Alain Barthélémy

Ferroelectric Control of Spin Polarization

Spin into Control Spintronics—the use of the spin direction of subatomic particles to control on and off states, instead of electric charge—has the potential to create low-power electronics, because less energy is needed to flip spin states than to flip switches to create voltage barriers. Theoretical work hints that spin-polarized electrons from a ferromagnetic electrode can be controlled by a change in polarization created in a ferroelectric thin film. Garcia et al. (p. 1106, published online 14 January) fabricated an iron-barium titanate junction on a lanthanum strontium manganate substrate that acts as a spin detector. Local control of spin polarization was observed in the ferroelectric layer, which retained its polarization without any applied power. Ferroelectric tunnel junctions control the spin polarization of electrons emitted from iron electrodes. A current drawback of spintronics is the large power that is usually required for magnetic writing, in contrast with nanoelectronics, which relies on “zero-current,” gate-controlled operations. Efforts have been made to control the spin-relaxation rate, the Curie temperature, or the magnetic anisotropy with a gate voltage, but these effects are usually small and volatile. We used ferroelectric tunnel junctions with ferromagnetic electrodes to demonstrate local, large, and nonvolatile control of carrier spin polarization by electrically switching ferroelectric polarization. Our results represent a giant type of interfacial magnetoelectric coupling and suggest a low-power approach for spin-based information control.

Propagation soliton et auto-confinement de faisceaux laser par non linearité optique de kerr

Abstract An experimental demonstration is presented of the stable monomode self-trapping of laser beams with adequate shape and power when they are propagating through homogeneous transparent dielectrics whose refractive index exhibits fluctuations proportional to the local intensity. The field evolution in such material is governed by a nonlinear scalar wave equation, that possesses stable solutions called solitons. Those solutions correspond to the exact balance of the natural diffraction divergence of the beam by the convergence of the self-induced refractive index gradient.

Picosecond steps and dark pulses through nonlinear single mode fibers

We experimentally observed transmission of “negative pulses” through a single mode optical fiber in conditions where self-phase modulation, due to self induced variations of silica refractive index, may be balanced by the positive group velocity dispersion (dVg/dδ#62;0). Soliton propagation could be expected at power levels where exact compensation of these two distorsions arises. The duration of the output dark pulses, determined by SHG correlation techniques, decreases when the launched power is increased, as predicted by computer simulation. For an input peak power of 0.2 W, corresponding to the theoretical soliton peak power, the emerging dark pulse looks like the input one.

Efficient coherent combining of widely tunable fiber lasers

We have experimentally demonstrated coherent combining of 2 and then 4 fiber lasers, with respectively 99% and 95% combining efficiency. The combining method investigated here is based on a multi-arm resonator of interferometric configuration. In spite of its interferometric nature, the multi-arm laser operates without significant power fluctuations, even in an unprotected environment. This occurs when the arm length difference is large enough to introduce spectral modulations of period smaller than the laser bandwidth. We have also experimentally shown that the combining method is compatible with wavelength tuning. A Mach- Zehnder Fiber Laser was tuned over a wide spectral range of 60nm. Theoretically then, we confirm that the combining method can be scaled to a large number of lasers without decreasing the combining efficiency.

Non-linear optical properties of chalcogenide glasses measured by Z-scan

Abstract To explore the potentialities of chalcogenide glasses for ultrafast optical switching, the non-linear refractive indices of different compositions are measured. Several binary glasses in the [Ge–Se] system, As2S3 and As2Se3 glasses, as well as ternary glasses in the [Ge–Se–As] system have been characterized. Non-linear measurements have been performed by the Z-scan technique, at different intensities, with the picosecond pulses emitted by a 10 Hz Q-switched mode-locked Nd-YAG laser at 1064 nm, in adequate conditions to characterize ultrafast non-linearities. Non-linear refractive indices as high as 22×10 −18 m 2 / W have been obtained in the ternary system. In the case of As2S3 and As2Se3 glasses, a significant variation of the value of the non-linear index of refraction with the laser intensity has been observed which is attributed to a fifth order non-linearity.

Measurement of phase shifts introduced by nonlinear optical phenomena on subpicosecond pulses.

A new method for the coherent characterization of subpicosecond optical pulses after propagation through a nonlinear medium (i.e., with frequency broadening) is described. The phase shift on a single pulse introduced by the nonlinearity can be measured with a high dynamic range. We report as one application of this technique a measurement of the self-phase modulation incurred by an 800-fsec pulse after the pulse propagates along a dispersive single-mode optical fiber.

Spatial-soliton-induced guided waves in a homogeneous nonlinear Kerr medium

It has been predicted that an intense pump beam can induce the focusing of a weak probe beam at a different wavelength that is simultaneously propagating in a Kerr homogeneous material through cross-phase modulation. In Kerr-type nonlinear material, spatial solitons are the only stable waves able to propagate at high-intensity levels without leading to catastrophic breakdown. We propose the use of a soliton beam to induce the stable guiding of a probe beam. Experimental results obtained with picosecond pulses are reported that evidence spatial-solitoninduced guiding.

Observation of quadratic spatial solitons in periodically poled lithium niobate

We report what are to our knowledge the first observation and characterization of quadratic spatial solitons in periodically poled lithium niobate (PPLN). Using a Nd:YAG (1064-nm) source, we observed self-trapped solitary-wave propagation over more than six diffraction lengths as a result of the cascaded quadratic nonlinear process. Low-threshold operation (5-kW peak power) was measured for the quasi-phase-matched PPLN structure designed for frequency doubling.

Passive phase locking of an array of four fiber amplifiers by an all-optical feedback loop

We report the passive phase locking of an array of four fiber amplifiers in a unidirectional ring cavity. The feedback loop consists of a single-mode fiber that filters intracavity the far-field pattern of the four emitted beams. The pointing of the laser output can be managed by the intracavity filtering.

Magnetoresistance and spin electronics

We review several topics in the field of spin electronics: (i) giant magnetoresistance observed in magnetic multilayers; (ii) magnetization reversal by spin injection and (iii) spin-polarized tunneling in magnetic tunnel junctions combining electrodes of ferromagnetic transition metal and half-metallic oxide.