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Publicly Available Published by De Gruyter May 18, 2019

1.5 μm photoluminescence and upconversion photoluminescence in GeGaAsS:Er chalcogenide glass

Vit Prokop, Lukas Strizik, Jiri Oswald, Milan Vlcek, Ludvik Benes, Spyros N. Yannopoulos, Bozena Frumarova and Tomas Wagner

Abstract

The paper reports on ≈1.5 μm Stokes photoluminescence (PL) emission and upconversion photoluminescence (UCPL) emission in the visible and near-infrared spectral region in Er3+-doped Ge25Ga8As2S65 chalcogenide glasses at pumping wavelengths of 980 and 1550 nm. The ≈1.5 μm PL emission spectra are broadened with increasing concentration of Er ions which is discussed in terms of radiation trapping and UCPL dynamics affecting the Er3+: 4I13/2 level lifetime. The UCPL emission was observed at ≈530, ≈550, ≈660, ≈810 and ≈990 nm and its overall intensity as well as red-to-green UCPL emission intensity ratio increases with increasing Er concentration. To explore the UCPL dynamics we measured double logarithmic dependency of green (≈550 nm) and red (≈660 nm) UCPL emission versus pump power at pumping wavelength of 975 nm. Moreover, we measured quadrature frequency resolved spectroscopy (QFRS) on green UCPL emission (≈550 nm) using 975 nm pumping wavelength and various excitation powers. The QFRS spectra on green UCPL were analyzed in term of QFRS transfer function for three-level model from which we deduced energy transfer upconversion rate w11 (s−1) originating from Er3+: 4I11/2, 4I11/24F7/2, 4I15/2 transitions.

Introduction

Amorphous chalcogenides doped with rare-earth (RE) ions are promising materials in many applications due to their unique properties [1], [2], [3] such as low phonon energy, high refractive index, broad transparency from the visible to the mid-infrared spectral region with relatively good solubility of RE3+ ions in chalcogenide glasses (ChG) containing Ga [4], Al [5], In [6]. ChG can be relatively easily prepared as bulks or thin films and in variety of shapes [1].

Among the most studied and promising chalcogenide hosts for RE3+ doping to achieve the visible UCPL belong sulfides as Ge-Ga-S [7], Ge-Ga-Sb-S [8], Ge-Ga-As-S [9], and Ga-La-S [10]. Compared to Ge-Ga-S glasses, the glasses of the Ge-Ga-As-S system show improved glass-forming ability [9] and thermal stability [8], [11] with still good solubility of RE3+ ions [9]. The Ge-Ga-Sb-S glasses have higher thermal stability compared to Ge-Ga-As-S however, their transparency is markedly reduced in the visible spectral region [8], [12], [13]. Studied materials show potential for optical fibers [14], optical fiber amplifiers [15], non-linear optical waveguides [16], laser devices [17], [18] or chemical sensors and detectors [19], [20].

In this article we present photoluminescent and structural properties of Er3+-doped Ge25Ga8As2S65 (GGAS) chalcogenide glasses as promising candidates for UCPL from the green visible to the near-infrared spectral region. We measured emission spectra and lifetimes of ≈1.5 μm PL Stokes emission and UCPL emission spectra at pumping wavelengths of 980 and 1550 nm. Moreover, the green UCPL emission (≈550 nm) at 975 nm pumping wavelength and various pump powers was investigated by QFRS to deduce the energy transfer upconversion rate parameter w11 for 0.01 and 0.5% Er-doped GGAS glasses.

Experimental

GGAS: x at% Er3+ chalcogenide glasses, where x=0.01–1 at%, were synthesized by the melt-quenching technique from high purity elements of Ge (5N), Ga (5N), As (5N), S (4N) and Er (3N). The synthesis was carried out in the rocking furnace at 1050°C for 24 h. Melt was quenched into water and ampoule with raw glass was subsequently annealed near the glass transition temperature (Tg–20)°C for 3 h to relax the mechanical strain. After that, the glass samples were slowly cooled down to room temperature, cut into ≈1 mm thick rectangles and both-side polished into the optical quality.

