Er3+-doped tellurite waveguides deposited by excimer laser ablation

Materials Science and Engineering B105 (2003) 65–69

Er3+-doped tellurite waveguides deposited by excimer laser ablation A.P. Caricato a , M. Fernández a , M. Ferrari b , G. Leggieri a,∗ , M. Martino a , M. Mattarelli c , M. Montagna c , V. Resta a , L. Zampedri b,d , R.M. Almeida e , M.C. Conçalves e , L. Fortes e , L.F. Santos e a Dipartimento di Fisica, INFM and Università di Lecce, 73100 Lecce, Italy CNR-IFN, Istituto di Fotonica e Nanotecnologie CSMFO Group, 38050 Povo-Trento, Italy c Dipartimento di Fisica CSMFO Group, INFM and Università di Trento, 38050 Povo-Trento, Italy d Dipartimento Ingegneria dei Materiali, Università di Trento, via Mesiano 44, 38050 Povo-Trento, Italy Departamento de Engenharia de Materiais, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b

e

Abstract This paper reports on the optical properties of Erbium-doped zinc–tellurite (TeO2 –ZnCl2 –ZnO) oxyhalide glass waveguides, deposited by reactive pulsed laser deposition (RPLD) on silica substrates. Er3+ -doped zinc–tellurite glass (ZT) targets were ablated in oxygen dynamical flow at two different pressure values of 5 and 10 Pa by ArF excimer laser at the fluence of 3.7 J/cm2 . The waveguiding properties of the deposited films were investigated by the m-line technique. The TE0 mode excitation was used for photoluminescence (PL) and Raman measurements, in order to study the Erbium ion 4 I13/2 → 4 I15/2 transition and structural properties of the deposited films, respectively. Optical band gap and wavelength dependence of the real and imaginary parts of the refractive index were estimated from transmission spectra. © 2003 Elsevier B.V. All rights reserved. Keywords: Laser ablation; Erbium-doped tellurite thin film; Optical properties; Waveguides

1. Introduction Erbium doped silica based fibres are commonly used in optical networks to achieve optical amplification in C-band, exploiting the Er3+ 4 I13/2 → 4 I15/2 transition at 1.5 ␮m [1]. As the optical properties (emission bandwidth, lifetimes, quantum efficiency) of Er3+ ions in glass depend on the matrix where they are embedded, different materials are studied in order to satisfy the demand for larger bandwidth required for the development of wavelength division multiplexing (WDM) systems [2]. In this respect, tellurite glasses (TG) have definite advantages over other non-silica glasses. In fact, they combine a wide transmission region (0.35–6 ␮m), low phonon energy, good glass stability and high corrosion resistance [3]. TG show also a high rare-earth (RE) ion solubility [4], a feature required in order to develop Er3+ -doped amplifiers for integrated optics. Moreover, (RE) ions in TG show an enhancement ∗ Corresponding author. Tel.: +39-0832-320497; fax: +39-0832-320505. E-mail address: [email protected] (G. Leggieri).

0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.08.017

of the radiative transition rates due to their high refractive index and a broadening of the emission bands due to the variety of sites available for the optical ions. This opens the possibility of using TG for broadband Er3+ -doped fibre amplifiers [4–7]. In particular, the most studied configuration is the planar one, where a film is deposited on a suitable substrate, which is then patterned. This could allow the use of low cost devices in the promising field connected to the development of metropolitan area networks [8]. Many techniques were proposed to deposit thin film waveguides, based both on chemical and physical methods [9–14]. Pulsed laser deposition (PLD) is emerging as a reliable method for thin film deposition of different kinds of materials [15]. The deposition can be performed in vacuum (PLD), or in a low-pressure atmosphere, to promote a chemical reaction between the ablated material and the ambient atmosphere (reactive pulsed laser deposition (RPLD)). PLD and RPLD are very suitable techniques to fabricate optoelectronic and photonic devices, because they permit the stoichiometric transfer of complex bulk materials to a thin film in a single step, and the realisation of structures with a step refractive index profile.

A.P. Caricato et al. / Materials Science and Engineering B105 (2003) 65–69

The purpose of the present study is to investigate the optical properties of Er3+ -doped ZT (TeO2 –ZnCl2 –ZnO) thin films deposited on silica substrates, at room temperature, by the RPLD technique. The structural characteristics of the films are discussed. Optical properties, including TE and TM modes and photoluminescence of the deposited thin films, are also given.

