Fast response to short-pulse optical excitation in intrinsic and Ce-doped garnet scintillators G. Tamulaitis, A. Augulis, V. Gulbinas, S. Nargelas, E. Songaila, A. Vaitkevicius, Vilnius University, Lithuania E. Auffray, M.T. Lucchini CERN M. Korjik, A. Borisevich, A. Fedorov, V. Mechinsky, Research Institute for Nuclear Problems, Minsk, Belarus Task 14.2 Test infrastructure for innovative calorimeters with optical readout

Outline Motivation Experimental Photoluminescence in PWO Photoluminescence in GAGG:Ce Conclusions Outlook

10 ps TARGET: To develop a test station for the characterization of timing properties of scintillators

10 ps TARGET: OBJECTIVES: To develop a test station for the characterization of timing properties of scintillators OBJECTIVES: Fast optical response in scintillators Novel principles of optical readout

10 ps TARGET: OBJECTIVES: RESULTS: To develop a test station for the characterization of timing properties of scintillators OBJECTIVES: Fast optical response in scintillators Novel principles of optical readout Two-photon absorption. Significant and sensitive to irradiation in PWO, weak in garnets. RESULTS: Main publications: E.Auffray et al., NIMA, 804, 194 (2015)

10 ps TARGET: OBJECTIVES: RESULTS: To develop a test station for the characterization of timing properties of scintillators OBJECTIVES: Fast optical response in scintillators Novel principles of optical readout Photoluminescence rise time. In subpicosecond domain in PWO and YAGG:Ce, nanoseconds in GAGG:Ce Free carrier absorption. Fast in PWO and YAGG:Ce, strong influence of Gd sublattice on hole dynamics in GAGG:Ce. Two-photon absorption. Significant and sensitive to irradiation in PWO, weak in garnets. RESULTS: E.Auffray et al., available online in J. Lumin. E.Auffray et al., submitted to Optical Materials Main publications: E.Auffray et al., NIMA, 804, 194 (2015)

Setup for study of nonlinear absorption (pump and probe, two-photon absorption, free carrier absorption, light-induced transient grating techniques)

Time-resolved photoluminescence spectroscopy EXCITATION: DETECTION: Hamamatsu streak camera. FWHM of the instrumental response function : 2.95 ps Yb:KGW oscillator (Light Conversion Ltd.) emitting at 1030 nm 80 fs pulses at 76 MHz repetition rate. Harmonics generator (HIRO, Light Conversion Ltd.); the third 343 nm (3.64 eV) and fourth 254 nm (4.9 eV) harmonics

New results on photoluminescence in PWO New features have been discovered 20 years after the material approval for collider experiments!

Luminescence kinetics PWO Initial stage of spectrally integrated photoluminescence kinetics (dots), the instrumental response function (green line), and the best fit obtained using a bi-exponential decay function (red solid line) The rise time is below the time resolution at the instrumental response function with FWHM of 2.95 ps Both regular WO42- and defect-related WO3 luminescence centers show subpicosecond leading edge of the luminescence transient

PWO Decay times: 1 = 3.8 ps and 2 = 683 ps @ 343 nm exc. 1 = 5.9 ps and 2 = 824 ps @ 254 nm exc. Initial part of photoluminescence kinetics at 343 nm excitation (blue line) and 254 nm excitation with pulse energy of 15 mJ/cm2 (red solid line) and 1.5 mJ/cm2 (green dotted line). Both at 343 nm and 254 nm excitation, the kinetics of luminescence within 400-500 nm and 500-600 nm are identical The fast components observed open new opportunities for fast timing with PWO!

Photoluminescence in GAGG:Ce

GAGG:Ce Luminescence spectra at two excitation wavelengths At the same excitation pulse energy and absorption coefficient , PL intensity is considerably lower after predominant excitation of Gd3+ ions  nonradiative recombination during the excitation transfer to radiative recombination centers is important

GAGG:Ce Photoluminescence kinetics Two rise components: Shorter than the instrumental response function; Slow component with with the rise time of 8 ns @ 254 nm exc. 2.5 ns @ 343 nm exc. This is in consistence with the 2 ns rise time observed in GAGG:Ce under gamma irradiation [M.T. Lucchini et al., NIMA, 816, 176 (2016)]. The instrumental function with FWHM of 100 ps PL decay is monoexponential Decay times: 60 ns at 254 nm and 70 ns at 343 nm

Conclusions An experimental setup enabling the study of fast processes in scintillators in sub-picosecond domain is developed. A sub-picosecond rise time of luminescence in intrinsic PWO scintillation crystal is demonstrated. Three luminescence components are revealed in PWO. In addition to the decay component with the decay time of ~10 ns, fast and intermediate components with decay times of 4-6 ps and 600-800 ps have been observed. The rise of the luminescence response to short pulse excitation in GAGG:Ce takes several nanoseconds even at the direct resonant excitation of Ce3+ ions. This feature is explained by the position of the excited Ce3+ levels in the conduction band. The luminescence response time is influenced by the capture of nonequilibrium carriers by matrix-building Gd3+ ions.

Outlook Combining our optical test bench with pulsed X-ray tube will be very important for future study of fast phenomena in scintillators; We are seeking for collaborators for this combination and additional funds to get a ns pulse X-ray tube, which is commercially available. Combining the optical test bench and X-ray tube will enable us to study: - fast processes under the conditions of ionization; - fast processes in inorganic and organic scintillators; - fast processes in semiconductors including wide-band-gap diamond.