Zhiren Zheng

Measurement of transit time for femtosecond-laser-driven shock wave through aluminium films by ultrafast microscopy

Shock waves in unconfined aluminium films of varying thicknesses from 3 to 10 µm induced by a single fs-laser-pulse with a full width half maximum width of about 130 fs at λ = 800 nm wavelength have been investigated by using ultrafast microscopy. With successive sub-picosecond-resolved micrographs of the aluminium/α-quartz interface taken at various probing times, the shock transit time in aluminium films of different thicknesses was measured accurately. From the best linear fit of the transit time versus the thickness of aluminium films, the shock velocity was measured to be 9.0 ± 0.4 km s−1 at the laser intensity of 7.84 × 1013 W cm−2. From the shock velocity D the shock pressure P and temperature T were calculated to be 69 ± 5 GPa and 1852 ± 400 K, respectively. Experimental results in good agreement with previous theoretical and experimental works indicate that the time-resolved ultrafast microscopy technique is a fairly accurate and feasible technique for studying fs-laser-driven shock waves in opaque thin films.

Upconversion luminescence of CdTe nanocrystals by use of near-infrared femtosecond laser excitation

We study the steady-state and time-resolved luminescent properties of CdTe nanocrystals by one- and two-photon excitation with a femtosecond laser. We observe that 1208 nm excitation causes a shift of the emission peak of about 20 nm to the infrared compared with 400 nm laser excitation. It is found that upconversion luminescence is composed of a photoinduced trapping and a band edge excitonic state and produces the observation of biexponential decay kinetics. We conclude that the redshift of the emission peak is caused by the relative change in luminescence intensity between excitonic and trapping states.

Red upconversion emission in LiNbO 3 codoped with Er 3+ and Eu 3+

Red upconversion (UC) emission at 626 nm is obtained from a LiNbO(3) crystal codoped with Er(3+) and Eu(3+) under 800 nm femtosecond laser excitation. Energy transfer from ((2)H(11/2,),(4) S(3/2)) levels of Er(3+), which are excited by excited state absorption, to (5)D(1) of Eu(3+) followed by rapidly relaxing to (5)D(0) nonradiatively leads to this red UC emission. The energy transfer efficiency and Er-Eu transfer microparameter of approximately 30% is obtained in LiNbO(3):Er(3+)(1.0 mol%),Eu(3+)(0.1 mol%). These initial experimental results indicate that the red UC emission can be obtained from Er(3+)/Eu(3+) codoped system under diode laser excitation.

Effect of End Groups on the Raman Spectra of Lycopene and β-Carotene under High Pressure

The Raman spectra of all-trans-lycopene in n-hexane were measured under high pressure, and the results compared with those of β-carotene. The different pressure effects on Raman spectra are analyzed taking into account the different structures of lycopene and β-carotene molecules. It is concluded that: (a) the vibronic coupling between the S1 and S0 states of β-carotene is stronger than that of lycopene, (b) the diabatic frequency increment of the ν1 mode is more susceptible to pressure than that of the ν2 mode for lycopene, and (c) β-rings rotation can relieve the pressure effect on the C=C bond length in β-carotene. This work provides some insights for elucidating the structural and environmental effects on Raman spectra of carotenoids.

Visible and ultraviolet upconversion emission in LiNbO3 triply doped with Tm3+, Yb3+, and Nd3+

Visible and ultraviolet upconversion (UC) emission is observed under 800 nm femtosecond laser excitation in LiNbO3 crystals triply doped with Tm3+, Yb3+, and Nd3+ at room temperature. Energy transfer (ET) from Nd3+ to Yb3+ then to Tm3+ is very important in this UC emission process. The overlapping between the emissions of D12→F34 and G14→H36, which makes up of blue emission band, is confirmed by transient investigation. From the pump energy dependence investigation, it is known that the dominant populating mechanism for the G14 state is the two-photon process, and that for D12 is the three-photon process. In our UC emission model, the G14 state is populated by the ET of F25/2(Yb3+)+H34(Tm3+)→F27/2(Yb3+)+G14(Tm3+), D12 state is populated by the ET of F32+H34→D12+H36 among Tm3+ ions. For LiNbO3 crystals doped with Tm3+ to the concentration of 0.9 mol %, the measured lifetimes of G14 and D12 are ∼80 and 4 μs.

