www.elsevier.com/locate/optmat

Optical Materials 27 (2004) 343–349

Spectroscopy of potential laser material Yb3+(4f13) in NaBi(WO4)2

Larry D. Merkle a,*, Mark Dubinskii a, Bahram Zandi a, John B. Gruber b,Dhiraj K. Sardar c, Edvard P. Kokanyan d, Vahan G. Babajanyan d,

Gagik G. Demirkhanyan d, Radik B. Kostanyan d

a ARL/Adelphi Laboratory Center, 2800 Powder Mill Road, Adelphi, MD 20783-1197, USAb Department of Physics, San Jose State University, San Jose, CA 95192-0106, USA

c Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX 78249-0663, USAd Institute for Physical Research, Armenian National Academy of Sciences, Ashtarak 378410, Armenia

Received 12 November 2003; accepted 20 February 2004

Available online 14 August 2004

Abstract

We report the solid-state synthesis, growth, and spectroscopic properties of single-crystal NaBi(WO4)2 containing Yb3+ ions. The

tetragonal crystal phase is disordered, resulting in substantial inhomogeneous broadening of spectra. The site symmetry for the Yb3+

ions can be approximated by D2d symmetry according to a quasi-center model, which has recently proven quite satisfactory for the

analysis of Er3+ crystal field splittings in the same host. We use the Er3+ results to estimate crystal field parameters for Yb3+, and

compare the splittings predicted for these parameters with observed spectra. We also report the fluorescence lifetime at cryogenic

and room temperatures, and give estimates for the stimulated emission cross-section and radiative lifetime of Yb3+ emission.

The results indicate that this material may have promise as a gain medium for a smoothly tunable 1 lm laser.

� 2004 Elsevier B.V. All rights reserved.

PACS: 42.70.HgKeywords: Yb3+; Absorption; Luminescence; Crystal field; Tungstate

1. Introduction

Trivalent ytterbium (Yb3+) incorporated into various

host crystals demonstrates stimulated emission near

1 lm, often tunable, and self-frequency-doubled green

laser emission from selected nonlinear host materials[1–7]. The ground-state electronic configuration of

Yb3+(4f13), consisting of the ground-state manifold,2F7/2, and the excited state manifold, 2F5/2 separated

by about 10,000 cm�1, provides a simple energy level

scheme that avoids many of the problems that are pre-

sent when other rare earth ions are used as the lasant.

These problems include excited state absorption, cross-

0925-3467/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2004.02.030

* Corresponding author.

E-mail address: Lmerkle@arl.army.mil (L.D. Merkle).

relaxation and upconversion [8–12]. High-power

InGaAs diode lasers are now available to pump the

excited 2F5/2 state in the 900–980 nm spectral range with

very small quantum defect between absorption and

emission (low-thermal loading) with the result that

numerous applications have appeared that call for1 lm high-energy and high-beam quality laser sources

with Yb3+ as the lasant [2,12].

In the present study, we report the solid-state synthe-

sis, growth, and spectroscopic properties of single-crys-

tal NaBi(WO4)2 doped with Yb3+ in concentrations

between 0.2 and 2.0 at.% in the melt. Absorption and

emission spectra obtained between 900 and 1100 nm

are reported at various temperatures from 8 to 300 K.Different valency cations Na+ and Bi3+ statistically

fill the S4 symmetry sites in the ratio of 1:1 in the

NaBi(WO4)2 lattice leading to a disordered tetrahedral

344 L.D. Merkle et al. / Optical Materials 27 (2004) 343–349

phase. This disorder is reflected in the inhomogeneous

broadening of the Yb3+ spectra. NaBi(WO4)2 has the

scheelite structure with space group C64h � I41=a and lat-

tice parameters a = 0.528 nm and c = 1.150 nm [13,14].

To interpret the crystal-field splitting of the multi-

plets, 2F7/2 and 2F5/2, we began from the crystal fieldparameters recently obtained from the analysis of Er3+

spectra in the same host [15]. The three-parameter the-

ory of Morrison and co-workers [16,17] was then used

to interpolate a set of crystal-field parameters, Bnm, from

Er3+ to Yb3+, yielding predicted splittings that can be

compared with observed values.

