Optical Materials 28 (2006) 231–240 www.elsevier.com/locate/optmat

Synthesis and characterization of diffusion-doped Cr2+:ZnSe and Fe2+:ZnSe Umit Demirbas a, Alphan Sennaroglu a

a,*

, Mehmet Somer

b

Laser Research Laboratory, Department of Physics, Koc¸ University, Rumelifeneri Yolu, Sariyer, 34450 Istanbul, Turkey b Department of Chemistry, Koc¸ University, Rumelifeneri Yolu, Sariyer, 34450 Istanbul, Turkey Received 18 May 2004; accepted 15 October 2004 Available online 9 February 2005

Abstract This paper provides a detailed description of the preparation and characterization of diffusion-doped Cr2+:ZnSe and Fe2+:ZnSe. In the experiments, Cr2+:ZnSe samples with peak absorption coefficients (1775 nm) of as high as 74 cm
1 and fairly good spatial uniformity were obtained. In the case of Fe2+:ZnSe, samples with a maximum absorption coefficient of 11.6 cm
1 at 2400 nm were synthesized. A three-dimensional analytical diffusion model was further developed to determine the diffusion coefficient D from absorption data. At 1000
C, D was determined to be 5.4 · 10
10 cm2/s and 7.95 · 10
10 cm2/s for Cr2+:ZnSe and Fe2+:ZnSe, respectively. In the case of Cr2+:ZnSe, we employed an alternative technique to determine the diffusion coefficient from position-dependent absorption data taken with a Cr4+:YAG laser. With this method, the temperature variation of D was further measured in the 800– 1100
C temperature ranges.  2005 Elsevier B.V. All rights reserved. PACS: 42.55.P,R; 42.70.H Keywords: Solid-state lasers; Mid-infrared lasers; Thermal diffusion; Transition metal ion-doped chalcogenides

1. Introduction Extensive spectroscopic studies performed over the last four decades showed that II–VI compound semiconductors doped with transition metal (TM) ions possess luminescence bands in the mid-infrared region of the electromagnetic spectrum between 2 and 5 lm [1–9]. More recently, researchers at Lawrence Livermore National Laboratory investigated the lasing potential of a subgroup of these materials, namely, zinc chalcogenides doped with Cr2+, Co2+, Ni2+, and Fe2+, and demonstrated efficient, room-temperature lasing action near 2.5 lm by using Cr2+:ZnSe and Cr2+:ZnS [10,11]. Since *

Corresponding author. Tel.: +90 212 338 1429; fax: +90 212 338 1559. E-mail address: [email protected] (A. Sennaroglu). 0925-3467/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.10.022

then, considerable attention has been focused on the development of mid-infrared TM ion-doped chalcogenide lasers [12–26]. In the case of Cr2+:ZnSe, the most extensively studied member of this class of tunable solid-state gain media, pulsed [10,11], continuous-wave [14], diode-pumped [27], mode-locked [15], random-lasing [25], and single-frequency [24] operations have been demonstrated. The broadest tuning range demonstrated to date extends from 2000 to 3100 nm [28]. Lasing action has also been obtained from other chromium-doped chalcogenide hosts such as ZnS [10], CdMnTe [29], and CdSe [30], to name a few. A second important member of the chalcogenide lasers is Fe2+:ZnSe. Pulsed lasing of this gain medium has also been demonstrated at sub-ambient temperatures in the 4–4.5 lm wavelength region [13]. Together, Cr2+- and Fe2+-doped chalcogenide lasers operate at wavelengths above the tuning

