Excitation Spectroscopy of Er3+

-doped α-Al2O3 Nanopowder 

and Ceramic

by T. Sanamyan, R. Pavlacka, G. Gilde, and M. Dubinskiy

ARL-TR-6126 August 2012

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Army Research Laboratory 

Adelphi, MD 20783-1197 

ARL-TR-6126 August 2012

Excitation Spectroscopy of Er3+-doped α-Al2O3 Nanopowder 

and Ceramic 

T. Sanamyan and M. Dubinskiy 

Sensors and Electron Devices Directorate, ARL 

R. Pavlacka and G. Gilde

Weapons and Materials Research Directorate, ARL 

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Excitation Spectroscopy of Er3+-doped α-Al2O3 Nanopowder and Ceramic

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T. Sanamyan, R. Pavlacka, G. Gilde, and M. Dubinskiy

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14. ABSTRACT

We have developed an excitation spectroscopy setup for investigating the spectroscopic properties of non-transparent samples, 

such as powders and ceramic, in the spectral range of 380–2700 nm and temperature range of 10–300 K. We present the results 

of a comprehensive spectroscopic study of successfully synthesized nanopowders and fully densified transparent ceramics of 

erbium ion (Er3+)-doped corundum (α-Al2O3). Fluorescence and excitation spectra in a range of temperatures 10–300 K were 

used to infer the energy level schemes of the ground state and the first three excited Er3+ multiplets (4I15/2, 

4

I13/2, 

4

I11/2, and 4I9/2) 

in this material. The observed Er3+ spectra in -Al2O3 nanopowders and ceramics exhibit sharp, well-resolved emission and 

excitation lines, indicating predominant crystalline single-site Er3+ occupation. Obtained experimental data also allowed us to 

assess the potential of this material as the gain medium for high power laser applications in anticipation that it becomes 

available in a low-loss (grain-oriented) ceramic or single crystalline form in the near future.

15. SUBJECT TERMS

Alumina, aluminum oxide, spectroscopy, excitation, ceramic

16. SECURITY CLASSIFICATION OF: 

17. LIMITATION

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 ABSTRACT

UU

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34

19a. NAME OF RESPONSIBLE PERSON

T. Sanamyan

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(301) 394-2044

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Prescribed by ANSI Std. Z39.18 
iii

Contents 

List of Figures iv

List of Tables v

Acknowledgments vi

1. Introduction 1

2. Experimental 3

3. Results 6

4. Conclusions 18

5. References 20

List of Symbols, Abbreviations, and Acronyms 23

Distribution List 25
iv

List of Figures 

Figure 1. Experimental setup for excitation spectroscopy of powder and ceramic. .......................4

Figure 2. Control interface in LabVIEW environment for the developed excitation 

spectroscopy of powder and ceramic. 1- spectrally and energetically corrected excitation 

spectrum of Er3+-doped α-Al2O3; 2,3 - the raw excitation spectrum and the fluorescence 

signal amplitude measured from the α-Al2O3 powder; 4, 5 - excitation spectrum and the 

signal amplitude from the reference sample (Er:YAG); 6,7 - the peak and average 

amplitudes of the reference pump signal, used for the energetic calibration of the 

excitation spectra. ......................................................................................................................5

Figure 3. Room temperature fluorescence spectra from 4I13/2 state for the two randomly 

chosen Er3+ doped α-Al2O3 experimental samples. ....................................................................7

Figure 4. Er3+-doped -Al2O3 powder pellet and Er3+-doped -Al2O3 transparent ceramic 

presented used in the present work. ..........................................................................................8

Figure 5. Room temperature excitation spectra corresponding to the Er3+ 4I15/2  4S3/2 

transitions at visible spectral range. ..........................................................................................9

Figure 6. Typical Er3+ 4I13/2 state fluorescence lifetime in Er3+ doped α-Al2O3 for three 

samples sintered at various temperatures. The measured values for each curve are labeled 

in the picture. ..........................................................................................................................10

Figure 7. Room temperature fluorescence spectra of Er3+ doped powders of α-Al2O3, Y2O3, 

YAlO3, and YAG. The color legend – at top right. ................................................................12

Figure 8. Fluorescence spectra of Er3+ doped α-Al2O3 corresponding to the 4

I13/2  4I15/2

transition in the temperature range of 10–300 K. Shown on the insets are the blow-ups of 

the weak spectral features in the 1430–1500 nm (left) and 1600‒1700 nm (right) spectral 

intervals. ...................................................................................................................................13

Figure 9. 10-K fluorescence spectrum of Er3+ 4I13/2  4

I15/2 transition in Er3+ doped α-Al2O3. 

The arrows indicate the spectral lines corresponding to the transitions from the lowest 

stark sublevel of the 4I13/2 multiplet to the lowest five sublevels of the 4I15/2 multiplet. 

