High-Resolution Optical Line Width Measurements as a Material

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High Resolution Optical Linewidth Measurements as a Materials Characterization Tool Nathalie Kunkel, John Bartholomew, Laurent Binet, Akio Ikesue, and Philippe Goldner J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03337 • Publication Date (Web): 07 Jun 2016 Downloaded from http://pubs.acs.org on June 14, 2016

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The Journal of Physical Chemistry

High Resolution Optical Linewidth Measurements as a Materials Characterization Tool. Nathalie Kunkel,∗,† John Bartholomew,† Laurent Binet,† Akio Ikesue,‡ and Philippe Goldner† PSL Research University, Chimie ParisTech, CNRS, Institut de Recherche de Chimie Paris, 11 rue Pierre et Marie Curie, 75005 Paris, France, and World Laboratory, Mutsuno, Atsuta-ku, Nagoya 456-0023, Japan E-mail: [email protected] Phone: +33 (0) 1 53 73 79 23. Fax: +33 (0) 1 55 42 74 89

∗

To whom correspondence should be addressed PSL Research University, Chimie ParisTech, CNRS ‡ World Laboratory, Japan †

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Abstract We present a case study on Eu3+ -doped Y2 O3 transparent ceramics where high resolution laser spectroscopy is used as a materials characterization tool. By comparing the results from coherent and incoherent optical spectroscopy with other characterization methods we show that optical techniques can deliver supplementary information about the local environment of the activator ions in materials. Thus, high resolution spectroscopy may be of interest for the investigation of a wider range of rare-earth doped optical materials beyond materials studied for quantum information technology. The refinement of optical spectroscopy for the study of narrow optical transitions in rare earth ion single crystals has demonstrated that these techniques are extremely sensitive tools for probing the local environment of the rare earth ion. These techniques, such as photon echo experiments, have been important in developing materials for quantum information technology and spectral filtering applications. Here, we apply these techniques to transparent ceramic samples and compare the results with information gained from conventional material characterization techniques. Our present study demonstrates the high sensitivity of laser spectroscopic methods to microstructural strain and the presence of defects. In particular, the sensitivity is sufficient to detect small changes introduced by different thermal treatments in nominally equivalent materials. The results of our work show that it is possible to relate high resolution optical measurements to defects and microstructural strain.

Introduction Rare earth doped materials play an important role in many optical applications. For example, they are used in lighting applications such as energy saving lamps and LEDs, plasma screens, lasers, scintillators, optical fibers for, e.g. telecommunication, and up-conversion layers in solar cells ( 1,2 and references therein). Trivalent rare earth ions often show remarkably narrow optical inhomogeneous and homogeneous linewidth. 3–6 The reduction of broadening to achieve narrow optical linewidths is of great interest in many areas of optics 2 ACS Paragon Plus Environment

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including quantum storage and processing applications 7,8 or spectral filtering used in radio frequency signal analysis 9 or laser-locking. 10 On the other hand, narrow optical linewidths are sensitive to the local environment and its perturbations. 4 However, previous studies, especially on homogeneous linewidths, did not clarify the nature of these perturbations. Inhomogeneous linewidths probe the frequency distribution of an ensemble. Time domain techniques, such as 2 pulse photon echoes, allow probing a chosen set of ions within the inhomogeneous line and reveal the homogeneous linewidths of these centers. These techniques are the optical equivalents of the spin echo techniques used in nuclear spin and electron paramagnetic resonance spectroscopy, initially developed by Hahn 11 and later extended to the optical domain. 12 Thus, photon echo measurements, that are most commonly applied in quantum information applications and related fields, could also complement the characterization of many materials doped with rare earth ions. While the inhomogeneous linewidths are very sensitive to static effects in the local environment of the activator ion, the homogeneous linewidths are a very sensitive probe for dynamical effects. With the help of different pulse techniques and the investigation of temperature and magnetic field dependences, perturbations such as defects, impurity ions, fluctuating spins and low frequency vibrational modes suggesting the presence of local disorder, can be detected. 4,8,13,14 Even though a rare earth ion is used as the local probe, many of the detected defects are independent of the rare earth ion doping so that a large part of the information that is gained is also relevant for non-doped materials or materials doped with other rare earth ions. The ability to apply the same techniques to the study of powders and other highly scattering samples 15,16 further extends their applicability for characterization. In the present work we examine Eu3+ -doped Y2 O3 transparent ceramics where we relate conventional studies of the materials properties with the optical properties obtained by high resolution and coherent laser spectroscopy. Choosing such samples is driven by the interest in the development of rare earth doped transparent ceramics as an alternative for single crystals. 17,18



