Search Results (25 total)

We propose a technique to obtain slow and fast light propagations based on coherent population oscillations forced by a modulated pump. This mechanism produces an enhancement of 1 order of magnitude of the delay or advancement of light signals. The relative phase between the pumps to the signal fields is used as a knob for changing light propagation from ultraslow group velocities to negative group velocities. The experimental realization of the phenomenon was carried out in an erbium-doped fiber amplifier at room temperature.

We analyze the propagation regime of an amplitude-modulated 1536  nm signal when traveling along a highly doped erbium fiber pumped at 977  nm as a function of the fiber length. A propagation-induced transition from superluminal to subluminal propagation takes place along the fiber length which allows a change in regime solely based upon increasing the signal modulation frequency. This peculiar behavior is due to the interplay between pump absorption and pump-power broadening of the spectral hole induced by coherent population oscillations. The effect of ion density on this frequency-dependent regime change has been investigated.

We measure polarization-resolved instantaneous patterns in a large-aspect ratio quasi-isotropic Nd:YAG laser. High correlation between the instantaneous orthogonal polarization patterns recorded at the earlier stages of the laser pulse has been found due to the strong cross saturation between both polarization modes.

We report time-resolved measurements of transverse dynamics in a pulsed custom-made Nd:YAG (yttrium aluminum garnet) broad-area laser, where a dynamic transition from order to turbulentlike patterns appears, without the modification of the Fresnel number. To our knowledge, it is the first time this dynamical transition is observed in a class-B laser. Higher values of the pumping and a higher ratio between the electric field and the inversion population decay rates diminish the time at which the transition takes place. A theoretical model based on the adiabatic approximation allows us to explain the main features of the observed dynamics and to compare the results with those previously seen, especially in broad-area CO₂ lasers.

We report a direct experimental observation of antiphase oscillations in population dynamics in lasers. We show that these population oscillations are intrinsically related to the well-known antiphase polarization dynamics, i.e., the antiphase oscillations of two orthogonal polarization laser field states. We have used a class B Nd:YAG (yttrium aluminum garnet) laser.

We analyze the effect of injected atomic coherence on transverse patterns of a broad area laser by means of the semiclassical two-level Maxwell-Bloch equations. A single longitudinal mode is considered. The injected atomic coherence forces a spatially homogeneous profile to appear and locks the field phase to a single value. Above a pump threshold value a very rich scenario of patterns is developed. Near threshold we find stationary patterns such as rhombic and hexagonal lattices. Well above threshold nonstationary patterns such as complex highly ordered vortex lattices traveling along the cross section, and nearly traveling waves appear.

The squeezing spectrum of the fluorescent light is investigated for a laser-driven three-level atom of the Λ configuration when quantum interference of the decay channels is accounted for. We show that when the two atomic transitions contribute to the detected fluorescence field, squeezing at certain frequency intervals is obtained in both the weak- and the high-Rabi-frequency regimes even for equally decay rates of the transitions. Unlike in two-level atoms in free space, squeezing can be obtained in both the in-phase and out-of-phase quadrature spectra although in different spectral regions. We also show that the squeezing spectrum can be controlled by an adequate selection of the Rabi frequencies and atomic detunings. Another remarkable effect is that squeezing can be achieved with proper relative phases of the driving fields. We provide an analytical description in the dressed basis which accounts for the main features of the squeezing spectra obtained from the numerical work.

In this paper we investigate the resonance fluorescence, the squeezing, and the absorption spectra of a laser-driven four-level atom consisting of three closely spaced upper levels decaying to a common lower level. The three upper levels are coupled by the same vacuum modes to the lower level leading to spontaneously generated coherence effects. High population inversion and extremely narrow emission lines are obtained with moderate Rabi frequencies, which are a direct consequence of decay-induced interferences. The fluorescent field is shown to exhibit two-mode squeezing in the in-phase quadrature. Squeezing in both quadratures simultaneously at different frequencies is greatly enhanced when quantum interference is accounted for. We also examine the absorption spectrum of a weak field and demonstrate that quantum interference can induce two prominent and nearly transparent holes where the slope of the refractive index is very steep. This special situation could allow the simultaneous propagation of two weak pulses with different frequencies, which could be feasibly exploited towards the realization of the entanglement of two photons.

