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CdSe/ZnS core-shell quantum dots have been embedded within microstructured polymer optical fibres. The emission properties of quantum dots within fibres have been explored to show that variation in concentration, sample length and pumping regimes effects the emission characteristics of these quantum dots.
©2009 Optical Society of America
1. Introduction
Polymethylmethacrylate (PMMA) is a transparent thermoplastic material with many uses in the opto-electronic and photonics industries. Due to its good optical and thermal properties, it is often used in the manufacture of micro- and nano-devices including semiconductor nanotube formation [1], nanoimprinting lithography [2], polymer optical fibres (POF) [3] and as a host matrix for electroluminescence devices [4]. Importantly, its properties can be modified and/or enhanced by ‘doping’ with a range of materials, including dyes [5], nanoparticles [6, 7], rare-earth compounds [8] and quantum dots (QD) [9]. The latter are semi-conductor nanocrystals whose diameter is smaller than that of the exciton Bohr radius. As such, QD can provide a wide range of desirable optical properties such as narrow emission spectra, non-linear properties, broad excitation spectra and an emission wavelength which can be readily ‘tuned’ by varying the characteristics and dimensions of the dots [10].
Dopants can also be incorporated into low-cost polymer optical fibres (POF) with the (relatively) low fabrication temperatures, allowing a diverse range of dopants to be embedded without being damaged. A versatile variant here are microstructured polymer optical fibres (mPOF) which guide light by a pattern of air channels that run the length of the fibre and are designed to achieve total internal reflection and low transmission losses [11–13]. By combining the mPOF concept with the use of such dopants, a powerful framework is available for tailoring the material properties of the final fibre [6, 7]. This makes the QD-mPOF combination an ideal platform for the studying of QDs and QD-doped PMMA as we are able to readily alter both the material composition and the microstructure to tailor various fibre properties such as numerical aperture to capture the maximum amount of emission and guide it to the detector. Potential application of quantum dots in POF include, current sensors using magneto-optical effects [14], biosensors [15], oxygen [16] and temperature sensors [17].
Here we report on the impact on QD emission properties brought about by changes in material thickness (fibre length), dopant concentration, and laser type (pulsed or CW). Evaluating the emission properties will provide better understanding of the effects of QDs embedded in PMMA thus allowing potential devices to be explored.
2. Fabrication of quantum dot doped fibres
The dopants used were CdSe/ZnS core-shell QD (EvidentTech) with a peak emission of either 520 or 620 nm (measured in a toluene suspension); the crystal diameters are 2.4 and 5.2 nm respectively. CdSe/ZnS QDs were chosen because their emission spectra lie within the visible wavelengths, which coincide with the low attenuation window of PMMA. PMMA (0.5 mm particles; Goodfellow) was ground to around 50 μm using a ball mill operating at 1800 rpm and mixed with either of the QD. The mixture was ground for a further 2 minutes to ensure homogeneity and to evaporate any residue toluene associated with the QD. It should be noted that the latter grinding of the doped powder has to be done with some care to minimise damage to the QD shell which results in the presence of a secondary emission [18], shown in Fig. 1(a) . Thus Fig. 1 compares the emission characteristics of doped PMMA powder before grinding, after gentle grinding (~120 rpm), and after intense grinding (~1680 rpm).
Fig. 1 (A) shows the emission for a rod fabricated with doped powder containing damaged QD from excessive grinding; (B) shows the emissions for a rod where the doped powder underwent gentle grinding; (C) show the emission from a doped powder before any subsequent grinding. The change in emission peak is due to different sample thickness. The QDs used were Cd/Se core, Zn/S shell dots with peak emission at 620 nm (in toluene).
Once a doped powder was prepared, it was placed in a mould to which heat and vacuum were applied so as to fuse the mixture into a 5mm diameter rod. For each QD, rods were fabricated in which the dopant concentrations on a weight basis were 0.0145, 0.025, 0.1 and 0.2%.
These doped rods were used to create a preform that was then drawn to fibre. The first step involved stacking 34 capillaries (each 5mm in diameter) between two hollow PMMA tubes, the smaller having 40/46 mm internal/external diameters with the latter having 60/70mm diameters as shown in Fig. 2 . This arrangement was then stretched down to an external diameter of 12.5mm. A QD-doped rod was inserted to form the preform which was drawn to fibre. This fibre contained a doped core surrounded by a microstructured cladding. During the fibre fabrication process, vacuum was applied to prevent air gaps from forming at the interface between the doped rod and the inner wall of hollow tube. Simultaneously pressure was applied to the capillaries to allow independent control over the size and shape of the cladding microstructure. Expanding the capillaries in this way eliminates any interstitial holes between the capillaries and the two containment tubes, resulting in very thin PMMA ‘bridges’ connecting the inner and outer tubes, as shown in Fig. 3 .The preform was drawn at around 250 °C with the fibres having an external diameter of 380 μm, a (doped) core diameter of 160 μm and a ‘bridge’ thickness of order 1 μm. Figure 3 shows a selection of fibres, noting that this particular design was chosen because of its high numerical aperture (NA), estimated to be ~0.3 in the visible [19], ensuring that the maximum amount of light emitted from the QD is captured and guided to a detector.
