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Abstract
Knowledge of the light attenuation by a material is essential for any photonic device, while liquids are insufficiently quantified, especially in the mid-infrared range. Here, we present a quantitative spectroscopic study reporting concrete attenuation values of the regions of low attenuation of selected organic and inorganic solvents up to 20 µm that is useful for light transport and nonlinear frequency conversion. In addition to prominent liquids, the study presents CBrCl3 as a promising candidate for optofluidics. Our study uncovers the potential of these solvents for waveguiding applications at long wavelengths and highlights the importance of careful liquid selection.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Nonlinear optofluidics has gained increased interest in recent years, primarily due to novel and unique optical properties offered by several organic and inorganic solvents, which can lead to unexplored nonlinear effects such as hybrid solitons and exceptionally high pulse-to-pulse correlation [1] or broadband supercontinua [2,3]. These outcomes suggest these liquids to be an integral part of many future nonlinear optical devices, particularly within fields that demand stable light sources with tailored properties, such as biomedical and environmental sciences [4–7].
In fiber optics, liquids are proving to be well suited as core material, offering many benefits. For example, liquid-core optical fibers (LCOFs) can act as promising waveguides for nonlinear light generation [2,8–11] exploiting the unique optical properties of several liquids, such as higher refractive index than silica [12], strong nonlinearity [13,14], increased control over fiber dispersion [15–18], power stability [19] and unique non-instantaneous responses [1,20,21]. In addition to these key features, it is well-known that several solvents, particularly with no C-H bonds, offer wide mid-infrared transparency [22], in contrast to their counterparts i.e. containing C-H bonds, that show high losses at mid-infrared wavelengths due to vibrational resonances of the molecular units [23,24]. Note that for waveguide-related applications such as LCOFs, it is not sufficient to have knowledge on the wavelengths of the absorption lines, but rather requires concrete numbers on the attenuation in the spectral domains of interest.
In light of this, some solvents such as carbondisulfide (CS2) [1,12,16,18,19,25–27], or carbonclorides incl. carbontetrachloride (CCl4) [15,28,29], tetrachloroethylene (C2Cl4) [15,29,30], or bromotrichloroethane (CBrCl3) [31,32] represent suitable candidates for nonlinear light generation in LCOFs, while the currently available literature does not provide quantitative information on the attenuation coefficients in mid-infrared spectral domain.
The aim of this work is to fill these knowledge gaps through presenting quantitative data on the attenuation of selected organic and inorganic solvents in the mid-infrared range in order to assess them for waveguide-based nonlinear frequency conversion applications. The analysis concentrates on specifically analyzing the low attenuation regions (<1 dB/cm) which is achieved by absorption spectroscopic experiments employing very long (up to 48.4 mm) liquid filled cuvettes. The paper is structured as follows: first, several relevant properties of selected organic and inorganic solvents are presented, followed by the attenuation measurement setup and the data analysis. Thereafter, the attenuation measurement results for the liquids mentioned are presented in comparison and are discussed in detail.
2. Selection of liquids
In this work, four solvents (CS2, C2Cl4, CCl4, CBrCl3) are selected based on their suitable properties in the context of mid-infrared nonlinear frequency conversion using LCOFs. As it can be seen from Table 1, the refractive indices of these liquids are higher than that of fused silica (i.e. $n$= 1.419@3 $\mathrm {\mu }$m) [33], allowing them to be used as core materials of silica-based LCOFs. It is worth mentioning that Table 1 additionally summaries the key properties of the selected liquids in the context of LCOFs, including viscosity, vapor pressure, melting and boiling points, all of which suggest these liquids to be suitable candidates for fiber-based applications. In addition to the liquids which are already used for nonlinear light generation using LCOFs (i.e. CS2, C2Cl4, CCl4), CBrCl3 represents an additional potential candidate due to its refractive index being higher than that of silica. Note that further measurements on the material dispersion in mid-infrared and on the nonlinear refractive index are required, which are the part of ongoing work in our group. All liquids were purchased from Sigma Aldrich and were handled in accordance with the safety data sheets provided by the company.
