Loukas Athanasekos

Diffractive optic sensor for remote-point detection of ammonia

Remote-point photonic sensors are fabricated and evaluated. They are based on nanocomposite thin films comprising NiCl(2) nanocrystals embedded in sol-gel silica matrix and are patterned using direct UV laser microetching techniques to form surface relief structures, which exhibit environment sensitive optical diffraction effects. A strong response to ammonia is detected via the alteration of diffraction efficiency of its orders upon exposure to the analyte. Detection of ammonia in the 2 ppm level with a typical response time of about 30 s in the ambient, 50% RH 20 degrees C, room environment is demonstrated.

Nanocomposite hybrid photonic media for remote point sensors

The use of lossless optical media exhibiting reversible optical interactions upon exposure to physical and chemical agents allows for the implementation of remote point photonic sensors. Hybrid nanocomposite media represent a new class of sensor materials. They are produced by flexible chemical synthesis methods and can potentially be tailored by design to implement specific functionalities. These materials combined with externally manipulated optical interfaces lead to an emerging class of diffractive photonic devices offering unique sensing potential and advantageous operational features at low cost. The first case studies on ammonia and methanol detection are discussed.

ArF excimer laser microprocessing of polymer optical fibers for photonic sensor applications

A study of polymer optical fiber microstructuring by use of deep ultraviolet excimer laser radiation at 193 nm wavelength is performed. The ablation characteristics of the fiber cladding and core materials are analyzed comparatively. The laser irradiation effects are dynamically studied by on-line monitoring of the laser ablation induced waveguiding losses, the latter being correlated with the spatial structuring parameters. The fiber surface is modified to incorporate cavities, which are subsequently employed as sensitive material receptors for the development of customized photonic sensors. The sensing capability of the microstructured plastic optical fibers is demonstrated by ammonia and humidity detection.

Novel Approach for Lysozyme Detection Employing Block Copolymer Overlayers on Plastic Optical Fibers

A novel approach on lysozyme sensing is proposed by employing Polymer Optical Fibers functionalized successfully with overlayers of block copolymer sensitive materials. The detection scheme is based on electrostatic interaction between lysozyme and the copolymer. Low concentration levels of lysozyme have been detected successfully with almost spontaneous response times.

Micro-fabrication by laser radiation forces: a direct route to reversible free-standing three-dimensional structures.

The origins and first demonstration of structurally stable solids formed by use of radiation forces are presented. By experimentally proving that radiation forces can indeed produce stable solid material forms, a novel method enabling two- and three-dimensional (2d and 3d) microfabrication is introduced: An optical, non-contact single-step physical operation, reversible with respect to materials nature, based on the sole use of radiation forces. The present innovation is elucidated by the formation of polyisoprene and polybutadiene micro-solids, as well as plasmonic and fluorescent hybrids, respectively comprising Au nanoparticles and CdS quantum dots, together with novel concepts of polymeric fiber-drawing by radiation forces.

Polymer based photonic sensors for physicochemical monitoring

The ability to engineer polymer materials that have special response to external factors as well as to incorporate nano- and/or micro- materials in these polymer matrices make these materials perfect candidates for physicochemical sensors. The incorporation of the nano-/micro-materials help the polymer materials become more sensitive to a variety of external factors. Different polymer designs for the detection of humidity, alcohols and hydrocarbons are described. Special diffractive photonics structures are implemented to offer increased sensitivity to physicochemical changes. The final photonic sensor design is based on an optimization of chemical as well as optical design. The chemical polymer designs are based on selecting and synthesizing the appropriate polymeric structure that will facilitate interaction with the analyte, through a number of physical and chemical processes (adsorption, solubilization, entrapment, coulombic interaction and hydrogen bonding). These functions are determined by the chemical and structural features of the polymer used i.e. functional groups, glass transition, porosity, etc. In the case of polymer/nano- and/or micro-inorganic hybrid materials, interactions of the polymer matrix with the inorganic component(s) and dispersion of the nanomaterials within the matrix have to be taken into account. Suitable photonic interfaces based on transmissive and/or diffractive techniques are designed to provide the medium with interface tailoring and interrogation methodologies. Novel photonic information processor prototype devices based on free space configurations are demonstrated to extract/recover the captured information from the sensing material. The advantageous characteristics of the presented integrated sensor are the fully reversible behavior, at ambient operating conditions, without the need for additional heating or light exposure.

Multilayer metal/metal-oxide diffractive structure for photonic temperature sensing

We designed and fabricated multilayer metal/metal-oxide surface relief diffractive grating structures by growing alternating Pt and SnO(x) layers. Optical interrogation at 633 nm reveals the temperature dependence of their reflection and transmission diffractive effects. This function is explored here in the context of a remote, spatially localized, photonic temperature sensing operation, achieving sensitivity of 10% per °C for the zeroth-order in the transmission mode. The experimental demonstration is found to be in good agreement with the results of rigorous coupled wave analysis of the composite metal/metal-oxide element.

Optimized design of remote point diffractive optical sensors

We present an optimization study of surface relief grating based sensors for use in optical remote point sensing. Our study includes the effects of geometrical characteristics of the photonic structure on the efficiencies of the first and second diffraction orders and the exhibited sensor responsivities of these orders. Our predictions are verified experimentally with measurements of surface relief gratings patterned by direct laser ablation on SiO2 sol–gel nanocomposite material with embedded NiCl2 nanocrystals developed for ammonia sensing.

Diffractive optical elements for photonic gas sensors

Diffractive optical elements are designed and demonstrated as elemental units in photonic gas sensors. Diffraction gratings are written on specially designed photosensitive polymers using photolithographic techniques, as well as on multilayer metal/metal oxide thin film structures. Photonic sensors are implemented using grating structures as the elemental units for the detection of the external agent. These gratings are designed from such materials that show response to the external agent and the sensitivity is increased through the design of the grating. The principle of operation is based on the gratings diffraction efficiency variations due to index of refraction alterations and/or geometrical changes of the grating structure (e.g., groove depth, groove spacing) to external factors. The advantageous characteristics of the presented integrated sensor are the fully reversible behavior at ambient operating conditions, without the need for additional heating or light exposure. Applications of these sensitive photonic sensors so far include water vapor, hydrocarbons, and alcohol detection. The optical designs are based on diffraction efficiency measurements, and incorporate a monochromatic optical source and simple optoelectronic detection components. The photonic sensor integration is based on bulk optics approach.

Direct ArF excimer laser microfabrication methods for polymer photonics

Direct laser ablative microfabrication methods are exploited in the development of polymer photonic structures. A dedicated experimental microfabrication facility using ArF excimer laser radiation at λ = 193 nm has been developed. It operates, in conjunction with soft-lithography, chemical post-processing and laser deposition methods, aiming to high quality materials processing. The flexibility of the alternative techniques developed here is demonstrated through the fabrication of exemplar photonic structures, including surface relief holograms, planar optical waveguide and polymer fiber structures. The versatility of the technique is verified by patterning geometrically complex and non-planar surfaces, towards a complete suite of alternative and flexible tools for fabricating polymer photonics.