Indicated on the right inset (in blue) is the blow-up of the weak spectral feature in the 

1620–1700 nm spectral range. Shown in red is the same spectrum averaged by using 

Savitzky-Golay filter. Indicated on the left inset is the energy level scheme of the 4I15/2

ground state multiplet inferred from the spectrum. .................................................................14

Figure 10. Emission cross section of the Er3+ 4I13/2  4

I15/2 transition at room (300 K) and 

cryogenic (77 K) temperatures in Er3+ doped α-Al2O3. ...........................................................15

Figure 11. Excitation spectra corresponding to the Er3+ 4I15/2  4I13/2 transitions in the 

temperature range 10–300 K in Er3+ doped α-Al2O3. Indicated on the inset is the energy 

level scheme of the 4I13/2 multiplet inferred from these spectra. ..............................................16

Figure 12. Emission (from Fuchtbaer-Ladenburgh), absorption (from emission through 

reciprocity) and excitation cross sections for the Er3+ 4I15/2  4I13/2 and 4I13/2  4I15/2 

transitions at room temperature in Er3+ doped α-Al2O3. ..........................................................17
v

List of Tables 

Table 1. Energy levels for Er3+:α-Al2O3

4

I15/2,

 4I13/2, 4

I11/2, 

4

I9/2 manifolds in wave numbers 

(cm–1). Weak transitions are marked by asterisks. ...................................................................18
vi

Acknowledgments 

We gratefully acknowledge financial support of this work by the High Energy Laser Joint 

Technology. 
1

1. Introduction 

Corundum (-Al2O3) is the most common form of crystalline alumina (also known as sapphire). 

The oxygen (O

2–

) ions form a nearly hexagonal close-packed structure with aluminum (Al3+)

ions filling two-thirds of the octahedral interstices. Each Al3+ center is octahedral. Corundum has 

a trigonal Bravais lattice with a space group of R

3

c with six formula units in the hexagonal unit 

cell (1). Due to its unique suite of thermal, optical, and electrical properties, rare earth (RE) 

doped aluminum oxide (RE:Al2O3) is the subject of ever-growing interest as the gain material for 

microelectronics, integrated on-chip devices for telecommunication, and most recently, for high 

power laser applications. While there have been recent significant achievements in power scaling 

of yttrium aluminum garnet (Y3Al5O12 or YAG) based cryogenically cooled lasers with nearly 

diffraction limited beam quality (2), including power scaling toward laser fission application (3), 

the extreme pump power densities combined with the unavoidable non-radiative losses in the 

pump-lase process introduce severe thermally induced optical distortions in the aggressively 

pumped and aggressively cooled gain medium. Regardless of the sophistication of heat removal 

techniques and their efficiency, the insufficient thermal conductivity of the currently used gain 

media is the bottleneck for non-distortive heat removal. The development of innovative gain 

media with extremely high thermal conductivity remains the most critical technological 

challenge preventing power scaling of bulk high energy solid-state lasers with high beam quality. 

Single crystal -Al2O3 (sapphire) has a liquid nitrogen temperature (LNT) thermal conductivity 

of well over 1000 W/m·K (4), which is ~40 times higher than the LNT thermal conductivity of 

YAG (the most prevalent solid-state laser host) and approximately two orders of magnitude 

higher than the room temperature thermal conductivity of YAG. Due to the above and the 

favorable combination of other LNT parameters (dn/dT = 0.19x10–5 K

–1

, and 

CTE = 0.34x10–6 K

–1

), which define the thermal figure of merit of the cryo-cooled doped -

Al2O3 laser material, it can be pumped far more aggressively with far less thermally induced 

optical distortion. Thus, a cryo-cooled RE doped sapphire (RE:Sapphire) laser would possess 

extremely high power extraction capabilities, far exceeding those possessed by state-of-the-art 

solid-state laser gain media. 

To date, RE:Al2O3 devices have been synthesized predominantly as thin films for optoelectronics 

applications using multiple thin-film deposition techniques. Ytterbium (Yb)-Er co-doped Al2O3

films were prepared on silicon dioxide (SiO2)/silicon (Si) substrate using a medium frequency 

magnetron sputtering system (5, 6). Erbium-doped Al2O3 films have been deposited in a singlestep process by pulsed laser deposition using independent ablation of Al2O3 and Er targets (7). 

Reactive co-sputtering allows for straightforward deposition of Er3+ doped Al2O3 on a variety of 

substrates (8). In most instances these films are amorphous aluminum oxide layers deposited on 
2

Si or oxidized Si substrates aiming at applications in the future monolithic integrated optics, 

interconnects, and integrated optoelectronics devices in general. Amorphous RE:Al2O3, however, 

is not very useful for high power bulk solid-state laser applications where crystallinity is 

necessary to access the highest possible thermal conductivity as well acceptably high absorption 

and emission cross sections. The latter are largely due to the narrowband single-crystalline 

absorption and emission features as opposed to wideband features in amorphous materials with 

much lower peak cross sections. 

Successful efforts in producing single-crystalline thin films of Nd3+:Al2O3 grown by plasmaassisted molecular beam epitaxy (MBE) have been recently reported (9). Though MBE growth 

cannot be volume scaled as required for high power laser applications, these results confirmed 

that (1) single-crystalline sapphire can be doped with RE3+ ions to a usable dopant level; 

(2) spectral features of the RE:Sapphire are, as expected, “crystalline-narrow;” (3) the spectra are 

consistent with single-site doping of the RE3+ ion into the host crystal; and (4) RE:Sapphire 

exhibits sufficient peak transition cross sections, exceeding those of RE3+-doped YAG 

Currently, there is no available technology to produce RE:Sapphire in the dimensions required 

for scalable high energy laser applications. Undoped sapphire can be grown in large sizes by a 

variety of melt-growth techniques. However, due to the significant ionic radius mismatch 

between the RE ions and the Al ion (ligand) of the host, RE elements exhibit negligible solubility 

within the sapphire crystal structure and tend to not incorporate into the growing crystal. 

Therefore, a non-traditional approach to RE:Sapphire synthesis must be adopted. 

