Professor Markus Pollnau

SrAl2O4 that is optically activated by Eu2+, often additionally co-doped with Dy3+, is a non-radioactive persistent phosphor which is known for its excellent afterglow properties. It has found various applications, e.g. in the watch industry, for security signs, in medical diagnostics, and in photovoltaics. The monoclinic SrAl2O4 was synthesized in polycrystalline form and structurally characterized. Its luminescence and afterglow properties were studied. Wavelength-dependent thermoluminescence experiments were performed on SrAl2O4:Eu and SrAl2O4:Eu,Dy polycrystalline samples. Substitution of Sr2+ by Eu2+ on two different Sr sites in the crystal is associated with blue and green Eu2+ emission. Excitation at 445 nm allows to selectively excite one of the two different Eu2+ ions, whereas excitation at 375 nm excites both Eu2+ ions. Incorporation of dysprosium increases significantly (by a factor of about 4 to 8) the total number of traps involved in the afterglow of this persistent phosphor. Increasing the temperature at which the samples are irradiated (loaded) from 173 K to 248 K reveals that many new traps can only be occupied or activated at higher temperatures, leading to a strong increase of the integrated thermoluminescence intensity, in particular for the Dy-codoped samples. The results of this study reveal that the diversity of traps leading to the long afterglow is much larger than previously reported in the literature. We propose that the presence of dysprosium induces an excitation-induced charge-transfer reaction Eu2+ + Dy3+ -> Eu3+ + Dy2+. However, the principal traps responsible for the efficient afterglow are temperature-activated and appear to be associated with the green-emitting Eu2+ ion on the Sr2 site coupled to a nearby dysprosium ion.

By exploiting Einstein's rate-equation approach [A. Einstein, Phys. Z. 18, 121 (1917)] to Planck's law of blackbody radiation [M. Planck, Ann. Phys. 309, 553 (1901)], we obtain a simple relation between the population densities of the two energy levels of the atomic oscillators in the walls of the black body, as assumed by Einstein in his paper from 1917, and the occupation numbers in a photonic excited and ground state. This relation establishes a quantum principle for photons and, more generally, all bosons, which has the same physical relevance as Pauli's exclusion principle [W. Pauli, Z. Phys. 31, 765 (1925)], which is the quantum principle for fermions. We demonstrate the equivalence of these two quantum principles by inserting either of them into the Boltzmann distribution, thereby transforming the Boltzmann distribution into either the Fermi-Dirac [E. Fermi, Rendiconti Lincei 3, 145 (1926)] and Bose-Einstein [Bose, Z. Phys. 26, 178 (1924)] distribution.

Wavelength-dependent thermoluminescence (TL) experiments were performed on SrAl2O4:Eu, SrAl2O4:Eu,B, SrAl2O4:Eu,Dy and SrAl2O4:Eu,Dy,B polycrystalline samples. Excitation at 445 nm allows to selectively excite one of the two different Eu2+ ions substituting for Sr in the crystal, whereas excitation at 375 nm excites both Eu2+ ions. Incorporation of boron generates the deepest traps which contribute to the very long afterglow in this material, while dysprosium increases significantly (by a factor of about 4–8) the total number of traps involved in the afterglow of this persistent phosphor. Increasing the temperature at which the samples are irradiated (loaded) from 173 K to 248 K reveals that many new traps can only be occupied or activated at higher temperatures, leading to a strong increase of the integrated TL intensity, in particular for the Dy-containing samples. Boron does not appear to contribute to these thermally-activated traps significantly responsible for the long afterglow of SrAl2O4:Eu,Dy,B. The results of this study reveal that the diversity of traps leading to the long afterglow is much larger than previously reported in the literature. We propose that boron stabilizes F centers (which absorb in the far UV), while the presence of dysprosium induces an excitation-induced charge-transfer reaction Eu2+ + Dy3+ → Eu3+ + Dy2+. However, the principal traps responsible for the efficient afterglow are temperature-activated and appear to be associated with the green emitting Eu2+ ion on the Sr2 site coupled to a nearby dysprosium ion. [Display omitted] •Detailed thermoluminescence experiments were done on differently doped SrAl2O4 powders.•Thermally activated traps are found for Dy-doped samples.•An excitation-induced charge transfer between Eu and Dy ions is postulated.

