Spectroscopic study of the incommensurate phase of ThBr4 via the optical and magnetooptical properties of U4+

U4+ is used to probe the incommensurate structure of ThBr 4 via absorption and magnetic circular dichroism (MCD) measurements. Below Tc = 95 K, the crystal field experienced by U 4+ ions is either of D2d or D2 symmetry. This can be simulated by varying a phase angle ϕ form 0(D 2d) to ± π/2 (D2). One very neat advantage of MCD over absorption is that the D2d centres show up very clearly in the former due to the occurrence of a derivative-like singularity. We present a model to simulate absorption, Zeeman and MCD profiles. It accounts satisfactorily for most of our data and we arrive at the following main conclusions : (i) it is probable that the phase is truly incommensurate since the lower limit of the number of sites is found to be 80; (ii) for a low concentration of U4+ (2 x 10-4), we found no evidence of a pinning of the incommensurate modulation by the impurities. MCD provides also a definite proof that the ground state of U 4+ in ThBr4 is Γ4 with a D2 d symmetry. Finally, we determine the Landé factors of five excited states, our findings being in fair agreement with the results of recent crystal field calculations

The CAT Imaging Telescope for Very-High-Energy Gamma-Ray Astronomy

The CAT (Cherenkov Array at Themis) imaging telescope, equipped with a very-high-definition camera (546 fast phototubes with 0.12 degrees spacing surrounded by 54 larger tubes in two guard rings) started operation in Autumn 1996 on the site of the former solar plant Themis (France). Using the atmospheric Cherenkov technique, it detects and identifies very high energy gamma-rays in the range 250 GeV to a few tens of TeV. The instrument, which has detected three sources (Crab nebula, Mrk 421 and Mrk 501), is described in detail.Comment: 24 pages, 15 figures. submitted to Elsevier Preprin

Mesure du dichroïsme circulaire. Description de différents montages a très hautes performances

A number of devices are considered which perform circular dichroism measurements by a null-method. A theoretical study shows that they all possess similar characteristics. One particular actual achievement is fully described and its performance is illustrated with a number of magnetic circular dichroism curves. The spectral range 2 100-7 500 Å can be investigated. Our machine has both a good resolution (1-6 Å) and high accuracy. The smallest measurable dichroïc optical density is as small as 10-6 or 10-5 according to the resolution considered (6 Å or 1 Å respectively).On considère plusieurs dispositifs de mesure du dichroïsme circulaire par une méthode de zéro. Leur étude théorique montre qu'ils ont tous sensiblement les mêmes caractéristiques. L'une de nos réalisations expérimentales est ensuite décrite et ses performances sont illustrées par des courbes de dichroïsme circulaire magnétique. Notre appareil possède une excellente résolution (variable de 1 à 6 Å) et une grande sensibilité. Son domaine spectral s'étend de 2 100 à 7 500 Å. La densité optique dichroïque mesurable peut devenir inférieure à 10-6 lorsqu'on utilise une bande passante de 6 Å et reste de l'ordre de 10-5 pour une bande passante de 1 Å

Magneto-optical evidence of magnon sidebands in Cs3CoCl5

A magneto-optical study of the spin-forbidden 4A2 → 2E transition of Cs3CoCl5 below its antiferromagnetic ordering temperature ( TN = 0.52 K) has been conducted under high resolution conditions using a new type of He3 optical cryostat. Our experimental results are in agreement with those observed previously but cannot be interpreted by using the single ion model. Careful analysis of the temperature variation of both absorption and magnetic circular dichroism spectra, between 0.8 TN and 50 TN, leads to the conclusion that the spin forbiddeness is removed via an exchange induced electric dipole mechanism. The experimentally deduced exchange energy |J'/ k| = 0.11 K is in agreement with the known value J'/k = — 0.111 K and the zero field splitting of the 2E state is less than 1 cm-1 instead of the 28 cm-1 value deduced from the single ion model.Nous effectuons une étude magnétooptique à haute résolution et en utilisant un nouveau type de cryostat optique He3 de la transition interdite de spin 4A2 → 2E de Co2+ dans Cs3CoCl5. Nos résultats expérimentaux étendus jusqu'à une température inférieure à la température d'ordre du cristal (TN = 0,52 K) sont en accord avec ceux précédemment obtenus à plus haute température mais ne peuvent être interprétés en utilisant le modèle de l'ion simple. L'étude détaillée de la variation de l'intensité d'absorption et du dichroisme circulaire magnétique des différentes raies, en fonction de la température (entre 0,8 et 50 fois TN), montre que le mécanisme dipolaire électrique d'échange est à l'origine de la levée d'interdiction de la transition. L'énergie d'échange déduite de nos expériences |J'/k| = 0,11 K est en accord avec la valeur connue J'/k = — 0,111 K. La séparation en champ nul de l'état excité 2E est moins de 1 cm-1 au lieu de la valeur 28 cm-1 estimée en utilisant le modèle de l'ion simple

Second harmonic generation by scanning near-field optical microscopy

International audienc

Imaging subwavelength holes in chromium films in scanning near-field optical microscopy. Comparison between experiments and calculation

Near-field optical signals are imaged in the vicinity of nano-holes using two different near-field optical microscopes. The experimental results are compared with electromagnetic field calculations based on a modal approximation. It turns out that an optical fibre detects the Poynting vector whereas the apertureless tip is sensitive to the field amplitude