Ew Erik Bogaart

All-optical switching due to state filling in quantum dots

We report all-optical switching due to state filling in quantum dots (QDs) within a Mach–Zehnder interferometric switch (MZI). The MZI was fabricated using InGaAsP/InP waveguides containing a single layer of InAs/InP QDs. A 1530–1570 nm probe beam is switched by optical excitation of one MZI arm. By exciting below the InGaAsP band gap, we prove that the refractive index nonlinearity is entirely due to the QDs. The switching efficiency is 5 rad/(μW absorbed power), corresponding to a 6 fJ switching energy. Probe wavelength insensitivity was obtained using a broad size distribution of QDs.

Ultrafast carrier capture at room temperature in InAs∕InP quantum dots emitting in the 1.55μm wavelength region

The energy and excitation density dependence of the carrier dynamics in self-assembled InAs∕InP quantum dots (QDs), emitting in the 1.55μm wavelength region, is investigated by means of time-resolved pump-probe differential reflection spectroscopy at room temperature. We observe ultrafast carrier capture and subsequential carrier relaxation into the QD ground state within 2.5 ps. The carrier lifetime in the QDs strongly depends on the QD optical transition energy within the QD ensemble as well as the carrier density, and ranges from 560 up to 2600 ps.

Picosecond time-resolved bleaching dynamics of self-assembled quantum dots

Picosecond bleaching dynamics of vertically stacked self-assembled quantum dots (QDs) is investigated by means of time-resolved pump-probe differential reflection spectroscopy (TRDR) at room temperature (RT). We observe that the absorption spectrum, which represents the QD density of states at RT, is strongly shifted with respect to the photoluminescence spectrum. This shift can not be interpreted by carrier emission and re-trapping alone. TRDR allows us to study the dynamics of the pump generated carriers within the QDs. From the time-resolved measurements, we detect that the bleaching decay time has a strong energy dependence and is dominated by radiative and nonradiative recombination at low energy and by carrier emission at high energy.

Coherent acoustic phonons in strain engineered InAs∕GaAs quantum dot clusters

Coherent excitation of the quasilongitudinal and quasitransverse acoustic phonon mode in strain engineered InAs∕GaAs quantum dot (QD) clusters grown on (311)B GaAs is monitored by means of time-resolved differential reflection spectroscopy. Carrier capture within the ordered QD clusters initiate coherent acoustic phonon excitation, which induces a transient modulation of the local strain-induced piezoelectric field within the QD clusters. The excited acoustic phonons then modulate the optical properties of the QDs through the quantum-confined Stark effect, causing distinct oscillations of the differential reflection signal.

Dichroic reflection of InAs∕GaAs quantum dots

Polarization-resolved reflection measurements are performed on nearly circular InAs∕GaAs quantum dots (QDs) by means of time-resolved differential reflection spectroscopy. We observe linear polarization anisotropy of the differential absorption, revealing the dichroic character of the QD reflection. The observed magnitude of the dichroism is Θ[11¯0]∕Θ[110]=1.07. The polarization has a preferential direction orientated along the [11¯0] crystal axis, which is confirmed by polarization-resolved photoluminescence. We observe that the polarization anisotropy of the reflectivity is strongly dependent on the pump excitation density, decreasing from ρ=0.14 at low excitation to ρ=0.06 at high excitation. The pump power dependence is described by a binomial model taking into account the statistics of carrier capture into a limited number of QDs.

Anomalous exciton lifetime by electromagnetic coupling of self-assembled InAs/GaAs quantum dots

We report on the experimental observation of a hitherto ignored long-range electromagnetic coupling between self-assembled InAs/GaAs quantum dots (QDs). A 12 times enhancement of the QD exciton lifetime is observed by means of time-resolved differential reflection spectroscopy. The enhancement is due to local field effects within the QD ensemble. The electromagnetic coupling of the QDs results in a collective polarizability, and is observed as a suppression of the emission rate. Our results reveal that the mutual coupling strength can be optically tuned by varying the pump excitation density. This enables us to optically tune the exciton lifetime.

Observation of a Long‐Range Interaction between Semiconductor Quantum Dots

We demonstrate electromagnetic interaction between distant quantum dots (QDs), as is observed from transient pump‐probe differential reflectivity measurements. The QD‐exciton lifetime is measured as a function of the probe photon energy and shows a strong resonant behavior with respect to the QD density of states. The observed exciton lifetime spectrum reveals a subradiance‐like coupling between the QD, with a 12 times enhancement of the lifetime at the center of the ground state transition. This effect is due to a mutual electromagnetic coupling between resonant QDs, which extends over distances considerably beyond the nearest neighbor QD‐QD separation.