Light-induced Drift Research Articles

Optical manipulations without light forces: deep superlattice for trapping and rotation of impurity particles in a buffer gas

We propose a scheme to obtain a deep optical superlattice (OSL) for resonant impurity particles in a buffer gas. In contrast to the well-known traditional methods of forming optical lattices (supelattices) that are based on the gradient force of radiation pressure, the presented scheme is based on the effect of a light induced drift (LID) in interfering light waves. The principle of its operation is based on the conception of so-called rectified forces induced by interfering bichromatic optical fields, which was developed earlier in the theory of resonant radiation-pressure forces. In the scheme under consideration, a deep OSL (with a period much larger than the optical wavelength) is formed due to the action of the effective rectified force on atomic particles which is proportional to the difference between the frequencies of transport collisions of excited and unexcited particles with buffer gas atoms.

A hybrid mechanism of 3D optical trapping of particles immersed in a cold buffer gas

A novel mechanism of the formation of a deep optical superlattice (OSL) for three-dimensional (3D) trapping of resonant impurity atoms immersed in a transparent cold buffer gas is proposed and investigated theoretically. This mechanism is associated with the joint action of two factors of different physical nature: rectification effect of the gradient force of light pressure and effect of the light induced drift (LID) of atoms (a consequence of the difference in gas-kinetic cross sections of excited and unexcited atoms). Such a situation can be realised when a gas mixture is irradiated by a bichromatic combined light beam which is a special coaxial superposition of cosine-Gaussian optical beams. Here, the RcGF creates a periodic 1D array of deep potential wells (OSL) along the beam axes, and an effective force, causing LID, pulls resonant particles into the combined light beam and provides their transverse (with respect to the beam axes) confinement in OSL. As a result, there can occur a giant accumulation of impurity particles in the OSL cells, accompanied by a strong spatial particle localization far away from the walls of the reservoir, in which the gas mixture is located.

Influence of Ground State Hyperfine Pumping on Light-Induced Drift Velocity of Atoms

Influence of Ground State Hyperfine Pumping on Light-Induced Drift Velocity of Atoms

Development of a stretched absolute wavelength standard using dichroic atomic vapor spectroscopy towards observation of light-induced drift

An absolute wavelength standard broader than the Doppler width of the atomic absorption signal was developed by using a method called dichroic atomic vapor spectroscopy (DAVS). DAVS is known as a method to obtain an error signal for stabilization of laser wavelength, called dichroic atomic vapor laser locking (DAVLL). In the DAVS, a magnetic field is applied to the atomic vapor cell and a stretched signal is obtained by the Zeeman effect. The DAVS experiment was performed by using ring-type permanent magnets and a cesium vapor cell at room temperature, and the placement and number of magnets were optimized. By measuring the height of the DAVS signal calibrated in advance, continuous monitoring of the absolute laser wavelength become possible. The developed system is suitable to be used as a stretched wavelength standard in the experiment of the light-induced drift.

Enhancement of the sensitivity of the light-induced drift effect to interatomic interaction potentials using a mixture of two buffer gases

Based on five different nonempirical (calculated ab initio) interaction potentials for pairs of colliding Li – Ne particles and three different interaction potentials for pairs of colliding Li – Ar particles, we have theoretically investigated the spectral features of the light-induced drift (LID) rate of Li atoms in the buffer Ne gas in a mixture of buffer Ne + Ar gases. The calculations of LID of Li atoms in the buffer Ne gas for two interaction potentials predict anomalous LID and, as a result, strong sensitivity of the spectral shape of the LID lines to the differences in these potentials. For three other potentials (out of the five in question), the shape of the LID line of Li atoms in Ne is insensitive to the shape of the potential, since calculations with these potentials predict a normal LID effect. In this case, as it turned out, by adding a small fraction (approximately 10 %) of Ar to Ne, one can go from normal LID to anomalous LID and thereby radically increase the sensitivity of the LID line shape of Li atoms to the difference in these interaction potentials. The results obtained enable high-precision testing of the interatomic interaction potentials in experiments on anomalous LID.

