Silicon lens breaks the diffraction limit in the near-infrared wavelength range
With increase in demand for fabricating as well as visualizing structures much smaller than the wavelengths of light, the resolving capabilities of lenses used in conventional microscopes need to be improved. Today, commercial lenses are limited in their resolution to the Abbe diffraction limit because of their inability to focus both the propagating and the evanescent waves. The evanescent waves, which are well known to contain intricate details of objects (much smaller than the wavelengths of light), decay with distance and hence, almost never restored at the focus. This limits the resolution of the lenses to the details provided only from the propagating waves. A certain class of metamaterials, known as left-handed metamaterials (LHMs) however, has been extensively researched and demonstrated to amplify evanescent waves, and therefore information contained in both propagating and evanescent fields can now be reconstructed into a ‘perfect’ image using LHM based lenses. This property of “superlensing” or, more rigorously, imaging beyond the diffraction limit, has recently been demonstrated in a series of experiments involving different classes of LHMs. In most of these experiments the LHMs consists of metallo-dielectric periodic microstructures that operate at microwave frequencies. Metal-based LHMs, however, show large absorption losses especially when scaled to visible or near-infrared wavelengths. Although recent remarkable efforts have examined important pathways for improvements to these losses, the current loss values can possibly act as practical limit on the resolution of sub-diffraction limit imaging at optical frequencies. A promising alternative is to use dielectric-based photonic crystal (PhC) LHMs, whose optical losses could be considerably smaller. In addition, these CMOS-compatible materials platform promises seamless integration with existing technologies and scalability to optical and near-infrared wavelengths, critical for nanolithography applications, imaging, and detection nanodevices. Thus, this first experimental observation of subdiffraction imaging in negative refraction photonic crystals at the near-infrared suggests interesting opportunities towards achieving these applications, where the commonly perceived diffraction limit is no longer a barrier.
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LH11494
The not-so-unsual stripe formation on surfaces
The simplest possible approach to making nanoscale patterns is for the
material to spontaneously order into the pattern. Such self-assembling
systems are known to occur if a material is deposited on a substrate. A
common physical origin of self-assemble is balancing the cost of breaking
bonds on the material with the deformations produced on the substrate.
Standard theories indicate that the periodicities of such patterns should
depend very strongly on the properties of deposit and substrate, so strongly,
in fact, that observing such patterns at all should be rare. In contrast, we
experimentally observe and study in real time by electron microscopy the
formation of self-assembling patterns in a very simple system: gold deposited
on tungsten. By modeling the formation of the patterns, we conclude that such
patterns should be common on metal surfaces.
***
LN11483
Controlling colloidal valence
Chemistry as we know it is realized binding atoms together to form
molecules, whose natural scale is in the nanometer range. However,
in 2002, David Nelson showed that a chemistry on the micrometer scale
should also be possible, using liquid crystal-covered colloidal particles
as ``atoms'' and DNA or polymer linkers as ``bonds''. In our study, we
propose to control the number of bonds per colloidal particle --- the
``colloidal valence'' --- by means of an electric field of appropriate
symmetry. Colloidal valence will greatly affect the actual architecture
of colloidal assemblies, which is crucial for their interaction with
light and hence for their application as photonic materials. As the
bonding linkers can only be attached to the so-called liquid crystal
defect points, we have thoroughly explored molecular ordering in the
liquid crystal layer covering each colloid. We show, via large-scale
computer simulations, that the number, position, and type of defect
points, and hence the colloidal valence, can be changed from the
value 4 in the absence of the electric field, to 2, 4, 8, or higher,
depending on field strength and symmetry.
***
LN11300
A single molecule as an electro-mechanical system
When passing electric current through a single-molecule bridging between
two metallic needles, some of the electrons can activate molecular
motion (vibrations). We have found that the conductance of the molecule
can either increase or decrease by the activation of such vibration. By
measuring the molecule conductance and the current noise across the
molecule we discovered that a crossover between conductance enhancement
to conductance suppression takes place when the main probability of
electrons to cross the molecule exceeds 1/2. The effect of molecular
vibrations on the conductance of a single-molecule is a central issue in
the field of molecular electronics and has been recently much debated in
theory. Our findings provide the first experimental support for several
theoretical models that predict such a crossover.
***LN11503
Exploding Molecules with Light!
