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There have been multiple demonstrations of amplified spontaneous emission (ASE) and lasing using colloidal semiconductor nanocrystals. However, it has been proven difficult to achieve low thresholds suitable for practical use of nanocrystals as gain media. Low-threshold blue ASE and lasing from nanocrystals is an even more challenging task. Here, we show that colloidal nanoplatelets (NPLs) with electronic structure of quantum wells can produce ASE in the red, yellow, green, and blue regions of the visible spectrum with low thresholds and high gains. In particular, for blue-emitting NPLs, the ASE threshold is 50 μJ/cm(2), lower than any reported value for nanocrystals. We then demonstrate red, yellow, green, and blue lasing using NPLs with different thicknesses. We find that the lateral size of NPLs does not show any strong effect on the Auger recombination rates and, correspondingly, on the ASE threshold or gain saturation. This observation highlights the qualitative difference of multiexciton dynamics in CdSe NPLs and other quantum-confined CdSe materials, such as quantum dots and rods. Our measurements of the gain bandwidth and gain lifetime further support the prospects of colloidal NPLs as solution-processed optical gain materials.

Lead chalcogenide quantum dots (QDs) have been considered promising gain media in the near‐infrared region. Precipitation of lead chalcogenide QDs into glasses can significantly improve their stabilities and processabilities using mature fiber‐optic technologies and has great potential for application as a broad‐band optical amplification in optical telecommunication. However, the net optical gain of lead chalcogenide QD‐doped glasses has not been realized upon continuous‐wave (CW) pumping, hindering their application as fiber‐optic amplifiers. Here, the study reports a heterostructured PbS/NaCl QD‐doped glass and realizes a net optical gain in the near‐infrared region upon CW pumping. The photoluminescence quantum efficiencies of these QD‐doped glasses are boosted to 43–75% with significantly improved photo/thermal stabilities, and the gain threshold is reduced to ∼single exciton level due to the passivation and charge doping of the PbS QDs by the NaCl wrapping. As a result, net optical gain in the conventional band of telecommunication is realized from the 1 mm thick PbS QD‐doped glasses under CW pumping. [ABSTRACT FROM AUTHOR]

A systematic study of a series of InGaAs/GaAs lasers in the 1–1.3 μm optical range based on quantum wells (2D), quantum dots (0D), and quantum well-dots of transitional (0D/2D) dimensionality is presented. In a wide range of pump currents, the dependences of the lasing wavelength on the layer gain constant, a parameter which allows comparing lasers with different types of active region and various waveguide designs, are measured and analyzed. It is shown that the maximum optical gain of the quantum well-dots is significantly higher, and the range of lasing rawavelengths achievable in edge-emitting lasers without external resonators is wider than in lasers based on quantum wells and quantum dots. [ABSTRACT FROM AUTHOR]

Fiber laser field is typically dominated by rare earth ions as gain material in the core of a silica optical waveguide. Due to their specific emission wavelengths, rare‐earth‐doped fiber lasers are available only at few predefined wavelengths. However, quantum dots (QDs) are materials which show tunable emission with change in size and composition. Due to such tunability, QDs seem to be promising candidates for obtaining fiber lasers at a spectrum of wavelengths which is not possible using rare earth ions. To replace rare earth ions with QDs, it is of paramount importance that QDs show signatures of optical gain. Herein, the synthesis of asymmetric pod‐shaped CdSe/CdS QDs is reported, which demonstrate efficient gain on pumping. The intrinsic gain properties of the QDs are evaluated through transient absorption (TA) spectroscopy. Later, the exquisite QDs are used to fabricate specialty fibers from which amplified spontaneous emission (ASE) is obtained using continuous wave laser pump at room temperature. Finally, emission signal stability is checked by studying photobleaching and controlling the concentration of the QDs. [ABSTRACT FROM AUTHOR]

Fiber laser field is typically dominated by rare earth ions as gain material in the core of a silica optical waveguide. Due to their specific emission wavelengths, rare‐earth‐doped fiber lasers are available only at few predefined wavelengths. However, quantum dots (QDs) are materials which show tunable emission with change in size and composition. Due to such tunability, QDs seem to be promising candidates for obtaining fiber lasers at a spectrum of wavelengths which is not possible using rare earth ions. To replace rare earth ions with QDs, it is of paramount importance that QDs show signatures of optical gain. Herein, the synthesis of asymmetric pod‐shaped CdSe/CdS QDs is reported, which demonstrate efficient gain on pumping. The intrinsic gain properties of the QDs are evaluated through transient absorption (TA) spectroscopy. Later, the exquisite QDs are used to fabricate specialty fibers from which amplified spontaneous emission (ASE) is obtained using continuous wave laser pump at room temperature. Finally, emission signal stability is checked by studying photobleaching and controlling the concentration of the QDs.

