Markus Wermuth

Mechanisms of near-infrared to visible upconversion in CsCdBr3:Ho3+

Abstract Crystals of CsCdBr 3 doped with x % Ho 3+ ( x =0.35, 2.25) were synthesized and studied by high-resolution absorption, upconversion luminescence and excitation spectroscopy in the temperature range 295–4 K. The time evolution of the upconversion luminescence, within 1.5 ms of a pulsed excitation, was recorded. Two NIR to VIS upconversion mechanisms were identified. Below 100 K upconversion occurs by energy transfer within Ho 3+ pairs. Above 100 K an excited-state absorption is dominant. It occurs whenever the energies of the ground- and excited-state absorption steps coincide. At room temperature this latter process overall is 5 times more efficient than the energy-transfer process.

Up-conversion excitation of sharp Cr3+2E emission in YGG and YAG codoped with Cr3+ and Yb3+

The 2 E→ 4 A 2 emission of Cr 3+ in crystals and glasses is of great importance in science and technology. We found a new excitation mode for this emission in the garnets YGG and YAG codoped with Cr 3+ and Yb 3+ . It is based on an up-conversion mechanism using 2 F 5/2 of Yb 3 as an intermediate state. Excitation of the Yb 3+ in the near infrared around 1000 nm leads to visible Cr 3+ 2 E→ 4 A 2 emission around 700nm at low temperatures. The up-conversion luminescence intensity shows a strong temperature dependence due to an intrinsic loss mechanism. The up-conversion excitation spectra follow the Yb 3+2 F 7/2 → 2 F 5/2 absorptions. From its time dependence the up-conversion mechanism is found to be dominated by an energy transfer step: Two excited Yb 3 + ions simultaneously transfer their 2 F 5/2 excitation to Cr 3+ .

Upconversion excitation of Cr3+2E emission in Y3Ga5O12 codoped with Cr3+ and Yb3+

Abstract The 2 E → 4 A 2 emission of Cr3+ in crystals and glasses is of great importance in science and technology. We report a new excitation mode for this emission in Y3Ga5O12 (YGG) codoped with Cr3+ and Yb3+. It is based on an upconversion mechanism using 2 F 5/2 of Yb3+ as an intermediate state. Excitation in the near-IR at 969.6 nm leads to visible luminescence around 700 nm at low temperature. In YGG:2%Cr3+,1%Yb3+ at 15 K and for a laser power of 150 mW the efficiency of the process is 5.5%. From its time dependence the upconversion mechanism is found to be dominated by an energy transfer step: Two excited Yb3+ ions simultaneously transfer their 2 F 5/2 excitation to Cr3+.

Upconversion luminescence in a 5d transition-metal ion system: Cs2ZrCl6 : Os4+

Abstract The first upconversion luminescence in a 5d electron system is reported. At temperatures below 100 K excitation of Cs 2 ZrCl 6 : Os 4+ in the NIR at 11226 cm −1 leads to VIS and NIR luminescences between 17000 and 11500 cm −1 . The intraconfigurational character within (t 2g ) 4 of the d–d transitions involved is thought to be mainly responsible for this unusual phenomenon in a 5d transition metal complex.

Photon avalanche in Cs2ZrCl6: Os4+

The visible (VIS) upconversion luminescence of Cs2ZrCl6: x% Os4+ (x=0.1,1) upon excitation in the near infrared (NIR) is presented and analyzed. The crystals of Cs2ZrCl6: x% Os4+ (x=0.1,1) were synthesized by the Bridgman technique. VIS luminescence around 17 000 cm−1 is induced by NIR excitation between 10 500 and 14 500 cm−1. The three excited states Γ4 (3T1g), Γ5 (1T2g), and Γ1 (1A1g) with 15 K lifetimes of 20 ms, 8 μs, and 4 μs, respectively, in Cs2ZrCl6: 1% Os4+ participate in the upconversion. Excitation at the Γ4 (3T1g)→Γ1 (1A1g) energy shows the typical fingerprints of a photon avalanche. The dynamics of this excitation are calculated with a rate-equation model, and the observed concentration and temperature dependence of the processes are analyzed. The upconversion behavior of Cs2ZrCl6: x% Os4+ (x=0.1,1) is compared with the bromide analog Cs2ZrBr6: 0.2% Os4+.

Chemical tuning of transition metal upconversion properties

Abstract The upconversion properties of Ti 2+ , Ni 2+ and Os 4+ doped chloride and bromide host lattices are summarized and discussed. These systems are used to illustrate the field of transition metal based upconversion systems and their perspectives, since most upconversion materials investigated so far involve rare earth ions. Three strategies to tune the upconversion properties of transition metal ions by means of chemistry have been studied. They involve a chemically induced crossover in a metastable intermediate excited state, the use of metal–metal exchange interactions to enhance absorption oscillator strengths, as well as the suppression of efficient multiphonon relaxation channels in heavier halide host lattices.

Luminescence spectroscopy and NIR to VIS upconversion of Cs2GeF6: 2% Re4+

We present and analyse high-resolution absorption, luminescence and upconversion-luminescence spectra of Cs2GeF6: 2% Re4+. The crystals of Cs2GeF6: 2% Re4+ were grown from solution. Upon excitation around 11 000 cm−1 in the near infrared, visible upconversion luminescence around 17 000 cm−1 is observed. On increasing the temperature from 15 K to room temperature the integrated upconversion luminescence intensity decreases to 2%. This is attributed to the decreasing absorption cross-section at the monochromatic laser-excitation energy. The upconversion excitation spectra and the results obtained from time-dependent measurements indicate a dominant energy-transfer upconversion mechanism.

Lifetime determination of intermediate excited electronic states via up-conversion using sequences of square-wave excitation pulses

Abstract Methods to determine lifetimes of intermediate excited electronic states from up-conversion-luminescence transients induced by square-wave excitation pulses are presented. The methods are based on an excited-state absorption mechanism promoting population from a long-lived electronic state in the infrared to a short-lived emitting state in the visible. The methods are introduced for a three-level model system and applied to measure intermediate excited state lifetimes in three experimental examples of rare-earth or transition-metal ion doped insulating materials, namely CsCdBr 3 :Ho 3+ , Cs 2 ZrBr 6 :Os 4+ , and Cs 2 ZrCl 6 :Os 4+ .