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Abstract
We describe the treatment of new hydrogels with nine different chemical compositions by femtosecond laser writing. The phase change induced in the wavefront when passing through the written areas was measured in all of these materials. The induced phase change is negative, which is attributed to the higher presence of water in the written regions and confirmed via Raman spectroscopy. The largest induced change in a single layer at 100 mm/s was −3.69 waves at 543 nm. These results show a strong dependence of the phase change on the concentration of some components and their molar ratio. We propose that some components are essential for the nonlinear energy absorption (“dopants”), while other components (“quenchers”) are essential in redirecting the absorbed energy to cause chemical reactions that profoundly change the polymer structure.
© 2017 Optical Society of America
1. Introduction
Femtosecond laser technology in the nonlinear absorption regime gives the user the ability to deposit energy inside of a material within a highly localized volume [1]. This highly localized energy deposition can, under certain conditions, result in highly localized changes to the refractive index within the bulk of the material. By carefully controlling the exposed regions, the laser power, and the scanning speed as the scanning takes places, it is possible to create optical devices. This idea has been extensively used in literature for the successful creation of 3D optical storage devices, diffraction gratings, directional couplers, waveplates, among others [1–4].
One of the fields where this technology has been applied is in the field of vision correction. Over the last decade, there has been an effort from different research groups to induce refractive index changes in biocompatible hydrogels [5–11]. Such effort can successfully lead to the writing of visual refractive devices on materials commonly used for contact lenses and/or intraocular lenses [7–11].
Regardless of the application, all femtosecond writing techniques greatly benefit of using materials that yield large refractive index change when the scanning speed is large. Having a large scanning speed allows for a shorter writing time, which is ultimately more cost effective and makes the scale up of the process more favorable.
The refractive index change in a material leads to phase changes induced in the wavefront. The refractive index change inside of a material and the phase change induced in the wavefront are related by the following equation
where Δϕ is the induced phase change, Δn is the induced refractive index change, d is the axial length parallel to the optical axis where the refractive index change takes place, and λ is the wavelength of light [8]. Equation (1) states the phase change in number of waves [8].The ability of inducing one wave of change at the design wavelength is of great utility, as this allows the user to write phase-wrapped structures, like Fresnel lenses, while keeping the monofocality at this wavelength [12]. Given that the phase change induced in a single layer is small and well below one wave, this goal is usually achieved by the writing of several layers [11]. In this context, a “layer” refers to the refractive index modulations induced in the material at a certain depth below the surface of the sample, and induced with a single scan of the laser beam’s focus over the plane defined by that depth. Using this definition for “layer,” inducing a localized refractive index change in a material by scanning two times the laser beam’s focus at a certain depth over the same area should be considered as writing two distinct layers with a layer separation of 0 μm. Being able to write structures with a smaller number of layers brings similar benefits as writing at larger speeds, which is writing optical devices faster, more cost effectively, and with more scalability.
It has been reported extensively in literature that the phase change (or refractive index change) induced will be highly dependent on the writing parameters used, such as the writing speed, the laser beam power, the numerical aperture (NA), the line separation, etcetera [7-8, 11]. The phase change induced will also be a function of the characteristics of the femtosecond laser used to do the writing, such as the pulse rate, pulse width, central wavelength, etcetera [13–16]. Finally, the phase change induced will also be a function of the material properties, such as the material’s water content [5, 16–20] and the material’s chemical composition [6, 9-10, 16, 21].
In this manuscript, we present the results of a study in which we tested the effect of femtosecond laser treatment on hydrogels of different chemical compositions, while keeping everything else constant. Differences in chemical composition resulted in different phase changes induced with femtosecond writing. All tested hydrogels are based on covalently crosslinked copolymers of 2-Hydroxyethyl methacrylate (HEMA). Poly(HEMA) is a biocompatible polymer that is widely used in the manufacturing of contact lenses and intraocular lenses [6, 21–23]. In this study, the studied copolymers of 2-HEMA are made of three different types of compounds. The first type of compound is responsible for forming the backbone of the polymer and accounts for more than 95% of the molar composition in all of the tested hydrogels. The compounds forming the polymer backbone are HEMA, Ethylene glycol dimethacrylate (EGDMA), and Triethylene glycol dimethacrylate (TEGDMA). The second type of compound in these polymers is made of UV-absorbent molecules. The UV-absorbent compounds used in this study introduce UV-absorbent pendant groups to the polymer and they are: 4-Methacryloxy-2-hydroxybenzophenone (MOBP), 7-((2-Acryloyloxy)ethyloxy)coumarin (AEC), and 3-(3-(tert-butyl)-4-hydroxy-5-(5-methoxy-2H-benzo[d] [1–3]triazol-2-yl)phenoxy)propyl methacrylate (BTAM). The last type of compound in the chemical composition of the tested hydrogels is made exclusively of methacrylic acid (MAA), which introduces a pendant carboxylate group to the polymer. MAA was found to help in the femtosecond laser writing by increasing the magnitude of the phase change induced and is probably responsible for increasing the threshold at which the polymer starts to burn or to char in the compositions possessing it. In this manuscript, we will refer to the UV-absorbent compounds as “dopants,” and to the MAA as the “quencher”. We found that the magnitude of the phase change induced, in general, tends to increase with increasing the molar fraction of dopant and quencher and that the presence of both is required for achieving high phase shift in the hydrogel. We also found that MOBP and BTAM greatly enhance the induced phase change, while AEC has a significantly smaller effect. The phase change induced measured in all of the tested materials was negative, and therefore, the induced refractive index change is negative as well.
