Tailoring the mode-switching dynamics in quantum-dot micropillar lasers via time-delayed optical feedback (original) (raw)
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Intensity fluctuations in bimodal micropillar lasers enhanced by quantum-dot gain competition
Physical Review A, 2013
We investigate correlations between orthogonally polarized cavity modes of a bimodal micropillar laser with a single layer of self-assembled quantum dots in the active region. While one emission mode of the microlaser demonstrates a characteristic s-shaped input-output curve, the output intensity of the second mode saturates and even decreases with increasing injection current above threshold. Measuring the photon auto-correlation function g (2) (τ ) of the light emission confirms the onset of lasing in the first mode with g (2) (0) approaching unity above threshold. In contrast, strong photon bunching associated with super-thermal values of g (2) (0) is detected for the other mode for currents above threshold. This behavior is attributed to gain competition of the two modes induced by the common gain material, which is confirmed by photon crosscorrelation measurements revealing a clear anti-correlation between emission events of the two modes. The experimental studies are in excellent qualitative agreement with theoretical studies based on a microscopic semiconductor theory, which we extend to the case of two modes interacting with the common gain medium. Moreover, we treat the problem by an extended birth-death model for two interacting modes, which reveals, that the photon probability distribution of each mode has a double peak structure, indicating switching behavior of the modes for the pump rates around threshold.
Mode-switching induced superbunching in quantum-dot microlasers
The super-thermal photon bunching in quantum-dot (QD) micropillar lasers is investigated both experimentally and theoretically via simulations driven by dynamic considerations. Using stochastic multi-mode rate equations we obtain very good agreement between experiment and theory in terms of intensity profiles and intensity-correlation properties of the examined QD micro-laser's emission. Further investigations of the time-dependent emission show that super-thermal photon bunching occurs due to irregular mode-switching events in the bimodal lasers. Our bifurcation analysis reveals that these switchings find their origin in an underlying bistability, such that spontaneous emission noise is able to effectively perturb the two competing modes in a small parameter region. We thus ascribe the observed high photon correlation to dynamical multistabilities rather than quantum mechanical correlations.
Modeling quantum dot lasers with optical feedback: sensitivity of bifurcation scenarios
physica status solidi (b), 2010
We present a systematic study of the complex dynamics of a quantum dot (QD) laser subjected to optical feedback from a short external cavity. Our model consists of a Lang-Kobayashi like model for the electric field combined with a microscopically based rate equation system. We separately treat electron and hole dynamics in the QDs and the surrounding wetting layer (WL). By tuning the phase-amplitude coupling and the optical confinement factor we are able to discuss various scenarios of the dynamics on the route towards conventional quantum well (QW) lasers. Due to the optical feedback, multistability occurs in our model in form of external cavity modes (ECMs) or delayinduced intensity pulsations. In dependence of the feedback strength we analyze complex bifurcation scenarios for the intensity of the emitted laser light as well as time series, power spectra, and phase portraits of all dynamic variables in order to elucidate the internal dynamics of the laser.
Physical Review B, 2010
Ultrafast changes of the statistical properties of light emission are studied for quantum-dot micropillar lasers. Using pulsed excitation with varying power, we follow the time-evolution of the second-order correlation function g (2) (t, τ = 0) reflecting two-photon coincidences and compare it to that of the output intensity. The previously impossible time resolution of a few picoseconds gives insight into the dynamical transition between thermal and coherent light emission. The g (2) results allow us to isolate the spontaneous and stimulated emission contributions within an emission pulse, not accessible via the emission intensity dynamics. Results of a microscopic theory confirm the experimental findings.
Mode-switching induced super-thermal bunching in quantum-dot microlasers
New Journal of Physics, 2016
The super-thermal photon bunching in quantum-dot (QD) micropillar lasers is investigated both experimentally and theoretically via simulations driven by dynamic considerations. Using stochastic multi-mode rate equations we obtain very good agreement between experiment and theory in terms of intensity profiles and intensity-correlation properties of the examined QD micro-laser's emission. Further investigations of the time-dependent emission show that super-thermal photon bunching occurs due to irregular mode-switching events in the bimodal lasers. Our bifurcation analysis reveals that these switchings find their origin in an underlying bistability, such that spontaneous emission noise is able to effectively perturb the two competing modes in a small parameter region. We thus ascribe the observed high photon correlation to dynamical multistabilities rather than quantum mechanical correlations.
Mode Competition Induced by Optical Feedback in Two-Color Quantum Dot Lasers
IEEE Journal of Quantum Electronics, 2000
We investigate theoretically the impact of optical feedback on quantum dot two-color laser dynamics when emitting from both the excited and the ground states. Detailed analysis of the laser dynamics is provided by numerical integrations and continuation techniques, as well as by analytical methods. As the feedback strength is increased the quantum dot laser undergoes a sequence of bifurcations involving steady-states, external cavity modes, self-pulsations and chaos. We furthermore report on two interesting mode competition results: 1/ the optical feedback favors the ground state emission; hence an increase of the feedback strength will generally lead to an increase of the ground state emission output power. 2/ The optical feedback can select one lasing state or induce bistable switchings between different steady-states depending on the feedback strength and the injection current.
