User Guide for WOLFSIM
Contents
1. the tables should be 200 x 1000 Remember the width of the custom structure must be same as the system period In the current version of WOLFSIM only one custom structure can be added Still there is no limit of a combination with multiple built in structures 5 4 Iterative Simulation The new WOLFSIM provides options for simulations with variable parameters such as periods and incident angles When the option for multiple simulations in the Iteration tab two different options will appear to set the minimum and maximum values for the simulation variable and the step size between simulations as well The current version includes this option for period and incident angle but we are welcome to get your suggestions for additional options 5 5 Output Options Simulation results can be stored by selecting options in the Output tab One of the main change in the new WOLFSIM is the output data format in HDF5 HDF5 is one of the most advanced scientific data format To access the data you can use another software called HDF View or you can import the HDF5 files in Matlab or many other softwares such as Maple or Mathematica More information can be found in another documentation Installation Guide r About Settings Model Definition Iteration Output Import Export rIterative Simulation Run Multiple Simulations vi f9 Variation of Period Minimum Grating Period um 2 Variation of Incident Angle Max2. Layer Figure 4 Structure Modeling and Preview You can also define arbitrary structure using the Custom structure First you need to define the thickness of the structure Then you can specify the directory containing material parameters complex refractive indices and Euler angles Total of nine data files in the text format txt are required to define the custom structure Each of data files contains information of material properties at every grid point in the area where the custom structure is placed The permittivity tensor at a grid point x y i j can be defined by three complex refractive indices n ix and three Euler angles a 3 7 5 i Exg i j Ery i j xz i 7 LS egsa wua alii T oq Pay Yan O emp 0 T o 5 Bas Ye zz i j zy i j zz i j 0 0 3 4 3 2 where T is the transformation matrix given by cos a cos 5 cos y sina sin y sinacos cosy cosasiny sin p cos y T cosacos O siny sinacosy sinqa cos siny cosacosy sinfsiny E 3 cos asin 5 sin o sin 5 cos O where 15 3 1123 Rios n1 23 and 123 are real and imaginary parts of the complex refractive in dices respectively corresponding to the principal axes Data for ni 2 3 412 3 and Euler angles a 3 7 should be provided from external data files named n1 txt n2 txt n3 txt k1 txt k2 txt k3 txt
3. alpha txt beta txt and gamma txt in the specified directory The optical conductivities 012 3 are calculated from the values of k1 23 by the relationship 01 23 47 of n12 3K1 2 3 F pm I7 a Add Laye eoe Open Type Custom Structure L WOLFSIM 2 0 0 Gi Date Modified i Custom Thursday November 15 2007 6 25 PM Layer Parameters Thickness um 5 Definition Directory Browse Add Layer Cancel g Library Tuesday October 30 2007 11 39 PM For lossy dielectrics dielectric constant epsilon n 2 k 2 must be same or greater than 1 3 Result Thursday November 15 2007 6 42 PM Li A WOLFSIM GUI Thursday November 15 2007 9 31 PM v File Format Text files HJ Cancel Choose Figure 5 Custom Structure We should note that for lossy dielectrics with anisotropy in absorption the WOLFSIM can model the media only when the orientation of anisotropy lies in one of the major axes x y or z Therefore the Euler angles must be zero when amp 4 3 are nonzero In addition the WOLFSIM can model only dielectric media whose dielectric constant 1 2 3 is same as or greater than 1 0 so that n12 3 gt K1 2 3 should be satisfied One can generate these tables using Excel or Matlab The dimension of these tables must be matched to that of the grid space For example if the structure is 2um thick and 10um wide and the grid spacing Au 10nm