More significantly, in this study we report the result of having obtained multiple waves of phase change in the visible region in a single layer at scanning speeds of 100 mm per second. In one instance, the magnitude of the phase change induced was larger than three waves of change. Most bodies of work in literature tend to report their results in terms of refractive index change, and common results in glass and polymers vary from 10−4 to 10^−2 [5]. In 2006, Ding et al. reported a refractive index change as high as 0.06 [5]. However, even when the refractive index change has such a high value, this refractive index change tends to happen over a small axial length, which gives the value of d in Eq. (1) a small value. Ding et al. reported a d value of 3 μm, which ultimately leads to a delta phase of 0.28 waves at 633 nm [5]. To the extent of our knowledge, the work here described is the first time a single-layer induced phase change of multiple waves in the visible region is reported.
Furthermore, in the field of femtosecond laser writing of refractive structures, having the ability of inducing large phase changes is only one of the challenges. Another challenge is explaining the physical and/or chemical phenomenon that yields such refractive index change. Depending on the material and on the writing conditions used, the phenomena presented in literature to explain the refractive index change is cross-linking [9-10, 16-17, 24-25], material densification [25–27], color centers [27], phase separation [2, 28], polymer unzipping [29], and the creation of hydrophobic or hydrophilic functional groups [19-20] (some of these explanations are compatible with each other). Some of these works have made use of Raman spectroscopy to study the femtosecond-laser-written regions to gain an understanding of the material changes that took place in those regions [13, 16-17, 19, 26, 30]. Raman spectroscopy has also been used to quantify water content in hydrogel materials, including pHEMA [31, 32]. As such, we proceeded to take the Raman spectra of the written regions and compare it to the Raman spectra of the unwritten regions. Through the use of Raman spectroscopy, we found out that the written regions exhibit larger water content than the unwritten regions, which is consistent with the negative sign measured for the phase change and refractive index change. Furthermore, we found that the higher the water content in the written region (as indicated by the intensity of the Raman broad peak in the 3420 cm−1 region), the higher the magnitude of the phase change, indicating that some water concentration increase in the written region is directly responsible for the negative phase change observed.
The rest of this manuscript is structured as follows. In section 2, we describe the hydrogels used in this study and their characterization before femtosecond writing. In section 3, we describe the setup used to carry out the femtosecond laser writing, as well as the metrology done on the samples after the writing. Sections four, five, and six are the results, discussion, and summary respectively.
2. Hydrogels used in the study and their characterization
Several hydrogel discs were prepared with nine different chemical compositions. For ease of reference, each different polymer composition was labeled with a different letter, those letters being C, D, E, F, G, H, J, K, and L. Concentrations of dopants (MOBP, BTAM, or AEC) and the quencher (MAA) are given in Table 1. Notice that materials C, D, E, F, G, H, and L are made with the same dopant and quencher, albeit in different quantities.
Table 1. Content of compounds and their molar ratio [in % mol] in different materials
The synthetic procedure occurred as follows. The samples were prepared by thermal polymerization using 2,2’-azobis-2-methyl-propanimidamide dihydrochloride (AAPH), 0.14% w/w, as the initiator. A monomer mixture (210 ± 1 mg), as indicated by Table 1, and the appropriate amount of initiator were weighted into a mold using an analytical balance and covered with a Teflon lid with a 1 mm diameter hole. The Teflon lid reduces the monomer mixture evaporation during thermal polymerization and it simultaneously enables an inert gas inlet. The molds with the monomers were closed into a steel chamber connected to a nitrogen inlet. The system was left to blow through with nitrogen at ambient temperature for 30 minutes. Then, the chamber with samples was inserted into the heater and samples were left to polymerize in an inert atmosphere at 80 °C for 2 hrs. The xerogels were left to cool down for 30 minutes and hydrated according to the procedure described in the next paragraph.
The first step of the hydration process is placing the xerogel in contact with 2 mL of demineralized water for 12 to 24 hours. This transforms the xerogel into a hydrogel by water absorption and facilitates sample removal from the mold. The second step is to put the hydrogel in contact with 2 mL of a solution 99.5% w/w demineralized water and 0.5% w/w NaHCO3 for 12 to 24 hours. This previous step transforms the methacrylic acid into sodium methacrylate in the polymer network. Then, the hydrogel is placed in contact with 2 mL of phosphate-buffered saline (PBS) solution for 3 to 18 hours. PBS is an aqueous solution of 0.830%-wt. NaCl, 0.053%-wt. NaH2PO4·2H2O, and 0.599%-wt. Na2HPO4·12H2O with pH of 7.4 ± 0.1. The last step is repeated 8 more times. The nine PBS exchanges serve for equilibrating the sample in PBS and they also help for removing residual agents from the sample. After this procedure, the sample is stored in PBS, at which point the sample is ready for the experiments.
