Spontaneous Recombination Rate Research Articles

Impact of Carrier Transport and Capture on VCSEL Dynamics

Using a vertical-cavity surface-emitting laser (VCSEL) equivalent circuit model based on two carrier rate equations to include effects of carrier dynamics, we study the impact of carrier transport and capture on the small- and large-signal modulation response of high-speed VCSELs. The model also accounts for parasitics, current-induced self-heating, and gain compression. A variation of the effective capture time from 1 to 15 ps is found to have a large impact on the small-signal modulation response, with the 3 dB bandwidth decreasing from 40 to 15 GHz and the response transitioning from under-damped to over-damped. This is primarily due to the increasing low frequency parasitic-like roll-off with increasing effective capture time. A significant effect on the optical waveforms produced by the VCSEL under 56 Gbit/s on-off keying (OOK) non-return-to-zero (NRZ) and pulse-amplitude modulation 4 (PAM4) modulation is observed, with a short effective capture time leading to horizontal eye closure caused by timing jitter (TJ) and intersymbol interference (ISI) and a long effective capture time leading to vertical eye closure caused by long rise- and fall-times. However, for high modulation speed, a short effective capture time is needed and the photon lifetime should be set for clear eye opening. We also show the impact of the effective capture time on the output power vs current characteristics and map the dependence of internal temperature, carrier densities, carrier escape and leakage rates, and spontaneous recombination rates on current for different effective capture times.

Intrachromosomal Recombination in Yeast.

Spontaneous and induced mitotic recombinations are driven by lesions such as single-strand nicks and gaps and double-strand breaks in the genome. For regions of the genome that are not repetitive, spontaneous recombination rates are too low to be detected by simple screening and require reporters where a recombination product can be selected. This chapter describes commonly used types of reporters where a gene is duplicated as direct repeats and both copies are mutated with different mutations, rendering the cell defective for the gene and auxotrophic for the gene product. Recombination between the two defective copies can result in a wild-type gene and a prototrophic phenotype for the cell. Methods to use these types of reporters to determine recombination rates between the two gene copies are described, and their use in monitoring both increased and decreased recombinations is discussed.

Identifying the variables that drive tamoxifen-independent CreERT2 recombination: Implications for microglial fate mapping and gene deletions.

Ligand-dependent Cre recombinases such as the CreERT2 system allow for tamoxifen-inducible Cre recombination. Important examples are the Cx3cr1-CreERT2 and Sall1-CreERT2 lines that are widely used for fate mapping and gene deletion studies of brain macrophages. Our results now show that both CreERT2 lines can exhibit a high rate of tamoxifen-independent "leaky" excision with some reporter strains, while this is not observed with others. We suggest that this disparity is determined by the length of the floxed transcriptional STOP cassette that is incorporated in the various reporter lines. In addition, the rate of spontaneous recombination was also determined by the CreERT2 expression levels and the longevity of the CreERT2-expressing cells. The implications of these results are discussed in the context of fate mapping and inducible gene deletion studies in macrophages and microglia.

Influence of quantum well design on light polarization switching in AlGaN ultraviolet emitters

Polarization properties from AlGaN quantum well (QW) strongly determine the efficiency of deep ultraviolet (UV) light-emitting diodes (LEDs), hence knowing the critical Al-content at which the light polarization switches is essential for high-efficiency deep UV LED designs. This work theoretically investigates the influence of QW design on the light polarization switching in AlGaN-based UV LEDs. The physics analysis by using the self-consistent 6-band k·p model shows that the Al-content for valence subbands crossover presents an increasing trend as AlGaN QW thickness increases with consideration of polarization electric field, carrier screening effect and strain state. On the other hand, the critical Al-content where the transverse-electric-polarized spontaneous emission recombination rate (Rsp) is equal to the transverse-magnetic-polarized Rsp has the maximum value at the QW thickness of ∼1.5 nm. The difference between the two types of critical Al-contents can be explained by the quantum confined stark effect and the band mixing effect. The light polarization properties from reported AlGaN-based UV emitters show a similar trend to our theoretical results on critical Al-contents, indicating the importance on the understanding of QW design for high-efficiency deep-UV emitters.

