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We present an experimental study of the hysteretic behavior of an Yb mode-locked fiber laser (MLFL) in a multi-pulsing regime. Our main focus is on the internal nonlinear transmission curve of the laser, which is fundamental to obtain the mode-locking regime. We have also observed hysteresis in the temporal separation between dispersive bound solitons. These results have been interpreted in terms of the negative and positive feedback and the redistribution of the solitons energy.

Mode-locked fiber lasers are rich dynamical systems that may present several different types of pulsed operating modes, depending on external control parameters such as pump power. A systematic experimental characterization of such regimes is a challenging problem. Here single pulse regimes of ytterbium mode-locked fiber laser are explored and related to the nonlinear amplitude modulation affecting the pulses inside the laser cavity. To understand the full dynamics of this system, average power measurements (slow time scale) have been performed and compared to simultaneous time-resolved measurements (fast time scale). The average nonlinear transmission resulting from the amplitude modulation is measured, allowing to relate the transitions between dynamical regimes to the system's nonlinearity., Comment: 15 pages, 5 figures

The storage of broadband single photons from a parametric-down-conversion source is a capability with the potential to foster significant development in the field of quantum information. A particular challenge to this problem, however, is the mismatch between the short-lived photon and the long-lived memories, which translates into quite different frequency bands for the two systems. Ultimately, this difficulty can be mapped into the problem of how a narrowband medium can efficiently absorb a broadband pulse of light. Here we present a detailed approach to this problem focusing on the absorption of photons at 800 nm, a common choice for parametric-down-conversion sources, by hot vapors of rubidium atoms. For this, we employ a stronger control field to drive a sequential two-photon transition on the atoms, which is intrinsically broadband, together with a weak signal field consisting of a femtosecond pulse of light. We describe then how to measure small absorptions of the signal pulse and how to improve this absorption through the various parameters of the problem. Our results are modeled by a perturbative theory suitable to our present weak-absorption regime. Finally, we provide a road map with different strategies to achieve larger absorptions.

In spite of recent conceptual and experimental advances, the ultimate nature of the wave function in quantum theory remains nebulous. Is it a mathematical device that describes our knowledge on a physical system or is it a physical field in any tenable sense? In this work we report on significant experimental steps toward a test on the reality of the wave function, which does not address the duality “epistemic versus ontic” in the usual way. Instead of considering inequalities derived from the partial indistinguishability of non-orthogonal states, we focus on the possibility of discriminating instantaneous reduction from finite-time wave function collapse. The former can only be associated with a Bayesian update, while the latter would be compatible with realistic interpretations. We employ non-maximally entangled photons which must be finely synchronized before detection. The required partial entanglement of the pairs is characterized by the violation they produce in the Clauser–Horne–Shimony–Holt inequality and the synchronization is obtained via time-scan measurements. This realization allows us to better understand the necessary instrumental conditions to the execution of a final, conclusive test.

We report experimental measurements of structural soliton pairs in a mode-locked Yb-doped fiber laser. The quantization of the temporal separation and relative phase between the pair of dissipative solitons is clearly observed and, most interestingly, the transitions between different states as the pump power is varied show abrupt jumps. This is a clear signature of the discrete nature of the stable pair states. The typical separations are of the order of 1 ps and the separation changes are of the order of 100 fs, while the relative phase of the pulses jumps between π/2 → π/4 → π/2 as the pump power is increased.

We experimentally study the nonlinear dynamics of a femtosecond ytterbium doped mode-locked fiber laser. With the laser operating in the pulsed regime a route to chaos is presented, starting from stable mode-locking, period two, period four, chaos and period three regimes. Return maps and bifurcation diagrams were extracted from time series for each regime. The analysis of the time series with the laser operating in the quasi mode-locked regime presents deterministic chaos described by an unidimensional Rossler map. A positive Lyapunov exponent $\lambda = 0.14$ confirms the deterministic chaos of the system. We suggest an explanation about the observed map by relating gain saturation and intra-cavity loss.

