1 Introduction

Post processing of photonic crystal fibers (PCF) has recently drawn a lot of attention. It increases the range of applications of PCFs even further. PCFs [1, 2] provide a broad base for the development of groundbreaking optical devices, and hence a lot of progress was made over the past years. Moreover, the structure of such fibers leads to both a higher degree of freedom of the optical properties and for new applications in a wide range of technology and research which cannot be achieved by conventional fibers. Optical sensors for biomolecules and gases are one of the potentially novel applications of PCFs [3]. The major advantages of optical sensing with PCFs are the long effective interaction length, the small mode volume, and the high evanescent field at the position of the sample. Another promising application area is the light guidance in spectral ranges which are not available with common optical fibers [4]. In fact, microstructured fibers provide the opportunity to guide light in both the mid- and far-infrared spectral range, and likewise in the terahertz region. In addition such fibers have made a major impact in nonlinear optics, where the tunable properties of the group velocity dispersion in combination with high nonlinearities are used for spectral broadening and, therefore, for the generation of new frequency components [5].

Various post processing techniques allow for further tailoring of the temporal and spatial dispersion of the PCFs. Tapering [6] is one promising post processing method, which was directly adapted from the techniques used for conventional fibers [7]. The air holes of PCFs provide an excellent possibility to be filled with other optical materials such as polymers [8], ferrofluids [9], gases [10], liquid crystals [11, 12], semiconductors [13], low melting compound glasses [14] or even metals [15, 16]. Liquid-filled PCFs [17–19] have been utilized for tunable fiber optics, such as optical filters [20] or thermo-optical switches [11], for nonlinear applications [21, 22] or for refractive index sensing [23, 24]. Rather than filling the air holes altogether, only selected strands can be filled with optical materials [26]. Selectively liquid-filled PCFs can for example combine the large interaction length of the fibers with the high nonlinearity of several liquids for spectral broadening [25–29]. The influence of the environment of waveguides on the light propagation renders selectively filled PCFs extremely sensitive to variations in the refractive index of liquids, caused for example by temperature changes [30]. Another potential application could be the generation of spatiotemporal solitonic waves [31], so-called light bullets, which were predicted by Silberberg [32] and first experimentally observed by Minardi et al. [33] in a hexagonal array of conventional fibers. In the linear propagation regime, selectively liquid-filled PCFs hold large promises for tunable highly birefringent fibers [34], which can be used for adjustable filters or sensors. Asymmetric liquid filling in the microstructure of a PCF causes polarizing dependent behavior, as one polarization direction leaks out, whereas the other polarization is still guided by the fiber core [35].

Different manufacturing techniques allow fabrication of liquid-filled PCFs [36–44]. All methods use one common procedure: holes that are not to be filled are blocked first, and afterward the open holes are filled with liquids using capillary force. One possibility for closing the fiber selectively is to exploit the different filling velocities of monomers, which are dependent on the hole diameters [36, 38]. Hence, ultraviolet curable polymers propagate over different distances into the fiber, so that after curing and recleaving the fiber only those air holes are closed where the hardened polymer propagated over a larger distance. Another possibility is to use a glass tip and selectively spread UV-curable polymer over the cross sectional area of the fiber [39, 40]. After curing, only the selected holes are closed. Additionally, using a fusion splicer and collapsing several air strands is another alternative technique [37, 38].

In a recent publication [26], we demonstrated ultrafast temporal solitonic supercontinuum generation with a selectively liquid-filled PCF and sketched the fabrication process. Here, we present this versatile fabrication approach in detail and demonstrate the flexibility of the liquid-filling process by means of manufacturing a large mode area selectively liquid-filled PCF. Our device is illustrated schematically in Fig. 1, where the mode image in the inset is calculated numerically by a full-vectorial finite element method at a wavelength of 1.3 μm. Instead of using doped silica cores like in multicore fibers [45], our post-processed PCF consists of 19 neighboring liquid-filled strands that were embedded into the hexagonally microstructured area of a PCF. This enables easy tailoring of the coupling strength between the liquid cores both by temperature and mixing ratio of different liquids. We experimentally demonstrate the transition between isolated wave guiding and evanescently coupled guiding related to the wavelength by means of a PCF that was selectively filled with a mixture of toluene and ethanol. The adjustability of this transition by tuning the temperature or the mixing ratio is demonstrated by numerical full-vectorial finite element method simulations.

