Dark Lattice Research Articles

Acoustic dark lattices generated by composite spiral plates for multi-particle trapping

The interactions of two Bessel acoustic vortices (BAVs) with different topological charges (TCs) have been investigated in detail. The theoretical results show that, with changing the TCs of two BAVs, the intensity distributions of the coupled-BAVs can switch among the petal mode, the acoustic dark lattice (ADL) mode, and the crescent mode, where the positions of the dark spots coincide with the phase singular points. We concentrate on the ADL modes and a composite spiral plate (CSP) is well designed to realize them. It is shown both numerically and experimentally that the CSP can create the ADL modes in air, which also exhibit strong self-healing. Additionally, the CSP is employed to generate ADLs in water. Four-order and six-order ADLs produced by two steel CSPs are used, respectively, to achieve multi-particle trappings in water experimentally. The capacity of the ADLs generated by the simple CSPs to trap multiple micro-particles in desired positions may find applications in biomedicine, including microarrays, tissue engineering, and regenerative medicine.

Near-band-gap photoinduced nuclear spin dynamics in semi-insulating GaAs: Hyperfine- and quadrupolar-driven relaxation

Understanding and manipulating spin polarization and transport in the vicinity of semiconductor-hosted defects is a problem of present technological and fundamental importance. Here, we use high-field magnetic resonance to monitor the relaxation dynamics of spin-3/2 nuclei in semi-insulating GaAs. Our experiments benefit from the conditions created in the limit of low illumination intensities, where intermittent occupation of the defect site by photoexcited electrons leads to electric field gradient fluctuations and concomitant spin relaxation of the neighboring quadrupolar nuclei. We find indication of a heterogeneous distribution of polarization, governed by different classes of defects activated by either weak or strong laser excitation. Upon application of a train of light pulses of variable repetition rate and on/off ratio, we uncover an intriguing regime of mesoscale nuclear spin diffusion restricted by long-range, nonuniform electric field gradients. Given the slow time scale governing nuclear spin evolution, such optically induced polarization patterns could be exploited as a contrast mechanism to expose dark lattice defects or localized charges with nanoscale resolution.

Experimental Evidence of Dark Periodic Lattices in Nonlinear Liquid Medium

In this work experimental measurements showing formation of dark periodic lattices in negative nonlinear liquid medium are reported. These lattices were formed by the propagation of periodic patterns through a nonlinear liquid media. These periodic patterns were induced by interference of three plane waves. Here three interesting cases showing the evolution of bright into dark periodic lattices are reported. In the first case periodic lattices formed by dark spots were observed. In the second case a periodic lattice formed by a set of stripes was observed. Finally in the third case a periodic lattice showing a combination of stripes and dark spots is reported.

Purely loss-coupled distributed feedback lasers based on electromagnetically induced absorption in active photonic band gaps

We utilize electromagnetically induced absorption (EIA) in a one-dimensional functional (active) photonic band gap structure to propose a coherently generated purely loss-coupled distributed feedback (DFB) laser. Such a laser happens when interaction of the photonic band structure with a control laser field totally removes its refractive index perturbation and replaces it with periodic regions of EIA, forming an optical dark lattice. Such a lattice does not have any stop band, but scatters light coherently at the Bragg wavelength. We also show that before the optical dark lattice happens, the photonic band gap structure can act as a coherently generated partly loss-coupled DFB laser with dominant lasing mode at the longer wavelength side of the stop band. We expect such a distinct feature makes this laser quite stable.

Optical switching of one-dimensional photonic band gaps and coherent generation of dark and bright optical lattices in quantum wells

We propose one-dimensional bright and dark optical lattices where Bragg scattering occurs, respectively, via periodic generation of gain without inversion and electromagnetically induced absorption in the absence of any refractive index perturbation. The absorption and gain processes are caused by quantum interference in the conduction intersubband transitions of a corrugated $n$-doped quantum well structure interacting with an intense infrared laser. The quantum well structure is embedded between two optical confinement layers, forming a periodic structure with a Bragg wavelength similar to that of the intersubband transition between the second and third conduction subbands of the quantum well. We show that in the absence of the intense infrared laser this transition is transparent, and the periodic structure forms a passive one-dimensional photonic gap generated by the background refractive index contrast of the corrugated region. In the presence of this laser, the intersubband transition is influenced by the quantum coherence effects, causing dramatic changes in the photonic band gap. We show that when one adjusts the intensity and wavelength of such a laser properly these effects can (i) destroy the one-dimensional photonic band gap, and (ii) form periodic regions of gain without inversion or electromagnetically induced loss, while the index perturbation is completely canceled out. Such bright and dark optical lattices can scatter a light at Bragg wavelength when its frequency is the same or close to that of the intersubband transition.

