4,271 research outputs found
Pulsatile microvascular blood flow imaging by short-time Fourier transform analysis of ultrafast laser holographic interferometry
We report on wide-field imaging of pulsatile microvascular blood flow in the
exposed cerebral cortex of a mouse by holographic interferometry. We recorded
interferograms of laser light backscattered by the tissue, beating against an
off-axis reference beam with a 50 kHz framerate camera. Videos of local Doppler
contrasts were rendered numerically by Fresnel transformation and short-time
Fourier transform analysis. This approach enabled instantaneous imaging of
pulsatile blood flow contrasts in superficial blood vessels over 256 x 256
pixels with a spatial resolution of 10 microns and a temporal resolution of 20
ms.Comment: 4 page
Magnetic Resonance Lithography with Nanometer Resolution
We propose an approach for super-resolution optical lithography which is
based on the inverse of magnetic resonance imaging (MRI). The technique uses
atomic coherence in an ensemble of spin systems whose final state population
can be optically detected. In principle, our method is capable of producing
arbitrary one and two dimensional high-resolution patterns with high contrast
Flux density measurements of GPS candidate pulsars at 610 MHz using interferometric imaging technique
We conducted radio interferometric observations of six pulsars at 610 MHz
using the Giant Metrewave Radio Telescope (GMRT). All these objects were
claimed or suspected to be the gigahertz-peaked spectra (GPS) pulsars. For a
half of the sources in our sample the interferometric imaging provides the only
means to estimate their flux at 610 MHz due to a strong pulse
scatter-broadening. In our case, these pulsars have very high dispersion
measure values and we present their spectra containing for the first time
low-frequency measurements. The remaining three pulsars were observed at low
frequencies using the conventional pulsar flux measurement method. The
interferometric imaging technique allowed us to re-examine their fluxes at 610
MHz. We were able to confirm the GPS feature in the PSR B182313 spectrum and
select a GPS candidate pulsar. These results clearly demonstrate that the
interferometric imaging technique can be successfully applied to estimate flux
density of pulsars even in the presence of strong scattering.Comment: 7 pages, 3 tables, 5 figures, accepted for publication in MNRA
Phase Fluctuations in Bose-Einstein Condensates
We demonstrate the existence of phase fluctuations in elongated Bose-Einstein
Condensates (BECs) and study the dependence of those fluctuations on the system
parameters. A strong dependence on temperature, atom number, and trapping
geometry is observed. Phase fluctuations directly affect the coherence
properties of BECs. In particular, we observe instances where the phase
coherence length is significantly smaller than the condensate size. Our method
of detecting phase fluctuations is based on their transformation into density
modulations after ballistic expansion. An analytic theory describing this
transformation is developed.Comment: 11 pages, 7 figure
Spin squeezing, entanglement and quantum metrology with Bose-Einstein condensates
Squeezed states, a special kind of entangled states, are known as a useful
resource for quantum metrology. In interferometric sensors they allow to
overcome the "classical" projection noise limit stemming from the independent
nature of the individual photons or atoms within the interferometer. Motivated
by the potential impact on metrology as wells as by fundamental questions in
the context of entanglement, a lot of theoretical and experimental effort has
been made to study squeezed states. The first squeezed states useful for
quantum enhanced metrology have been proposed and generated in quantum optics,
where the squeezed variables are the coherences of the light field. In this
tutorial we focus on spin squeezing in atomic systems. We give an introduction
to its concepts and discuss its generation in Bose-Einstein condensates. We
discuss in detail the experimental requirements necessary for the generation
and direct detection of coherent spin squeezing. Two exemplary experiments
demonstrating adiabatically prepared spin squeezing based on motional degrees
of freedom and diabatically realized spin squeezing based on internal hyperfine
degrees of freedom are discussed.Comment: Phd tutorial, 23 pages, 17 figure
Distinguishing non-resonant four-wave-mixing noise in coherent stokes and anti-stokes Raman scattering
A method of examining a sample comprises exposing the sample to a pump pulse of electromagnetic radiation for a first period of time, exposing the sample to a stimulant pulse of electromagnetic radiation for a second period of time which overlaps in time with at least a portion of the first exposing, to produce a signal pulse of electromagnetic radiation for a third period of time, and interfering the signal pulse with a reference pulse of electromagnetic radiation, to determine which portions of the signal pulse were produced during the exposing of the sample to the stimulant pulse. The first and third periods of time are each greater than the second period of time
Photon-induced near-field electron microscopy (PINEM): theoretical and experimental
Electron imaging in space and time is achieved in microscopy with timed (near relativistic) electron packets of picometer wavelength coincident with light pulses of femtosecond duration. The photons (with an energy of a few electronvolts) are used to impulsively heat or excite the specimen so that the evolution of structures from their nonequilibrium state can be followed in real time. As such, and at relatively low fluences, there is no interaction between the electrons and the photons; certainly that is the case in vacuum because energy–momentum conservation is not possible. In the presence of nanostructures and at higher fluences, energy–momentum conservation is possible and the electron packet can either gain or lose light quanta. Recently, it was reported that, when only electrons with gained energy are filtered, near-field imaging enables the visualization of nanoscale particles and interfaces with enhanced contrast (Barwick et al 2009 Nature 462 902). To explore a variety of applications, it is important to express, through analytical formulation, the key parameters involved in this photon-induced near-field electron microscopy (PINEM) and to predict the associated phenomena of, e.g., forty-photon absorption by the electron packet. In this paper, we give an account of the theoretical and experimental results of PINEM. In particular, the time-dependent quantum solution for ultrafast electron packets in the nanostructure scattered electromagnetic (near) field is solved in the high kinetic energy limit to obtain the evolution of the incident electron packet into a superposition of discrete momentum wavelets. The characteristic length and time scales of the halo of electron–photon coupling are discussed in the framework of Rayleigh and Mie scatterings, providing the dependence of the PINEM effect on size, polarization, material and spatiotemporal localization. We also provide a simple classical description that is based on features of plasmonics. A major part of this paper is devoted to the comparisons between the theoretical results and the recently obtained experimental findings about the imaging of materials and biological systems
Contrast enhanced spectroscopic optical coherence tomography
A method of forming an image of a sample includes performing SOCT on a sample. The sample may include a contrast agent, which may include an absorbing agent and/or a scattering agent. A method of forming an image of tissue may include selecting a contrast agent, delivering the contrast agent to the tissue, acquiring SOCT data from the tissue, and converting the SOCT data into an image. The contributions to the SOCT data of an absorbing agent and a scattering agent in a sample may be quantified separately
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