The degree of spectral coherence characterizes the spectral purity of light. It can be equivalently expressed in the time domain by the decay time z or the quality factor Q of the light-emitting oscillator, the coherence time tau(coh) or length l(coh) of emitted light or, via Fourier transformation to the frequency domain, the linewidth Delta nu of emitted light. We quantify these parameters for the reference situation of a passive Fabry-Perot resonator. We investigate its spectral line shapes, mode profiles, and Airy distributions and verify that the sum of all mode profiles generates the corresponding Airy distribution. The Fabry-Perot resonator is described, as an oscillator, by its Lorentzian linewidth and finesse and, as a scanning spectrometer, by its Airy linewidth and finesse. Furthermore, stimulated and spontaneous emission are analyzed semi-classically by employing Maxwell's equations and the law of energy conservation. Investigation of emission by atoms inside a Fabry-Perot resonator, the Lorentz oscillator model, the Kramers-Kronig relations, the amplitude-phase diagram, and the summation of quantized electric fields consistently suggests that stimulated and spontaneous emission of light occur with a phase 90 degrees in lead of the incident field. These findings question the quantum-optical picture, which proposed, firstly, that stimulated emission occurred in phase, whereas spontaneous emission occurred at an arbitrary phase angle with respect to the incident field and, secondly, that the laser linewidth were due to amplitude and phase fluctuations induced by spontaneous emission. We emphasize that the first derivation of the Schawlow-Townes laser linewidth was entirely semi-classical but included the four approximations that (i) it is a truly continuous-wave (cw) laser, (ii) it is an ideal four-level laser, (iii) its resonator exhibits no intrinsic losses, and (iv) one photon is coupled spontaneously into the lasing mode per photon-decay time tau(c) of the resonator, independent of the pump rate. After discussing the inconsistencies of existing semi-classical and quantum-optical descriptions of the laser linewidth, we introduce the spectral-coherence factor, which quantifies spectral coherence in an active compared to its underlying passive mode, and derive semi-classically, based on the principle that the gain elongates the photon-decay time and narrows the linewidth, the fundamental linewidth of a single lasing mode. This linewidth is valid for lasers with an arbitrary energy-level system, operating below, at, or above threshold and in a cw or a transient lasing regime, with the gain being smaller, equal, or larger compared to the losses. By applying approximations (i)-(iv) we reproduce the original Schawlow-Townes equation. It provides the hitherto missing link between the description of the laser as an amplifier of spontaneous emission and the Schawlow-Townes equation. Spontaneous emission entails that in a cw lasing mode the gain is smaller than the losses. We verify that also in the quantum-optical approaches to the laser linewidth, based on the density-operator master equation, the gain is smaller than the losses. We conclude this work by presenting the derivation of the laser linewidth in a nut shell.

In the literature one finds several conflicting accounts of the phase difference of stimulated and spontaneous emission, as well as absorption, with respect to an existing (triggering) electromagnetic field. One of these approaches proposes that stimulated emission and absorption occur in phase and out of phase with their driving field, respectively, whereas spontaneous emission occurs under an arbitrary phase difference with respect to an existing field. It has served as a basis for explaining quantum-mechanically the laser linewidth, its narrowing by a factor of 2 around the laser threshold, as well as its broadening due to amplitude-phase coupling, resulting in Henry’s α-factor. Assuming the validity of Maxwell’s equations, all three processes would, thus, violate the law of energy conservation. In semi-classical approaches, we investigate stimulated emission in a Fabry-Pérot resonator, analyze the Lorentz oscillator model, apply the Kramers-Kronig relations to the complex susceptibility, understand the summation of quantized electric fields, and quantitatively interpret emission and absorption in the amplitude-phase diagram. In all cases, we derive that the phase of stimulated emission is 90° in lead of the driving field, and the phase of absorption lags 90° behind the transmitted field. Also spontaneous emission must obey energy conservation, hence it occurs with 90° phase in lead of an existing field. These semi-classical findings agree with recent experimental investigations regarding the interaction of attosecond pulses with an atom, thereby questioning the physical explanation of the laser linewidth and its narrowing or broadening.

Pr3+-doped yttria (Y2O3) nanocrystal layers embedded in between pure yttria thin films were prepared on four different substrates. Pulsed laser deposition was used to fabricate this sandwich-like structure. An exhaustive structural and optical characterization of the initial nanocrystals was performed to study their preservation once incorporated between the deposited thin films. We demonstrate that the prepared Y2O3:Pr3+ nanocrystals can be integrated into the thin films after the pulsed laser deposition process, retaining their original crystal structure and luminescent features regardless of the number of deposition cycles and the nature of the substrate. In this sense, we present a novel method to embed and protect the luminescent material, paving the way for developing future optoelectronic applications.

A simple recursive method based on the circulating field approach to obtain the exact electric-field and intensity distributions in an arbitrary multi-resonator structure is presented. Reflectivity curves obtained via this method and the coupled-mode theory are compared.

Rare-earth-doped solid-state lasers utilize distributed-feedback ( DFB) resonators to generate ultra- narrow-linewidth emission down to a few kHz. The longitudinal modes of a single Fabry-Perot and a DFB resonator are influenced by the structure, the optical excitation configuration, and external feedback, as has been observed experimentally. Here we investigate the influence of bi-directional launching of light and external feedback on the longitudinal modes of single Fabry-Perot, double Fabry-Perot, and DFB resonators. These results can be exploited for the technological advancement of DFB lasers.

Ridge waveguides in amorphous Al 2 O 3 :Yb 3+ are produced by reactive co-sputtering and reactive-ion etching. Their spectroscopic properties, optical gain, and cooperative upconversion are studied and explained based on a model of distinct ion classes.

Periodic optical structures are employed in numerous photonic applications. The coupled-mode theory was developed to calculate the electric-field distributions within such structures. However, for more complex, nonperfectly periodic optical structures it merely provides approximated solutions. The characteristic-matrix approach provides exact solutions but is cumbersome to apply. We introduce a simple method to obtain the exact electric-field and intensity distributions in an arbitrary multi-resonator structure by considering the structure as a combination of multiple Fabry-Pérot resonators of various lengths and refractive indices. The circulating-field approach can be applied recursively to obtain the electric-field distribution of such structures. As each resonator is considered separately, this method can be easily applied to structures with non-uniform resonator lengths and refractive indices, such as chirped and tapered gratings, thereby greatly simplifying their analysis. We apply this method to the calculation of reflectivity spectra and electric-field, intensity, and phase distributions of Bragg gratings and distributed-feedback (DFB) structures.