Creating, detecting and locating ultracold molecules in a surface trap

Ultracold molecules have been produced by photoassociation of Cs atoms trapped in a mirror magneto-optical trap. The molecules were detected by resonantly enhanced multi-photon ionization followed by time-of-flight mass spectroscopy. The time-of-flight ofatomic and molecular ions was investigated in the presence of a dc bias voltageapplied to the conducting mirror. This technique provides a new tool for determining the distance between the cold molecules and the mirror surface. This revised version was published online in August 2005 with a corrected cover date.     

Photoisomerization dynamics study on cis-azobenzene derivative using ultraviolet-to-visible tunable femtosecond pulses

We performed the transient absorption measurement and the first rate equation (RE) analysis for cis isomer of 4-carboxy-2′,6′-dimethylazobenzene to clarify the quantitative difference between the photoisomerization process and the thermal relaxation process from the excited state. The RE analysis enabled us to determine the cis-to-trans photoisomerization rate per each pump pulse to be 3% under the condition of the 430 nm, 150 fs pump pulse with energy of 200 nJ. Moreover, the signal due to the yielded trans molecules appearing in the transient absorption was assigned from the following observed result: the transient absorbance change at the 380 nm probe mostly decreased within 300 fs after the 430 nm pulse pumping and then slowly decreased to zero, while the absorbance change at the 350 nm probe had a positive constant component in the over one picosecond time region. The RE analysis showed that this constant component is due to the yielded trans molecules, and its positive value is due to the fact that the absorption cross-section of the -to- transition in their trans molecules is larger than that of the -to- transition in the original cis molecules.     

Optimizing the production of metastable calcium atoms in a magneto-optical trap

We investigate the production of long-lived metastable (4³ P ₂ ) calcium atoms in a magneto-optical trap (MOT) operating on the 4¹ S ₀ ?4¹ P ₁ transition at 423 nm. For excited 4¹ P ₁ atoms a weak decay channel into the triplet states 4³ P ₂ and 4³ P ₁ exists via the 3¹ D ₂ state. The undesired 4³ P ₁ atoms decay back to the ground state within 0.4 ms and can be fully recaptured if the illuminated trap volume is sufficientlylarge. We obtain a flux of above 10¹⁰ atoms/s into the 4³ P ₂ state. We find that our MOT lifetime of 23 ms is mainly limited by this loss channel, and thus the 4³ P ₂ production is not hampered by inelastic collisions. If we close the loss channel by repumping the 3¹ D ₂ atoms with a 671 nm laser back into the MOT cycling transition, a non-exponential 72 ms trap decay is observed, indicating the presence of inelastic two-body collisions between 4¹ S ₀ and 4¹ P ₁ atoms. Received: 10 July 2001 / Revised version: 22 October 2001 / Published online: 23 November 2001     

Formation of ultracold molecules via photoassociation with blue detuned laser light

We propose a new possibility to form ultracold molecules, via photoassociation of a pair of cold atoms into vibrational levels of the external well of an excited electronic state located at intermediate interatomic distance ( ≈ 20 Bohr radii), and embedded in the dissociation continuum above its dissociation limit. The existence of such a well is demonstrated by conventional free-free absorption spectroscopy at thermal energies. Estimation for cold atom photoassociation and cold molecule formation rates are obtained within a perturbative approach [Drag et al., IEEE J. Quant. Electr. 36, 1378 (2000)], and are found observable for usual conditions of photoassociation experiments. Received 1st March 2001     

Coherent control of ultracold molecule dynamics in a magneto-optical trap by use of chirped femtosecond laser pulses

We have studied the effects of chirped femtosecond laser pulses on the formation of ultracold molecules in a Rb magneto-optical trap. We have found that application of chirped femtosecond pulses suppressed the formation of (85)Rb and (87)Rb(2) a(3)sigma(+)(u) molecules in contrast to comparable nonchirped pulses, cw illumination, and background formation rates. Variation of the amount of chirp indicated that this suppression is coherent in nature, suggesting that coherent control is likely to be useful for manipulating the dynamics of ultracold quantum molecular gases.     

