Two-photon Excited-state Absorption Research Articles

RETRACTED: Near-infrared-to-visible upconverting luminescence of Er3+-doped CdSe nanocrystals grown by chemical bath

Abstract CdSe thin films were prepared by chemical bath and doped in situ with Er3+ ions. Three impurity levels were prepared by changing the relative volume of the salt solution containing Er3+ ions in the CdSe growing solution. Changes in the grain size (∼5.5–3.5 nm) and band gap energy (∼1.80–2.25 eV) were observed in the undoped and doped CdSe films, respectively. Photoluminescence studies displayed room temperature emission exhibiting NIR-to visible upconversion. The transition bands 4I13/2 → 4I15/2, 2I9/2 → 4I15/2 and 2F9/2 → 4F15/2 in the ∼700–850 nm region were investigated. Upconversion emissions were observed from the CdSeEr sample under light excitation (325 nm). The upconversion emission intensity ratio of these transitions is attributed to the variation of the local structure around Er3+ ions. These results confirm that visible upconversion emissions of Er3+ in the CdSeEr nanocrystals are mainly produced via two-photon excited-state absorption and energy transfer upconversion processes.

Infrared to visible upconversion luminescence in Er 3+/Yb 3+ doped titanate glass prepared by containerless processing

Er 3+ doped TiO 2–La 2O 3 glasses modified by ZrO 2 have been successfully fabricated by the containerless method with incorporated Yb 3+ ions as sensitizers. Under the excitation of 980 and 808 nm diode lasers, visible emissions centered at 534, 554 and 674 nm are observed, which are assigned to the Er 3+ transitions of 2H 11/2→ 4I 15/2, 4S 3/2→ 4I 15/2 and 4F 9/2→ 4I 15/2, respectively. The emission signals are so strong that they can be observed by naked eyes even at pumping power as low as 20 mW. Measurements of pump-power dependent intensity and time-resolved decay behavior of upconversion luminescence show that two-photon excited state absorption (ESA) and energy transfer (ET) between rare earth ions are the predominant mechanisms for upconversion emissions. Besides, the intensity of upconversion luminescence has been enhanced by increasing the concentration of ZrO 2 in these rare earth doped bulk titanate glasses.

Visible and infrared luminescence properties of Er 3+-doped Y 2Ti 2O 7 nanocrystals

Er 3+-doped Y 2Ti 2O 7 nanocrystals were fabricated by the sol–gel method. While the annealing temperature exceeds 757 °C, amorphous pyrochlore phase Er x Y 2− x Ti 2O 7 transfers to well-crystallized nanocrystals, and the average crystal size increases from ∼70 to ∼180 nm under 800–1000 °C/1 h annealing. Er x Y 2− x Ti 2O 7 nanocrystals absorbing 980 nm photons can produce the upconversion (526, 547, and 660 nm; 2 H 11/2→ 4 I 15/2, 4 S 3/2→ 4 I 15/2, and 4 F 9/2→ 4 I 15/2, respectively) and Stokes (1528 nm; 4 I 13/2→ 4 I 15/2) photoluminescence (PL). The infrared PL decay curve is single-exponential for Er 3+ (5 mol%)-doped Y 2Ti 2O 7 nanocrystals but slightly nonexponential for Er 3+ (10 mol%)-doped Y 2Ti 2O 7 nanocrystals. For 5 and 10 mol% doping concentrations, the mechanism of up-converted green light is the two-photon excited-state absorption. Much stronger intensity of red light relative to green light was observed for the sample with 10 mol% dopant. This phenomenon can be attributed to the reduced distance between Er 3+–Er 3+ ions, resulting in the enhancement of the energy-transfer upconversion and cross-relaxation mechanisms.

Yellow-to-violet, blue, and green frequency upconversions in Nd3+-doped PbWO4 single crystal

Upconversion emission properties of Nd3+ ions doped in PbWO4 crystal are investigated by pulsed laser excitation into the G5∕24+G2(1)7∕2 levels at temperatures from 10to295K. The main upconversion emission consists of strong blue emission bands in the wavelength region of 435–447nm corresponding to the P1∕22→I9∕24 transitions. Some UV bands located in the wavelength region of 356–397nm corresponding to the D3∕24→I9∕24, I11∕24, and I13∕24 transitions and weak green emission at 525.2nm corresponding to the G7∕24→I9∕24 transition are observed. Several upconversion blue emissions from the P1∕22 level and the absoprtion lines due to the I9∕24→P1∕22 transition indicate multisite structure of the Nd3+ ions in the PbWO4 lattices. We investigate the excitation spectra of the upconversion emissions and their pump power dependence and lifetimes from which the probable upconversion mechanisms are proposed and analyzed. The dominant mechanisms for the upconversion emission originating from the D3∕24 level (UV emission) and the P1∕22 level (blue emission) are proven to be the sequential two-photon excited-state absorption, while the energy transfer upconversion process is responsible for the green upconversion emission from the G7∕24 level of Nd3+.