4. to end the simulation manually 0606 WOLFSIM Input File Generator About Settings Model Definition Iteration Output Import Export Simulation Spatial Grid Resolution 20 Wavelength Grid spacing Auto Terminate Y Angle of Incidence 0 degrees Period of System 10 um Polarization Angles yw 0 degrees x 10 degrees Refractive Index Incoming Media 1 0 Outgoing Media 1 0 Source Parameters Y Select Source Type Monochromatic Wave Gaussian Pulse Figure 2 Simulation Settings The WOLFSIM employs the split field method which is one of the field transform techniques for periodic boundaries at oblique angles The angle of incidence can be defined in this tab The computation space is terminated by applying PBC and PML absorbing boundary conditions The system width can be determined from the common periodicity of whole structures to be modeled Once you define the system period you can set the period of each element when you define them in the Model Definition tab Polarization analysis is the most important feature of the WOLFSIM You can define the incident po larization by two polarization angles of the polarization ellipse the orientation angle w and the ellipticity angle x The simulation space can be split into three areas incoming area problem area and outgoing area The current version of WOLFSIM assumes transmissive elements as problem structures The input wave is excited i
5. WOLFSIM Wideband Optical FDTD Simulator Chulwoo Oh and Michael J Escuti email to mjescuti ncsu edu November 18 2007 Contents 1 Introduction 2 Download and Installation Sl INDO sca sendira ee REESE RES Se dE 2 2 lnstelblOH 2 464266 4664468 dE ESEC EE ERO e WO Se X we 0 XE Ee ew 3 A Short Introduction of the FDTD method 4 Key Features 4 1 Periodic Boundaries at oblique incidence 2 2 a a e 4 2 Wideband Simulation llle 4 3 Multiple Layered Structures 22 4 4 Polarization Analysis lt lt sonrie 5 Graphical User Interface GUI for WOLFSIM EL WOLFE O o ee OEC Ra ERE e a d domu a o RIS a a e RES Dol DMR SEU CK XR e SEARA ra AAA AAA 5 3 Model Definition iu o nos oem oom mox m deu bee eet bee OR x xm ho Ros od xo X REOR n 94 Iterative Simulation lt lt sa sa sa cuo bd X ADC o0 REE ER 9 3 ex a o Output ONO o aoo aea ee ee ERR EE d om E DA EA X Xo c O 5 8 Input Pile Generation 2 222599 ooo hor 9 mox ok Eo Mog X BOR Ea E X pov 6 Applications of WOLFSIM Bl Boat Pa Crating uuo udo se ER ee EMBRAER TRS eS HERS 6 2 Dielectric Slab with AR Coatings lle 6 3 Twisted Nematic Liquid Crystal Cell rre s 04 Polarization Graiing 225499 X 993 Xd EGER E E woe RA AAA m X ox o 1 Introduction WOLFSIM is an Open Source software package developed by OLEG at NCSU for numerical modeling of electromagnetic systems especially 1D and 2D periodic structures in anisotrop
6. ay not be working for certain cases but one can determine the termination time manually instead of using the automatic option One also can select the single frequency modeling usually for obtaining field map images In this case monochromatic light is illuminated to the problem structure The simulation will end when the system reach the steady state condition This auto termination can be disabled and one can enforce to terminate the simulation at a fixed time step 4 3 Multiple Layered Structures WOLFSIM provides convenience of implementing stacked structures Once structures are defined by built in functions or external text files which contain material properties of media each structure can be added by simple commends in Input txt We will explicitly show how to add structures in Section and 4 4 Polarization Analysis WOLFSIM is designed for anisotropic media where the polarization state of light is in the main consid erations One can define the polarization state of incident light from the two characteristic angles of the polarization elllipse the orientation angle Y and the ellipticity angle x WOLFSIM provides the Stokes parameters which describe the polarization state of light for both the input and the output as simulation results 5 Graphical User Interface GUI for WOLFSIM WOLFSIM requires a separate data file Input txt including simulation parameters The new version of WOLFSIM version 2
7. e DFT will start automatically by detecting the energy passing through the sampling line e 27 fot 1 Source Parameters Gaussian Pulse E Peak Wavelength 1 0 um Pulse Width 3 fsec Set the pulse width close to the time period T at the center wavelength wl e g T wl c 3 333 fsec at wl 1 um Figure 3 Generation of Gaussian Pulse 5 3 Model Definition The new WOLFSIM provides the built in definitions for ten different structures Gradient Anti Refraction AR coating Circular Polarization Grating PG Linear Polarization Grating PG Dielectric Slab Lossy Dielectric Slab Twisted Nematic Liquid Crystal TNLC cells Sinusoidal Phase Grating Binary Phase Grating Waveplate and Micro Prism films You can add structures simply by defining dimensions and material parameters that are required to define each type of structures Once you define each structure you can easily review the properties of individual structures by selecting in the scroll window The preview highlights the structure that you select and shows the type of the structure and its dimensions and material properties You can also edit remove structures if needed O ee E WOLFSIM Input File Generator e WOLFSIM Input File Generator About Settings Model Definition Iteration Output About Settings Model Definition Iteration Output Import Export Import Export Add Remove Media Add Remo
