Radiative Electron-hole Recombination Research Articles

Dynamics of femtosecond lasers and induced plasma in non-Kerr nonlinear transparent materials: Competing effects of electron–hole radiative recombinations and single-electron diffusions

Modern laser micromachining utilizes ultrashort optical fields, such as femtosecond lasers, to perform high-precision processings on solid materials, including cutting, drilling, ablation, polishing, and scripturing. Femtosecond laser-based material processings can induce a plasma of free electrons whose density depends on physical phenomena such as single-electron diffusions, multiphoton ionization, and electron–hole radiative recombinations. In this work, we examine the dynamics of femtosecond lasers in transparent materials with non-Kerr nonlinearity, taking into account the generation of an electron plasma. In these specific materials, a balance between the nonlinearity and the group-velocity dispersion of the optical medium can favor the formation of optical filaments propagating with a permanent shape by virtue of their “solitonic” features. We are interested in the effects of the competition between electron–hole radiative recombination and single-electron diffusion processes on the spatiotemporal profiles of the propagating optical field and of the plasma density. The model features a complex Ginzburg–Landau equation with an optical nonlinearity of a general saturable form and a Kth-order nonlinearity term accounting for K-photon ionization processes, coupled to a rate equation for the electron plasma density where the present terms are representing avalanche ionizations, single-electron diffusion, and electron–hole radiative recombination processes. The modulational-instability analysis suggests that the continuous-wave regime will be stabilized by strong electron–hole radiative recombination processes for a fixed value of the single-electron diffusion coefficient, a stability enhanced by an increase in K. In the nonlinear regime, numerical simulations of the model equations for different combinations of the nonlinearity-saturation exponents and different values of the photon number K unveil soliton train structures forming from the laser field propagation and the time evolution of the plasma density. These structures turn out to be either dissipative soliton trains in the absence of electron–hole radiative recombinations or soliton crystals when electron–hole radiative recombination processes are taken into consideration to balance the damping effect caused by single-electron diffusions.

Amphi-Luminescent MoS2 nanostructure for photocatalytic splitting of water and removal of Methylene Blue

Chemical dyes used in the textile industries are one of the major pollutants in water. Methylene blue (MB) is a commonly seen dye that creates hazardous health problems. In this article the photocatalytic degradation of MB by the nanocatalyst MoS2 (Nano-MoS2) and carbon dot (C Dots) incorporated MoS2 (Nano-CD-MoS2) is reported. The photocatalytic degradation of MB is analyzed based on the electron-hole recombination rate of the catalyst. Photoluminescence emission exhibited by the catalyst is used as a key indicator to probe the electron-hole recombination rate. Nano-MoS2 was synthesized hydrothermally at 180 0C for 8 h from ammonium tetra thiomolybdate (ATTM). C Dot was prepared following a green root from ash guard extract which later mixed with Nano-MoS2 and kept in an autoclave at a temperature 140 °C for 4 h to get Nano-CD-MoS2. The photoluminescence (PL) and photocatalytic behavior of Nano-MoS2 and Nano-CD-MoS2 and their application for water splitting and water purification are reported. The incorporation of graphene and artificial C Dot into MoS2 nanostructures are reported to increase the conductivity and active edge sites of MoS2 that enhances the photocatalytic action. Since green C Dots are eco-friendly and easily synthesizable than artificial C Dots, as a novel study, this article investigated the influence of green C Dots on the PL and photocatalytic performance of nanosized MoS2. Nano-MoS2 and Nano-CD-MoS2 exhibited both upconversion and downconversion PL; accordingly the nanostructures were termed as amphi-luminescent. The amphi-luminescence property widens the photon absorption range and hence enhances the catalytic degradation of dyes. Nano-MoS2 which exhibited lesser intensity of amphi-luminescence emission compared to Nano-CD-MoS2 showed better results in degradation of MB. C Dots may bind with the valence band electrons of MoS2, resulting in the reduction of dangling bonds. Dangling bonds can trap photo-induced excitons to hinder the rate of electron-hole recombination. So, fast electron-hole recombination occurs in Nano-CD-MoS2 than Nano-MoS2. Fast electron-hole recombination supports radiative electron-hole recombination while suppresses the non-radiative energy transfer of electrons and causes high PL intensity. However, according to the energy level diagram, Nano-MoS2 with minimal electron-hole recombination rate is more favorable for O2/O2–,.OH/ OH– and.OH/H2O reactions that facilitate MB degradation. Photocatalytic activity of catalysts were confirmed by measuring the photocurrent from a simple custom-made two-electrode water photolysis cell where the nanocatalysts were dispersed in electrolyte. Lead and steel rods were used as electrodes. Multimeter was used to measure current. Nano-MoS2 exhibited better performance with a maximum photocurrent of 141 µA. Influence of green C Dots in energy levels, PL and photocatalysis of MoS2 and mechanisms of PL and degradation of MB are thoroughly investigated in this article.

