1. Introduction

Precise knowledge about the absorption of light in liquids is important for many applications, especially within the areas of bioanalytics, medicine and environmental sciences [1–4]. Due to unique properties such as reconfigurability via exchanging and mixing as well as the ability for doping, liquids are progressively employed in photonics, examples of which include tunable photonic elements [5–8] or dye lasers [9]. One emerging research direction is to guide light in the liquid core of an optofluidic waveguide, yielding new applications in areas such as environmental or biomedical spectroscopy [2–4] or nonlinear light generation [10–16]. The latter gained substantial attention during recent times, since liquids can provide large material nonlinearities and sophisticated non-instantaneous temporal response [17].

One key issue for many of the outlined applications is to obtain a sufficiently low optical attenuation. Water, for instance, operates only in a very narrow spectral domain due to strong OH-vibrational resonances for wavelengths > 1 μm, discarding it from applications in the near-infrared (NIR) [18]. Alternatives with moderate attenuation are organic solvents without OH-bonds like benzenes and their derivatives. Even though providing moderate attenuation on the short-wavelength side of the NIR, these solvents exhibit high losses for wavelengths > 1.2 mm due to absorption of vibrational overtones of the carbogen (CH)-unit [18–21]. Such high attenuation is a severe problem in polymer optical fibers and has been massively reduced by deuteration, having led to the development of polymeric systems such as PMMA-d8 [19,22].

Here we present the potential of deuteration to substantially lower the absorption of organic solvents as one promising pathway to circumvent the abovementioned issues. Deuteration relies on replacing the hydrogen atoms in an organic system by heavier deuterium, which imposes higher effective masses on the vibrational oscillations and thus red-shifted as well as less intense fundamental and overtone resonances. In detail, we spectroscopically examine the visible (VIS) and infrared absorption of the deuterated as well as non-deuterated configurations of chloroform, dimethyl sulfoxide (DMSO), toluene and nitrobenzene.

2. Theoretical background of absorption

The absorption of light passing through a homogeneous medium is mathematically described by the Lambert-Beer law, which relates the attenuation to the properties of the surrounding material:

2.1. Physics behind absorption and motivation for deuteration

As a general rule, the absorption features of organic substances result from various CX-bonds (X = H, D, Cl, S, N, …), which are decoupled and possess characteristic vibrational frequencies that are independent from the remaining parts of the molecule [19]. The potential of each of these molecular bonds can be approximated by a Morse-potential, revealing that the fundamental resonance frequency ν₀ is associated with the effective oscillator mass µ and force constant $k({\nu }_0~\sqrt{k/\mu })$. The effective mass includes the molecular masses of both atoms involved and is given by µ = m₁m₂/(m₁ + m₂).

In particular for the carbogen-based solvents, experiments show that the main absorption features in the NIR result from comparably low-weight CH-groups, i.e., from the oscillators with the lowest value of µ. This imposes strong absorption resonances at the long-wavelength side of the IR spectrum, which create strong overtone resonances in the NIR. As a consequence, exchanging the lightweight hydrogen by heavier atoms shifts the fundamental resonances to longer wavelengths, overall reducing NIR overtone absorption.

The most elementary substitution with only minor impact on the chemical properties is deuteration, replacing the hydrogen atoms by deuterium. The effective mass of the CH-bond increases from µ_CH = 0.92m_H (m_H: molecular weight of hydrogen) to µ_CD = 1.71m_H, leading to a red-shift of the fundamental vibration resonances (and overtones) by a factor of ${\nu }_{\text{CD}}/{\nu }_{\text{CH}}=\sqrt{{\mu }_{\text{CH}}/{\mu }_{\text{CD}}}\approx 0.74$.