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U. Demirbas et al. / Optical Materials 28 (2006) 231–240

range of already existing tunable solid-state media such as Co2+:MgF2 [31] and Tm3+:YLF [32], and have important potential applications in vibrational spectroscopy, trace gas detection, and medicine [33,34]. Laser-active TM ions can be introduced into the chalcogenide hosts by using various methods. These include doping during crystal growth [6], diffusion doping [6,35], physical vapor transport technique [36], and pulsed laser deposition [37]. The simplest and most cost-effective technique is diffusion doping. Diffusion doping technique has been used to prepare laser-active samples with single-crystal as well as polycrystalline hosts [35,38–40]. In the particular case of Cr2+:ZnSe lasers, diffusiondoped polycrystalline samples have also been used since they give power performance comparable to that obtained from single crystals [17]. In previous studies, Burger and co-workers further investigated the role of growth conditions on the spectroscopic characteristics of chromium-doped ZnSe [35,40]. As an extension of previous work, we present the results of a systematic study which investigates the synthesis of Cr2+:ZnSe and Fe2+:ZnSe samples by thermal diffusion. In particular, we have measured how the diffusion parameters such as temperature and time influence the chromium and iron concentrations inside the host and determined the diffusion coefficient for both Cr2+:ZnSe and Fe2+:ZnSe. In the case of Cr2+:ZnSe, we have further characterized the spatial transverse uniformity of the diffusion-doped samples. The paper is organized as follows. Section 2 describes the synthesis and the characterization methods used in the experiments. In Section 3.1, we discuss a three-dimensional, analytical diffusion model that was used to relate the measured absorption coefficient to diffusion parameters. Section 3.2 describes the analysis of continuous-wave saturation data to determine the absorption cross-section ra for ZnSe. ra was then used for the estimation of doping concentrations from absorption data. The results of the study are presented in Section 4. In the synthesis experiments, polycrystalline samples were used. In the case of Cr2+:ZnSe, we describe two different methods for the analysis of diffusion doping. In the first case, the absorption at the center of cylindrical samples prepared at 1000
C and for different diffusion periods was measured as a function of diffusion time to determine the diffusion coefficient. In the second case, position-dependent

absorption data were measured by using a Cr4+:YAG laser and the variation of the diffusion coefficient was determined as a function of temperature between 800 and 1100
C. Cr+2:ZnSe samples with peak central absorption coefficient of up to 74 cm
1 at 1775 nm and corresponding Cr+2 ion concentration of 5 · 1019 ion/ cm3 could be prepared. In the case of Fe2+: ZnSe, the maximum absorption coefficient obtained at 2400 nm was 11.6 cm
1 (corresponding average iron concentration = 7.7 · 1019 ions/cm3). In ZnSe, the average diffusion coefficient of Cr2+ was determined to be 5.4 · 10
10 cm2/s at 1000
C by using time and position-dependent absorption data. In the case of Fe2+:ZnSe, diffusion was analyzed at 1000
C and the diffusion coefficient was determined to be 7.95 · 10
10 cm2/s.

2. Experimental All starting dopant and host materials were purchased commercially: ZnSe (polycrystalline tablets, 10 mm dia · 2.0 mm tablets from Crystran Ltd.), Cr (powder, 99.5%, Aldrich), CrSe (Aldrich), Fe (powder, 99.99%, Aldrich), FeSe (Alfa Aeser). The syntheses of the samples were carried out in a silica ampoule depicted in Fig. 1, in which the dopant (metals or metal selenides) and the host (ZnSe) are placed in different compartments separated by open pointing. The spatial separation ensures that the deposition of the dopant on the ZnSe will occur only via gas phase. By this means, the less volatile impurities such as metal oxides remained as solid and the contamination of the samples could be kept at a minimum level. In a typical experiment, 0.35 g dopant is placed in compartment I of the reaction ampoule containing the pre-encapsulated ZnSe tablet (compartment II). The ampoule is then evacuated and sealed under high-vacuum (P < 10
5 mbar). The samples were placed in a tube furnace and heated up within 1 h to the specific reaction temperature, annealed for 1–43 days and cooled down to room temperature during 5 h. Special care was taken to position the reaction ampoules in the center of the furnace to ensure temperature uniformity. With Cr2+:ZnSe, two types of measurements were performed at room temperature. First, the absorption coefficient at the center of each cylindrical sample was

Fig. 1. Reaction ampoule (silica) used for the synthesis of chromium and iron-doped ZnSe.