We have also measured the fluorescence spectra at

cryogenic and room temperatures. We used the resultingenergy levels and absorption data to predict an approx-

imate peak stimulated emission cross-section and a radi-

ative lifetime, for comparison with observed lifetimes.

Comparison of the predicted and observed emission

spectra gives additional evidence of the inhomogeneity

in this material, but also suggests its potential as a tun-

able laser material.

Wavelength (nm)900 950 1000 1050 1100

Abs

orpt

ion

Stre

ngth

(Arb

itrar

y U

nits

)

0

2

4

6

8

300 K200 K120 K8 K

Fig. 1. Unpolarized absorption spectrum of Yb3+:NaBi(WO4)2obtained at 8, 120, 200, and 300 K. The melt Yb concentration is

0.8 at.%, corresponding to approximately 1.15 · 1020 Yb/cm3 in the

crystal. Baselines are offset for clarity.

2. Experimental

The solid phase synthesis of NaBi(WO4)2 was car-

ried out by sintering stoichiometric mixtures of high-

purity materials NaCO3, Bi2O3, and WO3 in two

stages. The first thermal treatment was done at

450 �C for 5 h followed by grinding the mixture anda second stage that involved heating at 720 �C for 7

h. Single crystals were grown by the Czochralski meth-

od from the melt in air using platinum crucibles. A ver-

tical type Fibrothal resistant heater-furnace was used

with a 0.1 �C precision controller and with a vertical

temperature gradient of 15–25 �C/cm above the melt

in the growth zone. Khantal wire was used as a heating

element for an active after heater. The melting temper-ature for NaBi(WO4)2 is 920 ± 20 �C. The pulling rate

was varied between 0.5 and 2 mm/h depending on the

dopant concentration, and the rotation rate was 10

rpm. The Yb ions were added to the melt in the form

of NaYb(WO4)2 and the mixture retained at a temper-

ature about 20 �C higher than the melting point for 5

h. Crystals having a diameter of 15 mm and length of

30 mm were grown from oriented seeds along the a-axis. X-ray diffraction analysis of both powders and

crystals confirmed the reported scheelite structure

[13]. Crystals were grown with Yb concentrations be-

tween 0.2 and 2.0 at.% in the melt [18]. To determine

the amount of Yb in the crystal grown with 0.8 at.%

in the melt, we had a sample analyzed for Yb by Gal-

braith Laboratories, Knoxville, TN. They reported 0.44

wt.% Yb, corresponding to 1.85 at.% or 1.15 · 1020 Yb/cm3 in the crystal. This indicates a distribution coeffi-

cient of about 2.3 for Yb in NaBi(WO4)2, somewhat

smaller than that reported by Subbotin et al., but still

strikingly large [19].

Absorption spectra were obtained from an upgraded

Cary Model 14 R spectrophotometer. The spectral

bandwidth was set at 0.2 nm, and the instrument was

calibrated internally to an accuracy better than 0.3nm. Samples of Yb3+-doped crystals were cut and pol-

ished as plates of various thicknesses for spectroscopic

measurements. Fluorescence spectra were obtained

using a Spex 1680 double monochromator with gratings

blazed at 1 lm. A cooled Hamamatsu R5108 (S-1-like)

photomultiplier tube was used as the detector. Fluores-

cence was excited by a Xe arc lamp or a Lambda Physik

ScanMate optical parametric oscillator. Spectra wereobtained at various temperatures between 8 K and room

temperature by mounting the sample on the cold finger

of a CTI Model 22 closed-cycle helium cryogenic refrig-

erator. The sample temperature was monitored with a

silicon-diode sensor attached to the base of the sample

holder and maintained by using a Lake Shore tempera-

ture control unit. For fluorescence lifetime measure-

ments, the sample was excited at 954 nm using thesecond Stokes Raman shift in high-pressure hydrogen

gas of the frequency doubled output of a Continuum

Surelite Nd:YAG laser. The fluorescence was detected

using the same photomultiplier as for the spectra,

and analyzed using a Tektronix TDS7104 digital

oscilloscope.

