Overview of published articles & reports


  • Title: Damping of superfluid flow by a thermal cloud

    Authors: R. Meppelink, S.B. Koller, J.M. Vogels, H.T.C. Stoof, and P. van der Straten
    Abstract: One of the principal signatures of superfluidity is the frictionless flow of a superfluid through another substance. Here we study the flow of a Bose-Einstein condensate (the superfluid) through a thermal cloud and study its damping for different harmonic confinements and temperatures. The measured damping rates close to the collisionless regime are found to be in good agreement with Landau damping and become smaller for more homogeneous systems. In the hydrodynamic regime we observe additional damping due to collisions and we discuss the implications of these findings for superfluidity in this system.
    Published: Phys. Rev. Lett. 103, 265301 (2009)
    Download: oop6.pdf
    Archive: 0909.0886


  • Title: Phase contrast imaging of Bose condensed clouds

    Authors: R. Meppelink, R.A. Rozendaal, S.B. Koller, J.M. Vogels and P. van der Straten
    Abstract: Phase contrast imaging is used to observe Bose-Einstein condensates (BECs) at finite temperature \insitu. The imaging technique is used to accurately derive the absolute phase shift of a probe laser beam due to both the condensate and the thermal cloud. The accuracy of the method is enhanced by using the periodicity of the intensity signal as a function of the accumulated phase. The measured density profiles can be described using a two relevant parameter fit, in which only the chemical potential and the temperature are to be determined. This allows us to directly compare the measured density profiles to different mean-field models in which the interaction between the condensed and thermal atoms is taken into account to various degrees.
    Published: Phys. Rev. A 81, 053632 (2010)
    Download: pci.pdf
    Archive: 0909.4429


  • Title: Observation of shock waves in a large Bose-Einstein condensate

    Authors: R. Meppelink, S.B. Koller, J.M. Vogels, P. van der Straten, E.D. van Ooijen, N.R. Heckenberg, H. Rubinsztein-Dunlop, S.A. Haine, M.J. Davis
    Abstract: We observe the formation of shock waves in a Bose-Einstein condensate containing a large number of sodium atoms. The shock wave is initiated with a repulsive, blue-detuned light barrier, intersecting the BEC, after which two shock fronts appear. We observe breaking of these waves when the size of these waves approaches the healing length of the condensate. At this time, the wave front splits into two parts and clear fringes appear. The experiment is modeled using an effective 1D Gross-Pitaevskii-like equation and gives excellent quantitative agreement with the experiment, even though matter waves with wavelengths two orders of magnitude smaller than the healing length are present. In these experiments, no significant heating or particle loss is observed.
    Published: Phys. Rev. A 80, 043606 (2009)
    Download: shockwave.pdf
    Archive: 0907.3989


  • Title: Sound propagation in a Bose-Einstein condensate at finite temperatures

    Authors: R. Meppelink, S.B. Koller, and P. van der Straten
    Abstract: We study the propagation of a density wave in a magnetically trapped Bose-Einstein condensate at finite temperatures. The thermal cloud is in the hydrodynamic regime and the system is therefore described by the two-fluid model. A phase-contrast imaging technique is used to image the cloud of atoms and allows us to observe small density excitations. The propagation of the density wave is used to determine the speed of sound as a function of the temperature. We find the speed of sound to be in good agreement with calculations based on the Landau two-fluid model.
    Published: Phys. Rev. A 80, 043605 (2009)
    Download: sound3.pdf
    Archive: 0909.3455


  • Title: Enhanced Heat Flow in the Hydrodynamic Collisionless Regime

    Authors: Meppelink, R; van Rooij, R; Vogels, JM; van der Straten, P
    Abstract: We study the heat conduction of a cold, thermal cloud in a highly asymmetric trap. The cloud is axially hydrodynamic, but due to the asymmetric trap radially collisionless. By locally heating the cloud we excite a thermal dipole mode and measure its oscillation frequency and damping rate. We find an unexpectedly large heat conduction compared to the homogeneous case. The enhanced heat conduction in this regime is partially caused by atoms with a high angular momentum spiraling in trajectories around the core of the cloud. Since atoms in these trajectories are almost collisionless they strongly contribute to the heat transfer. We observe a second, oscillating hydrodynamic mode, which we identify as a standing wave sound mode.
    Published: Phys. Rev. Lett 103, 095301 (2009)
    Download: thermal_conductivity2.pdf
    Archive: 0908.1659


  • Title: Atom lithography without laser cooling

    Authors: B. Smeets, P. van der Straten, T. Meijer, C.G.C.H.M. Fabrie, K.A.H. van Leeuwen
    Abstract: Using direct-write atom lithography, Fe nanolines are deposited with a pitch of 186 nm, a full width at half maximum (FWHM) of 50 nm, and a height of up to 6 nm. These values are achieved by relying on geometrical collimation of the atomic beam, thus without using laser collimation techniques. This opens the way for applying direct-write atom lithography to a wide variety of elements.
    Published: Appl. Phys. B 98, 697-705 (2009)
    Download: atom_litho.pdf
    Archive: 0908.2733


  • Title: Cruising through molecular bound-state manifolds with radiofrequency

    Authors: F. Lang, P. v. d. Straten, B. Brandstatter, G. Thalhammer, K. Winkler, P.S. Julienne, R. Grimm and J. Hecker Denschlag
    Abstract: The production of ultracold molecules with their rich internal structure is currently attracting considerable interest. For future experiments, it will be important to efficiently transfer these molecules from their initial internal quantum state at production to other quantum states of interest. Transfer tools such as optical Raman schemes, radiofrequency transitions or magnetic field ramping exist, but are either technically involved or limited in their applicability. Here, we demonstrate a simple, highly efficient hybrid transfer method that overcomes a number of the previous limitations. The scheme is based on magnetically tuned mixing of two neighbouring molecular levels, which enables otherwise forbidden radiofrequency transitions between them. By repeating this process at various magnetic fields, molecules can be successively transported through a large manifold of quantum states. Applying nine transfers, we convert very weakly bound Feshbach molecules to a much more deeply bound level with a binding energy corresponding to 3.6 GHz.As an important spin-off of our experiments, we demonstrate a high-precision spectroscopy method for investigating level crossings.
    Published: Nature Physics 4, 223 (2008)
    Download: nphys838.pdf

