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Liquid Core ARROW Waveguides: a Promising Photonic Structure for Integrated Optofluidic Microsensors

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Liquid Core ARROW Waveguides: a Promising Photonic Structure for Integrated Optofluidic Microsensors micromachines Review Liquid Core ARROW Waveguides: A Promising Photonic Structure for Integrated Optofluidic Microsensors Genni Testa *, Gianluca Persichetti and Romeo Bernini Istituto per il Rilevamento Elettromagnetico dell’Ambiente, Consiglio Nazionale delle Ricerche (IREA-CNR), Via Diocleziano 328, 80124 Naples, Italy; [email protected] (G.P.); [email protected] (R.B.) * Correspondence: [email protected]; Tel.: +39-08-1762-0643; Fax: +39-08-1570-5734 Academic Editors: Shih-Kang Fan, Da-Jeng Yao and Yi-Chung Tung Received: 16 January 2016; Accepted: 7 March 2016; Published: 11 March 2016 Abstract: In this paper, we introduce a liquid core antiresonant reflecting optical waveguide (ARROW) as a novel optofluidic device that can be used to create innovative and highly functional microsensors. Liquid core ARROWs, with their dual ability to guide the light and the fluids in the same microchannel, have shown great potential as an optofluidic tool for quantitative spectroscopic analysis. ARROWs feature a planar architecture and, hence, are particularly attractive for chip scale integrated system. Step by step, several improvements have been made in recent years towards the implementation of these waveguides in a complete on-chip system for highly-sensitive detection down to the single molecule level. We review applications of liquid ARROWs for fluids sensing and discuss recent results and trends in the developments and applications of liquid ARROW in biomedical and biochemical research. The results outlined show that the strong light matter interaction occurring in the optofluidic channel of an ARROW and the versatility offered by the fabrication methods makes these waveguides a very promising building block for optofluidic sensor development. Keywords: liquid ARROW; optofluidics; microfluidics; optical sensors; integrated silicon technology 1. Introduction Optofluidics is an emerging research field that combines the advantages of microfluidics and optics on the same platform towards highly functional and compact devices [1,2]. More precisely, optofluidic approaches provide high-yield integration of fluidics by including fluidic elements as parts of the photonic structure. This solution confers to the optofluidic systems a high level of compactness and reconfigurability as the fluids provide an effective means to tune and reconfigure the photonic architecture. In the field of optical sensor technologies, optofluidics offers innovative design solutions in order to integrate all the required microfluidic and optical functionalities on a single chip for high-throughput analysis. Optofluidic sensors have been extensively developed for biomedical and biochemical analysis for healthcare purposes (i.e., diagnostic), pharmaceutical research, and environmental monitoring [3]. Whatever optical method is used, highly-sensitive detection requires a very careful design of both optical and microfluidic parts in order to push the device sensitivity to the limit of the theoretical capability. Furthermore, innovative design strategies need to be developed to overcome the reduced interaction length owing to devices size miniaturization. In this context, optofluidics answer to the growing need for miniaturized and portable sensing devices, highly integrated, and extremely sensitive [4]. Optofluidic waveguides represent one of the simplest examples of optofluidic integration, as the optical and fluidic structures are tightly integrated into each other. In particular, an optofluidic Micromachines 2016, 7, 47; doi:10.3390/mi7030047 www.mdpi.com/journal/micromachines Micromachines 2016, 7, 47 2 of 19 Optofluidic waveguides represent one of the simplest examples of optofluidic integration, as Micromachinesthe optical and2016 ,fluidic7, 47 structures are tightly integrated into each other. In particular, an optofluidic2 of 19 waveguide is a photonic structure able to perform optical confinement of the light through a fluid [5]. waveguideAmong others, is a photonic liquid cores structure antiresonant able to perform reflecting optical optical confinement waveguides of the light(ARROWs) through have a fluid been [5]. Amongrevealed others, as a very liquid powerful cores antiresonant building-block reflecting for optofluidic optical waveguides sensors [6]. (ARROWs) In an ARROW have been the revealed light is asguided a very through powerful the building-blockliquid core by means for optofluidic of highly sensors reflective [6]. mirrors In an ARROWsuitably realized the light on is the guided core throughsidewalls. the Since liquid both core the by light means and of the highly liquid reflective are guided mirrors through suitably the same