Control of the radiative properties via photon-plasmon interaction in Er 3 +-Tm 3 +-codoped tellurite glasses in the near infrared region

The novelty of this paper is that it reports on the tuning of the spectral properties of Er-Tm ions in tellurite glasses in the near-infrared region through the incorporation of silver or gold nanoparticles. These noble metal nanoparticles can improve the emission intensity and expand the bandwidth of the luminescence spectrum centered at 1535 nm, covering practically all the optical telecommunication bands (S, C + L and U), and extended up to 2010 nm wavelength under excitation by a 976 nm laser diode. Both effects are obtained by the combined emission of Er and Tm ions due to efficient energy transfer processes promoted by the presence of silver or gold nanoparticles for the (Er)I11/2→(Tm)H5, (Er)I13/2→(Tm)H4 and (Er)I13/2→(Tm)F4 transitions. The interactions between the electronic transitions of Er and Tm ions that provide a tunable emission are associated with the dynamic coupling mechanism described by the variations generated by the Hamiltonian HDC in either the oscillator strength or the local crystal field, i.e. the line shape changes in the near-infrared emission band. The Hamiltonian is expressed as eigenmodes associated with the density of the conduction electron generated by the different nanoparticles through its collective free oscillations at each resonance frequency of the nanoparticle and their geometric dependence. A complete description of photon-plasmon interactions of noble metal nanoparticles with the Er and Tm ions is provided. ©2014 Optical Society of America OCIS codes: (160.2750) Glass and other amorphous materials; (160.5690) Rare-earth-doped materials; (250.5403) Plasmonics; (300.6280) Spectroscopy, fluorescence and luminescence. References and links 1. M. Yamane and Y. 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Introduction
There has been an intense scientific and technological interest in the development of new materials as near-infrared (NIR) super-broadband and intense luminescence sources for the advancement of optical amplifiers technology, tunable lasers and amplified signals transmission [1][2][3].Rare-earth (RE) ion -doped glasses are an alternative solution since the demand for optical devices that operate in the NIR region has been increasing.
Bulk glasses with high solubilities of RE ions represent a large optical confinement (medium gain [2,4,5]), which offers an attractive way of producing transparent, small and high-emission quantum yield devices.In contrast, the glasses fabrication with low contaminants content is still limited by the traditional technique of melting the powders precursor in crucible, despite the recent progresses observed in this area [6].Specific properties of the host matrix, such as high refractive index, large optical transmission (visible and NIR regions) and low phonon energies are required for the achievement of high emission yield.In this sense, tellurite glasses (whose major component is TeO 2 ) have appeared as optimal candidates among the oxide glasses because of their wide transparency range (from 400 to 5500 nm), high refractive index (~1.8-2.0),relatively low phonon energy (~800 cm −1 ) in comparison to silicate glasses [7][8][9] and good chemical and mechanical resistance [10,11].Nevertheless, there exists a technological limitation to the use of RE ions for optical emission and amplification, since each RE ion covers the O-, E-, S-, C-, L-and U-bands of the optical communications separately (in the 1000 -1680 nm region) [1,12].The emission bands of the trivalent RE ions are difficult to surpass since the f-f transitions of the 4f orbitals are confined in the inner-shell and their nature is usually mediated by the electric dipole transitions which are sensitive to local environmental [13,14].Several approaches are currently under development for a full use of the optical bandwidth through glassy systems, such as (i) doping with metal ions, e.g., bismuth [15][16][17][18], nickel [19][20][21], bismuth-nickel codoped [22], and, chromium [23], (ii) generation of supercontinuum light in highly non-linear optical fibers [24][25][26] and (iii) co-doping with RE ions [27][28][29].Therefore, the expansion in the gain bandwidth in glassy materials for the manufacture of more efficient optical devices in the transmission network is a key factor for the present and future development of optical communications.
