Institute for Physical Research

We present an experimental measurement of the cooperative Lamb shift and the Lorentz shift using a nanothickness atomic vapor layer with tunable thickness and atomic density. The cooperative Lamb shift arises due to the exchange of virtual photons between identical atoms. The interference between the forward and backward propagating virtual fields is confirmed by the thickness dependence of the shift, which has a spatial frequency equal to twice that of the optical field. The demonstration of cooperative interactions in an easily scalable system opens the door to a new domain for nonlinear optics.

We study the interaction of a weak probe field, having two orthogonally polarized components, with an optically dense medium of four-level atoms in a tripod configuration. In the presence of a coherent driving laser, electromagnetically induced transparency is attained in the medium, dramatically enhancing its linear as well as nonlinear dispersion while simultaneously suppressing the probe field absorption. We present the semiclassical and fully quantum analysis of the system. We propose an experimentally feasible setup that can induce large Faraday rotation of the probe field polarization and therefore be used for ultrasensitive optical magnetometry. We then study the Kerr nonlinear coupling between the two components of the probe, demonstrating a novel regime of symmetric, extremely efficient cross-phase modulation, capable of fully entangling two single-photon pulses. This scheme may thus pave the way to photon-based quantum information applications, such as deterministic all-optical quantum computation, dense coding, and teleportation.

The thermal conductivity and electrical resistivity of a suspended nickel nanowire have been measured for T=15–300K. The temperature dependence of the thermal conductivity and the Lorenz number strongly differ from the bulk. The comparison of the transports in the Ni nanowire shows, that at temperatures 75<T<300K Wiedemann–Franz (WF) law holds, whereas at temperatures T<75K the WF law is violated, indicating that thermal current in this material is suppressed more than electrical current. The results are explained by combined effect of confined dimension, enhanced disorder, and grown contribution of N-processes.

The main objective of this contribution is to point out the potentialities of cerium doped LuAG single crystal as pixels and fibers. We first show that after optimization of growth conditions using Bridgman technology, this composition exhibits very good performances for scintillating applications (up to 26 000 photons/MeV). When grown with the micropulling down technology, fiber shapes can be obtained while the intrinsic performances are preserved. For the future high energy experiments requiring new detector concepts capable of delivering much richer informations about x- or gamma-ray energy deposition, unusual fiber shaped dense materials need to be developed. We demonstrate in this frame that cerium doped LuAG is a serious candidate for the next generation of ionizing radiation calorimeters.

In a thin cell of dilute vapour, the absorption spectrum exhibits sub-Doppler features due to the relative enhancement of the slow atom contribution, with respect to the transient nature of the interaction with moving atoms. For a two-level system in the linear regime, the narrowest response is predicted to be found for a λ/2 thickness, as an effect of the coherent character of the dipole response as early described by Romer and Dicke (Phys. Rev., 99 (1955) 532) in the microwave regime. We report here on the direct observation of this effect in the optical regime in an ultra-thin vapour cell. This effect is shown to vanish for a thickness equal to λ, and a revival is observed at 3λ/2, as expected from the predicted λ-periodicity. The experiment is performed on the D1 resonance line of Cs vapour (λ = 894 nm), in a specially designed cell, whose thickness varies locally.

Inorganic scintillators with high density and high light yield are of major interest for applications in medical imaging and high energy physics detectors. In this work, the optical and scintillation properties of Mg co-doped Ce:Gd3Al2Ga3O12 crystals, grown using Czochralski technique, have been investigated and compared with Ce:Gd3Al2Ga3O12 ones prepared with identical technology. Improvements in the timing performance of the Mg co-doped samples with respect to Ce:Gd3Al2Ga3O12 ones have been measured, namely a substantial shortening of the rise time and scintillation decay components and lower afterglow were achieved. In particular, a significantly better coincidence time resolution of 233 ps FWHM, being a fundamental parameter for TOF-PET devices, has been observed in Mg co-doped crystals. The samples have also shown a good radiation tolerance under high doses of γ-rays, making them suitable candidates for applications in harsh radiation environments, such as detectors at future collider experiments.