Nanostructuring Multilayer Hyperbolic Metamaterials for Ultrafast and Bright Green InGaN Quantum Wells.

Semiconductor quantum well (QW) light-emitting diodes (LEDs) have limited temporal modulation bandwidth of a few hundred MHz due to the long carrier recombination lifetime. Material doping and structure engineering typically leads to incremental change in the carrier recombination rate, whereas the plasmonic-based Purcell effect enables dramatic improvement for modulation frequency beyond the GHz limit. By stacking Ag-Si multilayers, the resulting hyperbolic metamaterials (HMMs) have shown tunability in the plasmonic density of states for enhancing light emission at various wavelengths. Here, nanopatterned Ag-Si multilayer HMMs are utilized for enhancing spontaneous carrier recombination rates in InGaN/GaN QWs. An enhancement of close to 160-fold is achieved in the spontaneous recombination rate across a broadband of working wavelengths accompanied by over tenfold enhancement in the QW peak emission intensity, thanks to the outcoupling of dominating HMM modes. The integration of nanopatterned HMMs with InGaN QWs will lead to ultrafast and bright QW LEDs with a 3 dB modulation bandwidth beyond 100 GHz for applications in high-speed optoelectronic devices, optical wireless communications, and light-fidelity networks.

Effect of InGaN thickness on assisted trap recombination and behaviour of InGaN/AlGaN double heterostructure LED

This work is dedicated to the study of InGaN based LED on the thickness variation effect. The operating voltage, total emission rate, efficiency droop and spontaneous recombination rate was improved by increasing the thickness of InGaN layer of InGaN/AlGaN electron blocking layer (EBL). Based on optical and electrical results, the thicker InGaN layer could have lower operating voltage and higher total emission rate. LED with 5nm thick InGaN layer also could prevent electron leakage into p-region and improve hole injection efficiency. As a result it decreases 60% of efficiency droops and reduces the trap assisted recombination in InGaN/AlGaN active light emitting region. This indicates that InGaN layer may decrease the non-radiative recombination

Activation of Checkpoint Kinase Chk1 by Reactive Oxygen Species Resulting from Disruption of wat1/pop3 in Schizosaccharomyces pombe.

DNA double-strand breaks are critical lesions that can lead to chromosomal aberrations and genomic instability. In response to DNA damage, Chk1, a serine/threonine kinase, is responsible for cell cycle arrest to prevent damaged cells from progressing through the cell cycle. Here, we report that the disruption of wat1, a WD repeat-containing protein, leads to the phosphorylation of Chk1. The double-deletion of chk1 and wat1 had a grave effect on the survival of fission yeast cells, and the spontaneous recombination rate was also high upon double-deletion of wat1 and chk1, as compared to the single-mutant. In the absence of wat1, the cells exhibited a high level of nuclear fragmentation that resulted in the accumulation of Rad22 yellow fluorescent protein foci. Furthermore, we show that wat1 is required for the regulation of the oxidative stress response. We observed elevated levels of reactive oxygen species (ROS) generation in wat1-null mutant that led to a high degree of propidium iodide staining at nonpermissive temperature. Based on the results presented here, we hypothesize that ROS production in wat1-null mutant cells generates DNA fragmentation that could trigger a checkpoint response and that, in the absence of checkpoint kinase Chk1, the cells exhibit severe growth defects leading to a synthetic lethal phenotype.

Potential Pitfalls of the Mx1-Cre System: Implications for Experimental Modeling of Normal and Malignant Hematopoiesis.

SummaryConditional knockout mice are commonly used to study the function of specific genes in hematopoiesis. Different promoters that drive Cre expression have been utilized, with the interferon-inducible Mx1-Cre still being the most commonly used “deleter strain” in experimental hematology. However, different pitfalls associated with this system could lead to misinterpretation in functional studies. We present here two of these issues related to the use of Mx1-Cre: first, a high spontaneous recombination rate when applying commonly used techniques in experimental hematology, and second, undesired short-term consequences of the use of polyinosinic:polycytidylic acid, including changes in cellular phenotypes that, however, resolve within days. Our studies emphasize therefore that proper controls are crucial when modeling gene deletion using the Mx1-Cre transgene.