The refractive index of each sample was measured prior to femtosecond writing using refractometer CLR 12-70 in PBS at 25 °C. Samples were incubated at 25 °C for 24 hours prior to measurement.
Equilibrium water content (EWC) for each hydrogel composition was determined in samples that were incubated at 25 °C for at least 24 hours prior to measurement. The EWC was measured by, first, weighting the sample equilibrated in PBS. Then, the sample was dried at 105 °C for 24 hours to xerogel and weighted again. The water content is calculated by using Eq. (2).
where mh is the mass of the hydrogel and mx is the mass of the xerogel.Thickness of hydrated sample was measured by micrometer on the equilibrated hydrogel sample enclosed between two microscope glasses (with thickness of the two glasses subtracted). The equilibrium water contents, refractive indices, and material thicknesses are summarized in Table 2.
Table 2. Water content, refractive indices, and sample thicknesses for the different materials.
Transmittance spectra of the prepared hydrogels were measured using one beam spectrometer SCINCO S-3100 with a photodiode array detector in range of wavelength from 220 nm to 800 nm. The transmission spectra of all these materials are shown in Fig. 1. Laser spectrum used for writing is shown in Fig. 1(a).
Fig. 1 Transmission spectra for materials (a) with no dopant (H and D), (b) with moderate MOBP concentration (E, C, and L), (c) with high MOBP concentration (F and G), and (d) with alternative dopant (J and K). Refer to Table 2 for sample thicknesses.
3. Setup and methodology
The writing setup is similar to the one we have previously described [8]. A diagram of the femtosecond laser writing system used in this study is shown in Fig. 2(a). The first element of the writing system is a Spectra-Physics Mai Tai HP laser, which produces 100 fs femtosecond pulses with a repetition rate of 80 MHz, a central wavelength of 800 nm, and with an average power of 3.0 W. Directly after the MAI TAI laser in the beam path there is an SHG (Model: UHG-2-FS from Spectra Physics) which doubles the frequency of the beam to 400 nm, which is the wavelength used for carrying out the femtosecond writing. The SHG removes the unconverted 800 nm light from the laser beam. After the SHG in the beam path, there is an attenuator composed of a half-wave plate (HWP) mounted in a rotation mount and a polarizing beamsplitting cube (PBC) directly afterwards. By rotating the half wave plate, the user controls how much power reaches the sample plane with a precision of ± 1.5 mW. Directly after the PBC, there is a Keplerian telescope made of two singlets. The purpose of this Keplerian telescope was to allow the use of an electronic shutter with a small opening to have binary control of the power. The electronic shutter (Uniblitz model LS2T controlled with a Uniblitz model T132 shutter driver) allows blocking the beam or letting it go through in real time as the writing takes place. The beam is then sent to a 2-mirror galvanometer (SC2000 Digital Scanner, GSI Lumonics). The galvanometer is then relayed to the entrance pupil of the microscope objective used to do the writing (40 X, NA 0.60, LUCPlanFLN, Olympus). Relaying the galvanometer mirrors to the entrance pupil of the objective is a necessary step to avoid beam walk at the entrance pupil plane as a function of angle, which would introduce vignetting and a power-dependence as a function of field [8]. The beam optimally filled the microscope objective’ entrance pupil.
Fig. 2 (a) A diagram displaying the femtosecond writing system. (b) Diagram of Mach-Zehnder interferometer used with two wavelengths for phase change measurements.
The sample is placed directly after the microscope objective. The writing takes place with the sample height adjusted so that the focal volume is within the hydrogel, 140 μm below the hydrogel surface closest to the microscope objective. The pulses were measured to have a pulse width of 415 fs FWHM after the microscope objective with a Mini USB TPA-UV autocorrelator by Applied Physics & Electronics Incorporated. During the writing process, the sample is sandwiched between two borosilicate glass coverslips, and the sample is in contact with the PBS solution to ensure the sample is hydrated at all times. The sample is mounted on a kinematic mount used to remove sample tilt with respect to the optical axis. This kinematic mount is attached to a linear motor stage configuration (VP-25XA, Newport Corp.), which allows XYZ control of the coordinates of sample [8]. A CCD camera is used to monitor the obliquely scattered light coming from the sample during the writing process.
The writing takes place by commanding the galvanometer mirrors to move as to create a raster pattern with a line spacing of 0.5 μm, which yields an almost continuous refractive index change [8]. The electronic shutter is synchronized with the galvanometer scanning. By opening and closing the shutter while the galvo raster scanning is taking place, we wrote rectangles of 33 μm by 252 μm dimensions at constant power and speed. We measured the phase change induced in the wavefront by these rectangles for the different chemical compositions using the same exposure parameters in order to test which chemical composition yielded the maximum phase change upon femtosecond writing.
Many rectangles can be written in the same sample, as once the writing of an individual rectangle is finished, we use the motorized stages to move the sample to a new location, where a new rectangle can be written. Five writing conditions were applied to each of the nine different chemical compositions. Those writing conditions were 118 mW, 133 mW, 148 mW, 163 mW, and 178 mW with a speed of 100 mm/s for all of those powers. At least 13 rectangles were written in each material at each writing condition. All of the rectangles written within an individual sample were written in a well-recorded pattern. By writing in a well-recorded pattern, it is possible to know the exposure parameters applied to each rectangle based on their location with respect to fiducial markers and to the other rectangles.