Physicists often like to blow up microscopic objects and study what comes out. This approach has certainly proved effective in high energy particle physics and, in the less energetic world of atomic collisions, new experimental techniques are now enabling physicists to view molecular interactions in unprecedented detail. A case in point probes what happens when polarized light, of sufficient energy, is absorbed by the simplest molecule H2 resulting in both electrons being ejected simultaneously. Once the glue that held the molecule together has gone, the positively charged nuclei rapidly fly apart in opposite directions creating a so-called “Coulomb Explosion”. Understanding the details of this fundamental process has been a hot topic for both theory and experiment in recent years. The new collaborative work of Reddish et al (2008) Phys Rev Letts (LN11503), using state-of-the-art theoretical methods and 3-dimensional momentum imaging techniques, effectively views this double ionisation process as the H2 molecule - fixed in space at specific orientations - vibrates, i.e. as a function of internuclear separation. They have been able to unambiguously analyse and interpret the complex 4-particle dynamics during the Coulomb explosion by comparing their results with the simpler problem of photoionization of H2+.
***
LL11508
"Quantum Carpentry": The Spirit Level goes Quantum
We propose a quantum version of the commonly known engineering
instrument, the ``Spirit Level''. This instrument is typically
used to determine if a given surface is horizontal or plumb. While the
spirit level uses an air bubble within a spirit (usually ethanol)
filled glass tube, the proposed version on the other hand uses
a bubble of quantum fluid within another quantum fluid. We have shown
that by using a quantum fluid that is created by Bose condensing
trapped gases, the quantum version of the spirit level, so called
``Quantum Level'', will allow experimentalists to achieve sensitivity
20,000 times better than the most sensitive engineering spirit
level. This implies, in simple terms, measuring an incline approximately
of the size of an human hair in 100 Kilometers.
***
LQ11331
Quantum jumps with memory
Quantum mechanical system can be in a superposition of two or more states whose coexistence is forbidden by classical physics. When a quantum system interacts with its environment, superpositions get destroyed due to decoherence. This process can be described by quantum jumps which change the system state suddenly and indicate the flow of information from the system to its environment. However, quantum systems which have memory are able to regain the information which they leaked earlier. We show that this leads to a concept of quantum jumps that counterintuitively restore superpositions instead of destroying them. Earlier, quantum jumps have been widely used to describe quantum dynamics without memory. However, the use of the same theoretical description for systems with memory leads to a serious problem: negative probability for a quantum jump to occur. Our results solve this problem with an interesting consequence. Namely, a negative probability for a usual jump, which
destroys superpositions, actually corresponds to positive probability for a novel quantum jump which restores quantum superpositions. The solution allows to design efficient simulation algorithm for open quantum systems and demonstrates what happens when the direction of the information flow between the system and the environment gets reversed due to memory and the system regains the information it leaked earlier.
***
LN10907
Visualizing Motion of Non-Resolvable Objects in a Microscope
It is well known that objects smaller than the wavelength of light (about 1/2000 of mm) can not be resolved individually in a microscope. In this paper we show that the motion of sub-wavelength particles can still be measured very precisely with an ordinary microscope without resorting to track individual particles. This is realized by an advanced image processing technique that isolates efficiently the signal due to the particle motion in a time series of microscopy images.
Our technique, named Differential Dynamic Microscopy (DDM), is a complement to techniques which probe the dynamics of particles, such as dynamic light scattering or particle video tracking. DDM can be used to determine the object size and shape or to measure the mechanical properties of the host fluid, such as for example its viscosity or elasticity. We envision its use in a wide range of systems, such as cells, macromolecules, emulsions, foams, colloidal glasses and gels.
***
LN11239
How fast can one tunnel into chaos?
Tunneling through a barrier is a striking and well studied consequence of
quantum mechanics. In contrast, tunneling from regular to chaotic dynamics
is barely understood. In this paper, we provide a theory predicting tunneling
rates in billiards. Microwave experiments with a mushroom-shaped billiard
confirm these predictions. Playing billiards with photons and electrons is
relevant for applications in microlasers and semiconductor nanostructures.
***
LK11063
The soft strength of Surface Plasmons
Using optical forces to manipulate with laser light small objects – such as living cells- at the surface of a chip is a fascinating aspiration of photonics. In this paper (LK11063), we show how surface plasmon fields engineered at a surface patterned with gold disks enable trapping micro-objects with ultra gentle forces down to a few tens of femto-Newtons. Surface plasmon tweezers not only are the softest optical tweezers ever reported but have the peculiarity of being tunable. In particular, we have discovered that a suitable adjustment of the laser illumination parameters (such as polarization and incident angle) enables controlling their selectivity and achieving trapping of a specific object out of a mix. This novel generation of integrated optical tweezers opens new perspectives for the implementation of future optically driven lab-on-a-chip devices with inestimable applications to biomedicine.
