1. Introduction

Quantum dots (QDs) have size-dependent photoluminescent properties from visible to infrared wavelengths. In particular, IV-VI semiconductors such as lead chalcogenides have large exciton Bohr radii and therefore are suitable for use in tunable optoelectronic devices such as color converters and optical amplifiers [1,2]. As hosts for QDs, glass materials are attractive because of their high mechanical strengths, chemical stabilities and ease of fabrication.

Several methods to grow QDs in glasses have been reported. The most common way is thermal treatment of bulk glass doped with precursors; this method exploits diffusion-controlled phase decomposition of over-saturated solid solutions [3]. However, precise control of the size and spatial distribution of QDs within the matrix remains a challenge. Several other methods such as ion implantation [4] and femtosecond laser irradiation [5,6] have been reported with limited success. Recently, PbS QDs have been precipitated by beaming a 532-nm continuous-wave (cw) laser at glasses that contained Ag nanoparticles (NPs) [7]; this method enables site-selective precipitation of QDs in glasses.

Attempts were also made to convert these glasses into waveguide structures. For example, ~1 m lengths of fiber were drawn from a glass rod that contained PbS QDs [8], but uncontrolled growth of QDs during the drawing process could not be easily suppressed at the temperature used (~750 °C). Planar waveguides have been fabricated by ion exchange of PbS-doped glasses [2], but half of the waveguide had to be exposed to air and therefore suffered from unwanted environmental influence.

We report use of a cw laser to fabricate channels containing PbS QDs for use in buried waveguides. The buried channels were fabricated with no post-heat treatment and short processing time and due to the precipitation of PbS QDs the glass develops graded refractive index n for light-guiding and emission in the near-infrared. This range is compatible with a wide tunable spectral range in telecommunication windows. The channels can be expected to function as active waveguides.

2. Experimental procedure

Composition (mol%) of the glass was 50SiO₂-25Na₂O-10BaO-5Al₂O₃-8ZnO-2ZnS with additional 0.8 PbO. Source powders were melted in an alumina crucible at 1350 °C for 30 min, then quenched by pouring onto a brass mold. Glasses were then annealed at 350 °C for 3 h. A glass plate with a thickness of 4 mm was polished using suspensions of 0.05-μm alumina grains, then dipped into molten AgNO₃ at 270 °C for 1 min to incorporate Ag⁺ ions into the both side of the glass. Then one of two sides of the specimen was polished away to allow transmission of laser light into the specimen for channel fabrication. The glass was heat-treated at 400 °C for 3 h to precipitate Ag NPs by thermal reduction of Ag⁺ ions [9]. Absorption spectra were recorded to identify the formation of Ag NPs by detection of the surface plasmon resonance (SPR) peak with a UV/Vis/NIR spectrophotometer (Lambda 750, Perkin Elmer, USA).

A cw laser beam (Millennia Pro 60s, Spectra-Physics, USA) of λ = 532 nm was shone at the glass using a setup in Fig. 1. The laser beam was irradiated from the opposite side of the ion-exchanged region and focused on the region that contained Ag NPs. The focal point of the beam was then moved to the interior of the glass specimen by moving the stage at 6 μm•s⁻¹. Laser intensity was fixed at 0.6 W with beam diameter < ~30 μm.

 

Fig. 1 A schematic drawing of irradiation process. The stage is moved parallel to beam direction with a speed of 5 ~8 μm•s⁻¹

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Uniformity of the channels was examined under an optical microscope. Photoluminescence (PL) spectra were recorded using a lateral excitation with λ = 800 nm from a Ti-sapphire laser (Coherent 890 turnable laser, Coherent, USA) pumped by the 532-nm cw laser. A combination of mechanical chopper, monochromator, InSb detector, and lock-in amplifier was used to record the PL spectra. Nanocrystals in the channel were examined using a scanning electron microscope (SEM, JEOL, JSM-7401F, Japan) with a back-scattered electron detector and energy dispersive spectrometer (EDS). A portion of the channels at ~40 μm from the center was further examined using a transmission electron microscope (TEM, JEOL, JEM-2200FS, Japan). The profile of n in the channels was measured using a preform analyzer with a resolution of 0.0002 (P106, Photon Kinetics, USA). We made a rod of 15 cm in length and 1.4 cm in diameter and the specimen for the measurement was prepared by immersing into the epoxy resin.

3. Results and discussion

During Ag⁺ exchange and heat treatment at 400 °C, the glass specimen changed from yellow to brown. An absorption band observed at ~425 nm is associated with the localized SPR as shown previously [7]. Irradiation of the focused laser beam onto the ion-exchanged region caused precipitation of PbS quantum dots [7]. Precipitation of PbS quantum dots in glasses is associated with the diffusion-controlled over-saturated solid solution. At high temperatures, the over-saturated precursors form quantum dots through nucleation and subsequent growth mechanisms [3]. Ag NPs seem to absorb the laser energy by SPR, and to re-radiate the energy as heat, which enables the movement of atoms, so PbS quantum dots precipitated at the irradiated spot [7]. As the focal point of the beam was moved inside the glass in Fig. 1, the initial black spot expanded following the direction of the beam. Several scan speeds between 5 ~8 μm•s⁻¹ were used; when the scan speed was 6 μm•s⁻¹, the channel could be made as long as the 4 mm that was allowed by our experimental set-up. Channel diameters were 36 ~52 μm, and were uniform except at the ends in Fig. 2. The channel with the most uniform diameter was obtained with a scan speed of 6 μm•s^-1 while others resulted in relatively large fluctuations in diameters. in Fig. 2. In non-irradiated region, any change of specimen color was not observed.

