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Abstract
Advances in 2-photon lithography have enabled in-lab production of sub-micron resolution and millimeter scale 3D optical components. The potential complex geometries are well suited to rapid prototyping and production of waveguide structures, interconnects, and waveguide directional couplers, furthering future development and miniaturization of waveguide-based imaging technologies. System alignment is inherent to the 2-photon process, obviating the need for manual assembly and allowing precise micron scale waveguide geometries not possible in traditional fused fiber coupler fabrication. Here we present the use of 2-photon lithography for direct printing of multi-mode waveguide couplers with air cladding and single mode waveguide couplers with uncured liquid photoresin cladding. Experimental results show reproducible coupling which can be modified by selected design parameters.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Fused fiber couplers are passive optical components which split or combine optical signals and are commonly used in a wide range of applications, from telecommunications to biomedical imaging. A fiber coupler functions by placing two fiber cores in close proximity such that when a photon is totally internally reflected, the electromagnetic field of the photon extends beyond the waveguide boundary, called the evanescent wave, and the photon can transfer from one fiber core to the other [1]. The energy exchange is controlled by the length of the coupling region and the distance between the cores. For a multi-mode fiber, the electronic field E of the evanescent wave decays exponentially as the distance increases from the fiber core as given by equations [2] 1 and 2, where z is the distance from the core edge, ${d_p}$ is the penetration depth, ${\theta _i}$ is the contact angle, λ is the wavelength, and n21 = n2/n1.
The amount of light transferred can be controlled by changing fiber proximity and waveguide properties. This can be accomplished by application of some combination of heat and elongating force [3,4]. Two common methods are thermal core expansion and physical tapering. Thermal core expansion involves heating a fiber to elevated temperatures, 2000°C for silica [4], such that the core dopants diffuse into the cladding, enlarging the mode field diameter. Physical tapering also requires elevated temperatures, although less than thermal expansion, and physical force to elongate and decrease the fiber’s diameter.
Both methods require laborious processing, as control of transition characteristics is very important to minimizing losses and maintaining beam quality, and small deviations in design can result in large losses or even catastrophic failure due to localized heating in high power applications [4]. Additionally, alignment with other optical components is critical as sub-micron lateral misalignment or angular misalignment from cleaving can result in drastic losses [4,5]. These limitations make iterative prototyping, reproducible designs, and system integration difficult.
Two photon polymerization (2PP) based 3D printing is a technology enabling creation of three-dimensional structures using a focused laser beam. The process utilizes femtosecond laser pulses to induce non-linear absorption of photons at the focal point, polymerizing a photoresin with sub-micron resolution [6]. It is particularly well suited to the creation of complex geometries that exist within a volume, rather than on a surface, enabling the creation of optical components with unique properties beyond the capabilities of grayscale lithography. 2PP has been shown to fabricate lenses [6–9], waveguides [5,6,9–12], chip level spectrometers [13], and various other optical components or systems. Waveguides especially benefit from direct printing, as a waveguide core with air as cladding can achieve extremely small core to core pitch while preserving intricate architectures, enabling high resolution imaging [12]. 2PP enables an ease of design and speed of iterative development that can greatly benefit the development of micro-optics. Additionally, 2PP enables a high degree of control over design of waveguides as the bulk parts of the waveguide such as cladding and mechanical housing can be optimized for mass, fabrication speed, and meta-mechanical properties [14].
In this paper, we present a novel approach for fabricating high aspect ratio, customizable waveguide directional couplers using 2PP. The ability to directly integrate with other optical components and the high degree of control over form offered by 2PP make it a promising alternative to traditional fabrication methods. We present highly reproducible air cladding multi-mode waveguide couplers and liquid IP-S cladding single mode waveguides. In subsequent sections below, design principles governing 2PP fabrication are explained and then coupler designs are presented. Single mode waveguide coupling is designed using mode solving software. The coupling ratio is measured by launching a HeNe laser into one waveguide, imaging the output, and comparing the intensity values of output of the two waveguides. Finally, we discuss the results and future directions.
2. Design considerations
2PP is an attractive 3D printing technology for optical fabrication as it achieves high resolution and requires no post-processing other than a wash step to produce optical quality surfaces and components. The goal here was to use a commercial printer to fabricate waveguide couplers with accessible processing and designs. However, the format has several inherent limitations such as printing field-of-view (FOV), print speed, and single material printing, as well as process parameters such as print resolution (slicing and hatching size), stitching zone artifacts, and post-processing effects which must be considered when designing a print [6] and are discussed below.
