Wenfei Zhang

An efficient and stable fluorescent graphene quantum dot–agar composite as a converting material in white light emitting diodes

Graphene quantum dots (GQDs) have attracted great attention due to their unique optoelectronic properties. There remains a critical challenge to utilize the water-soluble GQDs for device applications. Here we report a facile method to fabricate a GQD–agar composite. The composite exhibits excellent optical stability and no luminescence quenching is observed. The composite is successfully applied as a colour converting material in blue light-emitting diodes (LEDs) to achieve white light emission. The luminous efficiency and light conversion efficiency of the white LED are 42.2 lm W−1 and 61.1% respectively. The light conversion efficiency of the WLED is stable for over 100 hours of continuous operation.

Observation of Lasing Emission from Carbon Nanodots in Organic Solvents

Lasing is observed from carbon nanodots (C-dots) dispersed into a layer of poly(ethylene glycol) coated on the surface of optical fibers under 266 nm optical excitation. This is due to the enhancement of photoluminescence intensity via the esterification of carboxylic groups of the C-dots, and the formation of high-Q cylindrical microcavities to support second-type whispering gallery modes.

Confining energy migration in upconversion nanoparticles towards deep ultraviolet lasing

Manipulating particle size is a powerful means of creating unprecedented optical properties in metals and semiconductors. Here we report an insulator system composed of NaYbF4:Tm in which size effect can be harnessed to enhance multiphoton upconversion. Our mechanistic investigations suggest that the phenomenon stems from spatial confinement of energy migration in nanosized structures. We show that confining energy migration constitutes a general and versatile strategy to manipulating multiphoton upconversion, demonstrating an efficient five-photon upconversion emission of Tm3+ in a stoichiometric Yb lattice without suffering from concentration quenching. The high emission intensity is unambiguously substantiated by realizing room-temperature lasing emission at around 311 nm after 980-nm pumping, recording an optical gain two orders of magnitude larger than that of a conventional Yb/Tm-based system operating at 650 nm. Our findings thus highlight the viability of realizing diode-pumped lasing in deep ultraviolet regime for various practical applications.

Phonon-Assisted Population Inversion in Lanthanide-Doped Upconversion Ba2LaF7 Nanocrystals in Glass-Ceramics

The effective population inversion of 2 H11/2 from 4 S3/2 state of Er3+ ions can be achieved through the annihilation of phonons; random lasing action from BLF films embedded with Yb3+ /Er3+ codoped BLF nanocrystals is demonstrated and high ambient temperature (>433 K) operation lasers with a very low excitation threshold (<530 nJ cm-2 ) are realized.

Wide-bandwidth lasing from C-dot/epoxy nanocomposite Fabry–Perot cavities with ultralow threshold

We show that by maximizing the amount of organosilane functional groups, the quantum yield of surface functionalized carbon nanodots (C-dots) dispersed in epoxy can be enhanced to 68% under optical excitation at 450 nm wavelength. This is the highest quantum yield ever recorded for C-dots at such an excitation wavelength. Lasing emission can also be demonstrated from a Fabry–Perot cavity by using C-dots as the gain medium. The lasing threshold is found to be ∼200 W cm−2 which is 2 orders of magnitude lower than that provided in the recent reports. Furthermore, tunable single-mode lasing over a bandwidth of ∼60 nm wide is achieved from the Fabry–Perot cavity in the Littrow configuration.

Random lasing in Eu3+ doped borate glass-ceramic embedded with Ag nanoparticles under direct three-photon excitation

We report the observation of random lasing from Eu(3+) doped borate glass ceramic films embedded with Ag nanoparticles through three-photon absorption at room temperature. Under 1179 nm ultrashort femtosecond pulse excitation, discrete sharp peaks with linewidth ∼0.4 nm emerge randomly from a broad emission band with peak wavelength at ∼612 nm. In addition, the number of sharp peaks increases with the increase of excitation power. We also show that the emission spectrum varies with different observation angles and the corresponding lasing threshold is dependent on the excitation area. Hence, we verify unambiguously that the Eu(3+) doped borate glass ceramic film supports random lasing action via three-photon absorption excitation. In addition, Ag nanoparticles, which act as light scatterers, allow the formation of random microcavities inside the bulk film.