Unveiling the Radiative Electron-Hole Recombination of MoS2 Nanostructures at Extreme pH Conditions.

Nanostructures of MoS2 are in wide research for optoelectronic, energy and biological applications. Opto-electronic and biological applications requires the tuning of photoluminescence properties of MoS2 nanostructures. In this article, nanosized MoS2 is hydrothermally synthesized, and photoluminescence at extreme pH conditions (pH 1 and 13) is examined. As the photoluminescence gives a key to probe the radiative electron-hole recombination, here, photoluminescence emissions are used as an indicator to suggest the pattern of electron-hole recombination in the material at extreme pH conditions. Raman spectroscopy, dynamic light scattering, Scanning electron microscopic image and energy dispersive x-ray analysis are done for material confirmation. At pH 1 and 13 as-synthesized nanostructured MoS2 exhibited both upconversion and downconversion photoluminescence. The intensity of photoluminescence is varied with respect to pH. Excitation-dependent photoluminescence mechanisms and preliminary understanding on the ratio of quantum yields and life span of excited state of as-synthesized nanostructured MoS2 are unveiled here.

Electron Oscillation-Induced Splitting Electroluminescence from Nano-LEDsfor Device-Level Encryption.

Data security is a major concern in digital age, which generally relies on algorithm-based mathematical encryption. Recently, encryption techniques based on physical principles are emerging and being developed, leading to the new generation of encryption moving from mathematics to the intersection of mathematics and physics. Here, device-level encryption with ideal security is ingeniously achieved using modulation of the electron-hole radiative recombination in a GaN-light-emitting diode (LED). When a nano-LED is driven in the non-carrier injection mode, the oscillation of confined electrons can split what should be a single light pulse into multiple pulses. The morphology (amplitude, shape, and pulse number) of those history-dependent multiple pulses that act as carriers for transmitted digital information depends highly on the parameters of the driving signals, which makes those signals mathematically uncrackable and can increase the volume and security of transmitted information. Moreover, a hardware and software platform are designed to demonstrate the encrypted data transmission based on the device-level encryption method, enabling recognition of the entire ASCII code table. The device-level encryption based on splitting electroluminescence provides an encryption method during the conversion process of digital signals to optical signals and can improve the security of LED-based communication.

Performance Improvement of InGaN-Based Red Light-Emitting Diodes via Ultrathin InN Insertion Layer

The serious separation of electron–hole wavefunctions, which is caused by the built-in electric field, prevents electron–hole radiative recombination in quantum wells (QWs) in high-In-content InGaN-based red light-emitting diodes (LEDs). Here, we propose a staggered structure that inserts an ultrathin InN layer in the single quantum well (SQW) to reduce the piezoelectric polarization and suppress the quantum confined Stark effect (QCSE). We have numerically simulated the effects of SQW with the InN insertion layer (IL) on the energy band structure and electron–hole wavefunctions of the red LED. Owing to alleviated piezoelectric polarization and improved overlaps of electron–hole wavefunctions, the simulation results have revealed that the internal quantum well (IQE) of the red LED with InN IL exhibits 42% higher than that of the red LED with a square-shaped QW (SSQW) at 60 A/cm2, and the efficiency droop ratio of red LED with InN IL is 48% lower than that of red LEDs with SSQW. Furthermore, we have found that the position of InN IL can affect the energy states of carriers, which has a great influence on the IQE and peak emission wavelength of red LEDs.

Lattice-matched AlInN/GaN bottom DBR impact on GaN-based vertical-cavity-surface-emitting laser diodes: systematical investigations.