The amorphous state of Er3+-doped GGAS glasses was studied by X-ray diffraction (XRD) analysis using diffractometer Bruker AXS D8 Advance with Cu radiation in the range of 2θ from 10° to 70°. The chemical composition of prepared samples was determined by the energy dispersive X-ray (EDX) microanalyzer IXRF System with a detector GRESHAM Sirius 10 and at accelerating voltage of the primary electron beam ≈20 kV. The room temperature Raman spectra were measured by the Raman FRA-106 Bruker as a part of the FT-IR spectrophotometer IFS 55 using the excitation source of the YAG: Nd3+ laser (λexc≈1064 nm) in the 50–200 cm−1 spectral region. The Raman spectra were reduced by the method of Gammon and Shuker [21] and then decomposed by a sum of eight Gaussians using the Fityk program v. 0.9.8 [22].

The refractive index nG of the glasses was analyzed by the variable angle spectroscopic ellipsometry VASE®, J.A. Woollam Co., Inc. in the spectral region of 500–2300 nm measured with a step of 20 nm and at angles of light incidence 65°, 70° and 75°. Ellipsometric data were fitted to the Sellmeier model, nG2=A+Bλ2/(λ2C) [23]. The concentration of Er3+ ions ρ (cm−3) in glassy samples was calculated from density measurements and chemical composition. Transmittance T was measured by double-beam UV-Vis-NIR spectrophotometer JASCO V-570 in the spectral region of 400–1800 nm with step of 1 nm. The absorption cross-section σa spectrum was calculated by the equation σa=ln10 [−logT+log (1−R)]/ρd, where R is reflectance determined from refractive index as R=[(nG+1)/(nG−1)]2 and d is sample thickness in cm units. The absorption cross-section spectrum of 0.5% Er-doped GGAS glass was used for application of Judd–Ofelt (JO) theory to calculate the intensities of Er3+ intra-4f electronic transitions. Er3+ ≈1.5 μm PL emission spectra were acquired at pumping wavelength of 980 nm, the UCPL emission spectra in the spectral region of 400–1050 nm were acquired at 980 nm (~9 W cm−2) and 1550 nm (~13 W cm−2) excitation wavelengths using the diode lasers.

The Er3+: 4I13/2 level lifetimes (at λ≈1535 nm) for Er-doped GGAS glasses excited by 980 nm were analyzed by time-resolved spectroscopy from 1/e value or by using the Eq. 1:

(1) N 1 = N 1 ( 0 ) e k 1 t + N 2 ( 0 ) β k 2 k 1 k 2 ( e k 2 t e k 1 t ) ,

where Ni(0) is the initial population density, ki is relaxation rate at level 4I13/2 (i=1) and 4I11/2 (i=2), β is branching ratio of the 4I11/24I13/2 transitions and t is time. Equation 1 takes into account the delayed population of the 4I13/2 energy level from the pumped 4I11/2 level.

The QFRS spectra on green UCPL emission (λ≈550 nm) were measured at pumping wavelength of 975 nm and at various pump powers using the QFRS system specified in the previous articles [7], [24], [25], except that we replaced the focusing lens to focal length of ≈25 mm and we used photomultiplier (PMT) tube Hamamatsu H10723-20 as a detector preset to 0.7 V. The QFRS spectra were acquired at various pump powers ranging from 0.79 to 42.5 mW. Pump power P≈1 mW corresponds to photon-flux density Φ~2.9×1021 cm−2 s−1. The excitation power dependence of green (≈550 nm) and red (≈660 nm) UCPL emission was measured by the same instrument using the lock-in detection at chopping frequency of 30 Hz.

Results and discussion

Figure 1a presents the XRD diffractograms of 0.5 and 1.0% Er-doped GGAS samples. The samples doped with 0.5 and less at% of Er3+ ions are amorphous however, the 1 at% Er-doped sample is partially crystalline as indicate the sharp diffraction lines in Fig. 1a. This is also suggested by SEM images of GGAS samples doped with 0.5 or 1 at% of Er ions (Fig. 1c and d) where the presence of at least two phases in heavily Er-doped sample is observed. The presence of crystallites in 1 at% Er-doped GGAS sample is also featured by sharp-shaped Er3+: 4I13/24I15/2 (≈1.5 μm) emission spectrum presented in Fig. 1b due to appearance of Stark levels thus indicating that the crystalline phase occurs in close proximity to the Er3+ ions [26], [27].

