This paper studies two different approaches for evanescent wave optical sensing: an horizontal one and a vertical one. In horizontal waveguides, the evanescent wave is distributed on the upper cladding. While in a vertical configuration, the evanescent wave is distributed on the left and right sides of the waveguide. In an horizontal configuration the evanescent wave can be also on both sides of the waveguide in order to increase the optical energy for sensing if the substrate under the waveguide is locally removed. However, in this configuration to achieve sensitive devices, the layers have to be freestanding and thin  limiting practical implementations of such approaches. Furthermore, very few materials can be defined as tall and thin in the case of a vertical configuration, as the deposition techniques often used (PECVD/LPCVD) are meant for films in the couple of micron range. In the following we will investigate the properties of the materials used but also the fabrication feasibility for both configurations.
We present Si microlenses fabricated using dry ICP plasma etching of silicon and thermal photoresist reflow. The process is insensitive to thermal reflow time and it can be easily incorporated into fabrication flows for complex optical systems. Using this process, we were able to fabricate microlenses with diameter of 150 μm, radius of curvature of 682 μm and with a surface roughness of only 25 nm.
In an optomechanical cavity the optical and mechanical degree of freedom are strongly coupled by the radiation pressure of the light. This field of research has been gathering a lot of momentum during the last couple of years, driven by the technological advances in microfabrication and the first observation of quantum phenomena. These results open new perspectives in a wide range of applications, including high sensitivity measurements of position, acceleration, force, mass, and for fundamental research. We are working on low frequency pondero-motive light squeezing as a tool for improving the sensitivity of audio frequency measuring devices such as magnetic resonance force microscopes and gravitational-wave detectors. It is well known that experiments aiming to produce and manipulate non-classical (squeezed) light by effect of optomechanical interaction need a mechanical oscillator with low optical and mechanical losses. These technological requirements permit to maximize the force per incoming photon exerted by the cavity field on the mechanical element and to improve the element’s response to the radiation pressure force and, at the same time, to decrease the influence of the thermal bath. In this contribution we describe a class of mechanical devices for which we measured a mechanical quality factor up to 1.2 × 106 and with which it was possible to build a Fabry-Perot cavity with optical finesse up to 9 × 104. From our estimations, these characteristics meet the requirements for the generation of radiation squeezing and quantum correlations in the ∼ 100kHz region. Moreover our devices are characterized by high reproducibility to allow inclusion in integrated systems. We show the results of the characterization realized with a Michelson interferometer down to 4.2K and measurements in optical cavities performed at cryogenic temperature with input optical powers up to a few mW. We also report on the dynamical stability and the thermal response of the system.
Microlens Arrays in silicon are suitable for an important wavelength range within the IR spectrum, since silicon features relatively high refractive index and is transparent at the aimed wavelengths, leading to microlenses with a focal length short enough to allow compact systems, offering an alternative for applications where miniaturization and reduction of alignment and packaging costs are necessary. The microlenses are meant to sample and focus an IR beam on a focal plane array, which might be an image sensor, or a dedicated IR sensor, as for instance lab-on-chip, or a selective gas detection system. Nowadays refractive microlenses are manufactured using sophisticated techniques with relatively high costs and complexity of well controlled steps, like thermal reflow, and grayscale lithography. We hereby propose an alternative solution for microfabrication of silicon microlens arrays, with a single-mask step using KOH anisotropic etching of Si. The proposed technique solves many current demands, like achieving high reproducibility, fill-factor close to 100%, and higher precision of focal axes alignment. We have made optical profilometric measurements to estimate the shape, roughness and the focal distance. We have also observed the focal points imaging in the IR spectrum, proving that the silicon microlenses actually yield the results expected.
We have studied, for the first time, the sensing capabilities of plasma-enhanced chemical vapor deposition (PECVD) SiC-SiO2-SiC horizontal slot waveguides. Optical propagation losses were measured to be 23.9 dB/cm for the quasi-transverse magnetic mode. To assess the potential of this device as a sensor, we simulated the confinement factor in the slot. This simulation revealed that SiC-based slot waveguides can be used, advantangeously, for sensing as the confinement strongly varies with the refractive index of the slot material. A confinement factor change of 0.15/refractive index units was measured for different slot materials.