realized microchannel, on the core ultra-high sidewalls. Sincesensitivity both can the lightbe achieved and the by liquid exploiting are guided the direct through interaction the same of microchannel, the entire optical ultra-high waveguide sensitivity mode canwith be the achieved fluid samples. by exploiting Such a strong the direct optical interaction coupling, of combined the entire with optical the waveguideultra-small modeliquid withvolume, the fluidenables samples. sensitivity Such down a strong to single optical molecule coupling, level, combined which with is the the ultimate ultra-small goal liquidof analytical volume, methods. enables sensitivityARROWs downcan be to easily single fabricated molecule level, using which silicon is thetechnologies; ultimate goal more of analyticalover, like methods.others optofluidic ARROWs canwaveguides, be easily fabricatedsuch as the using photonic silicon crystal technologies; [7,8] and moreover, the slot like wave othersguides optofluidic [9,10], waveguides,ARROWs are, such in asessence, the photonic planar and, crystal hence, [7,8 ]highly and the attractive slot waveguides for planar [optofluidic9,10], ARROWs integration are, in [11,12]. essence, As planar a matter and, of hence,fact, starting highly from attractive the first forplanar demonstration optofluidic of integrationthe ARROWs’ [11,12 potentialities]. As a matter in of the fact, field starting of optical from thesensing, first demonstrationthere has been ofa growing the ARROWs’ interest potentialities for them as inbasic the elements field of optical in optofluidic sensing, therechip for has sensing been a growingapplications. interest for them as basic elements in optofluidic chip for sensing applications. In the following, we report onon optofluidicoptofluidic sensorssensors realizedrealized withwith liquidliquid ARROWs.ARROWs. Starting from the description of the working principle and fabrication methods of liquid ARROWs, we review the state ofof thethe art art of of on on chip chip integration integration for sensingfor sensin applications,g applications, highlighting highlighting specific specific advantages advantages gained throughgained through the optofluidic the optofluidic integration. integration. 2. Operation Operation Principles Principles and and Simulations Simulations 2.1. 1-D Liquid ARROW ARROW waveguideswaveguides enable enable light light confinement confinement and propagation and propagation thanks to thanks interference to interference phenomena occurringphenomena in occurring the cladding in structure,the cladding made structure, up of alternating made up high-of alternating and low- high- refractive and low- index refractive (RI) thin layers.index (RI) The thin working layers. principle The working of an principle ARROW of waveguide an ARROW can waveguide be easier can understood be easier byunderstood considering by aconsidering schematic one-dimensionala schematic one-dimensional structure, as shown structure, in Figure as shown1. For simplicity,in Figure we1. For consider simplicity, only two we claddingconsider only layers two with cladding refractive layers index withn1 andrefractiven2 (n1 index> n2). n Light1 and propagates n2 (n1 > n2). throughLight propagates the core by through means ofthe Fresnel core by reflections means of at theFresnel cladding reflections interfaces. at the The cladding entrapped interfaces. light in each The cladding entrapped layer light experiences in each interferencecladding layer due experiences to repeated interference reflections atdue cladding to repeat interfaces.ed reflections In order at cladding to intensify interfaces. the light In reflectedorder to backintensify into the the light core, reflected each interference back into the cladding core, each must interference be designed cladding to fulfill must a specific be designed condition. to fulfill By consideringa specific condition. the analogy By considering of a cladding the layer analogy with aof Fabry–Pérot a cladding layer cavity, with such a conditionFabry–Pérot corresponds cavity, such to thecondition anti-resonance corresponds state ofto thethe cavity, anti-resonance which performs state of the the maximum cavity, reflectivity.which performs The reflectivity the maximum of the multilayerreflectivity. stack The constitutedreflectivity of by the multiple multilayer interference stack constituted claddings, generallyby multiple called interference Fabry–Pérot claddings, mirror, cangenerally be very called high, Fabry–Pérot up to 99%. mirror, can be very high, up to 99%. Figure 1. 1-D structure of ARROW waveguide. A detailed description of the ARROW confinement phenomenon is reported in [13], where the phase accumulated by the fundamental mode at each round trip in the interference claddings is clearly Micromachines 2016, 7, 47 3 of 19 defined. By setting the specific condition of antiresonance, a very useful expression to calculate
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