LQ11930
Cold atoms are lasing.
A new laser has been developed at the Institut Non
Linéaire de Nice (France). This laser uses a cloud of laser-cooled rubidium
atoms (~100 µK) as the gain medium, placed inside a cavity. As there is no
stationary population inversion in such a medium, gain is obtained with
trickier mechanisms, corresponding to multi-photons transitions. The team
led by Robin Kaiser has demonstrated lasing action with three different
mechanisms, namely Mollow gain, Raman gain, and four-wave-mixing. Up to 300
µW laser power have been produced. The gain mechanism that produces the
laser depends on the pumping parameters (detuning, intensity,
polarization…). The laser can thus be tuned continuously from one regime to
another. This laser could find applications in quantum optics or in the
study of laser dynamics, but the primary goal of the Nice team is to
combine one of these gain mechanisms with multiple scattering in the atom
cloud. This should lead to the first realization of a random laser (when
the feedback is due to multiple scattering in the gain medium itself) with
cold atoms. The present result, published in Phys. Rev. Lett. (W. Guerin,
F. Michaud and R. Kaiser, …) is the first step towards this goal. The next
step is thus to preserve the laser... without the cavity !
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BSR1107
Spin-polarized current plays an important role in
nano spin-electronics because it can change the direction of
magnetizations or move magnetic walls in magnetic nano structures.
However, the ratio between the coefficients of adiabatic and
non-adiabatic components of the torque exerted by the spin-polarized
current on magnetizations has been a controversial issue for many years.
In this paper, the authors derived the equation for the magnetization
dynamics in the presence of a spin current based on the non-equilibrium
thermodynamics. They showed that the coefficient of non-adiabatic
torque is not equal to the damping coefficient called the Gilbert
damping constant, in general. The equality holds when the relaxation
time of the fluctuating magnetic field is very short compared to the
time scale of magnetization dynamics. They applied the theory to
current-induced magnetization reversal in magnetic multi-layers and
showed that the switching time is a decreasing function of the
relaxation time.
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EQ10326
Tracing hidden signal inside plasma
First time, we have shown that the plasma medium can be useful to detect
untraceable subthreshold signal applied externally in the plasma medium,
by applying external noise, whereas, previously only autonomous dynamics
of the plasma was detectable using noise. Usually noises are disturbance
for the modern communication devices. In this experiment it is shown that
for suitable parametric regions and configurations of the system, untraceable
signal can be traced using noise and inherent nonlinearity of the plasma.
This technique may be used in plasma antenna to detect hidden signal.
We have also devised very simple statistical tool (AMD) to quantify stochastic
resonance that is free from computational difficulties of usual statistical
tools like Signal to noise ration (SNR) or cross-correlation techniques.
Vedio 1: Oscilloscopic view of stochastic resonance in plasma. Lower trace
is the applied subthreshold signal plus noise and upper trace is the output
signal. The video shows that initially for low amplitude of noise applied
signal was not traceable (noise has been increased continuously) and with
increase in the noise system detects the hidden signal. Maximum transmission
is at optimum noise level and for high level noise, transmission becomes noisy.
