Photon Upconversion Spectroscopy
391
UC emission in organic molecules. Lanthanide based UC emission rely on 4f-4f transitions while TTA based UC emission is observed through triplet states of moleculebased chromophores. The mechanism of UC emission is different in these two cases
and is given in detail in the following sections.
1.2 Photon Upconversion Mechanism in Lanthanides
There are several mechanisms involved in lanthanide based UC emission process
namely: ground state absorption (GSA)/excited state absorption (ESA), energy
transfer upconversion (ETU), photon avalanche (PA), cooperative energy transfer
(CET) based UC, and cross relaxation (CR) process, etc. The details of all these
processes are as follows.
1.2.1 Ground State Absorption (GSA)/Excited State Absorption (ESA)
Process
GSA/ESA is a single ion process which was first introduced by Bloembergen in
1959 by using NIR photons for sequential excitation [17]. ESA process seems to be
a simple and basic process but its requirement is meticulous. In this process, ions in the
ground state absorb two or more low energy photons successively and then promoted
to a high lying excited state. From excited state it undergoes radiative transition to
ground state by emitting a high energy photon. Herein, the absorption cross section
of the intermediate state should be high and it should be a metastable state. This
process can be understood by using three level energy systems as shown in Fig. 2a.
When a resonant photon having energy equal to the energy difference between the
two levels 1 and 2 is incident on the sample, the ions in level 1 are promoted to
level 2 via GSA. If the level 2 is a metastable level, the ions in level 2 get sufficient
time to absorb another incident photon and thus they may be lifted to level 3 through
ESA process. The emission from level 3 to 1 gives the photon of higher energy than
that of the incident photons. Possibility of absorptions of photons in level 3 to go to
further excited state is also possible. Several trivalent lanthanide ions (Ln
3+ ) having
ladder-like energy levels can achieve efficient ESA. For example, Ho
3+ , Er
3+ , Nd
3+
and Tm
3+ are a few such lanthanide ions which give efficient ESA based UC under
975 and/or 808 nm excitations [18–20].
1.2.2 Energy Transfer Upconversion (ETU) Process
ETU process recognized at first by Auzel in the year 1966 [21]. This is a two ion
process, in which one is known as sensitizer and the other is activator. Herein, the
absorption cross section of the sensitizer ion remains usually larger than the activator
ion. In this process, energy transfer, as shown in Fig. 2b, may take place from excited
391
UC emission in organic molecules. Lanthanide based UC emission rely on 4f-4f transitions while TTA based UC emission is observed through triplet states of moleculebased chromophores. The mechanism of UC emission is different in these two cases
and is given in detail in the following sections.
1.2 Photon Upconversion Mechanism in Lanthanides
There are several mechanisms involved in lanthanide based UC emission process
namely: ground state absorption (GSA)/excited state absorption (ESA), energy
transfer upconversion (ETU), photon avalanche (PA), cooperative energy transfer
(CET) based UC, and cross relaxation (CR) process, etc. The details of all these
processes are as follows.
1.2.1 Ground State Absorption (GSA)/Excited State Absorption (ESA)
Process
GSA/ESA is a single ion process which was first introduced by Bloembergen in
1959 by using NIR photons for sequential excitation [17]. ESA process seems to be
a simple and basic process but its requirement is meticulous. In this process, ions in the
ground state absorb two or more low energy photons successively and then promoted
to a high lying excited state. From excited state it undergoes radiative transition to
ground state by emitting a high energy photon. Herein, the absorption cross section
of the intermediate state should be high and it should be a metastable state. This
process can be understood by using three level energy systems as shown in Fig. 2a.
When a resonant photon having energy equal to the energy difference between the
two levels 1 and 2 is incident on the sample, the ions in level 1 are promoted to
level 2 via GSA. If the level 2 is a metastable level, the ions in level 2 get sufficient
time to absorb another incident photon and thus they may be lifted to level 3 through
ESA process. The emission from level 3 to 1 gives the photon of higher energy than
that of the incident photons. Possibility of absorptions of photons in level 3 to go to
further excited state is also possible. Several trivalent lanthanide ions (Ln
3+ ) having
ladder-like energy levels can achieve efficient ESA. For example, Ho
3+ , Er
3+ , Nd
3+
and Tm
3+ are a few such lanthanide ions which give efficient ESA based UC under
975 and/or 808 nm excitations [18–20].
1.2.2 Energy Transfer Upconversion (ETU) Process
ETU process recognized at first by Auzel in the year 1966 [21]. This is a two ion
process, in which one is known as sensitizer and the other is activator. Herein, the
absorption cross section of the sensitizer ion remains usually larger than the activator
ion. In this process, energy transfer, as shown in Fig. 2b, may take place from excited