Yao Li

Efficient single-photon frequency upconversion at 1.06 μm with ultralow background counts

We demonstrated an efficient single-photon frequency upconversion system for the infrared photons at 1.06u2009μm with ultralow background counts. By pumping the system at a longer wavelength than that of the single-photon signal, the background counts caused by the pump-induced parametric fluorescence in the nonlinear crystal were reduced markedly. We achieved so far the lowest noise of ∼150/s for a near unity conversion efficiency.We demonstrated an efficient single-photon frequency upconversion system for the infrared photons at 1.06u2009μm with ultralow background counts. By pumping the system at a longer wavelength than that of the single-photon signal, the background counts caused by the pump-induced parametric fluorescence in the nonlinear crystal were reduced markedly. We achieved so far the lowest noise of ∼150/s for a near unity conversion efficiency.

Temporal and spectral control of single-photon frequency upconversion for pulsed radiation

We demonstrate efficient single-photon detection at 1.04u2002μm with quite low background counts by using the frequency upconversion detection technique with pulsed single-photon source and strong pump field prepared from two synchronized fiber lasers. Temporal and spectral control of the pump and signal lasers enabled an upconversion efficiency of 81.1%. As the system was pumped at a wavelength longer than that of the signal, the corresponding background counts were reduced down to 1.5×103u2002s−1 due to efficient suppression of parametric fluorescence in the periodic poled lithium niobate crystal.

Temperature dependence of optical band gap in ferroelectric Bi3.25La0.75Ti3O12 films determined by ultraviolet transmittance measurements

Optical properties of ferroelectric Bi3.25La0.75Ti3O12 (BLT) films on quartz have been investigated using ultraviolet-infrared transmittance spectra in the temperature range of 77–500K. The spectra can be divided into three distinctive photon regions between 1.1 and 6.5eV. It is found that the band gap Eg decreases from 3.88to3.77eV with the temperature. The parameters aB and ΘB of the Bose-Einstein model are 30.3meV and 218.7K, respectively. The band narrowing coefficient dEg∕dT is −2.65×10−4eV∕K at room temperature. The present results can be crucial for future application of ferroelectric BLT-based electro-optic and high temperature optoelectronic devices.

Properties of highly (100) oriented Pb(Mg1∕3,Nb2∕3)O3–PbTiO3 films on LaNiO3 bottom electrodes

The 70%Pb(Mg1∕3,Nb2∕3)O3–30%PbTiO3 (PMNT) films have been fabricated on LaNiO3 (LNO) coated silicon substrate. The conductive LNO films act as a seed layer for the growth of PMNT films, which depresses the formation of pyrochlore phase and induces the high (100) preferred orientation of perovskite PMNT films. Compared with the PMNT films grown on platinum bottom electrode, the ferroelectric properties of PMNT films grown on LNO are enhanced. The frequency dependence of complex permittivity from PMNT films on LNO is the conjunct result of polarization relaxation and movement of oxygen vacancy, which can be fitted by the function containing Debye and universal dielectric response models, respectively.

Effects of LaNiO3 bottom electrode on structural and dielectric properties of CaCu3Ti4O12 films fabricated by sol-gel method

CaCu3Ti4O12 (CCTO) thin films are prepared by a sol-gel method on LaNiO3-coated silicon and Pt∕TiO2∕SiO2∕Si substrate. Compared with the films on Pt, the CCTO on LaNiO3 exhibits a (400) orientation. Dielectric loss of CCTO on LaNiO3 is lower than 0.25 within 100Hz–10kHz, lower than the reported value of CCTO grown on Pt∕TiO2∕SiO2∕Si by pulse laser deposition. Possible reason is that LaNiO3 acts as seed layer for the growth of CCTO. The crystallinity of CCTO is improved and the dielectric properties are enhanced. Complex impedance spectrum of CCTO on LaNiO3 is discussed according to grain boundary barrier layer capacitance model.

High-Speed Single-Photon Frequency Upconversion With Synchronous Pump Pulses

Single-photon signals at 1.56 ¿m were converted into the visible region by sum-frequency generation, with synchronous pump pulses at 1.03 ¿m in a periodically poled lithium niobate crystal. The signal and pump sources were provided by two synchronized mode-locked fiber lasers at the operation rate of 7.88 MHz. Such a high-speed single-photon upconversion system has shown an overall detection efficiency of 10.4%. The corresponding background noise was measured to be 3.3 × 103 s-1. The noise-equivalent power in this scheme was 9.93 × 10-17 W/Hz1/2, which was the lowest compared to the other upconversion systems with similar pump and signal wavelengths.

Preventing eavesdropping with bright reference pulses for a practical quantum key distribution

We analyze application of bright reference pulses to prevent the photon-number-splitting attack in weak-pulse quantum key distribution. Under the analyzed eavesdropping strategy, the optimal parameters of bright reference and signal pulses can ensure a secure transmission distance up to 146 km. To realize the quantum key distribution scenario with up-present techniques, we present an experimentally feasible scheme to create a large splitting ratio between bright reference and signal pulses, and to switch the bright reference pulses away from signal pulses to avoid the after-pulse disturbance.

Fiber-coupled near-infrared single-photon spectrum analyzer based on a InGaAs/InP avalanche photodiode single-photon detector

A highly sensitive fiber-coupled single-photon spectrum analyzer is built based on a InGaAs/InP avalanche photodiode single-photon detector operated in Geiger mode. This device is capable of analyzing weak light signals with less than 3.6 × 10−4 photon pulse−1 in the spectral range from 1390 to 1600 nm. We have used it to measure the spectrum of the weak nonlinear signal of the fiber caused by the clock pulse in the quantum key distribution experiment. The spectral information will help to eliminate the nonlinear background in the fiber and improve the performance of the system.