In this paper, by using advanced numerical models, we investigate the impact of the AlN/GaN distributed Bragg reflector (DBR) and AlInN/GaN DBR on stimulated radiative recombination for GaN-based vertical-cavity-surface-emitting lasers (VCSELs). According to our results, when compared with the VCSEL with AlN/GaN DBR, we find that the VCSEL with AlInN/GaN DBR decreases the polarization-induced electric field in the active region, and this helps to increase the electron-hole radiative recombination. However, we also find that the AlInN/GaN DBR has a reduced reflectivity when compared with the AlN/GaN DBR with the same number of pairs. Furthermore, this paper suggests that more pairs of AlInN/GaN DBR will be set, which helps to even further increase the laser power. Hence, the 3dB frequency can be increased for the proposed device. In spite of the increased laser power, the smaller thermal conductivity for AlInN than AlN results in the earlier thermal droop in the laser power for the proposed VCSEL.

Self-starting dynamics of continuous-waves stability in a generalized complex nonlinear transparent media for passively mode-locked lasers with fast saturable absorbers

The dynamics and stability of continuous-waves in a K-order complex nonlinear transparent media for passively mode-locked lasers with fast saturable absorbers is theoretically and systematically studied. The Benjamin modulational-instability techniques was used, assuming that in the mode-locked regime, the laser will find full expression just after the continuous-waves (cws) become unstable. A global analysis of the operation regimes are established. The model is described by a K-order term generated by K-photon ionization processes of a complex Ginzburg-Landau (CGL) equation, and plasma generation coupled to a first order differential Drude’s equation for the electron plasma density with electron-hole radiative recombination effects. Basis conditions were established for various stability regime for cw. From this, we observed that an increase in the number of photon rate K induced self-starting for normal dispersion regime. We were also able to establish that an increase in the laser input field intensity which essentially favored the self-starting laser process.

Luminescence and nonlinear optical properties of stable MAPbBr3 quantum dots in SiO2 mesopores

MAPbBr3 quantum dots in SiO2 mesopores ([Formula: see text][Formula: see text]nm) were prepared by the spin-coating method, and their luminescence and nonlinear optical properties were studied by time-resolved photoluminescence (TRPL) and Z-scan techniques. The results showed that the absorption and photoluminescence peaks are at 464[Formula: see text]nm and 476[Formula: see text]nm, respectively. The TRPL spectroscopy showed two relaxation processes, a short lifetime (1.04[Formula: see text]ns) and a long lifetime (4.49[Formula: see text]ns), attributed to the trap–capture recombination and the electron–hole radiative recombination, respectively. Two-photon absorption (TPA) coefficient was 529[Formula: see text]cm/GW at 800[Formula: see text]nm. The nonlinear signal changed from TPA to saturable absorption with increase in light intensity.

Ultraweak electron-phonon coupling strength in cubic boron arsenide unveiled by ultrafast dynamics

We report a time-resolved ultrafast quasiparticle dynamics investigation of cubic boron arsenide ($c$-BAs), which is a recently discovered highly thermally conducting material. The excited-state ultrafast relaxation channels dictated by the electron-phonon coupling (EPC), phonon-phonon scattering, and radiative electron-hole recombination have been unambiguously identified, along with their typical interaction times. Significantly, the EPC strength is obtained from the dynamics, with a value of ${\ensuremath{\lambda}}_{{T}_{2}}=0.008$ (corresponding to $\ensuremath{\lambda}\ensuremath{\langle}{\mathrm{\ensuremath{\Omega}}}^{2}\ensuremath{\rangle}=1.18\ifmmode\pm\else\textpm\fi{}0.08\phantom{\rule{0.16em}{0ex}}\mathrm{p}{\mathrm{s}}^{--2}$), demonstrating an unusually weak coupling between the electrons and phonons. As a comparison, an ultraweak EPC strength for graphene is also expected. We propose that preserving an ultrasmall EPC strength may be a prerequisite for exhibiting an ultrahigh thermal conductivity. Our investigation provides insight for searching and designing ultrahigh thermal conductivity materials. Notably, during our analysis we have generalized the fluence-dependence method for obtaining the EPC strength to room temperature, which can be applied to many other types of quantum materials in the future.

