A. V. Savelyev

Gain compression and its dependence on output power in quantum dot lasers

The gain compression coefficient was evaluated by applying the frequency modulation/amplitude modulation technique in a distributed feedback InAs/InGaAs quantum dot laser. A strong dependence of the gain compression coefficient on the output power was found. Our analysis of the gain compression within the frame of the modified well-barrier hole burning model reveals that the gain compression coefficient decreases beyond the lasing threshold, which is in a good agreement with the experimental observations.

On the gain properties of “thin” elastically strained InGaAs/InGaAlAs quantum wells emitting in the near-infrared spectral region near 1550 nm

The results of experimental studies of the gain properties of “thin” (3.2 nm thick) elastically strained InGaAs/InGaAlAs quantum wells emitting in the near-infrared spectral region near 1550 nm are presented. The results of studying the threshold and gain characteristics of stripe laser diodes with active regions based on “thin” quantum wells with a lattice–substrate mismatch of +1.0% show that the quantum wells under study exhibit a high modal gain of 11 cm–1 and a low transparency current density of 46 A/cm2 per quantum well.

Simultaneous multi-state stimulated emission in quantum dot lasers: experiment and analytical approach

The theoretical investigation of the double-state lasing phenomena in InAs/InGaAs quantum dot lasers has been carried out. The new mechanism of the ground-state lasing quenching, which takes place in quantum dot (QD) laser operating in double-state lasing regime at high pump level, was proposed. The difference between electron and hole capture rates causes the depletion of the hole levels and consequently leads to the decrease of an output lasing power via QD ground state with the growth of injection. Moreover, it was shown that the hole-to-electron capture rates ratio strongly affects both the light-current curve and the key laser parameters. The model of the simultaneous lasing through the ground and excited QD states was developed which allows to describe the observed quenching quantitatively.

Spectral Width of Laser Generation in Quantum Dot Lasers: An Analytical Approach

An analytical approach to description of broad spectra of lasing in quantum-dot lasers has been developed. It is shown that the spectral width of the laser generation is determined by three parameters: the spectral width of the gain spectrum, homogeneous broadening of the line, and the effective parameter of the gain saturation. As a result, the dependence of the spectral width of lasing on the laser output power is shown to be universal and can be described as a function of a single dimensionless parameter.

Effect of modulation p-doping level on multistate lasing in InAs/InGaAs quantum dot lasers having different external loss (Conference Presentation)

Significant interest in compact InAs/InGaAs quantum dot (QD) lasers emitting near 1.3 mkm is caused by the diversity of their applications including non-invasive medicine and ultra-fast data transmission. In such lasers, lasing typically starts at the ground-state (GS) optical transitions of QDs. A further increase in injection may result in the appearance of an additional, short-wavelength spectral line associated with the excited-state (ES) optical transitions of QDs – a simultaneous lasing via QD GS and ES, i.e. multi-state lasing, takes place. The appearance of the ES-line may sufficiently affect the useful GS component. As injection current exceeds the multi-state lasing threshold, a decrease and even a complete quenching of GS-lasing may take place. As it was shown in [V.V. Korenev et. al, Appl. Phys. Lett. 102, 112101 (2013)], the usage of modulation p-doping has a positive influence on the hole concentration in QDs making GS-lasing quenching less pronounced. However, the influence of the concentration of p-dopant on multi-state lasing in general – and on the GS-lasing quenching in particular – has not been yet studied. To clarify this question experimentally, a series of InAs/InGaAs QD laser wafers was grown by molecular beam epitaxy. The active region of each sample was comprised of 10 layers of InAs/InGaAs QDs separated by 35 nm-thick GaAs spacers. Each spacer was p-doped into its central part of 10 nm using carbon atoms. Dependent on the sample, the carbon concentration was equal to 0, 3·10^17 cm^(-3), or 5·10^17 cm^(-3). A series of light-current curves corresponding to the GS component of output power was studied both theoretically and experimentally for “short” (0.5-mm-long) and for “long” (1.0-mm-long) samples. The experiment shows that in case of the short samples, the increase in p-doping level from 0 to 5·10^17 cm^(-3) results in the increase in maximum output power corresponding to the GS of QDs (WGS) from 0.8 to 2.2W, while in case of the longer samples the situation is opposite and WGS decreases from 4.5 to 3.7W correspondingly [V.V. Korenev et. al, Appl. Phys. Lett. 111, 132103 (2017)]. Qualitatively, such a discrepancy can be explained as follows. In case of the short samples, the higher p-doping level results in the faster hole capture into QDs mitigating the competition for the common holes between GS and ES optical transitions, which is an important reason for the GS-lasing quenching. In longer samples, optical loss is small and GS gain is far from its saturated value. Consequently, the ES energy level is weakly occupied and p-doping is not necessary to apply. However, even a small increment in the internal loss due to the p-dopant may lead to a noticeable decrease in laser`s differential efficiency. As a result, the higher p-doping level does not necessarily lead to the higher GS power as it was previously expected. However, if the sample is sufficiently short, the usage of modulation p-doping increases GS power. For a given cavity length, there is a certain p-doping level improving GS lasing characteristics.

