180 research outputs found
Statistical mechanics of Floquet systems: the pervasive problem of near degeneracies
The statistical mechanics of periodically driven ("Floquet") systems in
contact with a heat bath exhibits some radical differences from the traditional
statistical mechanics of undriven systems. In Floquet systems all quasienergies
can be placed in a finite frequency interval, and the number of near
degeneracies in this interval grows without limit as the dimension N of the
Hilbert space increases. This leads to pathologies, including drastic changes
in the Floquet states, as N increases. In earlier work these difficulties were
put aside by fixing N, while taking the coupling to the bath to be smaller than
any quasienergy difference. This led to a simple explicit theory for the
reduced density matrix, but with some major differences from the usual time
independent statistical mechanics. We show that, for weak but finite coupling
between system and heat bath, the accuracy of a calculation within the
truncated Hilbert space spanned by the N lowest energy eigenstates of the
undriven system is limited, as N increases indefinitely, only by the usual
neglect of bath memory effects within the Born and Markov approximations. As we
seek higher accuracy by increasing N, we inevitably encounter quasienergy
differences smaller than the system-bath coupling. We therefore derive the
steady state reduced density matrix without restriction on the size of
quasienergy splittings. In general, it is no longer diagonal in the Floquet
states. We analyze, in particular, the behavior near a weakly avoided crossing,
where quasienergy near degeneracies routinely appear. The explicit form of our
results for the denisty matrix gives a consistent prescription for the
statistical mechanics for many periodically driven systems with N infinite, in
spite of the Floquet state pathologies.Comment: 31 pages, 3 figure
Ac Stark Effects and Harmonic Generation in Periodic Potentials
The ac Stark effect can shift initially nonresonant minibands in
semiconductor superlattices into multiphoton resonances. This effect can result
in strongly enhanced generation of a particular desired harmonic of the driving
laser frequency, at isolated values of the amplitude.Comment: RevTeX, 10 pages (4 figures available on request), Preprint
UCSBTH-93-2
Trion Species-Resolved Quantum Beats in MoSe2
Monolayer photonic materials offer a tremendous potential for on-chip
optoelectronic devices. Their realization requires knowledge of optical
coherence properties of excitons and trions that have so far been limited to
nonlinear optical experiments carried out with strongly inhomogenously
broadened material. Here we employ h-BN encapsulated and electrically gated
MoSe2 to reveal coherence properties of trion-species directly in the linear
optical response. Autocorrelation measurements reveal long dephasing times up
to T2=1.16+-0.05 ps for positively charged excitons. Gate dependent
measurements provide evidence that the positively-charged trion forms via
spatially localized hole states making this trion less prone to dephasing in
the presence of elevated hole carrier concentrations. Quantum beat signatures
demonstrate coherent coupling between excitons and trions that have a dephasing
time up to 0.6 ps, a two-fold increase over those in previous reports. A key
merit of the prolonged exciton/trion coherences is that they were achieved in a
linear optical experiment, and thus are directly relevant to applications in
nanolasers, coherent control, and on-chip quantum information processing
requiring long photon coherence.Comment: 21 pages, 6 figures, 2 SOI figure
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