Laser dynamics and stability of continuous-waves in nonlinear optical transparent medium with saturable absorber: competing effects between Kerr nonlinearity, saturable absorber, and electron–hole radiative recombination processes

Laser dynamics and stability of continuous-waves in nonlinear optical transparent medium with saturable absorber: competing effects between Kerr nonlinearity, saturable absorber, and electron–hole radiative recombination processes

Evidence of auger heating in hot carrier cooling of CsPbBr3 nanocrystals

Hot Carrier (HC) Dynamics in Lead halide perovskites shows a unique hindered hot carrier cooling. In the present study, we use femtosecond transient absorption technique to interrogate CsPbBr3 to study the role of different carrier cooling mechanisms in HC cooling process. This is accomplished here by considering fitting of the high-energy region of photoinduced bleach with a Fermi-Dirac distribution instead of Maxwell-Boltzmann Distribution. This approach reveals important time dependent parameters like quasi fermi level, apart from carrier temperature. Under low carrier densities with N < <1, the dynamics revealed bleach recovery in time scales > 1 ns revealing dynamics is governed by electron-hole radiative recombination. The HC cooling dynamics under low carrier density (2 *1017/cm3) revealed the cooling is predominantly dominated by short lifetime component of ~0.26 ps. This time is significantly elongated as compared to lifetime of longitudinal optic (LO) phonon lifetime in perovskites, clearly indicative of Froehlich interaction in play along with large polaron formation. However, under higher carrier densities > 1018/cm3, additional lifetime component contributes to HC cooling with a time constant of ~17 ps. This time constant could be assigned to Auger heating. The role of LO phonon filling and Auger heating was ascertained also on the basis energy loss rate vs carrier temperature fitting with these individual cooling models. To ascertain the role of Auger heating we also used different sized samples which showed the scaling of Auger heating times in agreement with size dependent Auger recombination times as reported in the literature. Such long HC cooling opens avenues to possibly extract hot carriers before cooling leading to development of HC photovoltaics.

Temperature Characterization of Unipolar-Doped Electroluminescence in Vertical GaN/AlN Heterostructures

An electroluminescence (EL) phenomenon in unipolar-doped GaN/AlN/GaN double-barrier heterostructures—without any p-type contacts—was investigated from 4.2 K to 300 K. In the range of 200–300 K, the extracted peak photon energies agree with the Monemar formula. In the range of 30 to 200 K, the photon energies are consistent with A-exciton emission. At 4.2 K, the exciton type likely transforms into B-exciton. These studies confirm that the EL emission comes from a cross-bandgap (or band-to-band) electron-hole radiative recombination and is excitonic. The excitons are formed by the holes generated through interband tunneling and the electrons injected into the GaN emitter region of the GaN/AlN heterostructure devices.

Laser dynamics in nonlinear transparent media with electron plasma generation: effects of electron–hole radiative recombinations

The performance of optical devices manufactured via laser micromachining on nonlinear transparent materials usually relies on three main factors, which are the characteristic laser parameters (i.e. the laser power, pulse duration and pulse repetition rate), the characteristic properties of host materials (e.g. their chromatic dispersions, optical nonlinearities or self-focusing features) and the relative importance of physical processes such as the avalanche impact ionization, multiphoton ionization and electron–hole radiative recombination processes. These factors act in conjunction to impose the regime of laser operation; in particular, their competition determines the appropriate laser operation regime. In this work a theoretical study is proposed to explore the effects of the competition between multiphoton absorption, plasma ionization and electron–hole radiative recombination processes on the laser dynamics in transparent materials with Kerr nonlinearity. The study rests on a model consisting of a K-order nonlinear complex Ginzburg–Landau equation, coupled to a first-order equation describing time variation of the electron plasma density. An analysis of the stability of continuous waves, following the modulational instability approach, reveals that the combination of multiphoton absorption and electron–hole radiative recombination processes can be detrimental or favorable to continuous-wave operation, depending on the group-velocity dispersion of the host medium. Numerical simulations of the model equations in the fully nonlinear regime reveal the existence of pulse trains, the amplitudes of which are enhanced by the radiative recombination processes. Numerical results for the density of the induced electron plasma feature two distinct regimes of time evolution, depending on the strength of the electron–hole radiative recombination processes.

