Patrick Vincent

Gratings in nonlinear optics and optical bistability

A new use of gratings in nonlinear optics is presented, i.e., the realization of bistable components. The device studied here consist of grating couplers ruled on a Kerr nonlinear medium, which use the guided-wave resonance to increase the local field and thus the nonlinearities. The result is that they are intrinsic bistable optical systems with high-speed, low pumping thresholds, and geometry well adapted to optical integration. First, a linear electromagnetic study of the devices is presented. It follows optimizing the grating parameters in order to get the best coupling between the incident beam and the guided mode inside the corrugated waveguide. Then a graphical construction is given that demonstrates the bistable character of the system in nonlinear optics. Next a nonlinear analysis of the devices is rigorously derived from Maxwell equations. It states precisely the predictions of the graphical construction and allows comparison of the effectiveness of the guided-wave resonance with the surface plasmon resonance in order to reduce the threshold of bistability.

Phosphatidylserine delivery to endoplasmic reticulum-derived vesicles of plant cells depends on two biosynthetic pathways

Vesicles formed from endoplasmic reticulum (ER) by a cell‐free system of leek cells (Allium porrum) are enriched in phosphatidylserine (PS), especially species containing very long chain fatty acids (VLCFA, at least 20 carbon atoms). In plant cells, PS is formed either by PS synthase or the serine exchange enzyme, although it is not known which pathway(s) contribute(s) to PS delivery in the ER‐derived vesicles (EV), nor to what extent this occurs. Taking advantage of a cell‐free system, we have shown that PS enrichment originates mainly from the serine exchange enzyme which is the only pathway that synthesizes the VLCFA‐PS species. On the other hand, both enzymes synthesize PS with long chain fatty acids (up to 18 carbon atoms), but these species are given to the EV by PS synthase.

One of the origins of plasma membrane phosphatidylserine in plant cells is a local synthesis by a serine exchange activity

In plant cells, as in animal cells, the endoplasmic reticulum (ER) is considered to be the major site of phospholipid synthesis, and it has been shown that phosphatidylserine (PS) reaches the plasma membrane via the vesicular ER‐Golgi‐plasma membrane pathway in leek cells. However, it has never been determined whether the plasma membrane of leek cells is able to synthesize PS. We have analyzed the distribution of PS synthesizing enzymes along the vesicular pathway. In ER, Golgi and plasma membrane fractions isolated from leek cells, we have measured the activity of the two biosynthetic pathways leading to the synthesis of PS, i.e. serine exchange and CTP cytidylyltransferase plus PS synthase. We have found a high serine exchange activity in the plasma membrane fraction, and then determined that this membrane is able to synthesize both long chain fatty acid‐ and very long chain fatty acid‐containing PS. Therefore, the PS in the plasma membrane of leek cells has two different origins: the intracellular vesicular pathway from the ER and a local synthesis in the plasma membrane.

X-ray gratings: the GMS mount.

Using rigorous electromagnetic theory, the interest of the specific off-plane mounting for x-ray gratings is discussed in detail. The optimization of the blaze angle for ruled gratings is considered, as well as the extension of the mounting to sinusoidal and lamellar profiles.

Differential theory of gratings: answer to an objection on its validity for TM polarization

The rigorous character of the differential method for diffraction gratings for TM polarization was questioned by Depine Simon [ J. Opt. Soc. Am. A4, 834 ( 1987)]. They believe that they have found an argument that explains the mismatch of the numerical results found for deep metallic gratings from both integral and differential method, and they think that this argument enables them to say that the differential theory is not a rigorous electromagnetic method. We show here that the proof of their argument is not established.

Fast Pockels light modulator using guided wave resonance.

A new type of optical modulator is proposed in which a guided wave is resonantly excited by a grating coupler. The guiding layer is made of electrooptic (EO) material. The excitation of the resonance is controlled by a modulation voltage across the EO layer. This device acts either as a bulk EO modulator or an EO waveguide modulator.

Differential theory for metallic gratings in nonlinear optics: second-harmonic generation

The differential formalism previously developed to study the diffraction by gratings in linear optics is generalized to metallic gratings in nonlinear optics for TM polarization. It is applied to the study of the enhancement of second-harmonic generation at a silver grating, linked with the resonant excitation of surface plasmons.

Computation of the efficiencies and polarization effects of XUV gratings used in classical and conical mountings

Abstract A new formalism has been recently developed in our laboratory to investigate the diffraction of an electromagnetic wave by a grating. It not only includes the effect of the finite conductivity of the surface, which is essential for XUV gratings, but it allows the wave vector of the incident field not to be perpendicular to the grating grooves. As an application, we show that the use of the so-called G.M.S. mount gives very high efficiencies for wavelengths as short as 250 A. Our theoretical results, for this mounting, have been compared to measurements and have shown a surprising agreement.

On a General Theory of Anomalies and Energy Absorption by Diffraction Gratings and Their Relation with Surface Waves

The authors generalize a previous theory to study the phenomenon of absorption of plane waves by a grating working with several spectral orders. The theory allows predicting, with the aim of a simple formula, the shape of various kinds of grating anomalies, as well as the shape of the total diffracted energy curve. The use of a convenient matrix, called S*S, leads to the introduction of the efficiency hyperellipsoid. This enables one to predict in a simple way a phenomenon of total absorption of a finite number of plane waves by a grating.

Optical bistability by photothermal displacement in grating coupler configuration: a theoretical interpretation

The bistable behavior of an external grating coupler on a planar waveguide, which was previously discovered experimentally, is investigated by means of the electromagnetic theory of gratings. The theory accounts for all shapes of the observed hysteresis loops. The various shapes of the hysteresis loops, including the butterflylike shapes, are explained.