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Experimental details The transparent ceramics were prepared from high purity Y2 O3 (99.999%) and Eu2 O3 (99.99%) nano powders (see

18

for details). In some samples, 0.5 % ZrO2 was added as

a sintering aid. After ball milling the powders in ethanol for 15 hours, the slurry was dried, crushed into powder and the powders uniaxially pressed into a pellet. The pellets were cold isostatically pressed at 98 MPa, presintered under vacuum at 1500◦ C for 3 hours, and afterwards hot isostatically pressed at 1600◦ C and 147 MPa for 2 hours under argon gas. After the hot isostatic pressing (HIP) step, transparency was already achieved and the samples showed a slight grey colouration. Some of the prepared samples were then annealed under air at 1100◦ C (1 hour). Annealing led to a change in colour to a yellow tinge. After cutting and surface polishing, all samples had a diameter of 12 mm and a thickness of 6 mm. X-ray diffraction data were collected on a Bruker D8 Advance with focusing Bragg-Brentano geometry and showed that all samples were single phase. The linewidths of the X-ray reflections were extremely narrow which indicates that the samples have a very low degree of microstructural strain. Y2 O3 crystallizes in the cubic space group Ia¯3 with two cation sites with the local symmetry C2 and S6 . All high resolution optical measurements were carried out on the C2 site. Inhomogeneous linewidths of the 7 F0 → 5 D0 transition were measured in a closed-cycle helium cryostat at 15 K in transmission using a Coherent 899-21 cw dye laser with a 1-MHz linewidth for lines narrower than 50 GHz and a Varian Cary 5000 spectrometer otherwise. The laser was centered at 580.87(5) nm (vacuum) and a low laser intensity (∼2 mW/cm2 ) was chosen in order to prevent hole burning and saturation effects. A Thorlabs PDB150A amplified photodiode was used for signal detection. Homogeneous linewidths were measured by photon echo experiments on the 7 F0 → 5 D0 transition in a helium bath cryostat (Janis CTI-Cryogenics Model CCS-150, LakeShore model 330 temperature controller) at temperatures between 1.7 and 15 K and different laser powers. Exciting and rephasing pulses of 3.1 and 4 µs were used. 4 ACS Paragon Plus Environment

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Photoluminescence spectra were recorded with a tunable optical parametric oscillator pumped by a Nd:YAG laser (Ekspla NT342B-SH with 6 ns pulse length) as excitation source, a Jobin-Yvon HR250 monochromator, and a PI-Max ICCD camera for detection. The excitation wavelengths, gate widths and gate delays were varied in order to see transitions with different lifetimes. Decay curves for the Eu3+ 5 D0 state were recorded at 611 nm with an excitation wavelength of 581 nm using a Jobin-Yvon HR250 monochromator and a photomultiplier tube. For thermoluminescence measurements the samples were excited at 10 K for 5 min by a UV lamp with an emission at 254 nm. Then, the samples were heated with a rate of 10K/min up to 600 K and the luminescence was monitored between 235 and 751 nm using a Princeton Instruments monochromator and a PI-100 CCD camera. Electron paramagnetic resonance spectra were recorded with a Bruker ELEXSYS E500 and an ELEXSYS Super High Sensitivity Probe Head in X-band. Spectra of the defect centers were recorded at room temperature and the microwave power was 0.25 mW to avoid saturation of the EPR signal. A calibration of the magnetic field was carried out using lithium particles in an electron-irradiated LiF standard sample exhibiting a conduction electron EPR line at g = 2.0023. In order check for the presence of Eu2+ , spectra were also recorded at low temperatures.