2.2. Material classification

The various absorption features of the solvents investigated here can be classified into three categories according to the underlying molecular vibrations: (i) stretching modes, (ii) bending modes, and (iii) CH_x-deformations. The first dominates in saturated carbons (i.e., alkanes) such as chloroform (Cl₃ ≡ C − H) and DMSO (H₃ ≡ C – S…), where significant absorption arises from single carbogen group with a fundamental resonance at around 3.33–3.51 μm [19, 20]. The second absorption mechanism strongly appears in benzene derivatives with aromatic ring structure (C₆H₆) such as toluene (C₆H₅CH₃) or nitrobenzene (C₆H₅NO₂), with one hydrogen atom being replaced by a different functional group (toluene: H → methyl CH₃, nitrobenzene: H → nitro NO₂). Here CH-bending vibrations around 11.11 μm to 14.49 μm dominate over CH-stretching resonances due to their total amount (which relates to the six-fold symmetry) and constructive ring twisting. Thirdly, additional absorption bands emerge from CH_x-deformations with resonance wavelengths between 6.9 mm and 8.06 mm (e.g. in DMSO and toluene) resulting from the attached CH₃-units. This classification, which is related to the underlying vibrational mode also applies to the corresponding deuterated substances, with the resonance wavelengths scaled by $\sqrt{{\mu }_{\text{CH}}/{\mu }_{\text{CD}}}\approx 0.74$. Vibrations resulting from molecular bonds with higher weight (e.g., C-Cl or C-S) possess resonances at even longer wavelengths deep within the mid-IR and therefore their respective overtone absorption is typically not visible in the NIR.

3. Measurement setup

All investigated liquids were purchased from Sigma Aldrich with about a factor 2–10 higher expenses for the deuterated derivatives. The information provided in the safety sheets indicated that the deuterated substances reveal no additional toxicity compared to their non-deuterated counterparts; thus no additional precautions exceeding those provided in the sheets (e.g. work under a hood, safety gloves and glasses) were enforced in our experiments. The spectroscopic characterization relies on a setup consisting of a broadband supercontinuum source (NKT Photonics, SuperK COMPACT, 450 nm – 2.4 μm), solvent-filled cuvettes and an optical spectrum analyzer (Instrument Systems, Spectro320). The collimated probe light passed through a solvent-filled quartz cuvette (cuvette lengths: 1, 2, 5, 10, 20, 50, 100 mm) and was free-space coupled into a transport fiber with low OH-content (core diameter 1 mm) for delivery to the spectrometer. Two apertures before and after the samples were used to align the cuvettes in the optical beam. A spectral resolution of 1 nm with a scanning speed of 50 ms/nm was used throughout all measurements. To resolve the various spectral features accurately, appropriate sample lengths were chosen to result in high dynamic ranges. All spectra were normalized to the transmission of a corresponding empty cuvette, taking into account the transmission of the various interfaces of filled and empty cells given by

4. Results

To demonstrate the concept of absorption reduction via deuteration, we have analyzed two non-aromatic solvents (chloroform and DMSO) as well as two benzene derivatives (toluene and nitrobenzene), and compared the deuterated spectra to those of their non-deuterated counterparts.

4.1. Non-aromatic solvents

Visible and NIR absorption in non-aromatic substances mainly results from overtone absorption of the CH-stretching and CH_x-deformation modes, generally giving rise to distinct peaks in the absorption spectra.

As a first example, we investigate chloroform (Fig. 1(a)), where the non-deuterated configuration shows strong absorption in the NIR caused by overtones of the CH-stretching mode (1.15 μm and 1.69 μm). Additional absorption features at 1.41 μm and 1.86 μm arise from CH-deformation resonances. Overtones of C-Cl modes are not visible in the measurements due to the three times higher molecular mass of Cl, shifting the fundamental C-Cl stretching mode far into the mid-IR. The results are in good agreement with [18, 19] and confirm that chloroform is reasonably transparent. Particular in the VIS, losses < 0.2 dB/cm are obtained resulting from the comparably simple chemical structure of this solvent. Even lower absorption is measured for deuterated chloroform (chloroform-d), showing absorption values < 0.1 dB/cm across the entire spectral domain from 450 nm to 1.5 μm. The dominant absorption features are red-shifted by a factor of 1/0.75, being in good agreement with the scaling of the effective oscillator masses. Both peak and base absorption are substantially lower in chloroform-d, with at least one order of magnitude lower peak absorption for wavelengths below 2.2 μm. This highlights chloroform-d as a highly transparent substance and interesting candidate for liquid-core waveguides and spectroscopic applications.

 

Fig. 1 Spectral distribution of visible to near-infrared absorption of two examples of deuterated (red) and non-deuterated (blue) non-aromatic solvents: (a) Chloroform (CHCl₃), and (b) DMSO (C₂H₆SO). The top (bottom) plots show the spectra of the respective liquid in linear (logarithmic) scale. The black arrows in the lower plots indicate the red-shift of the respective overtone absorption features. The semitransparent shading refers to the absolute measurement error imposed by the noise of the spectrometer. See Data File 1, Data File 2, Data File 3 and Data File 4 for underlying values.

