Core/Shell ZnO/ZnS Nanoparticle Electron Transport Layers Enable Efficient All-Solution-Processed Perovskite Light-Emitting Diodes.

Solution-processed perovskite-based light-emitting diodes (PeLEDs) are promising candidates for low-cost, large-area displays, while severe deterioration of the perovskite light-emitting layer occurs during deposition of electron transport layers from solution in an issue. Herein, core/shell ZnO/ZnS nanoparticles as a solution-processed electron transport layer in PeLED based on quasi-2D PEA2 Csn-1 Pbn Br3n+1 (PEA = phenylethylammonium) perovskite are employed. The deposition of ZnS shell mitigates trap states on ZnO core by anchoring sulfur to oxygen vacancies, and at the same time removes residual hydroxyl groups, which helps to suppress the interfacial trap-assisted non-radiative recombination and the deprotonation reaction between the perovskite layer and ZnO. The core/shell ZnO/ZnS nanoparticles show comparably high electron mobility to pristine ZnO nanoparticles, combined with the reduced energy barrier between the electron transport layer and the perovskite layer, improving the charge injection balance in PeLEDs. As a result, the optimized PeLEDs employing core/shell ZnO/ZnS nanoparticles as a solution-processed electron transport layer exhibit high peak luminance reaching 32 400 cd m-2 , external quantum efficiency of 10.3%, and 20-fold extended longevity as compared to the devices utilizing ZnO nanoparticles, which represents one of the highest overall performances for solution-processed PeLEDs.

In-Situ Chemical Thinning and Surface Doping of Layered Bi2Se3.

As a promising topological insulator, two-dimensional (2D) bismuth selenide (Bi2Se3) attracts extensive research interest. Controllable surface doping of layered Bi2Se3 becomes a crucial issue for the relevant applications. Here, we propose an efficient method for the chemical thinning and surface doping of layered Bi2Se3, forming Se/Bi2Se3 heterostructures with tunable thickness ranging from a few nanometers to hundreds of nanometers. The thickness can be regulated by varying the reaction time and large-size few-layer Bi2Se3 sheets can be obtained. Different from previous liquid-exfoliation methods that require complex reaction process, in-situ and thickness-controllable exfoliation of large-size layered Bi2Se3 can be realized via the developed method. Additionally, the formation of Se nanomeshes coated on the Bi2Se3 sheets remarkably enhance the intensity of Raman vibration peaks, indicating that this method can be used for surface-enhanced Raman scattering. The proposed chemical thinning and surface-doping method is expected to be extended to other bulk-layered materials for high-efficient preparation of 2D heterostructures.

All-Inorganic Quantum Dot Light-Emitting Diodes with Suppressed Luminance Quenching Enabled by Chloride Passivated Tungsten Phosphate Hole Transport Layers.

Although excellent performance such as high efficiency and stability have been achieved in quantum dot (QD)-based light-emitting diodes (QLEDs) possessing an organic/inorganic hybrid device structure, the highly expected all-inorganic QLEDs remain at the bottleneck stage in recent years, resulting from the luminance quenching of QDs caused by inorganic hole transport layer (HTL) and unbalanced charge injection due to large energy barrier for injecting holes from HTL to QDs. Here, it is reported that the solution-processed inorganic environmentally friendly chloride (Cl)-passivated tungsten phosphate (Cl@TPA) films serve as HTL. The incorporation of Cl in TPA effectively passivates the oxygen vacancies, which not only avoids the luminescence quenching of QDs by reducing carrier concentration but also facilitates the hole injection from HTL to QDs with a favorable electronic band alignment, thus achieving the record external quantum efficiency of ≈9.27%, among all previous reports about all-inorganic QLEDs. Most importantly, the resulting all-inorganic QLEDs with Cl@TPA exhibit a substantial improvement in the operational lifetime (T50  > 105 h under an initial luminance of 100 cd m-2 ), which is almost 30-fold higher than the devices with TPA HTL. This work furnishes a promising strategy for highly efficient and stable QLEDs based on inorganic device structure.

High-power tunable mid-infrared fiber gas laser source by acetylene-filled hollow-core fibers.

