Electron-hole Plasma Recombination Research Articles

Lasing emission from ZnO hierarchical spherical microcavity

We investigated lasing characteristics of ZnO hierarchical micro-spherical particles with diameters of 1–5 μm . The lasing emission consists of a small number of discrete laser peaks unlike conventional random lasing from ZnO nanopowder-assembly. Theoretical calculations based on a many-body theory reveal that the optical gain is achieved at the observed lowest lasing threshold value, 4 mJ⋅cm−2⋅pulse−1 , which corresponds to the excited carrier density ∼2.4×1025 m−3 . Because the carrier density is much higher than the Mott density, the gain origin for lasing is electron-hole plasma recombination. The lasing frequency mode shift ( ∼1.2 meV ) is due to the refractive index change induced by exciting high carrier density up to 6.1×1025 m−3 . By changing hierarchical structures via controlling growth condition and performing the annealing treatment and performing the calculation of scattering efficiency of ZnO particles, we found that the hierarchy of the micro-spherical particle plays a crucial role of the lasing feedback: the strong light scattering at the interface between outer nanoparticles with the sizes of 100–200 nm and smaller ones with the sizes of 10–60 nm consisting in the micro-spherical particle results in a light confinement. Furthermore, it has been confirmed that each discrete lasing mode shows strong gain competition with each other probably due to spatial overlap between the modes. These results suggest that both random scattering and microcavity modes contribute to the lasing oscillation.

Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2.

The density-driven transition of an exciton gas into an electron-hole plasma remains a compelling question in condensed matter physics. In two-dimensional transition metal dichalcogenides, strongly bound excitons can undergo this phase change after transient injection of electron-hole pairs. Unfortunately, unavoidable nanoscale inhomogeneity in these materials has impeded quantitative investigation into this elusive transition. Here, we demonstrate how ultrafast polarization nanoscopy can capture the Mott transition through the density-dependent recombination dynamics of electron-hole pairs within a WSe2 homobilayer. For increasing carrier density, an initial monomolecular recombination of optically dark excitons transitions continuously into a bimolecular recombination of an unbound electron-hole plasma above 7 × 1012 cm-2. We resolve how the Mott transition modulates over nanometer length scales, directly evidencing the strong inhomogeneity in stacked monolayers. Our results demonstrate how ultrafast polarization nanoscopy could unveil the interplay of strong electronic correlations and interlayer coupling within a diverse range of stacked and twisted two-dimensional materials.

Solution-processed whispering-gallery-mode microsphere lasers based on colloidal CsPbBr3 perovskite nanocrystals

Perovskite nanocrystals (NCs) have emerged as attractive gain materials for solution-processed microlasers. Despite the recent surge of reports in this field, it is still challenging to develop low-cost perovskite NC-based microlasers with high performance. Herein, we demonstrate low-threshold, spectrally tunable lasing from ensembles of CsPbBr3 NCs deposited on silica microspheres. Multiple whispering-gallery-mode lasing is achieved from individual NC/microspheres with a low threshold of ∼3.1 μJ cm−2 and cavity quality factor of ∼1193. Through time-resolved photoluminescence measurements, electron–hole plasma recombination is elucidated as the lasing mechanism. By tuning the microsphere diameter, the desirable single-mode lasing is successfully achieved. Remarkably, the CsPbBr3 NCs display durable room-temperature lasing under ∼107 shots of pulsed laser excitation, substantially exceeding the stability of conventional colloidal NCs. These CsPbBr3 NC-based microlasers can be potentially useful in photonic applications.

A Unified Pervasive Linebroadening Function for Quantum Wells in Light Emitting Diodes

The broadening functions for quantum wells in LEDs and laser diodes below the lasing threshold are examined. Inhomogeneous and homogeneous broadening mechanisms are included. Hydrogen-atom-like exciton and the electron-hole plasma recombination models are considered. Material disorder and the Urbach tail are reviewed as the main reasons for the inhomogeneous broadening. Charge carrier scattering and relaxation times in the conduction and valence bands are examined as the origin for the homogeneous lifetime broadening. Two homogeneous lineshapes are compared using the momentum relaxation times obtained from the electron and hole mobilities available for GaAs. In addition to crystal disorder, the mutual collision of charge carriers in the quantum wells is examined as the reason for the relaxation time shortening. The analogy to pressure broadening in gases is used to combine the proper homogeneous and inhomogeneous broadening functions to a unified quantum well lineshape.

