1. Introduction

Colloidal nanoparticles (NPs) have been the focus of intensive research as they exhibit unique properties, which lead to their various applications in areas like biology, energy conversion, and photonics. Among different type of NPs, semiconductors quantum dots (QDs) have been paid special attention because the quantum confinement effect provides them with exceptional optical properties, making them strong candidates for a variety of photonic applications, such as low-dimensional optical amplifier and laser medium. Up to the present, optical gain and laser actions in a wide spectrum range have been realized with metal chalcogenides QDs incorporated in planar waveguide structures and colloidal solutions [1–3]. However, due to their toxicity [4], and vulnerability under continuous pumping that may result in optical photo-degradation [5–6], application of QDs for commercial use still seems impractical at present. Compared with QDs, rare earth (RE) doped NPs is environmentally benign and do not undergo photo-degradation under continuous excitation. These advantages together with the long radiative lifetimes of RE ions make them potential nano-scaled candidates for optical gain and laser medium [7–9]. Recently, optical gain at 1530 nm and stimulated emission at 863 nm have been demonstrated with planar waveguides containing 20wt%-50wt% of Yb³⁺-Er³⁺, and Nd³⁺ doped fluorides nanocrystals [10, 11], respectively. However, unlike these polymer-based or sol-gel derived planar waveguide that are capable of incorporating large amount of NPs (usually >10wt%), for colloidal solution system it is difficult to disperse such a high concentration of NPs, meanwhile maintain high stability and low light loss due to Rayleigh scattering by NPs. In addition, the strong quenching effect of hydroxy and organic groups will reduce markedly the emission intensity especially in the near-infrared (NIR) region. To our knowledge, there is still no report on colloidal solution amplifier or laser medium activated with RE doped NPs. In this article, we present experimental result of optical amplifier at the communication wavelength (1550 nm) with a transparent colloidal solution of Yb³⁺-Er³⁺ codoped NaYF₄ nanocrystals. The doping concentration of RE ions and the concentration of NPs in the solution were optimized so as to achieve efficient NIR emission. This colloid may also find applications in micro-fluidic devices, and might be a potential substitute to the widely used dye solution as liquid laser medium that is free of photo-bleaching for high power laser systems.

2. Experimental

The NPs of Yb³⁺-Er³⁺ codoped NaYF₄ (a host of low vibration energy) were synthesized by a solvo-thermal route [12]. In a typical process, 10 ml aqueous solution (0.05 M) of rare earth nitrate was added to a mixture of 10 mL ethanol, 10 mL oleic acid (OA) and 0.6g NaOH. After mild stirring for 30 min, the milky solution was transferred to a Teflon-lined stainless steel autoclave (50 mL) and heated at 160°C for 5 h. After natural cooling to ambient temperature, the nanocrystals deposited at the bottom of the Teflon bottle was washed with ethanol and collected by centrifugation. The obtained nanocrystals can be readily redistributed in organic solvents like cyclohexane, benzene and carbon tetrachloride (CCl₄). In the present work CCl₄ was used as the solvent based on following three factors: (1) it does not exhibit absorption bands in NIR region because of its low fundamental vibration energy, (2) it eliminates any residual H₂O that serves as an effective quencher for NIR emission, and (3) it has a relatively high refractive index compared with other non-polar solvents and therefore minimizes loss of light due to Rayleigh scattering. The phase composition and the morphology of the products were characterized by X-ray diffraction (XRD, Rigaku D/MAX-RA) and transmission electron microscopy (TEM, Hitachi Model H-600), respectively. A quartz vessel of 10×10×50 mm³ filled with the colloid solution of the NPs was used for the spectroscopic measurement and optical amplification experiment. The absorption and the emission spectra were measured with a Jasco V570 UV/VIS/NIR spectrophotometer, and a Zolix SBP 300 spectrofluorometer equipped with an InGaAs detector, respectively.

