609 research outputs found
The Role of Spatial Coherence and Orbital Angular Momentum of Light in Astronomy
The orbital angular momentum (OAM) of light is potentially interesting for
astronomical study of rotating objects such as black holes, but the effect of
reduced spatial coherence of astronomical light sources such as stars is
largely unknown. In a lab-scale experiment, we find that the detected OAM
spectrum depends strongly on the position of the light-twisting object along
the line of sight. We develop a simple intuitive model to predict the influence
of reduced spatial coherence in astronomical observations, and discuss
line-of-sight and intensity issues.Comment: updated versio
Microcavity resonance condition, quality factor, and mode volume are determined by different penetration depths
The penetration depth in a Distributed Bragg Reflector (DBR) co-determines
the resonance condition, quality factor, and mode volume of DBR-based
microcavities. Recent studies have used an incomplete description of the
penetration depth and incorrect equations. We present a complete analysis that
involves three different penetration depths. We also present a series of
experiments on microcavities to accurately determine the frequency and modal
penetration depth of our DBRs and compare these results with theoretical
predictions. The obtained results are relevant for anyone who models a DBR as
an effective hard mirror if lengths of the order of the wavelength are
relevant, as is the case for microcavities.Comment: 16 pages, 7 figure
Optimisation of sample thickness for THz-TDS measurements
How thick should the sample be for a transmission THz-TDS measurement? Should
the sample be as thick as possible? The answer is `no'. Although more thickness
allows T-rays to interact more with bulk material, SNR rolls off with thickness
due to signal attenuation. Then, should the sample be extremely thin? Again,
the answer is `no'. A sample that is too thin renders itself nearly invisible
to T-rays, in such a way that the system can hardly sense the difference
between the sample and a free space path. So, where is the optimal boundary
between `too thick' and `too thin'? The trade-off is analysed and revealed in
this paper, where our approach is to find the optimal thickness that results in
the minimal variance of measured optical constants.Comment: 13 pages, 11 figure
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Oratie uitgesproken door Prof.dr. Martin P. van Exter bij de aanvaarding van het ambt van hoogleraar in de Optica aan de Universiteit Leiden op vrijdag 18 november 2016Oratie uitgesproken door Prof.dr. Martin P. van Exter bij de aanvaarding van het ambt van hoogleraar in de Optica aan de Universiteit Leiden op vrijdag 18 november 201
Observation of Goos-H\"{a}nchen shifts in metallic reflection
We report the first observation of the Goos-Hnchen
shift of a light beam incident on a metal surface. This phenomenon is
particularly interesting because the Goos-Hnchen shift
for polarized light in metals is negative and much bigger than the positive
shift for polarized light. The experimental result for the measured shifts
as a function of the angle of incidence is in excellent agreement with
theoretical predictions. In an energy-flux interpretation, our measurement
shows the existence of a backward energy flow at the bare metal surface when
this is excited by a polarized beam of light.Comment: The parer was published on Optics Express. The new version is
modified according to the reviewers suggestion
Quantum dot nonlinearity through cavity-enhanced feedback with a charge memory
In an oxide apertured quantum dot (QD) micropillar cavity-QED system, we
found strong QD hysteresis effects and lineshape modifications even at very low
intensities corresponding to less than 0.001 intracavity photons. We attribute
this to the excitation of charges by the intracavity field; charges that get
trapped at the oxide aperture, where they screen the internal electric field
and blueshift the QD transition. This in turn strongly modulates light
absorption by cavity QED effects, eventually leading to the observed hysteresis
and lineshape modifications. The cavity also enables us to observe the QD
dynamics in real time, and all experimental data agrees well with a power-law
charging model. This effect can serve as a novel tuning mechanism for quantum
dots.Comment: 7 pages, 6 figure
Binary central stars of planetary nebulae with long orbits: the radial velocity orbit of BD+33.2642 (PN G052.7+50.7) and the orbital motion of HD112313 (PN LoTr5)
We study the impact of binary interaction processes on the evolution of low-
and intermediate-mass stars using long-term monitoring of their radial
velocity.
Here we report on our results on the central stars of two planetary nebulae
(PNe): the well-studied spectrophotometric standard BD+33.2642 (central star of
PNG 052.7+50.7) and HD112313 (central star of PN LoTr5), the optical light of
which is dominated by a rapidly rotating G star.
The high-resolution spectra were cross-correlated with carefully selected
masks of spectral lines. The individual masks were optimised for the spectral
signatures of the dominant contributor of the optical light.
We report on the first detection of orbital motion in these two objects. For
BD+33.2642 we sampled 1.5 cycles of the 1105 +/- 24 day orbital period. For HD
112313 a full period is not yet covered, despite our 1807 days of monitoring.
The radial-velocity amplitude shows that it is unlikely that the orbital plane
is co-planar with the one defined by the nebular waist of the bipolar nebula.
To our knowledge these are the first detections of orbits in PNe that are in a
range from several weeks to a few years.
The orbital properties and chemical composition of BD+33.2642 are similar to
what is found in post-AGB binaries with circumbinary discs. The latter are
probably progenitors of these PNe. For LoTr5 the Ba-rich central star and the
long orbital period are similar to the Ba star giants, which hence serve as
natural progeny. In contrast to the central star in LoTr5, normal Ba stars are
slow rotators. The orbits of these systems have a low probability of occurrence
according to recent population synthesis calculations.Comment: 4 pages, 3 figures, Astronomy and Astrophysics, letter
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