Fig. 1
figure 1

Schematic drawing of the large mode area liquid-filled PCF. The 19 liquid-filled strands embedded into the mircostructured area of the fiber guide the light. The mode image is calculated numerically by a fully vectorial finite element method. In the simulation, the fiber is filled with a mixture of toluene and ethanol in a fraction of 4:1. The wavelength is 1.3 μm

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Our fabrication method is based on a three-dimensional femtosecond direct laser writing technique [46, 47], which enables us to close only selected holes of a PCF [26]. In contrast to other closing techniques [36–40, 43, 44], our procedure can be used for fibers with a ratio of hole diameter to hole distance of nearly one, as well as for any arbitrary pattern. After closing a desired pattern the unblocked holes can be filled by various liquids. Other materials, such as quantum dots, gases, liquid crystals, polymers, semiconductors, or metals can be used as well as filling media.

2 Fabrication

For our approach, we use commercially available PCFs, where one cleaved fiber end is coated with photoresist and afterward selectively exposed with a modelocked Ti:Sapphire femtosecond laser oscillator with 80 MHz repetition rate. We use the commercially available negative resist SU-8 from MicroChem [48], which is common for nanolithography. The strong crosslinking of this photoresist causes an extremely high chemical and thermal resistance. In order to coat one end with photoresist, the fiber is first dipped into a drop of the resist (Fig. 2(a)) and afterward dipped onto a clean glass plate. This technique obviates the need of a post exposure bake, as the thin layer and small amount of resist ensures that the organic solvent diffuses out immediately. Hence, after coating the two-photon exposure process can immediately be carried out (Fig. 2(b)). The exposure process of every individual hole is realized by moving the fiber up and down to expose the photoresist over a distance of 100 μm by a computer controlled piezoelectric stage (P-563 PIMars XYZ Piezo System, Physik Instrumente (PI), Karlsruhe, Germany). In order to obtain a consistent exposure process the stage moves with a constant velocity of 100 μm/s. The position of the laser focus relative to the single holes of the PCF is controlled at each step by an image recognition algorithm using spatial cross-correlation technique. In order to seal a set of holes reliably and reproducibly, the fabrication process is automated. The laser power is adjusted before the beam enters into the microscope. For this application, an input power of 60 mW is used which corresponds to about 35 mW at the position of the sample. The wavelength is set to be around 774 nm. For this center wavelength, the photoresist is nearly perfectly transparent for the one-photon transition, but the tightly focused laser beam and the short pulses allow for two-photon absorption. The pulse duration is about 175 fs, measured at the position of the sample, using a modified autocorrelator (Micro$cor, NT&C, Marnheim, Germany). For the exposure process, a microscope objective with a numerical aperture of 0.8 is utilized. Focusing provides a volume element (voxel) in which the two-photon absorption process takes place with a diameter of approximately the same size as the hole diameter. To control whether the individual exposure process was successful, the exposure is directly observed by a CCD camera during the process. After the exposure, the resist has to be developed. The crosslinking process of the photoresist occurs during the post-exposure bake. Afterward, the unexposed photoresist is removed by rinsing with a common developer. This step is crucial. If the fiber stays in the solvent for too long, the latter can infiltrate into the freshly opened holes. Our fabrication technique is not limited to a special kind of PCF or a particular pattern. By the utilization of capillary forces, the unblocked strands in the PCF can be filled with the designated liquid without requiring external pressure (Fig. 2(c)). Therefore, the structured fiber end is held into the liquid reservoir, while the opposite end is observed under a microscope until the liquid flows along the whole fiber length. Some examples of our fabrication technique are shown in Fig. 3. The six-fold propeller pattern is fabricated into two different PCFs, namely an LMA-8 fiber (Fig. 3(a)) and an NL-2.3-790 fiber (Fig. 3(b)). The two microscope images show the versatility of this approach concerning the fiber type; even fibers with a ratio of hole diameter to hole distance of nearly one (NL-2.3-790 fiber) can be processed. Figure 3(c) shows the excellent and homogenous closing of a checkerboard pattern fabricated into the NL-2.3-790 fiber. In the colored scanning electron microscope image the blue marked holes are closed with a photoresist, and the core is marked yellow. The liquid-filled PCF that is analyzed in this paper is shown in Fig. 3(d). The bright holes are the liquid-filled ones, which act as a coupled waveguide array.