The Pr4+ ions in Mg doped PrGaO3 perovskites

In this study, the valence state of Pr in Mg doped PrGaO3 perovskites was investigated. It is found that Pr4+ ions are created in samples doped with low-valence ions, for example, with Mg2+. The color of the doped samples changes from black to light green after being treated in H-2 atmosphere, which shows that the Pr4+ ions in the sample are reduced to Pr3+ ions. The open circuit voltage of the solid oxide fuel cell (SOFC) using PrGa0.95Mg0.05O3 as the electrolyte increases with time and the X-ray photoelectron spectroscopy (XPS) data of PrGa0.9Mg0.1O3 also show the existence of Pr4+ ions in Mg doped PrGaO3

Measurement of infrared level lifetime by upconversion luminescence

Based on repetition frequency-dependent excited state absorption (ESA) upconversion (UC) luminescence, a method to measure the lifetime of an IR intermediate level is proposed so long as ESA UC luminescence can occur in the rare earth ions. The feasibility of this idea is demonstrated via a theoretical simulation. A Er(3+):LiNbO₃ crystal ESA UC luminescence under femtosecond laser excitation is used to illustrate this measurement method, and the obtained 1.5 μm lifetime of 2.31 ms is shorter than previous reported values. This method can obviate the influence of radiation trapping effect on lifetime measurement, which is crucial in the traditional pulse sampling technique.

Effect of solvent on absorption spectra of all-trans-β-carotene under high pressure

The absorption spectra of all-trans-beta-carotene in n-hexane and carbon disulfide (CS(2)) solutions are measured under high pressure at ambient temperature. The common redshift and broadening in the spectra are observed. Simulation of the absorption spectra was performed by using the time-domain formula of the stochastic model. The pressure dependence of the 0-0 band wavenumber is in agreement with the Bayliss theory at pressure higher than 0.2 GPa. The deviation of the linearity at lower pressure is ascribed to the reorientation of the solvent molecules. Both the redshift and broadening are stronger in CS(2) than that in n-hexane because of the more sensitive pressure dependence of dispersive interactions in CS(2) solution. The effect of pressure on the transition moment is explained with the aid of a simple model involving the relative dimension, location, and orientation of the solute and solvent molecules. The implication of these results for light-harvesting functions of carotenoids in photosynthesis is also discussed.

Two-photon-excited luminescence from a Eu3+-doped lithium niobate crystal pumped by a near-infrared femtosecond laser

Blue (487.6 nm), green (544.1 nm), yellow (582.1 nm), and red (623.6 nm) upconversion (UC) luminescences are achieved in a Tb(3+)-doped lithium niobate crystal when an 800 nm femtosecond laser is loosely focused onto the sample at room temperature. The relationship between UC luminescence intensity and the pump energy indicates that a two-photon excitation process is dominant in this UC luminescence phenomenon. The Tb(3+) sensitive temperature dependence of the luminescence intensity is demonstrated via an obvious reduction of luminescence intensity with durative laser irradiation.

Planar nanosecond shock wave generation and propagation in poly vinyl alcohol investigated by CARS

Planar nanosecond shock waves were generated with ultrafast Gaussian laser pulses by using saturable dye-doped polymer film as shock layer. Ultrafast spatially resolved Coherent Anti-Stokes Raman (CARS) spectra of shock compression of a polycrystalline anthracene optical gauge were measured, and temporal profile and velocity of the shock pulse were obtained. Detailed measurements showed the shock pressure, as indicated by the blueshift of an anthracene vibrational transition, and the shock velocity, as indicated by the arrival time of shock wave at the anthracene gauge layer, remains constant within better than 5% over the central region probed by CARS.