3. Absorption and emission spectra

The unpolarized absorption spectra obtained be-

tween 900 and 1100 nm at different temperatures are

given in Fig. 1. At the lowest temperature, polarized

L.D. Merkle et al. / Optical Materials 27 (2004) 343–349 345

spectra were also measured, and were nearly the same as

in Fig. 1. The spectrum at 8 K consists of two bands

with structure near 940 and 960 nm and a sharp intense

peak at 974 nm with a shoulder on the short wavelength

side. The inhomogeneous broadening and structure is

likely the result of the disorder due to random Na+

and Bi3+ occupation of the 4a crystallographic positions

[20]. The Yb3+ ions probably replace Bi3+ ions at these

4a sites. The several peaks in Table 1 reflect this struc-

ture. The levels labeled Y1, Y2 and Y3 lie at the approx-

imate centers of the three main bands.

In Table 1 the absorption spectra can be interpreted

using the quasi-center concept proposed by Kaminskii

and co-workers [20,21] for the spectroscopic analysisof Nd3+ in NaBi(WO4)2. The temperature-dependent

‘‘hot band’’ spectra obtained at 120, 200, and 300 K

(see Fig. 1) represent transitions from at least one ex-

cited Stark level of the ground-state multiplet manifold,2F7/2. In Table 1 the crystal-field splitting of 2F5/2 is labe-

led as Y1, Y2, and Y3. The levels of 2F7/2 are labeled as

Z1, Z2, Z3, and Z4. The Z1 � Z2 splitting is established

from the hot band data as approximately 202 cm�1.The upper levels of 2F7/2 are determined from emis-

sion spectra. The low-temperature spectra of nominally

0.8 at.% Yb3+ in NaBi(WO4)2, excited near 940 nm by

an optical parametric oscillator, are shown in Fig. 2.

The disordered nature of this host is evident in the

breadth of the emission features, only partially resolved

even at low temperature. This disorder is probably also

the cause of the very weak polarization dependence ob-served. The spectra are corrected for system response,

including the relative intensities of the different polariza-

tions. It is evident that the two polarizations are of com-

parable strength, and that no line is completely

polarized. At room temperature, the r- and p-polarizedspectra are nearly identical.

Despite broadening of the emission spectra, it is pos-

sible to discern peaks in the low-temperature spectrumat about 974, 998, and 1010 nm. There is also a shoulder

on the long wavelength side, near 1020 nm. Deconvolu-

tion using Origin software gives strong peaks at 974.7,

996.3, 1009.8 and 1019.9 nm, and weaker peaks at

966.0 and 983.5 nm. The four strong peaks indicate

the crystal-field splitting of the 2F7/2 manifold, with

Stark-level energies of 0, 222, 357, and 455 cm�1, which

are included in Table 1. The 222 cm�1 value for Z2 issomewhat different from that found in hot band absorp-

tion. Given the inhomogeneity of the spectra and the

breadth of the hot absorption band, the difference is

not unduly large. Of the two weaker peaks, the one at

983.5 nm is about 130 cm�1 below the 996.3 nm line,

very similar to the spacing between the prominent 962

and 974 nm low-temperature absorption lines. The weak

feature yielding a deconvolution position of 966 nmexhibits structure at 962 and 964.5 nm, again closely

similar to absorption features. Thus, a plausible inter-

pretation of these weaker emission features is emission

from the Y2 level to Z1 and Z2. Observable population

of Y2 would indicate a sample temperature significantly

above the nominal 9 K, which is possible since the sam-

ple was farther from the cold head than was the temper-

ature sensor.Note from Fig. 2 that Y1 ! Z2 (998 nm) and

Y1 ! Z3 (1010 nm) are much stronger than the transi-

tion Y1 ! Z1 (974 nm). Despite its being only a shoul-

der, the same also appears true of the Y1 ! Z4

transition (1021 nm). With so much of the emission

strength in a smooth distribution at long wavelengths,

where the absorption is relatively small even at room

temperature, Yb:NaBi(WO4)2 may be of interest as atunable laser.