Two body loss rate in a magneto-optical trap of metastable He

We have measured the two body loss rate in a magneto-optical trap containing triplet metastable He atoms. We find a rate constant cm³/s at a -8 MHz detuning, with an uncertainty of a factor 2. This measurement is in disagreement with a recent experiment which measures the absolute, ion-producing collision rate, but agrees with several other published measurements. Received 20 April 1999 and Received in final form 12 July 1999     

Superradiant Rayleigh scattering from a Bose-Einstein condensate with the incident laser along the long axis

We have realized the superradiant Rayleigh scattering in an elongated Bose-Einstein condensate, where the pump laser pulse travels along the long axis. In this incident configuration the spatial asymmetries and the backward scattering pattern are reported. The spatial and temporal evolution of the superradiant process is analyzed by our semiclassical model.     

Rosen-Zener model in cold molecule formation

The Rosen-Zener model for association of atoms in a Bose-Einstein condensate is studied. Using a nonlinear Volterra integral equation, we obtain an analytic formula for final probability of the transition to the molecular state for weak interaction limit. Considering the strong coupling limit of high field intensities, we show that the system reveals two different time-evolution pictures depending on the detuning of the frequency of the associating field. For both limit cases we derive highly accurate formulas for the molecular state probability valid for the whole range of variation of time. Using these formulas, we show that at large detuning regime the molecule formation process occurs almost non-oscillatory in time and a Rosen-Zener pulse is not able to associate more than one third of atoms at any time point. The system returns to its initial all-atomic state at the end of the process and the maximal transition probability 1/6 is achieved when the field intensity reaches its peak. In contrast, at small detuning the evolution of the system displays large-amplitude oscillations between atomic and molecular populations. We find that the shape of the oscillations in the first approximation is defined by the field detuning only. Finally, a hidden singularity of the Rosen-Zener model due to the specific time-variation of the field amplitude at the beginning of the interaction is indicated. It is this singularity that stands for many of the qualitative and quantitative properties of the model. The singularity may be viewed as an effective resonance-touching.     

Cold inelastic collisions between lithium and cesium in a two-species magneto-optical trap

We investigate collisional properties of lithium and cesium which are simultaneously confined in a combined magneto-optical trap. Trap-loss collisions between the two species are comprehensively studied. Different inelastic collision channels are identified, and inter-species rate coefficients as well as cross-sections are determined. It is found that loss rates are independent of the optical excitation of Li, as a consequence of the repulsive Li^*-Cs interaction. Li and Cs loss by inelastic inter-species collisions can completely be attributed to processes involving optically excited cesium (fine-structure changing collisions and radiative escape). By lowering the trap depth for Li, an additional loss channel of Li is observed which results from ground-state Li-Cs collisions changing the hyperfine state of cesium. Received 28 December 1998 and Received in final form 16 February 1999     

Light-induced effect on the density distribution in a sample of cold trapped atoms

As it is known [1] an intense laser field can induce atom-atom interaction according to a dipole-dipole R ^–3 law. Such an interaction depends on the angle between light polarization and interatomic vector-position R. This angular dependence may produce an anisotropy in the spatial density distribution of the confined sample of cold atoms. We develop the main relations and apply them to the case of an atomic cloud of cold trapped neutral atoms with the density higher than or of the order of ^–3, where is the wavelength of light. The results presented here show the effect of such an interaction in a density regime of high experimental interest.     

High-order resonances in a parametrically excited magneto-optical trap

We present analytical and numerical study of high-order parametric resonance in a driven magneto-optical trap of cold atoms. We have obtained the general solutions for parametric resonance of arbitrary order. In particular, the amplitude and phase of atomic limit-cycle motion is expressed as a function of the modulation amplitude and frequency. Moreover, the atomic dynamics for high-order parametric resonance is investigated in terms of the Hamiltonian approach, which is useful in studying transitions between attractors. We find that the analytical results are in good agreement with the numerical calculations.     

Decay rate measurement of lithium in a magneto-optical trap

A detailed account of various experimental techniques developed during the study on the decay rate coefficient of laser trapped ⁷Li atoms are presented. The frequency of a dye laser is stabilized using a simple sealed-off cell specially designed for Li vapor. The accurate number of trapped atoms are obtained by measuring the fluorescence intensity and the population ratio between the ground and the excited states by absorption coefficient measurement. The absolute value of the collisional lossrate coefficient of trapped ⁷Li atoms is determined by analyzing the temporal change of the fluorescence intensity when the supply of the Li beam is turned off.     

