Unusual photoluminescence phenomena: New insights in Stokes and anti-Stokes emission

Abstract

Photoluminescent materials are applied in many devices that we use in our daily lives. For example in fluorescent lamps and LED-lamps, photoluminescent materials convert the source light to create white light. Photoluminescent materials can also play a role in more complicated devices, as for example in medical imaging. New applications of photoluminescent materials require some unusual photoluminescence properties and some of these properties are investigated in this thesis. First, the ambiguity about the relation between the Huang-Rhys parameter and Stokes shift of a photoluminescent material is resolved. The Stokes shift is the energy difference between the absorbed and emitted photons of a material. This shift is a consequence of changes in chemical bonding within a luminescent center during excitation and emission, which is quantified in the Huang-Rhys parameter. Two mutually exclusive relations between the Huang-Rhys parameter and Stokes shift are prevalent in literature. This thesis clarifies this ambiguity. Second, an upconverting photoluminescent material based on divalent thulium is investigated. Upconversion is the absorption of two low-energy photons, followed by the emission of one high-energy photon. This photon conversion is useful for the use of low-energy photons in photovoltaic cells. Quantum chemical calculations and a two-color two-photon upconversion experiment show that the excited states of Tm2+ have parallel potential energy surfaces, which explains why Tm2+ can function as an efficient upconversion material. Third, a new experiment is developed and applied with the aim to confirm that some photoluminescent materials emit two low-energy photons after absorption of one high-energy photon. This phenomenon, known as photon cutting, was so far difficult to detect. Photon cutting can convert the solar spectrum in such a way that the high-energy photons in the spectrum are used more efficiently in photovoltaic cells. The experiment described in this thesis utilizes the photon cutting emission property that photons are emitted in pairs. When the photon time correlation of the emitted photons shows bunching in pairs, this is an unambiguous proof of photon cutting. The correlation experiment was applied to two well-known photon cutting materials based on the lanthanides Pr3+ and Gd3+-Eu3+ as a proof of concept. Last, this thesis describes a photoluminescent material based on divalent bismuth. Whereas most photoluminescent materials are based on excitation of electrons within a d- or f-subshell of from a f- to a d-subshell in the electron cloud of the atom, the orange-red photoluminescence of Bi2+ finds its origin in an electronic excitation within the p-subshell of the bismuth ion. With quantum chemical calculations it was confirmed that such an excitation is indeed responsible for the Bi2+ photoluminescence. Furthermore, the shape of the emission band fine structure was explained and it was shown that Bi2+ emission can be tuned to any orange or red color by changing the dopant host lattice.