372 research outputs found
The Hanle and Zeeman Effects in Solar Spicules: A Novel Diagnostic Window on Chromospheric Magnetism
An attractive diagnostic tool for investigating the magnetism of the solar
chromosphere is the observation and theoretical modeling of the Hanle and
Zeeman effects in spicules, as shown in this letter for the first time. Here we
report on spectropolarimetric observations of solar chromospheric spicules in
the He I 10830 \AA multiplet and on their theoretical modeling accounting for
radiative transfer effects. We find that the magnetic field in the observed
(quiet Sun) spicular material at a height of about 2000 km above the visible
solar surface has a strength of the order of 10 G and is inclined by
approximately with respect to the local vertical direction. Our
empirical finding based on full Stokes-vector spectropolarimetry should be
taken into account in future magnetohydrodynamical simulations of spicules.Comment: 12 pages and 2 figure
Modelling the incomplete Paschen-Back effect in the spectra of magnetic Ap stars
We present first results of a systematic investigation of the incomplete
Paschen-Back effect in magnetic Ap stars. A short overview of the theory is
followed by a demonstration of how level splittings and component strengths
change with magnetic field strength for some lines of special astrophysical
interest. Requirements are set out for a code which allows the calculation of
full Stokes spectra in the Paschen-Back regime and the behaviour of Stokes I
and V profiles of transitions in the multiplet 74 of FeII is discussed in some
detail. It is shown that the incomplete Paschen-Back effect can lead to
noticeable line shifts which strongly depend on total multiplet strength,
magnetic field strength and field direction. Ghost components (which violate
the normal selection rule on J) show up in strong magnetic fields but are
probably unobservable. Finally it is shown that measurements of the integrated
magnetic field modulus are not adversely affected by the Paschen-Back
effect, and that there is a potential problem in (magnetic) Doppler mapping if
lines in the Paschen-Back regime are treated in the Zeeman approximation.Comment: 8 pages, 10 figures, to appear in MNRA
Single mode terahertz quantum cascade amplifier
A terahertz (THz) optical amplifier based on a 2.9 THz quantum cascade laser (QCL) structure has been demonstrated. By depositing an antireflective coating on the QCL facet, the laser mirror losses are enhanced to fully suppress the lasing action, creating a THz quantum cascade (QC) amplifier. Terahertz radiation amplification has been obtained, by coupling a separate multi-mode THz QCL of the same active region design to the QC amplifier. A bare cavity gain is achieved and shows excellent agreement with the lasing spectrum from the original QCL without the antireflective coating. Furthermore, a maximum optical gain of ∼30 dB with single-mode radiation output is demonstrated
Single mode terahertz quantum cascade amplifier
A terahertz (THz) optical amplifier based on a 2.9 THz quantum cascade laser (QCL) structure has been demonstrated. By depositing an antireflective coating on the QCL facet, the laser mirror losses are enhanced to fully suppress the lasing action, creating a THz quantum cascade (QC) amplifier. Terahertz radiation amplification has been obtained, by coupling a separate multi-mode THz QCL of the same active region design to the QC amplifier. A bare cavity gain is achieved and shows excellent agreement with the lasing spectrum from the original QCL without the antireflective coating. Furthermore, a maximum optical gain of ∼30 dB with single-mode radiation output is demonstrated
Scattering polarization of hydrogen lines in the presence of turbulent electric fields
We study the broadband polarization of hydrogen lines produced by scattering
of radiation, in the presence of isotropic electric fields. In this paper, we
focus on two distinct problems: a) the possibility of detecting the presence of
turbulent electric fields by polarimetric methods, and b) the influence of such
fields on the polarization due to a macroscopic, deterministic magnetic field.
We found that isotropic electric fields decrease the degree of linear
polarization in the scattered radiation, with respect to the zero-field case.
On the other hand, a distribution of isotropic electric fields superimposed
onto a deterministic magnetic field can generate a significant increase of the
degree of magnetic-induced, net circular polarization. This phenomenon has
important implications for the diagnostics of magnetic fields in plasmas using
hydrogen lines, because of the ubiquitous presence of the Holtsmark,
microscopic electric field from neighbouring ions. In particular, previous
solar magnetographic studies of the Balmer lines of hydrogen may need to be
revised because they neglected the effect of turbulent electric fields on the
polarization signals. In this work, we give explicit results for the
Lyman-alpha and Balmer-alpha lines.Comment: 15 pages, 6 figure
Cr:LiSrAlF6 channel waveguides as broadband fluorescence sources
We report on the production and fluorescence of active channel waveguides in Cr:LiSrAlF6. We have produced ∼10μm wide and 5μm high channel waveguides by He+ ion implantation, lithographic patterning and subsequent Ar+ ion sputtering. Diode-pumped waveguides emitted 13μW of fluorescence light with a spectrum ranging from 760nm to 900nm at a pump power of 165mW and a pump wavelength of 660nm. The compact and cheap optical pump source is a main advantage of this fluorescence material. This makes Cr:LiSrAlF6 channel waveguides a suitable candidate for a broadband fluorescence source in low-coherence interferometry and other applications in the near-infrared wavelength rang
Terahertz s-SNOM with > λ/1000 resolution based on self-mixing in quantum cascade lasers
Near-field imaging techniques have great potential in many applications, ranging from the investigation of the optical properties of solid state and 2D materials to the excitation and direct retrieval of plasmonic resonant modes, to the mapping of carrier concentrations in semiconductor devices. Further to this, the capability of performing imaging with non-ionizing terahertz (THz) radiation on a subwavelength scale is of fundamental importance in biological applications and healthcare. The implementation of stable, compact solid state sources such as quantum cascade lasers (QCLs) in apertureless scanning near field optical microscopes (s-SNOM), instead of bulkier gas lasers, has been already reported with a resolution ≥ 1 μm [1] based on metallic tips. Here we report on the realization of an s-SNOM, based on tuning fork sensors [2], to maintain a constant sample/tip distance in tapping mode, and using quantum cascade lasers emitting around 3 THz as both source and detector in a self-mixing scheme [3]. The implementation of a fast and efficient feedback mechanism allowed the achievement of a spatial resolution lower than 100 nm, as shown in Fig. 1, thus achieving the record resolution with a QCL better than λ/1000. The self-mixing approach allows an extremely sensitive and fast detection scheme, which overcomes the slow response of traditional THz detectors, by monitoring the scattered signal fed back into the QCL cavity, modulating the power or the bias. In order to enhance the sensitivity of the whole apparatus, as well as the collection of the scattered light, silicon lenses have been attached to the QCLs with an antireflection parylene coating which was thick enough to strongly reduce the laser emission, but still allowed enough power for alignment. Figure 1 reports the topography a) and the THz voltage signal on the QCL b) of Au square features (top-left square corner) over a Si substrate, exhibiting an enhanced scattering. As the reference voltage used for subtraction from the QCL voltage was placed lower than the QCL voltage, the THz signal dropped on the Au square
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