Joël Charrier

Chalcogenide glass optical waveguides for infrared biosensing.

Due to the remarkable properties of chalcogenide (Chg) glasses, Chg optical waveguides should play a significant role in the development of optical biosensors. This paper describes the fabrication and properties of chalcogenide fibres and planar waveguides. Using optical fibre transparent in the mid-infrared spectral range we have developed a biosensor that can collect information on whole metabolism alterations, rapidly and in situ. Thanks to this sensor it is possible to collect infrared spectra by remote spectroscopy, by simple contact with the sample. In this way, we tried to determine spectral modifications due, on the one hand, to cerebral metabolism alterations caused by a transient focal ischemia in the rat brain and, in the other hand, starvation in the mouse liver. We also applied a microdialysis method, a well known technique for in vivo brain metabolism studies, as reference. In the field of integrated microsensors, reactive ion etching was used to pattern rib waveguides between 2 and 300 μm wide. This technique was used to fabricate Y optical junctions for optical interconnections on chalcogenide amorphous films, which can potentially increase the sensitivity and stability of an optical micro-sensor. The first tests were also carried out to functionalise the Chg planar waveguides with the aim of using them as (bio)sensors.

Ultra-low reflection porous silicon nanowires for solar cell applications

High density vertically aligned Porous Silicon NanoWires (PSiNWs) were fabricated on silicon substrate using metal assisted chemical etching process. A linear dependency of nanowire length to the etching time was obtained and the change in the growth rate of PSiNWs by increasing etching durations was shown. A typical 2D bright-field TEM image used for volume reconstruction of the sample shows the pores size varying from 10 to 50 nm. Furthermore, reflectivity measurements show that the 35% reflectivity of the starting silicon wafer drops to 0.1%, recorded for more than 10 μm long PSiNWs. Models based on cone shape of nanowires located in a circular and rectangular bases were used to calculate the reflectance employing the Transfert Matrix Formalism (TMF) of the PSiNWs layer. Using TMF, the Bruggeman model was used to calculate the refractive index of PSiNWs layer. The calculated reflectance using circular cone shape fits better the measured reflectance for PSiNWs. The remarkable decrease in optical reflectivity indicates that PSiNWs is a good antireflective layer and have a great potential to be utilized in radial or coaxial p-n heterojunction solar cells that could provide orthogonal photon absorption and enhanced carrier collection.

Optical loss study of porous silicon and oxidized porous silicon planar waveguides

We have studied optical losses as a function of the wavelength for planar waveguides formed from porous silicon or oxidized porous silicon. Scattered light from the surface of samples was also observed. This observation reveals the influence of porous silicon dissolution front fluctuations called waviness on propagation. After oxidation, the measured losses decreased strongly and attained a value equal to 0.5 dB/cm in the near infrared. Surface and volume scattering losses were modeled in order to determine their principal contributions to overall losses. For porous silicon waveguides obtained from a P+ silicon substrate, the losses were mainly due to absorption by the material; whereas, for oxidized porous silicon waveguides, the principal contribution depends on the used wavelength. In the visible spectrum, losses due to volume scattering were predominant while in the near infrared, surface scattering was responsible for most of the losses.

Sulphide GaxGe25−xSb10S65(x=0,5) sputtered films: Fabrication and optical characterizations of planar and rib optical waveguides

We report the fabrication and the physical and optical characterizations of sulphide GaxGe25−xSb10S65(x=0,5) rib waveguides. High quality films fabricated on SiO2/Si wafer substrates were obtained using the sputtering magnetron rf deposition method. The slab waveguides obtained without annealing present propagation losses of about 0.6 dB/cm at 1550 nm. These optical losses are not important for implementation in optical devices based on silicon-on-insulator or polymer, for instance, atomic force microscopy measurements revealed low interface roughness between the different media (substrate/film and film/air). Reactive ion etching was used to pattern rib waveguides between 2 and 300 µm wide. The parameters were optimized to obtain a dry etching process that had low surface roughness, vertical sidewalls, etch depth of more than 1 µm, and reasonable etching rate. This technique was used to fabricate Y optical junctions for optical interconnections on chalcogenide amorphous films. Their optical transmission was demonstrated by optical near field of guided modes and optical losses were measured and discussed.

