We have fabricated fiber-coupled superconducting single-photon detectors (SSPDs), designed for quantum-correlationtype
experiments. The SSPDs are nanostructured (~100-nm wide and 4-nm thick) NbN superconducting meandering
stripes, operated in the 2 to 4.2 K temperature range, and known for ultrafast and efficient detection of visible to nearinfrared
photons with almost negligible dark counts. Our latest devices are pigtailed structures with coupling between
the SSPD structure and a single-mode optical fiber achieved using a micromechanical photoresist ring placed directly
over the meander. The above arrangement withstands repetitive thermal cycling between liquid helium and room
temperature, and we can reach the coupling efficiency of up to ~33%. The system quantum efficiency, measured as the
ratio of the photons counted by SSPD to the total number of photons coupled into the fiber, in our early devices was
found to be around 0.3 % and 1% for 1.55 &mgr;m and 0.9 &mgr;m photon wavelengths, respectively. The photon counting rate
exceeded 250 MHz. The receiver with two SSPDs, each individually biased, was placed inside a transport, 60-liter
liquid helium Dewar, assuring uninterrupted operation for over 2 months. Since the receiver's optical and electrical
connections are at room temperature, the set-up is suitable for any applications, where single-photon counting capability
and fast count rates are desired. In our case, it was implemented for photon correlation experiments. The receiver
response time, measured as a second-order photon cross-correlation function, was found to be below 400 ps, with
timing jitter of less than 40 ps.
A model is presented that successfully predicts electro-optical properties of
Lanthanide materials, irrespective whether these materials are inorganic or
organic, diluted or concentrated, metallic, semi-conducting or insulating. The
model is firmly based on recent experimental data revealing that the variation
in 4f and 5d energies relative to the valence band over the Ln series (La, Ce,
Pr,.. ,Lu) is universal. Application to LnS and the oxides LnO, Ln2O3 and LnO2
demonstrates its potential by correctly predicting the ground state electron
configuration, metallic, insulating or semi-conducting behavior, Ln ion valence
state and band-gap of these model Ln systems.
A method that has proven succesful in locating the energy levels of divalent and trivalent lanthanide ions (Ce, Pr,...,
Eu,...Yb, Lu) in wide band gap inorganic compounds like YPO4 and CaF2 is applied to locate lanthanide levels in the
wideband semiconductors GaN, AlN, their solid solutions AlxGa1-xN, and ZnO. The proposed schemes provide a
description of relevant optical and luminescence properties of these lanthanide doped semiconductors. Especially, the
relation between thermal quenching of Tb3+ emission and the location of the energy levels is explained.
Five elpasolite structure chlorides and bromides: Cs2NaYCl6, Cs2NaLaCl6, Cs2NaGdCl6, Cs2LiLaCl6 and Cs2LiYBr6 doped by Ce3+ ions have been investigated and results of scintillation and luminescent properties of studied crystals under optical, X- and (gamma) -ray excitation are presented. The position and splitting of Ce3+ 5d levels in all compounds were determined and result has a good agreement with theory.