Columnar Quantum Dots Research Articles

Twofold gain enhancement by elongation of QDs in polarization preserving QD-SOAs

The impact of quantum dot (QD) elongation on key parameters of QD-based semiconductor optical amplifiers (SOAs) is investigated using a combination of 8-band -theory including strain and piezoelectricity up to second order and a rate equation model describing the population of QD ground, excited and wetting layer states. By considering columnar QDs of selected aspect ratios, we show that chip gain and saturation gain can be enhanced by up to +3.6 dB via an increased elongation of the individual QDs while retaining polarization preserving amplification and gain recovery times below 700 fs. Our results enable the optimization of polarization preserving QD-SOA devices which combine ultrafast gain recovery with high gain and low power consumption.

Theoretical study of polarization dependence of carrier-induced refractive index change of quantum dot.

The influences of dot material component, barrier material component, aspect ratio and carrier density on the refractive index changes of TE mode and TM mode of columnar quantum dot are analyzed, and a multiparameter adjustment method is proposed to realize low polarization dependence of refractive index change. Then the quantum dots with low polarization dependence of refractive index change (<1.5%) within C-band (1530 nm - 1565 nm) are designed, and it shows that quantum dots with different material parameters are anticipated to have similar characteristics of low polarization dependence.

Tailoring Energy Band Alignment of Vertically Aligned InGaAs Quantum Dots Capped with GaAs(Sb)/AlGaAsSb Composite Structure after Thermal Annealing Treatment

The effects of thermal annealing treatment on a vertically aligned InGaAs/GaAs(Sb)/AlGaAsSb quantum dot (QD) structure with the purpose of tailoring energy band alignment are studied. In contrast to the typical blue-shift in the emission upon annealing because of In–Ga intermixing in the typical InGaAs/GaAs QDs, thermally annealed InGaAs/GaAs(Sb)/AlGaAsSb QDs exhibit a red-shift upon annealing at 700 °C, owing to Sb aggregation on top of the InGaAs QDs, resulting in tailoring of the band alignment and strain reduction for the reduced emission energy. Power-dependent and time-resolved photoluminescence were utilized herein to extend the carrier lifetime from 1.63 ns to 6.38 ns and to elucidate mechanisms of the aggregation of antimony that cause the energy band alignment modifying from type I to type II after rapid thermal annealing. In addition, the thermal stability of the columnar QDs was improved by capping QDs with a GaAsSb overgrown layer, because In–Ga intermixing was suppressed, helping to preserve...

Single photon emission up to liquid nitrogen temperature from charged excitons confined in GaAs-based epitaxial nanostructures

We demonstrate a non-classical photon emitter at near infrared wavelength based on a single (In,Ga)As/GaAs epitaxially grown columnar quantum dot. Charged exciton complexes have been identified in magneto-photoluminescence. Photon auto-correlation histograms from the recombination of a trion confined in a columnar dot exhibit sub-Poissonian statistics with an antibunching dip yielding g(2)(0) values of 0.28 and 0.46 at temperature of 10 and 80 K, respectively. Our experimental findings allow considering the GaAs-based columnar quantum dot structure as an efficient single photon source operating at above liquid nitrogen temperatures, which in some characteristics can outperform the existing solutions of any material system.

Four Wave Mixing in Quantum Dot Semiconductor Optical Amplifiers

We report an extensive experimental characterization of four wave mixing (FWM) in InAs/InGaAsP/InP columnar quantum dot semiconductor optical amplifiers (QD-SOAs). This paper includes a characterization of the QD-SOA and of the single and double pump FWM performance. The double pump case is studied in both the orthogonal and parallel pumps configurations. Unprecedented ultrabroadband and efficient wavelength conversion are found including, for the two pumps case, 100-nm conversion span in the telecom bands around 1550 nm (S/C/L bands) with positive conversion efficiency and more than 30-dB output optical signal-to-noise ratio.

