Spontaneous Emission Coupling Research Articles

Continuous-wave quantum dot photonic crystal lasers grown on on-axis Si (001)

Semiconductor III–V photonic crystal (PC) laser is regarded as a promising ultra-compact light source with unique advantages of ultralow energy consumption and small footprint for the next generation of Si-based on-chip optical interconnects. However, the significant material dissimilarities between III-V materials and Si are the fundamental roadblock for conventional monolithic III-V-on-silicon integration technology. Here, we demonstrate ultrasmall III-V PC membrane lasers monolithically grown on CMOS-compatible on-axis Si (001) substrates by using III-V quantum dots. The optically pumped InAs/GaAs quantum-dot PC lasers exhibit single-mode operation with an ultra-low threshold of ~0.6 μW and a large spontaneous emission coupling efficiency up to 18% under continuous-wave condition at room temperature. This work establishes a new route to form the basis of future monolithic light sources for high-density optical interconnects in future large-scale silicon electronic and photonic integrated circuits.

Benchmarking semiclassical and perturbative methods for real-time simulations of cavity-bound emission and interference

We benchmark a selection of semiclassical and perturbative dynamics techniques by investigating the correlated evolution of a cavity-bound atomic system to assess their applicability to study problems involving strong light-matter interactions in quantum cavities. The model system of interest features spontaneous emission, interference, and strong coupling behavior and necessitates the consideration of vacuum fluctuations and correlated light-matter dynamics. We compare a selection of approximate dynamics approaches including fewest switches surface hopping (FSSH), multitrajectory Ehrenfest dynamics, linearized semiclassical dynamics, and partially linearized semiclassical dynamics. Furthermore, investigating self-consistent perturbative methods, we apply the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy in the second Born approximation. With the exception of fewest switches surface hopping, all methods provide a reasonable level of accuracy for the correlated light-matter dynamics, with most methods lacking the capacity to fully capture interference effects.

Photonic “hourglass” design for efficient quantum light emission

We propose a novel “hourglass”-shaped design for highly efficient generation and collection of quantum light. The design features a quantum dot in a photonic nanowire sandwiched between tapered Bragg reflectors. For a Purcell factor of 9, the design features a spontaneous emission coupling of 0.993 to the cavity mode enabled by the strong dielectric screening of radiation modes. Thanks to a highly reflecting bottom mirror, we furthermore demonstrate a collection efficiency of 0.95 to a Gaussian profile. Finally, this photonic structure features a broad operation bandwidth, as large as 11 nm.

Lasing thresholds and photon statistics in high-βburied multiple quantum well photonic crystal nanocavity lasers

We investigate the lasing thresholds and photon statistics of buried multiple quantum well (MQW) photonic crystal (PhC) lasers with a high spontaneous emission coupling coefficient $\ensuremath{\beta}$ and a large carrier transparency number ${N}_{0}$. Using measured input-output curves and delay-dependent second-order photon correlations ${g}^{(2)}(\ensuremath{\tau})$, we estimate that our population inversion threshold is near the kink threshold of the input-output curve, which indicates that the buried MQW PhC lasers operate close to the ``lasers without inversion'' (intensity jump without inversion) regime proposed by Yamamoto and Bj\ork. With respect to static photon statistics ${g}^{(2)}(0)$, the super-Poissonian photon statistics ${g}^{(2)}(0)g1$ are observed even one order of magnitude above the kink threshold. The super-Poissonian photon statistics are specific to the class-B lasers where the photon decay rate ${\ensuremath{\gamma}}_{c}$ is much larger than that of carriers ${\ensuremath{\gamma}}_{\ensuremath{\parallel}}$ and the relaxation oscillation occurs. We also propose that the pump power needed to maximize the damping time of the relaxation oscillation could be the noise threshold at which the photon statistics change from thermal to Poissonian.

