Photon echoes produced by switching electric fields

We demonstrate photon echoes in Eu3+:Y2SiO5 by controlling the inhomogeneous broadening of the Eu3+ 7F0<-->5D0 optical transition. This transition has a linear Stark shift, and we induce inhomogeneous broadening by applying an external electric field gradient. After optical excitation, reversing the polarity of the field rephases the ensemble, resulting in a photon echo. This is the first demonstration of such a photon echo, and its application as a quantum memory is discussed.     

Optical decoherence times and spectral diffusion in an Er-doped optical fiber measured by two-pulse echoes, stimulated photon echoes, and spectral hole burning

Two-pulse and stimulated photon echoes and spectral hole burning were measured on the transition from the lowest component of the ⁴I_15/2 manifold to the lowest component of ⁴I_13/2 of Er³⁺ in a silicate optical fiber at 1.6 K. The two-pulse echo decays gave decoherence times as long as 230 ns for magnetic fields above 2 T. A large field dependent contribution to the homogeneous line width of >2 MHz was found and interpreted in terms of coupling to magnetic tunneling modes (TLS) in the glass. The stimulated echoes measured at 2 T showed spectral diffusion of 0.8 MHz/decade of time between 0.4 and 500 μs. Spectral diffusion in this high field region is attributed to coupling to elastic TLS modes which have a distribution of flip rates in glasses. Time-resolved spectral hole burning at very low field showed stronger spectral diffusion of 5.7 MHz/decade of time, attributed to coupling to magnetic spin-elastic TLS modes.     

Two-photon emission competing with cooperative and nonlinear processes

Stimulated non-degenerate two-photon emission from a pencil of lithium vapour is demonstrated on three decay cascades of the Li atom, namely accompanied with transitions from the 3s, 3d and 4s levels to the 2s level, upon pulsed two-photon excitation of the Li 4f and 4d levels. It competes with superfluorescence and parametric emission which show up at slightly shifted wavelengths. In the cascade via the 3d level, which is strongest under the experimental conditions, also two-photon Stokes scattering of the excitation light is identified.     

Surface photon echoes in the infrared range

We discuss the possibility of observing photon echoes in the infrared range from a surface monolayer. Three different detection schemes are considered: direct infrared detection, external up-conversion, and in-situ up-conversion. The external up-conversion scheme appears most promising, while the in-situ up-conversion scheme has the advantage of being highly surface specific. Dephasing relaxations of surface vibrational excitations should be measurable.     

Coherent control of optical precursors in a warm atomic system with Doppler effects

We theoretically study coherent control of optical precursors via active Raman gain (ARG) in an N-type warm atomic system. When a step pulse passes through an ARG window, main fields are advanced due to fast-light effect and constructively interference with optical precursors, then an enhanced transient pulse appears. As the control field decreases, the interference effect is strengthened, and the transient pulse builds up and becomes narrow. Moreover, its peak intensity is inverse to the system temperature and also determined by the input-pulse form. The scheme may be useful in designing optical devices in optical communication.     

Intensity correlations in a coherently prepared Rb vapor in a magnetic field

We have studied the intensity correlations between two orthogonally linearly polarized components of a laser field propagating through a resonant atomic medium. These experiments have been performed in a rubidium atomic vapor. We observe that the correlations between the orthogonally polarized components of the laser beam are maximal in the absence of a magnetic field. The magnitude of the correlations depends on the applied magnetic field, and the magnitude first decreases and then increases with increasing magnetic field. Minimal correlations and maximal rotation angles are observed at the same magnetic fields. The width of the correlation function is directly proportional to the excited state lifetime and inversely proportional to the Rabi frequency of laser field.     

Sub-Doppler spectral resolution in a resonantly driven four-level coherent medium

Sub-Doppler spectral resolution has been studied in Doppler-broadened multi-level ⁸⁷Rb atoms coherently coupled by two strong laser fields. Narrow spectral features of absorption or gain are observed in the center or sides of the Doppler-broadened absorption profile. Analytical and numerical calculations based on a four-level N type model are presented to explain the experimental results.     