8. fractive index n 1 5 at both air slab interfaces The thickness of both AR layers is set to d 0 8um 6 3 Twisted Nematic Liquid Crystal Cell The next example is a 90 twisted nematic liquid crystal TN LC cell The twisted nematic structure can be implemented as stratified structures thickness Au of uniaxially anisotropic media with variation of the optical axis along the thickness The cell thickness is d 2um and LC material parameters are the ordinary index n 1 4 and the extraordinary index n 1 6 birefringence An 0 2 Again we use a Gaussian 10 1 MET s REC CE y ho m e al e m E 0 8 d c 0 8F a o 0 6 q 0 61 E m c i E T 0 4 S 0 4 E E 0 2 0 2r put l 0 0 5 1 1 5 2 0 0 5 1 1 5 2 And A f f 0 a Gradient AR Coating b 90 TNLC Cell Figure 10 Transmittance spectra a a AR coated dielectric slab and a 90 b TNLC cell with crossed polarizers pulse as an input source centered at Ag 0 8mm To minimize the effect of the Fresnel losses at air LC boundaries gradient index AR layers are added at both interfaces The thickness of both AR layers is set to d 0 8um 6 4 Polarization Grating The last example is modeling of a special anisotropic grating known as polarization grating PG Unlike conventional phase gratings PG can have only three diffracted orders 0 and 1 orders and the maximum efficiency can reach ideally 100 when And 1 24 Mo
9. hich can affect numerical properties A shift of spectral response to longer wavelengths can appear as one of typical numerical characteristics of the FDTD method Obviously one simple solution to reduce numerical errors including the numerical dispersion is smaller grid spacings More details and advances of the FDTD method can be found in Ref 2 4 Key Features 4 1 Periodic Boundaries at oblique incidence WOLFSIM is specially designed for analysis of optical elements with periodic structures like gratings Bloch periodic boundaries reduce the computation space into one period of the structure The periodic boundary conditions PBC at a general angle of incidence are implemented in WOLFSIM using the split field update technique In addition non diagonal permittivity tensor is integrated with PBC 4 2 Wideband Simulation One of key advantages of WOLFSIM is its capability of spectral analysis in a single simulation with wideband source such as a Gaussian pulse Along with PBC this wideband feature suggests WOLFSIM as an efficient simulation tool for analysis of periodic anisotropic media With a Gaussian incident pulse the analysis can be done in the same frequency range of the pulse To obtain a frequency response of the system the program performs the discrete Fourier transformation DFT to the sampled field values during time marching The time duration for the DFT will be automatically determined However this automatic decision m
10. ic media WOLFSIM provides the wideband spectral response near field and far field from a single simulation of one period of the structure two dimensional spatial grid is combined with a full three dimensional permittivity tensor to allow for 2D simulation Oblique source incidence is explicitly incorporated using a modified split field FDTD method A full description of this package was published in Optics Express 1 More information can be found in the following link http www ece ncsu edu oleg wolfsim html WOLFSIM was initiated by a graduate student Chulwoo Oh in OLEG director Dr Michael Escuti at NCSU We must mention that WOLFSIM is a software tool for university research purpose and the current version of WOLFSIM is not final and still developing We are grateful to your suggestions for improvement of WOLFSIM 2 Download and Installation 2 1 Download A package of computer codes for WOLFSIM is freely downloadable at http www ece ncsu edu oleg tools The package is distributed as a zipped file i e WOLFSIM X X X zip which contains the main code WOLFSIM c and libraries h In addition this user manual and an installation guide are available to download at http www ece ncsu edu oleg wolfsim download html Since WOLFSIM is still developing only the latest version will be available through these links You may contact us to obtain the previous versions 2 2 Installation WOLFSIM is written in s