Results Inhomogeneous linewidths The broadening of the inhomogeneous linewidth is a measure of the static perturbations in the local surrounding of the activator ion, such as point defects and microstructural strain. 4,19,20 For all samples the peak absorption coefficient and the inhomogeneous linewidth obtained from transmission experiments are listed in Table 1 and in Fig. 1 the inhomogeneously broadened absorption spectra of Eu3+ :Y2 O3 transparent ceramics with and without additives are depicted. 5 ACS Paragon Plus Environment


Table 1: Absorption coefficients of Eu3+ :Y2 O3 transparent ceramics for the 7 F0 → 5 D0 line and inhomogeneous linewidth at 15 K, 5 D0 excited state lifetimes T1 and transmission [%] at approx. 580 nm at room temperature, off-resonant (maximum theoretical value approx. 80%). [Zr4+ ] (%) 0.5 0.5 0.5 0.5

-1

)

[Eu3+ ](%) 0.5 0.5 0.5 0.5 1.0 1.0 1.0 1.0

A b s o r p tio n c o e ffic ie n t ( c m

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Annealing x x x x

α (cm−1 ) 1.0 < 0.1 1.0 0.2 1.6 0.3 1.3 0.6

Γinh (GHz) 19.5 ∼ 100 21.4 ∼ 100 24.2 ∼ 100 40.5 ∼ 100

1 ,0

T1 (µs) 947 943 944 948 986 947 1000 978

Transm. [%] 48 64 56 75 42 73 67 74

n o a d d itiv e w ith Z r O 2

0 ,8

0 ,6

0 ,4

0 ,2

0 ,0 -1 0 0

-5 0

0

5 0

1 0 0

F re q u e n c y (G H z )

Figure 1: Absorption spectra of Eu3+ :Y2 O3 (0.5%, no post HIP annealing) transparent ceramics with and without additives.

As can be seen in Table 1, the inhomogeneous linewidths of ceramic samples without the additive ZrO2 are narrow; comparable to the inhomogeneous linewidths obtained for pedestal grown fiber crystals 13 and not much broader than for high-quality single crystals. 18 The addition of ZrO2 , which is usually applied to obtain higher degrees of transparency, 21 leads to much broader inhomogeneous linewidths. While the samples without the additive show linewidths in the range of 20 GHz, those with the additive are found to be approximately 100 GHz or more. For one of the annealed ZrO2 -free samples a sightly larger linewidth of 40 6 ACS Paragon Plus Environment