Phase-Transparent Amplification of 16 QAM Signals in a QD-SOA

We study the nonlinear phase distortions in a columnar quantum dot semiconductor optical amplifier (QD-SOA) for coherent 16 quadrature amplitude modulation (QAM) signals. A few ps gain recovery in the QD-SOA enables suppression of the pattern dependent phase noise, which is a major performance limiting issue in conventional SOAs. A 25 dB phase and intensity transparent amplification of 40 Gb/s 16 QAM signals is experimentally demonstrated by error vector magnitude measurement.

Study of carrier dynamics of vertically aligned InAs/GaAsSb quantum dot structures

AbstractThe carrier dynamics of vertically aligned InAs/GaAsSb Type‐II quantum dot (QD) structure are comprehensively analyzed by time‐resolved photoluminescence (TRPL) in this study. A columnar InAs QD structure with an overgrown layer of GaAsSb is proposed to enhance the thermal stability and carrier lifetime through the Type‐II energy band alignment and carrier tunneling effect. The carrier lifetimes of columnar QD structures were correspondent with the PL spectra in wavelength‐dependent TRPL measurements because of the electronic coupling effect and the dot‐size dependent oscillator strength. The improved results in this work make columnar Type‐II QDs the promising candidates for novel optoelectronic device application. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Coherent Wavelength Conversion in a Quantum Dot SOA

We experimentally demonstrate wavelength conversion of coherent signals in an InAs/InGaAsP/InP columnar quantum dot semiconductor optical amplifier by using low power single pump four-wave-mixing. We report highly efficient conversion, i.e., 30 nm conversion span with a conversion efficiency larger than -10 dB and output optical signal to noise ratio larger than 20 dB. Transparency to coherent modulation formats is demonstrated in a 25 nm conversion of 10 GBaud quadrature phase shift keying (QPSK), 8-phase shift keying (PSK) and 16-quadrature amplitude modulation (QAM) signals. We found 0.9 to 1.1 dB optical signal to noise ratio penalty at forward error correction error free limit of 2 × 10-3 bit error rate for all the signals.

Temperature Dependence of Spin Relaxation Time in InAs Columnar Quantum Dots at 10 to 150 K

We have investigated carrier spin relaxation in InAs columnar quantum dots (CQDs) using time-resolved photoluminescence measurement. The CQDs were formed by depositing a 1.8 monolayer InAs seed dot layer and a short-period GaAs/InAs superlattice (SL). The spin relaxations of the 3- and 35-period SL CQDs show double exponential decay up to 50 and 130 K, respectively. The spin relaxation times of the fast component, whose amplitudes are 4–11 times larger than that of the slow component, are around 100 ps for the two samples. For the 3-period SL CQDs, the fast spin relaxation time shows no temperature dependence up to around 50 K, indicating the relevance of the Bir-Aronov-Pikus process. The slow spin relaxation time of the 35-period SL CQDs was found to decrease from 3.42 ns at 10 K to 0.849 ns at 130 K. This large change may be explained by the Elliott–Yafet process considering acoustic phonon scattering.

Temperature Dependence of Spin Relaxation Time in InAs Columnar Quantum Dots at 10 to 150 K

Temperature Dependence of Spin Relaxation Time in InAs Columnar Quantum Dots at 10 to 150 K

Effects of Interface Grading on Electronic States in Columnar Type-II Quantum Dots

Columnar type-II GaSb quantum dots (QDs) in GaAs are studied theoretically to clarify how electronic states are affected by the interface grading caused by the interdiffusion of Sb and As. Truncated-cone-shaped QD stacks are analyzed as a function of the diffusion length Ld, where the stacking dot number N is varied from 1 to 4. The energies of heavy- and light-hole ground states Ehh and Elh, respectively are calculated. The heavy hole is the lowest state when N = 1 and 2, and Elh is lower than Ehh for N = 4. When N = 3, the lowest state changes from light holes to heavy holes as Ld increases. We also evaluate the overlap Θ between the electron and hole wave functions, which determines the strength of optical transition. It is found that Θ changes by 2–3 orders of magnitude depending on N and Ld.