11. ile Format Text files E PA VA 4 4 Figure 8 Load and Save of Input File 6 Applications of WOLFSIM In this section we will show modeling of several optical elements using WOLFSIM We hope that these examples give you an idea of how to use WOLFSIM in a more practical sense 6 1 Binary Phase Grating We model a binary grating with rectangular grooves Grating parameters of the binary grating are given by the average index Nay 1 5 the index modulation n 0 2 the grating period A 8um and the thickness d 2um The fill factor FF is 0 5 For our FDTD simulation we use a Gaussian pulse as an input source centered at Ay 0 8um The incoming area where the source is excited is filled with air and the outgoing media is filled with dielectric media n 1 7 d t 400At e t 500At f t 600At Figure 9 Snap shots of the near field image 6 2 Dielectric Slab with AR Coatings We model a dielectric slab with anti reflecction AR coatings to evaluate the performance of AR layers The index profile of AR layers is defined as n t n ng n1 10t 15t 6t where n and n are refractive indices of the incoming and outgoing media respectively and t is the position within 0 d d is the thickness of AR layers For our FDTD simulation we use a Gaussian pulse as an input source centered at Ao 0 8um Two AR layers are integrated with a 2um slab of dielectric media with the re
12. imum Grating Period um 10 Step Size um 2 Figure 6 Iterative Simulation 0698 WOLFSIM Input File Generator About Settings Model Definition Iteration Output Import Export WOLFSIM Output Options NOTE Source type must be specified Frequencies vi Permittivity Tensor Y Impermittivity Tensor Far Field Output File Type Efficiency Y HDES vi EM Field Values ASCII Stokes Parameters rNear Field EM Field Values Immediately After Model PQ Field Values Immediately After Model Final EM field Map Animation EM Field Values Every Specified Time Step V WARNING Heavy Memory Consumption Time Step 100 Figure 7 Output Options The WOLFSIM without HDF5 is also available for users who don t want to use HDF5 In this case the ASCII is the default data format for output files 5 6 Input File Generation The final step is to save the input file Input txt You can generate the input file using the button named Generate File in the Import Export tab You also can load the pre existing input file from your local directory Now you are ready to run a simulation with the WOLFSIM The following section shows examples with different structures iQ 00 WOLFSIM Input File Generator l 0 206 WOLFSIM Input File Generator A About Settings Model Definition Iteration Output Import Export Lo A About gt Settings Model Definition Iteration Out
13. m grating period and step size Output Tab Figure 1 WOLFSIM GUI 5 2 Simulation Setting In the second tab Settings you can define the simulation parameters including the spatial grid resolution and the automatic termination and the source parameters as an input WOLFSIM normalize all dimensions by the wavelength of incident light The wavelength Ao is defined as the peak wavelength of a Gaussian pulse of course a single wavelength for monochromatic light The spatial grid resolution AN is defined as the ratio of the center wavelength Ao to the spatial grid spacing Au Note that the WOLFSIM employs a square grid structure The grid spacing must be chosen with care because the sufficiently small grid spacing is critical to get accurate results with less numerical dispersion errors Typically Au is determined by the shortest wavelength of interest and the largest refractive index of the media as Au Amin 20Nmaz For example when Amin Ao 2 and Nmar 1 5 the grid resolution N 60 is a good number for accurate simulation results The new WOLFSIM has an option for the auto termination This option allows to terminate the simula tion by detecting the energy passing through the sampling line immediately after the problem structure It is useful only when the problem structure is transparent or not absorbing too much and when the Gaussian pulse is defined well Otherwise you may want to disable the option and set the maximum time step