On the Possibility of the Precise Test of Interatomic Interaction Potentials Using the Effect of Anomalous Light-Induced Drift

The spectral features of the velocity of light-induced drift (LID) of lithium atoms in a neon buffer gas have been theoretically studied using four known ab initio calculated interatomic interaction potentials for the Li–Ne system of colliding particles. The calculations with each of these four potentials predict a strong temperature dependence of the spectral shape of the LID signal of Li atoms in the Ne atmosphere in various temperature intervals. The results allow the high-precision test of the interatomic interaction potentials in experiments on anomalous LID.

Segregation of Calcium Isotopes in the Atmospheres of CP Stars as a Consequence of Light-Induced Drift

A mechanism for the segregation of calcium isotopes in the atmospheres of chemically peculiar (CP) stars due to light-induced drift (LID) of singly charged 48Ca+ ions is discussed. One peculiarity of Ca+ is that an adequate description of the effect of LID requires taking into account several energy levels of Ca+, and thus several pairs of relative differences (ν i − ν k )/ν i for the transport frequencies for collisions of levels i and k with neutral atoms (hydrogen, helium). The known real (calculated ab initio) interaction potentials are used to numerically calculate the factors (ν i − ν k )/ν i for several states of Ca+ for collisions with H and He atoms. These computations show that, at the temperatures characteristic of the atmospheres of CP stars, T = 6600−12 000 K, fairly high values are obtained for Ca+ ions, (ν i − ν k )/ν i ≈ 0.4−0.6. Simple, transparent computations demonstrate that the LID rates of Ca+ ions in the atmospheres of cool CP stars (Teff = 6600 K) exceed the drift rate due to light pressure by two orders of magnitude. The LID is directed upward in the stellar atmosphere, and the heavy isotope 48Ca is pushed into upper layers of the atmosphere. This can explain the observed predominance of the heavy isotope 48Ca in the upper atmospheric layers of CP stars; according to the radiative-diffusion theory, the action of light pressure alone (in the absence of LID) would lead to sinking of the isotope 48Ca deeper into stellar atmosphere, following the lighter main isotope 40Ca. The 48Ca+ LIDrate decreases and its drift rate due to light pressure increases with growth of the effective temperatures in the atmospheres of CP stars. The manifestations of LID and light pressure are roughly comparable in the atmospheres of CP stars with effective temperatures near Teff = 9500 K.

Influence of laser linewidth on light-induced drift velocity of atom

Light-induced drift has many applications in astrophysics, semiconductor physics, and isotope separation. Light-induced drift velocity is a key parameter to characterize the effect of light-induced drift. Laser linewidth exerts a great influence on light-induced drift velocity through influencing the velocity selectivity of atomic excitation, so it is an important factor that cannot be ignored in the study of light-induced drift. However, in existing theoretical studies, the influence of laser linewidth is seldom considered and the exciting light is always treated as monochromatic light. Furthermore, in a few theoretical studies about laser linewidth, the numerical model adopted does not include all the factors of light-induced drift, such as energy level degeneracy, hyperfine structure, and collision model, which will cause the error of calculation. In order to study the influence of laser linewidth on light-induced drift velocity, a four-level rate equation model is established to describe the atomic energy level transition in the process of light-induced drift. In the theoretical model, we introduce strong collision model to describe collisions between atoms and buffer gas. The influences of energy level degeneracy and hyperfine structure are also taken into account. Numerical method is used to calculate the four-level rate equation. According to the calculation results, the influence of laser linewidth on drift velocity of alkali metal atoms is analyzed. The results show that as the linewidth increases, the value of drift velocity first increases and then decreases. There is an optimal linewidth that maximizes the drift velocity. For the best light-induced drift effect, the laser should work under the optimal linewidth condition. When the laser linewidth fluctuates near the optimum linewidth, the laser linewidth should be set to be slightly wider than the optimal linewidth. This can reduce the influence of fluctuation and obtain a better drift effect. In addition, as the laser linewidth increases, the optimum power density corresponding to the maximum drift velocity decreases. When the laser linewidth is narrow, small fluctuations near the optimal laser power density will not have great influence on drift velocity. When the laser linewidth is wide, the power density should be set strictly. The optimum linewidth is related to laser power density, temperature and buffer gas pressure. As the laser power density increases, the value of optimum linewidth first increases rapidly and then decreases slowly. The value of optimal linewidth also increases linearly with the increase of temperature, and it decreases with the increase of buffer gas pressure. In conclusion, the laser linewidth does play a key role in the process of light-induced drift. The results of this study can provide a theoretical basis for future experiments, and be a good reference to the selection of exciting light.

