Search results for "FREQUENCY"
showing 10 items of 2158 documents
Harmonic generation from nanorings driven by a two-color laser field
2014
We study the high harmonic generation and the polarization of the harmonics emitted by a nanoring driven by two laser fields of angular frequency ?1 and ?2, with ?2?=?2?1, and ?1 resonant between the ground and the first excited state. We show that by varying parameters, such as laser intensity, photon energy and the delay between the two laser pulses, we can control the number of harmonics and the polarization of the radiation. In particular we show that with this choice of two-laser photon frequency rate we obtain more harmonics with respect to other configurations. We also calculated the average absorbed energy and the average angular momentum acquired by the electron as a function of th…
Nonlinear magneto-optical resonances atD1excitation ofRb85andRb87in an extremely thin cell
2010
Nonlinear magneto-optical resonances have been measured in an extremely thin cell (ETC) for the ${D}_{1}$ transition of rubidium in an atomic vapor of natural isotopic composition. All hyperfine transitions of both isotopes have been studied for a wide range of laser power densities, laser detunings, and ETC wall separations. Dark resonances in the laser induced fluorescence (LIF) were observed as expected when the ground-state total angular momentum ${F}_{g}$ was greater than or equal to the excited-state total angular momentum ${F}_{e}$. Unlike the case of ordinary cells, the width and contrast of dark resonances formed in the ETC dramatically depended on the detuning of the laser from th…
Experimental and numerical study of noise effects in a FitzHugh–Nagumo system driven by a biharmonic signal
2013
Abstract Using a nonlinear circuit ruled by the FitzHugh–Nagumo equations, we experimentally investigate the combined effect of noise and a biharmonic driving of respective high and low frequency F and f. Without noise, we show that the response of the circuit to the low frequency can be maximized for a critical amplitude B∗ of the high frequency via the effect of Vibrational Resonance (V.R.). We report that under certain conditions on the biharmonic stimulus, white noise can induce V.R. The effects of colored noise on V.R. are also discussed by considering an Ornstein–Uhlenbeck process. All experimental results are confirmed by numerical analysis of the system response.
Probing the internal solar magnetic field through g-modes
2006
The observation of g-mode candidates by the SoHO mission opens the possibility of probing the internal structure of the solar radiative zone (RZ) and the solar core more directly than possible via the use of the p-mode helioseismology data. We study the effect of rotation and RZ magnetic fields on g-mode frequencies. Using a self-consistent static MHD magnetic field model we show that a 1% g-mode frequency shift with respect to the Solar Seismic Model (SSeM) prediction, currently hinted in the GOLF data, can be obtained for magnetic fields as low as 300 kG, for current measured modes of radial order n=-20. On the other hand, we also argue that a similar shift for the case of the low order g…
Absolute frequency measurement of rubidium 5S-7S two-photon transitions.
2013
We report the absolute frequency measurements of rubidium 5S-7S two-photon transitions with a cw laser digitally locked to an atomic transition and referenced to an optical frequency comb. The narrow, two-photon transition, 5S-7S (760 nm) insensitive to first order in a magnetic field, is a promising candidate for frequency reference. The performed tests yield the transition frequency with accuracy better than reported previously.
Scalar Dark Matter in the Radio-Frequency Band: Atomic-Spectroscopy Search Results
2019
Among the prominent candidates for dark matter are bosonic fields with small scalar couplings to the Standard-Model particles. Several techniques are employed to search for such couplings and the current best constraints are derived from tests of gravity or atomic probes. In experiments employing atoms, observables would arise from expected dark-matter-induced oscillations in the fundamental constants of nature. These studies are primarily sensitive to underlying particle masses below $10^{-14}$ eV. We present a method to search for fast oscillations of fundamental constants using atomic spectroscopy in cesium vapor. We demonstrate sensitivity to scalar interactions of dark matter associate…
The UWB-OFDM Channel Analysis in Frequency
2009
In this paper, the ultra-wideband channel with orthogonal frequency division multiplexing (UWB-OFDM) is analyzed in the frequency domain. For UWB-OFDM channels with log-normal fading in the time domain, we show that the amplitude of each subcarrier can be approximated by a Nakagami-m random variable, where the fading parameter, the mean power and the correlation coefficient are expressed in terms of the following parameters: time arrival of the clusters, inter-arrival time of rays inside clusters, and power decay constants of rays and clusters.
Generation of High-Repetition-Rate Dark Soliton Trains and Frequency Conversion in Optical Fibers
1998
Induced modurational polarization instability in birefringent fibers leads to trains of dark soliton-like pulses. Optimal large-signal cw and soliton frequency conversion is also analysed.
Laser Speckle Size And Temporal Transfer Function In Human Vision
1988
Using a blue laser stimulus we measured TMTF with speckle sizes of 19.1 µm, 3.3 µm and absence of speckle. Our results indicate, if we compare the TMTF curves obtained for different speckle sizes, a gain of modulation due fundamentally to the presence of the spec kle; in the low frequency region this gain of modulation can be explained in terms of inhibitory effects. On the other hand, we observe by using the diffusion-inhibition model, that the presence of speckle in the test-field produces a delay of 12 ms and a reduction of the height of the response, with respect to that obtained in the uniform test (with absence of speckle).
Determination of the threshold of the break-up of invariant tori in a class of three frequency Hamiltonian systems
2001
We consider a class of Hamiltonians with three degrees of freedom that can be mapped into quasi-periodically driven pendulums. The purpose of this paper is to determine the threshold of the break-up of invariant tori with a specific frequency vector. We apply two techniques: the frequency map analysis and renormalization-group methods. The renormalization transformation acting on a Hamiltonian is a canonical change of coordinates which is a combination of a partial elimination of the irrelevant modes of the Hamiltonian and a rescaling of phase space around the considered torus. We give numerical evidence that the critical coupling at which the renormalization transformation starts to diverg…