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GHz was found and can be attributed to possible small variations of the annealing process (also see Discussion). The observation of wider inhomogeneous linewidths in Zr-doped samples can be attributed to a higher level of local microstructural strain, induced by the incorporation of Zr4+ into the Y2 O3 lattice. The increase in local strain can be attributed to three factors: the different ionic radii, the charge compensation that is needed when Zr4+ replaces the trivalent Y3+ and the possible changes in the coordinations spheres in the vicinity of the activator ion Eu3+ , which may be caused by the necessary charge compensation while adding Zr4+ . Zr4+ (IR 0.72 A, VI) has a slightly smaller ionic radius than Y3+ (IR 0.892 A, VI), whereas the ionic radius of Eu3+ (IR 0.950 A, VI) is slightly larger than that of Y3+ . 22 At the same time the influence of Zr4+ is much more important than that of Eu3+ (as can be seen in Table 1), suggesting that disorder due to charge compensation is a more significant contribution to the inhomogeneous line broadening. In an earlier work on nanocrystalline Eu3+ :Y2 O3 , it was estimated that at a doping concentration of 0.5 %, strain due to Eu3+ was responsible for 6 GHz of the total linewidth of 12 GHz. 23 The dependence of the inhomogeneous linewidth on the concentration of impurity ions or defects can be used as a characterization tool and the detection limit can be estimated for a given type of impurity ion. Given the observed 100 GHz linewidths for a ZrO2 concentration of 0.5 % and assuming a linear relationship between the concentration of defects and line broadening, 19,20,24 it can be estimated that the excess broadening due to the presence of Zr4+ can be resolved down to concentrations of approximately 0.02 %. Since it is known that the Eu3+ concentration itself also contributes to the broadening, it can be expected that for samples with lower europium concentrations an even lower Zr4+ concentration may still be detected. This is much more sensitive than, for example, the estimated changes in the lattice parameters due to the presence of additives or impurity ions that can be carried out by the analysis of laboratory X-ray diffraction powder data. About 15 % of ZrO2 can be introduced into



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the Y2 O3 structure before the occurence of significant structral changes. For a decrease of ˚ to the Y3+ content from 100% to 90% a decrease of the unit cell parameters from 10.6040 A ˚ was observed. 25 Assuming a linear dependence of the lattice parameter on the zir10.5652 A conium concentrations and a change in the lattice parameter of 0.0003 nm as significant, this would yield a limit of approximately 1% of zirconium that can still be detected by a change in the lattice parameter. However, the concentration dependence of the lattice parameters is actually not linear, which makes the determination of the zirconium concentration from the changes in the lattices parameters more difficult.

Homogeneous linewidths In contrast to the inhomogeneous linewidth, the homogeneous linewidth Γh depends on dynamic processes in the surroundings of the activator ion. It is determined by several contributions, given in Eq. 1. Since the different components are independent to a large extent, they can be probed individually to extract the maximum amount of information. Γh can be split into the individual components 17,26,27 according to the following equation:

Γh = Γpop + Γion−ion + Γion−spin + Γphonon + ΓTLS ,

(1)

which in our case can be rewritten as

Γh =

1 + αISD P + Γion−spin + αphonon T 7 + αT LS T n 2πT1

(2)

with n ≈ 1. 13 Γpop is the broadening due to the excited state lifetime T1 of the 5 D0 level via Γpop =

1 . 2πT1

Since the excited state lifetime is ≈ 1 ms (see Table 1 or Ref. 13 ), it contributes only approximately 170 Hz. This linewidth is more than a magnitude lower than the linewidths found in the present samples. The variations for T1 across samples are not significant and possible 8 ACS Paragon Plus Environment

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resulting variations in the T1 contribution to Γh are negligible compared to other factors influencing the homogeneous linewidths. Γion−ion is the contribution of the instantaneous spectral diffusion (ISD), which depends linearly on the excitation density. As a Eu3+ ion has a different electric dipole moment in the ground and excited states, high excitation powers in the photon echo sequence result in an additional broadening through dipole-dipole interactions. In order to minimize its contribution, 2 pulse photon echoes were recorded for different excitation powers and the resulting data were extrapolated to zero laser power. Both the extrapolated values for Γh and the corresponding coherence times T2 (Γh = πT1 2 ) are given in Table 2. The terms Γphonon and ΓTLS take into account the phonon contribution and dynamic fluctuations due to nearly equivalent configurations in the local lattice. This phenomenon is also known as TLS (two levels system) and usually significant in disordered materials such as glasses. Their contribution can be investigated by studying the temperature dependence of the homogeneous linewidth. If the energy gaps between the ground state and the excited state as well as other possible crystal field levels are large enough, as it is the case for the europium 7 F0 → 5 D0 transition, direct processes will not play a role and the only contribution will be that of the Raman process. 13 It is characterized by a T7 behavior and does not play a role below approximately 7-8 K (see Fig. 2 and 3). Samples without post HIP annealing show a linear increase and decrease of Γh in the region up to approximately 8 K. For a magnetic perturbation that has a characteristic rate R that increases strongly with temperature, an increase in homogeneoud broadening is observed until R is comparable to the time scale of the photon echo measurements. For higher temperatures, R becomes sufficiently large to induce a motional averaging effect and a decrease im homogeneous linewidth. A similar temeprature dependence has been observed for spin transitions in electron paramagnetic resonance spectroscopy. 28 A model to describe this anomalous temperature dependence and calculate the characteristic fluctuation rate R will be presented in detail in another work. Samples that were annealed under air after HIP show a linear increase of Γh in the low tem-



