Nonradiative Triplet Loss Suppressed in Organic Photovoltaic Blends with Fluoridated Nonfullerene Acceptors.

In organic photovoltaic (OPV) blends, photogenerated excitons dissociate into charge-separated electrons and holes at donor/acceptor interfaces. The bimolecular recombination of spin-uncorrelated electrons and holes may cause nonradiative loss by forming the low-lying triplet excited states (T1) via the intermediate charge-transfer triplet states. Here, we show that such a spin-related loss channel can be suppressed in the OPV blends with fluorinated nonfullerene acceptors (NFAs). By combining ultrafast optical spectroscopy and triplet sensitization measurements, the T1 states at the acceptors have been observed to generate from the charge-separated electrons and holes in the OPV blends with a same polymer donor and two sets of NFAs with and without fluorination. The triplet formation is largely suppressed and the lifetime of charge carrier is markedly prolonged in the blends with fluorinated NFAs. The fluorination effect on the charge dynamics can be ascribed to the modified energy alignment between the triplet excited states of charge-transfer and locally excited characters as supported by quantum chemical computation. Our findings explain the mechanism responsible for the improved photocurrent generation in the OPV blends with fluorinated NFAs, suggesting that manipulating the energy landscape of triplet excited states is a promising strategy for further optimizing OPV devices.

The Scarier the Better: Maximizing Exposure Therapy Outcomes for Spider Fear.

BACKGROUND: While exposure therapy effectively reduces anxiety associated with specific phobias, not all individuals respond to treatment and some will experience a return of fear after treatment ceases. AIMS: This study aimed to test the potential benefit of increasing the intensity of exposure therapy by adding an extra step that challenged uncontrollability (Step 15: allowing a spider to walk freely over one's body) to the standard fear hierarchy. METHOD: Fifty-one participants who had a severe fear of spiders completed two 60-min exposure sessions 1 week apart in a context that was either the same or different from the baseline and follow-up assessment context. Participants were categorized into groups based on the last hierarchy step they completed during treatment (Step 14 or fewer, or Step 15). RESULTS: Those who completed Step 15 had greater reductions in fear and beliefs about the probability of harm from baseline to post-treatment than those who completed fewer steps. Although completing Step 15 did not prevent fear from returning after a context change, it allowed people to maintain their ability to tolerate their fear, which earlier steps did not. Despite some fear returning after a context change, individuals who completed Step 15 tended to report greater reductions in fear from baseline to the follow-up assessment than participants who completed 14 or fewer steps. CONCLUSIONS: Overall, these results suggest that more intensive exposure that directly challenges harm beliefs may lead to greater changes in fear and fear beliefs than less intensive exposure.

Highly Flexible and Efficient All-Polymer Solar Cells with High-Viscosity Processing Polymer Additive toward Potential of Stretchable Devices.

Considering the potential applications of all-polymer solar cells (all-PSCs) as wearable power generators, there is an urgent need to develop photoactive layers that possess intrinsic mechanical endurance, while maintaining a high power-conversion efficiency (PCE).Herein a strategy is demonstrated to simultaneously control the intercalation behavior and nanocrystallite size in the polymer-polymer blend by using a newly developed, high-viscosity polymeric additive, poly(dimethylsiloxane-co-methyl phenethylsiloxane) (PDPS), into the TQ-F:N2200 all-PSC matrix. A mechanically robust 10wt% PDPS blend film with a great toughness was obtained. Our results provide a feasible route for producing high-performance ductile all-PSCs, which can potentially be used to realize stretchable all-PSCs as a linchpin of next-generation electronics.

Composite Nd:YAG-SiC-bonding laser with orthogonal-linear-polarization output.

We report a multiple-gain-element Nd:YAG laser where the gain media (three pieces of slab crystal) are alternately bonded to two optical quality 4H-SiC wafers. Such composite gain configuration can efficiently remove waste heat from the gain medium, preventing thermal lensing and heat-induced birefringence/distortion under high power laser operation. Through near Brewster's angles incidence designing and polarization discrimination, two orthogonally linearly polarized (P and S polarized) laser beams are generated simultaneously from different parts of the same system. Based on a T = 3% output coupler, this continuous wave laser produces maximum power of 5.34 W (0.83 W) with a slope efficiency of 21.1% (3.6%) for the S (P) polarized laser beam. At the 5-W level, the S polarized beam has a beam quality of M2~1.2. The wavelengths of these two perpendicularly polarized laser beams differ about 0.6 nm (1063.7 and 1064.3 nm). Polarized output behavior dependent on the output-coupler transmission is also studied, and it is found that increasing the transmission leads to steady growth of the P polarized laser beam; when a T = 1.3% output coupler is adopted, more than 99% of the output is the S polarized beam. The highest total output power is 6.75 W obtained with the T = 1.3% output coupler, corresponding to slope efficiency of 25.7%. This composite laser scheme, bonding multiple gain media with high-thermal-conductivity materials, opens a new avenue for high-power high-beam-quality solid-state lasers with multiple-polarization output beams.

