Optical traps

We investigate the collisional stability of a sample of 40 K atoms immersed in a tunable spin mixture of 6 Li atoms. In this three-component Fermi-Fermi mixture, we find very low loss rates in a wide range of interactions as long as molecule formation of 6 Li is avoided. The stable fermionic mixture with two resonantly interacting spin states of one species together with another species is a promising system for a broad variety of phenomena in few-and many-body quantum physics.

We experimentally reporton optical binding of many glass particles in air that levitate in a single optical beam. A diversity of particle sizes and shapes interact at long range in a single Gaussian beam. Our system dynamics span from oscillatory to random and dimensionality ranges from 1 to 3D. The low loss for the center of mass motion of the beads could allow this system to serve as a standard many body testbed, similar to what is done today with atoms, but at the mesoscopic scale.

Efimov physics is a universal phenomenon in quantum threebody systems. For systems with resonant two-body interactions, Efimov predicted an infinite series of three-body bound states with geometric scaling symmetry 1. These Efimov states, first observed in cold caesium atoms 2 , have been recently reported in a variety of other atomic systems 3-13. The intriguing prospect of a universal absolute Efimov resonance position across Feshbach resonances remains an open question. Theories predict a strong dependence on the resonance strength for closed-channel-dominated Feshbach resonances, whereas experimental results have so far been consistent with the universal prediction. Here we directly compare the Efimov spectra in a 6 Li-133 Cs mixture near two Feshbach resonances which are very di erent in their resonance strengths, but otherwise almost identical. Our result shows a clear dependence of the absolute Efimov resonance position on Feshbach resonance strength and a clear departure from the universal prediction for the narrow Feshbach resonance. The first Efimov resonance position is crucial to understanding Efimov physics, as from it one may derive the absolute scale of parameters for subsequent Efimov resonances. Previous experiments in homonuclear systems yield an interesting observation: the first Efimov resonance position a − appears to follow a universal formula a − ≈ −9 r vdW (ref. 14), where r vdW is the van der Waals length of the molecular potential 15. Calculations from universal theory based on a single-channel model confirm this 'van der Waals universality' for Feshbach resonances with s res > 1(refs 16-20), where the resonance strength parameter s res characterizes the strength of the coupling between open and closed channels of the Feshbach resonance 15. Similar theories also give universal but modified predictions for heteronuclear systems 21,22. Further calculations predict significant deviations from the universal prediction for resonances with s res 1 (refs 18,23-25). Experiments reaching down to s res = 0.11, however, have shown little or no dependence on s res (ref. 9). Experimental results, including this work, are summarized in Fig. 1. In this Letter, we compare Efimov spectra in a 6 Li-133 Cs mixture near one broad (s res = 0.66) and one very narrow (s res = 0.05) interspecies Feshbach resonance at B 0 = 889 and 893 G, respectively. These two Feshbach resonances, shown in Fig. 2a, differ significantly in resonance strength, but only slightly in Cs-Cs scattering length (a CsCs = 200 and 260a 0 , where a 0 is the Bohr radius). Therefore, this pair of resonances is an excellent case to assess the influence of the Feshbach resonance strength on Efimov physics while keeping other parameters nearly identical. We observe a significant difference in Efimov resonance position between the two Feshbach resonances. Our measurements also show that the resonance associated with the first Efimov state is suppressed near both Feshbach resonances.