perature region, suggesting the presence of TLS (for values of αT LS see Table 2) and thus, the presence of weak disorder. This is similar to the behavior found for a number of crystals obtained by laser-heated pedestal growth. 13,29 The last remaining contribution Γion−spin is caused by magnetic field fluctuations due to electron and nuclear spin flips. It may include a contribution of the Y nuclear spins (expected to be 200-300 Hz 3 ) as well as that of possible defect centers. Even though the inhomogeneous linewidth of the Zr4+ -doped samples show a large broadening compared to Zr4+ -free samples, the homogeneous linewidths do not vary so strongly. This was also observed for different europium concentrations in Eu3+ :Y2 SiO5 24 and Eu3+ :Y2 O3 crystals co-doped with Sc3+ . 30 H o m o g e n e o u s lin e w id th ( k H z )

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2 5

2 0

1 5

1 0 2

3

4

5

6

7

8

9

1 0

1 1

T (K )

Figure 2: Temperature dependence of Γh for the 7 F0 → 5 D0 line in Eu3+ :Y2 O3 transparent ceramics with 0.5 % europium. Open symbols: with addtive ZrO2 . Dotted lines: fit to the data with Γh = Γh,0 + αphonon T7 , ignoring the data points in the non-linear region of the low temperature regime.


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H o m o g e n e o u s lin e w id th ( k H z )

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2 0

1 5

1 0

5 2

3

4

5

6

7

8

9

1 0

T (K )

Figure 3: Temperature dependence of Γh for the 7 F0 → 5 D0 line in a Eu3+ :Y2 O3 annealed transparent ceramics with 0.5 % europium. Open symbols: with addtive ZrO2 . Solid lines: fit to the data with Γh = Γh,0 + αT LS T + αphonon T7 . Table 2: Coherence lifetimes T2 of the 7 F0 → 5 D0 transition in Eu3+ :Y2 O3 transparent ceramics at 3.5 K extrapolated from laser power dependence, Γh extrapolated from laser power dependence at 3.5 K, αT LS (a TLS exponent n=1 was assumed). [Eu3+ ](%) 0.5 0.5 0.5 0.5 1.0 1.0 1.0 1.0

[Zr4+ ] (%) 0.5 0.5 0.5 0.5

Annealing x x x x

T2 (µs) 42 36 62 61 42 22 65 59

Γh [kHz] αT LS [Hz/K] 7.4(1) 8.7(3) 5.1(2) 560(40) 5.2(1) 520(50) 7.4(2) 14.0(6) 4.9(1) 350(50) 5.4(1) 320(40)

Luminescence Besides the well-known f-f emission of Eu3+ (e.g. 31,32 ), samples without post HIP annealing or additives also showed a very short-lived broad band emission around 450 nm (see Fig.4). To distinguish between the short lived emission and the regular f-f emission, which is of much higher intensity, different gate widths and delays were used. The short lived emission could be observed a few hundred nanoseconds after excitation, whereas the f-f emission was still 11 ACS Paragon Plus Environment


