LETTERS PUBLISHED ONLINE: 20 APRIL 2014 | DOI: 10.1038/NMAT3927 Optical Fano resonance of an individual semiconductor nanostructure Pengyu Fan 1 *, Zongfu Yu 2 , Shanhui Fan 2 and Mark L. Brongersma 1 * Fano resonances with a characteristic asymmetric line shape can be observed in light scattering, transmission and reflec- tion spectra of resonant optical systems 1 . They result from interference between direct and indirect, resonance-assisted pathways. In the nanophotonics field, Fano eects have been observed in a wide variety of systems, including metal- lic nanoparticle assemblies 2 , metamaterials 2,3 and photonic crystals 4,5 . Their unique properties find extensive use in applications, including optical filtering, polarization selectors, sensing, lasers, modulators and nonlinear optics 6–11 . We report on the observation of a Fano resonance in a single semiconductor nanostructure, opening up opportunities for their use in active photonic devices. We also show that Fano- resonant semiconductor nanostructures aord the intriguing opportunity to simultaneously measure the far-field scattering response and the near-field energy storage by extracting photogenerated charge. Together they can provide a complete experimental characterization of this type of resonance. The advances in the field of plasmonics have made it possible to engineer the optical coupling in metallic nanoparticle assemblies to produce strong Fano resonances 12–14 . This is accomplished by spectrally placing a broad and a narrow scattering resonance of a system on top of each other. The coherent interference between the two scattering pathways produces a characteristic asymmetric Fano line shape 15 . Fano resonances obtained from subwavelength scatterers are of particular interest as an analysis of the line shape can reveal many insights into the coherent, near- field optical interactions in such nanoscale systems that typically remain hidden in a far-field light scattering experiment 16 . To obtain a complete picture behind the origin and nature of a Fano resonance, it is required to quantify the frequency-dependent energy storage that occurs in the resonant scattering pathway. The stored energy in metallic nanostructures is dissipated as heat and can be accessed only by challenging near-field optical techniques 17 or nonlinear generation 18 . In this work, we show that Fano resonances can be obtained in single semiconductor nanostructures. Moreover, we demonstrate that these structures facilitate a convenient simultaneous measurement of the far-field scattering and near-field energy storage to enable a complete experimental characterization of these nanophotonic resonators. Semiconductor nanowires support a series of optical resonances 19,20 that give rise to strong light scattering and absorption. These properties have been applied for structural colour 21 , light localization 22 , compact and efficient photodetection 23 , thermal emitters 24 , solar cells 25 and other new optoelectronic devices 26 . The resonant wavelengths in the nanowires can be tuned by changing their size 20 , cross-sectional shape 25 and environment 21 . Figure 1a illustrates how a one-dimensional semiconductor nanostructure affords simultaneous measurement of its scattering and absorption resonances. When the structure is illuminated from the top, the scattered light can be analysed in bright- and dark-field configurations in an optical microscope. With electrical contacts at the two terminations, the structure is turned into a metal–semiconductor–metal (MSM) photodetector whose photocurrent provides a direct measure of the light absorption in the semiconductor region. The light absorption in the wire with a volume V at an illumination angular frequency ω is in turn linked to the optical energy storage in the nanowire as: (A = R ω · Im (ε) ·|E | 2 · dV ). Here, Im(ε) is the imaginary part of the dielectric constant that determines the intrinsic light absorption properties of the semiconductor and E is the electric field of the light. Figure 1b shows a scanning electron microscope (SEM) image of a 220-nm-wide and 50-nm-high polycrystalline silicon (Si) nanostripe photodetector generated by photolithographic means (see Methods). We analyse the Fano resonance produced by this device. The observation of a Fano resonance in a subwavelength semiconductor nanostructure has thus far been elusive. This can be explained by the fact that most light scattering studies have focused on spherical and cylindrical nanorod geometries for which the observation of such resonances is challenging 2 . We start by providing intuition why reshaping a semiconducting cylinder into a judiciously shaped stripe can result in the observation of a clear Fano resonance in the total scattering efficiency Q TS . This quantity is defined as the ratio of the total scattering cross-section and the geometrical cross-section, taken as the width of the stripe. We will then also show that this enables experimental observation of a Fano resonance in a differential, backscattering efficiency Q BS measured in our experiments 19 . From Mie theory, it is known that the spectrum of the total scattering efficiency of a cylinder shows a series of resonances related to the excitation of orthogonal modes with different angular momenta, termed the dipolar, quadrupolar and so on, channels 19 . Owing to the mode-orthogonality, the spectra of the total scattering efficiency do not show any effects of inter-channel interference between the distinct angular-momentum scattering channels. As a result, the spectrum of Q TS can be obtained by linearly summing the contributions from all of the relevant scattering channels. For completeness, it is worth mentioning that inter-channel interference effects can be observed in the differential back- and forward-scattering spectra 2 . For our work, it is important to realize that each angular momentum channel supports a resonant and non-resonant pathway and a Fano resonance can thus, at least in principle, occur in the spectrum of Q TS as a result of intra-channel interference. 1 Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, USA, 2 Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA. *e-mail: pengyu.fan@gmail.com; brongersma@stanford.edu NATURE MATERIALS | VOL 13 | MAY 2014 | www.nature.com/naturematerials 471 © 2014 Macmillan Publishers Limited. All rights reserved