 

Fig. 2 Optical microscope image of the channel formed with a scan speed 6 μm•s⁻¹. Due to the limitation of the setup, maximum total length of the channel was 4 mm.

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The cross-sectional area of the channel was examined using a backscattered electron image in Fig. 3(a). A black area in the center appeared to be a pure glassy phase, and white particles appeared and became increasingly common with distance from the center of the channel. These particles were identified as PbS by fast Fourier transformation analysis of the TEM image in Fig. 3(b) and compositional distribution using EDS in Fig. 3(c). The measured distances of 0.345 and 0.291 matched well with the interplanar distances of the (220), (110) planes of PbS crystals, respectively (0.3429, 0.2969, JCPDS #03-8293). The fast Fourier transformation (FFT) was indexed based on these assignments and was consistent with the diffraction pattern of bulk PbS crystal along the $[0\overline{1}1]$ zone axis. Furthermore, analysis of the composition shows strong signals of Pb and S from a white particle in Fig. 3(c). Closer to the center the white particles had diameter (D) ~120 nm, and As distance from the center of increased, the D decreased to below the resolution of SEM, and appear as a white rim in Fig. 3(a). This trend is consistent with the hypothesis that the high temperature near the center of radiation forces the elements to diffuse away [9]. Formation of NPs in the channel can cause increase in n following the Maxwell-Garnett equation that was designed for composite materials that consist of homogeneous glass and embedded small semiconductor spheres with diameters less than the light wavelength [10]. In Fig. 4(a), n inside the channel increased by as much as ~0.002 due to precipitation of PbS. Measurements were repeated 5 times and the error bar was ~± 0.0006. Contribution of Ag ⁺ ions on the change of refractive index seems minimal mainly because of limited diffusion of Ag ⁺ ions in a short time (< 7 min). The PL spectrum in Fig. 4(b) recorded from the channel shows a band with a peak wavelength at ~1290 nm. As evidenced from the large size distribution of the crystals in SEM micrograph in Fig. 3, the broadness of the PL band is most likely due to the large size distribution of PbS QDs.

 

Fig. 3 (a) Cross-sectional area of a channel formed with 6 μm•s⁻¹ scan speed at the center. The image was recorded by back-scattered electron detector of Scanning electron microscope. White particles: nanoparticles. (b) Transmission electron microscopic image and fast Fourier transformation pattern of QD (c) An enlarged image of a white particle closer to the center; Inset: the compositional profile around the white particle measured by EDS (d) Expanded image of the area marked as a rectangular shape in (a).

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Fig. 4 (a) Refractive index profile. The channel with 6 μm•s⁻¹ was located at the center (x = 0). (b) Photoluminescence spectrum from the channel excited laterally at 800 nm.

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The mechanism responsible for channel growth along the glass matrix has been considered. PbS QDs formed under 532-nm cw laser irradiation only when Ag NPs were present [7]. Similarly, the channel was not initiated unless the laser beam was focused on the ion-exchanged side where Ag NPs had precipitated. However, the channel can be extended by moving the focusing point of the laser beam to the interior of the glass where no Ag NP is present. One possible explanation for these observations is that Ag⁺ diffuse through the channel to form Ag NPs, but this explanation is unlikely because the mechanism requires that an Ag ⁺ ion should diffuse along the full 4 mm length of the channel in 7 min. Another possibility is that phases such as PbO₃ and PbSO₄ form on the PbS powders due to oxidation [11], absorb the laser beam and reradiate it as heat [12]. But this explanation is also unlikely because formation of these particles must be difficult inside the oxide glass matrix since the glass matrix lacks free oxygen and free sulfur. Furthermore, the 532-nm laser beam cannot be significantly absorbed by the glass matrix, because it is transparent at this wavelength. Steady-state excitation in QDs shows blinking behavior and photodarkening because non-radiative Auger recombination to the defect centers usually appears on the surface of NPs [13]. Therefore, one possible mechanism for formation of PbS QDs inside the channel away from the Ag NPs is that photon energy is converted to heat energy by this non-radiative process on the surfaces of QDs.

The QDs had broad size distributions due to the Gaussian distribution of beam intensity which lead to the formation of NPs that had larger radius than exciton Bohr radius. Nevertheless, cw-laser-induced channels due to precipitation of PbS QDs inside glasses grow controllably in a relatively short time. These channels may be exploited as buried waveguides.

4. Conclusions

Cw laser irradiation (λ = 532 nm) was used to form buried optical channels containing PbS QDs in glass. A channel with a diameter of 36 ~52 μm and length up to the 4-mm limit of our experimental setup was formed when the laser scan speed was 6 μm•s⁻¹. PL centered at λ = 1290 nm was recorded from the channel and the change in n was ~0.002. Ag NPs contributed to the initial precipitation of PbS QDs, but subsequent precipitation of PbS QDs in the forming channel is probably due to the heat generated by recombination at defects on the surface of QDs. These channels can be used as active buried waveguides, if the size distribution of PbS QDs inside the channel can be controlled more precisely.