Fig. 1: 
          Effect of Er content on glass-forming ability.
          (a) XRD diffractograms, (b) Er3+: 4I13/2→4I15/2 (~1.5 μm) PL emission spectra under 980 nm excitation and (c, d) SEM images for the respective 0.5 at% and 1 at% Er-doped GGAS samples.

Fig. 1:

Effect of Er content on glass-forming ability.

(a) XRD diffractograms, (b) Er3+: 4I13/24I15/2 (~1.5 μm) PL emission spectra under 980 nm excitation and (c, d) SEM images for the respective 0.5 at% and 1 at% Er-doped GGAS samples.

The experimentally determined chemical composition of all Er-doped GGAS samples shown in Table 1 agrees well with theoretical composition. However, as presented by SEM image of Fig. 1d, the heavily Er-doped sample contains two phases: dark phase and light phase. The chemical composition of dark phase corresponds to theoretical chemical composition of prepared glasses however, the light phase is Er-enriched phase (see Table 1). Thus, the Er solubility limit in Ge25Ga8As2S65 glasses lies between 0.5 and 1 at% of Er ions which agrees well with the previously published work [27], where for the homogenous distribution of RE ions in chalcogenide glasses the Ga/RE atomic ratio is proposed to be ≥10.

Table 1:

The chemical composition of the synthesized GGAS:Er samples determined by the EDX spectroscopy.

Er in Ge25Ga8As2S65 Chemical composition
Ga (at%) Ge (at%) As (at%) S (at%) Er (at%)
0.01 at% 7 26 2 65
0.1 at% 7 26 2 65
0.5 at% 7 26 2 65
1 at% (light phase) 4 12 1 64 19
1 at% (dark phase) 7 26 2 65

  1. The experimental error is ±1 at%.

The structure of the GGAS glass was investigated by the Raman scattering. A representative reduced Raman spectrum shown in the Fig. 2 was decomposed to 8 Gaussian-type lines centered approximately at 159, 281, 331, 345, 375, 412, 439 and 485 cm−1. An attempt to interpret the origin of these lines to specific vibrational modes is based on previous works [28], [29], [30], [31], [32], [33]. The bands at 159, 345 and 412 cm−1 can be assigned to corner-sharing ν4, ν1 and ν3 vibrations of the [GeS4] tetrahedra, while contributions forming the corresponding modes of the [GaS4] tetrahedra cannot be excluded. The band at 375 cm−1 is assigned to ν1-[GeS4] and [GaS4] vibrations of edge-shared tetrahedral units. This line is practically the companion ν1c vibration mode related to edge-shared tetrahedral of [GeS4] at 370 cm−1. The band at 281 cm−1 can be assigned to the vibrational mode of the S3Ge(Ga)-Ge(Ga)S3 structural units containing homonuclear (semi)metal–(semi)metal bonds. The band centered at 331 cm−1 could be possibly assigned to ν1-[GaS4] vibrations of tetrahedral units. The band at 439 cm−1 is still an unsolved issue, but could be probably assigned to companion ν3c vibration modes related to edge-shared tetrahedral units [GeS4] and [GaS4]. The band around 485 cm−1 has been assigned to the existence of disulfide bonds (or two-membered S chains) in form of S2As-S-S-As2S and S3Ge(Ga)-S-S-Ge(Ga)S3.

Fig. 2: 
          Reduced Raman spectrum of the GGAS glass. Open circles are experimental data, bold black line is sum of Gaussians used for spectrum decomposition.

Fig. 2:

Reduced Raman spectrum of the GGAS glass. Open circles are experimental data, bold black line is sum of Gaussians used for spectrum decomposition.

Figure 3 presents absorption-cross section spectrum of Er3+ in GGAS glass composed of seven Er3+ absorption bands. These bands are attributed to ground state absorption (GSA) transitions from Er3+: 4I15/2 to Er3+: 4I13/2 (≈1552 nm), 4I11/2 (≈991 nm), 4I9/2 (≈817 nm), 4F9/2 (≈666 nm), 4S3/2 (≈552 nm), 2H11/2 (≈530 nm) and 4F7/2 (≈491 nm) levels. The inset of the Fig. 3 shows the dispersion of refractive index of GGAS:Er3+ measured by spectroscopic ellipsometry in transparent spectral region. The refractive index is practically identical for all studied samples (nG≈2.10 at λ=1550 nm), except that of 1 at% Er-doped sample which was partially crystalline and therefore was not analyzed.

