Tm3+-Codoped LiNbO3

Article pubs.acs.org/crystal

Influence of Factors on the Growth of the Er3+/Tm3+-Codoped LiNbO3 Crystal by Er3+/Tm3+ Codiffusion De-Long Zhang,*,†,‡,§ Cong-Xian Qiu,†,‡ Ping-Rang Hua,†,‡,§ Dao-Yin Yu,†,‡ and Edwin Yue-Bun Pun§ †

Department of Opto-electronics and Information Engineering, School of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China ‡ Key Laboratory of Optoelectronic Information Technology, Ministry of Education (Tianjin University), Tianjin 300072, China § Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China

ABSTRACT: The influence of factors on the growth of the Er3+/Tm3+-codoped LiNbO3 (LN) crystal by codiffusion of stacked Er and Tm metal thin films was studied. These factors include the thicknesses and the coating sequence of the Er and Tm metal films, and the growth is characterized by the diffusivity and solubility. To achieve the goal, Er3+/Tm3+-codoped LN crystals were grown by codiffusion in air at 1130 °C of stacked Er and Tm metal thin films coated onto the surface of Z-cut congruent LN plates. The metal films have different thicknesses and coating sequences. After the growth, the Er3+ and Tm3+ profiles were analyzed by secondary ion mass spectrometry. The Er3+/Tm3+ diffusivity and surface concentration were obtained from the measured profiles. The diffusivity is influenced by neither the thickness nor the coating sequence of metal films. The solubility consists of Er3+ and Tm3+ components, and the two components equal the respective concentrations in the diffusion reservoir and change with the initial metal film thickness. Nevertheless, their sum, i.e., the solubility, remains a constant at a given temperature and shows a weaker effect of either the crystal cut or the coating sequence of Er and Tm films, together with the two components. In addition, the emission characteristics of the Er3+/Tm3+ diffusion-codoped LN were investigated. The results show that the codoping allows the combination of the wavelength emissions of both ions, and the resultant emission band in the telecommunication window around 1.5 μm is as wide as 150 nm, providing the possibility of S+C+L broadband amplification by employing the commercial 980 and 795 nm laser diodes as the pump sources.

I. INTRODUCTION 3+

the development of some novel optical-damage-resistant devices, such as laser diode (LD)-pumped infrared amplifiers and/or lasers working at 1.76 μm (Tm3+), 2.7 μm (Er3+), and at S+C+L band [1.45 μm (Tm3+) + 1.5 μm (Er3+)], upconversion lasers working in violet/blue [370, 450, and 475 nm (Tm3+)] and green [560 nm (Er3+)] regions, and various quasi-phasematching devices that can be pumped and/or can operate in both visible and near-infrared regimes. These devices do not suffer from optical damage. One can choose a low-cost 980/795 nm LD as the pump source, and the devices can operate at high light intensity. In addition, the Ti:Tm:LN waveguide may also find use in quantum communication.12−14

3+

The singly Er - or Tm -doped LiNbO3 (LN) crystal is a promising substrate material for active integrated optics as it combines the Er3+ or Tm3+ laser property with the excellent electro-optic, acousto-optic, and nonlinear optical properties of the LN crystal. Such an effective combination, together with the possibility of producing a high-quality waveguide of low loss, allows the broadband amplification and lasing in the telecommunication wavelength region around 1.5 μm. In recent years, a family of Ti (or ZnO)-diffused singly Er3+- or Tm3+doped LN waveguide lasers (amplifiers) and integrated devices have been described.1−11 Er3+ and Tm3+ codoping not only allows the combination of the emissions of both ions but also allows the utilization of the optical-damage-resistant merit of Tm3+ dopants.10 Such a crystal is a more promising material. The realization of a Ti:Er:Tm:LN strip waveguide would allow © 2013 American Chemical Society

