1. Introduction

The architecture of photonic-crystal fibers (PCFs) [1] suggests a variety of strategies for optical sensing. The most common approach involves using the evanescent field of PCF modes, confined to the high-index material in a PCF, for the detection and analysis of gas- and liquid-phase species in air holes of PCF cladding [2, 3]. This protocol of optical sensing has been employed for the experimental demonstration of compact, practical, and efficient PCF sensors of gas-phase media [4–6] and biomolecules in aqueous solution [7].

Myaing et al. [8] have demonstrated another method of biosensing using PCFs. In those experiments, a double-clad PCF served to improve the efficiency of two-photon fluorescence detection of biomolecules with the fluorescence signal delivered to the detector in the backward direction through the same fiber. The possibility of extending the concept of fiber-grating-based sensing to PCFs has been demonstrated by Eggleton et al. [9].

Air-guided modes of hollow-core PCFs have been shown to be ideally suited for gas-phase sensing using linear [10] and nonlinear [11] optical methods. Hollow-core PCFs adapted to transmit high-energy laser pulses [12, 13], initiating laser-induced ablation on solid surfaces in laser technologies and biomedicine, can simultaneously collect radiation emitted by laser-produced plasma on metal surfaces or dental tissues and deliver it in the backward direction to spectrum analyzers and detectors [14].

In this work, we show that a PCF with a special design can realize different protocols of optical sensing, simultaneously serving, whenever necessary, for the collection and on-line monitoring of liquid-phase samples. Such a PCF integrates an optical sensor with a micropipette, a sample collector, and a microfluidic polycapillary array in a single fiber-optic component, which is ideally suited for lab-on-chip applications, biosensing, and in vivo work in medicine.

2. Experimental technique and protocols

Photonic-crystal fibers used in our experiments were produced of fused silica (Figs. 1(a), 1(b)) and soft glasses (Figs. 1(c), 1(d)). Description of fabrication processes, based on standard PCF technologies [1], and characterization of optical properties for these types of PCFs have been published earlier [14, 15]. Two protocols of optical sensing employed in our experiments are sketched in Figs. 2(a) and 2(b). In the first protocol (Fig. 2(a)), a dual-cladding PCF is used to sense luminescent species in a distant liquid-phase sample. Diode-laser radiation is delivered to a sample through the central core of the fiber with a diameter ranging in our experiments from 1 up to 10 µm (Figs. 1(a)–1(d)). The fiber cladding with a substantially larger diameter (about 200 µm in Fig. 1(a) and 350 µm in Fig. 1(b)) serves to collect the fluorescent response from the sample and to guide it in the backward direction to a detector. This mode of fluorescent signal collection with a dual-cladding PCF has been earlier shown to enhance biosensing based on two-photon fluorescence [8]. The small effective mode area of the laser beam, delivered through a small-diameter PCF core, allows to achieve higher intensities in the probed solution of biomolecules, giving rise, due to the nonlinear nature of two-photon fluorescence, to higher levels of the fluorescent signal. In our work, the sensing of liquid-phase species was based on the one-photon fluorescent response of molecules. Still, the use of a double-cladding PCF offers important advantages for optical sensing. Delivery of laser radiation through a small-diameter PCF core provides a high-precision positioning of the pump beam on a sample that can be located at large distances from the radiation source. The large collection aperture of the outer PCF cladding, on the other hand, increases the solid angle of fluorescence collection, eventually improving the detection sensitivity of the method.

 

Fig. 1. Cross-section views of photonic-crystal fibers made of (a, b) fused silica and (c, d) soft glasses.

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In this scheme of optical sensing, the detector is placed next to the radiation source, while the probed medium can be located at a large distance from the laser and the detector. This advantage makes this mode of optical sensing attractive for various applications, including biothreat detection and biomolecule and biospecies detection on substrate and biochip surfaces. The length of the PCF sensor was about 15 m in our experiments, with the maximum distance of sensing determined by PCF losses, estimated as 70 dB/km for the best of our PCF samples.