Novel Methods of Genetic Modification of Human Pluripotent Stem Cells

Genomic engineering has enormous potential along basic research, drug discovery and cell therapeutics. Many existing methods for targeted gene knockout mutagenesis or integration rely on homologous recombination. The low rate of spontaneous recombination in nearly all mammalian cell types, as well as the scale of screening, effort and time required to isolate the targeted events during genome modification, have hindered progress in this field. The present review has the objective to present latest improvements of technology to develop genetic modification to a clinical grade level so it can be used in human therapy. Keywords: Genomic editing, naive pluripotent stem, patient-specific, pluripotent, stem cell/genetic therapy.

Antenna-coupled photon emission from hexagonal boron nitride tunnel junctions.

The ultrafast conversion of electrical signals to optical signals at the nanoscale is of fundamental interest for data processing, telecommunication and optical interconnects. However, the modulation bandwidths of semiconductor light-emitting diodes are limited by the spontaneous recombination rate of electron-hole pairs, and the footprint of electrically driven ultrafast lasers is too large for practical on-chip integration. A metal-insulator-metal tunnel junction approaches the ultimate size limit of electronic devices and its operating speed is fundamentally limited only by the tunnelling time. Here, we study the conversion of electrons (localized in vertical gold-hexagonal boron nitride-gold tunnel junctions) to free-space photons, mediated by resonant slot antennas. Optical antennas efficiently bridge the size mismatch between nanoscale volumes and far-field radiation and strongly enhance the electron-photon conversion efficiency. We achieve polarized, directional and resonantly enhanced light emission from inelastic electron tunnelling and establish a novel platform for studying the interaction of electrons with strongly localized electromagnetic fields.

Characterization of InN - In0.25Ga0.75N Quantum Well Laser Structure for 1330 nm Wavelength

In this work, a nitride based wurtzite-strained single quantum well laser has been designed and characterized at 1330 nm wavelength. Here, 12Å InN layer has been used as well material and 15Å In0.25Ga0.75N layer has been used as barrier material along with GaN as separate confinement heterostructure for better carrier and optical confinement. To determine the electronic properties of the designed structure, a developed self-consistent method has been solved. Conduction subband calculation has been performed with the solution of Schrodinger equation formed by single band Hamiltonian followed by Poisson's equation. 6-bands k . p Hamiltonian for wurtzite semiconductor has been solved for valence band Schrodinger's equation with Poisson's equation considering band mixing effect, strain due to lattice mismatch and spontaneous and piezoelectric polarization. The interband momentum matrix elements, optical gain, spontaneous emission rate, spontaneous radiative recombination rate and radiative current density have been calculated to analyze the optical properties of the designed laser and a good agreement with previously published works is obtained. All analysis has been performed using injection carrier density of 6×10-19 cm-3 at T = 300 K. Due to the strain effect, the wave-function overlap integral has been obtained as 43.27%. From the electronic properties analysis, it has been found that, the designed structure is TE polarized with C1-HH1 and C1-LH1 dominating transitions. From the analysis of optical properties, the obtained spontaneous emission rate is 5945.2 s-1 at 1329.55 nm and spontaneous emission rate per energy interval per unit volume is 7.21×1027 s-1cm-3eV-1 at same wavelength. The obtained radiative recombination rate is 7.77×1029 s-1cm-3 and radiative current density is 149.19 Acm-2. The optical gain for TE polarization of the designed QW structure is 5261.52cm-1 at 1336.7 nm. Figure 1

Nonequilibrium plasmons with gain in graphene

Graphene supports strongly confined transverse-magnetic sheet plasmons whose spectral characteristics depend on the energetic distribution of Dirac particles. The question arises whether plasmons can become amplified when graphene is pumped into a state of inversion. In establishing a theory for the dynamic non-equilibrium polarizability, we are able to determine the exact complex-frequency plasmon dispersion of photo-inverted graphene and study the impact of doping, collision loss, and temperature on the plasmon gain. We calculate the spontaneous emission spectra and carrier recombination rates self-consistently and compare the results with approximations based on Fermi's golden rule. Our results show that amplification of plasmons is possible under realistic conditions but inevitably competes with ultrafast spontaneous emission, which for intrinsic graphene, is a factor 5 faster than previously estimated. This work casts new light on the nature of non-equilibrium plasmons and may aid the experimental realization of active plasmonic devices based on graphene.