Received: August 1, 2013 Revised: October 10, 2013 Published: October 16, 2013 5316

dx.doi.org/10.1021/cg401165m | Cryst. Growth Des. 2013, 13, 5316−5323

Crystal Growth & Design

Article

Table 1. Summary of Initial Er and Tm Metal Film Thicknesses Coated, Temperatures, and Durations, Evaluated Li2O Contents, erfc-Fitted Parameters, and Diffusivity and Solubility Components for Er3+/Tm3+ Codiffusion Growth in Z-Cut Congruent LN Plates under an Atmosphere of Surrounding Air sample 1 cut of crystal metal thickness temp (°C) duration (h) Li2O content Er3+ profile type Ii(j0) (cps) j0 (μm) INi (cps) dj (μm) Tm3+ profile type Ii(j0) (cps) j0 (μm) INi (cps) dj (μm) Er3+ diffusivity (×10−2 μm2/h) Tm3+ diffusivity (×10−2 μm2/h) τi for Er (nm) τi for Tm (nm) Er3+ solubility component (×1020 ions/cm3) Tm3+ solubility component (×1020 ions/cm3) a

sample 2

sample 3

sample 4

sample 5

sample 6

sample 7

Z-cut 10 nm Er, 35 nm Tma 1130 30 48.4 mol %,b 48.4 mol %c

Z-cut 15 nm Er, 30 nm Tm 1130 30 48.4 mol %, 48.4 mol %




Z-cut 15 nm Tm, 30 nm Er 1130 30 48.4 mol %, 48.4 mol %

Z-cut 30 nm Tm, 15 nm Er 1130 30 48.4 mol %, 48.4 mol %

erfc 27 0.5 0.0 3.2 ± 0.2

erfc 10 0.5 0.1 3.2 ± 0.2

erfc 23 0.5 0.1 3.2 ± 0.2

erfc 20 0.5 0.1 3.2 ± 0.2

erfc 32 0.5 0.2 3.2 ± 0.2

erfc 24 0.5 0.0 3.2 ± 0.2

erfc 7 0.5 0.1 3.2 ± 0.2

erfc 122 0.5 2.0 3.2 ± 0.1 8.0 ± 0.8

erfc 100 0.5 1.0 3.2 ± 0.1 8.0 ± 0.8

erfc 89 0.5 1.0 3.2 ± 0.1 8.0 ± 0.8

erfc 64 0.5 0.3 3.3 ± 0.1 8.0 ± 0.8

erfc 98 0.5 1.0 3.3 ± 0.1 8.0 ± 0.8

erfc 75 0.5 1.0 3.3 ± 0.1 8.0 ± 0.8

erfc 70 0.5 0.4 3.2 ± 0.1 8.0 ± 0.8

8.0 ± 0.5

8.0 ± 0.5

8.0 ± 0.5

8.3 ± 0.5

8.3 ± 0.5

8.3 ± 0.5

8.0 ± 0.5

3.5 12.2 0.64 ± 0.10

4.8 10.4 0.89 ± 0.17

8.0 7.8 1.50 ± 0.14

9.6 4.9 1.80 ± 0.20

12.4 3.6 2.29 ± 0.20

9.5 4.8 1.84 ± 0.20

5.1 10.6 1.00 ± 0.17

2.32 ± 0.20

1.97 ± 0.24

1.48 ± 0.16

0.90 ± 0.17

0.66 ± 0.10

0.88 ± 0.17

2.02 ± 0.23

A 10 nm thick Er film was coated prior to a 35 nm thick Tm film. bData for the Er3+/Tm3+-doped part of surface. cData for the undoped part.

As an alternative, an Er3+/Tm3+-codoped LN crystal can be produced by the codiffusion growth of stacked Er and Tm metal films. Material preparation and spectroscopic properties are fundamental knowledge for device design and performance optimization. Next, we give a short review of these two aspects for both cases of Er3+/Tm3+ single-diffusion and codiffusion growth. For the diffusion growth of rare-earth-doped LN, the growth is characterized by the parameters of diffusivity and solubility of rare-earth ions. It is crucial to fully understand the factors influencing the growth. For the case of Er3+ or Tm3+ singly doped LN, the possible factors include the growth temperature, growth atmosphere (such as wet or dry air, O2, Ar, or Li-rich and Li-poor atmosphere), material Li composition, and crystal cut. Many papers have been published over the past several years, and the effects of these factors are already clear.15−22 For the case of Er3+/Tm3+ codiffusion, some additional factors may affect the growth, besides those mentioned above. These include the thicknesses and the coating sequence of the initial Er and Tm metal films. In our earlier paper,22 we reported preliminary work on the codiffusion characteristics of Er3+/ Tm3+ in the LN. The work in that paper focused on the temperature dependence of the growth parameters of diffusivity and solubility components, as well as the comparison with the single diffusion. Although the initial Er and Tm metal film thickness effect on the solubility component was mentioned in that paper, the relevant conclusion was drawn from a few data and hence made only supposedly because of a lack of systematic experimental data. Moreover, the coating conse-