Noble metal nanoparticles (NPs) embedded in RE-doped tellurite glasses can either improve or quench the RE spectroscopic properties for optical applications in telecommunication bands [30][31][32][33][34] and light emission by upconversion processes [35][36][37].The full understanding of the interactions involved in NPs and RE ions is still under debate.Some researchers have ascribed such an emission enhancement to the energy transfer (ET) between NP:RE, while others have attributed it to an improvement in the local field near the NPs (which exponentially decays from the NP surface).In fact, both interaction effects are produced from a coherent collective oscillation of the surface electrons of the metal NP (nonpropagating excitations), also known as localized surface plamon resonance (LSPR) [38,39], which enables the confinement of the electromagnetic energy ( ( ) ( ) ) into volumes smaller than the diffraction limit (λ 0 /n) 3 , where 0 ε is the permittivity of the free space, n is the electron density, e and m are the charge and mass of the electron, respectively, n is the refractive index of the surrounding medium and λ 0 is the incident excitation wavelength.Moreover the polarization of the NP can be either longitudinal (electric dipole parallel to the NP:RE axis) or transverse (electric dipole perpendicular to the NP:RE axis), depending on the NP shape, e.g.non-spherical NP, nanorods and others [40][41][42][43].Such electric dipole coupling is formed when both NP:RE have the same potential, but opposite polarity [38].The controlled production of anisotropic NPs embedded in glassy materials [44], like tellurite glasses [30][31][32][33][34][35][36][37]43] and the comprehension of the basic concepts involved constitute a current challenge that may bring us new technological applications.
The present research is focused on the emission enhancement and widening of broadbands in the NIR region in Er 3+ -Tm 3+ -doped tellurite glass by means of gold or silver NPs embedded in the glass for gain amplification.Such an emission bandwidth is obtained under excitation at 976 nm laser diode through an integrated emission between the Er 3+ and Tm 3+ ion levels via efficient ET processes: (Er 3+ ) 4 I 11/2 →(Tm 3+ ) 3 H 5 , (Er 3+ ) 4 I 13/2 →(Tm 3+ ) 3 H 4 and (Er 3+ ) 4 I 13/2 →(Tm 3+ ) 3 F 4 , which cover all the S-, C + L-and U-bands of the optical telecommunications.These ETs are mediated by either silver or gold NPs (SNPs or GNPs).The NIR emission in the 1450-2010 nm region is ascribed to the following electronic transitions of Er 3+ ions: 4 I 13/2 → 4 I 15/2 (1530 nm), 2 H 11/2 → 4 I 9/2 (1680 nm) and 4 S 3/2 → 4 I 9/2 (1720 nm), and the electronic transitions of Tm 3+ ions: 3 H 4 → 3 F 4 (1490 nm) and 3 F 4 → 3 H 6 (1810 nm).Meanwhile, the NPs produce the following effects: (i) increment in the ET quantum yield, resulting in an emission improvement, and (ii) variations in the oscillator strength (local crystal field), resulting in a blue/red band shift.The latter may change the line shape and broaden the NIR emission band.Furthermore, the most outstanding characteristic of glassy materials and more particularly tellurite glasses has arisen from their potential applications as optical fiber amplifiers or fiber laser covering the entire telecommunication window.

Experimental section
Glasses of 74TeO 2 -5ZnO-15Na 2 O-5GeO 2 -1Er 2 O 3 :xTm 2 O 3 (mol%) nominal composition with x = 0.05 or 0.06 mol% and containing 0.25 mol% of Ag or Au added in the form of chloride (AgCl or AuCl 3 ) were prepared by the traditional melt-casting technique.More details about the preparation are provided in a previous paper [29].For clarity, the studied glass samples are labeled as TxMy, where M represents the noble metal (S or G for silver and gold, respectively), x = 5 or 6 for the concentration of Tm 2 O 3 (0.05 or 0.06 mol%, respectively), and y is the annealing time at 300 °C (y = 2, 4, 6, 8, 10 and 12 hours) -for the samples without NPs, i.e. 5 hours.All the glass samples were prepared from high purity starting materials (3N and above).Silver or gold NPs were generated in the host matrix by thermal mobility according to the annealing time after the decomposition of the metal chloride during the glass melting [31][32][33]43,45].The final samples were cut into 10 × 30 × 1 mm 3 square pieces and polished until obtaining adequate transparency for the optical characterizations.