To elucidate low-dimensional effects on thermoelectric materials, bismuth telluride film and nanowires array were fabricated by potentiostatically electrodeposition. Both materials are slightly Te-rich, n-type Bi2Te3, exhibiting preferred orientation in rhombohedral strcture. For both the Seebeck coefficient S ≈ −70 μV/K at 300 K decreases linearly with decreasing temperature, showing a diffusive nature of current flow. The temperature dependence of resistivity (=1/σ) of nanowires obtained from the data of a nanowires array and a single-nanowire reveals a better electric conductivity than that of the bulk. By coupling temperature-dependent thermal diffusivity and heat capacity data with a modified effective medium theory, a thermal conductivity κ of 0.75 W/(m K) was obtained at 300 K. The ZT was calculated to be 0.45 at 300 K and 0.9 at 350 K for Bi2Te3 nanowires.

We discuss a decoherence insensitive method to create many-particle entanglement in a spin system with controllable collective interactions and propose an implementation in an ion trap. An adiabatic change of parameters allows a transfer from separable to a large variety of entangled eigenstates. We show that the Hamiltonian can have a supersymmetry permitting an explicit construction of the ground state at all times. Of particular interest is a transition in a nondegenerate ground state with a finite energy gap since here the influence of collective as well as individual decoherence mechanisms is substantially reduced. A lower bound for the energy gap is given.

Transient and steady-state measurements of photoinduced birefringence in single-domain and periodically poled lithium niobate (LN) crystals containing different nonphotorefractive impurities are presented. The birefringence change is induced by a 532 nm beam and probed by a 633 nm beam. Data were taken at 25 and 50 °C. We find that doping by HfO2 is very effective in reducing the photorefraction. This is interesting also because it is known that hafnium-doped LN crystals can be periodically poled during growth. The analysis of the rise and decay of the induced birefringence shows that doping considerably increases both the photoconductivity and the dark conductivity of the LN crystals.

We compare the behavior of absorption and of resonance fluorescence spectra in an extremely thin Rb vapor cell as a function of the ratio of $L∕\ensuremath{\lambda}$, with $L$ the cell thickness $(L\ensuremath{\sim}150--1800\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ and $\ensuremath{\lambda}$ the wavelength of the Rb ${D}_{2}$ line $(\ensuremath{\lambda}=780\phantom{\rule{0.3em}{0ex}}\mathrm{mn})$. The Dicke-type coherent narrowing [G. Dutier et al., Europhys. Lett. 63, 35 (2003)] is observed only in transmission measurements, in the linear regime, with its typical collapse and revival, which reaches a maximum for $L=(2n+1)\ensuremath{\lambda}∕2$ ($n$ integer). It is shown not to appear in fluorescence, whose behavior-amplitude, and spectral width, is more monotonic with $L$. Conversely, at high-intensity, the sub-Doppler saturation effects are shown to be the most visible in transmission around $L=n\ensuremath{\lambda}$.

Absorption, reflection as well as luminescence emission, excitation, and decay curves for single crystals of and grown by the Bridgman technique have been measured at various temperatures. The fluorescence spectra photo-excited over a wide energy domain ranging from the UV to the x-ray region, and the kinetics are typical of the cerium and praseodymium ions. These experimental results show that the exciton transfer to the dopant occurs at around 8 eV, and the energy transfer via sequential hole and electron trapping is dominant at higher energy. This process must be considered as the main scintillation mechanism in this crystal. The high efficiency of this mechanism is explained by the small energy difference between the 4f level of the dopant and the top of the valence band, estimated from XPS measurements.

We have grown high-quality LuAG:Yb3+ crystals with 0.75, 3.8, 10, 12, 15, 20, and 50 at. % concentrations by the vertical Bridgman method. With low-temperature spectroscopy the Stark sublevel structure of the 2F7/2 ground state and the 2F5/2 excited state has been determined. With room-temperature spectroscopy, the emission cross section was found to be 3×10−20 cm2, being 1.5 times the YAG:Yb3+ emission cross section. The luminescence quantum efficiency was measured in samples with different Yb3+ concentrations. Its value was found to be 90% for 3.8 and 10 at. %, 84% for 20 at. %, and 70% for 50 at. % Yb3+. The laser-emission tunability under diode pumping was found to extend from 1045 up to 1095 nm in a 12 at. % sample with 3.15 mm thickness.

We demonstrate electromagnetically induced transparency in a four-level cascade system where the upper level is a Rydberg state. The observed spectral features are sub-Doppler and can be enhanced due to the compensation of Doppler shifts with AC Stark shifts. A theoretical description of the system is developed that agrees well with the experimental results, and an expression for the optimum parameters is derived.

