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American Journal of Physics

Impurity level lifetime measurements using a lock-in amplifier

Kenneth M. Edmondson, Steve Agoston, Radha Ranganathan

Published Jun 1, 1996• 10 citations
Publisher

Abstract

Lifetimes of metastable levels of chromium in ruby are determined using an unusual technique. The fluorescence from the chromium is measured using an ac technique in which the excitation is modulated and the fluorescence is detected synchronously. The novelty lies in the measurement of the dependence of the signal on the modulation frequency and extracting the lifetime from the frequency dependence. The experiments and data analysis demand a detailed understanding of the methods of lock-in amplification and synchronous detection and hence they can serve as a useful primer on the subject. The observed values for the lifetimes show excellent agreement with previous measurements.

DOI
10.1119/1.18176

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At the time this investigation was undertaken the published experimental evidence indicated that the lifetime 1 4 of the T 2 state in pink ruby was approximately 5 0 ns. 4 Observation of fluorescence from the T 2 state had been reported but no details of the emission had been published.In this investigation it was demonstrated by direct 4 measurements that the T 2 state lifetime is less than 2 ns at temperatures between 2° K and 3 0 0° K.No fluorescent 4 emission from the T 2 state was observed although extremely high excitation powers and sensitive detection equipment were used.It was demonstrated that the nonradiative 4 2 transitions T 2 -► E occur with a frequency which is greater by a factor of at least 3 x 10^ than that of the fluorescent 4 4 4 4 Tj A 2 transitions.If the T 2 -* ■ A 2 transitions are considered to be purely electronic, it is shown that the 4 lxfetime of the T 2 state must be less than 45 p s .In the more likely event that the transitions are vibrationally assisted it is shown that the lifetime must be less than 3.5 p s .The author is indebted to the members of his doc toral committee who have been extremely generous with their time and facilities.Without such generosity the project could never have been completed.Professor C. Kikuchi was co-chairman of the committee

•Jan 1, 1969•0 citations
Lifetimes of metastable levels of chromium in ruby are determined using an unusual technique. The fluorescence from the chromium is measured using an ac technique in which the excitation is modulated and the fluorescence is detected synchronously. The novelty lies in the measurement of the dependence of the signal on the modulation frequency and extracting the lifetime from the frequency dependence. The experiments and data analysis demand a detailed understanding of the methods of lock-in amplification and synchronous detection and hence they can serve as a useful primer on the subject. The observed values for the lifetimes show excellent agreement with previous measurements.
Abstract Achieving the radiation decay rate with high accuracy is crucial for all light emitters and absorbers. Therefore, a wide variety of measurement approaches have been developed for the measurement of radiation decay rate. Herein, we introduce a method to measure the decay rate with a common photoluminescence intensity setup without any modification. The square wave created by simply turning the output of the laser on and off is used as the excitation waveform, and a lock-in amplifier is used to gather the detected signal, making it a cost-effective way. Moreover, the constructed method can be used in a wide range of frequencies, detection voltages, and lifetimes. Finally, we applied the derived method to measure the photoluminescence lifetime of the first excited state of erbium ions in upconverting erbium-ytterbium silicate samples.
A spatially periodic distribution of excited electronic states is produced in ruby which is placed into the interference region of two light beams derived from a pulsed laser (wavelength 530 nm). The spatially modulated population density acts like an optical grating and is detected by diffraction of a quasi-cw laser beam (488 nm). The lifetimes of the population density gratings are measured for ruby crystals with 0.05 to 5% Cr-doping and compared with the fluorescence lifetimes of the R-lines (694 nm). For the 0.05% ruby both lifetimes are nearly equal. At higher Cr-concentrations the grating decay is considerably faster than the fluorescent decay. The grating lifetimes are independent of the grating period A = 1 to 100 μm. From this observation an upper limit of the diffusion constant for energy transfer between excited electronic states in ruby is estimated.
Techniques for the measurement of non-radiative decay rates between the optically excited states in solid-state laser materials are described. Two crystals have been investigated, ruby (Cr3+:Al2O3) and Sm2+:CaF2. A review of the spectroscopy of these materials is presented with particular emphasis on aspects relevant to non-radiative decay. An experiment is described using a second harmonic Nd3+ Nd:glass Q-switched laser to pump ruby crystals. Limiting non-radiative relaxation times were derived from the rise of fluorescence. Rubies varying in concentration between 0.05% and 0.4% were studied over a range of temperatures, 77 -> 7000 K. 4T2 -> 2E(E + 2A), 4T2 -> 2T1 (Eb) and 4T2 -> 2T1 (2A + Ea) non-radiative relaxations were all found to be <4ns in good agreement with the experimental results of other workers. Two experiments employing a ruby laser to pump Sm2+ :CaF2 are described. The first used a Q-switched laser and, apart from some improvements was similar to the ruby experiment. An upper limit of 4f5 5d pump -> metastable non-radiative decay of 2ns was found for Sm2+:CaF2, The fluorescence decay constant at 295 K was also measured to be 63ns. The second experiment employed a mode-locked laser and the Sm2+:CaF2 non-radiative decay time was <0.5ns. A new effect - provisionally called 'surface emission' - discovered during the ruby measurements, has been examined in detail and the results of this investigation are presented. A phase-shift technique for the measurement of non-radiative decay times is examined in detail. A pilot experiment using a self-mode-locked He-Ne laser at 633 nm to pump Sm2+:CaF2 is described. The non-radiative relaxation time found by this method was <33ps. Suggestions for improved experiments, both pulsed and phase-shifts, are made and the application of the techniques to other materials is discussed.
In this work we expose a recently developed theoretical model for the analysis of the measurements obtained with the so-called pinhole method, for the determination of the upper level lifetime in materials where the superimposition between fluorescence and absorption spectra determines significant radiation trapping effect. Under fairly general conditions the fluorescence decay curve after a pulsed excitation of the sample obtained with this experimental set-up can be conveniently described by a double exponential functional form. The faster of the two decay times is close to the intrinsic fluorescence lifetime, with a difference that can be calculated with geometrical considerations. The theoretical results were tested with a suitably designed experiment, where the upper level lifetime of the laser transition of some Yb doped samples were investigated, obtaining results that are consistent with literature data and in good agreement with the theoretical results as for the temporal dependence of the fluorescence decay.
At the time this investigation was undertaken the published experimental evidence indicated that the lifetime 1 4 of the T 2 state in pink ruby was approximately 5 0 ns. 4 Observation of fluorescence from the T 2 state had been reported but no details of the emission had been published.In this investigation it was demonstrated by direct 4 measurements that the T 2 state lifetime is less than 2 ns at temperatures between 2° K and 3 0 0° K.No fluorescent 4 emission from the T 2 state was observed although extremely high excitation powers and sensitive detection equipment were used.It was demonstrated that the nonradiative 4 2 transitions T 2 -► E occur with a frequency which is greater by a factor of at least 3 x 10^ than that of the fluorescent 4 4 4 4 Tj A 2 transitions.If the T 2 -* ■ A 2 transitions are considered to be purely electronic, it is shown that the 4 lxfetime of the T 2 state must be less than 45 p s .In the more likely event that the transitions are vibrationally assisted it is shown that the lifetime must be less than 3.5 p s .The author is indebted to the members of his doc toral committee who have been extremely generous with their time and facilities.Without such generosity the project could never have been completed.Professor C. Kikuchi was co-chairman of the committee