Photon Upconversion Spectroscopy
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pump photons and they reach to a transient state from where the combined energy
is transferred to one activator ion simultaneously. The UC is observed via radiative
emission from the acceptor ion. The basic condition to get CET process is that, the
activator emission energy should be twice to that of the energy of sensitizer emission.
1.2.4 Photon Avalanche Process
The proposal of photon avalanche process was devised by Chivian in 1979 with
a model example of Pr
3+ doped LaCl 3 /LaBr 3 materials [22]. This process is relatively complex compared to ESA and ETU processes as it requires a threshold pump
density to begin with. This is due to fact that, the energy gap between intermediate
excited state and the ground state and the energy of pump photons show considerable mismatch. Usually, energy of pump photons is lower. The mechanism of photon
avalanche is explained with the help of three level systems, shown in Fig. 2d. In
the first step, ions are pumped to the intermediate state 2 through a weak and nonresonant GSA process. This is followed further by a resonant excitation to level 3
by ESA process. Subsequently, an energy transfer between the ion in level 3 and the
neighboring ground state ion takes place through resonant cross relaxation process
which ultimately populates two ions in the intermediate state. Thus, population in
the intermediate state increases exponentially with repetition of cross relaxation
beyond a threshold. This thereby causes a multiple enhancement of population in
level 3 through strong ESA process. Thus a strong UC emission from level 3 to
ground state is observed through avalanche process. The necessary conditions for
the photon avalanche process are (i) excitation energy should not be in resonance
with the ground state absorption, (ii) excited state absorption cross section should
be very high, and (iii) concentration of rare earth ions should be high enough for
ion-ion interaction which may be efficient for the cross-relaxation process.
1.2.5 Cross Relaxation (CR) Process
Cross relaxation is a process of energy transfer between two identical ions or two
ions having similar type of energy separation between two levels. The fundamental
reason for CR is ion-ion interaction. The mechanism of cross-relaxation process
is demonstrated with the help of three level systems as shown in Fig. 2e. The deexcitation energy of the ion 1 from level 2 to 1, is transferred to the ion 2 to excite
it from level 2 to 3 and vice-versa. The process of energy transfer can be expressed
as [(3, 1) ↔ (2, 2)]. The condition for CR process requires a high concentration of
ions.
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pump photons and they reach to a transient state from where the combined energy
is transferred to one activator ion simultaneously. The UC is observed via radiative
emission from the acceptor ion. The basic condition to get CET process is that, the
activator emission energy should be twice to that of the energy of sensitizer emission.
1.2.4 Photon Avalanche Process
The proposal of photon avalanche process was devised by Chivian in 1979 with
a model example of Pr
3+ doped LaCl 3 /LaBr 3 materials [22]. This process is relatively complex compared to ESA and ETU processes as it requires a threshold pump
density to begin with. This is due to fact that, the energy gap between intermediate
excited state and the ground state and the energy of pump photons show considerable mismatch. Usually, energy of pump photons is lower. The mechanism of photon
avalanche is explained with the help of three level systems, shown in Fig. 2d. In
the first step, ions are pumped to the intermediate state 2 through a weak and nonresonant GSA process. This is followed further by a resonant excitation to level 3
by ESA process. Subsequently, an energy transfer between the ion in level 3 and the
neighboring ground state ion takes place through resonant cross relaxation process
which ultimately populates two ions in the intermediate state. Thus, population in
the intermediate state increases exponentially with repetition of cross relaxation
beyond a threshold. This thereby causes a multiple enhancement of population in
level 3 through strong ESA process. Thus a strong UC emission from level 3 to
ground state is observed through avalanche process. The necessary conditions for
the photon avalanche process are (i) excitation energy should not be in resonance
with the ground state absorption, (ii) excited state absorption cross section should
be very high, and (iii) concentration of rare earth ions should be high enough for
ion-ion interaction which may be efficient for the cross-relaxation process.
1.2.5 Cross Relaxation (CR) Process
Cross relaxation is a process of energy transfer between two identical ions or two
ions having similar type of energy separation between two levels. The fundamental
reason for CR is ion-ion interaction. The mechanism of cross-relaxation process
is demonstrated with the help of three level systems as shown in Fig. 2e. The deexcitation energy of the ion 1 from level 2 to 1, is transferred to the ion 2 to excite
it from level 2 to 3 and vice-versa. The process of energy transfer can be expressed
as [(3, 1) ↔ (2, 2)]. The condition for CR process requires a high concentration of
ions.