| 1. |
- Hagman, Henning, 1981-, et al.
(författare)
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Directed transport with real-time external steering and drifts along pre-designed paths using a Brownian motor
- 2011
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 83:2
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Tidskriftsartikel (refereegranskat)abstract
- We have realized real-time steering of the directed transport in a Brownian motor based on cold atoms in optical lattices and demonstrate drifts along predesigned paths. The transport is induced by spatiotemporal asymmetries in the system, where we can control the spatial part, and we show that the response to changes in asymmetry is very fast. In addition to directional steering, a real-time control of the magnitude of the average drift velocity and an on-off switching of the motor are also demonstrated. We use a noninvasive real-time detection of the transport, enabling feedback control of the system.
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| 2. |
- Hagman, Henning, 1981, et al.
(författare)
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Plasmon-enhanced four-wave mixing by nanoholes in thin gold films
- 2014
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Ingår i: Optics Letters. - : Optical Society of America. - 0146-9592 .- 1539-4794. ; 39:4, s. 1001-1004
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Tidskriftsartikel (refereegranskat)abstract
- Nonlinear plasmonics opens up for wavelength conversion, reduced interaction/emission volumes, and nonlinear enhancement effects at the nanoscale with many compelling nanophotonic applications foreseen. We investigate nonlinear plasmonic responses of nanoholes in thin gold films by exciting the holes individually with tightly focused laser beams, employing a degenerated pump/probe and Stokes excitation scheme. Excitation of the holes results in efficient generation of both narrowband four-wave mixing (FWM) and broadband multiphoton excited luminescence, blueshifted relative to the excitation beams. Clear enhancements were observed when matching the pump/probe wavelength with the hole plasmon resonance. These observations show that the FWM generation is locally excited by nanoholes and has a resonant behavior primarily governed by the dimensions of the individual holes.
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| 3. |
- Kastberg, Anders, 1963-, et al.
(författare)
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A three-dimensional Brownian motor, realised with symmetric optical lattices
- 2009
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 246:5, s. 999-1005
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Tidskriftsartikel (refereegranskat)abstract
- A three-dimensional Brownian motor is realised using lasercooled caesium atoms trapped in a system of two static, and individually symmetric, optical lattices; a so-called double optical lattice. Isotropic fluctuations, emanating from light scattering, are rectified, and the diffusion of the ensemble of atoms is biased, with a resulting constant velocity that is controllable both in direction and magnitude. The working principle of the Brownian motor can be seen as a pulsation between two different potentials, both symmetric but around different points. The correlation between interferometric spatial offsets, and imbalance in optical pumping rates, leads to a spatio-temporal asymmetry sufficient for generating a controlled, directed motion.
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| 4. |
- Satapathy, Nandan, et al.
(författare)
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Theoretical investigation of quantum walks by cold atoms in a double optical lattice
- 2009
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 80:1, s. 012302-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the feasibility of carrying out quantum walks with cold atoms in a double optical lattice. Monte Carlo simulations of time-of-flight (TOF) detection and absorption imaging were carried out, focusing on a specific experimental implementation. These indicate that absorption imaging would be best suited for detection of quantum walks. With typical experimental parameters a few hundred quantum walk steps will be needed for an unambiguous detection of the quantum walk signature. We show that in special cases, few-step quantum walks can also be detected in our system if one measures the relative population of the atoms in their internal states rather than their displacement in space, that is, measurements are made in the space of the coin operator rather than in that of the displacement operator.
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| 5. |
- Zelan, Martin, 1981-, et al.
(författare)
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Experimental measurement of efficiency and transport coherence of a cold-atom Brownian motor in optical lattices
- 2011
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - : American Physical Society. - 1539-3755 .- 1550-2376. ; 83:2
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Tidskriftsartikel (refereegranskat)abstract
- The rectification of noise into directed movement or useful energy is utilized by many different systems. The peculiar nature of the energy source and conceptual differences between such Brownian motor systems makes a characterization of the performance far from straightforward. In this work, where the Brownian motor consists of atoms interacting with dissipative optical lattices, we adopt existing theory and present experimental measurements for both the efficiency and the transport coherence. We achieve up to 0.3% for the efficiency and 0.01 for the Peclet number.
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| 6. |
- Zelan, Martin, 1981-, et al.
(författare)
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Fluctuation-induced drift in a gravitationally tilted optical lattice
- 2010
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 82:3
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Tidskriftsartikel (refereegranskat)abstract
- Experimental and theoretical studies are made of Brownian particles trapped in a periodic potential, which is very slightly tilted due to gravity. In the presence of fluctuations, these will trigger a measurable average drift along the direction of the tilt. The magnitude of the drift varies with the ratio between the bias force and the trapping potential. This can be closely compared to a theoretical model system, based on a Fokker-Planck equation formalism. We show that the level of control and measurement precision we have in our system, which is based on cold atoms trapped in a three-dimensional dissipative optical lattice, makes the experimental setup suitable as a testbed for fundamental statistical physics. We simulate the system with a very simplified and general classical model, as well as with an elaborate semiclassical Monte Carlo simulation. In both cases, we achieve good qualitative agreement with experimental data.
