1,053 research outputs found
Adaptive pumping for spectral control of random lasers
A laser is not necessarily a sophisticated device: Pumping energy into an
amplifying medium randomly filled with scatterers, a powder for instance, makes
a perfect "random laser." In such a laser, the absence of mirrors greatly
simplifies laser design, but control over emission directionality or frequency
tunability is lost, seriously hindering prospects for this otherwise simple
laser. Lately, we proposed a novel approach to harness random lasers, inspired
by spatial shaping methods recently employed for coherent light control in
complex media. Here, we experimentally implement this method in an optofluidic
random laser where scattering is weak and modes extend spatially and strongly
overlap, making individual selection a priori impossible. We show that control
over laser emission can indeed be regained even in this extreme case by
actively shaping the spatial profile of the optical pump. This unique degree of
freedom, which has never been exploited, allows selection of any desired
wavelength and shaping of lasing modes, without prior knowledge of their
spatial distribution. Mode selection is achieved with spectral selectivity down
to 0.06nm and more than 10dB side-lobe rejection. This experimental method
paves the way towards fully tunable and controlled random lasers and can be
transferred to other class of lasers.Comment: 23 pages, 7 figure
Optofluidic random laser
An active disordered medium able to lase is called a random laser (RL). We
demonstrate random lasing due to inherent disorder in a dye circulated
structured microfluidic channel. We consistently observe RL modes which are
varied by changing the pumping conditions. Potential applications for on-chip
sources and sensors are discussed.Comment: 3 pages, 4 figure
The sound of violets: the ethnographic potency of poetry?
This paper takes the form of a dialogue between the two authors, and is in two halves, the first half discursive and propositional, and the second half exemplifying the rhetorical, epistemological and metaphysical affordances of poetry in critically scrutinising the rhetoric, epistemology and metaphysics of educational management discourse.
Phipps and Saunders explore, through ideas and poems, how poetry can interrupt and/or illuminate dominant values in education and in educational research methods, such as:
âą alternatives to the military metaphors â targets, strategies and the like â that dominate the soundscape of education;
âą the kinds and qualities of the cognitive and feeling spaces that might be opened up by the shifting of methodological boundaries;
âą the considerable work done in ethnography on the use of the poetic: anthropologists have long used poetry as a medium for expressing their sense of empathic connection to their field and their subjects, particularly in considering the creativity and meaning-making that characterise all human societies in different ways;
âą the particular rhetorical affordances of poetry, as a discipline, as a practice, as an art, as patterned breath; its capacity to shift phonemic, and therewith methodological, authority; its offering of redress to linear and reductive attempts at scripting social life, as always already given and without alternative
Cooperative scattering and radiation pressure force in dense atomic clouds
We consider the collective scattering by a cloud of two-level atoms
driven by an uniform radiation field. Dense atomic clouds can be described by a
continuous density and the problem reduces to deriving the spectrum of the
atom-atom coupling operator. For clouds much larger than the optical
wavelength, the spectrum is treated as a continuum, and analytical expressions
for several macroscopic quantities, such as scattered radiation intensity and
radiation pressure force, are derived. The analytical results are then compared
to the exact -body solution and with those obtained assuming a symmetric
timed Dicke state. In contrast with the symmetric timed Dicke state, our
calculations takes account of the back action of the atoms on the driving field
leading to phase shifts due to the finite refraction of the cloud
The role of Mie scattering in the seeding of matter-wave superradiance
Matter-wave superradiance is based on the interplay between ultracold atoms
coherently organized in momentum space and a backscattered wave. Here, we show
that this mechanism may be triggered by Mie scattering from the atomic cloud.
We show how the laser light populates the modes of the cloud, and thus imprints
a phase gradient on the excited atomic dipoles. The interference with the atoms
in the ground state results in a grating, that in turn generates coherent
emission, contributing to the backward light wave onset. The atomic recoil
'halos' created by the scattered light exhibit a strong anisotropy, in contrast
to single-atom scattering
Targeted mixing in an array of alternating vortices
Transport and mixing properties of passive particles advected by an array of
vortices are investigated. Starting from the integrable case, it is shown that
a special class of perturbations allows one to preserve separatrices which act
as effective transport barriers, while triggering chaotic advection. In this
setting, mixing within the two dynamical barriers is enhanced while long range
transport is prevented. A numerical analysis of mixing properties depending on
parameter values is performed; regions for which optimal mixing is achieved are
proposed. Robustness of the targeted mixing properties regarding errors in the
applied perturbation are considered, as well as slip/no-slip boundary
conditions for the flow
Resonances in Mie scattering by an inhomogeneous atomic cloud
Despite the quantum nature of the process, collective scattering by dense
cold samples of two-level atoms can be interpreted classically describing the
sample as a macroscopic object with a complex refractive index. We demonstrate
that resonances in Mie theory can be easily observable in the cooperative
scattering by tuning the frequency of the incident laser field or the atomic
number. The solution of the scattering problem is obtained for spherical atomic
clouds who have the parabolic density characteristic of BECs, and the
cooperative radiation pressure force calculated exhibits resonances in the
cloud displacement for dense clouds. At odds from uniform clouds which show a
complex structure including narrow peaks, these densities show resonances, yet
only under the form of quite regular and contrasted oscillations
Experimental perspectives for systems based on long-range interactions
The possibility of observing phenomena peculiar to long-range interactions,
and more specifically in the so-called Quasi-Stationary State (QSS) regime is
investigated within the framework of two devices, namely the Free-Electron
Laser (FEL) and the Collective Atomic Recoil Laser (CARL). The QSS dynamics has
been mostly studied using the Hamiltonian Mean-Field (HMF) toy model,
demonstrating in particular the presence of first versus second order phase
transitions from magnetized to unmagnetized regimes in the case of HMF. Here,
we give evidence of the strong connections between the HMF model and the
dynamics of the two mentioned devices, and we discuss the perspectives to
observe some specific QSS features experimentally. In particular, a dynamical
analog of the phase transition is present in the FEL and in the CARL in its
conservative regime. Regarding the dissipative CARL, a formal link is
established with the HMF model. For both FEL and CARL, calculations are
performed with reference to existing experimental devices, namely the
FERMI@Elettra FEL under construction at Sincrotrone Trieste (Italy) and the
CARL system at LENS in Florence (Italy)
Models with short and long-range interactions: phase diagram and reentrant phase
We study the phase diagram of two different Hamiltonians with competiting
local, nearest-neighbour, and mean-field couplings. The first example
corresponds to the HMF Hamiltonian with an additional short-range interaction.
The second example is a reduced Hamiltonian for dipolar layered spin
structures, with a new feature with respect to the first example, the presence
of anisotropies. The two examples are solved in both the canonical and the
microcanonical ensemble using a combination of the min-max method with the
transfer operator method. The phase diagrams present typical features of
systems with long-range interactions: ensemble inequivalence, negative specific
heat and temperature jumps. Moreover, in a given range of parameters, we report
the signature of phase reentrance. This can also be interpreted as the presence
of azeotropy with the creation of two first order phase transitions with
ensemble inequivalence, as one parameter is varied continuously
Chaos in free electron laser oscillators
The chaotic nature of a storage-ring Free Electron Laser (FEL) is
investigated. The derivation of a low embedding dimension for the dynamics
allows the low-dimensionality of this complex system to be observed, whereas
its unpredictability is demonstrated, in some ranges of parameters, by a
positive Lyapounov exponent. The route to chaos is then explored by tuning a
single control parameter, and a period-doubling cascade is evidenced, as well
as intermittence.Comment: Accepted in EPJ
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