Dark-field microscopic images were obtained by an optical microscope (Olympus BX53 with a Darkfield dry condenser 10 × NA 0.8) coupled to a digital monochrome camera (Olympus XM10) and using a 75 W Xe lamp as a light source.A transmission dark-field objective (Olympus UPlanSApo 60x/1.2WNA 0.9 -NPLan) was used to focus the image of NPs embedded in the glasses.
The thermal properties of the glass samples were examined by differential scanning calorimetry (Netzsch DSC 404F3 Pegasus) in sealed Al pans at 10 °C/min heating rate.The density of the samples was measured by an MD-300S (Alfa Mirage) electronic densimeter and the Archimedes method using distilled water as the immersion liquid.Multiple density measurements were performed for each sample composition so as to obtain accurate values ( ± 5 mg/cm3 precision).
The transmission spectra were recorded from 350 to 2000 nm on a Varian Cary 500Scan UV-VI-NIR double beam spectrophotometer of ± 0.3 nm resolution.Steady-state luminescence spectra were obtained from a Nanolog spectrofluorimeter from Horiba Jobin Yvon equipped with a liquid-nitrogen-cooled Symphony InGaAs near-infrared detector.A pig-tailed diode laser at 976 nm (power 40 mW) was used as an external excitation source.The spectral slit width was 10 nm for the emission signal and the data acquired were corrected by instrumental factors.The samples were excited by a Pico Quant pulsed diode laser at 972 nm (model LDH-P-C) with pulse train of 60 ps and dead time of 20 ms for the measurements of the 4 I 13/2 excited state lifetime.The time resolved emission signal centered at 1535 ± 4 nm was measured by a Hamamatsu NIR-PMT module detector coupled to the Nanolog system by employing the method of Time Correlated Single-Photon Counting (TCSPC).Decay analysis software (DAS6) was used for obtaining the lifetime values by fitting a single exponential function.Measurements of both steady-state emission and lifetime were taken at room temperature.The laser beam was always collimated and focused at the center of the glass surface with a lens of 50 mm focal length.

Results and discussion
The glass transition temperatures Tg, determined for T5 and T6 samples are Tg = 311 ± 2°C and 309 ± 2°C, respectively.The annealing temperature used to generate and grow the NPs in both glasses was below their glass transition temperature Tg, at 300 °C.Such an experimental procedure was chosen so as to prevent a possible interaction between the glass network and the NPs mobile precursors [46], and a slow diffusion of the metallic Ag 0 or Au 0 particles [33].Besides, the glass transition temperatures of the samples containing NPs were also determined: Tg = 303 ± 2°C and Tg = 301 ± 2°C for T5Sy and T6Sy samples, respectively, and Tg = 305 ± 2°C and Tg = 304 ± 2°C for T5Gy and T6Gy samples, respectively.The measured density of the samples, the calculated number density N for the Er 3+ , Tm 3+ , Ag 0 and Au 0 species, and the average distance r a-b between them [33,43]

3+
-Ag 0 distance from the T5Sy and T6Sy samples are observed.The Ag 0 and Au 0 atoms are closer to the Er 3+ ions than to the Tm 3+ ions, and the Tm 3+ ions are closer to the Au 0 atoms than to the Ag 0 atoms.Besides, an increased r Tm

3+
-M 0 distance with the increases of the N Tm 3+ was observed.We have assumed that each Ag 0 or Au 0 atom has become a nucleation center and formed one NP during the annealing process (the growth is governed by the glass viscosity and the Ag 0 or Au 0 atoms mobility [45]) of specific size and shape.To maintain a thermodynamic and electronic charge equilibrium we can assume that the separation distance between RE and NP remains similar to the distance between RE ions and Ag 0 /Au 0 atoms.