My wife, Yang Li, deserves a special position in my acknowledgment list. She is a brave woman. She is constantly supportive throughout our marriage and especially during my PhD study at UT. She undertook most of the tasks and met the needs of the family. She provided me a warm home. She consoled me when I felt frustrated. Most importantly, she brought our vii two daughters, Sannie and Jenna, to this family. Without her love, I would have done little. I would like to thank my parents, my brother, my sister and other members in the family, for their enduring love and encouragements. I also appreciate the support and understanding of my parents-in-law, Fu-Cai Yang and Zhong-Fang Cang. They have been very generous to us during the years of hardship and frustration. The author would like to give special thanks to Kirsten Viering, Isaac

When linearly polarised light is transmitted through a spinning window, the plane of polarisation is rotated. This rotation arises through a phase change that is applied to the circularly polarised states corresponding to the spin angular momentum (SAM). Here we show an analogous effect for the orbital angular momentum (OAM), where a differential phase between the positive and negative modes (±) is observed as a rotation of the transmitted image. For normal materials, this rotation is on the order of a micro radian, but by using a slow-light medium, we show a rotation of a few degrees. We also note that, within the bounds of our experimental parameters, this rotation angle does not exceed the scale of the spatial features in the beam profile.

We investigate the effects of 1 st order spherical aberration and defocus upon the stiffness of an optical trap tens of μm into the sample. We control both these aberrations with a spatial light modulator. The key to maintain optimum trap stiffness over a range of depths is a specific non-trivial combination of defocus and axial objective position. This optimisation increases the trap stiffness by up to a factor of 3 and allows trapping of 1μm polystyrene beads up to 50μm deep in the sample.

In investigating the binding interactions between the human telomeric RNA (TERRA) G-quadruplex (GQ) and its ligands, it was found that the small molecule carboxypyridostatin (cPDS) and the GQ-selective antibody BG4 simultaneously bind the TERRA GQ. We previously showed that the overall binding affinity of BG4 for RNA GQs is not significantly affected in the presence of cPDS. However, single-molecule mechanical unfolding experiments revealed a population (48 %) with substantially increased mechanical and thermodynamic stability. Force-jump kinetic investigations suggested competitive binding of cPDS and BG4 to the TERRA GQ. Following this, the two bound ligands slowly rearrange, thereby leading to the minor population with increased stability. Given the relevance of G-quadruplexes in the regulation of biological processes, we anticipate that the unprecedented conformational rearrangement observed in the TERRA-GQ-ligand complex may inspire new strategies for the selective stabilization of G-quadruplexes in cells. Non-canonical nucleic acid structures such as G-quadruplexes have recently attracted significant attention for their potential roles in the regulation of biological processes. [1] Gquadruplexes are formed with a minimum of four guanine (G)-rich repeats in DNA or RNA sequences. [2] They consist of

We study the dynamics of an initially degenerate homogeneous Bose gas after an interaction quench to the unitary regime at a magnetic Feshbach resonance. As the cloud decays and heats, it exhibits a crossover from degenerate- to thermal-gas behavior, both of which are characterized by universal scaling laws linking the particle-loss rate to the total atom number N. In the degenerate and thermal regimes, the per-particle loss rate is ∝N^{2/3} and N^{26/9}, respectively. The crossover occurs at a universal kinetic energy per particle and at a universal time after the quench, in units of energy and time set by the gas density. By slowly sweeping the magnetic field away from the resonance and creating a mixture of atoms and molecules, we also map out the dynamics of correlations in the unitary gas, which display a universal temporal scaling with the gas density, and reach a steady state while the gas is still degenerate.

Conical diffraction of linearly polarised light in a biaxial crystal produces a beam with a crescent-shaped intensity profile. Rotation of the plane of polarisation produces the unique effect of spatially moving the crescent-shaped beam around a ring. We use this effect to trap microspheres and white blood cells and to position them at any angular position on the ring. Continuous motion around the circle is also demonstrated. This crescent beam does not require an interferometeric arrangement to form it, nor does it carry optical angular momentum. The ability to spatially locate a beam and an associated trapped object simply by varying the polarisation of light suggests that this optical process should find application in the manipulation and actuation of micro-and nano-scale physical and biological objects.