Fig. 2
figure 2

Scheme of the fabrication process with the two-photon direct laser writing technique and our filling process. In a first step (a), the fiber is coated with a UV photoresist, which is exposed by two-photon absorption of a near-infrared femtosecond laser subsequently (b). After development, the unblocked holes can be filled (c) with different optical materials, such as liquids, gases, metals, low melting compound glasses, or quantum dots. In contrast to other fabrication techniques, this method can be used for nearly every kind of PCF as well as for every pattern realizable in the microstructure of a PCF

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Fig. 3
figure 3

Results of the fabrication and filling process. Microscope images of the (a) LMA-8 and the (b) NL-2.3-790 fibers, each of them prepared and closed with a six-fold propeller pattern. The bright holes are the sealed holes and the dark ones are open. (c) Colored scanning electron microscope images of a selectively closed NL-2.3-790 fiber with a checkerboard pattern, which shows the accuracy and the versatility of this technique. The blue marked holes are closed with photoresist and the core is marked yellow. (d) Microscope image of the 19 liquid strands embedded into the structured region of the LMA-8

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3 Simulation method

The modes in the fibers were simulated by a full-vectorial finite element method using the commercial finite element Maxwell-solver software Comsol Multiphysics. A two-dimensional mode solver was used to obtain the mode images and the resulting effective mode area of the fundamental supermode. The numerically analyzed PCF is a selectively liquid-filled LMA-8 fiber, where the complete structure is used for the simulations. As parameters, we have utilized a pitch value of 5.6 μm and a hole diameter of 2.7 μm [49, 50]. The refractive index of the glass structure is calculated using its Sellmeier equation [51], as well as for the refractive indices of ethanol [52] and toluene [53]. For the temperature behavior of the refractive index of the liquids, a linear characteristic

$$ n (\varDelta T ) = n_0 + \frac{dn}{dT} \cdot \varDelta T.$$
(1)

was assumed. Here, n 0 denotes the refractive index calculated by the Sellmeier equation. The temperature difference ΔT is the difference between the absolute temperature T and the temperature T 0 at which the Sellmeier coefficients are given. The thermooptical coefficients dn/dT amount to −5.5⋅10−4/K for toluene and −4⋅10−4/K for ethanol [54]. Furthermore, we assumed constant values of the thermooptical coefficients with respect to the wavelength and temperature dependence. In contrast to the value of the liquids the thermooptical coefficient of fused silica is negligible and was therefore not considered. In order to obtain the refractive index of the liquid mixtures, the Lorentz–Lorenz equation was used [55]

$$ \frac{n^2-1}{n^2+2} = \phi\frac{n_1^2-1}{n_1^2+2} +\phi_2 \frac {n_2^2-1}{n_2^2+2}.$$
(2)

Here, n, n 1, and n 2 are the refractive index of the solution and the constituents, respectively. ϕ=ϕ 1 and ϕ 2 are the volume fractions of the constituents. In (2), ϕ 2 can be replaced by (1−ϕ).