  • Title: Coherent Optical Transfer of Feshbach Molecules to a Lower Vibrational State

    Authors: K. Winkler, F. Lang, G. Thalhammer, P. v. d. Straten, R. Grimm, and J. Hecker Denschlag
    Abstract: Using the technique of stimulated Raman adiabatic passage (STIRAP) we have coherently transferred ultracold 87Rb2 Feshbach molecules into a more deeply bound vibrational quantum level. Our measurements indicate a high transfer efficiency of up to 87%. Because the molecules are held in an optical lattice with not more than a single molecule per lattice site, inelastic collisions between the molecules are suppressed and we observe long molecular lifetimes of about 1 s. Using STIRAP we have created quantum superpositions of the two molecular states and tested their coherence interferometrically. These results represent an important step towards Bose-Einstein condensation of molecules in the vibrational ground state.
    Published: Phys. Rev. Lett. 98, 043201 (2007)
    Download: stirap.pdf

  • Title: Resonant superradiant backward-scattering as a source for many-particle entanglement

    Authors: K.M.R. van der Stam, R. Meppelink, J.M. Vogels, J.W. Thomsen, P. van der Straten
    Published: Phys. Rev. Lett. (submitted)
    Archive: 0707.1465


  • Title: Reaching the hydrodynamic regime in a Bose-Einstein condensate by suppression of avalanches

    Authors: K. M. R. van der Stam, R. Meppelink, J. M. Vogels, and P. van der Straten
    Abstract: We report the realization of a Bose-Einstein condensate (BEC) in the hydrodynamic regime. The hydrodynamic regime is reached by evaporative cooling at a relatively low density suppressing the effect of avalanches. With the suppression of avalanches a BEC containing more than 108 atoms is produced. The collisional opacity can be tuned from the collisionless regime to a collisional opacity of more than 2 by compressing the trap after condensation. In the collisional opaque regime a significant heating of the cloud at time scales shorter than half of the radial trap period is measured, which is a direct proof that the BEC is hydrodynamic.
    Published: Phys. Rev. A 75, 031602 (2007)
    Download: Hydro.pdf
    Archive: cond-mat/0609172


  • Title: On the role of Penning ionization in photoassociation spectroscopy

    Authors: E van der Zwan, D van Oosten, D Nehari, P van der Straten and H T C Stoof
    Abstract: We study the role of Penning ionization on the photoassociation spectra of He(2 3^S_1)-He(2 ^3S_1). The experimental set-up is discussed and experimental results for different intensities of the probe laser are shown. For modelling the experimental results we consider coupled-channel calculations of the crossing of the ground state with the excited state at the Condon point. The coupledchannel calculations are first applied to model systems, where we consider two coupled channels without ionization, two coupled channels with ionization, and three coupled channels, for which only one of the excited states is ionizing. Finally, coupled-channel calculations are applied to photoassociation of He(2 ^3S_1)-He(2 ^3S_1) and good agreement is obtained between the model and the experimental results.
    Published: J. Phys. B 39, S825-S847 (2006)
    Download: multi-chan.pdf
    Archive: physics/0603268


  • Title: Large atom number Bose-Einstein condensate of sodium

    Authors: K. M. R. van der Stam, E. D. van Ooijen, R. Meppelink, J. M. Vogels, and P. van der Straten
    Abstract: We describe the setup to create a large Bose-Einstein condensate containing more than 120 10^6 atoms. In the experiment a thermal beam is slowed by a Zeeman slower and captured in a dark-spot magneto-optical trap MOT. A typical dark-spot MOT in our experiments contains 2.0 10^10 atoms with a temperature of 320 microK and a density of about 1.0 10^11 atoms/cm3. The sample is spin polarized in a high magnetic field before the atoms are loaded in the magnetic trap. Spin polarizing in a high magnetic field results in an increase in the transfer efficiency by a factor of 2 compared to experiments without spin polarizing. In the magnetic trap the cloud is cooled to degeneracy in 50 s by evaporative cooling. To suppress the three-body losses at the end of the evaporation, the magnetic trap is decompressed in the axial direction.
    Published: Rev. Sci. Instr. 78, 013102 (2007)
    Download: LargeNbec.pdf
    Archive: physics/0609028


  • Title: Spin-polarizing cold sodium atoms in a strong magnetic field

    Authors: K. M. R. van der Stam, A. Kuijk, R. Meppelink, J. M. Vogels, and P. van der Straten
    Abstract: The efficiency of evaporative cooling, which is used for the creation of a Bose-Einstein condensate, depends strongly on the number of particles at the start of the evaporation. A high efficiency can be reached by filling the magneto-optical trap with a large number of atoms and subsequently transferring these atoms to the magnetic trap as efficiently as possible. In our case, for sodium, this efficiency is limited to 1/3, because the magnetic substates of the F=1 state, which is used in the trapping process, are equally populated. This limit can be overcome by spin-polarizing the sample before the transfer. For sodium atoms, however, the improvement is very small when it is done in a small magnetic field due to the large number of optical transitions in combination with the high optical density. In this paper, we describe spin-polarizing sodium atoms in a high magnetic field. The transfer efficiency is increased by a factor of 2. The high magnetic field makes the process also more robust against variations in the magnetic field, the laser frequency, and the polarization of the laser beam.
    Published: Phys. Rev. A 73, 063412 (2006)
    Download: spinpol.pdf
    Archive: physics/0603255


  • Title: Atom lithography of Fe

    Authors: E. te Sligte, B. Smeets, K. M. R. van der Stam, R. W. Herfst, P. van der Straten, H. C. W. Beijerinck, and K. A. H. van Leeuwen
    Abstract: Direct write atom lithography is a technique in which nearly resonant light is used to pattern an atom beam. Nanostructures are formed when the patterned beam falls onto a substrate. We have applied this lithography scheme to a ferromagnetic element, using a 372 nm laser light standing wave to pattern a beam of iron atoms. In this proof-of-principle experiment, we have deposited a grid of 50-nm-wide lines 186 nm apart. These ultraregular, large-scale, ferromagnetic wire arrays may generate exciting new developments in the fields of spintronics and nanomagnetics. © 2004 American Institute of Physics.
    Published: App. Phys. Lett. 85, 4493 (2004)
    Download: smeets.pdf