quence issue was not addressed in that paper. It is thus essential to conduct an independent, systematic study on the initial metal film thickness and coating sequence effects on growth. One objective of this work is to make the effects clear. With respect to spectroscopic properties, much work has been done for the singly Er3+- or Tm3+-doped LN and many classical papers have been published in recent years. Until now, the Er3+ spectroscopic knowledge tends to be complete. Researchers have reported various aspects of Er3+ spectroscopic properties such as absorption and emission (including upconversion) characteristics,23−33 Judd−Ofelt theoretical data,34−39 site occupation in the lattice,40−46 and absorption and emission cross sections of the Er3+ ion in both bulk material6,47−51 and waveguide structure.52−58 Some papers about the growth, optical, and Tm3+ spectroscopic properties of homogeneously doped LN have also been published.46,59−66 For the Er3+/Tm3+ codoping case, however, few papers could be found in the literature. Although two previous papers involved with the (bulk) Er3+/Tm3+-codoped LN,45,46 the study concentrated on Er3+ site occupation. It is unclear if the merit of combination of wavelength emissions of both Er3+ and Tm3+ ions is true for the case of diffusion codoping. It is thus essential to make this argument clear. This is another objective of this work.

II. EXPERIMENTAL SECTION Z-Cut commercial congruent LN crystal plates with 1 mm thickness and with optical grade surfaces were used in this study. The ordinary and extraordinary refractive indices at the surface of each as-grown 5317



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plate, which are used to evaluate the Li composition in the crystal, were measured first. Both Er (99.9%) and Tm (99.4%) metal films with thicknesses of 10, 15, 22, 30, and 35 ± 2 nm were then coated onto part of the surface of each plate, and the uncoated part is for reference. After the Tm/Er metal film deposition, the plates were annealed in air. A diffusion temperature of 1130 °C was chosen for all plates, and a diffusion duration of 30 h was chosen. Table 1 summarizes the Er/Tm metal thin film thickness coated and the diffusion parameters adopted for each plate. Note that these samples have different thicknesses and coating sequences of Er and Tm metal films. For samples 1−5, the Er metal film was coated prior to the Tm metal film. The thicknesses of Er and Tm metal films change relatively. As the sample goes from sample 1 to sample 5, the Er thickness increases while the Tm thickness decreases. Nevertheless, the sum of the thicknesses of the two films remains constant (45 nm). For samples 6 and 7, the coating order is just reversed; i.e., the Tm film was coated prior to the Er film. For sample 6, a Tm metal film with a smaller thickness of 15 nm was coated prior to an Er metal film with a larger thickness of 30 nm. For sample 7, however, a Tm metal film with a larger thickness of 30 nm was coated prior to an Er metal film with a smaller thickness of 15 nm. To obtain a complementary error function (erfc) profile, from which the solid solubility of Tm3+ or Er3+ in LN can be obtained,15,16 a thicker rare-earth metal film (45 nm in total) was coated, and a shorter diffusion duration of 30 h was adopted. After the diffusion growth, the surface refractive indices of the doped and undoped parts were measured again. The refractive index was measured at the 1311 and 1553 nm wavelengths using a commercial Metricon 2010 prism coupler (Metricon Corp., Pennington, NJ), which is based on the working principle of measuring the critical angle of total reflection. The refractive index measured by this method is the value at the crystal surface because the total reflection takes place there. For an LN plate, it is convenient to choose either the transverse magnetic or electric polarization scheme to measure the ordinary or extraordinary index, depending on the cut of the plate to be measured. All measurements were taken at room temperature (25 ± 0.1 °C). Secondary ion mass spectrometry (SIMS) was used to analyze the depth profile of the diffused Er3+ and Tm3+ ions. The analysis was accomplished by a time-of-flight second ion mass spectrometry [ToF SIMS V (ION-TOF GmbH, Münster, Germany)]. A Cs+ beam (∼45 μm in diameter) with 22 or 30 nA at 3 keV was used to sputter a crater of 120 μm × 120 μm, and a pulsed bismuth ion beam of 1 pA at 25 keV was used to analyze the secondary ions 6Li, 93Nb, 167Er, and 169Tm as a function of time. Positive secondary ions were detected, and ions from a central area from 20.5 μm × 20.5 μm to 25.4 μm × 25.4 μm inside the crater were detected. During the analysis, a low-energy pulsed electron gun was used to neutralize the positive charges and lower the degree of surface charge accumulation. For the same purpose, a 30 nm thick Ag film was coated on the diffused surface of each sample to be analyzed before the analysis. The trace and depth of each erosion crater were measured by a Tencor Alpha Step 200 profilometer (KLA-Tencor Corp., Milpitas, CA). The depth resolution was determined mainly by the roughness of the crater under analysis and is better than 5 nm in our case. As a representative, sample 2 was selected and the spectroscopic investigation was conducted. The emission spectra were recorded by a HORIBA Jobin-Yvon Fluorolog-3 double-grating-excitation spectrofluorometer equipped with a standard photomultiplier for visible detection and an NIR photomultiplier for near-infrared detection (0.9−1.6 μm) and a PbS photodetector for mid-IR detection (1−3 μm). A 450 W continuous xenon lamp was used as the excitation source. The Er3+ ions were excited by the 980 nm wavelength light (∼20 mW). The Tm3+ ions were excited by the 795 nm wavelength light. We also attempted to simultaneously excite the two ions using the 795 nm excitation. A couple of 1200 grooves/mm gratings blazed at the wavelength of 750 nm were used to disperse the excitation light. The (near-infrared) fluorescence was dispersed by a near-infrared instrument (600 grooves/mm). The set entrance and exit slit widths lead to a spectral resolution of 14 and 50 nm, respectively. The

