Low Threshold Gain Research Articles

Optimized waveguide coupling of an integrated III-V nanowire laser on silicon

The recent integration of III-V semiconductor nanowire (NW) lasers on silicon waveguides marked a key step toward their usage as coherent light sources for future silicon photonics applications. However, the low index contrast between III-V semiconductors and silicon results in a weak modal reflectivity, calling for improved design structures that enable both low-threshold lasing and good in-coupling efficiency into waveguides. Here, we perform numerical simulations to explore how the alternating refractive index of a silicon waveguide with a thin SiO2 interlayer can be used to significantly improve the reflectivity at the III-V–silicon interface to values of up to 83%. We further investigate the frequency dependencies of the end-facet reflectivity and in-coupling efficiency as a function of the nanowire and waveguide dimensions. Our results are kept general by the normalization to the nanowire radius R and show for a waveguide width of 2.75⋅R a maximum coupling efficiency of 50%. Variations in waveguide height or SiO2 interlayer thickness by ±0.1⋅R increase the coupling efficiency by a factor of 2, with little effect on the end-facet reflectivity. Ultimately, a prototypical NW-laser structure consisting of a 1.3-μm emitting InGaAs MQW active region in a core-multishell structure was simulated, showing an optimized low-threshold gain of <500 cm−1 for a TE01 mode with a coupling efficiency of ∼13%. By simplified approximations, we illustrate that these analyses can be adapted to a variety of material systems and serve as guidelines in the construction of optimized nanowire lasers on silicon-on-insulator waveguides for future on-chip optical interconnects.

Low Optical Loss Amplified Spontaneous Emission and Lasing in a Solution‐Processed Organic Semiconductor

AbstractMeeting the challenges of realizing organic electrically pumped laser, low threshold gain materials with minimum optical loss are highly demanded, especially those with insignificant nonradiative processes during lasing. Herein, the low optical loss amplified spontaneous emission (ASE) and high Q‐factor (2,700) lasing are discovered in a solution‐processed benzothiadiazole‐cored multifluorene derivative. The excellent lasing properties of this molecule inspire to focus on the origin of its intrinsic optical loss. The ultrafast time‐resolved spectroscopy studies reveal that the low intrinsic optical loss originates from a relatively “simple” excited state interaction including just singlet–singlet absorption and stimulated emission, excluding additional loss processes such as intermolecular charge‐separation and triplet–triplet absorption that are detrimental to achieve high quality lasing. Quantitative descriptions on ASE process are also discussed based on both transient absorption spectra and transient photoluminescence spectra. This study shows new options and deeper understanding for designing low threshold gain materials.

Semiconductor Nanoplatelet Excimers.

Excimers, a portmanteau of "excited dimer", are transient species that are formed from the electronic interaction of a fluorophore in the excited state with a neighbor in the ground state, which have found extensive use as laser gain media. Although common in molecular fluorophores, this work presents evidence for the formation of excimers in a new class of materials: atomically precise two-dimensional semiconductor nanoplatelets. Colloidal nanoplatelets of CdSe display two-color photoluminescence resolved at low temperatures with one band attributed to band-edge fluorescence and a second, red band attributed to excimer fluorescence. Previously reasonable explanations for two-color fluorescence, such as charging, are shown to be inconsistent with additional evidence. As with excimers in other materials systems, excimer emission is increased by increasing nanoplatelet concentration and the degree of cofacial stacking. Consistent with their promise as low-threshold gain media, amplified spontaneous emission emerges from the excimer emission line.

A study of low threshold and high gain Nd3+ ions doped SiO2-B2O3-Na2CO3-NaF-CaF2 glasses for NIR laser applications

Fluoroborosilicate glasses of composition 35SiO2-25B2O3-10Na2CO3-15NaF-15CaF2-xNd2O3 (where x = 0.1, 0.5. 1.0, 2.0 mol%) were prepared by melt quenching technique and various physical properties have been calculated. From the absorption spectra J-O Intensity parameters Ωλ (λ = 2, 4, 6) and radiative properties are evaluated by using J-O theory. The high values of Ω2 = 4.213 × 10−20 cm2, Ω4 = 5.345 × 10−20 cm2, Ω6 = 5.526 × 10−20 cm2 suggest that among the prepared glasses 0.5 mol% Nd glass is more asymmetric, more covalent and rigid in nature. The emission spectra were recorded with 808 nm laser as excitation source. The strong NIR emissions were observed at 876 nm, 1056 nm, 1328 nm corresponding to the transitions 4F3/2 → 4I9/2, 4F3/2 → 4I11/2, 4F3/2 → 4I13/2 respectively. Stimulated emission cross -section (σemi) and Gain bandwidth (σemi × Δλeff) were calculated. For 0.5 mol% Nd these values are found to be 3.30 × 10−20 cm2, 11 × 10−26 cm2. From the decay curve analysis the lifetime values for 4F3/2 level have been determined and these values are decreased with increase in Nd3+ ions concentration. These results may suggest that the prepared SBNCNd05 (Nd = 0.5 mol%) glass could be useful for 1056 nm laser applications.