2. Experimental RPLD was carried out in a stainless steel chamber, which was evacuated down to 10−5 Pa. ZT films were deposited in a dynamical flow of oxygen at pressure values of 5 (sample T1) and 10 Pa (sample T2). The target was ZT glass of nominal molar composition 60TeO2 –20ZnO–20ZnCl2 :1ErCl3 . The doped glass was prepared from dry mixtures of 99.99% purity TeO2 , ZnO and ZnCl2 and 99.9% purity ErCl3 . Batches of about 10 g were weighed into a vitreous silica crucible. Melting was performed in a muffle furnace inside a nitrogen-filled glove box, for 1 h at ∼800 ◦ C, after a pre-heating of 2 h at ∼200 ◦ C, to improve dehydration. Regular stirring of the crucible prevented bubble formation and assured a homogeneous melt. The melt was poured onto a stainless steel mould, previously heated to 240 ◦ C. Finally, the glass samples (∼3 mm thick) were annealed for about 12 h at 240 ◦ C [16]. The target was ablated using an ArF excimer laser, operating at the wavelength of 193 nm. The laser fluence and pulse duration were 3.7 J/cm2 and 22 ns, respectively. Series of 20,000 pulses at a repetition rate of 10 Hz were directed on the target at 45◦ incidence angle. In order to obtain as uniform as possible irradiation conditions, the target was spinned and vertically spanned during the process. The films were deposited on beforehand cleaned SiO2 (2 × 2 cm2 ) substrates at room temperature. The target–substrate distance was 40 mm. After the deposition, the samples were characterized with several diagnostic techniques. The transmission spectra were recorded by using an UV-Vis Perkin–Elmer Lambda 900 spectrophotometer. They were used to determine the real and imaginary parts of the complex refractive index vs. wavelength, as well as the film thickness, by using a home-made computer code. From the absorption edge, the optical band gap was determined. The thickness of the waveguides and the refractive index at 632.8 and 543.5 nm were also measured, in TE and TM polarizations, by an m-line apparatus based on the prism coupling technique [9]. The losses at 632.8 nm were evaluated by photometric detection of the light intensity scattered out of the waveguide plane, by exciting the TE0 mode [17]. The TE0 mode excitation was used both for Raman and photoluminescence (PL) measurements, detecting the scattered light from the front of the waveguide. The Raman spectra were collected in VV polarization, upon excitation with an Ar+ ion laser, operating at 457.9 nm. The signal was selected by a double monochromator and analysed by a photon-counting system. PL spectroscopy, in

the region of the 4 I13/2 → 4 I15/2 transition of Er3+ ions, was performed using the 514.5 nm line of an Ar + ion laser as excitation source. The luminescence was dispersed by a 320 mm single-grating monochromator with a resolution of 2 nm. The light was detected using an InGaAs photodiode and the standard lock-in technique [9].

3. Experimental results and discussion In Fig. 1, we report the Raman spectra of the bulk target and the deposited films. No crystalline peaks have been detected by Raman analysis in both deposited films. The band at 660 cm−1 is assigned to the stretching vibrations of the TeO4 trigonal bypiramidal groups. They are linked through Te–O–Te, with O in a position alternatively axial and equatorial, and form the backbone of pure TeO2 . The presence of a modifier ion such as Zn leads to the creation of TeO3 and TeO3+1 polyhedra that are responsible for the band at 770 cm−1 [18]. The band at 420 cm−1 is assigned to bending vibrations of the Te–O–Te bonds at corner sharing sites, whose intensity can be considered a measure of the connectivity of the network [4]. In the low frequency range, we distinguish the boson peak at 45 cm−1 and two shoulders, at 120 and 280 cm−1 . Following Mazzuca et al. [19], we ascribe the first shoulder to vibrations due to TeO4 –ZnO6 –TeO3 chain structures, where Zn plays a glass network-forming role, in agreement with the vibrational frequencies of ZnTeO3 and Zn2 Te3 O8 crystals. The 280 cm−1 shoulder, on the other hand, can be assigned to vibrations of non-bridging Cl− ions bonded to Zn species [20]. The cut-off frequency of the optical modes is 900 cm−1 , like in other tellurite systems [4–6], leading to a decrease in multiphonon relaxation of excited ions with respect to a silica based matrix, where the cut-off occurs at around 1100 cm−1 . After the deposition, we find some significant differences with respect to the target glass. (i) In the region at higher frequency, there is a

Intensity [arbitrary units]

66

Target T1 (p=5 Pa) T2 (p=10 Pa)

-500

0

500

1000

1500

2000

-1

Raman shift [cm ] Fig. 1. Normalized Raman spectra of the target glass (60TeO2 – 20ZnO–20ZnCl2 :1ErCl3 ) and of the T1 and T2 planar waveguides, collected in the VV polarization, by exciting at 457.9 nm. The excitation of the waveguide was in the TE0 mode.

