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Optical properties of PMMA doped with erbium(III) and ytterbium(III) complexes.

INTRODUCTION

In recent years, great attention has been focused on erbium-doped optical materials because of erbium's unique emission performance around 1530 nm, which corresponds to that of the silica fibers used in telecommunications systems (1). This property is utilized in the development of both erbium-doped optical fiber amplifiers (EDFA) (2) and erbium-doped planar waveguide amplifiers (EDPWA) (3). Erbium-doped amplifiers can be directly pumped into the first excited manifold using an optical source operating at either a wavelength of 1480 nm or at one of the higher absorption bands (e.g., 980 nm). At 980 nm. an erbium laser system with emission around 1530 nm corresponds to [.sup.4][I.sub.[13/2]] [right arrow] [.sup.4][I.sub.[15/2]] transition in the three-level system. Erbium-ytterbium co-doping of EDFAs has been proposed as a method for the production of high-power amplifiers in very short waveguides (4). The application of optical materials to the development of EDFAs and EDPWAs has been studied intensively, e.g., silica (5), phosphate glass (6), zirconia (7), sodium-calcium-silicate glass (8), sapphire (9), yttrium oxide (10), and others (11).

Nowadays, new materials, such as polymers, are being investigated as alternatives to the aforementioned glassy and crystalline materials because of their suitable optical properties (e.g., low optical absorption), feasible fabrication processes, and low costs (12), (13). Among them, polymethylmethacrylate (PMMA) is the most convenient one to handle. Therefore, we have focused on the study of suitability of this material especially in the range of development and construction of optical active devices such as optical amplifiers or optical sources. In this article, we are going to report on the doping of the PMMA with [Er.sup.3+] and mixture of [Er.sup.3+] + [Yb.sup.3+] ions.

EXPERIMENTAL

Small pieces of PMMA (Goodfellow) were left to dissolve in chloroform for a few days before being used in the fabrication of PMMA layers. The layers were formed by the solution either being spin-coated onto silicon and glass substrates or by being poured into bottomless molds placed on a glass substrate and left to dry. For [Er.sup.3+] doping, solutions whose content ranged from 1.0 to 20.0 at% erbium were added to the PMMA. For [Er.sup.3+]/[Yb.sup.3+] co-doping, the erbium(III) tris(2,2,6,6-tetramethyl-3,5-heptanedionate) (Sigma-Aldrich) and the ytterbium(III) tris(2,2,6,6-tetramethyl-3,5-heptanedionate) (Goodfellow) were together dissolved in chloroform. Samples containing 1.0 at% erbium were co-doped with ytterbium in amounts also ranging from 1.0 to 20.0 at%. The molecular structure of erbium(III) tris(2,2,6,6-tetramethyl-3,5-heptanedionate) and ytterbium(III) tris(2,2,6,6-tetramethyl-3,5-heptanedionate) is shown in Fig. 1.

[FIGURE 1 OMITTED]

RESULTS AND DISCUSSION

Infrared Spectra of the RE-Doped PMMA

The fabricated samples were investigated by infrared spectroscopy (FTIR). Infrared reflectance and ATR spectra were obtained using a Bruker IFS 66/v FTIR spectrometer equipped with a broadband MCT detector, to which 128 interferograms were added with a resolution of 4 [cm.sup.-1] (Happ-Genzel apodization). Figure 2 displays the FTIR spectra of pure PMMA obtained at room temperature in the wavelength range from 400 to 2400 [cm.sup.-1]. These spectra are in good agreement with the published data (14). The strong broad bands occurring around 2998 and 2950 [cm.sup.-1] were caused by the stretching vibrations of [CH.sub.3] and [CH.sub.2], and indicate a high content of hydrogen-rich [CH.sub.x] in the PMMA. The weak absorption band at about 3439 [cm.sup.-1] corresponds to the O--H vibrations of the PMMA layers. Unfortunately, the overton of that vibration occurs at ~1450 nm, i.e., the wavelength where one of the absorption bands of Er(III) also occurs is responsible for its 1530 nm photoluminesce. In consequence, the presence of O--H groups in the matrix containing erbium ions thus causes problems by hindering emission in the region around 1530 nm, and that this has implications for the amplifying effect (see e.g., Ref. 15). The low intensity of the O--H vibration band in our samples suggests that the concentration of O--H groups is sufficiently low to predict the strong luminescence performance of our materials. However, the presence of the O--H groups in this type of the material will be always a problem as the chemicals used for the rare earth (RE) doping are rather hygroscopic. This disadvantage can be overcome by performing all these treatments in protective ambient, e.g., argon, atmosphere.