Effects of Interface Grading on Electronic States in Columnar Type-II Quantum Dots

Columnar type-II GaSb quantum dots (QDs) in GaAs are studied theoretically to clarify how electronic states are affected by the interface grading caused by the interdiffusion of Sb and As. Truncated-cone-shaped QD stacks are analyzed as a function of the diffusion length Ld, where the stacking dot number N is varied from 1 to 4. The energies of heavy- and light-hole ground states Ehh and Elh, respectively are calculated. The heavy hole is the lowest state when N = 1 and 2, and Elh is lower than Ehh for N = 4. When N = 3, the lowest state changes from light holes to heavy holes as Ld increases. We also evaluate the overlap Θ between the electron and hole wave functions, which determines the strength of optical transition. It is found that Θ changes by 2–3 orders of magnitude depending on N and Ld.

Study of multiple-stacking growth of 1.55 μm InAs columnar quantum dots with modulated tensile-strained InGaAsP barrier layers

An investigation was performed of columnar InAs quantum dots (CQDs) with modulated tensile-strained InGaAsP barriers in which the amount of tensile strain in the upper parts was higher than in the lower parts, the dots being deposited on an InP substrate grown by metalorganic vapor phase epitaxy. The smaller tensile strain of the barrier layers in the lower parts made the photoluminescence (PL) wavelength longer while the larger tensile strain of the barrier layers in the upper parts increased the strain compensation of the CQDs. Compared to CQDs with uniformly tensile-strained barriers, 1.55 μm emission was obtained at a higher average strain of barrier layers. By utilizing modulated tensile-strained barriers, triple-stacking of 12-fold CQDs with a PL wavelength of 1.55 μm using 30-nm-thick spacer layers was achieved with good crystallinity, indicating suitability for fabrication of high density CQDs.

All-Optical Wavelength Multicasting in a QD-SOA

We experimentally demonstrate multicast wavelength conversion by exploiting cross-gain modulation and cross-phase modulation in an InAs/InGaAsP/InP columnar quantum dot semiconductor optical amplifier (CQD-SOA). We report wavelength multicasting of 40 and 80 Gb/s return-to-zero signals on four 400-GHz spaced channels. We find in all cases a moderate conversion penalty which we attribute mostly to four-wave mixing (FWM) among the equally spaced converted signals. We study the effect of the FWM impairments and find that the critical number of equally spaced consecutive channels giving significant penalty is 4. Instead, two- and three-channel conversion is possible with a null or negative Q2 factor penalty.

Design of columnar quantum dots for polarization-independent emission using 8–band k·p method

Control of the polarization of the emitted light can be highly beneficial for certain optoelectronic applications such as optical amplifiers. It has been recently demonstrated experimentally that semiconductor quantum dots with large height to base length aspect ratio are able to emit polarization-independent light from the edge of the wafer. However, analysis of the physics responsible for the observed polarization properties of such nano–objects (like columnar quantum dots or quantum rods) is still rather limited. In particular, the role of the material surrounding the columnar QD on the strain and thus on the polarization properties has not been considered previously. We report here, based on original software, the results of eight–band k·p calculations of the electronic and polarization properties of columnar InyGa1-yAs quantum dots (CQD) with high aspect ratio (up to 6) embedded in an InxGa1-xAs/GaAs quantum well. We calculate the relative intensities of transverse-magnetic (TM) and transverse-electric (TE) linear polarized light emitted from the edge of the semiconductor wafer as a function of the two main factors affecting the heavy hole – light hole valence band mixing and hence the polarization dependent selection rules for the optical transitions, namely i) the composition contrast y/x between the dot material and the surrounding well, and ii) the dot aspect ratio. Our numerical results show, in contrast to the previously reported expectations, that the former is the main driving parameter for tuning the polarization properties. This is explained analyzing the biaxial strain in the CQD, based on which it is possible to predict on the TM to TE intensity ratio.

