Raman spectroscopic characterization of crater walls formed upon single-shot high energy femtosecond laser irradiation of dimethacrylate polymer doped with plasmonic gold nanorods

The changes in the bonding configuration on the crater walls formed in urethane dimethacrylate based polymer doped with plasmonic gold nanorods upon irradiation with a single-shot high energy femtosecond laser pulse has been studied by Raman spectroscopy. New Raman bands were detected in the 2000-2500 cm-1 region of the Raman spectrum the intensities of which show strong dependence on the concentration of plasmonic nanoparticles and the energy of the laser pulse. Based on model calculations of the Raman frequencies of the polymer these peaks were attributed to carbon-deuterium and nitrogen-deuterium vibrations and their appearance indicates the occurrence of nuclear reactions in the laser field amplified by plasmonic nanoparticles, involving the polymer atoms.


Introduction
The use of high energy short laser pulses is gaining significance in numerous applications.
Localized surface plasmon polaritons (LSPP) can for example be excited efficiently with these pulses even up to very high laser intensities. One of the reasons to do so is that with the help of plasmonic nanoparticles the electromagnetic field of the lasers can be focused to the nanoscale resulting in electromagnetic fields orders of magnitude higher, than that of the laser pulse. Our motivation to explore this amplification effect has been to use these high fields to realize tabletop plasmonic nano-fusion processes [1]. Ti:Sa femtosecond laser pulses have been used to excite LSPPs in a polymer sheet, containing resonant plasmonic gold nanorods, with laser intensities up to a few times 10 17 W/cm 2 . The first step in our studies has been the analysis of the craters formed by individual laser shots in the material [2]. It has been found, that the volume of these craters is always significantly higher in the polymer containing the gold nanoparticles than in the same polymer without them. The volume increase can be attributed only to nuclear processes, and it has been found, that the crater volumes depend linearly on the energy of the laser pulses [2]. Since this energy production has been found to be comparable, or even larger, than that of the energy of the laser pulse, it has been decided to use vibrational spectroscopic method (Raman spectroscopy) to characterize structural changes occurred in the polymeric structure forming the crater walls created upon irradiation with single-shot high-energy femtosecond laser pulse. The irradiation with different pulse energies has been performed on pure polymer and that doped with plasmonic gold nanorods (in two different concentrations) having plasmon resonance at the wavelength of the femtosecond laser, and the Raman measurements were performed on both types of samples.
Raman spectroscopy is widely used to characterize the bonding configuration of different polymeric materials, including dimethacrylates. This method can be used to determine the degree of conversion in these types of polymers, as well, through the changes and ratio of the Raman peak of the C=C double bonds to that of a reference band belonging to a bond not affected by the polymerization [3][4][5]. In addition to the bonds of the polymer frame it allows to study the different C-H and N-H groups as well.

Materials and sample preparation
The photopolymerizable dimethacrylate resin mixture consists of urethane dimethacrylate The preparation method of undoped (UDMA-X) and doped with gold nanorods (UDMA-Au) polymer samples is described in detail elsewhere [6]. In short, the mixture of the UDMA and TEGMA monomers and the photoinitiator (and the gold nanorods) was placed on a glass slide in a thin template allowing to obtain a thin layer of the resin. Then the polymerization mixture was irradiated with a standard dental curing lamp emitting blue light with a constant 3 min exposure time. The obtained samples of disc shape were clear (non-doped polymer) or had pink color (Au doped samples). The gold nanorods were added to the monomer mixture in 2 different concentrations of 0.124 m/m% and 0.182 m/m%, and the corresponding samples are marked as UDMA-Au1 and UDMA-Au2 in this manuscript, respectively.