Cascade conical refraction occurs when a beam of light travels through two or more biaxial crystals arranged in series. The output beam can be altered by varying the relative azimuthal orientation of the two biaxial crystals. For two identical crystals, in general the output beam comprises a ring beam with a spot at its centre. The relative intensities of the spot and ring can be controlled by varying the azimuthal angle between the refracted cones formed in each crystal. We have used this beam arrangement to trap one microsphere within the central spot and a second microsphere on the ring. Using linearly polarized light, we can rotate the microsphere on the ring with respect to the central sphere. Finally, using a half wave-plate between the two crystals, we can create a unique beam profile that has two intensity peaks on the ring, and thereby trap two microspheres on diametrically opposite points on the ring and rotate them around the central sphere. Such a versatile optical trap should find application in optical trapping setups.

The diffraction limited resolution of light focused by a lens was derived in 1873 by Ernst Abbe. Later in 1952, a method to reach sub-diffraction light spots was proposed by modulating the wavefront of the focused beam. In a related development, super-oscillating functions, i.e. band limited functions that locally oscillate faster than their highest Fourier component, were introduced and experimentally applied for super-resolution microscopy. Up till now, only simple Gaussian-like sub-diffraction spots were used. Here we show that the amplitude and phase profile of these subdiffraction spots can be arbitrarily controlled. In particular we utilize Hermite-Gauss, Laguerre-Gauss and Airy functions to structure super-oscillating beams with sub-diffraction lobes. These structured beams are then used for high resolution trapping and manipulation of nanometer-sized particles. The trapping potential provides unprecedented localization accuracy and stiffness, significantly exceeding those provided by standard diffraction limited beams.

We report a double-sided two-mirror neodymiumdoped KGd(WO 4 ) 2 /Nd:KGW conical refraction laser which produces a linearly polarised output from the flat output coupler and a cone-refracted output from the curved output coupler. The linearly polarised output had a polarisation extinction ratio of 20:1 with a Gaussian beam profile and a measured M 2 = 1. The unpolarised output had the intensity and polarisation distribution corresponding to a cone-refracted Gaussian beam with extinction ratio of 2.7:1.

We investigate the collisional stability of a sample of 40 K atoms immersed in a tunable spin mixture of 6 Li atoms. In this three-component Fermi-Fermi mixture, we find very low loss rates in a wide range of interactions as long as molecule formation of 6 Li is avoided. The stable fermionic mixture with two resonantly interacting spin states of one species together with another species is a promising system for a broad variety of phenomena in few-and many-body quantum physics. PACS numbers: 34.50.-s, 67.85.Lm, 05.30.Fk The groundbreaking achievements in experiments with ultracold Fermi gases have opened up unprecedented possibilities to study new regimes of strongly interacting quantum matter [1, 2, 3]. Recent experiments have opened up two important new research frontiers with fermionic atoms that go beyond the two-component spin mixtures so far exploited in the field. Mixtures involving three different spin states [4, 5] and mixtures of different fermionic species [6, 7, 8] have produced first exciting results like the demonstration of fermionic Efimov states [9, 10] and the creation of Fermi-Fermi molecules .

Sub-micron ring, pillar and wall structures were written by two-photon polymerisation of a sol–gel resin using a femtosecond laser beam which was shaped using internal conical diffraction. The ring structure was written using a demagnified image of the ring-shaped beam which arises in conical diffraction of a narrow light beam. The pillar and wall structures were produced by imaging the Bessel beam formed by conical diffraction in combination with a converging lens.

Conical diffraction of linearly polarised light in a biaxial crystal produces a beam with a crescent-shaped intensity profile. Rotation of the plane of polarisation produces the unique effect of spatially moving the crescent-shaped beam around a ring. We use this effect to trap microspheres and white blood cells and to position them at any angular position on the ring. Continuous motion around the circle is also demonstrated. This crescent beam does not require an interferometeric arrangement to form it, nor does it carry optical angular momentum. The ability to spatially locate a beam and an associated trapped object simply by varying the polarisation of light suggests that this optical process should find application in the manipulation and actuation of micro-and nano-scale physical and biological objects.