Observation of bubble-involving spontaneous gas dissolution in superheated Al alloy melt

We present a direct visualization of spontaneous gas dissolution in Al-7.7 mass% Ca eutectic alloy melt during superheating using high-brilliance synchrotron X-ray imaging. A bubble-involving gas dissolution process was observed, which can be understood within the framework of adsorption-diffusion-dissolution mechanism. The heterogenous nucleation and combined effect of hydrogen diffusivity and solubility results in the growth of individual bubbles in a stochastic way with Gaussian distribution. This also applies to the behavior of group bubbles in early stage, while which in final stage can be treated as reverse Ostwald ripening dominated by Lifshitz-Slyozov-Wagner diffusion mechanism when pure diffusive condition is satisfied.

Large-area highly crystalline WSe2 atomic layers for ultrafast pulsed lasers

Large-area and highly crystalline transition metal dichalcogenides (TMDs) films possess superior saturable absorption compared to the TMDs nanosheet counterparts, which make them more suitable as excellent saturable absorbers (SA) for ultrafast laser technology. Thus far, the nonlinear optical properties of large-scale WSe2 and its applications in ultrafast photonics have not yet been fully investigated. In this work, the saturable absorption of chemical vapor deposition (CVD) grown WSe2 films with large-scale and high quality are studied and the use of WSe2 films as a broadband SA for passively mode-locked fiber lasers at both 1.5 and 2 μm ranges is demonstrated. To enhance the light-material interaction, large-area WSe2 film is tightly transferred onto the side wall of a microfiber to form a hybrid structure, which realizes strong evanescent wave interaction between light and WSe2 film. The integrated microfiber-WSe2 device shows a large modulation depth of 54.5%. Using the large-area WSe2 as a mode-locker, stable soliton mode-locked pulse generation is achieved and the pulse durations of 477 fs (at 1.5 μm) and 1.18 ps (at 2.0 μm) are demonstrated, which suggests that the large-area and highly crystalline WSe2 films afford an excellent broadband SA for ultrafast photonic applications.

Large-area and highly crystalline MoSe2 for optical modulator

Transition metal dichalcogenides (TMDs) have been successfully used as broadband optical modulator materials for pulsed fiber laser systems. However, the nonlinear optical absorptions of exfoliated TMDs are strongly limited by their nanoflakes morphology with uncontrollable lateral size and thickness. In this work, we provide an effective method to fully explore the nonlinear optical properties of MoSe2. Large-area and high quality lattice MoSe2 grown by chemical vapor deposition method was adopted as an optical modulator for the first time. The large-area MoSe2 shows excellent nonlinear optical absorption with a large modulation depth of 21.7% and small saturable intensity of 9.4 MW cm-2. After incorporating the MoSe2 optical modulator into fiber laser cavity as a saturable absorber, a highly stable Q-switching operation with single pulse energy of 224 nJ is achieved. The large-area MoSe2 possessing superior nonlinear optical properties compared to exfoliated nanoflakes affords possibility for the larger-area two-dimensional materials family as high performance optical devices.

Ultraviolet Lasing Characteristics of ZnS Microbelt Lasers

Investigation on the room-temperature ultraviolet lasing characteristics of a single ZnS microbelt laser is presented. Lasing emission with peak wavelength at round 335 nm is observed from the hexagonal-wurtzite phase of ZnS microbelt under optical excitation. This is due to the Fabry-Perot resonance along the length of the microbelt. By studying the low-temperature and time-resolved photoluminescence, it is verified that the corresponding lasing characteristics are attributed to the excitonic optical gain process. Furthermore, the rectangular cross-sectional nanostructure of ZnS microbelt suppresses TM polarization for excitation power lower than ~1.4 times the threshold. Hence, ZnS microbelts can be a promising building block to realize ultraviolet semiconductor lasers with control of laser polarization.