Laser irradiation experiments
Femtosecond laser illumination was implemented by a Ti:Sapphire-based chirped-pulse two-stage amplifier-laser system (Coherent Hydra) delivering pulses with 40 fs pulse length at 795 nm central wavelength with 10 Hz repetition rate and 25 mJ maximum pulse energy. The sample treatment was achieved by single pulses of different energy that were illuminating different positions of the sample surface. The beam was focused by a lens with 50 cm focal length, and the sample was shifted laterally after each illumination. The laser irradiation experiments were performed under vacuum conditions for avoiding nonlinear processes in air as for example filamenting. The pressure in the vacuum chamber was in the range of 10 −6 mbar.
Since the energy density is above the ablation threshold of the material, crater formation was observed at the focal spot.

Raman spectroscopic measurements
A Renishaw InVia micro-Raman spectrometer connected to a LeicaDM2700 microscope was used for the Raman spectroscopic measurements. The Raman spectra were recorded on the wall of the formed crater with 532 nm excitation in backscattering geometry, with the laser focused into a spot of ~1.3 microns diameter on the sample surface by using an 50X/0.5 NA objective. The laser power was ~6 mW that equals to 5-10% of the maximum intensity of the laser source. The spectra were recorded in the 1000 -2600 cm -1 spectral region. Due to the low intensity of the investigated Raman bands the accumulation time was set to 2 hours in each spot.
Before the measurements, calibration was done by using a silicon wafer and its characteristic peak at 520 cm -1 Raman shift.
Since this study is about the evaluation and comparison of small intensity Raman peaks recorded on different craters formed upon single-shot laser irradiation, a special care was given to the processing of the measured Raman spectra. The experimental data were processed with the Origin 2019 software and a custom Matlab code. The first step was a background subtraction by using the Asymmetric Least Squares method [7]. Then, the integral intensity of the peaks of interest were determined by determining the area under the curve for the Raman bands observed in the 2000-2500 cm -1 region. In addition, the integral intensity of the C-C peak at 1460 cm -1 was also determined. This Raman band used for the normalization of the Raman intensities of the other peaks in the 2000-2500 cm -1 region.

2.4.Modeling and calculations of selectively deuterized UDMA monomer
The chemical structure of urethane-dimethacrylate (UDMA) monomer is shown in   The self-consistent density functional theory (DFT) field method using the hybrid B3LYP functional consisting of a linear combination of the pure corrected exchange functional by Becke [8] and the three-parameter gradient-corrected correlation functional by Lee et al. [9] was applied for geometry optimization of UDMA model and Raman spectra calculations using the Gaussian-09 program package [10]. The triple zeta valence (TZV) Pople 6-311++G(d,p) basis set was used for all atoms [11]. The total energy of the gas-phase model was optimized using the Berny optimization algorithm. The vibrational frequencies of model calculated using the same method and basis set verified the optimized geometry as true energy minimum structure. The Raman activities of the vibrational modes of UDMA model were also calculated. To study the effect of selective deurerization on the Raman spectra of UDMA model, the C1H2, C2H2, C3H2, C4H2 and N1H groups (hereafter denoted as C1, C2, C3, C4 and N1 sites) were used for hydrogento-deuterium substitution ( Figure 2). The optimized structure of UDMA models were used for further vibrational mode frequency calculations of selectively deuterated structures. For this reason the exact mass of the deuterium isotope (2.01410 a.m.u.) was used for hydrogen atoms bonded to the selected site (centre).
To simulate the Raman spectra of the model a Lorentz-shape function with the intensity proportional to the calculated Raman activity and with full width at half-height (FWHH) of 20 cm -1 , modeling the natural band width of the experimental spectra, was applied for each of the computed Raman modes. Figure 3 shows the Raman spectra recorded on several different craters formed upon singleshot irradiation of the UDMA polymer with and without gold nanorods with laser pulses of different energy in the 1950-2600 cm -1 spectral region. While both groups of spectra show a relatively strong photoluminescence background, some well-resolved bands can also be observed in the spectra of the UDMA-Au samples. The two broad peaks have maxima around 2100 cm -1 and 2400 cm -1 and their intensity seems to be dependent on the laser pulse energy. These features are difficult to observe in the spectra of the non-doped samples (however a weak and broad band between 2050 and 2500 cm -1 can be resolved there as well).