3. Measurement setup and data analysis
The focus of these measurements is to detect low-loss spectral regions ($\leq$ 1 dB/cm) of CS2, CCl4, C2Cl4 and CBrCl3 requiring long path-lengths of the liquid-filled cuvette. The attenuation measurements are conducted using an FTIR-spectrometer (vertex 80v) from Bruker optics. The samples were illuminated with a MIR light source and the spectra were collected with a FTIR room temperature detector, deuterated L-Alanine doped triglycine sulphate (DLaTGS) detector. The spectra were recorded in the spectral range 1.5–25 $\mathrm {\mu }$m with a resolution of $2~\text {cm}^{-1}$, however the data is only presented upto 20 $\mathrm {\mu }$m wavelength due to high losses of zinc selenide at >20 $\mathrm {\mu }$m. A commercially available cuvette from Specac (omni-cell) is used for preparing liquid-filled cuvette samples which are compatible with the holder of the sample compartment of the FTIR allowing for precise and reproducible positioning of the sample in the beam path. The cuvette has a pair of zinc selenide windows which are transparent in the mid-infrared spectral domain [37]. The cuvette offers variable path lengths, which can be adjusted by exchanging the Polytetrafluoroethylene (PTFE) spacers that can have different thicknesses between the zinc selenide windows. The maximum allowed path length of the cuvette provided by the company is 1 mm which is insufficient for our application. Therefore, we have implemented some design changes of the cuvette which allows using thicker PTFE spacers between the windows so that longer path lengths can be achieved (see Fig. 1).
Fig. 1. Design of the length-adjustable cuvette where (a) is the un-assembled cuvette with all the constituent parts, (b) is the the assembled cuvette with short and (c) long spacer.
Figure 1 demonstrates the design of the length-adjustable cuvette used for absorption measurements of liquids. The cuvette consists of a front plate with filling ports and luer plugs allowing for liquid filling using a syringe, two zinc selenide windows at the front and back of the cuvette, PTFE spacers of different thicknesses and a back plate with a cell nest. The back (and front) plate contains neoprene gaskets (PTFE gasket) for a good contact between the plate and the windows avoiding any leakage. Figure 1(a) shows the un-assembled cuvette with all the constituent parts and Fig. 1(b-c) are the assembled liquid-filled cuvettes demonstrating two different path lengths. In the presented experiments we have measured four different path lengths of the liquid-filled cuvettes ($L=$ 6 mm, 10 mm, 29.2 mm, 48.4 mm). As the light beam of the FTIR spectrometer has a large transverse extension and is focused into the center of the sample compartment, it is difficult to avoid any ’cropping’ of the light beam, specially when using longer cuvette. Therefore, an aperture (diameter: 4 mm) was inserted into the beam path to reduce the size of the beam in front of the cuvette.
3.1 Attenuation coefficient
Based on Beer-Lambert law [38], the attenuation coefficient $\alpha _{i}$ of liquid samples is calculated as:
Here, the subscripts $i$ ($= 1, 2, 3$) refer to three different path lengths of the cuvette, ($L_1$ = 10 mm, $L_2$ = 29.2 mm, $L_3 = 48.4~\text {mm}$) $\Delta L_i$ is defined as $\Delta L_{i}=L_i-L_\mathrm {ref}$ where $L_\mathrm {ref}$ is the path length of the shortest liquid-filled cuvette (i.e. $L_\mathrm {ref}=$ 6 mm) which is used as a reference. The empty cuvette is not used as a reference due to higher losses caused by the higher Fresnel reflections at the glass-air interfaces compared to the filled-cuvette where the Fresnel losses are less due to smaller refractive index contrast at the glass-liquid interface. The transmittance $T_i$ is defined as;
where $P_i$ is the transmitted spectral power through the cuvette with three different path lengths and $P_{\mathrm {ref}}$ is that of the reference. $P_{\mathrm {D, ave}}$ refers to the average (of 50 individual measurements) measured spectral powers when the light is completely blocked (see Supplement 1 Fig. S3), i.e., the dark noise of the detector which is subtracted from the measured transmitted power. The attenuation coefficients $\alpha _{i}$ for each path-length of the cuvette are computed using Eq. (1).3.2 Determination of measurement error
The error of the measurements is determined by the propagation of uncertainty method [39] and can be calculated as
In the following, Eq. (4) is used to calculate the standard deviation of the attenuation coefficient $\alpha$ caused by the dark noise of the detector.
Since the light beam is mildly focused in the sample chamber and thus slightly diverges in the cuvette, it is relevant to reveal the impact of beam divergence on the attenuation coefficient. In the Supplement 1, a conservative estimation that is based on considering two selected beams (central beam and beam with the highest divergence angle) is presented. In summary, within the context of the data evaluation used in this work, a negligible influence of beam divergence on the attenuation coefficient was found, which is why this source of error is disregarded in the following. An additional estimation related to error propagation confirms the mentioned fact that the beam divergence can be neglected.
