Quantum-dot Micropillar Lasers Research Articles

Noise-induced dynamics and photon statistics in bimodal quantum-dot micropillar lasers

Emission characteristics of quantum-dot micropillar lasers (QDMLs) are located at the intersection of nanophotonics and nonlinear dynamics, which provides an ideal platform for studying the optical interface between classical and quantum systems. In this work, a noise-induced bimodal QDML with orthogonal dual-mode outputs is modeled, and nonlinear dynamics, stochastic mode jumping and quantum statistics with the variation of stochastic noise intensity are investigated. Noise-induced effects lead to the emergence of two intensity bifurcation points for the strong and the weak mode, and the maximum output power of the strong mode becomes larger as the noise intensity increases. The anti-correlation of the two modes reaches the maximum at the second intensity bifurcation point. The dual-mode stochastic jumping frequency and effective bandwidth can exceed 100 GHz and 30 GHz under the noise-induced effect. Moreover, the noise-induced photon correlations of both modes simultaneously exhibit super-thermal bunching effects () in the low injection current region. The -value of the strong mode can reach over 6 in the high injection current region. Photon bunching () of both modes is observed over a wide range of noise intensities and injection currents. In the presence of the noise-induced effect, the photon number distribution of the strong or the weak mode is a mixture of Bose–Einstein and Poisson distributions. As the noise intensity increases, the photon number distribution of the strong mode is dominated by the Bose–Einstein distribution, and the proportion of the Poisson distribution is increased in the high injection current region, while that of the weak mode is reduced. Our results contribute to the development preparation of super-bunching quantum integrated light sources for improving the spatiotemporal resolution of quantum sensing measurements and enhancing the security of optical communication.

Spin‐Lasing in Bimodal Quantum Dot Micropillar Cavities

AbstractSpin‐controlled lasers are highly interesting photonic devices and have been shown to provide ultrafast polarization dynamics in excess of 200 GHz. In contrast to conventional semiconductor lasers their temporal properties are not limited by the intensity dynamics, but are governed primarily by the interaction of the spin dynamics with the birefringent mode splitting that determines the polarization oscillation frequency. Another class of modern semiconductor lasers are high‐β emitters, which benefit from enhanced light–matter interaction due to strong mode confinement in low‐mode‐volume microcavities. In such structures, the emission properties can be tailored by the resonator geometry to realize for instance bimodal emission behavior in slightly elliptical micropillar cavities. This attractive feature is utilized to demonstrate and explore spin‐lasing effects in bimodal high‐β quantum dot micropillar lasers. The studied microlasers with a β‐factor of 4% show spin‐laser effects with experimental polarization oscillation frequencies up to 15 GHz and predicted frequencies up to about 100 GHz, which are controlled by the ellipticity of the resonator. These results reveal appealing prospects for very compact, ultrafast, and energy‐efficient spin‐lasers and can pave the way for future purely electrically injected spin‐lasers enabled by short injection path lengths.

Random number generation with arrays of coupled quantumdot micropillar lasers

The paper investigates the results of random number generation with arrays of coupled quantum-dot micropillar lasers. The micropillars array laser generation is modeled based on the rate equations for quantum dot lasers. The numerical simulation of the dynamics for the arrays of coupled quantum-dot micropillar lasers is carried out utilizing the semi-implicit Euler method, implemented in Julia programming language. The algorithm of random bit sequence generation consists of the following steps: sampling the values of the total field intensity for coupled micropillar lasers; normalizing and discretizing the obtained values per resolution of the 8-bit analog-to-digital converter; extracting the four least significant bits from the bit representation of the sampled values; concatenating the bit values in a single sequence. The possibility of the bit sequences generation having an equiprobable distribution of zeros and ones with a performance of up to 400 Gbit/s was shown utilizing a random number generator based on an array of coupled quantum-dot micropillar lasers for sequences with a length of 14285716 bits at a sampling rate of 100 gigasamples per second and four least significant bits extraction. The resulting bit sequences successfully passed 14 NIST 800-22 statistical tests for the p-value equal to 0.01. The proposed method can be applied to develop random number generators based on larger arrays of coupled quantum-dot micropillar lasers. The results can be utilized in the experimental implementation of random number generators based on arrays of coupled quantum-dot micropillar lasers.

