Quantum Dot Microlasers Research Articles

High-β lasing in photonic-defect semiconductor-dielectric hybrid microresonators with embedded InGaAs quantum dots

We report an easy-to-fabricate microcavity design to produce optically pumped high-β quantum dot microlasers. Our cavity concept is based on a buried photonic-defect for tight lateral mode confinement in a quasi-planar microcavity system, which includes an upper dielectric distributed Bragg reflector (DBR) as a promising alternative to conventional III–V semiconductor DBRs. The cavities show distinct emission features with a characteristic photonic-defect size-dependent mode separation and Q-factors up to 17 000. Comprehensive investigations further reveal lasing operation with a systematic increase (decrease) of the β-factor (threshold pump power) with the number of mirror pairs in the upper dielectric DBR. Notably, due to the quasi-planar device geometry, the microlasers show high temperature stability, evidenced by the absence of temperature-induced redshift of emission energy and linewidth broadening typically observed for nano- and microlasers at high excitation powers. The device exhibits remarkable lasing performance, maintaining efficacy even under elevated temperatures of up to 260 K.

A kinetic model of a quantum-dot microlaser

The results of a qualitative analysis of a semiclassical model of light generation in low-dimensional solidstate lasers, including quantum-dot microlasers and, on its basis, a numerical modelling of the regular pulsation regime that occurs under conditions of nonlinear shift and broadening of the resonant spectral gain line due to the influence of dipoledipoles interaction and absorption in quasi-resonant transitions on the dielectric susceptibility of the active medium are herein presented. The modelling of lasing was carried out for the parameters of semiconductor quantum-dot structures.

Defect-assisted, spray-printed colloidal quantum dot microlasers for biosensing.

This study successfully implements spectrally distinguishable CdSe-ZnS core-shell colloidal quantum dot (CQD) microlasers by a simple, efficient spray printing technique and demonstrates its potential in biosensing. We have systematically characterized the optical properties of printed microring lasers with diameters less than 60 µm. The smallest structure that can be excited has a diameter as small as 30 µm, which is much smaller than the counterparts prepared by piezoelectric ink-jet printing. The detection sensitivity of 4.54 nm/min/refractive index unit is verified in glucose sensing using a printed CQD microlaser. Biosensing of diverse glucose and bovine serum albumin solutions using printed microlasers with the assistance of defects demonstrates a new, to the best of our knowledge, prototype for the development of high-performance, low-cost on-chip microcavity sensors.

Regulated Photon Transport in Chaotic Microcavities by Tailoring Phase Space.

Manipulating light dynamics in optical microcavities has been made mainly either in real or momentum space. Here we report a phase-space tailoring scheme, simultaneously incorporating spatial and momentum dimensions, to enable deterministic and insitu regulation of photon transport in a chaotic microcavity. In the time domain, the chaotic photon transport to the leaky region can be suppressed, and the cavity resonant modes show stronger temporal confinement with quality factors being improved by more than 1 order of magnitude. In the spatial domain, the emission direction of the cavity field is controlled on demand through rerouting chaotic photons to a desired channel, which is verified experimentally by the far-field pattern of a quantum-dot microlaser. This work paves a way to insitu study of chaotic physics and promoting advanced applications such as arbitrary light routing, ultrafast random bit generation, and multifunctional on-chip lasers.

Ultrastable low-cost colloidal quantum dot microlasers of operative temperature up to 450\u2009K

Quantum dot microlasers, as multifunctional optical source components, are of great importance for full-color high-pixel display, miniaturized coherent lighting, and on-chip integrated photonic and electronic circuits. Since the first synthesis of colloidal quantum dots (CQD) in the 1990s, motivation to realize high-performance low-cost CQD micro-/nanolasers has been a driving force for more than three decades. However, the low packing density, inefficient coupling of CQDs with optical cavities, and the poor thermal stability of miniaturized complex systems make it challenging to achieve practical CQD micro-/nanolasers, especially to combine the continuous working ability at high temperatures and the low-cost potential with mass-produced synthesis technologies. Herein, we developed close-packed CQD-assembled microspheres and embedded them in a silica matrix through the rapid self-aggregation and solidification of CdSe/ZnS CQD. This technology addresses the core issues of photoluminescence (PL) quenching effect and low optical gain in traditional CQD laser research. High-efficiency low-threshold CQD microlasers are demonstrated together with long-playing (40 min) working stability even at 450 K under pulsed laser excitation, which is the highest operational temperature for CQD lasers. Moreover, single-mode CQD microlasers are obtained with tunable wavelengths across the entire visible spectral range. The chemosynthesis process supports the mass-produced potential of high-density integrated CQD microlasers, promoting CQD-based low-cost high-temperature microdevices.

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.