After the writing, a 20X differential interference contrast (DIC) microscope objective mounted in an Olympus BX51 microscope system was used to photograph the written regions and assess the general quality of the writing.
The phase change induced in the wavefront passing through the written regions was measured with a Mach-Zehnder interferometer very similar to the one we described earlier [8]. The Mach-Zehnder interferometer used has identical microscope objectives (40X, NA 0.60, LUCPlanFLN, Olympus) in the sample and reference arms. The written patterns are brought to a plane conjugate to the detector plane. The interferometer made use of two sources: a 633 nm HeNe laser and a 543 nm HeNe laser, and a diagram of it is shown in Fig. 2(b). These two different sources were coaligned and they were used to do multi-wave interferometry. The phase map corresponding to each interferogram was calculated using carrier fringe methods [33]. Therefore, we introduced tilt between the reference and sample arm. The phase in these phase maps was unwrapped with the Goldstein’s branch cut unwrapping algorithm [34-35]. The phase map of the local background corresponding to each rectangle was subtracted [8]. In order to do so, we assumed little to no phase variation across neighboring regions coming from the sample [8]. Then, the phase map from a neighboring region to each rectangle (~200 μm of separation) was measured and taken to be the local background corresponding to each rectangle. As such, four interferograms were measured per each rectangle: the interferogram of the area with the rectangle at 543 and 633 nm and an interferogram of the local background at 543 nm and 633 nm. Subtracting the phase map of the local background removes phase variations introduced by aberrations in the incoming wavefront and phase variations introduced by the interferometer itself due to small differences between both arms of the interferometer [8]. Quantitative measurements of the phase change in the Mach-Zehnder started around 2.5 hours after the femtosecond writing finished for all of the samples.
The sign of the phase change was measured by carrying out the wedge experiment we described earlier [8]. This technique consists of leaving the Mach-Zehnder in the null configuration (removing tilt or any other difference between both arms of the interferometer). Then, the sample is placed under a wedge or any configuration that introduces a linear phase ramp. This wedge introduces tilt fringes in the interferogram. At this point, it is important to note in the interferogram which direction corresponds to the direction of more optical path length. Then, the rectangles are brought into the imaged area. By observing the direction in which the fringes inside of the rectangle jump, one can assign the sign to the phase change induced [8].
Given that we wrote rectangles of almost constant phase change with sharp phase discontinuities with respect to the unwritten background, extra care has to be taken to properly measure the phase change of each rectangle. Interferometry done with a single wavelength cannot be used to get the integer number of wavelengths of phase change in a discontinuity; only the fractional part of the answer is to be relied. For instance, if one measures a phase change of −0.3 waves with a single wavelength, this phase change could correspond to a phase change of −0.3 waves, or −1.3 waves, or −2.3 waves, or −3.3 waves, etcetera. A measured phase change of −0.3 waves could also arise due to a phase change of 0.7 waves, or 1.7 waves, or 2.7 waves, etcetera. However, if the wedge experiment has already been performed and the sign of the phase change induced is known, the phase change alternatives with the wrong sign are discarded.
We followed two independent and different strategies for assigning the integer number of waves to each written rectangle. The first strategy consists of measuring the phase change in small power increments (at the same speed), and then stitching the results in power-space. For instance, if the following results are obtained as the power is increased in small increments: −0.2 λ, −0.4 λ, −0.6 λ, −0.9 λ, and −0.2 λ then the last phase change represents a real value of −1.2 λ. This strategy uses the fact that the magnitude of the induced phase change is known to increase monotonically as the laser power increases. The second strategy is the use of multi-wave interferometry. The idea behind this technique is that the ambiguity in the results can be removed by imaging at two different wavelengths. Given that one wave of phase change implies different optical path lengths for different wavelengths, only the correct selection of the integer number of waves of phase change can be compatible with the phase maps obtained at both wavelengths [36]. The answer picked is the one that makes the phase change at 543 nm to be the closest to a factor of (633/543) larger than the phase change at 633 nm. This approach ignores material dispersion, which has to be present in the modified region. This is not a problem, as it can be shown that the contribution of material dispersion to the ratio of the induced phase change at both wavelengths will be very small compared to the effect to this ratio from an error in the integer number of wavelengths.
An additional aspect of the second strategy for assigning the integer number of waves to each rectangle is as following. The phase change measured in each rectangle is not perfectly constant. The phase change inside of each rectangle has some variations, as shown by the individual pixels inside the obtained phase map. Due to variations within each rectangle, it is possible for the range of possible values to imply phase change combinations for both wavelengths that would result in different answers for the integer number of wavelengths. However, this ambiguity only arises when the phases from the different pixels inside of each rectangle are averaged first at each wavelength and then correlated to the answer obtained at the second wavelength. If individual pairs of the same pixels are compared in the phase map at both wavelengths, this ambiguity disappears. One of the pixels in each pair has to be inside of the written region and the other one has to be outside of the written region. This idea is shown in Fig. 3. This figure shows the phase map retrieved for the same rectangle at both wavelengths. The arrows in the figure point at three pairs of pixels: pixels A1 and B1, pixels A2 & B2, and pixels A3 and B3. By comparing the phase change between the written and unwritten region as indicated by these pair of pixels, only one answer for the integer number of waves satisfies the data given by these pixels pairs. A program was written to carry out these calculations for many pixels pairs automatically.