U. Demirbas et al. / Optical Materials 28 (2006) 231–240

measured by using a commercial spectrophotometer (Shimadzu UV–VIS–NIR 3101 PC) in the 300– 3200 nm wavelength range. In the second set of measurements, the radial variation of the absorption coefficient was analyzed with a continuous-wave home-built Cr4+:YAG laser operating at 1510 nm.

3. Theory 3.1. Model for thermal diffusion doping During the diffusion process, the concentration N ð~ r; tÞ of the transition metal ions inside the host satisfies the well-known diffusion equation oN ð~ r; tÞ ¼ Dr2 N ð~ r; tÞ; ot

ð1Þ

where t is the diffusion time, ~ r gives the position inside the host, and D is the diffusion coefficient. All of the samples used in our study were cylindrical in shape as shown in Fig. 2, where R is the radius and L is the thickness. By expressing the Laplacian operator in cylindrical coordinates (r, z) and by assuming azimuthal symmetry, Eq. (1) becomes

 1 o oN ðr; z; tÞ o2 N ðr; z; tÞ 1 oN ðr; z; tÞ r ¼ 0: þ
r or or oz2 D ot ð2Þ Eq. (2) can be solved exactly with the following set of initial/boundary conditions: N ðr; z; t ¼ 0Þ ¼ 0; N ðr ¼ R; z; t > 0Þ ¼ n0 ;
 L N r; z ¼  ; t > 0 ¼ n0 : 2

ð3Þ

Here, n0, assumed to be constant, is the TM ion concentration on all sides of the sample. In this particular case, the solution for N(r, z, t) can be written in terms of a double sum according to

z r=R z = L/2 y x z = -L/2

Fig. 2. Sketch of the cylindrical sample with thickness L and radius R (
L/2 < z < L/2, 0 6 r < R). The center of the sample coincides the origin.

233

N ðr; z; tÞ


 1 X 1 8n0 X ð
1Þn J 0 ðram Þ ð2n þ 1Þp z cos ¼ n0
L pR m¼1 n¼0 ð2n þ 1Þam J 1 ðRam Þ ! ! 2 ð2n þ 1Þ p2 2 exp
þ an Dt ; ð4Þ L

where n and m are summation indices, J0(x) is the Bessel function of order zero of the first kind, Ram is the mth positive root of J0(x) = 0 and J1(x) is the Bessel function of order one of the first kind [41]. The differential absorption coefficient of the samples can be expressed in terms of the doped TM ion concentration N(r, z, t) and the absorption cross-section ra(k) according to aðr; z; tÞ ¼ ra ðkÞN ðr; z; tÞ:

ð5Þ

The transmission T of the samples can then be expressed in terms of these parameters as ! Z L=2 T ¼ exp
aðr; z; tÞ dz : ð6Þ
L=2

In the experiments, we measured the absorbance (A) of the samples in the 300–3200 nm range. A is related to the transmission through A =
log(T). After determining the background losses Ab due to surface imperfections and Fresnel reflections, the average absorption coefficient aav was determined by using aav ¼

1 A
Ab : L ln 10

ð7Þ

The absorption band of the TM ion doped chalcogenides has a Gaussian profile and the wavelength dependence of absorption cross-section can be described by 2 !2 3 k
k 0 pffiffiffiffiffiffiffiffiffiffiffi 5: ð8Þ ra ðkÞ ¼ r0 exp 4
4 Dk= lnð2Þ In Eq. (8), r0 is the peak of the absorption cross-section curve, k is the wavelength, k0 and Dk are the central wavelength and width (FWHM) of the absorption band, respectively [35]. Experimental absorption measurements taken with Cr:ZnSe samples show that k0 and Dk do not vary as a function of concentration. Best fit between experimental data and Eq. (8), gave k0 = 1775 nm, and Dk = 365 nm for Cr+2:ZnSe. The model described above can be used to investigate the time and position dependence of the absorption coefficient in the diffusion-doped samples. As an example, Fig. 3 shows the calculated variation of aav as a function of the radial coordinate r, for three different values of the diffusion coefficient (5 · 10
11, 9 · 10
10, and 3 · 10
9 cm2/s). Here the crystal was assumed to have a cylindrical shape with L = 2.1 mm, and R = 5 mm, and n0r0 was chosen to be 10,000 cm
1. The diffusion time was 10 days. Fig. 3 clearly shows how the