excitation beam was oriented along the direction perpendicular to the sample surface, i.e., parallel to the Z axis of the crystal. Because of the limitations by various factors, which will be discussed later, only the fluorescence at 1.45 μm (Tm3+) and 1.5 μm (Er3+) could be measured. All spectra were recorded at room temperature.

III. RESULTS AND DISCUSSION A. Influence of Er and Tm Metal Film Thickness and Coating Sequence on Growth. The model for the codiffusion growth of Er3+/Tm3+-codoped LN crystal has been described and justified previously.22 Here, we give only a brief description for it for the convenience of discussion. Consider a one-dimensional Cartesian coordinate system fixed at the diffused surface with x (z) axis along the depth direction of the X (Z)-cut plate. It has been justified that the Er3+ or Tm3+ planar diffusion growth process can be described by two independent one-dimensional Fick-type equations, given by i i ∂C RE (j , t ) ∂ 2C RE (j , t ) i = DRE [C(Li 2O)] ∂t ∂j 2

(1)

CiRE(j,t) 3+

where (i = X or Z; j = x or z) denotes the rare-earth (RE) ion Er or Tm3+ concentration at a depth position x or z after a diffusion time t and DiRE[C(Li2O)] is the Er3+ or Tm3+ diffusivity, which is assumed to be independent of the Er3+ and/ or Tm3+ concentration but is dependent on the Li2O content, C(Li2O). Out-diffusion of Li+ in the crystal usually accompanies in-diffusion of an ion into the LN crystal and results in the decrease in the Li2O content of the crystal and hence the Li2O content-dependent diffusivity DiRE[C(Li2O)]. It is crucial to know the Li2O content at the Er3+/Tm3+-doped and undoped parts of the crystal surface. The refractive index measurement method was used to evaluate the Li2O content in the Er3+/ Tm3+-doped and undoped parts on the surfaces of the plates studied. The results show that the Er3+/Tm3+ doping contribution to the refractive index is on the order of 10−4 and can be ignored. From the measured refractive indices, the corresponding Li2O content can be evaluated using the Li2O content-dependent Sellmeier equation.67 Table 1 summarizes the results of the doped (data labeled by superscript a) and undoped (data labeled by superscript b) parts of the crystal surfaces. One can see that after the diffusion growth process the Li2O content at the Er 3+/Tm3+-doped surface can be considered to be the same as that of the undoped surface, and the 30 h long diffusion growth process resulted in an only slight 0.2 ± 0.1 mol % Li2O content reduction at both the doped and undoped surfaces. We conclude that Li out-diffusion can be ignored for all of the crystals studied, and the Er3+ and Tm3+ ions diffusion growth can be described by eq 1 with a Li2O content-independent diffusivity. A description for the solution to eq 1 has been detailed previously.22 For a given Er or Tm metal film thickness and at a given diffusion growth temperature, there exists a time parameter t1. When the diffusion growth time t < t1, the Er3+ or Tm3+ diffusion reservoir is not exhausted, and the solution to eq 1 is an erfc with a constant surface concentration, named CiRE (RE = Er or Tm). For the Er3+ and Tm3+ codiffusion growth case, the solubility is equal to CiEr + CiTr. For the sake of convenience, CiRE is named the Er3+ or Tm3+ solubility component. As a representative, Figure 1 shows the depth profiles, on a semilogarithmic scale, of 6Li, 93Nb, 167Er, and 169Tm SIMS signals detected from samples 2, 4, 6, and 7, which have different thicknesses and coating sequences but the same total Er and Tm metal film thickness of 45 nm. The black solid line 5318