3.3 Data processing based on error analysis
As mentioned earlier, the measurements for each liquid are conducted using three effective path lengths ($\Delta L_{1}$ = 4 mm, $\Delta L_{2}$ = 23.2 mm, $\Delta L_{3}$ = 42.4 mm). Based on measured spectral powers $P_i$, $P_{\mathrm {ref}}$ and $P_{\mathrm {D}}$, the attenuation coefficient $\alpha _i$ and standard deviation $\sigma _{\alpha _{i}}$ are computed using Eqs. (1) and (4) respectively, for each relative cuvette length giving three sets of data for each liquid at any of the wavelength considered. The $\alpha _{i}$ value with the smallest standard deviation $\sigma _{\alpha _{i}}$ is then selected at every wavelength. Hence, the resulting spectrum is a composition of the most reliable measurement values. For example, $\alpha$ computed from cuvette with longer path-length will be selected for low attenuation regions whereas shorter path lengths are considered for the high attenuation regions. Theoretically, more loss values can be measured with this method, unlike the cut-back method, which always uses all loss values at each wavelength, and thus it is not able to handle inappropriate transmittance values.
4. Results
In this section, the experimental results of attenuation measurements of the four selected solvents are presented and discussed. Specifically, Fig. 2(a-d) depicts the spectral distribution of the attenuation coefficient for CS2, C2Cl4, CCl4 and CBrCl3 in linear scale, while Fig. 3(a-d) shows that of in logarithmic scale, from 1.5 $\mathrm {\mu }$m to 20 $\mathrm {\mu }$m (see Supplement 1 Fig. S1 for raw attenuation measurements of CS2 as an example). The gray regions in every plot are considered as ’invalid regions’ due to exceedingly high attenuation that even for smallest cuvette length zero transmission is measured. Note that shorter cuvettes could principally be used (0.5 mm) to resolve these regions, while this is not the focus of this work, as we are only interested in low-attenuation regions that are important from the waveguiding perspective. In the areas of high absorption, the attenuation values exceed those appropriate for a waveguide over centimeter distances and are thus beyond the scope of this study. The bright-colored regions in the plots depict the attenuation coefficients $\alpha$, whereas the dark-colored domains refer to the standard deviations of the attenuation coefficients $\sigma _{\alpha }$. The grey dotted-line indicates a level of attenuation of 1 dB/cm. It can be noted that in every plot the standard deviation increases towards longer wavelengths, which is due to the detector which suffers from higher dark noise at increasing wavelength (see Supplement 1 Fig. S3).
Fig. 2. Attenuation spectra of (a) CS2, (b) C2Cl4, (c) CCl4 and (d) CBrCl3 in linear scale. The bright-colored regions show the attenuation coefficients $\alpha$ as a function of wavelength, while dark-colored regions refer to the standard deviations ($\sigma _{\alpha }$). The horizontal gray dotted-line in each plot emphasizes an attenuation level of 1 dB/cm. The gray regions refer to ’invalid regions’ as no data was recorded in these domains due to exceedingly high attenuation. Note that parts of these regions have been trimmed several times to improve the visibility of the measured data (see Dataset 1 [40]).
Fig. 3. Attenuation spectra of (a) CS2, (b) C2Cl4, (c) CCl4 and (d) CBrCl3 in logarithmic scale. Similar to the previous figures, the bright domains refer to the spectral distribution of the attenuation coefficients $\alpha$, while the dark-colored regions show the standard deviations ($\sigma _{\alpha }$). Grey dotted-lines indicate an attenuation level of 1 dB/cm. Within the gray regions, no data was recorded in these regions due to exceedingly high attenuation. Note that parts have been trimmed several times to improve the visibility of the measured data.
As shown in Fig. 2(a-d), all liquids show specific regions of low attenuation that are separated by a defined number of strong resonances which result from vibrational oscillations of the respective molecule. Note that since our work concentrates on the domains of low damping and not on the vibrational resonances, we have refrained here from identifying the individual transitions and assigning them to specific vibration modes (for more details on that see [22,35]). To quantify the attenuation behavior more precisely with respect to waveguide-related applications, the attenuation data is presented in logarithmic scale in Fig. 3 clearly showing that within the domains of low attenuation, the liquids typically reveal losses in the order of 1 dB/cm while overall increasing towards longer wavelength. The liquids investigated have low losses in the mid-infrared range, which in principle enables waveguide-related applications that employ devices of tens of centimeters. These losses are comparable to typically employed soft glasses such as, tellurite (0.1 dB/m@1.97 $\mathrm {\mu }$m) and chalcogenide (5 dB/m @2 $\mathrm {\mu }$m) [41,42]. For wavelength <5 $\mathrm {\mu }$m, the broadest transmission windows are found for CCl4 and CBrCl3, making them interesting candidates for fiber-related applications that include composite glasses as cladding or address spectral broadening effect that employ ultrafast pump lasers at $\lambda$=1.55$\mathrm {\mu }$m. In addition, we would like to highlight the liquid CBrCl3, which shows broadband transmission while having a larger refractive index ($n$ = 1.506 @ $\lambda$ = 589 nm) in contrast to for instance CCl4. For longer wavelengths (>5 $\mathrm {\mu }$m), the wide transmission bands of CS2 should be highlighted, which can have bandwidths of up to 2 $\mathrm {\mu }$m with losses of the order of $\leq$1 dB/cm and allow for deep mid-IR application. The other liquids investigated mostly reveal a substantial number of resonances in that spectral domain, which makes them difficult to employ in broadband nonlinear frequency conversion schemes. Nevertheless, they can be used in situations where specific narrow-band spectral domains are considered, as for instance done in the context of parametric oscillation. Here we would like to mention the comparable narrow-band resonances of C2Cl4 up to 8 $\mathrm {\mu }$m in that context.