3. Results and discussion

Figure 1(a) shows the X-ray diffraction (XRD) pattern of the Yb³⁺-Er³⁺ codoped NaYF₄ NPs. All the diffractions peaks can be well indexed to a cubic structure of Fm3m (PDF No. 39-0724). Figure 1(b) presents the TEM images of the nanocrystals. It is observed that all of the NPs crystallized into well-defined nanocubes with an average dimension of approximately 25 nm×25 nm, as shown in Fig. 1(c). With a more detailed observation we find that the NPs are covered with thin sheath of approximately 3 nm in thickness. This might be the OA layer formed in situ during solvo-thermal synthesis and attached to the surface of the nanocrystals, leaving its alkyl chains outside. As a result, the NPs are provided with hydrophobic surfaces and became soluble and stable in different organic solvents.

 

Fig. 1. Structural characteristics of the Yb3⁺-Er³⁺ codoped NaYF₄ nanoparticles: (a) XRD pattern, (b) typical TEM image, insert is an image of a single nanocube covered by thin sheath of OA, and (c) size distribution of the nanoparticles obtained by counting the size of approximately 500 separate nanoparticles.

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The nanocubes were dispersed in CCl₄ to form a stable colloid solution with a concentration of 1.0wt%. This colloid is highly transparent as observed by naked eyes and remain stable without of noticeable precipitation of NPs for over 48 h. The concentration of 1.0wt% for NPs-colloid system was carefully determined considering the balance between the transparency and NIR emission intensity of the colloid. Figure 2 shows the transmission spectrum of the colloid solution in the wavelength region of 200 nm - 2000 nm. The absorption peak due to the Yb³⁺: ² F _7/2→² F _5/2 transition that located at around 980 nm can be clearly identified. In contrast, the f-f transition peaks of Er³⁺ can not be observed because of its low concentration and absorption section. Other weak peaks located at around 1400 nm, 1700 nm, and 1800 nm can be assigned to the double-frequency of the different vibration modes of alkyl chains [13, 14] that cover the surfaces of the nanocubes as have been confirmed by TEM in Fig. 1(b).

 

Fig. 2. Transmission spectrum of the colloidal solution containing 1wt.% Yb³⁺-Er³⁺ codoped NaYF₄, insert is the graph of a quartz vessel (10×10×50 mm³) filled with the nanocube solution.

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Fig. 3. Emission spectra of the solution containing 1wt.% of NaY_80%-x%Yb20%Er_x%F₄ (x=0.5, 1, 2, 5, 10) upon excitation by a 980 nm LD, insert shows the decay curves of colloids with Er³⁺ concentration in the nanocubes of x=2, and 10.

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The dependence of the NIR emission intensity on the doping concentration of Er³⁺ due to its ⁴ I _13/2+⁴ I _15/2 transition was measured with the excitation at 980 nm, and the results are given in Fig. 3. For the Yb³⁺-Er³⁺ ions pair, the 980 nm pumping light is absorbed mainly by Yb³⁺ through the transition of ² F _7/2→² F _5/2 as it has a high absorption cross section at this wavelength compared with Er³⁺. Afterwards, energy transfer from Yb³⁺ to Er³⁺ occurs, resulting in the population of ⁴ I _11/2 of Er³⁺. The emission at 1550 nm then occurs after Er³⁺ relaxes nonradiatively to its ⁴ I _13/2 level. Here, for the Yb³⁺-Er³⁺ codoped NaYF₄ (NaY_80%-x%Yb_20%Er_x%F₄), we fixed the Yb³⁺ concentration at 20%, and changed the Er³⁺ concentration (x%) from 0.5% to 10%. We obtained the highest emission intensity with Er³⁺ concentration of 2% with a full width at half maximum (FWHM) of 85 nm (approximately 360 cm^-1 or 0.044 eV), and this colloid was used for the demonstration of optical amplification. A higher Er³⁺ concentration results in a decrease in emission intensity, and an increase in the decay rate of the NIR luminescence. This behavior can be understood by considering self-absorption of Er³⁺ ions, and enhanced possibility of cross relaxation between nearby Er³⁺ ions that leads to a high nonradiative rate under high Er³⁺ concentration. The average lifetimes estimated by (where I(t) stands for the intensity at time t [15]) are 1.22 ms and 0.41 ms for colloidal solutions of nanocubes with the Er³⁺ concentration corresponding to x=2 and x=10, respectively. These lifetimes are close to that of the Er³⁺ doped LaF₃ nanocrystals reported by Veggel et al. [7]. Additionally, the decays curves shown in Fig. 3 deviate notably from single exponential decay function, suggesting that the radiative transition is accompanied by rapid nonradiative process probably due to strong coupling between the nanocubes and the different vibration modes of the solvent and the organic adsorbents.