Optical gain and resistance against photo-bleaching are key material parameters for practical applications in microlasers and microphotonics. Here, we present studies of optical amplification and photo-bleaching in a wide range of optical gain materials, including fluorescent dyes, quantum dots and rods, organic and inorganic room temperature polaritons, and nanodiamonds with emission range from 430 to 680 nm. We used amplified spontaneous emission (ASE) for each material to measure the optical gain. Robustness against photo-bleaching was determined by measuring the intensity of spontaneous emission of the material as a function of the number of excitation femtosecond pulses (106–1010) at various excitation energy densities. We show that pyrromethene laser dyes are the best organic emitters in terms of low excitation energy and good optical gain, whereas solid-state polariton materials are better in terms of high optical gain and stability. We found that all organic and inorganic optical gain materials bleach completely after 106–109 illumination pulses with typical 0.1–0.6 GW/cm2 peak power density, with an exception of nanodiamonds. We show that nanodiamonds are the only optical gain material that shows no photo-bleaching beyond the 1010 excitation pulses of 300 fs pulse duration and with 0.16 GW/cm2 peak power density. [ABSTRACT FROM AUTHOR]

In quantum dots, where confinement is strong, interactions between charge carriers play an essential role in the performance of semiconductor materials for optical gain. Therefore, understanding this phenomenon is critical for achieving new devices with enhanced features. In this context, the current study examines the optical properties of an exciton confined in a spherical InAs quantum dot under the influence of magnetic and intense laser fields. We investigate the oscillator strength, exciton lifetime, and optical gain, considering the effects of both external fields. We also pay particular attention to the influence of quantum dot size on the results. Our calculations show that the two external fields have opposite effects on our findings. Specifically, the applied magnetic field increases the oscillator strength while the intense laser reduces it. In addition, the optical gain peaks are redshifted under the application of the intense laser, whereas the magnetic field causes a blueshift of the peak threshold. We also find that both external perturbations significantly influence the exciton lifetime. Our study considers the outcomes of both the exciton's ground ( 1 s ) and first excited ( 1 p ) states. The theoretical results obtained in this study have promising implications for optoelectronic devices in the ∼3–4 μ m wavelength range only through the control of quantum dot sizes and external perturbations. [ABSTRACT FROM AUTHOR]

Although traditional quantum-confined nanostructures e.g. regular quantum wells or quantum dots have achieved huge success in the field of semiconductor lasers for past decades, these traditional nanostructures are encountering the difficulty of enhancing device performance to a higher level due to their inherent gain bottleneck. In this paper, we are proposing a new super-gain nanostructure based on self-assembled well-wire complex energy-band engineering with InGaAs-based materials to break through the existing bottleneck. The nanostructure is constructed by utilizing the special strain-driven indium (In)-segregation and the growth orientation-dependent on-GaAs multi-atomic step effects to achieve the distinguished ultra-wide and uniform super-gain spectra. The structural details and its luminescence mechanism are investigated by multiple measurement means and theoretical modeling. The polarized gain spectra with the max fluctuation of <3 cm−1 in 904 nm–998 nm for transverse electric (TE) mode and 904 nm–977 nm for transverse magnetic (TM) mode are simultaneously obtained with this nanostructure. It enables an ultra-low output power fluctuation of <0.7 dB and a nearly-constant threshold power throughout an ultra-wide wavelength range under a fixed injection level. It was difficult to realize these in the past. Therefore, the described super-gain nanostructure brings a brand-new chance of developing high performance of tunable laser diodes. [ABSTRACT FROM AUTHOR]

Colloidal quantum dots (CQDs) have emerged as promising candidates for solution‐processed optical gain materials. Despite the substantial progress made with Cd or Pb‐contained CQDs, lasing from the environmentally friendly CQDs remains underexplored. Here, the highly emissive ZnSeTe/ZnSe/ZnS and ZnSe/ZnS CQDs is developed. The optical gain and lasing action from such Te‐alloyed and intrinsic ZnSe/ZnS CQDs across the blue spectral region is achieved. Te‐alloyed ZnSe core manifests a quasitype‐II band alignment with the ZnSe intermediate layer, favorable for suppressing the Auger recombination and also lasing wavelength tuning. The transient absorption spectroscopy reveals evidence of a blue optical gain with a threshold of one exciton per dot and a lifetime of 171 ps in the alloyed CQDs. Under a quasicontinuous‐wave pumping, lasing is observed from the blue CQDs in Fabry–Pérot cavities with a lower threshold fluence of 12.5 µJ cm−2 as compared with the traditional counterparts with heavy metals. The demonstration of heavy‐metal‐free CQD lasing opens up new opportunities in the application of CQDs as efficient and environmental‐friendly gain media. [ABSTRACT FROM AUTHOR]

Colloidal semiconductor quantum dots (QDs) can be considered a promising material platform for solution‐processed laser diodes. However, due to some fundamental challenges, the realization of electrically pumped lasing based on QDs remains unresolved. Here, a binary blend of QDs and ZnO nanocrystals is employed, which serve as nano‐sized scatterers to facilitate waveguide gain in ultra‐thin films. By carefully engineering the electric field in these films, an infrared amplified spontaneous emission in a record thin colloidal gain medium is observed, with a thickness of 16 nm, at a wavelength of 1675 nm. Employing these binary blends as a gain medium, an optically pumped infrared stimulated emission in a full‐stacked light‐emitting diode (LED) device is demonstrated. The functional LED device, which comprises a single layer of graphene as an anode electrode, shows strong electroluminescence under electrical injection. This study suggests a promising device for realizing infrared QD‐based laser diodes. [ABSTRACT FROM AUTHOR]