Fig. 3: 
          GSA cross-section spectrum of the 0.5 at% Er-doped GGAS glass. Inset: Dispersion of refractive index for GGAS glasses doped with 0.01, 0.1 and 0.5 at% of Er.

Fig. 3:

GSA cross-section spectrum of the 0.5 at% Er-doped GGAS glass. Inset: Dispersion of refractive index for GGAS glasses doped with 0.01, 0.1 and 0.5 at% of Er.

Figure 4a and b show normalized absorption and emission spectra of the Er3+: 4I13/24I15/2 (λ≈1.5 μm) transitions in GGAS glasses at pumping wavelength of 980 nm. The ≈1.5 μm emission band is broadened with increasing concentration of Er ions, while the absorption band of all studied glasses samples exhibits no significant changes in shape. For better understanding of this behavior, lifetimes of Er3+: 4I13/2 energy level as a function of Er concentration in GGAS glasses were analyzed. The decay curves of the 4I13/24I15/2 transitions under 980 nm pumping wavelength are presented in Fig. 4c. The Er3+: 4I13/2 level lifetime τ=1/k1 was analyzed by fitting of decay curves in Fig. 4c using the Eq. 1 with fixed parameters of k2=696 s−1 and β=0.136 which were obtained by Judd–Ofelt theory with inclusion of multiphonon relaxation rate [24]. Lifetime values determined by using Eq. 1 or from 1/e values are presented in the inset of Fig. 4c. It can be seen that lifetimes determined from 1/e values are notably higher than those obtained by using the Eq. 1 nevertheless, they holds same trends. The Er3+: 4I13/2 lifetime value τ≈2.76 ms determined by Eq. 1 for 0.01% Er-doped GGAS sample matches better the lifetime τJO≈2.16 ms calculated by Judd–Ofelt theory than that τ≈3.35 ms determined from 1/e value. The 4I13/2 lifetime is firstly prolonged with increasing Er concentration from 0.01 to 0.1 at% and subsequently decreases with further increase of Er content. However, the mechanism behind this is still unclear. The lifetime may be prolonged e.g. due to radiation trapping processes [34], [35], [36] or by cross-relaxation CR2 processes Er3+: 2H11/2, 4I15/24F9/2, 4I13/2 [37], [38]. Contrary to that, the lifetime shortening may be explained by the concentration quenching [39] or by the Er3+: 4I11/2, 4I13/24F9/2, 4I15/2 energy transfer upconversion ETU2 processes [40], [41], [42], [43]. The considerable presence of cross-relaxation and energy transfer upconversion processes is discussed later in the manuscript.

Fig. 4: 
          Er concentration dependence on the 4I15/2↔4I13/2 transitions.
          (a, b) Are respective normalized emission and absorption spectra of the Er3+: 4I13/2↔4I15/2 transitions in GGAS:Er glass under 980 nm pumping wavelength. (c) The decay curves of Er3+: 4I13/2→4I15/2 transitions in GGAS glass under 980 nm excitation; inset: 4I13/2 lifetimes determined from 1/e values τ1/e or by using the Eq. 1 τEq.1 with maximal error of ±0.05 ms. (Color online).

Fig. 4:

Er concentration dependence on the 4I15/24I13/2 transitions.

(a, b) Are respective normalized emission and absorption spectra of the Er3+: 4I13/24I15/2 transitions in GGAS:Er glass under 980 nm pumping wavelength. (c) The decay curves of Er3+: 4I13/24I15/2 transitions in GGAS glass under 980 nm excitation; inset: 4I13/2 lifetimes determined from 1/e values τ1/e or by using the Eq. 1 τEq.1 with maximal error of ±0.05 ms. (Color online).