All waveguides described here were printed on the QuantumX 2-photon lithography system (Nanoscribe Gmbh). The system utlizes ultra-short light pulses at 780 nm with a 0.8 NA 25x objective to polymerize a voxel of photoresin [15]. The 25x objective was used in an immersion dip-in laser lithography (DiLL) mode with IP-S photoresin. This configuration is optimized for smooth surfaces and the fabrication of micro-optics, allowing feature design in the 100’s of nanometers and optical quality surface roughness Ra of 10-20 nm . The DiLL mode allows only single material printing, restricting the 2PP fabricated features to a single photoresin. Nanoscribe’s DescribeX software was used to generate the 3D print files. Nanoscribe’s GrayscribeX software enables print voxel modulation for improved surface quality and a reduction of stitching artifacts, but it is currently only available for 2.5D structures [15] and so it could not be used for the 3D architecture of a waveguide coupler. However, 3D voxel modulation has been recently demonstrated [16], and future deployment will improve structure quality and print time while decreasing the time and effort of print optimization.
The 25x objective has a FOV diameter of 700 µm. Any structure which exceeds the xy limits imposed by this FOV will be printed in increments of the largest circumscribed square, i.e. 495 × 495µm2. The objective has a working distance of 380 µm for a total incremental print volume of 495 × 495 × 380µm3. For current system configurations and DescribeX software, print features that cross an xy boundary will have a lateral stitching artifact. While xy artifacts can be mitigated [17], ensuring all print features reside within one xy print section streamlines testing and improves print quality by removing the possibility for lateral or axial stitching artifacts. Thus, all prints were designed to fit within one xy print zone, with one end of the waveguides starting attached to the substrate and rising vertically in the z direction. The maximum print height of the QuantumX is the travel distance of the stage (20 mm) and was not a limitation for this project.
Additionally, any design must account for the post processing. When the print is finished, the stage retracts from the overhead objective, subjecting the print to the viscous photoresin flowing downwards as the meniscus between the objective and the substrate is broken. Then, the print is washed for 20 minutes in SU-8 developer and then 20 seconds in IPA, removing all exposed unpolymerized photoresin and subjecting the print to surface tension forces as the liquids dry. These forces can damage and deform microstructures, requiring a compensating mechanical design or an augmented post-processing procedure [18]. As discussed below, the waveguide structures presented here either mechanically compensate for surface tension forces or protect print features by printing waveguides within a sealed box configuration.
The liquid IP-S photoresin has a refractive index of 1.4874 at 632.8 nm [19], and an index of 1.510 at 632.8 nm when polymerized [19]. Thus, when fabricating a waveguide, the core is formed from polymerized IP-S as it has the highest refractive index available, and the cladding can be air, unpolymerized IP-S, or an externally added epoxy.
Using air as cladding affords a high degree of design flexibility as the large refractive index difference results in an NA of 1 and a maintenance of total internal reflection (TIR) for angles smaller than of 48.56°. However, any method that uses air as cladding exposes the print features to surface tension forces during post processing, requiring waveguide diameters sufficiently large to ensure mechanical stability during processing in order to maintain the precise configurations coupling requires. We have found waveguide diameters of 10 µm are mechanically robust enough to avoid processing damage and serve as good starting point for prototyping and later optimization.
Unpolymerized IP-S can be used as a cladding if the entire structure is printed to be enclosed, which protects internal cladding from being removed during the SU-8 developer wash. Unpolymerized IP-S as cladding resulting in a waveguide NA of 0.2996 and a maintenance of TIR for angles smaller than 11.44°. The protected internal environment enables preservation of small and sensitive print features, as well as much smaller waveguide diameters. The decreased refractive index difference and ability to fabricate small (<5 µm) waveguide diameters enables this method to produce single mode waveguides. Waveguide diameters less than 1.6 µm will have a normalized frequency V number less than 2.4 for wavelengths over 550 nm.
