Chemical sensing is usually achieved in photonics platforms by monitoring spectral changes on the output of a passive photonic element due to the modulation of the refractive index of core and cladding. Therefore, compact interferometers are usually sought for the embodiment of refractometer sensors. We present our work on refractive index sensors based on arrayed waveguide interference, which are built on a Silicon-On-Insulator (SOI) platform. A comparative study of two configurations, resonant and non-resonant is presented. In both cases the main design is based on a set of closely placed single mode waveguides. The distance between waveguides is such that directional coupling occurs. Moreover, when the distance between the waveguides is small comparatively to the transversal exponential decay length of the eigenmode of the waveguide, there is an enhancement effect of the electric field in the region between the waveguides, as usually seen for slotted waveguides. The reported sensors include multiple parallel slotted waveguides which are the core of the sensor. Non-resonant configuration incorporates straight waveguides from which the output can be directly imaged onto a CCD array for direct sensor read-out, while the resonant layout presents a set of concentric racetrack waveguides designed for light extended lifetime, enhancing the sensor sensitivity. A top polymer cladding is used to encapsulate the waveguides providing a permeable low index material. This cladding material acts as the transducer element, changing its optical properties when in contact with a chemical of interest, therefore allowing for high sensitivity and chemical selectivity.
KEYWORDS: Photonics, Waveguides, Lab on a chip, Integrated optics, Sensors, Industrial chemicals, Biological and chemical sensing, Chemical species, Photonic integrated circuits, Prototyping
Sensors based on optical waveguides for chemical sensing have attracted increasing interest over the last two decades, fueled by potential applications in commercial lab-on-a-chip devices for medical and food safety industries. Even though the early studies were oriented for single-point detection, progress in device size reduction and device yield afforded by photonics foundries have opened the opportunity for distributed dynamic chemical sensing at the microscale. This will allow researchers to follow the dynamics of chemical species in field of microbiology, and microchemistry, with a complementary method to current technologies based on microfluorescence and hyperspectral imaging.
The study of the chemical dynamics at the surface of photoelectrodes in water splitting cells are a good candidate to benefit from such optochemical sensing devices that includes a photonic integrated circuit (PIC) with multiple sensors for real-time detection and spatial mapping of chemical species.
In this project, we present experimental results on a prototype integrated optical system for chemical mapping based on the interaction of cascaded resonant optical devices, spatially covered with chemically sensitive polymers and plasmon-enhanced nanostructured metal/metal-oxide claddings offering chemical selectivity in a pixelated surface. In order to achieve a compact footprint, the prototype is based in a silicon photonics platform. A discussion on the relative merits of a photonic platform based on large bandgap metal oxides and nitrides which have higher chemical resistance than silicon is also presented.
KEYWORDS: Optical lithography, Lithography, Optical fibers, Near field optics, Photoresist materials, Silicon, Zone plates, Optical components, Electron beam lithography, Ion beams
In this work, nanolithographic patterning by means of a nanostencil inscribed on an optical fiber tip is presented. Oneshot registration of multiple-sized features within a 4 μm diameter patterning circle has been experimentally tested on photoresist AZ5214E coated silicon substrate, with features as small as 160 nm beign obtained, replicating the original stencil with excellent agreement. The nanostencil was created by focused ion beam (FIB) milling, although other techniques such as femtosecond laser ablation or pattern transfer to fiber tip can also be employed. Stencils can be arbitrary or based on optical elementary designs such as line patterns, photonic crystals, Fresnel zone plates or photon sieve. Exact transfer of the inscribed pattern is obtained while in contact lithography, while proximity exposure enables complex modulation of the optical near-field by the phase and/or amplitude stencil mask. This allows for optical interference to occur, in full 3D space, rendering sub-wavelength spot focusing, annular pattern formation, as well as the formation of 3D complex shapes. Experimentally, a 405 nm laser beam with 17 mW power was launched into the core of UV-Visible single mode fiber (S405-XP) on which end a photon sieve was previously inscribed by FIB. This tip was scanned over the photoresist. Patterning consisted of 1Dscans, for which a minimum line width of 350 nm was obtained.Additionally, step-and-repeat patterning of the photon sieve fiber tip stencil was performed with, all features down to 160 nm being clearly resolved.
