Computing the photonic local density of states with scuff-ldos

scuff-ldos is a tool for computing the electromagnetic local density of states (LDOS) at points inside or outside compact or extended material bodies.

The inputs you supply to scuff-ldos calculation are

  • A .scuffgeo file describing your geometry.

  • A list of evaluation points at which you want to know the LDOS.

  • One or more angular frequencies at which to perform calculations.

  • Optionally, for periodic geometries, you may additionally specify a list of Bloch wavevectors at which to evaluate wavevector-resolved contributions to the LDOS. If you do not specify such a list, scuff-ldos will evaluate an integral over the Brillouin zone to compute the total LDOS at each point.

The outputs you get back from a scuff-ldos calculation may include

  • The LDOS at each point.

  • For periodic geometries, the contributions of individual Bloch wavevectors to the LDOS. If you supplied a list of Bloch vectors as an input, you will get wavevector-resolved information for each point in your list; otherwise, you will get wavevector-resolved information for each point chosen automatically by scuff-ldos in its numerical-cubature evaluation of the Brillouin-zone integral.

  • Optionally, the full Cartesian components of the scattering parts of the dyadic Green's functions (DGFs) [the electric / magnetic LDOS is proportional to the imaginary part of the trace of the electric / magnetic DGFs].

For testing purposes, scuff-ldos also includes built-in functionality to compute the LDOS for some geometries that may be handled analytically (specifically, infinite-area PEC ground planes and infinite-area dielectric half-spaces).

1. What scuff-ldos actually computes

Some technical details on the calculation performed by scuff-ldos may be found in this memo. The long story short is as follows:

What scuff-ldos actually computes is the scattering part of the dyadic Green's functions (DGFs) of the geometry in question. For a given (angular frequency, evaluation point) pair , these are matrices giving the scattered fields at due to point sources at , with all fields and sources having time dependence . (The full definition may be found in the memo above). The LDOS is obtained from the imaginary part of the traces of the DGFs.

For non-periodic geometries, scuff-ldos does 6 scattering calculations for each point---specifically, scattering calculations in which the incident field is the field of an electric or magnetic point source oriented in each of the 3 cartesian directions. (Because these calculations involve the same scattering geometry at the same frequency, just with different incident fields, they are fast in a BEM solver like scuff-em because the BEM matrix need not be recomputed anew for each new incident field.) The results for the LDOS at each point are reported in the .LDOS output file.

For periodic geometries, scuff-ldos does many scattering calculations for each point. Indeed, the DGFs at are defined as the response of the system to a single point source at ; however, in in scuff-em calculations for periodic geometries, all currents and fields, including incident fields, must be Bloch-periodic, a condition which is not satisfied by the fields of a single point source at . Instead, what scuff-em can compute is the response of the system to a phased array of point sources---that is, an infinite collection of point sources located at points with phases ; here ranges over all lattice vectors in a 1D or 2D lattice, and is a 1D or 2D Bloch wavevector that ranges over the Brillouin zone (BZ) of the reciprocal lattice. By performing these calculations at all possible Bloch vectors and adding up the results---that is, by performing an integration over the BZ, effectively an inverse Fourier transform---we recover the fields of just the single point source at . For periodic geometries, scuff-ldos performs this BZ integration by numerical cubature for each point. This involves sampling the integrand (that is, computing Bloch-periodic DGFs) at large numbers of points; these samples, which provide Bloch-vector-resolved information on the LDOS and DGFs of the system, are reported by scuff-ldos in the.byOmegakBloch output file, while results for the full (BZ-integrated) LDOS are written to the .LDOS output file.

2. scuff-ldos command-line options

Common options

scuff-ldos recognizes the following subset of the [list of commonly accepted options to scuff-em command-line codes][CommonOptions].

--geometry --EPFile --Omega --OmegaFile --OmegakBlochFile --AbsTol --RelTol --FileBase --Cache --ReadCache --WriteCache

Of these options, --geometry and --EPFile are always mandatory, while one of --Omega, --OmegaFile, or --OmegakBlochFile must also be specified. All other command-line arguments are optional.

If you specify --Omega or --OmegaFile in a calculation involving a periodic geometry, then scuff-ldos will perform a numerical cubature over the Brillouin zone for each value. (The options --BZSymmetry, --AbsTol, and --RelTol control the parameters of this cubature.) Samples of the integrand at the cubature points will be written to the .byOmegakBloch file, while the full integrated resuts will be written to the .LDOS file.

Alternatively, if you use --OmegakBlochFile to specify a list of () points, then scuff-ldos will skip the numerical BZ cubature and instead perform computations at just the points you specified. In this case you wil get back a .byOmegakBloch file, but not an .LDOS file.

Options requesting analytical LDOS calculations

```` --GroundPlane

--HalfSpace PEC --HalfSpace Aluminum

--SkipBZIntegration ````

As noted above, for testing purposes scuff-ldos incorporates

The first option here instructs scuff-ldos to bypass the usual LDOS calculation it would otherwise perform and instead to compute the LDOS of an auxiliary geometry in which the half-space region lying below the plane (the region ) is filled with a homogeneous material described by the given scuff-em material designation. If this material is PEC, then the calculation is performed using the image-source method. Otherwise, the calculation is performed using the analytical plane-wave decomposition outlined in the scuff-ldos memo.

As illustrated by this example, the --HalfSpace option is intended to be tacked on to an otherwise complete scuff-ldos command-line containing options such as --geometry and --EPFile. With --HalfSpace, scuff-ldos performs the same calculation that it would do without that option---using the same evaluation points and the same frequency options---but just does the calculation a different way. (Although the surface meshes specified in the .scuffgeo file are not referenced in this case, a valid .scuffgeo file must still be supplied; the LATTICE...ENDLATTICE section of this file is used to define the lattice used for the analytical calculation.)

3. scuff-ldos output files

The .log file

Like all command-line codes in the scuff-em suite, scuff-ldos produces a text output file named FileBase.log that you can follow to monitor the status of your calculation.

The .LDOS file

This file reports values of the electric and magnetic LDOS for each angular frequency and each evaluation point you requested. This file is always produced for calculations on non-periodic geometries. For calculation on periodic geometries, this file is produced only if you specified the --Omega and/or --OmegaFile command-line options.

The .byOmegakBloch file

This file reports Bloch-vector resolved versions of the information reported by the .LDOS file. This file is only produced for calculations on periodic geometries.

4. Examples of calculations using scuff-ldos