Upconversion photoluminescence (UCPL) emission spectra at pumping wavelengths of 980 or 1550 nm are presented in Fig. 5. There are observed five emission bands originating from electronic transitions from Er3+ upper levels of 2H11/2 (≈530 nm), 4S3/2 (≈550 nm), 4F9/2 (≈660 nm), 4I9/2 (≈810 nm) and 4I11/2 (≈990 nm) to Er3+: 4I15/2 ground level. The overall UCPL emission intensity as well as red-to-green UCPL emission intensity ratio, i.e. ratio of the red UCPL emission intensity of the Er3+: 4F9/24I15/2 (≈660 nm) transitions to that of the green UCPL emission intensity of the Er3+: 2H11/2/4S3/24I15/2 (≈530+550 nm) transitions, increases with increasing Er3+ concentration. Such behavior may be explained by the exploration of the UCPL dynamics.

Fig. 5: 
          UCPL emission spectra of the Er3+ doped GGAS chalcogenide glasses excited. (a) By the 980 nm or (b and c) 1550 nm laser. (Color online).

Fig. 5:

UCPL emission spectra of the Er3+ doped GGAS chalcogenide glasses excited. (a) By the 980 nm or (b and c) 1550 nm laser. (Color online).

There are three basic UCPL mechanisms adopted in literature [44]: (I) ground state absorption (GSA) followed by excited state absorption (ESA), i.e. GSA/ESA; (II) ground state absorption (GSA) followed by energy transfer upconversion (ETU), i.e. GSA/ETU; and (III) ground state absorption (GSA) followed by cross-relaxation (CR) processes, i.e. photon avalanche (PA) upconversion. The GSA/ESA mechanism may originate within single Er3+ ion thus, it is independent on the Er–Er interionic distance [44]. In contrast, the GSA/ETU or PA UCPL requires energy transfer between two neighboring Er3+ ions therefore they are dependent on the interionic distance which is usually adjusted by concentration of Er3+ ions inside of a host matrix [44]. Since the green UCPL emission intensity increases rapidly and nonlinearly with increasing concentration of Er3+ ions, the additional GSA/ETU mechanism to GSA/ESA may stay behind the UCPL dynamics in highly Er-doped GGAS glasses, i.e. mixed GSA/ESA+ETU [44], [45]. Thus, we investigated the green UCPL dynamics (λ≈550 nm) using the QFRS for three-level model approximation (see Fig. 7) at pump wavelength of 975 nm and various pump powers P (mW) or photon-flux densities Φ (cm−2 s−1). Further details can be found in the recently published paper [24].

Fig. 6: 
          The double logarithmic plots of green (≈550 nm) and red (≈660 nm) UCPL emission intensity vs. pump power at pumping wavelength of 975 nm.

Fig. 6:

The double logarithmic plots of green (≈550 nm) and red (≈660 nm) UCPL emission intensity vs. pump power at pumping wavelength of 975 nm.

Fig. 7: 
          QFRS spectroscopy.
          
            P-evolved QFRS spectra of the green UCPL for the (a) 0.01 at% and (b) 0.5 at% Er-doped GGAS glasses measured under 975 nm excitation. (c) Φ-evolved lifetimes τ1 and τ2 for 0.01 and 0.5 at% Er-doped GGAS glasses under 975 nm pumping wavelength; inset: Detail of Φ-evolved τ2.

Fig. 7:

QFRS spectroscopy.

P-evolved QFRS spectra of the green UCPL for the (a) 0.01 at% and (b) 0.5 at% Er-doped GGAS glasses measured under 975 nm excitation. (c) Φ-evolved lifetimes τ1 and τ2 for 0.01 and 0.5 at% Er-doped GGAS glasses under 975 nm pumping wavelength; inset: Detail of Φ-evolved τ2.

The double logarithmic dependency of UCPL emission intensity versus pump power is presented for green and red UCPL emission in Fig. 6 and comprises of two linear segments, where the slope of each segment provides the number of photons involved in the UCPL process [44]. Theoretically, the slope n of low-power linear segment is n=2 for both GSA/ESA and GSA/ETU UCPL mechanisms, and it scales as n=1–2 for GSA/ESA or n=1 for GSA/ETU processes at high-power region [44]. The low-power and high-power linear segments are divided by a kink point Pk (or Φk) which was found to be ~3 mW for green UCPL and ~2.8 mW for red UCPL. The experimentally determined slope values n≈2 at P<Pk indicate a presence of two-photon UCPL processes.