  • Title: Laser collimation of an Fe atomic beam on a leaky transition

    Authors: B. Smeets, R.W. Herfst, L.P. Maguire, E. te Sligte, P. van der Straten, H.C.W. Beijerinck, and K.A.H. van Leeuwen
    Abstract: We report results of the first laser collimation of a thermal beam of Fe atoms on the leaky 5D4 - 5F5 transition, with both parallel linear pxpx and crossed linear pxpy laser polarization configurations. The measured atomic beam divergence is compared to a rate-equation model and a quantum Monte Carlo model. The experimental values for the divergence are limited by the finite laser line width, which is comparable to the natural line width of the Fe atom. In general, flux decreases with higher intensities, showing the effect of the leaky transition. At the best beam collimation a RMS = 0.17 mrad, which is for a detuning of d = -gamma and a saturation parameter of s = 6, the flux decreased to approximately 70%. Highest flux was measured for a detuning of d = -2 gamma and s = 4, reaching 135% of the uncooled value. From our measurements we estimate the total leak rate to be 1/(240 ± 40), which is in good agreement with the literature value of 1/244. The crossed linear polarization configuration is the better choice, with a slightly better collimation but the same atomic beam flux. Plugging of the largest leak would increase the flux to at least 80% of the closed transition value, resulting in better contrast for atom lithography.
    Published: Appl. Phys. B 80, 833-839 (2005)
    Download: smeets2.pdf

  • Title: Barrier-limited surface diffusion in atom lithography

    Authors: E. te Sligte, K. M. R. van der Stam, B. Smeets, P. van der Straten, R. E. Scholten, H. C. W. Beijerinck, and K. A. H. van Leeuwen
    Abstract: Thermally activated surface diffusion has a strong influence on structure widths in atom lithography. We investigate the effects of two barriers to thermally activated atomic diffusion on atom lithography: a thermally activated Ehrlich-Schwoebel barrier, and pollution from the residual gas in the vacuum system. We performed kinetic Monte Carlo simulations using a one-dimensional surface grid. We find that the ES barrier fails to explain the lack of temperature dependence observed experimentally [W. R. Anderson et al., Phys. Rev. A 59, 2476 1999]. The dependencies of the structure width on temperature, vacuum conditions, and beam characteristics can be explained using the pollutant adatom hypothesis. Only the variation of structure width with deposition duration was not entirely reproduced by this model. We attribute this to the one-dimensional nature of our simulations. These results demonstrate that barrier-limited diffusion can play an important role in atom lithography, and that pollutant adatoms are a likely candidate barrier.
    Published: J. App. Phys. 43, 41-44 (2005)
    Download: sligte.pdf

  • Title: Bose-Einstein condensatie

    Authors: E.D. van Ooijen, K.M.R. van der Stam, D. van Oosten, F. Ditewig, J. van der Weg, J.M. Vogels en P. van der Straten
    Published: Nevac blad, Jaargang 43, nr. 2, p. 41-44 (2005)
    Download: nevac.pdf

  • Title: Atomic Trapping and Cooling

    Authors: P. van der Straten and H.J. Metcalf
    Published: Wiley-VCH Encyclopedia of physics
    Download: A29a.pdf

  • Title: Laser cooling simulation based on the semiclassical approach

    Authors: B. Smeets, R.W. Herfst, E. te Sligte, P. van der Straten, H.C.W. Beijerinck, and K.A.H. van Leeuwen
    Abstract: We investigate the region of validity of the semiclassical approach to simulating laser cooling. We conclude that for the commonly used pi_x-pi_y polarization-gradient configuration, the semiclassical approach is valid only for transitions with recoil parameters omega_r on the order of 10-4 or less. For the standard laser-cooling transitions only the transitions in Rb and Cs satisfy this condition. For the Doppler and sigma+-sigma- polarization-gradient configuration the semiclassical approach is valid for most of the commonly used transitions; however, the expected gain in execution speed compared with quantum Monte Carlo calculations has been realized only in part. A drastic reduction in calculation time is to be expected by implementing an analytical approach to the long-term contribution of the diffusion coefficient
    Published: J. Opt. Soc. Am. B 22, 2372 (2005)
    Download: simu.pdf

  • Title: Rotationally induced Penning ionization of ultracold photoassociated helium dimers

    Authors: J. Leonard, A. P. Mosk, M. Walhout, M. Leduc, M. van Rijnbach, D. Nehari and P. van der Straten
    Abstract: We have studied photoassociation of metastable 23S1 helium atoms near the 23S1-23P2 asymptote by both ion detection in a magneto-optical trap and trap-loss measurements in a magnetic trap. A detailed comparison between the results of the two experiments gives insight into the mechanism of the Penning ionization process. We have identified four series of resonances corresponding to vibrational molecular levels belonging to different rotational states in two potentials. The corresponding spin states become quasi-purely quintet at small interatomic distance, and Penning ionization is inhibited by spin conservation rules. Only a weak rotational coupling is responsible for the contamination by singlet spin states leading to a detectable ion signal. However, for one of these series Bose statistics does not enable the rotational coupling and the series detected through trap-loss does not give rise to su±cient ionization for detection.
    Published: Eur. Phys. Lett. 70, 190-196 (2005)
    Download: ensutrecht.pdf
    Archive: cond-mat/0502401


  • Title: Inelastic light scattering from a Mott insulator

    Authors: D. van Oosten, D. B. M. Dickerscheid, B. Farid, P. van der Straten, and H. T. C. Stoof
    Abstract: We propose to use Bragg spectroscopy to measure the excitation spectrum of the Mott in- sulator state of an atomic Bose gas in an optical lattice. We calculate the structure factor of the Mott insulator taking into account both the selfenergy corrections of the atoms and the corresponding dressing of the atom-photon interaction. We determine the scattering rate of photons in the stimulated Raman transition and show that by measuring this scattering rate in an experiment, in particular the excitation gap of the Mott insulator can be determined.
    Published: Phys. Rev. A, 71, 021601(R) (2005)
    Download: inelastic.pdf
    Archive: cond-mat/0405492