It has been demonstrated that bulk ceramics possess the capability to accommodate “lessfriendly” dopants and/or significantly higher dopant levels than melt-grown crystals while 

maintaining sufficient optical quality for laser applications (10). This indicates that this solidstate approach allows for more “non equilibrium” dopant incorporation than traditional meltgrowth techniques. In many cases (at least for cubic hosts, e.g., YAG), a highly transparent, low 

loss, ceramic is a suitable replacement for a single crystal for laser applications. Due to its 

uniaxial crystal structure, however, laser-quality transparency cannot be achieved in 

polycrystalline -Al2O3 unless some technique of ceramic grain alignment, like magnetic grain 

alignment (11), is used. Additionally, the thermal conductivity of polycrystalline -Al2O3 is far 

lower than that of sapphire. Therefore, RE-doped-Al2O3 ceramics are not appropriate for high 

power laser gain media. On the other hand, RE-doped-Al2O3 ceramics can potentially be used 

as a precursor to RE:Sapphire. One possibility is that RE-doped-Al2O3 ceramics can be used as 

a feedstock for laser-heated pedestal growth (LHPG). Like polycrystalline ceramics, crystals 

grown by LHPG have been shown to be capable of “non equilibrium” dopant incorporation (12). 

It is possible that melting and solidification occur so rapidly in LHPG as to prevent the RE 

segregation that occurs during traditional melt-growth techniques. Another possibility would be 

to apply solid-state crystal conversion (SSCC) technique, wherein a ceramic is converted to a 
3

single crystal via the growth of a single grain to produce RE:Sapphire. SSCC has already been 

used to produce large sapphire crystals (13). Crystals grown by SSCC have also been 

demonstrated to be capable of “non equilibrium” dopant incorporation. Therefore, the synthesis 

and spectroscopic characterization of RE-doped-Al2O3 ceramics may enable the development 

of single-crystalline RE:Sapphire in dimensions and dopant concentrations adequate for high 

energy laser applications. 

This report details the results of comprehensive spectroscopic study of Er3+-doped α-Al2O3

(Er:Al2O3) per our first attempt toward transparent bulk Er3+-doped sapphire gain material via 

ceramic route with the objective to develop a new approach to RE:Sapphire as a laser gain 

medium. We have achieved successful incorporation of sufficient amounts of Er3+ into both 

highly crystalline α-Al2O3 powders and fully densified transparent α-Al2O3 ceramics. Presented 

here are the results of detailed spectroscopic investigation of Er3+-doped α-Al2O3 at all 

development stages. The detailed spectroscopic analysis of the ground state and the first three 

excited multiplets of Er3+ ions in α-Al2O3 host are presented for the first time. These data can be 

successfully used for further development of high power Er3+-doped α-Al2O3 based lasers once 

the high quality Er3+-doped sapphire material is available. The processing strategies developed to 

(1) enhance Er3+ incorporation and (2) sinter Er3+-doped α-Al2O3 to transparency are addressed in 

reference 14. 

2. Experimental 

The layout of the developed experimental setup for excitation spectroscopy of the RE doped 

powder and ceramic is shown in figure 1. The ACTON 2500i spectrometer was used to obtain 

fluorescence spectra in the spectral range of 500–1200 nm, and Nicolet 6700 Fourier transform 

infrared (FTIR) spectrometer was used to obtain fluorescence spectra in the wavelength range 

beyond 1200 nm. Both spectrometers were equipped with a standard commercial cryogenic 

dewar (Cryo Industries of America, Inc.), allowing temperature control and measurements in the 

temperature range of 10‒300 K. 
4

Figure 1. Experimental setup for excitation spectroscopy of powder and ceramic.

To achieve low thermal resistance of the pulverized samples (for temperature uniformity) and 

minimize the self-absorption effects in Er3+ doped α-Al2O3, the powder samples were immersed 

in the thermoconductive paste and sandwiched between the two sapphire windows. The 

thickness of the samples used in all spectroscopic measurements was less than 100 μm. The 

sandwiched samples were attached to the copper holder and mounted on the cold finger of the 

cryostat. The spectral resolution of the measurement setup was better than 1 nm at room 

temperature and 0.2 nm at cryogenic temperatures. Excitation of Er3+ energy levels around 

10000‒12000 cm–1 was typically accomplished by diode lasers emitting at ~800 and ~980 nm. 

Occasionally, a tunable pulsed titanium (Ti)-Sapphire laser was used for fluorescence excitation 

at shorter wavelengths. The spectral correction of the fluorescence spectra taken with both 

spectrometers has been performed using a standard calibrated quartz-halogen tungsten lamp 

(Optronic Laboratories, LLC). The majority of samples under investigation were Er3+-doped αAl2O3 powders. Currently available sintered transparent ceramic samples were less than 0.5 mm 

thick; therefore, the information on Er3+:Al2O3 absorption properties was obtained using the 

excitation spectroscopy technique. In this case, a tunable Continuum Panther EX optical 

parametric oscillator (OPO) pumped by a SpectraPhysics PRO Series Laser was used as a 

fluorescence excitation source. The OPO was capable of delivering a few millijoules of short, 

~10-ns pulses in the spectral range of 400–1600 nm at a 10-Hz repetition rate. In order to 

compensate for pulse-to-pulse energy fluctuations of the excitation source, the excitation spectra 

were normalized to the reference signal from a photodiode, which was directly proportional to 

the excitation pulse energy. The overall spectral resolution of the excitation setup was limited by 
5

the OPO output spectral width and was found to be better than ~0.3 nm across the entire range of 

the excitation wavelengths. In order to double-check the validity of spectral correction, along 

with the Er3+:Al2O3 excitation spectra, the excitation spectra of Er:YAG, a material well known 

spectroscopically, excited by the same source have been collected as well (see the details in 

figure 1 and figure 2). The Er3+:Al2O3 lifetime measurements in the temperature range of 

10–300 K were conducted by using the ~10-ns pulses from tunable Ti:Sapphire laser for 

excitation and detected by a high speed InGaAs or Si detector attached to the ACTON 2500i 

monochromator. The signal from the detector was processed using a TDS 2014 digital 

oscilloscope. 

