Near-infrared emitting colloidal solution of nanocrystals for multi-band optical amplification

Shengzhi Sun; Bin Qian; Zhenyan Wang; Xiaofeng Liu; Jianrong Qiu

doi:10.1364/OME.9.002523

1. Introduction

Rare-earth (RE) ions activated inorganic compound nanocrystals (NCs) have been paid growing attention because of their excellent optical properties, low toxicity and high stability against photo-degradation, which lead to their application in various fields [1–10]. RE ions, such as Pr³⁺, Er³⁺ and Tm³⁺, which are capable of emitting near-infrared (NIR) light in the communication wavelength region, have aroused special interest as they can be used as activator ions for fiber amplifiers and planar waveguide amplifiers [11–16]. Since the NIR emissions of aforementioned RE ions are all f-f transitions and their bandwidths are narrow in nature, they can not meet the requirement of broadband fiber amplifiers and tunable lasers in the NIR region, which are indispensable in dense wavelength-division-multiplexing (WDM) network systems [17–19]. At present, searching for a broadband NIR emitter has prompted intensive research into the optical properties of transition metal ions (such as Ni²⁺ and Cr⁴⁺), bismuth activated glasses and crystals [20–30]. However, the emissions of these ions are strongly correlated with the host. Their efficiencies as well as gain properties still can not rival that of RE-activated materials.

RE ions are still attractive NIR emitters with well-understood electronic transitions. To broaden the NIR emission of RE ions activated materials, one may simply think that this can be realized by means of codoping. However, this strategy fails in most circumstance. For emission of different RE ions in the same host, quenching often occurs due to interaction among different electronic transitions of the activators. To circumvent this problem, here we propose a simple strategy to realize simultaneous and efficient multi-band NIR emission from Pr³⁺, Tm³⁺ and Er³⁺ that covers the entire communication wavelength region (from O to U band) by employing a nanocrystals-solvent colloidal system. Within the colloid system, each NC contains only one type of NIR emitting ion, and different types of NCs which doped with different ions are spatially separated by distances long enough from each other, so that interaction among different RE ions that located in different NCs can be inhibited. With appropriate pumping, the colloid exhibits a multi-band NIR emission spectrum contributed from the respective transitions of RE ions located in different NCs. This NIR emitting colloid is expected to be a potential multi-band liquid gain medium for WDM system.

2. Experimental section

Synthesis of nanocrystals and the preparation of colloidal dispersion: Nanocrystals of RE doped NaYF₄ were synthesized by a solvothermal route using the liquid-solid-solution strategy [31]. In a typical procedure, 1.0 g NaOH was dissolved in a solution of 10 mL ethanol and 5 mL H₂O, to which 15 mL oleic acid was added subsequently, and stirred continuously until the solution become transparent. Then, aqueous solution of 1.0 mL RE³⁺ (0.5 M in total, in the form of RECl₃) and 2.5 mL (1.0 M) NaF were added into the above solution and vigorously stirred for another 20 min. The ratio of different RE ions Y:Yb:Er (Pr or Tm) varies according to the composition of the target nanocrystals. The milky solution was finally transfered into a Teflon-lined stainless steel autoclave and heated to 150°C for 240 min with a electric oven. After natural cooling to ambient temperature, the nanocrystals deposited at the bottom of the Teflon container was collected and washed with ethanol. Colloidal solutions were prepared by dispersing the as-synthsized nanocrystals in less-polar or non-polar organic solvent such as cyclohexane, benzene and carbon tetrachloride (CCl₄) under ultrasonic agitation for around 20 min. The as-prepared colloidal solutions are stable without noticeable precipitation (for 24 h) at concentrations up to 1.0 wt.%. For preparing solution with multi-band NIR emission, we just mix the three solutions that contain single type of NCs (NaYF₄:Yb, Er or NaYF₄:Yb, Tm or NaYF₄:Yb, Pr) and adjust their fractions to optimize emission spectrum.