Surface Plasmon Enhanced Hot Exciton Emission in Deep UV‐Emitting AlGaN Multiple Quantum Wells

The surface plasmon (SP)‐exciton coupling effect has attractive potential applications in nitride semiconductor‐based light‐emitting diodes (LEDs) with improved performances by enhancing the spontaneous radiative recombination rate, especially for AlGaN‐based deep UV LEDs with an intrinsic low internal quantum efficiency (IQE). In this work, significant deep UV emission enhancement is demonstrated on AlGaN‐based multiple quantum well (MQW) structure by introducing the coupling of local surface plasmons (LSPs) from Al nanoparticles (NPs) with the excitons in AlGaN MQWs. By precisely manipulating the size of the Al NPs, the evolution of emission wavelength and enhancement ratio for the SP‐coupled AlGaN MQWs along with the change of the LSP properties is investigated in detail. With respect to the bare MQWs, strong emission enhancements accompanied by large shifts towards shorter wavelengths are observed from the AlGaN MQW structure decorated with Al NPs, which is proposed to originate from the suppressing of the ground state exciton emission and the strongly enhanced emission from the high‐order QW exciton states by the LSP–exciton coupling process. Theoretical calculations confirm that the higher energy sub‐band e2–hh2 transition from the MQWs is the maximum enhanced emission channel through the SP coupling process.

Genome stability in the uvh6 mutant of Arabidopsis thaliana

Plant XPD homolog UVH6 is the protein involved in the repair of strand breaks, and the excision repair and uvh6 mutant is not impaired in transgenerational increase in HRF. While analyzing the transgenerational response to stress in plants, we found that the promoter and gene body of Arabidopsis thaliana (Arabidopsis) XPD homolog UVH6 underwent hypomethylation and showed an increase in the level of transcript. Here, we analyzed the mutant of this gene, uvh6-1, by crossing it to two different reporter lines: one which allows for analysis of homologous recombination frequency (HRF) and another which makes it possible to analyze the frequency of point mutations. We observed that uvh6-1 plants exhibited lower rate of spontaneous homologous recombination but higher frequencies of spontaneous point mutations. The analysis of strand breaks using ROPS and Comet assays showed that the mutant had a much higher level of strand breaks at non-induced conditions. Exposure to stresses such as UVC, heat, cold, flood and drought showed that the mutant was not impaired in an increase in somatic HRF. The analysis of spontaneous HRF in the progeny of control plants compared to that of the progeny of stressed plants demonstrated that uvh6-1 was mildly affected in response to temperature, UV and drought. Our data suggest that UVH6 may be involved in the repair of strand breaks and excision repair, but it is unlikely that UVH6 is required for transgenerational increase in HRF.

An approach to achieve significantly faster luminescence decay of thin-film scintillator by surface plasmons

A fast component of 2.2 ns from the LSO thin-film scintillator was achieved through coupling of scintillator with surface plasmons of silver nanoparticles. From the emission spectra, the observed fast component is from the transition of 5d to 4f level of Ce3+ in LSO. The fast component is attributed to the enhanced spontaneous recombination rate due to the surface plasmons. The present demonstration provides an interesting approach to improve the timing resolution of scintillator, which is distinguished from these conventional methods.

Photoluminescence, recombination rate, and gain spectra in optically excited n-type and tensile strained germanium layers

We theoretically investigate the optical properties of photo-excited biaxially strained intrinsic and n-type doped Ge semi-infinite layers using a multi-valley effective mass model. Spatial inhomogeneity of the excess carrier density generated near the sample surface is considered. Strain effects on the band edges, on the band dispersions, and on the orbital compositions of the near gap states involved in radiative recombinations are fully taken into account. We obtain, as a function of the distance from the sample surface, the energy resolved absorption/gain spectra resulting from the contribution of the radiative direct and phonon-assisted band-to-band transitions and from the intra-band free carrier absorption. Photoluminescence spectra are calculated from the spatially dependent spontaneous radiative recombination rate, taking into account energy-dependent self-absorption effects. For suitable combinations of doping density, strain magnitude, pump power, and emitted photon polarization, we find gain values up to 5800 cm−1.

