735 research outputs found
Rational expectations and the paradox of policy-relevant natural experiments
Policy experiments using large microeconomic datasets have recently gained ground in macro- economics. Imposing rational expectations, we examine robustness of evidence derived from ideal natural experiments applied to atomistic agents in dynamic settings. Paradoxically, once experi-mental evidence is viewed as suÂą ciently clean to use, it then becomes contaminated byex post endo- geneity: Measured responses depend upon priors and the objective function into which evidence is fed. Moreover, agentsĂpolicy beliefs become endogenously correlated with their causal parameters, severely clouding inference, e.g. sign reversals and non-invertibility may obtain. Treatment-control di§erences are contaminated for non-quadratic adjustment costs. Constructively, we illustrate how inference can be corrected accounting for feedback and highlight factors mitigating contamination
Photon-energy dissipation caused by an external electric circuit in "virtual" photo-excitation processes
We consider generation of an electrical pulse by an optical pulse in the
``virtual excitation'' regime. The electronic system, which is any
electro-optic material including a quantum well structure biased by a dc
electric field, is assumed to be coupled to an external circuit. It is found
that the photon frequency is subject to an extra red shift in addition to the
usual self-phase modulation, whereas the photon number is conserved. The Joule
energy consumed in the external circuit is supplied only from the extra red
shift.Comment: 4 pages, 1 fugur
Charge transport and phase transition in exciton rings
The macroscopic exciton rings observed in the photoluminescence (PL) patterns
of excitons in coupled quantum wells (CQWs) are explained by a series of
experiments and a theory based on the idea of carrier imbalance, transport and
recombination. The rings are found to be a source of cold excitons with
temperature close to that of the lattice. We explored states of excitons in the
ring over a range of temperatures down to 380 mK. These studies reveal a sharp,
albeit continuous, second order phase transition to a low-temperature ordered
exciton state, characterized by ring fragmentation into a periodic array of
aggregates. An instability at the onset of degeneracy in the cold exciton
system, due to stimulated exciton formation, is proposed as the transition
mechanism.Comment: 8 pages including 4 figure
Sum-over-states vs quasiparticle pictures of coherent correlation spectroscopy of excitons in semiconductors; femtosecond analogues of multidimensional NMR
Two-dimensional correlation spectroscopy (2DCS) based on the nonlinear
optical response of excitons to sequences of ultrafast pulses, has the
potential to provide some unique insights into carrier dynamics in
semiconductors. The most prominent feature of 2DCS, cross peaks, can best be
understood using a sum-over-states picture involving the many-body eigenstates.
However, the optical response of semiconductors is usually calculated by
solving truncated equations of motion for dynamical variables, which result in
a quasiparticle picture. In this work we derive Green's function expressions
for the four wave mixing signals generated in various phase-matching directions
and use them to establish the connection between the two pictures. The formal
connection with Frenkel excitons (hard-core bosons) and vibrational excitons
(soft-core bosons) is pointed out.Comment: Accepted to Phys. Rev.
Parity forbidden excitations of Sr2CuO2Cl2 revealed by optical third-harmonic spectroscopy
We present the first study of nonlinear optical third harmonic generation in
the strongly correlated charge-transfer insulator Sr2CuO2Cl2. For fundamental
excitation in the near-infrared, the THG spectrum reveals a strongly resonant
response for photon energies near 0.7 eV. Polarization analysis reveals this
novel resonance to be only partially accounted for by three-photon excitation
to the optical charge-transfer exciton, and indicates that an even-parity
excitation at 2 eV, with a_1g symmetry, participates in the third harmonic
susceptibility.Comment: Requires RevTeX v4.0beta
Photonic crystals of coated metallic spheres
It is shown that simple face-centered-cubic (fcc) structures of both metallic
and coated metallic spheres are ideal candidates to achieve a tunable complete
photonic bandgap (CPBG) for optical wavelengths using currently available
experimental techniques. For coated microspheres with the coating width to
plasma wavelength ratio and the coating and host
refractive indices and , respectively, between 1 and 1.47, one can
always find a sphere radius such that the relative gap width (gap
width to the midgap frequency ratio) is larger than 5% and, in some cases,
can exceed 9%. Using different coatings and supporting liquids, the width
and midgap frequency of a CPBG can be tuned considerably.Comment: 14 pages, plain latex, 3 ps figures, to appear in Europhys. Lett. For
more info on this subject see
http://www.amolf.nl/research/photonic_materials_theory/moroz/moroz.htm
Third-person knowledge ascriptions: a crucial experiment for contextualism
In the past few years there has been a turn towards evaluating the empirical foundation of epistemic contextualism using formal (rather than armchair) experimental methods. By-and-large, the results of these experiments have not supported the original motivation for epistemic contextualism. That is partly because experiments have only uncovered effects of changing context on knowledge ascriptions in limited experimental circumstances (when contrast is present, for example), and partly because existing experiments have not been designed to distinguish between contextualism and one of its main competing theories, subject-sensitive invariantism. In this paper, we discuss how a particular, âthird-personâ, experimental design is needed to provide evidence that would support contextualism over subject-sensitive invariantism. In spite of the theoretical significance of third-person knowledge ascriptions for debates surrounding contextualism, no formal experiments evaluating such ascriptions that assess contextualist claims have previously been conducted. In this paper, we conduct an experiment specifically designed to examine that central gap in contextualismâs empirical foundation. The results of our experiment provide crucial support for epistemic contextualism over subject-sensitive invariantism
Decoherence and Programmable Quantum Computation
An examination of the concept of using classical degrees of freedom to drive
the evolution of quantum computers is given. Specifically, when externally
generated, coherent states of the electromagnetic field are used to drive
transitions within the qubit system, a decoherence results due to the back
reaction from the qubits onto the quantum field. We derive an expression for
the decoherence rate for two cases, that of the single-qubit Walsh-Hadamard
transform, and for an implementation of the controlled-NOT gate. We examine the
impact of this decoherence mechanism on Grover's search algorithm, and on the
proposals for use of error-correcting codes in quantum computation.Comment: submitted to Phys. Rev. A 35 double-spaced pages, 2 figures, in LaTe
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Ultrabroadband 50-130 THz pulses generated via phase-matcheddifference frequency mixing in LiIO3
We report the generation of ultrabroadband pulses spanningthe 50-130 THz frequency range via phase-matched difference frequencymixing within the broad spectrum of sub-10 fs pulses in LiIO_3. Modelcalculations reproduce the octave-spanning spectra and predict few-cycleTHz pulse durations less than 20~;fs. The applicability of this scheme isdemonstrated with 9-fs pulses from a Ti:sapphire oscillator and with 7-fsamplified pulses from a hollow fiber compressor as pumpsources
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