Oscar G. Calderón

Optical bistability using quantum interference in V -type atoms

The behaviour of a V-type three-level atomic system in a ring cavity driven by a coherent field is studied. We consider a V configuration under conditions such that interference between decay channels is important. We find that when quantum interference is taken into account, optical bistability can be realized with a considerable decrease in the threshold intensity and the cooperative parameter. On the other hand, we also include the finite bandwidth of the driving field and study its role in the optical bistable response. It is found that at certain linewidths of the driving field optical bistability is obtained even if the system satisfies the trapping condition and the threshold intensity can be controlled. Furthermore, a change from the optical bistability due to quantum interference to the usual bistable behaviour based on saturation occurs as the driving field linewidth increases.

Observation of large 10-Gb/s SBS slow light delay with low distortion using an optimized gain profile

An optimum SBS gain profile is designed to achieve better slow-light performance. It consists of a nearly flat-top profile with sharp edges. Tunable delays up to 3 pulse widths for 100-ps-long input pulses, corresponding to 10 Gb/s data rates, are found while keeping an output-input pulse-width ratio below 1.8. Bit-error-rate (BER) measurements performed for a non-return-to-zero modulation format demonstrates 28 ps of delay under error-free operation.

Optical bistability in V-type atoms driven by a coherent field in a broadband squeezed vacuum

We study the optical bistability (OB) induced in a coherently-driven V-type three-level atom, when the cancellation of the spontaneous emission produced by quantum interference in the two possible decay channels to the ground sublevel takes place. In addition the atoms interact with a beam in a broadband squeezed vacuum state. New features are found in this situation. In particular, vacuum-induced optical bistability (VIOB) may be controlled by the amplitude of the squeezed vacuum or its phase relative to the phase of the external field.

Slow and fast light based on coherent population oscillations in erbium-doped fibres

In this paper we review the main results on slow and fast light induced by coherent population oscillations in optical fibres doped with erbium ions. We explain the physics behind this technique and we describe the experimental realization. Finally, we summarize some recent advances in this field and future goals.

Plasmon-enhanced terahertz emission in self-assembled quantum dots by femtosecond pulses

A scheme for terahertz (THz) generation from intraband transition in a self-assembled quantum dot (QD) molecule coupled to a metallic nanoparticle (MNP) is analyzed. The QD structure is described as a three-level atom-like system using the density matrix formalism. The MNP with spherical geometry is considered in the quasistatic approximation. A femtosecond laser pulse creates a coherent superposition of two subbands in the quantum dots and produces localized surface plasmons in the nanoparticle which act back upon the QD molecule via dipole-dipole interaction. As a result, coherent THz radiation with a frequency corresponding to the interlevel spacing can be obtained, which is strongly modified by the presence of the MNP. The peak value of the terahertz signal is analyzed as a function of nanoparticles size, the MNP to QD distance, and the area of the applied laser field. In addition, we theoretically demonstrate that the terahertz pulse generation can be effectively controlled by making use of a train of femtosecond laser pulses. We show that by a proper choice of the parameters characterizing the pulse train a huge enhancement of the terahertz signal is obtained.

Quantum interference effects in resonance fluorescence and absorption spectra of a V-type three-level atom damped by a broadband squeezed vacuum

The resonance fluorescence spectrum (RFS) and the absorption of a weak probe beam of a V-type three-level atom damped by a broadband squeezed vacuum and driven by a single coherent field which simultaneously couples the two upper states are studied. We consider the interplay between the squeezed field and the vacuum-induced coherence (VIC) between the two possible decay channels. The RFS for a degenerate excited doublet exhibits a high narrowing of the central line. In the situation in which the excited doublet is non-degenerate, we found that the squeezed field breaks the trapping condition attained when induced interference is maximal. A explanation of the main features in terms of dressed atomic states is given. We have also studied the probe absorption spectrum for a resonant driving field. It is shown that the probe may be amplified or absorbed over a wide range of frequencies when both quantum interference and the squeezed vacuum are considered. The sensitivity of the spectrum to the squeezed phase allows the control of the amplification/absorption of the probe signal.

Laser emission of a flash-lamp pumped Rhodamine 6 G solid copolymer solution

A Rhodamine 6 G solid solution in a copolymer of 2 hydroxyethyl methacrylate and methyl methacrylate have been flash-lamp pumped in an untuned laser cavity. The pumping energy threshold to obtain laser emission is 20 percent higher than in equivalent methanol solutions. With a flash-lamp electrical excitation average energy of 150 joules per pulse, no photodegradation was observed in the laser emission after more than 30 pumping pulses. There was an absence of local irregular fluctuations found typically in high Fresnel number dye lasers. An interpretation based on the assumption of stereospecificity in the lodging cavity of dye molecules, is consistent with the high photostability and the absence of local fluctuations obtained with this polymeric solid-state laser.

Superradiance from an ultrathin film of three-level V-type atoms: interplay between splitting, quantum coherence and local-field effects

We carry out a theoretical study of the collective spontaneous emission (superradiance) from an ultrathin film comprised of three-level atoms with V configuration of the operating transitions. As the thickness of the system is small compared to the emission wavelength inside the film, the local-field correction to the averaged Maxwell field is relevant. We show that the interplay between the low-frequency quantum coherence within the subspace of the upper doublet states and the local-field correction may drastically affect the branching ratio of the operating transitions. This effect may be used for controlling the emission process by varying the doublet splitting and the amount of low-frequency coherence.

Modulation-frequency-controlled change from sub- to superluminal regime in highly doped erbium fibers

We report a change from sub- to superluminal propagation upon increasing the modulation frequency of an amplitude-modulated 1,550 nm signal when propagating through highly doped erbium fibers pumped at 980 nm. We show that the interplay between the pump absorption and the pump-power broadening of the spectral hole induced by coherent population oscillations may drastically affect the fractional advancement or delay of the signal for the considered fibers.

Competition between the modulation instability and stimulated Brillouin scattering in a broadband slow light device

We observe competition between the modulation instability (MI) and stimulated Brillouin scattering (SBS) in a 9.2 GHz broadband SBS slow light device, in which a standard 20 km long single-mode LEAF fibre is used as the SBS medium. We find that MI is dominant and depletes most of the pump power when we use an intense pump beam at ∼1.55 μm, where the LEAF fibre is anomalously dispersive. The dominance of the MI in the LEAF-fibre-based system suppresses the SBS gain, degrading the SBS slow light delay and limiting the SBS gain-bandwidth to 125 dB GHz. In a dispersion-shifted highly nonlinear fibre, the SBS slow light delay is improved due to the suppression of the MI, resulting in a gain-bandwidth product of 344 dB GHz, limited by our available pump power of 0.82 W.