Passively Q-switched Ho,Pr:LiLuF4 laser with graphitic carbon nitride nanosheet film.

A few-layer graphitic carbon nitride (g-CN) nanosheet film on an yttrium aluminum garnet substrate was fabricated and employed as saturable absorber for a passively Q-switched Ho,Pr:LiLuF4 laser at 2.95 µm. Under an absorbed pump power of 3.89 W at a pump wavelength of 1.15 µm, a maximum average output power of 101 mW was realized with a pulse duration of 420 ns and a repetition rate of 93 kHz. Even shorter pulse durations of 385 ns were obtained at a reduced output coupler transmission.

Diode-end-pumped Ho, Pr:LiLuF4 bulk laser at 2.95 µm.

A diode-end-pumped continuous-wave (CW) and passively Q-switched Ho, Pr:LiLuF4 (Ho, Pr:LLF) laser operation at 2.95 µm was demonstrated for the first time, to the best of our knowledge. The maximum CW output power was 172 mW. By using a monolayer graphene as the saturable absorber, the passively Q-switched operation was realized, in which regimes with the highest output power, the shortest pulse duration, and the maximum repetition rate were determined to be 88 mW, 937.5 ns, and 55.7 kHz, respectively. The laser beam quality factor M2 at the maximum CW output power were measured to be Mx2=1.48 and My2=1.47.

Developing carbon-nitride nanosheets for mode-locking ytterbium fiber lasers.

Graphitic carbon nitrides (CNs) have appeared as a new type of photocatalyst for water splitting, but their optical properties (e.g., nonlinear absorption), to the best of our knowledge, have been seldom explored. Here, we report the saturable absorption effects of novel 2D carbon-nitride-type nanosheets and use them as saturable absorbers to passively mode-lock Yb-doped fiber lasers. The CN-based saturable absorber is manufactured by solution coating of 2D CN nanosheets on a gold mirror and has a modulation depth and saturation intensity of 12.5% and 7.5 MW/cm2, respectively. Two different output couplers are employed to construct ring laser cavities. With the 10% coupler, the mode-locked fiber laser produces pulses with duration of ∼310 ps, average power of 1.24 mW, and repetition rate of 7.65 MHz. The laser spectrum is centered at 1066 nm with a bandwidth of 2.4 nm. Increasing the coupling ratio to 50% improves the output power to 2.58 mW but at the same time broadens the pulse width to 420 ps. As a new kind of 2D material with strong saturable absorption, CN nanosheets will open a new way for novel photonic and optoelectronic devices.

Precise manipulation on spike train of uneven duration or delay pulses with a time grating system.

In this paper, we proposed a time grating system to achieve spike train of uneven duration or delay (STUD) pulses, and theoretically study their features under various modulation conditions. This time grating scheme, which is a temporal analogy of spatial grating, introduces great degree of freedom for controlling the output pulse characteristics (pulse width, repetition rate, pulse shape, etc.) through simply tuning the electronics elements and the programmable phase modulation function. The narrowest pulse width is highly determined by the modulation parameters and the branch number N, and the numerically obtained value is around tens of femtoseconds in the current case. When super-Gaussian pulses are modulated with an optimized and modified trapezoidal function, the pulse rising/falling edge can be greatly compressed to form a clean nearly-square wave (with edges less than 10 fs). STUD pulses generated with this time grating system have high-degree controllability and are very beneficial for suppressing parametric instabilities in laser driven inertial confinement fusion.

High power tandem-pumped thulium-doped fiber laser.

We propose a cascaded tandem pumping technique and show its high power and high efficient operation in the 2-µm wavelength region, opening up a new way to scale the output power of the 2-µm fiber laser to new levels (e.g. 10 kW). Using a 1942 nm Tm(3+) fiber laser as the pump source with the co- (counter-) propagating configuration, the 2020 nm Tm(3+) fiber laser generates 34.68 W (35.15W) of output power with 84.4% (86.3%) optical-to-optical efficiency and 91.7% (92.4%) slope efficiency, with respect to launched pump power. It provides the highest slope efficiency reported for 2-µm Tm(3+)-doped fiber lasers, and the highest output power for all-fiber tandem-pumped 2-µm fiber oscillators. This system fulfills the complete structure of the proposed cascaded tandem pumping technique in the 2-µm wavelength region (~1900 nm → ~1940 nm → ~2020 nm). Numerical analysis is also carried out to show the power scaling capability and efficiency of the cascaded tandem pumping technique.