High-power tunable pulsed and CW mid-infrared fiber gas laser sources in acetylene-filled hollow-core fibers, to the best of our knowledge, are demonstrated for the first time. By precisely tuning the wavelength of the pump source, an amplified tunable 1.5 µm diode laser, to match different absorption lines of acetylene, the laser output is step-tunable in the range of 3.09~3.21 µm with a maximum pulse average power of ~0.3 W (~0.6 µJ pulse energy) and a maximum CW power of ~0.77 W, making this system the first watt-level tunable fiber gas laser operating at mid-infrared range. The output spectral and power characteristics are systemically studied, and the explanations about the change of the ratio of the P over R branch emission lines with the pump power and the gas pressure are given, which is useful for the investigations of mid-infrared fiber gas lasers.

Ionization degree measurement in the gain medium of a hydrocarbon-free rubidium vapor laser operating in pulsed and CW modes.

Although the diode pumped alkali laser (DPAL) works in a three-level scheme, higher energy-state excitation and ionization processes exist during operation, which may lead to deleterious effects on laser performance. In this paper, we report the ionization degree measurement in the gain medium of an operational hydrocarbon-free Rb DPAL by using the optogalvanic method. The results show that, at the pulsed mode with a duration of ~1 ms, a maximal ionization degree of ~0.06% is obtained at a pump power of 140 W. While in the CW mode, the plasma reaches an ionization degree as high as ~2% at a pump power of 110 W, which is mainly due to the enough time for sufficient plasma development. A comparison with our previous work [Opt. Lett.39, 6501 (2014)] as well as modeling results is made and discussed. The influences of different population transfer channels on laser performance are simulated and analyzed. The results show that, for a typical hydrocarbon-free Rb laser (pump intensity of 15 kW/cm2, helium pressure of 10 atm and cell temperature of 438 K), all the high-energy excitation effects give an overall negative influence on laser efficiency of ~3.78%, while the top two influencing channels are the photoionization (~1.8%) and the energy pooling (~1.53%). The work in this paper experimentally reveals the influence of the macroscopic ionization evolution process on an operational DPAL for the first time, which would be helpful for a more comprehensive understanding of the physics in DPALs.

Real-time measurement of temperature rise in a pulsed diode pumped rubidium vapor laser by potassium tracing atom based absorption spectroscopy.

In this paper, we first propose and demonstrate a novel tracing atom based absorption spectroscopy method for the real-time measurement of the temperature rise inside the pump region of a pulsed diode pumped alkali laser (DPAL). By artificially adding potassium atoms into the gain medium of an operational rubidium laser, the information of the temperature rise can be obtained from the variation of the potassium absorption signal. Some important influencing factors are studied. Typical results show that, as the pump power (2 ms duration) increases from 22 W to 92 W, the temperature rise increases from 103 K to 227 K. As the pulse duration increases from 1ms to 5 ms, the temperature rise increases from 128 K to 314 K, and the heat relaxation time increases from 3.8 ms to 8.1 ms. The method is favored for its ability for real-time detection and high sensitivity, which provides a useful way for DPAL diagnostics.

High efficiency quasi-three level thin film laser enabled on a sinusoidal grating substrate.

The excitation and emission properties of optical materials can be adjusted by nanostructures and to achieve high optical efficiency in the optically pump laser, especially for lasers with short absorption length and high pump threshold. We present and theoretically investigate a Yb-doped thin film on a 1D grating structure in this paper. High reflectivity at the pump and emission wavelength are realized simultaneously and in terms of the guided-mode resonance theory, the local field of high reflected light is enhanced which will increase the absorption of associated laser wavelength. We analyze parameters of the nanostructure in detail based on rigorous coupled-wave theory and an appropriate structure is decided. Finally, we demonstrate that this designed structure can effectively improve the optical efficiency of optically pump solid state laser.

Method for remotely measuring the spin-damping time of mesospheric sodium.

By using the return flux enhancement factor due to repumping, the spin-damping time of mesospheric sodium can be estimated. Because the absolute return photon number does not need to be measured, this method is independent of sodium abundance. An example of how to find the spin-damping time using this method is given. As a result, it is shown that this method is sensitive and has the potential to improve the precision of the spin-damping time estimations of mesospheric sodium. Finally, the impact of the geomagnetic field on this method is analyzed.