14. n the incoming area and sampling for analysis is done in the outgoing area Additional options for reflective elements will be included in the next version WOLFSIM also assumes homogeneous dielectric media in both the incoming and outgoing areas Finally WOLFSIM has two incident mode monochromatic and Gaussian The monochromatic sinu soidal source is useful when you want to obtain the field map images in the near field region as well as the far field information A spectral response of the system can be obtained with a Gaussian incident pulse 2 t t P Po exp where Po is the complex amplitude to is the time delay T is the time domain pulse width and fo is the peak frequency of the pulse To define the Gaussian pulse you need to set the peak wavelength Ao and the pulse width We recommend to set the pulse width close to the time period at the peak wavelength which normally generate useful data within a frequency range fo 2 lt f lt 2fo or a spectral range Ag 2 lt lt 2X9 For example when Ay lum the pulse width T zz Ag c 3 3337 sec The frequency information is extracted by applying the discrete Fourier transformation DF T for the far field information If your pulse width is too short you may encounter numerical dispersion errors If your pulse width is too long the auto termination option may not work properly In this case disable the auto termination option and set the end point for the simulation Still th
15. om a simulation in one period of the structure by applying periodic boundary conditions PBCs We developed an efficient FDTD algorithm incorporating PBCs with non diagonal permittivity tensors at a general angle of incidence using the split field update technique 1 Note that WOLFSIM assumes a transversal periodicity but it does not take account of a longitudinal periodicity The two remaining boundaries are terminated by the uniaxial perfectly matched layer UPML absorbing boundary conditions 4 Since the FDTD algorithm approximates the solution of Maxwell s equations in discrete time and space a motion of the electromagnetic wave in the grid space may differ from that in continuous space Two main numerical characteristics of the FDTD algorithm are numerical stability and numerical dispersion For a numerically stable finite difference scheme the Courant stability condition which is necessary must be satisfied the temporal resolution At must be selected below a special number known as the Courant factor The Courant stability factor can be determined by the von Neumann method Second non physical dispersions often called the numerical dispersion can affect the accuracy of the simulation results To minimize numerical errors the spatial resolution Au should be selected to be sufficiently small in consideration of frequencies of interest dimensions of the structure material properties structure shapes and any other factors w
16. put Import Export Load Input Settings Load Input Settings LFSIM WOLFSIM 2 0 0 Input txt Load File LFSIM WOLFSIM 2 0 0 Input txt Load File File Loaded eoe Save e Open Generate Input File 2 WOLFSIM 2 0 0 Eb Save As Input txt a WOLFSIM 2 0 0 KA gt Custom Thursday November 15 2007 6 25 PM Status t M HDF5 Read m Thursday November 007 6 16 PM Loading simulation Nan Date Modified Loading source paf custom Thursday November 15 2007 6 25 PM 2 INSTALL htm Thursday November 15 2007 10 08 PM Loading layer parat Bb HDFS Read m Yee MESURES 20 E PM Library Tuesday October 30 2007 11 39 PM pr pd B inputit Friday November 16 1 49 AM LICENSE txt Tuesday December 5 2006 5 11 AM EUR NN icra ter Abbo Vi MR WR SEXE gt Result Thursday November 15 2007 6 42 PM File written EE IN n irsday November 15 2007 1 M a E sae gt Library Tuesday October 30 2007 11 39 PM esu nursday November Po Fn LICENSES Tuesday Decembe 2006 5 11 AM MB Kesult mat Thursday November 15 2007 6 47 PM 3 Result Thursday November 15 2007 6 42 PM 3 VERSION txt Thursday November 15 2007 10 11 PM 3 E t h5 Thursda Noven f 5 607 15 PM m im Thursday Novembe 15 2007 6 PM 5 E Y nbs ni 7 PM Ly WOLFSIM 2 zu Thursday November 15 2007 1 17 N BR ur gt zi WOLFSIM GUI Thursday November 15 2007 9 31 PM E VE 14 File Format Text files reJ F
17. re details of PG can be found in Ref 1 Grating parameters are the grating pitch A 8um the birefringence An 0 2 and the thickness d 4um A Gaussian pulse is used as an input source centered at Ag 0 8um To minimize the effect of the Fresnel losses at air LC boundaries gradient index AR layers are added at both interfaces The thickness of both AR layers is set to d 0 8um 11 Polarization Grating a t 100At b t 300At c t 500At d t 700At e t 900At f t 1100At Figure 11 Snap shots of the near field image References 1 C Oh and M J Escuti Time domain analysis of periodic anisotropic media at oblique incidence an efficient FDTD implementation Opt Express 14 11 870 11 884 2006 2 A Taflove and S C Hagness Computational electrodynamics finite difference time domain method 2nd ed Artech House Norwood MA 2000 3 K S Yee Numerical solution of initial boundary value problems involving Maxwell s equations in isotropic media IEEE Trans Antennas Propag 14 302 307 1966 4 S D Gedney An anisotropic perfectly matched layer absorbing medium for the truncation of FDTD lattices IEEE Trans Antennas Propag 44 1630 1639 1996 5 G B Arfken and H J Weber Mathematical methods for physicists 4th ed Academic Press San Diego 1995 12
