Sealed liquid-core photonic crystal fibers for practical nonlinear optics, nanophotonics and sensing applications

A. Bozolan1,2,*, R. M. Gerosa1, C. J. S. de Matos1, M. A. Romero2 and C. M. B. Cordeiro3

1 Grupo de Fotônica – Universidade Presbiteriana Mackenzie – São Paulo – Brazil

2 Depto de Engenharia Elétrica – Universidade de São Paulo – USP – São Carlos – Brazil 3 Instituto de Física “Gleb Wataghin” – Universidade Estadual de Campinas – UNICAMP –

Campinas – Brazil

ABSTRACT

Photonic crystal fibers have been the subject of several studies for potential application in areas such as sensing, nonlinear optics, telecommunication and nanophotonics. Many applications are enabled by the possibility of selectively inserting gases, liquids, polymers and colloids into the internal microstructure, which results in efficient interaction with the guided light, allowing for the development of, e.g., sensitive chemical sensors also, the insertion of materials can be exploited to modify waveguide characteristics such as modal field distributions, the nonlinear coefficient and the chromatic dispersion. Experimentally, the insertion of liquids is particularly straightforward and enables many of the envisaged studies. However, evaporation is an important limiting issue, which ultimately prevents the realization long-term practical applications. Also, in some cases contact of the liquid with the external environment may degrade its properties. To address these issues, we experimentally demonstrate a new technique to selectively seal a liquid-filled hole of a photonic crystal fiber. The characteristics of the sealed fibers remained stable for at least a few weeks. Two experiments were, then, carried out to demonstrate the potential of the technique. In the first experiment, a water-core photonic crystal fiber was used for supercontinuum generation, with the generated spectrum not showing degradation over time. In the second experiment, a colloid of CdSe nanoparticles was inserted into the core of a fiber and stable photoluminescence was observed.

Keywords: Photonic Crystal Fibers, Optical fibers filled by liquids, Nonlinear Optics, Nanophotonics.

1. INTRODUCTION

Photonic crystal fibers (PCFs) are specialty optical fibers1, the holes of which can be filled with liquids and colloids for several applications such as sensing2, nonlinear optics3,4 and nanophotonics5. One of the main problems with liquid filled PCFs, however, is that the low volumes required to fill a hollow core or a cladding hole, together with the fairly high evaporation rate of liquids such as water, toluene and ethanol, leads to millimeters of length being emptied over a few minutes6. For liquid-core PCFs, evaporation makes experiments impractical for most applications. To circumvent this difficulty, the use of liquid reservoirs in both fiber ends3 as well as the insertion of the fiber in a viscous liquid polymer7 have been reported. Other authors reported the selective sealing of specific PCF holes with glue, but aiming at taper fabrication and little detail was given about the sealing method8. However, while the former approach means that the sample volume is significantly increased (negating one of the main advantages of liquid-filled PCFs, i.e. the use of nanoliter volumes), the latter results in bulky fiber terminations and often leads to air plugs between the polymer solution and the liquid, which results in considerable losses.

*bozolan@usp.br, phone: 55-11-2114-8869; fax 55-11-2114-8711

2nd Workshop on Specialty Optical Fibers and Their Applications (WSOF-2),edited by Juan Hernández-Cordero, Ismael Torres-Gómez, Alexis Méndez, Proc. of SPIE

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Here, we demonstrate a technique to close both facets of a liquid-filled hole in a PCF with a curable polymer (NOA- 73, refractive index ~1.56), which prevents evaporation with minimal impact to the fiber ends. As the polymer is directly deployed to the hole extremities with a micropipette, the length of undesired air plugs is minimized. Two proof-of-principle experiments are, then, described, demonstrating the benefits of the reported technique.

2. METHOD DESCRIPTION AND EXPERIMENTAL SETUPS

The technique is based on the use of a micropipette, Figure 1(a), to selectively insert, by pressure, the liquid pre-polymer into the hole of choice. The micropipette presented a tip with an external diameter of 3 to 4 µm and a ~1 µm internal diameter. It was attached to a XYZ positioner which directed it into the liquid-filled hole to be sealed. Once inside the hole, the pipette tip was pressed against the inner wall of the selected hole, in such way that the air had a route to escape and could be expelled by the polymer, thus minimizing the formation of air plugs. The pre-polymer was then cured using a UV lamp. This procedure was subsequently repeated for the opposite fiber end. The created polymer plugs had an estimated length of tens of microns.

Two different commercially-available hollow-core PCFs were sealed after their cores were filled with water, mineral oil or toluene. One of them was the model HC-1550-02, by NKT Photonics, which has a 10.9 μm core diameter and ~3.78 μm diameter cladding holes. The other was the model HC-19-1550, by NKT Photonics, which presents a ~20 μm core diameter and ~4 μm cladding hole diameter.

The first proof-of-principle experiment consisted of reproducing the supercontinuum generation experiment in a water-core PCF, as previously demonstrated by us9. 40-fs pulses with a repetition rate of 1 kHz from an Optical Parametric Amplifier (OPA) operating at ~980-nm were coupled into 7-cm of a sealed water-core PCF (HC-1550-02). The high cladding air-filling fraction (>90%) ensured that light was guided through total internal reflection in the liquid-core. In addition, water’s low material dispersion at the pump wavelength facilitated nonlinear spectral broadening.