KEYWORDS: Optical fibers, Etching, Ion beams, Optical lithography, Zone plates, Diffraction, Picosecond phenomena, Optical tweezers, Near field optics, Infrared cameras
Focused ion beam (FIB) patterning of 3D topography on optical fiber tips for application in stand-alone, rugged and simplified setups for optical tweezers cell sorters, optical near-field lithography and optical beam profile engineering are reported. We demonstrate various configurations based on single-step FIB patterning, multiple-step FIB processing and hybrid approaches based on optical fiber pre- and post-FIB treatment with either etching, fusion splicing, photopolymerization or electroplating steps for optical fiber texture, topography and composition engineering. Different conductive coatings for minimal charge accumulation and beam drift are studied with the relative merits compared. Furthermore optimal beam parameters for accurate pattern replication and positioning are also presented. Measured experimental field profiles are compared with numerical simulations of fabricated optical fiber tips for fabrication accuracy evaluation. Applications employing these engineered fiber tips in the field of optical tweezers, optical vortex generation, photolithography, photo-polymerization and beam forming are presented.
Multilayer (ML) thin films, based on indium molybdenum oxide (IMO) and aluminum zinc oxide (AZO), having different stacking were deposited using RF sputtering at room temperature (RT). The total-layer thickness of the MLs ranges between 93 nm and 98 nm. The deposited films were characterized by their structural, electrical, microstructural, and optical properties. X-ray diffraction (XRD) peaks obtained at 2θ of around 30.6° and 34.27° are matched with cubic-In2O3 (222) and hexagonal-ZnO (002), respectively. The MLs have both nano-crystalline and polycrystalline structures depending on the layer properties. A conspicuous feature of XRD analysis is the absence of diffraction peak from 50 nm thick IMO layer when it is stacked below 50 nm thick AZO, whereas it appears significantly when the stacking is reversed to place IMO above AZO layer. Hall measurements confirmed that the deposited MLs are n- type conducting and the electrical properties are varied as a function of layer properties. The deposited MLs show high shortwavelength infrared transmittance (SWIRT) even at 3300 nm, which is ranging as high as 75 % - 90 %. Overall, the MLs show high transmittance in the entire Vis-SWIR region. The optical band gap (Eg) calculated using the absorption coefficient (α) and photon energy (hν) of the deposited MLs is ranging between 3.19 eV and 3.56 eV, depending on the layer properties. Selected as- deposited films were annealed in open air at 400 °C for 1 h; the transmittance of annealed films was improved but their electrical properties deteriorated. Atomic force microscopy (AFM) analysis shows that the root-mean-square (RMS) roughness of the MLs ranges between 0.8 nm and 1.5 nm.
We report an optical fiber chemical sensor based on a focused ion beam processed optical fiber. The demonstrated sensor is based on a cavity formed onto a standard 1550 nm single-mode fiber by either chemical etching, focused ion beam milling (FIB) or femtosecond laser ablation, on which side channels are drilled by either ion beam milling or femtosecond laser irradiation. The encapsulation of the cavity is achieved by optimized fusion splicing onto a standard single or multimode fiber. The empty cavity can be used as semi-curved Fabry-Pérot resonator for gas or liquid sensing. Increased reflectivity of the formed cavity mirrors can be achieved with atomic layer deposition (ALD) of alternating metal oxides. For chemical selective optical sensors, we demonstrate the same FIB-formed cavity concept, but filled with different materials, such as polydimethylsiloxane (PDMS), poly(methyl methacrylate) (PMMA) which show selective swelling when immersed in different solvents. Finally, a reducing agent sensor based on a FIB formed cavity partially sealed by fusion splicing and coated with a thin ZnO layer by ALD is presented and the results discussed. Sensor interrogation is achieved with spectral or multi-channel intensity measurements.
Piezomachined ultrasonic transducer (PMUT) arrays are commonly found in applications in the field of ultrasonography
and gesture recognition systems. Their application for bio and chemical sample preparation is another possibility, based
on their beam steering and acoustic field manipulation capabilities. Post-fabrication non-destructive measurement of key
device temporal and spatial parameters is required in order to adjust either simulation models or tune fabrication steps. In
this work we report an optical testing setup for measuring the acoustic spectrum of PMUT devices and arrays,
characterize maximum deflection of PMUTs and piezopumps and investigate the load effect of electrical contacts on the
spatial and temporal oscillation behavior of these piezoelectric structures. Spatial parameters are evaluated with digital
holography and temporal parameters with single point Doppler shift and frequency-shifted. We employ this testing setup
to measure our own designed PMUT structures which were fabricated at IME-Singapore, evaluating the relative merits
of the PMUT design parameters.
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