Improved hybrid plasmonic microcavity laser

In this paper, an improved hybrid surface plasmon nanolaser with a gain medium ridge and a layer of air gap is proposed. In order to achieve low propagation loss and sub-wavelength field confinement, a triangular air gap and a 50 nm microcavity end face silver mirror are adopted in this structure, and the combination of this particular triangular structure and silver mirror effectively improves the performance of nano-laser. In this paper, we numerically simulate the waveguide by using the finite-element method. The COMSOL multiphysics software is a superior numerical simulation software to simulate the real physical phenomena based on the finite element method. On the basic of the COMSOL multiphysics software, a two-dimensional cross-section model and a three-dimensional model are built, the transmission performance and microcavity performance of the improved structure are analyzed in detail at a working wavelength of 1550 nm. Some quantities including the electric field distribution, transmission length, normalized mode field area, average energy density, foundation modal volume, quality factor of the structure, threshold gain, quality factor, effective modal volume, and Purcell factor are considered here which are dependent on the dielectric constant and geometrical parameters. The results indicate that on a two-dimensional scale, the contradiction between transmission loss and transmission distance can be effectively solved by the guidance of Fom value, and the IHPM laser structure with optimal transmission characteristics is obtained under the guidance of quality factor and foundation modal volume. A deep sub-wavelength constraint on light is achieved:the propagation length of the electromagnetic mode reaches a millimeter level and the longest distance can reach 1.29 mm. When testing the microcavity performance of the laser separately on a two-dimensional scale and three-dimensional scale, the high quality factor, low gain threshold, ultra-small effective mode volume of 0.001092 μm3 and ultra-high Purcell factor of 8.29×105 are obtained by adjusting the structural parameters and plating a 50 nm-thick silver layer on the end face of the laser microcavity. Compared with the previous structure without air gaps, the designed structure has a low laser lasing threshold and strong micro-cavity local capability when these two structural parameters are unified. The designed hybrid surface plasmon nanolaser may serve as a fundamental building block for various functional photonic components and can have applications such as in sensing, nanofocusing, and nanolasing.

Low-threshold optically pumped lasing in highly strained germanium nanowires

The integration of efficient, miniaturized group IV lasers into CMOS architecture holds the key to the realization of fully functional photonic-integrated circuits. Despite several years of progress, however, all group IV lasers reported to date exhibit impractically high thresholds owing to their unfavourable bandstructures. Highly strained germanium with its fundamentally altered bandstructure has emerged as a potential low-threshold gain medium, but there has yet to be a successful demonstration of lasing from this seemingly promising material system. Here we demonstrate a low-threshold, compact group IV laser that employs a germanium nanowire under a 1.6% uniaxial tensile strain as the gain medium. The amplified material gain in strained germanium can sufficiently overcome optical losses at 83 K, thus allowing the observation of multimode lasing with an optical pumping threshold density of ~3.0 kW cm−2. Our demonstration opens new possibilities for group IV lasers for photonic-integrated circuits.

High-Efficiency Optical Gain in Type-II Semiconductor Nanocrystals of Alloyed Colloidal Quantum Wells.

Colloidal nanocrystals having controlled size, tailored shape, and tuned composition have been explored for optical gain and lasing. Among these, nanocrystals having Type-II electronic structure have been introduced toward low-threshold gain. However, to date, their performance has remained severely limited due to diminishing oscillator strength and modest absorption cross-section. Overcoming these problems, here we realize highly efficient optical gain in Type-II nanocrystals by using alloyed colloidal quantum wells. With composition-tuned core/alloyed-crown CdSe/CdSexTe1-x quantum wells, we achieved amplified spontaneous emission thresholds as low as 26 μJ/cm2, long optical gain lifetimes (τgain ≈ 400 ps), and high modal gain coefficients (gmodal ≈ 930 cm-1). We uncover that the optical gain in these Type-II quantum wells arises from the excitations localized to the alloyed-crown region that are electronically coupled to the charge-transfer state. These alloyed heteronanostructures exhibiting remarkable optical gain performance are expected to be highly appealing for future display and lighting technologies.