Robust single-mode lasers based on hexagonal CdS microflakes

We have achieved single-mode whispering-gallery-mode lasing in CdS microflakes with sharp linewidth (∼0.12 nm) and high quality factor (∼4200). Such lasers are superior to previous CdS lasers in these lasing parameters. Through time-resolved photoluminescence measurements, electron–hole plasma recombination is established to be the lasing mechanism. The radiative recombination rate of CdS microflakes is enhanced by a factor of ∼4.7 due to the Purcell effect.

Double threshold behavior in a resonance-controlled ZnO random laser

We observed unusual lasing characteristics, such as double thresholds and blue-shift of lasing peak, in a resonance-controlled ZnO random laser. From the analysis of lasing threshold carrier density, we found that the lasing at 1st and 2nd thresholds possibly arises from different mechanisms; the lasing at 1st threshold involves exciton recombination, whereas the lasing at 2nd threshold is caused by electron-hole plasma recombination, which is the typical origin of conventional random lasers. These phenomena are very similar to the transition from polariton lasing to photon lasing observed in a well-defined cavity laser.

Stimulated emission via electron-hole plasma recombination in fully strained single InGaN/GaN heterostructures

The optical properties of fully coherently grown single InGaN/GaN heterostructures for 12<In%<17 were investigated under low and high density excitations. At lower density, S-shape temperature dependence of the main emission peak, associated with localized exciton recombination was observed. The activation energy of the localized excitons remains invariable in the given range of In-contents. Most interestingly, under high density pulse-excitations, stimulated emission by electron-hole plasma recombination was observed for temperatures up to 295 K.

Low threshold, single-mode laser based on individual CdS nanoribbons in dielectric DBR microcavity

Abstract Single-mode lasers with low threshold are attractive for their potential applications in many areas, such as optical communication, signal processing and displays. Here we report a nanoscale single-mode laser with CdS nanoribbons (NRs) sandwiched between two dielectric distributed Bragg reflectors (DBRs). Under optical pumping, the band edge emission of CdS ribbons can be effectively confined and give lasing in the DBR microcavity, with the lasing threshold as low as ~8 μJ/cm 2 , which is one order lower than those of bare CdS ribbons for comparison. More importantly, the CdS-DBR laser can realize single-mode emission, for the length of the resonance cavity can support a mode spacing larger than the bandwidth of the optical gain. Light polarization measurements demonstrate the single-mode lasing has a polarization degree as high as 97.4%. Time-resolved photoluminescence (TRPL) measurements further reveal that the lasing action comes from the electron-hole plasma recombination process. Moreover, the wavelength of the lasing mode can be broadly tuned by changing the thickness of CdS NRs. This low-threshold single-mode laser may find applications in highly integrated photonics devices and systems.

Origins of lasing emission in a resonance-controlled ZnO random laser

We investigate the origins of lasing emission in a scatterer-resonance-controlled random laser made of ZnO nanopowder over a wide temperature range (20–300 K). At higher temperatures ( K), the lasing emission appears around exciton recombination energies and the lasing threshold carrier density is comparable to the Mott density, indicating that the resonance-controlled random laser is going toward showing excitonic lasing; at lower temperatures, random lasing is caused by usual electron–hole plasma recombination because of the threshold carrier density being much larger than the Mott density.

Electron-hole plasma lasing in a ZnO random laser

We investigate the random lasing behaviors of ZnO nanopowder over a wide temperature range from 20 to 300 K. The temperature dependencies of the lasing peak energy and threshold carrier density are measured. It is found that these dependencies agree very well with theoretical calculations based on quantum many-body theory. We conclude from the analysis of the temperature-dependent lasing characteristics that the lasing emission of the ZnO nanopowder originates from the recombination of electron-hole plasma over the entire temperature range investigated.

