)
)
iSOM-images of fibroblast and living neurons.
Recently we have develloped a fibered high-resolution optical microscope, named interferometric Scanning Optical Microscope (iSOM). Its working principle is based on the interference of the internally reflected light at an optical fiber tip with the light reflected by the surface facing the tip. iSOM is complementary to the other observation techniques such as AFM, SEM or NSOM. It is working in the optical far field, i.e. at tip-sample distances of the order of a few microns. Samples with topography in the micron range can thus be imaged without any mechanical contact, which is of particular importance for the observation of living cells. Moreover, closed-loop tracking of the fiber tip height, as done in AFM or NSOM, is not required. This important feature allows less complex electronic control units to be used at much faster recording speeds.
The achieveable resolution is typically half the wavelength laterally to the fiber tip and better than 10 nm axially for highly reflective samples. iSOM can be developed at reduced costs and is well suited for integration with other measure- ment techniques, as only an optical fiber has to be positioned near the sample surface. There- fore, lengthy optical alignments can be avoided. Because iSOM can work in various environments, such as liquids, vacuum, or even low temperatures, we expect it to cover a broad range of applications.
The investigation of the optical excitation of collective charge modes at certain metal surfaces, commonly named "plasmonics", is actually a very busy research field because of its great interest for bio-sensors, local spectroscopy, opto-electonics or more generally for sub-wavelength optics. In each of these domains the challenge is to confine, enhance and control the light on distances below the actual wavelength and even down to nano-meter scale. In fact, in contrast to electrons, light cannot be guided or confined in these dimensions because of optical diffraction. A possible solutions are optically excited surface plasmons (SP) which allows to confine the electromagnetic energy on sub-wavelength and even nano-meter scale. They also allows to realise very strong electromagnetic energy concentration.
In this framework we are more particularly working on the filed enhancement of SP excitation in metallic micro/nano periodic structures. We have studied theoretically and experimentally the linear response of such systems. We know the nature of their different resonances, where some of them are linked to strong electromagnetic fields. Recently we have shown that the implementation of lack of the periodicity indices new resonances for which the field enhancement is even higher.
The in depth study of a birefringent waveguides requires an experimental tool allowing vectorial characterisation of the waveguide modes as a function of the propagation direction. In this context we have developed the hemispheric/ ellipsométric m-line technology. Our set-up allows to measure the refractive index and the polarisation angle of each mode as a function of the propagation direction.Using a high index glass hemisphere for light coupling the propagation direction can be easily modified by rotating the sample. The precision, resolution, reproducibility, and speed of measurement is very good. The ellipsometric function is implemented by a la mbda/2 plate to control the incident polarisation and and polariser to analyse the reflected polarisation. Our hemispheric/ ellipsometric m-line set up has shown its performance in the study of ion-implanted KTP waveguides , allowing the first direct measurement of mode hybridisation in birefringent waveguides.
We have developed a set of theoretical simulation tools for birefringent slab waveguides based on the 4x4 matrix or transfer matrix approach. They allow to determine the effective indexes of the waveguide modes for any propagation direction in particular for off-axis propagation, e.g. not parallel to the material dielectric axis. The main results are: all off-axis modes are hybrid, e.g. they contain TE and TM contributions; the walk-off between the phase vector and the Poynting vector is of the same order than for not-guided propagation; in case of mainly TM modes (TM') the walk-off direction can change with the waveguide depth, leading to mode distortion; for a wide range of propagation directions the TM' are leaking into the substrate.
The guided mode dispersion shows two types of crossings between TE' and TM' modes. The crossings between different parity modes (e.g. TE'2 and TM'1) are characterised by strong mode coupling leading to total hybridization and the opening of a local gap. In case of same parity modes their is no mode coupling and the effective index curves are crossing without any interaction.
In this project we investigate rare earth-doped thin films for applications such as integrated optical amplifiers of lasers. The originality of this work resides in the chemical elaboration process: sol gel and Pechini. This gentle chemistry techniques allows the elaboration of glassy materials which is not feasible with other techniques. The work is based on a collaboration with V.R.Mastelaro, A.C. Hernandes (University of São Paulo, Brasil), and A. Ibanez (Institut Néel, Grenoble) who are responsible for the elaboration and the morphological characterization of the samples.
Our investigation of alumino-phosphate and alumino-borate glasses is original as these compositions were not yet elaborated as thin films. This project follows our strategy to study glasses with compositions similar to good laser crystals. This kind of material allows to prevent rare earth clustering and thus to obtained enhanced amplification efficiency. We have chosen to work with erbium because of its emission near 1540 nm used in optical telecommunications and in the centre of the eye-save spectral region.
The sol-gel process is widely used for the elaboration of oxide or hybrid glasses. Its principle advantages are the relative low elaboration temperature, the use of ultra-pure precursors, the large possibility of doping, the possibility of film deposition on nearly any substrate, and finally its low investment costs compared to other techniques. The Pechini technique is much less used. Its similar to sol-gel, but uses more common precursors which are thus less expensive. Our results show that the quality of materials obtained by Pechini are comparable to those obtained by sol-gel.
The sol-gel technique is a very convenient technique for optical grade thin films elaboration. Its great flexibility allows the deposition of a great number of compositions, including rare earth-doped glasses. The great interest in these active materials is motivated by the search for materials with enhanced spectroscopic properties in order to realise advanced integrated optics amplifiers or light sources. At IMEP, we were studying two original approaches to overcome common problems with rare earth-doped materials: YETO solid solutions and organic nano-cage doping. The investigated samples are elaborated in the group of M. Langlet at LMGP (INP Grenoble). The thin films are deposited using the patented Aerosol-Gel process.