Transient and steady-state measurements of photoinduced birefringence in Hf-doped congruent lithium–niobate crystals are presented. Through a systematic investigation of crystals with different dopant concentrations, we find that Hf doping induces a progressive decrease of photorefraction up to a threshold concentration of about 4 mol %. Such a decrease is correlated to a corresponding increase in photoconductivity. The interpretation of measurements has to take into account heating effects that may be attributed to nonlinear absorption processes.

We present an experimental measurement of the refractive index of high density Rb vapor in a gaseous atomic nanolayer. We use heterodyne interferometry to measure the relative phase shift between two copropagating laser beams as a function of the laser detuning and infer a peak index $n=1.26\ifmmode\pm\else\textpm\fi{}0.02$, close to the theoretical maximum of 1.31. The large index has a concomitant large index gradient creating a region with steep anomalous dispersion where a subnanosecond optical pulse is advanced by $>100\text{ }\text{ }\mathrm{ps}$ over a propagation distance of 390 nm, corresponding to a group index ${n}_{g}=\ensuremath{-}(1.0\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{5}$, the largest negative group index measured to date.

We have investigated the influence of post-deposition annealing on the optical and electrical properties of c-axis oriented zinc oxide films prepared on sapphire substrates by electron beam evaporation. The ZnO films as-deposited and annealed in air were colourless and transparent in visible range and had sharp ultraviolet absorption edges. It is found that the optical bandgap energy of the films lies in the range of ∼3.27 to ∼3.30 eV depending on the annealing regime. From the analysis of the Urbach tail at the absorption edge, the width of the tail of localized states extending into the bandgap was obtained and a value of 44 meV can be achieved by annealing in air.

Detailed spectral physical investigations of the (Y1-xErx)Al5O12 and (Lu1−xEr3)3Al5O12 garnet systems in a wide Er3+ ion concentration range (x = 0.003 to 1) is carried out. The calculation of the ωt intensity parameters in terms of the induced electric dipole transition theory is completed, and the influence of the data on hypersensitive transitions, as well, as the role of the TR3+ ion wavefunction choice to the results of said calculation is discussed. The estimation of the contribu-tion, stipulated by the induced oscillating dipole moments of the ligands in the value of the ω2 parameter is carried out. Intermanifold luminescence branching ratios for all initial laser states of the Er3+ ions connected with the stimulated emission process are calculated. The possibility of using the concept of “spectroscopic quality” of an activated medium in the case when the intensity parameter ω2 ≠ 0 is discussed. The precise measurements of 3 μm stimulated emission spectral composition at ∼ 110 and 300 K are carried out. The energy of the Stark levels of the 4I11/2 and 4I13/2 manifolds is defined. All 3 μm induced transitions are identified and the laser kinetics at their wavelengths is studied. [Russian Text Ignored].

Optical detection of Rydberg states using electromagnetically induced transparency (EIT) enables continuous measurement of electric fields in a confined geometry. In this paper, we demonstrate the formation of radio frequency (rf)-dressed EIT resonances in a thermal Rb vapour and show that such states exhibit enhanced sensitivity to dc electric fields compared to their bare counterparts. Fitting the corresponding EIT profile enables precise measurements of the dc field independent of laser frequency fluctuations. Our results suggest that space charges within the enclosed cell reduce electric field inhomogeneities within the interaction region.

The van der Waals atom-surface attraction, scaling as C3 z-3 for z the atom-surface distance, is expected to be valid for ~ 1-1000 nm, covering 8-10 orders of magnitudes in the interaction energy. Thanks to a Cs vapor nanocell, we analyze the spectroscopic modifications induced by the atom-surface attraction on the 6P3/2->6D5/2 transition. The C3 value, extracted independently for various thicknesses ranging from 40 nm to 130 nm, is found to be independent of the thickness. It agrees, but only within a factor of 2, with an elementary theoretical prediction, whose validity is discussed.

We describe the so-called λ-Zeeman method to investigate individual hyperfine transitions between Zeeman sublevels of atoms in an external magnetic field of 0.1mT–0.25T. Atoms are confined in a nanocell with thickness L=λ, where λ is the resonant wavelength (794 or 780nm for D1 or D2 line, respectively, of Rb). Narrow resonances in the transmission spectrum of the nanocell are split into several components in a magnetic field; their frequency positions and transition probabilities depend on the B field. Possible applications are described, such as magnetometers with nanometric spatial resolution and tunable atomic frequency references.