We study the dispersion and absorption spectra of a weak probe in a Λ-type three-level atomic system with closely ground sublevels driven by a strong field and damped by a broadband squeezed vacuum. We analyze the interplay between the spontaneous generated coherence and the squeezed field on the susceptibility of the atomic system. We find that by varying the intensity of the squeezed field the group velocity of a weak pulse can change from subluminal to superluminal. In addition we exploit the fact that the properties of the atomic medium can be dramatically modified by controlling the relative phase between the driving field and the squeezed field, allowing us to manipulate the group velocity at which light propagates. The physical origin of this phenomenon corresponds to a transfer of the atomic coherence from electromagnetically induced transparency to electromagnetically induced absorption. Besides, this phenomenon is achieved under nearly transparency conditions and with negligible distortion of the propagation pulse.

Pattern formation in large-aspect-ratio single-mode inhomogeneously broadened lasers is studied by means of the integro-differential Maxwell-Bloch equations. As the inhomogeneous linewidth increases, the neutral stability curve shows a relaxation in the traveling wave selection allowing structures with different sizes to grow. We have performed numerical simulations with a simplified model based on a few discrete groups of atoms at different resonant frequencies to observe transverse dynamics above threshold for good and bad cavity configurations. We obtain, in general, that close to threshold the inhomogeneous broadening leads to a more complex pattern in comparison to the homogeneous broadening case. At higher pumpings, a stabilization in the number of spatial frequencies taking part in the transverse pattern is found. The influence of the laser aperture on temporal dynamics is also studied. It is shown how the typical self-pulsing regime present in single-mode inhomogeneously broadened lasers is destroyed for a wide enough aperture.

We report the first direct experimental observation of the fast dynamics (nanosecond scale) of complex two-dimensional transverse patterns in broad area lasers. The laser emission bright peaks forming the transverse patterns are observed to be aperiodically flashing in time with different growing rates. These optical filaments do not move along the cross section during their lifetime, which is close to 2 ns. The experimental observations have also been reproduced by numerical integration of the Maxwell-Bloch equations.

Phase domains and phase solitons in two-level amplifying media damped by a squeezed vacuum are predicted for the first time. Two different types of pattern formation are found depending on the relative value of the cavity detuning to the squeezed parameter: the usual one in lasers via a supercritical Hopf bifurcation and a new one via pitchfork bifurcation.

In this paper we analyze the steady-state populations and gain lineshape of a V-type three-level atom with a closely spaced excited doublet. The atom is driven by a strong coherent field, a weak probe, and a single broadband squeezed vacuum. We focus our attention in the interplay between the quantum interference and the squeezed field on the probe gain. It is shown that the relative phases between the two coherent fields and the squeezed field play an important role in the optical properties of the atom. Specifically, we find that the probe can experience gain without population inversion for proper values of the parameters characterizing the squeezed field and in the absence of incoherent pumping. The system can be tailored to exhibit multiple dispersion regimes accompanied by negligible gain or absorption over a large bandwidth, a desirable feature for obtaining propagation of pulses with negligible distortion.

The spatiotemporal evolution of field-induced structures in very dilute polarizable colloidal suspensions subject to rotating magnetic fields has been experimentally studied using video microscopy. We found that there is a crossover Mason number (ratio of viscous to magnetic forces) above which the rotation of the field prevents the particle aggregation to form chains. Therefore, at these high Mason numbers, more isotropic clusters and isolated particles appear. The same behavior was also found in recent scattering dichroism experiments developed in more concentrated suspensions, which seems to indicate that the dynamics does not depend on the volume fraction. Scattering dichroism experiments have been used to study the role played by the volume fraction in suspensions with low concentration. As expected, we found that the crossover Mason number does not depend on the volume fraction. Brownian particle dynamics simulations are also reported, showing good agreement with the experiments.

Transverse effects in the laser dynamics due to near dipole-dipole interactions are studied considering the presence of permanent electric dipole moments. The semiclassical two-level Maxwell-Bloch equations are used and a single longitudinal mode is assumed. A traveling wave is selected at threshold when the sum of the cavity detuning and the near dipole-dipole parameter is larger than zero. As a consequence of this, transverse pattern can occur even when the laser frequency is larger than the frequency of the atomic transition. Also, a cutoff in the laser field spectrum arises. We found that the near dipole-dipole interactions significantly modify the stability picture of the traveling waves. Numerical simulations have been carried out and the effect of the near dipole-dipole on the pattern formation is addressed.