  • Title: Laser frequency stabilization using Doppler-free bichromatic spectroscopy

    Authors: E.D. van Ooijen, G. Katgert, and P. van der Straten
    Abstract: We report on a method of laser stabilization, Doppler-free bichromatic lock (DFBL) in which we use the differential signal of two Lamb-dips shifted in frequency by means of an AOM. We show that this method provides us with a tool for precision locking to a molecular or atomic transition and is relatively simple to implement in a diode or dye laser system without the use of a frequency modulation of the laser or lock-in amplifier. A simple model description of this method is presented and the experimental realization is shown for a dye laser at the Na D1 and D2 line.
    Published: App. Phys. B 79, 57-59 (2004)
    Download: apb1522.pdf

  • Title: Analysis of photoassociation spectra for giant helium dimers

    Authors: J. Léonard, A. P. Mosk, M. Walhout, P. van der Straten, M. Leduc, and C. Cohen-Tannoudji
    Abstract: We perform a theoretical analysis to interpret the spectra of purely long-range helium dimers produced by photoassociation (PA) in an ultracold gas of metastable helium atoms. The experimental spectrum obtained with the PA laser tuned closed to the 2 3S12 3P0 atomic line has been reported in a previous paper. Here, we first focus on the corrections to be applied to the measured resonance frequencies in order to infer the molecular binding energies. We then present a calculation of the vibrational spectra for the purely long-range molecular states, using adiabatic potentials obtained from perturbation theory. With retardation effects taken into account, the agreement between experimental and theoretical determinations of the spectrum for the 0u+ purely long-range potential well is very good. The results yield a determination of the lifetime of the 2 3P atomic state.
    Published: Phys. Rev. A 69, 032702 (2004)
    Download: longrangehelium.pdf
    Archive: quant-ph/0310006


  • Title: Mott insulators in an optical lattice with high filling factors

    Authors: D. van Oosten, P. van der Straten, and H.T.C. Stoof
    Abstract: We discuss the superfluid to Mott insulator transition of an atomic Bose gas in an optical lattice with high filling factors. We show that also in this multi-band situation, the long-wavelength physics is described by a single-band Bose-Hubbard model. We determine the many-body renormalization of the tunneling and interaction parameters in the effective Bose-Hubbard Hamiltonian, and consider the resulting model at nonzero temperatures. We show that in particular for a one or two-dimensional optical lattice, the Mott insulator phase is more difficult to realize than anticipated previously.
    Published: Phys. Rev. A 67, 033606 (2003)
    Download: numberfluct.pdf
    Archive: cond-mat/0205066


  • Title: Photoassociation of ultracold sodium atoms

    Authors: A. Amelink and P. van der Straten
    Abstract: Using ultra-cold atoms a new technique of photoassociation spectroscopy has been developed. The technique has an unprecedented resolution and can be used to obtain high-resolution data on singly and doubly excited states of diatomic molecules. We describe the principle of the technique and show results for one specific case, namely the Na_2 molecule.
    Published: Phys. Scr. 68, C82-89 (2003)
    Download: camp.pdf

  • Title: Laser frequency stabilization using an Fe-Ar hollow cathode discharge cell

    Authors: B. Smeets, R. C. M. Bosch, P. van der Straten, E. te Sligte, R.E. Scholten, H. C. W. Beijerinck and K.A.H. van Leeuwen
    Abstract: Polarization spectroscopy of an Fe-Ar hollow cathode discharge cell was used to lock a frequency doubled Ti:Sapphire laser to the 372 nm 5D4->5F5 transition of 56Fe. The discharge cell produced a density of 10^18 m^-3 ground state 56Fe atoms at a temperature of 650 K, a density comparable to a conventional oven at 1500 K. Saturated absorption spectroscopy and two schemes of polarization spectroscopy were compared with respect to signal-to-background ratio and the ef- fect of velocity changing collisions. The laser was locked within 0.2 MHz for hours by feedback of the dispersive polarization spectroscopy signal.
    Published: submitted to Appl. Phys. B (2003)
    Download: hollow_cathode.pdf

  • Title: Laser cooling and trapping of atoms

    Authors: H.J. Metcalf and P. van der Straten
    Abstract: This article presents a review of some of the principal techniques of laser cooling and trapping that have been developed during the past 20 years. Its approach is primarily experimental, but its quantitative descriptions are consistent in notation with most of the theoretical literature.
    Published: The Optics Encyclopedia, pg. 975-1014, eds. Brown, Creath, Kogelnik, Kriss, Schmit, Weber, Wiley-VCH, Weinheim, Germany, 2004.
    Download: vch.pdf

  • Title: Laser cooling and trapping of atoms

    Authors: H.J. Metcalf and P. van der Straten
    Abstract: This paper presents a review of some of the principal techniques of laser cooling and trapping that have been developed during the past 20 years. Its approach is primarily experimental, but its quantitative descriptions are consistent in notation with most of the theoretical literature. It begins with a simplified introduction to optical forces on atoms, including both cooling and trapping. Then its three main sections discuss its three selected features of 1) quantization of atomic motion, 2) effects of the multilevel structure of atoms, and 3) the effects of polychromatic light. Each of these features is an expansion in a different direction from the simplest model of a classical, two level atom moving in a monochromatic laser field.
    Published: J. Opt. Soc. Am. B 20, 887 (2003)
    Download: visby.pdf

  • Title: The Quest for BEC

    Authors: Peter van der Straten and Harold Metcalf
    Abstract: Laser cooling of atoms was suggested in the 1970's as a way to perform better laser spectroscopy. Since it has been very successful in producing cold, dense sample of atoms it is nowadays used in many atomic physics experiments. In this lecture note we will concentrate on those techniques, that are used to cool and trap atoms to phase space densities sufficient to observe a phase transition of the atoms to a Bose-Einstein condensation.
    Published: Interaction in Ultracold Gases, ed. M. Weidemuller and C. Zimmerman, pg. 1-63, Wiley-VCH, Germany, 2003
    Download: heidelberg.pdf