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Very similar effects are found for DMSO (Fig. 1(b)). In its natural form, it shows very strong overtone absorptions resulting from CH-stretching vibrations (1.18 μm and 1.70 μm) and CH₃-deformations (1.39 and 1.81 mm). This material is also highly transparent in the VIS (< 0.05 dB/cm), however, due to its more complex molecular structure (two attached methyl (CH₃) groups) DMSO is associated with a substantially higher IR absorption compared to chloroform. C-S and S=O based overtones are not visible, due to the comparably heavier weight of the sulfur atom. The deuterated version of DMSO (DMSO-d6) exhibits at least one order of magnitude lower absorption almost throughout the entire spectral domain of interest, with a wide VIS and NIR transmission window up to 1.4 μm. Again, the red-shift of the overtone resonances agrees with the mass-relation factor of 1/0.75.

4.2. Aromatic hydrocarbons

In addition to stretching- and deformation-based overtones, further absorption features arise in aromatic solvents from CH-bending modes of the benzene ring with fundamental resonance located between 11.11 μm and 14.49 μm [20].

As first benzene derivative we analyze the absorption of toluene (Fig. 2(a)), which shows strong overtones resulting from CH-bending modes (1.14 μm, 1.38 μm, 1.68 μm and 2.14 μm) and from CH-deformations of the methyl group (1.19 μm and 1.75 μm). Even though the CH-stretching resonances have lower overtone orders (and thus are more intense), they are not visible in the absorption spectra due to the overlapping bending resonances resulting from the six-fold symmetry of the benzene ring. No features related to C-C and C=C overtones have been identified because of their weak intensities [20], i.e., the heavy weight of the carbon atoms. In good agreement with Ref. [18], this material shows high transparency in the VIS (0.1 dB/cm) but suffers more strongly from NIR overtone absorptions than non-aromatic systems. The deuterated version of toluene (toluene-d8) has a substantially lower absorption almost across the entire spectral domain of interest with a low-loss transmission band (0.1 dB/cm) ranging from 450 nm to 1.5 μm, resulting from the red-shifted fundamental vibrational resonance. Particularly intriguing in toluene-d8 is the invisibility of the 7^th CH-bending overtone at 1.68 μm, which is highly prominent in its non-deuterated counterpart.

 

Fig. 2 Spectral distribution of visible to near-infrared absorption of two examples of deuterated (red) and non-deuterated (blue) benzene derivatives: (a) Toluene (C₆H₅CH₃), and (b) Nitrobenzene (C₆H₅NO₂). The top (bottom) plots show the spectra in linear (logarithmic) scale. The black arrows in the lower plots indicate the red-shift of the respective overtone absorption features. The semitransparent shading refers to the absolute measurement error imposed by the noise of the spectrometer. See Data File 5, Data File 6, Data File 7 and Data File 8 for underlying values.

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The absorption spectrum of nitrobenzene (Fig. 2(b)) is entirely dominated by overtone absorption of the CH-bending mode (1.13 μm, 1.40 μm, 1.66 μm and 2.14 μm), with a slightly lower absorption than toluene towards longer wavelengths. This can be explained by the exchange of the methyl group by the three times heavier nitro group (NO₂). The increasing absorption for wavelengths below 450 nm can be attributed to electronic absorption of the nitro group, which limits the transmission window more compared to the other investigated solvents. Similar to toluene, deuterated nitrobenzene shows at least one order of magnitude lower NIR absorption due to the red-shifted and less intense CD-bending resonances, leading to losses < 1 dB/cm almost everywhere up to a wavelength of 2.2 μm. The electronic absorption at the blue side of the visible spectrum, however, shifts to longer wavelengths too, making the use of deuterated nitrobenzene as a highly transparent liquid in the visible more restricted.