Successive oscillatory fields at radio to optical frequencies

The methods of separated or successive oscillatory fields (often called Ramsey methods) are described for radio, microwave and optical coherent radiation. The methods are initially described qualitatively. The quantitative theories of the methods are also given. For a two-level system at radio frequencies exact expressions for the transition probability amplitudes for an arbitrary number of successive oscillatory regions can be found in terms of products of successive transformation matrices. The expressions are greatly simplified if the oscillatory-field regions are very short. If there are only two oscillatory-field regions the situation is similar to a Young's two-slit interference pattern except that the two paths are separated in spin space rather than normal space. Some results from more than two oscillatory regions are discussed. The extensions of the methods and the theories to optical frequencies are described. The advantages and disadvantages of the methods are given as are the many extensions of the method beyond the original concept.I am honored to write an article in celebration of the sixtieth birthday of Herbert Walther. He has established one of the world's best atomic-physics laboratories and has made great contributions to physics, ranging from precision spectroscopy and laser cooling to measurements of fundamental atomic and quantum properties. He has also been a helpful consultant, adviser, and friend to many of us in the field. Happy birthday!!     

Upper level and phase controlling group velocity in V-type system

With the external field coupling the two upper levels, we investigate the light pulse propagation properties with weak probe field in a V-type system. Due to the external upper level (UL) coupling field, the dispersion of the system has been influenced by the relative phase. It is shown that the UL field and the relative phase can be regarded as switches to manipulate light propagation between subluminal and superluminal.     

Atomic spectroscopy with squeezed light for sensitivity beyond the vacuum-state limit

A frequency tunable source of squeezed light has been developed which is suitable for a variety of spectroscopic applications. In initial experiments continuous tunability over a range of 2 GHz has been achieved with a directly observed nonclassical noise reduction of 6 dB relative to the vacuum-state limit in a balanced homodyne detector. A process of light-induced absorption in the nonlinear crystal has been identified as the principal loss mechanism which prevents the observation of yet larger degrees of squeezing. Although our source is potentially broadly tunable over the range of wavelengths from 840 to 970 nm, the current research centers on the performance at 852 nm for spectroscopy of the D ₂ line of atomic cesium. For frequency-modulated (FM) saturation spectroscopy in a vapor cell, an improvement of 3.1 dB in sensitivity relative to the usual quantum limit is demonstrated for the detection of Doppler-free resonances. When corrected for the thermal noise of the detector, the enhancement in signal-to-noise ratio brought by the squeezed field is 3.8 dB relative to the shot-noise limit set by the vacuum fluctuations of the probe field.     

The study of the stability of 1D confined Bose-Einstein condensates based on one novel orthogonal basis

One novel orthogonal basis of the 1D Gross-Pitaevskii equation with square well, describing the properties of the nonuniform Bose-Einstein condensates, was introduced. Based on this orthogonal basis, we investigated the dynamical stability of the ground state and the first excited state for the nonuniform condensates. We found that the ground state is stable even for large negative nonlinear interaction. But the first excited state performs dynamical instability when the nonlinear interaction is over the critical value. Our numerical results support our analytical analysis. Some underlying physics are discussed.     

Collision induced and resonantly enhanced extra resonances in four-wave mixing in inhomogeneously broadened four level systems

Role of collisions in inducing and enhancing extra resonances in non-linear generation by four-wave mixing in inhomogeneously broadened four level systems is studied. It is shown that collision induced three and two-photon resonances can be enhanced further by tuning to exact one photon (non-absorbing) transitions between unpopulated upper levels. In addition we also find existence of collision induced interference dip at the three-photon resonance that arises due to complete cancellation of signal at the line center.     

Sub-Doppler features due to double dark resonances

A sharp intense sub-Doppler feature is predicted in the probe absorption spectrum of an atomic four-level system when a hyperfine-split state interacts with a single pump. The sub-Doppler feature, which is flanked by two power-broadened transparency windows, is obtained when the frequency of the pump is greater than that of the probe, and the residual Doppler broadening is significant.     

Quantum beats in fluorescence for multi-level atomic system

For Λ-type three-level atomic systems we have clarified using diagram that (1) it is impossible to observe quantum beats due to the ground state sublevels by measuring the time dependence of the fluorescence intensity, and (2) why it is physically possible to observe and how we can observe quantum beats in the ground state sublevels by using fluorescence. Generalization of the results shows that we can determine from which state (the ground state or the excited state) the quantum beats are originated. Analytical result is shown for four-level atomic systems.     