Regenerative Amplification by Using Self-Phase Modulation in a Quantum-Dot SOA

We experimentally demonstrate regenerative amplification of 10-, 20-, and 40-Gb/s return-to-zero signals exploiting a simple scheme based on self-phase modulation (SPM) in a columnar quantum-dot (CQD) semiconductor optical amplifier (SOA) and the use of a red-shifted off-set filter. We study the signal Q 2-factor improvement as a function of the input power to the amplifier. We also report results using a quantum-well (QW) SOA for comparison. We find that the SPM-based regenerative amplification is only possible in the quantum-dot amplifier.

Eight-band k⋅p calculations of the composition contrast effect on the linear polarization properties of columnar quantum dots

We present eight-band k⋅p calculations of the electronic and polarization properties of columnar InzGa1−zAs quantum dots (CQD) with high aspect ratio embedded in an InxGa1−xAs/GaAs quantum well. Our model accounts for the linear strain effects, linear piezoelectricity, and spin-orbit interaction. We calculate the relative intensities of transverse-magnetic (TM) and transverse-electric (TE) linear polarized light emitted from the edge of the semiconductor wafer as a function of the two main factors affecting the heavy hole—light hole valence band mixing and hence, the polarization dependent selection rules for the optical transitions, namely, (i) the composition contrast z/x between the dot material and the surrounding well and (ii) the dot aspect ratio. The numerical results show that the former is the main driving parameter for tuning the polarization properties. This is explained by analyzing the biaxial strain in the CQD, based on which it is possible to predict the TM to TE intensity ratio. The conclusions are supported by analytical considerations of the strain in the dots. Finally, we present the compositional and geometrical conditions to achieve polarization independent emission from InGaAs/GaAs CQDs.

Polarization Properties of Columnar Quantum Dots: Effects of Aspect Ratio and Compositional Contrast

We report a systematic theoretical and experimental investigation of the polarization properties of columnar quantum dots (CQDs) with extremely large aspect ratio and high compositional contrast. A strong variation of the polarization of the dominant interband transitions as a function of the CQD composition and aspect ratio is predicted by the theory and observed experimentally. In optimized InAs CQDs grown on GaAs, dominant emission and lasing is obtained in the transverse-magnetic polarization, which shows the potential of this approach for polarization-independent amplifiers on GaAs.

Spin Relaxation in InAs Columnar Quantum Dots

We have investigated carrier spin dynamics in InAs columnar quantum dots (CQDs) by time-resolved photoluminescence (PL) measurement. The CQDs were formed by depositing a 1.8 monolayer InAs seed dot layer and a short-period GaAs/InAs superlattice (SL). The spin relaxation time was found to be prolonged from 1.6 to 5.3 ns by increasing the number of SL periods from 3 to 35. The PL decay time also increased from 0.93 to 1.9 ns, indicating a decrease in the spatial overlap of electron and hole wave functions. The changes in both the spin relaxation time and the PL decay time suggest that the Bir–Aronov–Pikus process is the main spin relaxation mechanism.

Characterization of Columnar InAs/GaAs Quantum-Dot Structures Using Grazing Incidence X-ray Diffraction

Grazing incidence X-ray diffraction (GIXD) measurement using equipment available for laboratories was realized for the evaluation of columnar InAs/GaAs quantum dots (QDs). The QDs were grown by direct stacking of small self-assembled islands in the growth direction on (001) GaAs substrates, and we prepared some samples by changing the number of units stacked. Using an imaging plate for a detector, we successfully observed that QD-related signals appeared around a GaAs(220) peak. The QD signals extended in the direction of the 2θ axis, and the extension increased with increasing number of stacks in the columnar QD growths. The results of the measurements were evaluated by a simulation based on the finite element method. Using a simple model that divides the QDs into an InAs columnar core and an InGaAs transition domain, we found that the core was expanded according to the increase in the stacking number. We also found that the lattice constant increased almost uniformly toward the upper part inside the QD. GIXD evaluation in laboratories will enhance the speed of the development of QD devices.