This approach is based on the fact that compared to the amorphous environment of a glass host, a crystalline environment has some important advantages for rare earth doping.The uniform environment of the active ions results in absorption and emission cross sections concentration into sharp lines. Moreover, the crystalline structure prevents from ion clustering and the controlled ion-ion distance limits co-operative effects. But crystalline structures have also some inherent drawbacks when used in integrated optics. Single crystalline films are difficult to grow and have strong polarisation dependent waveguiding properties or are even birefringent. This problem can be solved by using nanocrystalline films. But scattering losses at the crystal boundaries are very high. We found that a good compromise consist in amorphous films of a compositions which is able to form a crystalline phase. Thus the short distance environment of the active ions is optimised, without the negative influence of crystallisation on the waveguiding properties. Practically, this state can be obtained by annealing the films slightly below their crystallisation temperature.
At IMEP we wer styduing the YxEr2-xTi2O7 (YETO) pyrochlore phase. The crystallisation of our films is clearly accompanied by photoluminescence emission (PL) narrowing and increase of the PL lifetime from 1.3 to 7 ms. We want to mention that, one specificity of our work is that all the spectroscopic measurements are done in a waveguiding geometry. Straight waveguides were realised in YETO by using strip loaded and inversed waveguiding geometries and the expected lower losses of the amorphous films was evidenced. Moreover, YETO films were used to realise a vertical microcavity. The coupling of the emission modes to the cavity modes was evidenced by the narrowing of the emission peak and an enhancement of the emission maximum by a factor of 30.
This approach fully uses the low temperature potential of sol-gel chemistry. It consists in the preparation of amorphous glass layers doped with organo-metallic rare earth compounds in such a way that the organic ligand is not broken up during the layer preparation. This approach aims to use the very strong absorption of the organic ligand in the near ultraviolet. Indeed, pumping in the UV allows effective pumping of the rare earth ion via light absorption of the organic ligand followed by energy transfer towards the rare earth ion. Recent work showed that this type of indirect pumping can be much more effective than direct pumping (energy transfers efficiency > 50%).
Because of its relative chemical and thermal stability, the ligand can also play another essential role. During the elaboration process, the rare earth ion remains linked to the organic group. Thus, it screens the active ion against interactions with other ions (co-operative effects) and from interactions with the host matrix (luminescence quenching by matrix phonons or residual OH groups). So, the organic ligands form a protective nano-cage around the rare earth ion.
Recent work on terbium doped silica-titania glasses co-doped with SSA showed a clear increase of the overall PL-emission intensity without affecting the PL lifetime. This result confirms our approach and opens a wide range of possibilities for the fabrication of enhanced rare earth-doped integrated optics devices.
The increasing interest in chalcogenide glasses is based on their exceptional properties. They exhibits excellent optical transparency in the near and far infrared spectral region, their phonon energy is very low, and their refractive index is high. All these properties makes them very good host materials for rare earth doping. Thus, the use of a chalcogenide glass should allow the realisation of improved integrated optical amplifiers or light sources. One important condition for the use of chalcogenide glasses in integrated optics is the elaboration of thin films and the realisation of confined waveguides. Different techniques such as wet or dry etching or the photosensitivity are used for this purpose and some good results are already obtained. At IMEP we are studying a different approach: local metal doping. The diffusive nature of this process should allow the fabrication of low loss devices. Moreover, by using photo-assisted doping the realisation of waveguide gratings becomes very straightforward. Besides photo-doping we are also studying thermal doping for waveguide fabrication. In this context, we have observed the first straight waveguide obtained by thermal silver doping of an arsenic-tri-sulphide thin film. Furthermore we have fabricated diffraction gratings by photo-doping using a Llyod mirror configuration.
These first results show the feasibility of our approach. In parallel to this device fabrication we are working on the understanding of the doping process in order to optimise its parameters. As an example, we have developed a straightforward theory to use the interference fringes observed in optical transmission spectra for the determination of the film thickness and the complex dispersion relation.
The optical properties of semiconductor nanoparticles are radically modified with respect to the corresponding bulk materials. This modification is due to the quantum confinement of free carriers inside the particle volume. Two of the most interesting resulting features are the blue shift of the absorption edge and the enhanced nonlinear properties.
At IMEP we are working on semiconductor-doped glass films elaborated by sol-gel in the group of A. Martucci at Padova University. We have intensively studied the nonlinear properties of lead sulphide and cadmium sulphide-doped films. For this purpose we have used the degenerated four-wave mixing set-up (DFWM) of GONLO (IPCM, Strasbourg) and the non-linear m-lines experiment at IMEP. This second technique, which was developed within the IMEP Photonics group, is particularly well-adapted for thin film samples. We have measured high, reversible, and reproducible nonlinearities. In general, the resonant nonlinearity was found to be 10 times higher than the off resonance one. The experimental results could be described in the framework saturated absorption model.
Recently, we have enlarged our work to photoluminescence studies of PbS-doped thin films. PbS is of great interest for optical amplifiers or light sources. Its bulk band gap is situated in the near infrared at 3 µm and can be shifted to the visible by well choosing the particle size. Thus, the near infrared, which is of particular interest for telecommunication and sensor devices, can be easily covered. Moreover, by well controlling the particle size dispersion large emission for broadband amplifiers or narrow emission for laser sources can be obtained. In our work we observed strong photoluminescence emission at 1 µm. The surface quality was identified to be the main limitation for strong emission. Thus, the ongoing work focus on surface passivation and also on a better control of the size dispersion.