MPB User Tutorial

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MIT Photonic Bands

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MPB manual

Introduction

Installation

User Tutorial

Data Analysis Tutorial

User Reference

Developer Information

Acknowledgements

License and Copyright

In this section, we'll walk through the process of computing the band structure and outputting some fields for a two-dimensional photonic crystal using the MIT Photonic-Bands package. This should give you the basic idea of how it works and some of the things that are possible. Here, we tell you the truth, but not the whole truth. The MPB User Reference section gives a more complete, but less colloquial, description of the features supported by Photonic-Bands. See also the MPB Data Analysis Tutorial in the next section for more examples, focused on analyzing and visualizing the results of MPB calculations. For more advanced functionality, see the Simpetus projects page.

The ctl File

The use of the Photonic-Bands package revolves around the control file, abbreviated "ctl" and typically called something like foo.ctl (although you can use any filename extension you wish). The ctl file specifies the geometry you wish to study, the number of eigenvectors to compute, what to output, and everything else specific to your calculation. Rather than a flat, inflexible file format, however, the ctl file is actually written in a scripting language. This means that it can be everything from a simple sequence of commands setting the geometry, etcetera, to a full-fledged program with user input, loops, and anything else that you might need.

Don't worry, though—simple things are simple (you don't need to be a Real Programmer), and even there you will appreciate the flexibility that a scripting language gives you. (e.g. you can input things in any order, without regard for whitespace, insert comments where you please, omit things when reasonable defaults are available...)

The ctl file is actually implemented on top of the libctl library, a set of utilities that are in turn built on top of the Scheme language. Thus, there are three sources of possible commands and syntax for a ctl file:

• Scheme, a powerful and beautiful programming language developed at MIT, which has a particularly simple syntax: all statements are of the form (function arguments...). We run Scheme under the GNU Guile interpreter (designed to be plugged into programs as a scripting and extension language). You don't need to learn much Scheme for a basic ctl file, but it is always there if you need it; you can learn more about it from these Guile and Scheme links.
• libctl, a library that we built on top of Guile to simplify communication between Scheme and scientific computation software. libctl sets the basic tone of the user interface and defines a number of useful functions. See the libctl pages.
• The MIT Photonic-Bands package, which defines all the interface features that are specific to photonic band structure calculations. This manual is primarily focused on documenting these features.

It would be an excellent idea at this point for you to go read the libctl manual, particularly the [[[libctl Basic User Experience]] section, which will give you an overview of what the user interface is like, provide a crash course in the Scheme features that are most useful here, and describe some useful general features. We're not going to repeat this material (much), so learn it now!

Okay, let's continue with our tutorial. The MIT Photonic-Bands (MPB) program is normally invoked by running something like:

unix% mpb foo.ctl >& foo.out

which reads the ctl file foo.ctl and executes it, saving the output to the file foo.out. (Some sample ctl files are provided for you in the mpb-ctl/examples/ directory.) However, as you already know (since you obediently read the libctl manual, right?), if you invoke mpb with no arguments, you are dropped into an interactive mode in which you can type commands and see their results immediately. Why don't you do that right now, in your terminal window? Then, you can paste in the commands from the tutorial as you follow it and see what they do. Isn't this fun?

Our First Band Structure

As our beginning example, we'll compute the band structure of a two-dimensiona