ESA processes responsible for infrared pumped, green and violet luminescence in low-concentrated Er: YAG

In this paper we discuss the up-conversion processes responsible for violet luminescence from the high energy erbium levels 2P3/2 (located at ∼31 000 cm−1) and 2H29/2 (∼24 000 cm−1) as well as for green luminescence from 4S3/2 level (∼18 000 cm−1) in low-concentrated (0.1 at%) Er: YAG crystals, at room temperature, excited with a cw Ti: Sapphire laser (pump transition 4I15/2→4I9/2). At this concentration excited state absorption (ESA) is the dominant up-conversion process. Several possibilities for populating the 2P3/2,2H29/2, and 4S3/2 levels, whose relative importance changes with the pump wavelength, are allowed by the energy level scheme of Er3+ in YAG. Thus, the 2P3/2 level can be populated by three-step processes (GSA + two ESA), the 2H29/2 level can be populated by both two- and three-step processes, while the 4S3/2 can be populated mainly by two-step processes. In order to put into evidence the specific chain of ESA which populate the levels 2P3/2, 2H29/2, and 4S3/2 for a given wavelength, we used absorption, fluorescence, and excitation spectroscopy as well as intensity measurements (fluorescence intensity vs. pump intensity). The comparison of the experimental excitation spectra with the simulated excitation spectra (SES) permitted us to identify the actual ESA processes which lead to the population of 2P3/2, 2H29/2, and 4S3/2 levels. The SES were constructed as products of GSA and ESA spectra. The intensity measurements confirmed that 2P3/2 level is populated by three-step mechanisms and shown that 2H29/2 level is populated mainly by a two-step one.