Figure 2. Control interface in LabVIEW environment for the developed excitation spectroscopy of powder and

ceramic. 1- spectrally and energetically corrected excitation spectrum of Er3+-doped α-Al2O3; 2,3 - the 

raw excitation spectrum and the fluorescence signal amplitude measured from the α-Al2O3 powder; 4, 5 -

excitation spectrum and the signal amplitude from the reference sample (Er:YAG); 6,7 - the peak and 

average amplitudes of the reference pump signal, used for the energetic calibration of the excitation 

spectra.

Figure 2 shows the excitation spectra data acquisition control interface in the LabVIEW 

environment. The chart 1 in figure 2 shows the final spectrally and energetically corrected 
6

excitation spectrum taken from the Er3+-doped α-Al2O3 powder. Charts 2,3 show the raw 

excitation spectrum and the fluorescence signal amplitude, respectively, for the α-Al2O3 powder 

sample. Charts 4,5 show the excitation spectrum and signal amplitude measured for the reference 

signal (Er:YAG for the current measurement) and charts 6,7 show the peak and average 

amplitudes of the reference pump signal, used for the energetic calibration of the excitation 

spectra. The fluorescence spectrum in most instances was detected by a cooled InGaAs detector 

combined with a Stanford Research System SR 850 digital lock-in amplifier. The equipment 

control and data acquisition were integrated into a simple LabVIEW based program. For signal 

acquisition, equipment control and data processing, a NI DAQmx USB 6210 device was used in 

the LabVIEW environment. 

Due to the fact that all spectral measurements were performed on samples that were either 

powders or sintered ceramics with random grain orientation, the measured fluorescence and 

excitation spectra were all unpolarized (polarization unresolved), though the Er3+-doped α-Al2O3

is a uniaxial spectroscopic object. This merely means that all of the presented spectra can be 

treated as a superposition of the σ- and π- polarized spectral components. 

Er3+-doped α-Al2O3 powder was synthesized by the co-precipitation of high purity metal nitrate 

and acetate solutions using a mixture of ammonium bicarbonate and ammonium hydroxide as the 

precipitating agent. Stoichiometry, as determined by the metal nitrate ratio prior to precipitation, 

was set at Er0.01Al1.99O3. The amorphous precipitated powder was calcined in air and sintered in 

vacuum in a field-assisted sintering technology unit (FAST), which is also often referred to as 

spark plasma sintering (SPS). Samples were sintered at 1200 °C for 5 min. Further details of the 

powder synthesis and processing are addressed in reference 14. 

3. Results 

Most of the samples under investigation were α-Al2O3 powders or ceramics sintered at various 

temperatures, with and without co-dopants. We have implemented magnesium oxide (MgO) 

and/or nickel oxide (NiO) as co-dopants in order to maximize Er3+ doping concentration in the αAl2O3 as well as obtain the samples with the Er3+ spectra indicating purely crystalline 

environment (with no admixture of “glassiness”) as well as nearly single-site nature of Er3+

activation. This resulted in an Er3+ doping level of 0.1‒0.3 at.% in most samples. Detailed 

analysis of material synthesis conditions and material compositions leading to the desirable Er3+

spectra along with the relevant spectroscopic analysis at all development stages are presented in 

reference 14. All the samples obtained under optimized conditions exhibited sharp, well-resolved 

emission lines characteristic of RE3+ ions, also indicating predominant crystalline single-site Er3+

occupation. Typical room temperature (RT) 4I13/2  4I15/2 fluorescence spectrum for three 
7

randomly chosen experimental samples of Er3+:Al2O3 with such well-defined spectral structure is 

presented in figure 3. Similar spectra were previously observed in Er3+-doped Al2O3 powders 

prepared using the sol-gel technique (15). All experimental data presented and analyzed in this 

report have been obtained from Er3+-doped α-Al2O3 powders and transparent ceramics with such 

well-defined, crystalline, highly reproducible, nearly single-site fluorescent spectra. No 

discernible difference was observed in spectral behavior of properly prepared powders and 

sintered ceramics. Single-phase nature of each and every sample was carefully monitored by xray diffraction (XRD) in order to confirm that we are working with phase-pure α-Al2O3. Figure 4 

displays a picture of both the Er3+-doped α -Al2O3 powder pellet and Er3+-doped α-Al2O3

transparent ceramic used in our spectroscopic study. The XRD measurements (14) also indicated 

that the powder have successfully converted to the desired alpha phase.

Figure 3. Room temperature fluorescence spectra from 4I13/2 state for the two randomly chosen Er3+ doped α-Al2O3 

experimental samples.
8

Figure 4. Er3+-doped -Al2O3 powder pellet and Er3+-doped -Al2O3 transparent ceramic presented 

used in the present work.

