Photoelastically induced light modulation in gradient index lenses

A new photoelastic light modulator is demonstrated based on the modulation of the birefringence and of the index profile in graded index lenses. Using the birefringence modulation we obtained 35% modulation depth in a quarter-pitch lens and 65% using a half pitch lens at acoustic frequencies up to the MHz range. Using the index profile modulation in a half-pitch lens as a fibre-to-fibre connector we obtained 15% modulation without the incorporation of any polarizer

Experimental determination of the Stokes parameters using a dual photoelastic modulator system

We develop a more general methodology for a dual photoelastic modulator (PEM) system that is used for the determination of the Stokes parameters of an arbitrary light beam. This allows for a degree of arbitrariness in the setting of the retardation amplitudes of both PEMs, thus permitting a robust and effective optimization of the detection system. Various experimental issues are considered and a calibration procedure is described that ensures accurate measurement of the absolute values of the Stokes parameters. Measurements of the Faraday rotation of a CoPt multilayer film are provided as a demonstration of the sensitivity of the dual-PEM system

L'optique non linéaire

The timing of light sources of very high power permits investigations into the field of optics where phenomena no longer depend linearly on the electric or magnetic fields. In particular, the polarisation of the dielectric can be expressed as a quadratic function of the electric field which leads to the emission of a harmonie radiation of the incident radiation. Theorically these phenomena are as yet relatively unknown and often difficult to understand. We shall examine them microscopically in order to discover their origin and also macroscopically in order to study the linking between incident radiation and induced radiation. On the expérimental side, very many experiments have been carried out. These experiments consists of research for conditions favorable for obtaining the second harmonie, of systematic studies of substances which permit high proportions of the second harmonic to be obtained and also of very varied research with a view to obtaining a better understanding non-linear phenomena.La mise au point de sources lumineuses de très haute puissance (LASERS) permet l'investigation du domaine de l'optique où les phénomènes ne dépendent plus linéairement des champs électriques ou magnétiques. En particulier la polarisation du diélectrique peut être exprimée comme une fonction quadratique du champ électrique, ce qui conduit à l'émission d'un rayonnement harmonique du rayonnement incident. Sur le plan théorique ces phénomènes sont encore mal connus et souvent très difficiles à interpréter. Nous les examinerons à l'échelle microscopique (pour étudier le couplage entre les rayonnements incidents et induits dans la matière). Sur le plan expérimental de très nombreuses recherches ont été effectuées ; elles concernent des recherches des conditions favorables à l'obtention du second harmonique, des recherches systématiques de corps permettant l'obtention de taux élevés de second harmonique, et des recherches très diverses permettant de mieux interpréter les phénomènes non linéaires

Vérification expérimentale des relations de Kramers-Kronig entre indices de réfraction et d'extinction

The existence of a Kramers-Kronig type relationship between the refractive and extinction indices has been checked experimentally. The experiments on n and k have been conducted with a highly performing photoelectric refractometer and a commercially available spectrophotometer respectively. A large number of compounds have been studied — potassium, ferricyanide and bichromate, rare-earth trichlorides (RE = Nd, Er, Eu), ferrous orthophenanthroline, phenanthrene and anthracene — as solutions in the spectral range 2 200-6 000 Å. A single crystal of lead chloride was also run in the solid phase. In all cases, the Kramers-Kronig transform of the curve k(λ) fits correctly the experimentally measured curve n(λ). We are thus lead to the conclusion that the transform is valid, at least in all the above examples.Nous vérifions expérimentalement les relations de Kramers-Kronig, entre les indices de réfraction et d'extinction. Pour ceci nous mesurons ces deux indices sur le même échantillon à l'aide respectivement d'un réfractomètre photoélectrique de hautes performances et d'un spectrophotomètre commercial. La comparaison de la dispersion de l'indice de réfraction mesurée expérimentalement et de la transformée de Kramers-Kronig de l'indice d'extinction montre que les relations de Kramers-Kronig sont valables dans tous les cas étudiés. Ceux-ci sont relatifs au ferricyanure et bichromate de potassium, aux chlorures de néodyme, erbium et europium, à l'ortho-phénanthroline ferreuse, au phénanthrène et à l'anthracène, tous ces corps étant en solution, et à un monocristal de chlorure de plomb. Le domaine spectral étudié s'étendant de 2 200 à 6 000 Å. Dans tous les cas nous montrons que la transformée de Kramers-Kronig de l'indice d'extinction est identique, à la précision des mesures près, à l'indice de réfraction mesuré. Ceci confirme la validité des relations de Kramers-Kronig, au moins pour tous les exemples ci-dessus

