24 research outputs found
Superfluorescence from Erbium-doped crystals
No full textIn Superfluorescence process, an initially incoherent ensemble of excited atoms (molecules, etc) gives rise to a macroscopic polarization due to spontaneously synchronization of the atomic dipoles. In this coupled system, every single dipole is correlated with all the others and the atomic transition rate is enhanced by a factor μN, with N the number of correlated atoms and μ a geometrical factor that accounts for the interference effects. The cooperative radiative decay results in the emission of coherent photon bunches whose peak intensity is proportional to μN^2 and the duration scales with (μN)^−1. In this thesis, we present the clear signatures of the Superfluorescence from Erbium-doped Y2SiO5 and YLiF4 crystals at liquid helium temperature. The process is observed for two transitions and involves the remarkable number of 10^12 Erbium ions, accelerating the radiative decay by millions of times. Several aspects of Superfluorescence are deeply investigated and a remarkable agreement with the theoretical expectation is reported
Cathodo- and radioluminescence of Tm3+: YAG and Nd3+: YAG in an extended wavelength range
No full textWe have studied the cathodo- and radioluminescence of Nd:YAG and of Tm:YAG single crystals in an extended wavelength range up to ≈5μm in view of developing a new kind of detector for low-energy, low-rate energy deposition events. Whereas the light yield in the visible range is as large as ≈104photons/MeV, in good agreement with literature results, in the infrared range we have found a light yield ≈5×104photons/MeV, thereby proving that ionizing radiation is particularly efficient in populating the low lying levels of rare earth doped crystals
A new technique for infrared scintillation measurements
No full textWe propose a new technique to measure the infrared scintillation light yield of rare earth doped crystals by comparing it to near UV–visible scintillation of a calibrated Pr:(Lu0.75Y0.25)3Al5O12 sample. As an example, we apply this technique to provide the light yield in visible and infrared range up to 1700 nm of this crystal
Optical coherence and spin population dynamics in <sup>171</sup>Yb<sup>3+</sup>:Y<sub>2</sub>SiO<sub>5</sub> single crystals
No full textYb3+171-doped Y2SiO5 (YSO) crystals are a promising platform for optical quantum memories in long-distance quantum communications. The relevance of this material lies in Yb171 long optical and spin coherence times, along with a large hyperfine splitting, enabling long quantum storage over large bandwidths. Mechanisms affecting the optical decoherence are, however, not precisely known, especially since low-temperature measurements have so far focused on the 2 to 4 K range. In this work, we performed two- and three-pulse photon echoes and spectral hole burning to determine optical homogeneous linewidths in two Yb171:YSO crystals doped at 2 and 10 ppm. Experiments were performed in the 40 mK to 18 K temperature range, leading to linewidths between 320 Hz, among the narrowest reported for rare-earth ions, and several MHz. Our results show that above ∼6 K, the homogeneous linewidth Γh is mainly due to an elastic two-phonon process which results in a slow broadening with temperature, with Γh reaching only 25 kHz at 10 K. At lower temperatures, interactions with Y89 nuclear spin flips, paramagnetic defects or impurities, and also Yb-Yb interactions for the higher concentrated crystal are likely the main limiting factor to Γh. In particular, we conclude that the direct effect of a spin and optical excited state lifetime is a minor contribution to optical decoherence in the whole temperature range that is studied. Our results indicate possible paths and regimes for further decreasing homogeneous linewidths or maintaining narrow lines at higher Yb171 concentration
Misure di luminescenza nel vicino infrarosso e nel visibile di Nd3+:YAG eccitati per impatto elettronico
No full textSi è misurata la luminescenza infrarossa (Evis) di cristalli di Nd3+:YAG eccitati per bombardamento con elettroni ai fini dello sviluppo di un rivelatore di radiazioni ionizzanti. Lo spettro osservato è confrontato con quello ottenuto per eccitazione laser. Si è misurata anche l'efficenza di emissione (IR).ope
Development of the Infrared Quantum Counter concept for low-threshold particle detection
No full textAn investigation on the feasibility of a new class of low-threshold, high efficiency particle detectors is carried out under the Axioma Project at the Laboratori Nazionali INFN in Legnaro. The present project is aimed at the detection of low-rate, low-energy deposition events, both for axionic dark matter searches and for coherent neutrino-nucleon scattering studies.
