1,299 research outputs found
Weak-wave advancement in nearly collinear four-wave mixing
We identify a new four-wave mixing process in which two nearly collinear pump
beams produce phase-dependent gain into a weak bisector signal beam in a
self-defocusing Kerr medium. Phase matching is achieved by weak-wave
advancement caused by cross-phase modulation between the pump and signal beams.
We relate this process to the inverse of spatial modulational instability and
suggest a time-domain analog.Comment: 7 pages, 3 figure
Quantum Noise and Superluminal Propagation
Causal "superluminal" effects have recently been observed and discussed in
various contexts. The question arises whether such effects could be observed
with extremely weak pulses, and what would prevent the observation of an
"optical tachyon." Aharonov, Reznik, and Stern (ARS) [Phys. Rev. Lett., vol.
81, 2190 (1998)] have argued that quantum noise will preclude the observation
of a superluminal group velocity when the pulse consists of one or a few
photons. In this paper we reconsider this question both in a general framework
and in the specific example, suggested by Chiao, Kozhekin, and Kurizki [Phys.
Rev. Lett., vol. 77, 1254 (1996)], of off-resonant, short-pulse propagation in
an optical amplifier. We derive in the case of the amplifier a signal-to-noise
ratio that is consistent with the general ARS conclusions when we impose their
criteria for distinguishing between superluminal propagation and propagation at
the speed c. However, results consistent with the semiclassical arguments of
CKK are obtained if weaker criteria are imposed, in which case the signal can
exceed the noise without being "exponentially large." We show that the quantum
fluctuations of the field considered by ARS are closely related to
superfluorescence noise. More generally we consider the implications of
unitarity for superluminal propagation and quantum noise and study, in addition
to the complete and truncated wavepackets considered by ARS, the residual
wavepacket formed by their difference. This leads to the conclusion that the
noise is mostly luminal and delayed with respect to the superluminal signal. In
the limit of a very weak incident signal pulse, the superluminal signal will be
dominated by the noise part, and the signal-to-noise ratio will therefore be
very small.Comment: 30 pages, 1 figure, eps
Photonic crystal polarizers and polarizing beam splitters
We have experimentally demonstrated polarizers and polarizing beam splitters
based on microwave-scale two-dimensional photonic crystals. Using polarized
microwaves within certain frequency bands, we have observed a squared-sinusoid
(Malus) transmission law when using the photonic crystal as a polarizer. The
photonic crystal also functions as a polarizing beamsplitter; in this
configuration it can be engineered to split incident polarizations in either
order, making it more versatile than conventional, Brewster-angle
beamsplitters.Comment: 7 pages, 3 figures, published Journal Applied Physics 93, 9429 (2003
Quantum Optics
Quantum optics, i.e. the interaction of individual photons with matter, began with the discoveries of Planck and Einstein, but in recent years it has expanded beyond pure physics to become an important driving force for technological innovation. This book serves the broader readership growing out of this development by starting with an elementary description of the underlying physics and then building up a more advanced treatment. The reader is led from the quantum theory of thesimple harmonic oscillator to the application of entangled states to quantum information processing. An equally impo
Microwave measurements of the photonic bandgap in a two-dimensional photonic crystal slab
We have measured the photonic bandgap in the transmission of microwaves
through a two-dimensional photonic crystal slab. The structure was constructed
by cementing acrylic rods in a hexagonal closed-packed array to form
rectangular stacks. We find a bandgap centered at approximately 11 GHz, whose
depth, width and center frequency vary with the number of layers in the slab,
angle of incidence and microwave polarization.Comment: 8 pages, 3 figures, submitted to Journal of Applied Physic
Hyperspectral imaging of cuttlefish camouflage indicates good color match in the eyes of fish predators
Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 108 (2011):9148-9153, doi:10.1073/pnas.1019090108.Camouflage is a widespread phenomenon throughout nature and an important
anti-predator tactic in natural selection. Many visual predators have keen color
perception, thus camouflage patterns should provide some degree of color matching in
addition to other visual factors such as pattern, contrast, and texture. Quantifying
camouflage effectiveness in the eyes of the predator is a challenge from the
perspectives of both biology and optical imaging technology. Here we take advantage
of Hyperspectral Imaging (HSI), which records full-spectrum light data, to
simultaneously visualize color match and pattern match in the spectral and the spatial
domains, respectively. Cuttlefish can dynamically camouflage themselves on any
natural substrate and, despite their colorblindness, produce body patterns that appear
to have high-fidelity color matches to the substrate when viewed directly by humans
or with RGB images. Live camouflaged cuttlefish on natural backgrounds were
imaged using HSI, and subsequent spectral analysis revealed that most reflectance
spectra of individual cuttlefish and substrates were similar, rendering the color match
possible. Modeling color vision of potential di- and tri-chromatic fish predators of
cuttlefish corroborated the spectral match analysis and demonstrated that camouflaged
cuttlefish show good color match as well as pattern match in the eyes of fish predators.
