a b s t r a c tWe 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 summary Program title: ElecSus Catalogue identifier: AEVD_v1_0Program summary URL:

A light, compact optical isolator using an atomic vapor in the hyperfine Paschen-Back regime is presented. Absolute transmission spectra for experiment and theory through an isotopically pure 87 Rb vapor cell show excellent agreement for fields of 0.6 T. We show π/4 rotation for a linearly polarized beam in the vicinity of the D 2 line and achieve an isolation of 30 dB with a transmission > 95 %.

We investigate experimentally and theoretically the Faraday effect in an atomic medium in the hyperfine Paschen-Back regime, where the Zeeman interaction is larger than the hyperfine splitting. We use a small permanent magnet and a micro-fabricated vapour cell, giving magnetic fields of the order of a tesla. We show that for low absorption and small rotation angles, the refractive index is well approximated by the Faraday rotation signal, giving a simple way to measure the atomic refractive index. Fitting to the atomic spectra, we achieve magnetic field sensitivity at the 10 −4 level. Finally we note that the Faraday signal shows zero crossings which can be used as temperature insensitive error signals for laser frequency stabilization at large detuning. The theoretical sensitivity for 87 Rb is found to be ∼ 40 kHz • C −1 .

Here we report on measurements of the absolute absorption and dispersion properties of an isotopically pure 87 Rb vapour for magnetic fields up to and including 0.6 T. We discuss the various regimes that arise when the hyperfine and Zeeman interactions have different magnitudes, and show that we enter the hyperfine Paschen-Back regime for fields greater than 0.33 T on the Rb D 2 line. The experiment uses a compact 1 mm 3 microfabricated vapour cell that makes it easy to maintain a uniform and large magnetic field with a small and inexpensive magnet. We find excellent agreement between the experimental results and numerical calculations of the weak probe susceptibility where the line positions and strengths are calculated by matrix diagonalization.

We demonstrate an atomic bandpass optical filter with an equivalent noise bandwidth less than 1 GHz using the D1 line in a cesium vapor. We use the ElecSus computer program to find optimal experimental parameters, and find that for important quantities the cesium D1 line clearly outperforms other alkali metals on either D-lines. The filter simultaneously achieves a peak transmission of 77%, a passband of 310 MHz and an equivalent noise bandwidth of 0.96 GHz, for a magnetic field of 45.3 gauss and a temperature of 68.0• C. Experimentally, the prediction from the model is verified. The experiment and theoretical predictions show excellent agreement.The Faraday effect in atomic media has come to be used for a wide range of applications, including creating macroscopic entanglement [1], GHz bandwidth measurements [2], non-destructive imaging [3], magnetometry [4], off-resonance laser frequency stabilization [5,6], and creating an optical isolator [7].Another application of increasing interest is utilizing the Faraday effect to create ultra-narrow bandwidth optical filters [8], of the order of a GHz width. These atomic Faraday filters are imaging filters [9] with a large field of view [10], and can be engineered to be low loss at the signal frequency [11]. This makes them the filter of choice for many applications, for example, they are used in atmospheric lidar [11][12][13][14] [9,24,25,[31][32][33][34], potassium [18,35,36], rubidium [37][38][39], and cesium [23,40,41]. A Faraday filter on the cesium D 1 line (894 nm) could be useful for quantum optics experiments which utilize the the Cs D 1 line [42], and could aid filtering degenerate photon-pairs at 894 nm in a similar way to that shown for 795 nm [21].In this letter we demonstrate the technique of using computer optimization to find optimal working parameters for a Faraday filter. Using this technique we find that a Faraday filter working at the Cs D 1 line has superior performance when compared to similar linear Faraday filters working with different elements and/or transitions. Experimentally, we verify the prediction of the model, and achieve a linear Faraday filter with the best performance to date.