Coherent Population Oscillations Research Articles

Probing degenerate two-level atomic media by coherent optical heterodyning

We analyse how light-induced coherent population oscillations and ground-state Zeeman coherence in an atomic medium with degenerate two-level transitions can modify spectra of applied cw resonant radiation at the sub-mW power level. The use of mutually coherent optical fields and heterodyne detection schemes allows spectral resolution at a kHz level, well below the laser linewidth. We find that ground-state Zeeman coherence may facilitate nonlinear wave mixing, while coherent population oscillations are responsible for phase and amplitude modulation of the applied fields. Conditions for the generation of new optical fields by nonlinear wave mixing in degenerate two-level atomic media are formulated.

Slow Light in Molecular-Aggregate Nanofilms

We study slow-light performance of molecular aggregates arranged in nanofilms by means of coherent population oscillations. The molecular cooperative behavior inside the aggregate enhances the delay of input signals in the gigahertz range in comparison with other coherent population oscillation-based devices. Moreover, the problem of residual absorption present in coherent population oscillation processes is removed. We also propose an optical switch between different delays by exploiting the optical bistability of these aggregates.

Pulse-width-dependent subluminal and superluminal propagation in highly doped erbium fibers

We experimentally and theoretically study the propagation of a 1536 nm light pulse superposed on a continuous wave background in a highly doped erbium fiber pumped at 977 nm. We observe a transition from subluminal to superluminal propagation with the pulse bandwidth. Furthermore, an improvement of the pulse delay and pulse distortion when increasing the pulse peak and keeping constant the background power is reported. These results are due to the relation between the pump-broadened transparency hole (induced by the coherent population oscillations) and the competition between gain and absorption along the fiber.

Coherent and incoherent dynamics in excitonic energy transfer: Correlated fluctuations and off-resonance effects

We study the nature of the energy transfer process within a pair of coupled two-level systems (donor and acceptor) subject to interactions with the surrounding environment. Going beyond a standard weak-coupling approach, we derive a master equation within the polaron representation that allows for investigation of both weak and strong system-bath couplings, as well as reliable interpolation between these two limits. With this theory, we are then able to explore both coherent and incoherent regimes of energy transfer within the donor-acceptor pair. We elucidate how the degree of correlation in the donor and acceptor fluctuations, the donor-acceptor energy mismatch, and the range of the environment frequency distribution impact upon the energy transfer dynamics. In the resonant case (no energy mismatch) we describe in detail how a crossover from coherent to incoherent transfer dynamics occurs with increasing temperature [A. Nazir, Phys. Rev. Lett. 103, 146404 (2009)], and we also explore how fluctuation correlations are able to protect coherence in the energy transfer process. We show that a strict crossover criterion is harder to define when off-resonance, though we find qualitatively similar population dynamics to the resonant case with increasing temperature, while the amplitude of coherent population oscillations also becomes suppressed with growing site energy mismatch.

Generation and Transmission of a Quasi-Optical Single Sideband Signal for Radio-Over-Fiber Systems

Generation of a quasi-optical single sideband (OSSB) signal for a radio-over-fiber system utilizing the coherent population oscillation effects in a nonlinear semiconductor optical amplifier and transmission over a single-mode fiber (SMF) are investigated. The quasi-OSSB signal generator consists of an electroabsorption modulator and a nonlinear semiconductor optical amplifier. Characteristics of generated quasi-OSSB signals having different radio-frequency (RF) waves (1-50 GHz) transmitted over SMFs of different lengths (4-21 km) were investigated. Bit-error-rate performance of quasi-OSSB signals having 155-Mb/s differential phase-shift keying (DPSK) data transmitted over 0- to 55-km-long SMFs was also evaluated.

Two-photon and two-photon-assisted slow light

We show that light pulses propagating in two-photon absorbing systems may present time delays like slow light produced via coherent population oscillations in one-photon interactions. Two regimes are numerically studied for a simplified two-level system: (a) a light pulse at frequency ω/2 undergoes two-photon absorption (TPA) and is delayed by the absorbing system (two-photon slow light) and (b) a light pulse at frequency ω is delayed in a system prepared by TPA of a light pulse at frequency ω/2 (two-photon-assisted slow light). The study carried out in solutions of dyes and dendrites shows significant delays, low distortion, and good transmission for easily reachable experimental conditions. The working principle can be applied to other media and can be used in telecommunications technology.