Developing high-power hybrid resonant gain-switched thulium fiber lasers.

In this paper, we propose hybrid-pumped resonant gain-switched thulium fiber lasers to realize high-average-power and high-pulse-energy 2-µm laser emissions. Based on numerical simulation, laser dynamics (pulse peak power, pulse energy, pulse duration, etc.) of this kind of laser system are investigated in detail. By taking advantages of the 793 nm continuous wave pump and the 1900 nm pulsed pump, performance of the laser emission can be significantly improved, with the highest average power of 28 W, peak power of 3.5 kW, pulse energy of 281 µJ, and narrowest pulse duration of 92 ns, all of which can be further optimized through designing the cavity parameters and the pumping circumstance. Compared with the pump pulses, two times improvement in pulse energy and average power has been achieved. This hybrid resonant gain-switched system has an all-fiber configuration and high efficiency (low heat load), and can be steadily extended into the cladding pump scheme, thus paving a new way to realize high power (>100 W average power) and high pulse energy (>1 mJ) 2 µm thulium fiber lasers.

High signal-to-noise ratio, single-frequency 2 µm Brillouin fiber laser.

A high optical signal-to-noise ratio (OSNR) single-frequency 2 µm Brillouin fiber laser (BFL) with watt-level output and high transfer efficiency is demonstrated for the first time to the best of our knowledge. The Brillouin pump is constructed with a two-stage thulium-doped fiber amplifier (TDFA) seeded by a 2 µm laser diode, providing 4.02 W average power with 1 MHz linewidth. Using an optimized length of 14 m for the Brillouin ring cavity, the BFL works stably in single-mode region with 8 kHz linewidth because of the linewidth narrowing effect. The transfer efficiency is 51% with 1.08 W output power and 62 dB OSNR for 3.22 W pump power.

Efficient 1856 nm emission from Tm,Mg:LiNbO3 laser.

Efficient continuous-wave laser emission at 1856 nm from a Tm,Mg:LiNbO(3) crystal slab with high Tm(3+) doping concentration is reported. A maximum output power of 2.62 W is realized with a slope efficiency of 19.6% and the beam quality factor M(2) of 1.7 at room temperature. We believe that this is the first demonstration of watt-level laser operation in Tm,Mg:LiNbO(3) crystal and the output power is four orders of magnitude higher than that reported previously in Tm-doped LiNbO(3) crystal. Performance degradation due to the photorefractive effect under high intensity 1856 nm laser is not observed thanks to the co-doping of magnesium ions. Quantitative analysis about the long-term photorefractive effect is also provided. Multi-wavelength laser operation is realized by using different narrow-band output couplers. This demonstration opens up a viable pathway towards 2-µm integrated optic devices for achieving laser oscillation, electro-optic and nonlinear optical effects within just one sample simultaneously.

Efficient Ho:LuLiF4 laser diode-pumped at 1.15 µm.

We report the first laser operation based on Ho(3+)-doped LuLiF(4) single crystal, which is directly pumped with 1.15-µm laser diode (LD). Based on the numerical model, it is found that the "two-for-one" effect induced by the cross-relaxation plays an important role for the laser efficiency. The maximum continuous wave (CW) output power of 1.4 W is produced with a beam propagation factor of M(2) ~2 at the lasing wavelength of 2.066 µm. The slope efficiency of 29% with respect to absorbed power is obtained.

Efficient dual-wavelength Nd:LuLiF4 laser.

We report an efficient continuous-wave (CW) and passively Q-switched dual-wavelength Nd:LuLiF4 laser at 1314 and 1321 nm for the first time. Maximum CW output power of 6.08 W is obtained, giving an optical-to-optical conversion efficiency of 30.2% and a slope efficiency of 32.1%. Even using high doping V3+:YAG as the saturable absorber to passively Q-switch the laser, stable dual-wavelength operation remains. Maximum pulse energy extracted from the resonator is 108.7 µJ at 17.2 kHz pulse-repetition rate, and maximum peak power is 885 W.

High-power narrow-bandwidth thulium fiber laser with an all-fiber cavity.

We report diode pumped high power 2-µm Tm(3+) fiber lasers with an all-fiber configuration. The all-fiber configuration is completed by specially designed fiber Bragg gratings with similar structure parameters matched to the gain fiber. The maximum output power is 137 W with an optical-to-optical slope efficiency of 62% with respect to absorbed 793-nm pump power. The laser wavelength is stabilized at ~2019 nm with a spectral linewidth less than 3 nm across all output levels. To the best of our knowledge, this is the highest 2-µm laser output from a single narrow bandwidth all-fiber laser system.

Targeting mitochondrial one-carbon enzyme MTHFD2 together with pemetrexed confers therapeutic advantages in lung adenocarcinoma.