A.P. Caricato et al. / Materials Science and Engineering B105 (2003) 65–69

67

Fig. 3. Refractive indices, n, and extinction coefficients, k, calculated from transmission spectra vs. wavelength. Fig. 2. Experimental optical transmission spectra of Er3+ -doped tellurite glass films T2 (full line), T1 (dotted line) and the substrate.

shift and a change of relative intensity of the peaks at 660 and 770 cm−1 . This effect is more evident in the spectrum of sample T2, where, together with an increase of the ratio TeO4 /TeO3 , we find also a broad luminescence band, centred at about 19,500 cm−1 (1800 cm−1 Raman shift), due to defects [21]. (ii) The peak at 420 cm−1 shifts to higher frequency and reduces its intensity, compared to the 770 cm−1 peak. At low frequency, we note (iii) the disappearance of the shoulders at 120 and 280 cm−1 and (iv) the decrease in intensity and a blue-shift of the boson peak. Measurements are in progress, in order to establish if these differences between target and film are due to the different quenching processes, which could affect the structural properties of the films (e.g. losing the Te–O–Zn–O–Te linkages), or just to a non-stoichiometric deposition. In Fig. 2, the transmission spectra of the deposited films in the wavelength range of interest for optical applications (200–2500 nm) are reported. The deposited films exhibit a transmission higher than 80% for wavelengths longer than 300 nm. The development of interference fringes indicates that the film thickness is uniform. From the transmission spectra, the refractive index n(␭), the extinction coefficient k(␭) and film thickness t were calculated using a home-made computer code based on an optical characterization method [22]. In Fig. 3, refractive indices, n(␭), and extinction coefficients, k(␭), are reported. The film thickness was 770 nm and 650 nm, for T1 and T2, respectively. The values of the film optical bandgap were estimated from the transmission spectra, using the Mott and Davis model [23]. From this model it results that the relationship between the absorption coefficient ␣(␻) and frequency around the absorption edge is: α(ω)¯hω = B(¯hω − Eg )r

and 3/2 depending on the nature of the electronic transitions leading to light absorption. In this work, a value of two was assumed, in agreement with other studies on tellurite thin films [24]. α(ω) was calculated using a general equation obtained from Eq. (14) in ref. [22]. α(ω) =

1 2 2 ln {2Ti D/[A − (A

t

− 4Ti2 BD)1/2 ]}

(2)

where A = 16n2 s (s is the substrate refractive index); B = (n + 1)3 (n + s2 ); D = (n − 1)3 (n − s2 ) and Ti = 2TM Tm /[TM + Tm ] (TM and Tm are upper and lower envelope values, respectively). Fig. 4 shows the plots of [α(ω)¯hω]1/2 versus h ¯ ω and the values of Eopt determined by extrapolating the linear parts of the curves to (α¯hω)1/2 = 0. The Eopt values were ∼3.77 eV (T1), ∼3.55 eV (T2) and 3.38 eV (target); these values are close to those reported in literature for pure TeO2 glass (∼3.79 eV [25] and ∼3.37 eV

(1)

where ω = 2πν is the angular frequency, B a constant, h ¯ the Planck constant divided by 2␲ and r equal to 3, 2, 1/2

Fig. 4. Dependence of (α¯hω)1/2 on photon energy for Er3+ -doped ZT films.

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A.P. Caricato et al. / Materials Science and Engineering B105 (2003) 65–69

Table 1 T1 and T2 optical parameters obtained from m-line measurements

T1 T2

O2 pressure during deposition (Pa)

5 10

Loss @ 633 nm ± 0.5 (dB/cm)

Refractive index ± 0.002

TE0

@633 nm

0.8 11.2

TE1

3.1 11.5

[26]). The value of Eopt decreases when the oxygen pressure increases: this behaviour can be attributed to a possible deviation from oxide stoichiometry. The optical parameters of the guides measured by the m-line technique are reported in Table 1. The guides showed 3 and 4 modes, respectively at 632.8 and 543.5 nm, from which we could recover, by means of an Inverse Wentzel–Kramers–Brillouin (IWKB) method, the in-depth refractive index profile of the deposited films which appear as “step-index” (Fig. 5). From the measured parameters, we obtained that the waveguides were single mode at 1.55 ␮m. The confinement factor, defined as the ratio of the integrated intensity in the waveguide to the total intensity, which includes also the squared evanescent fields, was about 90%. The comparison of refractive index of films with that of the bulk (1.962 at 632.8 nm and 1.970 at 543.5 nm) confirms the different structures of the deposited films depending on the oxygen pressure, already evidenced in the variation of Eopt and in the Raman spectra. On the other hand, it is reported [27] that PLD leads to the fabrication of films denser than the original bulk and thus with a higher refractive index (like T1 with respect to the target), while a higher increasing pressure lowers the density of the films, because the plasma species lose energy in collisions with the oxygen molecules in the deposition chamber [14]. In addition, since the atomic species to be deposited have different scattering cross section, this could also account for a selective loss of atoms during deposition. The stoichiometry change is also assoIndex Profile T1