The detection process is mainly based on the Infrared Quantum Counter concept applied to rare-earth (RE) doped crystals, as proposed by the Nobel prize Bloembergen in 1959. An energetic particle excites the rare-earth ions from the ground state to a low-lying (< eV), metastable (lifetime ) state. Simultaneously, a tuned pump laser promotes the excited ion to a higher lever (upconversion), that relaxes to the ground state or to some other lower lying state emitting a visible fluorescence photon. A very high detection efficiency can be reached depending on several factors, including the energy level scheme of the specific combination of RE ion and crystal matrix that may lead to excitation recycling induced light amplification processes.
A first most important requirement is that the energy deposition by means of an energetic particle in the crystal leads to a high production of ions excited in the lowest metastable level above the ground state. A second important requisite is a high upconversion efficiency that depends on the laser light absorption cross section and lifetime of the excited ions. Finally, a third requisite is that the crystal is as transparent as possible to the laser if the ions are in the ground state.
Owing to the scant number of studies on these subjects that can be found in literature, the best crystal-RE ion combination has to be sought by direct investigation in our lab. To this goal we have started a systematic investigation campaign of the properties of several combinations of single crystal hosts doped with different RE ions of selected concentrations.
The efficiency of the production of excited ion levels is studied by exciting crystals by either electron impact or X-ray irradiation with a home-made electron gun. The emission spectrum can be recorded in the range by suitably merging the spectra recorded with different spectrometers, including a FT-IR (Fourier Transform-InfraRed) one.
The outcome of these measurements is the light yield in both the visible and infrared range. In particular, light yields as high as can be obtained. From the spectra we are also able to identify the levels involved in the excitation by both the position of the emitted wavelengths and by their time evolution. We are also exciting the crystals with different lasers (diode, tunable dye and Ti:Sa laser, Nd:YAG higher harmonics) in order to shed light on possible different excitation mechanisms with respect to particle excitation.
The upconversion efficiency and crystal transparency
are studied by irradiating the crystals with finely tunable lasers at low temperature. The upconversion efficiency is measured by simulating the particle excitation with an infrared source of selected wavelength band and using a Ti:Sa or a dye laser tunable with picometer accuracy to promote the excited ions to the even higher excited levels whose decay produces the visible fluorescence light collected by a photomultiplier. In the future we will be also using a small -source to excite crystals.
The crystal transparency is affected by non-resonant multiphonon-assisted absorption that results in a noise source that might degrade the overall detection efficiency. We are investigating this process by varying both crystal temperature and laser intensity.
All these studies should allow us to define the optimum characteristics of the crystal-dopant system and to select the optimum actual crystal for low-threshold particle detection.ope
Misure di luminescenza nel vicino infrarosso e nel visibile di Nd3+:YAG eccitati per impatto elettronico
No full textSi è misurata la luminescenza infrarossa (Evis) di cristalli di Nd3+:YAG eccitati per bombardamento con elettroni ai fini dello sviluppo di un rivelatore di radiazioni ionizzanti. Lo spettro osservato è confrontato con quello ottenuto per eccitazione laser. Si è misurata anche l'efficenza di emissione (IR)
New ideas on prospective low energy threshold detectors for dark matter searches
No full textLow energy threshold detectors are necessary in many frontier fields of experimental physics. In particular, these are extremely important for probing possible dark matter (DM) candidates. We present a novel detection approach that exploits the energy levels of atoms maintained at cryogenic temperature. We exploit laser-assisted transitions that are triggered by the absorption of the incident particle in the material and lead to the emission of a fluorescent photon or an electron. In this approach, the incident particle will in fact excite the first low-lying energy level that is then up-converted using an opportune narrow-band laser system. Two different detection schemes are thus possible in our active material: one is based on a photon signal while the other takes advantage of high efficiency in-vacuum charge detection
Axion dark matter detection by laser induced fluorescence in rare-earth doped materials
No full textWe present a detection scheme to search for QCD axion dark matter, that is based on a direct interaction between axions and electrons explicitly predicted by DFSZ axion models. The local axion dark matter feld shall drive transitions between Zeeman-split atomic levels separated by the axion rest mass energy mac2. Axion-related excitations are then detected with an upconversion scheme involving a pump laser that converts the absorbed axion energy (~hundreds of μeV) to visible or infrared photons, where single photon detection is an established technique. The proposed scheme involves rare-earth ions doped into solid-state crystalline materials, and the optical transitions take place between energy levels of 4fN electron confguration. Beyond discussing theoretical aspects and requirements to achieve a cosmologically relevant sensitivity, especially in terms of spectroscopic material properties, we experimentally investigate backgrounds due to the pump laser at temperatures in the range 1.9 - 4.2 K. Our results rule out excitation of the upper Zeeman component of the ground state by laser-related heating efects, and are of some help in optimizing activated material parameters to suppress the multiphonon-assisted Stokes fuorescence
