85 research outputs found
Collective Antenna Effects in the Terahertz and Infrared Response of Highly Aligned Carbon Nanotube Arrays
We study macroscopically-aligned single-wall carbon nanotube arrays with
uniform lengths via polarization-dependent terahertz and infrared transmission
spectroscopy. Polarization anisotropy is extreme at frequencies less than
3 THz with no sign of attenuation when the polarization is perpendicular
to the alignment direction. The attenuation for both parallel and perpendicular
polarizations increases with increasing frequency, exhibiting a pronounced and
broad peak around 10 THz in the parallel case. We model the electromagnetic
response of the sample by taking into account both radiative scattering and
absorption losses. We show that our sample acts as an effective antenna due to
the high degree of alignment, exhibiting much larger radiative scattering than
absorption in the mid/far-infrared range. Our calculated attenuation spectrum
clearly shows a non-Drude peak at 10 THz in agreement with the
experiment.Comment: 5 pages, 5 figure
Renormalized Energies of Superfluorescent Bursts from an Electron-Hole Magneto-plasma with High Gain in InGaAs Quantum Wells
We study light emission properties of a population-inverted 2D electron-hole
plasma in a quantizing magnetic field. We observe a series of superfluorescent
bursts, discrete both in time and energy, corresponding to the cooperative
recombination of electron-hole pairs from different Landau levels. The emission
energies are strongly renormalized due to many-body interactions among the
photogenerated carriers, exhibiting red-shifts as large as 20 meV at 15 T.
However, the magnetic field dependence of the lowest Landau level emission line
remains excitonic at all magnetic fields. Interestingly, our time-resolved
measurements show that this lowest-energy burst occurs only after all upper
states become empty, suggesting that this excitonic stability is related to the
`hidden symmetry' of 2D magneto-excitons expected in the magnetic quantum
limit.Comment: 5 pages, 4 figure
Nonlinear Dynamics in Semiconductor Ring Lasers: From Phase Turbulence to Solitons
The recent study of ring quantum cascade lasers [1] , [2] (QCLs, Fig. 1a ) revealed a new laser instability. It is triggered by phase turbulence akin to the wave instabilities that occur in other nonlinear systems such as fluids, superconductors and Bose-Einstein condensates. The choice of the ring geometry took inspiration from Kerr combs [3] , that are commonly generated in passive ring microresonators and have attracted great attention within the photonics community in the last years thanks to their rich physics
Do Evaporating Black Holes Form Photospheres?
Several authors, most notably Heckler, have claimed that the observable
Hawking emission from a microscopic black hole is significantly modified by the
formation of a photosphere around the black hole due to QED or QCD interactions
between the emitted particles. In this paper we analyze these claims and
identify a number of physical and geometrical effects which invalidate these
scenarios. We point out two key problems. First, the interacting particles must
be causally connected to interact, and this condition is satisfied by only a
small fraction of the emitted particles close to the black hole. Second, a
scattered particle requires a distance ~ E/m_e^2 for completing each
bremsstrahlung interaction, with the consequence that it is improbable for
there to be more than one complete bremsstrahlung interaction per particle near
the black hole. These two effects have not been included in previous analyses.
We conclude that the emitted particles do not interact sufficiently to form a
QED photosphere. Similar arguments apply in the QCD case and prevent a QCD
photosphere (chromosphere) from developing when the black hole temperature is
much greater than Lambda_QCD, the threshold for QCD particle emission.