The optical gain (OG) of quantum dots containing impurity is dealt with in the present investigation under the governance of applied Gaussian white noise. In this regard, various pertinent physical parameters are varied over a range and the resulting variations of OG are thoroughly monitored. Along with OG, the profiles of the gain radiative current density and the bandgap are also analyzed. The authors have come across that the presence/absence of noise, the mode of application of noise (additive/multiplicative), the particular physical parameter concerned, and the range of magnitude of the physical parameter, in unison, govern the features of the OG profiles in diverse ways. The study reveals that tuning of various physical parameters under applied noise in some preferred mode can be useful for the attainment of high OG and saturation gain. Tuning of emission wavelength can also be exploited to design tunable light sources. [ABSTRACT FROM AUTHOR]

We investigated the optical gain properties and lasing characteristics of a pulsed electrically pumped laser structure, which consists of a single layer of self-assembled InP quantum dots (QDs) assembled in (Al0.10Ga0.90)0.52In0.48P barriers. The optical gain and absorption spectra were obtained by analyzing the amplified spontaneous emission as a function of the excitation length. At a current density of 1.2 kA/cm2, an internal optical loss value of 5 ± 2 cm $^{{-1}}$ and a peak modal gain of 39.3 cm $^{{-1}}$ corresponding to a material gain of approximately 2675 cm $^{{-1}}$ per QD layer were determined at room temperature. For a 2.24-mm-long laser with uncoated facets emitting at 660 nm, a low threshold current density of 281 A/cm2 and an external differential quantum efficiency of 34.2% were determined. The internal quantum efficiency of 66% and the transparent current density of 65.8 A/cm2 for a single layer of QDs were also demonstrated. [ABSTRACT FROM AUTHOR]

Abstract In this paper, optical gain of quantum dot ensemble is investigated in presence of external electric field for different carrier densities. First, the wave functions and the energy eigenvalues of carriers are calculated by solving the Schrödinger equation for a quantum dot. Then these electric field dependent wave functions and energies are used to calculate the optical gain using the transition rates based on Fermi's golden rule. The analysis assumes the Gaussian distribution of the density of states to take into account the effect of inhomogeneous broadening due to the size-deviation of dots in ensemble and the Lorentz shape function to take into account the effect of homogeneous broadening. Results show that optical gain is enhanced with electric field depending on the carrier densities injected into the dots, indicating applications in optical sources with electrically tunable output power. Highlights • Inhomogeneous broadening in quantum dot (QD) ensemble play significant role in linewidth enhancement of optical source. • Optical gain of InAs/GaAs QD ensemble investigated theoretically as a function of electric field and injected carrier density. • Gain is enhanced with electric field along with a red-shift of the peak. • Emission dominates absorption only after a minimum injected carrier density to result in effective gain. • Results indicate applications for electrically tunable power of a broadband optical source as in sub-cellular tissue imaging. [ABSTRACT FROM AUTHOR]

Dilute bismide alloys and dilute nitride alloys have garnered increasing research interest over the past few years, as it promises increased engineering flexibility in the design of advanced compound semiconductor heterostructure devices. Key device parameters such as lattice constant, bandgap and band offsets can be controlled with greater precision, and this leads to better performance for a wide range of electronic and optoelectronic devices. For dilute nitride alloys, band anticrossing effects are produced due to the coupling of their resonant states with the conduction band states. This lowers the conduction band edge energy. Likewise, the coupling of the resonant states of dilute bismide with the valence band states results in the valence band anticrossing and the valence band edge energy rises. Additionally, the effective bandgap may fall below the spin-orbital-splitting energy, thus inhibiting Auger recombination. This makes them excellent candidates for optoelectronic device applications. In this work, the electronic bandstructure and optical gain of InNBiAs/InP pyramidal quantum dots are investigated using the 16-band k·p model with constant strain. The effective bandgap falls as we increase the composition of nitrogen and bismuth. With an appropriate choice of composition of doped atoms, we can tune the emission wavelength in the range of 2–5µm suitable for mid-infrared device applications. Additionally, we observe the size effect by tuning the size of quantum dot. [ABSTRACT FROM AUTHOR]

On the basis of the coupled rate equations of the carriers in different energy states, the optical gain of an InGaAs/GaAs quantum dot (QD) semiconductor optical amplifier (QD-SOA) related to both the standard deviation of QD relative size fluctuation and the temperature is numerically investigated with a fourth-order Runge–Kutta method. Our results show that the quasi-flat gain spectra with the full-width at half-maximum more than 150 nm overlapped by the optical gains of both the ground state (GS) and the excited state (ES) can be achieved by either injection current density or temperature. When a pump light is resonant to the GS transition, the peak gain of the GS is not only varied with pump power, but the peak gain of the ES is also changed with this one. The ES may exhibit abnormal gain saturation at room temperature (300 K), which is considerably different from those of bulk and quantum-well SOAs. We predict that the QD-SOA both acting as an optical amplifier is applied to dense wavelength division multiplexing systems and acting as an all-optical wavelength converter implements the wavelength conversion without inverse codes based on the cross gain modulation. [ABSTRACT FROM PUBLISHER]

