269 research outputs found
Spatial dynamics, thermalization, and gain clamping in a photon condensate
We study theoretically the effects of pump-spot size and location on photon
condensates. By exploring the inhomogeneous molecular excitation fraction, we
make clear the relation between spatial equilibration, gain clamping and
thermalization in a photon condensate. This provides a simple understanding of
several recent experimental results. We find that as thermalization breaks
down, gain clamping is imperfect, leading to "transverse spatial hole burning"
and multimode condensation. This opens the possibility of engineering the gain
profile to control the condensate structure.Comment: Further extended, including new figures. Now 10 figure
Thermalization and breakdown of thermalization in photon condensates
The authors acknowledge financial support from EPSRC program “TOPNES” (Grant No. EP/I031014/1) and EPSRC (Grant No. EP/G004714/2). P.G.K. acknowledges support from EPSRC (Grant No. EP/M010910/1).We examine in detail the mechanisms behind thermalization and Bose-Einstein condensation (BEC) of a gas of photons in a dye-filled microcavity. We derive a microscopic quantum model, based on that of a standard laser, and show how this model can reproduce the behavior of recent experiments. Using the rate-equation approximation of this model, we show how a thermal distribution of photons arises. We go on to describe how the nonequilibrium effects in our model can cause thermalization to break down as one moves away from the experimental parameter values. In particular, we examine the effects of changing cavity length, and of altering the vibrational spectrum of the dye molecules. We are able to identify two measures which quantify whether the system is in thermal equilibrium. Using these, we plot “phase diagrams” distinguishing BEC and standard lasing regimes. Going beyond the rate-equation approximation, our quantum model allows us to investigate both the second-order coherence g(2) and the linewidth of the emission from the cavity. We show how the linewidth collapses as the system transitions to a Bose condensed state, and compare the results to the Schawlow-Townes linewidth.Publisher PDFPeer reviewe
Spontaneous rotating vortex lattices in a pumped decaying condensate
Injection and decay of particles in an inhomogeneous quantum condensate can
significantly change its behaviour. We model trapped, pumped, decaying
condensates by a complex Gross-Pitaevskii equation and analyse the density and
currents in the steady state. With homogeneous pumping, rotationally symmetric
solutions are unstable. Stability may be restored by a finite pumping spot.
However if the pumping spot is larger than the Thomas-Fermi cloud radius, then
rotationally symmetric solutions are replaced by solutions with spontaneous
arrays of vortices. These vortex arrays arise without any rotation of the trap,
spontaneously breaking rotational symmetry.Comment: Updated title and introduction. 4 pages, 3 figure
A conjugate gradient minimisation approach to generating holographic traps for ultracold atoms
Direct minimisation of a cost function can in principle provide a versatile
and highly controllable route to computational hologram generation. However, to
date iterative Fourier transform algorithms have been predominantly used. Here
we show that the careful design of cost functions, combined with numerically
efficient conjugate gradient minimisation, establishes a practical method for
the generation of holograms for a wide range of target light distributions.
This results in a guided optimisation process, with a crucial advantage
illustrated by the ability to circumvent optical vortex formation during
hologram calculation. We demonstrate the implementation of the conjugate
gradient method for both discrete and continuous intensity distributions and
discuss its applicability to optical trapping of ultracold atoms.Comment: 11 pages, 4 figure
Raman scattering with strongly coupled vibron-polaritons
Strong coupling between cavity photons and molecular vibrations can lead to the formation of vibron-polaritons. In a recent experiment with PVAc molecules in a metal-metal microcavity [A.Shalabney et al., Ang.Chem.Int.Ed. 54 7971 (2015)], such a coupling was observed to enhance the Raman scattering probability by several orders of magnitude. Inspired by this, we theoretically analyze the effect of strong photon-vibron coupling on the Raman scattering amplitude of organic molecules. This problem has recently been addressed in [J.del Pino, J.Feist and F.J.Garcia-Vidal; J.Phys.Chem.C 119 29132 (2015)] using exact numerics for a small number of molecules. In this paper we derive compact analytic results for any number of molecules, also including the ultra-strong coupling regime. Our calculations predict a division of the Raman signal into upper and lower polariton modes,with some enhancement to the lower polariton Raman amplitude due to the mode softening under strong coupling.PostprintPeer reviewe
Excitonic spectral features in strongly-coupled organic polaritons
Starting from a microscopic model, we investigate the optical spectra of
molecules in strongly-coupled organic microcavities examining how they might
self-consistently adapt their coupling to light. We consider both rotational
and vibrational degrees of freedom, focusing on features which can be seen in
the peak in the center of the spectrum at the bare excitonic frequency. In both
cases we find that the matter-light coupling can lead to a self-consistent
change of the molecular states, with consequent temperature-dependent
signatures in the absorption spectrum. However, for typical parameters, these
effects are much too weak to explain recent measurements. We show that another
mechanism which naturally arises from our model of vibrationally dressed
polaritons has the right magnitude and temperature dependence to be at the
origin of the observed data.Comment: 14 pages, 6 figur
Suppressing and restoring the Dicke superradiance transition by dephasing and decay
We show that dephasing of individual atoms destroys the superradiance transition of the Dicke model, but that adding individual decay toward the spin down state can restore this transition. To demonstrate this, we present a method to give an exact solution for the N atom problem with individual dephasing which scales polynomially with N. By comparing finite size scaling of our exact solution to a cumulant expansion, we confirm the destruction and restoration of the superradiance transition holds in the thermodynamic limit.PostprintPeer reviewe
Atom-only theories for U(1) symmetric cavity-QED models
R.P. was supported by the EPSRC Scottish Doctoral Training Centre in Condensed Matter Physics (CM-CDT), Grant No. EP/L015110/1.We consider a generalized Dicke model with U(1) symmetry, which can undergo a transition to a superradiant state that spontaneously breaks this symmetry. By exploiting the difference in timescale between atomic and cavity dynamics, one may eliminate the cavity dynamics, providing an atom-only theory. We show that the standard Redfield theory cannot describe the transition to the superradiant state, but including higher-order corrections does recover the transition. Our work reveals how the forms of effective theories must vary for models with continuous symmetry, and provides a template to develop effective theories of more complex models.Publisher PDFPeer reviewe
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