From the simulated modes, the effective mode area \(A_{\text{eff}}\) is calculated by

$$ A_{\text{eff}} = \frac{( \int{|E|^2 \,dA})^2}{\int {|E|^4 \,dA}},$$
(3)

where E represents the two-dimensional electric field distribution of the mode. The dispersion of the group velocity is represented by the dispersion parameter D, defined as

$$ D = -\frac{\lambda}{c} \frac{d^2n_\text{eff}}{d\lambda^2}.$$
(4)

Here, \(n_{\text{eff}}\) is the effective mode index of the guided supermode, λ the wavelength and c is the speed of light.

4 Results and discussion

The selectively liquid-filled PCF that we investigated consist of 19 strands of the LMA-8 fiber filled with a mixture of toluene and ethanol in a fraction of 4:1 (ϕ(toluene:ethanol)=0.8). Hence, the refractive index of the liquid strands (n=1.4558 @ 1.0 μm, T=20∘C) is slightly higher than the one of the glass structure (n=1.4504 @ 1.0 μm, T=20∘C). The 19 hexagonally arranged liquid strands act as a two-dimensional evanescently coupled waveguide array. The coupling strength of the waveguides on the one hand depends on the structure of the PCF, i.e., on hole distance and hole diameter, and on the other hand on the temperature-dependent refractive index of the used liquid. The refractive index of our liquid mixture is tunable by adjusting the mixing ratio. Figure 4 shows the measured and simulated mode images of our exemplary device. In the measurement, the light is coupled into the central liquid-filled strand. In Fig. 4(a), the mode image is obtained by illuminating our selectively filled PCF with a halogen lamp. The spectrum of our light source is displayed as an inset in the figure. For the mode images in Fig. 4(b) and (c), the incoupled light stems from a filtered supercontinuum of a tapered fiber pumped by a Ti:sapphire laser oscillator [56]. The spectral width is about 10 nm. Figure 4(b) indicates weak coupling between the liquid strands at a wavelength of 550 nm. Several filled holes at the edges of the 19-strand area also guide the light in a higher order mode, which is also predicted by the calculations of the normalized frequency V=10.64 [57], which does not fulfill the single mode criterium (V≤3.14). The required effective refractive index of the microstructured area is numerically calculated by a fully analytical vectorial approach [58]. For short wavelengths, the weak coupling causes a low power transfer from the central liquid strand to the outer liquid-filled rings within the fiber length, but the guided light is still localized at the liquid strands. An increase in the wavelength leads to a reduced difference of the refractive indices between the fused silica and the liquid and, therefore, to an increased coupling. As a result, the light propagates in one evanescently coupled large area higher-order supermode, which is exemplary shown at 1.2 μm in Fig. 4(c) for a higher-order mode. The 4.8 cm long selectively liquid-filled fiber piece only shows a maximum transmittance of 5%, including coupling and confinement losses. The material losses in the visible range are insignificant, but in the near-infrared regime ethanol and toluene show overtones of the rotational-vibrational structure of the molecules that result in several absorption dips in the transmittance spectrum. Hence, the transmission of the fiber is mainly determined by the material absorption. The simulated mode images of the fundamental supermode are shown in Fig. 4(d) and (e) for a wavelength of 550 nm and 1.2 μm, respectively. In our selectively liquid-filled PCF, the light propagates in multiple supermodes. At a wavelength of 1 μm, the finite element method simulations show that there are 38 different modes guided by our device.

