Weak Probe Light Research Articles

Coherent population trapping resonances at lower atomic levels of Doppler broadened optical lines

We have detected and analysed narrow high-contrast coherent population trapping (CPT) resonances, which are induced in absorption of a weak monochromatic probe light beam by counterpropagating two-frequency pump radiation in a cell with rarefied caesium vapour. The experimental investigations have been performed by the example of nonclosed three level -systems formed by spectral components of the line of caesium atoms. The applied method allows one to analyse features of the CPT phenomenon directly at a given low long-lived level of the selected -system even in sufficiently complicated spectra of atomic gases with large Doppler broadening. We have established that CPT resonances in transmission of the probe beam exhibit not only a higher contrast but also a much lesser width in comparison with well- known CPT resonances in transmission of the corresponding two-frequency pump radiation. The results obtained can be used in selective photophysics, photochemistry and ultra-high resolution atomic (molecular) spectroscopy.

Permanent dipole moment induced stability and group index switching in a three-level molecule

The effect of permanent dipole moments on the optical bistability of weak probe light in a unidirectional ring cavity with a three-level molecule is investigated. It is found that the intensity threshold of optical bistability and optical multistability can be controlled by quantum coherence induced by permanent dipole moments. It is shown that switching from optical bistability to optical multistability and lasing without population inversion can be occurred by this coherence under different conditions.

Spatial weak-light ring soliton in self-assembled quantum dots

By using semiclassical theory combined with multiple-scale method, we analytically study the linear absorption and the nonlinear dynamical properties in a lifetime broadened Λ-type three-level self-assembled quantum dots. It is found that this system can exhibit the transparency, and the width of the transparency window becomes wider with the increase of control light field. Interestingly, a weak probe light beam can form spatial weak-light dark solitons. When it propagates along the axial direction, the soliton will transform into a steady spatial weak-light ring dark soltion. In addition, the stability of two-dimensional spatial optical solitons is testified numerically.

EIT enhanced self-Kerr nonlinearity in the three-level lambda system under Doppler broadening

Using density-matrix theory, an analytical expression of the self-Kerr nonlinear coefficient of a three-level lambda EIT medium for a weak probe light is derived. Influences of the coupling light and Doppler broadening on the self-Kerr coefficient are investigated and compared to experimental observation with a good agreement. The self-Kerr nonlinearity is basically modified and greatly enhanced in the spectral region corresponding to EIT transparent window. Furthermore, sign, slope, and magnitude of the self-Kerr coefficient can be controlled with frequency and intensity of the coupling light and temperature. Such controllable Kerr nonlinearity can find interesting applications in optoelectronic devices working with low-light intensity.

Stability and Group Index Switching in a Four-Level Atomic System

A four-level atomic system is proposed for stability and group index switching in the presence of spontaneously generated coherence. It is found that the transition from optical bistability (OB) to optical multistability (OM) and subluminal to superluminal can be obtained simultaneously. It is shown that the relative phase between applied fields can affect the stability and group index behaviors of weak probe light in the medium. Our proposed model provides a new scheme for simultaneous switching from OB to OM and (sub-to-super) luminal light propagation in an atomic system.

Electromagnetically induced transparency in an entangled medium.

We theoretically investigate light propagation and electromagnetically induced transparency in a quasi-one-dimensional gas in which atoms interact strongly via exchange interactions. We focus on the case in which the gas is initially prepared in a many-body state that contains a single excitation and conduct a detailed study of the absorptive and dispersive properties of such a medium. This scenario is achieved in interacting gases of Rydberg atoms with two relevant S states that are coupled through exchange. Of particular interest is the case in which the medium is prepared in an entangled spin-wave state. This, in conjunction with the exchange interaction, gives rise to a nonlocal susceptibility that--in comparison to conventional Rydberg electromagnetically induced transparency--qualitatively alters the absorption and propagation of weak probe light, leading to nonlocal propagation and enhanced absorption.

Enhancement of self-Kerr nonlinearity via electromagnetically induced transparency in a five-level cascade system: an analytical approach

We propose to use a five-level cascade system to enhance self-Kerr nonlinearity under an electromagnetically induced transparency (EIT) condition. Using density-matrix theory, an expression of the self-Kerr nonlinear coefficient for a weak probe light is derived. Variations of the self-Kerr coefficient with respect to the frequency and intensity of a strong coupling light are investigated. The Kerr nonlinearity is basically modified and enhanced greatly in the spectral regions corresponding to EIT transparent windows. Furthermore, the sign, slope, and magnitude of the self-Kerr coefficient can be controlled with the frequency and intensity of the coupling light. Such a controllable Kerr nonlinearity can find interesting applications in optoelectronic devices working with low light intensity at multiple frequencies.