Concealment of Time Delay Signature of Chaotic Semiconductor Nanolasers With Double Chaotic Optical Injections

The time delay signature (TDS) concealment properties of a novel semiconductor nanolaser subject to double chaotic optical injections are investigated numerically. The TDS concealment is evaluated by the peak size of the autocorrelation function (ACF). The effects of feedback rate, injection rate, frequency detuning, Purcell cavity-enhanced spontaneous emission factor, and spontaneous emission coupling factor on the TDS concealment are carefully examined. It is found that, with the double chaotic optical injections, the TDS can be concealed in wide ranges of feedback rate, injection rate, and frequency detuning. Moreover, better TDS concealment can be achieved for a smaller frequency detuning and a larger injection rate. The 2-D maps of peak size of ACF further show that the range of parameter space contributing to better TDS concealment is wider for a moderate Purcell cavity-enhanced spontaneous emission factor and a smaller spontaneous emission coupling factor. These results may be interesting for the enhanced chaotic sources on-a-chip based on novel semiconductor nanolasers.

Numerical Investigation on Feedback Insensitivity in Semiconductor Nanolasers

This paper presents numerical investigations into the effect of feedback phase on the stability of semiconductor nanolasers (SNLs) in the presence of the external optical feedback. For this purpose, numerical solutions are obtained from rate equations where the effects of Purcell cavity-enhanced spontaneous emission factor F and an enhanced spontaneous emission coupling factor β are included. In this way, a phase-insensitive stable SNL is identified when the feedback coupling fraction is below a critical threshold η c. Furthermore, the relationship between η c and two other important system parameters, namely the injection current ( I dc) and the initial external cavity length ( L 0), is studied. The results show that η c has a bi-exponential relationship with either I dc or L 0. Moreover, the influence of F on the η c is evaluated, and it is found that η c increases with the increase of F . The results presented in this paper provide practical guidelines for the design of phase-insensitive stable SNLs, which are useful for densely integrated photonic circuits based applications, such as optical communications and sensing.

Near-Infrared Lasing at 1 μm from a Dilute-Nitride-Based Multishell Nanowire.

A coherent photon source emitting at near-infrared (NIR) wavelengths is at the heart of a wide variety of applications ranging from telecommunications and optical gas sensing to biological imaging and metrology. NIR-emitting semiconductor nanowires (NWs), acting both as a miniaturized optical resonator and as a photonic gain medium, are among the best-suited nanomaterials to achieve such goals. In this study, we demonstrate the NIR lasing at 1 μm from GaAs/GaNAs/GaAs core/shell/cap dilute nitride nanowires with only 2.5% nitrogen. The achieved lasing is characterized by an S-shape pump-power dependence and narrowing of the emission line width. Through examining the lasing performance from a set of different single NWs, a threshold gain, gth, of 4100-4800 cm-1, was derived with a spontaneous emission coupling factor, β, up to 0.8, which demonstrates the great potential of such nanophotonic material. The lasing mode was found to arise from the fundamental HE11a mode of the Fabry-Perot cavity from a single NW, exhibiting optical polarization along the NW axis. Based on temperature dependence of the lasing emission, a high characteristic temperature, T0, of 160 (±10) K is estimated. Our results, therefore, demonstrate a promising alternative route to achieve room-temperature NIR NW lasers thanks to the excellent alloy tunability and superior optical performance of such dilute nitride materials.

Topological photonic crystal nanocavity laser

Topological edge states exist at the interfaces between two topologically distinct materials. The presence and number of such modes are deterministically predicted from the bulk band topologies, known as the bulk-edge correspondence. This principle is highly useful for predictably controlling optical modes in resonators made of photonic crystals (PhCs), leading to the recent demonstrations of microscale topological lasers. Meanwhile, zero-dimensional topological trapped states in the nanoscale remained unexplored, despite its importance for enhancing light–matter interactions and for wide applications including single-mode nanolasers. Here, we report a topological PhC nanocavity with a near-diffraction-limited mode volume and its application to single-mode lasing. The topological origin of the nanocavity, formed at the interface between two topologically distinct PhCs, guarantees the existence of only one mode within its photonic bandgap. The observed lasing accompanies a high spontaneous emission coupling factor stemming from the nanoscale confinement. These results encompass a way to greatly downscale topological photonics.