These findings (i) indicate the strong potential of HSI technology to enhance studies
3
of biological coloration, and (ii) provide supporting evidence that cuttlefish can
produce color-coordinated camouflage on natural substrates despite lacking color
vision.We gratefully acknowledge financial support from the
National Science Council of Taiwan NSC-98-2628-B-007-001-MY3 to CCC, from
the Network Science Center at West Point and the Army Research Office to JKW,
from the NDSEG Fellowship to JJA, and from ONR grant N000140610202 to RTH
Demonstration of superluminal effects in an absorptionless, non-reflective system
We present an experimental and theoretical study of a simple, passive system
consisting of a birefringent, two-dimensional photonic crystal and a polarizer
in series, and show that superluminal dispersive effects can arise even though
no incident radiation is absorbed or reflected. We demonstrate that a vector
formulation of the Kramers-Kronig dispersion relations facilitates an
understanding of these counter-intuitive effects.Comment: 6 pages, 3 figures, accepted on Physical Review Letter
Signal velocity, causality, and quantum noise in superluminal light pulse propagation
We consider pulse propagation in a linear anomalously dispersive medium where
the group velocity exceeds the speed of light in vacuum (c) or even becomes
negative. A signal velocity is defined operationally based on the optical
signal-to-noise ratio, and is computed for cases appropriate to the recent
experiment where such a negative group velocity was observed. It is found that
quantum fluctuations limit the signal velocity to values less than c.Comment: 4 Journal pages, 3 figure
Cultureâgene coevolution of individualismâcollectivism and the serotonin transporter gene
Cultureâgene coevolutionary theory posits that cultural values have evolved, are adaptive and influence the social and physical environments under which genetic selection operates. Here, we examined the association between cultural values of individualismâcollectivism and allelic frequency of the serotonin transporter functional polymorphism (5-HTTLPR) as well as the role this cultureâgene association may play in explaining global variability in prevalence of pathogens and affective disorders. We found evidence that collectivistic cultures were significantly more likely to comprise individuals carrying the short (S) allele of the 5-HTTLPR across 29 nations. Results further show that historical pathogen prevalence predicts cultural variability in individualismâcollectivism owing to genetic selection of the S allele. Additionally, cultural values and frequency of S allele carriers negatively predict global prevalence of anxiety and mood disorder. Finally, mediation analyses further indicate that increased frequency of S allele carriers predicted decreased anxiety and mood disorder prevalence owing to increased collectivistic cultural values. Taken together, our findings suggest cultureâgene coevolution between allelic frequency of 5-HTTLPR and cultural values of individualismâcollectivism and support the notion that cultural values buffer genetically susceptible populations from increased prevalence of affective disorders. Implications of the current findings for understanding cultureâgene coevolution of human brain and behaviour as well as how this coevolutionary process may contribute to global variation in pathogen prevalence and epidemiology of affective disorders, such as anxiety and depression, are discussed
- âŠ