Observation of Slow Light in the Noise Spectrum of a Vertical External Cavity Surface-Emitting Laser

The role of coherent population oscillations is evidenced in the noise spectrum of an ultralow noise laser. This effect is isolated in the intensity noise spectrum of an optimized single-frequency vertical external cavity surface-emitting laser. The coherent population oscillations induced by the lasing mode manifest themselves through their associated dispersion that leads to slow light effects probed by the spontaneous emission present in the nonlasing side modes.

Intermodulation and harmonic distortion in slow light Microwave Photonic phase shifters based on Coherent Population Oscillations in SOAs

We theoretically and experimentally evaluate the propagation, generation and amplification of signal, harmonic and intermodulation distortion terms inside a Semiconductor Optical Amplifier (SOA) under Coherent Population Oscillation (CPO) regime. For that purpose, we present a general optical field model, valid for any arbitrarily-spaced radiofrequency tones, which is necessary to correctly describe the operation of CPO based slow light Microwave Photonic phase shifters which comprise an electrooptic modulator and a SOA followed by an optical filter and supplements another recently published for true time delay operation based on the propagation of optical intensities. The phase shifter performance has been evaluated in terms of the nonlinear distortion up to 3rd order, for a modulating signal constituted of two tones, in function of the electrooptic modulator input RF power and the SOA input optical power, obtaining a very good agreement between theoretical and experimental results. A complete theoretical spectral analysis is also presented which shows that under small signal operation conditions, the 3rd order intermodulation products at 2Ω1 + Ω2 and 2Ω2 + Ω1 experience a power dip/phase transition characteristic of the fundamental tones phase shifting operation.

Slow light propagation in a ring erbium-doped fiber

Slow light propagation is demonstrated by implementing Coherent Population Oscillations in a silica fiber doped with erbium ions in a ring surrounding the single mode core. Though only the wings of the mode interact with erbium ions, group velocities around 1360 m/s are obtained without any spatial distortion of the propagating mode.

Stark-tuned Förster resonance and dipole blockade for two to five cold Rydberg atoms: Monte Carlo simulations for various spatial configurations

Results of numerical Monte Carlo simulations for the Stark-tuned F\"orster resonance and dipole blockade between two to five cold rubidium Rydberg atoms in various spatial configurations are presented. The effects of the atoms' spatial uncertainties on the resonance amplitude and spectra are investigated. The feasibility of observing coherent Rabi-like population oscillations at a F\"orster resonance between two cold Rydberg atoms is analyzed. Spectra and the fidelity of the Rydberg dipole blockade are calculated for various experimental conditions, including nonzero detuning from the F\"orster resonance and finite laser linewidth. The results are discussed in the context of quantum-information processing with Rydberg atoms.

Slow and Fast Light Effects and Their Applications to Microwave Photonics Using Semiconductor Optical Amplifiers

We provide a comprehensive review of the application of slow and fast light (SFL) techniques to the field of microwave photonics. Basic principles leading to the implementation of phase shifting and true time delay operations which are instrumental in this field are first considered. We then focus on the description of the main results obtained by our groups in the implementation of broadband, full 360 phase shifting using coherent population oscillations in semiconductor waveguides. Next, attention is given to the evaluation of the system impairments implied by these devices when included in analog links. Finally, the main results obtained for several microwave photonic applications such as filtering, arbitrary waveform generation and optoelectronic oscillators (OEOs) are reviewed, and other directions for future research in the field are discussed.

The superluminal propagation in an organic nonlinear optical glass at room temperature

We investigated the light propagation in an organic glass with phthalocyanine gallium (GaPc) dispersed in polymethyl methacrylate (PMMA) fabricated by injection molding. The GaPc/PMMA organic glass is a reverse saturation absorption medium. We observed the superluminal propagation in this organic glass by using the technique of coherent population oscillations at room temperature. The largest time advancement is 2.21 ms and the corresponding group velocity is −0.90 m/s at the wavelength of 532 nm and with the input power of 89 mW. The density matrix theory with four-level model was used to analyze the experimental data. The theoretical simulations showed that the time advancement not only depends on the modulation frequency but also on the lifetime of the first triplet-state of the GaPc. We compared the time advancements between the GaPc/PMMA organic glass and the C60/toluene solution. Although the value of the former is smaller, it is more suitable for practical applications such as optical information processing than the latter.