234

U. Demirbas et al. / Optical Materials 28 (2006) 231–240

3.2. Determination of ion concentrations

100 3 E-9

90

9 E-10

αav (cm-1)

80

5 E-11

70 60

D = 3 x 10-9 cm2/s

50 40 30

D = 9 x 10-10 cm2/s

20 10

D = 5 x 10-11 cm2/s

0 -5

-4

-3

-2

-1

0

1

2

3

4

5

Radial Distance (mm)

Fig. 3. Calculated variation of the average absorption coefficient aav as a function of the radial coordinate r, for three different values of the diffusion coefficient (5 · 10
11, 9 · 10
10, and 3 · 10
9 cm2/s). The crystal was assumed to have a cylindrical shape with L = 2.1 mm, and R = 5 mm, and n0r0 was chosen to be 10,000 cm
1. The diffusion time was 10 days.

As can be seen from Eq. (5), the value of the absorption cross-section ra is needed in order to determine the concentration of Cr2+ ions from absorption measurements. We used absorption saturation data taken with a 1580-nm NaCl:OH
laser for the determination of ra. In particular, effects due to absorption saturation and excited-state absorption were taken into account to calculate the transmission of the crystal as a function of the incident pump power and ra was varied to find the best fit between experiment and theory. In the presence of pump saturation and excited-state absorption, the spatial evolution of the pump intensity is described by the following differential equation [42]: " # I 1 þ fp I sap 1 oI p ¼
aav : ð9Þ I I p oz 1 þ Ip sa

strength of thermal diffusion influences the transverse homogeneity of the samples. In particular, we see that an increase in D reduces the size of the central homogeneously doped section of the samples. Fig. 4 further shows the calculated variation of aav at the center of samples as a function of diffusion time, for the same values of diffusion coefficient and crystal dimensions. As the value of D increases the diffusion time required to reach the maximum attainable absorption coefficient decreases. The trends displayed in Figs. 3 and 4 suggest two alternative methods for the determination of diffusion coefficients form experimental absorption data. One method involves the measurement of the absorption coefficient at the center of the crystals subjected to different diffusion times at the same temperature. Or, alternatively, position-dependent absorption measurements can be performed. D=3x

100

αav (cm-1)

80

3 E-9 9 E-10

60

5 E-11

40

D = 5 x 10-11 cm2/s

20

0 0

50

100

150

200

250

I p ðr; zÞ ¼ P p ðzÞUp ðr; zÞ;

ð10Þ

where Pp(z) is the pump power and the normalized intensity distribution Up(r, z) can be calculated in terms of the spot-size function xp as ! 2 2r2 Up ¼ exp
2 : ð11Þ px2p xp The differential equation satisfied by the pump power is obtained by integrating over the beam crosssection: " # Z 1 P U 1 þ fp Ipsa p dP p ¼
aav P p dr2prUp : ð12Þ P U dz 1 þ Ipsa p 0

10-9 cm2/s

D = 9 x 10-10 cm2/s

Here, Ip is the pump intensity at 1580 nm, aav is the average differential absorption coefficient at 1580 nm, Isa (Isa = hckp/rasf, h = PlanckÕs constant, c = speed of light, kp = pump wavelength, and sf = fluorescence lifetime) is the saturation intensity, and fp (fp = resa/ra, resa = excited-state absorption cross-section) is the normalized strength of excited-state absorption. In the calculations, it was assumed that the laser intensity has a Gaussian intensity profile given by

300

Diffusion Time (Days)

Fig. 4. Calculated variation of the average absorption coefficient aav at the center of samples as a function of the diffusion time, for three different values of the diffusion coefficient (5 · 10
11, 9 · 10
10, and 3 · 10
9 cm2/s). The crystal was assumed to have a cylindrical shape with L = 2.1 mm, and R = 5 mm, and n0r0 was chosen to be 10,000 cm
1.