In summary, we have observed that: (i) the introduction of the metallic NPs increases the separation distance between the RE ions; (ii) the separation distances between Er 3+ ions and SNP are lower than those of Er 3+ ions and GNP, independently of the Tm 3+ ions density N Tm 3+ , suggesting a larger interaction (coupling) of SNP:Er 3+ dipoles than GNP:Er 3+ , and in contrast; (iii) the separation distances between Tm 3+ ions and SNP are larger than those of Tm 3+ ions and GNP, suggesting a larger interaction of GNP:Tm 3+ dipoles in comparison to SNP:Tm 3+ .Dark-field illumination is a microscopy technique largely used by biologists to observe unstained samples by making them appear brightly lit against a dark background [47].The light scattered (transmitted) from the NP is directed to a camera image plane through an aperture of 0.1 mm diameter so as to reduce scattered light from other regions of the glass.The dark-field microscopic images obtained from T5S12 and T5G12 samples are shown in Fig. 1.From this, we can be observed that the size/shape and distribution those NPs are non-homogeneous inside the glasses, in agree with the literature [30][31][32][33][34][35][36][37].Although the GNPs or SNPs embedded in the glass do not show any fluorescence, the surface plasmon scattering they produce can be detected as a white light.The absence of fluorescence can be explained by the absorption of both Er 3+ and Tm 3+ ions present in the glass, which results in the colors emission turn-off of those NPs.The RE ions absorption bands can indeed overlap the NPs resonance wavelength, as verified in the glasses absorption spectra shown in Fig. 2. Nonetheless, from these dark-field images the different shapes of the NPs (spherical, elliptic and more complex) can be distinguished.Such an unorganized growth was predicted in [45], which demonstrates the optical properties of the NPs embedded in the glass can be explored.
The room temperature absorption spectra of the TxMy samples are shown in Fig. 2. Eight intense transitions from the ground state to excited states 4 I 15/2 →( 4 I 13/2 , 4 I 11/2 , 4 I 9/2 , 4 F 9/2 , 4 S 3/2 , 2 H 11/2 , 4 F 7/2 and 4 F 3/2 ) are attributed to the presence of Er 3+ ions and correspond to the bands centered at 1538, 978, 806, 649, 544, 520, 488 and 444 nm, respectively.Five subtle transitions from the ground state to excited states 3 H 6 →( 3 F 4 , 3 H 5 , 3 H 4 , 3 F 3 and 3 F 2 ) are also observed for the Tm 3+ ions and correspond to the bands centered at 1750, 1190, 785, 660 and 651 nm, respectively.Due to the small concentration of silver and gold (small atomic density N Ag 0 or N Au 0 ) and the overlap of the LSPR band with the Er 3+ or Tm 3+ ions absorption bands, no plasmon band is observed in the absorption spectra recorded in the TxMy glasses.Nevertheless, the activity of the plasmon band associated with the presence of GNPs or SNPs in tellurite glasses was demonstrated in our previous investigations into the growth and geometry dependence on the optical properties of LSPR modes [31,32,45].The luminescence spectra of the glasses in the 1400-2120 nm range under 976 nm laser diode excitation (with a diode power of 40 mW) are shown in Fig. 3.Note that the measurement reproducibility was carefully verified for all samples studied.
