Daniel Houde

Properties of LYSO and recent LSO scintillators for phoswich PET detectors

The luminescence and nuclear spectroscopic properties of the new cerium-doped rare-earth scintillator lutetium-yttrium oxyorthosilicate (Lu/sub 0.6/Y/sub 1.4/Si/sub 0.5/:Ce, LYSO) were investigated and compared to those of both recent and older LSO crystals. UV-excited luminescent spectra outline important similarities between LYSO and LSO scintillators. The two distinct Ce1 and Ce2 luminescence mechanisms previously identified in LSO are also present in LYSO scintillators. The energy and timing resolutions were measured using avalanche photodiode (APD) and photomultiplier tube (PMT) readouts. The dependence of energy resolution on gamma-ray energy was also assessed to unveil the crystal intrinsic resolution parameters. In spite of significant progress in light output and luminescence properties, the energy resolution of these scintillators appears to still suffer from an excess variance in the number of scintillation photons. Pulse-shape discrimination between LYSO and LSO scintillators has been successfully achieved in phoswich assemblies, confirming LYSO, with a sufficient amount of yttrium to modify the decay time, to be a potential candidate for depth-of-interaction determination in multicrystal PET detectors.

Scintillation light emission studies of LSO scintillators

UV and /spl gamma/-ray excited luminescence and nuclear spectroscopy were used to study the relationship between the scintillation mechanisms of LSO and the spectroscopic characteristics obtained with PMT and APD readouts at room temperature. No correlation was found between scintillation decay time and light output. Like other investigators, we observed the existence of two distinct luminescence centers, Ce1 and Ce2, that mainly give rise to short (420 nm) and long (440 nm) emission wavelengths. Our measurements showed that different LSO crystals excited by /spl gamma/-rays have emission spectra with largely different shapes and maxima depending on the relative population and luminescence efficiency of these centers. It was also found that the poor energy resolution of LSO and YSO scintillators is well correlated with the coexistence of the two competing luminescence mechanisms. The prevalence of either Ce1 or Ce2 luminescence tends to reduce the variance of light emission and, thus, to improve energy resolution. Inversely, the coexistence of the two centers increases variance and degrades energy resolution.

Subpicosecond excitation of strongly coupled porphyrin–phthalocyanine mixed dimers

Electrostatic heterodimers are formed in the liquid phase by pairing a zinc porphyrin with either a copper or an aluminium phthalocyanine bearing oppositely charged substitutents. The ground-state absorption spectra of such heterodimers are drastically changed with respect to the corresponding monomers, indicating the existence of a strong interaction between the two chromophores. Irrespective of the nature of the metal ions and of the peripheral substituents, the heterodimers present very similar ground-state spectra.The photophysical properties of the two heterodimers are investigated with the use of femto- and nano-second absorption spectroscopies. Excitation of the zinc porphyrin–aluminium phthalocyanine heterodimer at 565 or 620 nm is followed by a very efficient electron transfer from the porphyrin to the phthalocyanine moiety while a very efficient intersystem conversion takes place in the excited zinc porphyrin–copper phthalocyanine, leading to the final ‘triplet’ excimer. The difference in behaviour is analysed and explained in terms of the thermodynamics of the transfer process and of the peculiar properties of the paramagnetic copper phthalocyanine.

Improved characteristics of a terahertz set-up built with an emitter and a detector made on proton-bombarded GaAs photoconductive materials

We report the coherent generation and detection of terahertz radiation from antenna-type devices made by using proton-bombarded GaAs photoconductive materials. Our combined emitter/detector set-up allows us to obtain a large bandwidth going from 0.1 up to 2 THz. We compare the performance of antenna emitters fabricated using mono- and multi-energy proton implantation in semi-insulating GaAs. Improved emission of terahertz radiation with a comparable bandwidth has been obtained using multi-energy proton implantation. Our results show that creating more defects in the optical absorption region gives rise to higher damage threshold biasing and larger saturation optical pumping power levels.

Pulsed photoconductive antenna terahertz sources made on ion-implanted GaAs substrates

In this work we show that improved performances of terahertz emitters can be obtained using an ion implantation process. Our photoconductive materials consist of high-resistivity GaAs substrates. Terahertz pulses are generated by exciting our devices with ultrashort near-infrared laser pulses. The ion implantation introduces non-radiative centres, which reduce the carrier lifetime in GaAs. The presence of the charged defects also induces a redistribution of the electric field between the antenna electrodes. This effect has a huge influence on the amplitude of the radiated terahertz field. Results obtained as a function of the laser excitation power are discussed and a comparison of the performance of these devices with a conventional antenna-type emitter is given.

Two-photon absorption of copper tetrasulfophthalocyanine induces phototoxicity towards Jurkat cells in vitro

The feasibility to induce oxygen-independent tumour cell kill by two-photon excitation of copper tetrasulfophthalocyanine (CuPcS4) was studied in Jurkat cells in vitro. Following incubation with CuPcS4 cells were transferred to a closed cuvette and irradiated with 532 nm pulsed-laser or 680 nm continuous-laser light to evaluate the effect of either two- or one-photon excitation, respectively. Cell survival was measured using MTT and Trypan Blue exclusion tests. Cell viability decreased 10-20% following two-photon excitation while one-photon illumination did not affect cell survival. These data confirm that two-photon excitation of CuPcS4 to the upper excited triplet state results in the formation of toxic species suggesting its potential use as a sensitizer for the photodynamic treatment of poorly oxygenated tumours.