In the second protocol of optical sensing, sketched in Fig. 2(b), liquid sample is collected by the microcapillary array in the PCF cladding, giving rise to sample flows induced by surface tension. Dye molecules in these sample flows were interrogated with laser radiation guided in PCF modes. The fluorescence signal generated by dye molecules was detected in the forward direction (Fig. 2(b)). For higher fluid-collection efficiencies and for a better control of sample microfluidic flows in potential lab-on-a chip applications of PCF sensors, surface-tension forces can be supplemented by electroosmotic guiding [16]. Promising ways to adjust and manipulate fluid flows in microchannels of PCFs with a system of separately addressable actuators and pumps on the fiber surface have been recently demonstrated by Mach et al. [17].

The two protocols of optical sensing with PCFs described above impose different requirements to the PCF design. To realize the first scheme, a PCF should have a small-size central core surrounded by a microstructure cladding, designed to provide the maximum confinement of guided modes in the fiber core. Other important functions of the PCF cladding in this regime of sensing include providing a large aperture for the collection of the fluorescence light from the sample and to deliver this radiation back to the detector, placed next to the radiation source. Realization of the second approach requires an array of microcapillaries separated by glass or fused silica walls, forming an arbitrary structure (Figs. 1(a)–1(d)). The regime of waveguiding of interrogating laser light in this scheme of optical sensing is controlled by the ratio of refractive indices of the fluid sample and the material of the fiber. When the refractive index of the fluid is lower than the refractive index of the PCF material (glass or fused silica), sample flows are probed by the evanescent field of PCF modes. For the opposite ratio of refractive indices, the light field can be replaced from the fiber material to the fluid and guided in the fluid through total internal reflection.

 

Fig. 2. Two protocols of optical sensing with a photonic crystal fiber. (a) Diode-laser radiation is delivered to a sample through the central core of the fiber, while the fiber cladding serves to collect the luminescent response from the sample and to guide it to a detector. (b) Liquid sample is collected by the microcapillary array in the PCF cladding and is probed by the field of PCF modes.

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3. Results and discussion

Double-cladding fused silica PCFs shown in Figs. 1(a) and 1(b) can simultaneously support both protocols of optical sensing. The inner and outer parts of the microstructure cladding in this PCF include arrays of air holes with radically different sizes. The inner part of the cladding (Fig. 1(a)) includes a nearly periodic system of holes with a diameter of about 3.5 µm, providing a high air filling fraction of the structure and serving to support strongly confined guided modes in the fiber core. The functions of the outer part of the microstructure cladding include the collection of fluorescence radiation from the sample and the delivery of this signal back to the detector in first regime of sensing or the guiding of both laser radiation and the fluorescence signal, simultaneously inducing sample flows through surface tension, in the second scheme of sensing. Requirements to the outer part of the cladding are therefore much less rigorous. For the convenience of fabrication, the holes in this section of the cladding can be made much larger compared to the holes in the inner part of the cladding.

Several dye molecules dissolved in water, alcohol, and dimethyl sulfoxyl (DMSO) were used to illustrate the above-described protocols of optical sensing with PCFs. The refractive indices of aqueous and alcohol solutions of dyes are lower than the refractive indices of PCF materials. This relation between the refractive indices corresponds to evanescent-field sensing of sample fluids. The refractive index of DMSO at the wavelength of 532 nm is n ₁≈1.47, while the refractive index of PCF material at this wavelength is n ₂≈1.46. The interrogating laser radiation is guided in the fluid-filled holes of the PCF by total internal reflection in this case.

Figure 3 presents fluorescence spectra of thiacarbocyanine dye dissolved in DMSO measured with a PCF shown in Fig. 1(b) using the first (open circles) and second (filled squares) protocols of optical sensing. The dashed curve in Fig. 3 displays the spectrum of the same solution in a standard cell. Comparison of the data presented in Fig. 3 shows that both protocols of optical sensing with PCF give nearly identical results. Fluorescence spectra measured with a PCF, in their turn, coincide with a high accuracy with the spectrum of the fluid sample in a standard cell.