This work reports the surface functionalisation of evanescent waveguide sensors to immobilise E. coli. In biosensors, the
surface functionalisation is an important treatment to ensure that the sensor properly detects the cells of interest. In this
paper, we study the thin film surface functionalisation of a TiO2 evanescent waveguide sensor and their effect on light
transmission for the early detection of E. coli in post colon surgery. TiO2 deposited using atomic layer deposition (ALD)
is used as waveguide material. Four layers are used in the functionalisation : the self-assembled monolayer (SAM), the
protein, 1-ethyl-3-(3-dimethylaminopropyl) (EDC) and the antibodies. Aminopropyltriethoxysilane (APTES) is used as
SAM and reacts with -OH group (hydroxyl). The -OH group must be provided on substrate. In order to have the proper
-OH group we deposited 10 nm SiO2 on the waveguides using PECVD and then treated the samples in oxygen plasma
chamber for 2 minutes to create the groups. Afterward APTES is immediately applied on the surface after every layers of
the functionalisation process. The second layer (Protein A) of the functionalisation is then put on APTES as interlayer.
EDC is used as crosslink agent between APTES and antibodies. The light of Superluminescent light emitting diodes
(SLEDs) (λ = 1.3 μm, 400 mA) is channelled using an optical fibre into the functionalised waveguides. The transmitted
light is measured with a photodiode. The sensitivity of the sensor was evaluated using several different drain fluid
concentrations in medium.
We fabricated horizontal slot waveguides using two low temperature deposition techniques ensuring the
full compatibility of the processes with CMOS technology. Slots width as thin as 45 nm with smooth slot
surfaces can easily be fabricated with simple photolithographic steps. Fundamental TM-like slot mode in
which the E-field is greatly enhanced within slot showed a 23.9 dB/cm and a 18 dB/cm in a PECVD
SiC/SiO2/SiC and a ALD TiO2/Al2O3/TiO2 vertical slot waveguide, respectively.
Optical Coherence Tomography (OCT) has found applications in many fields of medicine and has a large
potential for the optical biopsy of tumors. One of the technological challenges impairing faster adoption of
OCT is the relative complexity of the optical instrumentation required, which translates into expensive and
bulky setups. In this paper we report an implementation of
Time-Domain Optical Coherence Tomography
based on Plasma Enhanced Chemical Vapor Deposition (PECVD) Silicon Carbide (SiC). The devices, with
a footprint of 0.3 cm2, are fabricated using rib waveguides defined in a SiC layer. While most of the
components needed are known when using this material , a fast delay line with sufficient scanning range
is a specific requirement of Time Domain (TD)-OCT. In the system reported here this is obtained making
use of the thermo-optical effect. Though the current implementation still requires external sources and
detectors to be coupled to the planar waveguide circuit, future work will include three-dimensional
integration of these components onto the substrate to achieve a fully autonomous and compact OCT chip.
With the potential to include the read-out and driving electronics on the same die, the reported approach
can yield extremely compact and low-cost TD-OCT systems in the visible, enabling a broad range of new
applications, including OCT devices for harsh environment.
Optical Coherence Tomography (OCT) is a promising medical imaging technique. It has found applications in many fields of medicine and has a large potential for the optical biopsy of tumours. One of the technological challenges impairing faster adoption of OCT is the relative complexity of the optical instrumentation required, which translates into expensive and bulky setups. In this paper we report an implementation of Time Domain OCT (TD-OCT) based on a silicon photonic platform. The devices are fabricated using Silicon-On-Insulator (SOI) wafers, on which rib waveguides are defined. While most of the components needed are well-known in this technology, a fast delay line with sufficient scanning range is a specific requirement of TD-OCT. In the system reported, this was obtained making use of the thermo-optical effect of silicon. By modulating the thermal resistance of the waveguide to the substrate, it is possible to establish a trade-off between maximum working frequency and power dissipation. Within this trade-off, the systems obtained can be operated in the kHz range, and they achieve temperature shifts corresponding to scanning ranges of over 2mm. Though the current implementation still requires external sources and detectors to be coupled to the Planar Lightwave Circuit (PLC), future work will include three-dimensional integration of these components onto the substrate. With the potential to include the read-out and driving electronics on the same die, the reported approach can yield extremely compact and low-cost TD-OCT systems, enabling a wealth of new applications, including gastrointestinal pills with optical biopsy capabilities.
