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The gain and refractive index dynamics of dilute nitride antimonide semiconductor optical amplifiers are studied using heterodyne pump probe spectroscopy, both in forward and reverse bias regimes. In the forward biased absorption regime, both gain and refractive index relax on the same timescale indicating that both quantities are linked to the same relaxation process, interband recombination. Above transparency, in the forward biased gain regime, the gain and phase exhibit differing timescales resulting in a dynamical alpha factor that varies strongly with time. Reversed bias measurements suggest a recombination dominated absorption recovery where the recovery timescale increases with increasing reversed bias, possibly due to charge separation effects. [ABSTRACT FROM AUTHOR]

In this paper, room temperature two-colour pump-probe spectroscopy is employed to study ultrafast carrier dynamics in type-II GaSb/GaAs quantum dots. Our results demonstrate a strong dependency of carrier capture/escape processes on applied reverse bias voltage, probing wavelength and number of injected carriers. The extracted timescales as a function of both forward and reverse bias may provide important information for the design of efficient solar cells and quantum dot memories based on this material. The first few picoseconds of the dynamics reveal a complex behaviour with an interesting feature, which does not appear in devices based on type-I materials, and hence is linked to the unique carrier capture/escape processes possible in type-II structures. [ABSTRACT FROM AUTHOR]

The ultrafast gain and refractive index dynamics of tunnel injected quantum dot based semiconductor optical amplifiers in the 1300 nm range are investigated using a heterodyne pump probe technique. In the gain regime, ground state wavelengths exhibit full gain recovery in less than 10 ps up to 3 times transparency, attributed to enhanced carrier refilling via the injector layer. The effect of the injector can also been seen in unusual phase dynamics at excited state wavelengths at this injection level. [ABSTRACT FROM AUTHOR]

Ego defense mechanisms were studied in three groups of early adolescents: diabetic patients, non-psychotic psychiatric patients, and healthy high school students. Defenses were assessed from ratings of open-ended, in-depth interviews. High levels of denial and low levels of asceticism were found in all three groups. Comparisons between groups indicated that psychiatric patients had a distinctive profile of defense usage, in comparison to adolescents from the other two groups. An independent measure of ego development was positively correlated with the defenses of altruism, intellectualization, and suppression, while it was negatively correlated with acting out, avoidance, denial, displacement, projection, and repression. The findings of substantial differences in defense usage between the psychiatric and non-psychiatric samples, and the size and directions of the correlations with ego development level, lend support to the validity of the defense codes. [ABSTRACT FROM AUTHOR]

Multi-pulse heterodyne pump-probe measurements are used to investigate the reverse bias dynamics of InAs/GaAs quantum dots in a waveguide structure. Using a femtosecond pulse, we simultaneously populate high energy ground states and low energy excited states and measure the resulting gain and phase dynamics over the bandwidth of the pulse. We identify a ∼5 ps timescale in the phase dynamics which can be associated with low energy ground states outside the pulse bandwidth and may provide an explanation for the deterioration of monolithic mode locked laser performance at high reverse voltages. [ABSTRACT FROM AUTHOR]

Two-color pump-probe measurements are used to study the carrier dynamics of InAs/GaAs quantum dots in a waveguide structure under reverse bias conditions. For the case of initially populating the ground state (GS), we find relaxation dynamics that include both absorptive and bleaching components in the excited state (ES) wavelength range. We reproduce the main features of this induced absorption dynamics using a simple model with an additional term for induced absorption at the ES due to carriers injected at the GS. The induced absorption dynamics includes multiple recovery timescales which can be attributed to phonon-assisted processes of GS/ES interaction. [ABSTRACT FROM AUTHOR]

The gain and phase dynamics of InAs/GaAs quantum dot amplifiers are studied using single and two-color heterodyne pump probe spectroscopy. The relaxation of the wetting layer carrier density is shown to have a strong effect on the phase dynamics of both ground and excited state transients, while having a much weaker effect on the gain dynamics. In addition, the dynamical alpha factor may also display a constant value after an initial transient. Such behavior is strongly encouraging for reduced pattern effect operation in high speed optical networks. [ABSTRACT FROM AUTHOR]

Single-color and two-color pump-probe measurements are used to analyze carrier dynamics in InAs/GaAs quantum dot amplifiers. The study reveals that hole recovery and intradot electron relaxation occur on a picosecond time scale, while the electron capture time is on the order of 10 ps. A longer time scale of hundreds of picoseconds is associated with carrier recovery in the wetting layer, similar to that observed in quantum well semiconductor amplifiers. [ABSTRACT FROM AUTHOR]

Carrier dynamics of a 1.3 μm InAs/GaAs quantum dot amplifier is studied using heterodyne pump-probe spectroscopy. Measurements of the recovery times versus injection current reveal a power law behavior predicted by a quantum dot rate equation model. These results indicate that Auger processes dominate the carrier dynamics. [ABSTRACT FROM AUTHOR]

Measurements of the linewidth enhancement factor (also termed the α-parameter) for quantum dot semiconductor lasers emitting at 1310 nm in both single and multiple transverse modes are presented. Values between 1.5 and 3.0 were measured depending on the device length. In addition, its spectral dependence within the inhomogeneously broadened ground and excited state is investigated. [ABSTRACT FROM AUTHOR]

Isolated InGaAsP microrings with an outer diameter of 5.8 µm, a width of 1 µm, and a thickness of 0.41 µm were fabricated by epitaxial separation. Individual devices were bonded to multimode optical fiber using Van der Waals forces and optically pumped through the fiber. Lasing around 1505 nm was measured under pulsed and cw pumping at room temperature. The threshold pump power for pulsed operation was estimated to be 38 and 80 µW for cw operation. Multiple radial and azimuthal modes were present due to strong, three-dimensional confinement. Under strong pulsed pumping thermal effects caused the emission wavelength to chirp. [ABSTRACT FROM AUTHOR]

In this letter, we present a simple broad-area semiconductor laser using a current spreading layer to modify the transverse gain profile. The device exhibits excellent spatial coherence to total output powers of 2.5 W under pulsed operation. Devices have been focused down to a spot size of approximately 5 μm full-width at half-maximum at 2.5 W, with the beam profile and position remaining stable over the entire range of operation. Under continuous-wave operation, thermal effects reduce spatial coherence, leading to a significantly increased spot size and loss of beam stability. This work demonstrates the advantages of modifying the transverse gain profile, and how it can be used to produce high brightness devices required for single-mode fiber coupling [ABSTRACT FROM PUBLISHER]