关键参数对锂原子光致漂移速率影响的数值研究

关键参数对锂原子光致漂移速率影响的数值研究

Separation of chemical elements in the atmospheres of CP stars under the action of light induced drift

A mechanism for the separation of chemical elements and isotopes in the atmospheres of chemically peculiar (CP) stars due to light-induced drift (LID) of ions is discussed. The efficiency of separation due to LID is proportional to the relative difference of the transport frequencies for collisions of ions of heavy elements located in the excited state (collision frequency ν e ) and ground state (collision frequency ν g ) with neutral buffer particles (hydrogen and helium), (ν e − ν g )/ν g . The known interaction potentials are used to numerically compute the relative difference (ν e H − ν g H )/νg H for collisions between the ions Be+, Mg+, Ca+, Sr+, Cd+, Ba+, Al+, and C+ and hydrogen atoms. These computations show that, at the temperatures characteristic of the atmospheres of CP stars, T = 7000−20 000 K, values of |ν e H −ν g H |/ν g H ≈ 0.1−0.4 are obtained. With such relative differences in the transport collision frequencies, the LID rate of ions in the atmospheres of coolCP stars (T T > 10 000 K). LID of heavy ions is manifest only weakly in very hot stars (T > 20 000 K).

Numerical analysis of highly efficient laser-based method of radioactive Cs isotope separation utilizing light-induced drift in D1 and D2 transitions in rare gases

ABSTRACTWe investigated the feasibility of separating radioactive Cs isotopes using the effects of light-induced drift (LID). LID is the massive flow of particles caused by the difference between the collision frequencies of the buffer particles in the optically connected ground and excited states. We numerically evaluated the LID velocities of Cs-133, Cs-135, and Cs-137 in rare gases based on the currently available experimental technologies. The calculations were performed utilizing the steady-state analytical model and assuming that all of the collisions were strong. The velocity-dependent collision cross-sections were estimated using the WKB approximation with the recently developed ab initio atom–atom interaction potentials. A maximum velocity of more than 10 m/s was obtained when He was utilized as the buffer gas, and the expected maximum annual throughput was approximately 600 g, which is sufficient for practical purposes. We also examined the negative effects that were neglected in the employed model.

Wavelength-sensitivity of mouse retinal ganglion cells recorded by a high-density micro electrode array (MEA)

Event Abstract Back to Event Wavelength-sensitivity of mouse retinal ganglion cells recorded by a high-density micro electrode array (MEA) Florian Jetter1, Gabriel Bertotti2, Roland Thewes3 and Günther Zeck4* 1 NMI at the University Tuebingen, Neurochip Research, Germany 2 Technische Universität Berlin, Chair of Sensor and Actuator Systems, Germany 3 Technische Universität Berlin, Chair of Sensor and Actuator Systems, Germany 4 NMI at the University Tuebingen, Neurochip Research, Germany Motivation: High-density CMOS-MEAs can be used to simultaneously record the electrical spiking activity from hundreds of neurons [Bertotti et al., 2014]. Neuronal spiking can be induced by electrical stimulation [Eickenscheidt and Zeck 2014], by light stimulation applied to a light-sensitive retina [Zeck et al. 2011], or by light stimulation of optogenetically transfected neurons [Herrmann et al. 2014]. In this work, we investigate the wavelength-sensitivity of different mouse retinal ganglion cells and the intrinsic wavelength-sensitivity of the response of high-density CMOS-MEAs. Material and Methods: A CMOS-based high-density MEA comprising 4225 recording sites is used for recording the ganglion cell activity in C57/Bl6 mouse retina during flickering light stimulation (1 and 5 Hz, respectively) with different wavelength and of different light stimulus sizes. Light stimuli presented on a DMD (µ-matrix, Rapp Optoelectronic, Germany) are focused through a microscope objective on the retina. The DMD is illuminated by an LED system commonly used for optogenetic activation (pe-4000, coolLED, UK). Here the results for four stimulation wavelengths are presented (405, 470, 525, and 635nm) at intensities as high as 2 mW/mm² (470 nm). Recorded data is sampled at 25 kHz. Results: Light stimulation (stimulus area: 1 mm2) evokes spiking in the interfaced retina. Based on the stimulus polarity retinal ganglion cells are broadly classified in ON or OFF type, depending on whether they respond to light on- or offset, respectively. In Fig. 1A filtered recordings are shown of the measured extracellular voltage from two selected sensors during 1 Hz stimulation. Both, ON and OFF cell types respond to the three highest wavelengths used here. The sensor site recording the ON transient cell also detects activity from a second OFF cell with smaller amplitude. In Fig.1B we present the unfiltered extracellular voltage traces recorded by a third selected sensor site for all four wavelengths. Light onset leads to a measurable change of the current in the sensing transistor (cf. Bertotti et al., 2014) or more generally speaking of the sensed recording site response, reflected as a low-frequency change of signal back-converted into the voltage domain. However, this does not prevent the detection of light-induced spiking. We note, that the ganglion cell shown in Fig. 1B is not activated by the 635 nm light stimulus. Ongoing experiments investigate the sensitivity of retinal ganglion cells to chromatic stimulation and to stimuli presented at various intensities. The induced variations of the sensor signals are wavelength-dependent, with the highest change obtained for the lowest wavelength (405 nm) and undetectable changes for red light (635 nm). Conclusion: It is shown that recording of cellular spiking activity with CMOS-based MEAs is possible during optical stimulation. Ganglion cells in the mouse retina have different wavelength-sensitivities. The light-induced low-frequency drift of the sensor signals can be completely removed for the tested wavelengths and intensities using high-pass filtering.

