Krew/sio2 waveguide platform for optical sensing applications

Resumo

Today's world is hyperconnected. Without discussing the reasons or origins, this is a fact. At the same time, it seems that the trend is towards greater connectivity. People are connected, devices are connected, and people are becoming increasingly connected to devices. Connectivity together with the possibility of managing large amounts of information will open the door to the possible decentralization of an enormous amount of daily activities, such as health monitoring. Given the growing concern that people are having for their health, it is not uncommon to find on most people's arms a bracelet or clock capable of measuring their heart rate and counting the steps they take daily. These can be connected to a mobile phone that monitors these variables along with the calories this person takes in daily, the hours of any sport they practice or their hours of sleep. All these measurements are carried out by sensors incorporated into wristbands, watches, and mobiles. It is not difficult to imagine that soon all these objects and others such as clothing, can take other sensors that monitor our health much more accurately, measuring many other variables such as our hydration level. At the same time, it does not seem strange to imagine a near future where it is not necessary to visit the doctor to perform a blood or urine analysis. Returning to the facts, in the past decades, the scientific community has made great efforts aimed at the miniaturization of a large number of sensors. An iconic example of this evolution towards more compact sensors is that of glucose sensors for diabetics. This greatly facilitated glucose control for diabetics and made their lives more comfortable in regards to this disease. In the last 20 years, the field of microfluidics has received increasing attention from the scientific community as it has great potential to solve some problems in very varied fields such as physics, chemistry, biology, medicine or engineering. Many of these advances in microfluidics have allowed advancement in the miniaturization of many chemical and biological sensors. The main advantages provided by the implementation of microfluidics in the sensors are: portability; lower sample volume required; high-throughput screening; the emergence of new phenomena and improvement of physical processes; ease to integrate optical (Optofluidics) and mechanical elements (MEMS); and compactness and integration of laboratory operations (lab-on-chip devices). Thus, it is of special interest in the implementation of microfluidics in existing sensors or the design of sensing platforms compatible with this technology, in which Integrated Optics (IO) has a vital role. The optical sensors have been applied to the field of sensors with very diverse objectives. Some examples of optical sensors are the flow cytometers and microplate readers which have long been used to analyze biological and chemical samples. New devices have emerged in the past decade, due to optical bulk sensor miniaturization, which has evolved to integrated optical microsystems such as on-chip waveguides and resonators. The high compatibility between integrated optic sensors and microfluidics has resulted in an extensive amount of studies. This thesis aims to address some of the needs in the field of optical sensors with the manufacture of an optical platform for sensing applications based on crystal-on-glass waveguides. A family of crystals known as potassium double tungstates (with KRE(WO4)2 as generic formula, where RE means Rare Earth) has been selected to act as a crystal core for the crystal-on-glass waveguides platform fabrication. KLu(WO4)2 (KLuW) crystal was selected among other crystals of this family based on: (i) the research group's extensive experience using this material, (ii) it provides a large refractive index contrast with SiO2 of around 0.4, (iii) it has a broad transmission window from visible up to 5.5 µm wavelength which provides the possibility to operate at the chemical and biological fingerprint region of several organic and inorganic molecules, (iv) it possesses high absorption and emission cross-sections when doped with active lanthanide ions, and can be doped with high concentrations of active ions for in-situ laser light generation; these large absorptions and emission cross-sections could be relevant if this crystal is doped with active ions, and their optical spectroscopic features are used as a base for the signal used in chemical sensing. One of the novelties of this KLuW/SiO2 waveguide platform is the selection of a tapered geometry for the waveguides which theoretically should solve common coupling difficulties that most of the similar IO devices have, and provide a sensitivity increase due to the evanescent-wave enhancement. Besides, the infrared spectral range (from 2.5 to 20 micrometers wavelength) was selected for the sensor operation because it contains numerous, strong and well-defined molecular fingerprints (these are molecular groups characteristic vibrations which would allow to specifically detect molecules with a certain molecular structure). With the aim of manufacturing this KLuW/SiO2 tapered waveguide platform for sensing applications operating at the IR, a three-technique fabrication approach was proposed consisting of: KLuW plate to glass substrate bonding, waveguides fabrication by ultra-precise dicing saw cuts, and Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) waveguides post-processing. The use of these three techniques together for the fabrication of crystal-on-glass waveguides is very innovative and the results al promising. This combination with or without using the last post-processing step has a great potential to become relevant to the Integrated Optics field for the fabrication of IO devices. The fabrication of the KLuW/SiO2 tapered waveguide platform was successful. High-quality waveguides were fabricated by using the three-technique approach. Firstly, the adhesion of KRE(WO4)2 crystal plates to glass substrates was studied from two perspectives: the Direct Bonding (DB) of the crystal plates to the glass substrates by means of an oxygen plasma surface activation, and the Adhesive Bonding (AB) where the crystal plates are bonded to glass by using a third material, in this case, an organic adhesive. The crucial parameters for a long-life high-quality joint were found to be: (i) the cleanliness of both, crystal and glass, surfaces, (ii) the surface roughness and flatness for DB, (iii) the adhesive nature for AB. Concluding from this section that DB has great potential but nowadays the best choice is AB, more specifically AB with a commercial adhesive known as NOA83H which shown the best bonding performance. Secondly, in addition to tapered waveguides fabrication, rib and ridge waveguides were successfully fabricated on the KLu(WO4)2/SiO2 platform using ultra-precise dicing saw cut technique. This technique has shown a high-performance on waveguides manufacturing on crystals in comparison with classic microfabrication techniques such as wet etching and dry etching techniques. The key parameters which will determine the success of waveguides fabrication by the ultra-precise dicing saw technique were identified to be the dicing blade election and the cutting variables (cutting speed and blade rotation speed), and by the correct selection of these three variables results in a durable and high-performance blade capable of fabricating low roughness and low defect concentration waveguides, maximizing the crystal cut performance. The ICP-RIE post-processing of the KLu(WO4)2/SiO2 results were acceptable. The aim of this post-processing step was to reduce the waveguide height and this was achieved but reducing the waveguides final quality (increasing the surface roughness and the number of defects present at the waveguides). Nonetheless, the KLu(WO4)2/SiO2 platform tapered waveguides for sensing proof-of-concept succeed. The demonstration of the viability of using the tapered waveguides as chemical sensors consisted of a real-time measurement experiment of the transient response of the mid-IR tapered waveguide to the contact of ethanol/water solutions. It was proved that the waveguides sensor signal didn't suffer hysteresis (i.e. the signal was recovered after the ethanol/water solution drop removal. The respond of the sensor to a 10 % ethanol/water solution was estimated to be 0.4 dB/cm. Finally, the sensor showed a linear response for ethanol-water dissolutions ranging from 4 to 10 vol.%. These results confirm the possibility to use KLuW-on-glass tapered waveguides platform for chemical sensing applications.