39 research outputs found
Unpolarized, incoherent repumping light for prevention of dark states in a trapped and laser-cooled single ion
Many ion species commonly used for laser-cooled ion trapping studies have a
low-lying metastable 2D3/2 state that can become populated due to spontaneous
emission from the 2P1/2 excited state. This requires a repumper laser to
maintain the ion in the Doppler cooling cycle. Typically the 2D3/2 state, or
some of its hyperfine components if the ion has nuclear spin, has a higher
multiplicity than the upper state of the repumping transition. This can lead to
dark states, which have to be destabilized by an external magnetic field or by
modulating the polarization of the repumper laser. We propose using
unpolarized, incoherent amplified spontaneous emission (ASE) to drive the
repumping transition. An ASE source offers several advantages compared to a
laser. It prevents the buildup of dark states without external polarization
modulation even in zero magnetic field, it can drive multiple hyperfine
transitions simultaneously, and it requires no frequency stabilization. These
features make it very compact and robust, which is essential for the
development of practical, transportable optical ion clocks. We construct a
theoretical model for the ASE radiation, including the possibility of the
source being partially polarized. Using 88Sr+ as an example, the performance of
the ASE source compared to a single-mode laser is analyzed by numerically
solving the eight-level density matrix equations for the involved energy
levels. Finally a reduced three-level system is derived, yielding a simple
formula for the excited state population and scattering rate, which can be used
to optimize the experimental parameters. The required ASE power spectral
density can be obtained with current technology
Broadband lasers for photo-ionization and repumping of trapped ions
A frequency-stable, broadband laser is presented for experiments on trapped
ions. Since the design is based on widely available semiconductor optical
amplifier technology, similar lasers can be realized for virtually any
wavelength in the near-infrared, and the coherence properties and output power
allow for efficient second harmonic generation. No closed-loop frequency
stabilization for addressing Doppler- or naturally-broadened, dipole-allowed
transitions is needed, and the light source can be turned on and off during a
measurement cycle with sub-microsecond response time. As a case study, a
921.7-nm laser with an output power of 20mW and a linewidth of 10GHz is
realized, which is then frequency doubled to 460.9nm for excitation of
strontium as the first step in photo-ionization. The excitation efficiency is
compared to that achievable with a narrow-linewidth distributed Bragg reflector
laser as well as to theory.Comment: JOSA B accepted manuscript (9 pages, 5 figures
Broadband, unpolarized repumping and clearout light sources for Sr single-ion clocks
Future transportable optical clocks require compact and reliable light
sources. Here, broadband, unpolarized repumper and state clearout sources for
Sr+ single-ion optical clocks are reported. These turn-key devices require no
frequency stabilization nor external modulators. They are fiber based,
inexpensive, and compact. Key characteristics for clock operation are
presented, including optical spectra, induced light shifts and required
extinction ratios. Tests with an operating single-ion standard show a clearout
efficiency of 100%. Compared to a laser-based repumper, the achievable
fluorescence rates for ion detection are a few tens of per cent lower. The
resulting ion kinetic temperature is 1--1.5 mK, near the Doppler limit of the
ion system. Similar repumper light sources could be made for Ca+ (866 nm) and
Ba+ (650 nm) using semiconductor gain media.Comment: 4 pages, 6 figure
Phase Measurement of Resonant Two-Photon Ionization in Helium
We study resonant two-color two-photon ionization of Helium via the 1s3p 1P1
state. The first color is the 15th harmonic of a tunable titanium sapphire
laser, while the second color is the fundamental laser radiation. Our method
uses phase-locked high-order harmonics to determine the {\it phase} of the
two-photon process by interferometry. The measurement of the two-photon
ionization phase variation as a function of detuning from the resonance and
intensity of the dressing field allows us to determine the intensity dependence
of the transition energy.Comment: 4 pages, 5 figures, under consideratio
Accumulation horizons and period-adding in optically injected semiconductor lasers
We study the hierarchical structuring of islands of stable periodic
oscillations inside chaotic regions in phase diagrams of single-mode
semiconductor lasers with optical injection. Phase diagrams display remarkable
{\it accumulation horizons}: boundaries formed by the accumulation of infinite
cascades of self-similar islands of periodic solutions of ever-increasing
period. Each cascade follows a specific period-adding route. The riddling of
chaotic laser phases by such networks of periodic solutions may compromise
applications operating with chaotic signals such as e.g. secure communications.Comment: 4 pages, 4 figures, laser phase diagrams, to appear in Phys. Rev. E,
vol. 7
Probing single-photon ionization on the attosecond time scale
We study photoionization of argon atoms excited by attosecond pulses using an
interferometric measurement technique. We measure the difference in time delays
between electrons emitted from the and from the shell, at
different excitation energies ranging from 32 to 42 eV. The determination of
single photoemission time delays requires to take into account the measurement
process, involving the interaction with a probing infrared field. This
contribution can be estimated using an universal formula and is found to
account for a substantial fraction of the measured delay.Comment: 4 pages, 4 figures, under consideratio
Photoemission time-delay measurements and calculations close to the 3s ionization minimum in Ar
We present experimental measurements and theoretical calculations of
photoionization time delays from the and shells in Ar in the photon
energy range of 32-42 eV. The experimental measurements are performed by
interferometry using attosecond pulse trains and the infrared laser used for
their generation. The theoretical approach includes intershell correlation
effects between the 3s and 3p shells within the framework of the random phase
approximation with exchange (RPAE). The connection between single-photon
ionization and the two-color two-photon ionization process used in the
measurement is established using the recently developed asymptotic
approximation for the complex transition amplitudes of laser-assisted
photoionization. We compare and discuss the theoretical and experimental
results especially in the region where strong intershell correlations in the 3s
to kp channel lead to an induced "Cooper" minimum in the 3s ionization
cross-section.Comment: submitted to PR
Validation of a low-cost virtual reality system for training street-crossing. A comparative study in healthy, neglected and non-neglected stroke individuals
Unilateral spatial neglect is a common consequence of stroke that directly affects the performance of activities of daily living. This impairment is traditionally assessed with paper-and-pencil tests that can lack correspondence to real life and are easily compensated. Virtual reality can immerse patients in more ecological scenarios, thus providing therapists with new tools to assess and train the effects of this impairment in simulated real tasks. This paper presents the clinical validation and convergent validity of a low-cost virtual reality system for training street-crossing in stroke patients with and without neglect. The performance of neglect patients was significantly worse than the performance of non-neglect and healthy participants. In addition, several correlations between the scores in the system and in the traditional scales were detected.This study was funded in part by Ministerio de Educacion y Ciencia Spain, Projects Consolider-C (SEJ2006-14301/PSIC), "CIBER of Physiopathology of Obesity and Nutrition, an initiative of ISCIII" and the Excellence Research Program PROMETEO (Generalitat Valenciana. Conselleria de Educacion, 2008-157).Navarro, MD.; Llorens Rodríguez, R.; Noé, E.; Ferri, J.; Alcañiz Raya, ML. (2013). Validation of a low-cost virtual reality system for training street-crossing. A comparative study in healthy, neglected and non-neglected stroke individuals. Neuropsychological Rehabilitation. 23(4):597-618. https://doi.org/10.1080/09602011.2013.806269S597618234Allegri, R. F. (2000). Atención y negligencia: bases neurológicas, evaluación y trastornos. Revista de Neurología, 30(05), 491. doi:10.33588/rn.3005.99645Appelros, P., Karlsson, G. M., Seiger, Åke, & Nydevik, I. (2002). 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