In this work we consider a current carrying two level quantum dot(QD) that is coupled to a single mode phonon bath. Using self-consistent Hartree-Fock approximation, we obtain the I-V curve of QD. By considering the linear response of our system to an incoming classical light, we see that depending on the parametric regime, the system could have weak or strong light absorption or may even show lasing. This lasing occurs at high enough bias voltages and is explained by a population inversion considering side bands, while the total electron population in the higher level is less than the lower one. The frequency at which we have the most significant lasing depends on the level spacing and phonon frequency and not on the electron-phonon coupling strength. [ABSTRACT FROM AUTHOR]

The group-III nitrides, InN, GaN, and AlN, and their alloys have emerged as one of the most important material classes for optoelectronic devices. The incorporation of quantum dots (QDs) as active material improves the performance of conventional optoelectronic devices, such as laser diodes. In this study, we present a theoretical analysis of the gain characteristics of InxGa1-xN/Al0.2Ga0.8N three-dimensional quantum box lasers, based on the density matrix theory of semiconductor lasers with relaxation broadening. The study is done on three samples of QDs: GaN/Al0.2Ga0.8N, In0.3Ga0.7N/Al0.2Ga0.8N, and In0.5Ga0.5N/Al0.2Ga0.8N. A comparative study of the gain spectra of GaN/Al0.2Ga0.8N-based quantum-well and QD lasers is also presented for various side lengths. The variation of peak gain on carrier density is presented as well. The effect of indium composition on the variation in modal gain versus current density and the threshold current with inverse cavity length is plotted. The results show that the In0.5Ga0.5N/Al0.2Ga0.8N QD laser emitting at red wavelength has a higher value of optical gain of 19,575 cm-1 and a lower threshold current density of 143.9 A/cm². [ABSTRACT FROM AUTHOR]

In this paper, the gain, phase, and refractive index dynamics of photonic crystal quantum-dot semiconductor optical amplifiers (PC-QDSOAs) are investigated for the first time. For this purpose, we establish a comprehensive nonlinear state space model with 1088 state variables to simulate the carrier dynamics of the inhomogeneously broadened InAs/GaAs QD active region embedded in a flat-band slow-light photonic crystal waveguide. The gain and phase recovery responses are monitored during the amplification of an ultrashort pump pulse. Simulation results show that changes in refractive index in the PC-QDSOA active region are much stronger than the index changes in the conventional QDSOAs. In addition, the gain and phase recovery time in PC-QDSOAs are similar to those in the conventional QDSOAs based on ridge waveguide structures. Also, we found that the injected current and the consumed power in PC-QDSOAs can be two orders of magnitude less than those values in the conventional QDSOAs. [ABSTRACT FROM AUTHOR]

We report on the optical gain and loss of waveguides containing lithographically defined quantum dots. Lasing action has previously been demonstrated in a nominally identical structure. Measurements are made by monitoring the transmission of a resonant pulse while varying the injection current. We measure a maximum modal gain of 1.8 cm-1 at the peak of the ground state emission for a two-layer structure. The peak gain is insufficient for ground state lasing to be achieved in a structure with as-cleaved facets, but the gain per dot is comparable with that demonstrated in self-assembled quantum dots. [ABSTRACT FROM PUBLISHER]

Abstract: To study the characteristics of quantum-dot semiconductor optical amplifiers (QD-SOA), a refined sectionalized method is developed and utilized to solve the rate equations for the first time based on the three-level model of QD-SOA. The calculation results prove that the operation speed of refined sectionalized method is about twice as fast as that of conventional sectionalized method in the same precision. In addition, the injection current density of each section can be dynamically set depending on the input optical power to keep high gain or prevent its declination. [Copyright &y& Elsevier]

The photoluminescence (PL) from carbon dots can be dramatically modified through the engineering of surface oxygen-containing functional groups. Despite the recent surge of interest in the non-metal doping of carbon dots, the photo-physical properties of metal-doped carbon dots remain unexplored. Here, we report the highly efficient deep blue PL and amplified spontaneous emission from sodium (Na)-ion-doped carbon dots. Based on the structure- and temperature-dependent PL and the temporal evolution of PL, we reveal that the introduction of Na ions into carbon dots not only passivate the electronegative oxygen related defects but also create new Na–O related defect centers in the band gap, leading to deep blue PL with dramatically reduced bandwidth and significantly enhanced quantum yield. We establish a luminescence model with four energy levels to interpret the PL dynamics of Na+-doped carbon dots. We demonstrate for the first time the ASE from such carbon dots, which exhibit a net optical gain coefficient of 47.8 cm−1 at 440 nm. Our results open new horizons for improving the PL of carbon dots and pave the way for realizing optoelectronic devices based on carbon dots. • Efficient deep blue PL with a narrower bandwidth is achieved in Na + -doped C-dots. • Na + -doping leads to an enhanced PL quantum yield and a reduced bandwidth. • Deep blue PL arises from Na–O related defect luminescent centers. • Na + -doped C-dots exhibit a net optical gain under ultraviolet excitation. [ABSTRACT FROM AUTHOR]