  • Title: Energy distributions of He^+ and He_2^+ ions formed in ultracold He(2 ^3S_1)+He(2 ^3P_2) collisions

    Authors: M. Pieksma, M. Cizek, J. W. Thomsen, P. van der Straten, and A. Niehaus
    Abstract: Energy distributions of He^+ and He_2^+ ions produced in ultracold He(2 ^3S_1)+He(2 ^3P_2) collisions are presented. These helium metastable atoms are trapped and cooled in a magneto-optical trap, and have a temperature of 1.160.2 mK. Initial kinetic energies of the product He^+ and He_2^+ ions are analyzed in energy, using a weak electrostatic extraction field in combination with a time-of-flight technique. A pronounced structure is observed in the He1 ion distribution, which can be well explained by a theoretical model that considers the avoided crossings between only three metastable ^3\Sigma_g^+ states of the transient He_2 molecule. The measured He_2^+ distribution has a width that is consistent with the maximum recoil energy of 1.1 meV.
    Published: Phys. Rev. A 66, 022703 (2002)
    Download: he_ion.pdf

  • Title: Magnetic nanodots from atomic Fe: Can it be done?

    Authors: E. te Sligte, R.C.M. Bosch, B. Smeets, P. van der Straten, H.C.W. Beijerinck, and K.A.H. van Leeuwen
    Abstract: Laser focusing of Fe atoms offers the possibility of creating separate magnetic structures on a scale of 10 nm with exact periodicity. This can be done by using the parabolic minima of the potential generated by a standing light wave as focusing lenses. To achieve the desired 10-nm resolution, we need to suppress chromatic and spherical aberrations, as well as prevent structure broadening caused by the divergence of the incoming beam. Chromatic aberrations are suppressed by the development of a supersonic Fe beam source with speed ratio S=11+-1. This beam has an intensity of 3 x 10^15 sr-1 s-1. The spherical aberrations of the standing light wave will be suppressed by aperturing with beam masks containing 100-nm slits at 744-nm intervals. The beam divergence can be reduced by application of laser cooling to reduce the transverse velocity. We have constructed a laser system capable of delivering over 500 mW of laser light at 372 nm, the wavelength of the ^5D_4->^5F_5 atomic transition of ^56Fe we intend to use for laser cooling. Application of polarization spectroscopy to a hollow cathode discharge results in a locking system holding the laser continuously within 2 MHz of the desired frequency.
    Published: Proc. Nat. Acad. Sci. 99, 6509 (2002)
    Download: nanodots.pdf

  • Title: Supersonic Fe beam source for chromatic aberration-free laser focusing of atoms

    Authors: R.C.M. Bosch, H.C.W. Beijerinck, P. van der Straten, and K.A.H. van Leeuwen
    Abstract: A monochromatic Fe beam is generated by heated supersonic expansion of argon seeded with Fe vapor. At a nozzle temperature of 1930 K and 800 torr argon inlet pressure the Fe beam has an axial velocity spread of 8% and intensity of 3 x 10^15 sr-1 s-1, corresponding to a deposition rate of 10 nm/h at 150 mm from the nozzle. The two-chamber alumina crucibles are chemically stable for liquid Fe. With 400 mm3 Fe we have operated for more than 200 hours without reloading. The power consumption at 1930 K is 750 W. Temperature stability at constant power (without feedback) is better than 30 K. The source is intended for deposition of nanostructures by laser focusing of the Fe beam. The small axial velocity spread virtually eliminates the increase in focal spot size due to chromatic aberration.
    Published: Eur. Phys. J. Appl. Phys., 18, 221 (2002)
    Download: Fe_source.pdf

  • Title: Laser cooling and trapping

    Authors: H.J. Metcalf and P. van der Straten
    Published: In Handbook of Optics III, ed. M. Bass, J.M. Enoch, E. van Strykland, and B. Wolfe, McGraw-Hill, New York, 2002
    Download: handpubl.pdf

  • Title: Polarization in Collisions between Ultra-cold Sodium Atoms

    Authors: H.G.M. Heideman, P. van der Straten and A. Amelink
    Abstract: We have studied polarization effects in the photoassociation of two colliding ultra-cold Na-atoms. In a first step the two colliding atoms absorb a laser photon to associate to a singly excited molecule in a bound state of the 0_g^- or 1_g potential. In a second step the excited molecule absorbs a second photon and is further excited to a bound level in a doubly excited potential (0_u^- or 1_u which are autoionizing). By measuring the produced ions with parallel and perpendicular polarizations of the two laser beams used possible polarization effects can be studied. We find strong polarization effects if a 0_g^- state is used as the intermediate state and very little or no effects at all if a 1_g state is used.
    Published: To be published in Phys. Rev. Lett.
    Download: pol_afh_pas.pdf

  • Title: Quantum phases in an optical lattice

    Authors: D. van Oosten, P. van der Straten, and H. T. C. Stoof
    Abstract: We present the zero-temperature phase diagram of bosonic atoms in an optical lattice, using two different mean-field approaches. The phase diagram consists of various insulating phases and a superfluid phase. We explore the nature of the insulating phase by calculating both the quasiparticle and quasihole dispersion relation. We also determine the parameters of our single band Bose-Hubbard model in terms of the microscopic parameters of the atoms in the optical lattice.
    Published: Phys. Rev. A 63, 053601 (2001)
    Download: hubbard.pdf
    Archive: cond-mat/0011108


  • Title: Construction of a low velocity metastable Helium atomic beam

    Authors: G.R. Woestenenk, J.W. Thomsen, M. van Rijnbach, P. van der Straten, and A. Niehaus
    Abstract: We have constructed an atomic beam of metastable helium atoms \Hetts\ with a mean velocity of 300 m/s (15 K) and a yield of 3\times10^12 atoms/s sr. The metastable atoms are produced in a DC-discharge in a cryogenic environment cooled by liquid helium. Using a hexapole magnetic lens we have increased further the beam intensity by focusing the metastable atoms. Initial studies show a factor of 2.5 increase in the beam flux but more is expected when the hexapole is constructed from permanent hexapole magnets. The \Hetts\ atoms are subsequently loaded into a magneto-optical trap (MOT).
    Published: Rev. Sci. Instr. 72, 3842 (2001)
    Download: source.pdf

