Raman-based imaging and thermal characterization in near-field laser heating

Micro/nanoparticle induced nearfield laser ultrafocusing and heating has been widely used in laser-assisted nanopatterning and nanolithography to pattern nanoscale features on a large-area substrate. Probing of the temperature, stress, and optical fields induced by the nanoscale nearfield laser heating remains a great challenge since the heating area is very small (~100 nm or less) and not immediately accessible for sensing. Raman scattering method is a promising tool for noncontact temperature and thermal stress measurements. In this work, the first experimental study is reported on nanoscale mapping and thermal probing of particleand fiber-induced thermal, stress, and optical fields by using a single laser for both near-field excitation and Raman probing. The mapping results based on Raman intensity variation, wavenumber shift, and linewidth broadening all give consistent conjugated thermal, stress, and near-field focusing effects with an accuracy of 20 nm (< /26, = 532 nm). Nanoscale mapping of near-field effects of monolayer microparticles, a single microparticle, and a single microfiber demonstrates the strong capacity of such technique. A new strategy has been developed to de-conjugate the effects of temperature, stress, and nearfield focusing from Raman mapping. The temperature rise and stress in the nanoscale heating region is evaluated at different particle diameters and laser energy levels. For stronger laser fluence and larger particle size, the corresponding temperature and stress are higher. With a laser fluence of 3.9×10 W/m and for a single 1.21 m silica particle induced laser heating, the maximum temperature rise and local stress are 58.5 K and 160 MPa, respectively. Experimental results are explained and consistent with three-dimensional high-fidelity xiv optical, thermal, and stress field simulations. Graphene has attracted great research interests owing to its unique mechanical and electronic properties. In its application, graphene is of high possibility to be supported by a bulk substrate in 3-D devices. The knowledge of the interfacial phonon coupling and the energy exchange capacity at graphene/substrate interfaces is critical in the heat dissipation of graphene-based devices. In this work, the interfacial thermal characterization at CVD graphene/Si, epitaxial graphene/SiC, and CVD graphene/glass interfaces are explored. Temperature differences of graphene layers and the adjacent substrates under laser heating are distinguished at the nanoscale simultaneously by Raman spectroscopy. Linewidth broadening yields interfacial thermal resistances of graphene/Si, graphene/SiC, and graphene/glass as 5.4610, 2.2710, and 3.7610 KmW, respectively. The experimental results are much higher than the molecular dynamics simulation results. The high thermal contact resistances indicate poor contact at the interfaces. The wavenumber method reveals consistent results with the linewidth method, suggesting little stress experienced in the graphene. The thermal resistances obtained by intensity methods are significantly smaller than that based on linewidth and wavenumber methods. Light interference at the air layer between graphene and substrate interprets the small thermal resistance values based on intensity, which further proves the rough contact. Strategies are developed to evaluate the increments of separation distances between graphene and substrates after laser heating. AFM images are taken to verify the corrugation of graphene on substrates and the increments of separation distances after heating.