 

Fig. 3. Fluorescence spectra of thiacarbocyanine dye in DMSO measured with a PCF shown in Fig. 1b using the first (open circles) and second (filled squares) protocols of optical sensing. The dashed curve shows the spectrum of the same solution in a standard cell. The residual signal related to pump radiation transmitted through the set of filters is seen at 532 nm. Inset 1 presents an end-face image of a PCF sensor filled with thiacarbocyanine dye in DMSO, fluorescing under the action of 532-nm pump. A typical spatial intensity profile of this fluorescence radiation from PCF holes is shown by filled circles connected by the dashed line in inset 2. The solid line in this plot represents the field intensity profile calculated for the fundamental mode guided in the same PCF channel filled with fluid.

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Cross-referencing measurements thus demonstrate the equivalence of spectroscopic data attainable with the two PCF-sensing protocols and measurements in a standard cell. Each of the PCF sensing methods, at the same time, has important advantages over measurements in a standard cell. The first protocol, as mentioned above, is ideally suited for remote sensing of fluid samples, with a detector placed close to the radiation source, which makes it possible to perform measurements on biothreat fluid samples, as well as on biomolecules and biospecies processed by biochips. The second protocol of PCF sensing, on the other hand, uses the sample fluid in a much more efficient way than a standard cell does. To illustrate this argument, we compare the waveguide formed by the higher-index sample fluid in holes of the PCF in our experiments with thiacarbocyanine dye molecules dissolved in DMSO solvent. The image of the output end of the PCF filled with this fluid sample is presented in inset 1 to Fig. 3. Higher-index areas of fluid sample tend to confine light in the holes of PCF cladding. Each of these fluid channels forms a waveguide with a typical diameter of about 25 µm. With the refractive indices of the core and the cladding equal to n _co≈1.47 and n _cl≈1.46, the waveguide V parameter for a circular channel with a diameter d≈25 µm is estimated as V=πdλ ^-1($n_{\text{co}}^2$-$n_{\text{cl}}^2$)^1/2≈20 (λ is the radiation wavelength). With such values of the V parameter, waveguiding is essentially multimode and the fraction of radiation guided outside the waveguide core is negligibly small. In an ideal situation when the entire input radiation energy is coupled into the fundamental mode of a circular waveguide, the fraction of radiation power guided outside the waveguide core, ζ≈(2.4)² V ^-3, is less than 0.1%. The analysis of radiation intensity profiles in fluid channels at the output end of the PCF (inset 2 in Fig. 3) confirms this expectation, showing that laser radiation is strongly confined by total internal reflection to the modes guided in fluid channels. In practice, because of optical inhomogeneities, a noticeable part of laser radiation (up to 20% in our experiments) escapes from fluid channels, thus reducing the efficiency of interrogating light utilization. With the remaining fraction of laser radiation confined to fluid channels, PCFs, in fact, serve as compact and efficient cells, providing an 80% overlap of interrogating radiation and sample fluid. For a 5-cm section of a PCF with a sample fluid providing a reliable fluorescence signal at the detector, the volume of sample fluid filling the holes in the PCF cladding was about 0.5 µl. The level of the fluorescence signal obtained with such a PCF suggests that a few nanoliters of sample fluid would be sufficient to detect dye molecules with our experimental arrangement.

4. Conclusion

Experimental results presented in this paper demonstrate a high potential of PCFs for optical sensing and sample collection. A diversified architecture of photonic-crystal fibers suggests various approaches to optical sensing. We compared two protocols of optical sensing with PCFs in this work. In the first protocol, diode-laser radiation is delivered to a sample through the central core of a dual-cladding PCF with a diameter of a few micrometers, while the large-diameter fiber cladding serves to collect the fluorescent response from the sample and to guide it to a detector in the backward direction. In the second scheme, liquid sample is collected by a microcapillary array in the PCF cladding and is interrogated by laser radiation guided in PCF modes. For sample fluids with refractive indices exceeding the refractive index of PCF material, fluid channels in PCFs can guide laser light by total internal reflection, providing an 80% overlap of interrogating radiation with sample fluid. Specially designed PCFs are shown to integrate an optical sensor with a micropipette, a sample collector, and a microfluidic polycapillary array in a single fiber-optic component, which is ideally suited for lab-on-chip applications, biosensing, and in vivo work in medicine.