Porosity Gradient Resulting from Localised Formation of Porous Silicon: The Effect on Waveguiding

Porous silicon formation on patterned substrates leads to a depth-dependent porosity. These porosity variations depend on the anodisation parameters and on the size of the open windows in the masking layer. During the anodisation at a constant current intensity, the interfacial reaction area increases and consequently the porosity decreases. Moreover, the porous silicon growth rate depends on crystallographic directions and induces a porosity gradient along the core/cladding interface. These porosity gradients could be crucial in some applications such as optical waveguides. Oxidised porous silicon waveguides were fabricated through a masking layer by applying two constant current intensities during anodisation. The measured near field distribution reveals that the light propagation is localised near the core/cladding interface. These observations confirm that a porosity gradient exists along vertical cross section of waveguides. This study deals with the porosity gradient estimations resulting from the electrochemical etching through an opened window in a masking layer.

Optical gain measurements in porous silicon planar waveguides codoped by erbium and ytterbium ions at 1.53μm

The on-off optical gain measurements as a function of the pump power were performed on porous silicon planar waveguides codoped by erbium and ytterbium ions. These measurements were obtained for different ratios of Yb concentration to Er concentration. The highest value of the gain was reached when the Yb concentration is three times higher than that of Er at a moderate 980nm pump power value equal to 70mW. Optical losses measurements have been performed on these waveguides and were equal to 2.1dB∕cm and an internal gain of about 6.4dB∕cm was obtained.

Design of praseodymium-doped chalcogenide micro-disk emitting at 4.7 μm

A compact amplifier based on chalcogenide Pr3+-doped micro-disk coupled to two ridge waveguides is designed and refined by means of a home-made computer code. The gain G ≈ 7.9 dB is simulated for a Pr3+ concentration of 10 000 ppm, input signal power of -30 dBm at the wavelength 4.7 µm and input pump power of 50 mW at the wavelength 1.55 µm. In the laser behavior, i.e. without input signal, the maximum slope efficiency S = 8.1 × 10-4 is obtained for an input pump power of 2 mW. This value is about six times higher than that simulated for an optimized erbium-doped micro-disk.

Optical characterization at 7.7 µm of an integrated platform based on chalcogenide waveguides for sensing applications in the mid-infrared

A selenide integrated platform working in the mid-infrared was designed, fabricated and optically characterized at 7.7 µm. Ge-Sb-Se multilayered structures were deposited by RF magnetron sputtering. Using i-line photolithography and fluorine-based reactive ion etching, ridge waveguides were processed as Y-junction, spiral and S-shape waveguides. Single-mode optical propagation at 7.7 µm was observed by optical near-field imaging and optical propagation losses of 2.5dB/cm are measured. Limits of detection of 14.2 ppm and 1.6 ppm for methane and nitrous oxide, respectively, could be potentially measured by using this platform as an evanescent field sensor. Hence, these technological, experimental and theoretical results represent a first step towards the development of an integrated optical sensor operating in the mid-infrared wavelength range.

Selenide sputtered films development for MIR environmental sensor

A micro-sensor based on selenide glasses for evanescent wave detection in mid-infrared spectral range was designed and fabricated. Ge-Sb-Se thin films were successfully deposited by radio-frequency magnetron sputtering. In order to characterize them spectroscopic ellipsometry, atomic force microscopy and contact angle measurements were employed to study near and middle infrared refractive index, surface roughness and the wettability, respectively. Selenide sputtered films were micro-patterned by means of reactive ion etching with inductively coupled plasma process enabling single-mode propagation at a wavelength of 7.7 µm for a waveguide width between 8 and 12 µm. Finally, optical waveguide surface was functionalized by deposition of a hydrophobic polymer, which will permit detection of organic molecules in water. Thus, the optical transducer is a ridge waveguide composed by cladding and guiding Ge-Sb-Se sputtered layers exhibiting a tailored refractive index contrast and a polymer layer onto its surface ready for environmental detections in middle infrared.

Theoretical study of the factor of merit of porous silicon based optical biosensors

Porous silicon is an attractive material for label-free optical biosensors because of its biocompatibility, its large internal surface area, its open pore network, and its widely tunable refractive index. Many structures using this material and exploring reflectometry can be used for biosensing. The sensor performances and sensitivity depends on the parameters of the porous silicon layers and its thermal treatment such as porosity, pore size, oxidation degree, and used wavelength. A theoretical framework to model the reflectance spectra of three optical nanostructures (monolayer, Bragg mirror, and microcavity based on porous silicon layers) before and after the functionalization step is used to study the merit parameters for each device. Based on this theoretical work, optimized conditions to fabricate glucagon biosensors are proposed. A microcavity formed by a period constituted of two porous layers of porosities equal to 95% and 65% with a pore size of 60 and 51 nm, respectively, and with 40% oxidation ...