The oscillator strength (f fi ) of the trivalent RE ion is a fundamental physical quantity in analytical spectroscopy which determines the sensitivity of the 4f-4f transitions due to the chemical nature and symmetry of the ligand environment.This phenomenon is referred to as hypersensitivity transition and has been correlated with the coordination numbers and symmetries of the RE ions in the host matrix.On the other hand, LSPR creates electromagnetic resonance fields that can improve the local field [38].Such fields are formed through oscillating dipoles (from the NPs) around the RE ion, hence an additional oscillating electric field produced near the RE ion, which may induce strong changes in 4f-4f transitions [31] (e.g. a blue/red band shift or an enhanced/quenched luminescence).The induced oscillating dipoles depend on the size, shape and isotropic dipolar polarizabilities of the NPs environment.Therefore, evident variations occur in the luminescence spectra due to the NPs embedded in the glass, Fig. 3.In the first approximation, we can use the dynamic coupling mechanism [48][49][50] and the interaction energy between the NP:RE is given by:   According to our experimental conditions, we must consider that: (i) as the growth of the NPs is governed by the thermal treatment, NPs can have different geometrical ratios (e.g.ellipsoidal NPs), therefore the polarizability is [51] ( ) ( ) ( ) ( ) where principal axes a 1 , a 2 and a 3 introduce geometrical depolarization factors L i (for spherical particles, ε ω and NP ε are, respectively, the dielectric and NP permittivity as a function of the excitation frequency; (ii) we can assume a couple of particles (NP and RE ions) forming an electric dipole of −V/2 and + V/2 potentials, respectively [33]; (iii) the ET between Er 3+ :Tm 3+ is mediated by the NPs [38,43]; (iv) the ET from the RE ion to NP is more probable than the ET from NP to the RE ion, since the excited state lifetime of NPs is extremely shorter (ns order) in comparison with the higher energy excited states of Er 3+ and Tm 3+ ions (μs or ms order) [29,52]; (v) the luminescence quenching can be described by the ET between NP:RE and re-absorption by the NPs which are in resonance with the emissions of RE ions [33,38,43].Such considerations are summarized in Fig. 4.About the point (ii), in the case with multiple interactions at different distances between NPs:REs, the charge from the RE ion is the same, because the inc E  employed to excite these particles will be absorbed for the RE ion due to the crosssection absorption those is more than the NPs [Fig.2], case contrary the light incident (λ 0 ≠λ spp -wavelength surface plasmon resonance) is scattered for the NPs, and this scattered light is absorbed for the RE ion which emits and this coupled/resonates with the NP.
Several interesting aspects about the oscillator strength obtained by dynamic coupling mechanisms between NP:RE can be discussed.We can write the transition probability from any excited state transition decaying to background (2→1) as when inc E  is present and the states of the REs have opposite parity [31,38].Figure 3 shows the magnitude of the H DC effects on the crystalline potential as changes in the spectral lines, their widths, and shapes.In glasses, since different sites can be occupied by the RE ion, the Stark splitting is not well determined.However, here the manufactured process was strictly the same for all the samples, therefore the Stark splitting must be invariant in our samples.In this manner, the presence of NPs in the host matrix can modify the bandwidth of the Stark energy levels, which results in a blue/red shift and/or a broadening/increase in the RE ions emission as well as the emission intensity.A subtle increase in N Tm 3+ induces a decrease in the emission intensity of the Er 3+ and Tm 3+ ions due to the ET from Er 3+ to Tm 3+ ions (donor-acceptor) and a cross-relaxation process in which the quenching reduces the luminescence quantum efficiency of the Er 3+ -Tm 3+ ions [Fig.4].A similar behavior was observed by V.A.G.Rivera [29].Besides, such ET process between the Er 3+ to Tm 3+ ions still remains in the TxMy samples since the separation distance between the RE ions does not change with decreasing Tm 3+ ions concentration, Table 1.
On the one hand, in comparison to the T5 glass sample, the T5S08 sample shows 282% and 238% improvements in the emission intensity near 1535 and 1795 nm for the Er 3+ :( 4 I 13/2 → 4 I 15/2 ) and Tm 3+ :( 3 F 4 → 3 H 6 ) transitions, respectively.On the other hand, in comparison with the T6 sample, the T6S10 sample shows 233% and 248% improvements in the emission  In this scenario, we can use a modal approach similar to that developed in recent quantum theories that describes the propagation of surface plasmon created from the LSPR [53,54].We consider displacements varying in time as harmonic (damped) oscillators, i.e.
( ) . The optical properties of an NP are ruled by the collective excitation of its free electron cloud and create an electric dipole in the glass with polarizability α j .In this way, as described in [43], the ET between NP:RE could have either a positive or a negative impact for a specific electronic transition and depends on the efficiency of the coupling.Let us consider a set of orthonormal vector functions ( ) p q r (eigenfunctions) representing the amplitude of a harmonic displacement [54] that could be coupled with the RE electronic transitions.We can write ( )  q , resulting in a plasmon-photon coupling [Fig. 6],which can be verified as an enhancement/quenching of luminescence, a widening of the broadband emission and a change in its line shape, as observed in the luminescence spectra in Figs. 3 and  5.
