Transcript lecture 1
Professor Sharon Weiss Overview of course Capabilities of photonic crystals Applications MW 3:10 - 4:25 PM Featheringill 300 What is a photonic crystal? Structure for which refractive index is a periodic function in space 1-D photonic crystal 2-D photonic crystal 3-D photonic crystal z y x y x y What is a photonic crystal? Propagation of light over a particular wavelength range is forbidden (called photonic band gap – PBG) PBG 80 wa/2pc Reflectance (%) 100 60 40 20 0 1000 1400 1800 Wavelength (nm) PBG How do you make a photonic crystal? Lithography (top down) • Photoresist patterning • Exposure by electronbeam or stepper • Pattern transfer by reactive ion etch Chemistry (bottom up) • Wet chemistry (opals) • Molecular beam epitaxy or thin film deposition (multilayer films) How do you make a photonic crystal? Lithography (top down) Chemistry (bottom up) 1.5 mm Grüning et al., Appl. Phys. Lett. 68, 747(1996) Blanco et al., Nature 405, 437(2000) Emergence of the field Seminal papers (theory) First expt. PBG demonstration http://phys.lsu.edu/~jdowling/pbgbib.html Size scales Wavelength range of photonic band gap directly related to feature size of photonic crystal Refractive index periodicity Photonic band gap wavelength 1 millimeter THz 1 micron Mid IR 0.5 micron Near IR 0.1 micron Visible Preview: photonic crystal geometries and potential applications • • • • • • • • • • • Bragg mirrors Microcavities 1-D PBG waveguides Omnidirectional mirrors 2-D PBG waveguides Add/drop filters Lasers Superprism Fiber Artificial opals 3-D PBG woodpile structure http://www.physics.utoronto.ca/~john/ Bragg mirrors • Earliest example of photonic crystal • Initial applications include mirrors for VCSELs (vertical cavity surface emitting lasers) • Consists of alternating quarter wavelength optical thickness high and low refractive index materials Reflectance (%) 100 80 PBG 60 40 20 0 1000 1400 Wavelength (nm) 1800 Effect of Photonic Crystal Composition nL = 1.5 nH = 2.6 Stopband width increases as index ratio of nH/nL increases Reflectance nH = 2.4 nH = 2.2 nH = 2.0 700 900 1100 1300 Wavelength (nm) 1500 Omnidirectional Mirrors • Completely reflect light for all angles of incidence and all polarizations A. Bruyant et al., Appl. Phys. Lett. 82, 3227 (2003) Omniguide – Commercial Company • Light guided in air core of hollow tube • Confinement based on multilayer films that constitute the cladding Y. Fink et al., J. Lightwave Technology 17, 2039 (1999) http://www.omni-guide.com Microcavities • Defect layer breaks periodicity of dielectric function and introduces allowed mode into PBG Reflectance (%) 100 80 60 40 20 0 800 1000 1200 1400 1600 Wavelength (nm) 1800 2000 1-D Photonic Crystal Waveguides • Feature size of 100 nm achieved by x-ray lithography • Light guided near 1.5mm • Missing hole in center enables resonance wavelength • Changing length of defect tunes resonance wavelength J. S. Foresi et al., Nature 390, 143 (1997) 2-D PBG Waveguides • Silicon waveguides fabricated by a combination of lithography and electrochemistry 2 mm M. Loncar et al., Appl. Phys. Lett. 77, 1937 (2000) F. Muller et al., J. Porous Materials 7, 201 (2000) Fabrication of 2-D Photonic Crystal Oxidation Spin photoresist Lithography KOH etching Buffered HF Reactive ion etching (CF3 and O2) crystalline silicon oxide Electrochemical etching photoresist Add/Drop Filters • Theoretically investigated, preliminary experiments • Design of missing holes and enlarged holes allow for light to selectively exit waveguide H. Takano et al., Appl. Phys. Lett. 86, 241101 (2005) Y. Akahane et al., Appl. Phys. Lett. 82, 1341 (2003) Photonic Crystal Lasers • Incorporation of 2-D photonic crystal with light emitting semiconductor quantum well provides confinement and gain necessary for lasing O. Painter et al., Science 284, 1819 (1999) Superprism Effect • Light path shows a extremely wide swing with a slight change of incident light angle • Based on highly anisotropic dispersion by photonic band (negative refraction) T. Sato et al., Phys. Rev. B 58, R10096 (1998) Photonic Crystal Fiber • Light guided in air core instead of traditional high refractive index core • Allows for lower losses • 2-D PBG confines light in fiber • Currently 1.2dB/km (traditional fiber 0.15dB/km) R. F. Cregan et al., Science 285, 1537 (1999) P. J. Roberts et al., Opt. Express 13, 236 (2004) Artificial Opals • Chemical synthesis using chemical vapor deposition and wet etch to form air spheres surrounded by silicon shells • Complete photonic band gap observed in near-IR • Easier to achieve smaller dimensions with bottom-up technology 1.5 mm Blanco et al., Nature 405, 437(2000) Woodpile Structure: 3-D PBG • Extremely complicated high tech lithography used to achieve 3-D PBG – Series of deposition, patterning, etching, and planarization steps • Light confined in all three dimensions S. Y. Lin et al., Nature 394, 251 (1998) http://www.sandia.gov/mstc/technologies/photonics/gallery003.html