InAs/GaAs Quantum Dot Microlasers Formed on Silicon Using Monolithic and Hybrid Integration Methods.

An InAs/InGaAs quantum dot laser with a heterostructure epitaxially grown on a silicon substrate was used to fabricate injection microdisk lasers of different diameters (15–31 µm). A post-growth process includes photolithography and deep dry etching. No surface protection/passivation is applied. The microlasers are capable of operating heatsink-free in a continuous-wave regime at room and elevated temperatures. A record-low threshold current density of 0.36 kA/cm2 was achieved in 31 µm diameter microdisks operating uncooled. In microlasers with a diameter of 15 µm, the minimum threshold current density was found to be 0.68 kA/cm2. Thermal resistance of microdisk lasers monolithically grown on silicon agrees well with that of microdisks on GaAs substrates. The ageing test performed for microdisk lasers on silicon during 1000 h at a constant current revealed that the output power dropped by only ~9%. A preliminary estimate of the lifetime for quantum-dot (QD) microlasers on silicon (defined by a double drop of the power) is 83,000 h. Quantum dot microdisk lasers made of a heterostructure grown on GaAs were transferred onto a silicon wafer using indium bonding. Microlasers have a joint electrical contact over a residual n+ GaAs substrate, whereas their individual addressing is achieved by placing them down on a p-contact to separate contact pads. These microdisks hybridly integrated to silicon laser at room temperature in a continuous-wave mode. No effect of non-native substrate on device characteristics was found.

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.

Numerical design and investigation of an optically pumped 1.55 μm single quantum dot photonic crystal-based laser

Quantum dot microlasers are promising for application in photonic integrated circuits and lab-on-a-chip systems. In this article, we sought to design an optically pumped single quantum dot photonic crystal microlaser for 1.55 μm telecom-band operation. In the proposed scheme, an InAs quantum dot placed in the point defect of an InP photonic crystal slab acts as a laser active region. The significance of the proposed laser is its cavity design that supports two high-quality resonant modes at the pump and lasing wavelengths. Hence, the pump resonantly excites the active region. Simulations demonstrate satisfactory lasing characteristics such as a lasing threshold power of 55 nW and a spectral linewidth of ∼0.019 nm for the designed quantum dot photonic crystal-based laser.

Spectral and directional properties of elliptical quantum-dot microlasers

We have fabricated microdisk lasers of colloidal quantum dots in circular and elliptical forms with different aspect ratios. By characterizing the laser emission spectrum under optical pumping and through mode simulation calculations, we demonstrate that the elliptical resonators can display two sets of whispering-gallery modes that interact to varying degrees depending on the aspect ratio. This causes significant mode splitting in the emission spectra when the mode interaction is at maximum. We also performed angular-dependent laser emission measurements for characterizing the emission pattern of the microlasers. We found that the emission pattern becomes less isotropic and more directional as the boundary deviates further from circular symmetry. In addition, we demonstrate that the emission directional properties of these microlasers can be further tailored by coupling pairs of elliptical microcavities together in different manners, where the long or short elliptical axis interacts most strongly.

Electrically Tunable Single-Photon Source Triggered by a Monolithically Integrated Quantum Dot Microlaser

We report on a quantum dot micropillar-based single-photon source demonstrating tunable emission energy via an applied electric field and driven by an on-chip, whispering-gallery-mode microlaser. In this concept the cavity-enhanced single-photon source is monolithically integrated with an electrically driven, coherent excitation source. The device concept features low laser-threshold currents of a few tens of μA, has a small footprint of ∼200 μm2 and demonstrates high single-photon purity with g(2)(0) as low as 0.07 ± 0.03 under pulsed electrical excitation of the microlaser. The electric field applied along the growth direction of the single-photon emitter allows the emission to be tuned by up to 1.1 meV via the quantum-confined Stark effect, bringing it into resonance with the fundamental mode of the surrounding micropillar resonator for enhanced emission via the Purcell effect.

On-Chip Integrated Quantum-Dot-Silicon-Nitride Microdisk Lasers.

Hybrid silicon nitride (SiN)-quantum-dot (QD) microlasers coupled to a passive SiN output waveguide with a 7 µm diameter and a record-low threshold density of 27 µJ cm-2 are demonstrated. A new design and processing scheme offers long-term stability and facilitates in-depth QD material and device characterization, thereby opening new paths for optical communication, sensing, and on-chip cavity quantum optics based on colloidal QDs.

Injection Locking of Quantum-Dot Microlasers Operating in the Few-Photon Regime

We experimentally and theoretically investigate injection locking of quantum dot (QD) microlasers in the regime of cavity quantum electrodynamics (cQED). We observe frequency locking and phase-locking where cavity enhanced spontaneous emission enables simultaneous stable oscillation at the master frequency and at the solitary frequency of the slave microlaser. Measurements of the second-order autocorrelation function prove this simultaneous presence of both master and slave-like emission, where the former has coherent character with $g^{(2)}(0)=1$ while the latter one has thermal character with $g^{(2)}(0)=2$. Semi-classical rate-equations explain this peculiar behavior by cavity enhanced spontaneous emission and a low number of photons in the laser mode.