Characterizations: Morphological observation of the NCs was performed with a JEM-200CX transmission electron microscope (TEM) equipped with a charge-coupled device (CCD) camera. The specimen for TEM observation was prepared by loading the NCs from the colloidal solution on a carbon-coated copper grid. Crystal structure of the NCs was examined with powder X-ray diffraction using a Rigaku D/MAX-RA system. Optical absorption spectra of the colloid with a NCs concentration of 0.5 wt.% (filled in a 1 cm × 1 cm × 5 cm quartz cuvette) was recorded by a JASCO V-570 spectrophotometer. Emission spectra were measured using a ZOLIX SPB-300 spectrofluorometer equipped with an InGaAs detector, which has a detective wavelength region from 800 to 1650 nm.

3. Results and discussion

In the present work, sodium yttrium tetrafluoride (NaYF₄) was chosen as the nanocrystalline host for RE ions. Since NaYF₄ has a relative low vibration energy, it allows RE ions to emit efficiently in the NIR spectral region. A solvothermal route based on a liquid-solid-solution strategy was employed here for the synthesis of NCs [31]. The experimental conditions, such as the reaction time and temperature, were carefully determined so as to prevent Oswald ripening during synthesis and to obtain a mono-dispersive size distribution of the NCs. We chose 150°C as the solvothermal temperature and keep it for 4 h. α-NaYF₄ NCs with a cubic crystal structure (space group Fm3 m, a = 0.548 nm) were obtained. The structure of NCs was revealed by X-ray diffraction (Fig. 5 in the Appendix).

From the transmission electron microscope (TEM) image (Fig. 1(a)), it can be clearly observed that the NCs crystallized into polyhedrons, mostly nanocubes, with a average size of approximately 25 nm (Fig. 1(b)). Furthermore, The surface of the NCs is decorated by a layer of long-chained alkyl groups formed in-site during the solvothermal synthsis [31–33], manifested by its favorable solubility in non-polar or less-polar organic solvent (such as cyclohexane, benzene and carbon tetrachloride). Here, we choose carbon tetrachloride (CCl₄) as the solvent for the NCs-colloid system, for it is optically inert in the entire visible and NIR range (400 ∼ 2500 nm) due to the low vibration energy of the C-Cl bond. The CCl₄ colloid of the NCs is optically transparent (Fig. 6 in the Appendix). Moreover, the colloidal system withstands a NCs concentration up to 1.0 wt.% without any discernable decrease in optical transmission and sedimentation of the NCs for over 24 h.

Fig. 1. (a) Typical TEM image of NaYF₄:RE NCs synthesized using the solvothermal route at 150°C for 4 h, and (b) size distribution of the NCs.

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RE ions of Pr³⁺, Er³⁺ and Tm³⁺ with NIR emissions covering O to U band, respectively, were selected as the activator ions. For the purpose of achieving multi-band NIR emission with the colloidal solution of RE-doped NCs, the first obstacle is caused by the fact that there is no electronic level with identical energy exists for these ions. Therefore, it is difficult to simultaneously excite the three different ions doped in different NCs to generate efficient NIR emission by single wavelength pumping. This problem is circumvented here by the use of an efficient sensitizer ion Yb³⁺, which has a large absorption cross section due to its ²F_7/2→²F_5/2 transition at around 980 nm. To maximize the fluorescence intensity from the NCs solution, in the first step we examine the dependence of NIR fluorescence intensity of NCs on the concentration of the sensitizer Yb³⁺ ion for the three different types of NCs doped with ion pairs of Yb³⁺-Pr³⁺, Yb³⁺-Tm³⁺, and Yb³⁺-Er³⁺ (Fig. 7 in the Appendix). The emission spectra are given in Fig. 2. For Yb³⁺-Pr³⁺ pair, energy transfer from ²F_5/2 to ¹G₄ takes place upon excitation at 980 nm, and this is followed by emission peaking at 1310 nm from Pr³⁺ by transition of ¹G₄→³H₅. Likewise, 1550 nm emission due to ⁴I_13/2 → ⁴I_15/2 of Er³⁺ occurs as a result of energy transfer from ²F_5/2 to ⁴I_11/2 after absorption of the 980 nm pumping light. The energy transfer process is quite different for Yb³⁺-Tm³⁺ pair, in which a consecutive excitation process occurs due to energy transfer from Yb³⁺ and excited state absorption: first the transition of ³H₆→³H₅, and then ³F₄→³F₂. Consequently, emission from Tm³⁺ comprises contributions from transition of ³H₄→³F₄ at approximately 1470 and ³F₄→³H₆ centered on 1620 nm (The maximum emission intensity for this transition is usually at around 1840nm, which is out of the detection range of the InGaAs detector used in this work) [34,35]. The spectral result shown in Fig. 2 indicates that the optimal concentrations of the sensitizer (Yb³⁺) is 5%, 50% and 20% for ion pairs of Yb³⁺-Pr³⁺, Yb³⁺-Tm³⁺, and Yb³⁺-Er³⁺, respectively. Hereinafter these nanocrystals with the optimal Yb³⁺ concentration exhibiting the strongest emission intensity are denoted as NC-YP, NC-YT, and NC-YE, respectively.