An experimental test for the wave function collapse is realized. The experimental results reaffirms the epistemic interpretation for the state vector and opposes the ontic view.

A large third-order nonlinear susceptibility was measured at 1500 nm from a self-assembled metasurface composed of a 2D superlattice of ligand-capped gold nanospheres. A self-defocusing nonlinear refractive index (n2=−1.05×10−10 cm2 W−1) and a nonlinear absorption coefficient (α2=3.00×10−6 cm W−1) were determined using the Z-scan technique with 70 fs pulses at 1 kHz. The nonlinearity response time was measured to be 1.42 ps using an Optical Kerr Gate configuration. The large n2 value is four orders of magnitude larger, in modulus, than in the silica glass substrate at 1500 nm. We show the nonlinear enhancement depends on the morphology of the metasurface and is due to collective near-neighbor coupling between the nanospheres. The physical origin of the gold nonlinearity in the short-wave infrared region is discussed.

Broadband single photons are usually considered not to couple efficiently to atomic gases because of the large mismatch in bandwidth. Contrary to this intuitive picture, here we demonstrate that the interaction of ultrashort single photons with a dense resonant atomic sample deeply modifies the temporal shape of their wavepacket mode without degrading their non-classical character, and effectively generates zero-area single-photon pulses. This is a clear signature of strong transient coupling between single broadband (THz-level) light quanta and atoms, with intriguing fundamental implications and possible new applications to the storage of quantum information., 5 pages, 4 figures

Zero area single photons are generated through the resonant interaction of a parametric-down-conversion single photon with a dense atomic ensemble. This indicates a new pathway for developing a quantum memory for broadband single photons.

We demonstrate that the interaction of ultrashort single photons with a dense resonant atomic sample deeply modifies the temporal shape of their wavepacket mode without degrading their non-classical character, and effectively generates zero-area single-photon pulses.

In this paper we consider the interaction of inhomogeneously broadened two-level atoms with a train of pulses from a femtosecond mode-locked laser. Analytical results are obtained for the case in which the atomic relaxation times are longer then the laser repetition period, leading to accumulation in both population and coherence. These results are adequate for arbitrary pulse areas, and saturation effects are discussed. We illustrate our analytical results with a comparison to a direct numerical integration of the Bloch equations.

We have investigated the third-order nonlinear optical response of a niobic–silicate glass–ceramic containing sodium niobate (NaNbO3) nanocrystals using picosecond Z-scan and femtosecond optical Kerr gate techniques at 1064 and 800 nm, respectively. The results show a large dependence of the nonlinear refractive index (n2) on the volume fraction occupied by NaNbO3 crystallites. Nonlinear absorption is negligible for all samples and the response time of the nonlinearity was shorter than 100 fs demonstrating that this material is a promising candidate for ultrafast optical switching.

We have studied the ultrafast electronic response of thin NiFe films by femtosecond transient reflectivity measurements. The experiments were performed on films with varying thicknesses, substrates, and pump fluences. It has been observed that for high excitation densities the electron cooling time depends strongly on the nature of the underlying substrate and we attribute our results to transport of hot carriers out of the excited region. In particular, we have observed that for NiFe over NiO, carrier transport should be less important than for NiFe over Si.

Frequency upconversion in Pr 3+ -doped silica based fibers has been obtained through the photon avalanche effect. A cw visible (670 nm) laser diode was the pump source used to obtain emission in the blue-green region. The critical power density to observe the effect was estimated to be 2 .