Refractive index

2.0

1.8

TM 632.8 TM 543.5

1.6 0.0

0.4

Thickness ± 0.03 (␮m)

@543.5 nm

TE

TM

TE

TM

1.969 1.942

1.969 1.941

1.987 1.957

1.991 1.960

0.71 0.64

Intensity [arbitrary units]

Sample

1400

Target T1 T2

1450

1500

1550

1600

1650

1700

Wavelength [nm] Fig. 6. Normalized room temperature photoluminescence spectra of the 4I 4 3+ ion, upon excitation at 514.5 nm for 13/2 → I15/2 transition of Er the T1 and T2 planar waveguides (TE0 mode) and for the target glass.

ciated to the growing of structural defects, responsible for the large luminescence band at 19,500 cm−1 in the Raman spectra, which have a strong effect on the optical loss. In fact, the loss in the sample T2 was much higher than in T1, where it had a value of only 0.8 dB/cm at 632 nm. This last value, obtained in tellurite waveguides, is perfectly comparable with the attenuation coefficients measured in the technologically more mature silica based waveguides fabricated by sol–gel [18] or rf-sputtering [17]. In Fig. 6, we report the comparison of the luminescence spectra of the bulk and deposited films in the 1.5 ␮m region, upon excitation with the 514.5 nm line of an Ar+ laser. After the deposition, the bandwidth of the radiative transition between the 4 I13/2 and 4 I15/2 energy levels of Er3+ ions changed from 62 to 48 nm. We still don’t know if this decrease is to be ascribed to a reduction of Erbium concentration in the waveguide [5], or to a residual self-absorption effect in the bulk measurements. The measured lifetime of the metastable 4 I13/2 level of 3+ Er was 5.7 ms in the bulk, while it could not be measured in the waveguides, because of the very low signal.

0.8

Depth (µm)

Fig. 5. Refractive index vs. depth profile of planar waveguide T1, reconstructed from modal measurements for TM polarisation at 632.8 and 543.5 nm. The effective indices of the TM modes at 632.8 nm (∗) and 543.5 nm (䊉) are shown.

4. Conclusions The present work demonstrates that the RPLD technique has a potential for fabricating Er3+ -doped zinc–tellurite thin

A.P. Caricato et al. / Materials Science and Engineering B105 (2003) 65–69

films. Thin films were produced by depositing on silica substrates the material ablated from an oxychloride glass target of nominal composition 60%TeO2 –20%ZnO–20%ZnCl2 , doped with 1%ErCl3 , at room temperature. The films resulted amorphous, but the structure changed with respect to that of the target. The calculated band gaps were consistent with the values obtained in other studies. The waveguides showed four well confined TE and TM propagating modes at 543.5 nm and one mode at 1.5 ␮m. The optical losses at 632 nm were less than 1 dB/cm for the waveguide deposited with an O2 pressure of 5 Pa. Erbium emission in the region of the C-band was observed by PL measurements in all the samples. Acknowledgements We are grateful to A. Luches for reading the manuscript and for the precious suggestions, to S. Fonti for the transmission spectra and to A. Fazzi for the development of the computer code. This research was performed in the context of ICCTI/CNR on “Optical Amplification” collaborative grant, FIRB “Nanotecnologie, Microtecnologie, Sviluppo Integrato di Materiali” and MIUR-COFIN 2002 “Materiali nanostrutturati per l’ottica integrata”, italian projects. References [1] E. Desurvire, Erbium Doped Fiber Amplifier: Principle and Application, Wiley, New York, 1994. [2] H. Ogoshi, S. Ichino, K. Kurotori, Furukawa Rev. 19 (2000) 17. [3] J.S. Wang, E.M. Vogel, E. Snitzer, Opt. Mat. 3 (1994) 187. [4] A. Jha, S. Shen, M. Naftaly, Phys. Rev. B 62 (2000) 6215. [5] A. Mori, T. Sakamoto, K. Kobayashi, K. Shikano, K. Hoshino, T. Kanamori, Y. Ohishi, M. Shimuzu, J. Lightwave Tech. 20 (2002) 822.

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