345 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
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
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
Tuning micropillar cavity birefringence by laser induced surface defects
We demonstrate a technique to tune the optical properties of micropillar
cavities by creating small defects on the sample surface near the cavity region
with an intense focused laser beam. Such defects modify strain in the
structure, changing the birefringence in a controllable way. We apply the
technique to make the fundamental cavity mode polarization-degenerate and to
fine tune the overall mode frequencies, as needed for applications in quantum
information science.Comment: RevTex, 7 pages, 4 figures (accepted for publication in Applied
Physics Letters
A Herschel study of Planetary Nebulae
We present Herschel PACS and SPIRE images of the dust shells around the
planetary nebulae NGC 650, NGC 6853, and NGC 6720, as well as images showing
the dust temperature in their shells. The latter shows a rich structure, which
indicates that internal extinction in the UV is important despite the highly
evolved status of the nebulae.Comment: 2 pages, 1 figure, 2012, proceedings IAU Symposium 283 Planetary
Nebulae: An Eye to the Futur
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