Electron-electron and electron-phonon thermalization processes are investigated in gold films using a high-sensitivity multiple-wavelength femtosecond pump-probe technique. A nonequilibrium electron distribution is excited by free-carrier absorption of an infrared femtosecond pulse and its relaxation dynamics followed by measuring the transient reflectivity and transmissivity with a visible probe pulse. Measurements performed in a thin gold film in the very low perturbation limit (\ensuremath{\Delta}${\mathit{T}}_{\mathit{e}}$\ensuremath{\sim}20 K) yield evidence for the existence of a non-Fermi electron distribution with an electron thermalization time of \ensuremath{\sim}500 fs and an electron-lattice interaction time of 1 ps. The apparent thermalization dynamics of the electron gas is much faster in optically thick samples and is shown to be dominated by transport effects.

Wavelength-tunable femtosecond absorption saturation measurements are combined with ensemble Monte Carlo simulations to investigate the dynamics of photoexcited carriers in ${\mathrm{Al}}_{0.1}$${\mathrm{Ga}}_{0.9}$As. Continuum-generation and Fourier-synthesis techniques are used to generate tunable 40-fs optical pulses for the experiments. Carriers are excited to different initial states in the conduction band depending upon the wavelength selected for the experiment. The results of measurements in which the photon energies are either above or below the threshold for scattering from the \ensuremath{\Gamma} valley to the L valley are discussed. Quantitative agreement between experiment and theory is demonstrated for a deformation potential ${\mathit{D}}_{\mathrm{\ensuremath{\Gamma}}\mathrm{\ensuremath{-}}\mathit{L}}$\ensuremath{\Gamma} of 5\ifmmode\times\else\texttimes\fi{}${10}^{8}$ eV/cm.

We report the observation of the interferometric response in parametric four-wave mixing in rubidium atoms excited by a pair of ultrashort pulses. For pulses with orthogonal polarizations and in the presence of the two-photon transition 5S-5D reveals a one-photon quantum interference, in contrast to the optical interference observed when the pulses have parallel polarization. For opposite circular polarizations no interference is observed in the fluorescence of the two-photon transition, in contrast to the one- and two-photon interferences exhibited in the nonlinear process.

We investigate the transmission of a cw laser interacting with rubidium vapor and a frequency comb. The results reveal various regimes of competition and the importance of optical pumping and power broadening of the lasers.

We study the magnetic transient dynamics induced by a femtosecond laser using Time-Resolved Magneto-Optical Kerr Effect to characterize the local anisotropy and the Gilbert damping constant of magnetic films of Fe permalloy.

We study the role of the anisotropy field in the laser-induced magnetization dynamics on thin films of Fe/MgO and Py/Si. We also characterize the local anisotropy and the Gilbert damping constant.

We present experimental results on the Two-Beam Coupling of femtosecond beams in atomic rubidium vapor. Propagation and accumulation effects due to the modelocked pulse train are investigated.

We describe two-beam coupling and temporal encoding experiments in barium titanate. Volume gratings are created in the photorefractive material by 50-fs optical pulses. Information in the writing pulses may be encoded as spatially distributed volume gratings in the crystal. Femtosecond temporal waveform reconstruction is demonstrated.

Experimental results are described for exploiting the fast and large nonlinear susceptibility of semiconductor-doped glasses (SDGs). Self-diffraction experiments were performed using incoherent broadband lasers with femtosecond time resolution yielding new information on the time response of the SDG nonlinearity. The use of backward nondegenerate wave-mixing to demonstrate yellow-to-green frequency conversion with femtosecond resolution is reported. In addition, results are presented which demonstrate the use of the SDG as a picosecond all optical gate at a 100-MHz repetition rate. >

Received 16 August 2006; revised manuscript received 15 December 2006; published 7 February 2007The interaction of externally controlled chirped femtosecond pulses in a two-photon material was investi-gated. Numerical simulations based on the solution of nonlinear Schrodinger equation, including two-photonabsorption and nonlinear propagation effects, were compared to experiments performed on CdS crystal. Theexperimental results for different chirps positive and negative are found to be in excellent agreement with thenumerical simulations. A temporal narrowing of the probe beam due to a cross-phase modulation from a pumpbeam was also observed.DOI: 10.1103/PhysRevB.75.052301 PACS number s : 78.47. p, 42.65.Re, 42.65.An