We study a single-mode laser system, whose active medium consists of molecules with a large difference between excited and ground electrical permanent-dipole moments. In this case, the Maxwell-Bloch equations are further coupled by nonlinear terms involving the ratio between this difference between the dipoles and the transition dipole moment. It is found that these new terms lead to multiple stationary solutions. From the linear stability analysis, we demonstrate the bistable (or multistable) character of the lasing solutions.

Recently a new method of controlling the pulse length of a short-pulse free-electron laser (FEL) has been developed. By modulating the synchronism between the optical and electron pulses in the FEL cavity, it was found that the output power and the micropulse length of the FEL beam oscillates at the modulation frequency. In this paper, we study theoretically the behavior of the micropulse length, both in the high loss (steady state) regime and the low loss (limit cycle) regime, when a modulated desynchronism is applied. In order to do this, we analyze the dynamics of a short-pulse FEL oscillator. The modulation frequency value plays an important role in the dynamics. We find that there is a resonantlike phenomenon between the externally applied desynchronism modulation and the limit cycle oscillation without modulation of a free-electron laser.

We have theoretically investigated the effects of self-phase-modulation on multiphoton nonlinear optical processes in semiconductors, utilizing multiple-order perturbation theory based on a set of coupled nonlinear-wave equations. Our results clearly demonstrate that the self-phase-modulation in the fundamental field induces spectral broadening in the harmonic fields (cross-phase modulation) and alleviates phase mismatch by providing a distribution of wave vectors. These results are consistent with our recent observation of extreme nonlinear optical behavior in bulk semiconductors under intense mid-infrared radiation.

We have observed extreme nonlinear optical phenomena produced by intense midinfrared (MIR) pulses in semiconductors. These phenomena include multiple off-resonance optical sidebands (up to ±3 MIR photons interacting with a near-infrared photon), multiple MIR harmonics (up to the seventh harmonic), and significant broadening and modification of MIR harmonic spectra. The generation of these extreme MIR nonlinear optical phenomena is primarily aided by cross-phase modulation.

We present evidence of stochastic resonance in free-electron lasers. In order to do that, we have analyzed theoretically the dynamics of a free-electron laser oscillator. A weak modulation and a noise source have been applied to the initial energy of the electron beam. We have found stochastic resonance for different frequencies and amplitudes of the modulation. A threshold crossing mechanism leads to the stochastic resonance in this system.

We present an experimental and theoretical study of the effect of desynchronism modulation on short pulse free-electron laser (FEL) oscillators. We find that the output power and the micropulse length of the FEL beam oscillate periodically at the modulation frequency and that the minimum micropulse length during the cycle can be significantly shorter than that which can be obtained without modulation. For example, when the desynchronism of our FEL is modulated at 40 kHz, the minimum measured micropulse length is 300 fs. Without modulation the minimum is about 700 fs. We show that when the desynchronism is modulated, the FEL can operate for part of the cycle in the normally inaccessible portion of the output power curve where the FEL gain is less than the cavity losses. It is even possible for the FEL to operate periodically in the region of negative desynchronism where gain, as normally defined, does not exist.

A study of the spatiotemporal evolution of a pulsed CO₂ laser is performed using an experimental setup which allows transverse pattern recordings with 1-ns resolution. A rich laser dynamics, hidden until now behind temporal averages, is uncovered. Features like spontaneous symmetry breaking, mode beating, and phase locking are observed and theoretically interpreted using Maxwell-Bloch equations. Numerical simulations by means of mode decompositions are performed, showing very good agreement with the empirical results. The influence of the curvature of the mirrors in the dynamics of the system is also theoreticaly analyzed.

Transverse effects in the laser threshold, originated by electronic-vibrational coupling in the active centers, are analyzed theoretically by means of the semiclassical two-level Maxwell-Bloch equations. A single longitudinal mode is considered. It is found that the first laser threshold suffers modifications depending on the electronic-vibrational coupling strength. This coupling imposes certain conditions for the selection of a particular transverse spatial state and provides the minimum wavelength that can appear in the transverse pattern. The nature of the bifurcation and the stability of the homogeneous and critical traveling waves are analyzed.

In this paper we analyze the transverse pattern-forming characteristics of a two-level and single-longitudinal-mode laser. The corrections to the classical empty cavity modes due to pumping and losses are specifically considered, leading to different excitation rates for the different transverse modes. The near-threshold excitation rates provide a selection mechanism that is studied and compared to the results from numerical simulations of the Maxwell-Bloch equations. © 1996 The American Physical Society.