Amplified spontaneous emission from a single organic microfiber fabricated by melt electrospinning

Abstract We study gain amplification processes in a single organic microfiber fabricated by melt electro-spinning method. Smooth, individual and cylindrical fiber is obtained. The gain-induced response of the microfiber is found to depend sensitively on the fiber structure. In the homogeneous single fiber where loop cavity forms, high net optical gain of up to 31.4 cm −1 results in spectral narrowing at the material gain peaks. In the case of strong optical feedback, which occurs in such a long microfiber, amplified spontaneous emission with a low threshold of 0.08 mW is obtained. The low threshold and high gain attribute also to the cylindrical cavity which supplies positive feedback.

Design of an 845-nm GaAs Vertical-Cavity Silicon-Integrated Laser with an Intracavity Grating for Coupling to a SiN Waveguide Circuit

A short-wavelength hybrid GaAs vertical-cavity silicon-integrated laser (VCSIL) with in-plane waveguide coupling has been designed and optimized using numerical simulations. A shallow etched silicon nitride (SiN) grating is placed inside the cavity of the hybrid vertical-cavity silicon-integrated laser to both set the polarization state of the resonant optical field and to enable output coupling to a SiN waveguide with high efficiency. The numerical simulations predict that for apertures of 4 and 6-μm oxide-confined VCSILs operating at 845-nm wavelength, a slope efficiency for the light coupled to the waveguide of 0.18 and 0.22 mW/mA is achievable, respectively, while maintaining a low threshold gain of 583 and 589 cm−1, respectively, for the lasing.

Near-infrared hybrid plasmonic multiple quantum well nanowire lasers.

The lasing characteristics of hybrid plasmonic AlGaAs/GaAs multiple quantum well (MQW) nanowire (NW) lasers beyond diffraction limit have been investigated by 3D finite-difference time-domain simulations. The results show that the hybrid plasmonic MQW NW has lower threshold gain over a broad diameter range in comparison with its photonic counterpart. Beyond the diffraction limit, the hybrid plasmonic MQW NW has a lowest threshold gain of 788 cm-1 at a diameter of 130 nm, and a cutoff diameter of 80 nm, half that of the photonic lasers. In comparison with the hybrid plasmonic core-shell NWs, the hybrid plasmonic MQW NWs exhibit significantly lower threshold gain, higher Purcell factor, and smaller cutoff diameter, which are attributed to the superior overlap between the hybrid plasmonic modes and gain medium, as well as a stronger optical confinement due to the grating-like effect of MQW structures. Moreover, the hybrid plasmonic MQW NW has a lower threshold gain than that of the core-shell NW over a broad wavelength range. The hybrid plasmonic MQW NW structure is promising for ultrasmall and low-consumption near-infrared nanolasers.

Impact of Bi2O3 on structural properties and lasing effects in Nd3 + doped bismuth phosphate glasses at 1.053 μm emission

Influence of addition of bismuth oxide (Bi2O3) on structural and optical properties of 0.5Nd3+doped different phosphate glasses prepared by melt quenching technique were reported. The structural properties were analysed by FT-IR and 31P MAS NMR techniques. 31P NMR results showed that, with the variation of Bi2O3 concentration in the prepared phosphate glass matrices there was no significant change in the structure. The optical properties have been analysed using Judd–Ofelt (J-O) theory. From absorption spectra, three phenomenological J–O intensity parameters (Ω2, Ω4, Ω6) have been calculated and these parameters were used to estimate the radiative properties. The J-O intensity parameters increased with increase in the concentration of Bi2O3, which confirmed the higher covalency, asymmetry and rigidity. The branching ratios (β) and emission cross-sections (σ) were calculated from the emission spectra of Nd3+ doped bismuth phosphate (BiP) glasses. Among different glass matrices 15mol% of Bi2O3 glass matrix has higher β and σ values which are useful for the laser applications. The improved emission cross-section value with the addition of Bi2O3 content results low threshold and high gain applications. The decay curves of all these BiP glasses showed single exponential behaviour.

Low Threshold Multiexciton Optical Gain in Colloidal CdSe/CdTe Core/Crown Type-II Nanoplatelet Heterostructures.