Metallic nanoparticles (NP) draw intense scientific interest due to their unique physical properties, which differ from those of bulk and atomic species. Nowadays, the aim of this research area is focused, for example, in the nonlinear (NL) optical properties of NP, the dynamics of confined electrons and the possibility of photonic applications. In this work the transient response of non-spherical nanoparticles are studied. Two differently prepared colloids are examined: one sample has a broad distribution of sizes and shapes of silver nanoparticles, while the second sample, which is processed by laser ablation, presents a more uniform size and shape distribution (greater fraction of spherical particles). The experiments are performed with a Ti:sapphire femtosecond laser using a two-color, polarization-resolved, pump-probe setup. The pump pulse at ≈ 800 nm excites intraband transitions, heating the electronic distribution, while the probe pulse is scanned around the peak of the Surface Plasmon Ressoance (SPR): ≈ 400 nm. It is observed that for the pristine colloid (non-spherical particles) the signal amplitudes of the probe polarizations parallel and perpendicular to the pump polarization present different behaviors as a function of the probe photon energy. The same effect is not observed for the laser ablated samples, which are isotropic. The temporal response of the samples is also different, with the laser ablated samples presenting faster electronic cooling rates. A model describing the induced dichroism that takes into account the shape of the colloid particles is presented. The effects of the size distribution are also discussed.

Time response and third-order susceptibility of a colloid containing silver sub-nanoparticles were studied. A giant optical nonlinearity and an oscillating response function were observed. The analysis of sub-nanoparticle response was based on a molecular approach.

We report the observation of intense frequency up-conversion in Nd3+-doped fluoroindate glasses pumped by the second harmonic of a cw mode-locked Nd : YAG laser. Mechanisms for generating the observed emissions are discussed.

Summary form only given. The coherent control of processes involving photoexcitation of atoms and molecules has seen many interesting developments. In these studies the main goal is to control the outcome of a certain process by the interference between different quantum paths, and it has been explored in resonant one- and two-photon excitation of atoms. Also, some studies about propagation effects indicate that it is interesting to consider the influence of high optical densities in the coherent control of sequential two-photon absorption processes when an intermediate level resonance occurs. We present results of preparing an atomic wave packet which generates a field that strongly reacts back on the atomic system. The experiment is done in rubidium vapor, where the laser, with 1OO fs and tuned to 778 nm, is double resonant with the sequential atomic transitions 5S-5P/sub 3/2/-5D. Femtosecond and picosecond beats are observed by monitoring the time-integrated emission at 420 nm in a second cascade, 5D-6P/sub 3/2/-5S. The sample is excited by pairs of collinear femtosecond pulses with variable relative delay, which can be scanned at slow or fast rates.

We investigate a time-resolved four-wave mixing when a two-photon resonance is involved. By varying the relative polarization of the pulse pairs, we are able to select the quatum pathways and distinguish optical and quantum interferences.

We report on efficient frequency upconversion in Er3+‐doped fluoroindate glasses. The process is observed under 1.48 μm laser diode excitation and results in the generation of strong blue (∼407 nm), green (∼550 nm), and red (∼670 nm) radiation. The main mechanism that allows for upconversion appears to be the energy transfer among Er3+ ions in excited states. The results illustrate the large potential of this new class of glasses for photonic applications.

The nonlinear optical behavior of a glass-ceramic containing sodium niobate (NaNbO/sub 3/) nanocrystals (GC-SNN) was studied in recent experiments where the third-order susceptibility, /spl chi//sup (3)/, at 532 nm was measured using nanosecond power limiting and picosecond Z-scan techniques. It was shown that the presence of NaNbO/sub 3/ nanocrystals influences the magnitude of the nonlinear (two-photon) absorption coefficient (/spl alpha//sub 2/) and the nonlinear refractive index (n/sub 2/) of the material. Here we extended our previous studies probing /spl chi//sup (3)/ at wavelengths where nonlinear absorption contributions are negligible.