observed for delay times greater than 1 ms after the excitation pulse. Similar emission has already been observed in Y2 O3 thin films 33 where several overlapping broad band emission peaks are reported. On reduced Y2 O3 thin films, previous electron energy loss spectroscopy studies show the presence of oxygen deficiencies that lead to a change in the density of states of the lower conduction band of Y2 O3 . 34 This resulted in a decrease of the electronic gap by about 0.8 eV. We thus attribute the emission observed in the present work to the anion sub-lattice, most likely to a F-center located on an oxygen vacancy. Even though Eu2+ can also show d-f broadband emission, its presence in our samples is highly unlikely since its characteristic lines 35 were not observed in EPR measurements (see below). Annealed samples did not show such broad band emission down to the detection limit of our experimental set-up. It can be concluded that most of the F-centers present in non-annealed samples are removed during a treatment under air due to the introduction of oxygen. In samples with post HIP annealing and all samples with the additive ZrO2 no broad band emission could be detected either. This can be explained by two effects. First, the introduction of Zr can reduce the number of the relevant defects. Secondly, it creates additional levels in the energy gap that lead to a system with different defects than it is observed for the Zr-free samples. 4 4 0 0 0

In te n s ity (a .u .)

4 2 0 0 0 4 0 0 0 0

In te n s ity ( a .u .)

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3 8 0 0 0

5 x 1 0

6

4 x 1 0

6

3 x 1 0

6

2 x 1 0

6

1 x 1 0

6

3 6 0 0 0 0

5 2 5

5 5 0

5 7 5

6 0 0

6 2 5

6 5 0

W a v e le n g th (n m )

3 4 0 0 0 3 2 0 0 0 3 0 0 0 0 4 0 0

4 5 0

5 0 0

5 5 0

6 0 0

6 5 0

W a v e le n g th ( n m )

Figure 4: Defect luminescence emission of a non-annealed Eu3+ :Y2 O3 ceramic without additives, 0.5% Eu3+ , excitation wavelength 390 nm, detected with a gate width of 500 ns and a gate delay of 100 ns. Insert: luminescence emission spectrum at an excitation of 465 nm, gate width 50 µs and gate delay 1 ms. 12 ACS Paragon Plus Environment

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Thermoluminescence Non-annealed Eu3+ :Y2 O3 ceramics without additives showed a thermoluminescence emission at around 420 K (see Fig.5) with an emission in the range of 550-650 nm. This is in agreement with our observation of a short lived broad band luminescence that was attributed to the presence of defects in the oxygen sub-lattice. In contrast, no thermoluminescence emission was observed for Eu3+ :Y2 O3 ceramics without additives after an annealing step under air. Thermoluminescence in this temperature range is for example reported for Tm3+ :Y2 O3 ceramics 36 and disappears almost completely after a short processing period under oxygen. This is in agreement with the fact that samples without post HIP annealing show a glow curve, but the glow curves disappears after a short treatment (1h) under air. This can be explained by an oxygen uptake that fills a large percentage of the oxygen vacancies. In, 36 a longer oxygen treatment leads to the re-appearance of a glow curve due to the incorporation of oxygen in interstitials. However, in the present work we do not use longer heat treatments under air or oxygen, because longer heat treatments lead to a deterioration of the transmission, probably due to grain growth. 1 9 9 0 0 0

1 9 8 0 0 0

In te n s ity ( a . u . )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 9 7 0 0 0

1 9 6 0 0 0

1 9 5 0 0 0

1 9 4 0 0 0 0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

T (K )

Figure 5: Thermoluminescence glow curve of a non-annealed 0.5 % Eu3+ :Y2 O3 ceramic without additives after excitation at 254 nm for 5 min at 10 K.



