Photonic Crystals Sensors

Transcription

1 Photonic Crystals Sensors Christopher J. Summers, Curtis W. Neff, Tsuyoshi Yamashita & C. P. Wong School of Materials Science and Engineering Georgia Institute of Technology Atlanta, Georgia th East Asia Conference on Chemical Sensors Guilin, China 6 9 th November, 2005

2 Outline/Motivation Review of Photonic Crystal Sensors 3D Opals 2D Photonic Crystal Waveguides Recent Studies of Photonic Crystal Structures Virtual waveguide photonic crystal - based on square lattice Photonic Crystal Fabry- Perot Etalons High refractive and dispersive photonic crystals Superlattices triangular lattice based Application of New PC Structures to Sensing Technologies Virtual waveguide devices to enhance interaction with analyte Non-Linear Superlattice Structures to enable spectral tuning Liquid Crystal Infiltration of 2D PC Non-linear & Electro-Optical (EO) materials

3 Background-Types of PC Sensors 3D Opals: Observe change in refractive index and/or sphere size from Bragg diffraction position --- when analyte attached or absorbed 2D Photonic Crystal Fiber Sensors Ideal for Gasses: NO x & Nerve gases: Use long pore lengths for index change 2D Photonic Crystal Waveguide Sensors Two-channel & Linear defect waveguides absorption & index change by incorporating fluid or gas in line-defect of holes IR (1550 nm) & terahertz region: Topol ancik et al. APL 82, 1143 (2003) Kurt & Citrin, APL 87, (2005) Nanocavity PC Laser Chem Detector: M. Loncar at al. APL 82, 4648 (2003) 2D PC Microcavity Sensor: E. Chow at al. Optics Lett. 29, 1093, nm, Observed sensitivity of n = 0.002: Sensing area 10 µm 2, requires only 1 fl of sample analyte. H z E x E y TE polarization

4 PC Sensors Issues Most detection schemes -- based on surface functionalization Observe change in refractive index when analyte attached/incorporated Identification dependent on accuracy of fictionalization Quantitative determinations dependent on induced index change: n~ No Direct identification of analyte leads to large positive false errors Need higher sensitivity and ability to analyze smaller samples Low divergence waveguides Need direct observation/identification in contrast to observing change in refractive index when analyte attached/incorporated High dispersion structures

5 PC Sensors Types - Advantages Advantages of 2D Photonic Crystal Sensors Compactness Sampling areas down to nl femtomoles Enhanced sampling interactions lengths slow light Design for E-field enhancement in holes Ultra fast real time detection 2D PC Spectroscopic Systems Potentially Offer: Unambiguous identification of analyte Dynamical tunability modulation/derivative spectroscopy Require high resolution, high signal-to-noise Volume sampling large number of holes

6 Virtual Waveguide Interconnect System Advantages of free-space optics No coupling Intersections allowed Broadband operation-30% BW Advantages of integrated optics Confined beams No hermetic packaging One lithography step Disadvantages/Advantage? Small feature sizes required (beam size ~15a) Compared to 2-3a for line defect PC waveguides PC mirrors have lower PBG 1555nm 9.97µm 1550nm 8.55µm 1545nm 7.12µm Collimation exploits same phenomena as sub-wavelength focusing

7 Photonic Band Gap Mirrors

8 Beam Properties Divergence Angle of 2D Gaussian Beam A Gaussian beam spreads in the paraxial approximation in an isotropic material as: θ k λ x = = 2 k πω 0 z 2ω 0 x θ k λ = x = 2 k πω z 0 z The divergence determines the coupling efficiency into the photonic crystal Thus: θ decreases with increasing beam width/wavelength ratio

9 Analysis of 2D Photonic Crystals Square Lattice nk 0 ( ω) Χ Μ Region of interest Band Gap Γ Χ Square lattice Hole diameter Refractive index Band Diagram Boundaries of the band surface Identification of band gaps Allowed Wave Vector Curve Equifrequency curves of the band surface Identification of propagation effects

10 Dispersion Curve Analysis of Square Lattice PC Isotropic Media 4/w 0 Photonic Crystal Normal Propagation Collimated Beam PC lattice designed to produce dispersion contour with a wide range of curvatures Concave defocusing Straight collimated beam: guiding Convex negative index, subwavelength focusing Negative Index Effect 2π/a ε µ z z = = ε µ z z = = negative negative Canceling of Z-component leads to self-collimation Effective negative index for the energy propagation obtained