Optical pumping of quantum dot micropillar lasers.

Arrays of quantum dot micropillar lasers are an attractive technology platform for various applications in the wider field of nanophotonics. Of particular interest is the potential efficiency enhancement as a consequence of cavity quantum electrodynamics effects, which makes them prime candidates for next generation photonic neurons in neural network hardware. However, particularly for optical pumping, their power-conversion efficiency can be very low. Here we perform an in-depth experimental analysis of quantum dot microlasers and investigate their input-output relationship over a wide range of optical pumping conditions. We find that the current energy efficiency limitation is caused by disadvantageous optical pumping concepts and by a low exciton conversion efficiency. Our results indicate that for non-resonant pumping into the GaAs matrix (wetting layer), 3.4% (0.6%) of the optical pump is converted into lasing-relevant excitons, and of those only 2% (0.75%) provide gain to the lasing transition. Based on our findings, we propose to improve the pumping efficiency by orders of magnitude by increasing the aluminium content of the AlGaAs/GaAs mirror pairs in the upper Bragg reflector.

Dynamic signatures of mode competition in optically injected high-β lasers

In this paper we theoretically investigate the statistical light-emission properties of an optically injected bimodal quantum-dot micropillar laser with high spontaneous emission rates. The nanostructured device is described in terms of a stochastic, semiclassically derived rate equation model. We focus on the stochastic switching dynamics between the two fundamental modes and correlate the results with an in-depth bifurcation analysis of the underlying deterministic dynamics. By analyzing different statistical measures, e.g. average intensity, auto- and cross-correlation functions, as well as dwell-time distributions, we give a road map on how to unravel the different dynamic regimes in the presence of large noise from experimentally accessible quantities.

Stochastic polarization switching induced by optical injection in bimodal quantum-dot micropillar lasers.

Mutual coupling and injection locking of semiconductor lasers is of great interest in non-linear dynamics and its applications for instance in secure data communication and photonic reservoir computing. Despite its importance, it has hardly been studied in microlasers operating at μW light levels. In this context, vertically emitting quantum dot micropillar lasers are of high interest. Usually, their light emission is bimodal, and the gain competition of the associated linearly polarized fundamental emission modes results in complex switching dynamics. We report on selective optical injection into either one of the two fundamental mode components of a bimodal micropillar laser. Both modes can lock to the master laser and influence the non-injected mode by reducing the available gain. We demonstrate that the switching dynamics can be tailored externally via optical injection in very good agreement with our theory based on semi-classical rate equations.

Mutual coupling and synchronization of optically coupled quantum-dot micropillar lasers at ultra-low light levels

Synchronization of coupled oscillators at the transition between classical physics and quantum physics has become an emerging research topic at the crossroads of nonlinear dynamics and nanophotonics. We study this unexplored field by using quantum dot microlasers as optical oscillators. Operating in the regime of cavity quantum electrodynamics (cQED) with an intracavity photon number on the order of 10 and output powers in the 100 nW range, these devices have high β-factors associated with enhanced spontaneous emission noise. We identify synchronization of mutually coupled microlasers via frequency locking associated with a sub-gigahertz locking range. A theoretical analysis of the coupling behavior reveals striking differences from optical synchronization in the classical domain with negligible spontaneous emission noise. Beyond that, additional self-feedback leads to zero-lag synchronization of coupled microlasers at ultra-low light levels. Our work has high potential to pave the way for future experiments in the quantum regime of synchronization.