Realization of a single-beam mini magneto-optical trap: A candidate for compact CPT cold atom-clocks

We have demonstrated the experimental realization of a single-beam mini magneto-optical trap of ⁸⁷Rb atoms, originally designed for a cold atom-clock with coherent population trapping (CPT). Only one beam is used as cooling, trapping and repumping beams rather than the three pairs of orthogonal beams of the standard magneto-optical trap. The core optics, which consists of a modified pyramidal funnel type mirror, a quarter-wave plate and a retroreflect mirror, is installed inside a mini titanium cubic chamber. The vacuum system, rubidium source, magnetic field coils and beam expander are designed in a compact geometry. As many as 1.1 × 10⁷ rubidium atoms are cooled and trapped, and thus the mini trap is ready for the implementation of a novel compact coherent population trapping cold atom-clock.     

Coherent enhancement of broadband frequency up-conversion in BBO crystal by shaping femtosecond laser pulses

An optimal feedback control of broadband frequency up-conversion in BBO crystal is experimentally demonstrated by shaping femtosecond laser pulses based on genetic algorithm, and the frequency up-conversion efficiency can be enhanced by ∼16%. SPIDER results show that the optimal laser pulses have shorter pulse-width with the little negative chirp than the original pulse with the little positive chirp. By modulating the fundamental spectral phase with periodic square distribution on SLM-256, the frequency up-conversion can be effectively controlled by the factor of about 17%. The experimental results indicate that the broadband frequency up-conversion efficiency is related to both of second harmonic generation (SHG) and sum frequency generation (SFG), where the former depends on the fundamental pulse intensity, and the latter depends on not only the fundamental pulse intensity but also the fundamental pulse spectral phase.     

Design and interpretation of laser pulses for the control of quantum systems

An extended theory of optimal control strategies for the design of feasible laser pulses is presented. The theoretical framework includes the control of population-transfer and expectation values. Selected studies for two-dimensional model systems providing aspects of uni- and bimolecular reactions are presented. Subsequent analysis of the obtained laser fields is performed to gain insight into the underlying reaction mechanisms. The effect of non-uniform spatial laser profiles and the orientation of the molecules versus the laser beam on controlling quantum dynamics is investigated with the main interest focussed on the achievable quantum yield and information on variable pathways stored in the laser field. Received: 10 November 1999 / Published online: 5 July 2000     

Structure formation on the surface of alloys irradiated by femtosecond laser pulses

Laser-induced periodic surface structures with different spatial characteristics have been observed after multiple linearly polarized femtosecond laser pulse (120 fs, 800 nm, 1 Hz to 1 kHz pulse repetition frequency) irradiation on alloys. With the increasing number of pulses, nanoripples, classical ripples and modulation ripples with a period close to half of classical ripples have all been induced. The generation of second-harmonic has been supposed to be the main mechanism in the formation of modulation ripples.     

Nonlinear mixing between the emission of a cw laser and a femtosecond laser

We mix the emission of a femtosecond Ti:sapphire laser with the emission of a continuous wave infrared laser in a beta-barium borate crystal. Green light with a center wavelength of 527 nm and a spectral width of 2.5 nm resulting from sum frequency generation is detected. An intensity study verifies that a nonlinear χ⁽²⁾ process is at the origin of the green light generation. The experimentally obtained conversion efficiency of 7 × 10⁻¹⁰ is in good agreement to simple theoretical considerations.     

Spectrally resolved coherent transient signal for ultracold rubidium molecules

We present spectrally resolved pump-probe experiments on the photoassociation of ultracold rubidium atoms with shaped ultrashort laser pulses. The pump pulse causes a free-bound transition leading to a coherent transient signal of rubidium molecules in the first excited state. In order to achieve a high frequency resolution the bandwidth of the pump pulse is reduced to a few wavenumbers. The frequency dependence of the transient signal close to the D1 atomic resonance is investigated for characteristic pump-probe delay times. The observed spectra, which show a pronounced dip for pump-probe coincidence, are interpreted using quantum dynamical calculations.     

Ultrafast control of coherent population transfer with a train of femtosecond pulse pairs

We investigate the ultrafast control of coherent population transfer in a Λ-type three-level system with a train of pump-Stokes femtosecond pulse pairs, where the pulse sequences can be produced either by optical delay line or by pulse shaping with sinusoidal phase modulation. It is shown that when the pump and Stokes pulses in each pair are applied in the counterintuitive order, similar to that in the stimulated Raman adiabatic passage technique, due to temporal quantum interference (besides optical interference in the case of overlapped subpulses), ultrafast control of coherent population transfer can be achieved by scanning the inter-pair time delay or by changing the sinusoidal phase modulation parameters. This method has potential applications in ultrafast control of chemical reactions and quantum information processing.