Fluorescence “turn-on” of silicon-containing nanoparticles for the determination of resorcinol

A fluorescent nanosensor based on silicon-containing nanoparticles (Si CNPs) with green fluorescence (FL) was prepared by one-step method. The prepared Si CNPs emitted green FL at 470nm under the excitation at 350nm. The FL signal of Si CNPs reveals an obvious enhancement in the presence of resorcinol (RC), due to the passivation of surface trap states of Si CNPs via the binding of OH group of RC with the NH group of Si CNPs, which allowed the formation of new radiative electron-hole recombination centers. This was confirmed by some analytical experiments performed on zeta potential, FL lifetime steady state, and the FTIR spectra. Most importantly, this nanosensor could selectively determine RC with high sensitivity and without interference from hydroquinone (HQ) and catechol (CT) as RC isomers. RC was detected in the linear range 0.05-40μM, with a detection limit of 0.012μM. The synthesized nanosensor was applied to the determination of RC in fresh fruit juice and water samples. The collected results confirmed the feasibility of our approach with high accuracy.

Carrier Dynamics in Thin Germanium–Tin Epilayers

The Si-based mid-infrared photonics is an emerging technology in which group-IV germanium–tin (Ge1–xSnx) binary alloys can play a fundamental role in the development of a Si-compatible photonic components including monolithically integrated coherent light sources and detectors, on the same Si or SOI substrate. Although the Ge1–xSnx-on-Si lasers, at low temperatures, have already been demonstrated, the knowledge of the material properties necessary for such device optimization and real-life usage is very limited. In particular, carrier relaxation kinetics, relaxation pathways, and accompanied physical mechanisms, important for the laser’s dynamics, have not been subjected to in-depth research and understanding. In this work, we present detailed spectroscopic studies on photoinjected carrier dynamics in Ge1–xSnx epilayers, as a function of Sn content (6–12%) and temperature (20–300 K), by utilizing time-resolved differential reflectivity and photoluminescence. The latter technique allowed us to track separated electron and hole dynamics with a femtosecond time resolution, while the former experiment exploited a joined electron–hole recombination. This experimental approach allowed us to identify (i) two initial electron relaxation processes after photoexcitation; (ii) radiative electron–hole recombination on below-band gap states; (iii) nonradiative carrier recombination involving the Shockley–Read–Hall mechanism; and (iv) nonradiative recombination through the surface states. The research results significantly expand the knowledge on the initial carrier relaxation dynamics in the Ge1–xSnx epitaxial material. It provides unknown up-to-date kinetic parameters of the initial stage of electron relaxation and further carrier recombination dynamics, unveils the critical role of band gap inhomogeneity for the relaxation dynamics, and highlights the role of below-band gap states that can participate in the light generation process in Ge1–xSnx epilayers.

Understanding the Synergistic Effect of Device Architecture Design toward Efficient Perovskite Light‐Emitting Diodes Using Interfacial Layer Engineering

AbstractMetal halide perovskite (MHP) light‐emitting diodes (LEDs) have been widely studied and have been reached to &gt;20% external quantum efficiency, owing to their attractive characteristics (e.g., solution processability, tunable bandgap and extremely high color purity, high mobility). During the rapid development of perovskite light‐emitting diodes (PeLEDs), modifying the device architecture has been widely studied as well as improving the crystal quality of MHP to achieve near‐unity photoluminescence quantum yield. However, efforts in device architecture engineering have received less attention despite their significance. Here, strategies are reviewed to enhance the efficiency of PeLEDs in terms of the device engineering by interfacial charge injection/transport, exciton‐quenching blocking, and defect passivation layers for enhancing radiative electron–hole recombination. Strategies are systematically classified for each layer in PeLEDs and discussed the synergetic effect between different strategies. Perspective is also provided on future research on PeLEDs focusing on their architecture.