Depending on the excitation wavelength, trivalent Er ions are capable of fluorescing in a wide 

spectral range extending from blue to mid-IR. We have attempted to excite and observe 

fluorescence from all Er3+ levels below 2H11/2 (6,000–19,000 cm–1photon energy range) by using 

for excitation 10-ns pulses from a tunable OPO (described earlier) in the range of 520–1600 nm. 

We observed fluorescence from 4I13/2 manifold through excitation of 4I9/2, as well as 4F9/2 and 

4

S3/2 manifolds. Figure 5 shows the excitation spectra of 4S3/2 in the vicinity of 550 nm in the 

temperature range of 10–100 K, with the details of the weak spectral transitions shown in the 

inset. Due to the relatively low Er3+ ion concentration and significant nonradiative quenching of 

all the states above 4I13/2, the quality of obtained fluorescence spectra was insufficient for their 

detailed spectroscopic analysis. For that reason, the data presented here are based on the 

fluorescence measurements from the excited state with the longest lifetime, 4I13/2 (

4

I13/2  4I15/2

transition). Moreover, the primary interest in our investigation was the Er 4I13/2, which are 

involved in Er based eye-safe laser operating at the 4I13/2  4I15/2 transition. It was found that the 

fluorescence decay from the 4I13/2 state is single-exponential, which is consistent with the low 

Er3+ concentration, when one can assume that inter-ionic interactions between the neighboring 

Er3+ ions, as well as between the Er3+ ions and possible uncontrollable impurity ions are 

negligible. The measured lifetime values for all studied samples are found to be in the range of 

2.2–7.7 ms, which is consistent with the lifetime measurements of this manifold in other Er3+-

doped oxides (16). The large lifetime difference are because of difference of sintering 

temperatures of the samples. Figure 6 shows typical fluorescence decay from the 4I13/2 level of 
9

Er3+:Al2O3 for three samples, sintered at different temperatures, when excited by ~10-ns pulses at 

~800 nm (excitation via the 4I15/2  4I9/2 absorption band). We have also investigated the 

temperature dependence of the 4I13/2 level fluorescence lifetime. In the temperature range of 

10–300 K we observed only slight variations in the lifetime, which can be explained by the 4I13/2

population redistribution and decay from the stark sublevels 4I13/2 with a larger probability of 

spontaneous emission. Much larger values of the 4I13/2  4I15/2 transition peak cross sections 

observed at lower temperatures (shown later in figure 10) indirectly support this statement.

Figure 5. Room temperature excitation spectra corresponding to the Er3+ 4I15/2  4S3/2 transitions 

at visible spectral range.
10

Figure 6. Typical Er3+ 4I13/2 state fluorescence lifetime in Er3+ doped α-Al2O3 for three samples 

sintered at various temperatures. The measured values for each curve are labeled in 

the picture.

The emission associated with the 4I13/2  4I15/2 transition of Er3+ ion in α-Al2O3 is of particular 

practical interest since efficient, high power, eye-safe lasers based on this transition have already 

been successfully demonstrated in other oxide laser hosts. It was shown that these lasers can 

efficiently operate at relatively low Er3+ concentrations at both room and cryogenic temperatures 

(17–19). The first report on successful doping of α-Al2O3 powder with trivalent Er was presented 

in reference 15. Later on, the authors of reference 20 indicated that the absorption and 

fluorescence spectra observed in reference 15, in fact, could have originated from the residual 

phase of the Er3+:yttrium orthoaluminate (YAlO3) compound. Moreover, numerous follow-on 

studies of Er-doped Al2O3 have shown that the residual phases of ErAlO3, ErAG, and Er2O3

compounds can be formed from starting materials as uncontrollable impurities during 

precipitation or calcination, and they may be responsible for the observed Er3+-like fluorescence 

in Al2O3 in the IR and visible spectral ranges. It is for that reason, in order to assure that the 

fluorescence and excitation spectra observed in this study undoubtedly originate from Er3+ ions 

in α-Al2O3, that along with testing our own Er-doped α-Al2O3 samples we have also performed 

the fluorescence and excitation testing of Er3+-doped YAG, Y2O3, and YAlO3 samples. Those 

samples were acquired as single crystalline and were pulverized for spectroscopic testing to 
11

make them identical to our own samples in terms of experimental conditions. We assumed Er3+-

doped YAG, Y2O3, and YAlO3 samples to be a spectroscopic substitute for ErAG, Er2O3, and 

ErAlO3. It is well known that, spectroscopically, Er3+-doped YAG, Y2O3, and YAlO3 are 

expected to be nearly identical or at least very similar to those of ErAG, Er2O3, and ErAlO3. 

Figure 7 shows the room temperature Er3+ 4

I13/2  4I15/2 emission spectra taken from Er3+-doped 

 -Al2O3, YAG, Y2O3, and YAlO3 powder samples. This direct comparison indicates that within 

the accuracy of our measurements there is no manifestation of any residual Er3+-doped YAG, 

Y2O3, and YAlO3 emissions (lines) in the Er3+-doped -Al2O3 test samples used in our work. As 

seen from figure 7 the fluorescence spectrum of our Er3+-doped α-Al2O3 powder is distinct in all 

of its structural details from the spectra obtained from Er3+-doped YAG, Y2O3, and YAlO3

powders. It also has a unique single sharp emission peak at 1535.4 nm. Figure 8 shows the 4I13/2 

 4I15/2 fluorescence spectra of our Er3+-doped α-Al2O3 samples for a range of temperatures 

between 10 and 300 K. While on a relative scale, figure 8 indicates that with temperature 

reduction from 300 to 10 K the maximum emission cross section is going up by a factor of six. 