Unconventional collective normal-mode coupling in quantum-dot-based bimodal microlasers
Physical Review A, 2015
We analyze the occurrence of normal-mode coupling (NMC) in bimodal lasers attributed to the collective interaction of the cavity field with a mesoscopic number of quantum dots (QDs). In contrast to the conventional NMC here we observe locking of the frequencies and splitting of the linewidths of the eigenmodes of the system in the coherent coupling regime. The theoretical analysis of the incoherent regime is supported by experimental observations where the emission spectrum of one of the orthogonally polarized modes of a bimodal QD micropillar laser demonstrates a two-peak structure.
Microscopic Dynamics of Quantum Dot Lasers
The exceptional performance of self-assembled Quantum Dot (QD) materials renders them extremely appealing for their use as optical communications devices. As lasers, they feature reduced and temperature independent threshold current and proper emission wavelength at the fiber telecommunication windows. These characteristics, together with the low linewidth enhancement factor and broad spectrum, make QD materials extremely attractive for application as light emitters or amplifiers. There exist, nevertheless, several unclear issues which prevent QDs from conquering the new generation of optoelectronic devices. Their differential efficiency is lower than expected. The output power of QD lasers is lower than that of their quantum well counterpart. Still, it is their dynamics which has incited the majority of studies. The modulation bandwidth of these devices seems to be limited by the relaxation of carriers from the upper energetic layers to the low levels within the dot. Besides, the electron-hole interaction is widely unknown, the extent of the electron-hole Coulombic attraction is not yet established. Throughout this thesis I present a theoretical and experimental study of the gain and phase dynamics of quantum dot lasers. I explain the appearance of different decay times observed in pump and probe experiments in QD amplifiers as a result of the different electron and hole relaxation times, by means of an electron-hole rate-equation model. The ultrafast hole relaxation first leads to an ultrafast recovery of the gain, which is then followed by electron relaxation and, on the nanosecond timescale, radiative and non-radiative recombinations. The phase dynamics is slower and is affected by thermal redistribution of carriers within the dot. Our results corroborate with spectral measurements of the dephasing and gain in QD amplifiers. Additionally, our work is compared with existing pump and probe results. Exploiting the capacity of QD lasers to emit at two different wavelengths corresponding to the ground state (GS) and excited state (ES), I present a theoretical study of the QD dynamics, based on a linearization of the QD rate-equations. The results predict the existence of single oscillation frequency of GS and ES, meaning that both states are highly coupled. In order to verify our theory, we perform two kinds of experiments. By modulating these lasers at high frequency, we measure separately the dynamics of GS and ES. However, in contradiction to our theory, two different modulation frequencies are found. Additional temporally-resolved measurements of the laser dynamics reveal a surprising effect. By injecting a sub-bandgap pump in an InAs/InGaAs QD laser, the emitted photons are depleted. Through additional transmission and photocurrent measurements, we relate this observation with carrier photoexcitation, which was so far only theoretically addressed. The role of carrier photoexcitation in our experimental laser dynamics is further supported by a rate-equation model. Impelled by this finding, we study the effect of carrier photoexcitation in the static and dynamic characteristics. We find that carrier photoexcitation reduces the efficiency of QD lasers, which is one of the major QD handicaps, and depletes the GS lasing after the ES threshold, as observed experimentally. Moreover, by adding carrier photoexcitation to our linearization of the rate-equations, we find that the theory predicts the appearance of two lasing resonance frequencies, in agreement with our previous experimental results. Additionally, we deal with the improvement of carrier relaxation. In tunnel injection devices, carriers are given an additional path towards the ground state of the dot by growing a quantum well layer close to the QD active plane. Through the quantum-mechanical tunneling effect, carriers relax from the nearby quantum well layer to the QDs, which speeds up relaxation. We aim at the increase of the modulation bandwidth while keeping the good performances quantum dot lasers have exhibited, such as low and temperature insensitive threshold current and proper emission wavelength. In the final part of this work, we present dynamical measurements of 1.5 μm InAs/InP tunnel injection and non-tunnel injection QD lasers, which display remarkable static characteristics. After proving with static measurements that tunnel injection is actually taking place in these structures, we show several dynamic measurements. Pump and probe measurements on QD devices show that the tunnel injection samples exhibit a slightly faster relaxation time than the non-tunnel injection samples used as reference, meaning that relaxation time is improved with tunnel injection. However, by probing the device with an ultrafast pump no improvement of the dynamic characteristics is observed. These results confirm that the laser dynamic properties of InP QD lasers, both standard and tunnel-injection designs, are actually not limited by relaxation of carriers. We point towards the size distribution of these quantum dash-like structures as the limiting factor of the modulation frequency.
Control of Nonlinear Dynamics of Quantum Dot Laser with External Optical Feedback
We examine the nonlinear dynamics of a semiconductor quantum-dot (QD) laser subject to external optical feedback by using dimensionless equations numerical model. In our QD laser model we employ dynamic for ground and excited state processes, additionally between the QDs and the wetting layer (WL). This enables us to tune the output of external cavity modes QDs by changing the bias current, delayed time and feedback strength to investigate how they affect the stability properties of the QD laser. Our results show that high bias current and small α-factor value lead to lower sensitivity of the laser towards optical feedback.