Room temperature continuous-wave nanolaser diode utilized by ultrahigh-Q few-cell photonic crystal nanocavities

Few-cell point-defect photonic crystal (PhC) nanocavities (such as LX and H1 type cavities), have several unique characteristics including an ultra-small mode volume (Vm), a small device footprint advantageous for dense integration, and a large mode spacing advantageous for high spontaneous-emission coupling coefficient (β), which are promising for energy-efficient densely-integratable on-chip laser light sources enhanced by the cavity QED effect. To achieve this goal, a high quality factor (Q) is essential, but conventional few-cell point-defect cavities do not have a sufficiently high Q. Here we adopt a series of modified designs of LX cavities with a buried heterostructure (BH) multi-quantum-well (MQW) active region that can achieve a high Q while maintaining their original advantages and fabricate current-injection laser devices. We have successfully observed continuous-wave (CW) lasing in InP-based L1, L2, L3 and L5 PhC nanocavities at 23°C with a DC current injection lower than 10 μA and a bias voltage lower than 0.9 V. The active volume is ultra-small while maintaining a sufficiently high confinement factor, which is as low as ~10-15 cm3 for a single-cell (L1) nanocavity. This is the first room-temperature current-injection CW lasing from any types of few-cell point-defect PhC nanocavities (LX or H1 types). Our report marks an important step towards realizing a nanolaser diode with a high cavity-QED effect, which is promising for use with on-chip densely integrated laser sources in photonic networks-on-chip combined with CMOS processors.

Ultrahigh Quality Upconverted Single‐Mode Lasing in Cesium Lead Bromide Spherical Microcavity

AbstractThe realization of ultrahigh quality upconverted single‐mode lasing in wavelength‐scale cavity is still a great challenge, because of limitation of microcavity quality, gain medium, and low spontaneous emission coupling efficiency β. In this work, it is demonstrated that inorganic cesium lead bromide (CsPbBr3) perovskite spheres with size of ≈780 nm serve as a gain medium and whispering‐gallery cavity simultaneously for two‐photon excited single‐mode laser. Lasing mode with a very narrow linewidth (≈0.037 nm) is realized, corresponding to an ultrahigh Q factor ≈1.5 × 104. Surprisingly, the lasing performance of such small microcavity is best among reported naturally formed semiconductor microcavities. Furthermore, room‐temperature single‐mode lasing at same wavelength can be obtained at any angle between −30° and 30°, showing the nearly uniform emission output of the spherical microcavity. This work paves a simple and straight way to obtain ultrahigh Q two‐photon excited single‐mode lasing in micro/nanostructures, holding great promise for practical applications in frequency upconversion, high sensitive sensing in nano‐optics.

Enhancement of stimulated emission by a metallic optofluidic resonator

Stimulated emission can be controlled by a material molecular energy band and the intensity of a pump laser, which can provide some population inversion and promote ground electron transition, respectively. We use a metallic optofluidic resonator to enhance stimulated emission intensity. The quality factor Q and the spontaneous emission coupling factor β of the metallic optofluidic resonator are discussed in detail to explain the enhancement mechanism. Experimental data demonstrate that the operated emission from rhodamin 6G solution can be observed due to the enhancement of stimulated emission from the optofluidic resonator.

Analysis of high-frequency oscillations in mutually-coupled nano-lasers.