18. tandard C C language The package should have one main code WOLF SIM c and library files i e FDTD IO h Once downloading the package it must be extracted into a new folder i e WOLFSIM or FDTD A sub folder named Library will be automatically created and this folder contains library files which are necessary to run WOLFSIM The folder name should not be changed unless the location of library files in the main code WOLFSIM c is modified The main code can be compiled with any C C compiler A text file named Input txt is required to run a simulation This text file provides information of simulation parameters for WOLFSIM A sample file of Input txt is included in the package More information of installation can be found in another documentation Installation Guide 3 A Short Introduction of the FDTD method WOLFSIM applies the finite difference time domain FDTD method 2 to analysis of optical elements with periodic structures in anisotropic media The FDTD simulation is directly solving Maxwell s equations in computational grid space during time marching Since Kane Yee 3 introduced an efficient and stable algo rithm often called the Yee algorithm the FDTD method has been one of the most successful approaches for analysis of electromagnetic systems WOLFSIM is designed specially for periodic structures in arbitrary anisotropic media A complete analysis of the system can be obtained fr
19. ve Media Anti Reflection Coating E Circular Polarization Grating Linear Polarization Grating Isotropic Dielectric Slab Lossy Dielectric Slab Twisted Nematic Liquid Crystal Sinusoidal Grating Binary Grating Waveplate Prismatic Film Custom Structure Preview Select Layer Type Type 0 Anti Reflection Coating Thickness 1 0 i o Edit Type 1 Circular Polarization Grating Thickness 2 5 Type 2 Linear Polarization Grating Thickness 2 5 Remove Type 3 Isotropic Dielectric Slab Thickness 2 0 Type 4 Lossy Dielectric Slab Thickness 2 5 MoveUp Type 5 Twisted Nematic Liquid Crystal Thickness 2 0 SS Type 6 Sinusoidal Grating Thickness 2 0 Move Down Type 7 Binary Phase Grating Thickness 3 0 Type 8 Waveplate Thickness 2 0 TiunofQl Driematie Cila 1 Driem Anala ON O Preview Tvpe 0 Anti Reflection Coatin e ircular Polarization Gratin ypel2 Linear Polarization Grating Type 4 Lossy Dielectric Slab Type 5 Twisted Nematic Liquid Crystal Type 6 Sinusoidal Grating Type 7 Binary Phase Grating Type 8 Waveplate Type 1 Circular Polarization Grating Num of Periods 1 0 Thickness um 2 5 Type 3 Isotropic Dielectric Slab 1 Ordinary Index 1 4 Birefringence 0 2 Pretilt degrees 0 0 Twist degrees 0 0 Relative Phase degrees 0 0 Type 9 Prismatic Film Type 10 Custom
20. x x includes the graphical user interface WOLFSIM GUI to manage the simulation parameters WOLFSIM GUI is a Java application so it is machine independent However you may need to install the latest version of Java on your system to run this application In this section we provide a brief instructions for WOLFSIM GUI There are five different parts which are separated by different tab windows Settings Model Definition Iteration Output and Import Export Simple examples for each part will be presented throughout the section 5 1 WOLFSIM GUI Start the Java GUI application with the name of WOLFSIM GUL jar In the first tab About you can find a brief introduction of the WOLFSIM and short descriptions for other four tabs e 06 WOLFSIM Input File Generator l About Settings Model Definition Iteration Output Import Export Opto Electronics amp Lightwave Engineering Group OLEG MSI LU AIM NA Electrical amp Computer Engineering The WOLFSIM Input File Generator is the primary means for controlling the WOLFSIM application s input parameters The application s tabs are utilized as follows Settings Tab General simulation variables as well as source selection and parameters Model Definition Tab Specification of the model to be simulated by WOLFSIM A preview of the model is also depicted Iteration Tab Allows the user to enable running multiple simulations across a specified minimum maximu
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