Two insets in figure 8 show the zoomed in fragments of the two relatively weak parts of 

fluorescence spectra in the same temperature range. The most detailed fluorescence spectrum at 

10 K is shown in figure 9. It clearly exhibits the five strong spectral components at 1535.4, 

1544.2, 1568.1, 1574.6, and 1578.3 nm, and the three much weaker components at 1662.2, 

1665.8, and 1672.2 nm, which are zoomed in and shown on the top right inset in figure 9. The 

energy level diagram of the Stark-split ground state multiplet 4I15/2 of Er3+ ion in α-Al2O3 inferred 

from the measured fluorescence spectrum at 10 K is presented on the top left inset in figure 9. 

For the three highest Stark levels of the energy level diagram, which were inferred from the 

mentioned three weakest fluorescence transitions, the energy (in cm–1) is marked by an asterisks 

next to it. Due to the low strength and diffuse nature of these emission lines, we could not 

identify these lines as related to the 4I13/2  4I15/2 transition of Er3+-doped Al2O3 with the 100% 

certainty. Therefore, with the high degree of confidence, we can state that we observe five out of 

eight theoretically expected spectral components corresponding to the inter-Stark transitions 

from the lowest Stark sublevel of the 4I13/2 multiplet to the first five lowest sublevels of the 4

I15/2

ground state multiplet. It should be noted that the energy diagram inferred from the fluorescence 

spectra in figure 9 substantially differs from that reported in reference 15. As mentioned in 

reference 20, the energy level diagram derived from the 10 K fluorescence spectra for Er3+ 4I13/2 

 4I15/2 in reference 15 can be partially related with the fluorescence of Er3+ in the residual 

YAlO3 compound, which was likely contained in their Er3+:Al2O3 powders. 
12

Figure 7. Room temperature fluorescence spectra of Er3+ doped powders of α-Al2O3, Y2O3, YAlO3,

and YAG. The color legend – at top right.
13

Figure 8. Fluorescence spectra of Er3+ doped α-Al2O3 corresponding to the 4I13/2  4I15/2 transition 

in the temperature range of 10–300 K. Shown on the insets are the blow-ups of the 

weak spectral features in the 1430–1500 nm (left) and 1600‒1700 nm (right) spectral 

intervals.
14

Figure 9. 10-K fluorescence spectrum of Er3+ 4I13/2  4I15/2 transition in Er3+ doped α-Al2O3. The arrows 

indicate the spectral lines corresponding to the transitions from the lowest stark sublevel of the 

4

I13/2 multiplet to the lowest five sublevels of the 4I15/2 multiplet. Indicated on the right inset (in 

blue) is the blow-up of the weak spectral feature in the 1620–1700 nm spectral range. Shown in 

red is the same spectrum averaged by using Savitzky-Golay filter. Indicated on the left inset is 

the energy level scheme of the 4I15/2 ground state multiplet inferred from the spectrum.

From the measured fluorescence spectrum and the radiative lifetime, τ of the 4I13/2 multiplet one 

can derive the stimulated emission cross section using the well-known Fuchtbauer-Ladenburg 

method (21):

5

2

8πn cτ λI(λ)dλ emi









(1)

where σemi is the emission cross section, η is the fluorescence quantum efficiency, λ is the

fluorescence wavelength, and n and c are the refractive index and the light velocity, respectively. 

It should be noted that for the cross section derivation the longest measured lifetime, 7.7 ms, was 

used. This is based on an assumption that in the samples exhibiting the longest fluorescence 

lifetime the nonradiative quenching processes are negligible and, thus, the observed 4I13/2  4I15/2

transition lifetime is purely radiative. For the same reason, with this longest lifetime chosen for 

an estimate, the quantum efficiency η of this transition is assumed to be close to 1. The emission 

cross section spectrum of the 4I13/2  4I15/2 transition obtained from the above derivation is 

presented in figure 10 for both RT and LNT. It can be seen that Er3+ ion in α-Al2O3 host has the 
15

cross section values quite adequate for an efficient laser. It should be suitable for high power 

laser operation based on the 4I13/2  4I15/2 transition. The highest laser power scaling potential is 

expected at cryogenic temperatures where the observed peak cross section is practically on par 

with that of RT Nd3+-doped YAG. The excitation spectra corresponding to the 4I15/2  4I13/2

inter-multiplet transitions are shown in figure 11. Similarly to what was observed in the 

fluorescence spectra, we found four strong excitation lines at 1509.4, 1516.1, 1522.3, and 1535.4 

nm and three much weaker components at 1455.8, 1457.1, and 1459.0 nm, shown on the top left 

inset of figure 11. The inferred energy level diagram of the 4I13/2 multiplet is shown on a second 

inset. For the reasons mentioned earlier in relevance to the 4I13/2  4I15/2 fluorescence spectra, for 

these weak transitions, the corresponding energy levels on the diagram have an asterisk next to 

the level energy.

Figure 10. Emission cross section of the Er3+ 4I13/2  4I15/2 transition at room (300 K) and cryogenic 

(77 K) temperatures in Er3+ doped α-Al2O3. 
16

Figure 11. Excitation spectra corresponding to the Er3+ 4I15/2  4I13/2 transitions in the temperature range 

10–300 K in Er3+ doped α-Al2O3. Indicated on the inset is the energy level scheme of the 4I13/2

multiplet inferred from these spectra.