Abstract: The electronic structures of self-assembled InAs1− x N x /GaAs nanostructures from quantum lens to quantum rings (QRs) are calculated using the 10-band k.p method and the valence force field (VFF) method. With the variation of shape of the nanostructure and nitrogen (N) content, it shows that the N and the strains can significantly affect the energy levels especially the conduction band because the N resonant state has repulsion interaction with the conduction band due to the band anticrossing (BAC). The structures with N and greater height have smaller transition energy, and the structures with N have greater optical gain due to its overwhelming greater value of factor . After analyzing the shape effect, we suggested that the nanostructures with volcano shape are preferred because the maximum optical gain occurs for quantum volcano. With our simulation result, researchers could select quantum dots (QDs) structures to design laser with better performance. [Copyright &y& Elsevier]

Abstract: The electronic structure and optical gain of InAsPN/GaP(N) quantum dots (QDs) are investigated in the framework of the effective-mass envelope function theory. The strain distribution is calculated using the valence force field (VFF) method. With GaP barrier, for smaller InAsPN QDs, the minimum transition energy may occur at a lower phosphorous (P) composition, but for larger QDs, the transition energy increases as P composition increases due to the increased bandgap of alloy QDs. When the nitrogen (N) composition increases, the transition energy decreases due to the stronger repulsion between the conduction band (CB) and the N resonant band, and the transition matrix element (TME) is more affected by the transition energy rather than N–CB mixing. To obtain laser materials with a lattice constant comparable to Si, we incorporated 2% of N into the GaP barrier. With this GaP0.98N0.02 barrier, the conduction band offset is reduced, so the quantum confinement is lower, resulting in a smaller transition energy and longer wavelength. At the same time, the TME is reduced and the optical gain is less than those without N in the barrier at a low carrier density, but the peak gain increases faster when the carrier density increases. Finally it can surpass and reach a greater saturation optical gain than those without N in the barrier. This shows that incorporating N into GaP barriers is an effective way to achieve desirable wavelength and optical gain. [ABSTRACT FROM AUTHOR]

We have solved the rate equations for InGaAs/GaAs self-assembled quantum-dot laser considering homogeneous and inhomogeneous broadening of optical gain numerically using fourth-order Runge-Kutta method. Dynamic-characteristics are analyzed; relaxation oscillation frequency, modulation bandwidth, and turn-on delay improve as injection current increases. With enhancing of the full width at half maximum (FWHM) of homogeneous broadening, threshold current and turn-on delay increase, because of elevating of central group density of states. Simulation results of static-characteristics show that slope-efficiency increases as the FWHM of homogeneous broadening heightens due to enhancing of central group carriers. Exceeding of the FWHM of homogeneous broadening from FWHM of inhomogeneous broadening results in degradation of slope-efficiency and static-characteristics because empty DOS increas. Nonlinearity appears in light-current characteristics at a special interval from ratio of FWHM of homogeneous broadening to FWHM of inhomogeneous broadening. Differential gain decreases as initial relaxation time and recombination times heighten. Consequently, relaxation oscillation frequency and modulation bandwidth decline. [ABSTRACT FROM AUTHOR]

Abstract: The shape of truncated square-based pyramid quantum dots (QDs) is similar to that of real QDs in experiments. The electronic band structures and optical gain of InAs1−x N x /GaAs QDs are calculated by using the 10-band model, and the strain is calculated by the valence force field (VFF) method. When the top part of the QD is truncated, greater truncation corresponds to a flatter shape of the QD. The truncation changes the strain distribution and the confinement in the direction. A flatter QD has a greater C1–HH1 transition energy, greater transition matrix element, less detrimental effect of higher excited transition, and higher saturation gain and differential gain. The trade-off between these properties must be considered. From our results, a truncated QD with half of its top part removed has better overall performance. This can provide guidance to growing QDs in experiments in which the proper growing conditions can be controlled to achieve required properties. [Copyright &y& Elsevier]

The study of the electron distribution and intersubband transitions in quantum cascade (QC) lasers provides fundamental insights into the laser operation. Measurements on intersubband transitions are often performed on QC lasers under nonoperating conditions, i.e., at zero electric field. In this study, a single-pass transmission technique that allows for probing of QC lasers under operating conditions was used. The measurements and analysis were done on QC lasers with vertical and diagonal transition active region designs operating at λ ~ 10 μm. The single-pass modal gain coefficients, extracted for the vertical transition design, were 24.04 cm/kA at 100 K, 21.36 cm/kA at 125 K, 14.25 cm/kA at 150 K, and 5.26 cm/kA at 175 K. [ABSTRACT FROM AUTHOR]

