Bernard Cretin

Scanning microdeformation microscopy

We have developed a new scanning microscope based on a vibrating contact tip and piezoelectric detection. Scanning the sample reveals surface topography and mainly, subsurface elastic properties. The preliminary images presented show surface and subsurface inhomogeneities in metallic samples. Lateral resolution is essentially related to the tip diameter as in near‐field microscopes.

Scanning microdeformation microscopy in reflection mode

A scanning microdeformation microscope based on a vibrating tip with piezoelectric detection in a transmission operating mode is upgraded to operate in reflection mode. Images which reveal subsurface inhomogeneities in silicon and metallic samples are presented. As in the first configuration the lateral resolution and the apparent penetration depth is related to the tip diameter.

Optical detection for scanning microdeformation microscopy

Scanning microdeformation microscopy is a kind of ac contact force microscopy sensitive to the variations of the local elastic constants of the investigated material. In this letter a new optical interferometer designed for detecting the small displacement of the cantilever is reported. The setup is described and some applications of the laser probe to scanning microdeformation microscopy are demonstrated.

Study of photothermal vibrations of semiconductor cantilevers near the resonant frequency

The elastic vibrations of semiconductor cantilevers, which were excited with a frequency-modulated laser, were studied theoretically and experimentally in this paper. The carrier density, temperature and elastic displacement distributions were obtained by using the theoretical model including plasma and thermoelastic effects. The results showed that the plasma wave had a significant influence on the vibrations. Using an interferometric setup the photothermal signals were investigated near the resonant frequency. The results showed that the experimental results were in good agreement with the theoretical ones.

Electronic and thermal generation of vibrations of optically excited cantilevers

The results of the theoretical analysis of the dynamic effects in the optically excited cantilevers were given. Theoretical model for dynamic elastic bending for two-layer cantilevers was derived including electronic and thermal elastic deformation effects which have the main influence on the dynamics of the cantilevers. The influence of the carrier transport characteristics (the carrier diffusion coefficient, the lifetime of photogenerated carriers, and the carrier recombination velocities) to the elastic vibrations of cantilevers was analyzed. Theoretical model was verified by comparing with the experimental results. The results of these investigations are important for sensors, actuators, and resonators based on the cantilevers.

Feedback-induced voltage change of a Vertical-Cavity Surface-Emitting Laser as an active detection system for miniature optical scanning probe microscopes.

We propose a novel detection technique for scanning probe microscopy based on the measuring of the feedback-induced voltage change of 780-nm VCSEL operating at constant current in far-field regime when we modulate mechanically the length of a coupled-cavity generating the feedback conditions. The voltage change of the VCSEL is produced by light back reflected from the sample to the laser cavity. Two-dimensional image probing is successfully demonstrated with high temporal resolution, offering a viable solution for miniature parallel scanning probe optical microscopes, such as confocal microscope, where the use of a photodetector is avoided. This approach opens the possibility to perform imaging tasks in a low cost and hand-held miniature device with much improved effective-space.

Towards dynamical force microscopy using optical probing of thermomechanical noise

Dynamic force microscopy is often used to investigate local tip–sample interaction in order to obtain the material elastic properties. In this letter, we demonstrate the feasibility of the detection of thermal noise of microcantilever in contact with hard samples. Optical sensing is based on a high sensitivity heterodyne interferometer. The detected power spectral density clearly shows the first flexural vibration mode of the microcantilever. The variation of the resonant frequency of this vibration mode is related to the local elastic properties of the sample.

Sensitivity optimization of the scanning microdeformation microscope and application to mechanical characterization of soft materials

In this article we present the study of the sensitivity optimization of our system of micromechanical characterization called the scanning microdeformation microscope. The flexural contact modes of vibration of the cantilever have been modeled. We discuss the matching between the cantilever stiffness and the contact stiffness which depends on the sample material. In order to obtain the best sensitivity, the stiffnesses must be the closest one to each other. Because the length of the cantilever directly affects its stiffness, the cantilever geometry can be optimized for different materials. We have validated this study with measurements on a soft material the polydimethylsiloxane with a cantilever optimized for materials of Youngs moduli of some megapascals. Experimental results obtained with two different samples have shown the high sensitivity of the method for the measurement of low Youngs moduli and have been compared with nanoindentation and dynamic mechanical analysis results.

DESIGN OF METALLIC MESH ABSORBERS FOR HIGH BANDWIDTH ELECTROMAGNETIC WAVES

In this paper, models of metallic absorbers for electromag- netic waves in the infrared to microwave frequency range are reported and discussed. The Hadleys formalism (1D model) of transmission, re∞ection and absorption for semi-inflnite layers, which allows to de- sign all conflgurations of unstructured absorber fllms and dielectrics is generalized. To make the micro-fabrication of the metallic absorbers easier (that means to have layers thick enough), the metallic layers need to be structured (grid for example). We developed a model that allows us to consider the structure of metal as a homogeneous layer, where the difiraction is negligible. This new layer can be used with the previous model. When difiraction efiects must be taken into account, we modifled an electrical model made by Ulrich. We further developed it for the conflguration of a dielectric before the metallic grid. The re- sults showed the importance to take into account all the dimensions of the grid, the dielectric layer parameters and the wavelength to design the best absorber.

Finite element analysis of the behavior of the scanning microdeformation microscope

The scanning microdeformation microscope, as many other scanning probe microscopes developed in the last years, is a kind of ac force microscope using the near-field acoustic interaction. The heart of the system is an electromechanical oscillator made of a silicon cantilever, a diamond or sapphire tip, associated with a bimorph piezoelectric transducer and a specific amplifier. The specificity of the system is the way of detection of the oscillation frequency performed electrically through the admittance of the piezoelectric transducer. In this paper, we describe the technique of detection involved in the microscope. A modelling of the complete behavior of the electromechanical oscillator performed with the finite element method (FEM) (of simulation) is presented. A comparison between experimental and theoretical behavior shows a very good agreement.