Differences in the Raman spectrum of the crater wall in samples with and without gold nanorods
Since the two UDMA samples used for the irradiation experiments were fabricated from the same monomer mixture, and the only difference is the addition of the gold nanorods to the monomer mixture, it can be assumed that the two Raman peaks appearing in the spectra of the UDMA-Au sample upon irradiation are related to the presence of the gold nanoparticles.  The relation between the Raman bands in the 2000-2500 cm -1 region is also supported by the variance in the peak intensity. Since the polymer contains the gold nanoparticles in a relatively small density, the Raman spectra are recorded in areas incorporating gold nanoparticles in different amounts. As a consequence, the band intensities will fluctuate in a broad range. On the other hand, the UDMA-X sample is completely homogeneous, it can be expected that the Raman peak intensity will be the same, independently on the place of the measurementand this can be bond is the least stable from the possible C-C bonds, therefore its formation upon laser irradiation is of small probability. The same is true for the azide group, since the UDMA monomer contains only two nitrogen atoms per monomer, so the conditions are not favorable for the formation of triple nitrogen chains. Formation of nitrile is possible, but this bond has a Raman peak around 2220-2260 cm -1 , which is right in-between the band positions observed in the Raman spectra (see Fig. 3). The different C-D bonds, however, have their Raman peak in the 2000-2200 cm -1 region, which is in good agreement with the position of one of the peaks observed in the Raman spectrum. This assignment suggests that deuterium atoms were formed during the interaction of the ultrashort laser pulse with the polymeric structure, when the local electromagnetic field was enhanced by the plasmonic gold nanorod antennas. These deuterium atoms are formed from hydrogen atoms being attached to the polymer during the reaction or are free atoms that substitute these hydrogens. As a consequence, carbon-deuterium groups form, and the corresponding peak can be detected in the Raman spectrum in the 2000-2200 cm -1 region.
Assuming this origin, the deuterons will be attached not only to the carbon atoms in the UDMA frame, but also to nitrogen. A simple estimation of the shift of the N-H Raman peak position at 3465 cm -1 upon the N-H to N-D substitution gives the latter to be around 2500 cm -1 , which is in good correlation with the position of the second peak observed in the Raman spectrum (see Fig.   2).
In order to verify this hypothesis, density functional theory calculations were performed to determine the Raman peak positions of the different C-D and N-D structural units. Figure 6 shows the spectral region characteristic of C-H and N-H stretching vibrations in the simulated Raman spectra of the four UDMA models (see Fig. 1). The C-H vibrations are in the 2800-300 cm -1 region, while the N-H band is around 3465 cm -1 . The tentative band assignments are given in Table 1. It can be seen that the bands in the 2800-3000 cm -1 region are related to symmetric and asymmetric vibrations of the CH2 group being in different bonding configurations. The NH group has only one vibrational frequency.   (Fig. 2) Table 1. It should be noted that in case of the natural model some of the CH modes are coupled, and for them the frequency change is not monotonic/proportional during H-to-D substitution.

Conclusions
Raman spectroscopic measurements were performed on the crater walls formed in urethane dimethacrylate -triethylene glycol dimethacrylate polymer containing plasmonic gold nanorods upon irradiation with a single-shot high energy femtosecond laser pulses of different energy.
New Raman bands were detected in the 2000-2500 cm -1 region of the Raman spectrum the intensity of which shows strong dependence on the presence/concentration of the plasmonic nanoparticles and the energy of the laser pulse. Based on model calculations of the Raman frequencies of the polymer these peaks were attributed to carbon-deuterium and nitrogendeuterium vibrations and their appearance indicates the occurrence of nuclear processes involving the polymer in the laser field amplified by plasmonic nanoparticles, forming hotspots at their surface. These results at these relatively low laser pulse energies (intensities) are encouraging for our plans to continue our studies at much higher energies to reach economically feasible tabletop nuclear fusion.