[Display omitted] • Effect of strain and exciton carrier confinement on SML QD size distribution is studied. • Sample splits with varying monolayer (ML) coverage, X (0.3, 0.5) and QD stacks, Y (4, 6, 8) were considered. • PL of 0.5 ML showed efficient carrier localization, while 0.3 ML by phonons. • Raman and HRXRD analysis helped us calculate the In% inside SA In x Ga 1− x As layers. • Best results were achieved for 0.5 ML with 4 QD stacks with GS PL peak at 1.19 eV and reduced defects. Strain – induced intermixing in sub – monolayer (SML) quantum dots (QDs) affects primarily the dot size distribution in an erratic way and also the inter – dot coupling efficiency for carrier transport. In this study, we have explored the role of strain and carrier confinement effects in order to control the dot size distribution by varying InAs QDs coverage (X: 0.3 and 0.5 ML) and stacks (Y: 4, 6, and 8) simultaneously. The size variation affects the position of localized levels inside QDs primarily as it is the inter – dot coupling that decides the luminescence efficiency. Next, through multiple stacking of QD layers, strain variation becomes inhomogenous that facilitate a higher degree of In – Ga intermixing which necessitates to estimate the amount of Indium (%) present inside such In–rich islands to progress for a superior optical performance. Hence, we combine and address these above concerns in great detail. The 20 K ground-state (GS) photoluminescence (PL) energy for 0.3 and 0.5 ML samples were centered at 1.24–1.33 eV, and 1.11–1.19 eV respectively. The PL linewidth and activation energy variation with temperature validated that there exists two possible thermal escape pathways for carrier recombination. Besides, PL – excitation (PLE) studies comprehended the influence of phonon and excited states (ES) in the samples. Raman analysis helped us calculate the In% inside QDs by figuring out the shift in InAs QD phonon modes (amount of compressive strain). Furthermore, the structural analysis by high-resolution X-ray diffraction (HRXRD) was done to calculate the strain moderated defect density and also the effect of in – plane compressive strain to the reduction in quantum confinement. The increase in compressive strain inside QDs caused more In–Ga intermixing in 0.3 ML samples and higher defect densities. Altogether we see that sample with 0.5 ML, 4 stacks showed vertically-correlated dot growth (from HR-XTEM) with minimum strain (−0.03383 a.u.), dislocation density (4.887 x 1011 cm−2) and higher In content (62.41%). This high degree of excitonic carrier confinement and strain tuning makes SML QDs an attractive candidate for room-temperature (RT) operable photodetector (PD) devices. [ABSTRACT FROM AUTHOR]

Colloidal CdTe quantum dots (QDs) of two different sizes (2.7 nm and 3.7 nm diameter) were self-assembled into mixed quantum dot donor-acceptor nanoclusters (QDANs) in aqueous solution. The ratio of small QDs (SQDs) to large QDs (LQDs) was 10:1, and the assembly was mediated by electrostatic attraction between oppositely-charged ligands (thioglycolic acid, TGA, and mercaptoethylamine, MA) on the QD surfaces. Under low excitation fluence (well below the threshold for biexciton formation), photoluminescence (PL) emission spectra and time-resolved decays in the QDANs revealed quenching in the SQDs and enhanced emission from the LQDs, providing evidence for resonance energy transfer (RET) from the wider band gap SQDs to the narrower band gap LQDs. The observed RET rate constant, determined from comparison of decays of free SQDs and those in the QDANs, was (3.4 ns)−1. Under these low-fluence conditions, the PL emission lifetimes of the LQDs in QDANs were the same as the free LQDs. Under higher excitation intensity, the decay of the LQDs in the QDAN showed a new, fast (sub-nanosecond), fluence-dependent component, suggesting formation of biexcitons and multiexcitons on the LQDs. The free LQDs required significantly (approximately 4-fold) higher excitation intensity than those in the QDANs to show similar evidence of biexciton formation through lifetime shortening. Since the QDANs contained a 10-fold excess of SQDs, the excitation energy that was initially distributed over the full population of QDs was funneled and concentrated into the small number of LQDs. These results support the idea that optical gain through biexciton formation can be enhanced by using RET, even when the rate constant for RET is much lower than that for biexciton decay. [Display omitted] • Nanocluster of CdTe quantum dots (QDs) assembled of two different sizes, small QDs (SQDs) and large QDs (LQDs). • Photoexcitation energy funnels in nanocluster from the SQDs to the LQDs, facilitating multiexciton formation in LQDs. • Multiexciton production in LQDs at lower-intensity pumping was proved by time-resolved photoluminescence. • This approach provides benefits in the design and development of new optical gain materials. [ABSTRACT FROM AUTHOR]