Picosecond generation of transient charge carriers in Langmuir–Blodgett films of semi-amphiphilic heterodimers

Langmuir–Blodgett homolayers are formed by deposition of semi-amphiphilic porphyrin–phthalocyanine heterodimers. The optical and photophysical properties of these dimers have been investigated and compared to the liquid-phase data. Excitation of the dimer results in an instantaneous formation of the singlet excited states, followed by a very efficient charge-transfer reaction. The oxidized porphyrin and reduced phthalocyanine moieties are formed within 2 ps and disappear in 70 ps. The triplet excited states of the porphyrin issued from the intersystem crossing decay pathway of the singlet excited states are formed with a very low quantum yield. They also undergo a charge-transfer reaction, leading to the formation of long-lived transient charge carriers. The photoprocesses determined in the Langmuir–Blodgett films of semi-amphiphilic porphyrin–phthalocyanine heterodimers are almost identical to those previously observed for the same dimers in the liquid phase.

Picosecond relaxation of charge transfer states in an asymmetric cerium double-decker sandwich compound

Abstract Subpicosecond time-resolved absorption studies have been carried out on a cerium porphyrin-phthalocyanine sandwich mixed dimer in the liquid and solid states. Following excitation, two relaxation processes having time constants of ≈ 1 and ≈ 40 ps are observed irrespective to the nature of the environment. The first process is attributed to the relaxation of the excited state of the dimer to a low-lying charge transfer state. The second one, much slower, accounts for the relaxation of the charge transfer state back to the ground state. The comparison of the liquid and solid phase data clearly allows the role of the solvent in the relaxation processes to be eliminated.

Oxygen Dependence of Two-photon Activation of Zinc and Copper Phthalocyanine Tetrasulfonate in Jurkat Cells

Photodynamic therapy (PDT), the use of light‐activated drugs, is a promising treatment of cancer as well as several nonmalignant conditions. However, the efficacy of one‐photon (1‐γ) PDT is limited by hypoxia, which can prevent the production of the cytotoxic singlet oxygen (1O2) species, leading to tumor resistance to PDT. To solve this problem, we propose an irradiation protocol based on a simultaneous, two‐photon (2‐γ) excitation of the photosensitizer (Ps). Excitation of the Ps triplet state leads to an upper excited triplet state Tn with distinct photochemical properties, which could inflict biologic damage independent of the presence of molecular oxygen. To determine the potential of a 2‐γ excitation process, Jurkat cells were incubated with zinc or copper phthalocyanine tetrasulfonate (ZnPcS4 or CuPcS4). ZnPcS4 is a potent 1O2 generator in 1‐γ PDT, while CuPcS4 is inactive under these conditions. Jurkat cells incubated with either ZnPcS4 or CuPcS4 were exposed to a 670 nm continuous laser (1‐γ PDT), 532 nm pulsed‐laser light (2‐γ PDT), or a combination of 532 and 670 nm (2‐γ PDT). The efficacy of ZnPcS4 to photoinactivate the Jurkat cells decreased as the concentration of oxygen decreased for both the 1‐γ and 2‐γ protocols. In the case of CuPcS4, cell phototoxicity was measured only following 2‐γ irradiation, and its efficacy also decreased at a lower oxygen concentration. Our results suggest that for CuPcS4 the Tn excited state can be populated after 2‐γ irradiation at 532 nm or the combination of 532 and 670 nm light. Dependency of phototoxicity upon aerobic conditions for both 1‐γ and 2‐γ PDT suggests that reactive oxygen species play an important role in 1‐γ and 2‐γ PDT.

Investigation of the properties of new scintillator LYSO and recent LSO scintillators for phoswich PET detectors

The luminescence and nuclear spectroscopic properties of the new cerium-doped rare-earth scintillator lutetium-yttrium oxyorthosilicate (Lu/sub 0.6/Y/sub 1.4/SiO/sub 5/:Ce, LYSO) were investigated and compared to those of both recent and older LSO crystals. UV-excited luminescent spectra outline important similarities between LYSO and LSO scintillators. The two distinct Ce1 and Ce2 luminescence mechanisms previously identified in LSO are also present in LYSO scintillators. The energy and timing resolutions were measured using avalanche photodiode (APD) and photomultiplier tube (PMT) readouts. The dependence of energy resolution on gamma-ray energy was also assessed to unveil the crystal intrinsic resolution parameters. In spite of significant progress in light output and luminescence properties, the energy resolution of these scintillators appears to still suffer from an excess variance in the number or scintillation photons. Pulse-shape discrimination between LYSO and LSO scintillators has been successfully achieved in phoswich assemblies, confirming LYSO to be a potential candidate for depth-of-interaction determination in multi-crystal PET detectors.