Fluorescence enhancing mechanism of optical repumping in sodium atoms for brighter laser guide star.

With quantum Bloch equations and sodium cell based experiment, we systematically resolved optical repumping mechanism of sodium atoms, a path to brighter sodium laser guide star (SLGS) that would be accepted by worldwide observatories. Besides the former studies, we detailed the population distribution of sodium atoms with and without repumping, which makes the repumping mechanism easy to understand. Experimental results based on a buffer gas free sodium cell and a single frequency laser implies that the optimum repumping frequency offset is 1712 MHz, and the repumping power fraction should be 10-18%. The light intensity depended character of repumping is also carried out. We learned that future SLGS pumped by higher effective light intensity would benefit more from repumping. To the best of our knowledge, this is the first systematic investigation of repumping mechanism using a single mode sum-frequency laser, which gives thorough support for previous numerical work and shows that the lab bench experiment could be used as an intermediate link between theoretical modeling and on-sky test.

Simulation of diffraction effects of anti-reflection microstructures used as intra-cavity optical elements.

The anti-reflection microstructures (ARMs) have been widely used due to their many advantages as compared with traditional films, such as high transmittance at required wavelength, high damage threshold, and resistance to corrosive environments etc. Some recent applications use ARMs as intra-cavity optical elements in laser systems. Due to the presence of microstructures, ARMs may also add diffraction effects on the features of the transmitted laser beams or the resonator's eigenmodes. In this paper, we simulate the diffraction effects of ARMs that used as intra-cavity optical elements, and propose some further considerations.

Experimental research of a chain of diode pumped rubidium amplifiers.

In this paper, we have set up a diode pumped rubidium MOPA system with a chain of two amplifiers. The experimental results show an amplified laser power of 26W with amplification factor of 16.3 and power extraction efficiency of 53% for a single amplifier, and an amplified laser power of 11W with amplification factor of 7.9 and power extraction efficiency of 26% for a chain of two amplifiers. The reason for lower performance of cascade amplification is mainly due to the limited total pump power, which will be not sufficient for efficient pumping when assigned from a single amplifier into two amplifiers. The situation could be well improved by increasing the seed laser power as well as the pump power for each amplifier to realize high efficient saturated amplification. Such MOPA configuration has the potential for scaling high beam quality alkali laser into high powers.

Experimental measurement of ionization degree in diode-pumped rubidium laser gain medium.

We use optogalvanic method to measure the ionization degree in the hydrocarbon-free rubidium DPAL gain medium. The results show that the ionization degree increases as pump intensity and helium pressure increase, and presents rollover as rubidium concentration increases. A maximal ionization degree of ∼6.45×10(-6) has been obtained with pump intensity of 0.82 kW/cm2, temperature of 150°C and helium pressure of 500 Torr. Theoretical estimation shows that energy pooling is the main process rather than photoexcitation for the subsequent ionization processes.

Study on photoionization in a rubidium diode-pumped alkali laser gain medium with the optogalvanic method.

We use the optogalvanic method to calculate the concentration of rubidium ions produced by photoionization in a Rb diode-pumped alkali laser gain medium. With bias voltage added across the electrodes of a rubidium hollow cathode lamp, the measured optogalvanic current is 2.3×10(-7) A. Further study shows that the rubidium ion concentration is proportional to the pump intensity, and the drift velocity of rubidium ions is proportional to the bias voltage. When the photoionization process reaches dynamic equilibrium, the rubidium ion concentration will not increase with growing rubidium atom density. The calculated rubidium ion concentration is 1.5×10(5)-10(6) according to the experiment, and the ionization degree is less than 2.4×10(-7).

Modeling of an optically side-pumped alkali vapor amplifier with consideration of amplified spontaneous emission.

Diode pumped alkali vapor amplifier (DPAA) is a potential candidate in high power laser field. In this paper, we set up a model for the diode double-side-pumped alkali vapor amplifier. For the three-dimensional volumetric gain medium, both the longitudinal and transverse amplified spontaneous emission (ASE) effects are considered and coupled into the rate equations. An iterative numerical approach is proposed to solve the model. Some important influencing factors are simulated and discussed. The results show that in the case of saturated amplification, the ASE effect can be well suppressed rather than a limitation in power scaling of a DPAA.