Letter pubs.acs.org/NanoLett
Stimulus-Responsive Light Coupling and Modulation with Nanofiber Waveguide Junctions Ilsun Yoon,† Kanguk Kim,† Sarah E. Baker,‡ Daniel Heineck,§ Sadik C. Esener,†,§ and Donald J. Sirbuly*,†,‡ †
Department of NanoEngineering, University of California, San Diego, La Jolla, California 92093, United States Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, United States § Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, California 92093, United States ‡
S Supporting Information *
ABSTRACT: We report a systematic study of light coupling at junctions of overlapping SnO2 nanofiber waveguides (WGs) as a function of gap separation and guided wavelength. The junctions were assembled on silica substrates using micromanipulation techniques and the gap separation was controlled by depositing thin self-assembled polyelectrolyte coatings at the fiber junctions. We demonstrate that the coupling efficiency is strongly dependent on the gap separation, showing strong fluctuations (0.1 dB/nm) in the power transfer when the separation between nanofibers changes by as little as 2 nm. Experimental results correlate well with numerical simulations using three-dimensional finite-difference time-domain techniques. To demonstrate the feasibility of using coupled nanofiber WGs to modulate light, we encased the junctions in an environment-responsive matrix and exposed the junctions to gaseous vapor. The nanofiber junctions show an ∼95% (or ∼80%) modulation of the guided 450 nm (or 510 nm) light upon interaction with the gaseous molecules. The results reveal a unique nanofiber-based sensing scheme that does not require a change in the refractive index to detect stimuli, suggesting these structures could play important roles in localized sensing devices including force-based measurements or novel chemically induced light modulators. KEYWORDS: Semiconductor nanowire, subwavelength waveguide, light modulation, evanescent field, sensor, nanophotonics
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evanescent field that can be used for strongly localized detection or to route light between cavities in close proximity.17,18 Scaling down the size of these optical fibers also allows light to be controllably injected into small areas (e.g., inside cells or microfluidic devices) and can improve the signal-to-noise of a detector that might require molecular-level sensitivity.18−20 A key component to uncovering novel applications for nanophotonic devices that utilize the evanescent field is to systematically study the transfer of light in photonic assemblies and determine the relationship between the sharp decaying optical field and the sensitivity to objects residing near the surface of the guiding cavity. If a fiber-based system could be encoded to show strong optical modulation in response to stimuli such as chemicals, mechanical forces, thermal energy, or molecular binding events, a highly versatile detection platform could be developed and integrated into current technologies. For this reason, coupled WG systems and other optical devices continue to be a major focus of research.21−24 It is anticipated that the coupling between high-index contrast WGs would be strongly dependent on the separation
ontrolling the flow of photons in devices smaller than the wavelength of light is central to the development of integrated optical circuits and novel biodetection platforms.1−5 Because of the many advantages of using photons over electrons in signal transduction, including high bandwidth signaling, various label-free sensing methodologies, and zeroelectron flow capabilities, nanophotonics has become an active area of research in the biomedical and life sciences communities. Important to this field are the significant advances that have been made in the ability to confine and route photons. Currently, various materials are being researched and developed including photonic band gap structures,6−8 metal-plasmonic architectures,9−12 lithographically defined optical cavities,13 and free-standing subwavelength optical waveguides (WGs).1,2 One class of materials, highaspect ratio (>102) semiconductor nanowires (NWs), have garnered interest as both active and passive materials in photonic devices.1,2,14,15 This is due in part to their capacity to guide broad optical signals ranging from the near-infrared (IR) to near-ultraviolet through the core of the WG while maintaining the ability to efficiently transfer light from one cavity to another. Because of the small cross-sectional dimensions of these NWs (∼100−300 nm), a large portion (∼10−40%) of the guided optical energy, as determined using step-index fiber modal power calculations,16 is located in the © 2012 American Chemical Society
Received: December 6, 2011 Revised: March 15, 2012 Published: March 26, 2012 1905
dx.doi.org/10.1021/nl2043024 | Nano Lett. 2012, 12, 1905−1911
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Figure 1. (a) Schematic of light coupling at a nanofiber junction created between two SnO2 waveguides (WGs). (b) Scanning electron micrograph of a nanofiber junction with a zero gap separation. (c) Dark-field (top) and emission (bottom) images of a nanofiber junction device. The emission image captures the WG outputs and junction scattering of the junction device in (b). (d) Experimental data showing the output ratio (as defined in the main text) as a function of crossing angle for the device shown in (c). There is no gap between WG(1) and WG(2).
