ElecSus: A program to calculate the electric susceptibility of an atomic ensemble

We present a computer program and underlying model to calculate the electric susceptibility of a gas, which is essential to predict its absorptive and dispersive properties. Our program focuses on alkali-metal vapours where we use a matrix representation of the atomic Hamiltonian in the completely uncoupled basis in order to calculate transition frequencies and strengths. The program calculates various spectra for a weak-probe laser beam in an atomic medium with an applied axial magnetic field. This allows many optical devices to be designed, such as Faraday rotators/filters, optical isolators and circular polarisation filters. Fitting routines are also provided with the program which allows the user to perform optical metrology by fitting to experimental data. Program title: ElecSus Catalogue identifier: AEVDᵥ1₀ Program summary URL: http: //cpc. cs. qub. ac. uk/summaries/AEVDᵥ1₀. html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Apache License, version 2 No. of lines in distributed program, including test data, etc.: 191 270 No. of bytes in distributed program, including test data, etc.: 309 4994 Distribution format: tar. gz Programming language: Python. Computer: Any single computer running Python 2. Operating system: Linux, Mac OSX, Windows. RAM: Depends on the precision required and size of the data set, but typically not larger than 50 MiB. Classification: 2. 2, 2. 3. External routines: SciPy library 1 0. 12. 0 or later, NumPy 1, matplotlib 2 Nature of problem: Calculating the weak-probe electric susceptibility of an alkali-metal vapour. The electric susceptibility can be used to calculate spectra such as transmission and Stokes parameters. Measurements of experimental parameters can be made by fitting the theory to data. Solution method: The transition frequencies and wavelengths are calculated using a matrix representation of the Hamiltonian in the completely uncoupled basis. A suite of fitting methods are provided in order to allow user supplied experimental data to be fit to the theory, thereby allowing experimental parameters to be extracted. Restrictions: Only describes a magnetic field parallel to the laser beam propagation direction. Results are only valid in the weak-probe regime. Running time: At standard precision less than a second for a theory curve, fitting will take 10 s to 20 min depending on the method used, the number of parameters to fit and the number of data points. References: 1 T. E. Oliphant, Comput. Sci. Eng. 9, 10 (2007). http: //www. scipy. org/. 2 J. D. Hunter, Comput. Sci. Eng. 9, 10 (2007). http: //matplotlib. org/.