Search results for "Bloch equations"
showing 10 items of 18 documents
Biexciton formation and exciton coherent coupling in layered GaSe.
2015
Nonlinear two-dimensional Fourier transform (2DFT) and linear absorption spectroscopy are used to study the electronic structure and optical properties of excitons in the layered semiconductor GaSe. At the 1s exciton resonance, two peaks are identified in the absorption spectra, which are assigned to splitting of the exciton ground state into the triplet and singlet states. 2DFT spectra acquired for co-linear polarization of the excitation pulses feature an additional peak originating from coherent energy transfer between the singlet and triplet. At cross-linear polarization of the excitation pulses, the 2DFT spectra expose a new peak likely originating from bound biexcitons. The polarizati…
Optimal control of the inversion of two spins in Nuclear Magnetic Resonance
2012
International audience; We investigate the optimal control of the inversion of two spin 1/2 particles in Nuclear Magnetic Resonance. The two spins, which differ by their resonance offset, are controlled by the same radio frequency magnetic field. Using the Pontryagin Maximum Principle, we compute the optimal control sequence which allows to reach the target state in a given time, while minimizing the energy of the magnetic field. A comparison with the time-optimal solution for bounded control amplitude realizing the same control in the same time is made. An experimental illustration is done using techniques of Nuclear Magnetic Resonance.
Optical non-contact electric field mapping by LIF in Cs vapor
2007
We present experimental and theoretical studies of the possibility of using cesium vapor as a tracer gas for optical non-contact electric field mapping. Optical images of electric field distributions have been obtained.
Optimal control design of preparation pulses for contrast optimization in MRI
2017
Abstract This work investigates the use of MRI radio-frequency (RF) pulses designed within the framework of optimal control theory for image contrast optimization. The magnetization evolution is modeled with Bloch equations, which defines a dynamic system that can be controlled via the application of the Pontryagin Maximum Principle (PMP). This framework allows the computation of optimal RF pulses that bring the magnetization to a given state to obtain the desired contrast after acquisition. Creating contrast through the optimal manipulation of Bloch equations is a new way of handling contrast in MRI, which can explore the theoretical limits of the system. Simulation experiments carried out…
Towards the time-optimal control of dissipative spin-1/2 particles in nuclear magnetic resonance
2011
International audience; We consider the time-optimal control of a spin 1/2 particle whose dynamics is governed by the Bloch equations with both longitudinal and transverse relaxation terms. We solve this control problem by using geometric optimal control techniques. We show the crucial role of singular extremals in the time-optimal synthesis. This role can mainly be attributed to the presence of dissipation. We also analyze the robustness of the optimal control sequence when both the maximum amplitude of the control field and the dissipative parameters are varied. Finally, we present an experimental implementation of the different solutions using techniques of Nuclear Magnetic Resonance.
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…
Ground-state magneto-optical resonances in cesium vapor confined in an extremely thin cell
2007
Experimental and theoretical studies are presented related to the ground-state magneto-optical resonance prepared in Cesium vapour confined in an Extremely Thin Cell (ETC, with thickness equal to the wavelength of the irradiating light). It is shown that the utilization of the ETC allows one to examine the formation of a magneto-optical resonance on the individual hyperfine transitions, thus distinguishing processes resulting in dark (reduced absorption) or bright (enhanced absorption) resonance formation. We report on an experimental evidence of the bright magneto-optical resonance sign reversal in Cs atoms confined in the ETC. A theoretical model is proposed based on the optical Bloch equ…
Two Applications of Geometric Optimal Control to the Dynamics of Spin Particles
2014
The purpose of this article is to present the application of methods from geometric optimal control to two problems in the dynamics of spin particles. First, we consider the saturation problem for a single spin system and second, the control of a linear chain of spin particles with Ising couplings. For both problems the minimizers are parameterized using Pontryagin Maximum Principle and the optimal solution is found by a careful analysis of the corresponding equations.
Instantaneous diffusion in spin-echo dynamics: A frequency-domain experimental investigation.
1992
We report an experimental investigation on the spectral content of the echo radiation emitted by an electron spin system excited by a two-pulse sequence. The measurements are carried out at liquid-helium temperature in a sample of quartz containing a very low concentration of [${\mathrm{AlO}}_{4}$${]}^{0}$ centers, in which instantaneous diffusion mechanism makes the main contribution to the phase relaxation. The experimental spectral shapes reproduce the theoretical ones, as calculated for an ideal system of noninteracting spins. However, the agreement is only qualitative, as the experimental spectral widths are found to be less than expected by a factor that depends on the exciting sequen…
A Coherent Master Equation for active mode locking in lasers
2015
We present the derivation of a new master equation for active mode locking in lasers that fully takes into account the coherent effects of the light matter interaction through a peculiar adiabatic elimination technique. The coherent effects included in our model could be relevant to describe properly mode-locked semiconductor lasers where the standard Haus’ Master Equation predictions show some discrepancy with respect to the experimental results and can be included in the modelling of other mode locking techniques too.