Electron paramagnetic resonance spectroscopy + Electron paramagnetic resonance spectra show the presence of interstitial O− 2 and F centers

(one electron on an oxygen vacancy). Fig. 6 shows a typical room temperature spectrum of a sample with post HIP treatment and without Zr4+ . The EPR spectra of all other 0.5% Eu3+ -doped samples can be found in the supporting information. We assign the signals with g-factors 2.0376(3) and 2.0029(3) to the parallel (gk ) and perpendicular (g⊥ ) component of + O− 2 . The signal at g = 1.9703(3) corresponds to the perpendicular component of an F -

center signal (g⊥ ). Depending on the thermal treatment and the presence of Zr4+ the signal strength shows a variation. Similar EPR signal have previously been observed in γ-irradiated Er:Y2 O3 . 37 We carried out a semi-quantitative analysis of the signal intensities (see Supporting Information Table S1) and found that the presence of Zr4+ leads to a strong reduction of the concentration of both defect centers. While the the O− 2 concentration strongly increased during the post HIP annealing process, the effect on the F+ center concentration is less significant. During the annealing process, the F+ center concentration increases only slightly. There was no evidence of Eu2+ in any of the samples. 2 0 1 5

F

+

c e n te rs

1 0

In te n s ity ( a .u .)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5 0 -5 -1 0 -1 5 -2 0 -2 5

O

2

-3 0 3 1 0 0 3 1 5 0 3 2 0 0 3 2 5 0 3 3 0 0 3 3 5 0 3 4 0 0 3 4 5 0 3 5 0 0 3 5 5 0 3 6 0 0

F ie ld ( G )

Figure 6: Electron paramagnetic resonance X-band spectrum of a sample with post HIP annealing, without additives and 0.5% Eu3+ recorded at RT.


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Discussion As shown in the Results section, high resolution spectroscopy of the inhomogeneous linewidth can reveal the presence of small concentrations of impurity ions or defects and the resulting strain effects. It can therefore serve as a very sensitive probe for microstructural strain. For our example, Eu3+ :Y2 O3 , the incorporation of a very low concentration of Zr4+ already has a large effect on the inhomogeneous broadening of the 7 F0 → 5 D0 absorption line. We explain this observation considering Zr4+ concentrations by strain effects that arise mainly due to charge compensation. Our estimations of the inhomogeneous linewidths are consistent for all samples except the sample with 1% Eu3+ and post HIP annealing that shows a linewidth of 40.5 GHz. This is about double than expected. However, the EPR spectra reveal a higher + O− concentration than in other samples. This may be caused by slightly varied 2 and F

annealing conditions, for example a slightly longer annealing time or higher oxygen partial pressure. The additional broadening can therefore be assigned to additional microstructural strain caused by a higher defect density. The 2 pulse photon echo measurements on the Eu3+ 7 F0 → 5 D0 transition deliver valuable information on dynamic effects in the proximity of the activator ion that lead to a good understanding of the material’s properties when it is combined with other conventional techniques. In our case study, samples without post HIP annealing do not show a linear increase of the homogeneous linewidth with increasing temperature in the low temperature regime, but rather an increase and decrease up to 8 K. At the same time, the overall homogeneous linewidth at low temperatures are ∼ 2 kHz broader than for samples with post HIP annealing and we also observed defect luminescence and thermoluminescence in these samples. Thus, we suggest that the observed behavior of the homogeneous linewidth is due to F and F+ centers. Here, a F center corresponds to two electrons on an oxygen vacancy with S=0 that cannot be detected by EPR, whereas a F+ center corresponds to one electron on an oxygen vacancy that also shows an EPR signal. Since the structure of Y2 O3 can be deduced from the fluorite 15 ACS Paragon Plus Environment


