Biomedical subjects

J L Bohn

Publications and source records attributed to J L Bohn.

8 recordsLinked to original sources

PubMed2006

Scattering length instability in dipolar Bose-Einstein condensates.

We predict a new kind of instability in a Bose-Einstein condensate composed of dipolar particles. Namely, a comparatively weak dipole moment can produce a large, negative two-body scattering length that can collapse the Bose-Einstein condensate. To verify this effect, we validate mean-field solutions to this problem using exact, diffusion Monte Carlo methods. We show that the diffusion Monte Carlo energies are reproduced accurately within a mean-field framework if the variation of the s-wave scattering length with the dipole strength is accounted for properly.

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PubMed2004

Observation of heteronuclear Feshbach resonances in a mixture of bosons and fermions.

Three magnetic-field induced heteronuclear Feshbach resonances were identified in collisions between bosonic 87Rb and fermionic 40K atoms in their absolute ground states. Strong inelastic loss from an optically trapped mixture was observed at the resonance positions of 492, 512, and 543+/-2 G. The magnetic-field locations of these resonances place a tight constraint on the triplet and singlet cross-species scattering lengths, yielding (-281+/-15)a(0) and (-54+/-12)a(0), respectively. The width of the loss feature at 543 G is 3.7+/-1.5 G wide; this broad Feshbach resonance should enable experimental control of the interspecies interactions.

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PubMed2004

Detection of spatial correlations in an ultracold gas of fermions.

Spatial correlations are observed in an ultracold gas of fermionic atoms close to a Feshbach resonance. The correlations are detected by inducing spin-changing rf transitions between pairs of atoms. We observe the process in the strongly interacting regime for attractive as well as for repulsive atom-atom interactions and both in the regime of high and low quantum degeneracy. The observations are compared with a two-particle model that provides theoretical predictions for the measured rf transition rates.

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PubMed2003

Tuning p-wave interactions in an ultracold Fermi gas of atoms.

We have measured a p-wave Feshbach resonance in a single-component, ultracold Fermi gas of 40K atoms. We have used this resonance to enhance the normally suppressed p-wave collision cross section to values larger than the background s-wave cross section between 40K atoms in different spin states. In addition to the modification of two-body elastic processes, the resonance dramatically enhances three-body inelastic collisional loss.

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PubMed2002

Rotational Feshbach resonances in ultracold molecular collisions.

In collisions at ultralow temperatures, molecules will possess Feshbach resonances, foreign to ultracold atoms, whose virtual excited states consist of rotations of the molecules. We estimate the mean spacing and mean widths of these resonant states, exploiting the fact the molecular collisions at low energy display chaotic motion. As examples, we consider the experimentally relevant molecules O2, OH, and PbO. Especially for polar species, the density of s-wave resonant states is quite high, implying potentially disastrous consequences for trapped molecules.

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PubMed2002

Resonant control of elastic collisions in an optically trapped fermi gas of atoms.

We have loaded an ultracold gas of fermionic atoms into a far-off resonance optical dipole trap and precisely controlled the spin composition of the trapped gas. We have measured a magnetic-field Feshbach resonance between atoms in the two lowest energy spin states, /9/2,-9/2> and /9/2,-7/2>. The resonance peaks at a magnetic field of 201.5+/-1.4 G and has a width of 8.0+/-1.1 G. Using this resonance, we have changed the elastic collision cross section in the gas by nearly 3 orders of magnitude.

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PubMed2001

Field enhancement in apertureless near-field scanning optical microscopy.

The near field of an apertureless near-field scanning optical microscopy probe is investigated with a multiple-multipole technique to obtain optical fields in the vicinity of a silicon probe tip and a glass substrate. The results demonstrate that electric field enhancements of >15 relative to the incident fields can be achieved near a silicon tip, implying intensity enhancements of several orders of magnitude. This enhancement arises both from the antenna effect of the elongated probe and from a proximity effect when the probe is near the substrate surface and its image dipoles play a role.

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PubMed1997

Laser trabeculoplasty: how little is enough?

BACKGROUND AND OBJECTIVE: The optimal number of laser applications for argon laser trabeculoplasty in patients with primary open-angle glaucoma has not been established. The purpose of this study was to determine the fewest number of burns necessary for clinically effective intraocular pressure reduction for these patients. PATIENTS AND METHODS: Using a prospective, randomized, collaborative study, the authors examined the relationship between the number of laser trabeculoplasty burns and intraocular pressure for patients with primary open-angle glaucoma. A total of 122 patients received either 50 burns to half of the trabecular meshwork (group 1) or 35 burns to one third of the trabecular meshwork (group 2). RESULTS: The mean baseline intraocular pressures were similar between group 1 (22.3 +/- 4.1 mm Hg [mean +/- SD]) and group 2 (22.8 +/- 5.0 mm Hg) (P = .58). Intraocular pressure reduction at 9 to 12 months, although significant in both groups (group 1: 4.4 +/- 3.0 mm Hg, P < .0001; group 2: 3.9 +/- 5.1 mm Hg, P < .0001), did not differ significantly between the two groups (P = .63). CONCLUSION: This study demonstrated that 35 burns may be as clinically effective as 50 burns in reducing the intraocular pressure in primary open-angle glaucoma patients.

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