4. Analysis of the spectra

The total of seven Stark levels, Y1–3 and Z1–4 appear-

ing in Table 1, is insufficient for a fitting analysis be-

tween the experimental and calculated levels toestablish a set of phenomenological crystal-field param-

eters, Bnm, for Yb3+ ions in a disordered (low-site sym-

metry) crystal. The situation can be made tractable

using the quasi-center concept. If the random distribu-

tion of Na+ and Bi3+ on the 4a crystallographic sites

were averaged, the 4a site point symmetry would be

S4. For two reasons, we chose to analyze the spectra

in terms of the still higher D2d symmetry. First, the veryweak polarization precluded symmetry label assign-

ments and identification of electric vs. magnetic dipole

transitions between individual Stark levels. Second, a

crystal-field Hamiltonian having S4 symmetry includes

both real and imaginary terms, resulting in a larger

number of crystal-field parameters than are associated

with D2d symmetry. The D2d symmetry limits the non-

zero Bnm terms to five, namely: B20, B40, B44, B60, andB64, with all parameters real and in units of cm�1. The

z-axis was chosen to coincide with the optical c-axis.

Some of us have recently applied this approach to the

analysis of Er:NaBi(WO4)2, with very satisfactory re-

sults [15]. The crystal field parameters for Er3+ can be

used in the three-parameter theory of Morrison and

co-workers [16,17] to estimate the crystal field parame-

ters for Yb3+ in the same host. The resulting set of crys-tal field parameters for Yb3+ is B20 = �240, B40 = 509,

B44 = 636, B60 = �735, and B64 = �138, in units of

cm�1. These Bnm in turn give the following energy levels

(adjusting only the centroid energies): E(Z1) = 0,

E(Z2) = 95, E(Z3) = 263, E(Z4) = 356, E(Y1) = 10,260,

E(Y2) = 10,414, E(Y3) = 10,548, also in cm�1.

Since these predicted splittings differ significantly

from those observed, we have also performed a least-squares fit of the energy levels. For E(Z2), we used the

average of the values inferred from absorption and from

Table 1

Absorption spectra of Yb3+ in NaBi(WO4)2a

Nob k (nm) 8 K a (cm�1) E (cm�1) obs. k (nm) 120 K a (cm�1) E (cm�1) obs. k (nm) 200 K a (cm�1) E (cm�1) obs. E (cm�1) fitc 2s+1Ljd

935.4 1.18 10,688 936.0 1.00 10,681 936.4 0.96 10,676

Y3 940.8 1.52 10,626 940.4 1.14 10,631 940.0 0.98 10,620 10,608

954(sh) 0.65 10,478 954(sh) 0.80 10,476 2F5/2

962 2.18 10,392 961.8 1.59 10,394 961.8 1.37 10,396 (10,410)

10,371

Y2 964.2 1.83 10,368 964.4 1.37 10,366 964.5 1.33 10,365

968(sh) 1.17 10,326

973.6(sh) 2.52 10,268

Y1 974.2 4.77 10,260 974.6 2.40 10,258 975 1.74 10,256 10,263

981(sh) 0.48 10,191 982(sh) 0.76 10,193

994.2 0.33 10,056 994.3 0.57 10,054

Z4 455e 467 2F7/2

Z3 357e 356

(259)

Z2 202, 222e 207

Z1 0 �6

a Sample nominally 0.8 at.% Yb3+.b Labels Yn represent absorption bands of the 2F5/2 multiplet manifold; transitions to the Stark levels of the 2F5/2 (Y1 through Y3) originate from Stark levels Z1 through Z4 of the ground-state

manifold, 2F7/2.c Splittings due to least-squares fit to observed levels, resulting in following crystal field parameters: B20 = �152, B40 = 819, B44 = 797, B60 = �449, B64 = �642, all in cm�1 (D2d symmetry).d Multiplet manifolds, 2F5/2 and

2F7/2; calculated centroids are given in parentheses.e Levels obtained from 9 K fluorescence spectra reported in Section 3.