Quantum-dot micropillar lasers subject to coherent time-delayed optical feedback from a short external cavity

We investigate the mode-switching dynamics of an electrically driven bimodal quantum-dot micropillar laser when subject to delayed coherent optical feedback from a short external cavity. We experimentally characterize how the external cavity length, being on the same order than the microlaser’s coherence length, influences the spectral and dynamical properties of the micropillar laser. Moreover, we determine the relaxation oscillation frequency of the micropillar by superimposing optical pulse injection to a dc current. It is found that the optical pulse can be used to disturb the feedback-coupled laser within one roundtrip time in such a way that it reaches the same output power as if no feedback was present. Our results do not only expand the understanding of microlasers when subject to optical feedback from short external cavities, but pave the way towards tailoring the properties of this key nanophotonic system for studies in the quantum regime of self-feedback and its implementation to integrated photonic circuits.

O-band InAs/GaAs quantum-dot microcavity laser on Si (001) hollow substrate by in-situ hybrid epitaxy

Recent years, the emergence of hyper-scale data centers boosted the research field of integrated silicon photonics. One of the major challenges for compact photonic integrated circuits is silicon based lasers. In this paper, we demonstrate optically pumped InAs/GaAs quantum-dot micropillar laser on exact Si (001) by (111)-faceted-sawtooth Si hollow structure via IV/III-V hybrid epitaxy. The lasing threshold of InAs/GaAs quantum-dot micropillar is as low as 20 μW with the pillar diameter of 15 μm. Moreover, the micropillar laser is capable of operating at maximum temperature up to 100 °C.

Determining the linewidth enhancement factor via optical feedback in quantum dot micropillar lasers.

The linewidth enhancement factor α is a key parameter determining the spectral and dynamical behavior of semiconductor lasers. Here, we propose and demonstrate a method for determining this parameter based on a direct measurement of variations in the laser gain and emission spectrum when subject to delayed optical feedback. We then use our approach to determine the pump current dependent linewidth enhancement factor of a high-β quantum dot micropillar laser. The validity of our approach is confirmed comparing it to two conventional methods, one based on the comparison of the linewidths above and below threshold and the other based on injection locking properties. Furthermore, the pump power dependence of α is quantitatively described by simulations based on a quantum-optical model.

Tailoring the mode-switching dynamics in quantum-dot micropillar lasers via time-delayed optical feedback.

Microlasers are ideal candidates to bring the fascinating variety of nonlinear complex dynamics found in delay-coupled systems to the realm of quantum optics. Particularly attractive is the possibility of tailoring the devices' emission properties via non-invasive delayed optical coupling. However, until now scarce research has been done in this direction. Here, we experimentally and theoretically investigate the effects of delayed optical feedback on the mode-switching dynamics of an electrically driven bimodal quantum-dot micropillar laser, characterizing its impact on the micropillar's output power, optical spectrum and photon statistics. Feedback is found to influence the switching dynamics and its characteristics time scales. In addition, stochastic switching is reduced with the subsequent impact on the microlaser photon statistics. Our results contribute to the comprehension of feedback-induced phenomena in micropillar lasers and pave the way towards the external control and tailoring of the properties of these key systems for the nanophotonics community.

Mode-switching induced super-thermal bunching 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.

Photon-Statistics Excitation Spectroscopy of a Quantum-Dot Micropillar Laser.

We introduce photon-statistics excitation spectroscopy and exemplarily apply it to a quantum-dot micropillar laser. Both the intensity and the photon number statistics of the emission from the micropillar show a strong dependence on the photon statistics of the light used for excitation of the sample. The results under coherent and pseudothermal excitation reveal that a description of the laser properties in terms of mean input photon numbers is not sufficient. It is demonstrated that the micropillar acts as a superthermal light source when operated close to its threshold. Possible applications for important spectroscopic techniques are discussed.

Nonlinear emission characteristics of quantum dot–micropillar lasers in the presence of polarized optical feedback

We report on electrically pumped quantum dot–microlasers in the presence of polarized self-feedback. The high-β microlasers show two orthogonal, linearly polarized emission modes which are coupled via the common gain medium. This coupling is explained in terms of gain competition between the two lasing modes and leads to distinct differences in their input–output characteristics. By applying polarized self-feedback via an external mirror, we are able to control the laser characteristics of the emission modes in terms of the output power, the coherence time and the photon statistics. We find that linearly polarized self-feedback stabilizes the lasing of a given mode, while cross-polarized feedback between the two modes reduces strongly the intensity of the other emission mode showing particular high-intensity fluctuations and even super-thermal values of the photon autocorrelation function g(2)(τ) at zero delay. Measurements of g(2)(τ) under external feedback also allow us to detect revival peaks associated with the round trip time of the external cavity. Analyzing the damping and shape of the g(2)(τ) revival peaks by a phenomenological model provides us insight into the underlying physics such as the effective exciton lifetime and gain characteristics of the quantum dots in the active region of these microlasers.