Fig. 3 (a) Phase map retrieved of region with rectangle written in Material C at 178 mW measured at 543 nm and at (b) 633 nm. The arrows point at three pairs of pixels, illustrating the concept that the phase change, as indicated by single pairs of pixels at the different wavelengths, can be used to unambiguously measure the phase change.
Both strategies for assigning the integer number of waves always yielded the same result for all rectangles, verifying the veracity of both approaches and of the results obtained.
A description of the setup used for collecting the Raman spectra of the different samples is as follows. Raman spectroscopy measurements were carried out using a Renishaw InVia Raman Microscope (Renishaw, Gloucestershire, UK), 488 nm excitation laser, and a 50X, NA 0.5 objective. The spectrometer was calibrated using the silicon F1g peak at 520.2 cm−1.
The Raman spectra were collected in material G (which exhibited the highest phase change) and in a material C (with a moderate phase change) by bringing the focal plane of the microscope objective to the plane of the written rectangle and focusing to obtain subjectively the sharpest image of the written rectangle. Spectra were taken from areas modified by the different laser powers used (i.e. writing power of 118 mW, 133 mW, 148 mW, 163 mW, and 178 mW) and in unmodified reference areas (i.e. writing power of 0 mW). Samples were immersed in PBS solution during the measurements. However, for rectangles written at 178 mW, the Raman spectra were also collected by scanning laterally across the rectangle in the plane of best focus using steps of 3 μm, and collecting the spectra at every step. This scanning experiment was done to directly observe how the Raman spectra change when the focal volume of the microscope objective moves in and out of a modified region.
Relative water content was assessed from ratio of the intensity of peak at 3420 cm−1, corresponding to stretching vibrational modes of the hydrogen bonded OH functional groups inside of water molecules [37], and the intensity of 2945 cm−1 peak, corresponding to stretching vibrational modes of the CH2 functional group [19, 38] and is thus associated with material of the hydrogel. The I3420/I2945 intensity ratio was reported to be correlated with water content in hydrogels [31]. Weak fluorescence background that was present in some spectra was removed by subtraction of a straight line that approximated the background. Raman spectra in this paper were normalized to the intensity of the peak at 2945 cm−1.
4. Results
Figure 4(a) shows a picture of two written rectangles under the DIC mode, while Fig. 4(b) shows a picture of the same rectangles taken under the bright field (i.e. intensity) mode.
Fig. 4 Image of two rectangles written at 400 nm in material L at 178 mW and 100 mm/s taken (a) under the DIC mode and (b) under the bright field mode. The laser dwelled momentarily at the end of the writing process, generating a small burn mark at the inferior left corner of each rectangle. Quantitative phase measurements were carried out using the central part of the rectangles, far from the short side of the rectangles.
Figure 5 shows a picture taken when the wedge experiment was carried out inside of a rectangle written in material L to measure the sign of the phase change. The arrow in this picture points in the direction of the increasing optical path length. The fringes inside of the written rectangle clearly jump in the direction of increasing optical path length. This indicates a negative phase change (and similarly a negative refractive index change) with respect to the bulk of the background, as the fringes need to jump in the direction of higher phase to compensate for the negative phase change acquired inside of the written rectangle. When measuring the magnitude of the phase change, the fringes for all materials and all writing conditions always jumped in the same direction as they did in material L, indicating the phase change to always be negative for all the materials and writing conditions used.
Fig. 5 Interferogram of rectangle in Material L written at 118 mW one month after writing with interferometer in null configuration and a wedge on top of sample. Arrow indicates direction of increasing optical path length.
The measured phase change at 543 nm as a function of power for all of the materials is shown in Fig. 6. As it can be seen, the magnitude of the induced phase change varied dramatically depending on the chemical composition of the material, going as high as 3.69 waves in material G at 178 mW, to as low as 0.01 waves in material H at 118 mW. The phase change measured in material H was so low, that the signal from this material is below the noise floor of our instrument. Material F burned at 148 mW and higher powers, which is the reason why those data points are not included in Fig. 6(b).
Fig. 6 Phase change magnitude measured at 543 nm for the rectangles written at 100 mm/s as a function of writing power for (a) material G, (b) material F, material K, material L, (c) material C, material E, (d) material J, material D, and material H. The error bars in these plots represent the standard deviation of each writing condition measurement.
Figure 7 shows the magnitude of the phase change measured at 133 mW as a function of chemical composition, that is, as a function of their MOBP and MAA molar fraction, for materials C, D, E, F, G, H, and L. This figure suggests that, in general, the higher the molar fraction of both of these components in the chemical composition, the higher the induced phase change obtained under the tested conditions.
Fig. 7 Phase change magnitude measured at 543 nm for the rectangles written at 100 mm/s and 133 mW as a function of MOPB and MAA molar fraction for materials C, D, E, F, G, H, and L.