Near-resonant dark optical lattice with increased occupation

A bichromatic near-resonant dark optical lattice (DOL) with rubidium atoms is demonstrated, which provides confinement in the Lamb-Dicke regime in all spatial dimensions. We apply spatially phase-matched optical potentials for each hyperfine ground state in order to enable improved Sisyphus cooling, undisturbed by optical hyperfine pumping processes. We also explore a method to increase the occupation of the DOL. Initially $2\ifmmode\times\else\texttimes\fi{}{10}^{9}$ rubidium atoms with a temperature of $70\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and a density of $5\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}\text{atoms}∕{\mathrm{cm}}^{3}$ are prepared in a magneto-optic trap (MOT) and a fraction of $5\ifmmode\times\else\texttimes\fi{}{10}^{7}$ atoms is loaded into a far detuned one-dimensional optical lattice (FOL). Subsequently, the MOT is replaced by the DOL, and the atoms become well localized within the microscopic light-shift potentials at a temperature of $10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ with a typical density of $3\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}\text{atoms}∕{\mathrm{cm}}^{3}$. We then apply alternating cycles of free evolution in the FOL and cooling and trapping in the DOL, obtaining a fourfold density increase to $1.2\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}\text{atoms}∕{\mathrm{cm}}^{3}$---i.e., 7.5% occupation---while maintaining a temperature of $10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$. In a final adiabatic cooling step we reduce the well depth to 75 times the single-photon recoil energy, which leads to a temperature of $2.8\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and a phase-space density of $1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. Despite the increased density, no excess heating or collisional losses are observed.

Interference technique by three equal-intensity umbrellalike beams with a diffractive beam splitter for fabrication of two-dimensional trigonal and square lattices

A systematic and comprehensive analysis of interference of three equal-intensity umbrellalike beams (ITUB) is provided based on the traditional interference theory. It shows that identical two-dimensional (2D) bright and dark lattices with spacing of the order of wavelength can be fabricated and a partial uniform contrast can be obtained by properly adjusting polarization of each beam. This study may lay a theoretical foundation for holographic fabrication of 2D periodical microstructures such as trigonal or square lattices.

Laser cooling of atoms in optical lattices including quantum statistics and dipole–dipole interactions

We develop a mean-field approach to include dipole-dipole interactions and quantum statistics in the atomic dynamics in bright and dark optical lattices including the proper spatial potentials instead of a simple δ-approximation. For classical distinguishable particles the results are even quantitatively similar to the properly scaled δ-function model. As the dominant effect bright and dark lattices exhibit opposite shifts in the lattice band energies and differ in their localisation properties as a function of density. The spatial-dependent potential allows strong modifications also in dark lattices, but the main conclusions obtained in the δ-approximation turn out to be still valid. Interestingly, important quantitative differences from the δ-model can occur as far as the effect of statistics in concerned, especially for fermions. We study the quantum statistical effects as a function of detuning and lattice well depths and identify the case of lattices with deep wells and large detunings as the preferred parameter region to observe them.

Mean-field approach to dipole-dipole interaction in an optical lattice

We compare the influence of quantum statistics and atom-atom interaction in bright and dark one-dimensional optical lattices for atoms using quantum rate equations. Via a Hartree-Fock--type mean-field approach we derive an effective nonlinear single-atom equation and solve it self-consistently for a prescribed atomic density. Using a simplified dipole-dipole interaction potential, we show that the atomic energy bands are very sensitive to the mean atomic density, even in a parameter regime where temperature and band populations exhibit only a very small density dependence. Bright lattices show energy shifts towards smaller band separations, while the effect is in the opposite direction and smaller for dark lattices. In general, dipole-dipole interaction strongly dominates the influence of quantum statistics, except for very large detunings, where the spontaneous emission rate is very small.

Dispersionless envelope lattice solitons on a D-dimensional nonlinear lattice

We show by using the real exponential approach that the d-dimensional discrete nonlinear Schrödinger equation has more general dispersionless envelope lattice soliton solutions than the known bright soliton and kink solutions. Depending on the values of the parameters, the new solutions can describe both bright and dark lattice solitons. Especially, we find novel “W”-like envelope lattice solitons.

On the possibility of using dark magneto-optical lattices to achieve Bose condensation of atoms

We show that in dark magneto-optical lattices, effects associated with the Bose statistics of atoms can be observed even at laser cooling temperatures (10−4–10−6 K), which exceed evaporative cooling temperatures in magnetic traps by several orders of magnitude. Quasicondensation occurs, i.e., the wave function is formed over the distances on which atoms are localized near the bottom of a separate potential well. In addition, switching off the magnetic field adiabatically reduces the temperature significantly, as a result of which Bose condensation in the entire volume of the gas can be observed. We propose a configuration of the light and magnetic fields in which the shape of the three-dimensional magneto-optical potential is independent of the phases of the emerging light waves.