  • Title: Penning Ionization in Optical Collisions

    Authors: G. Woestenenk, P. van der Straten, and A. Niehaus
    Abstract: We have studied binary collisions of cold He (2^3S_1) atoms under the influence of nearly-resonant light. The light is tuned below the atomic \sptrans\ transition. A semi-classical model is developed, calculating the absolute ionization rate as a function of the detuning of the light. The calculated ionization rate is compared with measurements and with values that have appeared in the literature. Good agreement is found between theory and measurements.
    Published: To be published in Phys. Rev. A (2001)
    Download: optcol.pdf

  • Title: Spectroscopy of autoionizing doubly excited states in ultracold Na_2 molecules produced by photoassociation

    Authors: A. Amelink, K.M. Jones, P.D. Lett, P. van der Straten and H.G.M. Heideman
    Abstract: We have performed photoassociation spectroscopy to study the doubly excited states of Na2 involved in the associative ionization of colliding Na(3P) atoms. Rovibrational levels of two doubly excited potentials connected to the P3/2+P3/2 asymptote, one of 0u+ and one of 1u symmetry, have been identified. These two doubly excited potentials dominate the route to ionization used in photoassociation experiments of cold Na. Binding energies and rotational constants for all the vibrational levels with binding energies <50 GHz have been measured. The asymptotic potentials and short-range ionization probabilities are extracted from the data.
    Published: Phys. Rev. A 61, 042707 (2000)
    Download: pseudo2col.pdf

  • Title: Photo-association spectroscopy of cold He(2~^3S) atoms

    Authors: N. Herschbach, P. J. J. Tol, W. Vassen, W. Hogervorst, G. Woestenenk, J. W. Thomsen, P. van der Straten, and A. Niehaus
    Abstract: We observe vibrational states by photo-association spectroscopy of cold He(2~^3S) atoms. Photo-association resonances are detected as peaks in the Penning ionization rate over a frequency range of 20~GHz below the atomic 2~^3S_1~-- 2~^3P_2 transition frequency. We have observed three vibrational series, of which two can be identified. A possible mechanism to explain the observed increase of the Penning ionization rate is discussed.
    Published: Phys. Rev. Lett. 84, 1874 (2000)
    Download: pashe.pdf

  • Title: Single-color photoassociative ionization of ultracold sodium: the region between -5 and 0 GHz

    Authors: A. Amelink, K.M. Jones, P.D. Lett, P. van der Straten and H.G.M. Heideman
    Abstract: We have measured the rate of production of Na_2^+ ions in collisions of ultracold Na atoms held in a magneto-optical trap (MOT) as a function of probe laser detuning using a single-color probe beam. The ion rate is measured with the atoms mainly in the 3S(f=2) level (``bright MOT'') or the 3S(f=1) level (``dark MOT''). Using recent experimental information about the doubly-excited autoionizing states of Na_2 we find that the large structures in the first 5 GHz red of the atomic transition frequency are due to a doubly-resonant excitation process with levels in the 0_g^- potential (asymptotically connected to 3S_1/2+3P_3/2) as the intermediate states, and levels in the 0_u^- and 1_u potentials (asymptotically connected to 3P_3/2+3P_3/2) as the final autoionizing states. We can account for nearly all of the observed structure in this region of the spectra for both the bright and dark MOT.
    Published: Phys. Rev. A 62, 013408 (2000)
    Download: 5GHz.pdf

  • Title: Laser cooling and trapping

    Authors: H.J. Metcalf and P. van der Straten
    Abstract: The Book on Laser Cooling and Trapping
    Published: Springer Verlag, New York, 1999

  • Title: Collisions between ultracold metastable He atoms

    Authors: G. Woestenenk, H.C. Mastwijk, J.W. Thomsen, P. van der Straten, M. Pieksma, M. van Rijnbach, A. Niehaus
    Abstract: We present experimental data on collisions between excited He-atoms occurring in a magneto-optical trap (MOT) at a temperature of 1.1 mK. He(23S)-atoms produced in a discharge are pre-cooled and trapped using the He(23S)-He(23P2) transition for laser manipulation. Measurements of the Penning ionization rate as a function of the MOT-laser frequency are presented and theoretically analyzed. The analysis, based on a model which is presented in detail for the first time, leads to a good understanding of the complex nature of optical collisions. Further, first and preliminary measurements of the kinetic energy distributions of He_2^+- and He^+-ions formed by Penning ionization in optical collisions are presented.
    Published: Nucl. Instr. and Meth. in Phys. Res. B 154, 194-203 (1999)
    Download: woestenenk-nimb.pdf

  • Title: Photo Associative Reactions of Laser Cooled Sodium

    Authors: P. van der Straten
    Abstract: We describe the novel technique of dynamical spectroscopy of transient molecules. We apply this technique to the study of ultra-cold collisions of Na atoms at 200 muK. By careful choice of laser intensities and detunings we can unravel the dynamics of these collisions and obtain information on interactions taking place at internuclear distances of 100-1000 a_0 and interaction energies below 1 mu-eV.
    Published: In Photonic, electronic and atomic collisions, ed. F. Aumayr and H. Winter, World Scientific, Singapore, 1998
    Download: icpeac97.pdf

  • Title: Optical collisions of cold, metastable helium atoms

    Authors: H.C. Mastwijk, J.W. Thomsen, P. van der Straten and A. Niehaus
    Abstract: We have studied the optical collisions of cold, metastable helium atoms in a magneto-optical trap. We have detected the rate of Penning and associative ionization of two metastable helium atoms at a temperature of 1 mK with and without nearly resonant light. We find that the associative ionization rate is increased with more than a factor 20 due to the presence of the light field. The absolute ionization rate near resonance of \EE{(1.9\pm0.8)}{-9} \ccps\ for the optical collision differs two orders of magnitude with the rate reported by Bardou \etal\ (\Journal{Europhys. Lett.}{20}{681}{1992}). Our experimental findings are in good agreement with theory. We present a simple, semi-classical model, which accounts for all the observed features.
    Published: Phys. Rev. Lett. 80, 5516 (1998)
    Download: optical.pdf