Figure 8(a) shows the normalized Raman spectra taken from rectangles in material C (with a moderate phase change) while Fig. 8(b) shows the normalized Raman spectra taken from rectangles in material G (with very large phase change). Spectra were also recorded from untreated areas in these materials (i.e. writing power of 0 mW). The ratio of 3420 cm−1 and 2945 cm−1 peak intensities I3420/I2945 that is associated with water content is higher in the written rectangles and becomes progressively large with increasing laser power. Increase in water content is especially pronounced in sample G. Note that the spectra of the unmodified regions in material G shows a larger I3420/I2945 ratio than the unmodified regions in material C, which is consistent with the fact that material G has larger water content than material C, as indicated in Table 2. Material C shows relatively small, but consistent increase in the I3420/I2945 ratio with the writing power.
Fig. 8 Raman spectrum for (a) material C and (b) material G recorded from rectangles written with various laser powers and also from outside of the rectangle. Spectra are normalized to peak 2945 cm−1 corresponding to νasCH2 stretching mode of hydrogel. The broadband centered at about 3420 cm−1 originating from νOH stretching vibration of water is stronger in rectangles written with a higher laser power. The increase in water content is much more prominent in material G, which also shows very high magnitude of phase change.
Raman spectra from the “fingerprint” region of pHEMA, from 500 cm−1 to 1800 cm−1, do not show any significant changes in the written rectangle compared to untreated areas (data not shown). This is consistent with Raman spectroscopy study of Ding et al. on hydrogels who observed no significant changes in characteristic Raman peaks in conditions that resulted in significant change of refractive index of the material but were still below the optical breakdown threshold [17].
The I3420/ I2945 ratio as a function of phase change magnitude in rectangles written with different laser powers is shown in Fig. 9. Notice that both, the phase change magnitude induced and the I3420/ I2945 ratio, increase with higher laser power in both characterized materials. This is consistent with the hypothesis that the phase change is associated with higher water content in the modified region. Water has a lower refractive index compared to the studied hydrogels, thus negative phase change of light passing through the written rectangles is observed.
Fig. 9 I3420/ I2945 ratio plotted as a function of the magnitude of the induced phase change in (a) material C and (b) material G. This ratio is proportional to water content of hydrogel. Higher water content is correlated with a higher magnitude of phase change. Error bars indicate standard deviation.
To demonstrate that the increased water content is confined only to the written rectangles, the Raman spectrum was collected along lateral scans across the width of the rectangles written with power 178 mW. The scan would start outside of a rectangle, continue across the rectangle, and finish on the other side of the rectangle. The Raman spectrum was collected every 3 µm along this lateral scan. From these spectra, the I3420/ I2945 ratio was calculated at each corresponding location. Figure 10 shows the resulting profile in material C and material G. The width of region with higher water content corresponds well to the expected width of written rectangle (~33 µm) and shows that higher water content is confined only to regions that were treated by the femtosecond laser. These results also show potential of Raman spectroscopy for investigation of highly localized changes in water content of hydrogels. It is also possible to use Raman spectroscopy for probing local water content with smaller step size to reveal finer detail.
Fig. 10 I3420/ I2945 ratio from Raman spectra collected across rectangle written using laser power of 178 mW in (a) material C and (b) material G. The width of written rectangles was 33 µm. The profiles show that higher water content, as indicated by I3420/ I2945 ratio, is confined only to areas treated with the laser.
5. Discussion
The fact that both, the I3420/ I2945 ratio and the magnitude of the phase change, increase as the writing power increases, speaks to the fact that increased water content in the written region is responsible for the change in the local refractive index. The negative sign of the phase change further adds validity to this hypothesis. Given this data alone, it is impossible to say what the reason is for the increased water content in the written region. One possible explanation is that the femtosecond writing creates polar hydrophilic functional groups that attract more water towards the written areas [19-20]. Another explanation could be that the writing process starts a depolymerization reaction [29], which could break the polymer into smaller fragments that diffuse out and leave a volume with increased water content. To discern which is the case will be the goal of further investigations. Furthermore, when we measured the sign of the phase change in the materials, which was done one month after the writing had taken place, we also noticed that the magnitude of such phase change had increased slightly relative to the phase change measured a few hours after the writing had finished. This indicates that the phase change induced in the material is, most likely, also a function of the time that has passed after the writing. The measurement of this effect will also be a subject of a future investigation.
As it can be seen in Fig. 6, material K yielded a high phase change, going as high as −1.01 λ at 178 mW when measured with 543 nm light, while material J yielded, at a higher molar concentration of dopant, a relatively much smaller phase change. Therefore, BTAM, similarly as MOBP, should be considered an efficient dopant for enhancing the performance of HEMA-based materials that will be used for femtosecond writing. Given that material J gave a significantly smaller result even at higher dopant concentration, AEC appears not to be an efficient dopant for femtosecond writing at 400 nm. The source of the difference between MOBP and BTAM on one hand and AEC on the other is not clear at the moment since all three compounds are similarly efficient UV absorbers.
