Metastable hard-axis polar state of a spinor Bose-Einstein condensate under a magnetic field gradient

We investigate the stability of a hard-axis polar state in a spin-1 antiferromagnetic Bose-Einstein condensate under a magnetic field gradient, where the easy-plane spin anisotropy is controlled by a negative quadratic Zeeman energy $q<0$. In a uniform magnetic field, the axial polar state is dynamically unstable and relaxes into the planar polar ground state. However, under a field gradient $B'$, the excited spin state becomes metastable down to a certain threshold $q_{th}$ and as $q$ decreases below $q_{th}$, its intrinsic dynamical instability is rapidly recalled. The incipient spin excitations in the relaxation dynamics appear with stripe structures, indicating the rotational symmetry breaking by the field gradient. We measure the dependences of $q_{th}$ on $B'$ and the sample size, and we find that $q_{th}$ is highly sensitive to the field gradient in the vicinity of $B'=0$, exhibiting power-law behavior of $|q_{th}|\propto B'^{\alpha}$ with $\alpha \sim 0.5$. Our results demonstrate the significance of the field gradient effect in the quantum critical dynamics of spinor condensates.Comment: 8 pages, 7 figure

Crossover from weak to strong quench in a spinor Bose-Einstein condensate

We investigate the early-time dynamics of a quasi-two-dimensional spin-1 antiferromagnetic Bose-Einstein condensate after a sudden quench from the easy-plane to the easy-axis polar phase. The postquench dynamics shows a crossover behavior as the quench strength (q) over tilde is increased, where (q) over tilde is defined as the ratio of the initial excitation energy per particle to the characteristic spin interaction energy. For a weak quench of (q) over tilde , the length scale of the initial spin excitations decreases, and we demonstrate that the long-wavelength instability is strongly suppressed for high (q) over tilde > 2. The observed crossover behavior is found to be consistent with the Bogoliubov description of the dynamic instability of the initial spinor condensate. ©2020 American Physical Society11sciescopu

Spin-driven stationary turbulence in spinor Bose-Einstein condensates

We report the observation of stationary turbulence in antiferromagnetic spin-1 Bose-Einstein condensates driven by a radio-frequency magnetic field. The magnetic driving injects energy into the system by spin rotation and the energy is dissipated via dynamic instability, resulting in the emergence of an irregular spin texture in the condensate. Under continuous driving, the spinor condensate evolves into a nonequilibrium steady state with characteristic spin turbulence, while the low energy scale of spin excitations ensures that the sample's lifetime is minimally affected. When the driving strength is on par with the system's spin interaction energy and the quadratic Zeeman energy, remarkably, the stationary turbulent state exhibits spin-isotropic features in spin composition and spatial spin texture. We numerically show that ambient field fluctuations play a crucial role in sustaining the turbulent state within the system. These results open up new avenues for exploring quantum turbulence in spinor superfluid systems.Comment: 9 pages, 9 figure

Metastable hard-axis polar state of a spinor Bose-Einstein condensate under a magnetic field gradient

We investigate the stability of a hard-axis polar state in a spin-1 antiferromagnetic Bose-Einstein condensate under a magnetic field gradient, where the easy-plane spin anisotropy is controlled by a negative quadratic Zeeman energy q<0. In a uniform magnetic field, the axial polar state is dynamically unstable and relaxes into the planar polar ground state. However, under a field gradient B′, the excited spin state becomes metastable down to a certain threshold qth, and as q decreases below qth its intrinsic dynamical instability is rapidly recalled. The incipient spin excitations in the relaxation dynamics appear with stripe structures, indicating the rotational symmetry breaking by the field gradient. We measure the dependences of qth on B′ and the sample size, and we find that qth is highly sensitive to the field gradient in the vicinity of B′=0, exhibiting power-law behavior of |qth|-B′α with α∼0.5. Our results demonstrate the significance of the field gradient effect in the quantum critical dynamics of spinor condensates. © 2019 American Physical Societ

Depth-of-field extension through focal plane oscillation and variable annular pupil

In this paper, a depth-of-field extension method is in troduced. The extension is realized by the variable annular aperture method previously proposed by the authors and focal plane oscillation method. By combining those methods, we see a synergetic effect that the depth-of-field is more extended than when each method is applied independently. The variable aperture and the focal plane oscillation are realized by a liquid crystal spatial light modulator and a deformable mirror, respectively. Simulation and experimental results are shown to verify the proposed method. © 2010 IEEE

Depth of Field Adaptation for Observation of Microscopic Objects by Using Variable Annular Aperture System

High magnification optical systems such as microscopes usually suffer from limited depth-of-field (DOF) problem. This hinders efficient observation of microscopic objects and prevents vision based approaches from being applied to automatic micromanipulation tasks. A DOF extension method using variable annular pupil was proposed by the authors&apos; previous publication, and a tradeoff between the depth extension range and image quality was observed. In this article, this tradeoff is numerically analyzed by a proposed wavelet-based image quality measure, and a proper DOF scheme to provide the best image for the given object distribution is proposed. The problem is reduced to finding the optimum pupil division parameter. The experimental results conducted for various imaging conditions with some MEMS objects verify that the system DOF can be adjusted according to their position in depth direction to provide the best visibility.

Depth-of-Field Extension Method Using Variable Annular Pupil Division

The depth-of-field (DOF) of an imaging system is closely related to the pupil function that describes the characteristics of the pupil such as size and shape. Shallow DOF is especially problematic in microscopic optical imaging systems because they have relatively large aperture size for high optical resolution. In this paper, a new DOF extension method is proposed. This is to acquire an image by integrating the incoming light while changing the size of annular pupil. By dividing the pupil into annular form, the effect of the defocus aberration is reduced. Theoretical background is explained and simulation and experimental results are shown for verification. The results show that the DOF can effectively be extended by this method. © 2006 IEEE.

Critical Energy Dissipation in a Binary Superfluid Gas by a Moving Magnetic Obstacle

© 2021 American Physical Society.We study the critical energy dissipation in an atomic superfluid gas with two symmetric spin components by an oscillating magnetic obstacle. Above a certain critical oscillation frequency, spin-wave excitations are generated by the magnetic obstacle, demonstrating the spin superfluid behavior of the system. When the obstacle is strong enough to cause density perturbations via local saturation of spin polarization, half-quantum vortices (HQVs) are created for higher oscillation frequencies, which reveals the characteristic evolution of critical dissipative dynamics from spin-wave emission to HQV shedding. Critical HQV shedding is further investigated using a pulsed linear motion of the obstacle, and we identify two critical velocities to create HQVs with different core magnetization.11Nsciescopu