Modeling and optimization of high index contrast gratings with aperiodic topologies SPIE 2013
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Transcript of Modeling and optimization of high index contrast gratings with aperiodic topologies SPIE 2013
Modeling and optimization of high index contrast gratings with aperiodic topologies
SPIE Optical Metrology 2013Modeling Aspects in Optical Metrology IV
Milan MaksimovicFocal‐Vision & Optics
Oldenzaal, The Netherlands
Modeling Aspects in Optical Metrology IV, SPIE Optical Metrology, Munich, Germany, 13‐14. May, 2013
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Outline
• Introduction– What is High Contrast Grating ?
• Modeling methods (brief outline)– Standard rigorous and selected approximate methods
• Motivation and central question:– Tailoring spectral response of HCG via optimization– How to generate complex (aperiodic) unit cell topology ?
• Selected examples obtained by numerical simulations:– Spectral response under different complex unit cell topologies
• Concluding remarks and (some) open questions
Introduction
High‐Contrast Grating: high index (bars) medium grating fully immersed in low index medium
Periodic sub‐wavelength structure: period smaller than the wavelength of the incident wave
Applications:• Ultrathin broadband reflectors for VCSEL (replacement for DBR reflectors)• High‐Q resonator mirrors• Resonant polarization sensitive filters and beam splitters• Planar flat focusing mirrors (wavefront shaping)• Hollow core waveguides (ultracompact optical coupler and filters)• ….
• Superstrate and substrate are simple low index media (air)• Refractive index is real and frequency independent • Only zero‐th diffraction order is propagating
Recent review ref.: Adv. Opt. Photon. 4, 379‐440 (2012).
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High‐Contrast Gratings: modeling usingRCWA/FMM
• RCWA/FMM– All quantities expanded in Fourier series,..
• Other rigorous numerical methods– FEM– FDTD– …
2( ) 2
1, ,TE TM zx z x zk
L
2 22
1TE x n x
k L
2 2
2 21 1
TM x xn x n xk n x
L
Decomposition:TE (with Ey ,Hx,Hz ) & TM (with Hy ,Ex,Ez ) ‐>leading non‐null components Φ(x,z) ‐ either Ey or Hy
( , ) ( ) ikzx z x e ( ) ( ) ikx x e 2( ) ( ) ( )TE TM x r x L
Outside the grating filed is expanded in finite number of propagating and evanescent waves!
Eigenvalue problem, pseudo‐periodicity,… sin
Grating Parameters:Λ ‐ period η ‐the dutytg ‐ grating heightn(x) ‐ refractive index
Plane Wave Excitation: Θ‐incidence angle k0=2π/λ – wave number
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High‐Contrast Gratings:modeling theories/methods
• Coupled (Bloch) Mode Theory: – Periodic waveguide‐array modes as an expansion basis– Several modes are excited in the wavelength range of interest– Coupling of waveguide‐array modes at interfaces– Dual‐mode regime responsible for most of novel phenomena:
(destructive/constructive ) interference between modes
• Effective Medium Theory:– Assumes single propagating mode: does not capture resonant phenomena in the intermediate regime!
• HCG as an Photonic Crystal– photonic bands originate from the “resonance” of the HCG supermodes,…
• HCG as an open resonant system – response in terms of resonant excitation of “leaky” modes
Scattering matrix:
Resonance representation of the scattering matrix (complex poles /zeros) decomposition
Spectral features =Non‐resonant background + Pole/zero resonance contribution
P. Lalanne, et. al. , J. LightwaveTechnol. 24, 2442 (2006).
V. Karagodsky , et al. Opt. Express 20, 10888‐10895 (2012).…..
Adv. Opt. Photon. 4, 379‐440 (2012)
High‐Contrast Gratings:simple unit cell
HCG with the: period Λ=0.716μm, duty cycle η=0.7,height tg=140nm, ng=3.48,n0=1;
HCG with the: period Λ=0.140μm,duty cycle η=0.3548, height tg=1494nm, ng=3.2,n0=1;
Adv. Opt. Photon. 4, 379‐440 (2012).