4.3. State of art comparison

In Fig. 3 the transmission properties of the eight solvents investigated here are compared with a few other important photonic materials, namely two frequently used glasses (fused silica (SiO₂) and arsenic trisulfide (As₂S₃) [23,24]) and water in its deuterated and non-deuterated versions (H₂O and D₂O) [18]. We define the transparency as a transmission greater > 50% over a length of 10 cm (equals to an absorption < 0.3 dB/cm) as a reasonable benchmark figure to indicate good transmission, particularly for spectroscopic and optical waveguide applications. All deuterated solvents substantially outperform their non-deuterated counterparts especially for wavelengths longer than 1550 nm, with the outstanding example of deuterated chloroform-d (CDCl₃) showing extremely broadband IR transmission far beyond 1.55 mm up to the IR-B domain at 2.15 μm.

 

Fig. 3 Comparison of transparency of the eight characterized organic solvents with other important photonic materials (silica (SiO₂), chalcogenide glass (As₂S₃), water (H₂O) and heavy water (D₂O)). The transparency is defined as a transmission > 50% over a length of 10 cm (equals to an absorption < 0.3 dB/cm).

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We believe deuterated solvents to have strong impact in the field of liquid based photonics including optofluidics, liquid-core waveguiding, spectroscopy, and nonlinear light generation. Particularly from the nonlinear optics perspective, being able to employ a broad variety of transparent materials yields a greater flexibility for ultrashort pulse propagation and frequency conversion. For example, the group-velocity dispersion (GVD) and the zero-dispersion wavelength (ZDW) of liquid core waveguides can be tuned and engineered to a great extend via mixing two or more liquids. Mixing liquids furthermore allows controlling the nonlinear response function of the resulting binary fluid, having the potential to enable new types of nonlinear states. For example, exchanging the core liquid of a single waveguide in real-time enables dynamical switching between Raman-based effects (e.g., dominant in carbon tetrachloride (CCl₄) and CHCl₃ [25]) and effects based on molecular reorientation (e.g., dominant in carbon disulfide (CS₂) and C₆H₅NO₂ [26]).

Liquids can also serve as hosts for optical dopants, e.g., for non-centrosymmetric nonlinear dye molecules which can introduce χ⁽²⁾ nonlinearities (e.g., Disperse Red 1 (DR1)) [27, 28]. Due to their large permanent dipole moments, high doping levels of organic nonlinear molecules generally demand strongly polar solvents. It is important to note that the lack of polarity has prevented the use of low-loss liquid materials such as carbon disulfide (CS₂) and carbon tetrachloride (CCl₄) in χ⁽²⁾-based nonlinear optics, creating a great demand for finding materials with low optical loss and high polarity.

In any nonlinear light generation application, the bulk material absorption principally limits the achievable spectral bandwidth. One example of absorption-limited white-light generation has been observed in water-filled hollow-core photonic crystal fibers [12, 29], with the generated bandwidth being terminated by the IR absorption of the water core, having prevented the observation of the interesting nonlinear states (e.g., solitons) in those systems. Another important aspect relates to the low toxicity of DMSO (and DMSO-d6) and the resulting possible biocompatibility, which might identify DMSO and its deuterated counterpart as interesting bio-host alternatives to water with substantially lower absorption in the NIR.

As a result, the improved absorption performance of deuterated solvents appears to be particular relevant for nonlinear light generation and spectroscopy, with the liquids investigated here extending the list of available options for liquid based photonics.

5. Conclusion

The increasing interest in liquids in photonics and spectroscopy requires as-low-as-possible optical absorption to ensure long light-matter interaction and negligible contributions of the host liquid. Organic solvents are interesting candidates for such applications but severely suffer from strong overtone absorptions particularly in the NIR. Here, we present the potential of deuteration to substantially improve the transmission of organic solvents. Deuteration generally relies on replacing the hydrogen atoms of organic molecules by deuterium, imposing red-shifted and less intense fundamental vibration resonances, which strongly reduces the impact of overtone absorptions in the VIS and NIR. By spectroscopically analyzing four solvents and their deuterated derivatives, we showed that the absorption of deuterated organic solvents drops by at least one order of magnitude compared to their non-deuterated counterparts below 1 dB/cm over the entire visible and near-infrared spectral domain. In particular, the deuterated configurations of chloroform and toluene show extremely broadband transmission windows from about 450 nm to 1.5 mm with losses as low as 0.1 dB/cm and below. We believe that deuteration is a useful approach for other organic systems and anticipate application of deuterated solvents in fields such as spectroscopy, bioanalytics, optofluidics and nonlinear optics.