Phase-Controlled Transparent Superluminal Light Propagation in a Doppler-Broadened Four-Level N-Type System

The propagation of a weak probe field in a four-level N-type quantum system in the presence of spontaneously generated coherence (SGC) is theoretically investigated. The optical properties of the system are studied and it is shown that the group velocity of light pulse can be controlled by relative phase of applied fields. By changing the relative phase of applied fields, the group velocity of light pulse changes from transparent subluminal to the transparent superluminal light propagation. Thus, the phase-controlled absorption-free superluminal light propagation is obtained without applying an incoherent laser fields to the system. The propagation of a weak probe light pulse is studied by solving the Maxwell's wave equation on numerical grid in space and time. Moreover, we study the third order self- and cross-Kerr susceptibility of probe field and calculate the nonlinear cross-phase shift for different values of intensity of applied fields. In addition, we take into account the effect of Doppler broadening on the light pulse propagation and it is found that a suitable choice of laser propagation directions allows us to preserve our results even in the presence of Doppler effect. It is demonstrated that by increasing the Doppler width of distribution to the room temperature, the dispersion changes from transparent subluminal to transparent superluminal light propagation which is our major motivation for this work.

Electromagnetically induced grating based on the giant Kerr nonlinearity controlled by spontaneously generated coherence

We propose a scheme for realizing electromagnetically induced grating via the giant Kerr nonlinearity in a coherently driven four-level system with spontaneously generated coherence. In the presence of spontaneously generated coherence, Kerr nonlinearity can be enhanced with vanishing linear absorption. Thus, with a standing-wave coupling field, one can achieve a pure absorption grating, which leads the probe field to gather the zero-order direction when the detuning of the coupling field is on resonance. Moreover, we can obtain a pure phase grating, which diffracts a weak probe light into the first-order direction and the second-order direction when the detuning of the coupling field is off resonance.

All-optical image switching in a double-Λ system

The coherent control of optical images has garnered attention because all information embedded in optical images is expected to be controlled in a parallel way. One of the most important control processes is switch for information delivery. We experimentally demonstrated phase-controlled optical image switching in a double-Λ system where the transmission of the image through a medium was switched. Two independent laser sources were adopted for a double-Λ system such that images inscribed in two weak probe light beams were incoherent with each other. Arbitrary phase was added to the optical images to show that switching could be accomplished just with the relative phase difference between the probe pixels.

Electromagnetically induced grating in an atomic system with a static magnetic field

We study the electromagnetically induced grating in an Fe=0↔Fg=1 transition interacting with a weak probe, a strong standing-wave coupling field and a static magnetic field. With a standing-wave coupling field, the phase modulation effectively diffracts a weak probe light into the first-order direction by applying a static magnetic field. Moreover, the diffraction pattern of probe beam could be dynamically tuned by manipulating the magnetic field, and the high efficiency of phase grating can be obtained by choosing the proper parameters.

Gain assisted large-scale tunable atomic filter based on double selective optical pump induced dichroism

A gain assisted large-scale tunable optical filter scheme via double selective optical pump induced dichroism is proposed. Theoretical model predicts that the filter provides a huge gain for the weak probe light, furthermore, its tunability can reach over gigahertz while its bandwidth is only several hundred megahertz, which is important for laser communication and lidar systems subjected to large Doppler shift.

Phase shift spectra of a fiber–microsphere system at the single photon level

We succeeded in measuring phase shift spectra of a microsphere cavity coupled with a tapered fiber using a weak coherent probe light at the single photon level. We utilized a tapered fiber with almost no depolarization and constructed a very stable phase shift measurement scheme based on polarization analysis using photon counting. Using a very weak probe light (n = 0.41), we succeeded in observing the transition in the phase shift spectrum between undercoupling and overcoupling (at gap distances of 500 and 100 nm, respectively). We also used quantum state tomography to obtain a 'purity spectrum'. Even in the overcoupling regime, the average purity was 0.982 ± 0.024 (minimum purity: 0.892), suggesting that the coherence of the fiber-microsphere system was well preserved. Based on these results, we believe this system is applicable to quantum phase gates using single light emitters such as diamond nitrogen vacancy centers.