In the measurements, we used a 2.5-mm-thick sample with aav = 8 cm
1 at 1580 nm. The fluorescence lifetime was taken as 5 ls [43]. The pump beam was focused to a 40-lm spot at the center of the crystal. The calculated and measured variation of the crystal transmission is shown in Fig. 5. Best-fit between experiment and theory was obtained for fp = 0.7 and ra = 6.94 · 10
19 cm2. The ra-value obtained in this analysis agrees very well with the previously reported value of 6.62 · 10
19 cm2 [6] obtained by using the peak absorption cross-section at 1775 nm and Eq. (8). In the case of Fe2+:ZnSe we used a ra-value of 1.5 · 10
19 cm2 at 2400 nm reported by Adams et al. [13].

U. Demirbas et al. / Optical Materials 28 (2006) 231–240 0.16

Transmission

0.155

0.15

0.145

0.14

0.135

0.13 0

0.3

0.6

0.9

1.2

1.5

1.8

Pump Power (W)

Fig. 5. Measured and calculated variation of the transmission of the Cr2+:ZnSe sample as a function of the incident power at 1580 nm. The best-fit value of the absorption cross-section was determined to be 6.94 · 10
19 cm2.

4. Results and discussion 4.1. Diffusion-doped Cr 2+–ZnSe In the experiments a total of 13 samples were prepared and characterized. Table 1 summarizes the preparation conditions used during the experiments. Also shown are the estimated peak absorption coefficients and Cr2+ ion concentrations measured at the center of the samples. Fig. 6 shows typical absorption spectra taken with undoped and chromium-doped ZnSe samples in the 300–3200 nm spectral range. The undoped material is transparent for wavelengths above 470 nm (dashed line in Fig. 6), corresponding to the well-known band-gap energy of 2.64 eV for ZnSe. Diffusion doping of Cr modifies the absorption spectra in two ways. First, the absorption band of the laser active Cr2+ ion forms in

235

the near-infrared region centered at 1775 nm (solid line) due to the optical transitions between the 5T2 and 5E levels of the Cr2+ ions in a tetrahedral crystal environment. In the particular case in Fig. 6, the sample was exposed to thermal diffusion at 1000
C for 4 days (sample 3 in Table 1). By using the absorption cross-section value determined in Section 3.2, the Cr2+ ion concentration at the center of the sample was estimated to be 5.1 · 1018 atoms/cm3. Second, the short wavelength absorption edge shifts to higher wavelengths due to optical transitions between different charge states of the chromium ion [8,44]. The amount of the spectral shift is related to the dopant concentration. Fig. 7 shows the measured variation of the shift in the absorption edge as a function of average chromium concentration. These measurements were performed by using samples prepared over different diffusion times. As can be seen from Fig. 7, a maximum shift of 160 nm was observed for an active Cr2+ ion concentration of 51.6 · 1018 ions/cm3. We used the model discussed in Section 3.1, to determine the diffusion coefficient for Cr2+ ions in the ZnSe host. In this study the average absorption coefficient at the center of the cylindrical samples was measured. Ten samples prepared at 1000
C for different diffusion times (samples 1, 2, 3, 6, 8, 9, 10, 11, 12 and 13 in Table 1) were used. Fig. 8(a) shows the measured variation of the average absorption coefficient at 1500 nm as a function of the diffusion time. At 1000
C, samples with absorption coefficient of as high as 13.7 cm
1 at 1500 nm could be produced, corresponding to an average absorption coefficient of 74.3 cm
1 at 1775 nm (see Eq. (8)). Least-squares fitting between experiment and diffusion model gave the best-fit values of 1.43 · 1022 atom/cm3 and 5.39 · 10
10 cm2/s for n0 and D, respectively. The diffusion coefficient determined in our study is lower than the previously reported values [35], possibly due the variation in the quality of the host

Table 1 Diffusion conditions and optical properties of Cr:ZnSe samples ZnSe sample no.

