Controlling the Two-Photon-Induced Photon Cascade Emission in a Gd(3+)/Tb(3+)-Codoped Glass for Multicolor Display.

We reported the first observation of the two-photon-induced quantum cutting phenomenon in a Gd(3+)/Tb(3+)-codoped glass in which two photons at ~400 nm are simultaneously absorbed, leading to the cascade emission of three photons in the visible spectral region. The two-photon absorption induced by femtosecond laser pulses allows the excitation of the energy states in Gd(3+) which are inactive for single-photon excitation and enables the observation of many new electric transitions which are invisible in the single-photon-induced luminescence. The competition between the two-photon-induced photon cascade emission and the single-photon-induced emission was manipulated to control the luminescence color of the glass. We demonstrated the change of the luminescence color from red to yellow and eventually to green by varying either the excitation wavelength or the excitation power density.

Three-photon-induced blue emission with narrow bandwidth from hot flower-like ZnO nanorods.

ZnO nanorods (NRs) self-organized into flowers were synthesized at different temperatures ranging from 100°C to 180°C by using the hydrothermal method. The existence of Zn interstitials (Zn(i)) was confirmed by X-ray photoelectron spectroscopy and a larger amount of Zn(i) was found in the ZnO NRs prepared at higher temperatures. A redshift of the emission peak of more than 15 nm was observed for the ZnO NRs under single photon excitation. The nonlinear optical properties of the flower-like ZnO NRs were characterized by using focused femtosecond laser light and strong three-photon-induced luminescence was observed at an excitation wavelength of ~750 nm. More interestingly, a large redshift of the emission peak was observed with increasing excitation intensity, resulting in efficient blue emission with a narrow bandwidth of ~30 nm. It was confirmed that the large redshift originates from the heating of the ZnO NRs to a temperature of more than 800°C and the closely packed ZnO NRs in the flowers play a crucial role in heat accumulation. The stable and efficient three-photon-induced blue emission from such ZnO NRs may find potential applications in the fields of optical display, high-temperature sensors and light therapy of tumors.

Efficient blue light emission from In0.16Ga0.84N/GaN multiple quantum wells excited by 2.48-µm femtosecond laser pulses.

We report on the efficient blue light emission from In0.16Ga0.84N/GaN multiple quantum wells excited by femtosecond laser pulses with long wavelengths ranging from 1.24 to 2.48 µm. It is found that the trap states in GaN barrier layers lead to an efficient cascade multiphoton absorption in which the carriers are generated through simultaneous absorption of n (n=1 and 2) photons to the trap states, followed by simultaneous absorption of m (m=3, 4, and 5) photons to the conduction band. The dependence of the upconversion luminescence on excitation intensity exhibits a slope between n and n+m, which is in good agreement with the prediction based on the rate equation model.

Controllable color display induced by excitation-intensity-dependent competition between second and third harmonic generation in ZnO nanorods.

We investigated the second and third harmonic generation (SHG and THG) in ZnO nanorods (NRs) by using a femtosecond laser (optical parametric amplifier with tunable wavelengths) with a long excitation wavelength of 1350 nm and a low repetition rate of 1 kHz. The damage threshold for ZnO NRs in this case was sufficiently large, enabling us to observe the competition between SHG and THG. The transition from red to blue emission and the mixing of red and blue light with different ratios were successfully demonstrated by simply varying excitation intensity, implying the potential applications of ZnO NRs in all-optical display.