This work constructed a kind of high-quality CdSe/CdS@ZnO QDs for the first time that enabled by inorganic ZnO sol as surface ligands with the improved PL stability and gain performance. The CdSe/CdS@ZnO QDs exhibited stable amplified spontaneous emission process, and finally as gain medium achieved high-performance single-mode lasing output with an ultralow threshold of ~3.3 μJ cm−2. [Display omitted] • High-quality CdSe/CdS@ZnO QDs enabled by inorganic ZnO sol as surface ligands. • ZnO sol possesses effective surface passivation capability and deionization effect. • CdSe/CdS@ZnO QDs exhibit the enhanced optical gain performance and PL stability. • All-inorganic CdSe/CdS@ZnO QDs lasers achieve high-performance lasing output. Colloidal semiconductor quantum dots (QDs) have attracted great attention in field of optoelectronic devices. However, the surface organic ligands of QDs involving in optical instability and limited passivation remain challenging, thus bringing additional difficulties in practical application. Here, the high-performance CdSe/CdS@ZnO QDs were successfully prepared by ZnO sol as surface ligands. Importantly, this kind of CdSe/CdS@ZnO QDs maintained the typical advantages of conventional QDs, such as superior monodispersity and long-term storage stability in solution. Because of the effective surface passivation provided by inorganic sol ligands and the deionization effect that electron transferring from surface state of QDs to ZnO acceptor, CdSe/CdS@ZnO QDs showed the remarkably enhanced photostability and biexciton lifetime, as well as further suppressed Auger process and PL blinking. Meanwhile, the CdSe/CdS@ZnO QDs demonstrated PL thermal stability much higher than that of pristine CdSe/CdS QDs under heating to 100 °C, which was efficiently improved 3 times that from ~20% to ~63% of their initial emission intensity. Moreover, a stable amplified spontaneous emission process was observed in CdSe/CdS@ZnO QDs film and still retained a lower ASE threshold (~28 μJ cm−2). Finally, we successfully fabricated a high-performance vertical cavity surface-emitting laser based on CdSe/CdS@ZnO QDs, which presented spatially directional single-mode lasing output with an ultralow threshold of ~3.3 μJ cm−2. We believe that this work could not only enrich the family of surface ligands passivated semiconductor QDs but also provide a novel strategy towards constructing stable and functionalized colloidal nanomaterials for all-inorganic optoelectronic devices. [ABSTRACT FROM AUTHOR]

Abstract: Optical gain measurements with femtosecond excitation were performed in CdSe colloidal quantum dots (QDs) dissolved in toluene using the variable stripe length method. Instead of ZnS-shell overcoating, we found that the spectral narrowing and intensity enhancement of the fluorescence can be achieved alternatively by excluding octadecene (ODE) at the preparation of QDs. The maximum gain (∼1.5mm−1) in the ODE-excluded CdSe QDs is comparable to that of CdSe/ZnS core–shell structures. Despite the uniform size distribution (), a broad gain spectrum appears in both cases due to the recombination of many excited levels between biexciton and exciton states. The remaining surface states in the ODE-excluded CdSe QDs also contribute to gain (1.0mm−1), which is involved with a three-level process. [Copyright &y& Elsevier]

An alternative segmented-contact method for accurate measurement of the optical gain and absorption of quantum-dot and quantum-dash active materials with small optical gain is reported. The usual error from unguided spontaneous emission is reduced by subtracting signals acquired from three independently controlled sections as opposed to just two found in the conventional technique. The quantum-dot gain spectra are measured to a precision of less than 0.2 cm-1 at nominal gain values below 2 cm-1, and gain spectrum of quantum-dash sample is calculated with an error less than 0.3 cm-1 at a gain less than 1 cm-1. These accuracies are checked with a self-calibrating method. The internal optical mode loss measurement is also described. [ABSTRACT FROM AUTHOR]

A simple, accurate, and fast multilayer feedforward artificial neural network (ANN) model for quantum-dot semiconductor optical amplifiers (QD-SOAs) is developed. The developed ANN model has demonstrated excellent precision and accurately Models the physical characteristics of QD-SOAs such as pulse amplification and four-wave mixing characteristics. Furthermore, the developed ANN model requires a very small computational time compared with numerical models, which is very attractive for computer-aided-design applications. The model is used to create very interesting design curves for QD-SOAs. In addition, the model is suitable for physical-parameter extraction from available measured data. [ABSTRACT FROM AUTHOR]

We have measured the passive modal absorption, modal gain, and spontaneous emission spectra of a quantum-dot system where the inhomogeneous broadening is sufficiently small so that the ground- and excited-state transitions can be spectrally resolved. Absorption by ground- and excited-state transitions is in the ratio 1:1.88 which is close to the ratio of 2 expected for dots with similar dimensions in two directions. The absorption cross section per dot is measured to be 1.1 × 10-14 cm². Optical gain from the ground-state saturates with current at a maximum value of one third of that predicted from the measured absorption if the system is fully inverted. The measured population inversion factor spectrum shows that the carrier distributions cannot be described only by a single global Fermi distribution and that the system is not in overall equilibrium. However, using parameters obtained by fitting the absorption spectrum, we find that for these particular samples the inversion factor spectrum can be described by a possible model where the ground- and excited-state occupancies are each described by a Fermi distribution but with different quasi-Fermi energy separations We speculate that photon mediation within the homogeneous linewidth could be one possible process which establishes quasi-equilibrium within each of the ground- and excited-state inhomogeneous distributions. [ABSTRACT FROM AUTHOR]

This paper deals with the simulation of the emission characteristics of self-assembled semiconductor quantum dash (QDash) active materials, characterized by high length-to-width and width-to-height ratios of the dash size and by a wide spreading of the dash dimensions. This significant size fluctuation requires to compute numerically the corresponding energy distribution of the electron and hole confined states. Furthermore, due to the long dash length, it is necessary to take into account the many longitudinal confined states that contribute to the emission spectrum. To implement a model that does not require excessive computation time, some simplifying assumptions have been introduced and validated numerically. Starting from good knowledge of the dash size, material composition, and optical waveguide dimensions, we have been able to simulate the amplified spontaneous emission and gain spectra of a quantum dash semiconductor optical amplifier with a good quantitative agreement with the measured data. As an application example, the model is used to predict the gain properties of different QDash ensembles having various size distributions. [ABSTRACT FROM AUTHOR]