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structure where two corners of the cube formed by the anions stay occupied, these defects can be located on different lattice sites, leading to different g-values in the EPR spectra. We especially relate the anomalous temperature dependence of the homogeneous linewidth in the low temperature regime to defect centers in close vicinity to the Eu3+ centers. During the post HIP annealing process, defect centers are filled with oxygen and O− 2 centers + are formed on interstitial lattice sites. Both the O− 2 as well as the remaining F centers could

be detected by EPR spectroscopy. The absence of defect luminescence and thermoluminescence can easily be explained by reduction of the F center concentration during the annealing process. The 2 pulse photon echo measurements yield narrower homogeneous linewidths at low temperatures for these samples and a linear increase of the homogeneous linewidth with temperature in the temperature regime below 8 K. We suggest that the decrease in the linewidth is caused by a reduction of defect centers in close proximity to the Eu3+ centers. The observation of a linear increase of the homogeneous linewidth in the low temperature regime is related with local disorder modes, which we attribute mainly to the presence of O− 2 on different interstitial lattice sites. While the presence of Zr4+ in both samples with and without post HIP annealing leads to a broadening of the inhomogeneous linewidth due to increased static lattice strain, it also prevents the observation of defect luminescence. The latter is caused by a preferred localization of the F centers close to the Zr4+ impurity leading to trapped electrons closed to the positive charge. Since the induced lattice strain is mostly static, the effect on the homogeneous linewidth is negligible.

Conclusions In summary, our high resolution spectroscopy study shows that small concentrations of impurity ions and the resulting strain effects cause a large additional broadening of the inho-


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mogeneous linewidths at low temperatures. At the same time, the homogeneous linewidths are extremely sensitive to defects and local disorder which is difficult to detect by other conventional techniques. Thus, high resolution laser and coherence spectroscopy represent a useful additional tool for the characterization of local environments in rare-earth doped materials. Even though, within this work, we focused on inhomogeneous linewidths measurements and two pulse photon echoes, it is also possible to apply three pulse photon echo schemes that allow for probing a longer time scale, investigate the magnetic field dependence of the homogeneous linewidths or study spectral hole burning. The inhomogeneous optical linewidth shows a much higher sensitivity for microstructural strain and defects than can be obtained with laboratory powder XRD experiments. A prerequisite for the use of photon echo techniques is the existence of a narrow optical transition corresponding to a narrow linewidths laser emission 8 and the coherence time and oscillator strength should be high enough to carry out pulse experiments in a time much shorter than the coherence time. These criteria are often fulfilled in the case of trivalent rare earth ions such as Eu3+ , Pr3+ , Tm3+ , Er3+ or Nd3+ . Especially Eu3+ and Pr3+ are very well suited for photon echo experiments due to the wide availability of Rhodamine 6G dye lasers, whereas Tm3+ , Nd3+ and Er3+ can be excited by diode lasers. The measurements of the homogeneous linewidth give access to weak local disorder and magnetic perturbations in the local environment of the rare earth ion. However, a large part of the defects and the local disorder that can be detected by coherent spectroscopy can be expected not to fundamentally depend on the rare earth metal ion used as the local probe, but is rather a materials property dependent on the preparation methods. The information gained by coherent spectroscopy can therefore also be valuable for non-doped materials or materials with other rare earth metal ions. Thus, high resolution laser and coherence spectroscopy can deliver useful information on the local environment of optical materials doped with trivalent rare-earths ions beyond applications in quantum information technology and could help in understanding their local structural properties and in improving the related optical properties.



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Acknowledgement Nathalie Kunkel thanks the Deutsche Forschungsgemeinschaft (DFG) for a Postdoctoral Research Fellowship (project no. KU 3427/1-1). Financial support by the ANR projects RAMACO (No. 12-BS08-0015-01) and DISCRYS (No. 14-CE26-0037-01), Idex ANR-10-IDEX0001-02 PSL? and Nano’K project RECTUS is gratefully acknowledged. The authors also thank Charles W. Thiel and Alban Ferrier for fruitful discussions, Bruno Viana and Yumiko Katayama for help with the thermoluminescence measurements and Patrick Aschehoug for technical support during the luminescence measurements.

Supporting Information Available Additional transmission, EPR spectra and EPR signal intensities: This material is available free of charge via the Internet at http://pubs.acs.org/.

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High-Resolution Optical Line Width Measurements as a Material

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