Photon Upconversion in Lanthanide-Activated Inorganic Luminescent Materials

Introduction

There is a great deal of research interest in development of the visible light emitting materials for different photonics, sensing and biomedical applications [Mader et al., 2010; Xu et al., 2010; Wade et al., 2003; Ye et al., 2010; Le et al., 2006; Kim et al., 2000]. Inorganic phosphors are very efficient materials for such task and used heavily by researchers working in the field of photonics. These materials can exhibit the desired emission by controlling several parameter e.g. materials ingredients, methods of preparation and process of pumping. Two main processes are famous for photon conversion in phosphor materials, namely, upconversion (UC) and downconversion (DC). Photon UC is the process of generating higher energy photon via pumping with a lower one. It is also termed as anti-stoke type emission whereas DC is the stoke type process, observed on excitation of material by high energy than the emitted radiations [Wang et al., 2009]. The term UC was proposed first time in 1959 by Bloembergen and then experimentally verified in 1966 by Auzel [Gao et al., 2004]. It is more effective way of generating visible emission by absorption of two or more low energy photons via different process such as, ground state absorption (GSA), excited state absorption (ESA), energy transfer (ET) etc. [Auzel, 2004]. Lanthanides (Ln³⁺) are very suitable candidates for the photon UC owing to their large number of energy levels, narrow emission spectra, long lifetime of the excited states, good chemical durability and they can be easily populated by the near infrared (NIR) radiations [Prasad, 2004; Dieke et al., 1968].

Figure 1.

Schematic representation of photon conversion process

Actually Ln³⁺ serves as activator and/or sensitizer and gives intense multicolour light emissions throughout visible region of electromagnetic spectrum, when embedded into a suitable host lattice. Mostly, the NIR diode lasers have been used as excitation source in UC process, which are less expensive with compare to the ultra violet (UV) lasers. Also, NIR radiation leads low autofluorescence, less scattering and absorption, and deep penetration to the target [Wang et al., 2009; Auzel, 2004]. A schematic representation of thetwo process of photon conversion is shown in Figure 1. It is clearly shown in the figure that NIR photon is converting in to the visible/UV by UC process. On the other hand, a UV or blue photon is converting into other visible or NIR photons by DC process. A three level energy scheme is also shown for both processes. In UC process, the atoms/ions (also known as luminescent-centers/active-centers) in ground state (E₁) are jump to excited state (E₂) via GSA process (by absorbing energy hν₁) and then to excited state (E₃) through ESA process (by absorbing a second photon of energy hν₂; hν₁ may equal to hν₂). A radiative transition from E₃ to E₁ gives the UC emission. In DC process, atoms in ground state (E₁) reaches directly to excited state (E₃) by absorbing a photon of energy hν₁ and relax back to ground state by releasing slightly lower photon hν₂ (hν₁> hν₂) involving some non radiative relaxation between intermediate level (E₂). A more clear strategy of UC processes will present in the next section of the chapter.