History of upconversion discovery and its evolution
Introduction
Before the sixties, all known anti-Stokes emissions would involve energies above excitation energies not higher than about kT. They were linked to thermal population of energy states above excitation states by about such an amount of energy difference. It was the well-known case of the fluorescence emissions from the so-called “thermal bands” or in the Raman effect for the “anti-Stokes bands”. Also Thermoluminescence, where traps are emptied by excitation energies of the order of kT, also constituted a field of anti-Stokes emission of its own. “Superexcitation”, that is raising an already excited electron to a still higher level was also known but with very weak intensity emissions. All these types of well-known anti-stokes processes were then presented in classical textbooks on luminescence [1].
Having been an actor of the upconversion field since its very begining now more than 50 years ago (1966), I shall present here some personal memories of the beginnings of this field.
Then I shall discuss the various efficient processes providing such large “anti-Stokes” emissions now called “upconversion processes” for which emissions are found to exceed excitation energies by 10–100 times kT and finally introduce the initial papers directing up-conversion towards their now wide biological applications.
Section snippets
Finding a thesis subject in 1965
In 1965, coming back to France-Telecom (FT) from my military service (1963–64) for the French Navy, I was looking to prepare a thesis in Pr. Otto Deutschbein Laboratory at CNET1 (Centre National d’Etudes des
The APTE effect some time called ETU for energy transfer upconversion
When active ions are situated at a sufficiently short distance for interactions between them to take place, two types of upconversion processes may occur:
- i)
summation of photon by energy transfers, I called the “APTE effect” (see above) [16];
- ii)
cooperative effects either by sensitization [25] or emission [26]. Both types, i) and ii), are often mistaken one for the other because they present several similarities and may be simultaneously present in a given system for a given excitation.
Of course the
Practical applications, evolution, and present day uses
As seen, among upconversion processes, APTE effects are the most efficient ones, because they involve at least one order of perturbation less than cooperative sensitization processes and other upconversion processes.
Yet, Cooperative luminescence effects, being only effective at the shortest distances, may be used very usefully as probe for RE ions clustering in laser materials and other RE-doped medium [28]. Finally it is worth saying that based on energy tranfers (APTE and Avalanche)
Conclusion
Upconversion, from basic physics evolved into an indispensable optical tool in low energy photon excitation which contrary to previously used UV and blue excitations avoids irradiation damages to the sample, minimizing autofluorescence and energy penetration of a too strong excitation light.
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