The coupling behavior of single mode waveguides in a traditional parallel configuration can be modeled via the Lumerical Mode Solutions software [20]. The bent multi-mode nature of the air cladding waveguides makes modeling more complex and will be explored in future work. Lumerical’s Finite Difference Mode (FDE) solver calculates the spatial profile of modes by solving Maxwell’s equations on a 2D mesh that spans the simulated waveguides. The power coupling as a function of length, P2, can then be calculated via Eq. (3), where P0 is the initial power in one waveguide, L is the length of the coupling region, $\mathrm{\Delta }n$ is the change in refractive index, and ${\lambda _0}$ is the wavelength (here 632.8 nm) [21].
3. Design
Based on the principles discussed above, two groups of structures were considered: (1) multi-mode air cladding waveguides and (2) single-mode unpolymerized IP-S cladding waveguides.
Multi-mode air cladding waveguides must be mechanically robust, as the waveguides are directly exposed to mechanical forces during processing. A waveguide diameter of 10 µm is sufficient for the waveguide to remain intact (verified experimentally). However, a traditional parallel waveguide coupler configuration with its high aspect ratio, is affected by surface tension leading to deviations from the specified spacings (Fig. 1(a-b)). The deformation can be ameliorated by repeating contact with external supports (Fig. 1(c)). However, the sensitivity of coupling to even nanometer-scale deviations makes the parallel air cladding waveguide coupler format unsuitable due to the random deformation caused by surface tension, as well as the undesirability of external contacts, necessitating investigation of alternative formats.
Fig. 1. Brightfield images of surface tension deformation on parallel waveguides with a 5 µm gap. Waveguides have a diameter of 10 µm and a coupling region length of 1.2 mm. 1 × 1 µm support spacings of (a) 1200 µm, (b) 300 µm, (c) 50 µm. Red arrows denote large waveguide deformation and white denote examples of supports. The scale bar in the lower right of each image represents 40 µm.
The inevitability of surface tension inducing random waveguide contact can be accounted for by specifying waveguide contact with a curving contact region to provide structural stability while avoiding excess external supports. The high resolution cylindrical surface of the waveguides ensures very little of the waveguides are in contact while providing a structurally robust nanometer width coupling region before and after the contact point. This format enables air cladding waveguides that require only routine processing and are mechanically robust.
The base air cladding coupler design features two waveguides which start and end 256 µm apart to facilitate input of light into one waveguide and output imaging for coupling measurements. The large bending angle enabled by air cladding allows the two waveguides to be quickly brought together to form a coupling region. The waveguides start with a 100 µm straight section, then a 45° turn inward for 100 µm, a 25 µm straight section and then a 100 µm 45° turn outward, and a 100 µm straight section (Fig. 2). The coupling region then consists of a variable number of 30° degree half circle turns which come into contact with the opposite waveguide. The number of contact points can be increased to increase coupling between waveguides.
Fig. 2. Schematic of input region of the air cladding coupler. This is followed by a variable contact coupling region and then an output region identical to the input region. (b) angled view of entrance to coupling region and the square 1 × 1 µm support.
As shown in Fig. 3, the design achieves fidelity between the designed Solidworks file and the print. External walls shelter the waveguides during processing and provide structural support to stay upright by meeting the waveguides at the top of the length. The design strategy enables fabrication of high aspect ratio structures, as the tallest structure has a 10 µm waveguide with a 1941 µm length, an aspect ratio of 194.1.
Fig. 3. Waveguide couplers with planned contact. Number above structure denotes number of waveguide contacts. (a) Solidworks models of design. Scale bar in lower left corner is 200 µm. (b) Brightfield images of printed structures with high fidelity to Solidworks structures with no unplanned waveguide contact. The scale bar for each image is 100 µm.
Waveguide structures with four or fewer contacts only require supports at the start and end of the coupling region; however, structures with 6 or more contact points require small 1 × 1 µm supports to center the two waveguides and not bring them into contact with the external walls (Fig. 4).
Fig. 4. Brightfield images of a 6-contact waveguide coupler with and without 1 × 1 µm supports in the center to counteract surface tension during processing. The scale bar in the lower left represents 60 µm.