Fig. 4
figure 4

Linear mode images of the 19 holes-core liquid-filled PCF, filled with a mixture of toluene and ethanol in a fraction of 4:1. (a) Mode image of the fiber illuminated by a halogen lamp. The input spectrum is shown in the inset. The measured mode images of the fundamental supermode at a wavelength of 550 nm (b) and a higher-order mode at 1200 nm (c). The illumination is carried out using 10 nm wide filtered supercontinua from a tapered fiber pumped by a Ti:sapphire oscillator. The corresponding mode images of the fundamental supermode simulated by FEM for 500 nm (d) and 1200 nm (e)

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Figure 5 shows the simulated behavior of the effective mode area of the fundamental supermode guided by our 19 strand liquid-filled PCF with respect to mixing ratio (Fig. 5(a)) and temperature (Fig. 5(b)). We simulated a temperature range from −10∘C to 25∘C and mixtures of toluene and ethanol with fractions in the range from 0.775 to 0.85. The wavelength-dependent evolution of the mode image for our 19-liquid-strand PCF, filled with toluene and ethanol in a fraction of 4:1 is shown as insets in Fig. 5(a). A steep rise of the mode area occurs as soon as the evanescent field of neighboring liquid-filled strands starts coupling. With our exemplary device an effective mode area of up to 317 μm2 can be achieved. The steep slope in the effective mode area can be shifted and also flattened by either the temperature or the mixing ratio or even both. Increasing the mixing ratio, i.e., raising the amount of toluene causes an increase in the refractive index which on the other hand degrades the coupling strength and therefore the effective mode area. Warming of our device causes a decrease of the refractive index of the liquid and, therefore, a stronger coupling between the liquid strands. The characteristics of the effective mode area nicely shows the transition from isolated to coupled spatial modes, because there is a drastic rise in the mode area, when the coupling between the single liquid-filled strands becomes stronger. The tunability of the effective mode area by the mixing ratio and the temperature of our selectively liquid-filled PCF can also be interesting for nonlinear applications, because the nonlinear coefficient is dependent on both the effective mode area and the nonlinear refractive index that is in liquids up to 200 times higher than in fused silica.

Fig. 5
figure 5

Simulated effective mode area behavior of the 19 strand liquid-filled PCF, filled with mixture of toluene and ethanol as a function of wavelength. The mixing ratio ϕ(toluene:ethanol) (a) and the temperature T (b) allow for tailoring the effective mode area as well as the dispersion properties

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Figure 6 shows how the dispersion parameter D of our selectively liquid-filled PCF varies with the wavelength for different temperatures. The zero dispersion wavelength for the simulated temperatures is in the region between 1.4 μm and 1.7 μm. The behavior of the D-parameter as a function of temperature at a wavelength of 1.55 μm is displayed as inset in Fig. 6. The high thermooptical effect of liquids makes it feasible that amongst the effective mode area the dispersion of the fiber can be also tuned.

Fig. 6
figure 6

Simulated group velocity dispersion (GVD) of the 19 strand liquid-filled PCF, filled with mixture of toluene and ethanol as a function of wavelength. As an inset, the temperature behavior of the GVD at the telecommunication wavelength of 1.55 μm is shown

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5 Conclusion

In conclusion, we presented a novel and versatile fabrication technique for selectively liquid-filled PCFs and demonstrated the strong tunability of such a device by temperature and mixing ratio of different liquids.

For our approach, we adapted a two-photon direct laser writing technique to close any desired structured pattern that is realizable in the microstructured area of a PCF. Subsequently, the post-processed fiber enables the possibility to fill the unblocked holes with any liquid, but also other optical material, for example, gases, liquids, liquid crystals, semiconductors, polymers, or metals.

The high thermooptical coefficients and the mixing of liquids allow for tuning a selectively filled PCF with respect to the optical properties of the device. As an example, we demonstrated a 19-liquid-strand PCF which displayed a strong dependence of the effective mode area on the temperature and the mixing ratio. A steep rise of the mode area between the isolated guiding for short wavelength and the coupled guiding for longer wavelength can be individually tailored. Our method will allow filling with other materials, such as gases, metal, liquid-crystals, polymers, low melting compound glasses, and quantum dots in the future. In nonlinear experiments, the observation of spatiotemporal effects should be feasible.