Luminescence Spectroscopy and Visible Upconversion Properties of Er3+ in ZnO Nanocrystals

Er3+-doped ZnO nanocrystals with hexagonal wurtzite structure were prepared by addition of LiOH to Zn−O−Er precursors in ethanol. The well-known green hot band from the 2H11/2 → 4I15/2 transition in the 520 − 540 nm region and Stokes emissions were investigated. And the blue, green, and red upconversion emissions were observed from the annealed samples under near-infrared light (808 nm) excitation. The upconversion emission intensity ratio of the hypersensitive 2H11/ 2 → 4I15/2 to 4S3/2 → 4I15/2 transition (I530nm/I550nm) in the sample annealed at 400 °C is different from that in the sample annealed at 700 °C, which is attributed to the variation of the local structure around Er3+ ions. The upconversion emission spectra and the pump power dependence of the luminescence intensities are provided to explain the upconversion mechanisms. These results confirm that visible upconversion emissions of Er3+ in the ZnO nanocrystals are mainly produced via two-photon excited-state absorption (ESA) and energy transfer...

Fluorescence Upconversion Properties of Er3+-Doped TiO2 and BaTiO3 Nanocrystallites

The effects of the erbium concentration, crystal size, crystal phase, and different processing temperatures on the upconverted emission of Er3+ in BaTiO3 and TiO2 nanocrystals are reported. A cw diode laser at 975 nm was used as a pump source for resonant sequential excitation of the 4I11/2 and 4F7/2 levels. Green and red upconversion emission at 550 and 670 nm were observed from these oxide nanocrystals with 975 nm excitation. With the same Er3+ concentration, the upconversion emission intensity from BaTiO3 was higher than that observed in the TiO2 host. In the TiO2 matrix, the maximum upconversion emission intensity of Er3+ was found for samples calcined at 800 °C where both the anatase and the rutile phases were present. The observed emission characteristics and the pump intensity dependence of the luminescence intensity confirm that the upconverted emission in these materials is produced by two-photon excited-state absorption (ESA) processes.

Effect of crystal nature on upconversion luminescence in Er3+:ZrO2 nanocrystals

The effects of crystal size and crystal phase on the upconverted emission of Er3+ in ZrO2 oxide nanocrystals are reported. Green (550 nm) and red (670 nm) upconversion emission were observed at room temperature from the S43/2 and F49/2 levels of Er3+:ZrO2 nanocrystals. It is found that at 850 mW of cw excitation power, the total luminescence was 11960 Cd/m2 for 1000 °C heated sample. We observed that the overall upconversion luminescence intensity depends on crystal structure and particle size. We have also confirmed that upconversion process in all these samples results from a two-photon excited-state absorption process.

Optical Spectroscopy and Upconversion Studies of Ho3+-Doped Bulk and Nanocrystalline Y2O3

In this paper we report on the optical spectroscopy and upconversion studies of Ho3+-doped nanocrystalline and bulk Y2O3, as a function of holmium concentration (0.1, 0.5, 1, 2, 5, and 10 mol %). Emission in the blue, green, red, and NIR portions of the spectrum is recorded after 457.9-nm excitation. Red (646 nm) pumping results in blue and green emission through a two-photon excited-state absorption (ESA) upconversion process. NIR (754 nm) pumping results in blue, green, and red emission. Overall luminescence of the nanocrystalline samples under one-photon excitation or 646-nm pumping is severely reduced when compared to that of the bulk sample, or nonexistent in the case of the 754-nm pumping, which is attributed to adsorbed atmospheric carbon dioxide and/or water on the surface of the nanocrystals. A quenching of the green (5F4, 5S2) → 5I8 emission is noted with increasing Ho3+ concentration that is attributed to a cross-relaxation mechanism involving two holmium ions.

NIR to Visible Upconversion in Nanocrystalline and Bulk Lu2O3:Er3+

In this paper we investigate the upconversion luminescence of bulk and nanocrystalline Lu 1 . 9 8 Er 0 . 0 2 O 3 following excitation into the 4 I 1 1 / 2 level using 980 nm radiation. After NIR continuous wave excitation, blue, green, and red emissions were observed for both samples under investigation with the green emission dominating the upconversion spectrum. A power study reveals that the 4 S 3 / 2 (green) and 4 F 9 / 2 (red) emitting levels are populated via two-photon excited-state absorption (ESA) and energy-transfer upconversion (ETU) processes. Upconversion from the 2 P 3 / 2 level is attained via a three-step phonon-assisted energy-transfer process. A marked temperature dependence of the excited-state lifetimes in both the bulk and nanocrystalline samples was evidenced.

Upconversion in Er3+:ZrO2 Nanocrystals

Erbium-doped ZrO2 nanoparticles are prepared by a sol−emulsion−gel technique. The effects of the Er3+ concentration and different codopants (Yb3+ and Y3+) in the ZrO2 matrix on the upconverted emission are reported. Green and red upconversion emission at 550 and 670 nm were observed from these oxide nanocrystals with 980 nm excitation. The overall intensity decreases with an increasing concentration of erbium in zirconia. The presence of codopants (Y3+ and Yb3+) also increases the overall intensity of the upconverted emission. The emission spectra and the pump intensity dependence of the luminescence intensities are used to understand the excitation mechanism. These results confirm that upconverted emission in these materials is due to a two-photon excited-state absorption (ESA)/energy transfer upconversion (ETU) process.