Fig. 2. Emission spectra of the colloidal solutions containing NCs of (a) NaY_(99.5-x)%Yb_x_%Pr_0.5%F₄ (x = 1∼10), (b) NaY_(99.5-x)%Yb_x_%Tm_0.5%F₄ (x = 5∼99), and (c) NaY_(99-x)%Yb_x_%Er_1%F₄ (x = 2∼50) upon excitation with a 980 nm laser diode. The electronic transitions corresponding to each of the emission are indicated in each figure.

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The second step involves the mixing of different NCs in a single colloid system with appropriate ratio to obtain the broadened NIR emission spectra. To clarify the feasibility of simultaneously observe NIR emission from all the three types of NCs in a solution, in which resonant energy transfer among different ions is negligible, we first give a preliminary analyses on the distance-dependent energy transfer process among different ions. From the Forster-Dexter model [36,37], the energy transfer rate from a donor to an acceptor is expressed by:

(1)$${P_{D - A}}(d - d) = \frac{{3{h^4}{c^4}{Q_A}}}{{4\pi {R^6}{n^4}{\tau _D}}}\int {\frac{{{f_D}(E) \cdot {F_A}(E)}}{{{E^4}}}} dE$$

where ${Q_A} = \int {\sigma (E)dE} $, $\sigma (E)$ is the absorption cross section of the acceptor, and the integral part stand for the spectral overlap between the emission band of the donor the and absorption band of the acceptor. According to Eq. (1), the energy transfer rate is inversely proportional to the ${\tau _D}$ (lifetime of the energy donor) and ${R^6}$($R$: distance between the two adjacent ions). With respect to the host lattice NaYF₄ (a = 0.548 nm), the nearest distance between two adjacent RE ions is around 0.387 nm [38,39], indicating strong interaction among different electronic transitions may occur within the space of approximately 15 × 10³ nm³ for a single NCs codoped with different RE ions, consequently leading to the quenching of emission. In comparison, for nanocrystals solutions, the average distance is estimated to be 150 nm (namely the average distance of two different RE ions is larger than 150 nm) for the colloid with a nanocrystals concentration of 0.5 wt.%. In such an environment, the interaction between different RE ions located in nearby nanocrystals can almost be neglected. These analyses are well supported by the spectral result given in Fig. 3. The NIR emission from all of the three RE ions is notably weakened for the nanocrystals codoped with ions of Yb³⁺-Pr³⁺-Er³⁺-Tm³⁺. Moreover, the emission for Tm³⁺ ions centered on 1470 nm and 1620 nm are completely quenched, probably due to NIR emission of Tm³⁺ involves a consecutive excitation process as compared with Pr³⁺ and Er³⁺ (Fig. 7 in the Appendix). A number of cross relaxation and energy transfer pathways among the four ions might be responsible for the quenching mechanisms [35]. In sharp contrast, as for the solution containing mixture of NC-YP, NC-YT, and NC-YE, all of the NIR emissions from electronic transitions of the three ions are preserved in the spectrum, implying that interaction between the three NIR emitting ions is effectively prevented.