Quantum dot laser optimization: selectively doped layers

Edge emitting quantum dot (QD) lasers are discussed. It has been recently proposed to use modulation p-doping of the layers that are adjacent to QD layers in order to control QDs charge state. Experimentally it has been proven useful to enhance ground state lasing and suppress the onset of excited state lasing at high injection. These results have been also confirmed with numerical calculations involving solution of drift-diffusion equations. However, deep understanding of physical reasons for such behavior and laser optimization requires analytical approaches to the problem. In this paper, under a set of assumptions we provide an analytical model that explains major effects of selective p-doping. Capture rates of elections and holes can be calculated by solving Poisson equations for electrons and holes around the charged QD layer. The charge itself is ruled by capture rates and selective doping concentration. We analyzed this self-consistent set of equations and showed that it can be used to optimize QD laser performance and to explain underlying physics.

Injection current minimization of InAs/InGaAs quantum dot laser by optimization of its active region and reflectivity of laser cavity edges

The ways to optimize key parameters of active region and edge reflectivity of edge- emitting semiconductor quantum dot laser are provided. It is shown that in the case of optimal cavity length and sufficiently large dispersion lasing spectrum of a given width can be obtained at injection current up to an order of magnitude lower in comparison to non-optimized sample. The influence of internal loss and edge reflection is also studied in details.

The analytical approach to the multi-state lasing phenomenon in undoped and p-doped InAs/InGaAs semiconductor quantum dot lasers

We introduce an analytical approach to the multi-state lasing phenomenon in p-doped and undoped InAs/InGaAs quantum dot lasers which were studied both theoretically and experimentally. It is shown that the asymmetry in charge carrier distribution in quantum dots as well as hole-to-electron capture rate ratio jointly determine laser’s behavior in such a regime. If the ratio is lower than a certain critical value, the complete quenching of ground-state lasing takes place at sufficiently high injection currents; at higher values of the ratio, our model predicts saturation of the ground-state power. It was experimentally shown that the modulation p-doping of laser’s active region results in increase of output power emitted via the ground-state optical transitions of quantum dots and in enhancement of the injection currents range in which multi-state lasing takes place. The maximum temperature at which multi-state lasing exists was increased by about 50°C in the p-doped samples. These effects are qualitatively explained in the terms of the proposed model.

Analytical model of ground-state lasing phenomenon in broadband semiconductor quantum dot lasers

We introduce an analytical approach to the description of broadband lasing spectra of semiconductor quantum dot lasers emitting via ground-state optical transitions of quantum dots. The explicit analytical expressions describing the shape and the width of lasing spectra as well as their temperature and injection current dependences are obtained in the case of low homogeneous broadening. It is shown that in this case these dependences are determined by only two dimensionless parameters, which are the dispersion of the distribution of QDs over the energy normalized to the temperature and loss-to-maximum gain ratio. The possibility of optimization of laser’s active region size and structure by using the intentionally introduced disorder is also carefully considered.