reversible and provides excellent verification that the nanofiber junctions can be utilized as a novel label-free detection platform or light modulation system. A schematic, along with an electron micrograph, dark-field, and emission images of a SnO2 WG junction is shown in Figure 1. The SnO2 WGs were synthesized via a thermal vaporization process as described elsewhere.1,27 The junction devices used in this work were fabricated using micromanipulation,4 but other assembly techniques such as optical,28 fluidic,29 or electricfields30 can also be used to assemble nanowire structures to study coupling effects. All nanofibers used in this study had cross-sectional dimensions of ∼100 × 300 nm, unless specified. The power transfer ratio can be close to unity when the WGs are close together and should be strongly dependent on the coupling angle (θ). To understand this effect in our WG system, light emissions at the end facet of the pumping WG [WG(1)], coupling WG [WG(2)], and the junction were monitored as a function of θ. The angle dependence (ranging from 20−80°) of the output ratio, defined as Ix/(I1 + I2 + Ij) (where Ix is the scattering emission intensity from the end facet of WG(1), WG(2), or the junction), of a typical nanofiber junction device is shown in Figure 1d. To avoid exposing the junction to direct UV excitation, WG(1) was excited at least 100 μm (>2× the beam diameter) away from the junction. As evident in the plot, the scattering loss becomes the dominant factor in reducing the power transfer ratio at crossing angles greater than θ ∼ 55°. For example, the scattering loss at the junction can rapidly reach ca. 60% as the crossing angle approaches 90°, which significantly affects the coupling efficiency (K′), defined as the intensity of WG(2) divided by the sum of the intensities of WG(1) and WG(2) [K′ = IWG(2)/ (IWG(1) + IWG(2))]. At angles of ∼30°, approximately 85% of the light guided in WG(1) is transferred to WG(2), and ≤3% is lost at the junction. The low scattering losses at shallow crossing angles allows the junction intensity to be neglected in the K′ calculations without accruing an error in excess of the
between WGs, leading to significant optical modulation in the WG outputs when the WGs move apart by only a few nanometers.25,26 To better understand light propagation and the decay of the optical field residing near the surface of dielectric WGs, we have fabricated nanofiber junctions and studied the power transfer occurring between coupled WGs as a function of coupling angle, gap separation, guided wavelength, and chemical stimulus. The approach involves cross-coupling nanofiber WGs with shallow coupling angles of ∼30° to produce small overlapping areas of 4 nm steps) from zero gap to a ∼50 nm gap by adding additional PE bilayers between the coupled WGs. White light (400−700 nm) was launched into the junction by exciting one end of pumping WG with above band gap (>3.6 eV) light that generates broad defect emission in the SnO2 crystal.1 By monitoring light intensity at the junction and two WG end facets with various band-pass filters, coupling efficiencies were quantified as a function of wavelength. Numerical simulations were also carried out using three-dimensional finite-difference time-domain (FDTD) techniques to compare to the experimental results. To demonstrate that a cross-coupled nanofiber structure can modulate light in response to a chemical stimulus, a poly(dimethylsiloxane) (PDMS) matrix was deposited at the junction to induce a gap separation upon exposure to pentane vapor. The slight gap separation and modulation of light caused by the swelling polymer matrix is 1906
dx.doi.org/10.1021/nl2043024 | Nano Lett. 2012, 12, 1905−1911
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experimental error (≤2%). There was no gap separation between WG(1) and WG(2) for all the angle dependent data collected in Figure 1d. To minimize scattering losses at the junction for the gap separation experiments, all crossing angles (as measured from SEM and optical microscopy) were held fixed at 30 ± 2°. Keeping the cross-angle at a shallow angle of ∼30°, small overlapping areas of 1000 s) is also likely amplified by our chamber’s large dead volume of 16.6 × 10−3 L. Gas exchanges in volumes this large, compared to the extremely small sampling volume of the nanofiber junction (