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| 7. |
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| 8. |
- Enejder, Annika, 1969, et al.
(författare)
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Nonlinear nearfield microscopy
- 2013
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Ingår i: Progress in Biomedical Optics and Imaging - Proceedings of SPIE. - : SPIE. - 1605-7422. - 9780819493576 ; 8588
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Konferensbidrag (refereegranskat)abstract
- Higher-order nonlinearity of light-matter interactions, such as second and third harmonic generation (SHG & THG) and Coherent anti-Stokes Raman Scattering (CARS) can be used for improving spatial resolution in microscopy as a consequence of the confinement of the nonlinear polarization to the high-intensity region of the focal volume. However, the resolution is limited to similar to 300 nm, not sufficient to resolve macromolecules or nanostructures of interest in the bio-, life- and nano-sciences. In the strive to push the resolution beyond the diffraction limit, allowing for nanoscale imaging, we have equipped a nonlinear optical microscope with a scanning-probe setup operated in tapping-mode feedback. A tapered, gold-coated, open-aperture tip with an aperture diameter of similar to 150 nm is scanned over the sample, probing the nonlinear nearfield generated by free-beam excitation. First nonlinear coherent Raman nearfield images of biological macromolecules and metallic nanostructures are shown. Limitations and future challenges with nonlinear nearfield microscopy are discussed.
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| 9. |
- Hagman, Henning, 1981-, et al.
(författare)
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Assessment of a time-of-flight detection technique for measuring small velocities of cold atoms
- 2009
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 105:8, s. 83109-
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Tidskriftsartikel (refereegranskat)abstract
- A low noise time-of-flight detection system for laser cooled atoms has been constructed and incrementally optimized. Here, a thorough description of the construction is presented along with an analysis of the capabilities of the system. The quality of the detection (the resolution, the reproducibility, the sensitivity, etc.) is crucial for, e.g., the ability to see details in the velocity distribution profile, which is of interest for fundamental studies of statistical physics and of the laser cooling processes, and for detection of small initial velocities of an atomic cloud, important, e.g., when studying small drifts induced by Brownian motors and ratchets. We estimate the signal-to-noise ratio of our signal to be better than 1000:1 for a typical single shot, and we discuss the effect of the initial atomic cloud size, the probe size, and the effects of the wave packet spread during the fall time on the measured quantities. We show that the shape of the velocity distribution is well conserved during the mapping done in the detection, i.e., in the convolution with the probe beam, and that velocities as small as a few percent of the single photon recoil velocity can be resolved.
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| 10. |
- Hagman, Henning, 1981-
(författare)
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Atomic transport in optical lattices
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis includes both experimental and theoretical investigations of fluctuation-induced transport phenomena, presented in a series of nine papers, by studies of the dynamics of cold atoms in dissipative optical lattices. With standard laser cooling techniques about 108 cesium atoms are accumulated, cooled to a few μK, and transferred into a dissipative optical lattice. An optical lattice is a periodic light-shift potential, and in dissipative optical lattice the light field is sufficiently close to resonance for incoherent light scattering to be of importance. This provides the system with a diffusive force, but also with a friction through laser cooling mechanisms. In the dissipative optical lattices the friction and the diffusive force will eventually reach a steady state. At steady state, the thermal energy is low enough, compared to the potential depth, for the atoms to be localized close to the potential minima, but high enough for the atoms to occasionally make inter-well flights. This leads to a Brownian motion of the atoms in the optical lattices. In the normal case these random walks average to zero, leading to a symmetric, isotropic diffusion of the atoms. If the optical lattices are tilted, the symmetry is broken and the diffusion will be biased. This leads to a fluctuation-induced drift of the atoms. In this thesis an investigation of such drifts, for an optical lattice tilted by the gravitational force, is presented. We show that even though the tilt over a potential period is small compared to the potential depth, it clearly affect the dynamics of the atoms, and despite the complex details of the system it can, to a good approximation, be described by the Langevin equation formalism for a particle in a periodic potential. The linear drifts give evidence of stop-and-go dynamics where the atoms escape the potential wells and travel over one or more wells before being recaptured. Brownian motors open the possibility of creating fluctuation-induced drifts in the absence of bias forces, if two requirements are fulfilled: the symmetry has to be broken and the system has to be brought out of thermal equilibrium. By utilizing two distinguishable optical lattices, with a relative spatial phase and unequal transfer rates between them, these requirements can be fulfilled. In this thesis, such a Brownian motor is realized, and drifts in arbitrary directions in 3D are demonstrated. We also demonstrate a real-time steering of the transport as well as drifts along pre-designed paths. Moreover, we present measurements and discussions of performance characteristics of the motor, and we show that the required asymmetry can be obtained in multiple ways.
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