346

L.D

.Merk

leet

al./Optica

lMateria

ls27(2004)343–349

Wavelength (nm)940 960 980 1000 1020 1040 1060 1080

Inte

nsity

(Arb

itrar

y U

nits

)

0.00

0.05

0.10

0.15

0.20

E ⊥ c E || c

T = 9 K EmissionExcited at 939 nm

Fig. 2. Observed emission spectra of Yb3+:NaBi(WO4)2 obtained at 9

K, with polarization states as labeled. Spectra are corrected for the

detection system�s wavelength and polarization dependence.

L.D. Merkle et al. / Optical Materials 27 (2004) 343–349 347

emission. The resulting best-fit energies and Bnm are

given in Table 1.

The stimulated emission cross-section as a function of

frequency, m, may be estimated from the observed

absorption spectra and the energy levels using the reci-procity method [2], where

rSEðmÞ ¼ rabsðmÞ � ðZ l=ZuÞ � exp½E0 � hm=kT �: ð1ÞIn Eq. (1) rSE(m) and rabs(m) are the stimulated emission

and absorption cross-sections, and Zl and Zu are the

lower and upper manifold partition functions (1.66and 1.77, respectively, when T equals room tempera-

ture); E0 is the energy difference between the lowest-

energy Stark levels of each manifold, and k is the

Boltzmann constant. The resulting stimulated emission

spectrum is shown in Fig. 3. The predicted peak cross-

section, about 1.5 · 10�20 cm2, occurs at 998 nm. Fig.

3 also shows the stimulated emission spectrum predicted

from the room temperature emission spectra by theFuchtbauer–Ladenburg method [2]. For this method,

Wavelength (nm)940 960 980 1000 1020 1040 1060

Cro

ss S

ectio

n (1

0-20 c

m2 )

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

ReciprocityFuchtbauer-Ladenburg

Fig. 3. Stimulated emission spectrum of Yb3+:NaBi(WO4)2 at room

temperature between 940 and 1060 nm as predicted from absorption

by reciprocity, and from emission by the Fuchtbauer–Ladenburg

method.

the emission spectrum and the index of refraction are

the averages over polarizations, and the radiative life-

time is estimated from the reciprocity prediction in the

next section.

Our measured room temperature emission spectra,

consistent with that reported by Subbotin et al. [19] donot fall off rapidly for wavelengths beyond the 998 nm

peak as does the cross-section spectrum predicted by

reciprocity. Indeed, the fluorescence peaks at about

1010 nm. Since the reciprocity method relies on very

weak absorption to predict emission at long wave-

lengths, its accuracy is inherently limited in this region.

To the extent that this difference is real it may indicate

that, among the randomly distributed Yb sites in thisdisordered material, emission occurs preferentially from

sites with lower 2F5/2 energies or with stronger electron–

phonon coupling. This possibility is supported by the

fact that we observe somewhat different emission spectra

for two different excitation wavelengths.

5. Fluorescence lifetimes

The peak stimulated emission cross-section and

bandwidth predicted by the reciprocity method enable

us to estimate the room temperature radiative lifetime

of the Yb3+(2F5/2) manifold. For simplicity, we use the

standard expression for a Gaussian band,

rSE;peakðmÞ ¼pðln 2=pÞ � k2=ð4pn2 � Dm� srÞ; ð2Þ

where k is the peak emission wavelength in vacuum, n is

the index of refraction (2.15) when averaged over polar-

izations [21], Dm is the full width of the stimulatedemission band at half maximum intensity, and sr is

the radiative lifetime. For a peak cross-section of

1.5 · 10�20 cm2, we obtain a value of 400 ls for sr atroom temperature.

We have measured the fluorescence lifetime in a sam-

ple with nominally 0.8 at.% Yb at room temperature

and at liquid nitrogen temperature. The fluorescence

decay waveforms are not purely exponential, but aredominated by a single lifetime component: 370 ls at

80 K and 550 ls at room temperature. The room tem-

perature value is comparable to that reported by Sub-

botin et al. [19].