Because of this process, the plasmon states on the RE ions are coupled, and the respective energies are modified, nevertheless, the occupation of a given plasmon mode on a given electronic level of the RE ion (Er 3+ or Tm 3+ , Fig. 6) will depend of the efficiency of the ET between NP and RE.Furthermore, when the higher energy levels of Er 3+ and Tm 3+ ions are coupled with the NP (the collective free oscillations with each resonance frequencyω p ), they can generate a white light, as initially observed in the NP in Fig. 1.
Figure 6 shows the schematic energy level diagrams in the vicinity of either an SNP [Fig.6(a)] or GNP [Fig.6(b)] of the Er 3+ -Tm 3+ ions doped tellurite glass.The excitation at 976 nm stimulates the Er 3+ ion from the ground level 4 I 15/2 to the 4 I 11/2 level and then to the 4 F 7/2 level through an excited state absorption process (ESA), in which the nonradiative decays populate the levels with lower energy of the Er 3+ ion.Likewise, the ET process from Er 3+ to Tm 3+ ions can populate the other energy levels of Tm 3+ ion and the levels of lower energy by nonradiative decays.Er 3+ :( 4 S 3/2 ) and Tm 3+ :( 3 H 4 ) excited states can also be populated by ET between two neighboring Er 3+ -Tm 3+ ions in the present system.Besides, the plasmon-photon coupling has been drawn as a red curved line (ET NP ) representing the displacement of a small volume charge  Another effect produced by the presence of NPs is the variation in the 4 I 13/2 lifetime, shown in Fig. 7, due to radiative/nonradiative ET after the incidence of the pump energy to excite the NPs.This means there occur an increment in the light absorption due to the presence of NPs [Fig.2] and results in a lifetime increase in the T5My and T6Sy samples in comparison with the T5 and T6 samples, respectively.A decrease in the lifetime is observed for the T6Gy samples and more particularly for T5G08, T5G10, T6G02 and T6G12, which also exhibit an expanded broadband [↔, Fig. 7] and higher emission intensity [↑, Fig. 7].In this manner, the increase or decrease of the lifetime is due to the presence of the NPs with a little or large damping ( 1/ NP γ τ = [33]), respectively, which favors or not the ET processes, Fig. 6.The T5G10 sample has shown improvements in both luminescence intensity and bandwidth broadening, which makes the sample an excellent candidate as an optical gain medium for potential applications in amplifying waveguides for plasmonic devices and other photonic nano-devices [4,55,56].
the T5Gy and T6Gy, and T5Sy and T6Sy samples and a clear increase in the r Tm
dipole moment induced in the NP by an incident electric field inc E  , i j r R −   is the separation distance between NP:RE, n j is the density of the conduction electrons inside a plasma characterized by carriers with charge N'e.

N 2
is the population in the excited level, h is the Planck constant and υ is the line frequency, and we have the following relation21 em I f ∝ .In this frame, the NPs contribute only to f 21

Fig. 4 .
Fig. 4. Schematic representation of the Er 3+ -Tm 3+ :NP interactions in two different scenarios: (a) the RE ions do not interact with each other, but with the NP and; (b) the interaction occurs between NP:RE and RE 1 :RE 2 .Equipotential surface (electric multipole coupling) with electric potentials −V/2 and + V/2 between NP:REs, 1,2 d

Table 1 . Measured and calculated physical properties of TxMy glass samples: glass density, number (ionic or atomic) density N and average distance between rare-earth- ions and metallic-atoms (M 0 ).
Vol. 22, No. 17 | DOI:10.1364/OE.22.021122| OPTICS EXPRESS 21131 coupling mechanism.Therefore, the states have different crystal-field representations, and more than one type of absorption/emission dipole can be simultaneously excited or emitted.