Geometry descriptions in buff-em

Geometries in buff-em are described by simple text files that are conventionally given the file extension .buffgeo.

Table of Contents

1. Syntax of the .buffgeo file

The .buffgeo file consists of one or more sections, delineated by the keywords OBJECT...ENDOBJECT. Each section defines a single compact object in a scattering geometry and takes the form

OBJECT Label
    KEYWORD argument
    KEYWORD argument
    ...
ENDOBJECT

where the various possible KEYWORD argument pairs are detailed below.

  • MESHFILE MyMeshFile.vmsh

Specifies the 3D (tetrahedra) volume mesh defining the object. The file should be present either in the current working directory or in the search path specified by the environment variable BUFF_MESH_PATH.

3D volume meshes are generated from a gmsh geometry file by running %gmsh -3 MyObject.geo; this will produce a file named MyObject.msh, which I typically rename to MyObject.vmsh to indicate that it is a volume mesh as opposed to the surface mesh files used by scuff-em.

  • MATERIAL GOLD
  • SVTENSOR MyMaterial.SVTensor

Specifies the material properties of the object.

The MATERIAL keyword is used for homogeneous isotropic bodies. In this case, GOLD should be a scuff-em material designation. This may be defined either in a database file (such as the file ${HOME}/.matprop.dat or ./matprop.dat) or on the fly in your .buffgeo file by including a MATERIAL...ENDMATERIAL section before your object definition.

The alternative SVTENSOR keyword is used for inhomogeneous and/or anisotropic materials. (It stands for "spatially-varying tensor.") In this case, MyMaterial.SVTensor should be a buff-em spatially-varying tensor file present either in the current working directory or in the search path defined by the environment variable BUFF_SVTENSOR_PATH.

  • DISPLACED 2.3 4.5 6.7
  • ROTATED 90 ABOUT 0 0 1

These options specify geometrical transformations to be performed on the object after it has been read in from the .vmsh file.

2. Examples of buff-em geometries

a. An SiO2 sphere coated with a layer of gold

OBJECT CoatedSphere
    MESHFILE CoatedSphere_4017.vmsh
    SVTENSOR CoatedSphere.SVTensor
ENDOBJECT

The file CoatedSphere.SVTensor is described here.

b. Two gold spheres, described by the same volume mesh but separated 3 length units in the z direction

MATERIAL GOLD
    wp = 1.37e16; 
    gamma = 5.32e13;
    Eps(w) = 1 - wp^2 / (w * (w + i*gamma));
ENDMATERIAL

OBJECT UpperSphere
    MESHFILE Sphere_677.vmsh
    MATERIAL Gold
ENDOBJECT

OBJECT LowerSphere
    MESHFILE Sphere_677.vmsh
    MATERIAL Gold
    DISPLACED 0 0 3
ENDOBJECT