Cold atoms preferentially occupy one of the dynamic double-well potentials, produced in the parametrically driven dissipative magneto-optical trap far from equilibrium, above a critical number of atoms. We find that the population asymmetry, which may be interpreted as the biased Brownian motion, can be qualitatively described by the mean-field Ising-class phase transition. This in situ study may be useful for investigation of dynamic phase transition or temporal behavior of critical phenomena.

Diffraction-limited optical dipole trap with a hollow optical fiber

We propose a novel three-dimensional diffraction-limited far-off-resonance optical dipole trap (DFORT) for neutral atoms operating in the Lamb–Dicke regime. Such a microscopic DFORT is generated by the diffracted LP01 mode of a hollow-core optical fiber near the fiber facet. For 87Rb and 133Cs atoms, we estimate that with a 100-mW trap-laser power, a cigar-shaped far-off-resonance optical dipole trap provides potential depth U0≃10xa0mK, radial (axial) trap frequency fr≃100xa0kHz(fz≃10xa0kHz), and trap volume Vtrap≃104λ3. The DFORT has the unique feature of a tightly focused trap with a large trap volume and convenient loading and cooling of the precooled atoms, which may be useful for optical Bose–Einstein condensation. A DFORT may be also operated as an elongated one-dimensional optical trap.

An optically-guided atomic fountain

Summary form only given. The optical guidance of cold atoms by the hollow laser beam (HLB) with blue detuning has been well understood experimentally and theoretically (Xinye Xu et al, 2000). Based on this clear understanding and the development of an efficient atomic guiding, we have performed the experiment of optically guided atomic fountain by using a blue-detuned cylindrical HLB. An HLB was generated from a micron-sized hollow optical fiber and collimated by the objective lens and propagates downwards to the center of a magneto-optical trap (MOT). We have demonstrated tenfold enhancement of the atom-funneling efficiency for the HLB-guided atomic fountain, which may lead to improved performance of atom optical experiments based on the atomic fountain such as the Rb atomic clock.

An Asymmetric Magneto-Optical Trap

Since the advent of the magneto-optical trap (MOT) in 1987 (Raab et al., 1987), it has been intensively studied and widely used as a pre-cooled atomic source for various experiments (Metcalf & van der Straten, 1999). In addition, there have been intensive studies on the MOT itself such as cold collisions (Walker & Feng, 1994), nonlinear optics (Tabosa et al., 1991), existence of sub-Doppler force (Wallace et al., 1994), or limit of density (Townsend et al., 1995). Nevertheless MOT itself is far from quantitative understanding and still keeps providing surprises as unexplored characteristics and applications are being developed. In the perspective of nonlinear dynamics in a MOT, there were several reports as follows: Sesko et al. observed several variations of atomic spatial distribution and abrupt change between the distributions when there exist laser beam misalignment, intensity imbalance or radiation trapping (Walker et al., 1990; Sesko et al., 1991). They explained the phenomena by optical torques exerted by the misaligned trapping lasers. Based on the studies of Sesko et al., Bagnato et al. have observed the limit cycles and some abrupt changes of atomic spatial distributions (Bagnato et al., 1993; Dias Nunes et al., 1996). Recently, Wilkowski et al. found the instability phenomena in a MOT and explained them by means of shadow effect (Wilkowski et al., 2000; di Stefano et al, 2003). In addition, MOT exhibits very unique collective effects and critical behaviors when the number of atoms increases such as instability-induced pulsation (Labeyrie et al., 2006), and plasma oscillations of ultracold neutral plasma (Kulin et al., 2000). In this article we present experimental and theoretical works on the applications of the magneto-optical trap by modifying the trap conditions, which is termed as an asymmetric magneto-optical trap (AMOT). This article is composed of three parts: In Sec. 2, we describe the parametric resonance achieved by the modulation of the trap laser intensities. When the modulation frequency is near twice the natural frequency of the trap and the modulation amplitude exceeds a threshold value, the parametric resonance can be excited; i.e., the trapped atoms are divided into two parts and oscillate in opposite directions. The various theoretical and experimental studies are presented. Section 3 is devoted to measurement of trap parameters by the method of parametric resonance and transient oscillation. By decreasing the modulation amplitude of the parametric excitation down to its threshold value one can measure the trap frequency. In the case of transient oscillation, the trap frequency and damping coefficient were obtained by measuring the trajectory of the atoms returning to the original trap center, after the applied uniform magnetic field, used for