Total diffusion time (days)

Diffusion temperature (
C)

Short wavelength abs. edge (nm)

Absorption coefficient at 1775 nm (cm
1)

Average Cr+2 concentration (·1018 ion/cm3)

Pure 1 2 3 4 5 6 7 8 9 10 11 12 13

0 1 2 4 5 5 5 5 6 15 20 20 30 43

– 1000 1000 1000 800 900 1000 1100 1000 1000 1000 1000 1000 1000

472 534 534 564 480 566 534 590 596 588 604 584 598 630

0 2.14 3.34 7.34 0.38 7.12 3.23 23.82 27.05 16.75 36.24 12.31 27.92 74.30

0 1.48 2.32 5.10 0.27 4.94 2.24 16.5 18.8 11.6 25.2 8.56 19.4 51.6

236

U. Demirbas et al. / Optical Materials 28 (2006) 231–240 Cr:ZnSe

25

20

Pure ZnSe

αav (cm-1)

4 day dif.

αav (cm-1)

8 6 4 2 0 1300 1500 1700 1900 2100 2300

15

Wavelength (nm)

10

5

0 300

600

900

1200

1500

1800

2100

2400

2700

3000

Wavelength (nm)

Fig. 6. Room temperature absorption spectra of pure ZnSe and Cr2+:ZnSe (sample 3 in Table 1) in the 300–3200 nm spectral range. Inset: Background subtracted absorption spectra of Cr2+:ZnSe.

18

Absorption Coefficient at 1500 nm (cm-1)

Short Wavelength Absoprtion Edge (nm)

Cr:ZnSe 650

600

550

500

16 14 12 10 8 6 4 2 0 0

450 0

10

20

30

40

50

60

5

10

(a)

15

20

25

30

35

40

45

Diffusion Time (days)

Avarage Cr Concentration ( x 1018 cm-3)

samples. Best-fit values of n0 and D can be used to predict the maximum possible absorption coefficient that can be obtained by diffusion doping of chromium. As can be seen from Fig. 8(b), our calculations predict that at a diffusion temperature of 1000
C, samples with absorption coefficient of up to 115 cm
1 (1775 nm) can be obtained, corresponding to an average chromium concentration of 8 · 1019 ions/cm3. As discussed in the experimental part, we used a second method by using a Cr:YAG laser to determine the spatial homogeneity and the position dependence of the absorption coefficient. In these experiments the output beam of the Cr:YAG laser operating at 1510 nm was focused to a 20-lm spot and scanned across the crystal

Absorption Coefficient at 1775 nm (cm-1)

140

Fig. 7. Measured variation of the short-wavelength absorption edge for Cr2+:ZnSe as a function of the average Cr2+ concentration.

120 100 80 60 40 20 0 0

(b)

100

200

300

400

Diffusion Time (days)

Fig. 8. (a) Measured and calculated variation of the absorption coefficient as a function of time at 1500 nm for Cr2+:ZnSe samples prepared at 1000
C. (b) Predicted variation of the average absorption coefficient as a function of diffusion time based on the best-fit values (1775 nm).