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6

Figure 2. Er3+ and Tm3+ diffusivities at 1130 °C (red balls for Er3+ and green balls for Tm3+).

93

Figure 1. Depth profiles, on a semilogarithmic scale, of Li, Nb, 167 Er, and 169Tm SIMS signals detected from samples 2, 4, 6, and 7. The black/magenta plot overlapped on each measured Er3+/Tm3+ (red and green balls) profile represents the best erfc fit.

diffusivity, and this is the case for all samples. Moreover, for samples 1−8, the Er3+ and Tm3+ diffusivities have the same value of ∼8 μm 2/h, implying that the Er3+ and Tm 3+ diffusivities are dependent on neither the thicknesses nor the coating sequence of the two metal films. For sample 9, the diffusivity has a slightly lower value of ∼7 μm2/h because of the anisotropy of the diffusion growth; i.e., the diffusion growth along the optical axis direction is faster than along the other directions. We conclude that, like in the case of single-diffusion growth,15−22 the diffusivity in the codiffusion growth case depends on only the growth temperature, material composition, and crystal cut, while it is influenced by neither the thickness nor the coating sequence of the metal film. i The Er3+ or Tm3+ solubility component CRE can be determined by applying the law of mass conservation to eq 3:

plots denote the 6Li and 93Nb substrate signals, and the red-ball and green-ball curves represent the measured Er3+ and Tm3+ ion profiles, respectively. Figure 1 shows that some of the Er3+ and Tm3+ ions have diffused into the bulk while a certain number of Er3+ and Tm3+ ions have not, remaining within an ∼0.5 μm thick shallow layer near the crystal surface, which acts as the diffusion reservoir (it is a temporary transition layer that becomes thinner as the rare-earth ions continuously diffuse into the bulk of the crystal and eventually disappears as all ions diffuse into the bulk15,68). All of the Er3+ and Tm3+ ion diffusion reservoirs have not been exhausted. All of the diffused Er3+ and Tm3+ profiles can be fit well by an erfc: i IRE (j) = I i(j0 ) erfc[(j − j0 )/dj] + INi

(2)

i C RE

IiRE(j)

where (i = X or Z; j = x or z) represents the yield of the secondary Er3+ or Tm3+ ion, dj denotes the Er3+ or Tm3+ diffusion depth, IiN is the background noise, and j0 represents the initial coordinate of the Er3+- or Tm3+-diffused layer. Table 1 brings together all the values of the fitting parameters. The black/magenta curve overlapped with each measured Er3+/ Tm3+ profile represents the best erfc fit, and there is excellent agreement between the fitting and measured curve. One can see from the parameter values of Ii(j0) and IiN that the background noise is within 1%. The Er3+ or Tm3+ ion concentration profile can be approximated as i i C RE (j) = C RE erfc[(j − j0 )/dj]