Because at the moment we do not have any transparent samples suitable for direct absorption 

measurements, it behooves us to compare the obtained excitation spectra for the 4I15/2  4I13/2

transition with the absorption cross section for the same transition. The absorption cross section 

can be inferred from the Fuchtbauer-Ladenburg derived emission cross section by reciprocity 

from the following well-known expression (22):

exp

ZL

U

abs emi

L

hc E

Z

Z kT

 

        

     

(2)

where σabs and σemi are absorption and emission cross sections; ZU and ZL are partition functions 

for the upper 4I13/2 and the lower 4I15/2 states; EZL is the zero line photon energy, i.e., the energy 

separation between the lowest stark sublevels of the 4I13/2 and 4I15/2 multiplets; k, c, and λ are the 

Planck constant, light velocity, and transition wavelength, respectively; and T is the temperature 

of the host. The calculation results are presented in figure 12. As can be seen from figure 12, 
17

there is a satisfactory agreement between the relative excitation peak strength and the absorption 

cross sections.

Figure 12. Emission (from Fuchtbaer-Ladenburgh), absorption (from emission through reciprocity) and 

excitation cross sections for the Er3+ 4I15/2  4I13/2 and 4I13/2  4I15/2 transitions at room 

temperature in Er3+ doped α-Al2O3. 

The energy structure of the 4I11/2 manifold is also of great practical importance since most of Erbased solid-state lasers are currently pumped through this level. This manifold provides 

sufficiently strong absorption in the spectral range of emission of commercially available 

InGaAs/AlGaAs) 980-nm semiconductor lasers. Moreover, this level in most oxide laser 

materials is metastable, with the lifetime varying from the shortest (~0.1 ms) in Er:YAG to the 

longest (~3.6 ms) in Er: yttrium oxide (Y2O3) (23). Lasers emitting around ~3 μm have been 

built based on a number of Er3+-doped oxide hosts (Er:YAG, Er:YAlO3 Er:Y2O3, etc.), where 

laser action is initiated from this level, and exploits the nominally self-terminated 4I11/2 4I13/2

laser transition (24). The fluorescence from this level in our Er3+-doped Al2O3 samples was too 

weak in order to be used for any sensible interpretation. The high multiphonon decay rate of the 

4

I11/2 level in this material is due to the fact that the energy gap between this and the next level 

down (4I13/2) is only about 3000 cm–1and the maximum phonon frequency of this host material is 

~750 cm–1(one of the highest among the oxide laser hosts) (25). The excitation spectrum taken 

at 10 K corresponds to the transitions from the lowest Stark sublevel of the ground state multiplet 
18

4

I15/2 to all Stark sublevels of the 4I11/2 state. At this temperature, we observed five out of six 

(expected theoretically) spectral components at 964.2, 965.0, 973.8, 975.2, and 979.5 nm. The 

Stark sublevels energy positions, inferred from the 10-K excitation spectra are shown in table 1. 

A very weak spectral component at 10322 cm–1, marked in the table by an asterisk, which can 

only be discerned after thorough data analysis and noise filtering in the 967‒969 nm spectral 

range can actually be interpreted as a missing line of the 4I15/2  4I11/2 transition at 968.8 nm. 

Table 1. Energy levels for Er3+:α-Al2O3

4

I15/2,

 4I13/2, 4

I11/2, 

4

I9/2 manifolds in wave 

numbers (cm–1). Weak transitions are marked by asterisks. 

4

I15/2

4

I13/2

4

I11/2

4

I9/2

0 6513 10209 12365

37 6569 10254 12376

136 6596 10269 12595

162 6625 10322* 12618

177 6854* 10363 12631

497* 6863* 10371

510* 6869*

533*

Normally, fluorescence from the 4I9/2 level of Er3+ ion is quenched by severe multiphonon 

relaxation (in almost all oxides) and only in a few hosts (the so-called low-phonon hosts) is it 

observable. The fluorescence signal from this level in Er3+-doped Al2O3 was also too weak to be 

used for any meaningful interpretation. Nevertheless, we have extensively explored the 

excitation spectra corresponding to the 4I15/2  4

I9/2 transition in a wide temperature range. 

Knowledge of the energy level diagram of this level is also important since the 4I15/2  4I9/2

absorption band of Er3+ is very suitable for pumping with another efficient commercially 

available InGaAs/AlGaAs semiconductor lasers emitting around 800 nm. We have observed all 

five absorption lines (expected theoretically) in the 10-K excitation spectrum. The energy level 

positions for the 4I9/2 state corresponding to the spectral range 788–823 nm, derived from the 

10-K excitation spectrum are shown in table 1. The absorption peaks at 791.7, 792.5, 793.9, 808,

and 808.7 nm are identified as corresponding to the 4I15/2  4I9/2 inter-multiplet transitions. 

4. Conclusions 

This work presents spectroscopic evidence that the trivalent Er ions can be successfully doped 

into α-Al2O3 host despite the significant ionic radius mismatch between the dopant and ligand. 

The observed Er3+ ion spectra in -Al2O3 nanopowders and fully densified ceramics are a 

manifestation of purely crystalline environment and also indicate the nearly single-site nature of 

Er3+ activation. So far we achieved Er3+ doping level of 0.1–0.3 at.% in most of our samples. The 

emission cross section (derived via Fuchtbauer-Ladenburg routine) followed by absorption cross 
19

section derived from reciprocity are presented for both room and cryogenic temperatures. 