 

Fig. 4. Optical amplification with the nanocube solution: (a) experimental setup for the measurement of optical gain: (1) 980 nm LD as the pumping beam, (2) 980 nm LD as the signal beam, (3) chopper, 200 Hz, (4) sample, quartz vessel filled with the nanocube solution, (5) filter, (6) InGaAs detector, and (7) digital oscilloscope, M1 and M2 are mirrors, and L1, L2 and L3 are lens; (b) comparison of the intensity for the amplified and the signal beam; (c) plot of gain coefficient as a function pumping power at 980 nm, and (d) plot of gain coefficient as a function of signal power.

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A traditional two-wave mixing method was employed for the measurement of optical amplification at 1550 nm of the colloid, as given in Fig. 4(a). The pump beam from a 980 nm laser diode (LD) and the signal beam from a 1550 nm LD was coupled and focused to the quartz vessel filled with the nanocube solution. The amplified signal was detected with an InGaAs detector and displayed in a digital oscilloscope. From Fig. 4(b) it is obvious that efficient optical gain is obtained for the pumping power of 200 mW and signal power of 0.3 mW. We define the internal gain coefficient as G =1/L· Log(I / I ₀′ ) , where L is the thickness of the solution, I is the intensity of the amplified signal, and I ₀′ = I ₀ /T ( I ₀ : signal intensity without pumping, and T: transmission). Figure 4(c) presents the dependence of gain coefficient on the pumping power of the 980 nm LD. The gain coefficient increases rapidly with the pumping power up to approximately 200 mW, where it reaches the highest value of 0.58 cm^-1, corresponding to a net gain of 0.58 dB for the colloid of 10 mm thickness in the present experimental condition. Further increase of the pumping power results in the gain saturation of solution. The gain saturation of the Yb³⁺-Er³⁺ system have been found in glasses as well as planar waveguide matrix, and it has been generally related to the excitation depletion, propagation loss, and nonlinear optical effects, such as enhanced upconversion of Er³⁺ and Yb³⁺ ions, whose intensity is proportional to the square of pumping density [10, 16]. As for the present case, the low concentration of the nanocubes in the solution (1wt% for nanocubes corresponding to about 0.17wt% for Yb³⁺ in the solution) that lead to saturation of absorption of the 980 nm pumping light may also play a key role in the resulting low saturation threshold of the solution. The factors responsible for gain saturation should be further elucidated so as to increase the gain coefficient of the colloidal solution. Figure 4(d) presents the relation of optical gain as a function of signal power. It can be seen that the optical gain decreases monotonously with the increase of the signal power, and optical gain can still be observed with signal power up to 3.0 mW. No optical gain is observed at the signal power higher than 3.0 mW because under this condition the optical loss and amplification reach a balance. Based on the above experimental results, we expect that laser action could be realized from the colloid by employing a similar system applied for the widely used dye lasers. Such investigation is in progress in our lab.

4. Conclusion

In conclusion we have realized successfully optical amplification at 1550 nm from a colloidal solution containing Er³⁺-Yb³⁺ codoped NaYF₄ nanocubes synthesized by a solvo-thermal route. We observed the highest optical gain coefficient at 1550 nm of 0.58 cm^-1 with the colloidal amplifier at the pumping power of 200 mW at 980 nm. The results implies that RE-doped nanocrystal solution represents a new family of liquid laser medium that is advantageous over dye solutions as well as colloidal quantum dots due to its high photo-stability as well as low toxicity.