P. Spencer

Persistent template effect in InAs/GaAs quantum dot bilayers

The dependence of the optical properties of InAs/GaAs quantum dot (QD) bilayers on seed layer growth temperature and second layer InAs coverage is investigated. As the seed layer growth temperature is increased, a low density of large QDs is obtained. This results in a concomitant increase in dot size in the second layer, which extends their emission wavelength, reaching a saturation value of around 1400 nm at room temperature for GaAs-capped bilayers. Capping the second dot layer with InGaAs results in a further extension of the emission wavelength, to 1515 nm at room temperature with a narrow linewidth of 22 meV. Addition of more InAs to high density bilayers does not result in a significant extension of emission wavelength as most additional material migrates to coalesced InAs islands but, in contrast to single layers, a substantial population of regular QDs remains.

Growth, optical properties and device characterisation of InAs/GaAs quantum dot bilayers

The growth and optical properties of InAs/GaAs quantum dot (QD) bilayers are investigated, where the strain interactions between closely spaced QD layers are exploited to tailor the optical properties of the system. The underlying (seed) layer acts as a template for subsequent growth of the upper layer, whose properties can then be modified due to the greater freedom in the choice of growth conditions. Extension of the emission wavelength of the QDs is observed, to 1400 nm at room temperature for GaAs-capped bilayers and extending to 1515 nm for InGaAs-capped bilayers. The QDs in the second layer are highly uniform, resulting in an inhomogeneous broadening of <20 meV and, for small separations between QD layers, efficient carrier tunnelling results in suppression of emission from the seed layer. Edge-emitting lasers incorporating QD bilayers operating either in the ground state or first excited state at 1340 nm at room temperature are demonstrated, showing comparable behaviour to QD lasers containing independent layers. Ground state lasing at 1425 nm at 250 K is also observed.

Ultrafast absorption recovery dynamics of 1300 nm quantum dot saturable absorber mirrors

We compare the performance of two quantum dot saturable absorber mirrors with one device operating at the quantum dot ground state transition whereas the other operates at the first excited state transition. Time-resolved photoluminescence and heterodyne four-wave mixing experiments demonstrate faster recovery of the excited-state device compared to the ground-state device. Femtosecond pulses were achieved with both devices, with the ground-state device producing 91 fs pulses and the excited-state device producing 86 fs pulses in a Cr:forsterite laser. The fast absorption recovery dynamics indicates the potential of devices exploiting excited-state transitions for use in high repetition rate lasers.

Experimental and theoretical study of polarization-dependent optical transitions in InAs quantum dots at telecommunication-wavelengths (1300-1500 nm)

The design of some optical devices, such as semiconductor optical amplifiers for telecommunication applications, requires polarization-insensitive optical emission at long wavelengths (1300–1550 nm). Self-assembled InAs/GaAs quantum dots (QDs) typically exhibit ground state optical emissions at wavelengths shorter than 1300 nm with highly polarization-sensitive characteristics, although this can be modified by the use of low growth rates, the incorporation of strain-reducing capping layers, or the growth of closely-stacked QD layers. Exploiting the strain interactions between closely stacked QD layers also affords greater freedom in the choice of growth conditions for the upper layers, so that both a significant extension in their emission wavelength and an improved polarization response can be achieved due to modification of the QD size, strain, and composition. In this paper, we investigate the polarization behavior of single and stacked QD layers using room temperature sub-lasing-threshold electrolumin...

The influence of size distribution on the luminescence decay from excited states of InAs/GaAs self-assembled quantum dots

We compare the time integrated and time resolved spectra of two samples having coincident ground state emission peaks: one consisting of highly uniform quantum dots, the other grown under conditions which produce a broad distribution of quantum dot sizes. The photoluminescence decay of the ground states in both samples is monoexponential from which we deduce a lifetime of ∼1100 ps independent of excitation power. The excited state decays for the two samples are biexponential with fast and slow components of ∼300 and ∼1100 ps, respectively. These are also independent of excitation power but their contribution to the decay curve changes with power. The data allow us to unequivocally associate the fast component with the excited state decay of larger dots and the slow component with the ground state decay of smaller dots which emit at the same energy. Furthermore, taking into account the degeneracy of the ground state and the optical selection rules for exciton recombination in a confined system we show that...

Toward 1550-nm GaAs-Based Lasers Using InAs/GaAs Quantum Dot Bilayers

By choice of appropriate growth conditions and optimization of the strain interactions between two closely stacked InAs/GaAs quantum dot (QD) layers, the emission wavelength of the QDs can be significantly extended, giving room-temperature emission from highly uniform QD ensembles in excess of 1500 nm. These QD bilayers are incorporated into edge-emitting laser structures and room-temperature ground-state lasing at 1420 nm and electroluminescence at 1515 nm are observed. Under high-bias conditions, asymmetric broadening of peaks in the laser gain spectra are observed, extending positive net modal gain from the devices to beyond 1500 nm, and the origin of this broadening is discussed.

O-band excited state quantum dot bilayer lasers

Bilayer InAs/GaAs quantum dot(QD) lasers operating in the excited state at wavelengths that span the O-band are demonstrated. The higher saturated gain and lower scattering time of the excited states of the ensemble of QDs offers the opportunity for fast direct-modulation lasers. We predict an increase in K-factor limited modulation bandwidth from QD lasers operating in the excited state due to a reduction in carrier transport and scattering times whilst maintaining high peak modal gain.

Optical spin-filtering effect in charged InAs/GaAs quantum dots

We present time resolved photoluminescence results using nonresonant polarized light which show that the electron spin-flip time is much longer than the recombination time for an ensemble of p-doped InAs/GaAs quantum dots. Under continuous wave excitation the degree of optical polarization of the ground state is found to be around 10%. However, the excited state polarization is twice this value. We attribute this effect to Pauli blocking of the injected spin population captured into the dots and show that the effect persists up to room temperature. For resonant excitation, values are nearly doubled in accordance with increased spin injection efficiency.

Influence of p-doping on the temperature dependence of InAs/GaAs quantum dot excited state radiative lifetime

The radiative lifetime of the excited state transition of undoped and p-doped InAs/GaAs quantum dots(QDs) is estimated from measurements of time-integrated and time-resolved luminescence from both ground and excited states. The radiative lifetime of the undoped QDs increases from 500 ps at 10 K to almost 3 ns at room temperature, consistent with a Boltzmann redistribution of holes over the available energy states. The rate of increase can be suppressed by a factor of ∼2 by p-doping the QDs to maintain a hole population in the lowest confined dot states to high temperatures.

Resolving Zeeman splitting in quantum dot ensembles

This letter presents a technique for the investigation of the fine structure and spin properties of quantum dot (QD) ensembles, allowing measurement of QD parameters previously accessible only from studies of individual QDs. We show how ∼μeV splittings can be deduced from information contained in the shape of the ensemble polarization spectra and demonstrate the effectiveness of this technique by measuring Zeeman splittings, g-factors, and sensitivity to QD fine structure effects.