  • Title: Photo induced collisions with laser cooled He* atoms

    Authors: H.C. Mastwijk, M. van Rijnbach, J.W. Thomsen, P. van der Straten and A. Niehaus
    Abstract: This paper presents an experimental investigation of cold collisions between metastable Helium atoms in an optical trap at 1mK. Penning (PI) and associative (AI) ionization reactions are distinguished using a mass spectrometer and studied under influence of near resonant laser light. Sensitive behavior of the ion rate is observed when the laser is tuned close to resonance. Experimental findings are well described, on an absolute scale, by a semi-classical model we have developed for optical collisions and by a modified Julienne-Vigue model.
    Published: Eur. Phys. J. D 4, 131-137 (1998)
    Download: phid.pdf

  • Title: Vibrational state distribution of Na_2^+-ions created in ultracold collisions

    Authors: J.J. Blange, J.M. Zijlstra, A. Amelink, X. Urbain, H. Rudolph, P. van der Straten, H.C.W. Beijerinck, and H.G.M. Heideman
    Abstract: The vibrational distribution P(v) of Na_2^+ ions created in ultracold collisions in a magneto-optical trap has been determined. Only two vibrational states with v = 2 and 3 are populated and we find P(2)=0.29\pm0.02 and P(3)=0.71\pm0.02. The results provide conclusive evidence that the ionization mechanism is photo-associative autoionization, and not photo-associative photoionization and will form a fundamental test for the theoretical description of the process.
    Published: Phys. Rev. Lett. 78, 3089 (1997)
    Download: vibratie.pdf

  • Title: Diagnostic technique for Zeeman-compensated atomic beam slowing: Technique and results

    Authors: P.A. Molenaar, P. van der Straten, H.G.M. Heideman and H. Metcalf
    Abstract: We have developed a new diagnostic tool for the study of Zeeman-compensated slowing of an alkali atomic beam. Our time-of-flight technique measures the longitudinal velocity distribution of the slowed atoms with a resolution below the Doppler limit of 30 cm/s. Furthermore it can map the position and velocity distribution of atoms in either ground hyperfine level inside the solenoid without any devices inside the solenoid. The technique reveals the optical pumping effects interfering with the slowing process and shows in detail how the slowing within the solenoid proceeds.
    Published: Phys. Rev. A 55, 605 (1997)
    Download: vspace.pdf

  • Title: Koud, kouder, koudst

    Authors: Peter van der Straten
    Abstract: In november 1997 maakte de Nobel commissie bekend, dat de Nobelprijs voor de natuurkunde voor 1997 uitgereikt zal worden aan Bill Phillips, Steven Chu en Claude Cohen-Tannoudji. Zij krijgen de prijs voor het werk, dat ze verricht hebben op het gebied van de laserkoeling van atomen. Hoewel de Nobelprijs zoals elk jaar aan personen wordt uitgereikt, wordt de prijs van dit jaar vooral beschouwd als een prijs voor het gehele gebied.
    Published: De vakidioot
    Download: vakidioot.pdf

  • Title: Dynamische spectroscopie van kortlevende moleculen

    Authors: Arjen Amelink, Peter van der Straten en Henk Heideman
    Abstract: De afgelopen 10 jaar hebben laserkoelings-technieken een geweldige ontwikkeling doorgemaakt. In laboratoria over de hele wereld worden nu routinematig atomen geproduceerd met temperaturen in het mK-regime en dichtheden van 10^12/cm^3. Het werkpaard in de laserkoeling is de magnetisch-optische val (magneto-optical trap, oftewel MOT), waarbij zes laserbundels op ingenieuze wijze gecombineerd met een niet homogeen magneetveld de atomen tegelijkertijd koelen en vangen. De lage temperatuur en de voldoende hoge dichtheid zorgen ervoor, dat in de MOT interacties kunnen worden bestudeerd in een nieuw regime, het regime van de ultra-koude botsingen.
    Published: Van A tot Q 11, 20, (1997)
    Download: vatq.pdf

  • Title: Long-Range Predissociation in Two-Color Photoassociation of Ultracold Na Atoms

    Authors: P.A. Molenaar, P. van der Straten and H.G.M. Heideman
    Abstract: We report two-color photo-associative ionization of sodium in a Magneto-Optical Trap. The experimental results yield information on both singly and doubly excited states. For the singly-excited states we find that the purely long range 0_g^- state can sustain up to 20 bound states. Higher bound states predissociate into the 3^2P_3/2 + 3^2S_1/2(F_g=1) dissociation continuum due to avoided crossings of the constituting hyperfine components of this potential with those of other molecular symmetries. Based on symmetry and energy considerations we argue that the doubly-excited 1_u state ( adiabatically connected to 2^3\Delta_u) remains autoionizing even when excited only a few GHz above the dissociation continuum.\\
    Published: Phys. Rev. Lett. 77, 1460 (1996).
    Download: twocolor.pdf

  • Title: Force, diffusion and channeling in sub-Doppler laser cooling

    Authors: M.D. Hoogerland, H.F.P. de Bie, H.C.W. Beijerinck, E.J.D. Vredenbregt, K.A.H. van Leeuwen, P. van der Straten and H.J. Metcalf
    Abstract: We present an extensive set of measurements on one-dimensional sub-Doppler cooling and channeling in counterpropagating light beams. The experimental method consists of the measurement of the profile of an initially sub-recoil collimated atomic beam, which is deflected by the interaction with the light field. The initial velocity of the atoms in the direction of the laser beams is varied in the range -1 \leq v_{\perp} (ms^-1) \leq 1 by changing the angle between atomic and laser beams. For the orthogonal circular polarization (\sigma^+ \sigma^-) sub-Doppler cooling configuration, the force and the diffusion coefficient characterizing the cooling process have been determined as a function of the initial velocity v_{\perp} from the average deflection and broadening of the atomic beam profile. We observe transient effects due to the slow evolution of the distribution over the magnetic sublevels to an equilibrium. The experimental results agree very well with Quantum Monte-Carlo simulations and semiclassical calculations. For the orthogonal linear polarization (\pi^x\pi^y) configuration, we demonstrate the validity of the well-known Sisyphus picture for the cooling mechanism by comparing the experimental data to the results of a simple semiclassical Monte-Carlo model incorporating only the dipole force and optical pumping. In weak standing waves of either circular or linear polarization, we demonstrate the characteristic features of channeling.
    Published: Phys. Rev. A 54, 3206 (1996)
    Download: neon_force.pdf

