W. J. Sandle

Optical bistability in a gaussian cavity mode

Abstract The mean field treatment of dispersive and absorptive bistability for two-level atoms in an optical cavity is extended to account for the gaussian envelope of the cavity mode. The principal modification to the plane wave cavity case is the raising of the critical threshold for absorptive bistability as compared to thresholds for dispersive bistability.

Self-focussing in a vapour of two-state atoms

Abstract Self-focussing in a vapour of two-state homogeneously broadened atoms is examined by numerical solution of the nonlinear wave equation describing propagation of a single frequency linearly polarized CW laser field. Close to resonance, effects due to saturation of the absorption (as well as saturation of the refractive index) are found to be important. A new phenomenon is predicted: focussing of a CW laser beam tuned to exact resonance. This is explained as being due to reshaping of the beam by the saturable absorber, followed by Fresnel diffraction.

A study of the transients in resonance fluorescence following a step or a pulse of magnetic field

Following (i) a sharp step or (ii) a pulse of applied magnetic field, the resonance fluorescence of an atomic vapour exposed to steady linearly polarized resonance radiation undergoes transient behaviour until a new steady state is reached; the form of the transient will be (i) modulation of the intensity with an exponential envelope, or (ii) simple exponential decay. From the modulation period the gJ value of the excited state is obtained. From the exponential decay the lifetime for the relaxation of atomic alignment is obtained directly. The technique, applied to the 63P1 state of mercury, gives 117.4+or-1.0 ns for the natural mean life.

Observation of polarization switching: D1 line of sodium in a Fabry-Perot

Abstract We report observation of the complete polarization switching sequence for a gas of atoms with a J = 1 2 to J = 1 2 transition, exposed to resonant or near resonant, linearly polarized laser excitation in an optical cavity. The sequence is from linearly polarized cavity output at low input power to predominantly circularly polarized output at intermediate power and back to linearly polarized output at high input power. The switching is abrupt, exhibits hysteresis and, when hyperfine optical pumping effects are minimized, is phenomenologically in accord with predictions from theory.

Polarization switching in a ring cavity with resonantly drivenJ=1/2 toJ=1/2 atoms

We have solved within the mean field limit for the steady state behaviour of a gas ofJ=1/2 toJ=1/2 model atoms in a ring cavity excited by an incident laser field of arbitrary polarization. Results are presented for the case of zero applied magnetic field and exact resonance between the laser frequency, the atomic transition and a cavity resonance (pure absorptive case). We find that the behaviour of the polarization switching between σ+ and σ− outputs depends on the values of the upper and lower state collisional relaxation rates, expressed via a single parameter β. The case of linearly polarized input is of particular interest since optical bistability is found to occur for β≦2, being pre-empted by polarization switching for β>2. The results are discussed in terms of an atomic feedback mechanism coupling the σ+ and σ− modes.

Observation of separated, polarised ring structures induced by nonlinear beam reshaping

Abstract A 400 mW elliptically polarised cw gaussian laser beam tightly focused in high density sodium vapour buffered by argon gas, and detuned by 10–30 GHz on the blue side of the sodium D 1 transition, is found to produce multiple rings of alternate circular polarisation at the cell exit window. A model which treats the coherent propagation of a polarised radiation field in a medium of homogeneously broadened J= 1 2 →J= 1 2 atoms is found to reproduce the essential features of the experiment, and suggests a simple physical explanation of the phenomenon.

Output coupling of a Bose-Einstein condensate formed in a TOP trap

Two distinct mechanisms are investigated for transferring a pure 87Rb Bose-Einstein condensate in the |F = 2, mF = 2 state into a mixture of condensates in all the mF states within the F = 2 manifold. Some of these condensates remain trapped whilst others are output coupled in the form of an elementary pulsed atom laser. Here we present details of the condensate preparation and results of the two condensate output coupling schemes. The first scheme is a radio-frequency technique which allows controllable transfer into available mF states, and the second makes use of Majorana spin-flips to populate all the manifold sub-states equally.

Observation of cw stimulated Raman emission in the neon 2p→1s manifold

Abstract We report previously unobserved cw laser emission from a commercial HeNe laser. This lasing is based on near-resonant stimulated electronic Raman scattering (SERS) involving NeI 1s5→2p→1s4, and 1s5→2p→1s2, electronic states when the laser cavity is pumped externally by cw dye laser radiation. Two of the emitted lines [2p2→1s4 (603.0 nm) and 2p2→1s2 (659.9 nm); both pumped at 588.2 nm (1s5→2p2)] are explored in some detail. Output power of the Raman laser has been measured as a function of the laser-atom detuning, the Ne discharge current and the pumping laser power. These Raman lines can be several times stronger than the 633 nm HeNe laser output itself. The observations are qualitatively consistent with calculations of Raman gain showing the possibility of all 14 dipole-allowed 2p→1s Stokes-Raman laser lines being observable with commercial HeNe lasers, given mirror coatings of sufficient spectral width.

Transients in mercury resonance fluorescence under pulsed electric fields: measurement of differential Stark shifts

A Stark effect experiment is presented, which involves the recording of transients in radiation resonantly scattered by a vapour, following the sudden application of an electric field pulse. The theory of this effect is developed from the case where a general step-function perturbation is applied. The transients have the form of damped sinusoidal oscillations: the damping is determined by the rate of radiative decay; the modulation frequencies depend directly on the energy-level splittings caused by the applied field. This technique has been to measure the magnitude and sign of the differential Stark shifts in the 63P1 level of 198Hg and 199Hg. These lead to a value for the tensor polarizability which is in good agreement with recent measurements. Results are presented of a calculation of the polarizabilities of the 63P1 and 61S0 levels based on experimental oscillator strengths.

Observation of a thermal phase-grating contribution to diffraction in erythrosin-doped gelatin films

Abstract Dynamic laser-induced gratings were written into gelatin films doped with the xanthene dye – erythrosin B. The diffraction signal relaxation time from the transient grating was analysed as a function of the pump beam fluence and grating period. Also, the relaxation time of the transient absorption signal was measured with a pump–probe technique to determine the triplet lifetime of the dye–host system. For writing beam fluences below 10 mJ/cm 2 , the diffraction relaxation time was found to be half of the triplet lifetime – consistent with a triplet-state population grating. However for fluences above this, the diffraction relaxation time had a quadratic dependence upon the grating period, thus giving strong evidence for a thermal grating contribution to the diffraction signal.