One possible cause for the observed temperature

dependence of the lifetime is reabsorption of emitted

light, often called radiative trapping, which is often sig-

nificant for Yb3+ [22]. A very simple model permits arough estimate of its effect on the observed lifetime, as

follows. A weighted average of the absorption cross-

section over the emission band at room temperature

yields values of 1.2 · 10�21 cm2 at 9 K and 3.7 · 10�21

cm2 at room temperature. The sample (0.8 cm · 0.65

cm · 0.35 cm) was excited fairly uniformly by not plac-

ing the sample exactly at the focus of the excitation

348 L.D. Merkle et al. / Optical Materials 27 (2004) 343–349

optics. Modeling the sample as a uniformly excited

sphere of the same volume, the observed lifetime for

Yb3+ ions near the center of the sphere is predicted to

be 5% longer than the intrinsic lifetime at 9 K due to

reabsorption, and 17% longer at room temperature.

Thus, the observed lifetimes predict only slightly shorterradiative lifetimes, about 350 ls at 9 K and 470 ls at

room temperature. This suggests that most of the ob-

served temperature dependence is caused by some other

mechanism, such as differences between the oscillator

strengths for transitions from the second 2F5/2 level,

Y2, relative to those from the first, Y1.

The room temperature lifetime estimate is reasonably

consistent with the radiative lifetime inferred from theabsorption data. The difference is probably not due to

nonradiative quenching, which is weak in Yb3+ systems,

as noted earlier. It may be due in part to the simple line

shape assumed in Eq. (2). Also, the evidence of inhomo-

geneity noted in regard to the emission spectra may also

result in the emitting ions having radiative lifetimes dif-

ferent from the full ensemble interrogated by absorption.

6. Discussion and conclusions

It is somewhat striking that the crystal field splittings

of Yb3+ in NaBi(WO4)2 predicted from Er3+ and the

three-parameter theory agree relatively poorly with the

observed spectra, whereas the splittings of Er3+ predicted

from Nd3+ spectra in this same host agree with experi-ment quite well [15]. The inhomogeneity of the emission

provides one possible reason. Since we have observed

that the emission varies with excitation source and differs

from the spectrum predicted from absorption, the energy

levels for 2F7/2 determined from emission may not repre-

sent the same statistical distribution of sites sampled in

the absorption experiments. Another possible cause is

relaxation of the ligands around the dopant ion due tothe large difference in ionic sizes between the dopant

and both Na+ and Bi3+ [23]. This difference is somewhat

greater for Yb3+ than for Er3+, so that the local environ-

ments of these two dopants may differ significantly.

The splitting of the 2F7/2 ground-state manifold in

Yb:NaBi(WO4)2 is seen to be considerably smaller than

that in Yb:YAG [12], and in several Yb-doped borate

and silicate crystals recently reported by Haumesseret al. [24]. This will make it more difficult to overcome

ground-state absorption and achieve room temperature

laser action. Yet, two features of the Yb3+ transitions

in this host appear sufficiently favorable to outweigh this

limitation.

The overall strength of transitions between 2F7/2 and2F5/2 levels in Yb:NaBi(WO4)2 is unusually large, as

indicated both by the relatively short fluorescence life-time and, perhaps even more strikingly, by the short

radiative lifetime predicted from absorption. As a

consequence, and despite considerable inhomogeneous

broadening, the predicted peak stimulated emission

cross-section is substantial. It is significantly larger than

that of Tm:YAG and those of all the materials reported

in Ref. [24], and indeed is comparable to that of

Yb:YAG.The other favorable factor arises from the evident

stronger emission at long wavelengths than predicted

from absorption by reciprocity. Due to ground-state

absorption, laser operation should only be expected at

wavelengths longer than perhaps 1020 nm, and in that

region reciprocity predicts relatively small stimulated

emission cross-sections. However, the stimulated emis-

sion spectrum predicted from the observed emission sug-gests that the cross-section is in fact quite reasonable in

the lower-absorption region beyond 1020 nm. Thus,

Yb:NaBi(WO4)2 may be quite promising as a tunable

laser having, unlike Yb:YAG [5], continuous and rela-

tively smooth tunability.

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