Light delivery and optical monitoring during photodynamic therapy (PDT) is often limited by the need for a physical
optical link between the light source and detection devices and the treatment volume. This can be critical when sources
need to be implanted within the body for extended periods. We report on the latest developments for a telemetric PDT
delivery and monitoring device that can dynamically vary the local illumination parameters based on the in-situ fluence
rate within the PDT target volume. Local light delivery and collection is achieved using solid-state optodes, microfabricated
on a silicon substrate. Photodiodes have been produced using a standard bipolar process. Chip-form LEDs are
then assembled into micro-machined pits adjacent to the light fluence rate detectors. The devices (1.2×1.2mm2) are
bonded to a flexible PCB together with the remaining electronics. Power coupling and communications are achieved by
means of an inductive link while light delivery and fluence rate monitoring are digitally managed using a
microcontroller. These devices are being tested in optical phantoms and in pre-clinical models. Our results show that it is
possible to manufacture solid-state optodes of suitable dimensions and that it is feasible to telemetrically deliver and
control the local fluence rate using them. It can also be concluded from our work that while the optode is sufficiently
small to be useful as a light delivery and monitoring device, digital control, read-out electronics and power coupling can
benefit from further optimization and miniaturization.
Detection peculiarities of an un-cooled (room temperature) 8x8 pixel array designed to image broadband THz radiation
were investigated. Each pixel consists of a thin conductive film absorber on a dielectric membrane with thermopile
temperature readout. It was designed and tested for four combinations of two different types of absorber and thermopile
materials. The photo-response profile, determined by scanning the pixels through the focus of a THz laser beam, was
wider than expected from a 2-D convolution of the Gaussian beam and the absorber surface. Also the time response did
depend on the position of the beam relative to the pixel. Simulations show that those properties are due to the fact that
also the thermopiles absorb THz radiation. For the best composition of absorber and thermopile, the responsivity, the
noise equivalent power, and the bandwidth were estimated to be of 28 V/W, 5x10-9 W/Hz1/2 and 50 Hz, respectively.
Planar silicon carbide (SiC) waveguides are proposed for fabrication on a silicon substrate with a oxide isolation layer. Using post deposition annealing it is possible to achieve low Polarization-Dependent Loss (PDL) within optical SiC waveguides fabricated using a low temperature deposition technique. Those waveguides have been successfully used in power splitters and cantilivers. These first devices open the way for photonic sensing in harsh environment using SiC.
This paper presents an on-going work to develop micromachined silicon-based strain sensor inspired from the campaniform sensillum of insects. We present simple optical setup for the characterisation of a membrane-in-recess structure as an early stage in mimicking the natural sensor. The microstructure is a 500 nm-thick SiO2/SiN circular membrane, burried 13 μm from the surface of a 3x3 mm, 525 μm thick Si-chip. The chip was attached to a 45x10x0.525 mm Si beam. The simple optical characterisation setup is based on imaging the reflected laser beam from the biomimetic structure. Since an optical cavity between the membrane and the Si beams beneath was formed, ideal flat-parallel Fabry-Perot interferometer equation was applied to interpret the results semi-quantitatively. We obtained 2-D interference fringe pattern having 3 orders of maxima from the middle to the edge of the circular apperture, as a result of an initial internal membrane stress. The pattern changed non-linearly as we applied flexural strain from behind the beam up to 50 μm, most probably caused by nonlinear deflection of the membrane (i.e. the membrane did not deflect similarly as the beam beneath it). This phenomena might explain one of the strain-amplifying properties of this biomimetic strain sensing microstructure.
