In this work, we report InGaAs based photodiodes integrating liquid crystal (LC) microcells resonant microcavity on their surface. The LC microcavities monolithically integrated on the photodiodes act as a wavelength selective filter for the device. Photodetection measurements performed with a tunable laser operating in the telecom S and C bands demonstrated a wavelength sweep for the photodiode from 1480 nm to 1560 nm limited by the tuning range of the laser. This spectral window is covered with a LC driving voltage of 7V only, corresponding to extremely low power consumption. The average sensitivity over the whole spectral range is 0.4 A/W, slightly lower than 0.6 A/W for similar photodiodes that do not integrate such a LC tunable filter. The quality of the filter integrated onto the surfaces of the photodiodes is constant over a large tuning range (70 nm), showing a FWHM of 1.5 nm.
We report on the optimized design of a polymer-based actuator that can be directly integrated on a VCSEL for vertical beam scanning. Its operation principle is based on the vertical displacement of a SU-8 membrane including a polymer microlens. Under an applied thermal gradient, the membrane is shifted vertically due to thermal expansion in the actuation arms induced by Joule effect. This leads to a modification of microlens position and thus to a vertical scan of the laser beam. Membrane vertical displacements as high as 8μm for only 3V applied were recently experimentally obtained. To explain these performances, we developed a comprehensive tri-dimensional thermo-mechanical model that takes into account SU-8 material properties and precise MOEMS geometry. Out-of-plane mechanical coefficients and thermal conductivity were thus integrated in our 3D model (COMSOL Multiphysics). Vertical displacements extracted from these data for different actuation powers were successfully compared to experimental values, validating this modelling tool. Thereby, it was exploited to increase MOEMS electrothermal performance by a factor higher than 5.
This paper reports on an alternative method for precise and uniform fabrication of 100μm-thick SU-8 microstructures on small-sized or non-circular samples. Standard spin-coating of high-viscosity resists is indeed known to induce large edge beads, leading to an air gap between the mask and the SU-8 photo-resist surface during UV photolithography. This results in a non uniform thickness deposition and in a poor pattern definition. This problem becomes highly critical in the case of small-sized samples. To overcome it, we have developed a soft thermal imprint method based on the use of a nano-imprint equipment and applicable whatever sample fragility, shape and size (from 2cm to 6 inches). After final photolithography, the SU8 pattern thickness variation profile is measured. Thickness uniformity is improved from 30% to 5% with a 5μm maximal deviation to the target value over 2cm-long samples.
In this work, we present a comparative study of optimized AZO electrodes deposited by Atomic Layer Deposition (ALD) with commercial ITO in terms of electrical, optical and structural properties. Despite a lower figure of merit mainly due to a higher sheet resistance, AZO-based OLEDs are shown to present a current density five times higher than ITO-based ones for the same applied voltage. These AZO electrodes fabricated by ALD could thus be promising substitutes for conventional ITO anodes in organic electronic devices.
We report on a simple method for the collective fabrication of polymer tunable microlens arrays suitable for
VCSEL active beam shaping. Its principle is based on a SU-8 suspended membrane, surmounted by a polymer
microlens, and thermally actuated to achieve a vertical displacement of lens plane. SU-8 resist presents many advantages
for MOEMS fabrication, as this resist allows for high aspect ratio patterns and high transparency. In addition, it exhibits
a thermal expansion coefficient suitable for thermal actuation. Moreover, this kind of polymer MOEMS can be
fabricated on VCSEL arrays with footprints as low as 500x500μm2 enabling a rapid, low cost and wafer-scale integration
technology. We have successfully fabricated this MOEMS on a glass substrate by means of a SU-8 double exposure
method and we report on a vertical displacement of 8μm under an applied power of 43mW (3V). A good agreement with
the theoretical thermo-mechanical behavior is found. Moreover, optical measurements of microlens focus displacement
under actuation are presented. We evaluate analytically the focus properties of the system under coherent laser
illumination, using the classical ABCD matrix formalism of Gaussian transformation optics. The same approach enables
one to assess its tolerance to opto-geometrical parameters, such as refractive index or dioptre curvature. As a wide range
of initial gaps between the membrane and the substrate can be chosen, this MOEMS technology opens new insights for
dynamic control of VCSEL beam or for tunable VCSELs fabrication.