0

+∞

i erfc[(j − j0 )/dj] dj = CsτRE

(4)

where Cs = ρNA/M, and ρ, NA, and M are the mass density of Er or Tm metal (9.06 g/cm3 for Er and 9.32 g/cm3 for Tm), Avogadro’s number (6.02 × 1023 atoms/mol), and the molecular weight of the Er or Tm metal, respectively. τiRE is the fraction of the coated total Er or Tm metal film thickness corresponding to the Er3+ or Tm3+ ions diffused into the bulk. It can be determined from the Er or Tm film thickness coated and the ratio of the area under the erfc profile to that under the entire Er3+ or Tm3+ profile, and its value is given in Table 1, together with the evaluated Er 3+ and Tm3+ solubility components. The Er3+/Tm3+ solubility component is (0.64 ± 0.10)/(2.32 ± 0.20), (0.89 ± 0.17)/(1.97 ± 0.24), (1.50 ± 0.14)/(1.48 ± 0.16), (1.80 ± 0.20)/(0.90 ± 0.17), (2.29 ± 0.20)/(0.66 ± 0.10), (1.84 ± 0.20)/(0.88 ± 0.17), and (1.00 ± 0.17)/(2.02 ± 0.23) × 1020 ions/cm3 for samples 1−7, respectively. The margin of error is 20% for both Er3+ and Tm3+. One can see that the solubility component of either Er3+ or Tm3+ changes from one sample to another. It appears that the change is associated with the thicknesses of the initial Er and Tm metal films, and the thicker the initial metal film, the higher the corresponding solubility component of this metal. This is obviously distinguished from the single-diffusion growth case, in which the solubility depends on only the growth temperature and material composition and is independent of the initial thickness of the metal film. The behavior in the codiffusion growth case in which the solubility component is related to the initial thickness of the metal film can be explained as follows in terms of the Er3+ and Tm3+ concentrations in the

(3) 3+

∫j

3+

The z0 parameter values imply that the Er and Tm ions in each sample are present in the same diffusion reservoir layer having a thickness of ∼0.5 μm. It can be also seen from Table 1 that the Er3+ and Tm3+ profiles have similar dj values of 3.2−3.3 μm for all samples because both the growth temperature and duration are identical for all samples. With the known dj, the Er3+/Tm3+ diffusivity can be calculated according to the relation dj = 2(DiREt)1/2, and the results are listed in Table 1. The relative error of diffusivity is within 10% for both Er3+ and Tm3+. For straightforwardness, the Er3+ and Tm3+ diffusivities are shown in Figure 2 for all of the samples studied here (red balls for Er3+ and green balls for Tm3+). For the completeness of data, the results of the two samples (one Z-cut and another X-cut) studied previously22 are also included in Figure 2 (see the data of samples 8 and 9). It is evident that the Er3+ and Tm3+ ions have the nearly same 5319



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Tm3+ solubility components (CiTm) for all of the samples studied here and the two samples (8 and 9) studied previously.22 Indeed, one can see that the sum is independent of the thicknesses and the coating sequence of the metal films, as well as the crystal cut, and can be considered as a constant of ∼3 × 1020 ions/cm3. This constant value is consistent with the solubility data of the Er3+ single-diffusion growth at the same temperature of 1130 °C.15 B. Er3+ and Tm3+ Spectroscopic Characteristics. Because the studied crystal is mainly used for optical device fabrication, the emission characteristics of the Er3+/Tm3+ diffusion-codoped LN should be investigated to verify the ability of the Er3+/Tm3+ codoping to combine the wavelength emissions of both ions. Figure 5 shows the energy-level diagram

diffusion reservoir. The solubility component depends on the surface concentration; the surface concentration equals the concentration in the reservoir, and the concentration in the reservoir depends on the initial thickness of the metal film coated. The thicker the initial metal film, the higher the concentration of this metal in the reservoir and hence the higher the corresponding solubility component of this metal. This argument is clarified by Figure 3, where the ratio of Er3+

Figure 3. Comparison of the ratio of Er3+ and Tm3+ solubility components with that of the initial Er and Tm metal film thicknesses.