11 FDTD Simulation of Self-Collimated Beam FDTD simulation of self-collimation - ω n = λ = 1.55mm Clear intensity confinement in photonic crystal ~25x longer propagation possible than in air No discernable beam spread for 120µm of propagation of a 8.5µm wide beam Beam spread decreased by an order of magnitude or more with beam sizes as small as 5-10 λ 0 Air/Dielectric media Photonic Crystal Photonic crystals can be designed with no beam spread at a particular frequency Applications include: Virtual waveguide interconnect system Miniaturization of conventional optical components for small beams

12 Test Structure Design for Collimation Output Waveguides Input Waveguide Photonic Crystal Fiber Small spacer Cleaved surface Principle of operation Gaussian like input from input waveguide Beam spread observable from number of lit output waveguides Region measured Quality requirement Smooth surfaces (<L s /20) Anisotropic sidewalls (<5 ) Uniform hole sizes in photonic crystal (<5% locally) Large area ~ 150 µm 2

13 Direct Top-View Measurements Photonic Crystal Input from the fiber Propagation thru test photonic crystal Output from the waveguides Infrared camera utilized to view scattered light from the device

14 Test Structures of Virtual Waveguide Input waveguide, photonic crystal, fan of waveguides for analysis Examples of photonic crystal fabrication.

15 Measurements of Virtual Waveguide Effect Test structure with no photonic crystal: Approximately 8 WGs lit up Test structure with virtual waveguide photonic crystal Only central WG lit up Χ Thus, beam collimation!! - As required for sensor applications - long path lengths and wide beam

16 Fabry-Perot Filters Incident beam Multiple reflections Multiple reflection and interference results in sharp transmission peaks Small beams spread inside of the filter resulting in lost intensity and spreading of the beam

17 Applications: Fabry-Perot Interferometer Beam spread degrades performance Beam size Intensity leaks backward (<16% center intensity) Photonic crystal confines beam >98% transmitted center intensity for mode near self-collimation Can control bandwidth for selecting number and intensity of transmitted beams Concept extends to other interferometers and bulk optical devices For sensor devices -- multiple holes in WG structure for sampling Photonic crystal No photonic crystal Transmitted intensity at center of beam

18 Advantages of Virtual Waveguide and FP-Interferometer Provide a wide waveguide, ~8.5 µm and extended path length possible by using multiple reflections --- also slow light propagation Can use integrated laser/led source with similar detector design FR-resonator structure can be used to enhance interaction FP-resonator tuning structure will allow high resolution wavelength scanning over restricted range. Make 2D PC structures of EO material to enhance wavelength tuning Theoretical and experimental techniques demonstrated

19 Propagation Effects in Superlattice PCs Giant refraction Superprism Tunable refraction

20 Two Dimensional PC: Triangular Lattice Simpler structure than 3D Top-down fabrication Integration with planar circuits Simpler analysis of optical properties than 3D Can have full PBG (light in plane of PC) Giant refraction effects Superprism effects H z E y E x TE polarization y Real Space x Band diagram: Plot of dispersion relationship, ω(k), along irreducible BZ boundary Reciprocal Space Band Diagram

21 Superlattice: Real & Reciprocal Space a 1 b 1 row i row j a a 2 b 2 Υ Γ Μ Χ Real Space Reciprocal Space Alternating rows posses different property ( r, n, or both) Unit cell definition with two holes per lattice point New BZ representation: hexagonal becomes rectangular BZ folding Symmetry reduction: six-fold to twofold

22 Dispersion Diagram Energy Dependence ZnS/Air Triangular Lattice M K Frequency (Energy) M K E = K Wave vector (ka/2π) Dispersion diagram: constant energy contour in 1 st BZ Homogeneous medium circle Γ M K E = 0.25 Photonic crystal complex in shape Propagation of light normal to the dispersion curve, in direction of energy flow

23 Wave Vector Diagram at PC Interface of a Triangular Lattice k 0 n 1 n 1 Interface Photonic Crystal

24 Static Superlattice Photonic Crystal Superlattice Strength: r 2 /r 1 Superlattice: hole radii, r 1 & r 2, in adjacent rows [i, j], respectively, Lattice vector a Increasing superlattice strength accomplished by increasing n or r between rows Strength of superlattice defined as: extra dielectric added when r 2 made smaller, r 2 /r 1 ratio In Si, for r 2 /r 1 =0.857, n eff =1.654 which is n=0.654 between rows of holes To increase the strength of the superlattice. The radius of the columns of row j is decreased down to r 2 =0.15a while r 1 is kept constant

