Chantal Fontaine

Precision Bragg reflectors obtained by molecular beam epitaxy under in situ tunable dynamic reflectometry control

Highly accurate layer thicknesses are required for multilayers involved in photonic devices, such as Bragg reflectors. In this letter, we demonstrate that precise, real‐time monitoring of molecular beam epitaxy growing layers can be achieved by near‐normal incidence dynamic reflectometry with a tunable sapphire–titanium laser used as a source. The advantage of this new technique lies in the possibility of synchronizing the material changes and the reflectivity extrema by selecting adequate analysis wavelengths. This technique is shown to provide 885 nm GaAs–AlAs Bragg reflectors with a layer thickness accuracy in excess of 1%.

Detection of lateral spontaneous emission for VCSEL monitoring

VCSELs (Vertical Cavity Surface Emitting Lasers) are nowadays more and more exploited in optoelectronic applications, monitoring their lasing power in a compact and low cost manner becomes crucial. To collect and control the output light, an external photodetector associated with an optical microlens array can be used. Integrated solutions based on the use of a bulk or QW photodetection section added in single-or double-cavity structures have also been proposed. Here, we have investigated a simpler solution based on a standard VCSEL array. Light emitted by a VCSEL has been electrically detected by adjacent VCSELs located in the same array, using in plane optical waveguiding of spontaneous emission in the intrinsic central zone of the devices. We show that the detected photocurrent can be related to the power of the emitting VCSEL. Signal intensity has been studied as a function of VCSELs distance. This method could lead to a more efficient way to monitor VCSEL emission.

Generalization of Bragg reflector geometry: Application to (Ga,Al)As‐(Ca,Sr)F2 reflectors

Bragg reflectors based on (Ca,Sr)F2 and (Ga,Al)As are studied. Modeling and fabrication of these structures by molecular‐beam epitaxy were performed. Quarter‐wave Bragg reflectors were found to present an excellent reflectance around the wavelength of interest, 870 nm, for only three periods of bilayers. However, the structures grown exhibited cracks after epitaxy due to thermal stress between both materials. To alleviate this problem, other reflector geometries were investigated consisting of deviations from the classical Bragg reflector. The new geometries enable one to reduce the absolute or relative fluoride thickness within the structure. The results obtained show that the use of adequate geometries allows one to overcome the stress problem, and good heteroepitaxial reflectors with a crack‐free surface morphology were obtained.

Integrated optical detection subsystem for functional genomic analysis biosensor

We present an optical microsystem aimed to be integrated into a nanomechanical biosensor for functional genomic analysis. The operation principle is based on a sub-nanometer resolution optical measurement of a cantilever deflection caused by a surface stress when the target nucleic acid sample hybridises to the nucleic acid probe on the active side of the cantilever. The resulting deflection, of the order of nanometers, is measured by an optical system, in which a laser beam reflects off the back of the cantilever to a position sensitive photo-detector. We report in this paper on the design, fabrication and test of the optical head associated with an optical coupling system which enables detection of the presence of target nucleic acid on the cantilever by amplifying the deflection caused by the stress.

Status Of Fluoride-Semiconductor Heteroepitaxial Growth

Molecular beam epitaxy of structures associating alkaline-earth fluorides and semiconductors has received considerable attention in the last few years. In this paper, we review the results published paying special attention to structures associating Ca, Sr, Ba fluorides and Si, GaAs, InP semiconductors. General trends for fluoride/semiconductor growth behavior are emphasized, as well as the problems encountered in the growth of semiconductor/fluoride heterostructures.

Integrated photodetection in VCSELs

We present the different approaches we have studied to integrate generically photo-detection in a VCSEL (vertical cavity surface emitting laser) thus increasing further the advantages of this laser device. Two main detection configurations for different applications are possible: either external detection (for telemetry, imagery, etc.) or internal detection (for power monitoring, feedback measurements, etc.). We have investigated these two possibilities in single-cavity devices with a standard geometry in order to keep the fabrication of optimised devices as simple as possible. In the first case (external detection) we have obtained an improvement of the detection quantum efficiency to 20% in a 850 nm-emitting VCSEL by chemical selective etching the first periods of the top Bragg reflector. In the second case, integrated detection for VCSELs monitoring, we have proposed to use an adjacent VCSEL to detect the part of guided light in the plane of the cavity. This signal comes mostly from the spontaneous emission escaping laterally from the VCSEL active zone, but can be nevertheless efficiently used to monitor the normal laser output power, since both signals are monotonously correlated. Furthermore, we propose a derived approach including a Schottky contact on the cavity. A significant value of photocurrent (> 100 /spl mu/A) is obtained and the slope variations of the collected current can be correlated to VCSEL threshold crossing and extinctions without the need of any signal treatment.

Integrated lateral detection in VCSELs for optical system monitoring

A new VCSEL design is proposed to combine emission and detection, based on a standard mesa process. Lateral guided optical signal is detected through a Schottky contact. Temperature, time-response and geometry-dependent characterizations are presented.

Integrated optical coupling element for functional genomic analysis biosensor

We present here a design of a coupling element aimed to be integrated into a nanomechanical biosensor for functional genomic analysis. The operation principle is based on a sub-nanometer resolution optical measurement of a cantilever deflection caused by a surface stress when the target nucleic acid sample hybridizes to the nucleic acid probe on the active side of the cantilever. The resulting deflection, of the order of nanometers, is measured by an optical system, in which a laser beam reflects off the back of the cantilever to a position sensitive photo-detector. We study in this paper three polymer optical coupling systems which could allow to detect the presence of target nucleic acid on the cantilever by amplifying the deflection caused by the stress.

Effects of selective wet etching on the spectral properties of a vertical-cavity surface-emitting laser/detector

In this paper, we present results on modelling, fabrication and characterization of oxide-confined VCSELs for short-distance alternative emission and photodetection at 820 nm. In the present designed single-cavity GaAs-based device, the buried oxide layer is not only used to obtain a single mode laser beam but also to enable decoupling between a small surface emission, limited by the aperture and a large surface detection, limited by the internal diameter of the top electrodes. The selective wet etching was found to improve simultaneously the detection sensitivity and the differential quantum efficiency without damaging the active surface.

Dual-purpose single-cavity oxide-confined VCSEL photodetector

The increasing interest for high-speed, compact and low cost devices for optoelectronic applications such as bi-directional optical interconnects, optical imaging or telemetry has recently led to focus on the ability for the vertical-cavity surface-emitting lasers to be used as resonant cavity enhanced photodetectors for dual-purpose applications. Here we present results on design, fabrication and characterization of an oxide-confined 830nm top-emitting laser for self-aligned emission and photodetection. In this single-cavity GaAs-based device, submitted alternatively to forward and reverse bias, the oxide layer is not only used to obtain a single mode emission but also to enable decoupling between a small surface emission and a large surface detection. However the optical path is observed to change because of the refractive index difference between the oxidized and non-oxidized zones of the structure. This leads to a detrimental blue-shift on the wavelength of the Fabry-Perot cavity mode. In this work, we demonstrate this effect in photodetection by the means of spatially localized photocurrent and reflectance spectra measurements. These results show that the photocurrent is correctly collected in the whole device despite of the presence of an oxide layer. The results obtained on selective etching for optimisation of this dual-purpose device are presented.