Magnetic circular dichroism study on YIG films

This paper presents the temperature dependence (300 — 1.8 K) of the magnetic circular dichroism (MCD) spectrum of YIG in the 19 000-33 000 cm -1 energy range. MCD data are analysed in terms of a model of two sublattices with additive contributions. This analysis provides strong support to the current picture that optical transitions involve simultaneous excitations on the two sublattices. Furthermore, our MCD data enable several new excited states to be located accurately.Nous présentons la dépendance en temperature (300 — 1,8 K) des spectres de dichroisme circulaire magnétique (DCM) de films de YIG entre 19 000 et 33 000 cm-1. Nos résultats sont analyses à l'aide d'un modele à deux sous-réseaux dont les contributions sont additives. Nous montrons ainsi que les transitions optiques du YIG mettent en jeu simultanément des excitations sur les deux sous-réseaux. Par ailleurs, nos spectres de DCM permettent de localiser avec precision des bandes non observées dans le spectre d'absorption

Wave propagation through a medium with static and dynamic birefringence

Photoelastic modulation (PEM) makes possible (with phase-sensitive detection) high precision measurement of small polarization changes in light that has interacted with matter. In an ideal modulator an oscillating birefringence (piezoelectrically induced in a silica crystal) leads to phase variations of the form cos(a1), sin(a1), where a1 = A sin(2πft); f is the oscillation frequency and A is the maximum retardation. This standard model cannot account for the results of recent PEM-based experiments to measure optical activity by means of light reflection.1 A more accurate theory, supported by experiment, is given that predicts phase variations of the form cos(b), sin(b)/ 2b, where 2 b = [ a 1 2 + a 2 2 + 2 a 1 a 2   cos ( 2 g ) ] 1 / 2 ; a2 is the retardation produced by a distributed static birefringence with principal axis inclined at angle g to the modulation axis. Theoretical analysis of the light flux for selected experimental configurations of spectroscopic and polarimetric interest leads to signals at f and 2f that differ markedly from the predictions of the standard model and a two-plate theory with discrete static and dynamic birefringence. Experimental procedures are suggested for circumventing the birefringence-related anomalies in PEM-based reflection spectroscopy of chiral materials.</jats:p

Absorption and fluorescence study of an antiferromagnet with alternating strong and weak coupling : RbMnCl3

The absorption spectrum of RbMnCl3 shows hot magnon satellites with frequencies of 75, 70 and 31 cm-1, and cold bands with frequences of 70, 65, 54 and 43 cm-1. The reduced zone boundary magnon energies (E/| J2 |) are expressed in terms of the ratio of the strong (J1) and weak (J2) exchange coupling constants. The fit of our experimental data leads to J 1/J2 ≃ 6 and J1 ≃ 26 cm-1, in serious disagreement with previous findings. Fluorescence measurements prove that optical excitations are completely localized.Le spectre d'absorption de RbMnCl3 présente des raies magnon chaudes (75, 70 et 31 cm-1) et froides (70, 65, 54 et 45 cm -1). Les énergies réduites (E/| J2 |) des magnons de bord de zone sont calculées en fonction du rapport J1/ J2 des deux intégrales d'échange caractérisant ce matériau. L'analyse de nos résultats conduit à J1/J2 ≃ 6 et J1 ≃ 26 cm-1, en sérieux désaccord avec des résultats antérieurs. Nos mesures de fluorescence prouvent que l'excitation optique reste localisée