HCG with spectral response optimized for broadband high reflectivity or high‐Q resonances
Broadband reflectivityHigh‐Q resonances
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HCG exhibiting high‐Q resonances:simple unit cell
Deep subwavelength regime
Near-wavelength regime
Diffraction regime
Interference of small number of waveguide‐array modes responsible for reflectivity pattern in “near‐wavelength” regime!
HCG with the: period Λ=0.716μm, duty cycle η=0.7, height tg=1.494μm, ng=3.48,n0=1; TE‐polarization
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Spectral response tailoring:how to generate unit cell topology?
Spectral response : resonances positions, number and shape, etc.
Initial design : one that already possesses desired (spectral ) features → reduction of computational burden in design and optimization tasks!
Fundamental grating parameters for optimization of spectral response (simple unit cell)
• period, thickness, duty cycle
Extended feature/parameter set (complex unit cell)• transition points in the unit cell• symmetry of the unit cell
How to generate initial unit cell topology?• Perturbations (“defects”) of the simple unit cell• Randomly generated transition points• Deterministic aperiodic sequence
OPTIMIZATION OF SPECTRAL RESPONSE
1. Initial parameters:excitation + structure
2. Computation: 1. Spectral Response2. Merit Function + Constraints
3. Stop or Modify parameters and repeat 2.4. Final Design
OPTIMIZATION (2 step approach)
1. Optimize structure with the simple unit cell2. Generate new complex unit cell topology
(global parameters fixed)3. Re‐optimize spectral response with
complex unit cell topology4. Final Design
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HCG with the unit cell topology derived from perturbations
Refractive index profile for:a) basic unit cell; b)‐ c) symmetrically perturbed unit cell; d) asymmetrically perturbed unit cell
1) Start with the simple unit cell (defined transition points)2) Generate new transition points by the “cut and split” procedure3) Transition points shift: Δx=pηΛ=pLH4) Parameter p controls the strength of perturbation
xxxxxxxxxxxx cp
cppp 43
022
011 ,,,
xxxxxxxxxxxxx pppp 2/,2/,, 01
026,5
013
022
011
Hc Lpxxpxxxx 01
02
02
01 ,2/
01x
02x
1px
cx
2px3
px 4px
ηΛ=LH
Λ
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HCG exhibiting broadband high reflectivity :symmetric perturbation of the unit cell
Example HCG optimized for broadband reflectivitySymmetric perturbations :• Spectral shape does not change (no
resonances)! • Bandwidth and central wavelength shift
Unit cell perturbations by “cut and split” procedure
HCG with the: period Λ=0.620 μm, duty cycle η=0.3548, height tg=0.140 μm, ng=3.2,n0=1; TE‐polarization
HCG exhibiting broadband high reflectivity :field localization
|Ey| |Hy| @ resonance
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HCG resonances with the symmetric and asymmetric perturbations of the unit cell
Transition points shift: Δx=pηΛ
a) p=0.025b) P= 0.05c) p= 0.075 d) p=0.1
HCG with the: period Λ=0.620 μm, duty cycle η=0.3548, height tg=0.140 μm, ng=3.2,n0=1; TE‐polarization
a)
b)
c)
d)
• Symmetry breaking ‐> spectral resonances! • Excitation of previously non‐excited modes…
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HCG exhibiting broadband high reflectivity :resonances under the oblique incidence
• Resonances (spectral reflectivity “anomalies”) appear under the oblique incidence!• Higher diffraction order(s) become(s) propagating under the oblique incidence!
HCG with the: period Λ=0.620 μm, duty cycle η=0.3548, height tg=0.140 μm, ng=3.2,n0=1; TE‐polarization
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HCG reflectivity: the symmetric and asymmetric perturbations of the unit cell
Spectral reflectivity : periodic structure (left), symmetrically perturbed structure with parameter p=0.1 (middle) and asymmetrically perturbed structure with the parameter p=0.1 (right)
Symmetry breaking ‐> split in the parameter space!
HCG reflectivity: randomly generated(asymmetric) transition points within unit cell
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• Transition points in the unit cell generated from uniform distribution in the interval (0,1)• Total thickness constrained to be within 1% difference from the initial unit cell
Resonances interaction ‐> crossing and anti‐crossing signature!