Given the experimental data that we obtained, we propose an explanation for the roles played by dopants (MOBP or BTAM) and MAA in the femtosecond writing process. This explanation accurately describes the relative success of the different compositions, as shown in Fig. 6 and Fig. 7. We would like to propose that, when doing femtosecond writing in a hydrogel, there need to be two different active components in the chemical composition of such hydrogel: a dopant and a quencher. We define the dopant to be the component with a high nonlinear absorption cross section, which is responsible for the nonlinear absorption of energy. We define the quencher to be the component that aids in redirecting the energy absorbed into the polymer main chain for the chemical reaction to take place. With these definitions in place, we propose that MOBP and BTAM mainly act as the dopants, while MAA acts mainly as the quencher. Therefore, it stands to reason that a material that has the highest concentration of quencher and dopant will lead to the highest phase change induced, as it was the case with material G. The higher the dopant concentration, the more energy that gets absorbed nonlinearly; and the higher the quencher concentration, the higher the fraction of that absorbed energy that successfully leads to the chemical change leading to a refractive index change.
This dopant/quencher explanation is also useful in explaining other observations such as the lower burning threshold of material F. Material F burned at 148 mW, whereas we could induce phase change without burning in every other material with power as high as 178 mW (which was the highest tested power). This means that, under the tested conditions, all other materials have a burning threshold that is, at least, 20% higher than that one of material F (and it could be much higher than that). Notice that material F has a relatively high dopant (i.e. MOBP) concentration, while it has a much smaller quencher (i.e. MAA) concentration than material G. Therefore, it could be proposed that the high dopant concentration in material F leads to a high amount of energy deposition at the focal volume. However, without a high concentration of quencher, the absorbed energy did not get redirected as efficiently into other parts of the polymer and this energy ended up being “accumulated” where it was absorbed, leading to a lower burning threshold.
Material D yielded a very low phase change. Material D is a material with quencher but no dopant. As such, it is to be understood that the reason why the phase change in such material was low is because having a quencher is of relatively little use if not much energy is being absorbed due to the lack of a good dopant. Material E is a material with dopant and no quencher. Yet, despise having no quencher, material E did give a significantly larger phase change than materials D and H, being −0.29 λ at 178 mW when measured with 543 nm light. This could be interpreted as MOBP acting as a dopant and, to a lesser extent, also as a quencher. This suggests that the same component can fulfill both roles at once, and that even in the materials with MAA and MOBP, both MOBP and MAA act as quenchers, albeit to different extents. Understandably, material H, with no dopant and no quencher, gave the worst results (smallest induced phase changes) among all the materials.
The transmission spectra shown in Fig. 1 shows that the addition of MAA to the composition did little to change the absorption spectra, and that the absorption spectra in the measured region are dominated by the dopant in the composition. This conclusion can be seen by comparing the spectra of material H and D, or the spectra of materials F and G. The pair of materials H and D and the pair of materials F and G are material pairs that have different amounts of MAA and HEMA, but the same amount of MOBP dopant. It seems like having the same amount of the same dopant yields almost identical spectra, as seen by comparing the spectra of materials H and D, or the spectra of F and G. Furthermore, by comparing the spectra of materials with no dopant (H and D) to materials with a dopant (E, C, F, G, L, K, and J), we notice that all dopants used absorb strongly in the ultraviolet section of the electromagnetic spectrum. Although such transmission properties do not guarantee large nonlinear absorption cross-sectional values for MOBP at 400 nm, these spectral characteristics do make such values feasible [21]. These spectral observations agree with the idea that MOBP is acting as the dopant in the femtosecond writing process, and with our proposed dopant/quencher hypothesis. Although the proposed dopant/quencher idea successfully explains the observed results, we emphasize the fact that it is only a hypothesis for the time being. Direct observation and validation of this hypothesis shall be the goal of future investigations.
Additionally, we would like to draw attention to the sheer magnitude of the maximum phase change obtained, which was −3.69 λ at 178 mW in material G when measured with 543 nm light. As far as we know, this phase change is the largest reported in literature in a single layer (i.e. no overwriting). The magnitude of the phase change induced is so large that even if the local refractive index inside of the written rectangles is as low as that of water (1.3333, which would yield the maximum phase change possible), the axial thickness of such affected region would still need to be as large 33.2 μm as calculated from Eq. (1). This is significantly larger than the Rayleigh range of the optical system (about 3 microns) used to write the modified layers. The long axial depth could be due to a variety of effects, such as self-focusing, (by the index of refraction instantaneously increasing before the chemical reaction takes place) [39], a temperature gradient affecting the nonlinear absorption threshold [17], some uncorrected spherical aberration [5], etcetera. To quantify the axial extent of the written region, study the reason behind its long axial extent, and measuring the order of the nonlinearity used (i.e. two-photon process vs. three-photon process vs. four-photon process, etc.), will also be the goal of future investigations. In particular, we would like to point out that the small index of refraction changes induced by Bille et al. require the writing of many layers [11, 19]. Hydrophobic materials such those used in those references cannot take advantage of the large refractive index shifts that we see in strongly hydrophilic materials.
