Bernard Andre

Effect of doping on global and local order in crystalline GeTe

Effect of nitrogen and carbon doping on the structure of GeTe has been investigated using x-ray diffraction and extended x-ray absorption fine structure (EXAFS) spectroscopies. While Bragg diffraction which probes the global structure exhibits a clear transition upon doping from the rhombohedral phase to the cubic (rocksalt) phase, the local structure probed by EXAFS remains rhombohedrally distorted across the compositions studied. The apparent inconsistency between the results of the two techniques used is attributed to disordering upon doping and the resulting order-disorder transition that is “seen” by site-averaging diffraction as a displacive rhombohedral-to-cubic transition.

Thermomechanical model of mirror laser damage at 1.06 μm: II. Flat bottom pits formation

Other defects than nodules are responsible for laser damage of High Reflection coatings at 1.06 micrometers . Among them, flat bottom pits (FBP) are particularly interesting because they are surface induced damages not related with preexisting cosmetic defects. The damage morphology observed with a Scanning Electron Microscope suggest that damages are related to absorbing nanometric centers. Following this idea, a model of FBP formation is developed in this paper. This model predicts a size dependence of the FBP versus the thickness of the buckled layers; this fact is experimentally confirmed. Two different kinds of FBP have been observed; the second one might be related with the seed of nodular defects.

Local structure of nitrogen in N-doped amorphous and crystalline GeTe

The local structure around nitrogen species in N-doped GeTe has been investigated using (soft) x-ray absorption spectroscopy and high-resolution x-ray photoemission spectroscopy. The obtained results demonstrate that while in as-deposited amorphous GeTe nitrogen is predominantly bonded to Ge atoms, upon crystallization the majority of nitrogen forms N2 molecules that are likely to be located in the grain boundaries, with only a small fraction of nitrogen species remaining bonded to Ge.

First Playback of High-Definition Video Contents from Super-Resolution Near-Field Structure Optical Disc

A super-resolution near-field structure (super-RENS) read-only-memory disc with an In–Sb thin film as a super-resolution active layer corresponding to a 50 Gbyte capacity per layer was fabricated. The carrier-to-noise ratio at a pit length of 75 nm exceeded 40 dB and a bit error rate (bER) of the order of 10-5 satisfying the 3.0×10-4 criterion was obtained. To the best of our knowledge, this is the first report on the successful and seamless playback performance of a high-definition video content from the super-RENS disc at a data transfer rate of either 36 or 72 Mbps. The bER results of fixed and adaptive partial response maximum likelihood detections are described.

Bit-Error-Rate Evaluation of Super-Resolution Near-Field Structure Read-Only Memory Discs with Semiconductive Material InSb

Bit-error-rate (bER) evaluation using hardware (H/W) evaluation system is described for super-resolution near-field structure (super-RENS) read-only-memory (ROM) discs fabricated with a semiconductor material, In–Sb, as the super-resolution active layer. bER on the order of 10-5 below a criterion of 3.0×10-4 is obtained with the super-RENS ROM discs having random pattern data including a minimum pit length of 80 nm in partial response maximum likelihood of the (1,2,2,1) type. The disc tilt, focus offset, and read power offset margins based on bER of readout signals are measured for the super-RENS ROM discs and are almost acceptable for practical use. Significant improvement of read stability up to 40,000 cycles realized by introducing the ZrO2 interface layer is confirmed using the H/W evaluation system.

Instability and Spontaneous Reconstruction of Few-Monolayer Thick GaN Graphitic Structures.

Two-dimensional (2D) semiconductors are a very hot topic in solid state science and technology. In addition to van der Waals solids that can be easily formed into 2D layers, it was argued that single layers of nominally 3D tetrahedrally bonded semiconductors, such as GaN or ZnO, also become flat in the monolayer limit; the planar structure was also proposed for few-layers of such materials. In this work, using first-principles calculations, we demonstrate that contrary to the existing consensus the graphitic structure of few-layer GaN is unstable and spontaneously reconstructs into a structure that remains hexagonal in plane but with covalent interlayer bonds that form alternating octagonal and square (8|4 Haeckelite) rings with pronounced in-plane anisotropy. Of special interest is the transformation of the band gap from indirect in planar GaN toward direct in the Haeckelite phase, making Haeckelite few-layer GaN an appealing material for flexible nano-optoelectronics.

Optical thin films serving biotechnology: fluorescence enhancement of DNA chip

Over the past years, DNA-chip technology has exploded. Yet scientists using such devices have to face many problems. One of them, due to the very low concentration of biological species to be detected, is the weakness of fluorescence signal collected through the reading system (microscope or scanner). To solve this problem, we proposed to use optical thin films technology. We studied the potentialities of this method step by step. The first step was to be able to understand, explain and forecast the fluorescence emitted by a DNA-chip in terms of fluorescence angular patterns. A theoretical and experimental study enabled us to master this issue even in the case of multi-layers substrates. Using this knowledge we were then able to explain, through simulations, the potentialities of this new type of substrates in terms of fluorescence enhancement. Thus we showed that a theoretical enhancement of twenty-fold (compare to a glass substrate) was achievable.

Amorphous InSb: Longer bonds yet higher density

Results of x-ray absorption studies of the structure of amorphous InSb are reported. We demonstrate that approximately 1% bond elongation in the amorphous phase (as compared to the crystalline phase) is accompanied by a counterintuitive increase (approximately 5%) in density. We argue that this controversy is due to the formation of wrong bonds in the amorphous phase with both Sb and In atoms effectively preserving their tetrahedral coordination. Our results additionally offer an alternative interpretation of the semiconductor-metal transition observed upon melting of InSb.

One-hundred Joule per square centimeter 1.06-μm mirrors

Very high damage threshold Hafnia Silica mirror coating have been developed successfully thanks to a clear understanding of the main limitations of the available mirrors. Coating with a 3ns pulse width threshold above 100J/cm2 are reported. These coatings proved to be much more resistant than very high quality bare silica substrate tested the same way. The limitation of these coatings is still extrinsic. The potential of this technology for large size high damage mirrors for fusion laser application is discussed.

Super-Resolution Optical Disc with Radial Density Increased by Narrowed Track Pitch Corresponding to Diffraction Limit

In–Sb-based super-resolution near-field structure read-only-memory discs with radial density increased by introducing a narrow track pitch corresponding to the diffraction limit of an optical system were developed. Using an optical system with a laser diode with a wavelength of 405 nm and an objective lens with a numerical aperture of 0.85, we confirmed that differential phase detection (DPD) could detect track errors from disc samples recorded random data including a minimum pit length of 75 nm in a 240 nm track period. It has higher capability of track error detection than push–pull detection at a narrowed track pitch. Moreover, bit error rates satisfying the criterion of 3.0×10-4 were experimentally obtained for 66.7-GB-capacity disc samples with a 240 nm track pitch through signal processing with the partial response maximum likelihood of the (1,2,2,1)-type, by applying DPD to tracking servo control. The feasibility of increasing the track density of the Blu-ray DiscTM physical format by 1.33 times was indicated.