Effects of Hydrogen Treatment and Air Annealing on Ultrafast Charge Carrier Dynamics in ZnO Nanowires Under in Situ Photoelectrochemical Conditions

ZnO nanowires (NWs) grown by the hydrothermal method on fluorine-doped tin oxide glass substrate were characterized by scanning electron microscopy, optical absorption, photoelectrochemical photocurrent density, and ultrafast transient absorption pump probe spectroscopy. The as-grown NWs were annealed in air, pure hydrogen atmosphere, or annealed first with air annealing followed by hydrogen treatment. By TA spectroscopy, the samples exhibited a triple exponential transient bleach recovery with lifetimes on the fast (10–15 ps), medium (100–200 ps), and slow (>1 ns) time scale, attributed to shallow donor mixed with electron hole plasma recombination, donor bound recombination, and donor–acceptor pair recombination (DAP), respectively. The as-grown samples were dominated by donor and DAP recombination. Air annealing improved the crystal structure but had little effect on hole trapping. Significantly, the hydrogen-treated NWs showed a reduction in hole trapping and DAP recombination. Hole trapping was attributed to zinc vacancies (VZn) and hydrogen was proposed to passivate these defects. The photocurrent density of the air annealed and cotreated NWs were measured, the latter of which showed improved performance which was attributed to, in part, decreased hole trap states and improved electrical conductivity. In situ ultrafast TA spectroscopy was used to study the photoanodes under working conditions as a function of applied bias. For both samples, the medium time constant became faster with increasing applied bias. A model was proposed to extract the electron–hole separation time constant.

Hydrothermal fabrication of well-ordered ZnO nanowire arrays on Zn foil: room temperature ultraviolet nanolasers

Well-ordered nanowires of the hexagonal wurtzite ZnO having an average diameter of 80 nm, a typical length of 12 μm, and a mean packing density of 7.5 nanowires μm−2 have been directly grown on Zn foil in a preferred [0001] direction by a hydrothermal process and employed for room temperature ultraviolet nanolasers. The lasing action of arrayed ZnO nanowires has been observed from 370 to 400 nm with threshold irradiance of 25 kW cm−2. Photoluminescence decays biexponentially: the fast component is attributed to free-exciton decay, and the slow one is to bound-exciton decay. The amplitude of the fast component increases whereas its lifetime decreases with the increment of threshold irradiance, suggesting that ZnO nanowire arrays undergo a change in the lasing mechanism from exciton–exciton scattering to electron–hole plasma recombination.

All-Optical High-Resolution Nanopatterning and 3D Suspending of Graphene

We introduce a laser-based technique capable of both imaging and patterning graphene with high spatial resolution. Both tasks are performed in situ using the same confocal microscope. Imaging graphene is based on the recombination of a laser-created electron-hole plasma yielding to a broadband up- and down-converted fluorescence. Patterning is due to burning graphene by local heating causing oxidation and conversion into CO(2). By shaping the laser beam profile using 1D phase-shifting plates and 2D vortex plates we can produce graphene dots below 100 nm in diameter and graphene nanoribbons down to 20 nm in width. Additionally, we demonstrate that this technique can also be applied to freely suspended graphene resulting in freely suspended graphene nanoribbons. We further present a way of freely hanging graphene vertically and imaging it in 3D. Taking advantage of having vertically hanging graphene for the first time, we measure the out-of-plane anisotropy of the upconversion fluorescence.

Ultrafast exciton dynamics in ZnO: Excitonic versus electron-hole plasma lasing

The use of ZnO bulk and especially nanolayer and nanowire structures for novel device applications has led to a renewal of interest in high-electron-density processes in ZnO, such as those occurring during lasing in ZnO. Using a pump-probe reflectometry technique, we investigate the ultrafast exciton dynamics of bulk ZnO under femtosecond laser excitation close to lasing conditions. Under intense excitation by 266-nm femtosecond (fs) pump pulses, the exciton resonance becomes highly damped and does not recover for several picoseconds. This slow recovery indicates a significant screening of the Coulomb interaction. Even below the lasing thresholds typically found for ZnO nanolayers and nanowires, we observe damping of the exciton resonance for several picoseconds, which indicates that the primary mechanism for lasing in ZnO induced by femtosecond laser pumping is electron-hole plasma recombination.