We present recent results on the integration of polymer microlenses on single mode Vertical-Cavity
Surface-Emitting Lasers (VCSELs) to achieve output beam control. We describe in particular low
cost and collective fabrication methods developed to allow for a self-alignment of the lens with the
laser source. These approaches are based either on surface tension effects or on a self-writing
process using novel Near Infra-Red (NIR) photopolymers. Results on beam collimation at 850nm are
presented and compared to a fully vectorial and three-dimensional optical model that takes into
account the complete geometry of laser resonator is used. Results on short distance focusing using
self-aligned microtips are presented. Considerations to achieve an active beam control by means of
polymer-based MEMS (Micro-electro-mechanical System) are also discussed. Potential applications
may concern the improvement of VCSEL insertion in optical interconnects or sensing systems, as
well as the fabrication of optical micro-probes for near-field microscopy.
Active control of VCSELs beam properties is a key issue to improve their integration in microsystems. We have
designed a micro-optical system that allows for a dynamic displacement of the VCSEL beam. It consists of a polymer
microlens associated to a SU-8 membrane vertically moved by means of a thermal actuator. This approach is suitable
with laser sources arrays. We present results on optical design demonstrating that a small deflection of the membrane
(2μm) could lead to a large displacement of the beam waist vertical position (in the millimetric range). Thermomechanical
modelling is performed to evaluate the maximum membrane displacement achievable with this system.
Finally, first feasibility results are presented.
Laser beam shaping is a key issue for the photonic integration of VCSEL sources. Most of the techniques
proposed to integrate micro-optics elements onto VCSEL devices imply either a hybrid assembly or a photolithography
step, whose precision limits the accuracy of lens alignment relatively to the VCSEL source. We present here a new
method for self-fabrication of microtips on Vertical-Cavity Surface-Emitting Lasers (VCSELs) by means of Near Infra-
Red (NIR) photo-polymerization. This approach is based on a single fabrication step, implementing novel
photopolymers sensitive at the lasing wavelength. Consequently the process is triggered by the laser source itself and can
be applied easily to VCSEL devices during their electro-optic characterization. The method we have developed for tips
fabrication is detailed as well as corresponding optical properties. The applications of this new and simple method
concern laser light focusing and collimation for integrated micro-systems, coupling to fibers for optical communications
as well as novel micro-probes fabrication for near-field optical microscopy.
We report on the design and fabrication of polymer microlenses fabricated on patterned SU-8 layers in view of
integrating microlenses on VCSEL arrays for laser beam shaping. For a standard top-emitting VCSEL, the lens has to be
fabricated on a thick intermediate layer (pedestal) whose optimal thickness can be modelled as a function of the initial
and of the aimed optical properties of the VCSEL beam. In this work, pedestals are fabricated with SU-8, which is a
negative-tone photoresist transparent at the lasing wavelength. Lens deposition is realized using a robotized silicon
microcantilever spotter technique after a simple SU-8 photolithography step in order to define high aspect ratio
cylindrical pedestals with wide range diameters [30-140μm]. The effect of pedestal diameter on the final contact angle
and curvature radius has been investigated using non contact optical profilometry and scanning electron microscopy. We
show that this technique leads to a complete delimitation of the polymer droplets and to a better control of the final lens
size. Moreover, lens positioning is fully ensured by the self-alignment of the droplet with the pillar center and
consequently with the VCSEL source, and allows for meeting the stringent requirements on alignments.
We report on the design and the fabrication of refractive microlenses using a polymer droplet deposition microsystem. The principle of this original technique consists in monomer droplets deposition using a robotized silicon-microcantilevers array. The advantages of this technique rely on the control of droplets dimensions and the positioning accuracy. Microlenses have been first modelled to optimize their geometrical parameters for VCSEL collimation. Results of lens optimization as well as the influence of the fabrication parameters fluctuations on the final divergence are detailed. First results on droplets deposition are presented, demonstrating the technique feasibility. Finally, the possibility of the modification of the surface energy to obtain the most suited contact angle before deposition is also discussed.
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.
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.
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.
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.
Microcavity light emitting diodes (MCLEDs) present several interesting features compared to conventional LEDs such as narrow linewidth, improved directionality and high efficiency. We report here on MCLEDs with a top emitting geometry. The MCLED layers were grown using molecular beam epitaxy on GaAs substrates. They consist of a 3-period Be- doped distributed Bragg reflector (DBR) centered at 950 nm wavelength, a cavity containing three InGaAs quantum wells and a 15-periods Si-doped DBR. Different values for the wavelength detuning between spontaneous emission line and Fabry-Perot cavity mode were explored, between -40 nm and +10 nm. Devices sizes ranged from 420 X 420 micrometers 2 to 22 X 22 micrometers 2. As expected from simulations, the higher efficiencies are obtained when the detuning is in the -20 to 0 nm range. The devices exhibit then up to 10% external quantum efficiency, measured for a 62 degree(s) collection half-angle. After correction for the surface shadowing due to the grid p-contact, the efficiency increases to 14% and is practically independent of device size.