The dynamics of mutually coupled nano-lasers has been analyzed using rate equations which include the Purcell cavity-enhanced spontaneous emission factor F and the spontaneous emission coupling factor β. It is shown that in the mutually-coupled system, small-amplitude oscillations with frequencies of order 100 GHz are generated and are maintained with remarkable stability. The appearance of such high-frequency oscillations is associated with the effective reduction of the carrier lifetime for larger values of the Purcell factor, F, and spontaneous coupling factor, β. In mutually-coupled nano-lasers the oscillation frequency changes linearly with the frequency detuning between the lasers. For non-identical bias currents, the oscillation frequency of mutually-coupled nano-lasers also increases with bias current. The stability of the oscillations which appear in mutually coupled nano-lasers offers opportunities for their practical applications and notably in photonic integrated circuits.

Phase control of the quantum statistics of collective emission

We report nonclassical aspects of the collective behaviour of two atoms in a cavity by investigating the photon statistics and photon distribution in a very broad domain of parameters. Starting with the dynamics of two atoms radiating in phase into the cavity, we study the photon statistics for arbitrary interatomic phases as revealed by the second-order intensity correlation function at zero time $g^{(2)}(0)$ and the Mandel $Q$ parameter. We find that the light field can be tuned from antibunched to (super-)bunched as well as nonclassical to classical behaviour by merely modifying the atomic position. The highest nonclassicality in the sense of the smallest $Q$ parameter is found when spontaneous emission, cavity decay, coherent pumping, and atom-cavity coupling are of comparable magnitude. We introduce a quantum version of the negative binomial distribution with its parameters directly related to $Q$ and $g^{(2)}(0)$ and discuss its range of applicability. We also examine the Klyshko parameter which highlights the nonclassicality of the photon distribution.

Ultralow-threshold continuous-wave lasing assisted by a metallic optofluidic cavity exploiting continuous pump.

We report an ultralow-threshold continuous-wave lasing via a metallic optofluidic resonant cavity based on the symmetrical metal-cladding waveguide. The high quality factor Q and spontaneous emission coupling factor β of the waveguide strengthen the interaction between the gain medium and the ultrahigh order modes (UOMs); hence, the room-temperature, narrowband lasing can be effectively pumped by a continuous laser of low intensity. Rhodamine 6G and methylene blue are chosen to verify the applicability of the proposed concept. Lasing is emitted from the chip surface when the pumped laser is well coupled into the UOMs. For methylene blue with a concentration of 2.57*10-13 mol/ml, the operated emission can be observed with the launched pump threshold as low as 2.1 μW/cm2.

A quantum optical study of thresholdless lasing features in high-\u03b2 nitride nanobeam cavities

Exploring the limits of spontaneous emission coupling is not only one of the central goals in the development of nanolasers, it is also highly relevant regarding future large-scale photonic integration requiring energy-efficient coherent light sources with a small footprint. Recent studies in this field have triggered a vivid debate on how to prove and interpret lasing in the high-β regime. We investigate close-to-ideal spontaneous emission coupling in GaN nanobeam lasers grown on silicon. Such nanobeam cavities allow for efficient funneling of spontaneous emission from the quantum well gain material into the laser mode. By performing a comprehensive optical and quantum-optical characterization, supported by microscopic modeling of the nanolasers, we identify high-β lasing at room temperature and show a lasing transition in the absence of a threshold nonlinearity at 156 K. This peculiar characteristic is explained in terms of a temperature and excitation power-dependent interplay between zero-dimensional and two-dimensional gain contributions.

Dynamical characteristics of nano‐lasers subject to optical injection and phase conjugate feedback

The dynamics of nano-lasers has been analysed using rate equations which include the Purcell cavity-enhanced spontaneous emission factor F and the spontaneous emission coupling factor β. It is shown that when subject to optical injection and phase conjugate feedback, nano-lasers may exhibit remarkably stable small-amplitude oscillations with frequencies of order 300 GHz. Critically, it is established that such oscillations persist when the effects of noise are taken into account. The appearance of such high-frequency oscillations is associated with the effective reduction of the carrier lifetime for larger values of the Purcell factor, F, and spontaneous coupling factor, β. The effects of the feedback distance and bias currents are also considered. As the optical injection strength increases for fixed phase conjugate feedback and relatively short feedback distances, the nano-laser displays periodic dynamics and then enters stable locking. As the feedback distance increases, the quasi-periodic dynamics dominates. Increased bias current can also induce quasi-periodic behaviour albeit this may be ameliorated by reducing the strength of the phase conjugate feedback.