Directional whispering gallery mode emission from Limaçon-shaped electrically pumped quantum dot micropillar lasers

We experimentally demonstrate directional far field emission from whispering gallery modes (WGMs) in electrically driven quantum dot micropillar lasers. In-plane directionality of whispering gallery mode emission is obtained by patterning micropillars with Limaçon-shaped cross-section and an upper air-bridge contact for current injection. The micropillar lasers with radii R0 down to 4.5 μm show Q-factors of 40 000 and threshold currents of 40 μA at low temperature. We achieved a far field divergence of about 30° and a directionality of 1.67 ± 0.15 for an optimal Limaçon deformation factor ɛ ≈ 0.5. Parameter dependent studies of the directional emission as a function of ɛ reveal good qualitative agreement with theoretical predictions.

Extrapolation of the intensity autocorrelation function of a quantum-dot micropillar laser into the thermal emission regime

We present investigations on the coherence of the emission from the fundamental mode of an AlGaInAs/GaAs quantum-dot microcavity laser. We measure the first-order field-correlation function g(1)(τ) with a Michelson interferometer, from which we determine coherence times of up to 20 ns for the highest pump powers. To fully characterize the coherence properties of the cavity emission, we apply a phenomenological model that connects the first- and second-order correlation functions. Hereby it is possible to overcome the limited sensitivity of the streak camera used for photon-correlation measurements, and thus to extend the accessible excitation-power range for g(2)(τ) down to the thermal regime.

Ultrafast tracking of second-order photon correlations in the emission of quantum-dot microresonator lasers

Ultrafast changes in 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 ${g}^{(2)}(t,\ensuremath{\tau}=0)$ 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.

Influence of the spontaneous optical emission factorβon the first-order coherence of a semiconductor microcavity laser

A systematic experimental and theoretical study of first-order coherence properties of high-$\ensuremath{\beta}$ quantum-dot micropillar lasers is presented. A nonlinear increase in the coherence length is found in the transition regime from spontaneous to dominantly stimulated emission. This increase is accompanied by a qualitative change in the first-order field-correlation function ${g}^{(1)}(\ensuremath{\tau})$ from a Gaussian-type profile to an exponential behavior, which is in excellent agreement with a microscopic semiconductor laser theory. Our results also demonstrate a decreasing coherence length with increasing spontaneous emission coupling $\ensuremath{\beta}$, thus raising questions about the practicability of high-$\ensuremath{\beta}$ lasers for device applications.

Low threshold electrically pumped quantum dot-micropillar lasers

We report on low threshold lasing in electrically pumped quantum dot-micropillar cavities. Lasing action associated with threshold currents as low as 8μA at 10K is observed for micropillar cavities with quality factors exceeding 10.000. Due to an optimized contact scheme lasing is achieved for pillar structures with diameters as small as 1.5μm, containing on average less than 100 quantum dots in the active layer. The transition from spontaneous to stimulated emission is confirmed by autocorrelation measurements which reveal pronounced photon bunching near threshold.

Photon Statistics of Semiconductor Microcavity Lasers

We present measurements of first- and second-order coherence of quantum-dot micropillar lasers together with a semiconductor laser theory. Our results show a broad threshold region for the observed high-beta microcavities. The intensity jump is accompanied by both pronounced photon intensity fluctuations and strong coherence length changes. The investigations clearly visualize a smooth transition from spontaneous to predominantly stimulated emission which becomes harder to determine for high beta. In our theory, a microscopic approach is used to incorporate the semiconductor nature of quantum dots. The results are in agreement with the experimental intensity traces and the photon statistics measurements.