35 research outputs found
Heterodyne interferometer with unequal path lengths
Laser interferometry is an extensively used diagnostic for plasma
experiments. Existing plasma interferometers are designed on the presumption
that the scene and reference beam path lengths have to be equal, a requirement
that is costly in both the number of optical components and the alignment
complexity. It is shown here that having equal path lengths is not necessary -
instead what is required is that the path length difference be an even multiple
of the laser cavity length. This assertion has been verified in a heterodyne
laser interferometer that measures typical line-average densities of with an error of .Comment: 15 pages, 9 figures, to be published in Rev. Sci. Instrum. 77 (2006
The Relation Between Optical beams Propagation in Free Space and in Strongly Nonlocal Nonlinear Media
The relation between optical beams propagation in strongly nonlocal nonlinear
(SNN) media and {propagation} in free space is {demonstrated using} the
technique of variable transformation. The governing equation, integral and
analytical solutions, and propagation properties in free space can be directly
transferred to their counterparts in SNN media through a one-to-one
correspondence. The one-to-one correspondence together with the Huygens-Fresnel
integral yields an efficient numerical method to describe SNN propagation. The
existence conditions and possible structures of solitons and breathers in SNN
media are described in a unified manner by comparing propagation properties in
SNN media with those in free space. The results can be employed in other
contexts in which the governing equation for the evolution of waves is
equivalent to that in SNN media, such as for quadratic graded-index media, or
for harmonically trapped Bose-Einstein condensates in the noninteracting limit.Comment: 10 pages, 2 figures, published in EP
A 12.5 GHz-Spaced Optical Frequency Comb Spanning >400 nm for near-Infrared Astronomical Spectrograph Calibration
A 12.5 GHz-spaced optical frequency comb locked to a Global Positioning
disciplined oscillator for near-IR spectrograph calibration is presented. The
comb is generated via filtering a 250 MHz-spaced comb. Subsequency nonlinear
broadening of the 12.5 GHz comb extends the wavelength range to cover 1380 nm
to 1820 nm, providing complete coverage over the H-band transmission widow of
Earth's atmosphere. Finite suppression of spurious sidemodes, optical linewidth
and instability of the comb have been examined to estmiate potential wavelength
biases in spectrograph calibration. Sidemode suppression varies between 20 db
and 45 dB, and the optical linewidth is ~350 kHz at 1550 nm. The comb frequency
uncertainty is bounded by +/- 30 kHz (corresponding to a radial velocity of +/-
5 cm/s), limited by the Global Positioning System disciplined oscillator
reference. These results indicate this comb can readily support radial velocity
measurements below 1 m/s in the near-IR.Comment: 16 pages, 12 figures, new file fixes some readability problems on
Mac
Nonlinear Spin Dynamics in Ferromagnets with Electron-Nuclear Coupling
Nonlinear spin motion in ferromagnets is considered with nonlinearity due to
three factors: (i) the sample is prepared in a strongly nonequilibrium state,
so that evolution equations cannot be linearized as would be admissible for
spin motion not too far from equilibrium, (ii) the system considered consists
of interacting electron and nuclear spins coupled with each other via hyperfine
forces, and (iii) the sample is inserted into a coil of a resonant electric
circuit producing a resonator feedback field. Due to these nonlinearities,
coherent motion of spins can develop, resulting in their ultrafast relaxation.
A complete analysis of mechanisms triggering such a coherent motion is
presented. This type of ultrafast coherent relaxation can be used for studying
intrinsic properties of magnetic materials.Comment: 1 file, LaTex, 23 page
Imaging Chromophores With Undetectable Fluorescence by Stimulated Emission Microscopy
Fluorescence, that is, spontaneous emission, is generally more sensitive than absorption measurement, and is widely used in optical imaging. However, many chromophores, such as haemoglobin and cytochromes, absorb but have undetectable fluorescence because the spontaneous emission is dominated by their fast non-radiative decay. Yet the detection of their absorption is difficult under a microscope. Here we use stimulated emission, which competes effectively with the nonradiative decay, to make the chromophores detectable, and report a new contrast mechanism for optical microscopy. In a pump-probe experiment, on photoexcitation by a pump pulse, the sample is stimulated down to the ground state by a time-delayed probe pulse, the intensity of which is concurrently increased. We extract the miniscule intensity increase with shot-noise-limited sensitivity by using a lock-in amplifier and intensity modulation of the pump beam at a high megahertz frequency. The signal is generated only at the laser foci owing to the nonlinear dependence on the input intensities, providing intrinsic three-dimensional optical sectioning capability. In contrast, conventional one-beam absorption measurement exhibits low sensitivity, lack of three-dimensional sectioning capability, and complication by linear scattering of heterogeneous samples. We demonstrate a variety of applications of stimulated emission microscopy, such as visualizing chromoproteins, non-fluorescent variants of the green fluorescent protein, monitoring lacZ gene expression with a chromogenic reporter, mapping transdermal drug distributions without histological sectioning, and label-free microvascular imaging based on endogenous contrast of haemoglobin. For all these applications, sensitivity is orders of magnitude higher than for spontaneous emission or absorption contrast, permitting nonfluorescent reporters for molecular imaging.Chemistry and Chemical Biolog
Nonlinear Spin Dynamics in Nuclear Magnets
A method is developed for solving nonlinear systems of differential, or
integrodifferential, equations with stochastic fields. The method makes it
possible to give an accurate solution for an interesting physical problem: What
are the peculiarities of nonlinear spin dynamics in nonequilibrium nuclear
magnets coupled with a resonator? Evolution equations for nuclear spins are
derived basing on a Hamiltonian with dipole interactions. The ensemble of spins
is coupled with a resonator electric circuit. Seven types of main relaxation
regimes are found: free induction, collective induction, free relaxation,
collective relaxation, weak superradiance, pure superradiance, and triggered
superradiance. The initial motion of spins can be originated by two reasons,
either by an imposed initial coherence or by local spin fluctuations due to
nonsecular dipole interactions. The relaxation regimes caused by the second
reason cannot be described by the Bloch equations. Numerical estimates show
good agreement with experiment.Comment: 1 file, 47 pages, LaTe