Effect of Photoinitiators Doped in PDMS for Femtosecond-Laser Writing: Characterization and Outcomes

Abstract

Herein, we have characterized in depth the effect of femtosecond (fs)-laser writing on various polydimethylsiloxane (PDMS)-based composites. The study combines systematic and nanoscale characterizations for the PDMS blends that include various photoinitiators (organic and inorganic agents) before and after fs-laser writing. The results exhibit that the photoinitiators can dictate the mechanical properties of the PDMS, in which Young's modulus of PDMS composites has higher elasticity. The study illustrates a major improvement in refractive index change by 15 times higher in the case of PDMS/BP-Ge [benzophenone (BP) allytriethylgermane] and Irgacure 184. Additional enhancement was achieved in the optical performance levels of the PDMS composites (the PDMS composites of Irgacure 184/500, BP-Ge, and Ge-ATEG have a relative difference of less than 5% in comparison with pristine PDMS), which are on par with glasses. This insightful study can guide future investigators in choosing photoinitiators for particular applications in photonics and polymer chemistry.

Figure 1

(a) UV–vis absorption spectra of the polymer composites incorporating different photoinitiators. (b) In the visible range for a transmittance spectrum of PDMS and PDMS blends. Note that all the measured samples consist of 1 mm thickness.

Figure 2

Refractive index vs wavelength of the studied materials before writing. A comparison between PDMS composites at five different wavelengths (473, 632.8, 964, 1311, and 1552 nm) was measured by the prism coupling method.

Figure 3

Fs-laser written waveguide. The corresponding refractive index profiles of (a) pristine PDMS with parameters: repetition rate 606 kHz, 8 passes, and 25 mm·s^–1; (b) with parameters: 101 kHz, 8 passes, and 20 mm·s^–1; (c) PDMS/BP with parameters: repetition rate 606 kHz, 2 passes, 5 mm·s^–1, and a near-field optical micrograph of the cross-section of the waveguide; (d) PDMS/Irgacure 184 with parameters: repetition rate 101 kHz, 2 passes, 5 mm·s^–1, and a near-field optical micrograph of the cross-section of the waveguide; and (e) PDMS/BP + Ge(ATEG) with parameters: repetition rate 606 kHz, 8 passes, 15 mm·s^–1, and a near-field optical micrograph of the cross-section of the waveguide.

Figure 4

(a) TGA curves of the polymer composites in air. All measurements were done under a 25 mL min^–1 stream of air, and the samples were heated up to 800 °C, at a heating rate of 10 °C min^–1. (b) DSC curves of the polymer composites and their corresponding T_g thermal events.

Figure 5

SEM images of (a) and (b) PDMS/BP + Ge composite cross-sections showing the laser-inscribed waveguides. (c) EDS mapping of the composite’s cross-section. A higher concentration of carbon and oxygen is noticeable at the waveguides.

Figure 6

AFM of the PDMS/BP + Ge composite. (a) Cross-section topography created during the cryo-fracture, and (b) its respective phase image. (c) Higher magnification of the central section of a waveguide shows the presence of particles of ca. 30–40 nm. (d) Phase image of (c).

Figure 7

(a) Raman spectroscopy of Sylgard-PDMS pristine sample writing grating parameters (repetition rate 606 kHz, 8 passes, and 25 mm·s^–1) at different temperatures in three regions. (b) Raman spectroscopy of PDMS blended with the BP sample with parameters: repetition rate 606 kHz, two passes, and 5 mm·s^–1 at different temperatures in three regions.

Figure 8

Modulation vs writing speed of PDMS/BP + Ge at T_f = 100 days, at two different repetition rates of 606 and 101 kHz based on (a) 2D and (b) 3D presentations combined with the refractive index change (RIU), the velocity of the writing of fs waveguide, and the number of passes.

Figure 9

Schematic route for the fabrication of photosynthesized PDMS composites of the various [A (organometallics) and B (organics)] photoinitiators incorporated in PDMS. (a) Transparent film, (b) fs-laser writing scheme, (c) written sample side by side, and (d) analysis figure.