[FIGURE 2 OMITTED]

Absorptance Spectra of the Er(III)- and Yb(III)-Doped Samples

Absorption measurements were performed using a UV-VIS-NIR Spectrometer (UV-3600 Shimadzu) in the spectral range from 350 to 1050 nm. The absorptance spectra of the erbium-doped samples are shown in Fig. 3a. and those of the ytterbium-doped samples in Fig. 3b. Figure 3a shows that the erbium content had a significant effect on the occurrence of the bands attributed to the [Er.sup.3+] ions. Two strong bands appeared at 377 nm ([.sup.4][G.sub.[11/2]]) and 519 nm ([.sup.2][H.sub.[11/2]]) for the sample containing 20.0 at% [Er.sup.3+], while the typical absorption band [.sup.2][F.sub.[5/2]] occurred at 976 nm for the sample containing 20.0 at% [Yb.sup.3+] (see Fig. 3b).

[FIGURE 3 OMITTED]

Table 1 presents the spectral data shown in Fig. 3, and the comparison of our results with the absorption bands attributed to pertinent transitions in erbium-doped fluoride glass in Ref. 15.
TABLE 1. Absorption levels of trivalent erbium in fluoride glass |15|
and PMMA.

                                             Wavelength (nm)

            Transition                 Fluoride glasses  PMMA

[.sup.4] [I.sub.[15/2]] [right arrow]         650        650 vw
[.sup.4] [F.sub.[9/2]]

[.sub.4] [S.sub.[3/2]]                        539        Not observed

[.sup.2] [H.sub.[11/2]]                       519        519 s

[.sub.4] [F.sub.[7/2]]                        485        Not observed

[.sup.4] [F.sub.[5/2]]                        448        448 vw

[.sup.4] [F.sub.[3/2]]                        441        Not observed

[.sup.2] [H.sub.[0/2]]                        405        405 w

[.sup.4] [G.sub.[11/2]]                       377        377 vs

[.sup.2] [G.sub.[9/2]]                        363        363 vw

[.sup.2] [G.sub.[7/2]]                        355        Not observed


Refractive Indices of the Er(III)-Doped Samples

Refractive indices were measured by using variable angle spectroscopic ellipsometry (J.A. Woollam & Co.) working in rotating analyzer mode. The measurements were carried out in the spectral range from 300 to 1700 nm. Figure 4 shows a dependence of the refractive indices of the [Er.sup.3+]-doped PMMA containing erbium in the concentrations from 1.0 to 20.0 at%. It is obvious that the increasing content of the [Er.sup.3+] increases the refractive indices of the material.

[FIGURE 4 OMITTED]

Photoluminescence

Semiconductor laser excitation (P4300 operating at [[lambda].sub.ex] = 980 nm with [E.sub.ex] = 500 mW; room temperature) was used to detect the sample luminescence in the range from 1450 to 1650 nm. Figure 5 presents spectra of the samples with four different concentrations of erbium (1.0, 5.0, 10.0. and 20.0 at%). Photoluminescence bands at 1530 nm, caused by the erbium transition [.sup.4][I.sub.[13/2]] [right arrow] [.sup.4][I.sub.15/2], apparently increased the intensity, from a negligible one hidden in the background at the lowest concentration of the RE to the highest one. The increasing concentration of the RE also has positive influence on the full width at half maximum (FWHM), which can be considered as one of the figures of merit for the utility of the optical active materials. Although the FWHM is rather small (below 15 nm) in the samples containing less than 10.0 at% of erbium, the FWHM reaches up to 35 nm in the samples containing 20.0 at% of the RE. which is a value expected at good quality optical active component.

[FIGURE 5 OMITTED]

There are few possibilities how to increase the intensity of the 1530 nm emission and one of them is illustrated in Fig. 6. It shows the well-known approach of co-doping the erbium-containing optical materials with ytterbium ions. The mechanism of the amplification of the 1530-nm erbium emission is described in Ref. 16. To test that effect, we used the samples containing the lowest concentration of the [Er.sup.3+] ions (1.0 at%) and co-doped them with various amounts of [Yb.sup.3+] ions (from 1.0 to 20.0 at%). Figure 6 shows that the low content of the co-dopant did not affect the 1530 nm luminescence, it remained hidden in the background. The higher concentrations, in this case 10.0 at%, increased the intensity of the luminescence to such an extent that it became clearly visible in the spectra. Figure 6 also shows that very high concentrations of the co-dopant are not needed as the 20.0 at% of the [Yb.sup.3+] did not result in any further enhancement of the 1530-nm luminescence.