and Tm3+ solubility components is compared with that of the initial Er and Tm metal film thicknesses for all of the samples studied here and the two samples (8 and 9) studied previously.22 One can see that the solubility component ratio matches well with the corresponding metal film thickness ratio; both can be considered the same, within error. We also note that from sample 1 to 5 the Er thickness increases while the Tm thickness decreases, and the Er3+ solubility component increases while the Tm3+ solubility component decreases. In particular, for sample 3, onto which similar initial Er and Tm metal film thicknesses were coated, as expected, the two ions have similar solubility components and the solubility component ratio can be thought to be unity within error. We also note from Figure 3 that the solubility component ratio is reasonably independent of the coating sequence of metal films, as well as the cut of the crystal plate. As stated above, at a given temperature and material composition, the solubility should be constant. This means that although the Er3+ and Tm3+ solubility components change with the initial thicknesses of the two metal films coated, their sum (CiEr + CiTm), i.e., the solubility, should remain constant at a given temperature. Figure 4 shows the sum of Er3+ (CiEr) and

Figure 5. Energy-level diagram of the Tm3+−Er3+ system and nearinfrared luminescence mechanisms. (1) ET1: resonant energy transfer of Tm3+ (3H4) → Er3+ (4I9/2). (2) ET2: phonon-assisted energy transfer of Er3+ (4I11/2) → Tm3+ (3H5) + phonons. (3) ET3: quasiresonant energy transfer of Er3+ (4I13/2) → Tm3+ (3F4). (4) CR1: cross-relaxation of Tm3+ (3H4) + Tm3+ (3H6) → Tm3+ (3F4) + Tm3+ (3F4). (5) CR2: cross-relaxation of Tm3+ (3H4) + Er3+ (4I15/2) → Tm3+ (3F4) + Er3+ (4I13/2). MPR and EM stand for multiphonon relaxation and energy migration, respectively.

of the Tm3+−Er3+ system and the near-infrared luminescence mechanism that involves various possible transition processes such as energy transfer (ET), cross relaxation (CR), multiphonon relaxation (MPR), and energy migration (EM). The important near-infrared fluorescence involves the 4I13/2 → 4I15/2 electronic transition of the Er3+ ion at 1.5 μm and the 3H4 → 3 F4 and 3F4 → 3H6 electronic transitions of the Tm3+ ion at 1.45 and 1.8 μm, respectively. The preferred pump source is the commercial high-power 980 nm LD for Er3+ and 795 nm LD for Tm3+. Figure 6 shows the near-infrared fluorescence spectra measured from representative sample 2. The spectrum shown in Figure 6a was measured under 980 nm wavelength excitation, and that shown in Figure 6b was measured under 795 nm wavelength excitation. The excitation beam was oriented along the direction perpendicular to the sample surface, i.e., parallel to the Z axis of the crystal. Under such an excitation scheme, there are fewer active ions excited in the diffusion-doped sample than excited in a homogeneously bulkdoped sample because in the former case the active ions are doped only in the very shallow layer (10−20 μm in depth) on the crystal surface while in the latter case the active ions are homogeneously doped in a whole plate. This means that the excited fluorescence of the diffusion-doped sample must be considerably weak in comparison with that of the homogeneously bulk doped crystal, and this speculation is verified experimentally (note that many more ions are excited as a strip waveguide is fabricated on the diffusion-doped surface and the

Figure 4. Solubility CiEr + CiTm (at 1130 °C) in all of the samples studied. 5320



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figure is only 50−60 nm for the single-Er3+ doping case) of the broadband emission around 1.5 μm, which covers the S+C+L telecommunication band. It would be possible that one realizes the S+C+L broadband amplification or lasing by employing just one 795 nm LD as the pump source. Note that this is only speculation and needs to be examined experimentally. We must envisage that the Er3+ absorption upon the 795 nm pump light is much weaker than that upon the 980 nm light, resulting in a low pump efficiency and hence no amplification of the 1.5 μm small signal. If so, one can choose another pumping scheme, which is not much trouble. That is employing two LDs as the pump sources. A 980 nm LD is responsible for Er3+ pumping and another 795 nm LD for Tm3+ excitation. In this case, light from 795 and 980 nm LDs can be combined via a special 795 nm/980 nm WDM coupler and pump the waveguide simultaneously. It is unfortunate that the 3F4 → 3H6 electronic transition of the Tm3+ ion at 1.8 μm could not be resolved because of the limitation of the facilities (the NIR photomultiplier detector that is part of the spectrofluorometer we employed has a spectral response upper limit of only 1.6 μm, while the PbS detector that is part of the spectrofluorometer does not work normally).