Another point worth mentioning regarding the transmission spectra shown in Fig. 1 is that the transmission spectra is not 100% for the samples at 400 nm (and it is actually very low for material K). This could potentially suggest that some fraction of the energy that is being absorbed linearly could be responsible, rather than a nonlinear process, for the induced phase change observed. This being the case, however, would not be consistent with the data collected as seen in Fig. 3, Fig. 4, Fig. 5, and Fig. 10. As it can be seen in all these figures, the regions of induced change in the xy plane are extremely sharp and well defined. Given that we use a large numerical aperture objective, structures so sharp and well defined can only be the product of a nonlinear process. This idea is shown qualitatively in Fig. 11. Figure 11 shows the volume of a sample that sees the cone of light as the beam scans the sample. A nonlinear process leads to changes only where the intensity is very high, which is at the focal plane or relatively close to it, which leads to a well-localized rectangle in the xy plane. This case is illustrated in Fig. 11(a). A linear process, however, would lead to changes everywhere in the sample that got exposed to the beam, leading to structures much larger and with not sharp discontinuities in the xy plane when seen from the top. This case is illustrated in Fig. 11(b). Given we only see phase change in the xy plane over the area of the exact size that was scanned by the focal spot, the process has to be caused by a nonlinear process. Furthermore, even though axial depth of the induced material change might be large in some cases, it does not extend over the entire sample (as a linear process would entail), as we know by the fact that we need to bring the rectangle to focus to be able to see it.
Fig. 11 (a) Diagram showing the affected volume under a nonlinear process. (b) Diagram showing the affected volume under a linear process.
Another argument against the possibility of the phase change being induced by a linear process is as follows. A linear process would predict material changes and phase changes that are directly proportional to the writing power, with 0 mW of writing power yielding 0 waves of phase change. However, if the phase changes reported in Fig. 6 were to be extrapolated back linearly until the extrapolation hits the x-axis, we would predict the unphysical result of needing to use large writing powers to obtain zero waves of phase change. Similarly, this linear extrapolation would predict large phase changes of the opposite sign at 0 mW of writing power. In order to correctly extrapolate back to 0 mW and obtain 0 waves of phase change at this writing power, it is necessary for the derivative of the phase change as a function of writing power to not be a constant. This type of response is a staple of nonlinear processes.
Finally, we will reiterate the fact that to write phase-wrapped structured such as Fresnel lenses, it is only necessary to induce one wave of phase change. Given that we have already shown the ability to induce almost 4 waves of change at speeds of hundreds of millimeters per second, it might be quite possible to use these materials, or materials with similar chemical composition, to reach one wave of induced phase change at scanning speeds as high as meters per second. At this point, the possibility of mass-producing refractive GRIN lenses created with femtosecond writing becomes much more feasible.
6. Summary
In this manuscript, we studied the effects of chemical composition when trying to induce large phase changes to the wavefront when passing through the femtosecond-laser-written areas. We have tested HEMA-based hydrogels with nine different chemical compositions.
After writing, the Raman spectra were taken from the written regions and compared to the spectra from the non-written regions. We paid particular attention to the I3420/I2945 ratio that can be used to assess water content in hydrogels. We observed that the water content was increased inside of the written rectangles and that a higher magnitude of phase change was correlated with higher water content. This indicates that the higher water content in femtosecond laser treated areas is directly responsible for the change in refractive index in these areas.
Two of the materials tested, K and G, yielded phase changes to the wavefront larger than one wave at 543 nm. Material K yielded −1.01 waves of phase change to the wavefront, while material G yielded −3.69 waves of phase change when measured at 543 nm. This shows the potential of MOBP, MAA, and BTAM in enhancing the material properties of materials to be used for femtosecond writing.
Seven of the nine materials synthesized (C, D, E, F, G, H, and L), had different combinations of MAA and MOBP. Their relative success indicates that, the larger the molar fraction of these chemicals in the chemical composition of the material, the higher the phase change induced. Based on these results, we propose that in femtosecond writing of hydrogels, two components need to be present: a dopant to absorb the energy nonlinearly and a quencher to redirect the absorbed energy into the polymer chain for the chemical reaction leading to a refractive index change to take place. We propose that MOBP acts mainly as the dopant, while MAA acts mainly as the quencher. This explanation can account for the relative success observed in each material, as well as explaining the lower burning threshold in material F. To directly test this hypothesis, as well as carrying out more characterization measurements in the written regions, will be the goal of further investigations. This work increases the feasibility and practicality of mass-producing refractive GRIN lenses created with femtosecond writing.
Funding
Center for Emerging and Innovative Sciences, a New York State-supported (NYSTAR) Center for Advanced Technology (award C090130); National Science Foundation (IIP: 1549700); Clerio Vision, Inc. (award 058149-002).
Acknowledgments
Development of the hydrogels used in this study, their synthesis, processing, and characterization, both before and after the laser treatment, was funded by MEDICEM Institute, s.r.o., part of the MEDICEM Group BV, NL. The authors declare the following interests: V.A. Stoy is Director of MEDICEM Institute, s.r.o., and MEDICEM Group BV, NL, and has a financial interest in a shareholder of MEDICEM Group, BV, that partially supported this research; W. H. Knox, has founder’s equity in Clerio Vision, Inc., which partially supported this research. W. H. Knox has no fiduciary responsibility in Clerio Vision, Inc.
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