From the application perspective, all liquids investigated reveal a moderate toxicity, which principally allows employing them in fiber-related experiments straightforwardly due to the typically small quantities used [18]. The application potential of the liquids in the context of ultrafast nonlinear frequency conversion can be evaluated on the basis of the quantities shown in Table 1. All liquids have a refractive index higher than that of silica (esp. CS2). This suggests their application in capillary-like silica-based LCFs, preventing the use of complex microstructured optical fibers in contrast to for instance water-based fibers [9]. It can be anticipated that the liquids discussed here can be used for supercontinuum generation in the mid-infrared, while high field concentrations in the core region and ultrashort pulses to reduce the power fraction in the cladding should be considered. It should be noted that liquid mixing provides a unique opportunity to precisely tune the refractive index of the core medium. As shown by the authors [17], mixing allows waveguide dispersion to be tuned very precisely in terms of number of modes supported and group velocity dispersion. Here, we would like to additionally highlight the very different molecular nonlinear refractive indices and molecular fractions. In that context, mixing provides a unique opportunity to tune the contribution of the molecular response of liquids and to achieve selected configurations of specific temporal nonlinear response, which is not feasible with solid-state materials. Furthermore, it is worth mentioning that all values of the thermo-optic coefficient are orders of magnitude higher than that of silica (Table 1), indicating a significant tuning potential regarding nonlinear frequency conversion. Here, we would like to highlight C2Cl4, which allows operation above 100$^{\circ }$C and has a low vapor pressure. All liquids have low viscosity, suggesting straightforward filling of micro-capillaries.
5. Summary
The exact knowledge of the optical attenuation of a material is crucial for any photonic device, and is particularly important in the context of nonlinear frequency conversion and waveguiding, both of which demand propagation across significant distances. In contrast to many solid-state materials, this essential property is not sufficiently quantified for liquids, especially at long wavelength, defining a severe knowledge gap that needs to be bridged in order to make liquids attractive for mid-infrared applications. In this work, we address this issue through a quantitative study of the regions of low attenuation of the several organic and inorganic liquids that are useful within the context of light transportation and nonlinear frequency conversion at mid-infrared wavelength. Through broadband spectroscopic characterization using tailored cuvettes, our study provides solid experimental data with concrete numbers for the optical attenuation of these materials, especially in the low-attenuation regions up to 20 $\mathrm {\mu }$m. Attenuation values are selected based on an error analysis, which allows the attenuation to be determined in broader spectral intervals compared to the commonly used cut-back approach. In addition to the characterization of prominent liquids, the study introduces the liquid CBrCl3 as a promising candidate, which has not been considered so far and especially combines a large refractive index with broadband transmission up to wavelengths of 6 $\mathrm {\mu }$m. Overall, to our knowledge, our study is the first to report measured attenuation values of the selected organic and inorganic solvents at mid-infrared wavelengths. By identifying and quantifying the spectral regions of low attenuation, our study demonstrates the potential of these solvents for waveguiding and especially for fiber-related applications at long wavelengths, as the losses determined are in the range of commonly used mid-infrared materials such as soft glasses. Our study clearly shows the importance of carefully selecting the liquid to be used in order to access the spectral range of interest for the desired application, which is particularly important in the field of liquid-based fiber optics and broadband nonlinear frequency conversion
Funding
BMBF funding program photonics research Germany (13N15464) integrated into the Leibniz Center for Photonics in Infection Research (LPI); Deutsche Forschungsgemeinschaft (259607349/GRK2101, JU3230/1-1, SCHM2655/11-1, SCHM2655/12-1, SCHM2655/3-2).
Acknowledgments
We acknowledge support by the German Research Foundation and the Open Access Publication Fund of the Thueringer Universitaets- und Landesbibliothek Jena Projekt-Nr. 433052568. The LPI initiated by Leibniz institute of Photonic Technology, Leibniz- Hans-Knöll-Institut, Jena University Hospital and Friedrich-Schiller University Jena is part of the BMBF national roadmap for research infrastructures.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available in Dataset 1, Ref. [40].
Supplemental document
See Supplement 1 for supporting content.
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