***
LN11227
Monsieur Poincaré, how long do I have to wait?
In 1890 the French mathematician Henri Poincaré proved
that conservative dynamical systems eventually return
to their initial neighborhood. But he could not tell how
long one has to wait for this to happen.
In the present work it is shown that the distribution of
these return times is universal for all Hamiltonian systems
with two degrees of freedom, like e.g. a double pendulum.
These are among the simplest systems that show chaotic dynamics
and the search for their universal behavior attracted the
interest of physicist and mathematicians for decades.
With the help of a newly proposed model and on the basis of
numerical studies this work elucidates the universal
character of the distribution of Poincaré return times.
***
LM11311
Scientists at NIST have created a new kind of "optical lattice" in which to
study ultracold atoms. Instead of each 100nm-scale site of the lattice having
a bucket-like well to hold a few atoms, Lundblad et al. created a lattice with a
ringlike "Mexican hat" geometry at each site. This means that the probability
distribution of the atom in the well is no longer the normal bell curve, but as
dictated by quantum mechanics, approaches a doughnut-like shape. Actual atomic
"doughnuts" were not observed, but the regime of squashed or dimpled
wavefunctions was approached, using a Bose-Einstein condensate as the source of
cold atoms to load the lattice. The mechanisms by which these new lattice
states decay was also studied. A lattice of sites with ringlike symmetry is of
interest to researchers using the tailored and customizable milieu of cold atoms
to explore condensed-matter physics analogues.
***
LP10959
Using Time Reversal to Understand Single-Molecule Mechanical Properties
NIH scientists have discovered a new way to understand the mechanical properties of individual molecules. Using technology such as optical tweezers, single molecules can be held at their ends and stretched, much like rubber bands. Beyond the technical difficulty of performing these precise measurements, there remains the theoretical challenge of recovering what their mechanical properties would be without such manipulation. For example, how likely is it that the molecule will have a certain end-to-end distance versus another? It has been previously understood how to calculate this property from repeatedly pulling the molecule in a single direction. Now, a new theory has been developed, based on optimally using the time reversal of paths that the molecule takes after being pulled in the opposite direction, to efficiently calculate the relative probabilities of molecular lengths. This theory should aid in the understanding of biology at the single-molecule level.
***
LF10925
Strange properties of cryogenic liquids:
From fundamental laws of fluids to technological applications.
Drop formation from the breakup of liquid jets is a universal feature of daily
life, as when water drops form at a dripping tap. It has been scrutinized for
centuries by scientists like Bohr, Eötvös, Laplace, Lenard, Rayleigh, Savart,
and Young. We have developed generators for periodic fluxes of mono-disperse
drops with diameters down to 10 mikrometers. When operated with liquid
Hydrogen or Nitrogen, we observe jets that are significantly more resistant
to breakup than predicted by well established theories. Moreover, at high
evaporation rates, axial symmetry of the dynamics is lost and spontaneous jet
bending is observed. Our findings illustrate how incomplete our understanding
remains of phenomena involving fluids with free boundaries. Our patentet
cooling method may also pave the way to novel technological applications,
like EUV sources for producting highly-integrated micro chips and laser-based
particle accelerators.
***
LL11771
The M1 resonance: a nuclear ingredient that might have been
overlooked in the synthesis of heavy elements in stars
The M1 resonance near the neutron threshold may be a nuclear
ingredient that has been overlooked in the past in the description of
the nuclear properties entering the complicated stellar
nucleosynthesis of the elements heavier than Fe. A systematic
measurement of zirconium isotopes (Zr-91,92,94) with monochromatic
photon beams has revealed the presence of a strong M1 resonance close
to the neutron threshold. This resonance is located in the energy
region in which stellar neutron capture and photodisintegration take
place most effectively. The E1 gamma-ray strength in the low-energy
tail of the giant E1 resonance has so far been considered as the main
excitation or de-excitation electromagnetic mode. Recently,
the emergence of a possible E1 pigmy resonance near the neutron
threshold was theoretically predicted in neutron-rich nuclei and
its clear signature was experimentally confirmed in a tin isotope
(Sn-132). In additon to the tail of the giant E1 resonance and the
pigmy E1 resonance, this research has shown that the M1 contribution
needs to be better understood and determined. Investigating resonance
with higher multipolarities of direct relevance to the synthesis of
heavy elements near neutron threshold may follow in the future.