Colloidal cadmium chalcogenide core/crown type-II nanoplatelet heterostructures, such as CdSe/CdTe, are promising materials for lasing and light-emitting applications. Their rational design and improvement requires the understanding of the nature of single- and multiexciton states. Using pump fluence and wavelength-dependent ultrafast transient absorption spectroscopy, we have identified three spatially and energetically distinct excitons (in the order of increasing energy): interface-localized charge transfer exciton (XCT, with electron in the CdSe core bound to the hole in the CdTe crown), and CdTe crown-localized XCdTe and CdSe core-localized XCdSe excitons. These exciton levels can be filled sequentially, with each accommodating two excitons (due to electron spin degeneracy) to generate one to six exciton states (with lifetimes of ≫1000, 209, 43.5, 11.8, 5.8, and 4.5 ps, respectively). The spatial separation of these excitons prolongs the lifetime of multiexciton states. Optical gain was observed in tri- (XXCTXCdTe) and four (XXCTXXCdTe) exciton states. Because of the large absorption cross section of nanoplatelets, an optical gain threshold as low as ∼43 μJ/cm2 can be achieved at 550 nm excitation for a colloidal solution sample. This low gain threshold and the long triexciton (gain) lifetime suggest potential applications of these 2D type-II heterostructures as low threshold lasing materials.

Ultralow-Threshold Single-Mode Lasing from Phase-Pure CdSe/CdS Core/Shell Quantum Dots.

The development of colloidal quantum dot (QD) lasers is blocked by Auger recombination (AR). Here, phase-pure wurtzite CdSe/CdS core/shell QDs with controlled shell thickness are reported, which possess nearly defect-free core/shell interfaces. Benefiting from increased volume, electron-hole partial spatial separation, and nearly defect-free alloyed interface, this series of QDs exhibit a greater than 3 orders of magnitude decrease in AR rates with increasing shell thickness. Consequently, the amplified spontaneous emission threshold of the QDs with an 11 monolayer CdS shell is found to reach a minimum of 16 μJ cm-2. A record long lifetime (>1000 ps) and extraordinarily large bandwidth (>170 nm) of optical gain are observed by employing ultrafast transient absorption spectroscopy. We leverage the low-threshold gain of the QDs to fabricate microlasers that display single-mode operation and an ultralow threshold of ∼2 μJ cm-2. Our results represent a valuable step toward practical QD lasers.

Ultrahigh Gain Polymer Photodetectors with Spectral Response from UV to Near‐Infrared Using ZnO Nanoparticles as Anode Interfacial Layer

Significant increase of photocurrent upon UV light exposure is demonstrated in a narrow‐bandgap polymer‐based photodetector using ZnO nanoparticles as anode interfacial layer. The phenomenon is attributed to the UV light illumination induced oxygen molecules desorption from surface of ZnO nanoparticles, which reduces the electron injection barrier at the anode interface. Ultrahigh external quantum efficiency of 140 000% and extremely low gain threshold voltage of 1.5 mV are achieved in this device with 30 s UV light irradiation. The gain mechanism is explained by the fast transit and replenishment of photogenerated electrons within their lifetime, which is prolonged by the electron‐only device structure, and the experiment results fit well with the proposed photoconductive model.

Lasing threshold characteristics of multi-wavelength Brillouin–erbium laser in the L-band region assisted by delay interferometer

This paper reports the simulation of L-band multi-wavelength Brillouin–erbium fiber laser that utilizes delay interferometer as a comb filter. The simulation is performed using OptiSystem. The structure exhibits threshold power in terms of Erbium doped fiber amplifier (EDFA) gain of 10[Formula: see text]mW at a very low Brillouin pump power of 0.00316[Formula: see text]mW for the generation of first Stokes. Thirty one output channels were produced with minimum EDFA gain of 1[Formula: see text]W at Brillouin pump power of 0.00316[Formula: see text]mW. In line with theory, it has been observed that higher Brillouin pump power requires low threshold gain and vice versa to generate the Brillouin Stokes.

Colloidal Nanoplatelets: Platelet‐in‐Box Colloidal Quantum Wells: CdSe/CdS@CdS Core/Crown@Shell Heteronanoplatelets (Adv. Funct. Mater. 21/2016)

On page 3570, H. V. Demir and co-workers synthesize CdSe/CdS@CdS core/crown@shell nanoplatelets (NPLs) with a novel 3D architecture resembling a platelet-in-box structure. This leads to increased photoluminescence quantum yield, enhanced absorption, and eliminated trap sites, enabling favorable excitonic properties for low-threshold gain. Carefully heterostructured NPLs will play a critical role in building high-performance colloidal optoelectronic devices.