The energy transfer rate parameter w11 (s−1) of the Er3+: 4I11/2, 4I11/24F7/2, 4I15/2 transition for green UCPL may be analyzed by QFRS on green UCPL at pump powers below Pk in combination with three-level model [24], [44]. Figure 7 shows the P-evolved QFRS spectra of the green UCPL for the 0.01 (a) and 0.5% (b) Er-doped GGAS glasses measured from pump powers P~0.79 to 42.5 mW or photon-flux densities from Φ~2.3×1021 to 1.2×1023 cm−2 s−1. The QFRS spectra plotted as a function of lifetime τ (s) in logarithmic scale are double-peaked, where long-lifetime component τ1 corresponds to Φ-evolved relaxation rate R1(Φ)=1/τ1(Φ) at the intermediate level 4I11/2 and short-lifetime component τ2 to Φ-evolved relaxation rate R2(Φ)=1/τ2(Φ) at the upper coupled manifolds 4F7/2/2H11/2/4S3/2. The lifetime distributions were obtained by deconvolution of QFRS spectra by a linear combination of two Gaussians. The τ1 component is rapidly shortened with increasing Φ from ≈1.14 to ≈0.43 ms while τ2 component is less sensitive to change of Φ with lifetime value of ~10 μs (Fig. 7c). Theoretically, the presence of mixed GSA/ESA+ETU process is featured by significant growth of the peak at τ2 component compared to peak at τ1 component with increasing Φ [24] which was observed experimentally in heavily 0.5 at% Er-doped GGAS sample (see Fig. 7).

Moreover, from QFRS spectra presented in Fig. 7 may be observed that the short lifetime τ2 rapidly decreased in heavily Er-doped GGAS sample. This may be explained by the closer proximity of fundamental absorption edge to thermally coupled Er3+ energy levels 4F7/2/2H11/2/4S3/2 (see Fig. 3) allowing the energy transfer processes from Er3+ ions to a host matrix [24], [25]. At certain low Φ, the energy transfer parameter w11 may be analyzed by the ratio γ (Φ→0) expressed by Eq. 2, which is ratio between peak a2 at the short-lifetime component τ2 and peak a1 at the long-lifetime component τ1 [24]:

(2) γ ( 0 ) = ( 1 k 1 k 2 ) ( 1 + 2 w σ 0 k 1 σ 1 ) 1 k 1 k 2 ,

where k1=R1(Φ→0) and k2=R2(Φ→0) are relaxation rates at the intermediate and upper coupled energy levels, respectively, σ0 is GSA cross-section and σ1 is ESA cross-section. We approximated the γ(0) ratio utilizing the parameters a1, a2, k1, k2 analyzed from the QFRS spectra at Φ<Φk (P~0.79 mW). To derive the energy transfer rate w11 (s−1) from ratio γ(0), the knowledge of σ0 and σ1 is required as well. These parameters were obtained by using Judd–Ofelt (JO) theory [46], [47], [48], [49], [50] with resulting JO intensity parameters Ω2=(13.02±0.07)×10−20 cm2, Ω4=(3.41±0.08)×10−20 cm2 and Ω6=(1.48±0.03)×10−20 cm2 from which were subsequently calculated σ0=6.6×10−20 cm2 and σ1=10.9×10−20 cm2 values. We found that the parameter w11=40 s−1 for the 0.01% Er-doped GGAS sample and w11=2960 s−1 for the 0.5% Er-doped GGAS glass. These parameters are slightly higher than those obtained for GeGaS:Er glasses [24] which is however attributed to energy transfer processes from Er3+: 4F7/2/2H11/2/4S3/2 levels to a host matrix due to their merging [24], [25] as is evident in Fig. 3. This is also supported by the extraordinary low values of short-lifetime component [25] τ2≈33 μs for 0.01% Er-doped sample and τ2≈26 μs for 0.5% Er-doped sample at the lowest Φ in comparison with lifetime value obtained by JO theory of τ2(JO) ≈168 μs.

