177 research outputs found
Gain narrowing in few-atom systems
Using a density matrix approach, we study the simplest systems that display
both gain and feedback: clusters of 2 to 5 atoms, one of which is pumped. The
other atoms supply feedback through multiple scattering of light. We show that,
if the atoms are in each other's near-field, the system exhibits large gain
narrowing and spectral mode redistribution. The observed phenomena are more
pronounced if the feedback is enhanced. Our system is to our knowledge the
simplest exactly solvable microscopic system which shows the approach to laser
oscillation
Interplay between multiple scattering, emission, and absorption of light in the phosphor of a white light-emitting diode
We study light transport in phosphor plates of white light-emitting diodes
(LEDs). We measure the broadband diffuse transmission through phosphor plates
of varying YAG:Ce density. We distinguish the spectral ranges where
absorption, scattering, and re-emission dominate. Using diffusion theory, we
derive the transport and absorption mean free paths from first principles. We
find that both transport and absorption mean free paths are on the order of the
plate thickness. This means that phosphors in commercial LEDs operate well
within an intriguing albedo range around 0.7. We discuss how salient parameters
that can be derived from first principles control the optical properties of a
white LED.Comment: 14 pages, 9 figure
Atomic Deuterium Adsorbed on the Surface of Liquid Helium
We investigate deuterium atoms adsorbed on the surface of liquid helium in
equilibrium with a vapor of atoms of the same species. These atoms are studied
by a sensitive optical method based on spectroscopy at a wavelength of 122 nm,
exciting the 1S-2P transition. We present a direct measurement of the
adsorption energy of deuterium atoms on helium and show evidence for the
existence of resonantly enhanced recombination of atoms residing on the surface
to molecules.Comment: 6 pages 4 figure
Collisional properties of ultracold K-Rb mixtures
We determine the inter-species s-wave triplet scattering length a3 for all
K-Rb isotopic mixtures by measuring the cross-section for collisions between
41K and 87Rb in different temperature regimes. The positive value
a3=+163(+57,-12)a0 ensures the stability of binary 41K-87Rb Bose-Einstein
condensates. For the fermion-boson mixture 40K-87Rb we obtain a large and
negative scattering length which implies an efficient sympathetic cooling of
the fermionic species down to the degenerate regime.Comment: 4 pages, 4 figures; revised version (references added and small
changes
In situ characterization of an optical cavity using atomic light shift
We report the precise characterization of the optical potential obtained by
injecting a distributed-feedback erbium-doped fiber laser (DFB EDFL) at 1560 nm
to the transversal modes of a folded optical cavity. The optical potential was
mapped in situ using cold rubidium atoms, whose potential energy was spectrally
resolved thanks to the strong differential light shift induced by the 1560 nm
laser on the two levels of the probe transition. The optical potential obtained
in the cavity is suitable for trapping rubidium atoms, and eventually to
achieve all-optical Bose-Einstein condensation directly in the resonator.Comment: 3 pages, 4 figure
Giant Helium Dimers Produced by Photoassociation of Ultracold Metastable Atoms
We produce giant helium dimers by photoassociation of metastable helium atoms
in a magnetically trapped, ultracold cloud. The photoassociation laser is
detuned red of the atomic line and produces strong heating
of the sample when resonant with molecular bound states. The temperature of the
cloud serves as an indicator of the molecular spectrum. We report good
agreement between our spectroscopic measurements and our calculations of the
five bound states belonging to a purely long-range potential well.
These previously unobserved states have classical inner turning points of about
150 and outer turning points as large as 1150 .Comment: 4 pages, 4 figure
Sympathetic Cooling with Two Atomic Species in an Optical Trap
We simultaneously trap ultracold lithium and cesium atoms in an optical
dipole trap formed by the focus of a CO laser and study the exchange of
thermal energy between the gases. The cesium gas, which is optically cooled to
K, efficiently decreases the temperature of the lithium gas through
sympathetic cooling. The measured cross section for thermalizing
Cs-Li collisions is cm, for both species in
their lowest hyperfine ground state. Besides thermalization, we observe
evaporation of lithium purely through elastic cesium-lithium collisions
(sympathetic evaporation).Comment: 4 pages 3 fig
Optical transmission matrix as a probe of the photonic interaction strength
We demonstrate that optical transmission matrices (TM) of disordered complex media provide a powerful tool to extract the photonic interaction strength, independent of surface effects. We measure TM of strongly scattering GaP nanowires and plot the singular value density of the measured matrices and a random matrix model. By varying the free parameters of the model, the transport mean free path and effective refractive index, we retrieve the photonic interaction strength. From numerical simulations we conclude that TM statistics is hardly sensitive to surface effects, in contrast to enhanced backscattering or total transmission based methods
Optical transmission matrix as a probe of the photonic strength
We demonstrate that optical transmission matrices (TM) of disordered complex media provide a powerful tool to extract the photonic interaction strength, independent of surface effects. We measure TM of strongly scattering GaP nanowires and plot the singular value density of the measured matrices and a random matrix model. By varying the free parameters of the model, the transport mean free path and effective refractive index, we retrieve the photonic interaction strength. From numerical simulations we conclude that TM statistics is hardly sensitive to surface effects, in contrast to enhanced backscattering or total transmission based methods.We acknowledge support from ERC grant 27948, NWOVici, STW, the Royal Society, and EPSRC through fellowship EP/J016918/1
Rotationally induced Penning ionization of ultracold photoassociated helium dimers
We have studied photoassociation of metastable \tripS helium atoms near the
\tripS-\tripP asymptote by both ion detection in a magneto-optical trap and
trap-loss measurements in a magnetic trap. A detailed comparison between the
results of the two experiments gives insight into the mechanism of the Penning
ionization process. We have identified four series of resonances corresponding
to vibrational molecular levels belonging to different rotational states in two
potentials. The corresponding spin states become quasi-purely quintet at small
interatomic distance, and Penning ionization is inhibited by spin conservation
rules. Only a weak rotational coupling is responsible for the contamination by
singlet spin states leading to a detectable ion signal. However, for one of
these series Bose statistics does not enable the rotational coupling and the
series detected through trap-loss does not give rise to sufficient ionization
for detection.Comment: 7 pages, 4 figures, submitted to EuroPhysics Letter
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