Jaroslaw Pulka

Intradot dynamics of InAs quantum dot based electroabsorbers

The carrier relaxation and escape dynamics of InAs/GaAs quantum dot waveguide absorbers is studied using heterodyne pump-probe measurements. Under reverse bias conditions, we reveal differences in intradot relaxation dynamics, related to the initial population of the dots’ ground or excited states. These differences can be attributed to phonon-assisted or Auger processes being dominant for initially populated ground or excited states, respectively.

Recovery time scales in a reversed-biased quantum dot absorber

The nonlinear recovery of quantum dot based reverse-biased waveguide absorbers is investigated both experimentally and analytically. We show that the recovery dynamics consists of a fast initial layer followed by a relatively slow decay. The fast recovery stage is completely determined by the intradot properties, while the slow stage depends on the escape from the dot to the wetting layer.

Refractive index dynamics of quantum dot based waveguide electroabsorbers

The refractive index dynamics of InAs/GaAs quantum dot based waveguide absorbers is studied using heterodyne pump-probe measurements. Absorption reduction due to the pump can be accompanied by either positive or negative refractive index changes depending on the wavelength used. This change in sign of the phase amplitude coupling can be understood by considering the atomlike nature of the quantum dot transitions involved.

Ultrafast response of tunnel injected quantum dot based semiconductor optical amplifiers in the 1300 nm range

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.

Ultrafast gain and refractive index dynamics in GaInNAsSb semiconductor optical amplifiers

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.

Induced absorption dynamics in quantum dot based waveguide electroabsorbers

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.

A Portable Sensing System for Impedance based Detection of Biotoxin Substances.

The study describes the development of a portable autonomous biosensing platform for impedance based detection of biotoxin substances. The platform implements a label-free approach, which is based on detection of the biosensor interfacial changes due to a bio recognition reaction. The interfacial changes are sensed by means of Electrochemical Impedance Spectroscopy in a frequency range from 10 Hz to 100 kHz. The platform comprises of an electrochemical biosensor, portable low-noise mix signal hardware and associated software incorporating signal processing algorithms for extraction biotarget concentration from the biosensor response. The biosensor is realized as an on-chip package-free three electrode micro electrochemical cell consisted of a counter electrode (CE), a reference electrode (RE) and a working electrode (WE) patterned on a single silicon chip. WE represents an array of 40 um diameter gold disks with 400 m center-to-center distance, which were undergone of corresponding surface modification for antibody immobilisation. The developed system was validated by an example of T-2 toxin detection. Performed calibration in the range of 0 – 250 ppm of T2 toxin concentrations confirmed that the system can provide successful detection of the toxin at the levels

Pump dependence of the dynamics of quantum dot based waveguide absorbers

The nonlinear two stage recovery of quantum dot based reverse-biased waveguide absorbers is investigated experimentally and analytically as a function of the initial ground state occupation probability of the dot. The latter is controlled experimentally by the pump pulse power. The slow stage of the recovery is exponential and its basic timescale is independent of pump power. The fast stage of the recovery is a logistic function which we analyze in detail. The relative strength of slow to fast components is highlighted and the importance of higher order absorption processes at the highest pump level is demonstrated.

Ultrafast Gain and Refractive Index Dynamics in AlInAs/AlGaAs Quantum Dot Based Semiconductor Optical Amplifiers Operating at 800 nm

The ultrafast gain and refractive index dynamics in AlInAs/AlGaAs quantum dot (QD) based semiconductor optical amplifiers is reported. Measurements in the forward bias regime indicate a complete gain recovery timescale of ~ 5 ps , while the phase dynamics occur over a much longer timescale. At increased pump powers, the impact of nonresonant carriers created by two-photon absorption is visible as an increased injection in both gain and phase dynamics. Reverse-biased measurements reveal a similar behavior to previous measurements on InAs QD devices.

Mixed state effects in waveguide electro-absorbers based on quantum dots

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.