The output surface and side profile of the multimode waveguide were imaged using an SEM and characterized using a Zygo NewView 5000 (Zygo, CT) white light interferometer (Fig. 5). The output surface was flat, as designed, but the side profile displayed a stair structure with steps of 300 nm (Fig. 5(e)). This is not unexpected as the hatching size selected was 300 nm. The surface roughness is difficult to measure as the 10 µm diameter of the fiber influences the Ra value. However, the Ra of the 10 µm diameter output was 2.8 nm and the Ra of a selected side profile 23 × 1.6 µm portion of straight waveguide was 4.4 nm.
Fig. 5. (a) SEM image of six contact coupler. (b) close look contact and fiber surface profile. (c) 1 × 1 µm support in coupling region. (d) Side profile of multimode waveguide. Staircase step height in (e) is 300 nm.
To investigate the influence of the 1 × 1 µm supports, the four contact coupler was fabricated with no supports, then 2 supports at the entrance and exit of the coupling region (Fig. 2(b)), and then one or two additional supports in the coupling region as with the six contact coupler. The addition of the two supports at the start and end of the coupling region resulted in a loss of 7% of light. However, additional supports inside the coupling region, as with the six contact coupler, resulted in an additional loss of 71% of light.
An additional coupler format was designed to simplify external supports and decrease waveguide bending angles, thus increasing the length of the coupling region. All angles the waveguides experiences were decreased to 10° (Fig. 6). The waveguides start 89.5 µm apart, begin with a 100 µm straight section, then a 50 µm 10° bend inward, a 100 µm straight section, and then a contact region with a 10° turn.
Fig. 6. Images of air cladding waveguides with only 10° bends with (a) 1-contact and (b) 2-contact structure. Microscope slide contact is on the left. Scale bars in the lower left represent 100 µm.
Single mode waveguides require a combination of a small refractive index difference and small waveguide diameter in order to be fabricated with 2PP, rendering air cladding waveguides unsuitable. As discussed above, the entire component can be printed as a sealed structure, thus decreasing the refractive index difference by using liquid IP-S as cladding and enabling small diameter waveguides, as well as the ability to utilize a traditional parallel waveguide configuration.
A diameter of 1.5 µm was chosen as it has a V number of 1.9382 at 632.8 nm, ensuring single mode behavior, and a critical bending radius of 204 µm. The waveguides start 45 µm apart with a 100 µm straight segment, a 50 µm 5° bend inward, a 200 µm straight segment, a 50 µm 5° bend outward, and then a variable coupling region of either 100 µm or 200 µm, and then a 5° bend outward, a 100 µm straight segment, a 5° bend inward and then a 100 µm straight section. The external housing was made to incorporate void areas so that stray light would leave the structure, as the entire structure can act as a waveguide in air (Fig. 7). Additionally, restriction of the volume of liquid IP-S may reduce internal turbulence which could deform waveguides. Application of an external contrast agent at the base of the print, in this case FolkArt 662 acrylic paint, also helps absorb any stray light and improve image contrast.
Fig. 7. (a) Solidworks design of single mode waveguide with a 200 µm coupling region length. (b) Transparent Solidworks image’s to display internal waveguide architecture. (c) SEM image of side profile of single mode coupler with (b) as a close up of the output. (c) top down view. (d) Alternate side profile.
To measure the losses for the two presented waveguides at 632.8 nm, straight waveguides were printed and the intensity was measured for lengths of 300 µm, 400 µm, and 500 µm. The 10 µm diameter multimode waveguides experienced a loss of 19.89% per 100 µm or -0.9631 dB/100 µm and the single mode waveguides experienced a loss of 3.48% per 100 µm or -0.1538 dB/100 µm.
A profile of two straight 1.5 µm diameter polymerized IP-S waveguides in a liquid IP-S cladding was simulated using Lumerical’s Mode solutions software. The effective refractive index difference was calculated for a coupling gap width of 500 nm and 0 nm, $\mathrm{\Delta }n$ = 0.002157 and $\mathrm{\Delta }n$ = 0.005549 respectively (Fig. 8). Then, using Eq. (3), the light transfer as a function of length can be calculated. The coupling length, the length at which the coupling region causes one hundred percent of power to transfer, is 1467 µm for a gap width of 500 nm and 570 µm for 0 nm. The predicted power coupled was calculated for a 500 nm gap with a 200 µm length, 21.25%, and a 0 nm gap with a 100 µm length, 27.20%, or a 200 µm length, 52.35%.
