Russell J. McLean

One-dimensional lattice of permanent magnetic microtraps for ultracold atoms on an atom chip

We report on the loading and trapping of ultracold atoms in a one-dimensional permanent magnetic lattice of period 10 µm produced on an atom chip. The grooved structure which generates the magnetic lattice potential is fabricated on a silicon substrate and coated with a perpendicularly magnetized multilayered TbGdFeCo/Cr film of effective thickness 960 nm. Ultracold atoms are evaporatively cooled in a Z-wire magnetic trap and then adiabatically transferred to the magnetic lattice potential by applying an appropriate bias field. Under our experimental conditions trap frequencies of up to 90 kHz in the magnetic lattice are measured and the atoms are trapped at a distance of less than 5 µm from the surface with a measured lifetime of about 450 ms. These results are important in the context of studies of quantum coherence of neutral atoms in periodic magnetic potentials on an atom chip.

Specular reflection of cold caesium atoms from a magnetostatic mirror

We have observed specular reflection and multiple bounces of a beam of laser-cooled caesium atoms from a magnetostatic mirror consisting of an array of rare-earth permanent magnets. Using a time-of-flight absorption technique, the reflection coefficient of the mirror for caesium atoms pumped toward the m=+4 state is determined to be . For a beam of unpolarized atoms the reflectivity is found to be unexpectedly high, , which is attributed to contributions from atoms in m = 0, -1, -2 states which are reflected at high magnetic fields close to the surface of the mirror as a result of the quadratic Zeeman effect.

Fast light in atomic media

Atomic media have played a major role in studies of fast light. One of their attractive features is the ability to manipulate experimental parameters to control the dispersive properties that determine the group velocity of a propagating light pulse. We give an overview of the experimental methods, based on both linear and nonlinear atom–light interaction, that have produced superluminal propagation in atomic media, and discuss some of the significant theoretical contributions to the issues of pulse preservation and reconciling faster-than-light propagation and the principle of causality. The comparison of storage of light, enhanced Kerr nonlinearity and efficient wave mixing processes in slow and fast light atomic media illustrates their common and distinct features.

Directional infrared emission resulting from cascade population inversion and four-wave mixing in Rb vapor

Directional infrared emission at 1.37 and 5.23 μm is generated in Rb vapors that are stepwise excited by low-power cw resonant light. The radiation at 5.23 μm originating from amplified spontaneous emission on the 5D(5/2)→6P(3/2) transition and wave mixing consists of forward- and backward-directed components with distinctive spectral and spatial properties. Diffraction-limited light at 1.37 μm generated in the copropagating direction only is a product of parametric wave mixing around the 5P(3/2)→5D(5/2)→6P(3/2)→6S(1/2)→5P(3/2) transition loop. This highly nondegenerate mixing process involves one externally applied and two internally generated optical fields. Similarities between wave mixing generated blue and 1.37 μm light are demonstrated.

Collimated blue light enhancement in velocity-selective pumped Rb vapour

The effect of hyperfine optical pumping on the parametric four-wave mixing process responsible for the generation of collimated blue light (CBL) in alkali atoms is explored. Over a wide range of atomic densities (4.5 ? 1010 cm?3 ? N ? 1.4 ? 1011 cm?3), a more than ten-fold enhancement of CBL is obtained from optical pumping produced by an additional laser. Pulsed optical pumping has been applied to investigate the temporal characteristics of CBL generation. It is shown that optical pumping offers efficient control of the mixing process and the collimated light generation that may be useful for probing a small number of atoms.

Enhanced atomic Kerr nonlinearity in bright coherent states

We show that long-lived bright Zeeman coherence that is responsible for absorption enhancement and steep anomalous dispersion in an atomic medium gives rise to a giant nonlinearity of the atomic susceptibility, in a similar manner to the dark coherence responsible for the well-known electromagnetically induced transparency. The intensity dependent refractive index of Cs vapour with a ground state Zeeman coherence in the vicinity of the D2 resonance line has been measured at low light intensity. The Kerr nonlinearity in the bright coherent state (n2 ≈ 1.0 × 10-6 cm2 mW-1) is higher than that in a dark state under the same experimental conditions. Higher order nonlinear components of the refractive index have also been estimated for both coherent states.

Periodic array of Bose-Einstein condensates in a magnetic lattice

We report the realization of a periodic array of Bose-Einstein condensates of 87Rb |F = 1; mF = -1> atoms trapped in a one-dimensional magnetic lattice close (8 micrometres) to the surface of an atom chip. A clear signature for the onset of BEC in the magnetic lattice is provided by in-situ site-resolved radiofrequency (RF) spectra, which exhibit a pronounced bimodal distribution consisting of a narrow component characteristic of a BEC together with a broad thermal cloud component. Similar bimodal distributions are found for various sites across the magnetic lattice. The realization of a periodic array of multiple BECs in a magnetic lattice represents a major advance towards the implementation of magnetic lattices to simulate many-body condensed matter phenomena in lattices of complex geometry and arbitrary period.

Perpendicularly magnetized, grooved GdTbFeCo microstructures for atom optics

Periodically grooved, micron-scale structures incorporating perpendicularly magnetized Gd10Tb6Fe80Co4 magneto-optical films have been fabricated and characterized. Such structures produce a magnetic field having flat equipotentials and whose magnitude decays exponentially with distance above the surface, making them attractive for manipulating ultracold atoms in atom optics. The GdTbFeCo films have been deposited on a Cr underlayer on a silicon (100) wafer and on a grooved silicon microstructure using DC magnetron sputtering. The films are found to have excellent magnetic properties for magnetic atom optics applications, including high remanent magnetization, high coercivity and excellent homogeneity. (Some figures in this article are in colour only in the electronic version)

Highly nonlinear atomic medium with steep and sign-reversible dispersion*

Experimental investigation of the optical properties of atomic media with bright and dark long-lived light-induced Zeeman coherence is reported. We have obtained a small negative value for the group velocity of light pulses in Cs vapour (Vg −c/6000), which agrees with direct anomalous dispersion measurements. The intensity dependent components of the refractive index of an atomic vapour with long-lived Zeeman coherence has been estimated. We have shown that such coherence can significantly enhance four-wave mixing, leading to the appearance of a multi-frequency comb-like spectrum at relatively low light intensity.

Broadband optical delay with a large dynamic range using atomic dispersion

We report on a tunable all-optical delay line for pulses with optical frequency within the Rb D2 absorption line. Using frequency tuning between absorption components from different isotopes, pulses of 10 ns duration are delayed in a 10 cm hot vapour cell by up to 40 ns while the transmission remains above 10%. The use of two isotopes allows the delay to be increased or decreased by optical pumping with a second laser, producing rapid tuning over a range of more than 40% of the initial delay at 110 °C. We investigate the frequency and intensity ranges in which this delay line can be realized. Our observations are in good agreement with a numerical model of the system.