Electroluminescence from monocrystalline silicon solar cell

The band edge emission with the peak at 1.15 μm is observed at room temperature from monocrystalline silicon solar cell at forward bias. The electroluminescence spectra can be fitted by electron hole plasma recombination model. The temporal response of electroluminescence is used to characterize the minority carrier lifetime by fitting the time evolution of radiative recombination using the Shockley–Read–Hall, radiative, and Auger recombination models. The minority carrier lifetime is almost constant (1.8 ms) for excess carrier density lower than 4×1015 cm−3, and then decreases at higher concentration.

Effect of chemical etching on the luminescent properties of zinc oxide nanorods

Vertically aligned ZnO nanorods grown on (100) Si substrates have been found to have a polycrystalline zinc oxide underlayer. After etching in a hydrochloric acid solution, the nanorods had smaller dimensions and pointed ends. As shown by exciton spectroscopy, the nanorods had a higher structural perfection and better luminescent properties in comparison with the underlayer. The 4.2-K luminescence spectra of the nanorods exhibit violet emission due to bound and interacting excitons. The low-temperature edge emission of the ZnO underlayer is shown to consist of lines due to electron-hole plasma recombination.

Piezoelectric gate for a two-dimensional electron system transport in LiNbO/sub 3/-GaAs/AlGaAs sandwich structures

Standing-wave piezoelectric fields in the LiNbO(3) driving plate are used to form depleted and accumulated electron densities in GaAs/AlGaAs quantum wells (QWs). The photoluminescence spectrum of the two-dimensional electron system varies both spatially and temporally, exhibiting an electron-hole plasma recombination and exciton and trion emissions at large and small electron densities, respectively. Controlling the piezoelectric field component perpendicular to the QW layers offers a versatile tool to achieve the spatially indirect exciton luminescence in double QW structures.

Stimulated emission at the second order stop-zone edge of the two-dimensional opal–zinc oxide photonic crystal

The fabrication of the 2D periodic structures in ZnO thin films by magnetron sputtering on the opal matrices was developed. The microstructures were characterized by AFM and SEM. The spontaneous and stimulated emissions of the ZnO layers on opal were studied at N 2 laser excitation ( λ = 337 nm). The stimulated emission near 397 nm was observed at room temperature from ZnO–opal structure. The threshold of the electron–hole plasma recombination laser process was ≅300 kW/cm 2 for this structure. This threshold is two orders of magnitude smaller of that one for the flat ZnO–SiO 2 films owing to DFB resonator effect in 2D structure.

Tuning the exciton luminescence in an acoustically depleted two-dimensional electron gas

Standing-wave piezoelectric fields can be used to vary spatially and temporally charge conditions in $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ quantum wells (QWs), offering a versatile tool to control the two-dimensional electron gas (2DEG) density in the well. A $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ piezoelectric resonator imparts a MHz-frequency oscillating piezoelectric field with a controllable ratio of the in-plane and the vertical field components to a 2DEG placed in close proximity to the plate surface. This allows us to dynamically tune the charge state in the plane of the QW and to influence the photoluminescence spectra. It is found that spatially distributed regions of depleted and accumulated electron densities are formed in the QW plane due to the in-plane components of the piezoelectric field. The photoluminescence spectrum then varies both spatially and temporally, exhibiting an electron-hole plasma recombination at large electron densities and exciton and trion emissions at the extreme of small densities. Controlling the piezoelectric field component perpendicular to the QW layers allows us to achieve the spatially indirect exciton luminescence in double quantum well structures.

Characteristic properties of ZnO random laser pumped by nanosecond pulses

Investigations of UV radiation spectra of powdered zinc oxide and its unordered film were conducted. The samples were pumped by 3-rd harmonics of the two-stage Nd:YAG-laser (λ=355 nm) with pulse duration ∼10 ns. Maximum density of energy of pumping pulses was about 150 mJ/cm2. With some of our samples, we achieved lasing. The threshold values of the pumping energy density occurred higher than those under picosecond pumping approximately for one-two orders of magnitude. It was shown that the threshold of lasing and the character of lasing spectra depend on the morphology of particles forming a powder. The long-wavelength shift of the maximum of lasing spectra with a rise of the pumping was observed. It was interpreted as a result of participation of electron–hole plasma recombination in the process of lasing. A chaotic character of lasing spectra was observed and partially analyzed.