We investigate coherent accumulation processes in three-level atoms excited by a train of ultrashort pulses in the case where the atomic relaxation times are greater than the laser repetition period. In this situation the resonances of the laser field with the atomic system are determined by the laser frequency comb rather than by the spectrum of a single pulse. Using the density matrix formalism, we develop a perturbative theory that is valid for arbitrary pulse shapes. The excitation of a Doppler-broadened atomic vapor by hyperbolic-secant pulses and $0\ensuremath{\pi}$ pulses is analyzed. It is shown that pulse shape has a great influence on the accumulation process and can change the spectral periodicity of the pattern impressed on the Doppler profile of the medium due to the two-photon absorption process. The effect of interpulse phases is also investigated, and we show that the atomic populations can vary by more than one order of magnitude with small variations of the laser repetition rate, while being insensitive to variations of the laser offset. Finally, the theory is adapted for the temporal-coherent-control technique, and its results are compared with previously reported experimental data.

Ultrafast light-induced dichroism in silver nanoparticles is studied. Two differently prepared colloids are examined: one sample has a broad distribution of sizes and shapes of silver nanoparticles, while the second sample, which is processed by laser ablation, presents a more uniform size and shape distribution (greater fraction of spherical particles). The experiments are performed with a femtosecond laser in a two-color, polarization-resolved, pump-probe setup. For the pristine colloid the signal amplitudes for the parallel and perpendicular polarizations present different behaviors as a function of the probe photon energy, which is not observed for the laser ablated samples. The temporal response of the samples is also different, with the laser ablated samples presenting faster electronic cooling rates. A model describing the induced dichroism that takes into account the shape of the colloid particles is presented. The effects of the size distribution are also discussed.

The optical nonlinearity of tungstate fluorophosphate glasses, synthesized in the NaPO3-BaF2-WO3 system, was investigated through experiments based on the third-order susceptibility, χ(3). Nonlinear (NL) refraction and NL absorption measurements in the picosecond regime were performed using the Z-scan technique at 532 nm. NL refractive index, n2∝Re χ(3), ranging from 0.4×10−14 cm2/W to 0.6×10−14 cm2/W were determined. The two-photon absorption coefficient, α2∝Im χ(3), for excitation at 532 nm, vary from 0.3 to 0.5 cm/GW. Light induced birefringence experiments performed in the femtosecond regime indicate that the response time of the nonlinearity at 800 nm is faster than 100 fs. The experiments show that χ(3) is enhanced when the WO3 concentration increases and this behavior is attributed to the hyperpolarizabilities associated to W–O bonds.

Measurements of the third-order nonlinear refractive index, ${n}_{2},$ and the two-photon absorption coefficient, ${\ensuremath{\alpha}}_{2},$ of a transparent niobic silicate nanocomposite are reported. Samples with various filling factors, $f,$ of sodium niobate nanocrystals were studied at 1064 nm, 800 nm, and 532 nm. $Z$-scan and Kerr gate techniques were applied using picosecond and femtosecond lasers. The response time of the nonlinearity was demonstrated to be smaller than 100 fs. An explanation is given for the observed spectral behavior of ${n}_{2}$ and ${\ensuremath{\alpha}}_{2},$ and a quantitative approach of their dependence with $f$ is presented in terms of a generalized Maxwell-Garnet model. Possible uses of the nanocomposite for all-optical switching applications are evaluated.

Nonlinear refraction, nonlinear absorption and light induced dichroism (LID) were investigated in silver colloids. The third-, fifth-, and seventh-order susceptibility were determined. The ultrafast dynamics of LID and the contribution of surface plasmons were characterized.