Tensile-strain and doping enhanced direct bandgap optical transition of n+ doped Ge/GeSi quantum wells

Band structures of tensile strained and n+ doped Ge/GeSi quantum wells (QWs) are calculated by multiple-band k·p method. The energy dispersion curves of the Γ and L conduction subbands are obtained. The effects of tensile strain and n+ doping in Ge on direct bandgap optical gain and spontaneous radiative recombination rate spectra are investigated including the electron leakage from Γ to L conduction subbands. Our results show that the optical gain and spontaneous radiative recombination rate can be significantly increased with the tensile strain, n-type doping concentration, and injection carrier density in the Ge QW. The free carrier absorption is calculated and cannot be ignored because of the heavily doped Ge. The pure TM mode polarized net optical gain up to 1153 cm−1 can be achieved for the Ge/Ge0.986Si0.014 QW with tensile strain of 1.61% and n-type doping concentration of 30 × 1018 cm−3.

Role of Cdc48/p97 as a SUMO-targeted segregase curbing Rad51–Rad52 interaction

Cdc48 (also known as p97), a conserved chaperone-like ATPase, plays a strategic role in the ubiquitin system. Empowered by ATP-driven conformational changes, Cdc48 acts as a segregase by dislodging ubiquitylated proteins from their environment. Ufd1, a known co-factor of Cdc48, also binds SUMO (ref. 6), but whether SUMOylated proteins are subject to the segregase activity of Cdc48 as well and what these substrates are remains unknown. Here we show that Cdc48 with its co-factor Ufd1 is SUMO-targeted to proteins involved in DNA double-strand break repair. Cdc48 associates with SUMOylated Rad52, a factor that assembles the Rad51 recombinase on chromatin. By acting on the Rad52-Rad51 complex, Cdc48 curbs their physical interaction and displaces the proteins from DNA. Genetically interfering with SUMO-targeting or segregase activity leads to an increase in spontaneous recombination rates, accompanied by aberrant in vivo Rad51 foci formation in yeast and mammalian cells. Our data thus suggest that SUMO-targeted Cdc48 restricts the recombinase Rad51 by counterbalancing the activity of Rad52. We propose that Cdc48, through its ability to associate with co-factors that have affinities for ubiquitin and SUMO, connects the two modification pathways for protein degradation or other regulatory purposes.

Modeling of current gain compression in common emitter mode of a transistor laser above threshold base current

We have obtained the expressions for the terminal currents in a heterojunction bipolar transistor laser the base of which contains a quantum well (QW). The emitter-base junction is assumed to be abrupt, leading to abrupt discontinuity in quasi-Fermi level at the interface. The expressions for the terminal currents as a function of collector-emitter and base-emitter voltages are obtained from the solution of the continuity equation. The current density in the QW located at an arbitrary position in the base is related to the virtual state current density. The threshold current density in the QW is calculated by using the expression for gain obtained from Fermi golden rule. The plot of collector current (IC) versus collector-emitter voltage (VCE) for different values of base current shows the usual transistor characteristics, i.e., a rising portion after a cut-in VCE, and then a saturation behavior. The dc current gain remains constant. However, as the base current exceeds the threshold, a stimulated recombination rate is added to the spontaneous recombination rate and the plots of collector currents become closer for the same increase in base current. This current gain compression is in agreement with the experimental observation. Our calculated values qualitatively agree with other experimental findings; however some features like Early effect do not show up in the calculation.

Energy efficient microcavity lasers with 20 and 40 Gb/s data transmission

Microcavity lasers (μCLs), reduced-size (≲3 μm aperture) vertical cavity surface-emitting lasers (VCSELs) defined by the buried-oxide process for current and field confinement (thus wide mode spacing), are demonstrated with low threshold current, sharp turn-on L-I characteristics, and wide bandwidth operation. Due to the enhanced spontaneous recombination rate at reduced mode and improved photon density, μCLs exhibit lower charge-field resonance peaks at a modulation bandwidth f−3 dB=18.7 GHz, thus permitting open-“eye” operation at 20 and 40 Gb/s data rates (I≲3 mA). The energy efficiency for 20 Gb/s data transmission is measured to be 4.84 Gb/s/mW, which is eight times better than 7 μm aperture VCSELs.