The increase of red-to-green UCPL emission intensity under 980 nm pumping wavelength with increasing Er concentration Fig. 8a may be explained on the basis of energy level model approximation depicted in Fig. 8b in accordance with Refs. [25], [40], [41], [42], [43], [51], [52], [53], [54], [55] as following. At low Er concentrations, the ETU or CR processes may be neglected [44]. Thus, the green emitting levels 4F7/2/2H11/2/4S3/2 are populated mainly via GSA/ESA processes, while the Er3+: 4F9/2 energy level may be populated by the radiative and/or nonradiative recombination from excited Er3+: 4F7/2/2H11/2/4S3/2 levels or by the radiative and/or nonradiative recombination from Er3+: 4I11/2 to 4I13/2 followed by ESA process Er3+: 4I13/24F9/2 [25]. As a result, the green UCPL emission dominates over red UCPL emission at low concentrations of Er ions, as was observed in Fig. 8a. However, at higher concentrations of Er ions, the ETU and CR processes become pronounced and may significantly modify above mentioned dynamics. In the case of green UCPL emission, there is proposed Er3+: 4I11/2, 4I11/24F7/2, 4I15/2 energy transfer [24] which was studied by the QFRS mentioned above. This ETU process significantly promotes the green UCPL emission intensity with increasing Er concentration. However, in the case of red UCPL emission, the red-emitting level 4F9/2 may be populated by the Er3+: 4I11/2, 4I13/24F9/2, 4I15/2 energy transfer and Er3+: 4F7/2, 4I11/24F9/2, 4F9/2 CR1 processes [43], [53], [54], [55] or by the Er3+: 2H11/2/4S3/2, 4I15/24I9/2, 4I13/2 CR2 followed by ESA2 or ETU2 processes [42], [51], [52]. Since 3–4 photons are required to obtain green (~530+550 nm) and red (~660 nm) UCPL emission at pumping wavelength of 1550 nm, the UCPL mechanism is more complex. In low Er3+-doped samples, the green-emitting levels (2H11/2, 4S3/2) are populated via the sequential GSA/ESA 4I15/24I13/24I9/22H11/2/4S3/2 transitions and the red-emitting level 4F9/2 by the subsequent radiative or nonradiative recombination 2H11/2/4S3/24F9/2 or by the phonon-assisted 4I15/24I13/24I9/24I11/2+phonon→4F9/2 GSA/ESA transitions. In contrast to 980 nm excitation wavelength, the red-to-green UCPL emission intensity ratio is higher for samples excited by 1550 nm which suggests the presence of phonon-assisted GSA/ESA processes mentioned above leading to population of red-emitting level Er3+: 4F9/2. The presence of the phonon-assisted process “4I9/24I11/2+ phonon” is also supported by the higher UCPL emission intensity of the Er3+: 4I11/24I15/2 (~990 nm) transitions compared with the Er3+: 4I9/24I15/2 (~810 nm) UCPL (see Fig. 5c). In heavily Er-doped samples the red UCPL emission dominates over the green UCPL emission which may be attributed to the presence of additional energy transfer Er3+: 4I13/2, 4I11/24I15/2, 4F9/2 populating the red-emitting level 4F9/2. Such energy transfer process should depopulate the Er3+: 4I11/2 level thus decrease the Er3+: 4I11/24I15/2 (~990 nm) UCPL emission intensity which is really lower in comparison with the Er3+: 4I9/24I15/2 (~810 nm) UCPL intensity in the 0.5% Er-doped GGAS glass (see Fig. 5c). Moreover, the UCPL emission spectrum for 1% Er-doped sample in Fig. 5c shows higher UCPL emission intensity at ~990 nm compared with those at ~810 nm which suggests the possible presence of additional cross-relaxation processes Er3+: 4I13/2, 4S3/24I11/2, 4F9/2 and Er3+: 4I9/2, 4S3/24F9/2, 4F9/2 all populating the red-emitting level 4F9/2 [56], [57], [58], [59]. So mostly, the red-emitting level is populated at the expense of population of green emitting level under the both 980 and 1550 nm excitation wavelengths [60]. Therefore, red-to-green UCPL emission intensity ratio increases with increasing Er concentration.

Fig. 8: 
          Green and Red upconversion photoluminescence.
          (a) Red-to-Green UCPL emission intensity ratio versus Er3+ concentration in GGAS at pumping wavelength of 980 and 1550 nm, (b) Schematic of energy-level diagram for green and red UCPL emission at pumping wavelength of 975 nm; the curved arrows indicate the multiphonon relaxation routes; Ni represent population densities with perturbed population densities ni at energy level i originating from a sinusoidal modulation φ of continuous-wave photon-flux density Φ; β is branching ratio under three-level model approximation. (Color online).