  • Title: Sideband generation using a Fiber-Optical Modulator (FOM)

    Authors: P.A. Molenaar, M.D. Steenbergen and P. van der Straten
    Abstract: We have constructed a Fiber Optical Modulator (FOM), which produces on the incoming laser frequency shifts of the order of 30 GHz. The modulator consist of a fiber of sufficient length, where the transmission is saturated by stimulated Brillouin scattering (SBS). The frequency shift is determined by the parameters of the fiber and is fixed. The ratio of intensity of the shifted frequency and the incoming frequency can be varied over a large range by varying the incoming intensity with respect to the threshold intensity for SBS.
    Download: sbs.pdf

  • Title: Quantum trajectory pictures of laser cooling

    Authors: G. Nienhuis, J. de Kloe and P. van der Straten
    Abstract: We have applied the method of single atom trajectories to study the mechanism behind some cooling schemes in laser cooling. In several cases we recognize the cooling mechanism as being due to a ``Sisyphus'' process, where the atoms move in a spatially varying light shift potential and are optically pumped towards the most light shifted states. In other cases we identify a ``Sisyphus'' process in time, where the light shift is constant and the force on the atom alternates between positive and negative. This process is interrupted by quantum jumps at random instants and in each case we depict the mechanism leading to a cooling force on the atom. In the special case of sub-Doppler laser cooling in a strong magnetic field we obtain 12 jump operators and identify the jump operators responsible for the cooling. The versatility of the single atom trajectory method allows it to be applied to any cooling process and is therefore a very valuable tool in unraveling the physical mechanisms behind cooling processes.
    Published: J. Opt. Soc. Am 12, 520 (1995)
    Download: mcwf.pdf

  • Title: Force measurements on sub-Doppler cooling processes

    Authors: M.D. Hoogerland, H.F.P. de Bie, H.C.W. Beijerinck, K.A.H. van Leeuwen, P. v.d. Straten, E. Vredenbregt, and H.J. Metcalf
    Abstract: We present the first direct measurements of the velocity dependence of the average force and diffusion constant for the metastable neon atoms in a one-dimensional sigma + sigma - laser cooling configuration. Force and diffusion are determined, respectively, from the deflection and broadening of a highly collimated atomic beam by its interaction with a pair of counter-running laser beams. The interaction time is much shorter than the characteristic damping time of the cooling process. The resulted force and diffusion measurements are compared with the results of both semiclassical and quantum Monte Carlo calculations.
    Published: Phys. Rev. Lett. 72, 3332 (1994)
    Download: neon_prl.pdf

  • Title: Cooling and Trapping of Neutral Particles

    Authors: H.J. Metcalf and P. v.d. Straten
    Abstract: This article describes recent experiments that are directed at reduction and control of thermal motion. There are many applications for a collection of slowly moving and/or monovelocity atoms. We can expect new and exquisitely more sensitive atomic beam experiments to probe the electrical neutrality of matter, the effects of parity violating interactions, gravitation, quantum statistics in optical absorption and emission, and other fundamental interactions. Such atoms could serve as an ideal target or beam for a variety of low energy and/or high energy-resolution scattering experiments, chemical reaction studies, or surface studies.
    Published: Phys. Rep. 244, 203 (1994)
    Download: phsrp.pdf

  • Title: Laser Cooling at Low Intensity in a Strong Magnetic Field

    Authors: P. van der Straten, S-Q. Shang, B. Sheehy, H. Metcalf and G. Nienhuis
    Abstract: We have studied theoretically and experimentally the effect of a relatively strong magnetic field on sub-Doppler laser cooling in a one-dimensional optical molasses. We used the operator description of laser cooling with the Larmor precession frequency \omega_Z much stronger then the optical pumping rate. We found velocity selective resonances (VSR) in the force at velocities v_r=n \omega_Z, with n=0,\pm 1, \pm 2 for both the scattering and redistribution force operators. These depend on the relative direction of the magnetic field and the polarization vectors of the light beams. Analytical results for the force on the atom are obtained in two cases that illustrate the effect of the VSR on the force. These formulas are compared with numerical calculations of the force. We also discovered a new redistribution mechanism that relies on the gradient of the eigenstates of the light shift operator, with eigenvalues that are independent of position so that a `Sisyphus' picture does not apply. The theory is compared with many experimental results and excellent agreement is found. We believe that all essential features of laser cooling at low intensity are well described by this operator theory.
    Published: Phys. Rev. A 47, 4160 (1993)
    Download: magcool.pdf

  • Title: Magnetically Induced Laser Cooling for Ne*: Approaching the Recoil Limit

    Authors: M.D. Hoogerland, H.C.W. Beijerinck, K.A.H. van Leeuwen, P. v.d. Straten, and H.J. Metcalf,
    Abstract: Magnetically induced laser cooling to temperatures close to the recoil limit is investigated in one dimension. For a metastable neon beam, we present high precision measurements investigating the actual temperature limit in this cooling process. Using time-of-flight techniques to reduce the effect of the longitudinal velocity spread, we observe cooling at small magnetic field toward v=0 with an rms width of the distribution of 5.4 cm/s, well below the Doppler limit. At a larger magnetic field (0.4 Gauss) the velocity-selective resonances are extremely sharp. Here we find the rms width of the distribution to be 3.4 cm/s, only 1.1 times the recoil speed hbar k/M, corresponding to a temperature T=2.7 muK.
    Published: Eur. Phys. Lett. 19, 669, (1992)
    Download: neon_recoil_paper.pdf

    Comments can be mailed to: P.vanderStraten@phys.uu.nl