[FIGURE 6 OMITTED]

CONCLUSIONS

We studied the fabrication of RE (RE = [Er.sup.3+] and [Er.sup.3+] + [Yb.sup.3+])-doped PMMA and evaluated their properties from the potential applicability point of view of in active optical devices. The samples showed several important features as listed below.

Refractive indices of the RE-doped samples increased with the increasing concentrations of the RE. All the RE-doped samples revealed only weak O--H absorption bands (so-called killers of the 1530 nm emission), although their intensity increased with increasing concentrations of the RE. It is clearly associated with the hygroscopic RE halogenides used for the doping. However, this problem can be overcome with the doping done in a protective ambient, e.g., under argon, atmosphere. The [Er.sup.3+] absorption bands, [.sup.4][G.sub.[11/2]] at 377 nm and [.sup.2][H.sub.[11/2]] at 519 nm. were quite strong and the typical [Yb.sub.3+] absorption band [.sup.2][F.sub.[5/2]] also appeared (at 976 nm). Although the 1530-nm luminescence bands that correspond to the erbium transition [.sup.4][I.sub.[13/2]] [right arrow] [.sup.4][I.sub.[15/2]] was very weak (almost hidden in the background) in the samples containing low concentrations of the RE. it improved substantially in the samples containing more than 10.0 at% of erbium. Such samples also revealed positive trend in broadening the FWHM reaching up to 35 nm. which is a good value for utilization in photonics. The addition of ytterbium had positive effect on the 1530-nm luminescence.

The erbium-doped samples showed an evident trend in improving their luminescence properties important for their active optical function (as widening of the FWHM and increasing the 1530 nm emission) with increasing concentrations of erbium in the samples. Actually, up to 20.0 at% of erbium can be added to the samples without the deterioration of their luminescence properties. This is a clear indication that no (or only negligible) clustering of the [Er.sup.3+] occurred when compared with silicate glasses, which are commonly accepted as suitable photonics materials but suffer from a strong tendency of the erbium ions to cluster. In the case of our PMMA samples it may mean that rather high amounts of the lasing RE ions can be incorporated into the matrix without dealing with the danger of deterioration of their function.

In our opinion, the RE-doped PMMA is a promising material for the utilization in many photonics devices as it is of low cost, easy to fabricate, and easy to handle with a great potential to improve its luminescence properties according to the particular application.

REFERENCES

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(12.) H. Ma, A.K.Y. Jen, and L.R. Dalton, Adv. Mater., 14, 19 (2002).

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(14.) F.Z. Tighilt, N. Gabouze. S. Sam, S. Belhousse, and K. Beldjilali, Surf. Sci., 601, 18 (2007).

(15.) J.F.M. Digonnet, Rare-Earth-Doped Fiber Lasers and Amplifiers, Marcel Dekker, Stanford, California (1993).

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Vaclav Prajzler, (1) Ivan Huttel, (2) Oleksiy Lyutakov, (2) Jiri Oswald, (3) Vladimir Machovic, (2) Vitezslav Jerabek (1)

(1) Department of Microelectronics, Faculty of Electrical Engineering, Czech Technical University, 16627 Prague 6, Czech Republic

(2) Institute of Chemical Technology, 16628 Prague 6, Czech Republic

(3) Institute of Physics, Academy of Sciences of the Czech Republic, 16253 Prague 6, Czech Republic

Correspondence to: Vaclav Prajzler; e-mail: xprajz1v@feld.cvut.cz

Contract grant sponsor: GA CR Grant (Czech Technical University. Prague); contract grant number: 102/09/P104: contract grant sponsor:

MSM: contract grant number: 6840770014.

DOI 10. 1002/pen.21418
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Title Annotation:polymethylmethacrylate
Author:Prajzler, Vaclav; Huttel, Ivan; Lyutakov, Oleksiy; Oswald, Jiri; Machovic, Vladimir; Jerabek, Vitezs
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1USA
Date:Sep 1, 2009
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