The femtosecond dynamics of hot carriers interacting with a cold high density electron‐hole plasma are investigated in GaAs and Al0.2Ga0.8As. Studies are performed using a three pulse pump‐probe technique where a cold plasma is first generated by a femtosecond pulse, then pump‐probe transmission measurements are performed after a few hundred picoseconds delay. The results indicate only a small increase of the hot carrier thermalization rate even for plasma densities as high as 1018 cm−3.

This letter introduces the general notion of significantly modulating the physical characteristics of a solid on a terahertz time scale with coherent lattice vibrations. We show, as an example, experiments in which coherent phonons are optically excited in single‐crystal Ti2O3, which is a narrow‐gap semiconductor at 300 K and gradually transforms into a semimetal at 600 K. Quantitative comparison of previous equilibrium measurements to our new transient measurements suggests that the induced coherent lattice vibration is significantly modulating the semiconductor‐to‐semimetal transition at 7 THz.

This paper analyzes the result of a temporal coherent control experiment using trains of ultrashort laser pulses in the situation where the laser repetition period is smaller than the relaxation times of the system. We focus on the accumulative effects present in sequential two-photon absorption. It is shown that stimulated emission due to the accumulation of population in the excited state give an important contribution to the coherently controlled process. Excellent agreement is obtained in the comparison between experiments in rubidium vapor and theory, where resonant pulse propagation is also taken into account.

The characterization and application of a new source of incoherent light for studies of ultrafast processes in condensed matter is described. It is based on the stimulated Raman effect in monomode optical fibers and has a coherence time of ∼80 fs. As an application, we have measured the homogeneous dephasing time of CdSxSe1−x‐doped glasses and obtained a value of 14±4 fs for above‐gap excitation at room temperature.

Femtosecond electron thermalization in metals was investigated using transient thermomodulation transmissivity and reflectivity. Studies were performed using a tunable multiple-wavelength femtosecond pump-probe technique in optically thin gold films in the low perturbation limit. An IR pump beam is used to heat the electron distribution and changes in electron temperature are measured with a visible probe beam at the d band to Fermi-surface transition. We show that the subpicosecond optical response of gold is dominated by delayed thermalization of the electron gas. This effect is particularly important far off the spectral peak of the reflectivity or transmissivity changes, permitting a direct and sensitive access to the internal thermalization of the electron gas. Using a simple rate-equation model, line-shape analysis of the transient reflectivity and transmissivity indicates a thermalization time of the order of 500 fs. At energies close to the Fermi surface, longer thermalization times \ensuremath{\sim}1--2 ps are observed. These results are in agreement with a more sophisticated model based on calculations of the electron-thermalization dynamics by numerical solutions of the Boltzmann equation. This model quantitatively describes the measured transient optical response during the full thermalization time of electron gas, of the order of 1.5 ps, and gives new insight into electron thermalization in metals.

We report on a detailed investigation of the coupling between a femtosecond-laser frequency comb and a cw diode laser interacting with an atomic medium of variable density. The comb is printed on a Doppler-broadened atomic transition and the frequency-dependent transmission of the cw laser is monitored as it is scanned over the inhomogenously broadened absorption profile. The printing process and its probing are analysed, experimentally and theoretically, as a function of both laser intensities and the atomic density. The results reveal the importance of optical pumping and power broadening by both lasers, allowing us to determine various regimes of competition between them.

We present experimental data which show that large amplitude coherent lattice vibrations can be impulsively excited using short optical pulses of light. Furthermore, using a quasi-equilibrium model, we show how the these coherent lattice vibrations suggest a ~ 7 THz modulation of the semiconductor-to-metal transition in Ti2O3.

Pairs of pulses from an incoherent source are used to investigate the time-resolved four-wave mixing response of atomic rubidium when a two-photon resonance is involved in the nonlinear process. By varying the relative polarization of the pulse pairs, we are able to select the quantum pathways and clearly distinguish optical and quantum interferences.