Fig. 8. (a-b) The two coupled modes for the 500 nm gap width coupling region displaying E field intensity. (c) Brightfield image of cross section of the same single mode coupler at the end of the coupling region. The scale bar represents 5µm
4. Methods
4.1 Fabrication procedure
As mentioned previously, Nanoscribe’s QuantumX with a 25x objective in DiLL mode with IP-S was used for all structures. The print parameters used were in line with recommended parameters [17] and were a slicing size of 0.2 µm, a hatching size of 0.3 µm, a laser power of 70 mW, and scanning speed of 100,000 µm/s. All prints were washed for 20 minutes in SU-8 developer, followed by a 20 second rinse in IPA, and left to air dry. Application of the 662 acrylic paint for contrast was performed with a 30 gauge needle by depositing a single droplet at the base of the structure.
4.2 Measurement procedure
Air cladding coupling measurements were performed using the system displayed in Fig. 9. A 5 mW HeNe laser was directed through a 0.6 ND filter, input into a beam expander to fill the NA of the 10x objective which focused the light into the waveguide. The top of the waveguide structure was imaged by a relay system which refocused the image onto the DMK sensor. For the multi-mode waveguides a 4x objective was used as the collimating lens and a 50 mm photographic lens refocused the image resulting in a relay magnification of 1x. To better image the small diameter single mode waveguides, a 20x objective was used as the collimating objective and a 4x objective used to refocus the image resulting in a relay magnification of 5.5x.
Fig. 9. Layout of the optical system used to input light to the waveguide coupler and reimage the waveguide output. (a) System used to image air cladding multimode waveguides. (b) Table listing components. (c) Image of a waveguide coupler illuminated with the laser. (d) Relay system used to image single mode waveguides.
5. Results
The air cladding waveguide couplers with a 10 µm diameter and a 30° contact angle were tested for 1,2,3,4, and 6 contacts (Table 1). As expected, the waveguide couplers experienced increased coupling with increasing number of contacts. A 1-contact waveguide experienced 9.93% coupling and the highest coupling was experienced by the 6-contact coupler with 21.74% coupled. The rate of coupling changes for each contact, which could be due to initial contact causing strong coupling of higher order modes which are preferentially populated due to waveguide bending and then are attenuated with increased length and contact, but further study is warranted. Three samples were tested for the 2-contact and 6-contact and two samples were tested for every other contact. The average standard deviation between prints is ±0.9%.
Table 1. Air cladding coupling ratio
With a reproducible and mechanically robust design established, coupler contact can be held constant at two contacts and waveguide parameters can be varied to affect coupling. Coupling appears to increase with increasing waveguide diameter, with 7.80 ± 0.25% coupled for a 7 µm diameter and 18.37 ± 0.60% for a 15 µm diameter (Fig. 10). This is consistent with the expectation that a larger diameter creates increased surface area for coupling. Two samples each were tested for the 7 µm and 15 µm diameter waveguides.
Fig. 10. Variance of waveguide parameters changes coupling. (a) Coupling increases as waveguide diameter increases. (b) Brightfield image of waveguides couplers with diameters of 7 µm and 15 µm diameter.
The 10° waveguide coupler was fabricated and compared against the 30° coupler in air. As expected from extending the coupling region with a shallower angle, both 1-contact and 2-contact 10° waveguide couplers showed increased coupling in air as compared to the 30° counterparts (Fig. 11). A 1-contact 10° had 17.42 ± 1.0% coupled, and a 2-contact had 29.82 ± 0.39% coupled. Three samples were tested for 1-contact 10° and two were tested for 2-contact.
The drive for greater design freedom unrestricted by surface tension resulted in our investigation of printing waveguides in a sealed box of polymerized IP-S with liquid unpolymerized IP-S as cladding. The protection from surface tension also enables waveguides of a smaller diameter, which, when paired with the decreased refractive index difference between the waveguide core and cladding, enables the fabrication of single mode waveguides. This format has the additional, important utility of being able to be modeled using mode solving software. Three coupling configurations were tested with 1.5 µm diameter waveguides: a 200 µm coupling region length with a 500 nm gap and a 100 µm coupling region length with 0 nm gap, and a 200 µm length region with a 0 nm gap (Table 2). The 200 µm length, 0 nm gap coupling region had one sample within 1.17% of the predicted value, and two samples within 3% or predicted values. The 100 µm coupling region length had two samples within 0.5% of predicted values and one sample 6% off the predicted values. The 200 µm length, 0 nm gap had all three samples within 5% of the predicted values.