U. Demirbas et al. / Optical Materials 28 (2006) 231–240

237 800 C

1000 C

25 20 15

D = 5.45 x 10

10

-10

2

cm /s

5 0

-5

-4.5

-4

-3.5

(a)

-3

-2.5

-2

-1.5

-1

-0.5

15

10

5

D = 2.4 x 10

10

D = 2.45 x 10

-10

2

cm /s

0 -4.5

-4

-3.5

-3

-2.5

2

cm /s

-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

-1

-0.5

0

Distance From the Center (mm) 1100 C

15

5

-10

0

900 C

20

-5

20

(b)

25

(c)

25

0

Distance From the Center (mm)

30

Absorption Coefficient at 1510 nm (cm-1 )

Absorption Coefficient at 1510 nm (cm-1)

30

Absorption Coefficient at 1510 nm (cm-1 )

Absorption Coefficient at 1510 nm (cm-1 )

35

-2

-1.5

-1

-0.5

Distance From the Center (mm)

0

30

25

20

15

10

D= 8.8 x 10

-10

2

cm /s

5

0

(d)

-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

Distance From the Center (mm)

Fig. 9. Measured and calculated radial variation of the absorption coefficient at 1510 nm for Cr2+:ZnSe samples prepared at (a) 1000
C, (b) 800
C, (c) 900
C, (d) 1100
C.

10.00

Diffusion Coefficient (x 10 -10 cm 2/s)

cross-section. Fig. 9(a) shows the measured and calculated variation of the average absorption coefficient as a function of the radial distance for the Cr:ZnSe sample subjected to diffusion at 1000
C for 10 days. Best-fit between experiment and theory was obtained for a D value of 5.45 · 10
10 cm2/s, in very good agreement with the time dependent result of 5.39 · 10
10 cm2/s. The advantage of this technique is that a single sample is enough for the determination of the diffusion coefficient. We also used samples prepared at 800, 900 and 1100
C to investigate the temperature dependence of the diffusion coefficient. Measured and calculated results are displaced in Fig. 9a–d. The transverse uniformity of the samples is related to both the diffusion coefficient and the diffusion time. In the particular case of the sample prepared at 1000
C for 20 days (Fig. 9a), the spatially uniform section of the sample extends over 65% of the sample cross-section. Finally Fig. 10 shows the variation of the best-fit value of the diffusion coefficient as a function of diffusion temperature between 800 and 1100
C. As expected the diffusion rate increases with increasing diffusion temperature.

8.00

6.00

4.00

2.00

0.00 700

800

900

1000

1100

1200

Temperature (˚C)

Fig. 10. Temperature dependence of diffusion coefficient for Cr2+ ions in ZnSe.

4.2. Diffusion-doped Fe2+–ZnSe Table 2 lists the growth conditions along with the estimated absorption coefficients and ion concentrations for the Fe2+:ZnSe samples prepared in our experiments.

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U. Demirbas et al. / Optical Materials 28 (2006) 231–240

Table 2 Diffusion conditions and properties of Fe:ZnSe samples ZnSe sample no.

Total diffusion time (days)

Diffusion temperature (
C)

Short wavelength abs. edge (nm)

Absorption coefficient at 2400 nm (cm
1)

Average Fe+2 concentration (·1018 ion/cm3)

Pure 1 2 3 4 5 6

0 2 4 6 13 20 30

– 1000 1000 1000 1000 1000 1000

472 476 476 478 522 546 550

0 0.116 1.31 2.46 4.76 8.94 11.6

0 0.8 8.7 16.4 31.7 59.6 77.3

Fe:ZnSe 25 Pure ZnSe

-1

α (cm )

20

4 day dif.

Fe:ZnSe

Short Wavelength Absoprtion Edge (nm)

Fig. 11 shows the absorption spectra for pure and irondoped ZnSe samples. The iron doped sample in Fig. 11 was exposed to thermal diffusion at 1000
C for 4 days. Similar to chromium doped ZnSe, doping with iron causes two observed modifications in the absorption spectra of the samples. Four holes in Fe+2(3d6) behaves almost in the same way as the four electrons in Cr+2(3d4), thus the tetrahedral crystal field of ZnSe splits the ground state term into the same energy-level structure consisting of the 5T2 and 5E levels. Optical transitions of the Fe+2 ions between these two energy levels gives rise to the formation of the absorption band around 3 lm (solid line in Fig. 11). From the absorption measurements taken at the center of the sample, the Fe2+ ion concentration was estimated to be 8.7 · 1018 ions/cm3. The second effect is the shift of the short wavelength absorption edge to higher wavelengths. Fig. 12 shows the measured variation of this shift as a function of average iron concentration. As can be seen from Fig. 12, a maximum shift of 78 nm was observed for an active Fe+2 ion concentration of 77.3 · 1018 ions/cm3. Diffusion coefficient for Fe2+:ZnSe was determined at 1000
C by using six samples prepared over different diffusion times (see Table 2). The results showing the mea-