Ab initio study of oxygen segregation in silicon grain boundaries: The role of strain and vacancies

Multi-crystalline silicon is widely used for producing low-cost and high-efficiency solar cells. During crystal growth and device fabrication, silicon solar cells contain grain boundaries (GBs) which are preferential segregation sites for atomic impurities such as oxygen atoms. GBs can induce charge carriers recombination significantly reducing carrier lifetimes and therefore they can be detrimental for Si device performance. We studied the correlation between structural, energetic and electronic properties of Σ3{111} Si GB in the presence of vacancies, strain and multiple O segregation. The study of the structural and energetic properties of GBs in the presence of strain and vacancies gives an accurate description of the complex mechanisms that control the segregation of oxygen atoms. We analysed tensile and compressive strain and we obtained that local tensile strain around O impurities is very effective for segregation. We also studied the role of multiple O impurities in the presence of Si vacancies finding that the segregation is favorite for those structures which have restored tetrahedral covalent bonds. The presence of vacancies attract atomic impurities in order to restore the electronic stability: the interstitial impurity becomes substitutional. This analysis was the starting point to correlate the change of the electronic properties in Σ3{111} Si GBs with O impurities in the presence of strain and vacancies. For each structure we analysed the density of states and its projection on atoms and states, the band gaps, the segregation energy and their correlation in order to characterise the nature of new energy levels. Actually, knowing the origin of defined electronic states would allow the optimization of materials in order to reduce non radiative electron-hole recombination avoiding charge and energy losses and therefore improving solar cell efficiency.

Interfacial Thermal Conductance between Monolayer WSe2 and SiO2 under Consideration of Radiative Electron-Hole Recombination.

This work reports the interfacial thermal conductance (G) and radiative recombination efficiency (β), also known as photoluminescence quantum yield (PL QY), of monolayer WSe2 flakes supported by fused silica substrates via energy-transport state-resolved Raman (ET-Raman). This is the first known work to consider the effect of radiative electron-hole recombination on the thermal transport characteristics of single-layer transition-metal dichalcogenides (TMDs). ET-Raman uses a continuous-wave laser for steady-state heating as well as nanosecond and picosecond lasers for transient energy transport to simultaneously heat the monolayer flakes and extract the Raman signal. The three lasers induce distinct heating phenomena that distinguish the interfacial thermal conductance and radiative recombination efficiency, which can then be determined in tandem with three-dimensional (3D) numerical modeling of the temperature rise from respective laser irradiation. For the five samples measured, G is found to range from 2.10 ± 0.14 to 15.9 ± 5.0 MW m-2 K-1 and β ranges from 36 ± 6 to 65 ± 7%. These values support the claim that interfacial phenomena such as surface roughness and two-dimensional (2D) material-substrate bonding strength play critical roles in interfacial thermal transport and electron-hole recombination mechanisms in TMD monolayers. It is also determined that low-level defect density enhances the radiative recombination efficiency of single-layer WSe2.

A “Turn-on” fluorescence perovskite sensor based on MAPbBr3/mesoporous TiO2 for NH3 and amine vapor detections

Recently, lead halide perovskites have attracted great interest in toxic gas detection, but as gas sensor, it is still a challenge to achieve their high-performance. Here, we propose a novel “turn-on” fluorescence NH3/amine vapor sensor by depositing the CH3NH3PbBr3 (MAPbBr3) on the mesoporous TiO2 (mp-TiO2) layer. The fluorescence intensity of the sensor increases rapidly within seconds upon exposure to NH3 gas (72 % at 5 ppm and 988 % at 100 ppm, respectively) with excellent reversibility, high gas selectivity, humidity insensitivity, and regeneration. Moreover, the sensor can be further used for detecting various amine vapors, including methylamine, ethylamine, propylamine, butylamine, hexylamine, octylamine, and decylamine. We presumed the “turn-on” fluorescence sensing mechanism is that the trace NH3 or amine molecules could decrease the surface trap defects and facilitate the formation of passivation layer. It will impede the electron-injection into mp-TiO2, which induces the radiative electron-hole recombination and fluorescence enhancement. This research demonstrates that it is feasible to design a “turn-on” fluorescence gas sensor based on the mp-TiO2-based perovskite.

Fe(III)-C3N4 hybrids photocatalyst for efficient visible-light driven nitrogen fixation

Solar-driven reduction of nitrogen to ammonia is a promising strategy for energy storage and conversion. Herein, a new strategy of modifying Fe(III) onto carbon nitride is reported showing enhanced photocatalytic nitrogen fixation. The Fe(III) and nitrogen vacancies narrow down the carbon nitride intrinsic bandgap and suppress the radiative electron-hole recombination. Fe(III)-C3N4 therefore shows potential as a new high-efficiency artificial photosynthesis ammonia.