Zero Crosstalk Regime Direct Modulation of Mutually Coupled Nanolasers

The modulation properties of dually modulated mutually coupled nanolasers have been analysed using rate equations which include the Purcell cavity-enhanced spontaneous emission factor F and the spontaneous emission coupling factor β . Specific attention is given to zero crosstalk operating regimes where the responses of the nanolasers are independent. Such regimes may provide either bidirectional isolation where both nanolasers operate independently or unidirectional-isolation where one nanolaser is immune to the other nanolaser but not vice versa. Access to the bidirectional isolation zero-crosstalk regime is, in general, enabled by strong driving or large modulation depth of the nanolasers. Conversely, strong injection coupling and low modulation depth lead to the unidirectional isolation regime. The modulation responses of the nanolasers in both bidirectional and unidirectional isolated zero crosstalk regimes are presented. The availability of bidirectional-isolated zero crosstalk regimes is seen to offer opportunities for exploitation in photonic integrated circuits.

Zero Cross-Talk Regimes in Dually Modulated Mutually-Coupled Nano-lasers

The modulation properties of dually modulated mutually-coupled nano-lasers have been analyzed using rate equations which include the Purcell cavity-enhanced spontaneous emission factor F and the spontaneous emission coupling factor β. Analysis of the dynamical response of modulated mutually-coupled nano-lasers reveals the existence of regimes of zero cross-talk wherein the response of one nano-laser is not impacted by the dynamics of the other nano-laser. The availability of zero-cross talk regimes is seen to offer opportunities for exploitation in photonic integrated circuits.

Modulated Mutually Coupled Nano-Lasers

The dynamics of mutually coupled nano-lasers subject to direct current modulation has been analyzed using rate equations, which include the Purcell cavity-enhanced spontaneous emission factor $F$ and the spontaneous emission coupling factor $\beta $ . Subject to two different modulation frequencies, the mutually coupled nano-lasers display two general types of response. The laser with the lower modulation frequency simply exhibits a response at that modulation frequency. Thus, we term a zero cross-talk response. On the other hand, at higher modulation frequencies the system displays a variety of dynamical responses which, in addition to zero cross-talk, includes a range of behaviors, which are classified from low cross-talk through to a complicated non-linear response. The precise behavior being dependent on the depth of modulation and the laser bias currents. The operational significance of the zero cross-talk regime is that it permits access to a simple periodic response at the modulation frequency. With a view to utilization, it is established that the region of zero cross-talk response enlarges with increasing modulation depth and increasing bias current. In this way, conditions are established, in which the lasers may act independently. The propensity for zero cross-talk response under stronger driving is consistent with previous analysis wherein modulated nano-lasers may have superior characteristics in the large-signal regime.

Excitonic lasing of strain-free InP(As) quantum dots in AlInAs microdisk

Formation, emission, and lasing properties of strain-free InP(As)/AlInAs quantum dots (QDs) embedded in AlInAs microdisk (MD) cavity were investigated using transmission electron microscopy and photoluminescence (PL) techniques. In MD structures, the QDs have the nano-pan-cake shape with the height of ∼2 nm, the lateral size of 20–50 nm, and the density of ∼5 × 109 cm−2. Their emission observed at ∼940 nm revealed strong temperature quenching, which points to exciton decomposition. It also showed unexpected type-I character, indicating In-As intermixing as confirmed by band structure calculations. We observed lasing of InP(As) QD excitons into whispering gallery modes in MD having the diameter of ∼3.2 μm and providing a free spectral range of ∼27 nm and quality factors up to Q∼13 000. Threshold of ∼50 W/cm2 and spontaneous emission coupling coefficient of ∼0.2 were measured for this MD-QD system.