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.

A Pulsed Nonclassical Light Source Driven by an Integrated Electrically Triggered Quantum Dot Microlaser

We present a novel compact nanophotonic device consisting of a nonclassical light source excited by a monolithically integrated and electrically driven quantum dot (QD) microlaser. Our device concept is based on self-assembled InAs QDs embedded in micropillar cavities and has many potential applications in the fields of quantum communication and quantum optics-based information processing. Electrically driven micropillars act as whispering gallery mode microlasers operable in both continuous and pulsed mode, with narrow pulse widths of 520 ps and decay constants as low as 160 ps observed. These microlasers are used as on-chip excitation sources to laterally excite individual QDs in nearby micropillars, which in turn act as vertically emitting nonclassical light sources. Our compact solid-state platform utilizes cavity-quantum electrodynamic effects to create antibunched light in continuous and pulsed operation with g (2)CW(0) = 0.76 ± 0.03 and g (2)Pulsed(0) = 0.78 ± 0.11, demonstrating its potential for the generation of triggered single photons in a highly integrated chip.

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.

On‐Chip Quantum Optics with Quantum Dot Microcavities

A novel concept for on-chip quantum optics using an internal electrically pumped microlaser is presented. The microlaser resonantly excites a quantum dot microcavity system operating in the weak coupling regime of cavity quantum electrodynamics. This work presents the first on-chip application of quantum dot microlasers, and also opens up new avenues for the integration of individual microcavity structures into larger photonic networks.

Observing chaos for quantum-dot microlasers with external feedback

Chaos presents a striking and fascinating phenomenon of nonlinear systems. A common aspect of such systems is the presence of feedback that couples the output signal partially back to the input. Feedback coupling can be well controlled in optoelectronic devices such as conventional semiconductor lasers that provide bench-top platforms for the study of chaotic behaviour and high bit rate random number generation. Here we experimentally demonstrate that chaos can be observed for quantum-dot microlasers operating close to the quantum limit at nW output powers. Applying self-feedback to a quantum-dot microlaser results in a dramatic change in the photon statistics wherein strong, super-thermal photon bunching is indicative of random-intensity fluctuations associated with the spiked emission of light. Our experiments reveal that gain competition of few quantum dots in the active layer enhances the influence of self-feedback and will open up new avenues for the study of chaos in quantum systems.

Light Trapped in a Photonic Dot: Microspheres Act as a Cavity for Quantum Dot Emission

Optical microcavities that confine the propagation of light in all three dimensions (3D) are fascinating research objects to study 3D-confined photon states, low-threshold microlasers, or cavity quantum electrodynamics of quantum dots in 3D microcavities. A challenge is the combination of complete electronic confinement with photon confinement, e.g., by linking a single quantum dot to a single photonic dot. Here we report on the interplay of 3D-confined cavity modes of single microspheres (the photonic dot states) with photons emitted from quantized electronic levels of single semiconductor nanocrystals (the quantum dot states). We show how cavity modes of high cavity finesse are switched by single, blinking quantum dots. A concept for a quantum-dot microlaser operating at room temperature in the visible spectral range is demonstrated. We observe an enhancement in the spontaneous emission rate; i.e., the Purcell effect is found for quantum dots inside a photonic dot.

Edge-emitting microlasers with one active layer of quantum dots

High performance edge-emitting microlasers with deeply etched distributed Bragg reflectors (DBRs) were fabricated on an AlGaAs-GaAs laser structure with a single GaInAs quantum dot (QD) active layer. Mirror reflectivities well above 90% were achieved by third-order narrow air-gap Bragg reflectors with /spl lambda//4 air-gaps. DBR lasers with 160-/spl mu/m-long cavities and cleaved mirrors on one side show differential efficiencies of 0.87 W/A and output powers of more than 50 mW at 980-nm emission wavelength in continuous wave (CW) operation at room temperature. With deeply etched DBRs on both sides of the cavity CW operating microlasers with cavity lengths down to 16 /spl mu/m could be realized with a minimum threshold current of 1.2 mA for a 30-/spl mu/m cavity length. All lasers are emitting at the QD ground state at room temperature. Twenty-/spl mu/m-long devices show CW threshold currents of about 3 mA, output powers above 1 mW, and single mode emission with >25 dB sidemode suppression ratios. First, high-frequency measurements mere performed proving that these QD microlasers are well suited for large-scale integrated high-speed optical data processing with modulation frequencies well above 10 GHz.