Additional QCD phenomenological arguments rule out the development of a
chromosphere around black hole temperatures of order Lambda_QCD. In all cases,
the observational signatures of a cosmic or Galactic halo background of
primordial black holes or an individual black hole remain essentially those of
the standard Hawking model, with little change to the detection probability. We
also consider the possibility, as proposed by Belyanin et al. and D. Cline et
al., that plasma interactions between the emitted particles form a photosphere,
and we conclude that this scenario too is not supported.Comment: version published in Phys Rev D 78, 064043; 25 pages, 3 figures;
includes discussion on extending our analysis to TeV-scale,
higher-dimensional black hole
On the role of continuum-driven eruptions in the evolution of very massive stars and Population III stars
We suggest that the mass lost during the evolution of very massive stars may
be dominated by optically thick, continuum-driven outbursts or explosions,
instead of by steady line-driven winds. In order for a massive star to become a
WR star, it must shed its H envelope, but new estimates of the effects of
clumping in winds indicate that line driving is vastly insufficient. We discuss
massive stars above roughly 40-50 Msun, for which the best alternative is mass
loss during brief eruptions of luminous blue variables (LBVs). Our clearest
example of this phenomenon is the 19th century outburst of eta Car, when the
star shed 12-20 Msun or more in less than a decade. Other examples are
circumstellar nebulae of LBVs, extragalactic eta Car analogs (``supernova
impostors''), and massive shells around SNe and GRBs. We do not yet fully
understand what triggers LBV outbursts, but they occur nonetheless, and present
a fundamental mystery in stellar astrophysics. Since line opacity from metals
becomes too saturated, the extreme mass loss probably arises from a
continuum-driven wind or a hydrodynamic explosion, both of which are
insensitive to metallicity. As such, eruptive mass loss could have played a
pivotal role in the evolution and fate of massive metal-poor stars in the early
universe. If they occur in these Population III stars, such eruptions would
profoundly affect the chemical yield and types of remnants from early SNe and
hypernovae.Comment: 4 pages, 1 figure, accepted by ApJ Letter
Coherent instabilities in a semiconductor laser with fast gain recovery
We report the observation of a coherent multimode instability in quantum
cascade lasers (QCLs), which is driven by the same fundamental mechanism of
Rabi oscillations as the elusive Risken-Nummedal-Graham-Haken (RNGH)
instability predicted 40 years ago for ring lasers. The threshold of the
observed instability is significantly lower than in the original RNGH
instability, which we attribute to saturable-absorption nonlinearity in the
laser. Coherent effects, which cannot be reproduced by standard laser rate
equations, can play therefore a key role in the multimode dynamics of QCLs, and
in lasers with fast gain recovery in general.Comment: 5 pages, 4 figure
Frequency combs induced by phase turbulence
Wave instability—the process that gives rise to turbulence in hydrodynamics1—represents the mechanism by which a small disturbance in a wave grows in amplitude owing to nonlinear interactions. In photonics, wave instabilities result in modulated light waveforms that can become periodic in the presence of coherent locking mechanisms. These periodic optical waveforms are known as optical frequency combs2–4. In ring microresonator combs5,6, an injected monochromatic wave becomes destabilized by the interplay between the resonator dispersion and the Kerr nonlinearity of the constituent crystal. By contrast, in ring lasers instabilities are considered to occur only under extreme pumping conditions7,8. Here we show that, despite this notion, semiconductor ring lasers with ultrafast gain recovery9,10 can enter frequency comb regimes at low pumping levels owing to phase turbulence11—an instability known to occur in hydrodynamics, superconductors and Bose–Einstein condensates. This instability arises from the phase–amplitude coupling of the laser field provided by linewidth enhancement12, which produces the needed interplay of dispersive and nonlinear effects. We formulate the instability condition in the framework of the Ginzburg–Landau formalism11. The localized structures that we observe share several properties with dissipative Kerr solitons, providing a first step towards connecting semiconductor ring lasers and microresonator frequency combs13
Enhancing Acceleration Radiation from Ground-State Atoms via Cavity Quantum Electrodynamics
When ground state atoms are accelerated through a high Q microwave cavity,
radiation is produced with an intensity which can exceed the intensity of Unruh
acceleration radiation in free space by many orders of magnitude. The cavity
field at steady state is described by a thermal density matrix under most
conditions. However, under some conditions gain is possible, and when the atoms
are injected in a regular fashion, the radiation can be produced in a squeezed
state
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