Carbon-bridged oligo(p-phenylenevinylene)s for photostable and broadly tunable, solution-processable thin film organic lasers.

Thin film organic lasers represent a new generation of inexpensive, mechanically flexible devices for spectroscopy, optical communications and sensing. For this purpose, it is desired to develop highly efficient, stable, wavelength-tunable and solution-processable organic laser materials. Here we report that carbon-bridged oligo(p-phenylenevinylene)s serve as optimal materials combining all these properties simultaneously at the level required for applications by demonstrating amplified spontaneous emission and distributed feedback laser devices. A series of six compounds, with the repeating unit from 1 to 6, doped into polystyrene films undergo amplified spontaneous emission from 385 to 585 nm with remarkably low threshold and high net gain coefficients, as well as high photostability. The fabricated lasers show narrow linewidth (<0.13 nm) single mode emission at very low thresholds (0.7 kW cm−2), long operational lifetimes (>105 pump pulses for oligomers with three to six repeating units) and wavelength tunability across the visible spectrum (408–591 nm).

Multifunctional Polymer Nanofibers: UV Emission, Optical Gain, Anisotropic Wetting, and High Hydrophobicity for Next Flexible Excitation Sources.

The use of UV light sources is highly relevant in many fields of science, being directly related to all those detection and diagnosis procedures that are based on fluorescence spectroscopy. Depending on the specific application, UV light-emitting materials are desired to feature a number of opto-mechanical properties, including brightness, optical gain for being used in laser devices, flexibility to conform with different lab-on-chip architectures, and tailorable wettability to control and minimize their interaction with ambient humidity and fluids. In this work, we introduce multifunctional, UV-emitting electrospun fibers with both optical gain and greatly enhanced anisotropic hydrophobicity compared to films. Fibers are described by the onset of a composite wetting state, and their arrangement in uniaxial arrays further favors liquid directional control. The low gain threshold, optical losses, plastic nature, flexibility, and stability of these UV-emitting fibers make them interesting for building light-emitting devices and microlasers. Furthermore, the anisotropic hydrophobicity found is strongly synergic with optical properties, reducing interfacial interactions with liquids and enabling smart functional surfaces for droplet microfluidic and wearable applications.

Fluorene- and benzofluorene-cored oligomers as low threshold and high gain amplifying media

Deliberate control of intermolecular interactions in fluorene- and benzofluorene-cored oligomers was attempted via introduction of different-length alkyl moieties to attain high emission amplification and low amplified spontaneous emission (ASE) threshold at high oligomer concentrations. Containing fluorenyl peripheral groups decorated with different-length alkyl moieties, the oligomers were found to express weak concentration quenching of emission, yet excellent carrier drift mobilities (close to 10−2 cm2/V/s) in the amorphous films. Owing to the larger radiative decay rates (&amp;gt;1.0 × 109 s−1) and smaller concentration quenching, fluorene-cored oligomers exhibited down to one order of magnitude lower ASE thresholds at higher concentrations as compared to those of benzofluorene counterparts. The lowest threshold (300 W/cm2) obtained for the fluorene-cored oligomers at the concentration of 50 wt % in polymer matrix is among the lowest reported for solution-processed amorphous films in ambient conditions, what makes the oligomers promising for lasing application. Great potential in emission amplification was confirmed by high maximum net gain (77 cm−1) revealed for these compounds. Although the photostability of the oligomers was affected by photo-oxidation, it was found to be comparable to that of various organic lasing materials including some commercial laser dyes evaluated under similar excitation conditions.

Controlling the Surface of Semiconductor Nanocrystals for Efficient Light Emission from Single Excitons to Multiexcitons

Semiconductor nanostructures have shown promise for light emission across various intensity regimes. Desired performance objectives of photoluminescence efficiency, low gain threshold, large gain lifetime and bandwidth have not been met by any one nanocrystal. A physical understanding of the design principles governing these objectives is also lacking. We show that a carefully engineered CdSe/Cd,Zn,S core/shell nanocrystal uniquely meets all criteria. The key factor allowing for these improvements is the gradual core/shell boundary, which decouples the surface electronic states.