To obtain an in-depth understanding of the interaction among RE ions for the colloid system, the decay profiles of the three different types of NCs-colloid systems are recorded at the emission wavelengths of 1310 nm, 1470 nm and 1550 nm that belong to radiative transitions of Pr³⁺, Tm³⁺ and Er³⁺, respectively (Fig. 4). It can be seen that the decay rates for the colloids containing single types of NCs and mixture of NCs are almost comparable, in striking comparison to the remarkably increased decay rates for the colloids containing NCs codoped with four types of ions. The decay of luminescence is characterized by an average lifetime that can be expressed by

(2)$$\tau = \int_0^\infty {t \cdot I(t )dt} /\int_0^\infty {I(t )dt} $$

where $I(t)$ stands for the intensity at time t. The obtained lifetimes of the solution containing mixture of NC-YP, NC-YT and NC-YE are far longer than that of the results obtained from the colloid contained the codoped NCs (Fig. 4). These results clearly indicate that the effect of nonradiative transition by means of cross relaxation among different ions dominants the decay process for the codoped NCs, whereas it is effectively suppressed for the solution containing mixture of NC-YP, NC-YT, and NC-YE.

Fig. 3. Emission spectra of solutions containing the three types NCs with a weight fraction ratio of NC-YP/NCYT/NC-YE = 20/30/1 (blue curve), and solutions containing NCs with the atomic concentration of NaY_0.88Yb_0.2Pr_0.005Tm_0.005Er_0.01F₄ (red curve). The colored background of the figure illustrating the communication wavelength region from O band (left) to U band (right).

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Fig. 4. Decay profiles recorded at (a) 1310 nm, (b)1470 nm and (c) 1550 nm, which belong to the emission of Pr³⁺, Tm³⁺ and Er³⁺, respectively. The black curves corresponds to the emission decays of colloidal solution containing a single type of NCs: (a) NC-YP, (b) NC-YT and (c) NC-YE; the red and the blue curves are the emission decays of colloidal solution containing mixture of three different types of NCs, and a single type of NC codoped with Yb³⁺-Pr³⁺-Tm³⁺-Er³⁺, respectively, corresponding to the emission spectrum in Fig. 3.

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To realize multi-band optical amplification in the communication wavelength region using the colloids containing NCs, it is indispensable to estimate the attenuation of light as a result of absorption and scattering. According to Beer-Lambert law the intensity of transmitted light can be expressed as:

(3)$${I_T} = {I_0} \cdot \exp [ - (\alpha + \sigma ) \cdot l]$$

where I_T and I₀ denote the intensities of the transmitted and the incident light, α is the absorption coefficient, σ is the turbidity which is caused by scattering. In the colloidal system, the concentration of the NIR emitting ions are estimated to be approximately 2.0 × 10¹⁷ ion/cm³ (for the solution with a NCs concentration of 0.5 wt.%), and the absorption cross section are in the magnitude of 10⁻²⁰ cm², therefore, the absorption coefficient is estimated to be ∼2.0 × 10⁻³ cm^-1. As of light scattering by colloidal system containing widely separated scattering centers, the total turbidity is given by:

(4)$$\sigma = ({2/3} )\cdot NV{k^4}{r^3}{(n\Delta n)^2}$$

where N is the nanocrystal number density, V is the volume of the nanocrystal, r is the radius of the nanocrystals (typically 20∼30 nm), k = 2π/λ (λ is the wavelength), n is the refractive index of the nanocrystal (approximately n = 1.6 for NaYF₄), and Δn is the index difference between the nanocrystal and the solvent [40,41]. Based on an estimation using the equation above, we found that the effect of scattering is almost negligible (σ∼10⁻⁷ cm^-1) compared with absorption in the NIR spectral region of 1000∼2000nm. Practically, the light attenuation for a colloidal solution (0.5wt%) with a thickness of 1 mm is around 1.3% - 2.5% in the NIR range, except for a few peaks due to the absorption by the capping agent (oleic acid).

