Ifan G. Hughes

Absolute absorption on rubidium D lines: comparison between theory and experiment

We study the Doppler-broadened absorption of a weak monochromatic probe beam in a thermal rubidium vapour cell on the D lines. A detailed model of the susceptibility is developed which takes into account the absolute linestrengths of the allowed electric dipole transitions and the motion of the atoms parallel to the probe beam. All transitions from both hyperfine levels of the ground term of both isotopes are incorporated. The absorption and refractive index as a function of frequency are expressed in terms of the complementary error function. The absolute absorption profiles are compared with experiment, and are found to be in excellent agreement provided a sufficiently weak probe beam with an intensity under one thousandth of the saturation intensity is used. The importance of hyperfine pumping for open transitions is discussed in the context of achieving the weak-probe limit. Theory and experiment show excellent agreement, with an rms error better than 0.2% for the D2 line at 16.5 degrees C.

The role of hyperfine pumping in multilevel systems exhibiting saturated absorption

We study pump–probe spectroscopy of Rb vapor. Absorption spectra are presented for a weak probe beam in a room temperature vapor subject to a strong counter propagating pump beam of identical frequency. The importance of hyperfine pumping in the formation of the sub-Doppler spectrum is explained. For typical experimental parameters we clarify why the standard designation of “saturated absorption” spectroscopy is a misnomer. In contrast to saturated absorption, the details of the transient solution are crucial and hyperfine pumping leads to a modification of the absorption for detunings of many tens of natural linewidths from resonance.

Cooperative Lamb Shift in an Atomic Vapor Layer of Nanometer Thickness

We present an experimental measurement of the cooperative Lamb shift and the Lorentz shift using an atomic nanolayer with tunable thickness and atomic density. The cooperative Lamb shift arises due to the exchange of virtual photons between identical atoms. The interference between the forward and backward propagating virtual fields is confirmed by the thickness dependence of the shift which has a spatial frequency equal to 2k, i.e. twice that of the optical field. The demonstration of cooperative interactions in an easily scalable system opens the door to a new domain for non-linear optics.

Optical isolator using an atomic vapor in the hyperfine Paschen–Back regime

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 87Rb 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 D2 line and achieve an isolation of 30 dB with a transmission >95%.

A gigahertz-bandwidth atomic probe based on the slow-light Faraday effect

The ability to control the speed and polarisation of light pulses will allow for faster data flow in optical networks of the future. Optical delay and switching have been achieved using slow-light techniques in various media, including atomic vapour. Most of these vapour schemes utilise resonant narrowband techniques for optical switching, but suffer the drawback of having a limited frequency range or high loss. In contrast, the Faraday effect in a Doppler-broadened slow-light medium allows polarisation switching over tens of GHz with high transmission. This large frequency range opens up the possibility of switching telecommunication bandwidth pulses and probing of dynamics on a nanosecond timescale. Here we demonstrate the slow-light Faraday effect for light detuned far from resonance. We show that the polarisation dependent group index can split a linearly polarised nanosecond pulse into left and right circularly polarised components. The group index also enhances the spectral sensitivity of the polarisation rotation, and large rotations of up to 15π rad are observed for continuous-wave light. Finally, we demonstrate dynamic broadband pulse switching, by rotating a linearly polarised nanosecond pulse from vertical to horizontal with no distortion and transmission close to unity. The phenomenon of reduced optical group velocity (slow light) is a topic of burgeoning interest [1]. In a slow-light medium, the group refractive index, ng, (the ratio of the speed of light in vacuo to the pulse velocity) is many orders of magnitude larger than the phase index, n. Hence an optical pulse propagates much more slowly than a monochromatic light beam. Large group indices of ∼ 10 are achievable in resonant optical processes, such as electromagnetically induced transparency (EIT), accompanied by a refractive index that is of the order of unity [2]. Such large group indices are the result of a rapid change

Degenerate four-wave mixing spectroscopy and spectral simulation of C2 in an atmospheric pressure oxy-acetylene flame

The d 3Πg↔a 3Πu Swan bands of C2 have been recorded with high resolution using DFWM in the nearly Doppler free, phase conjugate geometry. C2 was probed in a standard oxy-acetylene welding flame with excellent signal-to-noise ratio and spectral resolution. Theoretical spectra were simulated and fitted directly to the complex overlapping spectra. The good agreement obtained shows that DFWM holds promise to become a robust and reliable tool for flame thermometry. Current theories of DFWM are reviewed in context of the present work and advantages and disadvantages of the technique are discussed.

Absolute absorption on the rubidium D1 line including resonant dipole?dipole interactions

Here we report on measurements of the absolute absorption spectra of dense rubidium vapour on the D1 line in the weak-probe regime for temperatures up to 170 °C and number densities up to 3 × 1014 cm−3. In such vapours, modifications to the homogeneous linewidth of optical transitions arise due to dipole–dipole interactions between identical atoms, in superpositions of the ground and excited states. Absolute absorption spectra were recorded with a deviation of 0.1% between experiment and a theory incorporating resonant dipole–dipole interactions. The manifestation of dipole–dipole interactions is a self-broadening contribution to the homogeneous linewidth, which grows linearly with number density of atoms. Analysis of the absolute absorption spectra allows us to ascertain the value of the self-broadening coefficient for the rubidium D1 line: β/2π = (0.69 ± 0.04) × 10−7 Hz cm3, in excellent agreement with the theoretical prediction.

Hyperfine effects in electromagnetically induced transparency

The role of hyperfine structure in electromagnetically induced transparency (EIT) is investigated by studying the 5S1/2-5P3/2-5D3/2,5/2 cascade system in 85Rb and 87Rb. We show that if the hyperfine splitting Δhfs is larger than the Rabi frequency of the coupling beam, Ωc, then the observed EIT spectra can be modelled by adding terms corresponding to each hyperfine state. However, in the strong-coupling limit, Ωc>>Δhfs, a complete multilevel description is required.

Mobile atom traps using magnetic nanowires.

By solving the Landau-Lifshitz-Gilbert equation using a finite element method we show that an atom trap can be produced above a ferromagnetic nanowire domain wall. Atoms experience trap frequencies of up to a few megahertz, and can be transported by applying a weak magnetic field along the wire. Lithographically defined nanowire patterns could allow quantum information processing by bringing domain walls in close proximity at certain places to allow trapped atom interactions and far apart at others to allow individual addressing.

Maximal refraction and superluminal propagation in a gaseous nanolayer.

We present an experimental measurement of the refractive index of high density Rb vapor in a gaseous atomic nanolayer. We use heterodyne interferometry to measure the relative phase shift between two copropagating laser beams as a function of the laser detuning and infer a peak index n=1.26±0.02, close to the theoretical maximum of 1.31. The large index has a concomitant large index gradient creating a region with steep anomalous dispersion where a subnanosecond optical pulse is advanced by >100 ps over a propagation distance of 390 nm, corresponding to a group index n(g)=-(1.0±0.1)×10(5), the largest negative group index measured to date.