Separation and conversion dynamics of nuclear-spin isomers of gaseous methanol.

All symmetrical molecules with non-zero nuclear spin exist in nature as nuclear-spin isomers (NSIs). However, owing to the lack of experimental information, knowledge is rare about interconversions of NSIs of gaseous molecules with torsional symmetry. Here we report our separation and conversion observations on NSI-torsion-specific transition systems of gaseous methanol from a light-induced drift experiment involving partially spatial separation of the ortho and para isomers. We find that vibrationally excited molecules of the methanol spin isomer have a smaller collision cross-section than their ground-state counterparts. Interconversion of the enriched ortho isomer with the para isomer, which is generally considered improbable, has been quantitatively studied by sensitive detections of the spectral intensities. Rather counterintuitively, this reveals that the interconversion is inhibited with increasing pressure. Our results suggest that the spin conversion mechanism in methanol is via a quantum relaxation process with the quantum Zeno effect induced by molecular collisions.

Accurate measurement of the sticking time and sticking probability of Rb atoms on a polydimethylsiloxane coating

We present the results of a systematic study of Knudsen’s flow of Rb atoms in cylindrical capillary cells coated with a polydimethylsiloxane (PDMS) compound. The purpose of the investigation is to determine the characterization of the coating in terms of the sticking probability and sticking time of Rb on the two types of coating of high and medium viscosities. We report the measurement of the sticking probability of a Rb atom to the coating equal to 4.3 × 10−5, which corresponds to the number of bounces 2.3 × 104 at room temperature. These parameters are the same for the two kinds of PDMS used. We find that at room temperature, the respective sticking times for high-viscosity and medium-viscosity PDMS are 22 ± 3 μs and 49 ± 6 μs. These sticking times are about million times larger than the sticking time derived from the surface Rb atom adsorption energy and temperature of the coating. A tentative explanation of this surprising result is proposed based on the bulk diffusion of the atoms that collide with the surface and penetrate inside the coating. The results can be important in many resonance cell experiments, such as the efficient magnetooptical trapping of rare elements or radioactive isotopes and in experiments on the light-induced drift effect.

Numerical modeling of light-induced drift in a cloud of interstellar gas

We study velocity distribution of the gas in case of resonance interaction between directed radiation line and the gas in medium of non-resonating buffer gas. We have constructed a 6-parameter model numerically describing the effect of uniform quasi-stationary light-induced drift in a rarefied interstellar gas. Based on numerical calculations, we qualitatively determined the dependence of the light-induced drift velocity on the model parameters in the cases of optically thin and optically thick clouds. We checked that light-induced drift has resonance behavior and requires low collision rate of the gas. Also we showed and explained narrowing and shifting of emission line during propagation in cloud.