Fig. 1. (a) Micropipette with a droplet of polymer. (b) PCF cross-section with a sealed core filled with a red-emitting quantum dot

colloid.

In the second experiment, a fiber light source was developed based on a sealed PCF whose hollow core was filled with semiconductor nanocrystals. These nanocrystals have been studied for, among other applications, temperature sensing because their luminescent properties (emission intensity and spectrum) are temperature dependent9,10. We filled and sealed the hollow core of a PCF (HC-1550-02, 6.5 to 8.5 cm in length) with CdSe nanocrystals (~5 nm diameter) dispersed in mineral oil (250 μg/ml concentration) and measured the emission obtained when the waveguide was pumped by a solid-state CW laser at 473 nm with ~8,5 mW power. Figure 1(b) shows the image of the tip of one sample, in which some red emission generated by the microscope illumination can be observed.

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3. RESULTS AND DISCUSSIONS

Optical characterization of the sealed PCFs revealed that the net transmission (which takes into account all coupling, transmission and insertion losses) obtained in 5.5-m long water-core HC-1550-02 fibers was up to 15% (at 633 nm), indicating the ability to generate polymer plugs with reasonable optical qualities. When the core of the HC-19-1550 fiber was filled with toluene and sealed, the higher volatility of toluene led to a longer air plug between the polymer and the liquid and to a 0.5% net transmission in a 5.8-cm long PCF. Despite the high loss, this result indicates that the method is also attractive for use with highly volatile liquids, for which evaporation would otherwise absolutely prevent practical applications. Most importantly, in all cases the guiding properties of the filled fibers remained unchanged for at least a few weeks.

One issue that requires careful attention during the sealing process is the shape of the surface of the polymer plug at the fiber input, which induces a lensing effect. It was found that leaving the outer plug surface slightly inside the core hole resulted in a concave shape, allowing for the excitation of only lower order modes, as evidenced by the near field image of light leaving the opposite fiber tip shown in Fig. 2(a). On the other hand, when the plug was leveled with or exceeded the fiber tip, higher-order modes could not be avoided and the output light had the intensity distribution shown in Fig. 2(b). Clearly, for supercontinuum generation or other phase-sensitive applications the excitation of few modes is preferred. For other applications, such as spectroscopy, multi-mode excitation is not a problem and allows for higher net transmission values to be obtained.

Figure 2. (a) Guided light intensity distribution at the output of a water-core PCF showing the presence of a few lower-order modes (a) and many high-order modes (b) depending on the polymer plug outer surface shape.

Figure 3(a) depicts results obtained for the supercontinuum generation in a sealed water-core PCF. The shown spectra correspond to estimated pump peak powers in the 0.41 to 2.5 MW range (corresponding to 17.6 – 106.5 µW of average pump power). The widest spectral width at -20dB was ~260 nm. The main advantage of the current experiment over our previous work is the stability of supercontinuum generation because water evaporation is suppressed. With the sealed water core, the spectrum remained unchanged over up to about thirty minutes (limited by the mechanical instabilities in the experimental setup), while previously stability was maintained for about five minutes (limited by water evaporation). Also, the sealed fiber allowed for higher average pump powers to be used because thermal effects did not lead to evaporation, as observed previously by us.

Figure 3(b) shows the photoluminescence spectrum obtained with a sealed colloidal-core PCF as well as its comparison with an unsealed sample. The emission peak was at ~615 nm and the FWHM was ~25 nm for the sealed sample, while these characteristics were ~611 nm and ~23 nm, respectively, for the unsealed sample. The slight red shift observed for the sealed fiber is believed to be a consequence of this sample being slightly (2 cm) longer then the unsealed fiber. Also, the polymer plug may induce some pump input coupling loss. Both effects would cause photons emitted in first centimeters of fiber to be absorbed on the insufficiently pumped final centimeters, being reemitted with lower frequencies. Up to 2 µW of guided emission power was observed when the colloidal-core fiber was sealed while the unsealed PCF yielded up to 9 µW luminescence power. In comparison, a solid-core PCF with CdSe quantum dots deposited on the cladding hole inner walls had a measured luminescence power of only 0.18 µW. Both emission

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spectrum and power, thus, indicate that the insertion losses induced by the polymer seals are quite acceptable for the envisaged applications.

Fig. 3 (a) Supercontinuum generation in a sealed water-core PCF. (b) Emission luminescence spectra of sealed and unsealed CdSe colloidal-core PCFs.

4. CONCLUSION In summary, we demonstrated a practical and simple method to seal a liquid-filled PCF so that liquid evaporation and

colloid degradation is avoided. Two example applications were presented, which showed that the loss induced by the sealing process is acceptable and that the method indeed allows for long-term experiments to be performed. The method, thus, contributes to the practical use of liquid-filled fibers in sensing, nonlinear optics and nanophotonics applications.

ACKNOWLEDGMENTS

The financial support of Fundo Mackenzie de Pesquisa, CAPES, CNPq (including funding via Rede Nanofoton and Fotonicom) and FAPESP (Fotonicom) are gratefully acknowledged.

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