Fig. 2 The QD-doped rod was inserted into the centre of the microstructured cladding to form a preform that was drawn to fibre.
Fig. 3 QD-doped, high NA fibres. (A) 620 nm emission QD at 0.0145 wt %; (B) 620 nm emission QD at 0.025 wt %; (C) 520 nm emission QD at 0.1 wt %; (D) 520 nm emission QD at 0.2 wt %. The cracks are due to the fibre cleaving process and do not traverse the length of the fibre.
3. Fibre characterisation
Two laser sources were used for the characterization of the fibres. Either a 400 nm femtosecond laser (Spectra Physics Hurricane emitting 100 fs pulses at 1 kHz at 800nm with a maximum pulse energy of 1 mJ plus a frequency doubling unit) or an argon ion laser, 488 nm line, (Spectra Physics; model 177-G02) was used to excite the QD in the doped fibres. Each laser was coupled into the fibre core using a 25x objective lens while the emission from the fibre was coupled via a connecting fibre to a spectrometer (Ocean Optics) using a 10x objective lens. A 495 nm long-pass filter was placed at the fibre output to filter out any residue ‘pump light’ from the laser.
Fibre samples containing varying (520 nm QD) dopant concentrations were excited by pumping with the femtosecond laser employing an average power of 1.45 mW. Dopant levels of 0.0145, 0.1 and 0.2 wt % showed that an increase in concentration produced a red-shift in the peak emission wavelength from 517 to 522 to 528 nm, respectively, as shown in Fig. 4 . This shift is due to the overlap between the emission and absorption spectra of the QD, as shown in the Fig. 4 insert, with re-absorption by the QD of their own emission causing a shift to the red. This re-absorption also causes narrowing of the full width half maximum (FWHM) of the emission from 41 nm at 0.0145 wt %, to 29 nm at 0.1 wt %, and finally to 24 nm at 0.2 wt %.
Fig. 4 Peak emission of 520 nm QD-doped fibres (4 cm length) pumped with a femtosecond laser as a function of dopant concentration. The insert shows the overlap of the absorption (A) and emission (B) spectra.
Different lengths were also tested for the 0.2 wt % 520 nm QD-doped fibre. Lengths of 2.2, 4.0 and 5.9 cm were pumped with the femtosecond laser employing an average power of 1.7 mW. A shift in the peak wavelength from 524.2 to 528.5 to 531.4 nm occurred as the length was increased, as shown in Fig. 5 . The red shift observed is again due to re-absorption of the emission from the QD. The length increase was accompanied by a reduction in the FWHM from 27 to 25 to 22 nm, again the result of re-absorption.
Fig. 5 Peak emission of 520 nm QD-doped fibres (0.2 wt %) pumped with a femtosecond laser as a function of fibre length.
The high pump intensities (up to 23 μJ/pulse) from the femtosecond laser cause the QD to act as a saturable absorber, with the optical intensity causing a depletion of QD in their ground state. This phenomenon reduces the amount of re-absorption taking place and decreases the optical loss at shorter wavelengths. Figure 6(a) shows the peak emission wavelength as a function of average pump power from the laser. A blue-shift occurs in the peak as the pump power is increased. Furthermore, the increase in pump power, having reduced the extent of re-absorption, increases the FWHM of the emission from 25.5 to 30 nm, as shown in Fig. 6(b). To corroborate this effect as the result of the QD acting as a saturable absorber, the same type of fibre was pumped with a CW argon laser (488 nm), employing a range of average power equal to that for the pulsed laser. As expected, in this experimental setup, the peak emission wavelength and the FWHM of the emission spectra did not change with an increase in CW pump power.
Fig. 6 Peak emission wavelength as a function of average pump power using a femtosecond pulsed laser (a). An increasing pump power resulting in a decrease in peak wavelength is a known characteristic of a saturable absorber. This interpretation is confirmed by the FWHM increasing with average pump power (b).
4. Conclusions
CdSe/ZnS quantum dots were successfully used as a dopant within a PMMA matrix. This doped material was used in the fabrication of a microstructured polymer optical fibre whose photoluminescence was characterised. Both the emission peak wavelength and the FWHM of the embedded QD changed measurably with dopant concentration, fibre length and laser pump power. These changes can be attributed to the effects of re-absorption of the QD emission and the QD acting as saturable absorbers. A better understanding of changes in emission properties of QDs in polymer optical fibres is vital for the creation of potential applications [14–17].
Acknowledgements
This work was partly supported by The National Collaborative Research Infrastructure Strategy (NCRIS). The authors would also like to thank Richard Lwin for the assistance in fibre fabrication and John McGuire for his discussions on quantum dots.
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