623 research outputs found
Super-quadratic behavior of luminescence decay excited by energy-transfer upconversion
For several decades, energy-transfer upconversion (ETU) in rare-earth-ion doped systems [1,2] has attracted much attention, firstly, because of the fundamental interest in the physical nature of this process and, secondly, because of very practical considerations, namely the demonstration of near-infrared pumped visible light sources and, in reverse, the detrimental influence of ETU on the efficiency of infrared emitting systems.\ud
We investigate fundamentally the behavior of infrared luminescence emitted directly from a metastable level and visible luminescence emitted after ETU from this level to higher-lying levels. Although these two luminescences are connected by the same metastable level and influenced by the same ETU process, the infrared luminescence probes all ions, while the visible luminescence probes only the class of ions susceptible to ETU [3]. A simple analytical model [4] predicts that such luminescence decay curves exhibit a super-quadratic dependence of upconversion on direct luminescence decay.\ud
The Nd3+ ion can serve as a model system for such investigations. It exhibits strong ETU from the metastable 4F3/2 level. When doped into oxide matrices, the 4F3/2 level is the only metastable level. The Nd3+ energy levels excited by ETU decay by fast multiphonon relaxation and, hence, the weak visible fluorescence emitted from these levels represents a quasi instantaneous reaction on the dynamics of the 4F3/2 level. Experimental results obtained after pulsed laser excitation of Nd3+-doped oxide host materials show indeed a super-quadratic behavior of upconversion versus direct luminescence decay, in accordance with the model predictions [4].\ud
\ud
[1] F. Auzel, Proc. IEEE 1973, 6, 758\ud
[2] J.C. Wright, Top. Appl. Phys. 1976, 15, 239\ud
[3] M. Pollnau, D.R. Gamelin, S.R. Lüthi, H.U. Güdel, M.P. Hehlen, Phys. Rev. B 2000, 61, 3337\ud
[4] M. Pollnau, J. Alloys Compd. 2002, 341, 5
Superquadratic behavior of upconversion luminescence transients in rare-earth-ion doped laser crystals
Inhomogeneous active-ion distributions in laser materials lead to strong deviations of upconversion versus direct luminescence transients from the quadratic law of energy-transfer upconversion. Measured luminescence decay curves in LaSc3(BO3)4:Nd3+ and GdVO4:Nd3+ confirm experimentally the predicted deviations. Differences in energy migration within the metastable level of Nd3+ are identified
Mechanical oscillations in lasing microspheres
We investigate the feasibility of activating coherent mechanical oscillations
in lasing microspheres by modulating the laser emission at a mechanical
eigenfrequency. To this aim, 1.5% Nd3+:Barium-Titanium-Silicate microspheres
with diameters around 50 {\mu}m were used as high quality factor (Q>10^6)
whispering gallery mode lasing cavities. We have implemented a pump-and-probe
technique in which the pump laser used to excite the Nd3+ ions is focused on a
single microsphere with a microscope objective and a probe laser excites a
specific optical mode with the evanescent field of a tapered fibre. The studied
microspheres show monomode and multi-mode lasing action, which can be modulated
in the best case up to 10 MHz. We have optically transduced thermally-activated
mechanical eigenmodes appearing in the 50-70 MHz range, the frequency of which
decreases with increasing the size of the microspheres. In a pump-and-probe
configuration we observed modulation of the probe signal up to the maximum pump
modulation frequency of our experimental setup, i.e., 20 MHz. This modulation
decreases with frequency and is unrelated to lasing emission, pump scattering
or thermal effects. We associate this effect to free-carrier-dispersion induced
by multiphoton pump light absorption. On the other hand, we conclude that, in
our current experimental conditions, it was not possible to resonantly excite
the mechanical modes. Finally, we discuss on how to overcome these limitations
by increasing the modulation frequency of the lasing emission and decreasing
the frequency of the mechanical eigenmodes displaying a strong degree of
optomechanical coupling.Comment: 17 pages, 5 figure
Dependence of upconversion on direct luminescence decay in energy-transfer upconversion
