PHYSICAL REVIEW B 87, 155314 (2013) Imaging of high- Q cavity optical modes by electron energy-loss microscopy N. Le Thomas, 1,*,† D. T. L. Alexander, 2 M. Cantoni, 2 W. Sigle, 3 R. Houdr´ e, 1 and C. H´ ebert 2,4 1 Institut de Physique de la Mati` ere Condens´ ee, ´ Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), Station 3, CH-1015 Lausanne, Switzerland 2 Centre Interdisciplinaire de Microscopie ´ Electronique (CIME), ´ Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), Station 12, CH-1015 Lausanne, Switzerland 3 Max Planck Institute for Intelligent Systems, D-70569 Stuttgart, Germany 4 Laboratoire de Spectrom´ etrie et Microscopie ´ Electronique (LSME), ´ Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), Station 12, CH-1015 Lausanne, Switzerland (Received 24 January 2013; published 29 April 2013) We show a technique that images the intensity distribution and local state of polarization of the optical field of high-quality factor optical modes confined in dielectric planar photonic crystal nanocavities. Based on energy-loss spectroscopy of swift electrons, the technique gives a spatial resolution improved by a factor of 30 compared to the optical diffraction limit. Moreover, because the energy loss is induced by coupling of the moving charges with the local density of states of the dielectric cavity, it is sensitive to the entire volume of the confined electric field, not just its evanescent contributions. This three-dimensional sensitivity paves the way for a highly resolved tomography of confined modes in dielectric photonic nanostructures. DOI: 10.1103/PhysRevB.87.155314 PACS number(s): 79.20.Uv, 42.70.Qs, 42.82.Et I. INTRODUCTION Tiny confined optical modes of high-quality Q factor are fundamental building blocks in the field of photonics. Among other properties, their capability to enhance light- matter interaction is important for the development of ultrahigh sensitive sensors and quantum optical devices. 1 Imaging the electromagnetic field of such modes is intrinsically difficult as the main part of the information about the field is stored in evanescent contributions. Although techniques such as near-field scanning optical microscopy (SNOM) are able to investigate these evanescent contributions 2 they are restricted to surface imaging. Here, we demonstrate the imaging of high- Q modes confined in dielectric planar photonic crystal (PhC) nanocavities using high-energy resolution electron energy-loss spectroscopy. We retrieve spatially resolved information about the intensity distribution and the local state of polarization of the optical field at the dense core of the cavity, paving the way for a highly resolved tomography of photonic nanocavities. Electron energy-loss spectroscopy (EELS) acquired dur- ing scanning transmission electron microscopy (STEM) can obtain the electron energy-loss distribution with a sub-nm or even atomic spatial resolution, depending on the associated physical loss mechanism. Such a technique has recently been intensively used in the low energy-loss part of the spectrum for investigating electromagnetic modes confined in plasmonic nanostructures. 3,4 The coupling of the moving charge with the local optical density of states induces the electron energy loss, which is known as the Vavilov-Cherenkov (VC) effect in a homogeneous and transparent dielectric medium. 5 Imaging of local fields with the electron beam (e-beam) in metallic 6,7 and dielectric 8 structures has been demonstrated with a spatial res- olution of a few nm, surpassing by several order of magnitude the sub-μm diffraction limit intrinsic to conventional optical far-field techniques. In contrast to the previous studies, here we investigate a high-Q point defect mode in a subwavelength cavity. A particular difference is that we probe the mode inside the solid matrix as opposed to interpreting EELS signal only in the PhC holes. 8 Moreover, we show that the EELS signal is not subject to spurious residual luminescence in contrast to cathodoluminescence. 9 II. OPTICAL CAVITY PROBED WITH EELS Among the different types of subwavelength cavities, planar dielectric photonic crystal cavities defined in membranes of subwavelength scale thickness combine the advantage of a large Q factor with a strong confinement of the electromagnetic mode in a volume smaller than the wavelength cube. 10,11 We focus on a so-called L3 photonic crystal cavity that is defined by a line defect of three missing holes inside a triangular lattice of holes (lattice constant a = 460 nm). The three first holes located on both sides of the line defect have been slightly laterally shifted away from the core by a length of (0.17a,0.07a,0.05a) to optimize the Q factor. As shown in Fig. 1, the pattern of holes is etched in a 220 nm thick silicon membrane that is sufficiently thin to permit the transmission of the 200 keV e-beam. The lithographically prepared membrane was cut out of its substrate by using a focused ion beam, and then rendered as a TEM lamella by attaching it to a TEM grid using ion-beam-deposited amorphous carbon. The structure was patterned with deep UV lithography on a silicon on insulator (SOI) wafer and designed to operate at a wavelength around 1.5 μm (filling factor f = 35%). Light coupling into the cavity mode is carried out via a side-coupled photonic crystal waveguide located four lines of holes away from the cavity as explained in Ref. 12. The optical scattering spectrum of the cavity reveals a fundamental mode with a quality factor of 17900 after wet etching of the buried 2 μm thick silicon oxide insulator [see Fig. 1(d)]. The theoretical amplitude distribution of the electric field of the fundamental transverse electric (TE) cavity mode [Fig. 1(b)] exhibits a symmetric pattern of several maxima, whose corresponding local vector fields [Fig. 1(c)] are oriented along the y axis for the ones in the middle of the line defect and along the x axis 155314-1 1098-0121/2013/87(15)/155314(7) ©2013 American Physical Society