Sub-wavelength temperature probing in near-field laser heating by particles

Development of time-domain differential Raman for transient thermal probing of materials

A novel transient thermal characterization technology is developed based on the principles of transient optical heating and Raman probing: time-domain differential Raman. It employs a square-wave modulated laser of varying duty cycle to realize controlled heating and transient thermal probing. Very well defined extension of the heating time in each measurement changes the temperature evolution profile and the probed temperature field at μs resolution. Using this new technique, the transient thermal response of a tipless Si cantilever is investigated along the length direction. A physical model is developed to reconstruct the Raman spectrum considering the temperature evolution, while taking into account the temperature dependence of the Raman emission. By fitting the variation of the normalized Raman peak intensity, wavenumber, and peak area against the heating time, the thermal diffusivity is determined as 9.17 × 10(-5), 8.14 × 10(-5), and 9.51 × 10(-5) m(2)/s. These results agree well with the reference value of 8.66 × 10(-5) m(2)/s considering the 10% fitting uncertainty. The time-domain differential Raman provides a novel way to introduce transient thermal excitation of materials, probe the thermal response, and measure the thermal diffusivity, all with high accuracy.

Five orders of magnitude reduction in energy coupling across corrugated graphene/substrate interfaces

A normal full-contact graphene/substrate interface has been reported to have a thermal conductance in the order of 10(8) Wm(-2)K(-1). The reported work used a sandwiched structure to probe the interface energy coupling, and the phonon behavior in graphene was significantly altered in an undesirable way. Here, we report an intriguing study of energy coupling across unconstrained graphene/substrate interfaces. Using novel Raman-based dual thermal probing, we directly measured the temperature drop across the few nm gap interface that is subjected to a local heat flow induced by a second laser beam heating. The thermal conductance (Gt) for graphene/Si and graphene/SiO2 interfaces is determined as 183 ± 10 and 266 ± 10 Wm(-2)K(-1). At the graphene/Si interface, Gt is 5 orders of magnitude smaller than that of full interface contact. It reveals the remarkable effect of graphene corrugation on interface energy coupling. The measurement result is elucidated by atomistic modeling of local corrugation and energy exchange. By decoupling of graphene's thermal and mechanical behavior, we obtained the stress-induced Raman shift of graphene at around 0.1 cm(-1) or less, suggesting extremely loose interface mechanical coupling. The interface gap variation is evaluated quantitatively on the basis of corrugation-induced Raman enhancement. The interface gap could change as much as 1.8 nm when the local thermal equilibrium is destroyed.

Thermal probing in single microparticle and microfiber induced near-field laser focusing

Microparticle and microfiber induced near-field laser heating has been widely used in surface nanostructuring. Information about the temperature and stress fields in the nanoscale near-field heating region is imperative for process control and optimization. Probing of this nanoscale temperature, stress, and optical fields remains a great challenge since the heating area is very small (~100 nm or less) and not immediately accessible for sensing. In this work, thermal probing of a single microparticle and microfiber induced near-field focusing on a substrate with laser light is conducted experimentally and interpreted by high-fidelity simulations. The laser (λ = 532 nm) serves as both heating and Raman probing sources. It is very interesting to note that variation of the Raman intensity, wavenumber, and linewidth all can be used to precisely capture the size of the micro-size subject on the substrate. Nanoscale mapping of conjugated optical, thermal, and stress effects, and the de-conjugation of these effects are performed. The effect of the laser fluence on the temperature and stress in the nanoscale heating region is investigated. With laser fluence of 3.9 ×10(9) W/m(2) and for a 1.21 μm silica particle induced laser heating, the maximum temperature rise and local stress are 58.5 K and 160 MPa, respectively. For a 6.24 μm glass fiber, they are 33.0 K and 120 MPa, respectively. Experimental results are explained and consistent with three-dimensional high-fidelity optical, thermal and stress field simulation.

Nanoscale probing of thermal, stress, and optical fields under near-field laser heating

Micro/nanoparticle induced near-field laser ultra-focusing and heating has been widely used in laser-assisted nanopatterning and nanolithography to pattern nanoscale features on a large-area substrate. Knowledge of the temperature and stress in the nanoscale near-field heating region is critical for process control and optimization. At present, probing of the nanoscale temperature, stress, and optical fields remains a great challenge since the heating area is very small (∼100 nm or less) and not immediately accessible for sensing. In this work, we report the first experimental study on nanoscale mapping of particle-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 at a 20 nm resolution (<λ/26, λ = 32 nm). Nanoscale mapping of near-field effects of particles from 1210 down to 160 nm demonstrates the strong capacity of such a technique. By developing a new strategy for physical analysis, we have de-conjugated the effects of temperature, stress, and near-field focusing from the Raman mapping. The temperature rise and stress in the nanoscale heating region is evaluated at different energy levels. High-fidelity electromagnetic and temperature field simulation is conducted to accurately interpret the experimental results.