For several decades, energy-transfer upconversion (ETU) in rare-earth-ion doped systems [1,2] has attracted much attention, firstly, because of the fundamental interest in the physical nature of this process and, secondly, because of very practical considerations, namely the demonstration of near-infrared pumped visible light sources and, in reverse, the detrimental influence of ETU on the efficiency of infrared emitting systems. We investigate fundamentally the behavior of and interaction between infrared luminescence emitted directly from a metastable level and visible luminescence emitted after ETU from this metastable level to higher-lying levels. Although these two luminescences are connected by the same metastable level and influenced by the same ETU process, they probe different classes of ions. Whereas the infrared luminescence probes all ions, the visible luminescence probes only the class of ions susceptible to ETU [3]. A simple analytical model [4] predicts that such luminescence decay curves exhibit a super-quadratic dependence of upconversion on direct luminescence decay. The fraction of ions susceptible to ETU can be derived from this model. The Nd3+ ion can serve as a model system for such investigations. It exhibits strong ETU from the metastable 4F3/2 level. When doped into oxide matrices, the 4F3/2 level is the only metastable level within the 4f subshell. The Nd3+ energy levels excited by ETU decay by fast multiphonon relaxation and, hence, the weak visible fluorescence emitted from these levels represents a quasi instantaneous reaction on the dynamics of the 4F3/2 metastable level. Experimental results obtained after pulsed laser excitation of Nd3+-doped oxide host materials show indeed a super-quadratic behavior of upconversion versus direct luminescence decay, in accordance with the model predictions [4]. [1] F. Auzel, Proc. IEEE 6, 758 (1973) [2] J.C. Wright, Top. Appl. Phys. 15, 239 (1976) [3] M. Pollnau, D.R. Gamelin, S.R. Lüthi, H.U. Güdel, M.P. Hehlen, Phys. Rev. B 61, 3337 (2000) [4] M. Pollnau, J. Alloys Compd. 341, 51 (2002
Leaf water diffusion dynamics in vivo through a sub-terahertz portable imaging system
The development of terahertz based technology has given the opportunity for the realization of non destructive techniques capable of gaining meaningful information on delicate systems such as biological samples. Here, the health status of leaves in vivo has been monitored through a portable terahertz imaging system. The data have been extracted and analysed from the images acquired and compared with analogous results reported in the literature on similar systems. The possibilty of extracting additional information from the images regarding leaf details has also been explored
Dynamical response of the "GGG" rotor to test the Equivalence Principle: theory, simulation and experiment. Part I: the normal modes
Recent theoretical work suggests that violation of the Equivalence Principle
might be revealed in a measurement of the fractional differential acceleration
between two test bodies -of different composition, falling in the
gravitational field of a source mass- if the measurement is made to the level
of or better. This being within the reach of ground based
experiments, gives them a new impetus. However, while slowly rotating torsion
balances in ground laboratories are close to reaching this level, only an
experiment performed in low orbit around the Earth is likely to provide a much
better accuracy.
We report on the progress made with the "Galileo Galilei on the Ground" (GGG)
experiment, which aims to compete with torsion balances using an instrument
design also capable of being converted into a much higher sensitivity space
test.
In the present and following paper (Part I and Part II), we demonstrate that
the dynamical response of the GGG differential accelerometer set into
supercritical rotation -in particular its normal modes (Part I) and rejection
of common mode effects (Part II)- can be predicted by means of a simple but
effective model that embodies all the relevant physics. Analytical solutions
are obtained under special limits, which provide the theoretical understanding.
A simulation environment is set up, obtaining quantitative agreement with the
available experimental data on the frequencies of the normal modes, and on the
whirling behavior. This is a needed and reliable tool for controlling and
separating perturbative effects from the expected signal, as well as for
planning the optimization of the apparatus.Comment: Accepted for publication by "Review of Scientific Instruments" on Jan
16, 2006. 16 2-column pages, 9 figure
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