What may be expected from the “microwave resonant cavity technique” applied to rare‐earth‐doped insulating materials?

Abstract

The principle of measurements using the "microwave resonant cavity technique" applied to rare-earthdoped insulating materials is reviewed and the physical nature of the expected signals is discussed. Experimental results concerning both single crystal and powdered samples of Lu 2 SiO 5 : Ce 3+ are presented and discussed as typical examples. From measurements at various temperatures between 300 and 5K and under different pulsed laser beam powers, it is shown that detailed information on the rare earth photoionization process, photoconductivity dynamics and trapping effects may be gained, in addition to photoconductivity spectra.1 Introduction Photoconductivity studies of rare-earth (RE)-doped insulating materials using the microwave resonant cavity technique (MRCT) were conducted for the first time in our laboratory [1]. The primary goal of these investigations was indeed the determination of the RE photoionization thresholds, with the aim of locating the 4f n levels of RE ions with respect to the host conduction band (CB). As a matter of fact, the energy level diagrams of RE ions usually reported do not take this positioning into account and, consequently, lack the required information to help in understanding the complex processes associated with the absorption of high energy photons by RE-doped materials used as scintillators, ultraviolet lasing media or phosphors for fluorescent tubes and plasma display panels. MRCT was shown to be also helpful in gaining significant information on the photoionization process, photoconductivity dynamics, nature of the photoexcited electrons and trapping effects [2][3][4]. MRCT consists in detecting the dielectric response to a pulsed laser irradiation of a sample placed inside a microwave resonator. Let Q U and f 0 be respectively the cavity quality factor and resonance frequency when both the laser and microwave sources are not operating. Coupling the microwave guide to the cavity lowers its quality factor from Q U (U for unloaded cavity) to Q L (L for loaded cavity) defined by:

Understanding the scintillation efficiency of cerium-doped LSO, LYSO, YSO and LPS crystals from microwave study of photoconductivity and trapping

“…Indeed, the lifetime of a photoionized, mobile electron in the CB depends on the density of recombination centers in the material: the higher the density of trapping centers, the shorter the diffusion length and the time spent before recombination. This was already observed by comparing the photoconductivity dynamics obtained on a bulk crystal of LSO:Ce and on a microcrystalline powder of the same material [18]: it was shown that the electronic diffusion length was shorter in the powdered sample (60 nm in the grains, against 80 nm in the bulk), and this was ascribed to an enhanced density of electron traps in the powder, due to surface-related defects providing more recombination centers for the delocalized electrons.…”

Trends in Search for Bright Mixed Scintillators

“…The correlation between ( L ex /L veg ) max and the band gap difference Δ E g basing on E g values is plotted in Figure . Despite the strong LY enhancement in garnets and some halides with Δ E g > 0.9, large positive deviations of LY were observed as well in systems with weak E g changes.…”

Analysis of the photoconduction inCaF2:Eu2+crystals using the microwave resonant cavity technique

“…9 Of course, the decay of the absorption mode signal reflects the decay of the CB electronic population. This decay originates from the capture of electrons by Eu 3+ ions ͑either resulting from Eu 2+ photoionization or initially present in the sample͒, but also by traps.…”

“…The various potentialities of MRCT were recently reviewed. 9 MRCT was also successfully applied to powdered samples of insulating materials with grain diameters ranging from 1-5 m ͑Ref. 10͒.…”