F. Carreño

Dipole-dipole interaction between a quantum dot and a graphene nanodisk

We study theoretically the dipole-dipole interaction and energy transfer in a hybrid system consisting of a quantum dot and graphene nanodisk embedded in a nonlinear photonic crystal. In our model, a probe laser field is applied to measure the energy transfer between the quantum dot and graphene nanodisk, while a control field manipulates the energy transfer process. These fields create excitons in the quantum dot and surface plasmon polaritons in the graphene nanodisk which interact via the dipole-dipole interaction. Here, the nonlinear photonic crystal acts as a tunable photonic reservoir for the quantum dot, and is used to control the energy transfer. We have found that the spectrum of power absorption in the quantum dot has two peaks due to the creation of two dressed excitons in the presence of the dipole-dipole interaction. The energy transfer rate spectrum of the graphene nanodisk also has two peaks due to the absorption of these two dressed excitons. Additionally, energy transfer between the quantum dot and the graphene nanodisk can be switched on and off by applying a pump laser to the photonic crystal or by adjusting the strength of the dipole-dipole interaction. We show that the intensity and frequencies of the peaks in the energy transfer rate spectra can be modified by changing the number of graphene monolayers in the nanodisk or the separation between the quantum dot and graphene. Our results agree with existing experiments on a qualitative basis. The principle of our system can be employed to fabricate nanobiosensors, optical nanoswitches, and energy transfer devices.

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.

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.

Plasmonic control of nonlinear two-photon absorption in graphene nanocomposites

Nonlinear two-photon absorption in a quantum dot-graphene nanoflake nanocomposite system has been investigated. An external laser field is applied to the nanocomposite to simultaneously observe two-photon processes in the quantum dot and excite localized surface plasmons in the graphene nanodisk. This resonance condition can be achieved by tuning the plasmon resonance frequency in the graphene nanoflake via electrostatic gating. It is found that the strong local field of the graphene plasmons can enhance and control nonlinear optical processes in the quantum dot. Specifically, we show that the two-photon absorption coefficient in the quantum dot can be switched between single- and double-peaked spectra by modifying the graphene-quantum dot separation. Two-photon processes in the quantum dot can also be switched on or off by slightly changing the gate voltage applied to the graphene. Our findings indicate that this system can be used for nonlinear optical applications such as all-optical switching, biosensing and signal processing.

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.

Effect of ion pairs in fast-light bandwidth in high-concentration erbium-doped fibers

The effect of ion pairs in high-concentration erbium-doped fibers on slow- and fast-light propagation enabled by coherent population oscillations at room temperature has been experimentally investigated. We find that an increase of the erbium ion concentration increases the fractional advancement although it degrades the bandwidth of the modulated signals that propagate at superluminal velocities due to the presence of ion pairs in the fiber.

The characteristic matrix of a colour detection system

Colour detection systems can be specified in terms of a set of spectral functions (colour-matching functions in the case of human observers). By taking these functions into account we define the characteristic matrix associated with a given colour detection system. This definition provides a new way to evaluate the differences and similarities between the colorimetric behaviour of two colour detection systems. We have applied this formalism to the case of a set of colour-matching functions associated with human observers. The differences between the diagonal terms, , of the characteristic matrices associated with a given couple of observers provides us with information about the differences between their corresponding ith matching functions. The distance between two characteristic matrices, defined in terms of the norm, is a measure of the global difference between the colorimetric behaviour of the corresponding couple of associated observers. It seems to the authors that the characteristic matrix could play an important role in the characterization and design of colour detection and colour reproduction systems.

Phase-controlled slow and fast light in current-modulated semiconductor optical amplifiers

We present a theoretical study of the slow and fast light propagation in semiconductor optical amplifiers based on coherent population oscillations. By modulating the injection current to force the population oscillations, we can modify the delay or advancement of light signals. Specifically, it is shown that the relative phase of the optical signal to the bias current modulations can be used as a switch for changing the light propagation from delay to advancement. In addition, we analyse the effect of the modulation current for slow light in vertical cavity surface emission lasers by taking into account the cavity effects. It is shown that a change in the depth of the modulation allows us to tune the structural resonance, which in turn produces an enhancement of the delay.