25 Effect of SL Strength (r 2 /r 1 ) on Band Structure ( r) TE polarization Decreasing r 2 increases dielectric material in structure Stronger effect on air bands than dielectric bands Shifts bands to lower frequencies Decreases width of PBG Increases band splitting Similar effect in dynamic superlattice when changing n Evolution of a static superlattice band structure with radius ratio (a) r 2 /r 1 = 1, (b) r 2 /r 1 = 0.857, (c) r 2 /r 1 = 0.571

26 Energy Density of Static Superlattice lattice Μ point 2D PC-SL with strength a & b --- Degenerate states at bottom of air band at M-point -lattice c & d --- 3s and 3p states of the 2D PC-SL with strength 0.571

27 Dispersion Contours: Dependence on r 2 /r 1 Υ Μ Υ Μ r 2 /r 1 =1.0 ω n = r 2 /r 1 =0.571 ω n = Γ Χ Γ Χ Υ Μ Υ Μ For r = 0, (r 2 /r 1 =1), BZ folding scheme straight forward: curves converge to a single point at BZ boundaries. Radius modulation (r 2 /r 1 <1): curves diverge/repel at BZ boundaries Net result: relatively flat curvature in center of BZ high curvature near BZ boundaries high dispersion

28 Fabrication E-beam lithography ICP dry etching with Chlorine/C 4 F 6 recipe 1 mm 2 area written using smaller unit patterns Lattice constant: a=358 nm Silicon slab waveguide (SWG) ~300 nm 1 µm Si SiO 2 Si Triangular Lattice SL Lattice

29 Superlattice: Measured & Calculated Bands Measured bands FDTD calculated bands X M Dips in spectrum filtered and plotted as ω vs. k Full 3D FDTD calculations to match structure 400 nm Γ Y

30 Tunable Structures Concept: Dynamically change lattice property, i.e. refractive index, while under excitation Consequences: Active beam steering Tunable filtering Signal modulation

31 Superlattice Photonic Crystal Structures Based on Triangular Lattice Triangular Lattice Dynamic SL Dynamic Hybrid Dynamic : Row addressing scheme to modulate n (Park et al., PECS IV 2002) Static : Modulation in hole radius Static SL Static E/O SL allows tunability of optical properties (Neff et al., SPIE 2004) Dynamic liquid infiltration of holes Static holes of different diameter Hybrid combination of both structures Static Infiltrated Static E/O

32 Biased Unbiased Tunable Device Structures Static E/O Superlattice Static Infiltrate Superlattice Dynamic Hybrid Superlattice Static Infiltrate Superlattice biased/unbiased states

33 Band Structure and Dispersion Large Area Addressed Static Infiltrated Superlattice Band Structure Dispersion Contours Refraction Biased ε = 2.25 Unbiased ε = 2.89 Changing biased/unbiased state changes alignment of LC director changes ε Changes band structure Changes dispersion contours Changes refraction response

34 Refraction/Dispersion Properties of SSL PCs Static superlattice band structure with change in radius ratio: r 2 /r 1 = Clockwise rotation for T M defined as positive angle For: θ i = 0, θ r is positive ~ 10 o - Refraction at normal incidence For: θ i = 0 to 10 o θ r is positive For: θ i = negative, 0 to ~10 o θ r is positive - Negative refraction For: θ i = ~ - 10 to -40 o θ r = negative ranging from 0 to - 60 o - Positive refraction SSL exhibits high and tunable dispersion properties spectrometer!!

35 Summary Virtual Waveguide Structure Reduces beam divergence by over a factor of 10 Enhanced interaction lengths, larger sampling volumes Fabry-Perot Interferometer Increased interaction by multiple passes Tuning of spectral sensitivity with etalon ~ 30% 2D Superlattice PCs offer high refraction & dispersion Beam steering & Spectral scanning Direct identification of analyte Incorporation of Nonlinear or EO materials for tunability 2D PC offer new routes to Chemical & Biological Sensing NIR to Terahertz spectral range Compact, fast response times Issues: Source/detector developments, resolution/beam width

36 Acknowledgements Research Group Graduate Students Davy Gaillot Xudong Wang Swati Jain Postdoctoral Researcher Dr. Elton Graugnard Dr. Jeff King Collaborations with ARL: Drs. D. Morton, E. Forsythe &S. Blomquist U.S. Army Research Office - MURI Contract# DAAA

37 Thank You!!