Revealing the nature of morphological changes in carbon nanotube-polymer saturable absorber under high-power laser irradiation

Composites of single-walled carbon nanotubes (SWNTs) and water-soluble polymers (WSP) are the focus of significant worldwide research due to a number of applications in biotechnology and photonics, particularly for ultrashort pulse generation. Despite the unique possibility of constructing non-linear optical SWNT-WSP composites with controlled optical properties, their thermal degradation threshold and limit of operational power remain unexplored. In this study, we discover the nature of the SWNT-polyvinyl alcohol (PVA) film thermal degradation and evaluate the modification of the composite properties under continuous high-power ultrashort pulse laser operation. Using high-precision optical microscopy and micro-Raman spectroscopy, we have examined SWNT-PVA films before and after continuous laser radiation exposure (up to 40 hours) with a maximum optical fluence of 2.3 mJ·cm−2. We demonstrate that high-intensity laser radiation results in measurable changes in the composition and morphology of the SWNT-PVA film due to efficient heat transfer from SWNTs to the polymer matrix. The saturable absorber modification does not affect the laser operational performance. We anticipate our work to be a starting point for more sophisticated research aimed at the enhancement of SWNT-PVA films fabrication for their operation as reliable saturable absorbers in high-power ultrafast lasers.


X-ray photoelectron spectroscopy (XPS)
X-ray photoelectron spectroscopy was conducted using a Thermofisher ESCALAB 250 electron spectrometer equipped with a hemispherical sector energy analyser. A monochromated Al K α X-ray source was used for analysis at a source excitation Undamaged PVA/SDBS SWNT sample exposed for 7 hours SWNT sample exposed for 2 hours SWNT sample exposed for 17 hours PVA/SDBS exposed for 7 hours 0.5  Figure S1. XPS spectrum of SWNT-PVA sample a) before and b) after laser irradiation with a power density of 0.1 MW·cm −2 for 17 hours; c) Statistical analysis of the composition of laser affected and unaffected SWNT-PVA and pure PVA samples. energy of 15 KeV and emission current of 6 mA. The analyser pass energy was 20 eV with a step size of 0.1 eV and dwell time of 50 ms throughout the experiments. The base pressure within the spectrometer during examinations has been preserved at 5·10 −10 mbar level. This value ensured that all signals recorded are from the sample surface and no contamination has been introduced from the vacuum chamber. The resolution of the instrument is better than 0.4 eV. Curve synthesis was conducted using software embedded in the instruments Avantage data system. Analyses were conducted using an X-ray spot size of 120 µm and, where appropriate, the monochromated X-ray beam is positioned at the centre of a crater. Relative atomic concentrations were calculated from the intensities of the primary photoelectron spectral lines utilising codes incorporated in the instrument data system using Schofield cross sections.
For XPS analysis the size of the X-ray spot used during analysis is 120 µm in diameter, which is much larger than a typical crater. Therefore, the beam spot size was increased up to 250 µm to produce larger crater diameters, whilst preserving the launched optical fluence. The other set of laser ablated SWNT samples was produced at 20 nJ·cm −2 and a power density of 0.1 MW·cm −2 during a maximum of 17 hours.
During XPS analysis we investigated the region corresponding to carbon and its carbon-oxygen and carbon-carbon compounds (Fig. S1a) . During PVA production, vinyl acetate polymer is hydrolysed in the presence of catalysts to replace acetate groups by hydroxyl groups (-OH). However, by controlling the degree of hydrolysis, i.e. setting an amount of acetate groups, one can control water penetration in the resulting PVA matrix 1 . The PVA used has 86.5-89% degree of hydrolysis, which is consistent with XPS measurements. The C 1s satellite peak shows the presence of a carbon ring structure. Si is present as an organic Sisiloxane, which contamination we refer to index matching gel application, to fix the sample between optical fibre connectors. Therefore, its variation can be neglected in the analysis.
The XPS spectra of unaffected samples and samples after laser radiation exposure were compared. The polymer composite and sample with dispersed SWNTs have demonstrated a similar trend of modification. We can conclude that laser modified samples (both SWNT composites and PVA-SDBS) feature significant decreases of the C-O and O-C=O components; as well as an increase of carbon ring structures. However, in the case of SWNT composite one can observe a decrease of C-C component, while in PVA-SDBS sample it increases. No consistent variation of the carbon-oxygen compounds and overall surface composition after high-power laser exposure with a change of power and duration of irradiation has been observed.
The increase of C-satellite intensity in the XPS spectra together with broad photoluminescence discovered in the Raman spectrum of the laser ablated area of the SWNT-PVA sample both justify the formation of polycyclic aromatic hydrocarbons, acting as a robust laser irradiation coating for the SWNT-PVA. The formation of such a coating stabilises the depth and width of the crater during laser exposure.
Unfortunately majority of the signal from SWNT-PVA sample was overshadowed by signal from the index-matching gel, which was used for attaching polymer film onto fibre ferrule edge. XPS demonstrates main features of index gel XPS of organic siloxane at 102 (Si2p), 154 eV (Si2s), carbon at 285 eV and oxygen at 532 eV.Thus XPS cannot provide detailed information on the structural changes on the films surface.

Scanning electron microscopy (SEM)
To perform SEM measurements, samples were coated with a 4nm layer of Ir and analysed using SEM (JEOL 7100F). The samples are quite sensitive to the electron beam. Figure S2 shows clear "burn" mark around the crater as a consequence of imaging at high voltage. Therefore, to prevent film destruction, the energy of the electron beam has been decreased down to 1.5 kV, thus, the sample seemed more stable. Though the resolution of the measurements was not enough due to low energy of the electron beam, the apparent difference in the morphology at the bottom of the crater relative to the top surface can be observed. The top unexposed surface appearing largely featureless (or with some large features Fig. S2c), whereas the bottom of the crater has lots of small-domain size features (Fig. S2b). We cannot say definitively what these features are, but as we do not see any evidence of nanotubes, the observed change in morphology must be related to changes in the polymer (potentially from dehydration, cross-linking, forming PAHs and other carbon-rich structures).