Diffusional Separation of Calcium Isotopes in Chemically Peculiar Stellar Atmospheres

Abstract Diffusional separation of calcium isotopes in the atmospheres of hot chemically peculiar stars is studied. In addition to the usual radiative acceleration effect, the light-induced drift is taken into account. We propose that microturbulence in stable stellar atmospheres is generated by the interaction between plasma particles and radiative flux. Formulae for the microturbulent velocity and microturbulence diffusion coefficient are derived. Data on isotopic and hyperfine splitting of the calcium spectral lines have been collected as an input file. The equilibrium Ca isotope concentrations are found in model computations, iteratively correcting the radiative acceleration values. The general picture of Ca isotope stratification is found to be similar to our previous results obtained for Hg isotopes: dominating overabundance of the heaviest isotope. Diffusional stratification of Ca isotope concentrations in atmospheres of late B and early A spectral types are computed and visualized in figures. The isotope abundances on the inner boundary surface were fixed to be the solar ones. The computed Ca II infrared triplet line profiles are compared with the observed line profiles in a high-dispersion spectrum of HD 175640.

On the accuracy of a one-dimensional approach to the solution of kinetic equations with velocity-dependent collision frequencies

The solution of many problems in light-induced gas kinetics can be simplified significantly using quantum kinetic equations in the context of the so-called one-dimensional approximation, in which the initial equations are averaged over transverse (relative to the direction of radiation) velocities. The errors introduced in such an approach are usually assumed to be small; however, this has been confirmed quantitatively only on the basis of the simplest (two- and three-level) particle models. We analyze the accuracy of the one-dimensional approximation for multilevel particles quantitatively for the light-induced drift (LID) effect in cesium atoms in the atmosphere of inert buffer gases. It is shown that in the case of the so-called “normal” LID, one-dimensional kinetic equations can always be used instead of three-dimensional equations without a risk of losing some important fine details in the dependence of the drift velocity on the radiation frequency. In the case of anomalous LID, the error of the one-dimensional approximation is also insignificant, but it can be disregarded only in the case of light buffer particles. For comparable masses of resonant and buffer particles, the one-dimensional approximation may give a noticeable error in determination of drift velocity amplitudes; however, the positions of drift velocity zeros and extrema depending on radiation-frequency detuning can be described successfully. Results show that the error introduced by using the one-dimensional approximation for multilevel particles turns out to be more significant than for the simplest particle models.

Spectral anomalies of the effect of light-induced drift of caesium atoms caused by the velocity dependence of transport collision frequencies

The spectral features of the light-induced drift (LID) velocity of caesium atoms in inert buffer gases are studied theoretically. A strong temperature dependence of the spectral LID line shape of Cs atoms in Ar or Kr atmosphere in the vicinity of is predicted. It is shown that the anomalous LID of Cs atoms in binary buffer mixtures of two different inert gases can be observed at virtually any (including ambient) temperature, depending on the content of the components in these mixtures. The results obtained make it possible to precisely test the interatomic interaction potentials in the experiments on the anomalous LID.

Thermo-optical pressure difference in one-component gas

A new phenomenon—thermo-optical pressure difference in the gas (TOPD) is regarded. This effect is the steady state of the second order which arises in the gas located in a closed capillary in the presence of a fixed temperature gradient and a resonant optical radiation. TOPD is the result of imposition thermal transpiration and light-induced drift of gas in a capillary. The problem is solved on the basis of the linearized Boltzmann kinetic equations for excited and unexcited gaseous particles. Expressions for the kinetic coefficients and pressure drop in gas at the ends of the closed capillary are obtained. Possible cases of the steady state are regarded for atoms and molecules. Numerical estimates of this effect for atomic and molecular gases in the whole range of Knudsen numbers are given.

Spectral anomalies of the light-induced drift of rubidium atoms caused by the velocity dependence of transport collision frequencies

The spectral features of the light-induced drift (LID) velocity for rubidium atoms (85Rb and 87Rb) in an argon buffer medium and in binary buffer mixtures of noble gases (Ne + Ar, Ne + Kr, Ne + Xe, He + Ar, He + Kr, and He + Xe) have been investigated theoretically. A strong temperature dependence of the spectral shape of the LID signal for Rb atoms in an Ar atmosphere is predicted in the temperature range 450 K < T < 800 K. It is shown that the anomalous LID of Rb atoms in binary buffer mixtures of noble gases can be observed at almost any temperature (including the room one) depending on the fractions of neon or helium in these mixtures. The results obtained enable a highly accurate testing of the interatomic interaction potentials used to calculate the drift velocity for anomalous LID in LID experiments.