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HCG exhibiting high‐Q resonances:symmetrically perturbed unit cell
Reflectivity for a symmetrically perturbed structure with p=0.1 (solid) and unperturbed structure (dashed)
Filed localization: “hot spots”in the free space between grating bars!
Resonant line shape: Lorentzian ↔ Fano type!Unit cell symmetry preserved under perturbation!
HCG with the: period Λ=0.716μm, duty cycle η=0.7, height tg=1.494μm, ng=3.48,n0=1; TE‐polarization
|Ey| @ resonance
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HCG exhibiting high‐Q resonances:asymmetrically perturbed unit cell
Original structure operates in the “deep” subwavelength regime!
Symmetry breaking in the unit cell enables excitation of resonances!
|Ey| close to resonances 2.6966, 2.9337, 3.107 respectively.
Spectral reflectivity for periodic (dashed) and asymmetrically perturbed structure (solid) Angular dependence at resonance!
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a) basic periodic structure b) scaled structure according to 3rd generation of Thue-Morse sequence c) scaled structure according to “deformed” 4th generation of Thue-Morse sequence
HCG unit cell transition points deterministically generated from aperiodic sequence
Photonic aperiodic superlattices ( photonic quasi‐crystals) offer rich variety of complex patterns…
Aperiodic sequence of letters over finite alphabet:• Associate letters of the sequence with the high‐ or low‐ index material (H) and (L)• Example: Thue‐Morse (model aperiodic) sequence formed by substitutions L→LH and H → HL
• Several first generations ( initiator L): L, LH, LHHL, LHHLHLLH,….Generate transition points by re‐scaling simple unit cell (initial) structure:
1. Start with initial simple unit cell: η‐duty cycle, Λ –period 2. Select generation (n) with the NH=NL=2n (number of letters in the sequence) 3. Scaled (normalized) thicknesses: lH=η/NH and lL=(1‐η)/NH4. Generate transition points according to chosen sequence (repeated letters interpreted as double thicknesses)
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HCG reflectivity:3rd generation Thue‐Morse sequence based unit cell
• Additional resonances not present in the simple unit cell HCG• Localization of field (“hot spots”) outside the grating bars• Resonances moved into the (previously) deep subwavelength
domain
HCG with the: period Λ=0.716μm, duty cycle η=0.7, height tg=1.494μm, ng=3.48,n0=1; TE‐polarization
HCG reflectivity for “perturbed” 3rd generation Thue‐Morse sequence based unit cell
HCG with the: period Λ=0.620 μm, duty cycle η=0.3548, height tg=0.140 μm, ng=3.2,n0=1; TM‐polarization
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• Hierarchical fragmentation of the spectral response• Increasing the number of resonances in the given spectral range• Unit cell symmetry switches between subsequent generations
• What is the largest generation number accessible subject to fabrication resolution and tolerances?
HCG reflectivity for 3rd & “deformed” 4th generation Thue‐Morse sequence based unit cell
• Regular aperiodic structure ‐> generated according to deterministic aperiodic sequence• Deformed aperiodic structure ‐> re‐shuffled regular strucure
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Concluding remarks / Questions
We demonstrated on selected examples that:• HCG unit cell topology influences strongly spectral response and field distributions at resonances• Symmetry of the unit cell (perturbations) plays major role in shaping spectral response under normal and oblique
incidence (number and position of resonances)• Spectral resonance number, position, line shape and field localization may be controlled with complex unit cells:
– Lorentzian shape transforms into the Fano‐type resonance shape and vice versa under “perturbations”– Resonances introduced into the (previously) deep subwavelength region– Formation of the “hot‐spots “ in the free space
• Unit cell designed using deterministic aperiodic sequences may give rise to a highly fragmented spectral response displaying hierarchical structure
• Designed complex unit cell topology as a the efficient initial topology prior to global optimization procedure
Open questions:
• Is there an optimal choice of initial unit cell topology ?• What is the best choice of modeling method ?• Existence and stability of resonances under fabrication constrains, resolution and tolerances ?• Computational model, conditions and constraints for (robust) topology optimization ?• What is the influence of finite number of periods, material dispersion and absorption on HCG design?• …..
Milan [email protected]
SPIE Optical Metrology 2013Modeling Aspects in Optical Metrology
Modeling and optimization of high index contrast gratingswith aperiodic topologies