Bragg scattering of optical pulses from localized atoms in one-dimensional optical lattices

We outline the theory of a practical Bragg scattering technique for studying the long-range spatial order of atoms localized in one-dimensional optical lattices and we demonstrate the technique using and F = 1 → 1 transitions with lin ⊤ lin laser cooling fields. The latter transition gives a dark optical lattice. We show that the scattering by a spin grating, which may also be present, can be completely or partially filtered out by varying the polarizations of the cooling fields while holding the polarization of the probe and the detector analyser fixed. This allows the fraction of atoms that are localized to be measured. The probe field is treated as a pulse and so the technique also allows time-resolved details of the spatial structure to be studied.

Gauge structures in atom-laser interaction: Bloch oscillations in a dark lattice.

Atoms in dark atomic ground states do not interact with the laser light present and are therefore not susceptible to spontaneous processes. Laser cooling populates these states. Confinement of atoms in dark states in a (periodic) gauge potential allows for a lattice of atoms with a very low decay rate, a dark lattice. A dark lattice is a promising system to observe Bloch oscillations of neutral atoms, i.e., periodic excursion across the Brillouin zone in a uniform force field.

Bright and dark lattice solitary waves in a discrete nonlinear system

Exotic lattice solitary wave solutions of a modified version of the discrete modified KdV equation and its d-dimensional generalization are found. The solutions describe both bright and dark lattice solitary waves depending on the values of parameters. It is shown that there exists a critical value of the parameters which divides the parameter space into bright and dark solitary wave ranges.

Numerical calculations of solitary waves in Davydov's equations

Families of numerically very accurate bright and dark lattice solitary waves in Davydov's equations, which have been proposed to describe propagation of vibrational energy in α-helix protein are calculated using spectral collocation and path-following methods. Travelling solitary waves are found to exist only when the nonlinear coupling is less than some critical value which depends on the velocity of the solitary wave.

High-Resolution UHV Electron Microscopy of Reconstructed and Adsorbed Surfaces

High-resolution electron microscopy (HREM) has been applied for structure analyses of reconstructed and adsorbed surfaces at atomic resolution in transmission (TEM) and reflection (REM) mode by using an UHV electron microscope (modified 2000 FXV). In the last ICEM conference, we reported instrumental design of the new UHV electron microscopet[1] with a review on atomic structure study by UHV -EM[2]. In this review we show dynamic structural studies of surfaces which have been revealed by using a TV camera-and-video recording system. Details of each topic shown here are given in original papers[3-8].The UHV electron microscope of 200 keV has the theoretical point-to-point resolution of 0.21 nm. The microscope has two exchange system for evaporators and a gas-inlet system. Surface structural changes recorded on a video tape are resolved at time resolution of 1/30 sec. Figures shown are reproduced from the video frames.High-Resolution REMIn REM mode, we obtain lattice fringes of the surface superlattices by interference of reflections so that the position of the fringes vary with the excitation condition similarly to lattice fringes in TEM. In a certain reflection condition the lattice fringes can be regarded as the structure image. In case of Si(111)7×7 surface, positions of dark lattice fringes (0.23 nm spacing) of the 7×7 coincide almost with the steps[9,10].

The composition and structure of guinier-preston zones in lunar orthopyroxene

Pyroxenes (Ca,Mg,Fe)2Si2O6 are common rock-forming minerals containing both equilibrium and nonequilibrium precipitation products, the distribution, structure and occurrence of which present information necessary for deciphering the mineral1s thermal and mechanical history. One such feature is small zones of calcium-enrichment identified in both terrestrial and lunar orthorhombic pyroxene (Mg,Fe)2Si2O6 (Champness and Lorimer, 1974; and Nord et al.,1977). The zones are disc-shaped parallel to (100). Dark field lattice images obtained utilizing the hll reflections (Fig.l) indicate that the zones are coherent with the host and have similar lattice constants (d100=18.2Å for orthopyroxene). The maximum width of the zones in the terrestrial occurrences is 18Å (one-unit-cell wide) and have therefore been termed G.P. zones by Champness and Lorimer (1973). The lunar orthopyroxene ( from an olivine-anorthite rock, 76535), however, contains G.P. zones identical in all the above characteristics except that the zones are 5 to 6 unit cells wide (∼100Å, Fig. 2).