The electronic band structure of GaAsNBi spherical quantum dot (QDs) have been calculated under hydrostatically induced pressure using 16 band k.p model and considering the band anticrossing (BAC) interaction between the valence band (VB) of the host material with the N and Bi related resonant impurity level. It has a linear variation in bandgap with different Bi concentration and non-linear in bandgap with varying pressure, for 2.3% N in both the cases. However, small differences are observed in valance band offset (VBO), whereas more considerable changes can be seen in the conduction band offset (CBO) with an increasing hydrostatic pressure, which implies a stronger confinement of charge carriers. Finally, we also studied the percentage of strain distribution and its related properties of GaAsNBi spherical QDs along the radial direction, lattice misfit under biaxial and hydrostatic strain. These QDs are also being used in many applications, including selective frequency operation for optoelectronic devices by tuning distinct mole fraction and pressure. [ABSTRACT FROM AUTHOR]

Novel closed-form model for the threshold conditions and the optical gains of vertical dual-cavity semiconductor optical amplifier (VDC-SOA) is derived. Expressions for the transfer functions that relate the average photon density inside the two cavities to the incident photon density are derived. The model is simple, accurate and easy to use for designing dual-wavelength laser amplifiers. Good agreement is obtained between the analytical model and numerical calculations. The transmitted and reflective bistability characteristics of quantum dot VDC-SOA have been investigated. The threshold condition of each mode is expressed in terms of the top-cavity and the bottom-cavity currents. The dual modes λ o + and λ o - show different bistability behavior as a function of the top cavity current when the device operates at fixed operating point below the threshold curve. We find that the contrast ratio of mode λ o + is a weak function of the device currents, and the hysteresis width increases when the top cavity current is increased. For mode λ o - large contrast ratio and hysteresis width are obtained when the two cavities exhibit equal currents. Our analysis also reveals that the spectral characteristics of mode λ o + are different from that of mode λ o - . [ABSTRACT FROM AUTHOR]

The hydrostatic pressure induced band diagram of an InGaAsN/GaAs spherical quantum dot (QD) system is theoretically calculated using an expanded form of standard 8 band k · p Hamiltonian. The band parameters including energy gap, band offsets, and effective masses of electron and hole as a function of hydrostatic pressure are extracted using 10 band k · p model to calculate optical gain, threshold current density, and confined exciton binding energies of the QD system for the possible potential applications in 1.3–1.55 μm laser diodes. The optical gain increases with the increase in hydrostatic pressure up to 30 kbar and eventually decreases for further application of higher pressures. This effect is not usually observed in InGaAsN/GaAs quantum wells owing to the strong dependence of hole and electron effective masses with the hydrostatic pressure. A relative blue shift is observed in the gain spectra due to the increase in hydrostatic pressure or carrier concentration whereas a redshift is observed due to the increase in geometrical dimensions or impurity concentration which helps in estimating the range of operation of InGaAsN/GaAs QD based devices. The obtained binding energies, threshold current densities, and strain within the QDs help in understanding the influence of pressure and dot radius on these parameters. The hydrostatic pressure induced band diagram of InGaAsN/GaAs spherical quantum dots has been theoretically calculated using a 8 band k · p Hamiltonian. Nitrogen plays an important role in the reduction of the band gap in InGaAsN semiconducting material and leads to a larger threshold current due to the higher threshold carrier density with increase of hydrostatic pressure. For pressure values up to 30 kbar, there is an increase in the magnitude of optical gain. However, if the pressure is increased beyond 30 kbar, optical gain first saturates and thereafter decreases with the rise in pressure. [ABSTRACT FROM AUTHOR]

Since their inception, quantum dots have proven to be advantageous for light management applications due to their high brightness and well‐controlled absorption, scattering, and emission properties. As quantum dots become commercially available at large scale, the need for robust, stable, and flexible optical components continues to drive the development of robust and flexible quantum dot composite materials. In this review, after a thorough introduction to quantum dots, discussion delves into methods for fabricating quantum dot loaded composite optical elements such as thin films, microfabricated patterns, and microstructures. The importance of surface chemistry and ligand engineering, host matrixes, wet processing, and unique patterning methodologies is presented by considering photostability, aggregation, and phase separation of quantum dots in corresponding composites. With regard to prospective optical applications of quantum dot materials, emphasis is placed on light emitting and guiding composite materials for lasing applications, specifically whispering gallery mode‐based photonic microsystems. These developments will enable novel flexible, portable, and miniaturized optoelectronic devices such as light‐emitting diodes, flexible pixelated displays, solar cells, large‐area microwaveguides, omnidirectional micromirrors, optical metasurfaces, and directional microlasers. Herein the state of the art of quantum dot composite materials is described, with particular interest paid to light emitting and guiding technologies. Great attention is paid to micropatterning, focusing on the importance of surface chemistry and ligand engineering, host matrixes, and wet processing to achieve optimal photostability, aggregation, and phase separation of quantum dots in corresponding patterns and resulting devices. [ABSTRACT FROM AUTHOR]