Probe Transmission Spectrum Research Articles

STIMULATED SCATTERING IN AN OPTICAL LATTICE

We present a study of pump-probe spectroscopy experiments in an optical lattice. The probe transmission spectrum contains three types of resonances similar to those found in usual nonlinear media: Raman, Brillouin and Rayleigh resonances. We show evidence of the existence of a propagating elementary excitation in the lattice which involves no interaction between the particles and lead to a Brillouin-like scattering. We discuss the central Rayleigh resonance and show that its shape is sensitive to the radiation pressure.

Three-dimensional cooling of cesium atoms in four-beam gray optical molasses.

We report the observation of three-dimensional cooling of cesium atoms using a four-beam optical molasses tuned to the blue side of the ${\mathit{F}}_{\mathit{g}}$=3\ensuremath{\rightarrow}${\mathit{F}}_{\mathit{e}}$=2 hyperfine component of the 6${\mathit{S}}_{1/2}$\ensuremath{\rightarrow}6${\mathit{P}}_{3/2}$ transition. The cooling mechanism is a Sisyphus effect involving atomic states that are nearly uncoupled from the light field. Starting from a magneto-optical trap at 70 \ensuremath{\mu}K, the cooling time was on the order of 1 ms with almost 100% capture efficiency and temperature smaller than 5 \ensuremath{\mu}K measured. We also present a probe transmission spectrum showing that almost all the atoms are found in the nearly dark states.

Probe-transmission spectrum of a blue-detuned optical lattice.

The probe-transmission spectrum of a one-dimensional optical lattice is calculated. The optical lattice consists of atoms that are cooled by a pair of counterpropagating, linearly polarized fields, whose frequency \ensuremath{\omega} is detuned to the blue side of an atomic transition. A weak probe field of frequency ${\mathrm{\ensuremath{\omega}}}^{\ensuremath{'}}$ is assumed to be propagating in the same direction as one of the cooling fields. It is shown that, owing to the existence of dark and gray states in the lattice, the probe-transmission spectrum exhibits interesting features that are qualitatively different from those of a ``normal'' optical lattice with red-detuned cooling fields. Examples of these features include subrecoil linewidths and reversed absorption-amplification sidebands. It is shown that by monitoring the magnitudes of resonance signals in the spectrum, one can gain information on the evolution of the dark-state population in the lattice.

Probe transmission in a two-dimensional optical lattice.

We present a calculation of the probe-transmission spectrum of a two-dimensional optical lattice, in which atoms are cooled by a pair of standing-wave fields with perpendicular polarizations within the plane of incidence. A probe laser is directed along the axis of one of the cooling fields, with a polarization parallel to the incident plane of the cooling fields. The calculated probe-transmission spectrum exhibits both sidebands, which originate from stimulated Raman transitions between different vibrational states of the atoms in the molasses, and a central resonance structure, which originates from Rayleigh resonances among these vibrational states. We also include in our calculation a possible misalignment of the probe field relative to the axis of the cooling field. It is shown that the effrects of the probe misalignment can lead to significant modifications to the central resonance signal.

Multiphoton transitions in a spin-polarized 3D optical lattice.

We have observed novel resonances occuring at subharmonic frequencies in probe transmission spectra of ultracold atoms confined in the antinodes of a 3D optical standing wave by means of dipole forces. We interpret these resonances in terms of multiphoton Raman transitions between vibrational states of the trapped atoms.

Theory of coherent transients in semiconductor pump–probe spectroscopy

Probe-transmission spectra are calculated for a semiconductor that is excited by ultrashort pulses well above its band edge. Coherent coupling of the light field to the band-to-band transitions is assumed, neglecting the mixing of different momentum states. The transmission spectra exhibit pronounced oscillatory structures caused by the interaction of the probe pulse with the medium polarization, which is driven by the pump pulse. The details of the oscillations are determined by the temporal separation between pump and probe pulses and by the total temporal width of the pump pulse. Characteristic modifications of the spectra are obtained for strong pump fields (π and 2π pulses). Possible applications of the theoretical results for the determination of intraband scattering times in semiconductors are discussed.

Induced gain and modified absorption of a weak probe beam in a strongly driven sodium vapor

We present the results of an experimental and theoretical study of the gain (or absorption) experienced by a weak probe beam propagating through a sodium vapor in the presence of an intense pump field that is nearly resonant with the 3s → 3p atomic transition. The probe-transmission spectrum includes three distinct features that result from the modification of the atomic-level structure by the ac-Stark effect. Two of these features can lead to amplification of the probe wave. We measured the dependence of the probe spectrum on the detuning of the pump beam from resonance and on the pressure of a helium buffer gas. The experimentally obtained spectra are in good agreement with the predictions of a theoretical model based on the solution of the density-matrix equations of motion for a two-level atom and including the effects of Doppler broadening. The maximum gain measured in these experiments occurs at one of the Rabi sidebands and leads to a 38-fold increase in the intensity of the probe wave. Under some experimental conditions, we also observed two additional resonances, which are due to stimulated Raman scattering involving the sodium ground-state hyperfine levels.