In the final discussion, we made a preliminary calculation of the optical gain for the colloidal amplifier using the equation developed for the four-level system as express by

(5)$$g(l) = \frac{{{\sigma _e}{\tau _{Yb}}}}{{h{v_p}}}\frac{{{P_{abs}}}}{A}$$

The absorbed pump power P_abs can be written as

(6)$${P_{abs}} = {P_0}[1 - \exp ( - \alpha l)]$$

where σ_e is the emission cross section of the RE ions, and hv_p is the pump photon energy, and A is the cross section of the fiber core [42, 43]. We assume a fiber amplifier employing a capillary tube filled with the NCs colloid. For a fiber length of l = 2 cm, with the core diameter of 100 μm, the estimated optical gain is approximately in the range of 0.05∼0.5 dB/W at the peak wavelengths of 1310 nm, 1470 nm and 1550 nm. However, the presence of a large dip in the emission spectrum given in Fig. 3 centered at around 1360 nm indicate that broadband optical gain across the whole optical communication bandwidth is practically unattainable. These analyses suggest that the colloidal based capillary amplifier system might have the potential for practical multi-band optical amplification.

4. Conclusions

To summarize, we have investigated the NIR emission of colloids containing nanocrystals doped with ion pairs of Yb³⁺-Pr³⁺, Yb³⁺-Tm³⁺ and Yb³⁺-Er³⁺. The results indicated that interaction among different RE ions that lead to quenching of the NIR emission could be effectively inhibitted by solutions containing NCs singly doped with three different ion pairs. Our detailed analyses suggest that the multi-band NIR emitting solution of NCs mixtures, or an index-matched polymer waveguide incorporating the NCs, may find potential applications in multi-band optical amplifiers in the optical communication wavelength region.

Appendix

Fig. 5. Representative X-ray diffraction pattern of NaYF₄ nanocrystals synthesised at solvothermal conditions of 150°C and 4 h. All the diffraction peaks can be well indexed with a cubic crystal structure corresponding to JCPDS No. 77-2042. The XRD patterns of different RE ions doped NaYF₄ shows no observable change compared with the result obtained for non-doped NaYF₄ NCs.

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Fig. 6. Typical absorption spectrum of the nanocrystals-carbon tetrachloride colloid system with a concentration of 0.5 wt.% for a sample thickness of 1 mm. Before the measurement, we first recorded the baseline by using the same quartz cuvette filled with pure CCl₄. Therefore, in the transmission spectrum shown below, the optical loss by reflection at the air/quartz and quartz/CCl₄ interface is excluded and only scattering and absorption contribute to optical loss. The transitions of the RE ions are all invisible. The two absorption peaks located at approximately 1410 nm and 1700nm are caused by the surface capping agents (oleic acid) which used to stabilize the colloidal solution.

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Fig. 7. Part of the energy level diagrams illustrating sensitization process by Yb³⁺ for ion pairs of Yb³⁺-Pr³⁺ (a), Yb³⁺-Tm³⁺ (b) and Yb³⁺-Er³⁺ (c). The solid curves stand for the excitation process for RE ions, the dashed curves stand for energy transfer for Yb³⁺ to RE ions (RE = Pr, Tm or Er), and the dotted curves denote the phonon-assisted nonradiative transition between nearby electronic levels.

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Funding

National Natural Science Foundation of China (NSFC) (50872123, 51772270); State Key Laboratory of Precision Spectroscopy (SKLPS); State Key Laboratory of High Field Laser Physics; Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS) (SKL201706SIC); National Key R and D Program of China (2018YFB1107200).

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Near-infrared emitting colloidal solution of nanocrystals for multi-band optical amplification

Abstract

1. Introduction

2. Experimental section

3. Results and discussion

4. Conclusions

Appendix

Funding

References

Cited By

Figures (7)

Equations (6)

Optical Materials Express