Metabolic remodeling is the fundamental molecular feature of malignant tumors. Cancer cells require sufficient energy supplies supporting their high proliferative rate. MTHFD2, a mitochondrial one-carbon metabolic enzyme, is dysregulated in several malignancies and may serve as a promising therapeutic candidate in cancer treatment. Here, our data confirmed that MTHFD2 gene and protein was upregulated in the cancerous tissues of LUAD patients and was correlated with a poor survival in LUAD. MTHFD2 was involved in lung cancer cell proliferation, migration, and apoptosis by mediating its downstream molecules, such as DNA helicases (MCM4 and MCM7), as well as ZEB1, Vimentin and SNAI1, which contributed to tumor cell growth and epithelial-to-mesenchymal transition (EMT) process. Moreover, we identified that miRNA-99a-3p appeared to be an upstream mediator directly regulating MTHFD2 and MCM4 expression. Moreover, specific inhibition of MTHFD2 functions by siRNA or a chemical compound, improved anti-tumor sensitivities induced by pemetrexed in LUAD. Taken together, our study revealed the underlying molecular mechanisms of MTHFD2 in regulating cell proliferation and identified a novel therapeutic strategy improving the treatment efficacies in LUAD.

Mother-to-infant transmission of hepatitis B virus: a Chinese experience.

Over 90% of infants infected with hepatitis B virus (HBV) caused by mother-to-infant transmission will evolve to carrier status, and this cannot be prevented until widespread administration of the HB vaccine and hepatitis B immune globulin (HBIG) is implemented. This prospective study of 214 infants born to HBsAg-positive mothers was carried out to determine if either perinatal or intrauterine HBV transmission could be effectively prevented with HBIG and the HB vaccine. Peripheral blood was collected from mothers and from newborns before they received HBIG and the HB vaccine, as well as at 0, 1, 7, 24, and 36 months after birth. Infants born with an ratio of signal to noise(S/N) value of >5 for HBsAg (ABBOTT Diagnostic Kit) were defined as mother-to-infant transmission cases, those with an S/N between 5 and 50 were classified as perinatal transmission cases, and those with an S/N >50 were considered intrauterine transmission cases. Mother-to-infant transmission occurred in approximately 4.7% (10/214) of the infants; the perinatal transmission and intrauterine transmission rates were 3.7% (8/214) and 0.9% (2/214), respectively. The risk of mother-to-infant transmission increased along with maternal HBeAg or HBVDNA levels. After 36 months of follow-up, all perinatal cases became HBsAg-negative, whereas all intrauterine transmission cases evolved into carrier status. These results indicate that infants infected via intrauterine transmission cannot be effectively protected by HBIG and HB vaccine.

Continuous wave and passively Q-switched Nd:Lu xY1-x VO4 laser at 1.34 µm with V3+:YAG as the saturable absorber.

We demonstrated a diode-pumped continuous wave and passively Q-switched Nd:Lu(x)Y(1-x)VO4 laser at 1.34 µm with V3+:YAG as the saturable absorber. The crystal Nd:Lu(x)Y(1-x)VO4 with equal ionic ratio (x = 0.5) shows better laser performance. The maximum continuous wave output power of 1.45 W was obtained with the optical efficiency of 20.1% and the slope efficiency of 24.5%. For the pulsed operation, the minimum pulse width achieved was 42 ns with the pulse repetition frequency of 142 kHz, and the single pulse energy and the peak power were estimated to be 6.62 µJ and 157.6 W, respectively.

Efficient Q-switched Tm:YAG ceramic slab laser.

Characteristics of Tm:YAG ceramic for high efficient 2-µm lasers are analyzed. Efficient diode end-pumped continuous-wave and Q-switched Tm:YAG ceramic lasers are demonstrated. At the absorbed pump power of 53.2W, the maximum continuous wave (cw) output power of 17.2 W around 2016 nm was obtained with the output transmission of 5%. The optical conversion efficiency is 32.3%, corresponding to a slope efficiency of 36.5%. For Q-switched operation, the shortest width of 69 ns was achieved with the pulse repetition frequency of 500 Hz and single pulse energy of 20.4 mJ, which indicates excellent energy storage capability of the Tm:YAG ceramic.

High-power air-cooled SiC-clad Nd:YVO4 slab lasers.

We demonstrate a diode-pumped, air-cooled, 100 W class SiC-clad Nd:YVO(4) active slab laser based on diffusion bonding of two SiC plates to a thin Nd:YVO(4) slab. We obtained 83 W of cw output power with a slope efficiency of 27% without water cooling. This demonstration initiates a novel (to the best of our knowledge) cooling design for efficient removal of waste heat generated from the diode edge-pumped high-power slab laser at room temperature.