Figure 6. Unpolarized near-infrared emission spectra of Er3+/Tm3+ diffusion-codoped LN (sample 2). (a). Er3+ 4I13/2 → 4I15/2 electronic transition under 980 nm excitation. (b) Er3+ 4I13/2 → 4I15/2 (1.5 μm) and Tm3+ 3H4 → 3F4 (1.45 μm) electronic transitions under 795 nm excitation.

IV. CONCLUSION

excitation direction is along the waveguide axis). To detect the weak fluorescence, the entrance slit was set at the permitted maximal width and the exit slit was also set at a larger width. As a result, the spectral resolution is sacrificed, 50 nm only, and less spectral structure could be resolved as shown in Figure 6b. In addition, the measurement also suffers from another problem. That is the excitation-grating-induced diffraction of various high orders of the light from the xenon lamp, and the undesired diffracted light may excite the active ions. During the measurements, much effort has been spent to solve this problem. A special infrared filter with a cutoff wavelength of 1 μm was used for the excitation beam to avoid the possible excitation due to the high-order diffraction of visible light from the xenon lamp, which may impinge on the sample, together with the truly desired 795 or 980 nm near-infrared excitation light, and excite the active ions. The excitation light from the xenon lamp is already weak, only ∼20 mW for the 980 nm light. The use of the filter leads to additional attenuation of the excitation light and hence further weakening of the fluorescence. For these reasons, the recorded polarizationresolved spectrum shows poor resolution and only polarizationunresolved emission spectrum could be well resolved. Nevertheless, this does not prevent us from achieving our goal, i.e., verifying that Er3+/Tm3+ codoping allows the combination of the wavelength emissions of both ions. Figure 6a clearly shows the spectral feature of the 1.5 μm 4I13/2 → 4I15/2 electronic transition of the Er3+ ion in an LN crystal excited at the 980 nm wavelength, which corresponds to the 4I15/2 → 4I11/2 electronic transition (see Figure 5). The 795 nm wavelength excitation may result in the simultaneous near-infrared emissions of the two ions, including the 1.5 μm 4I13/2 → 4I15/2 electronic transition of the Er3+ ion and the 1.45 μm 3H4 → 3F4 electronic transition of the Tm3+ ion. Indeed, one can see from Figure 6b that the fluorescence originating from the two transitions is simultaneously resolved. As expected, the emission of the Tm3+ ion peaks at 1452 nm and that of the Er3+ ion peaks at 1532 nm. A rough evaluation shows that the codoping with Tm3+ ion results in an ∼150 nm full width at half-maximum (while the

We have demonstrated the factors influencing the growth of the Er3+/Tm3+-codoped LiNbO3 crystal by codiffusion of a stacked Er and Tm metal thin film coated onto part of the surface of an initially congruent LN crystal plate. Different from the Er3+ or Tm3+ single-diffusion growth case, for which possible factors influencing the growth include the already well-known diffusion temperature, diffusion atmosphere, material composition, and crystal cut, the factors in the codiffusion growth case also include the thickness and coating sequence of the Er and Tm metal films. This study shows that the Er3+ and Tm3+ diffusivities are similar at the same growth temperature and depend on neither the thickness nor the coating sequence of metal films. The solubility consists of Er 3+ and Tm 3+ components. The two components depend on their respective concentrations in the diffusion reservoir and hence the initial thicknesses of the metal films, and their ratio equals that of the corresponding initial metal film thicknesses. Although the two components change with the initial metal film thickness, their sum, i.e., the solubility, remains a constant at a given temperature and shows a weaker effect of either the crystal cut or the coating sequence of Er and Tm films, together with the two components. Indeed, the spectroscopic study shows that Er3+/Tm3+ codoping allows the combination of the wavelength emissions of both ions and results in an ∼150 nm broad emission band in the telecommunication window around 1.5 μm, offering the possibility of S+C+L broadband amplification or lasing by employing the commercial 980 and 795 nm LDs as the pump sources.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5321



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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China via Projects 61377060, 61077039, and 50872089, by the Key Program for Research on Fundamental to Application and Leading Technology, Tianjin Science and Technology Commission of China via Project 11JCZDJC15500, and by the Specialized Research Fund for the Doctoral Program of Higher Education of China via Project 20100032110052.

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Tm3+-Codoped LiNbO3

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