Group velocity coupling and synchronization-like phenomenon of two light beams based on coherent population oscillations

We report on the control of the slow and fast light propagation velocity of two light beams (at 1550 nm and 980 nm) through an erbium-doped fibre amplifier by means of coherent population oscillations. We modulate the amplitude of both beams with the same frequency so that the relative phase between both modulations allows us to control their group velocities. The same group velocity, in absolute value, is achieved when beams are modulated in phase or phase opposition. The experimental results are explained by a rate equation model.

Intermodulation distortion in microwave phase shifters based on slow and fast light propagation in semiconductor optical amplifiers

We show theoretically and validate experimentally the effect of filtering on the nonlinear behavior of slow and fast light links based on coherent population oscillations in semiconductor optical amplifiers. The existence of a dip in the power-versus-current characteristics for the fundamental frequency, as well as for the third-order intermodulation product, is clearly evidenced. These two dips occur at different bias currents. Their depths increase as the filtering strength of the red sideband is increased, and they completely vanish in the unfiltered case. Influence on the microwave photonics link is discussed.

Mechanical vibration-induced coherent optical spectroscopy in a single quantum dot coupled to a nanomechanical resonator

We theoretically propose a scheme to study coherent population oscillation (CPO) spectra in a system of a quantum dot coupled to a nanomechanical resonator. Due to nanomechanical vibrations CPO in a single quantum dot demonstrates novel features. In analogy with CPO in quantum optics, we refer to this effect as mechanically induced coherent population oscillation (MICPO). In this scheme, the vibration of the nanomechanical resonator makes a contribution to additional auxiliary energy levels so that the transparency phenomenon could be realized in such a system. The optical spectrum shows that the transparency based on MICPO can be controlled simply by the intensity of the optical field, the coupling strength and other relevant parameters. Furthermore, our technique provides detection in a real experiment to measure the decay rate of the nanomechanical resonator.

Experimental demonstration of enhanced slow and fast light by forced coherent population oscillations in a semiconductor optical amplifier

We experimentally demonstrate enhanced slow and fast light by forced coherent population oscillations in a semiconductor optical amplifier at gigahertz frequencies. This approach is shown to rely on the interference between two different contributions. This opens up the possibility of conceiving a controllable rf phase shifter based on this setup.

Harmonic Distortion in Microwave Photonic Phase Shifters Based on Coherent Population Oscillations in SOAs

We present a theoretical and experimental evaluation of any general order harmonic distortion in a slow and fast light propagation phase shifter based on coherent population oscillations in a semiconductor optical amplifier device. Different optical filtering implementations are proposed for enhancing the performance of microwave photonic phase shifters.

Wideband 360° microwave photonic phase shifter based on slow light in semiconductor optical amplifiers

In this work we demonstrate for the first time, to the best of our knowledge, a continuously tunable 360 degrees microwave phase shifter spanning a microwave bandwidth of several tens of GHz (up to 40 GHz). The proposed device exploits the phenomenon of coherent population oscillations, enhanced by optical filtering, in combination with a regeneration stage realized by four-wave mixing effects. This combination provides scalability: three hybrid stages are demonstrated but the technology allows an all-integrated device. The microwave operation frequency limitations of the suggested technique, dictated by the underlying physics, are also analyzed.

Spatial optical memory based on coherent population oscillations

We show that a system characterized by long-lived coherent population oscillations (CPOs), such as a two-level system that decays via a shelving state, can be used to construct a spatial optical memory. In the presence of a cw control field, a field is generated at the four-wave mixing (FWM) frequency with the same spatial profile as the initial probe. When the control field is switched off, these spatial profiles are encoded in the CPOs of the ground and storage states. When the control field is switched on again, the probe and FWM fields are retrieved simultaneously with the same spatial profiles as the initial probe.

Enhancing slow- and fast-light effects in quantum-dot semiconductor waveguides through ultrafast dynamics

We show that ultrafast carrier dynamics plays an important role on slow- and fast-light effects based on coherent population oscillations in quantum-dot semiconductor waveguides. Fast light in the gain regime and slow light in the absorption regime are found to be enhanced at frequencies beyond the usual limits of the carrier lifetime. The effects are investigated by a comprehensive model and are shown to originate from nonequilibrium dynamics within the quantum-dot carrier populations.