Photon Upconversion
Overview
This material consists of general introduction of the phenomenon "photon upconversion". I have attached a file for better understanding. There are also references for further studies.
“Photon Upconversion”
**(Please refer the attached file for better understanding)**
By: Kiran Hiremath
Contents:
They can be synthesized by-
1. Lanthanide based upconversion materials (Ln, Z=57 to Lu, Z=71) 2. Transition metal doped upconversion materials (Cr3+, Ti2+, Re4+,Os4+, and Mo3+) 3. Organic upconversion materials (Organic and organo-metallic chromophores)
1. Lanthanides based UCM:
2. Transition metal ion(TM) doped UCM:
3. Organic UCM:
Different mechanisms have been recognized, although the three basic mechanisms are – a. Excited state absorption (ESA) b. Energy transfer upconversion (ETU) and c. Photon avalanche (PA)
Involves multistep excitation by sequentially absorbing one or more photons from the ground state to intermediate reservoir stage, and finally populates at excited state, from which upconversion luminescence occurs. (Efficiency = ~10-5cm2/W)
There are certain requirements in this process: 1. Two ion system (sensitizer and activator) which lie in close in proximity to each other. 2. Long intermediate states. 3. High absorption of the senstizer. 4. Matching of intermediate energy states of sensitizer and activator. (Lanthanide pair : Yb+3/Tm+3 and Yb+3/Er+3) (Efficiency: ~10-3cm2/W)
It can be characterized by three distinct nonlinear behaviors: transmission, emission, and rise time on the pump power intensity with generally the existence of a critical pump threshold. It’s the combination of the previous two mechanism culminating in the release of several high energy photons.
Even though upconversion process seems promising in diverse fields, the insufficient emission intensity has been a problem that restricts its application.
a. Weak absorption of lanthanide ions. b. Complex energy levels may result in many non-radiative pathways. c. Concentration quenching. (cross relaxation and energy migration)
a. Host matrix - optically transparent and low lattice phonon energy. b. Sensitizer - high absorption cross-section. c. Selection of suitable activator. d. Building proper shell around the UCNPs to minimize the surface effects.
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