Fig. 8:

Green and Red upconversion photoluminescence.

(a) Red-to-Green UCPL emission intensity ratio versus Er3+ concentration in GGAS at pumping wavelength of 980 and 1550 nm, (b) Schematic of energy-level diagram for green and red UCPL emission at pumping wavelength of 975 nm; the curved arrows indicate the multiphonon relaxation routes; Ni represent population densities with perturbed population densities ni at energy level i originating from a sinusoidal modulation φ of continuous-wave photon-flux density Φ; β is branching ratio under three-level model approximation. (Color online).

Conclusions

We studied structural properties, ≈1.5 μm Stokes and upconversion photoluminescence in Er-doped Ge25Ga8As2S65 glasses at pumping wavelengths of 980 and 1550 nm. Since the 1 at% Er-doped GGAS sample was partially crystalline with Er-enriched phase and those doped 0.5 at% remained amorphous, it may be deduced that the Er solubility limit in studied glasses lies between 0.5 and 1 at%. The broadening of the Er3+: 4I13/24I15/2 (λ≈1.5 μm) emission spectrum under 980 nm excitation and change of 4I13/2 lifetime with increasing concentration of Er ions was discussed in terms of radiation trapping and upconversion processes. Upconversion photoluminescence was observed from visible green to near-infrared spectral region at pumping wavelength of 980 and 1550 nm. The overall UCPL emission intensity as well as red-to-green UCPL emission intensity ratio increases with increasing Er concentration which was discussed by changes of the UCPL dynamics including the energy transfer and the cross-relaxation processes at higher content of Er ions. The green UCPL emission (λ≈550 nm) was studied by the QFRS spectroscopy under 975 nm excitation and at various pump powers in term of QFRS transfer function for three-level model from which the energy transfer upconversion parameter w11 was deduced. The double-peaked QFRS spectra are composed of two lifetime components where the long-lifetime component τ1≈1.14–0.43 ms reflects the relaxation at the intermediate level Er3+: 4I11/2 while the short lifetime component τ2 of several tens of μs reflects the relaxation rate at coupled energy levels 4F7/2/2H11/2/4S3/2. The green UCPL dynamics in 0.01 at% Er-doped GGAS glass is mainly driven via the GSA/ESA processes while that in 0.5 at% Er-doped sample contains mixed GSA/ESA+ETU processes which is manifested by the greater energy transfer upconversion rate parameter w11=2960 s−1 for 0.5% Er-doped samples compared to negligible ETU rate w11=40 s−1 for 0.01% Er-doped sample. Such ETU rate parameters are slightly higher than those reported for GeGaS:Er glasses which is explained by the additional energy transfer processes from Er3+ ions to a host matrix due to merging of Er3+: 4F7/2/2H11/2/4S3/2 green emitting levels with fundamental absorption of a host matrix. Nevertheless, the GeGaAsS:Er glasses are promising candidates for UCPL applications as they have improved glass-forming ability and thermal properties compared to GeGaS:Er glasses and higher visible transparency than GeGaSbS:Er glasses.


Article note

A collection of invited papers based on presentations at the 13th International Conference on Solid State Chemistry (SSC-2018), Pardubice, Czech Republic, September 16–21, 2018.


Acknowledgements

This work was supported by the project CZ.1.05/4.1.00/11.0251 and by grant LM2015082 Center of Materials and Nanotechnologies from the Czech Ministry of Education, Youth and Sports of the Czech Republic and European Regional Development Fund-Project High sensitive sensors and low density materials based on polymeric nanocomposites – NANOMAT (No. CZ.02.1.01/0.0/0.0/17_048/0007376). Authors thank to Prof. Takeshi Aoki (Joint Research Center of High-Technology, Tokyo Polytechnic University, Japan) for development and perfected QFRS on UCPL, to Dr. Cyril Koughia (University of Saskatchewan, Canada) for discussion on radiation trapping processes and to Dr. Roman Svoboda (University of Pardubice, Czech Republic) for the differential scanning calorimetry measurements.

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Published Online: 2019-05-18
Published in Print: 2019-11-26

© 2019 IUPAC & De Gruyter, Berlin/Boston

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