Table 2. Single mode waveguide coupling
6. Conclusions
We have demonstrated the design and fabrication of two types of waveguide couplers using 2PP: the fabrication of high aspect ratio air cladding waveguide couplers and liquid IP-S cladding couplers which allow single mode waveguides. Both methods require only routine processing and allow rapid prototyping. Coupling was shown to be affected by several easily adjusted parameters, such as length and width of coupling region, number of contacts in the coupling region, waveguide diameter, and angle of contact. We have demonstrated 3D micro-optics that cannot be easily fabricated with other methods and with coupling reproducibly comparable to commercial directional couplers.
Air cladding waveguides achieve a high level of reproducibility with an average standard deviation of ±0.9, demonstrating the structures’ mechanical stability and overall performance. As discussed with single mode waveguides, nanometer spatial shifts can noticeably affect the coupling, emphasizing the robust mechanical stability and reproducibility of the air cladding waveguides. This could also be adapted for other high aspect ratio structures beyond waveguides to increase mechanical stability while decreasing external scaffolding. The observed changed in coupling rate could be due to initial transfer of higher order modes which are attenuated with increased length and contact. Further study is warranted, especially as modeling is expanded to encompass bent multi-mode waveguide structures.
Liquid IP-S cladding couplers enable fabrication of single mode waveguides, the behavior of which can be modeled and predicted. The observed single mode waveguide coupling is in general agreement with the predicted values. Two of the three 100 µm couplers are within 1% of expected values, and one 200 µm coupler is within 1.17%. The 200 µm coupling region length, 500 nm gap waveguide experienced two lower values of 18%, which can be explained by a waveguide drift of 86 nm for a total gap width of 585 nm, and the 100 µm value of 21% can be explained by a drift of 131 nm. The 200 µm coupling, 0 nm gap deviation can be similarity explained with the same order of drift. The observed drift is not entirely surprising as the 1.5 µm waveguides are extremely delicate and have no supports other than the connection at the coverslip and the top of the structure. Further optimization of these structures could add small supports at points supported by modeling to achieve higher reproducibility of single mode coupling.
Future work will include optimization of waveguide couplers towards applications, such as an integrated OCT, as well as leveraging the 2PP alignment with multi-component printing to ameliorate waveguide insertion losses. Significant potential remains for coupler format optimization as this paper sought to demonstrate high aspect ratio capabilities, but overall size and supports could be greatly reduced. Waveguides could also be printed horizontally along the substrate to reduce the vertical aspect ratio, though print optimization would be required to eliminate xy stitching artifacts. An alternative to direct printing of a solid waveguide, 2PP has been shown to fabricate photonic crystal waveguides with polarization splitting properties with a height of just 210 µm [11]. However, photonic crystal waveguides rely upon micron scale holes which require additional processing to remove unpolymerized resin, especially as length of the waveguide increases.
Continued investigation of liquid IP-S waveguides could improve coupling reproducibility for smaller diameter waveguides. Additionally, the refractive index difference between core and cladding could potentially be adjusted by progressive single photon curing of the entire structure to decrease the difference [19,22]. External coatings or filters can be applied to liquid IP-S couplers to protect from stray UV light polymerizing the sample, although as a photoresin designed for 2PP, the polymerization energy for IP-S is high and not prone to stray light polymerization. Further development of air cladding waveguides could pursue improved mechanical designs, alternative printing strategies, and alternative processing, such as critical point drying to enable small waveguide diameters and broaden the available configurations. Printing material and system parameters will be investigated to optimization waveguide properties such mechanical stability, coupling length, and losses. Coupling in multimode waveguides could additionally be adjusted by the application of an external epoxy to reduce the refractive index difference while protecting from perturbation, necessitating smaller angles waveguides such as the 10° waveguides.
Funding
National Institutes of Health (EB033160).
Acknowledgment
We would like to acknowledge all the members in Tkaczyk lab for the helpful discussions and assistance. Additional thanks to Erin Euliano for help with SEM imaging.
Disclosures
Dr. Tomasz Tkaczyk has financial interests in Attoris LLC focusing on applications and commercialization of hyperspectral imaging technologies.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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