Spatial weak-light ring dark solitons in an electromagnetically induced transparency medium

By using multiple-scale method，we analytically study the nonlinear dynamical properties of an electromagnetically induced transparency medium with three-level atoms and a strong coupling light field. It is shown that a weak probe light beam can form spatial weak-light dark solitons. When it propagates along axial direction，the dark soliton will transforms into a steady spatial weak-light ring dark soliton，which results from the balance of the interplay between beam diffraction and refractive nonlinearity.

Temporal Compression for a Weak Pulse Using Quantum Coherence

The absorptive and dispersive properties of an optical medium can be modified [1–4] by using quantum coherence and/or quantum interference. Recently electromagnetically induced transparency (EIT) has been of interest in the realization of rapid refractive index control to carry out a slow light propagation without absorption, which is advantageous to a dynamic optical buffer memory [5]. Recently, Boyd et al. applied a dynamic spectral hole burning technique, i.e., coherent population oscillation to realize the same effect of group velocity change in a room temperature solid [6]. Here we present a coherent light scheme to realize a dynamically variable photonic band gap in an atomic medium resulting in superluminal and ultraslow propagation of a weak light pulse without absorption. This is different from the EIT-based group index change, where the dispersion of the group velocity (second dispersion of the refractive index) is not zero but is enlarged. Nor is this the same as in Ref. 6 because all the populations are stayed in the ground states without variation during the interaction processes. Due to the band gap induced by quantum interference, a weak probe light pulse can be compressed. In this paper, a ∧-type three-level system is applied to investigate the modulation of the refractive index, re-

Optically induced Faraday effect in aΛconfiguration of spin-polarized cold cesium atoms

Polarization rotation of weak probe light induced by circularly polarized strong coupling light in a $\ensuremath{\Lambda}$ configuration is studied. We use spin-polarized cold cesium atoms trapped in a magneto-optical trap to remove complications from Zeeman distribution, Doppler broadening, and collisional decoherence. By using a very low probe intensity and short illumination period we work in a strictly linear regime. The probe and the coupling fields are optically phase locked to eliminate phase fluctuation and consequent atomic decoherence. Using this idealized situation we clarify the roles of optically induced Faraday rotation, circular dichroism, and electromagnetically induced transparency (EIT) in determining the final state of the probe light. In particular, we identify an experimental situation where the roles of atomic coherence and EIT are important.

Phase dependences of optical dispersion and group velocity in an Er3+-doped yttrium aluminium garnet crystal

A new method to control the dispersion property and furthermore the group velocity of a weak probe light by the use of the total closed interaction phase is proposed in erbium-doped yttrium aluminium garnet crystal. From numerical results, it is found that the flat dispersion, the normal dispersion and the anomalous dispersion can be achieved separately by controlling the phases of the applied fields. On this basis, the corresponding variations of the group velocity are discussed and we demonstrate that the subluminal group velocity and the superluminal group velocity can be separately achieved with different light phases and intensities.

Nonlinear properties of electromagnetically induced transparency in rubidium vapor

Nonlinear properties of electromagnetically induced transparency (EIT) with respect to a weak probe light are studied. We observe EIT in a rubidium vapor, measure the slow light group velocity and the EIT resonance width, and find that those parameters show a strong dependence on the probe power. We theoretically study the influence of the EIT probe dependence on the performance of the EIT-based optical delay lines and magnetometers.

Subluminal and superluminal propagation of light in an N-type medium

For a three-level electromagnetically induced transparency (EIT) atomic system, we show that, adding a third driving field coupled to a fourth state, the properties of the weak probe light propagation are greatly changed. Due to the increase of the driving field, when the driving and the coupling detunings are zero, the light propagation can be changed from subluminal to superluminal. Also, the analytical solution exhibiting superluminal group velocity is given at the zero probe detuning.

Spatial Weak-Light Solitons in an Electromagnetically Induced Nonlinear Waveguide

We show that a weak probe light beam can form spatial solitons in an electromagnetically induced transparency (EIT) medium composed of four-level atoms and a coupling light field. We find that the coupling light beam can induce a highly controllable nonlinear waveguide and exert very strong effects on the dynamical behavior of the solitons. Hence, in the EIT medium, it is not only possible to produce spatial solitons at very low light intensities but also simultaneously control these solitons by using the coupling-light-induced nonlinear waveguide.