Cavity ring-down spectroscopy of photochemically produced NaH for the determination of relative dipole transition moments

-30 Cm of isolated lines in the A¹Σ⁺←X¹Σ⁺ NaH transition band were measured. Received: 18 November 1996/Revised version: 10 May 1997     

Variance squeezing and information entropy squeezing via Bloch coherent states in two-level nonlinear spin models

The nonclassical squeezing effect emerging from a nonlinear coupling model (generalized Jaynes–Cummings model) of a two-level atom interacting resonantly with a bimodal cavity field via two-photon transitions is investigated in the rotating wave approximation. Various Bloch coherent initial states (rotated states) for the atomic system are assumed, i.e., (i) ground state, (ii) excited state, and (iii) linear superposition of both states. Initially, the atomic system and the field are in a disentangled state, where the field modes are in Glauber coherent states via Poisson distribution. The model is numerically tested against simulations of time evolution of the based Heisenberg uncertainty relation variance and Shannon information entropy squeezing factors. The quantum state purity is computed for the three possible initial states and used as a criterion to get information about the entanglement of the components of the system. Analytical expression of the total density operator matrix elements at t > 0 shows, in fact, the present nonlinear model to be strongly entangled, where each of the definite initial Bloch coherent states is reduced to statistical mixtures. Thus, the present model does not preserve the modulus of the Bloch vector.     

Parameter-dependent photon echo induced in CdSe/ZnS quantum dot quantum well

The two pulse photon echo (2PPE) phenomena induced by the 1s-1s electronic transition in CdSe/ZnS quantum dot quantum well (QDQW) has been studied by employing semiconductor Bloch equations. The energy eigenvalues and eigenfunctions of electrons and holes have been obtained by solving the stationary Schrödinger equation under effective-mass approximation. The Coulomb interaction, which changes with the size variation of QDQW, has been calculated and analyzed as a perturbation. The variations of the electric transition dipole moment and the energy interval with the changing of the size and structure of the QDQW have also been obtained. It has been shown from the numerical calculation results that the efficiency of 2PPE can be controlled by the variation of the size and structure of the QDQW and the mechanism has been explained in terms of the quantum size confined effect (QSCE) theory.     

Stabilization of three-dimensional light bullets by a transverse lattice in a Kerr medium with dispersion management

We demonstrate a possibility to stabilize three-dimensional spatiotemporal solitons (“light bullets”) in self-focusing Kerr media by means of a combination of dispersion management in the longitudinal direction (with the group-velocity dispersion alternating between positive and negative values) and periodic modulation of the refractive index in one transverse direction (out of the two). Assuming the usual model based on the paraxial nonlinear Schrödinger equation for the local amplitude of the electromagnetic field, the analysis relies upon the variational approximation (results of direct three-dimensional simulations will be reported in a follow-up). A predicted stability area is identified in the model’s parameter space. It features a minimum of the necessary strength of the transverse modulation of the refractive index, and finite minimum and maximum values of the soliton’s energy. The former feature is also explained analytically.     

High-resolution selective reflection spectroscopy in intermediate magnetic fields

We have measured the modifications of frequency-modulated selective reflection spectra of the CsD ₂ line recorded with circularly polarized light by a longitudinal magnetic field in the range 120–280 G. The spectra recorded with ⁺ and ^– polarizations were found to be qualitatively different, but are well described by a theoretical model based on the diagonalization of the hyperfine-Zeeman Hamiltonian. The technique presented here is a simple way for investigating fully resolved complex Zeeman spectra in moderate magnetic fields and may find applications in the investigation of anisotropies in long-range atom-surface interactions.     

Blue up-conversion with excitation into Tm ions at 808 nm in YVO4 crystals co-doped with thulium and ytterbium

The up-conversion of infrared radiation at 808 nm, emitted by a diode laser, into blue emission centered at 480 nm in 1 at.% Tm, 5 at.% Yb: YVO₄; 1 at.% Tm, 8 at.% Yb: YVO₄ and 2 at.% Tm, 5 at.% Yb: YVO₄ has been studied. The highest intensity of blue emission is found for the 1 at.% Tm, 8 at.%Yb: YVO₄ system. The power dependence of up-converted emission upon continuous-wave excitation as well as the time evolution of its intensity upon short-pulse excitation were found to be consistent with a two-step excitation mechanism in which the forward Tm³⁺–Yb³⁺ energy transfer is followed by the back Yb³⁺–Tm³⁺ energy transfer. The effect of dopant concentrations on the up-conversion process is interpreted taking into account dynamics of the excited states involved.