Presented detailed spectroscopic study of the ground state (4I15/2) and the first three excited Er3+

multiplets (4I13/2, 

4

I11/2, and 4I9/2) in this material can be successfully used for future design and 

development of high power Er:Sapphire based lasers. The currently planned crystal field 

calculations for Er3+ ion in an octahedral Al3+ position will answer the question of whether the 

observed spectral behavior supports the hypothesis of Er3+ ion incorporation into Al3+ site in -

Al2O3 single crystal structure. 
20

5. References

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Cr2O3 Structures. Zeitschrift für Kristallographie 1962, 117 (2–3), 235–237.

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Films Deposited by Radiofrequency Magnetron Co-sputtering. Opt. Mater. 2011, 33 (7), 

1135–1138.

6. Song, Q.; Li, C.; Li, J.; Ding, W.; Xu, J.; Deng, X.; Song, C. Photoluminescence Properties 

of the Yb : Er Co-doped Al2O3 Thin Film Fabricated by Microwave ECR Plasma Source 

Enhanced RF Magnetron Sputtering. Opt. Mater. 2006, 28 (12), 1344–1349.

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Erbium-doped Amplifiers with Silicon-on-insulator Waveguides. Optics Express 2010, 18

(26), 27703–27711. 

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(Al1−xGax)2O3 Films on Sapphire for Solid-state Waveguide Lasers. Opt. Lett. 2012, 35 (22), 

3793–3795. (or In Ref). 

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Express 2011, 1 (5), 1040–050.
21

12. Krebs, J. K.; Happek, U. Yb3+ Energy Levels in α-Al2O3. J. Lumin. 2001, 94–95, 65–68.

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Al2O3 into Single-Crystal Sapphire by Abnormal Grain Growth. J. American Ceramic 

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Vasilevskaya, T. N. Optical Spectra of Triply-charged Rare-earth Ions in Polycrystalline 

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Dependence of a Diode-Pumped Cryogenic Er:YAG Laser. Opt. Express 2009, 17 (9), 

7158–7169.

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513. 

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Eye-safe Er:Sc2O3 Laser. Optics Letters 2008, 33, 1524–1526. 

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22

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23

List of Symbols, Abbreviations, and Acronyms 

Al aluminum 

AlGaAs aluminum gallium arsenide 

AlO3 aluminum oxide 

α-Al2O3 alpha-phase aluminum oxide, also known as corundum 

cm

–1

 reverse centimeter 

CTE coefficient of thermal expansion 

DAQ data acquisition 

Er erbium 

FTIR Fourier transform infrared 

InGaAs indium gallium arsenide 

LHP laser heated pedestal growth 

LNT liquid nitrogen temperature 

MBE molecular beam epitaxy 

MgO magnesium oxide 

N refractive index 

NiO nickel oxide 

O oxygen 

OPO optical parametric oscillator 

RE rare earth 

RT room temperature 

Si silicon 

SiO2 silicon oxide 

SPS spark plasma sintering 
24

SSCC solid-state crystal conversion 

XRD x-ray diffraction 

Y2O3 yttrium oxide 

YAG yttrium aluminum garnet 

YAlO3 yttrium aluminum oxide or YAP

Yb ytterbium 
25

NO. OF 

COPIES ORGANIZATION 

 1 ADMNSTR 

 ELEC DEFNS TECHL INFO CTR 

 ATTN DTIC OCP 

 8725 JOHN J KINGMAN RD STE 0944 

 FT BELVOIR VA 22060-6218 

 1 US ARMY RSRCH DEV AND ENGRG CMND 

 ARMAMENT RSRCH DEV & ENGRG CTR 

 ARMAMENT ENGRG & TECHNLGY CTR 

 ATTN AMSRD AAR AEF T J MATTS 

 BLDG 305 

 ABERDEEN PROVING GROUND MD 21005-5001 

 1 US ARMY INFO SYS ENGRG CMND 

 ATTN AMSEL IE TD A RIVERA 

 FT HUACHUCA AZ 85613-5300

 1 US GOVERNMENT PRINT OFF 

 DEPOSITORY RECEIVING SECTION 

 ATTN MAIL STOP IDAD J TATE 

 732 NORTH CAPITOL ST NW

 WASHINGTON DC 20402 

 9 US ARMY RSRCH LAB 

 ATTN RDRL WMM E A SUTORIK 

 ATTN RDRL WMM E G A GILDE (4 COPIES) 

 ATTN RDRL WMM E R PAVLACKA (4 COPIES) 

 BLDG 4600 

 ABERDEEN PROVING GROUND MD 21005 

19 US ARMY RSRCH LAB 

 ATTN IMAL HRA MAIL & RECORDS MGMT 

 ATTN RDRL CIO LL TECHL LIB 

 ATTN RDRL CIO LT TECHL PUB 

 ATTN RDRL SEE M G NEWBURGH

 ATTN RDRL SEE M J ZHANG 

 ATTN RDRL SEE M L MERKLE 

 ATTN RDRL SEE M M DUBINSKIY (4 COPIES) 

 ATTN RDRL SEE M N TER-GABRIELYAN 

 ATTN RDRL SEE M T SANAMYAN (6 COPIES) 

 ATTN RDRL SEE M V FROMZEL 

 ATTN RDRL SEE M Z FLEISCHMAN 

 ADELPHI MD 20783-1197 

TOTAL: 32 (31 HCS, 1 ELEC) 
26

INTENTIONALLY LEFT BLANK.