570

540

510

480

450

0

2 4 6 8 Avarage Fe Concentration ( x 1019 cm -3)

10

Fig. 12. Variation of the short wavelength absorption edge with average Fe2+ ion concentration in Fe2+:ZnSe.

sured and the calculated variation of the absorption coefficient are depicted in Fig. 13(a). By using leastsquares fitting procedure, the diffusion coefficient was determined to be 7.95 · 10
10 cm2/s. The highest experimentally achieved absorption coefficient at 2400 nm was 11.6 cm
1, obtained by exposing the sample to thermal diffusion for 30 days. With the best fit-parameters, our model suggests that at the diffusion temperature of 1000
C, it should be possible to obtain samples with absorption coefficients of up to 22 cm
1 at 2400 nm corresponding to an average iron concentration of 14.7 · 1019 ions/cm3 (see Fig. 13(b)).

15

5. Conclusions 10

5

0 300

600

900

1200

1500

1800

2100

2400

2700

3000

Wavelength (nm)

Fig. 11. Room temperature absorption spectra of pure ZnSe and Fe2+:ZnSe sample 2 in the 300–3200 nm spectral range.

In conclusion, we presented a detailed account of the preparation and characterization of diffusion-doped Cr2+:ZnSe and Fe2+:ZnSe. In the experiments, Cr2+:ZnSe samples with peak absorption coefficients of as high as 74 cm
1 (1775 nm) and fairly good spatial uniformity could be obtained. In the case of Fe2+:ZnSe, the maximum absorption coefficient was 11.6 cm
1 at 2400 nm. A three-dimensional analytical diffusion model was further developed to determine the diffusion coeffi-

Absorption Coefficient at 2400 nm (cm-1)

U. Demirbas et al. / Optical Materials 28 (2006) 231–240

and Coherent Technologies, Inc. for providing the equipment used in the absorption saturation measurements. This project was supported in part by Tubitak (The Scientific and Technical Research Council of Turkey) under the grant TBAG-2030.

14 12 10 8 6

References 4 2 0 0

5

10

(a) Absorption Coefficient at 2400 nm (cm-1)

239

15

20

25

30

Diffusion Time (days)

25

20

15

10

(b)

5

0 0

100

200

300

400

Diffusion Time (days)

Fig. 13. (a) Measured and calculated variation of absorption coefficient with diffusion time at 2400 nm for Fe2+:ZnSe samples prepared at 1000
C. (b) Predicted variation of the average absorption coefficient as a function of diffusion time based on the best-fit values (2400 nm).

cient D from absorption data. At 1000
C, D was determined to be 5.4 · 10
10 cm2/s and 7.95 · 10
10 cm2/s for Cr2+:ZnSe and Fe2+:ZnSe, respectively. Absorption saturation data were analyzed to determine the absorption cross-section that was used in the estimation of the doping concentrations. In the case of Cr2+:ZnSe, an alternative technique was also employed to determine the diffusion coefficient from the position-dependent absorption data obtained with a Cr4+:YAG laser. With this method, the temperature variation of D was also measured in the 800–1100
C range. The results obtained in this study should provide useful guidelines for the synthesis and characterization of transition metal ion-doped chalcogenides for lasing and passive q switching applications.

Acknowledgments We are grateful to Muharrem Gu¨ler, Adnan Kurt, and Nuray Dindar for technical help during the experiments. We would also like to thank Clifford R. Pollock

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