We report dispersions resulting from a slot device (SD) in silica-based arrayed-waveguide grating (AWG). A SD is used to produce flattened passbands and we show the dependence on the bandwidth, the crosstalk, the ripple, and the chromatic dispersion (CD) of the passband in the presence of such a device. A comparison with other known techniques is also given. The device has been developed on a high index silica-based PLC platform but can be implemented on higher index contrast platforms.
The mechanism of direct bonding at room temperature has been attributed to the short range inter-molecular and inter-atomic attraction forces, such as Van der Waals forces. Consequently, the wafer surface smoothness becomes one of the most critical parameters in this process. High surface roughness will result in small real area of contact, and therefore yield voids in the bonding interface. Usually, the root mean square roughness (RMS) or the mean roughness (Ra) are used as parameters to evaluate the wafer bondability. It was found from experience that for a bondable wafer surface the mean roughness must be in the subnanometer range, preferentially less than 0.5 nm. When the surface roughness exceeds a critical value, the wafers will not bond at all. However RMS and Ra were found to be not sufficient for evaluating the wafer bondability. Hence one tried to relate wafer bonding to the spatial spectrum of the wafer surface profile and indeed some empirical relations that have been found. The first, who proposed a theory on the problem of the closing gaps between contacted wafers was Stengl. This gap-closing theory was then further developed by Tong and Gosele. The elastomechanics theory was used to study the balance between the decrease of surface energy due to the bonding and the increase of elastic energy due to the distortion of the wafer. They considered the worst case by assuming that both wafers have a waviness, with a wavelength (lambda) and a height amplitude h, resulting in a gap height of 2h in a head to head position. This theory is simple and can be used in practice, for studying the formation of the voids, or for constructing design rules for the bonding of deliberately structured wafers. But it is insufficient to know what is the real area of contact in the wafer interface after contact at room temperature because the wafer surface always possesses a random distribution of the surface topography. Therefore Gui developed a continuous model on the influence of the surface roughness to wafer bonding, that is based on a statistical surface roughness model Pandraud demonstrated experimentally that direct bonding between processed glass wafers is possible. This result cannot be explained by considering the RMS value of the surfaces only, because the wafers used show a RMS value larger than 1 nm. Based on the approach exposed in reference six, a rigorous analysis of wafer bonding of these processed glass wafers is presented. We will discuss the relation between the bonding process and different waveguide technologies used for implementing optical waveguides into one or both glass wafers, and give examples of optical devices benefiting from such a bonding process.
New methodologies in anisotropic wet-chemical etching of oriented silicon allowing useful process designs combined with smart mask-to crystal-orientation-alignment are presented. The described methods yield smooth, etch-step free surfaces as well as high-quality plan-parallel beams and membranes. With a combination of pre-etching at different depths and passivation steps, structures can be etched at different levels in a wafer. Design rules using the < 100 >-crystal orientation, supplemented with examples demonstrate the high potential of using < 100 > oriented wafers in microsystem design.
We propose a new fabrication method for monomode optical integrated devices. As an example, we show how a deeply buried planar waveguide can be obtained by assembling two half waveguides. These ones are realized by ion exchange and the assembling method is the wafer direct bonding (WDB). The optical properties are studied and compared with theory. The results prove that direct bonding, as in VLSI batch technology, is a low cost and high performance technology for optical devices fabrication.
The growing demand for complex components for integrated- optic requires fabrication methods allowing good reproducibility and a minimized number of processing steps. The ion-exchange technique is attractive because it can be used to make inexpensive and versatile waveguides in glass. To ensure reliable operation, the waveguides have to be buried beneath the surface. Various methods for making buried waveguides are based on field assisted ion exchange with one or more steps in the procedure. We propose a new assembling method in which a buried optical waveguide is obtained by direct bonding of two separate waveguides. We found that direct bonding of preprocessed wafers is a useful and versatile step in the fabrication of integrated optical components. Wafer direct bonding is compatible with VLSI batch technology and device miniaturization therefore low cost combined with high performance can be achieved. Furthermore, a large variety of symmetrical or asymmetrical index profiles is possible and the method allows simultaneous fabrication of many identical optical components. Optical properties of the component are studied and the advantages of this new process are summarized and compared with other techniques.